Synaptobrevin, a Major Protein of v-SNARE, Participates in Maintenance of Foot Processes of Podocytes: Downregulation of Synaptobrevin Is an Initiation Event Leading to Podocyte Injury in Minimal Change Nephrotic Syndrome (MCNS) Model

Background: Podocytes contain synaptic-like vesicle that contributes to maintaining specialized podocyte morphology and function. We reported synaptic vesicle protein 2B (SV2B) is expressed on the vesicle and plays a critical role in podocytes. However, the molecular basis and the function of the vesicle are not well understood. Synaptobrevin is a v-SNARE expressed on the synaptic vesicle and plays a central role in docking to plasma membrane. In this study we investigated the association with SV2B, expression and localization, and physiological and pathological roles of synaptobrevin in podocytes. Methods: Association with SV2B was analyzed with SV2B-deleted podocytes. The expression in normal and injured podocyte was analyzed. The function was elucidated with cultured podocyte treated with siRNA for synaptobrevin. Results: The expression of synaptobrevin was lowered in SV2B knockdown cultured podocyte (24.7±6.4%, p< 0.05). Synaptobrevin was restrictedly expressed in podocyte in glomeruli. In both rat and human kidney sections synaptobrevin was detected as a dot-like pattern along capillary loop. Major parts of synaptobrevin were co-stained with synaptopodin, a marker of foot processes. mRNA expression of synaptobrevin was decreased already at 1h in PAN nephropathy, a mimic of MCNS (37.9±16.4 % to normal) and the synaptobrevin staining became discontinuous on day 10, when proteinuria peaked and foot processes effacement was broadly detected (score: 2.23 ±0.03 vs. normal; 3.74±0.06, p<0.05). The expression of synaptobrevin in cultured podocytes increased with differentiating (2.88 times higher). If synaptobrevin was deleted, podocyte changed to round shape and the rate of podocytes possessing processes was lowered (19.3±4.7% vs. 34.0±2.6%, p<0.01), and mRNA expressions of slit diaphragm molecules were lowered (nephrin, 50.9±1.9%; CD2AP, 47.3±3.2%, p<0.05). Conclusion: Synaptobrevin participates in maintenance of foot processes of podocytes in cooperation with SV2B. Downregulation of synaptobrevin is involved in effacement of foot process and consequent downregulation of slit diaphragm molecules, which leading to proteinuria. Synaptobrevin can be an early marker and a therapeutic target of podocyte dysfunction. Funding: Government Support – Non-U.S.