Despite a key role for dense core vesicles (DCVs) in neuronal function, there are major gaps in our understanding of DCV biogenesis. We performed a genetic screen for C. elegans mutants with behavioral defects consistent with impaired DCV function and recovered five mutations in UNC-108 (Rab2). Electrophysiological analysis showed that acetylcholine release from small synaptic vesicles is normal in
unc-108 mutants. Genetic analysis showed that
unc-108 mutations impair a DCV function unrelated to neuropeptide release that, together with neuropeptide release, fully accounts for the role of DCVs in locomotion. EM analysis of DCVs in
unc-108 mutants, coupled with quantitative imaging of DCV cargo proteins, revealed that Rab2 acts in cell somas during DCV maturation to prevent the loss of soluble and transmembrane DCV cargo. In Rab2 null mutants, two-thirds of these cargo move to early endosomes via a PI(3)P - dependent trafficking pathway, whereas aggregated neuropeptides are unaffected. Other studies have shown a strong increase in tubulovesicular structures that correspond to early and recycling endosomes in
unc-108 mutant neuronal cell somas (Chun et al., 2008) and a long delay in the rate at which newly engulfed cell corpses move through an early endosome intermediate in
unc-108 mutants (Mangahas et al., 2008) We propose that, in the absence of Rab2, an expanded and/ or longer-lived early endosomal network interacts with maturing dense core vesicles and removes most of their soluble and transmembrane proteins before they exit the cell soma. These results, along with a complementary study (Sumacovic et al., submitted), reveal how neurons use the most highly conserved animal Rab protein to solve a challenging trafficking problem. Chun, D.K., J.M. McEwen, M. Burbea, and J.M. Kaplan. 2008. UNC-108/Rab2 Regulates Post-endocytic Trafficking in C. elegans. Mol Biol Cell. 19:2682-95. Mangahas, P.M., X. Yu, K.G. Miller, and Z. Zhou. 2008. The small GTPase Rab2 functions in the removal of apoptotic cells in Caenorhabditis elegans. J Cell Biol. 180:357-73. Sumakovic, M., Hegermann, J., Husson, S. J., Luo, L., Schwarze, K., Olendrowitz, C., Schoofs, L., Richmond, J. and S. Eimer. Submitted. UNC-108/RAB-2 and its effector RIC-19 are involved in dense core vesicle maturation in C. elegans.