iPLA2beta-mediated alternative splicing and beta-cell death in type 1 diabetes

Type 1 diabetes (T1D) accounts for approximately 5-10% of diabetes (> 20 million patients worldwide), and T1D patients are predicted to triple by 2050. It is therefore imperative to understand the mechanisms that contribute to T1D evolution, so that improved treatments can be developed to prevent and delay the onset and progression of the disease. In this regard, T1D is a consequence of autoimmune destruction of β-cells, and ER stress and cytokines play critical roles in this process. Importantly, we reported that the Ca2+-independent phospholipase A2β (iPLA2β), which in the islet is predominantly localized in β-cells, is a key participant in β-cell apoptosis in response to stimuli that induce ER stress (e.g., pro-inflammatory cytokines (CTKs) and hyperglycemia).

The iPLA2β catalyzes hydrolysis of the sn-2 fatty acyl substituent from membrane phospholipids to liberate arachidonic acid, which can be metabolized to eicosanoids (i.e., iPLA2β-derived lipids (iDLs)). We find that select iDLs are associated with the onset of diabetes in rodent models of T1D, and that inhibition or genetic ablation of iPLA2β promotes decreases in these iDLs, leading to preservation of β-cell mass and T1D amelioration. Mechanistically, ER stress and apoptosis are suppressed by factors such as MCL-1 and Bcl-x(L), Bcl-2 family members that promote β-cell survival. Many apoptotic factors such as these undergo alternative RNA splicing (AS) to generate splice variants with contrasting roles (e.g., the MCL-1S and Bcl-x(s) isoforms).

For example, we demonstrated that anti-apoptotic Bcl-x(L) protein is lost from β-cells undergoing apoptosis due to a shift in Bcl-x 5’ splice site (5’SS) selection and generating Bcl-x(s) RNA. We also find that AS of caspase-9 and RAGE towards pro-apoptotic variants correlates with iPLA2β expression and differences in the β-cell lipidome. Our preliminary studies reveal (a) candidate human islet β-cell-derived iDLs that regulate AS, (b) [ratio of Bcl-x(L)/x(s) inversely correlating with iPLA2β expression in NOD islet β-cells and T1D incidence and] (c) through expanded splicomic studies via biostatistical analyses of deep RNA sequencing, additional AS events [(RUVBL1)] regulated by these iDLs. These findings motivate us to assess the role of iDL-mediated AS in β-cells in T1D development.

Specifically, we propose to explore the hypothesis that iDLs contribute to apoptotic events in β-cells and that these lipid fingerprints act as early predictors of T1D onset/progression. Mechanistically, we hypothesize that iDLs modulate AS dis-favoring the generation of anti-apoptotic isoforms in β-cells. To interrogate our hypotheses, [we will utilize islets from β-cell and macrophage conditional NOD.iPLA2β-KO mice, human iPSC-derived β-cells, and human islets to]: SA1: Determine the ?-cell lipidome and iDLs that link ER stress, β-cell death and T1D development; SA2: Determine whether β-cell survival can be enhanced by modulation of specific AS induced by iDLs; SA3: Delineate the molecular mechanisms through which iDLs modulate AS. Overall, we posit that a more complete understanding of these iDLs and their mechanisms of action will lead to novel strategies to preserve β-cell viability and prevent the onset/progression of T1D.

Grant Number: 5R01DK126444-04
NIH Institute/Center: NIH
Principal Investigator: CHARLES CHALFANT