In situ structure of actin remodeling during glucose-stimulated insulin secretion using cryo-electron tomography.

Actin mediates insulin secretion from the pancreatic β-cell through a remodeling process. Previous studies have been hampered by limited resolution, providing an ambiguous depiction of actin remodeling as a process that begins with depolymerization into actin monomers, followed by repolymerization into actin filaments. Here, we report the in situ structure of actin remodeling in INS-1E β-cells during glucose-stimulated insulin secretion at nanoscale resolution. We demonstrate that actin remodeling occurs at the cell periphery rather than in the cell interior. The actin filament network at the cell periphery exhibits three marked differences after remodeling compared to those under basal conditions. First, approximately 12%of actin filaments reorient, their angle changing from 0-45° to 45-90° relative to the plasma membrane. Second, the actin filament network remains predominantly as cell-stabilizing bundles but partially reconfigures into a less compact arrangement. Third, actin filaments anchored to the plasma membrane reorganize from a "netlike" to a "blooming" architecture, featuring radial projections emanating from their anchor points. Remodeling precedes the transport of insulin secretory granulesto the plasma membrane and their release from it. Furthermore, the density of actin filaments and microtubules around insulin secretory granules is lowered after remodeling compared to the basal conditions, as expected for the subsequent granule transport and release. Finally, actin filaments and microtubules are more densely packed than under basal conditions. These findings advance our structural and functional understanding of actin remodeling during glucose-stimulated insulin secretion in pancreatic β-cells.