Research Article: Glucose Metabolism, Islet Architecture, and Genetic Homogeneity in Imprinting of [Ca2+]i and Insulin Rhythms in Mouse Islets

Date Published: December 23, 2009

Publisher: Public Library of Science

Author(s): Craig S. Nunemaker, John F. Dishinger, Stacey B. Dula, Runpei Wu, Matthew J. Merrins, Kendra R. Reid, Arthur Sherman, Robert T. Kennedy, Leslie S. Satin, Kathrin Maedler.

Abstract: We reported previously that islets isolated from individual, outbred Swiss-Webster mice displayed oscillations in intracellular calcium ([Ca2+]i) that varied little between islets of a single mouse but considerably between mice, a phenomenon we termed “islet imprinting.” We have now confirmed and extended these findings in several respects. First, imprinting occurs in both inbred (C57BL/6J) as well as outbred mouse strains (Swiss-Webster; CD1). Second, imprinting was observed in NAD(P)H oscillations, indicating a metabolic component. Further, short-term exposure to a glucose-free solution, which transiently silenced [Ca2+]i oscillations, reset the oscillatory patterns to a higher frequency. This suggests a key role for glucose metabolism in maintaining imprinting, as transiently suppressing the oscillations with diazoxide, a KATP-channel opener that blocks [Ca2+]i influx downstream of glucose metabolism, did not change the imprinted patterns. Third, imprinting was not as readily observed at the level of single beta cells, as the [Ca2+]i oscillations of single cells isolated from imprinted islets exhibited highly variable, and typically slower [Ca2+]i oscillations. Lastly, to test whether the imprinted [Ca2+]i patterns were of functional significance, a novel microchip platform was used to monitor insulin release from multiple islets in real time. Insulin release patterns correlated closely with [Ca2+]i oscillations and showed significant mouse-to-mouse differences, indicating imprinting. These results indicate that islet imprinting is a general feature of islets and is likely to be of physiological significance. While islet imprinting did not depend on the genetic background of the mice, glucose metabolism and intact islet architecture may be important for the imprinting phenomenon.

Partial Text: Although many genetic and environmental factors contribute to the development of type 2 diabetes, one of the key components is the failure of the pancreatic beta-cell to secrete insulin appropriately in the face of insulin resistance. In healthy individuals, beta-cells respond to glucose in a well-defined manner. As blood glucose levels rise, glucose is taken into the beta cell through glucose transporters and is metabolized through glycolysis and aerobic respiration, leading to an increase in ATP/ADP. An increase in ATP/ADP results in the closure of ATP-sensitive potassium channels (KATP), which triggers calcium influx through voltage-gated Ca2+ channels and results in insulin release. Following an initial burst of calcium influx and insulin secretion, beta-cells within the islets typically generate oscillations in intracellular calcium ([Ca2+]i) and insulin release [1]–[6] that continue as long as glucose remains elevated. These features of beta-cell function occur both in vitro and in vivo[7], [8].

We previously reported that islets from a population of mice display a bimodal distribution of periods, with peaks in the fast (<60 sec) range and slow (>2 minute) range, but that islets from a given mouse have tightly clustered periods that fall into just one of those ranges. We interpreted this finding to mean that islets in a mouse are imprinted by some factor or factors that harmonize their periods. In this study, we explored whether islet imprinting is restricted to the outbred Swiss-Webster mice used in the first study. We also investigated whether islets maintain imprinted patterns of insulin release, as they do for [Ca2+]i, using a novel microfluidics system which allowed insulin secretion to be measured from single islets with high temporal resolution. We showed further that imprinting is expressed in metabolic (NAD(P)H) oscillations as well as [Ca2+]i, oscillations. Finally, we asked whether intact islet structure is important for maintaining the tight distribution of oscillatory periods among islets from the same mouse that is characteristic of imprinting.



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