Vini Nagaraj

Overexpression of Alpha2A-Adrenergic Receptors Contributes to Type 2 Diabetes

Ratting Out a Diabetes Gene Inbred animals with inherited susceptibility to disease can be especially informative regarding pathogenetic mechanisms because they carry naturally occurring genetic variants of the same type that cause disease in humans. This principle is illustrated by Rosengren et al. (p. 217; published online 19 November), whose analysis of an inbred strain of rats prone to develop type 2 diabetes led to the discovery of a gene whose aberrant overexpression suppresses pancreatic insulin secretion in both rats and humans. The culprit gene, ADRA2A, encodes the alpha2A adrenergic receptor and is potentially a valuable lead for diabetes therapy because it can be targeted pharmacologically. Sequence variations in an adrenergic receptor gene cause reduced insulin secretion and contribute to type 2 diabetes. Several common genetic variations have been associated with type 2 diabetes, but the exact disease mechanisms are still poorly elucidated. Using congenic strains from the diabetic Goto-Kakizaki rat, we identified a 1.4-megabase genomic locus that was linked to impaired insulin granule docking at the plasma membrane and reduced β cell exocytosis. In this locus, Adra2a, encoding the alpha2A-adrenergic receptor [alpha(2A)AR], was significantly overexpressed. Alpha(2A)AR mediates adrenergic suppression of insulin secretion. Pharmacological receptor antagonism, silencing of receptor expression, or blockade of downstream effectors rescued insulin secretion in congenic islets. Furthermore, we identified a single-nucleotide polymorphism in the human ADRA2A gene for which risk allele carriers exhibited overexpression of alpha(2A)AR, reduced insulin secretion, and increased type 2 diabetes risk. Human pancreatic islets from risk allele carriers exhibited reduced granule docking and secreted less insulin in response to glucose; both effects were counteracted by pharmacological alpha(2A)AR antagonists.

Eukaryotic translation initiation factor 3 subunit e controls intracellular calcium homeostasis by regulation of cav1.2 surface expression.

Inappropriate surface expression of voltage-gated Ca2+channels (CaV) in pancreatic ß-cells may contribute to the development of type 2 diabetes. First, failure to increase intracellular Ca2+ concentrations at the sites of exocytosis impedes insulin release. Furthermore, excessive Ca2+ influx may trigger cytotoxic effects. The regulation of surface expression of CaV channels in the pancreatic β-cells remains unknown. Here, we used real-time 3D confocal and TIRFM imaging, immunocytochemistry, cellular fractionation, immunoprecipitation and electrophysiology to study trafficking of L-type CaV1.2 channels upon β-cell stimulation. We found decreased surface expression of CaV1.2 and a corresponding reduction in L-type whole-cell Ca2+ currents in insulin-secreting INS-1 832/13 cells upon protracted (15–30 min) stimulation. This internalization occurs by clathrin-dependent endocytosis and could be prevented by microtubule or dynamin inhibitors. eIF3e (Eukaryotic translation initiation factor 3 subunit E) is part of the protein translation initiation complex, but its effect on translation are modest and effects in ion channel trafficking have been suggested. The factor interacted with CaV1.2 and regulated CaV1.2 traffic bidirectionally. eIF3e silencing impaired CaV1.2 internalization, which resulted in an increased intracellular Ca2+ load upon stimulation. These findings provide a mechanism for regulation of L-type CaV channel surface expression with consequences for β-cell calcium homeostasis, which will affect pancreatic β-cell function and insulin production.

CART is overexpressed in human type 2 diabetic islets and inhibits glucagon secretion and increases insulin secretion

Aims/hypothesisInsufficient insulin release and hyperglucagonaemia are culprits in type 2 diabetes. Cocaine- and amphetamine-regulated transcript (CART, encoded by Cartpt) affects islet hormone secretion and beta cell survival in vitro in rats, and Cart−/− mice have diminished insulin secretion. We aimed to test if CART is differentially regulated in human type 2 diabetic islets and if CART affects insulin and glucagon secretion in vitro in humans and in vivo in mice.MethodsCART expression was assessed in human type 2 diabetic and non-diabetic control pancreases and rodent models of diabetes. Insulin and glucagon secretion was examined in isolated islets and in vivo in mice. Ca2+ oscillation patterns and exocytosis were studied in mouse islets.ResultsWe report an important role of CART in human islet function and glucose homeostasis in mice. CART was found to be expressed in human alpha and beta cells and in a subpopulation of mouse beta cells. Notably, CART expression was several fold higher in islets of type 2 diabetic humans and rodents. CART increased insulin secretion in vivo in mice and in human and mouse islets. Furthermore, CART increased beta cell exocytosis, altered the glucose-induced Ca2+ signalling pattern in mouse islets from fast to slow oscillations and improved synchronisation of the oscillations between different islet regions. Finally, CART reduced glucagon secretion in human and mouse islets, as well as in vivo in mice via diminished alpha cell exocytosis.Conclusions/interpretationWe conclude that CART is a regulator of glucose homeostasis and could play an important role in the pathophysiology of type 2 diabetes. Based on the ability of CART to increase insulin secretion and reduce glucagon secretion, CART-based agents could be a therapeutic modality in type 2 diabetes.

Complement inhibitor CD55 governs the integrity of membrane rafts in pancreatic beta cells, but plays no role in insulin secretion

CD55 is a glycosylphosphatidylinositol-anchored protein, which inhibits complement activation by acting on the complement C3 convertases. CD55 is widely localized in the cholesterol rich regions of the cell plasma membrane termed membrane rafts. CD55 is attached to these specialized regions via a GPI link on the outer leaflet of the plasma membrane. Membrane rafts anchor many important signaling proteins, which control several cellular functions within the cell. For example, we recently demonstrated that the membrane raft protein and complement inhibitor CD59 also controls insulin secretion by an intracellular mechanism. Therefore, we have in this study aimed at addressing the expression and function of CD55 in pancreatic beta cells. To this end, we observe that CD55 is highly expressed in INS1 832/13 beta cells as well as human pancreatic islets. Diabetic human islets show a tendency for increased expression of CD55 when compared to the healthy controls. Importantly, silencing of CD55 in INS1 832/13 cells does not affect their insulin secretory capacity. On the other hand, silencing of CD55 diminished the intensity of membrane rafts as determined by Atto-SM staining. We hence conclude that CD55 expression is affected by glycemic status in human islets and plays a critical role in maintaining the conserved structure of rafts in pancreatic islets, which is similar to that of the related complement inhibitor CD59. However CD55 does not interfere with insulin secretion in beta cells, which is in sharp contrast to the action of the complement inhibitor CD59.

Complement receptor C5aR is expressed in pancreatic islets and affects glucose-induced insulin and glucagon secretion

Background and aims: The association between type 2 diabetes and different forms of cognitive impairment is well established. The mechanism behind the association is however still unrevealed. We ha ...

Complement factors are expressed in pancreatic islets and affect glucose induced insulin secretion

Prevalence of lipid abnormalities before and after the introduction of lipid modifying therapy among Swedish patients with type 2 diabetes and/or coronary heart disease (PRIMULA Sweden)In the ACTION (A Coronary disease Trial Investigating Outcome with Nifedipine GITS) trial, the benefits of adding nifedipine GITS to the treatment of patients with stable symptomatic coronary artery disease were particularly apparent in those with concomitant hypertension. This further analysis has assessed whether or not the addition of nifedipine GITS is particularly beneficial in the treatment of patients with the combination of diabetes mellitus and chronic stable angina.Different sets of risk factors for the development of albuminuria and renal impairment in type 2 diabetes : the Swedish National Diabetes register (NDR)