Synaptic tetraspan vesicle membrane proteins are conserved but not needed for synaptogenesis and neuronal function in Caenorhabditis elegans

Abstract

Tetraspan vesicle membrane proteins (TVPs) comprise a major portion of synaptic vesicle proteins, yet their contribution to the synaptic vesicle cycle is poorly understood. TVPs are grouped in three mammalian gene families: physins, gyrins, and secretory carrier-associated membrane proteins (SCAMPs). In Caenorhabditis elegans, only a single member of each of these families exists. These three nematode TVPs colocalize to the same vesicular compartment when expressed in mammalian cells, suggesting that they could serve overlapping functions. To examine their function, C. elegans null mutants were isolated for each gene, and a triple mutant was generated. Surprisingly, these animals develop normally and exhibit normal neuronal architecture and synaptic contacts. In addition, functions of the motor and sensory systems are normal as determined by pharmacological, chemotaxis, and thermotaxis assays. Finally, direct electrophysiological analysis of the neuromuscular junction revealed no phenotype in the TVP mutants. We therefore conclude that TVPs are not needed for the basic neuronal machinery and instead may contribute to subtle higher order functions.integral membrane proteins ͉ phylogenesis ͉ vesicle trafficking ͉ synaptophysin ͉ synaptogyrin A mong the different synaptic vesicle proteins, those with four membrane-spanning domains [referred to as tetraspan vesicle membrane proteins (TVPs)] are a particularly abundant group whose function in neurotransmission is not understood (1, 2). These polypeptides belong to the physin, gyrin, and secretory carrier-associated membrane protein (SCAMP) families. Members are present in different combinations in synaptic vesicles and also in other transport vesicles of various mammalian cell types (2). Especially puzzling are the mild or even absent phenotypic defects in neurons of knockout mice lacking synaptophysin, synaptogyrin-1, or SCAMP-1 (3-6). A likely explanation is that other members of the same family or even members of other TVP families compensate for loss of a particular TVP. For example, synaptophysin-deficient mice exhibited no neuronal defects except in the photoreceptors that lack the related neuronal physin isoform synaptoporin (7). Furthermore, combined knockouts of the synaptic TVPs synaptophysin and synaptogyrin lead to changes in synaptic plasticity, whereas the single gene inactivations do not (6).Numerous publications propose roles for TVPs for practically all aspects of the synaptic vesicle cycle, including vesicle biogenesis, exocytosis, and endocytotic recycling (2). This notion is supported by the multiple interactions of TVPs with lipids, notably cholesterol (8) or various components of the recycling machinery including soluble N-ethylmaleimide-sensitive fusion attachment protein receptors (SNAREs) (9-11), dynamin (12, 13), adaptor proteins (14), and eps15 homology (EH)-domain proteins (15). In addition, it has been postulated that TVPs are directly responsible for microvesicle formation (8,16,17) and are involved in fusion pore formation (18-21).Thus,...

Nonredundant function of secretory carrier membrane protein isoforms in dense core vesicle exocytosis

“…For each of the knockdowns, the total number of exocytotic events appeared to decrease in relation to the control, indicating inhibition of exocytosis. 1 Supplemental material for this article is available at the American Journal of Physiology-Cell Physiology website. shows an example of a prespike foot (PSF) associated with many of the spikes and analyzed in all samples.…”

“…Presently, however, there is not a consistent picture of their roles and, particularly, whether different SCAMP isoforms have distinct functions. Whole organism knockouts of SCAMP-1 in the mouse and of the lone SCAMP isoform expressed in Caenorhabditis elegans resulted in little or no obvious phenotypic defects in neural function (1,9). In contrast, other findings clearly implicated selected mammalian SCAMPs (SCAMP-1 or -2) in the regulation of late events in exocytosis in mast cells and neuroendocrine cells and also suggested a role in constitutive clathrin-mediated endocytosis (1,8,9,14,23,24).…”

“…Whole organism knockouts of SCAMP-1 in the mouse and of the lone SCAMP isoform expressed in Caenorhabditis elegans resulted in little or no obvious phenotypic defects in neural function (1,9). In contrast, other findings clearly implicated selected mammalian SCAMPs (SCAMP-1 or -2) in the regulation of late events in exocytosis in mast cells and neuroendocrine cells and also suggested a role in constitutive clathrin-mediated endocytosis (1,8,9,14,23,24). Potentially, these discrepancies may reflect differences in the cell types in which functions were assayed and, for exocytosis in particular, the distinction between examining processes mediated by fast versus slow secretory events (14,23,24).…”

Abnormal Synaptic Vesicle Biogenesis inDrosophila SynaptogyrinMutants

“…Initial characterization of a Caenorhabditis elegans triple knockout of synaptophysin, synaptogyrin, and the tetraspanning membrane protein SCAMP revealed no detectable phenotypes besides a slight increase in clathrin-coated synaptic vesicles (Abraham et al, 2006). Closer examination of the C. elegans synaptogyrin mutant identified slightly altered motility and changes in sensitivity to drugs affecting GABAergic and cholinergic signaling (Abraham et al, 2011).…”

The architecture of an excitatory synapse

“…Trafficking proteins include synaptobrevin [also known as vesicle-associated membrane protein (VAMP)] and synaptotagmin (see below). In addition, SVs include a heterogeneous group of proteins that contain four transmembrane domains [such as synaptophysin, synaptogyrin and secretory carrier-associated membrane proteins (SCAMPs)] (Abraham et al, 2006), as well as the proteins synaptic vesicle glycoprotein 2 (SV2) and SV2-related protein (SVOP), which resemble transporters, but are proposed to be involved in trafficking.…”