numb mutants lack most the peripheral nervous system neurons. Neural and glial precursors are transformed into extra support cells in numb mutants (Uemura, 1989). In addition to affecting external sensory and chordotonal neurons, both numb and cut are required for the proper differentiation of multiple dendritic neurons. cut acts as a selector-type gene and is required to initiate the correct developmental program. numb is responsible for development of the neural fate (Uemura, 1989 and Brewster, 1995).

Each sensory organ precursor (SOP) cell undergoes two divisions. One daughter of the first division (IIa) gives rise to bristle and socket cells; and the other (IIb) gives rise to neuron and sheath (glial cell). Numb protein is asymmetrically distributed to the IIb daughter and is necessary to specify IIb cell fate (Knoblich, 1995, Spana, 1995 and Rhyu, 1994).

numb is also expressed in the central nervous system in specific neural precursors. The MP2 precursor develops from a proneural cluster and is morphologically identical to a neuroblast, but it has a much simpler cell lineage. MP2 divides asymmetically to produce a small ventral cell (vMP2) and a larger dorsal cell (dMP2). Subsequently, the neurons migrate to the same dorsoventral plane (vMP2 anterior to dMP2), and extend axons in opposite directions (the more anterior cell's axon extends in an anterior direction, and the more posterior cell's axon extends in a posterior direction). vMP2 is an interneuron with posterior axon projection. During MP2 mitosis, Numb is asymmetrically localized to the dorsal cortex of the MP2 and selectively partitioned into the dMP2 neuron. In the absence of Numb, the dMP2 is transformed into the vMP2 fate: conversely, ectopic Numb can transform vMP2 into dMP2. Numb acts as an intrinsic determinant of dMP2 fate (Spana, 1995).

Asymmetric cell division is a widespread mechanism in developing tissues that leads to the generation of cell diversity. For the most part the basis of asymmetric cell division has been analyzed in neuroblasts in the process by which neuroblast division yields another neuroblast and a secondary precursor cell: the ganglion mother cell (GMC). In the embryonic central nervous system of Drosophila melanogaster, GMCs divide and produce postmitotic neurons that take on different cell fates. The current study analyses the process of binary fate decision of two pairs of sibling neurons that occurs during cell division in GMCs. This process is accomplished through the intrinsic fate determinant, Numb. GMCs have apical-basal polarity; Numb localization and the orientation of division are coordinated to segregate Numb to only one sibling cell. The correct positioning of Numb and the proper orientation of division require Inscuteable (Insc). Loss of insc results in the generation of equivalent sibling cells. These results provide evidence that sibling neuron fate decision is nonstochastic and normally depends on the presence of Numb in one of the two siblings. Moreover, the data suggest that the fate of some sibling neurons may be regulated by signals that do not require lateral interaction between the sibling cells (Buescher, 1998).

The focus for the analysis of the roles of insc, numb, and components of the N-signaling pathway in fate specification, was on the only two pairs of GMC-derived neurons for which sibling relationships have been established: the RP2/RP2sib and the aCC/pCC neurons. These neurons are derived from two GMCs that can be identified unambigously by their specific expression of the nuclear protein Even-skipped (Eve). GMC1-1a divides into the aCC/pCC neurons that have approximately equal size and continue to express Eve. However, at later stages of development, aCC is distinguished from pCC by the expression of Zfh-1 and 22C10 (a membrane associated antigen). aCC is a motoneuron and forms an ipsilateral projection that pioneers the intersegmental nerve. GMC4-2a divides to form the sibling neurons RP2/RP2sib that are morphologically distinguishable. In 88% of the hemisegments, the newborn siblings show a significant difference in the size of their nuclei and cell bodies. This asymmetry appears to be initiated during cell division. In GA1019 mutant embryos, in which GMC4-2a fails to complete cytokinesis, cells are formed that contain one large and one small nucleus. This strongly suggests that the difference in size is generated early, prior to the completion of cytokinesis. The larger cell always adopts the RP2 fate, which is characterized by the expression of Eve, Zfh-1, and 22C10. RP2 forms an antero-ipsilateral projection. The smaller sibling always adopts the RP2sib fate, which is characterized by a further decrease in cell and nuclear size and the loss of Eve immunreactivity. Zfh-1 and 22C10 expression have not been shown in RP2sib. These observations suggest that the cell and nuclear size difference may serve as an early physical marker that will allow one to differentiate between the two progeny of GMC4-2a, irrespective of the molecular markers they express later (Buescher, 1998).

Mutations in mastermind (mam),sanpodo, and Notch equalize aspects of sibling cell fate but retain the difference in cell and nuclear size of sibling neurons. In mam mutant embryos, both progeny of GMC4-2a can adopt the RP2 fate with respect to Eve, Zfh-1, and 22C10 expression. However, despite this apparent change from the RP2sib to the RP2 cell fate, the unequal sizes of the GMC4-2a daughter cells remain; that is, their sizes are unaffected. mam is required for the correct fate specification of RP2sib and pCC but not for that of RP2 and aCC. The requirement for mam suggests that N signaling may be involved in the resolution of distinct sibling neuron cell fate. Mutations in mam and N result in similar defects and support the notion that N signaling is required for the resolution of sibling neuron fate. In inscuteable mutant embryos, GMC1-1a and GMC4-2a are correctly formed and express normal levels of Eve (and in the case of GMC4-2a, also Pdm-1). However, GMC1-1a divides to form two sibling neurons that both adopt the aCC fate (94%) with respect to marker gene expression. Similarly, GMC4-2a division results in two sibling cells, both of which adopt the RP2 fate (96%) with respect to expression of Eve, Zfh-1, and 22C10, as well as axon morphology. This strongly suggests that in wild-type embryos, the divisions of GMC1-1a and GMC4-2a are asymmetric in an insc-dependent manner and produce sibling cells that are intrinsically different; loss of insc function leads to the generation of sibling neurons with equivalent cellular identities. Moreover, in contrast to mam, sanpodo, and Notch mutant embryos, the duplicated RP2s seen in insc mutants are equal with respect to their cell and nuclear size. These observations are consistent with the idea that the size difference seen in wild-type embryos is generated by an insc-dependent process during the GMC cell division and occurs prior to the events mediated by mam, spdo, and N that presumably act at the level of the postmitotic sibling cells. No size asymmetry between the sibling neurons should be generated in an insc background regardless of whether the other functions (e.g., spdo) are present or not (Buescher, 1998).

Staining of wild-type embryos with anti-Insc antibody reveals that Insc is expressed in many, and possibly all, GMCs. During interphase, Insc protein is found cortically. In dividing GMCs, anti-Insc immunoreactivity is always seen as an apical/near apical crescent with respect to the surface of the embryo, that is, Insc is localized in the part of the cell located toward the ventral (outside) portion of the embryo. These observations suggest that GMCs possess apical-basal polarity and that their division may be asymmetric in an insc-dependent manner. If the asymmetry of GMC division leads to the generation of pairs of sibling cells that are nonequivalent, then which are the cell fate determinants that are segregated differentially? One such candidate is Numb: it is widely expressed in the developing CNS and has been shown to act as an intrinsic fate determinant in the SOP and MP2 lineages. Moreover, asymmetric Numb localization has been shown to be Insc-dependent in NBs (Buescher, 1998).

To see if Numb is asymmetrically localized in GMC1-1a and GMC4-2a in an Insc-dependent manner, wild-type and insc mutant embryos were stained with anti-Numb, anti-Eve, and DNA stain. In wild-type embryos, Numb always forms a crescent at or near the basal cortex of dividing GMCs, whereas in insc mutant embryos, crescents are either not formed or appear in basal and occasionally in lateral or apical positions. Although it is difficult to directly visualize the orientation of the mitotic spindle in GMCs, the positioning of the metaphase plate in wild-type strongly suggests a perpendicular orientation with respect to the apical surface of the embryo. Consistent with a horizontally placed cleavage plane, the newborn RP2/RP2sib cells are always oriented perpendicular or nearly perpendicular to the apical surface, with the larger cell located in the more dorsal (basal) position. It is concluded that during GMC4-2a division, Numb will be segregated predominantly or exclusively to the future RP2 neuron. In addition, the orientation of metaphase plate in dividing GMCs other than GMC4-2a is also horizontal with respect to the apical surface, indicating that the division of many, possibly all, GMCs are stereotyped in the same manner as NB divisions. Consistent with its function in NBs, loss of insc alters the orientation of GMC division: In 74% of the samples the metaphase plate is oriented perpendicular or close to perpendicular to the apical surface. Accordingly, the newborn RP2/RP2sib are frequently oriented horizontally with respect to the apical surface. Taken together, these results indicate that the apical-basal polarity, which is found in NBs is maintained in their daughter cells, the GMCs, and that Numb localization as well as spindle orientation are coordinated to ensure asymmetric segregation of Numb into one sibling only. So far, insc appears to be the only gene that is required for the apical-basal polarity of GMCs: the analysis of mam, spdo, and N mutant embryos reveals that none of these mutations alter the plane of GMC division. This observation is in agreement with the notion that these genes act later than insc in sibling neuron fate determination (Buescher, 1998).

It has been shown that the the correct positioning of Numb requires Inscuteable; Numb acts downstream of Insc. Taken together, the data show that asymmetric fate determination in the GMC4-2a lineage involves the same components as fate specification in the MP2 and SOP lineages and may be accomplished through productive (effective) N signaling in one sibling (RP2sib) and inhibited (ineffective N signaling) in the Numb-containing sibling (RP2). However, in the case of asymmetric fate determination in GMCs, Notch function is determined through an intracellular, Numb based mechanism, and not through a lateral inhibition mechanism. Consistent with this notion, the ectopic expression of N-intra (a constitutively active form of N) causes the same RP2 to RP2sib fate transformation as numb loss of function, implying that N-intra ectopic expression can override the effects of Numb (Buescher, 1998).

The loss of size asymmetry in insc mutant embryos is not restricted to the RP2/RP2sib lineage but is also observed in muscle progenitor cell divisions. For example, when the muscle progenitor P15 divides, the daughter cells are not equal in size; it is the larger of the two daughter cells that preferentially inherits the Numb that is asymmetrically localized in the dividing muscle progenitor. Similar to the RP2/RP2sib situation, removing insc function appears to equalize the size of the daughter cells derived from the P15 cell division. In contrast, this equalizing effect does not occur in NBs. NB division is highly asymmetric: each division generates a new NB and a GMC that is several times smaller than the NB. In insc mutants, NBs often bud off GMCs in lateral (rather than basal) positions, but the size asymmetry is retained. At present, it is not understood how size asymmetry is generated during progenitor cell division and if, and how, it might be linked to spindle orientation (Buescher, 1998).

During metazoan development, cell-fate diversity is brought about, in part, by asymmetric cell divisions. In Drosophila, bristle mechanosensory organs are composed of four different cells that originate from a single precursor cell (pI) after two rounds of asymmetric division. At each division, distinct fates are conferred on sister cells by the asymmetric segregation of Numb, a negative regulator of Notch signaling. The orientation of the mitotic spindles and the localization of the Numb crescent follow a stereotyped pattern. Mitosis of pI is oriented parallel to the anteroposterior axis of the fly. In all cases, Numb is distributed in an anterior crescent, and is segregated to the anterior daughter cell. The posterior daughter cell, pIIa, divides to generate the shaft and socket cells, and the anterior daughter cell, pIIb, divides to give rise to the neuron and the sheath cells. In all cases Numb accumulates in the anterior pole of pIIa, next to pIIb, and segregates to the anterior daughter, which differentiates into a shaft cell. pIIb divides soon after pIIa. Mitotic spindles of pIIb are oriented roughly orthogonal to the previous pIIa division axis. Numb is localized at the lateral pole, that is, away from the midline of the pIIb cell. As the lateral daughter cell inherits Numb, it is predicted that the lateral cell adopts a neuronal fate. The pattern of oriented mitosis is probable essential for sensory functions. Changing the identity of pIIb into a second pIIa by ectopic Notch signaling results in a second pIIa, which orients itself in a similar position to that of the original pIIa (Gho, 1998).

Signalling mediated by the Frizzled receptor polarizes pI along the A/P axis, thereby specifying the orientation of the mitotic spindle and positioning the Numb crescent. Mitoses in fz and dishevelled result in randomly oriented pI divisions in the epithelial plane. The Numb crescent also localizes in a random manner, however, its position is tightly correlated with the position of one pole of the misoriented spindle. Only pI cells respond to Fz/Dsh signaling. It is concluded that the polarity of the three mitotic cells in the bristle lineage are regulated by distinct mechanisms. Inscuteable is unlikely to be the pI organizer because clonal analysis shows that insc regulates neither bristle differentiation nor polarity in the notum. Thus the organizer acting downstream of Fz signaling (upstream of Numb) in planar divisions remains to be identified (Gho, 1998).

An important issue in Metazoan development is to understand the mechanisms that lead to stereotyped patterns of programmed cell death. In particular, cells programmed to die may arise from asymmetric cell divisions. The mechanisms underlying such binary cell death decisions are unknown. A Drosophila sensory organ lineage is described that generates a single multidentritic neuron in the embryo. This lineage involves two asymmetric divisions. Following each division, one of the two daughter cells expresses the pro-apoptotic genes reaper and grim and subsequently dies. The protein Numb appears to be specifically inherited by the daughter cell that does not die. Numb is necessary and sufficient to prevent apoptosis in this lineage. Conversely, activated Notch is sufficient to trigger death in this lineage. These results show that binary cell death decision can be regulated by the unequal segregation of Numb at mitosis. This study also indicates that regulation of programmed cell death modulates the final pattern of sensory organs in a segment-specific manner (Orgogozo, 2002).

The vmd1a neuron is located within a cluster of five multidendritic (md) neurons in the ventral region of abdominal segments A1-A7. The vmd1a neuron can be distinguished from the other ventral md neurons (vmd1-4) using the B6-2-25 enhancer-trap marker. The origin of this vmd1a neuron is not known. vmd1-4 neurons are generated by the four vp1-4 external sensory (es) organ primary precursor (pI) cells. Each vp1-4 pI cell follows a lineage called the md-es lineage. This lineage is composed of four successive asymmetric cell divisions that generate five distinct cells, the four cells of the es organ at the position where the pI cell has formed and one md neuron that will then migrate to the ventral md cluster. In the md-es lineage, the membrane-associated protein Numb is segregated into one of the two daughter cells at each cell division. Numb establishes a difference in cell fate by antagonizing Notch in the Numb-receiving cell. Because no es organ is found in the vicinity of the vmd1a neuron, this neuron is probably not generated by a md-es lineage (Orgogozo, 2002).

Cut has proved to be a useful lineage marker for establishing the md-es lineage since it is expressed in the pI cell and in all its progeny cells. To determine the origin of the vmd1a neuron, Cut was used as a marker since it accumulates in the vmd1a neuron. This analysis shows that the vmd1a neuron stems from a pI cell that divides asymmetrically twice. This vmd1a pI cell appears as an isolated Cut-positive cell located anterior to the pIIa-pIIb cell cluster present at the vp1 position. The vmd1a pI cell divides within the plane of the epithelium with Numb localized asymmetrically and segregating into one daughter cell. Surprisingly, at a later stage when three cells are present at the vp1 position, only a single Cut-positive cell is detected at the vmd1a position instead of the two seen earlier. This cell was named pIIb. The pIIb cell undergoes an asymmetric division oriented along the dorsal-ventral axis of the embryo, with the cell-fate determinants Numb and Prospero (Pros) segregating into the dorsal daughter cell. This second division in the vmd1a lineage produces a dorsal cell with high levels of Pros and a ventral cell with low levels of Pros. The ventral cell becomes undetectable around the time when the vp1 cluster is composed of five cells. In contrast, the dorsal cell, marked by high level of Pros, accumulates Elav, a neuronal marker. This cell is identified as the vmd1a neuron as it also expresses the enhancer-trap markers E7-2-36 (an md marker) and B6-2-25 (a vmd1a marker). It is concluded from this lineage study that the vmd1a neuron is born from a pI cell that divides asymmetrically twice to produce the vmd1a neuron and two daughter cells of unknown fate. The latter cells were named pIIa and pIIIb. The pIIa and pIIIb cells in the vmd1a lineage die via apoptosis. Consistently, TUNEL-positive nuclear fragments as well as ß-galactosidase-positive cytoplasmic fragments of CutA3-lacZ-expressing cells were observed at the position of the pIIa and pIIIb cells. These fragments are interpreted as the nuclear and cytoplasmic remnants of the two dying cells. In the md-es lineage, the pIIa and pIIIb cells generate the shaft/socket and the neuron/sheath cell pairs, respectively. In the absence of apoptosis, the pIIa and pIIIb cells in the vmd1a lineage were observed to generate ectopic shaft/socket and neuron/sheath cell pairs. It is concluded that in the absence of apoptosis, the vmd1a lineage is completely transformed into an md-es lineage (Orgogozo, 2002).

In the ventral region of segment A8, five Cut-positive md neurons are found. By contrast to segments A1-A7, no external sensory organs are observed in this ventral region. To test whether each of the five ventral A8 md neurons is generated via an apoptotic lineage similar to the one described for vmd1a, the ventral region of segment A8 was studied in embryos homozygous for the H99 deficiency. Remarkably, five ectopic es organs were observed in this region. These data indicate that five pI cells follow an apoptotic lineage similar to the vmd1a lineage in the ventral region of segment A8. Furthermore, analysis of wild-type stage 12 embryos shows that five Cut-positive pI cells form in the ventral region of segment A8 at positions corresponding to the vmd1a, vp1, vp2, vp4 and vp4a pI cells in segments A1-A7. It is therefore assumed that these A8 pI cells are homologous to the vmd1a, vp1, vp2, vp4 and vp4a pI cells of segments A1-A7. Together, these observations indicate that the main difference in sensory organ patterns in the ventral region between segment A8 and segments A1-A7 is that the pI cells at positions 1, 2, 4 and 4a follow an apoptotic lineage in segment A8 and an md-es lineage in segments A1-A7. A recent study has revealed that the homeotic Ultrabithorax (Ubx) gene acts at different steps in sensory organ development to regulate the bristle pattern in the thoracic legs. Indeed, Ubx controls the absence of two particular bristles in the third thoracic segment relative to the second thoracic segment by two distinct mechanisms. For the posterior sternopleural bristle, Ubx blocks the selection of the pI cell from the proneural cluster whereas for the apical bristle, it inhibits the differentiation of the pIIa and pIIb cells. Analysis suggests that homeotic genes may also regulate the final pattern of sensory organ by a third mechanism, i.e., by regulating the programmed cell death of the pIIa and pIIIb cells. Since Abdominal-B (Abd-B) regulates the homeotic identity of segment A8, it is proposed that Abd-B regulates cell death in sensory organ lineages in segment A8. It remains to be determined whether Abd-B acts in the proneural cluster or in the pI cell to specify its lineage or whether it more directly regulates the expression of pro-apoptotic genes in the pIIa and pIIIb cells (Orgogozo, 2002).

rpr and grim, but not hid, are expressed specifically in the pIIa and pIIIb cells of the vmd1a lineage. By contrast, these genes are not expressed in cells of the vp1-4 lineages. In embryos in which a pIIb cell divides at the vp1 position in at least one abdominal segment, most segments contain a vmd1a pIIa-pIIb pair with one cell expressing rpr or grim. This cell is the pIIa cell fated to die. In some other segments, neither of these two cells accumulates rpr (25%) or grim (8%). Since the development of segments is not perfectly synchronous, it is assumed that this represents a situation preceding the onset of rpr and grim expression in the pIIa cell. In the remaining segments, a single Cut-positive cell is detected indicating that the pIIa cell has died. In those segments, expression of rpr and grim is never detected in the remaining pIIb cell (Orgogozo, 2002).

During the pIIb division, Numb was shown to segregate into the dorsal pIIb daughter cell. This cell is not fated to die and differentiates as a vmd1a neuron. By contrast, it could not be directly determined which one of the two pI daughter cells inherits Numb. Indeed, since the orientation of the vmd1a pI cell division is random, the pIIa and pIIb cells could not be identified from their relative positions. Nevertheless the vmd1a pIIa and pIIIb cells appear to generate ectopic shaft/socket and neuron/sheath cell pairs when cell death is prevented. In the md-es lineage, these cell pairs are the progeny of the cells that do not inherit Numb. This suggests that both the vmd1a pIIIb cell and the pIIa cell do not inherit Numb. Thus, Numb appears to segregate in the cells that do not die in the vmd1a lineage (Orgogozo, 2002).

The role of Numb was tested in regulating rpr and grim expression as well as cell death in the vmd1a lineage. In numb mutant embryos in which a secondary precursor cell divides at the vp1 position in at least one abdominal segment, it was observed that the two Cut-positive vmd1a pI daughter cells accumulate rpr or grim transcripts (54% of the segments for rpr, 52% for grim). In other segments a single Cut-positive pI daughter cell was found accumulating rpr or grim. In these segments one pI daughter cell has already died and the other one is undergoing apoptosis. These two phenotypes are not seen in wild-type embryos. Thus, in the absence of numb, both pI daughter cells undergo programmed cell death. Consistently, no Cut-positive cell is observed at the vmd1a position in numb mutant embryos in most segments. It is concluded that numb is required to inhibit the expression of rpr and grim and to prevent cell death in the pIIb cell (Orgogozo, 2002).

To test whether numb is sufficient to prevent cell death, the progeny of the vmd1a pI cell was analyzed in arm-Gal4 UAS-numb embryos that express high levels of Numb. In wild-type embryos in which a vp1 pIIIb cell is dividing in at least one segment, one or two Cut-positive cells are observed at the vmd1a position. In contrast, four Cut-positive cells are observed in 50% of the segments in arm-Gal4 UAS-numb embryos at the same stage. In 8 out of the 9 segments with four cells, two cells accumulating high levels of Pros and two cells accumulating low levels of Pros are seen, suggesting that these cells are two vmd1a neurons and two pIIIb cells. These data indicate that the pIIa cell death is inhibited and that the pIIa cell is transformed into a pIIb-like cell (Orgogozo, 2002).

Numb is known to function by antagonizing Notch activity. This therefore suggests that Notch promotes cell death in the vmd1a lineage and that Numb blocks this activity of Notch. Unfortunately, the strong effect of Notch loss-of-function alleles on the selection of the vmd1a pI cell means that it was not possible to test directly whether Notch is required for cell death in the vmd1a lineage. Therefore the conditional Notchts1 allele was used. However, when Notchts1 embryos are shifted to a restrictive temperature (31°C) soon after the specification of the vmd1a pI cell (i.e., at 13-14.5 hours after egg laying at 19°C), no significant reduction was seen in the number of rpr- or grim-expressing pIIa cells. A stronger Notchts1/Notch55e11 combination causes the appearance of additional vmd1a pI cells even at the permissive temperature (19°C). It is therefore not possible to determine whether an increase in the number of rpr- or grim-negative cells results from a lack of Notch-dependent apoptosis or from an excess of vmd1a pI cells due to reduced Notch signaling during lateral inhibition (Orgogozo, 2002).

Therefore a test was performed to see whether an activated form of Notch, Nintra, can promote the death of the pIIb cell when expressed around the time of the vmd1a pI cell division. In 6% of the segments from embryos in which at least one segment shows a dividing vp1 pIIb cell, rpr or grim transcripts accumulate in both vmd1a pI daughter cells. In other segments, a single Cut-positive cell remains at the vmd1a position and accumulates rpr or grim. These expression patterns are not seen in heat-shocked control embryos. Importantly, these observations are similar to those made in numb mutant embryos. Thus, both loss of numb activity and ectopic Notch signaling lead to transcriptional activation of pro-apoptotic genes in the pIIb cell. Finally, a similar effect of Nintra on rpr and grim expression is seen in the vmd1a pIIb daughter cells when Nintra expression was induced at a later stage, i.e., when the vmd1a pIIb cell is dividing. Together, these results indicate that Notch signaling is sufficient to promote cell death in the vmd1a lineage (Orgogozo, 2002).

In summary, the lineage generating the vmd1a neuron has been described. This lineage is composed of two asymmetric divisions following which one daughter cell undergoes apoptosis. These two binary cell death decisions are regulated by the unequal segregation of Numb at mitosis. Therefore, the data provide the first experimental evidence that alternative cell death decision can be regulated by the unequal segregation of a cell fate determinant. The conserved role of Numb and Notch in neuronal specification in flies and vertebrates suggests that Numb-mediated inhibition of Notch may play a similar role in regulating cell death decisions in vertebrates (Orgogozo, 2002).

Bazooka, functioning independently of Inscuteable controls Numb asymmetric distribution and asymmetric divisions in the Drosophila embryonic CNS

Inscuteable is the founding member of a protein complex localized to the apical cortex of Drosophila neural progenitors that controls progenitor asymmetric division. Aspects of asymmetric divisions of all identified apicobasally oriented neural progenitors characterized to date, in both the central and peripheral nervous systems, require inscuteable. The generality of this requirement has been examined. Many identified neuroblast lineages, in fact, do not require inscuteable for normal morphological development. To elucidate the requirements for apicobasal asymmetric divisions in a context where inscuteable is not essential, focus was placed on the MP2 ---> dMP2 + vMP2 division. For MP2 divisions, asymmetric localization and segregation of Numb and the specification of distinct dMP2 and vMP2 identities require bazooka but not inscuteable. It is concluded that inscuteable is not required for all apicobasally oriented asymmetric divisions and that, in some cellular contexts, bazooka can mediate apicobasal asymmetric divisions without inscuteable (Rath, 2002).

Two obvious candidate molecules that might be responsible for mediating the MP2 asymmetric division are Baz and Partner of Inscuteable (Pins). pins appears not to be a major player since, in embryos lacking both maternal and zygotic pins, only 5.7% of the hemisegments show dMP2 duplication, as demonstrated by three odd-positive cells. Assessing the role of baz is problematic since loss of zygotic function has no effect, and removing both the maternal and zygotic baz results in embryos with severe morphological defects that prevent scoring of dMP2 and vMP2 fates in older embryos. These problems were circumvented by performing RNAi with baz double-stranded RNA on AJ96 embryos, which yielded ~25% embryos with reduced Baz protein but without the severe morphological defects that prevented scoring of vMP2 and dMP2 identities. In such embryos, vMP2>dMP2 transformations are observed in the great majority of hemisegments (57/60), as demonstrated by the presence of two cells double positive for ß-Gal and Odd. Moreover, localization of Pon and Numb becomes cortical in dividing MP2 (38/40). However, there does not appear to be a dramatic defect on the orientation of the cell division, with almost all of the MP2 divisions oriented within 45° of the A/B axis (32/34). These defects associated with baz RNAi cannot merely be due to secondary effects associated with a disruption of the epithelium since, in crumbs loss of function, Baz (12/12) and Pon (10/10) remain correctly localized to the MP2 apical and basal cortex, respectively. These results indicate that Baz is required for the asymmetric localization of Pon and Numb in the MP2 asymmetric division (Rath, 2002).

Although apical complex members, like Baz, Insc and Pins, are expressed and apically localized in both MP2 and NBs, their behavior appears to differ somewhat in the two cellular contexts. For example, when baz function is attenuated, Insc is in the cytosol of NBs while Insc remains localized as an apical crescent in most dividing MP2 cells; similar results are seen in dividing MP2s of embryos derived from bazXi106 germline clones, which lack both maternal and zygotic function. Moreover, it is interesting to note that in the absence of insc function, Baz and Pins can be localized to the apical cortex of metaphase MP2s, although the intensity of staining is always reduced compared with WT MP2s, and in some cases the weak apical crescent of Baz can be difficult to detect. These observations suggest that even a small amount of apically localized Baz is sufficient to mediate basal localization of Pon and direct the MP2 asymmetric division. Strikingly, Baz, but not Insc and Pins, seems to play a dominant role in mediating Pon/Numb basal localization in MP2. When baz function is attenuated, Pon/Numb become cortically localized even though Insc and Pins can remain apically localized. These observations indicate that the precise requirements for asymmetric protein localization differ between MP2s and NBs (Rath, 2002).

MP2 appears to be the only known A/B-oriented asymmetric division that does not require insc. Although MP2 delaminates from the neuroectoderm and divides in an apico-basal fashion like NBs, there are unique features that set MP2 apart from other neuroblasts. Unlike NBs that divide in a stem-cell-like mode, MP2 undergoes one differentiative division, making it more like a GMC or a pIIb division. Insc is present in both GMC/SOP lineages. A/B-oriented asymmetric GMC divisons, like those of GMC4-2a, require insc. While the first SOP division (the anterior-posterior pI > pIIa + pIIb) does not require insc, recent work has shown that the spindle orientation of the strikingly GMC-like A/B division of the pIIb cell is dependent on Insc. Finally, unlike NBs, Pros shows nuclear localization in MP2. There is evidence supporting the view that Pros acts to terminate cell proliferation during Drosophila neurogenesis. It is plausible that both GMCs and the MP2 precursor use nuclear Pros as a cue to reduce their mitotic potential and undergo a single differentiative division. Two recent reports have shown that planar asymmetric divisions undertaken by pI in the peripheral nervous system and epithelial cells with disrupted adherens junctions both require baz and not insc. It has been demonstrated in this study that insc is not required for all A/B-oriented asymmetric divisions. These findings further support the view that baz is a more general mediator of asymmetric divisions than insc, and can act to promote both A/B and planar asymmetric divisions in the absence of insc (Rath, 2002).

Sanpodo: a context-dependent activator and inhibitor of Notch signaling during asymmetric divisions

Asymmetric cell divisions generate sibling cells of distinct fates ('A', 'B') and constitute a fundamental mechanism that creates cell-type diversity in multicellular organisms. Antagonistic interactions between the Notch pathway and the intrinsic cell-fate determinant Numb appear to regulate asymmetric divisions in flies and vertebrates. During these divisions, productive Notch signaling requires sanpodo, which encodes a novel transmembrane protein. This study demonstrates that Drosophila sanpodo plays a dual role to regulate Notch signaling during asymmetric divisions - amplifying Notch signaling in the absence of Numb in the 'A' daughter cell and inhibiting Notch signaling in the presence of Numb in the 'B' daughter cell. In so doing, sanpodo ensures the asymmetry in Notch signaling levels necessary for the acquisition of distinct fates by the two daughter cells. These findings answer long-standing questions about the restricted ability of Numb and Sanpodo to inhibit and to promote, respectively, Notch signaling during asymmetric divisions (Babaoglan, 2009).

Work from many labs indicates that the state of Notch signaling determines daughter cell fate during asymmetric divisions - high-level Notch signaling induces the 'A' fate; low-level Notch signaling permits the 'B' fate. In this context, the current work demonstrates that spdo acts in both daughter cells to accentuate the difference between Notch signaling levels in the two cells - amplifying Notch signaling in the absence of Numb in the 'A' cell, and enabling Numb to inhibit Notch signaling in the 'B' cell. By exerting opposite effects on Notch signaling in a Numb-dependent manner, Spdo simultaneously ensures that Notch signaling exceeds threshold levels in the 'A' cell, yet remains well below such levels in the 'B' cell, thus enabling the faithful execution of asymmetric divisions (Babaoglan, 2009).

Why Numb can inhibit Notch signaling during asymmetric divisions but no other Notch-dependent event has long remained unclear. Genetic data demonstrate that numb acts through spdo to inhibit Notch signaling. As spdo is expressed exclusively in asymmetrically dividing cells, and Numb segregates exclusively into the 'B' daughter cell during asymmetric divisions, these results account for the specific ability of Numb to inhibit Notch signaling in 'B' daughter cells — the only cell type in Drosophila that co-expresses spdo and numb. spdo does not appear to enable Numb to inhibit Notch signaling by regulating the localization of Numb, as Numb localization is grossly normal in spdo mutant embryos (Babaoglan, 2009).

Why does productive Notch signaling require spdo function in 'A' daughter cells during asymmetric divisions, but not during any other Notch-dependent event in Drosophila? It was found that in the absence of Numb, Spdo amplifies but is not obligately required for transduction of Notch signaling. Thus, while Notch signaling can occur in 'A' daughter cells in the absence of spdo, spdo function is normally required to enable signaling levels to exceed the threshold required to induce the 'A' fate (Babaoglan, 2009).

The results indicate that limiting levels or activity of the Notch receptor probably underlies the sub-threshold nature of Notch signaling in 'A' daughter cells in the absence of spdo. Notch levels or activity may be limiting in 'A' daughter cells owing to the downregulation of proteins that localize to adherens junctions in asymmetrically dividing cells. Notch has been shown to localize preferentially to adherens junctions in epithelial cells, and asymmetrically dividing cells display reduced levels of Notch as well as other proteins that normally localize to adherens junctions. Some of these other proteins, such as Echinoid, are known to facilitate Notch signaling during lateral inhibition and other Notch-dependent events. Thus, reduced levels of Notch and facilitators of Notch signaling in asymmetrically dividing cells may account for the specific requirement for Spdo to amplify Notch signaling levels during asymmetric divisions (Babaoglan, 2009).

Consistent with a role for spdo in simply amplifying Notch signaling levels in the absence of Numb, the Notch-dependent 'A' fate develops at low frequency in some lineages in the absence of spdo. Thus, in the absence of spdo, Notch signaling levels appear close to, but usually below, the threshold required to induce the 'A' fate. Surprisingly, rare instances where Numb-dependent 'B' daughter cells adopt the 'A' fate were also observed in spdo mutant embryos, specifically in the development of Svp+ heart cells at 18°C. Such events have not been observed in wild type, and indicate that Numb requires Spdo in the 'B' cell to maintain Notch signaling levels reliably below the threshold required for the 'A' fate. Thus, the dual and opposing roles of spdo in the regulation of Notch signaling levels during asymmetric divisions are crucial for the unerring ability of the two daughter cells to adopt distinct fates (Babaoglan, 2009).

What is the molecular mechanism through which spdo regulates Notch signaling during asymmetric divisions? The results indicate that any mechanistic model for spdo function must account for the ability of spdo to boost Notch signaling in the absence of Numb and to reduce Notch signaling in the presence of Numb. Present models of spdo function, such as a postulated role for Spdo in promoting recycling of Delta in the 'B', do not fully address the duality of spdo function in the two daughter cells. Rather the genetic data, together with prior work on Spdo physical interactions and Numb-dependent localization, lead to the idea that in the absence of Numb, Spdo localizes to the cell membrane of the 'A' cell, where it increases Notch association with effectors, and in so doing boosts Notch signaling levels (Babaoglan, 2009).

How could Numb convert Spdo from an activator to an inhibitor of Notch signaling? Numb binds directly to Spdo and regulates its subcellular localization, preventing Spdo from localizing to the cell membrane. If either Notch or an effector is internalized with Spdo by Numb, a quantitative decrease in Notch signaling would result. However, the levels of Notch at the cell membrane appear roughly equivalent between the two daughter cells, suggesting that if numb functions in this manner it may do so by targeting a Notch effector rather than Notch itself along with Spdo. Alternatively, small changes in Notch receptor levels may be sufficient to decrease signaling levels below the threshold required to induce the 'A' fate. The elucidation of the precise mechanism through which Spdo exerts opposite effects on Notch pathway activity in the two daughter cells probably awaits the systematic identification of the factors that physically interact with Spdo during asymmetric divisions (Babaoglan, 2009).


The bristle mechanosensory organs of the adult fly are composed of four different cells that originate from a single precursor cell, pI, via two rounds of asymmetric cell division. The pattern of cell divisions in this lineage have been examined by time-lapse confocal microscopy using GFP imaging and by immunostaining analysis. pI divides within the plane of the epithelium and along the anteroposterior axis to give rise to an anterior cell, pIIb, and a posterior cell, pIIa. pIIb divides prior to pIIa (it has been previously reported that pIIa divides prior to pIIb) to generate a small subepithelial cell (not previously described) and a larger daughter cell, named pIIIb. This unequal division, oriented perpendicularly to the epithelium plane, has not been described previously. pIIa divides after pIIb, within the plane of the epithelium and along the AP axis, to produce a posterior socket cell and an anterior shaft cell. Then pIIIb divides perpendicular to the epithelium plane to generate a basal neuron and an apical sheath (glial) cell. The small subepithelial pIIb daughter cell (not previously described) has been identified as a sense organ glial cell: it expresses glial cell missing, a selector gene for the glial fate and migrates away from the sensory cluster along extending axons. It is proposed that mechanosensory organ glial cells, the origin of which has been until now unknown, are generated by the asymmetric division of pIIb cells. Both Numb and Prospero segregate specifically into the basal glial and neuronal cells during the pIIb and pIIIb divisions, respectively. This revised description of the sense organ lineage provides the basis for future studies on how polarity and fate are regulated in asymmetrically dividing cells (Gho, 1999).

The first detailed description of the sense organ lineage in the pupal notum of D. melanogaster had proposed that four cells are generated from a single pI cell via two rounds of asymmetric divisions. This first study also indicated the presence of a small BrdU-positive soma-sheath cell associated with the four BrdU-labelled sense-organ cells. Because this soma-sheath cell was often seen at some distance from the sensory cluster, it had been inferred that it derived from an unknown precursor, which also carried out its terminal DNA replication at approximately 16 hours APF. Soma sheath cells have previously been described in adult external sense organs as small, subepithelial, A101-positive cells found associated with the neuron and/or its axon. The data presented in the current study indicate that this soma-sheath cell most likely corresponds to the small pIIb daughter cell that differentiates as a glial cell. Earlier BrdU pulse-labelling experiments had indicated that the precursor of the shaft and socket cells, pIIa, replicated its DNA before the precursor of the neuron and sheath cells, named pIIb in this study. However, more recent studies indicate that the anterior Prospero-positive daughter cell, pIIb, enters mitosis prior to pIIa. The model proposed here suggests that pIIb does indeed divide prior to pIIa, while the precursor of the neuron and sheath cells, pIIIb, divides after pIIa (Gho, 1999).

This study confirms that pI and pIIa divide within the epithelial plane and along the AP axis. The orientation of the pI division is regulated by Frizzled signaling. By contrast, the orientation of the pIIa division relative to its sister cells does not require frizzled activity. The positioning of the mitotic spindle in pIIa might be influenced by cell signaling from anterior pIIb and/or pIIIb cells, or by cortical marks deposited during the previous pI division. Consistently, the mitotic spindle of pIIa is often tilted basally toward pIIIb. This study establishes that both pIIb and pIIIb divide perpendicular to the epithelial plane. This contrasts with a previous conclusion that pIIIb divides within the plane of the epithelium and perpendicular to the AP axis. Because horizontal sections were projected along the z-axis in the study, only mitotic spindles tilted relative to the apicobasal axis were recognized. This led to an erroneous conclusion. The previous observation that Numb localizes away from the midline, however, is consistent with the present finding that the most basal centrosome associated with the Numb crescent often occupies a lateral position (Gho, 1999 and references).

The current results confirm that Numb is asymmetrically distributed in dividing pI, pIIa and pIIIb cells, and is unequally inherited by the pIIb, shaft and neuron cells. It is also established that Numb forms a basal crescent in pIIb and segregates into the sense organ glial cell. In contrast with Numb, Prospero is not detected in dividing pI and pIIa. Prospero, like Numb, forms a basal crescent in pIIb and pIIIb, and preferentially segregates into the future glial cell and neuron. By contrast, two recent reports had indicated that Prospero is uniformly localized at the cell cortex in dividing pIIb. In these studies, the distribution of Prospero was examined in confocal sections perpendicular to the apicobasal axis of dividing pIIb. Therefore, it is possible that the basal distribution of Prospero could have escaped detection. A detailed co-localization analysis of Numb and Prospero in dividing pIIb and pIIIb has revealed that these two fate determinants do not strictly co-localize. In these cells, Prospero is mostly found at the basal pole, while Numb has also been found to accumulate in the cortical region of cell contact between sense organ cells. It will be interesting to examine how cell-cell interactions between sense organ cells regulate the activity of the protein complexes involved in the polar distribution of both Numb and Prospero (Guo, 1999).

The current analysis of the pIIb division reveals a striking analogy between the pIIb division in the notum and the neuroblast division in the embryo. (1) Both cells divide unequally to produce two cells of different size. (2) In both cases, the division is oriented along the apicobasal axis and the small daughter cell appears at the basal pole. (3) Numb and Prospero specifically localize at the basal pole and segregate into the small basal cell. It will thus be of interest to examine whether asymmetry is established by similar molecular mechanisms in both pIIb and neuroblast (Guo, 1999).

The basal pIIIb cell that inherits Numb and Prospero is proposed to be the neuron. As in dividing pIIb, Prospero has been found to localize asymmetrically at the basal pole of pIIIb, while Numb localizes in a basolateral crescent. Both proteins segregate preferentially into the basal daughter. Because Numb segregates into the basal daughter, it is proposed that the basal pIIIb daughter cell is the neuron. The apical pIIIb daughter must therefore be the sheath (glial) cell. This interpretation that the neuron corresponds to the basal pIIIb daughter cell implies that accumulation of Prospero in the neuron is only transient and that the high level accumulation of Prospero in the sheath cell is due to de novo synthesis. A transient accumulation of Prospero in the neuron would also be consistent with the hypothesis formulated by Manning (1999) that Prospero functions in the neuron to regulate axonal pathfinding (Guo, 1999).

Glial cells constitute a crucial component of the nervous system. They wrap the neuronal somata and axons and play a number of roles during normal neuronal activity and development, including axonal growth. Gliogenesis in the peripheral nervous system (PNS) of the adult fly has been best described in the wing. In this tissue, glial cells originate from regions of the ectoderm that also give rise to sense organs. Glial cells then migrate along the nerve following the direction taken by the axons. In addition, mutations that induce ectopic sense organs also lead to the emergence of ectopic glial cells. Conversely, mutations that reduce the number of sensory bristles result in a significant reduction of the number of glial cells. These observations have led to the hypothesis that gliogenesis is induced in the ectoderm by neighbouring sense organ cells. However, the exact origin of the glial cells is not known. The current finding that sense organ glial cells are produced by the asymmetric division of pIIb in the notum offers a novel interpretation for all these earlier observations and suggests that in the wing, glial cells originate from sensory lineages (Guo, 1999).

The division of pIIb is intrinsically asymmetric. It produces a small subepithelial cell that will adopt a glial fate and a larger pIIIb cell. The intrinsic nature of this division suggests that expression of gcm in the small subepithelial is a consequence of the initial asymmetry established in pIIb. Two fate determinants, Numb and Prospero, are unequally inherited by the future glial cell. This raises the possibility that they participate in activating gcm expression in the small pIIb daughter and act upstream of gcm in establishing a glial fate (Guo, 1999).

To test the function of Starry night in regulating spindle orientation and Numb protein localization during the SOP pI division, the effects were analyzed of both the loss of function and the overexpression of stan on the pupal notum. In stan (fmiE59/fmi71) transheterozygous mutant pupae, the Numb crescent is randomly positioned within the epithelial plane during the SOP pI division. The adapter protein Partner of numb (Pon), which controls Numb localization during SOP division (Lu, 1998) is also mispositioned, but the two proteins remain colocalized. Staining for tubulin reveals that spindle orientation and Numb crescent positioning are still tightly coupled. During the SOP pI division in apterous-Gal4;UAS-fmi pupae, the Numb crescent is also mispositioned and the mitotic spindle misoriented within the epithelial plane, but they remain aligned with each other. Therefore, loss of function and overexpression of stan both disrupts the cellular process that regulates mitotic spindle orientation and protein localization during the SOP pI division (Li, 1999).

Two types of asymmetric divisions in the Drosophila sensory organ precursor cell lineage

Asymmetric partitioning of cell-fate determinants during development requires coordinating the positioning of these determinants with orientation of the mitotic spindle. In the Drosophila peripheral nervous system, sensory organ progenitor cells (SOPs) undergo several rounds of division to produce five cells that give rise to a complete sensory organ. The asymmetric divisions that give rise to these cells have been visualized in developing pupae using green fluorescent protein fusion proteins. Spindle orientation and determinant localization are tightly coordinated at each division. Furthermore, two types of asymmetric divisions exist within the sensory organ precursor cell lineage: the anterior-posterior pI cell-type division, where the spindle remains symmetric throughout mitosis, and the strikingly neuroblast-like apical-basal division of the pIIb cell, where the spindle exhibits a strong asymmetry at anaphase. In both these divisions, the spindle reorients to position itself perpendicular to the region of the cortex containing the determinant. On the basis of these observations, it is proposed that two distinct mechanisms for controlling asymmetric cell divisions occur within the same lineage in the developing peripheral nervous system in Drosophila (Roegiers, 2001a).

Both the cell-fate determinant Numb and the adapter protein Partner of Numb (Pon) are expressed in the sensory organ precursor cell (pI). To visualize the localization of Pon and Numb proteins during interphase in the SOP lineage the Gal4-UAS system was used to express Numb and Pon proteins fused to GFP (UAS-Pon-GFP, UAS-Numb-GFP) under the control of the scabrous (sca ) promoter, which drives strong Gal4 expression in SOP cells. Both Pon-GFP and Numb-GFP show identical patterns of localization. Interphase SOP cells expressing Pon-GFP reveal that Pon is distributed on the cell surface and along processes extending from the cell body. SOPs expressing Numb-GFP also show a localization of the Numb protein at the cell surface and along processes (Roegiers, 2001a).

To confirm that these processes correspond to extensions of the plasma membrane, GAP-GFP was expressed in SOPs. GAP-GFP is a meristylation sequence fused to GFP that is targeted to the plasma membrane. GAP-GFP is strongly localized to the cell surface and cellular extensions. To visualize the dynamics of these cellular extensions during SOP mitosis, Pon-GFP and Tau-GFP were co-expressed. As the SOP enters mitosis and the centrosomes duplicate, these membrane processes are retracted. This retraction causes accumulations of Pon-GFP to gather on the cell surface, where Pon-GFP is quickly relocalized to the anterior cortex during metaphase (Roegiers, 2001a).

The dynamics of Numb-GFP and Pon-GFP were observed in mitotic SOPs. Numb-GFP and Pon-GFP show identical patterns of localization in the pI and pIIb cell division. The results discussed here were acquired with Pon-GFP, which has been used as a reporter for the dynamics of asymmetric divisions in the embryonic CNS. The first SOP division occurs roughly 15 h after the onset of pupae formation. The interphase SOP cell extends numerous processes that contain a weak Pon-GFP signal. In metaphase, Pon-GFP forms a strong crescent at the anterior cortex. Pon-GFP is then segregated to the anterior daughter cell (pIIb). For over 90 min after the first division, the anterior pIIb cell maintains a higher level of Pon-GFP expression than the posterior pIIa daughter. At this point, the pIIb cell enters mitosis. A crescent of Pon-GFP forms at a basal/posterior position. At anaphase, Pon-GFP is segregated to the basal posterior glial cell. The apical/anterior daughter of this division, the pIIIb cell, exhibits no GFP signal (Roegiers, 2001a).

Immediately after completion of the pIIb division, the posterior pIIa cell enters mitosis. In this cell, the crescent of Pon-GFP forms at the anterior cortex, much like the pI division. Pon-GFP is segregated to the anterior cell, which will give rise to the hair cell. The posterior daughter cell, which will give rise to the external socket cell, contains low levels of Pon-GFP. The division of the pIIIb daughter occurs ~90 min after the pIIa division. The Pon crescent forms and segregates to the basal/posterior daughter, which will differentiate into the neuron. The apical daughter, the sheath cell, is undetectable with Pon-GFP fluorescence. Together, these data show that Pon and Numb are asymmetrically segregated at each division of the SOP lineage. The divisions of the pI and pIIa cells occur in the plane of the epithelium and segregate Pon and Numb to the anterior daughter cell. The pIIb and pIIIb divisions occur in an apical-basal orientation, with a posterior bias. In these divisions Pon and Numb are localized to the basal/posterior daughter (Roegiers, 2001a).

The Gal4-UAS system was used to follow the dynamics of centrosome duplication and spindle orientation at each division in the SOP lineage, by expressing the Tau-GFP fusion protein in the SOP cell under the control of the scabrous promoter. In the pI division, centrosome duplication occurs within the plane of the epithelium, but its orientation is random relative to the A-P axis of the pupa. Spindles that form perpendicular to the A-P axis reorient until they are positioned parallel to the A-P axis (Roegiers, 2001a).

In SOPs in which centrosome duplication occurred along the A-P axis, spindles do not reorient, and remain parallel to the A-P axis. During the pIIb division most centrosomes duplicate along the apicobasal axis; however, the one pIIb cell that had centrosomes oriented in the plane of the epithelium reorients its spindle to the apical-basal orientation. This division gives rise to the smaller basal glial cell and the larger apical pIIIb cell. The apical centrosome is enriched in microtubules relative to the basal centrosome at telophase The posterior pIIa cell enters mitosis as anaphase is completed in the anterior pIIb cell. In the pIIa cell division, most spindles were oriented in the plane of the epithelium and two out of five spindles were oriented parallel to the A-P axis. By anaphase of the pIIa division most spindles (4/5) reoriented along the A-P axis. This division gives rise to the anterior hair cell and the posterior socket cell. The pIIIb cell divided between 80-120 min after the pIIa division. The spindles are typically oriented in the apical-basal orientation, as in the pIIb division. This division gives rise to the apical sheath cell and the basal neuron. The final cluster thus contains five cells. The glial cell migrates away from the cluster within 60 min after the final division (Roegiers, 2001a).

To observe the dynamics of spindle positioning and Pon crescent formation simultaneously, Pon-GFP and Tau-GFP were co-expressed in SOPs using the sca-Gal4 line. Pon crescent formation is a gradual process in the SOP and, during crescent formation, the spindle 'seesaws' back and forth as the crescent accumulates. When the crescent completes its accumulation at the anterior cortex, the spindle ceases its seesawing movement, and the cell enters anaphase with one spindle pole positioned in the center of the crescent. During anaphase, the cortex overlying the posterior centrosome pulls away from the centrosome, and there is no strong increase in astral microtubule number. In addition, the relative distances between the anterior and posterior centrosomes and the metaphase plate remain symmetric throughout mitosis and anaphase. The result of this division is the formation of a slightly smaller anterior pIIb cell that inherits the bulk of the accumulated Pon-GFP, and a larger posterior daughter -- the pIIa, which contains very low levels of Pon-GFP (Roegiers, 2001a).

To confirm that the anterior centrosome remains associated with the cell cortex at anaphase while the posterior centrosome detaches, Tau-GFP with hs-GFP-Moesin, an actin-binding protein, were co-expressed. During mitosis, the centrosomes are closely associated with the cell cortex, which was labelled with GFP-Moesin. At anaphase, the anterior centrosome remains closely associated with the cortex, while at the posterior the centrosome appears to pull away from the cortex. Together, these results indicate that the polarized accumulation of an anterior crescent of Pon-GFP precedes the final positioning of the spindle. In addition, the anterior cortex and centrosome are closely associated during the entire mitotic cycle of the pI, while the posterior cortex and centrosome are observed to separate at anaphase. In the pIIb division, the spindle is also observed to seesaw during accumulation of the Pon-GFP crescent. A high-resolution simultaneous analysis of spindle orientation and crescent formation in these cells is hindered by a high background and difficulty in acquiring rapid z series of this apical-basal division (Roegiers, 2001a).

The dynamics of Pon-GFP localization and the coordination of spindle positioning and Pon-GFP crescent localization were examined in frizzled (fz) mutant pupae. The Frizzled protein is required for maintaining planar polarity during SOP divisions and thus the proper orientation of sensory bristles on the thorax. During the pI division in wild-type pupae, Pon-GFP forms an anterior crescent and is segregated to the anterior pIIb cell after mitosis. The divisions of the pI cells on the pupal notum are asynchronous, but the A-P orientation of the divisions is invariant. In fzr54 mutant pupae, the pI division occurs with an orientation that is random with respect to the A-P axis. Moreover, in time-lapse confocal images of pI divisions in fz mutants, the Pon-GFP crescent had drifted between 45° and 90° from its initial position by late anaphase in four out of ten SOPs. This result suggests that Fz may have a role in stabilizing the positioning as well as orienting the Pon crescent during mitosis (Roegiers, 2001a).

The coordination of Pon crescent with spindle orientation was examined in the fz mutant background. Despite the mispositioning of the crescent relative to the A-P axis, the dynamics of spindle orientation are similar to those of the wild type. In most cases (90%), the mitotic spindle positions itself perpendicular to the Pon-GFP crescent, regardless of the relationship of the crescent to the A-P axis. The spindle has been observed to seesaw during the accumulation of Pon-GFP to one side of the cell; however, in about one-tenth of cases, the mitotic spindle is positioned parallel to the Pon-GFP crescent, but appeared to have both spindle poles anchored to the cortex. The spindles also have an abnormal curved morphology throughout mitosis. Notably, at telophase these cells appear to segregate the bulk of the Pon-GFP to one of the two daughter cells, despite the abnormal position of the spindle during metaphase and anaphase (Roegiers, 2001a).

Owing to this rescue of the asymmetric localization of Pon at anaphase, cell-fate transformations have never been reported in fz mutants, and no loss of external sensory structures have been observed that would indicate a mis-segregation of Numb protein in the pI division. In SOPs co-expressing Pon-GFP and Tau-GFP in a wild-type background, spindles are always oriented perpendicular to the Pon-GFP crescent at anaphase. The orientation of subsequent divisions within the SOP lineage does not depend on Fz. This analysis reveals that in 14 out of 18 SOPs, the division pattern of the SOP lineage occur in the same orientation relative to pI as in the wild type, despite the randomization of the pI division with respect to the A-P axis. In the 4 out of the 18 remaining SOPs, the divisions were asymmetric, but slight deviations from the wild-type orientation were observed, such as the pIIa division occurring at a 45° angle from the pI division. Together, these results indicate that Fz participates in establishing the correct orientation and stabilization of the position of the Pon-GFP crescent in the pI cell, and that it may participate in the coordination of spindle orientation and crescent formation (Roegiers, 2001a).

Both Miranda and Inscuteable (Insc) are expressed in the pIIb cell and Insc is required for proper apical-basal orientation of the mitotic spindle. The planar polarity gene fz is required for proper positioning of the Pon crescent at the anterior cortex of the pI cell during mitosis. Loss of Fz function has no effect on the proper apical-basal orientation of the pIIb division. This observation, along with the spindle asymmetry observed at anaphase in the pIIb division led to an exploration of the possibility that the pIIb cell division is a neuroblast-type cell division. Neuroblasts divide in the apical-basal orientation and require Insc to reorient the spindle along the apical basal axis. In addition, neuroblasts express Miranda, a linker protein that is asymmetrically localized to the basal cortex during mitosis. pIIb cells express Miranda and Inscuteable during mitosis, and Miranda forms a basal crescent. Insc is localized to the apical cortex of the pIIb cell (Roegiers, 2001a).

Since Insc has a central role in orienting the spindle in the neuroblast division, it was of interest to examine whether Insc is also involved in spindle orientation in the pIIb cell. Previous studies did not detect a role for Insc in the SOP lineage, but these studies did not focus on the pIIb division. Mitotic clones of the inscp72 mutant, which has been shown to disrupt asymmetric divisions in the embryonic neuroblast lineage, were generated within the SOP lineage to determine whether insc is required for proper apical-basal spindle orientation in the pIIb cell. The widely used p72 allele of insc is a small deletion, which removes the inscuteable gene product as well as removing skittles, a kinase involved in the phosphoinositol cycle. The skittles mutant has no effect on asymmetric divisions in the embryonic neuroblast lineage, thus the phenotypes observed are most probably caused by the loss of insc. In most inscp72 mutant clones in pIIb cells (5/6 cases), the spindle orients within the plane of the epithelium. In the remaining case, the spindle orients roughly along apical-basal axis. Interestingly, in the five cases mentioned above the spindle showed a clear bias along the A-P axis, and in each of these divisions the spindle was observed to reorient with respect to the initial orientation of the duplicated centrosomes. Loss of Insc function had no effect on spindle positioning in the pI cell division. Together, these results indicate that the pIIb cell division strongly resembles that of an embryonic neuroblast, but there appears to be residual cue in the absence of insc that orients the spindle along the A-P axis (Roegiers, 2001a).

These results suggest that there are two fundamental types of asymmetric divisions in the developing Drosophila nervous system. During Drosophila development these two types of divisions are reiterated in different tissues at different times to generate cell-fate diversity. The divisions of the sensory organ precursor cell provide a unique system for studying different types of asymmetric cell divisions within the same lineage and how they might be coordinated. The orientations of the divisions are tightly regulated: two divisions occur along the A-P axis, and two divisions occur in the apical-basal orientation. In the pI division, which occurs along the A-P axis, the spindle is symmetric and reorients to align perpendicular to the crescent of Pon-GFP, and fz is important for the proper orientation of the crescent and appears to contribute to the coordination of spindle orientation and crescent positioning. In contrast, the spindle in the pIIb cell orients along the apical-basal axis and exhibits a strong size asymmetry. insc, a gene of central importance in coordinating spindle orientation and crescent formation in embryonic neuroblast divisions, also has an important role in orienting the mitotic spindle in the pIIb cell. These findings provide strong evidence that the pIIb division is a neuroblast-like division. It will be interesting to know whether other genes known to be involved in controlling the asymmetric divisions of neuroblasts, such as bazooka or partner of inscuteable, are also required for the pIIb division. The results may reveal general mechanisms for generating cell-fate diversity in Drosophila as well as in other species (Roegiers, 2001a).

Lethal giant larvae acts together with Numb in Notch inhibition and cell fate specification in the Drosophila adult sensory organ precursor lineage

The tumor suppressor genes lethal giant larvae (lgl) and discs large (dlg) act together to maintain the apical basal polarity of epithelial cells in the Drosophila embryo. Neuroblasts that delaminate from the embryonic epithelium require lgl to promote formation of a basal Numb and Prospero crescent, which will be asymmetrically segregated to the basal daughter cell upon division to specify cell fate. Sensory organ precursors (SOPs) also segregate Numb asymmetrically at cell division. Numb functions to inhibit Notch signaling and to specify the fates of progenies of the SOP that constitute the cellular components of the adult sensory organ. In contrast to the embryonic neuroblast, lgl is not required for asymmetric localization of Numb in the dividing SOP. Nevertheless, mosaic analysis reveals that lgl is required for cell fate specification within the SOP lineage; SOPs lacking Lgl fail to specify internal neurons and glia. Epistasis studies suggest that Lgl acts to inhibit Notch signaling by functioning downstream or in parallel with Numb. These findings uncover a previously unknown function of Lgl in the inhibition of Notch and reveal different modes of action by which Lgl can influence cell fate in the neuroblast and SOP lineages (Justice, 2003).

The discovery that lgl function is required to specify cell fate within the SOP lineage, but does not affect asymmetric segregation of Numb, suggests that Lgl function is distinct from Dlg function in the SOP. Lgl function is most likely required after polarization of the SOP and somehow contributes to the selective inhibition of Notch activity that specifies the fate of the pIIb cell. How might Lgl fulfill this function? Lgl is a WD repeat-containing protein conserved in eukaryotes ranging from yeast to man. Similar to many other WD repeat-containing proteins, Lgl likely interacts with multiple partners in a dynamic manner. It binds type II myosins and t-SNAREs on the plasma membrane and is known to be involved in exocytosis in yeast and Drosophila by presumably targeting vesicles to the plasma membrane and thereby inserting membrane proteins at specific zones along the apical-basal axis of epithelial cells and releasing extracellular signaling molecules such as Dpp. The requirement for Lgl function, however, is not restricted to membrane proteins and secreted proteins that require vesicular transport. For example, formation of the basal crescent in neuroblasts involves cytoplasmic and cortical movements of globular proteins, such as Numb, Pon, Prospero, and Miranda, that attach to the cytoplasmic side of the membrane via lipid modifications or association with membrane proteins. One plausible scenario for the role of Lgl in mediating basal Numb crescent formation in neuroblasts is that Lgl and motor proteins form a complex that mediates basal transport of determinants. Such Lgl-containing adaptor complexes in the SOP must differ from those in embryonic neuroblasts under this scenario, given that anterior Numb crescent formation in the SOP is independent of Lgl (Justice, 2003).

A scenario is imagined in which Lgl is required to deliver components of the machinery required for Numb-mediated inhibition of Notch cannot be excluded. Alternatively, Lgl could directly participate in such a mechanism and could perhaps target endocytic vesicles containing Numb and Notch to the lysosome for degradation. A direct role for Lgl in the Notch pathway is supported by studies suggesting that vesicle trafficking of Notch and Delta plays a critical role during Notch pathway signaling. Lgl might bring Notch inhibitors to the plasma membrane or traffic endocytic vesicles in an inhibitory mechanism with Numb and alpha-Adaptin that specifies cell fates in the SOP lineage (Justice, 2003).

Numb function in sensory organ development: Regulation of membrane localization of Sanpodo by Lethal giant larvae and Neuralized in asymmetrically dividing cells of Drosophila sensory organs

In Drosophila, asymmetric division occurs during proliferation of neural precursors of the central and peripheral nervous system (PNS), where a membrane-associated protein, Numb, is asymmetrically localized during cell division and is segregated to one of the two daughter cells (the pIIb cell) following mitosis. numb has been shown genetically to function as an antagonist of Notch signaling, and also as a negative regulator of the membrane localization of Sanpodo, a four-pass transmembrane protein required for Notch signaling during asymmetric cell division in the central nervous system (CNS). lethal giant larvae (lgl) is required for Numb-mediated inhibition of Notch in the adult PNS. Sanpodo is expressed in asymmetrically dividing precursor cells of the PNS and Sanpodo internalization in the pIIb cell is dependent cytoskeletally-associated Lgl. Lgl specifically regulates internalization of Sanpodo, likely through endocytosis, but is not required for the endocytosis Delta, which is a required step in the Notch-mediated cell fate decision during asymmetric cell division. Conversely, the E3 ubiquitin ligase Neuralized is required for both Delta endocytosis and the internalization of Sanpodo. This study identifies a hitherto unreported role for Lgl as a regulator of Sanpodo during asymmetric cell division in the adult PNS (Roegiers, 2005).

This analysis of Sanpodo function in the adult PNS suggests that, as in the embryo, Sanpodo is expressed only in asymmetrically dividing precursor cells and is required for cell fates dependant on high levels of Notch signaling, perhaps through the direct interaction between Sanpodo and the full length Notch receptor. Sanpodo also interacts directly with Numb in vivo, and in both the embryonic CNS and the adult PNS, numb inhibits plasma membrane association of Sanpodo. Therefore, it appears that Sanpodo plays a similar role in asymmetrically dividing precursor cells in both the CNS and PNS in Drosophila (Roegiers, 2005).

Although there are many similarities between the mechanisms of asymmetric cell divisions in embryonic neuroblasts and adult sensory organ precursor cells, one difference involves the role of lgl. In neuroblasts, lgl is required along with another cortical tumor suppressor, dlg, to target Numb to a basal crescent during mitosis, whereas in pI cells, only dlg is required for Numb crescent formation. While lgl is dispensable for segregation of Numb to the pIIb cell following pI cell mitosis, lgl is required for the inhibition of Notch signaling in the pIIb cell. Based on the current study, it is proposed that Lgl functions with Numb to remove Sanpodo from the membrane, leading to down regulation of the Notch signaling pathway in the pIIb cell. Through what mechanism might Lgl regulate Sanpodo localization? Studies in Drosophila, yeast, and vertebrate cells have implicated Lgl as both a regulator of exocytosis, through its interaction with t-SNARES, and as cytoskeletal effector. In this study, no phenotypes suggesting gross defects in exocytosis were detected; in fact, increased accumulation of the membrane protein Sanpodo at the plasma membrane is seen in lgl mutants. Accumulation of Sanpodo at the plasma membrane in lgl mutants resembles the phenotype of three endocytic proteins Numb, alpha-Adaptin, and Shibire, suggested that lgl may have a broader role in vesicle traffic. Although a potential role for Lgl in endocytosis is observed, this role appears to be specific to Sanpodo, since endocytosis of Delta occurs normally in lgl mutants, suggesting that Lgl is not required for bulk endocytosis. Increasingly, selective endocytosis is being implicated as an important regulator of signaling pathways. Two recent studies demonstrate that Liquid facets, an endocytic epsin participates in the Neuralized-mediated Delta endocytosis, apparently by targeting mono-ubiquitinated Delta to a specific, activating, endocytic compartment. The Notch receptor is also subjected to an ubiquitin-mediated endocytic step required for activation via the E3 ubiquitin ligase Deltex, which targets Notch to the late endosome. However, the roles of Liquid facets and Deltex have not been explored in asymmetrically dividing neural precursors. One possible function for Lgl could be to direct Sanpodo toward a specific endocytic compartment. Alternatively, Lgl may be involved indirectly, by targeting molecules required for Sanpodo endocytosis to the membrane region. This scenario would be more consistent with Lgl's role as an exocytic regulator. An alternative hypothesis may be that Lgl regulates Sanpodo localization through its interaction with the cytoskeleton. Lgl functions as an inhibitor of non-muscle myosin II function in both Drosophila and yeast. The data suggests that cytoskeletal association of Lgl is required for regulating Sanpodo localization, because phosphorylation of Lgl by aPKC, which causes an autoinhibitory conformational change in Lgl that disrupts the association with the cytoskeleton, causes membrane accumulation of Sanpodo. It remains to be determined if Sanpodo endocytosis requires inhibition of myosin II activity (Roegiers, 2005).

Previously, Numb and Neuralized had been implicated in two complementary, and possibly independent, mechanisms to determine cell fate in PNS precursor cells. Numb functions to inhibit Notch autonomously by internalizing Sanpodo in the pIIb cell: while Neuralized acts on Delta in the pIIb cell to induce Notch signaling non-autonomously in the pIIa cell. Both neuralized-dependant uptake of Delta and Sanpodo internalization require dynamin function, suggesting that these steps rely on endocytosis. Unexpectedly, it was found that loss of neuralized function affects both Delta internalization and Sanpodo internalization. Failure to internalize Delta into the pIIb cell causes a cell fate transformation of the pIIa cell into a pIIb cell in neuralized mutants, and this transformation occurs despite the accumulation of Sanpodo at the membrane, suggesting that accumulation of Sanpodo at the membrane is not sufficient to induce Notch signaling in the pIIb cell in the absence of neuralized. It is unclear whether membrane accumulation of Sanpodo in neuralized mutants is due to a direct interaction between Neuralized and Sanpodo, perhaps through ubiquitination of Sanpodo, or through an indirect mechanism. Regardless, the data show that regulation of Sanpodo membrane localization is not completely independent of neuralized function. In summary, this study suggests that Sanpodo is regulated by both neuralized and lgl, while Delta is regulated by neuralized independently of lgl. In addition, this study shows that lgl appears to contribute to the endocytosis of Sanpodo, which suggests a broader role for lgl in vesicle trafficking, which may have important implications for its role as a tumor suppressor. Could the regulation of Notch signaling by Sanpodo, Lgl and Numb be conserved across species? Sequence analysis did not reveal any homologues of Sanpodo beyond other insect species. However, loss of function studies of the mouse homologues of Drosophila numb and lgl in the developing brain show strikingly similar phenotypes. Targeted numb/numblike knockouts in dorsal forebrain and Lgl1 knockouts cause profound disorganization of the layered regions of the cortex and striatum and formation of rosettelike accumulations of neurons. These phenotypes may indicate that Numb and Lgl function together to regulate Notch signaling in mouse neurogenesis as well as in Drosophila PNS development, but a functional homologue sanpodo has yet to be identified in the mouse (Roegiers, 2005).

Lineage-specific effects of Notch/Numb signaling in post-embryonic development of the Drosophila brain

Numb can antagonize Notch signaling to diversify the fates of sister cells. Paired sister cells acquire different fates in all three Drosophila neuronal lineages that make diverse types of antennal lobe projection neurons (PNs). Only one in each pair of postmitotic neurons survives into the adult stage in both anterodorsal (ad) and ventral (v) PN lineages. Notably, Notch signaling specifies the PN fate in the vPN lineage but promotes programmed cell death in the missing siblings in the adPN lineage. In addition, Notch/Numb-mediated binary sibling fates underlie the production of PNs and local interneurons from common precursors in the lAL lineage. Furthermore, Numb is needed in the lateral but not adPN or vPN lineages to prevent the appearance of ectopic neuroblasts and to ensure proper self-renewal of neural progenitors. These lineage-specific outputs of Notch/Numb signaling show that a universal mechanism of binary fate decision can be utilized to govern diverse neural sibling differentiations (Lin, 2010).

In contrast to MB lineages, in which GMCs divide to make two indistinguishable neurons, the three AL neuronal lineages examined produce GMCs that consistently undergo asymmetric cell division and yield daughter cells with distinct fates. This mechanism allows doubling of neuron types, as in the lAL lineage. However, in the adPN and vPN lineages, only one from each pair of daughter cells persists into the adult stage. They are both present as hemilineages. Notably, about 50% of central brain lineages exist as hemilineages, as revealed by clonal analysis with twin-spot MARCM using a pan-neuronal driver. Recovery of the missing hemilineages in the Drosophila VNC has implicated the Notch/Numb-mediated asymmetric cell division as a mechanism for divergent configuration of distinct insect brains. In sum, asymmetric cell division is broadly utilized; depending on the lineages, a GMC may divide to make two identical neurons, two distinct neurons, or only one mature neuron (Lin, 2010).

Notch and Numb underlie asymmetric cell division in diverse contexts, including the asymmetric cell divisions of diverse AL PN precursors. Notably, the output of Notch signaling is grossly opposite in the adPN versus vPN lineage. Each GMC in both lineages makes one PN and one mysterious sibling. Interestingly, Notch-on specifies the PN fate in the vPN lineage but antagonizes the PN fate in the adPN lineage. The cell-fate determinants for PNs of different lineages could be more distinct than their gross phenotypes suggest. In addition, the mysterious siblings of adPNs versus vPNs, upon rescued, might acquire very different fates. These lineage-dependent outputs of Notch signaling support the argument for its involvement in modulating cell differentiation, rather than specifying any de novo cell fate. It appears that two, possibly mutually exclusive, cell fates pre-exist in each precursor, and that Notch signaling, which occurs only in Numb-negative daughter cells, triggers cell differentiation along one rather than the other pre-programmed path (Lin, 2010).

Notch/Numb-dependent asymmetric cell division underlies the derivation of two complex lAL hemilineages that both persist into the adult stage. Distinct PN types are made along the Notch-off hemilineage, whereas diverse types of non-PNs, including various AL LNs and most Acj6-positive progeny, differentiate from Numb-negative daughter cells. As in other neuronal lineages, specific neuron types of the lAL lineage are made at specific times of development. However, it remains uncertain whether specific PN types consistently pair with specific non-PN types through the production of the sister hemilineages. Superficially, there exist many more non-PN types than the recognizable PN types in the lAL lineage, raising the possibility that neuronal temporal identity is altered in distinct paces between the two lAL hemilineages. Determining individual lAL GMCs and their derivatives is essential for resolving the detail and further elucidating how two parallel sets of temporal cell fates can be generated by a common progenitor through repeated self-renewal (Lin, 2010).

Besides governing neuronal cell fates following asymmetric cell division of GMCs, Numb, together with other basal complex proteins, including Brat and Prospero, is selectively segregated into GMCs during self-renewal of Nbs. However, in contrast with its essential role for preventing the transit-amplifying precursors from undergoing tumor-like overproliferation in PAN lineages, the function of Numb in restraining the basally situated Nb offspring from adopting Nb fate varies among non-PAN lineages and depends on the stage of development. Notably, Numb is required in certain non-PAN neuronal lineages, including the lAL lineage, for preventing production of ectopic Nbs. Although Notch is dispensable for maintaining the stem cell fate in lAL Nbs, it remains likely that loss of Numb leads to ectopic Notch signaling, which in turn promotes stem cell fate in otherwise GMCs. The differential requirement of Numb for proper specification of GMCs of different origins could be due to lineage- and/or stage-dependent variations in the abundance of Notch signaling components. Interestingly, the ectopic Nbs apparently maintain proper temporal identity and could make diverse neuron types as the endogenous progenitor. These raise the possibility that dynamic Notch signaling might be utilized in vivo to promote self-renewal versus amplification of Nbs (Lin, 2010).

Taken together, most neuron types in the Drosophila central brain are specified not only according to their lineage origin as well as birth order, but also depending on whether Numb exists to suppress Notch signaling in newly derived postmitotic neurons. It appears that postmitotic neurons are born with two opposing cell fates that were pre-determined in their immediate precursors based on their lineage and temporal origin. Notch signaling then suppresses the otherwise dominating fate. In addition, in certain neuronal lineages, Numb plays a subtle role in ensuring production of GMCs while Nbs undergo self-renewal. A conserved Notch/Numb-dependent mechanism probably governs diverse neural developmental processes through evolution (Lin, 2010).

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

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