Pancreatic beta cells in very old mice retain capacity for compensatory proliferation

The age-dependent regulation of pancreatic islet landscape is fueled by a HNF1a-immune signaling loop

Animal longevity is a function of global vital organ functionality and, consequently, a complex polygenic trait. Yet, monogenic regulators controlling overall or organ-specific ageing exist, owing their conservation to their function in growth and development. Here, by using pathway analysis combined with wet-biology methods on several dynamic timelines, we identified Hnf1a as a novel master regulator of the maturation and ageing in the adult pancreatic islet during the first year of life. Conditional transgenic mice bearing suboptimal levels of this transcription factor in the pancreatic islets displayed age-dependent changes, with a profile echoing precocious maturation. Additionally, the comparative pathway analysis revealed a link between Hnf1a age-dependent regulation and immune signaling, which was confirmed in the ageing timeline of an overly immunodeficient mouse model. Last, the global proteome analysis of human islets spanning three decades of life largely backed the age-specific regulation observed in mice. Collectively, our results suggest a novel role of Hnf1a as a monogenic regulator of the maturation and ageing process in the pancreatic islet via a direct or indirect regulatory loop with immune signaling.

Moderate beta-cell ablation triggers synergic compensatory mechanisms even in the absence of overt metabolic disruption

Regeneration, the ability to replace injured tissues and organs, is a phenomenon commonly associated with lower vertebrates but is also observed in mammals, in specific tissues. In this study, we investigated the regenerative potential of pancreatic islets following moderate beta-cell loss in mice. Using a rapid model of moderate ablation, we observed a compensatory response characterized by transient inflammation and proliferation signatures, ultimately leading to the recovery of beta-cell identity and function. Interestingly, this proliferative response occurred independently of inflammation, as demonstrated in ablated immunodeficient mice. Furthermore, exposure to high-fat diet stimulated beta-cell proliferation but negatively impacted beta-cell function. In contrast, an equivalent slower ablation model revealed a delayed but similar proliferative response, suggesting proliferation as a common regenerative response. However, high-fat diet failed to promote proliferation in this model, indicating a differential response to metabolic stressors. Overall, our findings shed light on the complex interplay between beta-cell loss, inflammation, and stress in modulating pancreatic islet regeneration. Understanding these mechanisms could pave the way for novel therapeutic strategies based on beta-cell proliferation.

Glycemic control releases regenerative potential of pancreatic beta cells blocked by severe hyperglycemia

Diabetogenic ablation of beta cells in mice triggers a regenerative response whereby surviving beta cells proliferate and euglycemia is regained. Here, we identify and characterize heterogeneity in response to beta cell ablation. Efficient beta cell elimination leading to severe hyperglycemia (>28 mmol/L), causes permanent diabetes with failed regeneration despite cell cycle engagement of surviving beta cells. Strikingly, correction of glycemia via insulin, SGLT2 inhibition, or a ketogenic diet for about 3 weeks allows partial regeneration of beta cell mass and recovery from diabetes, demonstrating regenerative potential masked by extreme glucotoxicity. We identify gene expression changes in beta cells exposed to extremely high glucose levels, pointing to metabolic stress and downregulation of key cell cycle genes, suggesting failure of cell cycle completion. These findings reconcile conflicting data on the impact of glucose on beta cell regeneration and identify a glucose threshold converting glycemic load from pro-regenerative to anti-regenerative.

Characterisation of an Atrx Conditional Knockout Mouse Model: Atrx Loss Causes Endocrine Dysfunction Rather Than Pancreatic Neuroendocrine Tumour

Simple Summary

ATRX and DAXX mutations occur in 30–40% of pancreatic neuroendocrine tumours (PanNETs), and there are no reports in the literature of any genetically engineered mouse model (GEMM) evaluating the effect of Atrx disruption as a putative driver event on PanNET initiation. We created a novel GEMM with Atrx conditional disruption in β cells. We observed that this genetic alteration, per se, was not tumourigenic, but we reported novel roles of Atrx on endocrine function, which resulted in dysglycaemia and the exacerbation of inflammageing (increased pancreatic inflammation and hepatic steatosis).

Abstract

ATRX is a chromatin remodeller that maintains telomere homeostasis. Loss of ATRX is described in approximately 10% of pancreatic neuroendocrine tumours (PanNETs) and associated with poorer prognostic features. Here, we present a genetically engineered mouse model (GEMM) addressing the role of Atrx loss (Atrx^KO) in pancreatic β cells, evaluating a large cohort of ageing mice (for up to 24 months (mo.)). Atrx loss did not cause PanNET formation but rather resulted in worsening of ageing-related pancreatic inflammation and endocrine dysfunction in the first year of life. Histopathological evaluation highlighted an exacerbated prevalence and intensity of pancreatic inflammation, ageing features, and hepatic steatosis in Atrx^KO mice. Homozygous floxed mice presented hyperglycaemia, increased weights, and glucose intolerance after 6 months, but alterations in insulinaemia were not detected. Floxed individuals presented an improper growth of their pancreatic endocrine fraction that may explain such an endocrine imbalance. A pilot study of BRACO-19 administration to Atrx^KO mice resulted in telomere instability, reinforcing the involvement of Atrx in the maintenance of β cell telomere homeostasis. Thereby, a non-obese dysglycaemic GEMM of disrupted Atrx is here presented as potentially useful for metabolic studies and putative candidate for inserting additional tumourigenic genetic events.

The pancreatic β-cell in ageing: Implications in age-related diabetes

The prevalence of type 2 diabetes (T2D) and impaired glucose tolerance (IGT) increases with ageing. T2D generally results from progressive impairment of the pancreatic islets to adapt β-cell mass and function in the setting of insulin resistance and increased insulin demand. Several studies have shown an age-related decline in peripheral insulin sensitivity. However, a precise understanding of the pancreatic β-cell response in ageing is still lacking. In this review, we summarize the age-related alterations, adaptations and/or failures of β-cells at the molecular, morphological and functional levels in mouse and human. Age-associated alterations include processes such as β-cell proliferation, apoptosis and cell identity that can influence β-cell mass. Age-related changes also affect β-cell function at distinct steps including electrical activity, Ca²⁺ signaling and insulin secretion, among others. We will consider the potential impact of these alterations and those mediated by senescence pathways on β-cells and their implications in age-related T2D. Finally, given the great diversity of results in the field of β-cell ageing, we will discuss the sources of this heterogeneity. A better understanding of β-cell biology during ageing, particularly at older ages, will improve our insight into the contribution of β-cells to age-associated T2D and may boost new therapeutic strategies.

The effects of aging on male mouse pancreatic β-cell function involve multiple events in the regulation of secretion: influence of insulin sensitivity

Aging is associated with a decline in peripheral insulin sensitivity and an increased risk of impaired glucose tolerance and type 2 diabetes. During conditions of reduced insulin sensitivity, pancreatic β-cells undergo adaptive responses to increase insulin secretion and maintain euglycemia. However, the existence and nature of β-cell adaptations and/or alterations during aging are still a matter of debate. In this study, we investigated the effects of aging on β-cell function from control (3-month-old) and aged (20-month-old) mice. Aged animals were further categorized in two groups: high insulin sensitive (aged-HIS) and low insulin sensitive (aged-LIS). Aged-LIS mice were hyperinsulinemic, glucose intolerant and displayed impaired glucose-stimulated insulin and C-peptide secretion, whereas aged-HIS animals showed characteristics in glucose homeostasis similar to controls. In isolated β-cells, we observed that glucose-induced inhibition of KATP channel activity was reduced with aging, particularly in the aged-LIS group. Glucose-induced islet NAD(P)H production was decreased in aged mice, suggesting impaired mitochondrial function. In contrast, voltage-gated Ca 2+ currents were higher in aged-LIS β-cells, and pancreatic islets of both aged groups displayed increased glucose-induced Ca 2+ signaling and augmented insulin secretion compared with controls. Morphological analysis of pancreas sections also revealed augmented β-cell mass with aging, especially in the aged-LIS group, as well as ultrastructural β-cell changes. Altogether, these findings indicate that aged mouse β-cells compensate for the aging-induced alterations in the stimulus-secretion coupling, particularly by adjusting their Ca 2+ influx to ensure insulin secretion. These results also suggest that decreased peripheral insulin sensitivity exacerbates the effects of aging on β-cells.

The Potential of Pancreatic Organoids for Diabetes Research and Therapy

The success of clinical transplantation of pancreas or isolated pancreatic islets supports the concept of cell-based cure for diabetes. One limitation is the shortage of cadaver human pancreata. The demand-supply gap could potentially be bridged by harnessing the self-renewal capacity of stem cells. Pluripotent stem cells and adult pancreatic stem cells have been explored as possible cell sources. Recently, a system for long-term culture of proposed adult pancreatic stem cells in a form of organoids was developed. Generated organoids partially mimic the architecture and cell-type composition of pancreatic tissue. Here, we review the attempts over the past decade, to utilize the organoid cell culture principles in order to identify, expand, and differentiate the adult pancreatic stem cells from different compartments of mouse and human pancreata. The development of the culture conditions, effects of specific growth factors and small molecules is discussed. The potential utility of the adult pancreatic stem cells is considered in the context of other cell sources.

Endoplasmic Reticulum Stress Induced Proliferation Remains Intact in Aging Mouse β-Cells

Aging is associated with loss of proliferation of the insulin-secreting β-cell, a possible contributing factor to the increased prevalence of type 2 diabetes in the elderly. Our group previously discovered that moderate endoplasmic reticulum (ER) stress occurring during glucose exposure increases the adaptive β-cell proliferation response. Specifically, the ATF6α arm of the tripartite Unfolded Protein Response (UPR) promotes β-cell replication in glucose excess conditions. We hypothesized that β-cells from older mice have reduced proliferation due to aberrant UPR signaling or an impaired proliferative response to ER stress or ATF6α activation. To investigate, young and old mouse islet cells were exposed to high glucose with low-dose thapsigargin or activation of overexpressed ATF6α, and β-cell proliferation was quantified by BrdU incorporation. UPR pathway activation was compared by qPCR of target genes and semi-quantitative Xbp1 splicing assay. Intriguingly, although old β-cells had reduced proliferation in high glucose compared to young β-cells, UPR activation and induction of proliferation in response to low-dose thapsigargin or ATF6α activation in high glucose were largely similar between young and old. These results suggest that loss of UPR-led adaptive proliferation does not explain the reduced cell cycle entry in old β-cells, and raise the exciting possibility that future therapies that engage adaptive UPR could increase β-cell number through proliferation even in older individuals.

In vivo screen identifies a SIK inhibitor that induces β cell proliferation through a transient UPR

It is known that β cell proliferation expands the β cell mass during development and under certain hyperglycemic conditions in the adult, a process that may be used for β cell regeneration in diabetes. Here, through a new high-throughput screen using a luminescence ubiquitination-based cell cycle indicator (LUCCI) in zebrafish, we identify HG-9-91-01 as a driver of proliferation and confirm this effect in mouse and human β cells. HG-9-91-01 is an inhibitor of salt-inducible kinases (SIKs), and overexpression of Sik1 specifically in β cells blocks the effect of HG-9-91-01 on β cell proliferation. Single-cell transcriptomic analyses of mouse β cells demonstrate that HG-9-91-01 induces a wave of activating transcription factor (ATF)6-dependent unfolded protein response (UPR) before cell cycle entry. Importantly, the UPR wave is not associated with an increase in insulin expression. Additional mechanistic studies indicate that HG-9-91-01 induces multiple signalling effectors downstream of SIK inhibition, including CRTC1, CRTC2, ATF6, IRE1 and mTOR, which integrate to collectively drive β cell proliferation.

SNAPIN Regulates Cell Cycle Progression to Promote Pancreatic β Cell Growth

In diabetes mellitus, death of β cell in the pancreas occurs throughout the development of the disease, with loss of insulin production. The maintenance of β cell number is essential to maintaining normoglycemia. SNAPIN has been found to regulate insulin secretion, but whether it induces β cell proliferation remains to be elucidated. This study aimed to explore the physiological roles of SNAPIN in β cell proliferation. SNAPIN expression increases with the age of mice and SNAPIN is down-regulated in diabetes. KEGG pathway and GO analysis showed that SNAPIN- interacting proteins were enriched in cell cycle regulation. B cell cycle was arrested in the S phase, and cell proliferation was inhibited after SNAPIN knockdown. The expression of CDK2, CDK4 and CCND1 proteins in the S phase of the cell cycle were reduced after SNAPIN knockdown, whereas they were increased after overexpression of SNAPIN. In addition, insulin protein and mRNA levels also increased or decreased after SNAPIN knockdown or overexpression, respectively. Conclusions: Our data indicate that SNAPIN mediates β cells proliferation and insulin secretion, and provide evidences that SNAPIN might be a pharmacotherapeutic target for diabetes mellitus.

The core clock transcription factor BMAL1 drives circadian β-cell proliferation during compensatory regeneration of the endocrine pancreas

Circadian clocks in pancreatic islets participate in the regulation of glucose homeostasis. Here we examined the role of these timekeepers in β-cell regeneration after the massive ablation of β cells by doxycycline-induced expression of diphtheria toxin A (DTA) in Insulin-rtTA/TET-DTA mice. Since we crossed reporter genes expressing α- and β-cell-specific fluorescent proteins into these mice, we could follow the fate of α- and β cells separately. As expected, DTA induction resulted in an acute hyperglycemia, which was accompanied by dramatic changes in gene expression in residual β cells. In contrast, only temporal alterations of gene expression were observed in α cells. Interestingly, β cells entered S phase preferentially during the nocturnal activity phase, indicating that the diurnal rhythm also plays a role in the orchestration of β-cell regeneration. Indeed, in arrhythmic Bmal1-deficient mice, which lack circadian clocks, no compensatory β-cell proliferation was observed, and the β-cell ablation led to aggravated hyperglycemia, hyperglucagonemia, and fatal diabetes.

Functional changes in beta cells during ageing and senescence

Insulin secretion from beta cells is crucial for maintaining euglycaemia and preventing type 2 diabetes, a disease correlated with ageing. Therefore, understanding the functional changes that beta cell function undergoes with age can reveal new therapeutic targets and strategies to delay or revert the disease. Herein, a systematic review of the literature agrees that, as humans age, their beta cell function declines, independently of peripheral insulin resistance, BMI and waist circumference. Rodent studies reveal that, with age, basal insulin secretion increases with either no change or an increase in stimulated insulin secretion, but the biological significance of this is unclear. The accumulation of senescent beta cells could explain some of these functional changes: transcriptional analysis of senescent and aged beta cells revealed parallel downregulation of several steps along the pathway linking glucose stimulation and insulin secretion. Moreover, specific deletion of senescent cells (senolysis) improved residual beta cell function, gene expression profile and blood glucose levels. In conclusion, cellular senescence could underlie the functional decline of beta cells during ageing and could represent a novel and promising approach for recovering insulin secretion. Graphical abstract.

The roles of first phase, second phase insulin secretion, insulin resistance, and glucose effectiveness of having prediabetes in nonobese old Chinese women

It has been established that prediabetes can causes significant comorbidities, particularly in the elderly. The deterioration of glucose metabolism are generally considered to be results of the impairment of the 4 factors: first, second insulin secretion (FPIS, SPIS, respectively), glucose effectiveness (GE), and insulin resistance. In this study, we enrolled older women to investigate their relationships with prediabetes.Five thousand four hundred eighty-two nonobese, nondiabetic women were included. They were divided into normal glucose tolerance and prediabetes groups. Receiver operating characteristic curve was performed to investigate the effects on whether to have prediabetes for each factors. Two models were built: Model 1: FPIS + SPIS, and Model 2: model 1 + GE. The area under the receiver operating characteristic (aROC) curve was used to determine the predictive power of these models.The aROC curve of GE was significantly higher than the diagonal line followed by SPIS and FPIS accordingly. The aROC curve of Model 1 (0.611) was not different from GE. However, Model 2 improved significantly up to 0.663. Based on this model, an equation was built (-0.003 × GE - 212.6 × SPIS - 17.9 × insulin resistance + 4.8). If the calculated value is equal or higher than 0 (≥0), then the subject has higher chance to have prediabetes (sensitivity = 0.607, specificity = 0.635).Among the 4 factors, GE is the most important contributor for prediabetes in older women. By building a model composed of FPIS, SPIS, and GE, the aROC curve increased significantly. The equation built from this model could predict prediabetes precisely.

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.

Long-term c-Kit overexpression in beta cells compromises their function in ageing mice

AIMS/HYPOTHESIS

c-Kit signalling regulates intracellular pathways that enhance beta cell proliferation, insulin secretion and islet vascularisation in mice up to 28 weeks of age and on short-term high-fat diet. However, long-term c-Kit activation in ageing mouse islets has yet to be examined. This study utilises beta cell-specific c-Kit-overexpressing transgenic (c-KitβTg) ageing mice (~60 weeks) to determine the effect of its activation on beta cell dysfunction and insulin secretion.

METHODS

Wild-type and c-KitβTg mice, aged 60 weeks, were examined using metabolic tests to determine glucose tolerance and insulin secretion. Pancreas histology and proteins in isolated islets were examined to determine the expression of beta cell transcription factors, proliferation and intracellular signalling. To determine the role of insulin receptor signalling in ageing c-KitβTg mice, we generated beta cell-specific inducible insulin receptor knockout in ageing c-KitβTg mice (c-KitβTg;βIRKO mice) and examined the ageing mice for glucose tolerance and islet histology.

RESULTS

Ageing c-KitβTg mice progressively developed glucose intolerance, compared with age-matched wild-type littermates, due to impaired insulin secretion. Increased beta cell mass, proliferation and nuclear forkhead box transcription factor O1 (FOXO1) expression and reduced exocytotic protein levels were detected in ageing c-KitβTg mouse islets. Protein analyses of isolated islets showed increased insulin receptor, phosphorylated IRS-1^Ser612 and cleaved poly(ADP-ribose) polymerase levels in ageing c-KitβTg mice. Ageing c-KitβTg mouse islets treated ex vivo with insulin demonstrated reduced Akt phosphorylation, indicating that prolonged c-Kit induced beta cell insulin insensitivity. Ageing c-KitβTg;βIRKO mice displayed improved glucose tolerance and beta cell function compared with ageing c-KitβTg mice.

CONCLUSIONS/INTERPRETATION

These findings indicate that long-term c-Kit overexpression in beta cells has a negative impact on insulin exocytosis and that temporally dependent regulation of c-Kit-insulin receptor signalling is important for optimal beta cell function.

Acceleration of β Cell Aging Determines Diabetes and Senolysis Improves Disease Outcomes

Type 2 diabetes (T2D) is an age-related disease. Although changes in function and proliferation of aged β cells resemble those preceding the development of diabetes, the contribution of β cell aging and senescence remains unclear. We generated a β cell senescence signature and found that insulin resistance accelerates β cell senescence leading to loss of function and cellular identity and worsening metabolic profile. Senolysis (removal of senescent cells), using either a transgenic INK-ATTAC model or oral ABT263, improved glucose metabolism and β cell function while decreasing expression of markers of aging, senescence, and senescence-associated secretory profile (SASP). Beneficial effects of senolysis were observed in an aging model as well as with insulin resistance induced both pharmacologically (S961) and physiologically (high-fat diet). Human senescent β cells also responded to senolysis, establishing the foundation for translation. These novel findings lay the framework to pursue senolysis of β cells as a preventive and alleviating strategy for T2D.

Glucocorticoids and checkpoint tyrosine kinase inhibitors stimulate rat pancreatic beta cell proliferation differentially

Cell therapy for diabetes could benefit from the identification of small-molecule compounds that increase the number of functional pancreatic beta cells. Using a newly developed screening assay, we previously identified glucocorticoids as potent stimulators of human and rat beta cell proliferation. We now compare the stimulatory action of these steroid hormones to a selection of checkpoint tyrosine kinase inhibitors that were also found to activate the cell cycle-in beta cells and analyzed their respective effects on DNA-synthesis, beta cell numbers and expression of cell cycle regulators. Our data using glucocorticoids in combination with a receptor antagonist, mifepristone, show that 48h exposure is sufficient to allow beta cells to pass the cell cycle restriction point and to become committed to cell division regardless of sustained glucocorticoid-signaling. To reach the end-point of mitosis another 40h is required. Within 14 days glucocorticoids stimulate up to 75% of the cells to undergo mitosis, which indicates that these steroid hormones act as proliferation competence-inducing factors. In contrast, by correlating thymidine-analogue incorporation to changes in absolute cell numbers, we show that the checkpoint kinase inhibitors, as compared to glucocorticoids, stimulate DNA-synthesis only during a short time-window in a minority of cells, insufficient to give a measurable increase of beta cell numbers. Glucocorticoids, but not the kinase inhibitors, were also found to induce changes in the expression of checkpoint regulators. Our data, using checkpoint kinase-specific inhibitors further point to a role for Chk1 and Cdk1 in G1/S transition and progression of beta cells through the cell cycle upon stimulation with glucocorticoids.

Glutamate decarboxylase 67 contributes to compensatory insulin secretion in aged pancreatic islets

Pancreatic islets play an essential role in regulating blood glucose levels. Age-dependent development of glucose intolerance and insulin resistance results in hyperglycemia, which in turn stimulates insulin synthesis and secretion from aged islets, to fulfill the increased demand for insulin. However, the mechanism underlying enhanced insulin secretion remains unknown. Glutamic acid decarboxylase 67 (GAD67) catalyzes the conversion of glutamate into γ-aminobutyric acid (GABA) and CO₂. Both glutamate and GABA can affect islet function. Here, we investigated the role of GAD67 in insulin secretion in young (3 month old) and aged (24 month old) C57BL/6J male mice. Unlike young mice, aged mice displayed glucose-intolerance and insulin-resistance. However, aged mice secreted more insulin and showed lower fed blood glucose levels than young mice. GAD67 levels in primary islets increased with aging and in response to high glucose levels. Inhibition of GAD67 activity using a potent inhibitor of GAD, 3-mercaptopropionic acid, abrogated glucose-stimulated insulin secretion from a pancreatic β-cell line and from young and aged islets. Collectively, our results suggest that blood glucose levels regulate GAD67 expression, which contributes to β-cell responses to impaired glucose homeostasis caused by advanced aging.

Forkhead box protein O1 (FoxO1) regulates hepatic serine protease inhibitor B1 (serpinB1) expression in a non-cell-autonomous fashion

FoxO proteins are major targets of insulin action, and FoxO1 mediates the effects of insulin on hepatic glucose metabolism. We reported previously that serpinB1 is a liver-secreted factor (hepatokine) that promotes adaptive β-cell proliferation in response to insulin resistance in the liver-specific insulin receptor knockout (LIRKO) mouse. Here we report that FoxO1 plays a critical role in promoting serpinB1 expression in hepatic insulin resistance in a non-cell-autonomous manner. Mice lacking both the insulin receptor and FoxO1 (LIRFKO) exhibit reduced β-cell mass compared with LIRKO mice because of attenuation of β-cell proliferation. Although hepatic expression of serpinB1 mRNA and protein levels was increased in LIRKO mice, both the mRNA and protein levels returned to control levels in LIRFKO mice. Furthermore, liver-specific expression of constitutively active FoxO1 in transgenic mice induced an increase in hepatic serpinB1 mRNA and protein levels in refed mice. Conversely, serpinB1 mRNA and protein levels were reduced in mice lacking FoxO proteins in the liver. ChIP studies demonstrated that FoxO1 binds to three distinct sites located ∼9 kb upstream of the serpinb1 gene in primary mouse hepatocytes and that this binding is enhanced in hepatocytes from LIRKO mice. However, adenoviral expression of WT or constitutively active FoxO1 and insulin treatment are sufficient to regulate other FoxO1 target genes (IGFBP-1 and PEPCK) but not serpinB1 expression in mouse primary hepatocytes. These results indicate that liver FoxO1 promotes serpinB1 expression in hepatic insulin resistance and that non-cell-autonomous factors contribute to FoxO1-dependent effects on serpinB1 expression in the liver.

Critical role for the Tsc1-mTORC1 pathway in β-cell mass in Pdx1-deficient mice

Mutations in the pancreatic duodenal homeobox (PDX1) gene are associated with diabetes in humans. Pdx1-haploinsufficient mice also develop diabetes, but the molecular mechanism is unknown. To this end, we knocked down Pdx1 gene expression in mouse MIN6 insulinoma cells. Pdx1 suppression not only increased apoptotic cell death but also decreased cell proliferation, which was associated with a decrease in activity of mechanistic target of rapamycin complex 1 (mTORC1). We found that in Pdx1-deficient mice, tuberous sclerosis 1 (Tsc1) ablation in pancreatic β-cells restores β-cell mass, increases β-cell proliferation and size, decreases the number of TUNEL-positive cells and restores glucose tolerance after glucose challenge. In addition, Tsc1 ablation in pancreatic β-cells increases phosphorylation of initiation factor 4E-binding protein 1 (4E-BP1) phosphorylation and 40S ribosomal protein S6, two downstream targets of mTORC1 indicating that Tsc1 mediates mTORC1 downregulation induced by Pdx1 suppression. These results suggest that the Tsc1-mTORC1 pathway plays an important role in mediating the decrease in β-cell proliferation and growth and the reduction in β-cell mass that occurs in Pdx1-deficient diabetes. Thus, mTORC1 may be target for therapeutic interventions in diabetes associated with reductions in β-cell mass.

Egr-1 transactivates WNT5A gene expression to inhibit glucose-induced β-cell proliferation

Selective β-cell loss is a characteristic of type 2 diabetes mellitus (T2DM). Inhibition of glucose-stimulated β-cell proliferation is one of the in vivo results of the lipotoxicity of saturated fatty acids (SFAs). However, the mechanism by which lipotoxicity inhibits β-cell proliferation is still unclear. In this study, we found palmitate, a saturated fatty acid, inhibited the β-cell proliferation induced by high glucose through the induction of Wnt5a expression in vitro and in vivo. We also found that Wnt5a was both sufficient and necessary for inhibition of β-cell proliferation. Additionally, Egr-1, but not NF-κB, FOXO1, Smad2, Smad3, SP1 or SP3 mediated the expression of Wnt5a. Deletion and site-directed mutagenesis of the WNT5A promoter revealed that activation of WNT5A gene transcription depends primarily on a putative Egr-binding sequence between nucleotides -52 to -44, upstream of the transcription start site. Furthermore, Egr-1 bound directly to this sequence in response to palmitate treatment, both in vitro and in vivo. Moreover, after mice islets were treated with Egr inhibitors, the expression of Wnt5a decreased significantly and the glucose-induced β-cell proliferation inhibited by palmitate was resumed. These findings establish Wnt5a as an Egr-1 target gene in β-cells, uncovering a novel Egr-1/Wnt5a pathway by which saturated free fatty acids block glucose-induced β-cell proliferation. Our study lends support for the potential of Egr-1 inhibitors or Wnt5a antibodies as therapeutics for the treatment of T2DM.

Pancreatic β Cell Regeneration as a Possible Therapy for Diabetes

Diabetes is the result of having inadequate supply of functional insulin-producing β cells. Two possible approaches for replenishing the β cells are: (1) replacement by transplanting cadaveric islets or β cells derived from human embryonic stem cells/induced pluripotent stem cells and (2) induction of endogenous regeneration. This review focuses on endogenous regeneration, which can follow two pathways: enhanced replication of existing β cells and formation of new β cells from cells not expressing insulin, either by conversion from a differentiated cell type (transdifferentiation) or differentiation from progenitors (neogenesis). Exciting progress on both pathways suggest that regeneration may have therapeutic promise.

Neonatal pancreatic pericytes support β-cell proliferation

OBJECTIVE

The maintenance and expansion of β-cell mass rely on their proliferation, which reaches its peak in the neonatal stage. β-cell proliferation was found to rely on cells of the islet microenvironment. We hypothesized that pericytes, which are components of the islet vasculature, support neonatal β-cell proliferation.

METHODS

To test our hypothesis, we combined in vivo and in vitro approaches. Briefly, we used a Diphtheria toxin-based transgenic mouse system to specifically deplete neonatal pancreatic pericytes in vivo. We further cultured neonatal pericytes isolated from the neonatal pancreas and combined the use of a β-cell line and primary cultured mouse β-cells.

RESULTS

Our findings indicate that neonatal pancreatic pericytes are required and sufficient for β-cell proliferation. We observed impaired proliferation of neonatal β-cells upon in vivo depletion of pancreatic pericytes. Furthermore, exposure to pericyte-conditioned medium stimulated proliferation in cultured β-cells.

CONCLUSIONS

This study introduces pancreatic pericytes as regulators of neonatal β-cell proliferation. In addition to advancing current understanding of the physiological β-cell replication process, these findings could facilitate the development of protocols aimed at expending these cells as a potential cure for diabetes.

Biphasic response as a mechanism against mutant takeover in tissue homeostasis circuits

Tissues use feedback circuits in which cells send signals to each other to control their growth and survival. We show that such feedback circuits are inherently unstable to mutants that misread the signal level: Mutants have a growth advantage to take over the tissue, and cannot be eliminated by known cell-intrinsic mechanisms. To resolve this, we propose that tissues have biphasic responses in and the signal is toxic at both high and low levels, such as glucotoxicity of beta cells, excitotoxicity in neurons, and toxicity of growth factors to T cells. This gives most of these mutants a frequency-dependent selective disadvantage, which leads to their elimination. However, the biphasic mechanisms create a new unstable fixed point in the feedback circuit beyond which runaway processes can occur, leading to risk of diseases such as diabetes and neurodegenerative disease. Hence, glucotoxicity, which is a dangerous cause of diabetes, may have a protective anti-mutant effect. Biphasic responses in tissues may provide an evolutionary stable strategy that avoids invasion by commonly occurring mutants, but at the same time cause vulnerability to disease.

Arginase-II Promotes Tumor Necrosis Factor-α Release From Pancreatic Acinar Cells Causing β-Cell Apoptosis in Aging

Aging is associated with glucose intolerance. Arginase-II (Arg-II), the type-II L-arginine-ureahydrolase, is highly expressed in pancreas. However, its role in regulation of pancreatic β-cell function is not known. Here we show that female (not male) mice deficient in Arg-II (Arg-II^-/-) are protected from age-associated glucose intolerance and reveal greater glucose induced-insulin release, larger islet size and β-cell mass, and more proliferative and less apoptotic β-cells compared with the age-matched wild-type (WT) controls. Moreover, Arg-II is mainly expressed in acinar cells and is upregulated with aging, which enhances p38 mitogen-activated protein kinase (p38 MAPK) activation and release of tumor necrosis factor-α (TNF-α). Accordingly, conditioned medium of isolated acinar cells from old WT (not Arg-II^-/-) mice contains higher TNF-α levels than the young mice and stimulates β-cell apoptosis and dysfunction, which are prevented by a neutralizing anti-TNF-α antibody. In acinar cells, our study demonstrates an age-associated Arg-II upregulation, which promotes TNF-α release through p38 MAPK leading to β-cell apoptosis, insufficient insulin secretion, and glucose intolerance in female rather than male mice.

Decrease in Ins+Glut2LO β-cells with advancing age in mouse and human pancreas

The presence and location of resident pancreatic β-cell progenitors is controversial. A subpopulation of insulin-expressing but glucose transporter-2-low (Ins⁺Glut2^LO) cells may represent multipotent pancreatic progenitors in adult mouse and in human islets, and they are enriched in small, extra-islet β-cell clusters (<5 β cells) in mice. Here, we sought to identify and compare the ontogeny of these cells in mouse and human pancreata throughout life. Mouse pancreata were collected at postnatal days 7, 14, 21, 28, and at 3, 6, 12, and 18 months of age, and in the first 28 days after β-cell mass depletion following streptozotocin (STZ) administration. Samples of human pancreas were examined during fetal life (22-30 weeks gestation), infancy (0-1 year), childhood (2-9), adolescence (10-17), and adulthood (18-80). Tissues were analyzed by immunohistochemistry for the expression and location of insulin, GLUT2 and Ki67. The proportion of β cells within clusters relative to that in islets was higher in pancreas of human than of mouse at all ages examined, and decreased significantly at adolescence. In mice, the total number of Ins⁺Glut2^LO cells decreased after 7 days concurrent with the proportion of clusters. These cells were more abundant in clusters than in islets in both species. A positive association existed between the appearance of new β cells after the STZ treatment of young mice, particularly in clusters and smaller islets, and an increased proportional presence of Ins⁺Glut2^LO cells during early β-cell regeneration. These data suggest that Ins⁺Glut2^LO cells are preferentially located within β-cell clusters throughout life in pancreas of mouse and human, and may represent a source of β-cell plasticity.

Forkhead Box Protein 1 (FoxO1) Inhibits Accelerated β Cell Aging in Pancreas-specific SMAD7 Mutant Mice

The mechanisms underlying the effects of exocrine dysfunction on the development of diabetes remain largely unknown. Here we show that pancreatic depletion of SMAD7 resulted in age-dependent increases in β cell dysfunction with accelerated glucose intolerance, followed by overt diabetes. The accelerated β cell dysfunction and loss of proliferation capacity, two features of β cell aging, appeared to be non-cell-autonomous, secondary to the adjacent exocrine failure as a "bystander effect." Increased Forkhead box protein 1 (FoxO1) acetylation and nuclear retention was followed by progressive FoxO1 loss in β cells that marked the onset of diabetes. Moreover, forced FoxO1 expression in β cells prevented β cell dysfunction and loss in this model. Thus, we present a model of accelerated β cell aging that may be useful for studying the mechanisms underlying β cell failure in diabetes. Moreover, we provide evidence highlighting a critical role of FoxO1 in maintaining β cell identity in the context of SMAD7 failure.

Individual Variation in Conditional β Cell Ablation Mice Contributes Significant Biases in Evaluating β Cell Functional Recovery

Despite the βDTA (Ins2-rtTA; Tet-DTA) mice have been developed as a valuable tool to study β cell regeneration, their individual variation in therapeutic efficacy has not been characterized. Here, we demonstrated that the βDTA mice exhibited significant variations in both spontaneous and acquired β cell regeneration. We found that doxycycline (DOX)-induced β cell death was sufficient to cause polydipsia, translating even subtle difference in drinking habit into large variations in actual DOX intake among individuals within the same group. Accumulating evidence shows that transient expression of VEGF_A enhances β cell functional recovery after injury. Therefore, we utilized the chemically modified mRNA (modRNA) technology to enable transient yet efficient VEGF_A expression in the pancreas after DOX-induced β cell death. Surprisingly, under optimized DOX dose permissive of β cell regeneration, VEGF_A modRNA only demonstrated marginal benefits on β cell functional recovery with large individual variations. We also revealed that the therapeutic efficacy of VEGF_A modRNA on β cell regeneration was dependent on the degree of β cell loss induced by the accumulated DOX intake. Therefore, our results highlight a significant contribution of individual variation in the βDTA model and call for attention in evaluating potential efficacy of therapeutic agents in β cell regeneration studies.

Single-Cell RNAseq Reveals That Pancreatic β-Cells From Very Old Male Mice Have a Young Gene Signature

Aging improves pancreatic β-cell function in mice. This is a surprising finding because aging is typically associated with functional decline. We performed single-cell RNA sequencing of β-cells from 3- and 26-month-old mice to explore how changes in gene expression contribute to improved function with age. The old mice were healthy and had reduced blood glucose levels and increased β-cell mass, which correlated to their body weight. β-Cells from young and old mice had similar transcriptome profiles. In fact, only 193 genes (0.89% of all detected genes) were significantly regulated (≥2-fold; false discovery rate < 0.01; normalized counts > 5). Of these, 183 were down-regulated and mainly associated with pathways regulating gene expression, cell cycle, cell death, and survival as well as cellular movement, function, and maintenance. Collectively our data show that β-cells from very old mice have transcriptome profiles similar to those of young mice. These data support previous findings that aging is not associated with reduced β-cell mass or functional β-cell decline in mice.

β-Cell dedifferentiation, reduced duct cell plasticity, and impaired β-cell mass regeneration in middle-aged rats

Limitations in β-cell regeneration potential in middle-aged animals could contribute to the increased risk to develop diabetes associated with aging. We investigated β-cell regeneration of middle-aged Wistar rats in response to two different regenerative stimuli: partial pancreatectomy (Px + V) and gastrin administration (Px + G). Pancreatic remnants were analyzed 3 and 14 days after surgery. β-Cell mass increased in young animals after Px and was further increased after gastrin treatment. In contrast, β-cell mass did not change after Px or after gastrin treatment in middle-aged rats. β-Cell replication and individual β-cell size were similarly increased after Px in young and middle-aged animals, and β-cell apoptosis was not modified. Nuclear immunolocalization of neurog3 or nkx6.1 in regenerative duct cells, markers of duct cell plasticity, was increased in young but not in middle-aged Px rats. The pancreatic progenitor-associated transcription factors neurog3 and sox9 were upregulated in islet β-cells of middle-aged rats and further increased after Px. The percentage of chromogranin A+/hormone islet cells was significantly increased in the pancreases of middle-aged Px rats. In summary, the potential for compensatory β-cell hyperplasia and hypertrophy was retained in middle-aged rats, but β-cell dedifferentiation and impaired duct cell plasticity limited β-cell regeneration.

Islet biology, the CDKN2A/B locus and type 2 diabetes risk

Type 2 diabetes, fuelled by the obesity epidemic, is an escalating worldwide cause of personal hardship and public cost. Diabetes incidence increases with age, and many studies link the classic senescence and ageing protein p16(INK4A) to diabetes pathophysiology via pancreatic islet biology. Genome-wide association studies (GWASs) have unequivocally linked the CDKN2A/B locus, which encodes p16 inhibitor of cyclin-dependent kinase (p16(INK4A)) and three other gene products, p14 alternate reading frame (p14(ARF)), p15(INK4B) and antisense non-coding RNA in the INK4 locus (ANRIL), with human diabetes risk. However, the mechanism by which the CDKN2A/B locus influences diabetes risk remains uncertain. Here, we weigh the evidence that CDKN2A/B polymorphisms impact metabolic health via islet biology vs effects in other tissues. Structured in a bedside-to-bench-to-bedside approach, we begin with a summary of the evidence that the CDKN2A/B locus impacts diabetes risk and a brief review of the basic biology of CDKN2A/B gene products. The main emphasis of this work is an in-depth look at the nuanced roles that CDKN2A/B gene products and related proteins play in the regulation of beta cell mass, proliferation and insulin secretory function, as well as roles in other metabolic tissues. We finish with a synthesis of basic biology and clinical observations, incorporating human physiology data. We conclude that it is likely that the CDKN2A/B locus influences diabetes risk through both islet and non-islet mechanisms.

Insulin-positive, Glut2-low cells present within mouse pancreas exhibit lineage plasticity and are enriched within extra-islet endocrine cell clusters

Regeneration of insulin-producing β-cells from resident pancreas progenitors requires an understanding of both progenitor identity and lineage plasticity. One model suggested that a rare β-cell sub-population within islets demonstrated multi-lineage plasticity. We hypothesized that β-cells from young mice (postnatal day 7, P7) exhibit such plasticity and used a model of islet dedifferentiation toward a ductal epithelial-cell phenotype to test this theory. RIPCre;Z/AP(+/+) mice were used to lineage trace the fate of β-cells during dedifferentiation culture by a human placental alkaline phosphatase (HPAP) reporter. There was a significant loss of HPAP-expressing β-cells in culture, but remaining HPAP(+) cells lost insulin expression while gaining expression of the epithelial duct cell marker cytokeratin-19 (Ck19). Flow cytometry and recovery of β-cell subpopulations from whole pancreas vs. islets suggest that the HPAP(+)Ck19(+) cells had derived from insulin-positive, glucose-transporter-2-low (Ins(+)Glut2(LO)) cells, representing 3.5% of all insulin-expressing cells. The majority of these cells were found outside of islets within clusters of <5 β-cells. These insulin(+)Glut2(LO) cells demonstrated a greater proliferation rate in vivo and in vitro as compared to insulin(+)Glut2(+) cells at P7, were retained into adulthood, and a subset differentiated into endocrine, ductal, and neural lineages, illustrating substantial plasticity. Results were confirmed using RIPCre;ROSA- eYFP mice. Quantitative PCR data indicated these cells possess an immature β-cell phenotype. These Ins(+)Glut2(LO) cells may represent a resident population of cells capable of forming new, functional β-cells, and which may be potentially exploited for regenerative therapies in the future.

Transient Suppression of TGFβ Receptor Signaling Facilitates Human Islet Transplantation

Although islet transplantation is an effective treatment for severe diabetes, its broad application is greatly limited due to a shortage of donor islets. Suppression of TGFβ receptor signaling in β-cells has been shown to increase β-cell proliferation in mice, but has not been rigorously examined in humans. Here, treatment of human islets with a TGFβ receptor I inhibitor, SB-431542 (SB), significantly improved C-peptide secretion by β-cells, and significantly increased β-cell number by increasing β-cell proliferation. In addition, SB increased cell-cycle activators and decreased cell-cycle suppressors in human β-cells. Transplantation of SB-treated human islets into diabetic immune-deficient mice resulted in significant improvement in blood glucose control, significantly higher serum and graft insulin content, and significantly greater increases in β-cell proliferation in the graft, compared with controls. Thus, our data suggest that transient suppression of TGFβ receptor signaling may improve the outcome of human islet transplantation, seemingly through increasing β-cell number and function.

PNA lectin for purifying mouse acinar cells from the inflamed pancreas

Better methods for purifying human or mouse acinar cells without the need for genetic modification are needed. Such techniques would be advantageous for the specific study of certain mechanisms, such as acinar-to-beta-cell reprogramming and pancreatitis. Ulex Europaeus Agglutinin I (UEA-I) lectin has been used to label and isolate acinar cells from the pancreas. However, the purity of the UEA-I-positive cell fraction has not been fully evaluated. Here, we screened 20 widely used lectins for their binding specificity for major pancreatic cell types, and found that UEA-I and Peanut agglutinin (PNA) have a specific affinity for acinar cells in the mouse pancreas, with minimal affinity for other major pancreatic cell types including endocrine cells, duct cells and endothelial cells. Moreover, PNA-purified acinar cells were less contaminated with mesenchymal and inflammatory cells, compared to UEA-I purified acinar cells. Thus, UEA-I and PNA appear to be excellent lectins for pancreatic acinar cell purification. PNA may be a better choice in situations where mesenchymal cells or inflammatory cells are significantly increased in the pancreas, such as type 1 diabetes, pancreatitis and pancreatic cancer.

Angptl4 links α-cell proliferation following glucagon receptor inhibition with adipose tissue triglyceride metabolism

Type 2 diabetes is characterized by a reduction in insulin function and an increase in glucagon activity that together result in hyperglycemia. Glucagon receptor antagonists have been developed as drugs for diabetes; however, they often increase glucagon plasma levels and induce the proliferation of glucagon-secreting α-cells. We find that the secreted protein Angiopoietin-like 4 (Angptl4) is up-regulated via Pparγ activation in white adipose tissue and plasma following an acute treatment with a glucagon receptor antagonist. Induction of adipose angptl4 and Angptl4 supplementation promote α-cell proliferation specifically. Finally, glucagon receptor antagonist improves glycemia in diet-induced obese angptl4 knockout mice without increasing glucagon levels or α-cell proliferation, underscoring the importance of this protein. Overall, we demonstrate that triglyceride metabolism in adipose tissue regulates α-cells in the endocrine pancreas.

A genetic mouse model for progressive ablation and regeneration of insulin producing beta-cells

The putative induction of adult β-cell regeneration represents a promising approach for the treatment of type 1 diabetes. Toward this ultimate goal, it is essential to develop an inducible model mimicking the long-lasting disease progression. In the current study, we have established a novel β-cell ablation mouse model, in which the β-cell mass progressively declines, as seen in type 1 diabetes. The model is based on the β-cell specific genetic ablation of the transcription initiation factor 1A, TIF-IA, essential for RNA Polymerase I activity (TIF-IA(Δ/Δ)). Using this approach, we induced a slow apoptotic response that eventually leads to a protracted β-cell death. In this model, we observed β-cell regeneration that resulted in a complete recovery of the β-cell mass and normoglycemia. In addition, we showed that adaptive proliferation of remaining β-cells is the prominent mechanism acting to compensate for the massive β-cell loss in young but also aged mice. Interestingly, at any age, we also detected β-like cells expressing the glucagon hormone, suggesting a transition between α- and β-cell identities or vice versa. Taken together, the TIF-IA(Δ/Δ) mouse model can be used to investigate the potential therapeutic approaches for type 1 diabetes targeting β-cell regeneration.

Connective tissue growth factor modulates adult β-cell maturity and proliferation to promote β-cell regeneration in mice

Stimulation of endogenous β-cell expansion could facilitate regeneration in patients with diabetes. In mice, connective tissue growth factor (CTGF) is expressed in embryonic β-cells and in adult β-cells during periods of expansion. We discovered that in embryos CTGF is necessary for β-cell proliferation, and increased CTGF in β-cells promotes proliferation of immature (MafA(-)) insulin-positive cells. CTGF overexpression, under nonstimulatory conditions, does not increase adult β-cell proliferation. In this study, we tested the ability of CTGF to promote β-cell proliferation and regeneration after partial β-cell destruction. β-Cell mass reaches 50% recovery after 4 weeks of CTGF treatment, primarily via increased β-cell proliferation, which is enhanced as early as 2 days of treatment. CTGF treatment increases the number of immature β-cells but promotes proliferation of both mature and immature β-cells. A shortened β-cell replication refractory period is also observed. CTGF treatment upregulates positive cell-cycle regulators and factors involved in β-cell proliferation, including hepatocyte growth factor, serotonin synthesis, and integrin β1. Ex vivo treatment of whole islets with recombinant human CTGF induces β-cell replication and gene expression changes consistent with those observed in vivo, demonstrating that CTGF acts directly on islets to promote β-cell replication. Thus, CTGF can induce replication of adult mouse β-cells given a permissive microenvironment.

Weaning triggers a maturation step of pancreatic β cells

Because tissue regeneration deteriorates with age, it is generally assumed that the younger the animal, the better it compensates for tissue damage. We have examined the effect of young age on compensatory proliferation of pancreatic β cells in vivo. Surprisingly, β cells in suckling mice fail to enter the cell division cycle in response to a diabetogenic injury or increased glycolysis. The potential of β cells for compensatory proliferation is acquired following premature weaning to normal chow, but not to a diet mimicking maternal milk. In addition, weaning coincides with enhanced glucose-stimulated oxidative phosphorylation and insulin secretion from islets. Transcriptome analysis reveals that weaning increases the expression of genes involved in replication licensing, suggesting a mechanism for increased responsiveness to the mitogenic activity of high glucose. We propose that weaning triggers a discrete maturation step of β cells, elevating both the mitogenic and secretory response to glucose.

Recent progress in studies of factors that elicit pancreatic β-cell expansion

The loss of or decreased functional pancreatic β-cell is a major cause of type 1 and type 2 diabetes. Previous studies have shown that adult β-cells can maintain their ability for a low level of turnover through replication and neogenesis. Thus, a strategy to prevent and treat diabetes would be to enhance the ability of β-cells to increase the mass of functional β-cells. Consequently, much effort has been devoted to identify factors that can effectively induce β-cell expansion. This review focuses on recent reports on small molecules and protein factors that have been shown to promote β-cell expansion.

Structural similarities and differences between the human and the mouse pancreas

Mice remain the most studied animal model in pancreas research. Since the findings of this research are typically extrapolated to humans, it is important to understand both similarities and differences between the 2 species. Beside the apparent difference in size and macroscopic organization of the organ in the 2 species, there are a number of less evident and only recently described differences in organization of the acinar and ductal exocrine tissue, as well as in the distribution, composition, and architecture of the endocrine islets of Langerhans. Furthermore, the differences in arterial, venous, and lymphatic vessels, as well as innervation are potentially important. In this article, the structure of the human and the mouse pancreas, together with the similarities and differences between them are reviewed in detail in the light of conceivable repercussions for basic research and clinical application.

Young capillary vessels rejuvenate aged pancreatic islets

Pancreatic islets secrete hormones that play a key role in regulating blood glucose levels (glycemia). Age-dependent impairment of islet function and concomitant dysregulation of glycemia are major health threats in aged populations. However, the major causes of the age-dependent decline of islet function are still disputed. Here we demonstrate that aging of pancreatic islets in mice and humans is notably associated with inflammation and fibrosis of islet blood vessels but does not affect glucose sensing and the insulin secretory capacity of islet beta cells. Accordingly, when transplanted into the anterior chamber of the eye of young mice with diabetes, islets from old mice are revascularized with healthy blood vessels, show strong islet cell proliferation, and fully restore control of glycemia. Our results indicate that beta cell function does not decline with age and suggest that islet function is threatened by an age-dependent impairment of islet vascular function. Strategies to mitigate age-dependent dysregulation in glycemia should therefore target systemic and/or local inflammation and fibrosis of the aged islet vasculature.

Chronic glucokinase activator treatment at clinically translatable exposures gives durable glucose lowering in two animal models of type 2 diabetes

BACKGROUND AND PURPOSE

Pharmacological activation of glucokinase (GK) lowers blood glucose in animal models and humans, confirming proof of concept for this mechanism. However, recent clinical evidence from chronic studies suggests that the glucose-lowering effects mediated by glucokinase activators (GKAs) are not maintained in patients with type 2 diabetes (T2D). Existing preclinical data with GKAs do not explain this loss of sustained glucose-lowering efficacy in patients. Here, we have assessed the effects of chronic (up to 11 months) treatment with two different GKAs in two models of T2D.

EXPERIMENTAL APPROACH

Two validated animal models of T2D, insulin-resistant obese Zucker rats and hyperglycaemic gk(wt/del) mice, were treated with two different GKAs for 1 or 11 months respectively at exposures that translate to clinical exposures in humans. Blood glucose, cholesterol, triglycerides and insulin were measured. GKA pharmacokinetics were also determined.

KEY RESULTS

Treatment with either GKA provided sustained lowering of blood glucose for up to 1 month in the Zucker rat and up to 11 months in hyperglycaemic gk(wt/del) mice, with maintained compound exposures. This efficacy was achieved without increases in plasma or hepatic triglycerides, accumulation of hepatic glycogen or impairment of glucose-stimulated insulin secretion.

CONCLUSIONS AND IMPLICATIONS

Chronic treatment with two GKAs in two animal models of diabetes provided sustained lowering of blood glucose, in marked contrast to clinical findings. Therefore, either these animal models of T2D are not good predictors of responses in human T2D or we need a better understanding of the consequences of GK activation in humans.

In vitro differentiation and expansion of human pluripotent stem cell-derived pancreatic progenitors

Recent progress in understanding stem cell biology has been remarkable, especially in deciphering signals that support differentiation towards tissue-specific lineages. This achievement positions us firmly at the beginning of an era of patient-specific regenerative medicine and human disease modeling. It will be necessary to equip the progress in this era with a reliable source of self-renewing progenitor cells that differentiate into functional target cells. The generation of pancreatic progenitors that mature in vivo into functional beta-cells has raised the hope for new therapeutic options in diabetes, but key challenges still remain including the production of sufficient numbers of cells for research and transplantation. Recent approaches to this problem have shown that the presence of organ- and stage-specific mesenchyme improves the generation of progenitors, from endoderm to endocrine cells. Alternatively, utilization of three-dimensional culture may improve the efficiency and yield of directed differentiation. Here, we review the current knowledge of pancreatic directed differentiation and ex vivo expansion of pancreatic progenitors, including recent advances in differentiation strategies for the generation of pancreatic progenitors, and we discuss persistent challenges which will need to be overcome before personalized cell-based therapy becomes a practical strategy.

Human β-cell regeneration: progress, hurdles, and controversy

PURPOSE OF REVIEW

Therapies that increase functional β-cell mass may be the best long-term treatment for diabetes. Significant resources are devoted toward this goal, and progress is occurring at a rapid pace. Here, we summarize recent advances relevant to human β-cell regeneration.

RECENT FINDINGS

New β-cells arise from proliferation of pre-existing β-cells or transdifferentiation from other cell types. In addition, dedifferentiated β-cells may populate islets in diabetes, possibly representing a pool of cells that could redifferentiate into functional β-cells. Advances in finding strategies to drive β-cell proliferation include new insight into proproliferative factors, both circulating and local, and elements intrinsic to the β-cell, such as cell cycle machinery and regulation of gene expression through epigenetic modification and noncoding RNAs. Controversy continues in the arena of generation of β-cells by transdifferentiation from exocrine, ductal, and alpha cells, with studies producing both supporting and opposing data. Progress has been made in redifferentiation of β-cells that have lost expression of β-cell markers.

SUMMARY

Although significant progress has been made, and promising avenues exist, more work is needed to achieve the goal of β-cell regeneration as a treatment for diabetes.

Adult tissue sources for new β cells

The diabetes pandemic incurs extraordinary public health and financial costs that are projected to expand for the foreseeable future. Consequently, the development of definitive therapies for diabetes is a priority. Currently, a wide spectrum of therapeutic strategies-from implantable insulin delivery devices to transplantation-based cell replacement therapy, to β-cell regeneration-focus on replacing the lost insulin-producing capacity of individuals with diabetes. Among these, β-cell regeneration remains promising but heretofore unproved. Indeed, recent experimental work has uncovered surprising biology that underscores the potential therapeutic benefit of β-cell regeneration. These studies have elucidated a variety of sources for the endogenous production of new β cells from existing cells. First, β cells, long thought to be postmitotic, have demonstrated the potential for regenerative capacity. Second, the presence of pancreatic facultative endocrine progenitor cells has been established. Third, the malleability of cellular identity has availed the possibility of generating β cells from other differentiated cell types. Here, we review the exciting developments surrounding endogenous sources of β-cell production and consider the potential of realizing a regenerative therapy for diabetes from adult tissues.

G0-G1 transition and the restriction point in pancreatic β-cells in vivo

Most of our knowledge on cell kinetics stems from in vitro studies of continuously dividing cells. In this study, we determine in vivo cell-cycle parameters of pancreatic β-cells, a largely quiescent population, using drugs that mimic or prevent glucose-induced replication of β-cells in mice. Quiescent β-cells exposed to a mitogenic glucose stimulation require 8 h to enter the G1 phase of the cell cycle, and this time is prolonged in older age. The duration of G1, S, and G2/M is ~5, 8, and 6 h, respectively. We further provide the first in vivo demonstration of the restriction point at the G0-G1 transition, discovered by Arthur Pardee 40 years ago. The findings may have pharmacodynamic implications in the design of regenerative therapies aimed at increasing β-cell replication and mass in patients with diabetes.

Glucose regulates rat beta cell number through age-dependent effects on beta cell survival and proliferation

Background

Glucose effects on beta cell survival and DNA-synthesis suggest a role as regulator of beta cell mass but data on beta cell numbers are lacking. We examined outcome of these influences on the number of beta cells isolated at different growth stages in their population.

Methods

Beta cells from neonatal, young-adult and old rats were cultured serum-free for 15 days. Their number was counted by automated whole-well imaging distinguishing influences on cell survival and on proliferative activity.

Results

Elevated glucose (10–20 versus 5 mmol/l) increased the number of living beta cells from 8-week rats to 30%, following a time- and concentration-dependent recruitment of quiescent cells into DNA-synthesis; a glucokinase-activator lowered the threshold but did not raise total numbers of glucose-recruitable cells. No glucose-induced increase occurred in beta cells from 40-week rats. Neonatal beta cells doubled in number at 5 mmol/l involving a larger activated fraction that did not increase at higher concentrations; however, their higher susceptibility to glucose toxicity at 20 mmol/l resulted in 20% lower living cell numbers than at start. None of the age groups exhibited a repetitively proliferating subpopulation.

Conclusions

Chronically elevated glucose levels increased the number of beta cells from young-adult but not from old rats; they interfered with expansion of neonatal beta cells and reduced their number. These effects are attributed to age-dependent differences in basal and glucose-induced proliferative activity and in cellular susceptibility to glucose toxicity. They also reflect age-dependent variations in the functional heterogeneity of the rat beta cell population.