Aim: Vitamin D-binding protein (DBP), also known as GC-globulin, transports vitamin D metabolites and is also a major actin scavenger. While DBP serum levels, gene polymorphisms and autoantigens have been associated with diabetes risk, the underlying mechanisms remain unknown. DBP is produced by the liver, but has recently been shown to be highly expressed in pancreatic α-cells. We therefore sought to investigate the role of DBP in α-cell identity and function using mice globally deleted for DBP (DBP−/−).
Results: 25(OH)D levels were low in DBP−/− mice, although no signs of vitamin D deficiency were present. Metabolic phenotyping revealed normal glucose tolerance but increased insulin sensitivity in DBP−/− animals compared to DBP+/+ littermates, despite similar body weights. DBP−/− islets showed loss of glucagon secretion in response to low glucose and epinephrine, although total glucagon content was unaffected. While α-cell mass was normal following loss of DBP, morphometric analysis showed an ˜30% decrease in α-cell size, which was associated with an increase in cell number. Suggesting changes in α-cell differentiation status, mRNA levels for Pax6, Arx, and Pou3f4 were decreased by 2-fold in islets from DBP−/− mice. Multicellular Ca2+ imaging of DBP−/− islets revealed impaired recruitment of α-cells by low glucose or epinephrine, but an increase in basal β-cell Ca2+ fluxes. Examination of the α-cell cytoskeleton revealed increased F-actin fiber thickness and density with decreased G-actin levels. Upon super-resolution analysis, single glucagon granules were found to be smaller and more diffusely scattered in DBP−/− vs. DBP+/+ islets, suggestive of defective maturation and/or trapping at the membrane. In sections from donors with type 1 diabetes, DBP expression was more heterogeneous compared to normal donors, with high and low DBP-expressing α-cells being present.
Conclusion: DBP is a critical regulator of α-cell phenotype and function, with implications for diabetes therapy and diagnosis.