Disrupted glucose homeostasis and glucagon and insulin secretion defects in Robo ßKO mice.

The axon guidance proteins, Roundabout (Robo) receptors play a critical role in morphogenesis of the islets of Langerhans. Mice with a ß cell-selective deletion of Robo (Robo ßKO), show severely disrupted spatial architecture of their islets, without defects in ß cell differentiation or maturity. We have recently shown that Robo ßKO mice have reduced synchronous glucose-stimulated ß cell calcium oscillations in their islets in vivo, likely disrupting their pulsatile insulin secretion. Here, we analyze whole-body metabolic regulation in Robo ßKO mice. We show that Robo ßKO mice have mild defects in glucose homeostasis, and altered glucagon and insulin secretion. However, we did not observe any severe whole-body glucoregulatory phenotype following the disruption of islet architecture in Robo ßKO. Our data suggest that islet architecture plays only a mild role in overall glucoregulation.

The Anna Karenina Model of ß-Cell Maturation in Development and Their Dedifferentiation in Type 1 and Type 2 Diabetes.

Loss of mature ß-cell function and identity, or ß-cell dedifferentiation, is seen in both type 1 and type 2 diabetes. Two competing models explain ß-cell dedifferentiation in diabetes. In the first model, ß-cells dedifferentiate in the reverse order of their developmental ontogeny. This model predicts that dedifferentiated ß-cells resemble ß-cell progenitors. In the second model, ß-cell dedifferentiation depends on the type of diabetogenic stress. This model, which we call the "Anna Karenina" model, predicts that in each type of diabetes, ß-cells dedifferentiate in their own way, depending on how their mature identity is disrupted by any particular diabetogenic stress. We directly tested the two models using a ß-cell-specific lineage-tracing system coupled with RNA sequencing in mice. We constructed a multidimensional map of ß-cell transcriptional trajectories during the normal course of ß-cell postnatal development and during their dedifferentiation in models of both type 1 diabetes (NOD) and type 2 diabetes (BTBR-Lepob/ob ). Using this unbiased approach, we show here that despite some similarities between immature and dedifferentiated ß-cells, ß-cell dedifferentiation in the two mouse models is not a reversal of developmental ontogeny and is different between different types of diabetes.

Reduced synchroneity of intra-islet Ca2+ oscillations in vivo in Robo-deficient ß cells.

The spatial architecture of the islets of Langerhans is hypothesized to facilitate synchronized insulin secretion among ß cells, yet testing this in vivo in the intact pancreas is challenging. Robo ßKO mice, in which the genes Robo1 and Robo2 are deleted selectively in ß cells, provide a unique model of altered islet spatial architecture without loss of ß cell differentiation or islet damage from diabetes. Combining Robo ßKO mice with intravital microscopy, we show here that Robo ßKO islets have reduced synchronized intra-islet Ca2+ oscillations among ß cells in vivo. We provide evidence that this loss is not due to a ß cell-intrinsic function of Robo, mis-expression or mis-localization of Cx36 gap junctions, or changes in islet vascularization or innervation, suggesting that the islet architecture itself is required for synchronized Ca2+ oscillations. These results have implications for understanding structure-function relationships in the islets during progression to diabetes as well as engineering islets from stem cells.