Elsevier

Molecular Metabolism

Volume 5, Issue 8, August 2016, Pages 656-668
Molecular Metabolism

Original article
The F-actin modifier villin regulates insulin granule dynamics and exocytosis downstream of islet cell autoantigen 512

https://doi.org/10.1016/j.molmet.2016.05.015Get rights and content
Under a Creative Commons license
open access

Highlights

  • Ica512-depletion reduces the genetic expression of the F-actin modifier villin.

  • Villin-depletion enhances basal insulin granule mobility and exocytosis.

  • Villin regulates the size of actin cages restraining insulin granules.

  • Villin acts downstream of insulin granule cargo Ica512.

  • The Ica512-villin genetic link enables granules to control cytoskeleton plasticity.

Abstract

Objective

Insulin release from pancreatic islet β cells should be tightly controlled to avoid hypoglycemia and insulin resistance. The cortical actin cytoskeleton is a gate for regulated exocytosis of insulin secretory granules (SGs) by restricting their mobility and access to the plasma membrane. Prior studies suggest that SGs interact with F-actin through their transmembrane cargo islet cell autoantigen 512 (Ica512) (also known as islet antigen 2/Ptprn). Here we investigated how Ica512 modulates SG trafficking and exocytosis.

Methods

Transcriptomic changes in Ica512−/− mouse islets were analyzed. Imaging as well as biophysical and biochemical methods were used to validate if and how the Ica512-regulated gene villin modulates insulin secretion in mouse islets and insulinoma cells.

Results

The F-actin modifier villin was consistently downregulated in Ica512−/− mouse islets and in Ica512-depleted insulinoma cells. Villin was enriched at the cell cortex of β cells and dispersed villin−/− islet cells were less round and less deformable. Basal mobility of SGs in villin-depleted cells was enhanced. Moreover, in cells depleted either of villin or Ica512 F-actin cages restraining cortical SGs were enlarged, basal secretion was increased while glucose-stimulated insulin release was blunted. The latter changes were reverted by overexpressing villin in Ica512-depleted cells, but not vice versa.

Conclusion

Our findings show that villin controls the size of the F-actin cages restricting SGs and, thus, regulates their dynamics and availability for exocytosis. Evidence that villin acts downstream of Ica512 also indicates that SGs directly influence the remodeling properties of the cortical actin cytoskeleton for tight control of insulin secretion.

Keywords

F-actin
Granules
Ica512
Insulin
Secretion
Villin

Abbreviations

D
diffusion coefficient
EGFP
enhanced green fluorescent protein
Ica512
islet cell autoantigen
IPGTT
intraperitoneal glucose tolerance test
IVGTT
intravenous glucose tolerance test
OGTT
oral glucose tolerance test
RT-DC
real-time deformability cytometry
SE
standard error
SG
secretory granules
TIRFM
total internal reflection fluorescence microscopy

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