Endoplasmic Reticulum Calcium in the Pathogenesis of Type 1 Diabetes

Abstract

Type 1 diabetes (T1D) accounts for 5-10% of all diabetes cases and results from immune-mediated destruction of pancreatic β-cells. Individuals with Darier Disease, which is caused by loss of function germline mutation(s) in the sarcoendoplasmic reticulum Ca2+-ATPase pump (SERCA2) gene, have an elevated risk of being diagnosed with T1D (risk ratio, 1.74; 95% CI, 1.13-2.69), suggesting a potential mechanistic relationship between SERCA2 and T1D pathogenesis. To determine the impact of reduced SERCA2 expression on T1D pathogenesis, we generated SERCA2 haploinsufficient mice by backcrossing C57BL6/J-S2+/- mice onto the non-obese diabetic (NOD) background (NOD-S2+/- mice). Female NOD-S2+/- mice showed accelerated T1D onset (14wks vs. 18wks, p<0.0001), elevated circulating anti-insulin antibodies, and increased immune cell infiltration into the islets compared to NOD-WT mice. Single-cell RNA sequencing (scRNA-seq) on islets and spatial proteomics on pancreatic lymph node (PLN) and spleen at 6 wks of age revealed increased immune cell presence in islets and enhanced B and T cell activation in PLN and spleen of NOD-S2+/- mice. Furthermore, scRNA-seq on isolated islets revealed temporal alterations in pathways related to mitochondria function in β cells, and mechanistic studies revealed decreased glucose-stimulated ATP production, reduced mitochondrial membrane potential, decreased islet expression of ATP synthase/mitochondrial complex III, increased mitochondrial Ca2+, and altered mitochondrial ultrastructure in NOD-S2+/- islets at 10 wks of age. In co-culture experiments, NOD-S2+/- B cells showed increased activation and NOD-S2+/- T cells showed increased proliferation and activation when cultured with NOD-WT islets. Interestingly, NOD-S2+/- islets induced B and T cell proliferation and T cell activation when cultured with NOD-WT immune cells. Lastly, administration of a small molecule SERCA activator in NOD-S2+/- mice decreased immune cell infiltration into the islet and delayed T1D onset. In summary, our results demonstrate a novel pathway whereby modulation of SERCA2 impacts islet mitochondrial function, islet immunogenicity, and immune cell proliferation and activation which fuel progression to T1D.