SYNAPTIC VESICLE PROTEIN SV2B, BUT NOT SV2A, IS PREDOMINANTLY EXPRESSED AND ASSOCIATED WITH MICROVESICLES IN RAT PINEALOCYTES

Microvesicles are endocrine counterparts of neuronal synaptic vesicles , and accumulate and secrete classic neurotransmitters. In mammalian p inealocytes, microvesicles accumulate L-glutamate through a vesicular glutamate transporter and secrete it through exocytosis, To characteri ze the molecular organization of microvesicles in more detail, we inve stigated in this study the expression and localization of synaptic ves icle protein 2 (SV2) in rat pinealocytes. RT-PCR analysis indicated th at transcripts specific for two isoforms, SV2A, a ubiquitous form pres ent in neuronal and endocrine cells, and SV2B, a neuron-specific form, are amplified in pineal RNAs, Northern blotting with specific transcr ipts indicated that the mRNA for SV2B is predominantly expressed, wher eas that for SV2A is below the detection limit. Site-specific antibodi es against SV2B recognized a single 72-kDa polypeptide in the pineal m embrane fraction, whereas anti-SV2A antibodies did not recognize any p olypeptides, Immunohistochemical analysis of cultured cells indicated that SV2B is expressed in pinealocytes but not in other types of cells . SV2B is present in somata and is especially rich in processes, which are filled with microvesicles. SV2B is colocalized with synaptophysin and synaptotagmin, markers for microvesicles, Immunoelectron microsco py indicated that SV2B is associated with microvesicles. These results indicated that SV2B, but not SV2A, is expressed in rat pinealocytes a nd associated with microvesicles, As SV2B is also expressed in culture d alpha TC6 clonal pancreatic alpha cells, SV2B is not a protein speci fic for neurons.