Critical role of gap junction coupled KATP channel activity for regulated insulin secretion

Mathematical model of morphogen electrophoresis through gap junctions

Gap junctional communication is important for embryonic morphogenesis. However, the factors regulating the spatial properties of small molecule signal flows through gap junctions remain poorly understood. Recent data on gap junctions, ion transporters, and serotonin during left-right patterning suggest a specific model: the net unidirectional transfer of small molecules through long-range gap junctional paths driven by an electrophoretic mechanism. However, this concept has only been discussed qualitatively, and it is not known whether such a mechanism can actually establish a gradient within physiological constraints. We review the existing functional data and develop a mathematical model of the flow of serotonin through the early Xenopus embryo under an electrophoretic force generated by ion pumps. Through computer simulation of this process using realistic parameters, we explored quantitatively the dynamics of morphogen movement through gap junctions, confirming the plausibility of the proposed electrophoretic mechanism, which generates a considerable gradient in the available time frame. The model made several testable predictions and revealed properties of robustness, cellular gradients of serotonin, and the dependence of the gradient on several developmental constants. This work quantitatively supports the plausibility of electrophoretic control of morphogen movement through gap junctions during early left-right patterning. This conceptual framework for modeling gap junctional signaling -- an epigenetic patterning mechanism of wide relevance in biological regulation -- suggests numerous experimental approaches in other patterning systems.

Hyperinsulinism in mice with heterozygous loss of K(ATP) channels

AIMS/HYPOTHESIS

ATP-sensitive K(+) (K(ATP)) channels couple glucose metabolism to insulin secretion in pancreatic beta cells. In humans, loss-of-function mutations of beta cell K(ATP) subunits (SUR1, encoded by the gene ABCC8, or Kir6.2, encoded by the gene KCNJ11) cause congenital hyperinsulinaemia. Mice with dominant-negative reduction of beta cell K(ATP) (Kir6.2[AAA]) exhibit hyperinsulinism, whereas mice with zero K(ATP) (Kir6.2(-/-)) show transient hyperinsulinaemia as neonates, but are glucose-intolerant as adults. Thus, we propose that partial loss of beta cell K(ATP) in vivo causes insulin hypersecretion, but complete absence may cause insulin secretory failure.

MATERIALS AND METHODS

Heterozygous Kir6.2(+/-) and SUR1(+/-) animals were generated by backcrossing from knockout animals. Glucose tolerance in intact animals was determined following i.p. loading. Glucose-stimulated insulin secretion (GSIS), islet K(ATP) conductance and glucose dependence of intracellular Ca(2+) were assessed in isolated islets.

RESULTS

In both of the mechanistically distinct models of reduced K(ATP) (Kir6.2(+/-) and SUR1(+/-)), K(ATP) density is reduced by approximately 60%. While both Kir6.2(-/-) and SUR1(-/-) mice are glucose-intolerant and have reduced glucose-stimulated insulin secretion, heterozygous Kir6.2(+/-) and SUR1(+/-) mice show enhanced glucose tolerance and increased GSIS, paralleled by a left-shift in glucose dependence of intracellular Ca(2+) oscillations.

CONCLUSIONS/INTERPRETATION

The results confirm that incomplete loss of beta cell K(ATP) in vivo underlies a hyperinsulinaemic phenotype, whereas complete loss of K(ATP) underlies eventual secretory failure.

Complementation rescue of Pdx1 null phenotype demonstrates distinct roles of proximal and distal cis-regulatory sequences in pancreatic and duodenal expression

The unique, well-demarcated expression domain of Pdx1 within the posterior foregut suggests that investigating its transcriptional regulation will provide insight into mechanisms that regionally pattern the endoderm. Previous phylogenetic comparison identified conserved noncoding regions that stimulate transcriptional activity selectively in cultured pancreatic beta cells. Characterization of these regulatory elements is helping to dissect the transcription factor networks that operate within beta cells, which is important for understanding the etiology of beta cell dysfunction and diabetes, as well as for developing methods to produce beta cells in vitro for cell-based therapies. We recently reported that deletion of three proximally located conserved areas (Area I-II-III) from the endogenous Pdx1 locus resulted in severely reduced expression of Pdx1 in the pancreas, and a milder decrease in other foregut tissues. Here, we report transgene-based complementation experiments on Pdx1 null mice, which reveal that the proximal promoter/enhancer region, including Area I-II-III, rescues the pancreatic defects caused by Pdx1 deficiency, but only weakly promotes expression of Pdx1 in the postnatal stomach and duodenum. These results reveal a role for distal cis-regulatory elements in achieving the correct level of extra-pancreatic Pdx1 expression, which is necessary for the production of duodenal GIP cells and stomach gastrin cells.

Connexin36 and pancreatic beta-cell functions

Most cell types are functionally coupled by connexin (Cx) channels, i.e. exchange cytoplasmic ions and small metabolites through gap junction domains of their membrane. This form of direct cell-to-cell communication occurs in all existing animals, whatever their position in the phylogenetic scale, and up to humans. Pancreatic beta-cells are no exception, and normally cross-talk with their neighbors via channels made of Cx36. These exchanges importantly contribute to coordinate and synchronize the function of individual cells within pancreatic islets, particularly in the context of glucose-induced insulin secretion. Compelling evidence now indicates that Cx36-mediated coupling, and/or the Cx36 protein per se, play significant regulatory roles in various beta-cell functions, ranging from the biosynthesis, storage and release of insulin. Recent preliminary data further suggest that the protein may also be implicated in the balance of beta-cell growth versus necrosis and apoptosis, and in the regulated expression of specific genes. Here, we review this evidence, discuss the possible involvement of Cx36 in the pathophysiology of diabetes, and evaluate the relevance of this connexin in the therapeutic approaches to the disease.

Oscillations, intercellular coupling, and insulin secretion in pancreatic beta cells

Insulin is a potent metabolic regulator that is released by pancreatic beta-cells, which respond to body glucose concentrations. Here the authors explain the physiological basis of insulin release.