UNC-119 suppresses axon branching in C. elegans.

The architecture of the differentiated nervous system is stable but the molecular mechanisms that are required for stabilization are unknown. We characterized the gene unc-119 in the nematode Caenorhabditis elegans and demonstrate that it is required to stabilize the differentiated structure of the nervous system. In unc-119 mutants, motor neuron commissures are excessively branched in adults. However, live imaging demonstrated that growth cone behavior during extension was fairly normal with the exception that the overall rate of migration was reduced. Later, after development was complete, secondary growth cones sprouted from existing motor neuron axons and cell bodies. These new growth cones extended supernumerary branches to the dorsal nerve cord at the same time the previously formed axons retracted. These defects could be suppressed by expressing the UNC-119 protein after embryonic development; thus demonstrating that UNC-119 is required for the maintenance of the nervous system architecture. Finally, UNC-119 is located in neuron cell bodies and axons and acts cell-autonomously to inhibit axon branching.

Mobilization of a Drosophila transposon in the Caenorhabditis elegans germ line.

Transposons have been enormously useful for genetic analysis in both Drosophila and bacteria. Mutagenic insertions constitute molecular tags that are used to rapidly clone the mutated gene. Such techniques would be especially advantageous in the nematode Caenorhabditis elegans, as the entire sequence of the genome has been determined. Several different types of endogenous transposons are present in C. elegans, and these can be mobilized in mutator strains (reviewed in ref. 1). Unfortunately, use of these native transposons for regulated transposition in C. elegans is limited. First, all strains contain multiple copies of these transposons and thus new insertions do not provide unique tags. Second, mutator strains tend to activate the transposition of several classes of transposons, so that the type of transposon associated with a particular mutation is not known. Here we demonstrate that the Drosophila mariner element Mos1 can be mobilized in C. elegans. First, efficient mobilization of Mos1 is possible in somatic cells. Second, heritable insertions of the transposon can be generated in the germ line. Third, genes that have been mutated by insertion can be rapidly identified using inverse polymerase chain reaction. Fourth, these insertions can subsequently be remobilized to generate deletion and frameshift mutations by imperfect excision.

Studies of synaptic vesicle endocytosis in the nematode C. elegans.

After synaptic vesicle exocytosis, synaptic vesicle proteins must be retrieved from the plasma membrane, sorted away from other membrane proteins, and reconstituted into a functional synaptic vesicle. The nematode Caenorhabditis elegans is an organism well suited for a genetic analysis of this process. In particular, three types of genetic studies have contributed to our understanding of synaptic vesicle endocytosis. First, screens for mutants defective in synaptic vesicle recycling have identified new proteins that function specifically in neurons. Second, RNA interference has been used to quickly confirm the roles of known proteins in endocytosis. Third, gene targeting techniques have elucidated the roles of genes thought to play modulatory or subtle roles in synaptic vesicle recycling. We describe a molecular model for synaptic vesicle recycling and discuss how protein disruption experiments in C. elegans have contributed to this model.

A post-docking role for active zone protein Rim.

Rim1 was previously identified as a Rab3 effector localized to the presynaptic active zone in vertebrates. Here we demonstrate that C. elegans unc-10 mutants lacking Rim are viable, but exhibit behavioral and physiological defects that are more severe than those of Rab3 mutants. Rim is localized to synaptic sites in C. elegans, but the ultrastructure of the presynaptic densities is normal in Rim mutants. Moreover, normal levels of docked synaptic vesicles were observed in mutants, suggesting that Rim is not involved in the docking process. The level of fusion competent vesicles at release sites was reduced fivefold in Rim mutants, but calcium sensitivity of release events was unchanged. Furthermore, expression of a constitutively open form of syntaxin suppressed the physiological defects of Rim mutants, suggesting Rim normally acts to regulate conformational changes in syntaxin. These data suggest Rim acts after vesicle docking likely via regulating priming.

An open form of syntaxin bypasses the requirement for UNC-13 in vesicle priming.

The priming step of synaptic vesicle exocytosis is thought to require the formation of the SNARE complex, which comprises the proteins synaptobrevin, SNAP-25 and syntaxin. In solution syntaxin adopts a default, closed configuration that is incompatible with formation of the SNARE complex. Specifically, the amino terminus of syntaxin binds the SNARE motif and occludes interactions with the other SNARE proteins. The N terminus of syntaxin also binds the presynaptic protein UNC-13 (ref. 5). Studies in mouse, Drosophila and Caenorhabditis elegans suggest that UNC-13 functions at a post-docking step of exocytosis, most likely during synaptic vesicle priming. Therefore, UNC-13 binding to the N terminus of syntaxin may promote the open configuration of syntaxin. To test this model, we engineered mutations into C. elegans syntaxin that cause the protein to adopt the open configuration constitutively. Here we demonstrate that the open form of syntaxin can bypass the requirement for UNC-13 in synaptic vesicle priming. Thus, it is likely that UNC-13 primes synaptic vesicles for fusion by promoting the open configuration of syntaxin.

Rules of nonallelic noncomplementation at the synapse in Caenorhabditis elegans.

Nonallelic noncomplementation occurs when recessive mutations in two different loci fail to complement one another, in other words, the double heterozygote exhibits a phenotype. We observed that mutations in the genes encoding the physically interacting synaptic proteins UNC-13 and syntaxin/UNC-64 failed to complement one another in the nematode Caenorhabditis elegans. Noncomplementation was not observed between null alleles of these genes and thus this genetic interaction does not occur with a simple decrease in dosage at the two loci. However, noncomplementation was observed if at least one gene encoded a partially functional gene product. Thus, this genetic interaction requires a poisonous gene product to sensitize the genetic background. Nonallelic noncomplementation was not limited to interacting proteins: Although the strongest effects were observed between loci encoding gene products that bind to one another, interactions were also observed between proteins that do not directly interact but are members of the same complex. We also observed noncomplementation between genes that function at distant points in the same pathway, implying that physical interactions are not required for nonallelic noncomplementation. Finally, we observed that mutations in genes that function in different processes such as neurotransmitter synthesis or synaptic development complement one another. Thus, this genetic interaction is specific for genes acting in the same pathway, that is, for genes acting in synaptic vesicle trafficking.

Mutations in synaptojanin disrupt synaptic vesicle recycling.

Synaptojanin is a polyphosphoinositide phosphatase that is found at synapses and binds to proteins implicated in endocytosis. For these reasons, it has been proposed that synaptojanin is involved in the recycling of synaptic vesicles. Here, we demonstrate that the unc-26 gene encodes the Caenorhabditis elegans ortholog of synaptojanin. unc-26 mutants exhibit defects in vesicle trafficking in several tissues, but most defects are found at synaptic termini. Specifically, we observed defects in the budding of synaptic vesicles from the plasma membrane, in the uncoating of vesicles after fission, in the recovery of vesicles from endosomes, and in the tethering of vesicles to the cytoskeleton. Thus, these results confirm studies of the mouse synaptojanin 1 mutants, which exhibit defects in the uncoating of synaptic vesicles (Cremona, O., G. Di Paolo, M.R. Wenk, A. Luthi, W.T. Kim, K. Takei, L. Daniell, Y. Nemoto, S.B. Shears, R.A. Flavell, D.A. McCormick, and P. De Camilli. 1999. Cell. 99:179-188), and further demonstrate that synaptojanin facilitates multiple steps of synaptic vesicle recycling.

Mutations in beta-spectrin disrupt axon outgrowth and sarcomere structure.

beta-Spectrin is a major component of the membrane skeleton, a structure found at the plasma membrane of most animal cells. beta-Spectrin and the membrane skeleton have been proposed to stabilize cell membranes, generate cell polarity, or localize specific membrane proteins. We demonstrate that the Caenorhabditis elegans homologue of beta-spectrin is encoded by the unc-70 gene. unc-70 null mutants develop slowly, and the adults are paralyzed and dumpy. However, the membrane integrity is not impaired in unc-70 animals, nor is cell polarity affected. Thus, beta-spectrin is not essential for general membrane integrity or for cell polarity. However, beta-spectrin is required for a subset of processes at cell membranes. In neurons, the loss of beta-spectrin leads to abnormal axon outgrowth. In muscles, a loss of beta-spectrin leads to disorganization of the myofilament lattice, discontinuities in the dense bodies, and a reduction or loss of the sarcoplasmic reticulum. These defects are consistent with beta-spectrin function in anchoring proteins at cell membranes.

UNC-13 is required for synaptic vesicle fusion in C. elegans.

We analyzed the synaptic physiology of unc-13 mutants in the nematode C. elegans. Mutants of unc-13 had normal nervous system architecture, and the densities of synapses and postsynaptic receptors were normal at the neuromuscular junction. However, the number of synaptic vesicles at neuromuscular junctions was two- to threefold greater in unc-13 mutants than in wild-type animals. Most importantly, evoked release at both GABAergic and cholinergic synapses was almost absent in unc-13 null alleles, as determined by whole-cell, voltage-clamp techniques. Although mutant synapses had morphologically docked vesicles, these vesicles were not competent for release as assayed by spontaneous release in calcium-free solution or by the application of hyperosmotic saline. These experiments support models in which UNC-13 mediates either fusion of vesicles during exocytosis or priming of vesicles for fusion.

The inositol trisphosphate receptor regulates a 50-second behavioral rhythm in C. elegans.

The C. elegans defecation cycle is characterized by the contraction of three distinct sets of muscles every 50 s. Our data indicate that this cycle is regulated by periodic calcium release mediated by the inositol trisphosphate receptor (IP3 receptor). Mutations in the IP3 receptor slow down or eliminate the cycle, while overexpression speeds up the cycle. The IP3 receptor controls these periodic muscle contractions nonautonomously from the intestine. In the intestinal cells, calcium levels oscillate with the same period as the defecation cycle and peak calcium levels immediately precede the first muscle contraction. Mutations in the IP3 receptor slow or eliminate these calcium oscillations. Thus, the IP3 receptor is an essential component of the timekeeper for this cycle and represents a novel mechanism for the control of behavioral rhythms.

Growth cones stall and collapse during axon outgrowth in Caenorhabditis elegans.

During nervous system development, neurons form synaptic contacts with distant target cells. These connections are formed by the extension of axonal processes along predetermined pathways. Axon outgrowth is directed by growth cones located at the tips of these neuronal processes. Although the behavior of growth cones has been well-characterized in vitro, it is difficult to observe growth cones in vivo. We have observed motor neuron growth cones migrating in living Caenorhabditis elegans larvae using time-lapse confocal microscopy. Specifically, we observed the VD motor neurons extend axons from the ventral to dorsal nerve cord during the L2 stage. The growth cones of these neurons are round and migrate rapidly across the epidermis if they are unobstructed. When they contact axons of the lateral nerve fascicles, growth cones stall and spread out along the fascicle to form anvil-shaped structures. After pausing for a few minutes, they extend lamellipodia beyond the fascicle and resume migration toward the dorsal nerve cord. Growth cones stall again when they contact the body wall muscles. These muscles are tightly attached to the epidermis by narrowly spaced circumferential attachment structures. Stalled growth cones extend fingers dorsally between these hypodermal attachment structures. When a single finger has projected through the body wall muscle quadrant, the growth cone located on the ventral side of the muscle collapses and a new growth cone forms at the dorsal tip of the predominating finger. Thus, we observe that complete growth cone collapse occurs in vivo and not just in culture assays. In contrast to studies indicating that collapse occurs upon contact with repulsive substrata, collapse of the VD growth cones may result from an intrinsic signal that serves to maintain growth cone primacy and conserve cellular material.

One GABA and two acetylcholine receptors function at the C. elegans neuromuscular junction.

We describe an electrophysiological preparation of the neuromuscular junction of the nematode C. elegans, which adds to its considerable genetic and genomic resources. Mutant analysis, pharmacology and patch-clamp recording showed that the body wall muscles of wild-type animals expressed a GABA receptor and two acetylcholine receptors. The muscle GABA response was abolished in animals lacking the GABA receptor gene unc-49. One acetylcholine receptor was activated by the nematocide levamisole. This response was eliminated in mutants lacking either the unc-38 or unc-29 genes, which encode alpha and non-alpha acetylcholine receptor subunits, respectively. The second, previously undescribed, acetylcholine receptor was activated by nicotine, desensitized rapidly and was selectively blocked by dihydro-beta-erythroidine, thus explaining the residual motility of unc-38 and unc-29 mutants. By recording spontaneous endogenous currents and selectively eliminating each of these receptors, we demonstrated that all three receptor types function at neuromuscular synapses.

UNC-11, a Caenorhabditis elegans AP180 homologue, regulates the size and protein composition of synaptic vesicles.

The unc-11 gene of Caenorhabditis elegans encodes multiple isoforms of a protein homologous to the mammalian brain-specific clathrin-adaptor protein AP180. The UNC-11 protein is expressed at high levels in the nervous system and at lower levels in other tissues. In neurons, UNC-11 is enriched at presynaptic terminals but is also present in cell bodies. unc-11 mutants are defective in two aspects of synaptic vesicle biogenesis. First, the SNARE protein synaptobrevin is mislocalized, no longer being exclusively localized to synaptic vesicles. The reduction of synaptobrevin at synaptic vesicles is the probable cause of the reduced neurotransmitter release observed in these mutants. Second, unc-11 mutants accumulate large vesicles at synapses. We propose that the UNC-11 protein mediates two functions during synaptic vesicle biogenesis: it recruits synaptobrevin to synaptic vesicle membranes and it regulates the size of the budded vesicle during clathrin coat assembly.

The Caenorhabditis elegans unc-49 locus encodes multiple subunits of a heteromultimeric GABA receptor.

Ionotropic GABA receptors generally require the products of three subunit genes. By contrast, the GABA receptor needed for locomotion in Caenorhabditis elegans requires only the unc-49 gene. We cloned unc-49 and demonstrated that it possesses an unusual overlapping gene structure. unc-49 contains a single copy of a GABA receptor N terminus, followed by three tandem copies of a GABA receptor C terminus. Using a single promoter, unc-49 generates three distinct GABAA receptor-like subunits by splicing the N terminus to each of the three C-terminal repeats. This organization suggests that the three UNC-49 subunits (UNC-49A, UNC-49B, and UNC-49C) are coordinately rescued and therefore might coassemble to form a heteromultimeric GABA receptor. Surprisingly, only UNC-49B and UNC-49C are expressed at high levels, whereas UNC-49A expression is barely detectable. Green fluorescent protein-tagged UNC-49B and UNC-49C subunits are coexpressed in muscle cells and are colocalized to synaptic regions. UNC-49B and UNC-49C also coassemble efficiently in Xenopus oocytes and HEK-293 cells to form a heteromeric GABA receptor. Together these data argue that UNC-49B and UNC-49C coassemble at the C. elegans neuromuscular junction. Thus, C. elegans is able to encode a heteromeric GABA receptor with a single locus.

C. elegans neuroscience: genetics to genome.

From their earliest experiments, researchers using Caenorhabditis elegans have been interested in the role of genes in the development and function of the nervous system. As the C. elegans Genome Project completes the genomic sequence, we review the accomplishments of these researchers and the impact that the Genome Project has bad on their research. We also speculate on future directions in this research that are enabled by the efforts of the Genome Project.

Identification and characterization of the vesicular GABA transporter.

Synaptic transmission involves the regulated exocytosis of vesicles filled with neurotransmitter. Classical transmitters are synthesized in the cytoplasm, and so must be transported into synaptic vesicles. Although the vesicular transporters for monoamines and acetylcholine have been identified, the proteins responsible for packaging the primary inhibitory and excitatory transmitters, gamma-aminobutyric acid (GABA) and glutamate remain unknown. Studies in the nematode Caenorhabditis elegans have implicated the gene unc-47 in the release of GABA. Here we show that the sequence of unc-47 predicts a protein with ten transmembrane domains, that the gene is expressed by GABA neurons, and that the protein colocalizes with synaptic vesicles. Further, a rat homologue of unc-47 is expressed by central GABA neurons and confers vesicular GABA transport in transfected cells with kinetics and substrate specificity similar to those previously reported for synaptic vesicles from the brain. Comparison of this vesicular GABA transporter (VGAT) with a vesicular transporter for monoamines shows that there are differences in the bioenergetic dependence of transport, and these presumably account for the differences in structure. Thus VGAT is the first of a new family of neurotransmitter transporters.

Caenorhabditis elegans rab-3 mutant synapses exhibit impaired function and are partially depleted of vesicles.

Rab molecules regulate vesicular trafficking in many different exocytic and endocytic transport pathways in eukaryotic cells. In neurons, rab3 has been proposed to play a crucial role in regulating synaptic vesicle release. To elucidate the role of rab3 in synaptic transmission, we isolated and characterized Caenorhabditis elegans rab-3 mutants. Similar to the mouse rab3A mutants, these mutants survived and exhibited only mild behavioral abnormalities. In contrast to the mouse mutants, synaptic transmission was perturbed in these animals. Extracellular electrophysiological recordings revealed that synaptic transmission in the pharyngeal nervous system was impaired. Furthermore, rab-3 animals were resistant to the acetylcholinesterase inhibitor aldicarb, suggesting that cholinergic transmission was generally depressed. Last, synaptic vesicle populations were redistributed in rab-3 mutants. In motor neurons, vesicle populations at synapses were depleted to 40% of normal levels, whereas in intersynaptic regions of the axon, vesicle populations were elevated. On the basis of the morphological defects at neuromuscular junctions, we postulate that RAB-3 may regulate recruitment of vesicles to the active zone or sequestration of vesicles near release sites.

Defective recycling of synaptic vesicles in synaptotagmin mutants of Caenorhabditis elegans.

Synaptotagmin, an integral membrane protein of the synaptic vesicle, binds calcium and interacts with proteins of the plasma membrane. These observations suggest several possible functions for synaptotagmin in synaptic vesicle dynamics: it could facilitate exocytosis by promoting calcium-dependent fusion, inhibit exocytosis by preventing fusion, or facilitate endocytosis of synaptic vesicles from the plasma membrane by acting as a receptor for the endocytotic proteins of the clathrin AP2 complex. Here we show that synaptic vesicles are depleted at synaptic terminals in synaptotagmin mutants of the nematode Caenorhabditis elegans. This depletion is not caused by a defect in transport or by increased synaptic vesicle release, but rather by a defect in retrieval or synaptic vesicles from the plasma membrane. Thus we propose that, as well as being involved in exocytosis, synaptotagmin functions in vesicular recycling.

Positive and negative control of the Antennapedia promoter P2.

To understand the nature of the regulatory signals impinging on the second promoter of the Antennapedia gene (Antp P2), analysis of its expression in mutants and in inhibitory drug injected embryos has been carried out. The maternally-active gene osk is identified as one of two general repressors of P2 which prevent Antp transcription until division cycle 14. Products of the zygotically-active segmentation genes ftz, hb, Kr, gt and kni then act as activators or repressors of Antp P2 in a combinatorial fashion. The timing of these events, and their positive versus negative nature, is critical for generating the expression patterns normal for Antp.

Function and misfunction of the two promoters of the Drosophila Antennapedia gene.

In the Antennapedia (Antp) gene of Drosophila melanogaster, structurally distinct RNAs arise from different transcription initiation sites. When the two sites are separated by a chromosome inversion, transcripts are produced from each fragment of the split Antp locus, and these RNAs initiate at the same nucleotide as in wild-type animals. Thus, the initiation sites are regulated by independent promoters. We show by in situ hybridization that transcripts from each promoter accumulate in spatially distinct patterns in a subset of wild-type imaginal discs. Importantly, these patterns are generally maintained in the inversion mutant. We conclude that the promoters possess independent and dissimilar regulatory elements for spatial activation. Finally, we have looked at transcription in seven different dominant Antp mutants, all of which show a transformation of head tissue to thoracic tissue. In each mutant, the second promoter is improperly activated in the eye-antennal imaginal disc. Because all but one of these mutations have inversion breakpoints distantly upstream of the activated promoter, they probably act via long-range euchromatic position effects. Our studies define how the dual promoters and chromatin structure of the Antp gene contribute to the generation of a complex pattern of transcription.