H3K27me3 Demethylases Maintain the Transcriptional and Epigenomic Landscape of the Intestinal Epithelium.

BACKGROUND & AIMS: Although trimethylation of histone H3 lysine 27 (H3K27me3) by polycomb repressive complex 2 is required for intestinal function, the role of the antagonistic process-H3K27me3 demethylation-in the intestine remains unknown. The aim of this study was to determine the contribution of H3K27me3 demethylases to intestinal homeostasis. METHODS: An inducible mouse model was used to simultaneously ablate the 2 known H3K27me3 demethylases, lysine (K)-specific demethylase 6A (Kdm6a) and lysine (K)-specific demethylase 6B (Kdm6b), from the intestinal epithelium. Mice were analyzed at acute and prolonged time points after Kdm6a/b ablation. Cellular proliferation and differentiation were measured using immunohistochemistry, while RNA sequencing and chromatin immunoprecipitation followed by sequencing for H3K27me3 were used to identify gene expression and chromatin changes after Kdm6a/b loss. Intestinal epithelial renewal was evaluated using a radiation-induced injury model, while Paneth cell homeostasis was measured via immunohistochemistry, immunoblot, and transmission electron microscopy. RESULTS: We did not detect any effect of Kdm6a/b ablation on intestinal cell proliferation or differentiation toward the secretory cell lineages. Acute and prolonged Kdm6a/b loss perturbed expression of gene signatures belonging to multiple cell lineages (adjusted P value < .05), and a set of 72 genes was identified as being down-regulated with an associated increase in H3K27me3 levels after Kdm6a/b ablation (false discovery rate, <0.05). After prolonged Kdm6a/b loss, dysregulation of the Paneth cell gene signature was associated with perturbed matrix metallopeptidase 7 localization (P < .0001) and expression. CONCLUSIONS: Although KDM6A/B does not regulate intestinal cell differentiation, both enzymes are required to support the full transcriptomic and epigenomic landscape of the intestinal epithelium and the expression of key Paneth cell genes.

mTORC1 to AMPK switching underlies ß-cell metabolic plasticity during maturation and diabetes.

Pancreatic beta cells (ß-cells) differentiate during fetal life, but only postnatally acquire the capacity for glucose-stimulated insulin secretion (GSIS). How this happens is not clear. In exploring what molecular mechanisms drive the maturation of ß-cell function, we found that the control of cellular signaling in ß-cells fundamentally switched from the nutrient sensor target of rapamycin (mTORC1) to the energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK), and that this was critical for functional maturation. Moreover, AMPK was activated by the dietary transition taking place during weaning, and this in turn inhibited mTORC1 activity to drive the adult ß-cell phenotype. While forcing constitutive mTORC1 signaling in adult ß-cells relegated them to a functionally immature phenotype with characteristic transcriptional and metabolic profiles, engineering the switch from mTORC1 to AMPK signaling was sufficient to promote ß-cell mitochondrial biogenesis, a shift to oxidative metabolism, and functional maturation. We also found that type 2 diabetes, a condition marked by both mitochondrial degeneration and dysregulated GSIS, was associated with a remarkable reversion of the normal AMPK-dependent adult ß-cell signature to a more neonatal one characterized by mTORC1 activation. Manipulating the way in which cellular nutrient signaling pathways regulate ß-cell metabolism may thus offer new targets to improve ß-cell function in diabetes.

Small Extracellular Vesicles Are Key Regulators of Non-cell Autonomous Intercellular Communication in Senescence via the Interferon Protein IFITM3.

Senescence is a cellular phenotype present in health and disease, characterized by a stable cell-cycle arrest and an inflammatory response called senescence-associated secretory phenotype (SASP). The SASP is important in influencing the behavior of neighboring cells and altering the microenvironment; yet, this role has been mainly attributed to soluble factors. Here, we show that both the soluble factors and small extracellular vesicles (sEVs) are capable of transmitting paracrine senescence to nearby cells. Analysis of individual cells internalizing sEVs, using a Cre-reporter system, show a positive correlation between sEV uptake and senescence activation. We find an increase in the number of multivesicular bodies during senescence in vivo. sEV protein characterization by mass spectrometry (MS) followed by a functional siRNA screen identify interferon-induced transmembrane protein 3 (IFITM3) as being partially responsible for transmitting senescence to normal cells. We find that sEVs contribute to paracrine senescence.

miR-17-92 and miR-106b-25 clusters regulate beta cell mitotic checkpoint and insulin secretion in mice.

AIMS/HYPOTHESIS: Adult beta cells in the pancreas are the sole source of insulin in the body. Beta cell loss or increased demand for insulin impose metabolic challenges because adult beta cells are generally quiescent and infrequently re-enter the cell division cycle. The aim of this study is to test the hypothesis that a family of proto-oncogene microRNAs that includes miR-17-92 and miR-106b-25 clusters regulates beta cell proliferation or function in the adult endocrine pancreas. METHODS: To elucidate the role of miR-17-92 and miR-106b-25 clusters in beta cells, we used a conditional miR-17-92/miR-106b-25 knockout mouse model. We employed metabolic assays in vivo and ex vivo, together with advanced microscopy of pancreatic sections, bioinformatics, mass spectrometry and next generation sequencing, to examine potential targets of miR-17-92/miR-106b-25, by which they might regulate beta cell proliferation and function. RESULTS: We demonstrate that miR-17-92/miR-106b-25 regulate the adult beta cell mitotic checkpoint and that miR-17-92/miR-106b-25 deficiency results in reduction in beta cell mass in vivo. Furthermore, we reveal a critical role for miR-17-92/miR-106b-25 in glucose homeostasis and in controlling insulin secretion. We identify protein kinase A as a new relevant molecular pathway downstream of miR-17-92/miR-106b-25 in control of adult beta cell division and glucose homeostasis. CONCLUSIONS/INTERPRETATION: The study contributes to the understanding of proto-oncogene miRNAs in the normal, untransformed endocrine pancreas and illustrates new genetic means for regulation of beta cell mitosis and function by non-coding RNAs. DATA AVAILABILITY: Sequencing data that support the findings of this study have been deposited in GEO with the accession code GSE126516.

ß-Cell DNA Damage Response Promotes Islet Inflammation in Type 1 Diabetes.

Type 1 diabetes (T1D) is an autoimmune disease where pancreatic ß-cells are destroyed by islet-infiltrating T cells. Although a role for ß-cell defects has been suspected, ß-cell abnormalities are difficult to demonstrate. We show a ß-cell DNA damage response (DDR), presented by activation of the 53BP1 protein and accumulation of p53, in biopsy and autopsy material from patients with recently diagnosed T1D as well as a rat model of human T1D. The ß-cell DDR is more frequent in islets infiltrated by CD45+ immune cells, suggesting a link to islet inflammation. The ß-cell toxin streptozotocin (STZ) elicits DDR in islets, both in vivo and ex vivo, and causes elevation of the proinflammatory molecules IL-1ß and Cxcl10. ß-Cell-specific inactivation of the master DNA repair gene ataxia telangiectasia mutated (ATM) in STZ-treated mice decreases the expression of proinflammatory cytokines in islets and attenuates the development of hyperglycemia. Together, these data suggest that ß-cell DDR is an early event in T1D, possibly contributing to autoimmunity.

Transcriptional Noise and Somatic Mutations in the Aging Pancreas.

The underlying mechanisms and functional significance of pancreatic ß cell heterogeneity are an intensive area of investigation. In a recent Cell paper, Enge and colleagues (2017) performed single-cell RNA sequencing of human pancreatic cells and concluded that with age, pancreatic cells become transcriptionally noisy and accumulate somatic mutations.

Metabolic Stress and Compromised Identity of Pancreatic Beta Cells.

Beta cell failure is a central feature of type 2 diabetes (T2D), but the molecular underpinnings of the process remain only partly understood. It has been suggested that beta cell failure in T2D involves massive cell death. Other studies ascribe beta cell failure to cell exhaustion, due to chronic oxidative or endoplasmic reticulum stress leading to cellular dysfunction. More recently it was proposed that beta cells in T2D may lose their differentiated identity, possibly even gaining features of other islet cell types. The loss of beta cell identity appears to be driven by glucotoxicity inhibiting the activity of key beta cell transcription factors including Pdx1, Nkx6.1, MafA and Pax6, thereby silencing beta cell genes and derepressing alternative islet cell genes. The loss of beta cell identity is at least partly reversible upon normalization of glycemia, with implications for the reversibility of T2D, although it is not known if beta cell failure reaches eventually a point of no return. In this review we discuss current evidence for metabolism-driven compromised beta cell identity, key knowledge gaps and opportunities for utility in the treatment of T2D.

PAX6 maintains ß cell identity by repressing genes of alternative islet cell types.

Type 2 diabetes is thought to involve a compromised ß cell differentiation state, but the mechanisms underlying this dysfunction remain unclear. Here, we report a key role for the TF PAX6 in the maintenance of adult ß cell identity and function. PAX6 was downregulated in ß cells of diabetic db/db mice and in WT mice treated with an insulin receptor antagonist, revealing metabolic control of expression. Deletion of Pax6 in ß cells of adult mice led to lethal hyperglycemia and ketosis that were attributed to loss of ß cell function and expansion of α cells. Lineage-tracing, transcriptome, and chromatin analyses showed that PAX6 is a direct activator of ß cell genes, thus maintaining mature ß cell function and identity. In parallel, we found that PAX6 binds promoters and enhancers to repress alternative islet cell genes including ghrelin, glucagon, and somatostatin. Chromatin analysis and shRNA-mediated gene suppression experiments indicated a similar function of PAX6 in human ß cells. We conclude that reduced expression of PAX6 in metabolically stressed ß cells may contribute to ß cell failure and α cell dysfunction in diabetes.

Pancreatic ß-Cells Express the Fetal Islet Hormone Gastrin in Rodent and Human Diabetes.

ß-Cell failure in type 2 diabetes (T2D) was recently proposed to involve dedifferentiation of ß-cells and ectopic expression of other islet hormones, including somatostatin and glucagon. Here we show that gastrin, a stomach hormone typically expressed in the pancreas only during embryogenesis, is expressed in islets of diabetic rodents and humans with T2D. Although gastrin in mice is expressed in insulin+ cells, gastrin expression in humans with T2D occurs in both insulin+ and somatostatin+ cells. Genetic lineage tracing in mice indicates that gastrin expression is turned on in a subset of differentiated ß-cells after exposure to severe hyperglycemia. Gastrin expression in adult ß-cells does not involve the endocrine progenitor cell regulator neurogenin3 but requires membrane depolarization, calcium influx, and calcineurin signaling. In vivo and in vitro experiments show that gastrin expression is rapidly eliminated upon exposure of ß-cells to normal glucose levels. These results reveal the fetal hormone gastrin as a novel marker for reversible human ß-cell reprogramming in diabetes.

Dynamical compensation in physiological circuits.

Biological systems can maintain constant steady-state output despite variation in biochemical parameters, a property known as exact adaptation. Exact adaptation is achieved using integral feedback, an engineering strategy that ensures that the output of a system robustly tracks its desired value. However, it is unclear how physiological circuits also keep their output dynamics precise-including the amplitude and response time to a changing input. Such robustness is crucial for endocrine and neuronal homeostatic circuits because they need to provide a precise dynamic response in the face of wide variation in the physiological parameters of their target tissues; how such circuits compensate their dynamics for unavoidable natural fluctuations in parameters is unknown. Here, we present a design principle that provides the desired robustness, which we call dynamical compensation (DC). We present a class of circuits that show DC by means of a nonlinear feedback loop in which the regulated variable controls the functional mass of the controlling endocrine or neuronal tissue. This mechanism applies to the control of blood glucose by insulin and explains several experimental observations on insulin resistance. We provide evidence that this mechanism may also explain compensation and organ size control in other physiological circuits.

p16(Ink4a)-induced senescence of pancreatic beta cells enhances insulin secretion.

Cellular senescence is thought to contribute to age-associated deterioration of tissue physiology. The senescence effector p16(Ink4a) is expressed in pancreatic beta cells during aging and limits their proliferative potential; however, its effects on beta cell function are poorly characterized. We found that beta cell-specific activation of p16(Ink4a) in transgenic mice enhances glucose-stimulated insulin secretion (GSIS). In mice with diabetes, this leads to improved glucose homeostasis, providing an unexpected functional benefit. Expression of p16(Ink4a) in beta cells induces hallmarks of senescence--including cell enlargement, and greater glucose uptake and mitochondrial activity--which promote increased insulin secretion. GSIS increases during the normal aging of mice and is driven by elevated p16(Ink4a) activity. We found that islets from human adults contain p16(Ink4a)-expressing senescent beta cells and that senescence induced by p16(Ink4a) in a human beta cell line increases insulin secretion in a manner dependent, in part, on the activity of the mechanistic target of rapamycin (mTOR) and the peroxisome proliferator-activated receptor (PPAR)-γ proteins. Our findings reveal a novel role for p16(Ink4a) and cellular senescence in promoting insulin secretion by beta cells and in regulating normal functional tissue maturation with age.

Loss of Liver Kinase B1 (LKB1) in Beta Cells Enhances Glucose-stimulated Insulin Secretion Despite Profound Mitochondrial Defects.

The tumor suppressor liver kinase B1 (LKB1) is an important regulator of pancreatic ß cell biology. LKB1-dependent phosphorylation of distinct AMPK (adenosine monophosphate-activated protein kinase) family members determines proper ß cell polarity and restricts ß cell size, total ß cell mass, and glucose-stimulated insulin secretion (GSIS). However, the full spectrum of LKB1 effects and the mechanisms involved in the secretory phenotype remain incompletely understood. We report here that in the absence of LKB1 in ß cells, GSIS is dramatically and persistently improved. The enhancement is seen both in vivo and in vitro and cannot be explained by altered cell polarity, increased ß cell number, or increased insulin content. Increased secretion does require membrane depolarization and calcium influx but appears to rely mostly on a distal step in the secretion pathway. Surprisingly, enhanced GSIS is seen despite profound defects in mitochondrial structure and function in LKB1-deficient ß cells, expected to greatly diminish insulin secretion via the classic triggering pathway. Thus LKB1 is essential for mitochondrial homeostasis in ß cells and in parallel is a powerful negative regulator of insulin secretion. This study shows that ß cells can be manipulated to enhance GSIS to supra-normal levels even in the face of defective mitochondria and without deterioration over months.

LKB1 and AMPK differentially regulate pancreatic ß-cell identity.

Fully differentiated pancreatic ß cells are essential for normal glucose homeostasis in mammals. Dedifferentiation of these cells has been suggested to occur in type 2 diabetes, impairing insulin production. Since chronic fuel excess ("glucotoxicity") is implicated in this process, we sought here to identify the potential roles in ß-cell identity of the tumor suppressor liver kinase B1 (LKB1/STK11) and the downstream fuel-sensitive kinase, AMP-activated protein kinase (AMPK). Highly ß-cell-restricted deletion of each kinase in mice, using an Ins1-controlled Cre, was therefore followed by physiological, morphometric, and massive parallel sequencing analysis. Loss of LKB1 strikingly (2.0-12-fold, E<0.01) increased the expression of subsets of hepatic (Alb, Iyd, Elovl2) and neuronal (Nptx2, Dlgap2, Cartpt, Pdyn) genes, enhancing glutamate signaling. These changes were partially recapitulated by the loss of AMPK, which also up-regulated ß-cell "disallowed" genes (Slc16a1, Ldha, Mgst1, Pdgfra) 1.8- to 3.4-fold (E < 0.01). Correspondingly, targeted promoters were enriched for neuronal (Zfp206; P = 1.3 × 10(-33)) and hypoxia-regulated (HIF1; P = 2.5 × 10(-16)) transcription factors. In summary, LKB1 and AMPK, through only partly overlapping mechanisms, maintain ß-cell identity by suppressing alternate pathways leading to neuronal, hepatic, and other characteristics. Selective targeting of these enzymes may provide a new approach to maintaining ß-cell function in some forms of diabetes.

Short-term overexpression of VEGF-A in mouse beta cells indirectly stimulates their proliferation and protects against diabetes.

AIMS/HYPOTHESIS: Vascular endothelial growth factor (VEGF) has been recognised by loss-of-function experiments as a pleiotropic factor with importance in embryonic pancreas development and postnatal beta cell function. Chronic, nonconditional overexpression of VEGF-A has a deleterious effect on beta cell development and function. We report, for the first time, a conditional gain-of-function study to evaluate the effect of transient VEGF-A overexpression by adult pancreatic beta cells on islet vasculature and beta cell proliferation and survival, under both normal physiological and injury conditions. METHODS: In a transgenicmouse strain, overexpressing VEGF-A in a doxycycline-inducible and beta cell-specific manner, we evaluated the ability of VEGF-A to affect islet vessel density, beta cell proliferation and protection of the adult beta cell mass from toxin-induced injury. RESULTS: Short-term VEGF-A overexpression resulted in islet hypervascularisation, increased beta cell proliferation and protection from toxin-mediated beta cell death, and thereby prevented the development of hyperglycaemia. Extended overexpression of VEGF-A led to impaired glucose tolerance, elevated fasting glycaemia and a decreased beta cell mass. CONCLUSIONS/INTERPRETATION: Overexpression of VEGF-A in beta cells time-dependently affects glycometabolic control and beta cell protection and proliferation. These data nourish further studies to examine the role of controlled VEGF delivery in (pre)clinical applications aimed at protecting and/or restoring the injured beta cell mass.

Conditional hypovascularization and hypoxia in islets do not overtly influence adult ß-cell mass or function.

It is generally accepted that vascularization and oxygenation of pancreatic islets are essential for the maintenance of an optimal ß-cell mass and function and that signaling by vascular endothelial growth factor (VEGF) is crucial for pancreas development, insulin gene expression/secretion, and (compensatory) ß-cell proliferation. A novel mouse model was designed to allow conditional production of human sFlt1 by ß-cells in order to trap VEGF and study the effect of time-dependent inhibition of VEGF signaling on adult ß-cell fate and metabolism. Secretion of sFlt1 by adult ß-cells resulted in a rapid regression of blood vessels and hypoxia within the islets. Besides blunted insulin release, ß-cells displayed a remarkable capacity for coping with these presumed unfavorable conditions: even after prolonged periods of blood vessel ablation, basal and stimulated blood glucose levels were only slightly increased, while ß-cell proliferation and mass remained unaffected. Moreover, ablation of blood vessels did not prevent ß-cell generation after severe pancreas injury by partial pancreatic duct ligation or partial pancreatectomy. Our data thus argue against a major role of blood vessels to preserve adult ß-cell generation and function, restricting their importance to facilitating rapid and adequate insulin delivery.

AMPK regulates ER morphology and function in stressed pancreatic ß-cells via phosphorylation of DRP1.

Experimental lipotoxicity constitutes a model for ß-cell demise induced by metabolic stress in obesity and type 2 diabetes. Fatty acid excess induces endoplasmic reticulum (ER) stress, which is accompanied by ER morphological changes whose mechanisms and relevance are unknown. We found that the GTPase dynamin-related protein 1 (DRP1), a key regulator of mitochondrial fission, is an ER resident regulating ER morphology in stressed ß-cells. Inhibition of DRP1 activity using a GTP hydrolysis-defective mutant (Ad-K38A) attenuated fatty acid-induced ER expansion and mitochondrial fission. Strikingly, stimulating the key energy-sensor AMP-activated protein kinase (AMPK) increased the phosphorylation at the anti-fission site Serine 637 and largely prevented the alterations in ER and mitochondrial morphology. Expression of a DRP1 mutant resistant to phosphorylation at this position partially prevented the recovery of ER and mitochondrial morphology by AMPK. Fatty acid-induced ER enlargement was associated with proinsulin retention in the ER, together with increased proinsulin/insulin ratio. Stimulation of AMPK prevented these alterations, as well as mitochondrial fragmentation and apoptosis. In summary, DRP1 regulation by AMPK delineates a novel pathway controlling ER and mitochondrial morphology, thereby modulating the response of ß-cells to metabolic stress. DRP1 may thus function as a node integrating signals from stress regulators, such as AMPK, to coordinate organelle shape and function.

Phosphorylation of ribosomal protein S6 attenuates DNA damage and tumor suppression during development of pancreatic cancer.

The signaling pathways that mediate the development of pancreatic ductal adenocarcinoma (PDAC) downstream of mutant Kras remain incompletely understood. Here, we focus on ribosomal protein S6 (rpS6), an mTOR effector not implicated previously in cancer. Phosphorylation of rpS6 was increased in pancreatic acinar cells upon implantation of the chemical carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) or transgenic expression of mutant Kras. To examine the functional significance of rpS6 phosphorylation, we used knockin mice lacking all five phosphorylatable sites in rpS6 (termed rpS6(P-/-) mice). Strikingly, the development of pancreatic cancer precursor lesions induced by either DMBA or mutant Kras was greatly reduced in rpS6(P-/-) mice. The rpS6 mutants expressing oncogenic Kras showed increased p53 along with increased staining of γ-H2AX and 53bp1 (Trp53bp1) in areas of acinar ductal metaplasia, suggesting that rpS6 phosphorylation attenuates Kras-induced DNA damage and p53-mediated tumor suppression. These results reveal that rpS6 phosphorylation is important for the initiation of pancreatic cancer.

A transgenic mouse marking live replicating cells reveals in vivo transcriptional program of proliferation.

Most adult mammalian tissues are quiescent, with rare cell divisions serving to maintain homeostasis. At present, the isolation and study of replicating cells from their in vivo niche typically involves immunostaining for intracellular markers of proliferation, causing the loss of sensitive biological material. We describe a transgenic mouse strain, expressing a CyclinB1-GFP fusion reporter, that marks replicating cells in the S/G2/M phases of the cell cycle. Using flow cytometry, we isolate live replicating cells from the liver and compare their transcriptome to that of quiescent cells to reveal gene expression programs associated with cell proliferation in vivo. We find that replicating hepatocytes have reduced expression of genes characteristic of liver differentiation. This reporter system provides a powerful platform for gene expression and metabolic and functional studies of replicating cells in their in vivo niche.

LKB1 regulates pancreatic beta cell size, polarity, and function.

Pancreatic beta cells, organized in the islets of Langerhans, sense glucose and secrete appropriate amounts of insulin. We have studied the roles of LKB1, a conserved kinase implicated in the control of cell polarity and energy metabolism, in adult beta cells. LKB1-deficient beta cells show a dramatic increase in insulin secretion in vivo. Histologically, LKB1-deficient beta cells have striking alterations in the localization of the nucleus and cilia relative to blood vessels, suggesting a shift from hepatocyte-like to columnar polarity. Additionally, LKB1 deficiency causes a 65% increase in beta cell volume. We show that distinct targets of LKB1 mediate these effects. LKB1 controls beta cell size, but not polarity, via the mTOR pathway. Conversely, the precise position of the beta cell nucleus, but not cell size, is controlled by the LKB1 target Par1b. Insulin secretion and content are restricted by LKB1, at least in part, via AMPK. These results expose a molecular mechanism, orchestrated by LKB1, for the coordinated maintenance of beta cell size, form, and function.

Pancreatic LKB1 deletion leads to acinar polarity defects and cystic neoplasms.

LKB1 is a key regulator of energy homeostasis through the activation of AMP-activated protein kinase (AMPK) and is functionally linked to vascular development, cell polarity, and tumor suppression. In humans, germ line LKB1 loss-of-function mutations cause Peutz-Jeghers syndrome (PJS), which is characterized by a predisposition to gastrointestinal neoplasms marked by a high risk of pancreatic cancer. To explore the developmental and physiological functions of Lkb1 in vivo, we examined the impact of conditional Lkb1 deletion in the pancreatic epithelium of the mouse. The Lkb1-deficient pancreas, although grossly normal at birth, demonstrates a defective acinar cell polarity, an abnormal cytoskeletal organization, a loss of tight junctions, and an inactivation of the AMPK/MARK/SAD family kinases. Rapid and progressive postnatal acinar cell degeneration and acinar-to-ductal metaplasia occur, culminating in marked pancreatic insufficiency and the development of pancreatic serous cystadenomas, a tumor type associated with PJS. Lkb1 deficiency also impacts the pancreas endocrine compartment, characterized by smaller and scattered islets and transient alterations in glucose control. These genetic studies provide in vivo evidence of a key role for LKB1 in the establishment of epithelial cell polarity that is vital for pancreatic acinar cell function and viability and for the suppression of neoplasia.