The actin binding protein villin controls accessibility of insulin granules to their docking sites

Aim:

F-actin regulates exocytosis of secretory granules (SGs) by restricting their access to the plasma membrane. We showed that F-actin modifier vilIin regulates insulin granule dynamics and exocytosis downstream of islet cell autoantigen 512. Here we investigated whether there is a linkage between villin and SNARE proteins mediated exocytosis, in particular the role of villin in the docking of SGs to their competent sites.

Results:

Computer simulations based on insulin secretion patterns in control vs. villin-depleted MIN6 cells lead us to propose a model in which the altered GSIS of the latter cannot be explained only by the greater mobility of the insulin SGs, but it depends also on an increased number of accessible docking sites for SG fusion at the plasma membrane. Using TIRF microscopy to analyze cells expressing syntaxin1-GFP and SNAP-tagged insulin we verified that the number of “docked” SGs was increased upon villin depletion, which in turn was inversely correlated with the total number of SGs under basal condition. Redistribution of SNARE protein SNAP25 upon villin depletion suggested a direct interaction of the two proteins. Data collected by FLIM and immnunoprecipitation assays corroborated this possibility. Finally, we found evidence that villin expression is enhanced by STAT5, which in turn lies downstream of ICA512 signaling following SG exocytosis.

Conclusions:

These data suggest that SGs are not passively regulated by the cortical cytoskeleton, but actively control the plasticity of the latter, and thus their own access to the plasma membrane and release by affecting villin expression and SNARE compartimentalization.