br Results br Discussion Fetal
Discussion Fetal development occurs under conditions of low oxygen tension and steady maternal glucose such that physiologic systems including the pancreas are functionally poised in the prenatal state. Postnatally, oxidative metabolism becomes dominant and intermittent feeding exposes the pancreas to dramatic changes in blood glucose. In particular, weaning has been reported to trigger a β cell maturation step characterized by enhanced glucose-stimulated oxidative phosphorylation and insulin secretion (Jacovetti et al., 2015, Stolovich-Rain et al., 2015). We demonstrate a direct linkage between the expression of the orphan nuclear receptor ERRγ and pre- and postnatal β cell metabolism. Based on selective expression in adult, but not fetal/neonatal β cells, our findings suggest that ERRγ is a key driver of the oxidative metabolic gene network in mature β cells, and that its postnatal induction orchestrates the metabolic maturation of β cells. Based on these findings, we posited that reversing the deficiency of ERRγ expression in iPSC-derived β-like cells may improve their ability to secrete insulin in response to a glucose challenge. Consistent with this notion, we show that targeted ERRγ expression in iPSC-derived β-like (iβeta) cells triggers a metabolic transformation that facilitates GSIS. Whereas significant variability and heterogeneity is observed in the human iβeta preparations (which we attribute to technical differences in ERRγ expression and differentiation conditions, e.g., Figure S5D and Table S6), transplantation of these cells into the kidney AR-M 1896 not only restores glucose homeostasis in STZ-induced type 1 diabetic mice, but re-establishes circadian rhythmicity to metabolic substrate usage, a process regulated by β cells. As a known activator of mitochondrial function including oxidative metabolism, this dependence on ERRγ expression suggests that very high cellular energy levels are needed to achieve and maintain glucose responsiveness (Dhawan et al., 2015, Jacovetti et al., 2015, Stolovich-Rain et al., 2015). Remarkably, as a single gene, ERRγ appears sufficient to induce a gene network necessary to overcome this metabolic roadblock in iPSC-derived β-like cells. In contrast, β cell-specific ERRγ-deficient mice are glucose intolerant and fail to appropriately secrete insulin in response to a glucose challenge. Thus, genome-orchestrated metabolic maturation seems to be a critical step in both the metabolic maturation of endogenous β cells in vivo and engineered β-like cells in vitro. Poor glucose management is associated with long-term diabetic consequences including diabetic retinopathy, nephropathy, and neuropathy. Although long-acting insulin formulations and programmable delivery pumps provide durable therapeutic utility, they fail to fully replicate the glucose-responsiveness of pancreatic β cells. Human islet transplantations offer superior glucose management, but require immunosuppressive drug regimens and are limited by the availability and viability of the transplanted cells. Although insulin independence can be achieved via islet transplantation, more than 50% of patients who received allotransplants and virtually all who received autotransplants are back on insulin therapy after 5 years. In both situations, transplantation of a larger mass of islets may alleviate some of the limitations. Patient-specific iPSC-derived β cells produced in unlimited supplies and potentially encapsulated in the newly developed alginate hydrogels (Vegas et al., 2016a, Vegas et al., 2016b) could resolve many of these concerns and is one of the major goals of stem cell replacement therapy. Despite recent advances, including generating functional β-like cells from hESCs (Pagliuca et al., 2014, Rezania et al., 2014) and the in vivo functional maturation of in vitro differentiated pancreatic progenitor cells (Kroon et al., 2008), the underlying mechanisms of β cell functional maturation remain poorly understood. Whereas dynamic chromatin remodeling (Xie et al., 2013) and sympathetic innervation stimuli (Borden et al., 2013) are implicated, our finding that ERRγ coordinates a transcriptional program regulating increased oxidative metabolism provides novel mechanistic insight into the functional limitations preventing β cell maturation (Figure 7). In support of these observations, genetic and epidemiology studies have independently implicated ERRγ as a risk factor in the development of diabetes (Chuang et al., 2008, Rampersaud et al., 2007, Tennessen and Thummel, 2011), although its functional relevance was not understood.