Pancreatic β-cells perform glucose-stimulated insulin secretion, a process at the center of type 2 diabetes etiology. Efforts to understand how β-cells behave in healthy and stressful conditions have revealed a wide degree of morphological, functional, and transcriptional heterogeneity. Sources of heterogeneity include β-cell topography, developmental origin, maturation state, and stress response. Advances in sequencing and imaging technologies have led to the identification of β-cell subtypes, which play distinct roles in the islet niche. This review examines β-cell heterogeneity from morphological, functional, and transcriptional perspectives, and considers the relevance of topology, maturation, development, and stress response. It also discusses how these factors have been used to identify β-cell subtypes, and how heterogeneity is impacted by diabetes. We examine open questions in the field and discuss recent technological innovations that could advance understanding of β-cell heterogeneity in health and disease.

SUMMARY We leverage the SM/J mouse to understand glycemic control in obesity. High-fat-fed SM/J mice initially develop poor glucose homeostasis relative to controls. Strikingly, their glycemic dysfunction resolves by 30 weeks of age despite persistent obesity. The mice dramatically expand their brown adipose depots as they resolve glycemic dysfunction. This occurs naturally and spontaneously on a high-fat diet, with no temperature or genetic manipulation. Removal of the brown adipose depot impairs insulin sensitivity, indicating that the expanded tissue is functioning as an insulin-stimulated glucose sink. We describe morphological, physiological, and transcriptomic changes that occur during the brown adipose expansion and remission of glycemic dysfunction, and focus on Sfrp1 (secreted frizzled-related protein 1) as a compelling candidate that may underlie this phenomenon. Understanding how the expanded brown adipose contributes to glycemic control in SM/J mice will open the door for innovative therapies aimed at improving metabolic complications in obesity.

52Disruption of glucose homeostasis increases the risk of type II diabetes, cardiovascular disease, stroke, 53 and cancer. We leverage a novel rodent model, the SM/J mouse, to understand glycemic control in 54 obesity. On a high fat diet, obese SM/J mice initially develop impaired glucose tolerance and elevated 55 fasting glucose. Strikingly, their glycemic dysfunction resolves by 30 weeks of age despite persistence of 56 obesity. A prominent phenotype is that they dramatically expand their brown adipose depots as they 57 resolve glycemic dysfunction. This occurs naturally and spontaneously on a high fat diet, with no 58 temperature or genetic manipulation. When the brown adipose depot is removed from normoglycemic 59 obese mice, fasting blood glucose and glucose tolerance revert to unhealthy levels, and animals become 60 insulin resistant. We identified 267 genes whose expression changes in the brown adipose when the 61 mice resolve their unhealthy glycemic parameters, and find the expanded tissue has a 'healthier' 62 expression profile of cytokines and extracellular matrix genes. We describe morphological, physiological, 63 and transcriptomic changes that occur during the unique brown adipose expansion and remission of 64 glycemic dysfunction in obese SM/J mice. Understanding this phenomenon in mice will open the door for 65 innovative therapies aimed at improving glycemic control in obesity.66 67 Significance Statement 68 Some obese individuals maintain normal glycemic control. Despite being obese, these individuals 69 have low risk for metabolic complications, including type-II diabetes. If we better understood why some 70 obese people maintain normoglycemia then we might develop new approaches for treating metabolic 71 complications associated with obesity. However, the causative factors underlying glycemic control in 72 obesity remain unknown. We discovered that, despite persistence of the obese state, SM/J mice enter 73 into diabetic remission: returning to normoglycemia and reestablishing glucose tolerance and improving 74 insulin sensitivity. A prominent phenotype is that they dramatically expand their brown adipose depots as 75 they resolve glycemic dysfunction. Understanding this phenomenon in mice will open the door for 76 innovative therapies aimed at improving glycemic control in obesity. 78An estimated 10-30% of obese individuals maintain glycemic control and some longitudinal 79 studies suggest their risk of developing type II diabetes is no greater than matched lean individuals (1). 80No causative factors underlying glycemic control in obesity have been discovered, however the strongest 81 predictors of impaired glycemic control in obesity are increased visceral fat mass and adipose tissue 82 dysfunction (2,3). Thus research efforts have focused on understanding the genetic and physiological 83 mechanisms of action of adipose. Recent research reveals that brown adipose activity is associated with 84 anti-diabetic properties. Cold exposure in both obese and lean individuals causes increased uptake...

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48Maintenance of functional β-cell mass is critical to preventing diabetes, but the physiological mechanisms that cause 49 β-cell populations to thrive or fail in the context of obesity are unknown. High fat-fed SM/J mice spontaneously 50 transition from hyperglycemic-obese to normoglycemic-obese with age, providing a unique opportunity to study β-51 cell adaptation. Here, we characterize insulin homeostasis, islet morphology, and β-cell function during SM/J's 52 diabetic remission. As they resolve hyperglycemia, obese SM/J mice dramatically increase circulating and 53 pancreatic insulin levels while improving insulin sensitivity. Immunostaining of pancreatic sections reveals that 54 obese SM/J mice selectively increase β-cell mass but not α-cell mass. Obese SM/J mice do not show elevated β-55 cell mitotic index, but rather elevated α-cell mitotic index. Functional assessment of isolated islets reveals that obese 56 SM/J mice increase glucose stimulated insulin secretion, decrease basal insulin secretion, and increase islet insulin 57 content. These results establish that β-cell mass expansion and improved β-cell function underlie the resolution of 58 hyperglycemia, indicating that obese SM/J mice are a valuable tool for exploring how functional β-cell mass can 59 be recovered in the context of obesity. 60 61 62 63 64 65 66 67 68 69 70 71 72