The role of UNC-108/RAB-2 in neuronal dense core vesicle maturation in C. elegans

Abstract

Small GTPases of the Rab family are involved in vesicular transport between different intracellular compartments. C. elegans mutant alleles of unc-108/rab-2 have been isolated based on their slow locomotion phenotype. This suggests defects in the synaptic transmission at the neuromuscular junction (NMJ) in C. elegans. In this work, I analyzed the functional defects causing reduced locomotion in unc-108/rab-2 mutant animals. In C. elegans, fast synaptic transmission at the NMJ is mediated by release of acetylcholine from synaptic vesicles. However, for efficient neurotransmission motorneurons also rely on the dense core vesicles (DCVs) that are coreleased with SVs at the NMJ. Mutants impaired in DCV secretion have strong locomotory defects, suggesting an important role of DCV signaling in neurotransmission. We show that slow movement of unc-108/rab-2 mutants is caused by impaired DCV signaling. We further show that RAB-2 is specifically required for neuronal DCV maturation. In unc-108/rab-2 mutants, specific cargo is inappropriately lost from maturing DCVs to endosomal/lysosomal degradation system. We demonstrate that a yet unidentified DCV factor is required in addition to neuropeptides for efficient neurotransmission. Finally, we show that RIC-19, the C. elegans ortholog of the human diabetes autoantigen ICA69, is also involved in DCV maturation and recruited to Golgi membranes by activated RAB-2. Thus, we conclude that RAB-2 and its effector RIC-19 are necessary for DCV maturation and normal neurotransmission at the neuromuscular junction. However, RAB-2 is not the only GTPase required for efficient DCV signaling, as we could demonstrate that expression of dominant active RAB-5 is able to block DCV secretion. This suggests that particularly DCV function in neurons heavily relies on Rab dependent intracellular trafficking.