Pluripotent stem cell (SC)-derived islets offer hope as a renewable source for β cell replacement for type 1 diabetes (T1D), yet functional and metabolic immaturity may limit their long-term therapeutic potential. Here, we show that limitations in mitochondrial transcriptional programming impede the formation of SC-derived β (SC-β) cells. Utilizing transcriptomic profiling, assessments of chromatin accessibility, mitochondrial phenotyping, and lipidomics analyses, we observe that SC-β cells exhibit reduced oxidative and mitochondrial fatty acid metabolism compared to primary human islets that are related to limitations in key mitochondrial transcriptional networks. Surprisingly, we find that reductions in glucose-stimulated mitochondrial respiration in SC-islets were not associated with alterations in mitochondrial mass, structure, or genome integrity. In contrast, SC-islets show limited expression of targets of PPARα, which regulate mitochondrial programming, yet whose functions in β cell differentiation are unknown. Importantly, treatment with WY14643, a potent PPARα agonist, induces expression of mitochondrial targets, improves insulin secretion, and increases the formation of SC-β cells both in vitro and following transplantation. Thus, PPARα-dependent mitochondrial programming promotes the differentiation of SC-β cells and may be a promising target to improve β cell replacement efforts for T1D. Here they show that PPARα-dependent mitochondrial programming promotes the differentiation of pluripotent stem cell-derived β cells. Targeting mitochondria has the potential to improve β cell replacement efforts for the treatment of type 1 diabetes.

Pluripotent stem cell (SC)-derived islets offer hope as a renewable source for β cell replacement for type 1 diabetes (T1D), yet functional and metabolic immaturity may limit their long-term therapeutic potential. Here, we show that limitations in mitochondrial transcriptional programming impede the formation of SC-derived β (SC-β) cells. Utilizing transcriptomic profiling, assessments of chromatin accessibility, mitochondrial phenotyping, and lipidomics analyses, we observed that SC-β cells exhibit reduced oxidative and mitochondrial fatty acid metabolism compared to primary human islets that are related to limitations in key mitochondrial transcriptional networks. Surprisingly, we found that reductions in glucose-stimulated mitochondrial respiration in SC-islets were not associated with alterations in mitochondrial mass, structure, or genome integrity. In contrast, SC-islets show limited expression of targets of PPAR⍺, which regulate mitochondrial programming, yet whose functions in β cell differentiation are unknown. Importantly, treatment with WY14643, a potent PPAR⍺ agonist, induced expression of mitochondrial targets, improved insulin secretion, and increased the formation of SC-β cells both in vitro and following transplantation. Thus, PPAR⍺-dependent mitochondrial programming promotes the differentiation of SC-β cells and may be a promising target to improve β cell replacement efforts for T1D.

Innate immune signaling is activated in immunometabolic diseases, including type 2 diabetes, yet its impact on glucose homeostasis is controversial. Here, we report that the E3 ubiquitin ligase TRAF6 integrates innate immune signals following diet-induced obesity to promote glucose homeostasis through the induction of mitophagy. Whereas TRAF6 was dispensable for pancreatic β cell function at baseline, TRAF6 was pivotal for insulin secretion, mitochondrial respiration, and mitophagy following metabolic stress in mouse and human islets. TRAF6 was critical for the recruitment and function of the ubiquitin-mediated (Parkin-dependent) mitophagy machinery. Glucose intolerance induced by TRAF6 deficiency following metabolic stress was reversed by concomitant Parkin deficiency by relieving obstructions in receptor-mediated (Parkin-independent) mitophagy. Our results establish that TRAF6 is vital for traffic through Parkin-mediated mitophagy and implicates TRAF6 in the cross-regulation of ubiquitin- and receptor-mediated mitophagy. Together, we illustrate that β cells engage innate immune signaling to adaptively respond to a diabetogenic environment.

Activation of innate immune signaling occurs during the progression of immunometabolic diseases, including type 2 diabetes (T2D), yet the impact of innate immune signaling on glucose homeostasis is controversial. Here, we report that the E3 ubiquitin ligase TRAF6 integrates innate immune signals following diet-induced obesity to promote glucose homeostasis through the induction of mitophagy. Whereas TRAF6 was dispensable for glucose homeostasis and pancreatic β-cell function under basal conditions, TRAF6 was pivotal for insulin secretion, mitochondrial respiration, and increases in mitophagy following metabolic stress in both mouse and human islets. Indeed, TRAF6 was critical for the recruitment and function of machinery within both the ubiquitin-mediated (Parkin-dependent) and receptor-mediated (Parkin-independent) mitophagy pathways upon metabolic stress. Intriguingly, the effect of TRAF6 deficiency on glucose homeostasis and mitophagy was fully reversed by concomitant Parkin deficiency. Thus, our results implicate a role for TRAF6 in the cross-regulation of both ubiquitin-and receptor-mediated mitophagy through the restriction of Parkin. Together, we illustrate that β-cells engage innate immune signaling to adaptively respond to a diabetogenic environment.

ABSTRACTMitochondrial damage is a hallmark of metabolic diseases, including diabetes and metabolic dysfunction-associated steatotic liver disease, yet the consequences of impaired mitochondria in metabolic tissues are often unclear. Here, we report that dysfunctional mitochondrial quality control engages a retrograde (mitonuclear) signaling program that impairs cellular identity and maturity across multiple metabolic tissues. Surprisingly, we demonstrate that defects in the mitochondrial quality control machinery, which we observe in pancreatic β cells of humans with type 2 diabetes, cause reductions of β cell mass due to dedifferentiation, rather than apoptosis. Utilizing transcriptomic profiling, lineage tracing, and assessments of chromatin accessibility, we find that targeted deficiency anywhere in the mitochondrial quality control pathway (e.g., genome integrity, dynamics, or turnover) activate the mitochondrial integrated stress response and promote cellular immaturity in β cells, hepatocytes, and brown adipocytes. Intriguingly, pharmacologic blockade of mitochondrial retrograde signalingin vivorestores β cell mass and identity to ameliorate hyperglycemia following mitochondrial damage. Thus, we observe that a shared mitochondrial retrograde response controls cellular identity across metabolic tissues and may be a promising target to treat or prevent metabolic disorders.

Mitochondrial damage is a hallmark of metabolic diseases, including diabetes, yet the consequences of compromised mitochondria in metabolic tissues are often unclear. Here, we report that dysfunctional mitochondrial quality control engages a retrograde (mitonuclear) signaling program that impairs cellular identity and maturity in β-cells, hepatocytes, and brown adipocytes. Targeted deficiency throughout the mitochondrial quality control pathway, including genome integrity, dynamics, or turnover, impaired the oxidative phosphorylation machinery, activating the mitochondrial integrated stress response, eliciting chromatin remodeling, and promoting cellular immaturity rather than apoptosis to yield metabolic dysfunction. Indeed, pharmacologic blockade of the integrated stress response in vivo restored β-cell identity following loss of mitochondrial quality control. Targeting mitochondrial retrograde signaling may therefore be promising in the treatment or prevention of metabolic disorders.

Proteotoxicity is a contributor to the development of type 2 diabetes (T2D), but it is unknown whether protein misfolding in T2D is generalized or has special features. Here, we report a robust accumulation of misfolded proteins within the mitochondria of human pancreatic islets in T2D and elucidate its impact on β cell viability. Surprisingly, quantitative proteomics studies of protein aggregates reveal that human islets from donors with T2D have a signature more closely resembling mitochondrial rather than ER protein misfolding. The matrix protease LonP1 and its chaperone partner mtHSP70 were among the proteins enriched in protein aggregates. Deletion of LONP1 in mice yields mitochondrial protein misfolding and reduced respiratory function, ultimately leading to β cell apoptosis and hyperglycemia. Intriguingly, LONP1 gain of function ameliorates mitochondrial protein misfolding and restores human β cell survival following glucolipotoxicity via a protease-independent effect requiring LONP1-mtHSP70 chaperone activity. Thus, LONP1 promotes β cell survival and prevents hyperglycemia by facilitating mitochondrial protein folding. These observations may open novel insights into the nature of impaired proteostasis on β cell loss in the pathogenesis of T2D that could be considered as future therapeutic targets.

Type 2 diabetes (T2D) is a metainflammatory disease characterized by impairments in mitochondrial function and ultrastructure that contribute to the overall disruption of β-cell function. Mitochondria rely on both the nuclear genome as well as their own 16.6 kilobase-pair circular genome to generate the machinery required for oxidative phosphorylation (OXPHOS). Recently, our group identified a reduction in mitochondrial DNA (mtDNA) copy number in islets from donors with T2D, indicating a disruption in mitochondrial genome stability. While mtDNA genome instability is implicated in several diseases, its impact on β-cell dysfunction in diabetes has yet to be explored. Here, we generated a mouse model prone to increased β-cell mtDNA deletions by selective expression of a dominant negative Twinkle (TwnkK320E) helicase mutant, which is essential for mtDNA maintenance. Beginning at 5 weeks, β-TwnkK320E mice exhibited impaired glucose tolerance, which progressively worsened with age. Circulating insulin concentrations following glucose stimulation were also reduced in β-TwnkK320E mice by 5 weeks of age, yet there was no difference in β-cell mass, suggestive of a β cell functional defect. Further, we observed a complete loss of MafA, a key regulator of β-cell maturity, from a subset of β cells in β-TwnkK320E mice. While mitochondrial mass was unchanged between groups, β‑TwnkK320E islets have altered expression of subunits of all OXPHOS complexes, as well as reduced glucose-stimulated oxygen consumption, indicative of impaired mitochondrial function. Together, these data suggest that the accumulation of mtDNA deletions diminishes β-cell function and support the importance of mitochondrial genome integrity to β-cell health. Disclosure R.K. Davidson: Employee; Eli Lilly and Company. J. Zhu: None. E.C. Reck: None. S. Soleimanpour: Advisory Panel; Novo Nordisk. Research Support; Ono Pharmaceutical Co., Ltd. Funding National Institutes of Health (T32DK101357)

Accumulating evidence indicates that type 2 diabetes (T2D) is a protein misfolding disease. LonP1 is an essential mitochondrial protease that mediates mitochondrial proteostasis through clearance of unfolded or misfolded proteins, but its role in β-cells is unknown. We generated mice bearing β-cell specific deletion of LonP1 (β-LonP1KO) and found that LonP1 deletion led to progressively impaired glucose tolerance with age and decreased glucose-stimulated insulin release (GSIS). LonP1 deficiency also induced β-cell apoptosis resulting in the loss of β-cell mass. In the mitochondria, β-LonP1KO mice exhibited diminished mitochondrial respiration, abnormal mitochondrial ultrastructure, reduced mitochondrial mass, and elevated ROS levels. An accumulation of misfolded mitochondrial proteins, defect of OXPHOS complex assembly, and/or induction of ROS could be primary triggers driving mitochondrial dysfunction and β-cell apoptosis in β-LonP1KO islets. Interestingly, β-cell specific overexpression of the antioxidant enzyme catalase targeted to mitochondria exerted only a modest, transient protective effect on glucose tolerance and β-cell mass in β-LonP1KO mice yet did not relieve accumulation of misfolded mitochondrial proteins, suggesting ROS alone could not account for the phenotype in LonP1-deficient mice. Concordantly, a similar effect of anti-oxidants on β-cell survival and mitochondrial protein misfolding was observed in human islets in the context of LonP1-deficiency. Thus, the LonP1 protease is vital for β-cell survival and mass by governing mitochondrial protein folding, which could be a potential therapeutic target to prevent β-cell failure during the development of T2D. Disclosure J. Li: None. J. Zhu: None. E.C. Reck: None. E.M. Walker: None. S. Soleimanpour: Advisory Panel; Novo Nordisk. Research Support; Ono Pharmaceutical Co., Ltd.

The dynamin-like GTPases Mitofusin 1 and 2 (Mfn1 and Mfn2) are essential for mitochondrial function, which has been principally attributed to their regulation of fission/fusion dynamics. Here, we report that Mfn1 and 2 are critical for glucose-stimulated insulin secretion (GSIS) primarily through control of mitochondrial DNA (mtDNA) content. Whereas Mfn1 and Mfn2 individually were dispensable for glucose homeostasis, combined Mfn1/2 deletion in β-cells reduced mtDNA content, impaired mitochondrial morphology and networking, and decreased respiratory function, ultimately resulting in severe glucose intolerance. Importantly, gene dosage studies unexpectedly revealed that Mfn1/2 control of glucose homeostasis was dependent on maintenance of mtDNA content, rather than mitochondrial structure. Mfn1/2 maintain mtDNA content by regulating the expression of the crucial mitochondrial transcription factor Tfam, as Tfam overexpression ameliorated the reduction in mtDNA content and GSIS in Mfn1/2-deficient β-cells. Thus, the primary physiologic role of Mfn1 and 2 in β-cells is coupled to the preservation of mtDNA content rather than mitochondrial architecture, and Mfn1 and 2 may be promising targets to overcome mitochondrial dysfunction and restore glucose control in diabetes.

Mitochondrial function is pivotal to β-cell competence. The mitochondrial life cycle balances mitochondrial biogenesis and turnover (mitophagy) to ensure optimal metabolic function. Human type 2 diabetic (T2D) β-cells are known to develop mitochondrial structural/functional defects, suggestive of a defective mitochondrial life cycle. However, it is unclear if these defects are a cause or consequence of T2D. Here, we observed that human T2D β-cells had reduced mitophagic flux, mtDNA content, and expression of mitochondrially encoded genes. To test the importance of the mitochondrial life cycle to drive β-cell failure in T2D, we developed 2 distinct β-cell specific mouse models: βTfamKO (to deplete mtDNA) and βClec16aΚΟ (to impair mitophagy). We observed age dependent loss of glucose tolerance, glucose stimulated insulin secretion and β-cell mass in both models, which was exacerbated by obesity in βClec16aΚΟ mice. Loss of β-cell mass was largely independent of changes in proliferation or apoptosis but rather, due to an induction of β-cell immaturity. Using lineage tracing approaches, we confirmed that βClec16aKO and βTfamKO mice induce formation of both insulin-negative immature β-cells and β-to-α cell transdifferentiation. Further, high-throughput gene expression profiling, biochemical, and metabolic assays highlighted that either Clec16a or Tfam deficiency induces an aberrant retrograde signaling program, manifested by reductions in cellular ATP and activation of the integrated stress response (ISR). Importantly, inhibition of the ISR in vivo ameliorated glucose intolerance, defective insulin secretion, β-cell immaturity, and loss of β-cell mass, suggesting that aberrant retrograde signaling may directly lead to loss of β-cell identity. Taken together, our studies illustrate that a unified mitochondrial lifecycle is necessary to maintain β-cell mass and identity and may be targeted to prevent β-cell failure in T2D. Disclosure G. Pearson: None. N. Lawlor: None. J. Zhu: None. E. M. Walker: None. E. C. Reck: None. M. L. Stitzel: None. S. Soleimanpour: None. Funding American Diabetes Association (1-19-PDF-063 to G.P.)

Inflammatory damage contributes to β cell failure in type 1 and 2 diabetes (T1D and T2D, respectively). Mitochondria are damaged by inflammatory signaling in β cells, resulting in impaired bioenergetics and initiation of proapoptotic machinery. Hence, the identification of protective responses to inflammation could lead to new therapeutic targets. Here, we report that mitophagy serves as a protective response to inflammatory stress in both human and rodent β cells. Utilizing in vivo mitophagy reporters, we observed that diabetogenic proinflammatory cytokines induced mitophagy in response to nitrosative/oxidative mitochondrial damage. Mitophagy-deficient β cells were sensitized to inflammatory stress, leading to the accumulation of fragmented dysfunctional mitochondria, increased β cell death, and hyperglycemia. Overexpression of CLEC16A, a T1D gene and mitophagy regulator whose expression in islets is protective against T1D, ameliorated cytokine-induced human β cell apoptosis. Thus, mitophagy promotes β cell survival and prevents diabetes by countering inflammatory injury. Targeting this pathway has the potential to prevent β cell failure in diabetes and may be beneficial in other inflammatory conditions.