Correlation of actin reorganization and granule movement in stimulated primary beta cells

Background and aims:

It has been shown that the depolarization by high K⁺ does not elicit the same functional consequences on the actin cytoskeleton as glucose. This raises the possibility that the actin cytoskeleton is targeted by the amplifying pathway of insulin secretion and regulates insulin granule mobility.

Methods:

All parameters were measured in perifused single mouse beta-cells. [Ca²⁺]_i was measured by the Fura technique, the mobility of the granules in the submembrane space and the actin cytoskeleton were imaged by TIRF microscopy of adenovirally transduced beta-cells. Actin was visualized by tagRFP-Lifeact and the insulin granules were visualized by Insulin-EGFP and C-peptide-emGFP.

Results:

40 mM KCl produced a fast and strong increase in [Ca²⁺]_i with an initial overshoot. 30 mM glucose led to a lower but more sustained elevation. To specifically measure the effect on granule mobility the values obtained during control perifusions (5 mM glucose throughout) were subtracted from those of the test perifusions. The mobility parameters which are indicative of granule turnover were significantly smaller for 30 mM glucose than for 40 mM KCl. Single beta cells as studied here had a remarkably thick cortical actin web as compared with beta cells within clusters. High glucose increased areas with low actin density, which was partially coupled to increased density of submembrane granules.

Conclusion:

K⁺ depolarization accelerates granule turnover in the submembrane space whereas glucose diminishes it. This correlates with actin reorganization in this compartment.