Differential regulation of embryonic and adult β cell replication

Context-dependent effects of CCN2 on β-cell mass expansion and indicators of cell stress in the setting of acute and chronic stress

Stimulation of functional β-cell mass expansion can be beneficial for the treatment of type 2 diabetes. Our group has previously demonstrated that the matricellular protein CCN2 can induce β-cell mass expansion during embryogenesis, and postnatally during pregnancy and after 50% β-cell injury. The mechanism by which CCN2 stimulates β-cell mass expansion is unknown. However, CCN2 does not induce β-cell proliferation in the setting of euglycemic and optimal functional β-cell mass. We thus hypothesized that β-cell stress is required for responsiveness to CCN2 treatment. In this study, a doxycycline-inducible β-cell-specific CCN2 transgenic mouse model was utilized to evaluate the effects of CCN2 on β-cell stress in the setting of acute (thapsigargin treatment ex vivo) or chronic [high-fat diet or leptin receptor haploinsufficiency (db/+) in vivo] cellular stress. CCN2 induction during 1 wk or 10 wk of high-fat diet or in db/+ mice had no effect on markers of β-cell stress. However, CCN2 induction did result in a significant increase in β-cell mass over high-fat diet alone when animals were fed high-fat diet for 10 wk, a duration known to induce insulin resistance. CCN2 induction in isolated islets treated with thapsigargin ex vivo resulted in upregulation of the gene encoding the Nrf2 transcription factor, a master regulator of antioxidant genes, suggesting that CCN2 further activates this pathway in the presence of cell stress. These studies indicate that the potential of CCN2 to induce β-cell mass expansion is context-dependent and that the presence of β-cell stress does not ensure β-cell proliferation in response to CCN2.NEW & NOTEWORTHY CCN2 promotes β-cell mass expansion in settings of suboptimal β-cell mass. Here, we demonstrate that the ability of CCN2 to induce β-cell mass expansion in the setting of β-cell stress is context-dependent. Our results suggest that β-cell stress is necessary but insufficient for CCN2 to increase β-cell proliferation and mass. Furthermore, we found that CCN2 promotes upregulation of a key antioxidant transcription factor, suggesting that modulation of β-cell oxidative stress contributes to the actions of CCN2.

Emerging diabetes therapies: Bringing back the β-cells

BACKGROUND

Stem cell therapies are finally coming of age as a viable alternative to pancreatic islet transplantation for the treatment of insulin-dependent diabetes. Several clinical trials using human embryonic stem cell (hESC)-derived β-like cells are currently underway, with encouraging preliminary results. Remaining challenges notwithstanding, these strategies are widely expected to reduce our reliance on human isolated islets for transplantation procedures, making cell therapies available to millions of diabetic patients. At the same time, advances in our understanding of pancreatic cell plasticity and the molecular mechanisms behind β-cell replication and regeneration have spawned a multitude of translational efforts aimed at inducing β-cell replenishment in situ through pharmacological means, thus circumventing the need for transplantation.

SCOPE OF REVIEW

We discuss here the current state of the art in hESC transplantation, as well as the parallel quest to discover agents capable of either preserving the residual mass of β-cells or inducing their proliferation, transdifferentiation or differentiation from progenitor cells.

MAJOR CONCLUSIONS

Stem cell-based replacement therapies in the mold of islet transplantation are already around the corner, but a permanent cure for type 1 diabetes will likely require the endogenous regeneration of β-cells aided by interventions to restore the immune balance. The promise of current research avenues and a strong pipeline of clinical trials designed to tackle these challenges bode well for the realization of this goal.

Multifunctional regulatory protein connective tissue growth factor (CTGF): A potential therapeutic target for diverse diseases

Connective tissue growth factor (CTGF), a multifunctional protein of the CCN family, regulates cell proliferation, differentiation, adhesion, and a variety of other biological processes. It is involved in the disease-related pathways such as the Hippo pathway, p53 and nuclear factor kappa-B (NF-κB) pathways and thus contributes to the developments of inflammation, fibrosis, cancer and other diseases as a downstream effector. Therefore, CTGF might be a potential therapeutic target for treating various diseases. In recent years, the research on the potential of CTGF in the treatment of diseases has also been paid more attention. Several drugs targeting CTGF (monoclonal antibodies FG3149 and FG3019) are being assessed by clinical or preclinical trials and have shown promising outcomes. In this review, the cellular events regulated by CTGF, and the relationships between CTGF and pathogenesis of diseases are systematically summarized. In addition, we highlight the current researches, focusing on the preclinical and clinical trials concerned with CTGF as the therapeutic target.

Mechanisms Underlying the Expansion and Functional Maturation of β-Cells in Newborns: Impact of the Nutritional Environment

The functional maturation of insulin-secreting β-cells is initiated before birth and is completed in early postnatal life. This process has a critical impact on the acquisition of an adequate functional β-cell mass and on the capacity to meet and adapt to insulin needs later in life. Many cellular pathways playing a role in postnatal β-cell development have already been identified. However, single-cell transcriptomic and proteomic analyses continue to reveal new players contributing to the acquisition of β-cell identity. In this review, we provide an updated picture of the mechanisms governing postnatal β-cell mass expansion and the transition of insulin-secreting cells from an immature to a mature state. We then highlight the contribution of the environment to β-cell maturation and discuss the adverse impact of an in utero and neonatal environment characterized by calorie and fat overload or by protein deficiency and undernutrition. Inappropriate nutrition early in life constitutes a risk factor for developing diabetes in adulthood and can affect the β-cells of the offspring over two generations. A better understanding of these events occurring in the neonatal period will help developing better strategies to produce functional β-cells and to design novel therapeutic approaches for the prevention and treatment of diabetes.

Mitogen Synergy: An Emerging Route to Boosting Human Beta Cell Proliferation

Decreased number and function of beta cells are a key aspect of diabetes mellitus (diabetes), a disease that remains an onerous global health problem. Means of restoring beta cell mass are urgently being sought as a potential cure for diabetes. Several strategies, such as de novo beta cell derivation via pluripotent stem cell differentiation or mature somatic cell transdifferentiation, have yielded promising results. Beta cell expansion is another promising strategy, rendered challenging by the very low proliferative capacity of beta cells. Many effective mitogens have been identified in rodents, but the vast majority do not have similar mitogenic effects in human beta cells. Extensive research has led to the identification of several human beta cell mitogens, but their efficacy and specificity remain insufficient. An approach based on the simultaneous application of several mitogens has recently emerged and can yield human beta cell proliferation rates of up to 8%. Here, we discuss recent advances in restoration of the beta cell population, focusing on mitogen synergy, and the contribution of RNA-sequencing (RNA-seq) to accelerating the elucidation of signaling pathways in proliferating beta cells and the discovery of novel mitogens. Together, these approaches have taken beta cell research up a level, bringing us closer to a cure for diabetes.

Circadian Regulation of the Pancreatic Beta Cell

Beta cell dysfunction is central to the development of type 2 diabetes (T2D). In T2D, environmental and genetic influences can manifest beta cell dysfunction in many ways, including impaired glucose-sensing and secretion coupling mechanisms, insufficient adaptative responses to stress, and aberrant beta cell loss through increased cell death and/or beta cell de-differentiation. In recent years, circadian disruption has emerged as an important environmental risk factor for T2D. In support of this, genetic disruption of the circadian timing system in rodents impairs insulin secretion and triggers diabetes development, lending important evidence that the circadian timing system is intimately connected to, and essential for the regulation of pancreatic beta cell function; however, the role of the circadian timing system in the regulation of beta cell biology is only beginning to be unraveled. Here, we review the recent literature that explores the importance of the pancreatic islet/beta cell circadian clock in the regulation of various aspects of beta cell biology, including transcriptional and functional control of daily cycles of insulin secretion capacity, regulation of postnatal beta cell maturation, and control of the adaptive responses of the beta cell to metabolic stress and acute injury.

Evidence of a developmental origin for β-cell heterogeneity using a dual lineage-tracing technology

The Cre/loxP system has been used extensively in mouse models with a limitation of one lineage at a time. Differences in function and other properties among populations of adult β-cells is termed β-cell heterogeneity, which was recently associated with diabetic phenotypes. Nevertheless, the presence of a developmentally derived β-cell heterogeneity is unclear. Here, we have developed a novel dual lineage-tracing technology, using a combination of two recombinase systems, Dre/RoxP and Cre/LoxP, to independently trace green fluorescent Pdx1-lineage cells and red fluorescent Ptf1a-lineage cells in the developing and adult mouse pancreas. We detected a few Pdx1⁺/Ptf1a^- lineage cells in addition to the vast majority of Pdx1⁺/Ptf1a⁺ lineage cells in the pancreas. Moreover, Pdx1⁺/Ptf1a⁺ lineage β-cells had fewer Ki-67⁺ proliferating β-cells, and expressed higher mRNA levels of insulin, Glut2, Pdx1, MafA and Nkx6.1, but lower CCND1 and CDK4 levels, compared with Pdx1⁺/Ptf1a^- lineage β-cells. Furthermore, more TSQ-high, SSC-high cells were detected in the Pdx1⁺Ptf1a⁺ lineage population than in the Pdx1⁺Ptf1a^- lineage population. Together, these data suggest that differential activation of Ptf1a in the developing pancreas may correlate with this β-cell heterogeneity.

Gestational exposure to metformin programs improved glucose tolerance and insulin secretion in adult male mouse offspring

Pancreatic β-cells are exquisitely sensitive to developmental nutrient stressors, and alterations in nutrient sensing pathways may underlie changes observed in these models. Here we developed a mouse model of in utero exposure to the anti-diabetic agent metformin. We have previously shown that this exposure increases offspring pancreatic β-cell mass at birth. We hypothesized that adult offspring would have improved metabolic parameters as a long-term outcome of metformin exposure. Virgin dams were given 5 mg/mL metformin in their water from E0.5 to delivery at E18.5. Body weight, glucose tolerance, insulin tolerance and glucose stimulated insulin secretion were analyzed in the offspring. When male offspring of dams given metformin during gestation were tested as adults they had improved glucose tolerance and enhanced insulin secretion in vivo as did their islets in vitro. Enhanced insulin secretion was accompanied by changes in intracellular free calcium responses to glucose and potassium chloride, possibly mediated by increased L channel expression. Female offspring exhibited improved glucose tolerance at advanced ages. In conclusion, in this model in utero metformin exposure leads to improved offspring metabolism in a gender-specific manner. These findings suggest that metformin applied during gestation may be an option for reprogramming metabolism in at risk groups.

Long-Term Outcomes of Hyperglycemic Preterm Infants Randomized to Tight Glycemic Control

OBJECTIVE

To determine whether tight glycemic control of neonatal hyperglycemia changes neurodevelopment, growth, and metabolism at school age.

STUDY DESIGN

Children born very low birth weight and randomized as hyperglycemic neonates to a trial of tight vs standard glycemic control were assessed at 7 years corrected age, including Wechsler Intelligence Scale for Children Fourth Edition, Movement Assessment Battery for Children 2, visual and neurologic examinations, growth measures, dual X-ray absorptiometry, and frequently sampled intravenous glucose tolerance test. The primary outcome was survival without neurodevelopmental impairment at age 7 years. Outcomes were compared using linear regression, adjusted for sex, small for gestational age, birth plurality, and the clustering of twins. Data are reported as number (%) or mean (SD).

RESULTS

Of the 88 infants randomized, 11 (13%) had died and 57 (74% of eligible children) were assessed at corrected age 7 years. Survival without neurodevelopmental impairment occurred in 25 of 68 children (37%), with no significant difference between tight (14 of 35; 40%) and standard (11 of 33; 33%) glycemic control groups (P = .60). Children in the tight group were shorter than those in the standard group (121.3 [6.3] cm vs 125.1 [5.4] cm; P < .05), but had similar weight and head circumference. Children in the tight group had greater height-adjusted lean mass (18.7 [0.3] vs 17.6 [0.2] kg; P < .01) and lower fasting glucose concentrations (84.6 [6.30] vs 90.0 [5.6] mg⋅dL^-1; P < .05), but no other differences in measures of body composition or insulin-glucose metabolism.

CONCLUSION

Tight glycemic control for neonatal hyperglycemia does not change survival without neurodevelopmental impairment, but reduces height, increases height-adjusted lean mass, and reduces fasting blood glucose concentrations at school age.

TRIAL REGISTRATION

ACTRN: 12606000270516.

β-cell insulin receptor deficiency during in utero development induces an islet compensatory overgrowth response

The presence of insulin receptor (IR) on β-cells suggests that insulin has an autocrine/paracrine role in the regulation of β-cell function. It has previously been reported that the β-cell specific loss of IR (βIRKO) leads to the development of impaired glycemic regulation and β-cell death in mice. However, temporally controlled βIRKO induced during the distinct transitions of fetal pancreas development has yet to be investigated. We hypothesized that the presence of IR on β-cells during the 2nd transition phase of the fetal murine pancreas is required for maintaining normal islet development.We utilized a mouse insulin 1 promoter driven tamoxifen-inducible Cre-recombinase IR knockout (MIP-βIRKO) mouse model to investigate the loss of β-cell IR during pancreatic development at embryonic day (e) 13, a phase of endocrine proliferation and β-cell fate determination. Fetal pancreata examined at e19-20 showed significantly reduced IR levels in the β-cells of MIP-βIRKO mice. Morphologically, MIP-βIRKO pancreata exhibited significantly enlarged islet size with increased β-cell area and proliferation. MIP-βIRKO pancreata also displayed significantly increased Igf-2 protein level and Akt activity with a reduction in phospho-p53 when compared to control littermates. Islet vascular formation and Vegf-a protein level was significantly increased in MIP-βIRKO pancreata.Our results demonstrate a developmental role for the β-cell IR, whereby its loss leads to an islet compensatory overgrowth, and contributes further information towards elucidating the temporally sensitive signaling during β-cell commitment.

Naringin Ameliorates Streptozotocin-Induced Diabetes through Forkhead Box M1-Mediated Beta Cell Proliferation

BACKGROUND

Adult pancreatic beta cells, though quiescent, can proliferate in response to physiological need. This inherent character is used in exploring the possibilities of expanding the beta cell mass in the treatment of diabetes. Forkhead box M1 (FoxM1) transcription factor is an important regulator in the proliferation and survival of adult beta cell mass. Naringin, a flavanone glycoside, is reported to have antidiabetic activity and exhibited an increase in insulin levels in diabetic animals.

OBJECTIVES

The present study tried to evaluate the role of naringin in the regulation of FoxM1 in the pancreas of diabetic rats and to reascertain its antilipidemic and antioxidant properties.

METHODS

Diabetes was induced in male rats using streptozotocin and treated with naringin (100 mg/kg) orally for 4 and 8 weeks. Serum biochemical parameters, insulin, gene and protein expression of FoxM1, and antioxidant markers in rat pancreas were analyzed.

RESULTS

Naringin administration reduced the blood sugar, urea, creatinine, and cholesterol values and improved the pancreatic antioxidant status in diabetic rats. Naringin-treated diabetic rats showed a significant increase in mRNA and protein expression of FoxM1 compared to the diabetic control rats, indicating regeneration of cells. It also increased the insulin immunopositive cells, indicating functional beta cells.

CONCLUSION

Naringin was found to upregulate the FoxM1 transcription factor in diabetic animals, which influenced the proliferation and functional status of beta cells.

Involvement of dying beta cell originated messenger molecules in differentiation of pancreatic mesenchymal stem cells under glucotoxic and glucolipotoxic conditions

Beta cell mass regulation represents a critical issue for understanding and treatment of diabetes. The most important process in the development of diabetes is beta cell death, generally induced by glucotoxicity or glucolipotoxicity, and the regeneration mechanism of new beta cells that will replace dead beta cells is still not fully understood. The aim of this study was to investigate the generation mechanism of new beta cells by considering the compensation phase of type 2 diabetes mellitus. In this study, pancreatic islet derived mesenchymal stem cells (PI-MSCs) were isolated from adult rats and characterized. Then, beta cells isolated from rats were co-cultured with PI-MSCs and they were exposed to glucotoxicity, lipotoxicity and glucolipotoxicity conditions for 72 hr. As the results apoptotic and necrotic cell death were increased in both PI-MSCs and beta cells especially by the exposure of glucotoxic and glucolipotoxic conditions to the co-culture systems. Glucotoxicity induced-differentiated beta cells were functional due to their capability of insulin secretion in response to rising glucose concentrations. Moreover, beta cell proliferation was induced in the glucotoxicity-treated co-culture system whereas suppressed in lipotoxicity or glucolipotoxicity-treated co-culture systems. In addition, 11 novel proteins, that may release from dead beta cells and have the ability to stimulate PI-MSCs in the direction of differentiation, were determined in media of glucotoxicity or glucolipotoxicity-treated co-culture systems. In conclusion, these molecules were considered as important for understanding cellular mechanism of beta cell differentiation and diabetes. Thus, they may be potential targets for diagnosis and cellular or therapeutic treatment of diabetes.

New insights into non-conventional epitopes as T cell targets: The missing link for breaking immune tolerance in autoimmune disease?

The mechanism by which immune tolerance is breached in autoimmune disease is poorly understood. One possibility is that post-translational modification of self-antigens leads to peripheral recognition of neo-epitopes against which central and peripheral tolerance is inadequate. Accumulating evidence points to multiple mechanisms through which non-germline encoded sequences can give rise to these non-conventional epitopes which in turn engage the immune system as T cell targets. In particular, where these modifications alter the rules of epitope engagement with MHC molecules, such non-conventional epitopes offer a persuasive explanation for associations between specific HLA alleles and autoimmune diseases. In this review article, we discuss current understanding of mechanisms through which non-conventional epitopes may be generated, focusing on several recently described pathways that can transpose germline-encoded sequences. We contextualise these discoveries around type 1 diabetes, the prototypic organ-specific autoimmune disease in which specific HLA-DQ molecules confer high risk. Non-conventional epitopes have the potential to act as tolerance breakers or disease drivers in type 1 diabetes, prompting a timely re-evaluation of models of a etiopathogenesis. Future studies are required to elucidate the disease-relevance of a range of potential non-germline epitopes and their relationship to the natural peptide repertoire.

Direct effect of glucocorticoids on glucose-activated adult rat β-cells increases their cell number and their functional mass for transplantation

Compounds that increase β-cell number can serve as β-cell replacement therapies in diabetes. In vitro studies have identified several agents that can activate DNA synthesis in primary β-cells but only in small percentages of cells and without demonstration of increases in cell number. We used whole well multiparameter imaging to first screen a library of 1,280 compounds for their ability to recruit adult rat β-cells into DNA synthesis and then assessed influences of stimulatory agents on the number of living cells. The four compounds with highest β-cell recruitment were glucocorticoid (GC) receptor ligands. The GC effect occurred in glucose-activated β-cells and was associated with increased glucose utilization and oxidation. Hydrocortisone and methylprednisolone almost doubled the number of β-cells in 2 wk. The expanded cell population provided an increased functional β-cell mass for transplantation in diabetic animals. These effects are age dependent; they did not occur in neonatal rat β-cells, where GC exposure suppressed basal replication and was cytotoxic. We concluded that GCs can induce the replication of adult rat β-cells through a direct action, with intercellular differences in responsiveness that have been related to differences in glucose activation and in age. These influences can explain variability in GC-induced activation of DNA synthesis in rat and human β-cells. Our study also demonstrated that β-cells can be expanded in vitro to increase the size of metabolically adequate grafts.

Epidermal Growth Factor Receptor Signaling Regulates β Cell Proliferation in Adult Mice

A thorough understanding of the signaling pathways involved in the regulation of β cell proliferation is an important initial step in restoring β cell mass in the diabetic patient. Here, we show that epidermal growth factor receptor 1 (EGFR) was significantly up-regulated in the islets of C57BL/6 mice after 50% partial pancreatectomy (PPx), a model for workload-induced β cell proliferation. Specific deletion of EGFR in the β cells of adult mice impaired β cell proliferation at baseline and after 50% PPx, suggesting that the EGFR signaling pathway plays an essential role in adult β cell proliferation. Further analyses showed that β cell-specific depletion of EGFR resulted in impaired expression of cyclin D1 and impaired suppression of p27 after PPx, both of which enhance β cell proliferation. These data highlight the importance of EGFR signaling and its downstream signaling cascade in postnatal β cell growth.

Pancreatic β Cell Mass Death

Type two diabetes (T2D) is a challenging metabolic disorder for which a cure has not yet been found. Its etiology is associated with several phenomena, including significant loss of insulin-producing, beta cell (β cell) mass via progressive programmed cell death and disrupted cellular autophagy. In diabetes, the etiology of β cell death and the role of mitochondria are complex and involve several layers of mechanisms. Understanding the dynamics of those mechanisms could permit researchers to develop an intervention for the progressive loss of β cells. Currently, diabetes research has shifted toward rejuvenation and plasticity technology and away from the simplified approach of hormonal compensation. Diabetes research is currently challenged by questions such as how to enhance cell survival, decrease apoptosis and replenish β cell mass in diabetic patients. In this review, we discuss evidence that β cell development and mass formation are guided by specific signaling systems, particularly hormones, transcription factors, and growth factors, all of which could be manipulated to enhance mass growth. There is also strong evidence that β cells are dynamically active cells, which, under specific conditions such as obesity, can increase in size and subsequently increase insulin secretion. In certain cases of aggressive or advanced forms of T2D, β cells become markedly impaired, and the only alternatives for maintaining glucose homeostasis are through partial or complete cell grafting (the Edmonton protocol). In these cases, the harvesting of an enriched population of viable β cells is required for transplantation. This task necessitates a deep understanding of the pharmacological agents that affect β cell survival, mass, and function. The aim of this review is to initiate discussion about the important signals in pancreatic β cell development and mass formation and to highlight the process by which cell death occurs in diabetes. This review also examines the attempts that have been made to recover or increase cell mass in diabetic patients by using various pharmacological agents.

Yap1 plays a protective role in suppressing free fatty acid-induced apoptosis and promoting beta-cell survival

Mammalian pancreatic β-cells play a pivotal role in development and glucose homeostasis through the production and secretion of insulin. Functional failure or decrease in β-cell number leads to type 2 diabetes (T2D). Despite the physiological importance of β-cells, the viability of β-cells is often challenged mainly due to its poor ability to adapt to their changing microenvironment. One of the factors that negatively affect β-cell viability is high concentration of free fatty acids (FFAs) such as palmitate. In this work, we demonstrated that Yes-associated protein (Yap1) is activated when β-cells are treated with palmitate. Our loss- and gain-of-function analyses using rodent insulinoma cell lines revealed that Yap1 suppresses palmitate-induced apoptosis in β-cells without regulating their proliferation. We also found that upon palmitate treatment, re-arrangement of F-actin mediates Yap1 activation. Palmitate treatment increases expression of one of the Yap1 target genes, connective tissue growth factor (CTGF). Our gain-of-function analysis with CTGF suggests CTGF may be the downstream factor of Yap1 in the protective mechanism against FFA-induced apoptosis.

Statistical approaches and software for clustering islet cell functional heterogeneity

Worldwide efforts are underway to replace or repair lost or dysfunctional pancreatic β-cells to cure diabetes. However, it is unclear what the final product of these efforts should be, as β-cells are thought to be heterogeneous. To enable the analysis of β-cell heterogeneity in an unbiased and quantitative way, we developed model-free and model-based statistical clustering approaches, and created new software called TraceCluster. Using an example data set, we illustrate the utility of these approaches by clustering dynamic intracellular Ca(2+) responses to high glucose in ∼300 simultaneously imaged single islet cells. Using feature extraction from the Ca(2+) traces on this reference data set, we identified 2 distinct populations of cells with β-like responses to glucose. To the best of our knowledge, this report represents the first unbiased cluster-based analysis of human β-cell functional heterogeneity of simultaneous recordings. We hope that the approaches and tools described here will be helpful for those studying heterogeneity in primary islet cells, as well as excitable cells derived from embryonic stem cells or induced pluripotent cells.

Exploiting Expression of Hippo Effector, Yap, for Expansion of Functional Islet Mass

Loss of pancreas β-cell function is the precipitating factor in all forms of diabetes. Cell replacement therapies, such as islet transplantation, remain the best hope for a cure; however, widespread implementation of this method is hampered by availability of donor tissue. Thus, strategies that expand functional β-cell mass are crucial for widespread usage in diabetes cell replacement therapy. Here, we investigate the regulation of the Hippo-target protein, Yes-associated protein (Yap), during development of the endocrine pancreas and its function after reactivation in human cadaveric islets. Our results demonstrate that Yap expression is extinguished at the mRNA level after neurogenin-3-dependent specification of the pancreas endocrine lineage, correlating with proliferation decreases in these cells. Interestingly, when a constitutively active form of Yap was expressed in human cadaver islets robust increases in proliferation were noted within insulin-producing β-cells. Importantly, proliferation in these cells occurs without negatively affecting β-cell differentiation or functional status. Finally, we show that the proproliferative mammalian target of rapamycin pathway is activated after Yap expression, providing at least one explanation for the observed increases in β-cell proliferation. Together, these results provide a foundation for manipulating Yap activity as a novel approach to expand functional islet mass for diabetes regenerative therapy.

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.

Estrogen Receptor α Regulates β-Cell Formation During Pancreas Development and Following Injury

Identifying pathways for β-cell generation is essential for cell therapy in diabetes. We investigated the potential of 17β-estradiol (E2) and estrogen receptor (ER) signaling for stimulating β-cell generation during embryonic development and in the severely injured adult pancreas. E2 concentration, ER activity, and number of ERα transcripts were enhanced in the pancreas injured by partial duct ligation (PDL) along with nuclear localization of ERα in β-cells. PDL-induced proliferation of β-cells depended on aromatase activity. The activation of Neurogenin3 (Ngn3) gene expression and β-cell growth in PDL pancreas were impaired when ERα was turned off chemically or genetically (ERα(-/-)), whereas in situ delivery of E2 promoted β-cell formation. In the embryonic pancreas, β-cell replication, number of Ngn3(+) progenitor cells, and expression of key transcription factors of the endocrine lineage were decreased by ERα inactivation. The current study reveals that E2 and ERα signaling can drive β-cell replication and formation in mouse pancreas.

Asparaginase treatment side-effects may be due to genes with homopolymeric Asn codons (Review-Hypothesis)

The present treatment of childhood T-cell leukemias involves the systemic administration of prokary-otic L-asparaginase (ASNase), which depletes plasma Asparagine (Asn) and inhibits protein synthesis. The mechanism of therapeutic action of ASNase is poorly understood, as are the etiologies of the side-effects incurred by treatment. Protein expression from genes bearing Asn homopolymeric coding regions (N-hCR) may be particularly susceptible to Asn level fluctuation. In mammals, N-hCR are rare, short and conserved. In humans, misfunctions of genes encoding N-hCR are associated with a cluster of disorders that mimic ASNase therapy side-effects which include impaired glycemic control, dislipidemia, pancreatitis, compromised vascular integrity, and neurological dysfunction. This paper proposes that dysregulation of Asn homeostasis, potentially even by ASNase produced by the microbiome, may contribute to several clinically important syndromes by altering expression of N-hCR bearing genes. By altering amino acid abundance and modulating ribosome translocation rates at codon repeats, the microbiomic environment may contribute to genome decoding and to shaping the proteome. We suggest that impaired translation at poly Asn codons elevates diabetes risk and severity.

Macrophages are essential for CTGF-mediated adult β-cell proliferation after injury

Objective

Promotion of endogenous β-cell mass expansion could facilitate regeneration in patients with diabetes. We discovered that the secreted protein CTGF (aka CCN2) promotes adult β-cell replication and mass regeneration after injury via increasing β-cell immaturity and shortening the replicative refractory period. However, the mechanism of CTGF-mediated β-cell proliferation is unknown. Here we focused on whether CTGF alters cells of the immune system to enhance β-cell replication.

Methods

Using mouse models for 50% β-cell ablation and conditional, β-cell-specific CTGF induction, we assessed changes in immune cell populations by performing immunolabeling and gene expression analyses. We tested the requirement for macrophages in CTGF-mediated β-cell proliferation via clodronate-based macrophage depletion.

Results

CTGF induction after 50% β-cell ablation increased both macrophages and T-cells in islets. An upregulation in the expression of several macrophage and T-cell chemoattractant genes was also observed in islets. Gene expression analyses suggest an increase in M1 and a decrease in M2 macrophage markers. Depletion of macrophages (without changes in T cell number) blocked CTGF-mediated β-cell proliferation and prevented the increase in β-cell immaturity.

Conclusions

Our data show that macrophages are critical for CTGF-mediated adult β-cell proliferation in the setting of partial β-cell ablation. This is the first study to link a specific β-cell proliferative factor with immune-mediated β-cell proliferation in a β-cell injury model.

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Following partial beta cell ablation, induction of CTGF in beta cells increases the number of pancreatic macrophages and T cells.

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Increased macrophages are found preferentially within islets.

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Expression of genes involved in macrophage and T cell chemoattraction is increased in islets following CTGF induction.

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Depletion of macrophages completely abrogates CTGF-mediated increases in beta cell proliferation after injury.

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.

Role of growth factors in control of pancreatic beta cell mass: focus on betatrophin

PURPOSE OF REVIEW

Betatrophin is a newly described hormone, which potently stimulates beta cell replication in mice. This discovery has engendered great hope that it could prove clinically important in the treatment of type 1 and type 2 diabetes.

RECENT FINDINGS

Betatrophin, a 198-amino acid protein secreted by liver and adipose tissue, stimulates growth of pancreatic beta cell mass in insulin-resistant mice. Betatrophin has previously been named RIFL, lipasin, and ANGPLT8, and its salutory effects on lipid metabolism have been described in mouse and human studies. Serum betatrophin levels in humans correlate with improved adipose tissue lipid storage and lower serum triglyceride levels in the fed state, but do not correlate with insulin resistance or carbohydrate tolerance in humans. Betatrophin has not yet been shown to have an effect on beta cell replication in human pancreatic islets.

SUMMARY

Many endocrine and paracrine factors, of which betatrophin is the newest described, increase beta cell mass in murine models. None of these factors, including betatrophin, have displayed the same activity in clinical studies. This may reflect a profound species difference in beta cell regeneration pathways in mice and humans.

β-Arrestin2 plays a key role in the modulation of the pancreatic beta cell mass in mice

AIMS/HYPOTHESIS

Beta cell failure due to progressive secretory dysfunction and limited expansion of beta cell mass is a key feature of type 2 diabetes. Beta cell function and mass are controlled by glucose and hormones/neurotransmitters that activate G protein-coupled receptors or receptor tyrosine kinases. We have investigated the role of β-arrestin (ARRB)2, a scaffold protein known to modulate such receptor signalling, in the modulation of beta cell function and mass, with a specific interest in glucagon-like peptide-1 (GLP-1), muscarinic and insulin receptors.

METHODS

β-arrestin2-knockout mice and their wild-type littermates were fed a normal or a high-fat diet (HFD). Glucose tolerance, insulin sensitivity and insulin secretion were assessed in vivo. Beta cell mass was evaluated in pancreatic sections. Free cytosolic [Ca(2+)] and insulin secretion were determined using perifused islets. The insulin signalling pathway was evaluated by western blotting.

RESULTS

Arrb2-knockout mice exhibited impaired glucose tolerance and insulin secretion in vivo, but normal insulin sensitivity compared with wild type. Surprisingly, the absence of ARRB2 did not affect glucose-stimulated insulin secretion or GLP-1- and acetylcholine-mediated amplifications from perifused islets, but it decreased the islet insulin content and beta cell mass. Additionally, there was no compensatory beta cell mass expansion through proliferation in response to the HFD. Furthermore, Arrb2 deletion altered the islet insulin signalling pathway.

CONCLUSIONS/INTERPRETATION

ARRB2 is unlikely to be involved in the regulation of insulin secretion, but it is required for beta cell mass plasticity. Additionally, we provide new insights into the mechanisms involved in insulin signalling in beta cells.

Altered pancreatic growth and insulin secretion in WSB/EiJ mice

These data suggest that insulin secretion in WSB mice is blunted specifically in vivo, either due to a reduced insulin requirement and/or due to factors that are absent or destroyed in vitro. These studies also highlight the role of post-natal growth in determining adult β-cell mass. Mice are important animal models for the study of metabolic physiology and the genetics of complex traits. Wild-derived inbred mouse strains, such as WSB/EiJ (WSB), are unrelated to the commonly studied mouse strains and are valuable tools to identify novel genes that modify disease risk. We have previously shown that in contrast to C57BL/6J (B6) mice, WSB mice fed a high fat diet do not develop hyperinsulinemia or insulin resistance, and had nearly undetectable insulin secretion in response to an intraperitoneal glucose challenge. As hyperinsulinemia may drive obesity and insulin resistance, we examined whether defects in β-cell mass or function could contribute to the low insulin levels in WSB mice. In young WSB mice, β-cell mass was similar to B6 mice. However, we found that adult WSB mice had reduced β-cell mass due to reduced pancreatic weights. Pancreatic sizes were similar between the strains when normalized to body weight, suggesting their pancreatic size is appropriate to their body size in adults, but overall post-natal pancreatic growth was reduced in WSB mice compared to B6 mice. Islet architecture was normal in WSB mice. WSB mice had markedly increased insulin secretion from isolated islets in vitro. These data suggest that insulin secretion in WSB mice is blunted specifically in vivo, either due to a reduced insulin requirement and/or due to factors that are absent or destroyed in vitro. These studies suggest that WSB mice may provide novel insight into mechanisms regulating insulin secretion and also highlight the role of post-natal growth in determining adult β-cell mass.

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.

Identification of Lgr5-independent spheroid-generating progenitors of the mouse fetal intestinal epithelium

Immortal spheroids were generated from fetal mouse intestine using the culture system initially developed to culture organoids from adult intestinal epithelium. Spheroid proportion progressively decreases from fetal to postnatal period, with a corresponding increase in production of organoids. Like organoids, spheroids show Wnt-dependent indefinite self-renewing properties but display a poorly differentiated phenotype reminiscent of incompletely caudalized progenitors. The spheroid transcriptome is strikingly different from that of adult intestinal stem cells, with minimal overlap of Wnt target gene expression. The receptor LGR4, but not LGR5, is essential for their growth. Trop2/Tacstd2 and Cnx43/Gja1, two markers highly enriched in spheroids, are expressed throughout the embryonic-day-14 intestinal epithelium. Comparison of in utero and neonatal lineage tracing using Cnx43-CreER and Lgr5-CreERT2 mice identified spheroid-generating cells as developmental progenitors involved in generation of the prenatal intestinal epithelium. Ex vivo, spheroid cells have the potential to differentiate into organoids, qualifying as a fetal type of intestinal stem cell.

Regulation of pancreatic function by connective tissue growth factor (CTGF, CCN2)

Connective tissue growth factor (CTGF/CCN2) is a cysteine-rich matricellular secreted protein that regulates diverse cell functions including adhesion, migration, proliferation, differentiation, survival, senescence and apoptosis. In the pancreas, CTGF/CCN2 regulates critical functions including β cell replication during embryogenesis, stimulation of fibrogenic pathways in pancreatic stellate cells during pancreatitis, and regulation of the epithelial and stromal components in pancreatic ductal adenocarcinoma. This article reviews the evidence establishing CTGF/CCN2 as an important player in pancreatic physiology and pathology, highlighting the specific cell types that are involved in each process and the importance of CTGF/CCN2 as a component of autocrine or paracrine signaling within or between these various cells. Translational applications, including the potential for CTGF/CCN2-based therapies in diabetes, fibrosis, or cancer, are discussed.