Glucagon secretion by pancreatic alpha-cells requires an intact tubulin-cytoskeleton-primary cilium axis.

BACKGROUND

Hyperglucagonemia is a hallmark of diabetes mellitus, resulting from the dysregulation of glucagon secretion by pancreatic alpha-cells. Although glucose sensing and insulin signaling are well-established regulatory processes, the pathways that govern glucagon secretion remain unclear. Recent evidences suggest that insulin-degrading enzyme (IDE) regulates glucagon secretion via an unknown pathway.

METHODS

Using IDE-immunoprecipitation proteomic data as a basis, we aimed to ascertain the molecular mechanisms downstream of IDE in the alpha-TC cell model. Knock-down studies of relevant genes involved in the functional pathways identified in the proteomic study, and its impact on glucagon secretion were performed. Primary cilium was stained and detected using confocal and STORM microscopies in alpha-TC cells and mouse pancreas.

RESULTS

Based on proteomic studies we focus our efforts on the relationship between IDE, tubulin cytoskeleton, and the primary cilium. Although IDE was not localized to the primary cilium of alpha-cells using confocal microscopy and STORM, its absence resulted in impaired ciliogenesis. Consistent with lower protein levels of the insulin receptor, the counterregulatory effect of insulin on glucagon secretion was reduced in IDE-deficient alpha-cells. Two cellular models of ciliary dysfunction (ARL13B-KD and IFT88-KD) resulted in impaired glucagon secretion, as well as a failure of insulin to suppress glucagon secretion in alpha-cells. Importantly, IDE, tubulin, ciliary markers (AcTubulin, ARL13B) and insulin receptor levels were diminished in glucose conditions of physiological glucagon repression.

CONCLUSIONS

IDE acts as a mechanistic link between glucose levels, tubulin, and the primary cilium, regulating glucagon secretion in alpha-cells. The dysregulation of the tubulin-primary cilium axis induces glucagon secretion impairment.