Investigating the Role of Islet Cytoarchitecture in Its Oscillation Using a New β-Cell Cluster Model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst fb, the synchronization index λ of the bursting β cells, the bursting period Tb, the plateau fraction pf, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells nβ, number of inter-β cell couplings of each β cell nc, and the coupling strength gc. We found that at low values of nβ, nc and gc, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at nβ∼100, nc∼6 and gc∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with nβ>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions

Multiple novel prostate cancer susceptibility signals identified by fine-mapping of known risk loci among Europeans

Genome-wide association studies (GWAS) have identified numerous common prostate cancer (PrCa) susceptibility loci. We have fine-mapped 64 GWAS regions known at the conclusion of the iCOGS study using large-scale genotyping and imputation in 25 723 PrCa cases and 26 274 controls of European ancestry. We detected evidence for multiple independent signals at 16 regions, 12 of which contained additional newly identified significant associations. A single signal comprising a spectrum of correlated variation was observed at 39 regions; 35 of which are now described by a novel more significantly associated lead SNP, while the originally reported variant remained as the lead SNP only in 4 regions. We also confirmed two association signals in Europeans that had been previously reported only in East-Asian GWAS. Based on statistical evidence and linkage disequilibrium (LD) structure, we have curated and narrowed down the list of the most likely candidate causal variants for each region. Functional annotation using data from ENCODE filtered for PrCa cell lines and eQTL analysis demonstrated significant enrichment for overlap with bio-features within this set. By incorporating the novel risk variants identified here alongside the refined data for existing association signals, we estimate that these loci now explain ∼38.9% of the familial relative risk of PrCa, an 8.9% improvement over the previously reported GWAS tag SNPs. This suggests that a significant fraction of the heritability of PrCa may have been hidden during the discovery phase of GWAS, in particular due to the presence of multiple independent signals within the same regio

Cx36 preferentially connects beta-cells within pancreatic islets

Previous studies have provided evidence for the transcripts of Cx43 and Cx45 within pancreatic islets. As of yet, however, it has proven difficult to unambiguously demonstrate the expression of these proteins by islet cells. We have investigated whether Cx36, a new connexin species recently identified in mammalian brain and retina, may also be expressed in pancreatic islets. Using probes that permitted the original identification of Cx36 in the central nervous system, we show that a transcript for Cx36 is clearly detectable in rat pancreatic islets. Using novel and affinity-purified polyclonal antibodies, we have found that Cx36 is actually expressed in pancreatic islets. Both in situ hybridization and immunolabeling indicated that this connexin is abundant in the centrally located insulin-producing beta-cells and is expressed much less, if at all, by the other endocrine cell types. This differential expression was further confirmed on fluorescence-activated cell sorter-purified preparations enriched in either beta- or non-beta-cells. The finding of a differential distribution of Cx36 within distinct regions of pancreatic islets creates the possibility that this connexin may provide the establishment of selective pathways of communication between the different types of endocrine cells comprising the pancreatic islet

Expression of connexin36 in the adult and developing rat brain

The distribution of connexin36 (Cx36) in the adult rat brain and retina has been analysed at the protein (immunofluorescence) and mRNA (in situ hybridization) level. Cx36 immunoreactivity, consisting primarily of round or elongated puncta, is highly enriched in specific brain regions (inferior olive and the olfactory bulb), in the retina, in the anterior pituitary and in the pineal gland, in agreement with the high levels of Cx36 mRNA in the same regions. A lower density of immunoreactive puncta can be observed in several brain regions, where only scattered subpopulations of cells express Cx36 mRNA. By combining in situ hybridization for Cx36 mRNA with immunohistochemistry for a general neuronal marker (NeuN), we found that neuronal cells are responsible for the expression of Cx36 mRNA in inferior olive, cerebellum, striatum, hippocampus and cerebral cortex. Cx36 mRNA was also demonstrated in parvalbumin-containing GABAergic interneurons of cerebral cortex, striatum, hippocampus and cerebellar cortex. Analysis of developing brain further revealed that Cx36 reaches a peak of expression in the first two weeks of postnatal life, and decreases sharply during the third week. Moreover, in these early stages of postnatal development Cx36 is detectable in neuronal populations that are devoid of Cx36 mRNA at the adult stage. The developmental changes of Cx36 expression suggest a participation of this connexin in the extensive interneuronal coupling which takes place in several regions of the early postnatal brain

Cx36 and the function of endocrine pancreas

The secretory, duct, connective and vascular cells of pancreas are connected by gap junctions, made of different connexins. The insulin-producing beta-cells, which form the bulk of endocrine pancreatic islets, express predominantly Cx36. To assess the function of this connexin, we have first studied its expression in rats, during sequential changes of pancreatic function which were induced by the implantation of a secreting insulinoma. We observed that changes in beta-cell function were paralleled by changes in Cx36 expression. We have also begun to investigate mutant mice lacking Cx36. The absence of this protein did not affect the development and differentiation of beta-cells but appeared to alter their secretion. We have studied this effect in MIN6 cells which spontaneously express Cx36. After stable transfection of a construct that markedly reduced the expression of this connexin, we observed that MIN6 cells were no more able to secrete insulin, in contrast to wild type controls, and differentially displayed a series of still unknown genes. The data provide evidence that Cx36-dependent signaling contributes to regulate the function of native and tumoral insulin-producing cells