The Role of Cystic Fibrosis Transmembrane Conductance Regulator in Insulin Secretion in Pancreatic Islet beta-cells.

摘要

Cystic fibrosis (CF) is a recessive autosomal genetic disease resulted from mutations of cystic fibrosis transmembrane conductance regulator (CFTR). CF affects critically the lung, liver, pancreas, intestine and reproductive tract. CF patients also exhibit a high percentage of diabetes, which almost reach 50% in adult. The pathological cause of diabetes in CF patients, also called CF related diabetes (CFRD), is still controversial. It has been reported that CFTR expressed in the islet β cells, which is responsible for insulin secretion. However, the exact role of CFTR in islet β-cell and its relation to diabetes have been ignored. The present study aims to elucidate the role of CFTR in the process of insulin secretion by pancreatic islet β cells.;In the second part of our study, mathematical model is built up to clarify the role of CFTR in the electrical activity during insulin secretion. It is shown that reduction of CFTR conductance hyperpolarizes the membrane of the β-cell, for which it requires a larger electrical stimulus to evoke an action potential, indicating the contribution of CFTR to the membrane potential as demonstrated by our experimental results. Increase in intracellular Cl- concentration and the conductance of CFTR result in higher frequency of membrane potential oscillations, demonstrating that Cl- is crucial for the membrane potential oscillations. The electrical spikes induced by increase of ATP/ADP in the model are abolished by decreasing CFTR conductance, which is consistent with our findings that CFTR is involved in the generation of action potentials induced by glucose. In other word, our model demonstrates that CFTR is crucial for insulin secretion by its contribution to membrane potential and participating in the generation of electrical spikes via conducting Cl- efflux, which confirms our findings in the experimental study.;Taken together, the present study reveals a previously unrecognized important role of CFTR in glucose-stimulated insulin secretion via contributing to the membrane potential and the participating in the generation of action potential in islet β cells. This finding sheds new light into the understanding of the pathogenesis of CFRD and may provide grounds for the development of new therapeutic approaches for CFRD.;Glucose-stimulated insulin secretion is associated with a complex electrical activity in the pancreatic islet β-cell, which is characterized by a slow membrane depolarization superimposed with bursts of action potentials. Closing ATP-sensitive K+ channels (KATP) in response to glucose increase is generally considered the initial event that depolarizes the β-cell membrane and activates the voltage-dependent Ca2+ channels, which constitutes the major depolarizing component of the bursting action potentials giving rise to the cytosolic calcium oscillations that trigger insulin release. While Cl- has been implicated in an unknown depolarization current of the β-cell, the responsible Cl- channel remains unidentified. In the first part of our study, we show functional expression of CFTR and its activation by glucose in the β-cell. Activation of CFTR by glucose was also demonstrated in CHO cell over-expression system. The glucose-elicited whole-cell currents, membrane depolarization, electrical bursts (both magnitude and frequency), Ca2+ oscillations and insulin secretion could be abolished or reduced by inhibitors/knockdown of CFTR in primary mouse β-cells or RIN-5F β-cell line, or significantly attenuated in isolated mouse islet β-cells from CFTR mutant mice compared to that of wildtype. Significantly increased blood glucose level accompanied with reduced level of insulin is found in CFTR mutant mice compared to the wildtype. The results strongly indicate a role of CFTR in the process of insulin secretion.