Communication of Islet Cells: Molecules and Functions

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Cell-matrix interactions improve beta-cell survival and insulin secretion in three-dimensional culture

Controlled matrix interactions were presented to pancreatic beta-cells in three-dimensional culture within poly(ethylene glycol) hydrogels. Dispersed MIN6 beta-cells were encapsulated in gel environments containing the following entrapped extracellular matrix (ECM) proteins: collagen type I, collagen type IV, fibrinogen, fibronectin, laminin, and vitronectin. In ECM-containing gels, beta-cell survival was significantly better than in gels without ECM over 10 days. Correspondingly, apoptosis in encapsulated beta-cells was less in the presence of each matrix protein, suggesting the ability of individual matrix interactions to prevent matrix signaling-related apoptosis (anoikis). MIN6 beta-cells cultured in gels containing collagen type IV or laminin secreted more insulin in response to glucose stimulation than beta-cells in all other experimental conditions. Variations in collagen type IV or laminin concentration between 10 microg/mL and 250 microg/mL did not affect insulin secretion. Finally, beta-cell function in hydrogels presenting both collagen type IV and laminin revealed synergistic interactions. With a total protein concentration of 100 microg/mL, three gel compositions of varying ratios of collagen type IV to laminin (25:75, 50:50, and 75:25) were tested. In the presence of 25 microg/mL of collagen type IV and 75 microg/mL of laminin, beta-cell insulin secretion was greater than with laminin or collagen type IV individually. These results demonstrate that specific, rationally designed extracellular environments promote isolated beta-cell survival and function.

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.