The Interactive Fly

Genes involved in tissue and organ development

Axonogenesis

Embryonic origins of a motor system: Motor dendrites form a myotopic map in Drosophila


Transcription factors
abrupt
motor axons and muscle

apterous
optic discs

asense
optic lobe

buttonless
transverse nerve outgrowth and bifurcation

castor
expressed in neuroblasts - effects commissures

drifter
midline glia

eagle
serotonin interneurons

extra machrochaete
motor neurons

fruitless
CNS axon tracts

huckebein
motor neurons

glass
eye

glial cells missing
glia

islet
dopamine and serotonin interneurons of the CNS

jing
expressed in CNS midline - mutants show disrupted axon scaffold

Lim1
CNS, mutants have motor coordination problems but no obvious defects in axon guidance

longitudinals lacking
central nervous system (ventral cord)

nerfin-1
zinc finger transcription factor required for the proper development of CNS commissural and connective axon fascicles

nervy
regulates repulsive axon guidance by linking the cAMP-dependent Protein kinase (PKA) to the Semaphorin 1a (Sema-1a) receptor Plexin A

Nkx6
CNS motoneurons

prospero
central nervous system

reversed polarity
photoreceptor axons

orthodenticle
ventral midline

pointed
ventral midline

scribbler (alternative name: brakeless)
essential for R1-R6 growth cone targeting

sequoia
functions in PNS neurons, photoreceptors, and motoneurons in the CNS, functions in confering dendritic morphology in the PNS

shuttle craft
segmental and intersegmental nerves

single minded
ventral midline

tinman
mesodermal effect on exit glia

trachealess
sensory axons


Cell surface and secreted

amalgam
CNS and PNS

beta amyloid protein precursor-like
neuromuscular junction

argos
eye

beaten path
motor axons, Bolwig's organ

breathless
midline glia

Cadherin-N
motor axon guidance

commissureless
axon guidance across the midline

capricious
involved in selective synapse formation - expressed in neurons and muscle

Connectin
a homophilic cell adhesion molecules expressed in muscles and the motoneurons that innervate them

derailed
axon guidance in the brain and CNS

Dlar
motor axon guidance

draper
required during metamorphosis for recognition and engulfment of degenerating axon branches by glia

Dscam
Bolwig's organ, brain and CNS

Eph receptor tyrosine kinase
expressed by interneurons of the ventral cord - protein expression confined to axons

Ephrin
functions as a ligand for Eph receptor - expressed on the surface of neuron cell bodies - acts to confine Eph receptor bearing axons to longitudinal tracts by repulsion

faint sausage
central nervous system (ventral cord): effect is secondary to defects in cell positioning

Fasciclin1
commissural axons

Fasciclin2
motor neurons

Fasciclin3
muscle and nerve

frazzled
motor axon guidance and commissures

Gliolectin
mutation disrupts the formation of commissural pathways and delays the completion of longitudinal pathfinding

Guanylyl cyclase at 76C
cGMP production - receptor-type protein possessing a single transmembrane domain - required for Semaphorin-1a-Plexin A directed repulsive axon guidance of motor neurons

kuzbanian
axon extention in CNS

Laminin A
CNS and Ocelli

Neurotactin
ocellar pioneer axons and interneurons

Netrin-A and Netrin-B
commissural axons, CNS neurons, and motor neurons

Notch
intersegmental nerves

off-track
CCK-4 family of 'dead' receptor tyrosine kinases, Ig-domains - required for lamina-specific targeting of R1-R6 axons -
associates with Plexin, the receptor for Semaphorin ligand

plexin A
CNS

plexin B
semaphorin domain protein, receptor for the secreted semaphorin Sema-2a -
regulates axon extension from the sensory neuron cell body in regions of direct contact with oenocytes

Protein tyrosine phosphatase 69D
central nervous system and eye

rhomboid
ventral midline

roughest
eye

roundabout
expressed on all longitudinally projecting growth cones and axons

semaphorin I
CNS

Semaphorin-2a
expressed in muscle and inhibits the neuronal growth cone from forming a synaptic arborization -
secreted from oenocytes and acts represses axon extension from the sensory neuron cell body

slit
ventral midline

Src oncogene at 42A
axons linking the larval eye (Bolwig's organ) to the optic lobe

Star
ventral midline

Syndecan

a heparan sulfate proteoglycan - a necessary component of Slit/Robo signaling required in Slit target cells

Toll
motor neurons

unc-5
multiple domain protein that functions as a repulsive netrin receptor

Wnt oncogene analog 5
required for the formation of the anterior of the two midline-crossing commissures present in each hemisegment


Others

Abl oncogene
neuronal growth cone pathfinding

abstrakt
ventral midline Bolwig's organ and CNS

APP-like protein interacting protein 1
JNK scaffolding scaffolding protein that is part of motor-cargo linkage complexes for both kinesin-1 and dynein - acts downstream of the JNK pathway
to affect axonal transport - binds APP family proteins

bifocal
functions downstream of misshapen to reorganize actin cytoskeleton in decelerating R cell growth cone motility

Calmodulin
axon guidance, including both defects in fasciculation and abnormal crossings of the midline

Calnexin 99A
misexpression causes embryonic axon guidance phenotype

chromosome bows
mediates the action of Slit and its receptors acting as a partner of the Abelson tyrosine kinase

COP9 complex homolog subunit 5
required for photoreceptor neurons (R cell) axon targeting in the optic lobe

crooked neck
mRNA splice factor - required in glial cells to control migration and axonal wrapping

Cyclin-dependent kinase 5
axon patterning

Disabled
axon pathfinding in the central nervous system

dreadlocks
axon pathfinding in the central nervous system and eye

Dynein heavy chain 64C
axon pathfinding and synaptogenesis

dunce
aberrant axons

embryonic lethal, abnormal vision (synonym: elav)
aberrant axons

enabled
central nervous system - axonal outgrowth and fasciculation

futsch
motor neurons

G protein oalpha 47A
motor neurons

kakapo
arborization and dendritic sprouting of motorneurons

kette (preferred name: HEM-protein)
a transmembrane protein that transduces information to the neuronal cytoskeleton affecting axon guidance

Liprin-α
scaffolding protein that physically interacts with LAR and is essential for R7 axon targeting

MICAL
flavoprotein monooxygenase - large, multidomain protein expressed in axons - interacts with the neuronal Plexin A -
enzymatic function required for Semaphorin 1a/PlexA-mediated repulsive axon guidance

mummy
acetylglucosamine diphosphorylase - functions in apical extracellular matrix formation by producing GlcNAc residues needed for
protein glycosylation - embryonic phenotypes in axon guidance are characteristic of defects in midline signaling

non-stop
ubiquitin-specific protease expressed in glia - involved in development of laminal glia

numb
motor neurons

Ornithine decarboxylase antizyme common alternative name: gutfeeling)
PNS growth cone guidance and fasciculation

p130CAS
SH3 domain and serine-rich domain protein that works together with integrins to direct axon guidance events

PAK-kinase
photoreceptor R cell guidance

Ptpmeg
protein tyrosine phosphatase involved in neuronal circuit formation the central brain - regulates establishment
and the stabilization of axonal projection patterns

RacGAP50C
functions during brain development to limit axon overextension

RhoGAP
part of an axon retraction pathway from Rho to myosin in mature neurons -
inactivation of RhoGAP leads to axon retraction - functions in mushroom bodies

pod-1
WD repeat protein - crosslinks actin and microtubules - proper levels of Pod-1 must be maintained in the growth cone for correct axon guidance

SCAR
promotes actin polymerization via Arp2/3 protein complex

sec5
promotes neurite outgrowth

sec15
functions in polarized exocytosis/secretion - involved in targeting photoreceptor axons that involves localization of specific cell adhesion and signaling molecules

Son of sevenless
a GEF that is recruited to the plasma membrane, where it forms a ternary complex with Roundabout
and Dreadlocks to regulate Rac-dependent cytoskeletal rearrangement in response to the Slit ligand

split ends
ventral midline

trio
motorneuron, mushroom body and photoreceptor axon pathfinding

twinstar
encodes Drosophila actin-depolymerizing and actin-severing protein cofilin - function in axon growth is inhibited by LIM kinase and activated by Slingshot phosphatase in convergent and divergent pathways from Rho GTPases to result in different developmental outcomes

unc-104 (common alternative name, immaculate connections or imac)
a Kinesin-3 family member that is essential for transporting synaptic vesicle precursors


Embryonic origins of a motor system: Motor dendrites form a myotopic map in Drosophila

The organisational principles of locomotor networks are less well understood than those of many sensory systems, where in-growing axon terminals form a central map of peripheral characteristics. Using the neuromuscular system of the Drosophila embryo as a model and retrograde tracing and genetic methods, principles underlying the organisation of the motor system have been uncovered. Dendritic arbors of motor neurons, rather than their cell bodies, are partitioned into domains to form a myotopic map, which represents centrally the distribution of body wall muscles peripherally. While muscles are segmental, the myotopic map is parasegmental in organisation. It forms by an active process of dendritic growth independent of the presence of target muscles, proper differentiation of glial cells, or (in its initial partitioning) competitive interactions between adjacent dendritic domains. The arrangement of motor neuron dendrites into a myotopic map represents a first layer of organisation in the motor system. This is likely to be mirrored, at least in part, by endings of higher-order neurons from central pattern-generating circuits, which converge onto the motor neuron dendrites. These findings will greatly simplify the task of understanding how a locomotor system is assembled. These results suggest that the cues that organise the myotopic map may be laid down early in development as the embryo subdivides into parasegmental units (Landgraf, 2003).

The analysis began by correlating the positions of motor neuron dendrites with the distribution of their muscle targets in the periphery. Motor neurons were retrogradely labelled in a pairwise fashion and the positions of their dendritic arbors were mapped. Because of an interest in the mechanisms that underlie the assembly of the motor system, focus was placed on stages when each motor neuron first establishes a characteristic domain of arborisation within the neuropile (early stage 17, 15h after egg-laying [AEL]) (Landgraf, 2003).

Motor axons project into the muscle field via two main nerves, the intersegmental (ISN) and the segmental nerve (SN). The transverse nerve (TN) runs along the segment border and has few motor axons. Choice of nerve root is one of several features that divide the motor neurons into two principal sets, the ISN and SN. (1) The cell bodies of SN motor neurons are located in the same segment as the muscles that they innervate, whereas ISN motor neuron somata are located in the segment next anterior (with the exception of the RP2 and two neuromodulatory efferent ventral unpaired median [VUM] neurons. (2) ISN motor neurons innervate internal muscles, which span a segment from anterior to posterior, whereas SN (and the TN) motor neurons innervate external muscles. External muscles are distinct from the internal set in several respects: (1) they are generally transverse; (2) unlike internal muscles, they require wingless (wg) signalling for their specification; (3) external (but not internal) muscles and their innervating motor neurons express the cell adhesion molecule (CAM) Connectin, with the single exception of muscle ventral transverse 1 (VT1) (Landgraf, 2003 and references therein).

In addition, ISN and SN motor neurons elaborate their dendrites in distinct regions of the neuropile. Dendrites of ISN motor neurons occupy a domain extending posteriorly from the posterior part of one neuromere into the anterior part of the next. SN motor neuron dendrites occupy a domain that lies between the domains of ISN motor neuron arbors (Landgraf, 2003).

Thus, the organisation of the body wall muscles into internal and external sets is reflected centrally in patterns of motor neuron arborisations. The innervating motor neurons project their axons through different nerves and elaborate their dendritic fields in distinct regions of the neuropile. Although dendritic arbors become progressively more elaborate and extensive over developmental time, their separate domains remain clearly recognisable and appear to be maintained at least until the motor system is fully functional (18 h AEL) (Landgraf, 2003).

Having established that there is a central representation of the muscle field, the organisation of the motor neuron dendrites was analyzed in greater detail. (1) The set of external muscles and their innervating (SN) motor neurons were examined. Muscles of similar anteroposterior positions, such as the ventral acute muscle (VA3) and the segment border muscle (SBM), are innervated by motor neurons whose dendritic arbors lie in a common region of the neuropile. Conversely, motor neurons supplying the anterior (lateral transverse 1-2 [LT1-LT2]) versus the posterior (SBM) muscles have dendritic arbors that are correspondingly separated in the anteroposterior axis of the CNS (Landgraf, 2003).

To put the idea of a regular map to the test, focus was placed on an unusual external motor neuron-muscle pair. Muscle VT1 is innervated by a TN rather than an SN motor neuron. However, VT1 lies at the same place in the anteroposterior axis as the SBM, although VT1 is ventral and the SBM more dorsal. The VT1 motor neuron dendritic field is found to overlaps with that of the SBM motor neuron. For the external set, it is concluded that differences in target muscle location in the anteroposterior axis are mapped centrally as regular differences in dendritic position, but dorsoventral distinctions are not (Landgraf, 2003).

It was next asked whether there is a similarly regular representation of the internal muscles in the developing CNS. While most external muscles are transverse and have unique anteroposterior locations, the internal muscles span the width of a segment so that positional distinctions between them are solely in the dorsoventral axis. It was found that the set of internal muscles is represented centrally by three dendritic domains. Motor neurons innervating ventral internal muscles elaborate their dendritic arbors in the anterior half of the ISN dendritic domain. Motor neurons with dorsolateral internal muscle targets (lateral longitudinal [LL] 1, dorsal acute [DA] 3, dorsal oblique 3-5 [DO3-DO5]) put their arbors into the posterior part of the ISN dendritic domain. Finally, dorsal muscles are represented by a motor neuron dendritic domain that lies between those representing ventral (anterior) and dorsolateral (posterior) internal muscle groups. Thus, the internal muscles are represented in the neuropile by three domains of dendritic arborisation that reflect their different dorsoventral locations in the periphery. Once again, it is concluded that there is a regular mapping of muscle position in the neuropile: in this case, it is positions in the dorsoventral axis peripherally that are represented centrally as differences in the anteroposterior locations of dendrites (Landgraf, 2003).

To test the idea that dendritic arbor positions relate to the distribution of muscles, an atypical motor neuron-muscle pair was examined. The RP2 motor neuron is reported to innervate dorsal muscle DA2, yet its dendrites span the domains that represent both dorsal and dorsolateral internal muscles. However, on careful analysis it was found that DA2 is, in fact, specifically innervated by a U neuron whose dendrites lie in the dorsal internal domain, whereas the RP2 axon forms endings generally on all dorsolateral and dorsal muscles by 19 h AEL. These seem to correspond to the type 1s boutons found in late larvae. Thus, the RP2 neuron puts its dendrites into a region of the neuropile that does indeed represent its targets, namely the dorsolateral and dorsal internal muscles (Landgraf, 2003).

Like the muscle field itself, the map of motor neuron dendrites is metamerically repeated. However, the boundaries of these two units are out of register with one another, since the dendrites of the motor neurons innervating internal muscles lie in the next anterior neuromere. The anterior border of the dendritic map, as defined by the extent of these anterior dendrites, coincides with the anterior margin of engrailed (en) expression. Thus, while the muscles are segmental in their organisation, the domains occupied by the dendrites of their innervating motor neurons are parasegmental (Landgraf, 2003).

To test whether genes that implement the parasegmental pattern in the epidermis are also required for the formation of the parasegmental organisation of the neuromuscular system, the formation of SN and ISN dendritic fields was studied in embryos singly mutant for the following segment polarity genes: en/invected (Df(enE)), wg (wgCX4), naked (nkd2), patched (ptc9), hedgehog (hh21), and gooseberry (Df2R(gsb)). Every one of the six different mutants that were analysed has partially aberrant patterns of neuroblasts (NBs). Nevertheless, SN and ISN motor neurons still form and can be identified by their characteristic axonal projections into the periphery. In addition, it was found that the fundamental separation between SN and ISN dendritic domains is present despite often severe perturbations in CNS structure. For example, in gsb mutant embryos, both nerve roots are frequently fused so that the SN and ISN share a common CNS exit point. Nevertheless, SN and ISN axons as well as their dendritic fields do not intermingle but remain separate. These results suggest that the subdivision of the neuropile into the principal ISN and SN dendritic domains is a robust feature of the system, which appears to be specified early in development, since the embryo subdivides into parasegmental units (Landgraf, 2003).

It was next asked what mechanisms underlie the formation of the myotopic map. Because ISN and SN motor neurons lie at different positions in the CNS and their axons grow out into the muscle field through different nerves, it is reasonable to suppose that at least the major subdivision of dendritic arborisations into internal and external domains could be a byproduct of the locations at which the motor neurons are generated and the paths taken by their growing axons. This ëpassive mapping' explanation can be excluded by considering a single motor neuron-muscle pair, namely dorsal transverse 1 (DT1) and its innervating motor neuron. DT1 is an external muscle (by position, orientation, wg dependence, and Connectin expression), yet its motor neuron is clustered with the internal muscle innervating set and its axon (uniquely for the external muscles) grows out through the ISN. Despite its packing within the ëinternal motor neuron' set, the DT1 motor neuron makes a long posterior projection through the internal muscle domain of the myotopic map to reach the external domain, where it arborises appropriately, reflecting the orientation and external nature of its target muscle. In contrast, motor neurons derived from the same NB as DT1 innervate neighboring internal muscles DO3-DO5 and put their dendrites in a more anterior region characteristic of the dorsolateral muscles. These findings strongly suggest that the mapping of the muscle field within the CNS is an active process of growth and arborisation that partitions dendrites into subdomains of the neuropile that are appropriate to their function, rather than a passive subdivision of available space by position of origin or axon trajectory (Landgraf, 2003).

Since dendritic arbors form after motor axons have reached their targets, the muscles could be instrumental in dictating the organisation of the central map. To test this idea, the UAS/GAL4 system was used to misexpress an activated form of Notch (Kidd et al. 1998) in the developing mesoderm, suppressing the formation of muscle founder cells while leaving other tissues intact. In such muscleless embryos, the main nerve trunks, SN and ISN, still form and project into the periphery. Retrograde labellings of these nerves show that SN and ISN motor neurons form relatively normal dendritic arbors that consistently conform to the characteristic separation of SN and ISN dendrites. Thus, the neuropile is partitioned into distinct fields of dendritic arborisation independently of the muscles. It is concluded that the mapping process is likely to be an autonomous property of the motor neurons and their neighboring cells (Landgraf, 2003).

It was next asked whether motor neuron dendritic fields could be patterned by the substrates on which they grow. In the Drosophila ventral nerve cord (VNC), motor neuron dendrites form in the dorsal-most region of the neuropile, sandwiched between longitudinal glia above and the underlying scaffold of axons. Glial cells can act as substrates for supporting and guiding axonal growth. To test whether they might also be required for the growth and spatial patterning of dendritic fields, dendritic arbors were analysed in glial cells missing (gcm) mutant embryos, which are defective in glial cell differentiation. Although the structure of the nervous system is disrupted in gcm mutant embryos and the dendritic arbors are abnormal, they continue to form in their characteristic locations and the fundamental distinction between the ISN and SN motor neuron dendritic fields is maintained. Remarkably, even the long posterior dendritic projection of the DT1 motor neuron forms and reaches its target region, the SN external muscle dendritic domain. These results suggest that the patterning of the neuropile into distinct motor neuron dendritic domains is a process that appears to be intrinsic to the motor neurons and their neighboring neurons, but does not require proper glial cell differentiation (Landgraf, 2003).

One likely explanation for the division of dendrites into separate domains is that there is a process of mutual exclusion between the arborisations of neighboring cells. Such a process of dendritic ëtiling' has so far only been documented between particular classes of sensory neurons, but could also occur in the motor system. The idea of tiling was tested by considering two groups of motor neurons whose axons have a common trajectory, but whose dendritic fields form in adjacent territories. The DO3-DO5 and DT1 motor neurons project their dendrites posteriorly, and at their most-anterior point, these dendrites meet the axons and dendrites of the anterior corner cell (aCC) and U/CQ neurons. To show whether the aCC and U/CQ axons and/or dendrites inhibit the growth of DO3-DO5 and DT1 dendrites anteriorly, these neurons (as well as RP2 and the posterior corner cell [pCC] interneuron) were selectively ablated. Using anti-Even-skipped (Eve) staining as a marker for aCC, RP2, and U/CQs (there are an additional two medially located eve-expressing interneurons, pCC and friend of pCC [fpCC], it was found that these neurons can be selectively ablated before they form dendrites (at approximately 11 h AEL): on average, by 10.5 h AEL all but 0.6 and by 12 h AEL all but 0.06 of the seven medially located eve-expressing neurons have been ablated per half-neuromere. In no instance was a concomitant anterior expansion of the DO3-DO5 and DT1 motor neuron dendrites into the regions vacated by the aCC and U/CQ dendrites observed. It is concluded that, at least in this instance, the initial dendritic territory of one set of motor neurons (DO3-DO5 and DT1) is not defined by a process of tiling, in which they are excluded by neighboring (aCC and U/CQ) dendritic arbors. However, it is possible that the elaboration of motor neuron dendritic arbors during later developmental stages may involve interactions between neighboring dendritic territories, activity-dependent processes, or both (Landgraf, 2003).

Thus, in summary, these results suggest that the mechanisms that subdivide the neuropile into distinct dendritc domains are very robust and refractory to perturbations. They further suggest that the cues that organise the map may be laid down early in development as the embryo subdivides into parasegmental units (Landgraf, 2003).

The patterning of the motor neuron dendritic arbors in the Drosophila embryo represents a first layer of organisation in the motor system. This is likely in part to be mirrored by the endings of higher-order neurons of central pattern generating circuits, which converge onto the myotopic map. While motor neuron cell body positions may, as has been proposed for vertebrate systems, relate to the ontogeny of target muscles, the operation of mature muscles is reflected by the allegiance of corresponding motor neuron dendrites to a particular territory in the neuropile. Thus, changes in muscle operation could be accommodated by a change of allegiance of the appropriate motor neuron dendrites from one domain to another (e.g., the DT1 motor neuron-muscle pair) without the need for rewiring the underlying higher-order circuitry. Such a model resolves the apparent discrepancy between the distributions of motor neuron cell bodies centrally and target muscles in the periphery. It also implies a considerable degree of flexibility, particularly at the level of motor output, yet suggests that elements of the underlying motor circuitry may have been highly conserved (Landgraf, 2003).

Reference

Landgraf, M., et al. (2003). Embryonic origins of a motor system: Motor dendrites form a myotopic map in Drosophila. PLoS Biol. 1(2):E41. 14624243


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