SUMMARY
Vitamin K is a micronutrient necessary for the γ-carboxylation of glutamic acids. This post-translational modification occurs in the endoplasmic reticulum (ER) and affects secreted proteins. Clinical studies have recently implicated vitamin K in the pathophysiology of diabetes, but the underlying molecular mechanism remains unknown. Here, we show that mouse β-cells lacking γ-carboxylation fail to adapt their insulin secretion in the context of age-related insulin resistance or diet-induced β-cell stress. In human islets, γ-carboxylase expression positively correlates with improved insulin secretion in response to glucose. We identified Endoplasmic Reticulum Gla Protein (ERGP) as a novel γ-carboxylated ER-resident calcium-binding protein expressed in β-cells. Mechanistically, γ-carboxylation of ERGP protects cells against calcium overfilling by diminishing STIM1 and Orai1 interaction and restraining store-operated calcium entry. These results reveal a critical role for vitamin K-dependent γ-carboxylation in the regulation of calcium flux in β-cells and in their capacity to adapt to metabolic stress.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵7 Co-first authors
-We are now showing that the gamma-carboxylated protein present in beta-cell is not junctate, but a new isoform of the Asph locus which we have named Endoplasmic Reticulum Gla Protein (ERGP). Fig. S6 - We have performed ratiometric cytosolic calcium measurements and confirmed our initial findings. Fig. 6 - We have assessed endoplasmic reticulum (ER) calcium level using a ratiometric FRET calcium sensor and confirmed reduced ER free calcium level in presence of gamma-carboxylated ERGP. Fig. 6 - We have performed FRET imaging experiments to assess the interaction between STIM1 and Orai1 and confirmed that gamma-carboxylated ERGP inhibits the formation of these protein complexes. Fig. 6 - Following a suggestion of reviewers, we are now showing that reducing cytosolic calcium in GGCX-deficient beta-cells using a calcium channel blocker (verapamil) corrects the fasting hyperinsulinemia of the Ggcxfl/fl; Ins1-Cre mice. Fig. 7 -We have assessed insulin secretion in response to glucose in isolated GGCX-deficient islets. Fig. 2