drifter


DEVELOPMENTAL BIOLOGY
ventral veins lacking is first expressed in stage 10 embryos (after 5 or 6 hours of development). The protein is distributed in a complex pattern including the tracheal system, the middle pair of midline glia, and selected CNS neurons. In the developing head region VVL is observed in the procephalon, clypeolabrum and labrium [Images] (Anderson, 1995).

During imaginal development, vvl is required for cell proliferation and the differentiation of the wing veins (de Celis, 1995).

vvl is expressed in cells fated to be tracheal placodes and wing veins before they undergo differentiation. Its absence prevents tracheal elongation and vein differentiation. Tracheal tree formation occurs by cell migration and cell fusion from the tracheal placodes. Migration of cells from the tracheal placodes is dependent on the breathless (btl) gene, the Drosophila homolog of FGF receptor. The same tracheal phenotype occurs in vvl and btl mutants, suggesting vvl and btl may act through similar downstream effector genes (de Celis, 1995). But the two genes are expressed independently so they appear to function in parallel pathways.

ventral veins lacking is expressed in future wing veins in both dorsal and ventral surfaces before the surfaces make contact. Therefore, vvl would appear to be required in vein formation to define or implement a vein differentiation program (de Celis, 1995). Integrins and laminin A may be involved. The torpedo receptor-tyrosine-kinase/rolled pathway, is involved in this instance in maintaining vvl expression.

From lineage to wiring specificity: POU Domain transcription factors control precise connections of Drosophila olfactory projection neurons

Axonal selection of synaptic partners is generally believed to determine wiring specificity in the nervous system. However, evidence has been found for specific dendritic targeting in the olfactory system of Drosophila: second order olfactory neurons (Projection Neurons) from the anterodorsal (adPN) and lateral (lPN) lineages send their dendrites to stereotypical, intercalating but non-overlapping glomeruli. POU domain transcription factors, Acj6 and Drifter, are expressed in adPNs and lPNs respectively, and are required for their dendritic targeting. Moreover, misexpression of Acj6 in lPNs, or Drifter in adPNs, results in dendritic targeting to glomeruli normally reserved for the other PN lineage. Thus, Acj6 and Drifter translate PN lineage information into distinct dendritic targeting specificity. Acj6 also controls stereotypical axon terminal arborization of PNs in a central target, suggesting that the connectivity of PN axons and dendrites in different brain centers is coordinately regulated (Komiyama, 2003).

Prior to this study of PNs, it was generally believed that synaptic connection specificity is conferred by selection of synaptic partners by presynaptic axons. Systematic lineage analysis strongly suggests that PN dendrites play an active role in establishing connection specificity. Specifically, a given PN's lineage and birth order predicts its glomerular target. However, the position of a given PN's target glomerulus is correlated neither with its neuroblast lineage nor with birth order. Thus, it is unclear how a PN's lineage contributes to its dendritic targeting specificity (Komiyama, 2003).

Molecular genetic evidence is provided that this active dendritic targeting is controlled by transcriptional programs within PNs. The data suggest that the observed dendritic targeting specificity is achieved in two steps: specification of a particular lineage and further intra-lineage specification. The POU domain transcription factors Acj6 and Dfr play critical roles in the first step (Komiyama, 2003).

Several lines of evidence support the idea that Acj6 and Drifter play analogous roles in translating lineage information into dendritic targeting specificity of adPNs and lPNs. (1) Acj6 and Dfr are mutually exclusively expressed in adPNs and lPNs; this lineage-specific expression could be used to regulate the distinct wiring specificity of these two PN lineages. (2) Loss-of-function phenotypes in neuroblast clones demonstrate that Acj6 and Dfr are required for proper dendritic targeting of at least a subset of PNs in their respective lineages. The neuroblast clone phenotypes likely underestimate the requirement of Acj6 or Dfr in PN dendritic targeting. Since each glomerulus is innervated by an average of 3 PNs, it might not be possible to detect inappropriate targeting if 1 or 2 PNs in the same class still innervate the glomerulus properly. This possibility is supported by the study of DL1 PNs. In neuroblast clone analysis, 11 out of 19 acj6-/- clones exhibited no detectable defects in DL1 glomerular innervation; in single-cell clone analysis with a higher resolution, each of the 11 clones showed significant phenotypes. Results from single-cell clone analysis of other PN classes support the generality of the DL1 phenotype -- failure to innervate one specific glomerulus (Komiyama, 2003).

(3) Misexpression of Acj6 in lPNs, or Dfr in adPNs, leads to dendritic targeting defects. In the case of Acj6 misexpression in lPNs, where the phenotypes are stronger (possibly due to a higher ratio of transgene to endogenous Acj6 expression than could be observed for Dfr transgene/endogenous Dfr), there are two qualitatively different mistargeting phenotypes. The first is non-specific accumulation of dendrites in the lateral part of the antennal lobe with associated glomerular organization defects. This phenotype is analogous to the non-specific accumulation of adPN dendrites in the dorsal part of the antennal lobe in acj6-/- adPN clones and may reflect a default response of dendrites deprived of targeting information. The second class of phenotypes is more revealing. In this case, lPN dendrites are mistargeted to well-defined dorsal landmark glomeruli distant from lPN cell bodies and areas of non-specific accumulation. Certain inappropriate glomeruli are specifically targeted, while their neighbors remain uninnervated; this observation argues against the alternative interpretation that misexpression simply causes non-specific dendritic spillover. The specificity of the mistargeting phenotypes caused by misexpression is further supported by the following two observations: (1) overexpression of Acj6 in adPNs, or Dfr in lPNs, never results in any phenotypes; and (2) specific mistargeting is not observed in loss-of-function mutants (Komiyama, 2003).

Taken together, these results strongly suggest that Acj6 and Dfr participate in instructing adPNs and lPNs to innervate a set of glomeruli appropriate to each lineage. At present, it remains probable that other transcription factors act in concert with Acj6 and Dfr to completely specify these lineage-dependent wiring programs. The existence of these other factors -- in addition to the likely underestimation of phenotypes in neuroblast clone analysis, or perdurance in the case of Dfr -- may explain why both loss-of-function and gain-of-function experiments affect only specific subsets of glomeruli (Komiyama, 2003).

It is important to note that Acj6 and Dfr alone cannot specify a particular PN to target its dendrites to a particular glomerulus. All adPNs express Acj6, yet they project their dendrites to a series of different glomeruli according to their birth order. There must be timing factors, probably also transcription factors, which further distinguish PNs within the same lineage based on their birth order. An elegant mechanism to specify different progeny from a common neuroblast has recently been described in the Drosophila embryonic CNS, where neuroblasts exhibit asymmetric cell division patterns similar to those giving rise to PNs. In the embryonic CNS, the neuroblast changes its transcription factor profile as a function of time, thereby specifying the fate of neurons born at different stages. It is suspected that analogous timing factors might exist in PN lineages. These timing factors, in collaboration with lineage-specific factors, will ultimately specify the expression of a repertoire of cell surface molecules that allow PNs to target their dendrites precisely to specific glomeruli (Komiyama, 2003).

Could the same hypothetical timing factors be used in both lineages? This was tested by attempting to switch the DL1 class of adPN to its lPN equivalent by simultaneously removing Acj6 and misexpressing Dfr. If the only differences between the DL1 adPN and its lPN equivalent are the POU domain lineage factors, it might be expected that the DL1 PNs lacking Acj6 but expressing Dfr now would target to a novel glomerulus. These PNs indeed acquire novel features compared to simple loss of Acj6. They no longer even partially innervate DL1. In a subset of clones, their axons also acquired novel branching patterns and terminal fields. However, a clear switch is not observed based both on these dendritic or axonal phenotypes. This could be due to inappropriate level and/or timing of transgene expression; it could also be because: (1) the hypothetical timing factors are not exactly the same in adPNs and lPNs; (2) Acj6 and Dfr are not the only factors distinguishing these two lineages, or (3) cell-cell interaction among PNs from the same lineage may play a role in determining targeting specificity (Komiyama, 2003).

Acj6 is necessary not only for PN dendritic targeting, but also for establishing highly stereotyped PN axon branching patterns and terminal fields in a higher olfactory center. This is best exemplified by the analysis of DL1 single-cell clones. acj6-/- DL1 PNs are defective specifically in the dorsal branch without affecting general axon growth and guidance. This specific phenotype suggests that Acj6 plays a role in selecting synaptic connections with specific third order neurons. Axon terminal arborizations of other classes of PNs are also likely to be regulated by Acj6, as revealed by phenotypes from neuroblast clones containing ~13 classes of adPNs. As for Dfr, there is no evidence from loss-of-function studies that it plays a role in PN axon terminal arborization because there is no equivalent in the lateral lineage to the DL1 PN, which can be unambiguously identified independent of its dendritic innervation. However, the fact that simultaneous loss of Acj6 and gain of Dfr in DL1 clones result in qualitatively different axonal phenotypes compared with simple loss of Acj6 suggests that Dfr also plays a role in regulating axon terminal arborization in the lateral horn (Komiyama, 2003).

These observations bring back the question of why PNs are prespecified to project their dendrites to specific glomeruli and thereby receive specific olfactory input, and to have axons exhibiting specific branching patterns and terminal fields, presumably allowing stereotyped connections with third order neurons. By making PNs genetically distinct at the outset, it is possible to coordinate the dendritic choices of different glomeruli and the specific connections made by axons in higher centers. This coordination may contribute to innate behavioral responses to odorant stimuli by allowing a highly stereotyped relaying of olfactory information from the periphery to higher olfactory centers. Mechanistically, it is possible that PNs use similar cell surface molecules, whose expression depends on specific transcription factors such as Acj6 and Dfr, to guide both dendrites and axons to appropriate targets. The dual Acj6 phenotypes (both axonal and dendritic) provide support for this hypothesis. In ongoing forward genetic screens and candidate tests to identify genes necessary for PN dendritic and axonal connectivity, additional mutants have been found with simultaneous defects in dendritic targeting and axonal arborization (Komiyama, 2003).

In theory, the dual phenotypes in dendrites and axons could be caused by primary defects in dendritic targeting, with axon arborization defects as a secondary consequence, or vice versa. However, two lines of evidence argue against such possibilities: (1) developmental studies indicate that there is not a sequential development of dendritic and axonal arborization; (2) different mutants exhibit different ranges and specificity in their axonal and dendritic phenotypes -- even for individual PNs with the same mutant genotype, there was no clear correlation between the severity of dendritic and axonal phenotypes. The possibility is thus favored that the correct targeting of PN axons and dendrites are both directly regulated events rather than a sequential process in which, for example, the correct targeting of dendrites then instructs the corresponding axonal arborization (Komiyama, 2003).

POU domain transcription factors are used widely in C. elegans, Drosophila, and mammalian development. In particular, classes III and IV POU domain proteins play a variety of important roles in neural development. C. elegans UNC-86, the founding member of the POU IV class, is expressed shortly after asymmetric division in one of the two daughter cells. In unc-86 mutants, the daughter neuroblast that usually expresses UNC-86 now acquires the fate of its parental neuroblast, resulting in reiterations of cell lineage. UNC-86 also regulates differentiation of a number of neuronal classes such as touch sensory neurons or HSN motor neurons. In mammals, 3 class IV and 4 class III POU domain proteins are widely expressed in the nervous system during development. Knockout experiments demonstrate their important functions in different developmental processes. Because there is genetic redundancy between members of the same class, however, phenotypes resulting from single gene knockouts tend to reflect defects in cells that uniquely express that particular POU domain protein (Komiyama, 2003).

Acj6 and Dfr are respectively the single existing members of the class IV and class III POU domain proteins in Drosophila. Both genes have been shown to play a variety of roles in development. In particular, photoreceptor axon targeting is disrupted in acj6 mutants, however this phenotype is not cell autonomous (Acj6 is not expressed in photoreceptors) and is probably due to a requirement for Acj6 in the target lamina neurons. By restricting genetic manipulations to a small subset of neurons with well-defined connection specificity, the requirement of Acj6 and Dfr in other developmental events is bypassed and focus was placed on their function in olfactory projection neurons. This study assigns a new function for POU domain proteins: regulating lineage-dependent wiring specificity down to specific synapse formation. Interestingly, PNs from two lineages utilize two POU domain proteins of different classes for analogous functions. It remains to be seen whether the large number of mammalian POU domain proteins could be used in this way to regulate the specificity of numerous connections necessary to assemble the mammalian nervous system (Komiyama, 2003).

Lastly, Acj6 functions in a subset of ORs to regulate the expression of olfactory receptors; it is possible that it also regulates other molecules including putative ORN axon targeting molecules (which are likely to be distinct from the ORs themselves). The demonstration that Acj6 is necessary for dendritic targeting specificity of a subset of PNs raises an intriguing possibility that Acj6 may regulate matching ORNs and PNs destined to form synaptic connections. In fact, Acj6 is also expressed in a subset of neurons whose cell bodies are located near the lateral horn, one of the two central targets of PN axons. Thus, it is even feasible that Acj6 also regulates matching of synaptic partners in the next olfactory center. Molecular markers and other genetic tools are currently being developed to test these intriguing possibilities (Komiyama, 2003).

Association of tracheal placodes with leg primordia in Drosophila and implications for the origin of insect tracheal systems

Adaptation to diverse habitats has prompted the development of distinct organs in different animals to better exploit their living conditions. This is the case for the respiratory organs of arthropods, ranging from tracheae in terrestrial insects to gills in aquatic crustaceans. Although Drosophila tracheal development has been studied extensively, the origin of the tracheal system has been a long-standing mystery. Tracheal placodes and leg primordia arise from a common pool of cells in Drosophila, with differences in their fate controlled by the activation state of the wingless signalling pathway. Early events that trigger leg specification have been elucidated and it is shown that cryptic appendage primordia are associated with the tracheal placodes even in abdominal segments. The association between tracheal and appendage primordia in Drosophila is reminiscent of the association between gills and appendages in crustaceans. This similarity is strengthened by the finding that homologues of tracheal inducer genes are specifically expressed in the gills of crustaceans. It is concluded that crustacean gills and insect tracheae share a number of features that raise the possibility of an evolutionary relationship between these structures. An evolutionary scenario is proposed that accommodates the available data (Franch-Marro, 2006).

The Drosophila tracheal system has a clearly metameric origin, arising from clusters of cells, on either side of each thoracic and abdominal segment, that express the tracheal inducer genes trachealess (trh) and ventral veinless (vvl). Conversely, the leg precursors can be recognized as clusters of cells that express the Distal-less (Dll) gene, on either side of each thoracic segment; these will give rise both to the Keilin's Organs (KOs, the rudimentary legs of the larvae) and to the three pairs of imaginal discs that will give rise to the legs of the adult fly (Franch-Marro, 2006).

To investigate whether there is a direct physical association between the leg and tracheal primordia, Drosophila embryos co-stained for the expression of trh and early markers of leg primordia were examined. Although Dll is one of the most commonly used markers for the leg primordia, it is not the earliest gene required for their specification. Instead, a couple of related and apparently redundant genes, buttonhead (btd) and Sp1, act upstream of Dll in the specification of these primordia (Estella, 2003). Examining the specification of tracheal cells with respect to btd expression, tracheal cells were observed to appear in close apposition to btd-expressing cells, from the earliest stages of their appearance (by stage 9/early stage 10). Interestingly, unlike Dll, btd is initially expressed both in the thoracic and abdominal segments, and its expression is restricted to the thoracic segments later, under the influence of the BX-C. Thus, the cells of the respiratory system in Drosophila always arise in close proximity to the cells that are fated to give rise to the legs (Franch-Marro, 2006).

To fully endorse this conclusion it is necessary to show that the btd-expressing cells in the abdomen correspond to cryptic leg primordia. This may be a key point because, although many of the genes required for leg development are already known, it has not yet been possible to induce leg development in abdominal segments (except by transforming these segments into thoracic ones). In particular, although the Dll promoter contains BX-C binding sites that repress its expression in the abdominal segments, no ectopic appendage has been reported by misexpressing Dll in the abdomen. These observations have lead to some doubts as to whether a leg developmental program is at all compatible with abdominal segmental identity (Franch-Marro, 2006).

Since the initial expression of btd in the abdominal segments is downregulated by the BX-C genes, it was reasoned that sustained expression of btd might overcome the repressive effect of the BX-C genes and force the induction of leg structures in the abdomen. To test this, a btd-GAL4 driver was used to drive btd expression, expecting that the perdurance of the GAL4/UAS system would ensure a more persistent expression of btd in its endogenous expression domain. No sign was ever obtained of ectopic Dll expression or KOs in the abdominal segments, but the increased expression of btd had an effect on the KOs of the thoracic segments, which had more sensory hairs than the three normally found in wild-type KOs. Thus, on its own, btd seems unable to overcome BX-C repression of leg development (Franch-Marro, 2006).

One possibility would be that the BX-C genes could suppress appendage development in the abdomen by independently repressing both btd and Dll in this region. To assess this possibility, the same btd-GAL4 driver was used to simultaneously induce the expression of both btd and Dll. Under these circumstances, it was observed that KOs develop in otherwise normal abdominal segments; as in the previous experiment, the newly formed KOs have more than three sensory hairs. These results suggest that expression of btd and Dll in the btd-expressing abdominal primordia is sufficient to induce the development of leg structures in the abdomen, overcoming the repressive effect of the BX-C genes. Furthermore, these results demonstrate that these clusters of btd-expressing cells in the abdomen are indeed cryptic leg primordia. These results clearly show that tracheal cells are specified in close proximity to the leg primordia, in both thoracic and abdominal segments (Franch-Marro, 2006).

Previous results have shown that the leg primordia are specified straddling the segmental stripes of wingless (wg) expression in the early embryonic ectoderm, whereas tracheal cells are specified in between these stripes. To investigate whether wg might play a role in determining the fate of these primordia, what happens when the normal pattern of wg expression is disrupted was studied. In wg mutant embryos, trh and vvl from the earliest stages of their expression are no longer restricted to separate clusters of cells; instead larger patches of expression add up to a continuous band of cells running along the anteroposterior axis of the embryo, while btd expression is suppressed in this part of the embryonic ectoderm. Conversely, ubiquitous expression of wg suppresses trh expression, while causing an expansion of btd expression along the embryo. Restricted activation or inactivation of the wg pathway by the expression of a constitutive form of armadillo or a dominant-negative form of dTCF, respectively, are also able to specifically induce or repress trh and btd expression. trh/vvl and btd seem to respond independently to wg signalling and there is no sign of cross-regulation among them, since btd expression is normal in trh vvl double mutants, and trh and vvl expression is normal in mutants for a deficiency uncovering btd and Sp1 (Franch-Marro, 2006).

The role of wg as a repressor of the tracheal fate is further illustrated by looking at the behaviour of transformed cells: the clusters of cells that have lost btd expression and gained trh and vvl expression in wg mutant embryos begin a process of invagination that is characteristic of tracheal cells. Furthermore, these cells also express the dof (stumps) gene, a target gene of both trh and vvl in the tracheal cells. Although further development of these cells is hard to ascertain because of gross abnormalities in wg- embryos, these results indicate that they have been specified as tracheal cells. Thus, wg appears to act as a genetic switch that decides between two mutually exclusive fates in this part of the embryonic ectoderm: the tracheal fate, which is followed in the absence of wg signalling; and the leg fate, which is followed upon activation of the wg pathway. Given that there are no cell lineage restrictions setting apart the cells of the tracheal and leg primordia, these two cell populations could be considered as a single equivalence group, with the differences in their fate controlled by the activation state of the wg signalling pathway (Franch-Marro, 2006).

A link between respiratory organs and appendages is also found in many primitively aquatic arthropods, like crustaceans, where gills typically develop as distinct dorsal branches (or lobes) of appendages called epipods. Following the current observations, which suggest a link between respiratory organs and appendages in Drosophila, whether further similarities could be found between insect tracheal cells and crustacean gills was examined. Specifically, whether homologues of the tracheal inducing genes might have a role in the development of appendage-associated gills in crustaceans was considered (Franch-Marro, 2006).

RT-PCR was used to clone fragments of the vvl and trh homologues from Artemia franciscana and from Parhyale hawaiensis, representing two major divergent groups of crustaceans (members of the branchiopod and malacostracan crustaceans, respectively). In the case of Artemia vvl, a fragment was cloned that corresponds to the APH-1 gene and an antibody was generated for immunochemical staining in developing Artemia larvae. It was observed that Artemia Vvl is initially absent from early limb buds; it becomes weakly and uniformly expressed while the limb is developing its characteristic branching morphology, and becomes strongly upregulated in one of the epipods as its cells begin to differentiate. Uniform weak expression persists in mature limbs, but expression levels in the epipod are always significantly higher. Expression of the trh homologue from Artemia appears to be restricted to the same epipod as Vvl. Similarly, homologues of vvl and trh were cloned from Parhyale hawaiensis and their expression was studied by in situ hybridization. Both genes are specifically expressed in the epipods of developing thoracic appendages. Besides epipods, the Artemia trh and vvl homologues are also expressed in the larval salt gland, an organ with osmoregulatory functions during early larval stages of Artemia development (Franch-Marro, 2006).

What is the significance of the two Drosophila tracheal inducer genes being specifically expressed in crustacean epipods/gills? One possibility is that the expression of these two genes was acquired independently in insect tracheae and in crustacean gills. Alternatively, tracheal systems and gills may have inherited these expression patterns from a common evolutionary precursor, perhaps a respiratory/osmoregulatory structure that was already present in the common ancestors of crustaceans and insects (Franch-Marro, 2006).

The latter possibility is considered unlikely by conventional views, because of the structural differences between gills and tracheae (external versus internal organs, discrete segmental organs versus fused network of tubes), and the difficulty to conceive a smooth transition between these structures. Yet, analogous transformations have occurred during arthropod evolution: tracheae can be organized as large interconnected networks or as isolated entities in each segment (as in some apterygote insects), invagination of external respiratory structures is well documented among groups that have made the transition from aquatic to terrestrial environments (terrestrial crustaceans, spiders and scorpions), and conversely evagination of respiratory surfaces is common in animals that have returned to an aquatic environment (tracheal gills or blood gills in aquatic insect larvae). A very similar (but independent) evolutionary transition is, in fact, thought to have occurred in arachnids, where gills have been internalised to give rise to book lungs, and these in turn have been modified to give rise to tracheae in some groups of spiders. Thus, a relationship between insect tracheae and crustacean gills is plausible (Franch-Marro, 2006).

A particular type of epipod/gill has also been proposed as the origin of insect wings, a hypothesis that has received support from the specific expression in a crustacean epipod of the pdm/nubbin (nub) and apterous (ap) genes - that have wing-specific functions in Drosophila. In fact, the Artemia nub and ap homologues are expressed in the same epipod as trh and vvl, raising questions as to the specific relationship of this epipod with either tracheae or wings. A resolution to this conundrum becomes apparent when one considers the different types of epipods/gills found in aquatic arthropods, and their relative positions with respect to other parts of the appendage (Franch-Marro, 2006).

The primary branches of arthropod appendages, the endopod/leg and exopod, develop straddling the anteroposterior (AP) compartment boundary, which corresponds to a widely conserved patterning landmark in all arthropods. Different types of epipods/gills, however, differ in their position with respect to this boundary. For example, in the thoracic appendages of the crayfish, some epipods develop spanning the AP boundary [visualized by engrailed (en) expression running across the epipod], whereas others develop exclusively from anterior cells (with no en expression). Given that wing primordia comprise cells from both the anterior and posterior compartments, wings probably derived from structures that were straddling the AP boundary. Conversely, given that tracheal primordia arise exclusively from cells of the anterior compartment (anterior to en and even wg-expressing cells), it seems probable that tracheal cells evolved from a population of cells that was located in the anterior compartment. In this respect, it is interesting to note that the former type of epipods express nub, whereas the latter do not (Franch-Marro, 2006).

In summary, it is suggested that the ancestors of arthropods had specific areas on the surface of their body that were specialized for osmoregulation and gas exchange. Homologues of trh and vvl were probably expressed in all of these cells and played a role in their specification, differentiation or function. Some of these structures were probably associated with appendages, in the form of epipods/gills or other types of respiratory surfaces. A particular type of gill, straddling the AP compartment boundary, is likely to have given rise to wings, whereas respiratory surfaces arising from anterior cells only may have given rise to the tracheal system of insects. Confirmation of this hypothetical scenario may ultimately come from the discovery of new fossils, capturing intermediate states in the transition of insects from an aquatic to a terrestrial lifestyle (Franch-Marro, 2006).

Intrinsic control of precise dendritic targeting by an ensemble of transcription factors

Proper information processing in neural circuits requires establishment of specific connections between pre- and postsynaptic neurons. Targeting specificity of neurons is instructed by cell-surface receptors on the growth cones of axons and dendrites, which confer responses to external guidance cues. Expression of cell-surface receptors is in turn regulated by neuron-intrinsic transcriptional programs. In the Drosophila olfactory system, each projection neuron (PN) achieves precise dendritic targeting to one of 50 glomeruli in the antennal lobe. PN dendritic targeting is specified by lineage and birth order, and their initial targeting occurs prior to contact with axons of their presynaptic partners, olfactory receptor neurons. A search was performed for transcription factors (TFs) that control PN-intrinsic mechanisms of dendritic targeting. Two POU-domain TFs, acj6 and drifter have been identified as essential players. After testing 13 additional candidates, four TFs were identified, (LIM-homeodomain TFs islet and lim1, the homeodomain TF cut, and the zinc-finger TF squeeze) and the LIM cofactor Chip, that are required for PN dendritic targeting. These results begin to provide insights into the global strategy of how an ensemble of TFs regulates wiring specificity of a large number of neurons constituting a neural circuit (Komiyama, 2007).

For technical simplicity, larval born GH146-Gal4-positive PNs, originating from three neuroblast lineages, anterodorsal (adPNs), lateral (lPNs), and ventral (vPNs), were studied. Out of ~25 classes defined by their glomerular targets, focus was placed on 17 classes whose target glomeruli are reliably recognized across different animals. The MARCM technique allows visualization and genetic manipulation of PNs in neuroblast and single-cell clones in otherwise heterozygous animals, so PN-intrinsic programs can be studied for dendritic targeting. GH146 is expressed only in postmitotic PNs (Komiyama, 2007).

acj6 and drifter have been identified as lineage-specific regulators of PN dendritic targeting. To identify additional transcription factors (TFs) that regulate dendritic targeting of different PN classes, candidates were tested that have been shown to regulate neuronal subtype specification and targeting specificity and have available loss-of-function mutants. The following was tested; (1) the expression of candidate genes in PNs at 18 hr after puparium formation (APF) when PN dendrites are in the process of completing their initial targeting, and/or (2) their requirement in PNs by examining dendritic targeting in homozygous mutant MARCM clones (Komiyama, 2007).

DL1 adPN expresses Acj6, an adPN lineage factor, but not Drifter or Cut. acj6−/− DL1 PNs typically have diffuse dendrites that always innervate, but are not limited to, DL1. drifter misexpression alone did not affect their dendritic targeting. However, when loss of acj6 and gain of drifter were combined, the dendrites completely missed DL1 and targeted anterior glomeruli (Komiyama, 2007).

Misexpression of Cut alone caused DL1 PNs to target part of DL1 and the vicinity, similar to acj6−/−. Notably, this diffuse phenotype was directional, because most mistargeted dendrites targeted medially to DL1 (Komiyama, 2007).

cut misexpression combined with loss of acj6 caused severe mistargeting of DL1 adPNs. The dendrites completely missed DL1 and occupied the medial to dorsomedial AL, typically VM2, DM6, and DC1. Interestingly, these glomeruli are all adPN targets near DM1 and DM2, the two glomeruli that most frequently fail to be innervated by cut−/− lPNs. One interpretation is that loss of acj6 made the DL1 adPN more sensitive to the instructive information of cut to target the medial AL, but the remaining lineage information kept the dendrites within the adPN glomeruli in the area. If this were true, adding a lPN lineage factor drifter may bring the dendrites to DM1 or DM2, since this might recreate, based on partial knowledge of the TF code, a code for targeting these glomeruli. Loss of acj6 and misexpression of cut and drifter were combined simultaneously in DL1 adPNs. Under this condition, the dendrites again mostly targeted the medial to dorsomedial AL. However, glomerular preferences were strikingly different: they frequently innervated 1, DM2, and DA2. Notably, DA2 and DM2 are lPN targets (Komiyama, 2007).

These results suggest that cut and drifter have qualitatively different instructive information, with cut controlling global targeting and drifter controlling local glomerular choice according to their lineage (Komiyama, 2007).

Concentric zones, cell migration and neuronal circuits in the Drosophila visual center

The Drosophila optic lobe comprises a wide variety of neurons, which form laminar neuropiles with columnar units and topographic projections from the retina. The Drosophila optic lobe shares many structural characteristics with mammalian visual systems. However, little is known about the developmental mechanisms that produce neuronal diversity and organize the circuits in the primary region of the optic lobe, the medulla. This study describes the key features of the developing medulla and reports novel phenomena that could accelerate understanding of the Drosophila visual system. The identities of medulla neurons are pre-determined in the larval medulla primordium, which is subdivided into concentric zones characterized by the expression of four transcription factors: Drifter, Runt, Homothorax and Brain-specific homeobox (Bsh). The expression pattern of these factors correlates with the order of neuron production. Once the concentric zones are specified, the distribution of medulla neurons changes rapidly. Each type of medulla neuron exhibits an extensive but defined pattern of migration during pupal development. The results of clonal analysis suggest homothorax is required to specify the neuronal type by regulating various targets including Bsh and cell-adhesion molecules such as N-cadherin, while drifter regulates a subset of morphological features of Drifter-positive neurons. Thus, genes that show the concentric zones may form a genetic hierarchy to establish neuronal circuits in the medulla (Hasegawa, 2011).

Concentric genes are expressed in a defined subset of medulla neurons throughout development, suggesting that a part of neuronal identities are pre-determined in the larval medulla primordium. The data suggest that Drf-positive neurons produce nine types of medulla neurons, including lobula projection and medulla intrinsic neurons, while Hth-positive neurons produce at least four types of neurons, including lamina projection and medulla intrinsic neurons. In Hth-positive neurons, Bsh is exclusively expressed in medulla intrinsic Mi1 neurons. A hth mutation caused the neuron to switch type, while a drf mutation affected subsets of morphological features of Drf-positive neurons. Thus, roles of concentric genes may be functionally segregated to form a genetic hierarchy. Apparently, other concentric genes must exist in addition to the four genes reported in this study. Because there are many neurons outside of the Drf domain in the larval medulla, some concentric genes may be expressed in the outer zones. Some transcription factors may have expression patterns that differ from those of concentric genes, and their combined expression may specify restricted subtypes of medulla neurons. For example, apterous (ap) and Cut are widely expressed in medulla neurons. Cut was co-expressed in subsets of Drf-positive neurons, while ap was expressed in all Drf- and Bsh/Hth-positive neurons (Hasegawa, 2011).

Early-born medulla neurons express the inner concentric genes, while late born neurons express the outer ones. Thus, concentric gene expression correlates with neuronal birth order. However, it is still unknown how concentric gene expression is specified. It would be possible to speculate that genes controlling temporal specification of neurons are expressed in NBs to control the concentric gene expression. However, the genes that are known to control neuronal birth order in the embryonic CNS were not expressed in larval medulla NBs. In addition to local temporal mechanisms, such as birth order, global and spatial mechanisms governed by morphogen gradient may also play a role in determining medulla cell type. In addition to birth order or a morphogen gradient, mutual repression among concentric genes may be essential in establishing defined concentric zones. Except for rare occasions, de-repression of other concentric genes was not induced in clones mutant for hth or drf. Additionally, ectopic hth expression did not compromise Drf and Run expression. These results may suggest that unidentified genes act redundantly with these genes to repress expression of other concentric genes and that weak Hth expression in NBs does not play roles in temporal specification of medulla neurons (Hasegawa, 2011).

Various types of cell migration play important roles during vertebrate neurogenesis. Although Drosophila has been a powerful model of neural development, extensive neuronal migrations coupled with layer formation found in this study have not been previously reported. The current findings may establish a model to understand molecular mechanisms that govern brain development via neuronal migrations (Hasegawa, 2011).

It is important to know whether the migration of medulla neurons occurs actively or passively. The distribution of cell bodies in the adult medulla cortex was not random, but organized according to cell type. In particular, the Mi1 neurons identified by Bsh expression migrated outwards and were eventually located in the outermost area of the adult medulla cortex, which was affected in hth mutant clones. The observation that defined localization of cell bodies is under the control of genetic program may not be explained by passive migration. Repression of apoptosis by expressing p35 under the control of elav-Gal4 did not compromise migration of Bsh- and Drf-positive neurons, suggesting that apoptosis is not a driving force of the migration. If neurons migrate actively in an organized manner, what regulates the pattern of migration? In many cases, glial cells play important roles in neuronal migration. Indeed, glial cells and their processes were identified in the medulla cortex. Glial cells or other cell types could provide cues for neuronal migration (Hasegawa, 2011).

The medulla neurons project axons near their targets forming subsets of dendrites in the larval brain; the cell bodies migrated in the presence of preformed neurites during pupal development. During or following cell body migration, additional dendrites were formed along the axonal shafts. Therefore, cell body migration may somehow contribute to circuit formation in the medulla. Indeed, similar strategies have been reported in sensory neurons of C. elegans and cerebellar granule cells in mammals. Thus, cell body migration in the presence of neurites may be a general conserved mechanism of circuit formation. Cell body migration may also allow developing cells to receive inductive cues provided by cells in the vicinity of the medulla cortex. For example, glial cells placed on the surface of the brain may trigger the expression of specific genes (e.g. ChAT) in Mi1 cells that are located in the outermost area of the adult medulla cortex (Hasegawa, 2011).

In adults, Mi1 neurons have arborization sites at M1 and M5, which coincide with the L1 lamina neuron terminals. In Golgi studies, Mi1 neurons were found in all parts of the retinotopic field. Indeed, the number of Bsh expressing medulla neurons was about 800, a figure similar to the number of ommatidial units. Therefore, the Mi1 neurons identified by Bsh expression are most probably columnar neurons with direct inputs from L1 neurons. Because L1 is known to have inputs from R1-6, which processes motion detection, Mi1 may participate in the motion detection circuit (Hasegawa, 2011).

If the genetic codes that specify each type of neuron are found, it may encourage the functional study of defined neurons. In the medulla, bsh-Gal4 is solely expressed by Mi1 neurons. Although the expression of Bsh is also observed in L4/5 lamina neurons, intersectional strategies such as split Gal4 may enable the activity of Mi1 to be specifically manipulated by inducing expression of neurogenetic tools like shibirets. This could provide insight into high-resolution functional neurobiology in the Drosophila visual system (Hasegawa, 2011).

Development of the mammalian central nervous system reiteratively establishes cell identity, directs cell migration and assembles neuronal layers, processes similar to the patterns observed during medulla development. In the cerebral cortex, neurons are generated within the ventricular or subventricular zones and migrate outwards, leaving their birthplace along the radial glial fibers. Later-born neurons migrate radially into the cortical plate, past the deep layer neurons and become the upper layers. The layers of the cortex are thus created inside-out. In the developing spinal cord, neuronal types are specified according to morphogen gradients. Within each domain along the dorsoventral axis, neuronal and glial types are specified according to their birth order. The spinal cord neurons then migrate extensively along the radial, tangential and rostrocaudal axes. Therefore, the initial organization of spinal cord neurons is disrupted in the mature system (Hasegawa, 2011).

The medulla shares intriguing similarities with the mammalian central nervous system. For example, the concentric zones established in the larval medulla resemble the dorsoventral subdivisions of the spinal cord. Extensive migrations of medulla neurons disrupt concentric zones, as observed in the spinal cord. However, this study found that the locations of cell bodies were organized according to neuronal type, a distribution that may be similar to the cortical organization of the cerebral cortex. Thus, the development of the medulla may share characteristics with various forms of neurogenesis found in the mammalian central nervous system. A comprehensive study of important features of neurogenesis will now be possible using the Drosophila visual center and powerful tools of Drosophila genetics. Unveiling all aspects of development in the medulla will not only shed light into the functional neurobiology of the visual system, but also elucidate the developmental neurobiology of vertebrates and invertebrates (Hasegawa, 2011).

A temporal mechanism that produces neuronal diversity in the Drosophila visual center

The brain consists of various types of neurons that are generated from neural stem cells; however, the mechanisms underlying neuronal diversity remain uncertain. A recent study demonstrated that the medulla, the largest component of the Drosophila optic lobe, is a suitable model system for brain development because it shares structural features with the mammalian brain and consists of a moderate number and various types of neurons. The concentric zones in the medulla primordium that are characterized by the expression of four transcription factors, including Homothorax (Hth), Brain-specific homeobox (Bsh), Runt (Run) and Drifter (Drf/Vvl), correspond to types of medulla neurons. This study examined the mechanisms that temporally determine the neuronal types in the medulla primordium. For this purpose, transcription factors were sought that are transiently expressed in a subset of medulla neuroblasts (NBs, neuronal stem cell-like neural precursor cells) and identified five candidates [Hth, Klumpfuss (Klu), Eyeless (Ey), Sloppy paired (Slp) and Dichaete (D)]. The results of genetic experiments at least explain the temporal transition of the transcription factor expression in NBs in the order of Ey, Slp and D. The results also suggest that expression of Hth, Klu and Ey in NBs trigger the production of Hth/Bsh-, Run- and Drf-positive neurons, respectively. These results suggest that medulla neuron types are specified in a birth order-dependent manner by the action of temporal transcription factors that are sequential ly expressed in NBs (Suzuki, 2013).

In the embryonic central nervous system, the heterochronic transcription factors suchas Hb, Kr, Pdm, Cas and Grh are expressed in NBs to regulate the temporal specification of neuronal identity. They regulate each other to achieve sequential changes in their expression in NBs without cell-extrinsic factors. However, expression of the embryonic heterochronic genes was not detected in the medulla NBs.Instead this study found that Hth, Klu, Ey, Slp and D are transiently and sequentially expressed in medulla NBs. The expression of Hth and Klu was observed in lateral NBs, while that of Ey/Slp and D was observed in intermediate and medial NBs, respectively. These observations suggest that the expression of heterochronic transcription factors changes sequentially as each NB ages, as observed in the development of the embryonic central nervous system (Suzuki, 2013).

This study demonstrates that at least three of the temporal factors Ey, Slp and D regulate each other to form a genetic cascade that ensures the transition from Ey expression to D expression in the medulla NBs. Ey expression in NBs activates Slp, while Slp inactivates Ey expression. Similarly, Slp expression in NBs activates D expression, while D inactivates Slp expression. In fact, the expression of Slp is not strong in newer NBs in which Ey is strongly expressed, but is up regulated in older NBs in which Ey is weakly expressed in the wildtype medulla. A similar relationship is found between Slp and D, supporting the idea that Ey, Slp and D regulate each other's expression to control the transition from Ey-expression to D-expression. In the embryonic central nervous system, similar interaction is mainly observed between adjacent genes of the cascade hb-Kr-pdm-cas-grh, and this concept may also be applied to the medulla primordium. The expression pattern and function of Ey, Slp and D suggest that they are adjacent to each other in the cascade of transcription factor expression in medulla NBs (Suzuki, 2013).

However, no such relationship was found between Hth, Klu and the other temporal factors.The sequential expression of Hth and Klu could be regulated by an unidentified mechanism that is totally different from the genetic cascade that controls the transition through Ey-Slp-D. Or, there might be unidentified temporal factors that are expressed in lateral NBs which act upstream of Hth and Klu to regulate their expression. It is necessary to identify additional transcription factors that are transiently expressed in medulla NBs (Suzuki, 2013).

The expression of concentric transcription factors in the medulla neurons correlates with the temporal sequence of neuron production from the medulla NBs (Hasegawa, 2011). In the larval medulla primordium, the neurons are located in the order of Hth/Bsh-, Run- and Drf-positive cells from inside to outside, and these domains are adjacent to each other (Hasegawa, 2011). Given that NBs generate neurons toward the center of the developing medulla, Hth/Bsh-positive neurons are produced at first, and then Run-positive and Drf-positive neurons. Thus Hth/Bsh, Run and Drf were used as markers to examine roles of Hth, Klu, Ey, Slp and D expressed in NBs in specifying types of medulla neurons. The continuous expression of Hth and Ey from NBs to neurons and the results of clonal analyses that visualize the progeny of NBs expressing each one of the temporal transcription factors suggest that the temporal windows of NBs expressing Hth, Klu and Ey approximately correspond to the production of Hth/Bsh-, Run- and Drf- positive neurons, respectively. Indeed, the results of the genetic study suggest that Hth and Ey are necessary and sufficient to induce the production of Hth/Bsh- and Drf-positive neurons,respectively (Hasegawa, 2011, 2013). Ectopic Klu expression at least induces the produc tion of Run-positive neurons (Suzuki, 2013).

Slp and D expression in NBs may correspond to the temporal windows that produce medulla neurons in the outer domains of the concentric zones, which are most likely produced after the production of Drf-positive neurons. The results at least suggest that Slp is necessary and sufficient and D is sufficient to repress the production of Drf-positive neurons. Identification of additional markers that are expressed in the outer concentric zones compared to the Drf-positive domain would be needed to elucidate the roles of Slp and D in specification of medulla neuron types (Suzuki, 2013).

D mutant clones did not produce any significant phenotype except for derepression of Slp expression in NBs. Drf expression in neurons was not affected either. Since D is a Sox family transcription factor, SoxN, another Sox family transcription factor, is a potential candidate molecule that acts together with D in the medulla NBs. However, its expression was found in neuroepithelia cells and lateral NBs that overlap with Hth-positive cells but not with D-positive cells. All the potential heterochronic transcription factors examined in this study are expressed in three to five cell rows of NBs. Nevertheless, one NB has been observed to produce one Bsh- positive and one Run-positive neuron (Hasegawa, 2011). Therefore, the expression pattern of the heterochronic transcription factors is not sufficient to explain the stable production of one Bsh-positive and one Run-positive neuron from a single NB.The combinatorial action of multiple temporal factors expressed in NBs may play important roles in the specification of Bsh- and Run- positive neurons (Suzuki, 2013).

Another possible mechanism that guarantees the production of a limited number of the same neuronal type from multiple rows of NBs expressing a temporal transcription factor could be a mutual repression between concentric transcription factors expressed in medulla neurons. For example, Hth/Bsh, Run and Drf may repress each other to restrict the number of neurons that express either of these transcription factors. However, expression of Run and Drf was not essentially affected in hth mutant clones and in clones expressing Hth (Hasegawa, 2011). Similarly, expression of Hth and Drf was not essentially affected in clones expressing run RNAi under the control of AyGal4, in which Run expression is eliminated. Hth and Run expression was not affected in drf mutant clones (Hasegawa, 2011). These results suggest that Hth/Bsh, Run and Drf do not essentially regulate each other during the formation of concentric zones in the medulla (Suzuki, 2013).

During embryonic development, the heterochronic genes that are expressed in NBs (hb-Kr-pdm-cas-grh) are maintained and act in GMCs to specify neuronal type. Similarly, Hth and Ey are continuously expressed from NBs to neurons, suggesting that their expression may also be inherited through GMCs (Hasegawa, 2011). However, this type of regulatory mechanism may be somewhat modified in the case of Klu, Slp and D (Suzuki, 2013).

Klu is expressed in NBs and GMCs, but not in neurons. Slp and D are predominantly detected in NBs and neurons visualized by Dpn and Elav, respectively. Occasionally, however, expression of D was found in putative GMCs, which are situated between NBs and neurons. Additionally, both D-positive and D-negative cells were found among Miranda-positive GMCs. Slp expression was not found in Miranda-positive GMCs. Finally, D is expressed in medulla neurons forming a concentric zone in addition to its expression in medial NBs. However, D expression was abolished in slp mutant NBs but remained in the mutant neurons, suggesting that D expression in medulla neurons is not inherited from the NBs. These results suggest that Slp and D expression are not maintained from NBs to neurons and that not all the temporal transcription factors expressed in NBs are inherited through GMCs. However, it is possible to speculate that Klu, Slp and D regulate expression of unidentified transcription factors in NBs that are inherited from NBs to neurons through GMCs (Suzuki, 2013).

Effects of Mutation or Deletion

ventral veins lacking mutants show a disruption of the developing tracheal tree as well as commissural defects in the developing CNS. These defects appear to be caused by a failure in proper migration of tracheal cells and midline glia (Anderson, 1995).

Since SOX2 can rescue Dichaete mutant phenotypes and is known to interact with the POU-domain transcription factor OCT-3, the possibility of a genetic interaction between Dichaete and the Drosophila POU-domain gene ventral veinless was examined. vvl is expressed in the midline and is required for correct MGL development. In embryos homozygous for the vvl ZM allele, which has reduced but detectable levels of Vvl, anterior and posterior commissures fail to separate correctly, the longitudinals are thinner and there are regions where they collapse towards the midline. As with Dichaete hypomorph, only a small number of neuromeres are affected. However, in Dichaete;vvl double mutants all phenotypes are far more pronounced and occur in almost every hemisegment demonstrating a strong synergistic effect on the development of the nerve cord. Staining with anti-Fasciclin II shows that most of the longitudinal axons cross the midline many times with a roundabout-like phenotype. The midline of the double mutant embryos was tested with anti-Slit and very few cells were found at stage 16. The few remaining cells stain very weakly and are found ventral to the commissures. Similar results are obtained with Argos mRNA. In single Dichaete and vvl mutants, Slit expression is only weakly affected. However, glial cells with reduced expression and aberrant morphology have been found. To support the contention that Dichaete and vvl interact, the consequences of ectopic expression of Dichaete and Vvl were examined. Ectopic expression of Vvl in segregating neuroblasts alone does not disrupt the neuropile. Expression of Dichaete alone causes weak defects in the commissures mainly thinning of posterior commissures and thickening of the anterior commissure. When Dichaete and Vvl are expressed together in neuroblasts, however, the neuropile phenotypes are far more severe. The longitudinals collapse toward the midline throughout the neuropile and, in some segments, commissures appear fused. In this case, unlike the Dichaete;vvl double mutant, anti-Fasciclin II staining shows collapse of the longitudinals toward the midline but they do not cross the midline. Taken together, these data suggest that, as in the mouse, SOX and POU domain transcription factors interact to regulate the expression of target genes (Soriano, 1998).

Cross-regulatory interactions among tracheal genes support a co-operative model for the induction of tracheal fates in the Drosophila embryo

The Drosophila tracheal system arises from clusters of ectodermal cells that invaginate and migrate to originate a network of epithelial tubes. Genetic analyses have identified several genes that are specifically expressed in the tracheal cells and are required for tracheal development. Among them, trachealess (trh) is able to induce ectopic tracheal pits and therefore it has been suggested that it would act as an inducer of tracheal cell fates; however, this capacity appears to be spatially restricted. The expression of the tracheal specific genes in the early steps of tracheal development and their crossinteractions have been examined. There is a set of primary genes including trh and ventral veinless (vvl) whose expression does not depend on any other tracheal gene and a set of downstream genes whose expression requires different combinations of the primary genes. The combined expression of primary genes is sufficient to induce some downstream genes but not others. These results indicate that there is not a single master gene responsible for the appropriate expression of the tracheal genes and support a model where tracheal cell fates are induced by the cooperation of several factors rather than by the activity of a single tracheal inducer (Boube, 2000).

trh and vvl appear to initiate or to act very early in the genetic hierarchy specifying tracheal development. vvl expression in the tracheal cells is independent of trh function. It is also found that trh expression in the tracheal cells is independent of vvl function indicating that the two genes act in parallel in the control of tracheal cell development. btl, a gene encoding an FGF receptor homolog required for tracheal migration, is a target of trh: btl requires vvl for the maintenance of its transcription. Transduction of the FGF signalling also requires the Downstream of FGF (Dof) protein, which is specifically expressed in the tracheal cells. However, dof is not a target gene activated as a result of FGF signaling as its expression is not affected in btl mutant embryos. Conversely, the results show that the specific expression of dof in the tracheal cells is dependent on trh and vvl activity. Thus, trh and vvl enable the tracheal cells to be competent to FGF signaling by regulating the expression of at least two elements (btl and dof) acting at different steps in the Btl pathway (Boube, 2000).

In contrast to the general requirement of the Btl pathway, the Dpp and EGF pathways are required for migration of certain branches of the tracheal system; competence of the tracheal cells to those signals depends on the specific tracheal expression in the tracheal cells of tkv and rho, respectively. Similarly to the btl pathway, rho expression in the tracheal cells depends both on trh and vvl function. However, while tracheal expression of tkv also depends on vvl, it appears to be independent of trh. The opposite appears to be the case for two other tracheal genes, tracheal defective (tdf) and pebbled (peb) [also known as hindsight (hnt)], which code for two putative transcription factors. Both genes appear to be targets of trh but they are present in the tracheal cells of vvl mutant embryos. Thus, some tracheal genes seem to be common targets of vvl and trh but others seem to depend only on one of them (Boube, 2000).

Whether knirps (kni) and knirps related (knrl) would fit in this genetic hierarchy was also analyzed. Both genes code for putative transcription factors that are expressed in overlapping patterns and share redundant functions during tracheal development; both genes have an early expression in the tracheal placodes and a later expression in a particular subset of tracheal branches. Quite surprisingly, it was found that the early tracheal expression of kni is not abolished in either trh or vvl mutant embryos and also that all the tracheal genes mentioned above are expressed in the tracheal cells of embryos that are mutant for a deficiency that uncovers both kni and knrl. In summary, there are some primary tracheal genes whose expression appears not to be regulated by any other tracheal gene. Subsequently, one or more of those primary genes are necessary for the appropriate expression of other downstream genes in the tracheal cells (Boube, 2000).

The complexity of regulatory interactions described above indicates that more than one gene can act as an inducer of the expression of downstream tracheal genes. This seems to contrast with earlier results suggesting that the trh gene acts as the master gene for tracheal fate. Evidence for this comes from experiments of ubiquitous expression of trh, which generates new tracheal pits at the correct position in anterior and posterior segments that normally do not form pits. The same result is observed when an UAS-trh construct is specifically expressed in the embryonic terminal regions by means of a sal-Gal4 line. In both cases, induction of extra tracheal pits can be visualized very early in development by the appearance of additional clusters of cells that express btl in more anterior and posterior segments. However, the capacity of trh to induce btl expression appears restricted to specific positions in the embryo. The restricted activation of btl by trh is not due to low levels of trh since the use of the Gal4/UAS system induces high levels of trh transcripts. Conversely, the alternative conclusion is favored: the activity of trh alone is not sufficient to induce btl expression, probably because other factors are also required in combination with trh. vvl is a good candidate for such a factor. vvl is required to induce some of the tracheal genes. In addition, vvl is expressed independent of trh in the tracheal placodes and in the analogous location within the segments that do not form tracheal pits. These are precisely the positions where trh can induce additional tracheal pits. Thus, it was asked whether both trh and vvl are required to instruct those cells to adopt a tracheal fate. On inducing an UAS-trh construct in the terminal regions of vvl mutant embryos (with the same sal-Gal4 line as above), ectopic induction of btl in new patches of cells is suppressed. This result indicates that it is the localized expression of vvl that accounts for the restricted induction of btl in a particular set of cells upon general expression of trh. The situation is different in the normal tracheal placodes where vvl is dispensable for induction of btl expression and is only required for its maintenance (Boube, 2000).

The above experiments indicate that vvl is required for the induction of extra tracheal pits in additional segments. However, general expression of vvl with an UAS-vvl construct does not induce additional tracheal pits or ectopic expression of btl. It was asked whether the co-expression of vvl and trh would be sufficient to induce tracheal fates, as monitored by induction of btl expression. Indeed, simultaneous expression of an UAS-vvl and an UAS-trh construct in the embryonic terminal regions under the common control of the sal-Gal4 line induces btl expression throughout both regions. Also, co-expression of vvl and trh in unrelated regions such as the distal leg primordia (directed by a Dll-Gal4 line) is sufficient to induce btl expression. Thus, vvl and trh are both required and their co-expression is sufficient to ectopically induce btl expression (Boube, 2000).

On the contrary, vvl and trh appear not to be sufficient for the expression of the remaining tracheal genes. While tracheal expression of dof, tdf, peb, tkv and rho require either vvl or trh, or both, no induction of any of these genes is observed upon ectopic expression of vvl and/or trh. Therefore, these results raise the possibility that full induction of tracheal fates requires one or more additional factors. In this regard it is worth noting that the tracheal branches generated from ectopic trh in vvl expressing cells are abnormal and do not fuse with the normal tracheal tree (Boube, 2000).

Expression of trh is repressed by sal in the terminal regions leading to the suggestion that this is the mechanism that accounts for the confinement of tracheal placodes to the central segments of the embryo. In contrast, vvl is expressed at the correct positions in segments that normally do not form tracheal placodes, although its expression in those sites is much weaker. Whether sal could also regulate vvl expression was investigated. Indeed, vvl expression is strongly increased in those sites in sal mutant embryos suggesting that vvl is downregulated by sal in the segments that do not form tracheal pits. Similarly, kni expression is also upregulated in the same sites in sal mutant embryos. Repression of vvl and kni by sal could in principle be attributed to the downregulation of trh by sal. However, this seems not to be the case because expression of vvl and early expression of kni in the tracheal placodes does not depend on trh. Therefore, sal seems to independently downregulate trh, vvl and kni in the most anterior and posterior embryonic regions (Boube, 2000).

Because trh and vvl are sufficient to activate btl, additional patches of btl expression were found in sal mutant embryos. dof and rho are expressed in additional patches of cells in sal mutant embryos. Repression of dof and rho by sal could also be attributed to the downregulation of trh and vvl by sal. However, this seems not to be the case because co-expression of trh and vvl in the sal domain is not sufficient to induce either dof or rho expression. Instead, sal could directly repress dof and rho or, alternatively, it could repress an additional factor necessary for their induction. In summary, many tracheal genes appear to be independently downregulated by sal in the terminal regions. Besides, the lack of sal expression does not have the same effect on the tracheal genes. In particular, some of the additional patches of trh expression are much weaker than the normal ones. This difference is not so pronounced in the case of vvl expression in sal mutant embryos. Also, one additional anterior pair of cell clusters for rho and dof expression is observed. Therefore, not all the tracheal placodes are equivalent in sal mutant embryos (Boube, 2000).

Drosophila p27Dacapo expression during embryogenesis is controlled by a complex regulatory region independent of cell cycle progression

In the embryonic epidermis, dacapo expression starts during G2 of the final division cycle and is required for the arrest of cell cycle progression in G1 after the final mitosis. The onset of dacapo transcription is the earliest event known to be required for the epidermal cell proliferation arrest. To advance an understanding of the regulatory mechanisms that terminate cell proliferation at the appropriate stage, the control of dacapo transcription has been analyzed. dacapo transcription is not coupled to cell cycle progression. It is not affected in mutants where proliferation is arrested either too early or too late. Moreover, upregulation of dacapo expression is not an obligatory event of the cell cycle exit process. During early development of the central nervous system, Dacapo cannot be detected during the final division cycle of ganglion mother cells, while it is expressed at later stages. The control of dacapo expression therefore varies in different stages and tissues. The dacapo regulatory region includes many independent cis-regulatory elements. The elements that control epidermal expression integrate developmental cues that time the arrest of cell proliferation (Meyer, 2002).

The pattern of stg expression anticipates and determines the embryonic cell division pattern. stg expression in the embryonic epidermis before mitosis 16, therefore, is highly similar to the pattern of dap expression, which also precedes this terminal mitosis 16. To analyze whether stg and dap expression before mitosis 16 are mechanistically coupled, the distribution of stg transcripts in ventral veins lacking (vvl) and trachealess (trh) embryos was examined. vvl and trh are expressed within the prospective tracheal pit regions and are known to co-operate for the specification of tracheal cell fate. The characteristic early dap expression in tracheal pits is not detected in vvl embryos and it is severely decreased in trh embryos. Interestingly, while the characteristic early expression of stg is not observed in trh embryos, it is normal in vvl mutants. As expected, progression through mitosis 16 also occurred in the normal pattern in these vvl mutants. The absence of the characteristic early dap expression in tracheal pits of vvl embryos, therefore, is not preceded by a change in the proliferation program. These findings indicate that the control of stg and dap expression is mechanistically distinct. Moreover, they suggest that regulators of developmental fates control dap expression directly and not indirectly by controlling the cell proliferation program via other cell cycle regulators (Meyer, 2002).

Requirement of ventral veinless in the development of the PNS

Most of the cells in the embryonic peripheral nervous system (PNS) of Drosophila are born in their final location. One known exception is the group of lateral chordotonal organs (lch5) whose precursors form in a dorsal position, yet the mature organs are located in the lateral PNS cluster. Mutations in the u-turn (ut) locus perturb the localization of lch5 neurons and result in a 'dorsal chordotonals' phenotype. ut is shown to be allelic to ventral veinless (vvl), also known as drifter. Vvl, a POU-domain transcription factor, has been shown to participate in the development of tracheae and CNS in the embryo, and in wing development in the adult; however, its role in PNS development has not been described. Characterization of the 'dorsal chordotonals' phenotype of vvl mutant embryos has revealed that in the absence of Vvl, cell fates within the lch5 lineage are determined properly and the entire organ is misplaced. Based on the positions of lch5 cells relative to each other in mutant embryos, and in normal embryos at different developmental stages, a two-step model is proposed for lch5 localization. lch5 organs must first rotate to assume a correct polarity and are then stretched ventrally to their final position. In this process, Vvl function is required in the ectoderm and possibly in the lch5 organs as well (Inball, 2003).

Vvl is also expressed in developing external sensory organs in the embryo and in the adult. In the embryo, loss of Vvl function results in increased apoptosis in specific es organs. Analysis of vvl mutant clones in adults reveals a requirement for Vvl in the control of cell number within the bristle lineage (Inball, 2003).

The ut gene was identified in a chemical mutagenesis screen for mutations affecting the development of the embryonic PNS. It was mapped by meiotic recombination to genetic position 3[26] on the left arm of the third chromosome. The approximate cytological location was determined to be 65A2;65E1. Three embryonic lethal alleles of ut were generated in that screen: utH599, utH76 and utM638. One of the candidate genes in these genomic regions is ventral veinless (vvl), which maps to 65C5. Two observations strongly suggest that ut and vvl could be allelic: (1) a cross between ut alleles and the In(3LR)282 chromosome, which has a breakpoint in the 65C-D region, yielded adult progeny that lacked the L4 wing veins (vvl is known to be required for wing vein formation) and (2) vvl mutant embryos exhibit a collapse of the anterior segments of the ventral nerve cord (VNC), a phenotype observed also in ut mutant embryos. Complementation tests between the embryonic lethal allele vvlGA3 and ut alleles demonstrate that ut and vvl are indeed allelic (Inball, 2003).

The question of how lch5 organs reach from their dorsal place of origin to their final lateral position has not been answered yet. All existing data regarding the so called 'lch5 migration' come from descriptions of abnormally positioned (and abnormally oriented) lch5 neurons in different mutant backgrounds. Based on these data, it has been suggested that concomitant with moving ventrally, the lch5 neurons turn approximately 145° counter clockwise (Inball, 2003).

By visualizing all the cells of lch5 organs located in a variety of positions between the dorsal and lateral clusters, it was possible to determine that the polarity of the neurons reflects the polarity of the whole organ. The mature ch organ is subepidermal and contains a neuron ensheathed by a scolopale cell, a ligament cell and a cap cell. The cap cells are connected to the ectoderm by attachment cells, which derive from the ch lineage. When the neurons point ventrally, the ligament cells are the most dorsal cells of the organ and the cap cells are the most ventral, whereas for the dendrites to point dorsally, the ligament cells must be ventral and the cap cells dorsal. In normal stage 12 embryos, the ligament cells can be detected at the dorsal part of the organ, whereas in older embryos they migrate ventrally to become the ventralmost cells of the organ. Based on these observations, a two-step model is proposed for the lateral localization of lch5 organs. In the first step, rotation of the organ takes place. The organ rotates around the attachment cells, which anchor it to the ectoderm and thus function as a pivot. The rotation results in both bringing the organ to its correct orientation and placing it in a more ventral position, closer to the lateral cluster. This step occurs during stage 12 and perhaps early stage 13. Once in their correct orientation the lch5 organs go through the second step, which involves ventral stretching into their final shape and position, as seen in stage 15 or older embryos. This model is further supported by the fact that when the thoracic dch3 are forced to descend to the lateral cluster by overexpressing abd-A, their orientation is reversed and the ligament cells are found at their ventral edge (Inball, 2003).

What makes the lch5 organs go through this process? It is possible that the ligament cells respond to a signal and migrate ventrally, thereby pulling with them all other cells of the organ. Interestingly, a similar change from a dorsal to ventral position has recently been shown for the PG3 cell, which like the ligament cells expresses the glial marker REPO. This may suggest that a common mechanism governs the change in position of both types of REPO-expressing cells. Another possibility is that the rotation step depends on the neurons and that the ligament cells are required only for stretching. Since the rotation of lch5 is completed by early stage 13, when axonal outgrowth begins, it is possible that the growing axons serve as a guide for the ligament cells. However, in vvl mutant embryos, the lch5 organs often fail to stretch even when their axonal outgrowth seems largely normal. This suggests that although the axons may play a part in guiding the ligament cells, other factor/s are required as well. The morphogenetic movements occurring during dorsal closure in stage 14 are likely to affect the dorsoventral position of the lch5 ectodermal attachment cell. However, dorsal closure alone cannot account for the stretching of lch5, which is not completed before late stage 15, well after dorsal closure is completed (Inball, 2003).

Mutations in three loci, abd-A, hth and sal, have been shown to perturb the lateral localization of lch5 organs. Mutations in these three loci result in both abnormal localization and abnormal number of lch5 organs. However, decisions of organ number and organ localization are not always coupled. For example, mutations in the EGFR pathway gene rhomboid and the EGFR pathway antagonist argos, affect the number of lch5 organs but only rarely affect their position. vvl is the first gene that affects the localization of the lch5 organs without affecting their number. The abnormal localization of lch5 organs in vvl mutant embryos is similar to the abnormal localization of these organs in hth and abd-A mutant embryos, suggesting these genes are required in the same developmental pathway. However, epistasis experiments did not provide evidence for direct genetic interactions between vvl and hth or vvl and abd-A (Inball, 2003).

Vvl, a class III POU-domain transcription factor, and its mammalian homologs, have been shown to be required for cell migration. Brn1 and Brn2, the mouse homologs of Vvl, have a crucial role in the migration of cortical neurons. In the CNS of Drosophila embryos, Vvl is required for the migration of midline glial cells. In the embryo, Vvl is also required for tracheal cell migration and in its absence the tracheal tree fails to form (Inball, 2003).

The mechanism by which Vvl affects the lateral localization of lch5 organs is not clear. Cell migration requires the existence of signals from the environment and the ability of the migrating cell to receive and respond to these signals. In tracheal development, Vvl functions autonomously and it was suggested to regulate the expression of cell surface molecules necessary for the migration of tracheal cells. In lch5 organs, Vvl expression is detected in the neurons. However, expressing Vvl under elav-Gal4 regulation in vvl mutant background could not rescue the mutant phenotype. This result suggests that the neuronal expression of Vvl is either not required or not sufficient for lch5 lateral localization. Driving Vvl expression with ato-Gal4 rescues the mutant phenotype; however, this occurs with a much lower efficiency than when Vvl is expressed under arm-Gal4 regulation. The major differences between these two drivers are that while ato-Gal4 drives strong expression in the lch5 lineage and in a small group of ectodermal cells, arm-Gal4 induces strong expression throughout the ectoderm and only weak expression in lch5 organs. Thus, the results of these experiments cannot determine unambiguously where Vvl is required during lch5 lateral localization, and suggest it could function in both lch5 organs and the surrounding ectoderm, or in the ectoderm alone. The more efficient rescue generated by arm-Gal4 may indicate that the ectodermal expression of Vvl is the main factor with regard to lch5 positioning. In the ectoderm, Vvl could be involved in the generation of a positional cue. Although the rescue of lch5 localization is achieved by ubiquitous expression of Vvl in the ectoderm, it should be noted that the normal ectodermal expression of Vvl during critical stages of lch5 positioning (stages 12 and early 13) is not uniform. Vvl is more strongly expressed in a dorsal domain of the embryo, from the position of the lateral cluster dorsally. Later during stage 13, Vvl expression becomes uniform throughout the ectoderm. It is not clear yet whether this differential expression is significant in the context of lch5 positioning. Vvl has been shown to interact with other transcription factors in the CNS and trachea. Thus, another possibility is that an unidentified partner of Vvl confers a spatial specificity to its activity (Inball, 2003).

Two additional cell types in the vicinity of the developing lch5 organs express high levels of Vvl: tracheal cells and oenocytes. The trachea is probably not involved in the process of lch5 localization, since trachealess mutants do not exhibit a 'dorsal ch' phenotype. The possible role of oenocytes in lch5 migration is intriguing. Impaired lch5 localization is many times accompanied by partial or complete loss of oenocytes, as seen in embryos mutant for abd-A, hth and vvl. However, rhomboid mutants lack oenocytes, yet their lateral ch organs (which consist of three, instead of five, scolopidia) are almost always positioned properly. Thus, it seems more probable that lch5 organs and oenocytes are independently affected by the same mutations (Inball, 2003).

In Drosophila, loss of es organ cells has been attributed to one of two reasons. Either the organ completely fails to form because of interference with the function of the proneural genes, or cell fate transformations occur between the cells comprising these organs. However, in vvl mutant embryos the decreased number of these cells is a result of increased apoptosis. Any of the cells of the organ could be affected, and the remaining cells express typical markers, suggesting that initial decisions of cell fates are not impaired. It is therefore possible that Vvl is required for cell survival in the developing es lineages. Another possibility is that Vvl is required for the differentiation of these organs, and that in its absence some of the cells fail to differentiate properly and go through apoptosis (Inball, 2003).

In mammals, POU-domain transcription factors play significant roles in survival of cells in the nervous system. Members of the class IV POU-factors are known to be essential for differentiation and survival of PNS cells. The most interesting of those in the context of Drosophila es organ development is Brn3c, which is required for maturation and survival of the inner ear hair cells. The vertebrate inner ear hair cells are mechanosensory organs, considered homologous to Drosophila bristles in many aspects. The parallelism between the two types of organs has been shown at the levels of function, structure and the molecular mechanisms responsible for their development. Mice deficient for Brn3c fail to develop inner ear hair cells and are completely deaf. A mutation in the human homolog of this gene has been shown to cause progressive hearing loss. The defects seen in the development of the hair cells in Brn3c-null mice are limited to maturation and survival of these organs (Inball, 2003 and references therein).

Although there is not sufficient evidence to consider a functional homology between Vvl, a class III POU-factor, and the mammalian class IV POU-factors, it will be interesting to determine whether the similarity of their loss-of-function phenotypes extends further at the molecular level (Inball, 2003).

vvl mutant clones in adult head tissue caused defects in bristle development that typically result in supernumerary cells. One possible explanation for an increase in the number of bristle cells is that too many precursors are formed as a result of inefficient lateral inhibition. In such a case, the appearance of complete ectopic organs would be expected. However, the supernumerary cells do not constitute separate organs, rather they increase the number of cells within a single es organ. This observation suggests that one or two extra cell divisions take place, resulting in the production of extra cells within the lineage. Thus, it is possible that Vvl is required in these cells for exit from the cell cycle (Inball, 2003).

Many abnormalities are also observed in the structure of the external support cells of the mutant bristles, especially in the shaft. Whether the structural defects are secondary to the abnormal pattern of cell division, or they represent another independent role for Vvl in the differentiation of these structures remains to be determined (Inball, 2003).

Loss of Vvl function in the embryonic and adult es lineages results in what seem to be two very different phenotypes: loss of cells in the embryo as opposed to overproduction of cells in the adult. However, it is possible that both phenotypes reflect a failure of the es cells to commence differentiation. These cells may behave differently when unable to differentiate properly, depending on their developmental context (Inball, 2003).

Specification of Drosophila motoneuron identity by the combinatorial action of POU and LIM-HD factors

In both vertebrates and invertebrates, members of the LIM-homeodomain (LIM-HD) family of transcription factors act in combinatorial codes to specify motoneuron subclass identities. In the developing Drosophila embryo, the LIM-HD factors Islet (Tailup) and Lim3, specify the set of motoneuron subclasses that innervate ventral muscle targets. However, since several subclasses express both Islet and Lim3, this combinatorial code alone cannot explain how these motoneuron groups are further differentiated. To identify additional factors that may act to refine this LIM-HD code, the expression of POU genes in the Drosophila embryonic nerve cord was analyzed. The class III POU protein, Drifter (Ventral veinless), is co-expressed with Islet and Lim3 specifically in the ISNb motoneuron subclass. Loss-of-function and misexpression studies demonstrate that the LIM-HD combinatorial code requires Drifter to confer target specificity between the ISNb and TN motoneuron subclasses. To begin to elucidate molecules downstream of the LIM-HD code, the involvement of the Beaten path (Beat) family of immunoglobulin-containing cell-adhesion molecules was analyzed. beat Ic genetically interacts with islet and Lim3 in the transverse nerve (TN) motoneuron subclass and can also rescue the TN fasciculation defects observed in islet and Lim3 mutants. These results suggest that in the TN motoneuron context, Islet and Lim3 may specify axon target selection through the actions of IgSF call-adhesion molecules (Certel, 2004).

In each abdominal hemisegment of the Drosophila embryo, the axons of ~40 motoneurons exit the ventral nerve cord and specifically synapse with 30 identified muscle fibers. The axons of these motoneurons form six discrete fascicles and exit the VNC together, extending into the periphery to innervate specific muscle groups (or fields). The ventral muscles are innervated by the motoneurons of three subclasses: the transverse nerve (TN), intersegmental nerve b (ISNb) and intersegmental nerve d (ISNd). In addition, the segmental nerve c (SNc) innervates a set of externally located ventral muscles. The two TN motoneurons contact ventral muscle 25 or the ventral process of the lateral bipolar dendritic neuron (LBD). The ISNb motoneurons innervate the ventral muscles 6, 7, 12, 13, 14, 28 and 30, and the ISNd motoneurons muscles 15, 16 and 17. The Drosophila LIM-HD genes, islet and Lim3, are co-expressed in a subset of CNS neurons, including several classes of motoneurons. Although Isl and Lim3 are required in these distinct motoneurons to specific neuronal identity, this two member 'LIM-code' cannot alone be responsible for the unique differentiation and function of each subclass. More specifically, since the TN and ISNb motoneuron subclasses both express a combination of Isl and Lim3, it is likely that other factors act to discriminate between these two subclasses (Certel, 2004).

To identify factors that act to further differentiate these motoneuron subgroups, the expression pattern of previously characterized transcription factors was examined. Isl and Lim3 are co-expressed with the class III POU factor, Drifter (Dfr), in a limited number of embryonic VNC neurons. Immunohistochemical experiments using double transgenic animals reporting on Isl and Lim3 expression and antibodies directed against Dfr reveal restricted triple expression specifically in the ISNb motoneuron subclass. Dfr, Isl and Lim3 expression is clearly observed in the RP1, RP3, RP4 and RP5 motoneurons that project out the ISNb fascicle to innervate ventral muscles 6, 7, 12 and 13. Dfr expression is not detected in the pair of Isl- and Lim3-expressing TN neurons. To confirm that the lateral co-expressing neurons belong to the ISNb subclass, embryos carrying the Lim3A-tau-myc transgene were double-labeled. This reporter construct drives expression in Isl-expressing motoneurons that project via the ISNb but not the ISNd fascicle. The lateral Dfr-expressing neurons do belong to the ISNb subclass since they also express the Tau-myc fusion protein. The co-expression of Dfr and Isl was further analyzed. In addition to the ISNb neurons, Dfr and Islet co-expression is also observed in two serotonergic EW neurons (Certel, 2004).

To verify the lack of Dfr expression in the ISNd subgroup, focus was placed on the well-characterized GW/7-3M motoneuron that innervates muscle 15 via the ISNd pathway. To identify this ISNd motoneuron, the enhancer-trap line, egP289, was used that drives lacZ expression in the 7-3M progeny. Although Dfr expression is seen in the EW interneurons, the ISNd GW/7-3M motoneuron does not express Dfr. The results of these labeling experiments demonstrate that Dfr expression can be further used to subdivide the ISNb from the TN and ISNd neuronal classes (Certel, 2004).

The ISNb fascicle contains motor axons from at least eight motoneurons innervating ventral muscles 6, 7, 12, 13, 14, 28 and 30. Loss of isl or Lim3 function in ISNb motoneurons affects axon targeting resulting in a reduction of target muscle innervation. The most common phenotype in both isl and Lim3 mutants is a failure to innervate the cleft between muscles 12 and 13. In Lim3 mutants, muscles 12/13 were not innervated by motor axons in 46% of hemisegments compared with 3% in wild-type hemisegments. In isl mutants, the lack of muscle contacts was also coupled with the ISNb motor axons, leaving the ventral muscle field and joining the TN (26% of hemisegments in isl mutants versus 0% in wild type) (Certel, 2004).

Dfr is functionally required in a variety of tissues, including the midline glia, trachea, wing veins, sensory neurons and antennal lobe projection neurons. The early lethality of null dfr alleles, and the VNC perturbations caused by midline glia defects made analyzing the requirement for Dfr function in ISNb neurons difficult. To circumvent this problem, and to examine whether Isl, Lim3 and Dfr are required to specify similar aspects of motor axon targeting, the ISNb fascicle was analyzed in trans-heterozygous combinations. Removing a single copy of isl, Lim3 and dfr results in motor axon targeting defects characterized by significant reductions in muscle innervation. The failure to innervate muscles 12 and 13 observed in both isl37Aa/+;dfrB129/+ and Lim3Bd1/+;dfrB129/+ trans-heterozygotes is easily quantifiable and observed in isl and Lim3 mutant embryos. Reducing the levels of Isl and Dfr also results in ISNb motor axons leaving the ventral muscle field and targeting the TN fascicle. Neither Dfr nor Isl is expressed in motoneurons that innervate the dorsal muscles or the externally located ventral muscles. In line with their restricted expression, no evidence was found of non-cell autonomous affects upon the targeting of the ISN and SNc motor axons in dfr, isl trans-heterozygotes (Certel, 2004).

In an attempt to address the specificity of the genetic interactions between POU and LIM-HD genes, genetic interactions were tested between dfr and another key regulator of ISNb identity, the Drosophila exex/hb9 gene. However, no evidence of ISNb axon pathfinding defects were found in embryos trans-heterozygotes for dfr and hb9 (dfrB129/hb9KK30). Therefore, results from genetic interaction studies indicate that Dfr may be required specifically in combination with Isl and Lim3 to specify the ISNb motoneuron subclass (Certel, 2004).

isl and Lim3 mutants display axon targeting defects manifested by a reduction in muscle innervation. Results from the trans-heterozygous genetic interaction studies suggest that Dfr is necessary in combination with Isl and Lim3 to specify ISNb axon targeting. If this hypothesis is correct, then a loss of Dfr function in ISNb motoneurons should also result in a reduction in muscle innervation. To address the possible role of Dfr in ISNb specification, RNA interference was used to reduce or eliminate the expression of Dfr protein specifically in neurons, without affecting its expression and function in midline glia. Transgenic flies were generated expressing double-stranded dfr RNA (UAS-dsdfr) under the control of neuronal-specific Gal4 drivers. To further assist in the removal of Dfr function, two independent UAS-dsdfr insertions were recombined onto a chromosome containing a null dfr allele, dfrB129. Although the protein produced by this allele is detected by the Dfr antiserum, it is non-functional because of a premature stop codon located before the DNA-binding POU domain. Several Gal4 drivers were used to analyze the effectiveness of the UAS-dsdfr transgenes. Using both the C155(elav)-Gal4 and the Lim3B-Gal4 drivers, Dfr protein was reduced or eliminated in the majority of ISNb motoneurons, including the RP motoneurons. As a control, the Dfr-expressing midline glia showed no loss of Dfr protein and, accordingly, no loss of the midline glia-specific marker Slit. This shows that UAS-dsdfr could be used together with neuronal-specific Gal4 drivers to address how loss of Dfr function affects ISNb neuron specification (Certel, 2004).

Using both early (ftzng-Gal4.20) and post-mitotic (Lim3B-Gal4) drivers to reduce Dfr activity, two axon outgrowth phenotypes were observed. (1) ISNb motor axons can now leave the ventral muscle field and contact or target the TN fascicle. This re-targeting suggests that losing Dfr leaves the ISNb neurons in an Isl/Lim3-only specified state generating a TN fate. (2) Reducing Dfr function causes an overall decrease in muscle innervation by the ISNb motor axons similar to defects observed in isl mutants. Both of these axon targeting and innervation phenotypes indicate that Dfr plays an important role in ISNb neuronal specification (Certel, 2004).

If Dfr functions as part of a LIM/POU combinatorial code to specify ISNb motoneurons, then ectopically expressing Dfr in the Isl/Lim3-expressing TN motoneurons would be predicted to alter TN axon pathfinding toward an ISNb-like behavior. To test this, Dfr was misexpressed in postmitotic Isl/Lim3-expressing TN neurons using the Lim3B-Gal4 driver. In wild-type development, the transverse nerve forms from the fasciculation of a sensory nerve axon (the lateral bipolar dendrite or LBD neuron) and the TMNp neuron. At late stage 15/16, these growth cones contact each other on the ventral interior muscle surfaces. The TN fascicle is on a different focal plane and in wild-type embryos does not come in contact with the ISNb fascicle (Certel, 2004).

Misexpression of Dfr protein in Lim3B-Gal4;UAS-dfr double transgenic embryos did not have any effect on TN axons exiting the ventral nerve cord or lead to random innervation in the periphery. Instead, adding Dfr function to the TN neurons caused the motor axons to target the ISNb muscle field in a significant number of hemisegments (52%). TN motor axons either send collaterals into ISNb muscle targets or in some cases even innervated ISNb muscles. This change in motor axon targeting appears to be specific to the TN motoneurons. Misexpressing Dfr in most, if not all, motoneurons via the ftzng-Gal4.20 did not result in any targeting changes or defects in the SNc or ISN fascicles; it did, however, result in a similar percentage of TMNp targeting defects (Certel, 2004).

The ability of the TN motoneurons to be respecified by Dfr misexpression was tested in isl mutant embryos. Without Isl function, the retargeting of TN motor axons did not occur, suggesting possible cooperative actions between these transcription factors. These results indicate that it is the addition of Dfr specifically to the TN Isl/Lim3 LIM-HD code that allows this motoneuron subclass to exhibit ISNb motoneuron characteristics (Certel, 2004).

In C. elegans touch receptor neurons, the POU protein UNC-86 directly regulates the expression of the LIM-HD gene, MEC-3. And in vertebrates, abLIM is a transcriptional target of the POU factor, Brn3.2. To determine whether Dfr, Isl and/or Lim3 are possible transcriptional targets of one another, the individual expression patterns were examined in each mutant background. The results indicate that in Drosophila Dfr, Isl and Lim3 must function at the same hierarchical level in ISNb motoneuron specification (Certel, 2004).

In Drosophila, Isl and Lim3 are co-expressed in a subset of CNS neurons including two neuron subclasses, the TN and ISNb motoneurons. Although Isl and Lim3 are required in these distinct motoneurons to specific motor axon pathway choice neuronal identity, these two factors alone cannot be responsible for the unique differentiation of each subclass. In this study, evidence has been provided that the class III POU domain protein Dfr functions in combination with this Isl/Lim3 LIM code, to specify the ISNb motoneuron class. Loss-of-function analyses indicate each of these transcription factors is required in the ISNb neurons for the specification of motor axon target selection. Without Dfr, Isl or Lim3, these motor axons fail to correctly innervate their designated muscle targets. In addition, genetic interaction studies suggest that this phenotype indicates a common aspect of motoneuron designation has been altered (Certel, 2004).

How might this LIM/POU code function in ISNb neurons? In C. elegans touch receptor neurons, the LIM-HD factor, MEC-3 and the POU protein UNC-86, physically interact to control specification. In the pituitary, the LIM domain of Lhx3 (P-Lim) specifically interacts with the Pit1 POU domain and is required for synergistic interactions with Pit1. Whether Dfr, Isl and or Lim3 physically interact to regulate ISNb motor axon target selection has not been determined. However, misexpression experiments indicate that the re-specification of transverse motoneurons by the addition of Dfr does require functional Isl protein. Although, this result does not distinguish between the possibilities of direct interactions between these proteins or the binding of a common transcriptional target, it does indicate that a functioning 'LIM code' is required for the re-specification of the TN neurons (Certel, 2004).

A second finding of the Dfr misexpression studies is that the target selection of postmitotic neurons can be robustly re-specified. The Lim3B-Gal4 line was used to add Dfr to the LIM-only transverse motoneurons. This Gal4 line does not activate reporter construct expression until stage 14 -- a post-mitotic stage even for the late developing transverse motoneurons. At this stage, the TN motor axons have exited the CNS and are navigating the periphery, although the TN motor axon and LBD fascicle have not come into contact. Misexpressing Dfr even at this relatively late stage of TN motoneuron differentiation can clearly alter axon pathfinding, and in a significant percentage of hemisegments, TN motor axons actually appear to ectopically innervate ISNb muscle targets. This result shows that these motoneurons remain plastic, even after becoming postmitotic and further indicate that the LIM/POU code may be acting directly on genes involved in axon targeting (Certel, 2004).

ventral veins lacking is required for specification of the tritocerebrum in embryonic brain development of Drosophila

The homeotic or Hox genes encode a network of conserved transcription factors which provide axial positional information and control segment morphology in development and evolution. During embryonic brain development of Drosophila, the Hox gene labial (lab) is essential for tritocerebral neuromere specification; lab loss of function results in tritocerebral cells that fail to adopt a neuronal identity, causing axonal pathfinding defects. Evidence is presented that the POU-homeodomain DNA-binding protein ventral veins lacking (vvl) acts genetically downstream of lab in the specification of the tritocerebral neuromere. In the embryonic brain, vvl expression is seen in all brain neuromeres, including the tritocerebral lab domain. Lab mutant analysis shows that vvl expression in the tritocerebrum is dependent on lab activity. Loss-of-function analysis focussed on the tritocerebrum reveals that inactivation of vvl results in patterning defects which are comparable to the brain phenotype caused by null mutation of lab. In the absence of vvl, mutant tritocerebral cells are generated and positioned correctly, but these cells fail to express neuronal markers. This indicates defects in neuronal differentiation. Moreover, longitudinal axon pathways in the tritocerebrum are severely reduced or absent and the tritocerebral commissure is missing in the vvl mutant brain. Genetic rescue experiments show that vvl is able to partially replace lab in the specification of the tritocerebral neuromere. These results indicate that vvl acts downstream of the Hox gene lab and regulates specific aspects of neuronal differentiation within the tritocerebral neuromere during embryonic brain development of Drosophila (Meier, 2006).

vvl is expressed in the embryonic brain from the extended germ band stage onwards. For an analysis of the protein distribution pattern of vvl in the embryonic brain, immunocytochemical experiments with a polyclonal antibody against the Vvl protein were carried out in combination with an anti-HRP antibody; anti-HRP immunoreactivity revealed the entire neural lineage of the developing CNS excluding the glial lineage. At late stage 11, vvl expression is first detected in few neuroblasts of the developing brain anlage, and by stage 13 becomes abundant in neuroblasts and their progeny within each brain neuromere. By stage 15, when neural progeny were generated and axonal projections are formed, Vvl protein is observed in specific cell clusters within all brain neuromeres. Accordingly, a prominent expression domain was also observed in the developing tritocerebrum (Meier, 2006).

Expression of vvl in the tritocerebrum suggested a possible overlap with the expression of the Hox gene lab. To investigate this, double-immunocytochemical experiments were carried out either on transgenic flies expressing a lab-lacZ reporter construct in which antibodies against Vvl were used together with anti-βgal antibodies, or on wildtype embryos in which antibodies against Vvl were used together with anti-Lab antibodies. These experiments revealed that the majority of cells expressing vvl in the tritocerebrum are located within the lab expression domain. Together with the observation that vvl appears to be differentially regulated by lab, the co-expression of lab and vvl in the tritocerebrum suggested that vvl activity might be lab-dependent in this neuromere. To study this further, whether mutational inactivation of lab affects vvl expression in the tritocerebrum was examined (Meier, 2006).

Mutational inactivation of lab results in regionalized axonal patterning defects which are due to both cell-autonomous and cell-nonautonomous effects. Thus, in the absence of lab, mutant cells are generated and positioned correctly in the brain, but these cells do not extend axons. Additionally, extending axons of neighboring wildtype neurons stop at the mutant domains or project ectopically, resulting in the disruption of the longitudinal connectives and a lack of the tritocerebral commissure. To characterize vvl expression in a lab-/- background, double-immunocytochemical experiments were carried out on homozygous lab null mutant embryos using antibodies against Vvl and HRP. These experiments revealed that vvl immunoreactivity is lacking in the tritocerebral lab mutant domain, in addition to the expected lack of anti-HRP immunoreactivity despite the persistence of cells in this region. This suggested that vvl expression in the posterior tritocerebrum is affected by loss of lab function during late stages of embryonic brain development, indicating that vvl expression in the tritocerebrum is lab-dependent (Meier, 2006).

To assess the functional role of vvl in tritocerebral neuromere formation, vvl null mutants were analyzed using immunocytochemical markers including anti-HRP, anti-ELAV, anti-REPO, and anti FASII, which label general neuronal (or glial) domains and tracts in the developing embryonic brain. In vvl loss-of-function mutants, a pronounced brain phenotype is observed in the late stage embryonic brain. Immunolabelling with neuron-specific anti-HRP and anti-FASII antibodies identified a gap separating the deutocerebral brain region from the neuromeres of the more posterior subesophageal ganglion. This dramatic phenotype is associated with severe axonal patterning defects in the embryonic brain. The longitudinal connectives that normally run from the deutocerebral and tritocerebral neuromeres to the subesophageal ganglion were severely reduced or missing and the tritocerebral commissure, which interconnects the brain hemispheres at the level of the tritocerebrum, was completely absent. Analysis of FASII immunoreactivity revealed that descending and ascending axons which in the wildtype normally project through the tritocerebrum in well formed fascicles, fail to project through this domain in vvl mutants. Moreover, a loss of anti-ELAV immunolabelling is observed in the tritocerebral domain, whereas glia-specific anti-REPO immunoreactivity revealed that glial cells are present in the vvl mutant but fail to be correctly localized in the affected region. In addition to the observed defects in the tritocerebral brain region, marked axonal patterning defects in the protocerebrum are also seen in vvl mutant embryos. Moreover the organization of the subesophageal ganglion and the VNC is affected in the vvl mutant. These latter two phenomena were not studied further (Meier, 2006).

At the gross histological level, the vvl mutant brain phenotype described above was, in part, reminiscent of the mutant brain phenotype observed for lab. Since lab and vvl show overlapping expression in the tritocerebral neuromere, and vvl expression is lacking in lab mutants, these findings suggest that either lab expression itself or lab expressing tritocerebral cells are affected in vvl mutant embryos. To investigate this, anti-Lab immunolabelling was carried out in late vvl loss-of-function mutant brains. Surprisingly, despite the expected lack of expression of neuronal differentiation markers, anti-Lab immunolabelling was detected in a wildtype-like pattern in the vvl mutant tritocerebral domain. This suggests that the expression of lab is not affected in the absence of vvl during late stages of embryonic brain development. Moreover, the lab expressing cells in the vvl mutant generally have the same relative position in the brain as does the normal lab expressing cells in the wildtype. Thus, despite the severe axonal patterning defects observed in this domain, mutant cells are generated and appear to be properly positioned in the developing tritocerebrum of the vvl null mutant. This, in turn, suggests that the pattern of proliferation in the tritocerebrum is initiated correctly in the absence of the vvl gene product, but that the cells that normally express vvl might become incorrectly specified in the vvl mutant leading to axogenesis defects. Moreover, the lack of anti-ELAV and anti-HRP immunolabelling together with the observed severe fasciculation defects in the vvl mutant tritocerebrum strongly suggest that mutational inactivation of vvl affects neuronal differentiation in the developing tritocerebrum (Meier, 2006).

These data imply that vvl might be involved in the specification of tritocerebral neuronal identity -- either by acting directly or indirectly downstream of tritocerebral lab activity. To further assess a possible lab-dependent vvl activity in the developing tritocerebrum, the potential of vvl to rescue the lab mutant brain phenotype was determined using the Gal4-UAS system. For this, a transgenic fly line carrying a Gal4 transcriptional activator under the control of the lab promoter together with CNS-specific upstream enhancer elements of the lab gene were used. By crossing this lab::Gal4 line to different UAS-responders it was possible to express the responder constructs in a pattern that corresponded to that of the endogenous lab gene. Using this approach, it has been shown that the lab mutant brain phenotype can be rescued by transgenic expression of the Lab protein in a lab null mutant background. To determine whether vvl might also be able, at least in part, to rescue the lab mutant brain defects, a transgenic UAS::vvl line was used in which the vvl coding sequence was placed under UAS control (Meier, 2006).

As a control, it was first determined whether lab::Gal4 driven misexpression of vvl in a lab+ background had any effects on the development and specification of the tritocerebral Lab domain. In none of these experiments were morphological abnormalities detected in the tritocerebrum or in any other part of the embryonic brain. The UAS::vvl responder was expressed under the control of the lab::Gal4 driver in the lab mutant domain. Remarkably, the Vvl protein was able to rescue specific lab mutant brain defects. Thus, the longitudinal pathways were restored and cells in the mutant domain showed wildtype-like anti-HRP immunolabelling. Moreover, FASII immunoreactivity revealed that descending and ascending axons from other parts of the brain again projected through the tritocerebral lab mutant domain. The vvl responder achieved a rescue efficiency (97.3%) which was comparable to the rescue efficiency of Lab, which was taken as 100%. In contrast, lab::Gal4-specific expression of vvl in the lab mutant domain did not rescue tritocerebral commissure formation, nor correct axonal projection of the frontal connective. This suggested that lab::Gal4 driven misexpression of vvl is sufficient to restore both neuronal marker gene expression like HRP and correct axonal patterning of longitudinal connectives in the lab mutant tritocerebrum. These findings together with the vvl mutant brain phenotype indicate that vvl acts genetically downstream of lab in the specification of the triocerebral neuromere (Meier, 2006).

Taken together, these findings demonstrate that vvl function is required for the specification of the developing tritocerebrum. The vvl gene is important for correct axon guidance and fasciculation of longitudinal connectives in the tritocerebral neuromere. In the absence of vvl, longitudinal and commissural axon pathways are severely affected. Comparable findings have been reported for the role of vvl in VNC development, where vvl mutant embryos exhibit aberrantly localized midline glia and axonal defects in that commissures are often fused and the longitudinal connectives are severly reduced or even disrupted. These findings suggest that vvl acts genetically downstream of the Hox gene lab in the control of regionalized neuronal identity and tritocerebral brain neuromere specification. In accordance with this notion is the successive time of lab and vvl expression in the tritocerebral neuromere. From stage nine onwards, lab expression commences in the intercalary segment and by early stage 11, lab is detected in all neuroblasts of the developing tritocerebrum. Accordingly, by late stage 11, vvl expression is first seen in the tritocerebral neuromere and thus succeeds initial lab expression. Interestingly, this time of initial vvl expression exactly coincides with the temporal requirement of lab for tritocerebral neuronal fate specification. Moreover, the results demonstrate that tritocerebral vvl expression is lab-dependent. In addition, in vvl mutants, lab is expressed normally in the tritocerebrum and yet cells in the affected tritocerebral domain phenocopy the lab mutant brain and do not express molecular markers characteristic of neuronal cells. Furthermore, the vvl gene can mediate neuronal specification and longitudinal connective formation in the absence of the Hox gene lab if expressed under appropriate spatiotemporal control. This indicates that vvl is required for the specification of neuronal identity in the tritocerebral lab domain and is sufficient to provide a permissive substrate for the migration of axons originating from outside this region. However, vvl cannot rescue tritocerebral commissure formation in the lab mutant brain. This indicates that lab exerts at least some of its effects on tritocerebral development through subordinate genes other than vvl (Meier, 2006).

Complex interplay of three transcription factors in controlling the tormogen differentiation program of Drosophila mechanoreceptors

This study has investigated the expression and function of the Sox15 transcription factor during the development of the external mechanosensory organs of Drosophila. Sox15 is expressed specifically in the socket cell, and the transcriptional cis-regulatory module has been identified that controls this activity. Suppressor of Hairless [Su(H)] and the POU-domain factor Ventral veins lacking (Vvl) bind conserved sites in this enhancer and provide critical regulatory input. In particular, Vvl contributes to the activation of the enhancer following relief of Su(H)-mediated default repression by the Notch signaling event that specifies the socket cell fate. Loss of Sox15 gene activity was found to severely impair the electrophysiological function of mechanosensory organs, due to both cell-autonomous and cell-non-autonomous effects on the differentiation of post-mitotic cells in the bristle lineage. Lastly, it was found that simultaneous loss of both Sox15 and the autoregulatory activity of Su(H) reveals an important role for these factors in inhibiting transcription of the Pax family gene shaven in the socket cell; shaven serves to prevent inappropriate expression of the shaft differentiation program. These results indicate that the later phases of socket cell differentiation are controlled by multiple transcription factors in a collaborative, and not hierarchical, manner (Miller, 2009).

After Su(H), Sox15 is the second transcription factor gene known to be activated specifically in the postmitotic socket cell of the Drosophila external sensory organ lineage. Three observations reported here indicate that although both genes come to be expressed at high levels in this cell, the underlying regulatory logic may be quite different (Miller, 2009).

The first is the distinct dynamics of autoregulatory socket enhancer (ASE)-stimulated Su(H) transcription versus Sox15 expression. Su(H) is immediately activated at high levels following the specification of the socket cell, due at least in part to the establishment of an autoregulatory loop working through the Su(H) ASE. Sox15 expression, however, exhibits a significant delay between socket cell specification and the time peak levels of transcript accumulation are achieved (Miller, 2009).

The second observation concerns the role played by Vvl in the activation of the Sox15 socket enhancer and the Su(H) ASE (Barolo, 2000). Conserved within the ASE lies a motif, CATAAAT, that might act as a weak Vvl binding site, suggesting the possibility that Vvl could play a part in the high-level activation of Su(H) in the socket cell. However, this appears not to be the case, since ASE-GFP is activated within the same temporal window, and just as strongly, in vvl mutant clones as in neighboring wild-type tissue. By contrast, while the long reporter fragment Sox7.5 > GFP, covering the whole intron, is also activated in vvl mutant sensory organs, there is a substantial delay in this expression, which is often not detectable until the socket cell has begun to divide aberrantly. At this time, neighboring wild-type sensory organs are already strongly expressing Sox7.5 > GFP. Vvl thus appears to be one factor present in the socket cell that is necessary for the full activation of Sox15, but not of Su(H) (Miller, 2009).

Finally, there is the observed role of N-activated Su(H) in contributing to the transcriptional activation of the Sox15 socket enhancer versus the Su(H) ASE. A major difference between the two genes is made apparent by the contrasting effects on reporter gene expression of mutating the high-affinity Su(H) site(s) in their respective socket cell enhancers. In the case of the Su(H) ASE, mutation of the Su(H) sites causes a strong reduction in socket cell activity at early times, along with ectopic activity in the shaft cell; by the adult stage, the mutant enhancer is inactive. Thus, N-activated Su(H) contributes critically to the transcriptional activation of the Su(H) ASE. The Su(H)-site-mutant Sox15 enhancer, on the other hand, shows no apparent diminution of its socket cell activity early (when it also drives ectopic expression in the shaft cell), and remains fully active in the pharate adult. In the case of Sox15, then, activation of Su(H) by the N signaling event appears to serve only the purpose of relieving Su(H)-mediated default repression; activation of the enhancer is evidently accomplished entirely through the action of other factors such as Vvl. This distinction in the role of N signaling in enhancer activation has been referred to as 'Notch instructive' [Su(H) ASE] versus 'Notch permissive' (Sox15 socket enhancer) (Miller, 2009).

This investigation of the loss-of-function phenotype of Sox15 has revealed that, like Su(H), it has an important role in controlling the socket cell differentiation program. Comparison of the phenotypic effects of losing Sox15 function, Su(H) function, or both, suggests an incomplete overlap in the target gene batteries regulated by the two factors. Loss of either Sox15 or Su(H) ASE activity causes a serious defect in mechanosensory organ function. The lack of Su(H) ASE activity confers the more severe phenotype, including significant reductions of both transepithelial potential (TEP) and mechanoreceptor current (MRC). The TEP defect signifies an inability of the socket cell to establish the receptor lymph cavity itself, the proper ionic composition of the receptor lymph, or a combination of the two. The genes required for these events have yet to be identified, but it is likely that Su(H) plays a role in regulating their expression in the socket cell. Sox15, on the other hand, does not appear to share this role, based on the apparent lack of a major TEP defect in Sox15 mutants. Instead, Sox15 appears to regulate targets that contribute to socket cell viability. Without these target factors, the cell eventually becomes necrotic. In addition, the principal physiological phenotype of Sox15 mutants is the MRC defect, which is also conferred by loss of Su(H) ASE function. Loss of MRC is indicative of a failure in neuronal function, yet both Sox 15 and the Su(H) ASE are active specifically in the socket cell. This apparent paradox indicates an important role for the socket cell as a support cell for the mechanosensory neuron. To date three proteins - Sox15 (this paper), Su(H), and the cytochrome P450 Cyp303a1 - expressed in and required specifically for socket cell differentiation appear to contribute to neuronal function in mechanosensation. Given that the socket cell envelops the other cells of the sensory organ as it develops, the socket may be intimately involved in their normal differentiation and in the establishment of structural and functional connectivity between them. Defects in these processes could readily manifest themselves in an MRC phenotype. Thus, the abnormal microtubule bundling in the sensory dendrite in Sox15 mutants may very well be the result of a defect in the socket cell's ability to contribute as it should to the neuron's normal development. It is unclear at this point if the dendrite defect is due to a failure to activate Sox15-dependent target genes directly involved in the socket cell's support function, or if it is an indirect consequence of the degeneration of the socket cell (Miller, 2009).

Previous studies have established that both daughters of the pIIa secondary precursor division are bipotent cells that can adopt either the shaft or socket cell fate. Asymmetric N signaling specifies that the posterior daughter expresses only the signal-dependent socket fate and the anterior daughter only the signal-independent shaft fate. Correspondingly, investigation of socket cell fate specification has largely focused on its positive aspects; i.e., those ways in which the N signaling event promotes the socket cell from the 'default' (signal-independent) shaft fate to the alternative fate, triggering its execution of the distinctive socket differentiation program. This study has shown that socket cell-specific activation of Sox15 expression is an important component of this program. But the present study has also revealed the other side of the coin, by showing that the N signaling event also results in the activation of a mechanism for suppressing in the socket cell the capacity to execute the shaft differentiation program. This suppression mechanism involves the combined action of Sox15 and Su(H) in inhibiting transcription of the sv gene, which encodes a Pax transcription factor that is a high-level activator of the shaft differentiation program. Without this inhibition, the socket cell generates both socket and shaft cuticular structures. It is clear, then, that much of the network circuitry necessary for the execution of the shaft differentiation program remains intact in the socket cell even after its fate has been specified. These results show that robust N-mediated cell fate specification in the mechanosensory bristle lineage involves not only promoting the signal-dependent fate, but also actively inhibiting the alternative program (Miller, 2009).

It is likely that at least Su(H)'s role in inhibiting sv expression in the socket cell is indirect, and occurs via an as yet unidentified repressor. An attractive candidate for this factor X would be one or more basic helix-loop-helix (bHLH) repressors encoded in the Enhancer of split Complex [E(spl)-C]. Multiple E(spl)-C bHLH repressor genes are activated directly by Su(H) in response to N signaling in a variety of developmental contexts. Consistent with this possibility, it was observed that socket cell-specific overexpression of E(spl)m7-VP16, a form of the E(spl)m7 bHLH repressor that has been converted to a strong activator, phenocopies the ectopic-shaft effect of sv overexpression in the same cell (Miller, 2009).

The results of this and earlier studies afford a glimpse of the regulatory architecture of the socket differentiation program, which is set in motion by the N signaling event that specifies the socket cell fate. It seems useful to distinguish two broad phases of this program, which no doubt overlap each other in time and are also very likely to share at least some components of the regulatory network. These two phases might be referred to as the earlier 'morphogenetic' and the later 'physiological' subdivisions of the socket program. The distinction is prompted by the observations of the phenotypes conferred by loss of the two socket cell-specific transcription factor activities identified so far, Su(H) and Sox15. In both cases, it was found that many characteristic aspects of the socket's cellular differentiation proceed completely normally, most notably the construction of the complex socket cuticular structure that surrounds the shaft structure (morphogenesis). By contrast, loss of Su(H) or Sox15 function in the socket cell results in major deficits in the electrophysiological capacity of the sensory organ (physiological differentiation). As described above, the specifics of these deficits differ for Su(H) versus Sox15 mutants, and include distinctive cell-autonomous defects in the socket cell and defects in other cells likely due to the failure of some aspects of the socket cell's support function. But the phenotypic commonalities (emphasizing the physiological and not the morphogenetic) are striking nonetheless. It is perhaps reasonable to speculate that transcription factors like Su(H) and Sox15 that are activated for the first time in the sensory organ lineage specifically in the socket cell will tend to function primarily in the later physiological phase of the differentiative program. By contrast, it may be expected that the earlier morphogenetic phase is controlled primarily by factors first expressed earlier in the lineage, at least in the pIIa precursor cell and perhaps in the SOP. Vvl exemplifies this notion: It is first expressed in the SOP, and loss of its activity causes visible defects in the socket cuticular structure, as well as aberrations in the mitotic status of the normally postmitotic socket cell. Investigation of the roles of additional transcriptional regulators in directing the socket differentiation program will test the viability of this broad conceptual framework (Miller, 2009).

Overall, this comparison of the roles of Sox15, Su(H), and Vvl in controlling aspects of the socket differentiation program indicates that they function largely in parallel, and collaboratively, rather than in a hierarchical fashion. This may suggest that the socket program will prove to be characterized by an ensemble of such parallel regulatory inputs that collectively direct the complex differentiation of the cell. It is perhaps useful to note that this picture contrasts already with what is known about the control of the shaft differentiation program, which is dominated by the function of Sv as a high-level regulator. Whether this reflects some important difference in how the differentiative programs of N-responsive versus N-non-responsive cell types are controlled will become clearer as more is learnt about the gene regulatory network that underlies mechanosensory organ development (Miller, 2009).

Regulation of locomotion and motoneuron trajectory selection and targeting by the Drosophila homolog of Olig family transcription factors

During the development of locomotion circuits it is essential that motoneurons with distinct subtype identities select the correct trajectories and target muscles. In vertebrates, the generation of motoneurons and myelinating glia depends on Olig2, one of the five Olig family bHLH transcription factors. This study investigated the so far unknown function of the single Drosophila homolog Oli. Combining behavioral and genetic approaches, this study demonstrates that oli is not required for gliogenesis, but plays pivotal roles in regulating larval and adult locomotion, and axon pathfinding and targeting of embryonic motoneurons. In the embryonic nervous system, Oli is primarily expressed in postmitotic progeny, and in particular, in distinct ventral motoneuron subtypes. oli mediates axonal trajectory selection of these motoneurons within the ventral nerve cord and targeting to specific muscles. Genetic interaction assays suggest that oli acts as part of a conserved transcription factor ensemble including Lim3, Islet and Hb9. Moreover, oli is expressed in postembryonic leg-innervating motoneuron lineages and required in glutamatergic neurons for walking. Finally, over-expression of vertebrate Olig2 partially rescues the walking defects of oli-deficient flies. Thus, these findings reveal a remarkably conserved role of Drosophila Oli and vertebrate family members in regulating motoneuron development, while the steps that require their function differ in detail (Oyallon, 2012).

The generation of coordinated muscle contractions, enabling animals to perform complex movements, depends on the assembly of functional neuronal motor circuits. Motoneurons lie at the heart of these circuits, receiving sensory input directly or indirectly via interneurons within the central nervous system (CNS) and relaying information to muscles in the periphery. During development neural precursors give rise to progeny that eventually adopt unique motoneuron subtype identities. Their axons each follow distinct trajectories into the periphery to innervate specific target muscles. Understanding of the molecular mechanisms that control the differentiation and respective connectivity of distinct neuronal subtypes is still limited (Oyallon, 2012).

The Olig family of basic Helix-Loop-Helix (bHLH) transcription factors in vertebrates includes the Oligodendrocyte lineage proteins Olig1-3, Bhlhb4 and Bhlhb5. All members play pivotal roles in regulating neural development. Olig2 controls the sequential generation of somatic motoneurons and one type of myelinating glia, the oligodendrocytes, from the pMN progenitor domain in the ventral neural tube. Olig2 mediates progenitor domain formation by cross-repressive transcriptional interactions and motoneuron differentiation upstream of the LIM-homeodomain containing transcription factors Lim3 (Lhx3) and Islet1/2 (Isl1/2). Downregulation of Olig2 enables Lim3 and Isl1/2 together with the proneural bHLH transcription factor Neurogenin2 (Neurog2) to activate the expression of Hb9, a homeodomain protein and postmitotic motoneuron determinant. In addition, Olig2 cooperates with the homeodomain protein Nkx2.2 to promote oligodendrocyte formation from uncommitted pMN progenitors. Olig1 mediates gliogenesis redundantly with Olig2, while Olig3 controls interneuron specification within dorsal neural tube progenitor domains. Recent studies uncovered important requirements of Bhlhb4 in retinal bipolar cell maturation, and Bhlhb5 in regulating the specification of retinal amacrine and bipolar cells, area-specific identity acquisition and axon targeting of cortical postmitotic neurons, as well as differentiation and survival of distinct interneuron subtypes in the spinal cord. In Drosophila, genome-wide data base searches identified one single family member, called Olig family (Oli)), and a recent study described Oli expression in the embryonic ventral nerve cord (VNC). However, despite the central roles of vertebrate Olig family members, the function of their Drosophila counterpart has not been investigated (Oyallon, 2012 and references therein).

In Drosophila, neurons are derived from stem cell-like neuroblasts (NBs). These divide asymmetrically to generate secondary precursor cells, the ganglion mother cells (GMCs), which divide once to produce two postmitotic neurons and/or glia. 15 of 30 embryonic NB lineages give rise to 36 motoneurons in addition to interneurons per abdominal hemisegment. Zfh1 regulates general motoneuron fate acquisition at the postmitotic level. The specification of ventrally projecting motoneuron subtypes is mediated by a combinatorial expression of five transcriptional regulators -- the fly orthologs of Isl, Lim3, Hb9 and Nkx6, as well as the POU protein Drifter (Dfr; Ventral veinless -- FlyBase). Many of these determinants are highly conserved, raising the question as to whether Oli functions as part of this genetic network that shapes motoneuron diversity. Although related molecules in vertebrates and invertebrates appear to mediate late aspects of glial function, factors that regulate early steps of gliogenesis and are molecularly and functionally conserved have so far not been identified. Olig2 is essential for oligodendrocyte development in vertebrates, and a recent study also implicated the C. elegans homolog Hlh-17 in regulating gliogenesis). Thus, Oli is also a potential candidate that could control early glial development in Drosophila (Oyallon, 2012).

This study provides insights into the so far unexplored function of the Oli bHLH transcription factor in the Drosophila nervous system. Oli is not required in glia; however, taking advantage of the well-defined embryonic motoneuron lineages and axonal projections, this study demonstrates that oli controls trajectory selection and muscle targeting of ventral motoneuron subtypes. Moreover, Oli is expressed in postembryonic lineages, which include glutamatergic leg-innervating motoneurons. Loss-of-function experiments revealed that oli is required for larval and adult locomotion. Chick Olig2 can partially rescue these defects in adults, highlighting at least one evolutionarily conserved role of Olig transcription factors in flies and vertebrates (Oyallon, 2012).

Oli protein is mainly expressed in postmitotic neurons, as well as in some GMCs during embryonic development. This is consistent with in situ hybridization labeling detecting high levels of oli mRNA in postmitotic progeny, in addition to transient expression in MP2 and 7.1 NBs. Oli is also expressed in postmitotic progeny of postembryonic lineages. By contrast, vertebrate Olig2 is required in progenitors to promote commitment to a general motoneuron identity. Also Olig1 and 3 largely function in progenitors. Interestingly, Bhlhb4 and Bhlhb5 are expressed and required in postmitotic progeny of the retina, brain and spinal cord. Thus, with respect to its primarily postmitotic expression, Drosophila Oli resembles more that of Bhlhb4 and Bhlhb5 than Olig1–3 in vertebrates (Oyallon, 2012).

The dynamic expression of Drosophila Oli is not consistent with that of a member of the temporal series of transcriptional regulators. With the latter, neurons largely maintain the determinant they expressed at the time of their birth. By contrast, Oli is widely expressed in newly born progeny, but subsequently levels decrease, and only some subtypes show high expression during late stages. Vertebrate Olig2 acts as a transcriptional repressor in homomeric and heteromeric complexes, and expression is downregulated in differentiating motoneurons to enable the activation of postmitotic determinants such as Hb9 by Lim3, Isl1/2 and Neurog2 . Strikingly in flies, Oli expression decreases in RP and lateral ISNb motoneurons during embryogenesis and prolonged high expression of Oli elicits muscle innervation defects, supporting the notion that Oli downregulation is critical for its function in some neurons. Oli could thus act in a dual mode to regulate the differentiation of neuronal subtypes. The first one may rely on downregulation and be a feature shared with vertebrate Olig2, the second one may require persistent activity, and possibly be a feature more in common with Bhlhb4 and Bhlhb5 family members (Oyallon, 2012).

The findings indicate that Drosophila Oli, unlike vertebrate Olig2, does not act as a general early somatic motoneuron determinant. It rather contributes to shaping ventral motoneuron subtype development as part of a postmitotic transcriptional regulatory network in concert with Drosophila Lim3, Isl, Hb9 and Dfr (Drifter/Vvl). This notion is supported by findings that (1) Oli is co-expressed in specific combinations with these determinants in differentiated ISNb and TN motoneuron subtypes; (2) similar to other ventral determinants, oli mutant embryos display distinct axonal pathfinding and muscle targeting defects; (3) oli does not act upstream of hb9, isl, lim3 or Dfr; and (4) oli and hb9 genetically interact, as loss of both enhances phenotypes in ISNb axons. Because of the proximity of oli, isl and lim3 genetic loci, it has so far not been possible to further extend these interaction assays. Some defects observed in oli mutants, such as failure to innervate the clefts of muscles 12/13 or aberrant contacts between ISNb and TN motoneurons are qualitatively similar to those observed in isl, lim3, hb9 and dfr, while the phenotype of isl-τ-myc-positive neurons abnormally exiting the VNC via the SN branch appears characteristic for oli. Moreover, the connectivity phenotypes observed in oli gain-of-function experiments were not reminiscent of trajectories of other motoneuron subtypes. This suggests that although Oli is a member of the combinatorial code, unlike for instance Dfr, it does not act as a simple switch between fates. It may rather act in concert or partially redundantly with these other determinants in regulating the stepwise process of axon guidance to ensure robustness of trajectory selection (Oyallon, 2012).

Individual transcription factors within an ensemble may regulate different biological properties to tightly coordinate the differentiation and synaptic connectivity of a given neuron subtype. As Oli does not act upstream of Isl, Lim3, Hb9 and Dfr, it may control the expression of other yet to be identified transcription factors, or - similar to dfr, Nkx6 and eve in Drosophila and Bhlhb5 in mice - axon guidance determinant or - as reported for Neurog2 - cytoskeletal regulators. Examining Fasciclin 3, N-Cadherin, PlexinA, and Frazzled, no obvious altered expression was observed in the absence of oli). Thus, future studies using approaches such as microarrays will be required to identify oli downstream targets that control subtype-specific axonal connectivity (Oyallon, 2012).

While the role of oli in controlling neuronal development linked to locomotion appears conserved in Drosophila and vertebrates, conservation does not extend to glia. Oli is neither expressed in glia during embryonic or postembryonic development, nor is it essential for basic glial formation in the embryonic VNC or required in glia for locomotion. This also applied to other parts of the nervous system, such as the 3rd instar larval visual system endowed with large glial diversity. hlh-17, the C. elegans Oli homolog, is expressed in cephalic sheath glia in the brain, and interestingly in some motoneurons in the larval CNS. However, as analysis of hlh-17 mutants could not pinpoint any requirement in glial generation and differentiation possibly due to redundancy with related factors, the precise role of the worm homolog remains elusive. Although ensheathing glia can be found in both invertebrates and vertebrates, myelinating glia have so far only been identified in vertebrates. This raises the possibility that the glial requirement of vertebrate Olig family members could be secondary, and Olig2 may have been recruited to collaborate with additional transcriptional regulators to promote the formation of myelinating glia. Indeed, Olig2 promotes motoneuron development together with Neurog2, and subsequently collaborates with Nkx2.2 to enable the generation of oligodendrocyte precursors and differentiating offspring from newly formed, uncommitted pMN progenitors. Interestingly in cell-based assays, Oli can physically interact with the Nkx2.2 homolog Ventral nervous system defective (Vnd). Together with the current observation that Oli is not essential for glial development, this suggests that the potential of these determinants to interact is evolutionarily conserved, while the steps depending on them diverged in flies and vertebrates (Oyallon, 2012).

The locomotion defects in oli mutant larvae are likely the consequence of embryonic wiring defects, whereas the adult phenotypes may be due to an additional or even sole postembryonic requirement. Unlike the so far identified widely expressed determinants Chinmo, Broad Complex or Castor in the postembryonic VNC, Oli expression is restricted to distinct lineages. That these include motoneurons is supported by observations that Oli is detected in postembryonic lineages 20-22 and 15, and expression overlaps with that of OK371-Gal4. Moreover, locomotion defects can be partially rescued by over-expressing oli in glutamatergic neurons with this driver. This initial characterization raises many new questions regarding the specific postembryonic role of Oli. Because of the expression in lineage 15, future experiments will need to specifically test, whether oli contributes to consolidating motoneuron subtype identity by regulating dendritic arbor-formation or leg muscle innervation with single cell resolution. The wider expression of Oli and the partial rescue with OK371-Gal4 further suggest a requirement of oli in lineages that are part of locomotion-mediating neural circuits beyond motoneurons. Because of the expression pattern and the severe walking defects of adult oli escapers, these observations open the door for future functional studies to unravel the mechanisms that shape neural circuits underlying adult locomotion (Oyallon, 2012).


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drifter: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 23 August 2014

 

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