Ventral Cord Neurons Research Articles

"Mitotic" kinesin-5 is a dynamic brake for axonal growth.

During neuronal development, neurons undergo significant microtubule reorganization to shape axons and dendrites, establishing the framework for efficient wiring of the nervous system. Previous studies from our laboratory demonstrated the key role of kinesin-1 in driving microtubule-microtubule sliding, which provides the mechanical forces necessary for early axon outgrowth and regeneration in Drosophila melanogaster. In this study, we reveal the critical role of kinesin-5, a mitotic motor, in modulating the development of postmitotic neurons. Kinesin-5, a conserved homotetrameric motor, typically functions in mitosis by sliding antiparallel microtubules apart in the spindle. Here, we demonstrate that the Drosophila kinesin-5 homolog, Klp61F, is expressed in larval brain neurons, with high levels in ventral nerve cord (VNC) neurons. Knockdown of Klp61F using a pan-neuronal driver leads to severe locomotion defects and complete lethality in adult flies, mainly due to the absence of kinesin-5 in VNC motor neurons during early larval development. Klp61F depletion results in significant axon growth defects, both in cultured and in vivo neurons. By imaging individual microtubules, we observe a significant increase in microtubule motility, and excessive penetration of microtubules into the axon growth cone in Klp61F-depleted neurons. Adult lethality and axon growth defects are fully rescued by a chimeric human-Drosophila kinesin-5 motor, which accumulates at the axon tips, suggesting a conserved role of kinesin-5 in neuronal development. Altogether, our findings show that at the growth cone, kinesin-5 acts as a brake on kinesin-1-driven microtubule sliding, preventing premature microtubule entry into the growth cone. This regulatory role of kinesin-5 is essential for precise axon pathfinding during nervous system development.

Activity of nested neural circuits drives different courtship songs in Drosophila

Motor systems implement diverse motor programs to pattern behavioral sequences, yet how different motor actions are controlled on a moment-by-moment basis remains unclear. Here, we investigated the neural circuit mechanisms underlying the control of distinct courtship songs in Drosophila. Courting males rapidly alternate between two types of song: pulse and sine. By recording calcium signals in the ventral nerve cord in singing flies, we found that one neural population is active during both songs, whereas an expanded neural population, which includes neurons from the first population, is active during pulse song. Brain recordings showed that this nested activation pattern is present in two descending pathways required for singing. Connectomic analysis reveals that these two descending pathways provide structured input to ventral nerve cord neurons in a manner consistent with their activation patterns. These results suggest that nested premotor circuit activity, directed by distinct descending signals, enables rapid switching between motor actions.

Peptidoglycan-dependent NF-κB activation in a small subset of brain octopaminergic neurons controls female oviposition.

When facing microbes, animals engage in behaviors that lower the impact of the infection. We previously demonstrated that internal sensing of bacterial peptidoglycan reduces Drosophila female oviposition via NF-κB pathway activation in some neurons (Kurz et al., 2017). Although we showed that the neuromodulator octopamine is implicated, the identity of the involved neurons, as well as the physiological mechanism blocking egg-laying, remained unknown. In this study, we identified few ventral nerve cord and brain octopaminergic neurons expressing an NF-κB pathway component. We functionally demonstrated that NF-κB pathway activation in the brain, but not in the ventral nerve cord octopaminergic neurons, triggers an egg-laying drop in response to infection. Furthermore, we demonstrated via calcium imaging that the activity of these neurons can be directly modulated by peptidoglycan and that these cells do not control other octopamine-dependent behaviors such as female receptivity. This study shows that by sensing peptidoglycan and hence activating NF-κB cascade, a couple of brain neurons modulate a specific octopamine-dependent behavior to adapt female physiology status to their infectious state.

Exposure of C. elegans eggs to a glyphosate-containing herbicide leads to abnormal neuronal morphology

Recent data demonstrate that chronic exposure of Caenorhabditis elegans (C. elegans) to a high-use glyphosate-containing herbicide, Touchdown (TD), potentially damages the adult nervous system. It is unknown, however, whether unhatched worms exposed to TD during the egg stage show abnormal neurodevelopment post-hatching. Therefore, we investigated whether early treatment with TD leads to aberrant neuronal or neurite development in C. elegans. Studies were completed in three different worm strains with green fluorescent protein (GFP)-tagged neurons to facilitate visual neuronal assessment. Initially, eggs from C. elegans with all neurons tagged with GFP were chronically exposed to TD. Visual inspection suggested decreased neurite projections associated with ventral nerve cord neurons. Data analysis showed a statistically significant decrease in overall green pixel numbers at the fourth larval (L4) stage (*p<0.05). We further investigated whether specific neuronal populations were preferentially vulnerable to TD by treating eggs from worms that had all dopaminergic (DAergic) or γ-aminobutyric acid (GABAergic) neurons tagged with GFP. As before, green pixel number associated with these discrete neuronal populations was analyzed at multiple larval stages. Data analysis indicated statistically significant decreases in pixel number associated with DAergic, but not GABAergic, neurons (***p<0.001) at all larval stages. Finally, statistically significant decreases (at the first larval stage, L1) or increases (at the fourth larval stage, L4) in superoxide levels, a developmental signaling molecule, were detected (*p<0.05). These data suggest that early exposure to TD may impair neuronal development, perhaps through superoxide perturbation. Since toxic insults during development may late render individuals more vulnerable to neurodegenerative diseases in adulthood, these studies provide some of the first evidence in this model organism that early exposure to TD may adversely affect the developing nervous system.

Unexpected Variation in Neuroanatomy among Diverse Nematode Species.

Nematodes are considered excellent models for understanding fundamental aspects of neuron function. However, nematodes are less frequently used as models for examining the evolution of nervous systems. While the habitats and behaviors of nematodes are diverse, the neuroanatomy of nematodes is often considered highly conserved. A small number of nematode species greatly influences our understanding of nematode neurobiology. The free-living species Caenorhabditis elegans and, to a lesser extent, the mammalian gastrointestinal parasite Ascaris suum are, historically, the primary sources of knowledge regarding nematode neurobiology. Despite differences in size and habitat, C. elegans and A. suum share a surprisingly similar neuroanatomy. Here, we examined species across several clades in the phylum Nematoda and show that there is a surprising degree of neuroanatomical variation both within and among nematode clades when compared to C. elegans and Ascaris. We found variation in the numbers of neurons in the ventral nerve cord and dye-filling pattern of sensory neurons. For example, we found that Pristionchus pacificus, a bacterial feeding species used for comparative developmental research had 20% fewer ventral cord neurons compared to C. elegans. Steinernema carpocapsae, an insect-parasitic nematode capable of jumping behavior, had 40% more ventral cord neurons than C. elegans. Interestingly, the non-jumping congeneric nematode, S. glaseri showed an identical number of ventral cord neurons as S. carpocapsae. There was also variability in the timing of neurodevelopment of the ventral cord with two of five species that hatch as second-stage juveniles showing delayed neurodevelopment. We also found unexpected variation in the dye-filling of sensory neurons among examined species. Again, sensory neuron dye-filling pattern did not strictly correlate with phylogeny. Our results demonstrate that variation in nematode neuroanatomy is more prevalent than previously assumed and recommend this diverse phylum for future “evo-devo-neuro” studies.

Regulation of axonal midline guidance by prolyl 4-hydroxylation in Caenorhabditis elegans.

Neuronal wiring during development requires that the growth cones of axons and dendrites are correctly guided to their appropriate targets. As in other animals, axon growth cones in Caenorhabditis elegans integrate information in their extracellular environment via interactions among transiently expressed cell surface receptors, their ligands, and the extracellular matrix (ECM). Components of the ECM undergo a wide variety of post-translational modifications that may affect efficacy of binding to neuronal guidance molecules. The most common modification of the ECM is prolyl 4-hydroxylation. However, little is known of its importance in the control of axon guidance. In a screen of prolyl 4-hydroxylase (P4H) mutants, we found that genetic removal of a specific P4H subunit, DPY-18, causes dramatic defects in C. elegans neuroanatomy. In dpy-18 mutant animals, the axons of specific ventral nerve cord neurons do not respect the ventral midline boundary and cross over to the contralateral axon fascicle. We found that these defects are independent of the known role of dpy-18 in regulating body size and that dpy-18 acts from multiple tissues to regulate axon guidance. Finally, we found that the neuronal defects in dpy-18 mutant animals are dependent on the expression of muscle-derived basement membrane collagens and motor neuron-derived ephrin ligands. Loss of dpy-18 causes dysregulated ephrin expression and this is at least partially responsible for the neurodevelopmental defects observed. Together, our data suggest that DPY-18 regulates ephrin expression to direct axon guidance, a role for P4Hs that may be conserved in higher organisms.

Sound-sensitive neurons innervate the ventro-lateral protocerebrum of the heliothine moth brain

Many noctuid moth species perceive ultrasound via tympanic ears that are located at the metathorax. Whereas the neural processing of auditory information is well studied at the peripheral and first synaptic level, little is known about the features characterizing higher order sound-sensitive neurons in the moth brain. During intracellular recordings from the lateral protocerebrum in the brain of three noctuid moth species, Heliothis virescens, Helicoverpa armigera and Helicoverpa assulta, we found an assembly of neurons responding to transient sound pulses of broad bandwidth. The majority of the auditory neurons ascended from the ventral cord and ramified densely within the anterior region of the ventro-lateral protocerebrum. The physiological and morphological characteristics of these auditory neurons were similar. We detected one additional sound-sensitive neuron, a brain interneuron with its soma positioned near the calyces of mushroom bodies and with numerous neuronal processes in the ventro-lateral protocerebrum. Mass-staining of ventral-cord neurons supported the assumption that the ventro-lateral region of the moth brain was the main target for the auditory projections ascending from the ventral cord.

Patterning of sexually dimorphic neurogenesis in the Caenorhabditis elegans ventral cord by Hox and TALE homeodomain transcription factors

Reproduction in animals requires development of distinct neurons in each sex. In C. elegans, most ventral cord neurons (VCNs) are present in both sexes, with the exception of six hermaphrodite-specific neurons (VCs) and nine pairs of male-specific neurons (CAs and CPs) that arise from analogous precursor cells. How are the activities of sexual regulators and mediators of neuronal survival, division, and fate coordinated to generate sex-specificity in VCNs? To address this, we have developed a toolkit of VCN markers that allows us to examine sex-specific neurogenesis, asymmetric fates of daughters of a neuroblast division, and regional specification on the anteroposterior axis. Here, we describe the roles of the Hox transcription factors LIN-39 and MAB-5 in promoting survival, differentiation, and regionalization of VCNs. We also find that the TALE class homeodomain proteins CEH-20 and UNC-62 contribute to specification of neurotransmitter fate in males. Furthermore, we identify that VCN sex is determined during the L1 larval stage. These findings, combined with future analyses made possible by the suite of VCN markers described here, will elucidate how Hox-mediated cell fate decisions and sex determination intersect to influence development of neuronal sex differences.

STR-33, a Novel G Protein-coupled Receptor That Regulates Locomotion and Egg Laying in Caenorhabditis elegans

Despite their predicted functional importance, most G protein-coupled receptors (GPCRs) in Caenorhabditis elegans have remained largely uncharacterized. Here, we focused on one GPCR, STR-33, encoded by the str-33 gene, which was discovered through a ligand-based screening procedure. To characterize STR-33 function, we performed UV-trimethylpsolaren mutagenesis and isolated an str-33-null mutant. The resulting mutant showed hypersinusoidal movement and a hyperactive egg-laying phenotype. Two types of egg laying-related mutations have been characterized: egg laying-deficient (Egl-d) and hyperactive egg laying (Egl-c). The defect responsible for the egg laying-deficient Egl-d phenotype is related to Gα(q) signaling, whereas that responsible for the opposite, hyperactive egg-laying Egl-c phenotype is related to Gα(o) signaling. We found that the hyperactive egg-laying defect of the str-33(ykp001) mutant is dependent on the G protein GOA-1/Gα(o). Endogenous acetylcholine suppressed egg laying in C. elegans via a Gα(o)-signaling pathway by inhibiting serotonin biosynthesis or release from the hermaphrodite-specific neuron. Consistent with this, in vivo expression of the serotonin biosynthetic enzyme, TPH-1, was up-regulated in the str-33(ykp001) mutant. Taken together, these results suggest that the GPCR, STR-33, may be one of the neurotransmitter receptors that regulates locomotion and egg laying in C. elegans.

Invertebrate insights into autism

Autism is a neurodevelopmental disorder characterized by deficits in social communication and interaction, impaired language development, and stereotyped repetitive behaviors. The broader category of autism spectrum disorders (ASDs) includes individuals with subsets of these symptoms ([Geschwind and

UNC-85, a C. elegans homolog of the histone chaperone Asf1, functions in post-embryonic neuroblast replication

Normal animal development requires accurate cell divisions, not only in the early stages of rapid embryonic cleavages, but also in later developmental stages. The Caenorhabditis elegans unc-85 gene is implicated only in cell divisions that occur post-embryonically, primarily in terminal neuronal lineages. Variable post-embryonic cell division failures in ventral cord motoneuron precursors result in uncoordinated locomotion of unc-85 mutant larvae by the second larval stage. These neuroblast cell division failures often result in unequally sized daughter nuclei, and sometimes in nuclear fusions. Using a combination of conventional mapping techniques and microarray analysis, we cloned the unc-85 gene, and find that it encodes one of two C. elegans homologs of the yeast Anti-silencing function 1 (Asf1) histone chaperone. The unc-85 gene is expressed in replicating cells throughout development, and the protein is localized in nuclei. Examination of null mutants confirms that embryonic neuroblast cell divisions occur normally, but post-embryonic neuroblast cell divisions fail. Analysis of the DNA content of the mutant neurons indicates that defective replication in post-embryonic neuroblasts gives rise to ventral cord neurons with an average DNA content of approximately 2.5 n. We conclude that UNC-85 functions in post-embryonic DNA replication in ventral cord motor neuron precursors.

Sex-Peptide-Regulated Female Sexual Behavior Requires a Subset of Ascending Ventral Nerve Cord Neurons

Male-derived Sex-peptide (SP) elicits egg laying and rejection of courting males in mated Drosophila females. Little is known about the genes that specify the underlying neuronal circuits and mediate this switch in female sexual behavior. Here we show that the egghead gene involved in glycosphingolipid biosynthesis provides an essential component to the SP response. We have isolated viable alleles of the vital egghead gene that abolish egghead expression from a distal promoter resulting in the absence of the largest transcript of this complex transcription unit. Temporally and spatially restricted expression of egghead revealed a requirement for egghead early in the development of apterous-expressing ventral nerve cord neurons to rescue the SP response. In viable egghead alleles, these ascending interneurons, three per abdominal and seven per thoracic hemisegment, fail to innervate the central brain. egghead expression in apterous neurons rescues neuronal targeting and the response to SP. Furthermore, neurotransmission in apterous neurons is required to elicit the SP response. Together with the former finding of SP binding to afferent nerves , these results suggest that SP-mediated modification of sensory input switches female sexual behavior from the virgin to the mated state.

Identification of cis-regulatory elements from the C. elegans Hox gene lin-39 required for embryonic expression and for regulation by the transcription factors LIN-1, LIN-31 and LIN-39

Expression of the Caenorhabditis elegans Hox gene lin-39 begins in the embryo and continues in multiple larval cells, including the P cell lineages that generate ventral cord neurons (VCNs) and vulval precursor cells (VPCs). lin-39 is regulated by several factors and by Wnt and Ras signaling pathways; however, no cis-acting sites mediating lin-39 regulation have been identified. Here, we describe three elements controlling lin-39 expression: a 338-bp upstream fragment that directs embryonic expression in P5-P8 and their descendants in the larva, a 247-bp intronic region sufficient for VCN expression, and a 1.3-kb upstream cis-regulatory module that drives expression in the VPC P6.p in a Ras-dependent manner. Three trans-acting factors regulate expression via the 1.3-kb element. A single binding site for the ETS factor LIN-1 mediates repression in VPCs other than P6.p; however, loss of LIN-1 decreases expression in P6.p. Therefore, LIN-1 acts both negatively and positively on lin-39 in different VPCs. The Forkhead domain protein LIN-31 also acts positively on lin-39 in P6.p via this module. Finally, LIN-39 itself binds to this element, suggesting that LIN-39 autoregulates its expression in P6.p. Therefore, we have begun to unravel the cis-acting sites regulating lin-39 Hox gene expression and have shown that lin-39 is a direct target of the Ras pathway acting via LIN-1 and LIN-31.

Conversion of Green Fluorescent Protein into a Toxic, Aggregation-prone Protein by C-terminal Addition of a Short Peptide

A non-natural 16-residue "degron" peptide has been reported to convey proteasome-dependent degradation when fused to proteins expressed in yeast (Gilon, T., Chomsky, O., and Kulka, R. (2000) Mol. Cell. Biol. 20, 7214-7219) or when fused to green fluorescent protein (GFP) and expressed in mammalian cells (Bence, N. F., Sampat, R. M., and Kopito, R. R. (2001) Science 292, 1552-1555). We find that expression of the GFP::degron in Caenorhabditis elegans muscle or neurons results in the formation of stable perinuclear deposits. Similar perinuclear deposition of GFP::degron was also observed upon transfection of primary rat hippocampal neurons or mouse Neuro2A cells. The generality of this observation was supported by transfection of HEK 293 cells with both GFP::degron and DsRed(monomer)::degron constructs. GFP::degron expressed in C. elegans is less soluble than unmodified GFP and induces the small chaperone protein HSP-16, which co-localizes and co-immunoprecipitates with GFP::degron deposits. Induction of GFP::degron in C. elegans muscle leads to rapid paralysis, demonstrating the in vivo toxicity of this aggregating variant. This paralysis is suppressed by co-expression of HSP-16, which dramatically alters the subcellular distribution of GFP::degron. Our results suggest that in C. elegans, and perhaps in mammalian cells, the degron peptide is not a specific proteasome-targeting signal but acts instead by altering GFP secondary or tertiary structure, resulting in an aggregation-prone form recognized by the chaperone system. This altered form of GFP can form toxic aggregates if its expression level exceeds the capacity of chaperone-based degradation pathways. GFP::degron may serve as an instructive "generic" aggregating control protein for studies of disease-associated aggregating proteins, such as huntingtin, alpha-synuclein, and the beta-amyloid peptide.

Expression of ‘segmentation’ genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed ( en), hedgehog ( hh), and wingless ( wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI–Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI–hh is expressed in a ring of the hindgut internal to that of CapI–Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto–mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.

Caenorhabditis elegans calnexin is N-glycosylated and required for stress response

Calnexin, a type I integral Ca 2+-binding protein in the endoplasmic reticulum (ER) membrane, has been implicated in various biological functions including chaperone activity, calcium homeostasis, phagocytosis, and ER stress-induced apoptosis. Caenorhabditis elegans CNX-1 is expressed in the H-shaped excretory cell, intestine, dorsal and ventral nerve cord, spermatheca, and head and tail neurons throughout development. A cnx-1 null mutant displays temperature-sensitive developmental and reproductive defects, and retarded growth under stress. Moreover, a double knockout mutant of calnexin and calreticulin exhibits additive severe defects. Interestingly, both cnx-1 transcript and protein levels are elevated under stress conditions suggesting that CNX-1 may be important for stress-induced chaperoning functions in C. elegans. Glycosidase treatment and site-directed mutagenesis confirmed that CeCNX-1 is N-glycosylated at two asparagine residues of Asn 203 and Asn 571. When transgenic animals from cnx-1 mutant were generated, a glycosylation defective construct failed to rescue phenotypes of cnx-1 mutant suggesting that glycosylation is important for calnexin’s functions in C. elegans.

Identification of evolutionarily conserved promoter elements and amino acids required for function of the C. elegans β-catenin homolog BAR-1

β-catenins are conserved transcription factors regulated posttranslationally by Wnt signaling. bar-1 encodes a Caenorhabditis elegans β-catenin acting in multiple Wnt-mediated processes, including cell fate specification by vulval precursor cells (VPCs) and migration of the QL neuroblast progeny. We took two approaches to extend our knowledge of bar-1 function. First, we undertook a bar-1 promoter analysis using transcriptional GFP reporter fusions and found that bar-1 expression is regulated in specific cells at the transcriptional level. We identified promoter elements necessary for bar-1 expression in several cell types, including a 321-bp element sufficient for expression in ventral cord neurons (VCNs) and a 1.1-kb element sufficient for expression in the developing vulva and adult seam cells. Expression of bar-1 from the 321-bp element rescued the Uncoordinated (Unc) phenotype of bar-1 mutants, but not the vulval phenotype, suggesting that a Wnt pathway may act in ventral cord neurons to mediate proper locomotion. By comparison of the 1.1-kb element to homologous sequences from Caenorhabditis briggsae, we identified evolutionarily conserved sequences necessary for expression in vulval or seam cells. Second, we analyzed 24 mutations in bar-1 and identified several residues required for BAR-1 activity in C. elegans. By phylogenetic comparison, we found that most of these residues are conserved and may identify amino acids necessary for β-catenin function in all species.

Neurons Finding Their Way with Kinase and Staying There Without

During nervous system development, growing axons must find their way to the appropriate regions and, once they have arrived, must remain in the proper place. Bülow et al. used genetic analysis to investigate the role of the Caenorhabditis elegans fibroblast growth factor receptor, EGL-15 (a receptor tyrosine kinase), in axonal pathfinding and maintenance of axonal position. C. elegans larvae that completely lacked EGL-15 activity showed defects in the extension (premature termination) and guidance (midline crossover defects) of ventral midline axons. Reducing EGL-15 activity with a hypomorphic allele or reducing the activity of the downstream guanosine triphosphatase LET-60 (Ras homolog) led to defects in axon guidance, whereas constitutively active let-60 allele rescued defects in axon extension in larvae lacking EGL-15 activity. Experiments with egl-15 gene reporters and with expression of egl-15 or let-60 under the control of tissue-specific promoters indicated that EGL-15 acted in the cells of the hypodermis (over which ventral nerve cord neurons grow) to affect axon outgrowth. Worms expressing a mutant allele that eliminated the activity of only one EGL-15 isoform, EGL-15(5A), showed no defects in embryonic axon extension or guidance but did exhibit position defects in adults, indicating a deficit in maintenance. A catalytically inactive mutant of EGL-15(5A) could not rescue the developmental defects that accompany loss of EGL-15 activity but could rescue the defects in maintenance produced by loss of egl-15(5A) . Further, a construct that encoded a secreted form of the EGL-15(5A) ectodomain rescued these maintenance defects. Thus, EGL-15 appears to affect axon navigation and positional maintenance through distinct mechanisms, one of which is independent of its tyrosine kinase activity. H. Bülow, T. Boulin, O. Hobert, Differential functions of the C. elegans FGF receptor in axon outgrowth and maintenance of axon position. Neuron 42 , 367-374 (2004). [Online Journal]

Converting Dendrites to Axons

One question in developmental neurobiology is how does, in most cases, only one of the many neurites become the axon for the neuron with the rest of the processes becoming dendrites. Hallam et al. used a green fluorescent protein fusion protein with synaptobrevin to label presynaptic nerve terminals of ventral motor cord neurons and screened Caenorhabditis elegans mutants to identify genes involved in axon specification (see the Perspective by Whited and Garrity). They found that mutants of the syd-1 gene resulted in decreased innervation to the ventral body muscles. The ventral motor neurons come in two types, DD and VD, with VD neurons arising during the larval stages and innervating the ventral muscles and DD neurons arising during embryogenesis and innervating the dorsal muscles. Morphological and colocalization studies with axonal markers suggested that, in syd-1 mutants, the VD neurons formed presynaptic specializations on both the dorsal and ventral processes, instead of being limited to the ventral processes. DD neurons at larval stages in syd-1 mutants showed polarity defects, but in the adult these defects were resolved. Neuronal process specification was also abnormal in another neuron, the ASI chemosensory neurons, in syd-1 mutant worms. Cloning of the syd-1 gene revealed that the protein has several domains: a PDZ protein interaction domain, a C2 domain, and a guanosine triphosphatase-activating (GAP) domain. The authors note that the GAP domain lacks conservation of key residues required for activity toward Rho, and activity toward Rho and Rac is not detectable. Thus, a new player in specifying the presynaptic specialization and the formation of axons has been identified, but SYD-1's exact role remains to be determined.S. J. Hallam, A. Goncharov, J. McEwen, R. Baran, Y. Jin, SYD-1 a presynaptic protein with PDZ, C2 and rhoGAP-like domains, specifies axon identity in C. elegans. Nature Neurosci. 5, 1137-1146 (2002). [Online Journal]J. L. Whited, P. A. Garrity, Specifying axon identity with Syd-1. Nature Neurosci. 5, 1107-1108 (2002). [Online Journal]

Converting Dendrites to Axons

One question in developmental neurobiology is how does, in most cases, only one of the many neurites become the axon for the neuron with the rest of the processes becoming dendrites. Hallam et al. used a green fluorescent protein fusion protein with synaptobrevin to label presynaptic nerve terminals of ventral motor cord neurons and screened Caenorhabditis elegans mutants to identify genes involved in axon specification (see the Perspective by Whited and Garrity). They found that mutants of the syd-1 gene resulted in decreased innervation to the ventral body muscles. The ventral motor neurons come in two types, DD and VD, with VD neurons arising during the larval stages and innervating the ventral muscles and DD neurons arising during embryogenesis and innervating the dorsal muscles. Morphological and colocalization studies with axonal markers suggested that, in syd-1 mutants, the VD neurons formed presynaptic specializations on both the dorsal and ventral processes, instead of being limited to the ventral processes. DD neurons at larval stages in syd-1 mutants showed polarity defects, but in the adult these defects were resolved. Neuronal process specification was also abnormal in another neuron, the ASI chemosensory neurons, in syd-1 mutant worms. Cloning of the syd-1 gene revealed that the protein has several domains: a PDZ protein interaction domain, a C2 domain, and a guanosine triphosphatase-activating (GAP) domain. The authors note that the GAP domain lacks conservation of key residues required for activity toward Rho, and activity toward Rho and Rac is not detectable. Thus, a new player in specifying the presynaptic specialization and the formation of axons has been identified, but SYD-1's exact role remains to be determined. S. J. Hallam, A. Goncharov, J. McEwen, R. Baran, Y. Jin, SYD-1 a presynaptic protein with PDZ, C2 and rhoGAP-like domains, specifies axon identity in C. elegans . Nature Neurosci. 5 , 1137-1146 (2002). [Online Journal] J. L. Whited, P. A. Garrity, Specifying axon identity with Syd-1. Nature Neurosci. 5 , 1107-1108 (2002). [Online Journal]