The Inhibitory G Protein α-Subunit, Gαz, Promotes Type 1 Diabetes-Like Pathophysiology in NOD Mice

Abstract

The α-subunit of the heterotrimeric Gz protein, Gαz, promotes β-cell death and inhibits β-cell replication when pancreatic islets are challenged by stressors. Thus, we hypothesized that loss of Gαz protein would preserve functional β-cell mass in the nonobese diabetic (NOD) model, protecting from overt diabetes. We saw that protection from diabetes was robust and durable up to 35 weeks of age in Gαz knockout mice. By 17 weeks of age, Gαz-null NOD mice had significantly higher diabetes-free survival than wild-type littermates. Islets from these mice had reduced markers of proinflammatory immune cell infiltration on both the histological and transcript levels and secreted more insulin in response to glucose. Further analyses of pancreas sections revealed significantly fewer terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL)-positive β-cells in Gαz-null islets despite similar immune infiltration in control mice. Islets from Gαz-null mice also exhibited a higher percentage of Ki-67-positive β-cells, a measure of proliferation, even in the presence of immune infiltration. Finally, β-cell-specific Gαz-null mice phenocopy whole-body Gαz-null mice in their protection from developing hyperglycemia after streptozotocin administration, supporting a β-cell-centric role for Gαz in diabetes pathophysiology. We propose that Gαz plays a key role in β-cell signaling that becomes dysfunctional in the type 1 diabetes setting, accelerating the death of β-cells, which promotes further accumulation of immune cells in the pancreatic islets, and inhibiting a restorative proliferative response.

Differential Effects of Prostaglandin E2Production and Signaling through the Prostaglandin EP3 Receptor on Human Beta-cell Compensation

“…In the INS-1 (832/13) and (832/3) rat insulinoma cell lines, adenoviral overexpression of wild-type human Gαz significantly reduced cAMP production and total cell number after a 2-3-day culture, synergizing with streptozotocin or IL-1β to promote beta-cell apoptosis [4]. Furthermore, in models of T1D, islets from Gαz null mice had significantly fewer TUNEL-positive beta-cells, in addition to a significantly enhanced beta-cell proliferation rate and functional response to glucose and Ex4 [4,5]. Yet, in the context of HFD-feeding, even wild-type islets from HFD-fed mice had few-to-no TUNEL-positive beta-cells, with the effects of Gαz loss being attributed solely to effects on beta-cell replication ( [2] and M.E.K., unpublished data).…”

“…150-200 islets from each human islet preparation were washed with PBS and used to generate RNA samples via Qiagen RNeasy Mini Kit according to the manufacturer's protocol. Copy DNA (cDNA) was generated and relative qPCR performed via SYBR Green assay using primers validated to provide linear results upon increasing concentrations of cDNA template, as previously described [5].…”

“…In beta-cells, EP3 is coupled specifically to the unique inhibitory G protein, Gz [2,3]. Islets from mice lacking the catalytic alpha-subunit of Gz (Gαz) constitutively produce more cAMP and secrete more insulin in response to glucose [2,[4][5][6]. Gαz-null mice are resistant to diabetes in a number of mouse models of the disease [2,4,5].…”

“…Islets from mice lacking the catalytic alpha-subunit of Gz (Gαz) constitutively produce more cAMP and secrete more insulin in response to glucose [2,[4][5][6]. Gαz-null mice are resistant to diabetes in a number of mouse models of the disease [2,4,5]. Gαz-null C57BL/6N mice are fully resistant to developing fasting hyperglycemia and glucose intolerance after a high-fat diet (HFD) feeding regimen-a model of pre-diabetes-due primarily to a synergistic effect of HFD feeding and the Gαz-null mutation on beta-cell proliferation [2].…”

Prostaglandin EP3 Receptor signaling is required to prevent insulin hypersecretion and metabolic dysfunction in a non-obese mouse model of insulin resistance

“…mouse models of the disease. In T1D models, Gαz-null mice display increased insulin secretion in reponse to glucose and the GLP1R agonist, exendin-4, an accelerated beta-cell replication rate, and improved beta-cell survival [10,11). Of direct relevance to this work, when fed a highfat diet, Gαz-null C57BL/6N mice are protected from glucose intolerance due to a significant enhancement of beta-cell replication and mass, increasing the insulin secretory capacity of the islet {Kimple, 2012 #7].…”