Regulation of Synaptic Transmission by RAB-3 and RAB-27 inCaenorhabditis elegans

Abstract

Rab small GTPases are involved in the transport of vesicles between different membranous organelles. RAB-3 is an exocytic Rab that plays a modulatory role in synaptic transmission. Unexpectedly, mutations in the Caenorhabditis elegans RAB-3 exchange factor homologue, aex-3, cause a more severe synaptic transmission defect as well as a defecation defect not seen in rab-3 mutants. We hypothesized that AEX-3 may regulate a second Rab that regulates these processes with RAB-3. We found that AEX-3 regulates another exocytic Rab, RAB-27. Here, we show that C. elegans RAB-27 is localized to synapse-rich regions pan-neuronally and is also expressed in intestinal cells. We identify aex-6 alleles as containing mutations in rab-27. Interestingly, aex-6 mutants exhibit the same defecation defect as aex-3 mutants. aex-6; rab-3 double mutants have behavioral and pharmacological defects similar to aex-3 mutants. In addition, we demonstrate that RBF-1 (rabphilin) is an effector of RAB-27. Therefore, our work demonstrates that AEX-3 regulates both RAB-3 and RAB-27, that both RAB-3 and RAB-27 regulate synaptic transmission, and that RAB-27 potentially acts through its effector RBF-1 to promote soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) function. INTRODUCTIONNeurotransmitter release is accomplished by the fusion of neurotransmitter-filled synaptic vesicles at the presynaptic nerve terminal. This process occurs through a series of highly regulated steps that include synaptic vesicle transport, docking/tethering, priming, fusion, endocytosis, recycling, and neurotransmitter refilling (Sudhof, 2004). Several genes have been assigned roles in the various steps of the synaptic vesicle cycle. Rab3 (termed RAB-3 in Caenorhabditis elegans), a member of the Rab family of small GTPases, regulates synaptic transmission, possibly through the docking, priming, or fusion steps Schluter et al., 2004).Rabs act in a variety of cell types and regulate vesicular transport between organelles. Rabs cycle on and off membranes via a GTP-dependent mechanism (Zerial and McBride, 2001). Rab activity is regulated by two proteins, which act in an antagonistic manner. The guanine nucleotide exchange factor (GEF) exchanges GTP for GDP, and the GTPase activating protein activates the intrinsic GTPase activity of a Rab (Bernards, 2003). GDP bound Rabs are held off of the membrane by a GDP dissociation inhibitor (Wu et al., 1996). The GTP/membrane-bound form of Rabs typically binds to a variety of effectors that regulate particular steps of membrane transport and cell signaling (Zerial and McBride, 2001;Spang, 2004).Rab3 was once thought to play a central role in regulating release. However, recent work has shown that a quadruple knockout of all four isoforms of Rab3 in mice only results in a 30% reduction in evoked synaptic response (Schluter et al., 2004). This is consistent with work in C. elegans showing that mutations in the single rab-3 gene cause only mild defects in synaptic transmission . Surprisingly, more dramatic phe...

Intestinal signaling to GABAergic neurons regulates a rhythmic behavior in Caenorhabditis elegans

“…aex-1, aex-3, aex-4, and aex-6 are each homologous to the genes that regulate exocytosis in secretory cells (11,12). Here, we present data that these genes regulate the secretion of a signal from the intestine to induce aBoc and Exp.…”

“…SNARE proteins (AEX-4) are thought to create a membrane fusion structure at the nerve terminal, with SNARE regulators (AEX-1) playing a critical role in exocytosis (31). Although the precise function of Rabs (AEX-6) in exocytosis, per se, is unclear, they play an important role in synaptic transmission (11). Interestingly, although the regulator for AEX-6, AEX-3, has a similar defecation phenotype when mutated, the effector of AEX-6, RBF-1, does not (11).…”

“…Although the precise function of Rabs (AEX-6) in exocytosis, per se, is unclear, they play an important role in synaptic transmission (11). Interestingly, although the regulator for AEX-6, AEX-3, has a similar defecation phenotype when mutated, the effector of AEX-6, RBF-1, does not (11). Perhaps AEX-6, also known as Rab27, functions through a novel effector to regulate the DMP.…”

“…It seems likely that studies of aex genes will give some insight into how the AVL and DVB neurons are activated, because the behavioral defects of aex mutants are reminiscent of animals whose AVL and DVB GABAergic neurons are laser ablated (7,8). aex-3 and aex-6 regulate synaptic transmission, probably by regulating exocytosis of neurotransmitter: aex-3 is a guanine nucleotide exchange factor that regulates Rab small guanosine triphosphatase function, and aex-6 (also known as rab-27) is a Rab small guanosine triphosphatase that regulates secretory vesicle exocytosis (10,11). aex-5 encodes a pro-protein convertase, an enzyme that is copackaged with propeptides and processes them to make mature secretory molecules (12,13).…”

Rab27b regulates exocytosis of secretory vesicles in acinar epithelial cells from the lacrimal gland

“…Several studies indicate related roles for Rab27 and Rab3 family proteins in diverse systems (19,28,36) while conversely other work indicates that Rab27a and Rab3A have different functions in insulin secretion (54). Our previous exploration of Rab3D, which is highly expressed in LG acinar cells, found that expression of DN Rab3D only modestly affected secretion and that CA Rab3D did not significantly alter secretion (14).…”