A critical phase in the development of the nervous system is the formation of connections between axons and their synaptic targets. These axon pathfinding events are controlled by both long- and short-range guidance cues that are expressed by intermediate targets which exist at various points along a given axonal trajectory. We have previously identified VEMA as a novel marker of two vertebrate midline structures that are important intermediate targets for midline-crossing axons: the floor plate of the ventral spinal cord and the optic chiasm of the ventral diencephalon. Interestingly, VEMA is also expressed in specific early-developing neuronal populations as they initiate axongenesis. The deduced amino acid sequence of VEMA contains a single transmembrane domain and several distinct sorting motifs that are thought to mediate trafficking of proteins between intracellular compartments and the plasma membrane. The C. elegans genome encodes for a single ortholog of VEMA that is 36 % identical to the amino acid sequence of vertebrate VEMA . We refer to the C. elegans ortholog of VEMA as
vem-1 . To elucidate the distribution of
vem-1 in embryonic and larval stage C. elegans we utilized a
vem-1::GFP transcriptional reporter line. Prior to the comma stage,
vem-1::GFP was expressed in distinct neurons of the head ganglia during the period of axonal outgrowth. During later stages of embryogenesis (1.5 fold to 3 fold stage),
vem-1::GFP was expressed in pioneer axons of the developing ventral nerve cord. Throughout the larval stages and into adulthood, expression of the
vem-1::GFP transgene was maintained on a specific subset of neurons located within a variety of head and tail ganglia. Interestingly, expression in the AVG and several of the PV neurons suggests a possible role for
vem-1 in regulating the outgrowth and/or guidance of axons which pioneer the ventral nerve cord. To test this hypothesis, we employed dsRNAi to disrupt VEMA function in a variety of GFP reporter lines. We utilized both the
unc-119::GFP reporter which is a pan-neural marker, and the
glr-1::GFP reporter, which labels interneurons and their axons (particularly the AV class) in the right ventral nerve cord to identify potential defects in ventral nerve cord structure. Importantly, these perturbations resulted in no observable change in cell body position, implying that
vem-1 has no major role in the migration of neuronal precursors. Strikingly, similar types of axonal defects were observed in the both the
glr-1::GFP and the
unc-119::GFP reporter lines (23 % and 17 % , respectively), thereby suggesting
vem-1 plays a role in the correct formation of the right ventral nerve cord. The defects most frequently observed were longitudinal breaks, a defasciculation phenotype in which an axon left the ventral nerve cord bundle for a short distance then eventually rejoined the fascicle, or lateral axons, which represent inappropriate projections from the ventral nerve cord into more lateral regions. The premature termination of an axon or the abnormal movement of an axon from the right fascicle across the midline to the left fascicle represent examples of less frequently observed defects. Most of the axonal defects occurred in the posterior ventral cord between the vulva and the tail. However, a few lateral axons were observed in anterior regions where an axon never joined the ventral nerve cord after extending through the nerve ring and instead traveled laterally above the retrovesicular ganglion. The use of more specific GFP reporter lines will be required in order to unambiguously identify the specific axons which are affected in these perturbations. Whether these perturbed axons are from neurons that express
vem-1 remains undetermined. Taken together, these RNAi results suggest that
vem-1 may play a critical role in regulating axon guidance and/or fascicle formation in the C. elegans ventral nerve cord. Furthermore, the restricted developmental expression pattern in nematodes and vertebrates suggests a possible evolutionarily-conserved role for VEMA/vem-1 in the developing central nervous system.