Conditional β1-integrin-deficient mice display impaired pancreatic β cell function

Vildagliptin inhibits high fat and fetuin-A mediated DPP-4 expression, intracellular lipid accumulation and improves insulin secretory defects in pancreatic beta cells

Dipeptidyl peptidase-4 (DPP-4), a ubiquitous proteolytic enzyme, inhibits insulin secretion from pancreatic beta cells by inactivating circulating incretin hormones GLP-1 and GIP. High circulating levels of DPP-4 is presumed to compromise insulin secretion in people with type 2 diabetes (T2D). Our group recently reported lipid induced DPP-4 expression in pancreatic beta cells, mediated by the TLR4-NFkB pathway. In the present study, we looked at the role of Vildagliptin on pancreatic DPP-4 inhibition, preservation of islet mass and restoration of insulin secretion. MIN6 mouse insulinoma cells incubated with palmitate and fetuin-A, a proinflammatory organokine associated with insulin resistance, showed activation of TLR4-NFkB pathway, which was rescued on Vildagliptin treatment. In addition, Vildagliptin, by suppressing palmitate-fetuin-A mediated DPP-4 expression in MIN6, prevented the secretion of IL-1beta and fetuin-A in the culture media. DPP-4 siRNA abrogated TLR4-NFkB pathway mediated islet cell inflammation. Vildagliptin also reduced palmitate-fetuin-A mediated intracellular lipid accumulation in MIN6 and isolated islets from high fat fed (HFD) mice as observed by Oil O Red staining with downregulation of CD36 and PPARgamma. Vildagliptin also preserved islet mass and rescued insulin secretory defect in HFD mice. Our results suggest that inhibition of DPP-4 by Vildagliptin protects pancreatic beta cells from the deleterious effects of lipid and fetuin-A, preserves insulin secretory functions and improves hyperglycemia.

Insulin secretion hot spots in pancreatic β cells as secreting adhesions

Pancreatic β cell secretion of insulin is crucial to the maintenance of glucose homeostasis and prevention of diseases related to glucose regulation, including diabetes. Pancreatic β cells accomplish efficient insulin secretion by clustering secretion events at the cell membrane facing the vasculature. Regions at the cell periphery characterized by clustered secretion are currently termed insulin secretion hot spots. Several proteins, many associated with the microtubule and actin cytoskeletons, are known to localize to and serve specific functions at hot spots. Among these proteins are the scaffolding protein ELKS, the membrane-associated proteins LL5β and liprins, the focal adhesion-associated protein KANK1, and other factors typically associated with the presynaptic active zone in neurons. These hot spot proteins have been shown to contribute to insulin secretion, but many questions remain regarding their organization and dynamics at hot spots. Current studies suggest microtubule- and F-actin are involved in regulation of hot spot proteins and their function in secretion. The hot spot protein association with the cytoskeleton networks also suggests a potential role for mechanical regulation of these proteins and hot spots in general. This perspective summarizes the existing knowledge of known hot spot proteins, their cytoskeletal-mediated regulation, and discuss questions remaining regarding mechanical regulation of pancreatic beta cell hot spots.

β1-Integrin-A Key Player in Controlling Pancreatic Beta-Cell Insulin Secretion via Interplay With SNARE Proteins

Shortcomings in cell-based therapies for patients with diabetes have been revealed to be, in part, a result of an improper extracellular matrix (ECM) environment. In vivo, pancreatic islets are emersed in a diverse ECM that provides physical support and is crucial for healthy function. β1-Integrin receptors have been determined to be responsible for modulation of beneficial interactions with ECM proteins influencing beta-cell development, proliferation, maturation, and function. β1-Integrin signaling has been demonstrated to augment insulin secretion by impacting the actin cytoskeleton via activation of focal adhesion kinase and downstream signaling pathways. In other secretory cells, evidence of a bidirectional relationship between integrins and exocytotic machinery has been demonstrated, and, thus, this relationship could be present in pancreatic beta cells. In this review, we will discuss the role of ECM-β1-integrin interplay with exocytotic proteins in controlling pancreatic beta-cell insulin secretion through their dynamic and unique signaling pathway.

Genome-wide CRISPR screen identified a role for commander complex mediated ITGB1 recycling in basal insulin secretion

OBJECTIVES

Pancreatic beta cells secrete insulin postprandially and during fasting to maintain glucose homeostasis. Although glucose-stimulated insulin secretion (GSIS) has been extensively studied, much less is known about basal insulin secretion. Here, we performed a genome-wide CRISPR/Cas9 knockout screen to identify novel regulators of insulin secretion.

METHODS

To identify genes that cell autonomously regulate insulin secretion, we engineered a Cas9-expressing MIN6 subclone that permits irreversible fluorescence labeling of exocytic insulin granules. Using a fluorescence-activated cell sorting assay of exocytosis in low glucose and high glucose conditions in individual cells, we performed a genome-wide CRISPR/Cas9 knockout screen.

RESULTS

We identified several members of the COMMD family, a conserved family of proteins with central roles in intracellular membrane trafficking, as positive regulators of basal insulin secretion, but not GSIS. Mechanistically, we show that the Commander complex promotes insulin granules docking in basal state. This is mediated, at least in part, by its function in ITGB1 recycling. Defective ITGB1 recycling reduces its membrane distribution, the number of focal adhesions and cortical ELKS-containing complexes.

CONCLUSIONS

We demonstrated a previously unknown function of the Commander complex in basal insulin secretion. We showed that by ITGB1 recycling, Commander complex increases cortical adhesions, which enhances the assembly of the ELKS-containing complexes. The resulting increase in the number of insulin granules near the plasma membrane strengthens basal insulin secretion.

Collagen IV-β1-Integrin Influences INS-1 Cell Insulin Secretion via Enhanced SNARE Protein Expression

β1-integrin is a key receptor that regulates cell-ECM interactions and is important in maintaining mature beta-cell functions, including insulin secretion. However, there is little reported about the relationship between ECM-β1-integrin interactions and exocytotic proteins involved in glucose-stimulated insulin secretion (GSIS). This study examined the effect of collagen IV-β1-integrin on exocytotic proteins (Munc18-1, Snap25, and Vamp2) involved in insulin secretion using rat insulinoma (INS-1) cell line. Cells cultured on collagen IV (COL IV) had promoted INS-1 cell focal adhesions and GSIS. These cells also displayed changes in levels and localization of β1-integrin associated downstream signals and exocytotic proteins involved in insulin secretion. Antibody blocking of β1-integrin on INS-1 cells cultured on COL IV showed significantly reduced cell adhesion, spreading and insulin secretion along with reduced exocytotic protein levels. Blocking of β1-integrin additionally influenced the cellular localization of exocytotic proteins during the time of GSIS. These results indicate that specific collagen IV-β1-integrin interactions are critical for proper beta-cell insulin secretion.

Organization and dynamics of the cortical complexes controlling insulin secretion in β-cells

Insulin secretion in pancreatic β-cells is regulated by cortical complexes that are enriched at the sites of adhesion to extracellular matrix facing the vasculature. Many components of these complexes, including bassoon, RIM, ELKS and liprins, are shared with neuronal synapses. Here, we show that insulin secretion sites also contain the non-neuronal proteins LL5β (also known as PHLDB2) and KANK1, which, in migrating cells, organize exocytotic machinery in the vicinity of integrin-based adhesions. Depletion of LL5β or focal adhesion disassembly triggered by myosin II inhibition perturbed the clustering of secretory complexes and attenuated the first wave of insulin release. Although previous analyses in vitro and in neurons have suggested that secretory machinery might assemble through liquid–liquid phase separation, analysis of endogenously labeled ELKS in pancreatic islets indicated that its dynamics is inconsistent with such a scenario. Instead, fluorescence recovery after photobleaching and single-molecule imaging showed that ELKS turnover is driven by binding and unbinding to low-mobility scaffolds. Both the scaffold movements and ELKS exchange were stimulated by glucose treatment. Our findings help to explain how integrin-based adhesions control spatial organization of glucose-stimulated insulin release.

Summary: Characterization of the composition of cortical complexes controlling insulin secretion, showing that their dynamics is inconsistent with assembly through liquid–liquid phase separation.

N-acetyl-L-cysteine treatment reduces beta-cell oxidative stress and pancreatic stellate cell activity in a high fat diet-induced diabetic mouse model

Obesity plays a major role in type II diabetes (T2DM) progression because it applies metabolic and oxidative stress resulting in dysfunctional beta-cells and activation of intra-islet pancreatic stellate cells (PaSCs) which cause islet fibrosis. Administration of antioxidant N-acetyl-L-cysteine (NAC) in vivo improves metabolic outcomes in diet-induced obese diabetic mice, and in vitro inhibits PaSCs activation. However, the effects of NAC on diabetic islets in vivo are unknown. This study examined if dosage and length of NAC treatment in HFD-induced diabetic mice effect metabolic outcomes associated with maintaining healthy beta-cells and quiescent PaSCs, in vivo. Male C57BL/6N mice were fed normal chow (ND) or high-fat (HFD) diet up to 30 weeks. NAC was administered in drinking water to HFD mice in preventative treatment (HFD^pNAC) for 23 weeks or intervention treatment for 10 (HFD^iNAC) or 18 (HFD^iNAC+) weeks, respectively. HFD^pNAC and HFD^iNAC+, but not HFD^iNAC, mice showed significantly improved glucose tolerance and insulin sensitivity. Hyperinsulinemia led by beta-cell overcompensation in HFD mice was significantly rescued in NAC treated mice. A reduction of beta-cell nuclear Pdx-1 localization in HFD mice was significantly improved in NAC treated islets along with significantly reduced beta-cell oxidative stress. HFD-induced intra-islet PaSCs activation, labeled by αSMA, was significantly diminished in NAC treated mice along with lesser intra-islet collagen deposition. This study determined that efficiency of NAC treatment is beneficial at maintaining healthy beta-cells and quiescent intra-islet PaSCs in HFD-induced obese T2DM mouse model. These findings highlight an adjuvant therapeutic potential in NAC for controlling T2DM progression in humans.

Fetuin-A secretion from β-cells leads to accumulation of macrophages in islets, aggravates inflammation and impairs insulin secretion

Elevated fetuin-A levels, chemokines and islet-resident macrophages are crucial factors associated with obesity-mediated type 2 diabetes (T2D). Here, the aim of the study was to investigate the effect of MIN6 (a mouse insulinoma cell line)-derived fetuin-A (also known as AHSG) in macrophage polarization and decipher the effect of M1 type pro-inflammatory macrophages in commanding over insulin secretion. MIN6 and islet-derived fetuin-A induced expression of the M1 type macrophage markers Emr1 (also known as Adgre1), Cd68 and CD11c (Itgax) (∼1.8 fold) along with increased cytokine secretion. Interestingly, suppression of fetuin-A in MIN6 successfully reduced M1 markers by ∼1.5 fold. MIN6-derived fetuin-A also induced chemotaxis of macrophages in a Boyden chamber chemotaxis assay. Furthermore, high-fat feeding in mice showed elevated cytokine and fetuin-A content in serum and islets, and also migration and polarization of macrophages to the islets, while β-cells failed to meet the increased insulin demand. Moreover, in MIN6 culture, M1 macrophages sharply decreased insulin secretion by ∼2.8 fold. Altogether our results support an association of fetuin-A with islet inflammation and β-cell dysfunction, owing to its role as a key chemoattractant and macrophage polarizing factor.

TUDCA receptors and their role on pancreatic beta cells

Bile acids have received increasing attention over the past years as their multiple alternative roles became clearer. Tauroursodeoxycholic Acid (TUDCA) in specific has generated special interest due to its ability to promote pancreatic survival and function, as well as reduce endoplasmic reticulum stress. However, there are few studies explaining the molecular mechanisms behind TUDCA's beneficial actions on pancreatic beta cells. In this review, we decided to review the literature in order to craft a primer for researchers on what is known about TUDCA's receptors and the molecular pathways involved in this bile acid's function in the endocrine pancreas. We review the studies that focused on G protein-coupled bile acid receptor (TGR5), Sphingosine-1-phosphate receptor 2 (S1PR2) and α5β1 Integrin function in pancreatic cells. Our hope is to provide a basis for future studies to expand upon, especially considering the current lack of studies focusing on the importance of these receptors, either through TUDCA signaling or other signaling molecules.

Activation of Pancreatic Stellate Cells Is Beneficial for Exocrine but Not Endocrine Cell Differentiation in the Developing Human Pancreas

Pancreatic stellate cells (PaSCs) are non-endocrine, mesenchymal-like cells that reside within the peri-pancreatic tissue of the rodent and human pancreas. PaSCs regulate extracellular matrix (ECM) turnover in maintaining the integrity of pancreatic tissue architecture. Although there is evidence indicating that PaSCs are involved in islet cell survival and function, its role in islet cell differentiation during human pancreatic development remains unclear. The present study examines the expression pattern and functional role of PaSCs in islet cell differentiation of the developing human pancreas from late 1st to 2nd trimester of pregnancy. The presence of PaSCs in human pancreata (8–22 weeks of fetal age) was characterized by ultrastructural, immunohistological, quantitative RT-PCR and western blotting approaches. Using human fetal PaSCs derived from pancreata at 14–16 weeks, freshly isolated human fetal islet-epithelial cell clusters (hIECCs) were co-cultured with active or inactive PaSCs in vitro. Ultrastructural and immunofluorescence analysis demonstrated a population of PaSCs near ducts and newly formed islets that appeared to make complex cell-cell dendritic-like contacts. A small subset of PaSCs co-localized with pancreatic progenitor-associated transcription factors (PDX1, SOX9, and NKX6-1). PaSCs were highly proliferative, with significantly higher mRNA and protein levels of PaSC markers (desmin, αSMA) during the 1st trimester of pregnancy compared to the 2nd trimester. Isolated human fetal PaSCs were identified by expression of stellate cell markers and ECM. Suppression of PaSC activation, using all-trans retinoic acid (ATRA), resulted in reduced PaSC proliferation and ECM proteins. Co-culture of hIECCs, directly on PaSCs or indirectly using Millicell^® Inserts or using PaSC-conditioned medium, resulted in a reduction the number of insulin⁺ cells but a significant increase in the number of amylase⁺ cells. Suppression of PaSC activation or Notch activity during the co-culture resulted in an increase in beta-cell differentiation. This study determined that PaSCs, abundant during the 1st trimester of pancreatic development but decreased in the 2nd trimester, are located near ductal and islet structures. Direct and indirect co-cultures of hIECCs with PaSCs suggest that activation of PaSCs has opposing effects on beta-cell and exocrine cell differentiation during human fetal pancreas development, and that these effects may be dependent on Notch signaling.

Blocking TLR4-NF-κB pathway protects mouse islets from the combinatorial impact of high fat and fetuin-A mediated dysfunction and restores ability for insulin secretion

Lipid mediated pancreatic β-cell dysfunction during Type 2 diabetes is known to be regulated by activation of TLR4 (Toll Like Receptor 4) and NF-κB (Nuclear factor kappa B). Recently we have reported that MIN6 cells (mouse insulinoma cells) secrete fetuin-A on stimulation by palmitate that aggravates β-cell dysfunction, but the mechanism involved in-vivo has not been demonstrated and thus remained unclear. Here we attempted to dissect the role of palmitate and fetuin-A on insulin secretion using high fat diet (HFD) fed mice model. HFD islets showed curtailed insulin secretion after 20 weeks of treatment with activated TLR4-NF-κB pathway. Further treatment of islets with palmitate raised fetuin-A expression by ~2.8 folds and cut down insulin secretion by ~1.4 folds. However, blocking the activity of TLR4, fetuin-A and NF-κB using specific inhibitors or siRNAs not only restored insulin secretion by ~2 folds in standard diet fed mice islets and MIN6 cells but also evoke insulin secretory ability by ~2.3 folds in HFD islets. Altogether this study demonstrated that blocking TLR4, fetuin-A and NF-κB protect pancreatic β-cells from the negative effects of free fatty acid and fetuin-A and restore insulin secretion.

The Role of Vascular Cells in Pancreatic Beta-Cell Function

Insulin-producing β-cells constitute the majority of the cells in the pancreatic islets. Dysfunction of these cells is a key factor in the loss of glucose regulation that characterizes type 2 diabetes. The regulation of many of the functions of β-cells relies on their close interaction with the intra-islet microvasculature, comprised of endothelial cells and pericytes. In addition to providing islet blood supply, cells of the islet vasculature directly regulate β-cell activity through the secretion of growth factors and other molecules. These factors come from capillary mural pericytes and endothelial cells, and have been shown to promote insulin gene expression, insulin secretion, and β-cell proliferation. This review focuses on the intimate crosstalk of the vascular cells and β-cells and its role in glucose homeostasis and diabetes.

Generation of insulin-secreting organoids: a step toward engineering and transplanting the bioartificial pancreas

Diabetes is a major health issue of increasing prevalence. ß‐cell replacement, by pancreas or islet transplantation, is the only long‐term curative option for patients with insulin‐dependent diabetes. Despite good functional results, pancreas transplantation remains a major surgery with potentially severe complications. Islet transplantation is a minimally invasive alternative that can widen the indications in view of its lower morbidity. However, the islet isolation procedure disrupts their vasculature and connection to the surrounding extracellular matrix, exposing them to ischemia and anoikis. Implanted islets are also the target of innate and adaptive immune attacks, thus preventing robust engraftment and prolonged full function. Generation of organoids, defined as functional 3D structures assembled with cell types from different sources, is a strategy increasingly used in regenerative medicine for tissue replacement or repair, in a variety of inflammatory or degenerative disorders. Applied to ß‐cell replacement, it offers the possibility to control the size and composition of islet‐like structures (pseudo‐islets), and to include cells with anti‐inflammatory or immunomodulatory properties. In this review, we will present approaches to generate islet cell organoids and discuss how these strategies can be applied to the generation of a bioartificial pancreas for the treatment of type 1 diabetes.

Oligomeric collagen as an encapsulation material for islet/β-cell replacement: effect of islet source, dose, implant site, and administration format

Replacement of islets/β-cells that provide long-lasting glucose-sensing and insulin-releasing functions has the potential to restore extended glycemic control in individuals with type 1 diabetes. Unfortunately, persistent challenges preclude such therapies from widespread clinical use, including cumbersome administration via portal vein infusion, significant loss of functional islet mass upon administration, limited functional longevity, and requirement for systemic immunosuppression. Previously, fibril-forming type I collagen (oligomer) was shown to support subcutaneous injection and in situ encapsulation of syngeneic islets within diabetic mice, with rapid (<24 h) reversal of hyperglycemia and maintenance of euglycemia for beyond 90 days. Here, we further evaluated this macroencapsulation strategy, defining effects of islet source (allogeneic and xenogeneic) and dose (500 and 800 islets), injection microenvironment (subcutaneous and intraperitoneal), and macrocapsule format (injectable and preformed implantable) on islet functional longevity and recipient immune response. We found that xenogeneic rat islets functioned similarly to or better than allogeneic mouse islets, with only modest improvements in longevity noted with dosage. Additionally, subcutaneous injection led to more consistent encapsulation outcomes along with improved islet health and longevity, compared with intraperitoneal administration, whereas no significant differences were observed between subcutaneous injectable and preformed implantable formats. Collectively, these results document the benefits of incorporating natural collagen for islet/β-cell replacement therapies.

Regulation of integrin α6A by lactogenic hormones in rat pancreatic β-cells: Implications for the physiological adaptation to pregnancy

AIM

During pregnancy, the maternal β-cell mass is increased in order to adapt to the physiological changes in insulin demand. Lactogenic hormones stimulate rodent β-cell attachment and proliferation in vitro. The aim of this study was to identify adhesion molecules involved in expansion of the β-cell mass during pregnancy in the rat.

METHODS

Quantitative RT-PCR was used to evaluate the expression of several integrins and laminins in isolated neonatal rat islets in response to growth hormone (GH) and prolactin (PRL) treatment. Double-immunofluorescence staining of rat pancreas was used to localize the expression of integrin α6β1. β-cell proliferation was evaluated by incorporation of bromodeoxyuridine (BrdU). The role of STAT5 phosphorylation was tested by addition of STAT5 mutants.

RESULTS

We found that the mRNA level of integrin-α6A, was upregulated 2.5-fold by PRL or GH. During pregnancy, a biphasic 3.4-4.5-fold increase of integrin-α6A and B mRNA levels was detected. A disintegrin peptide (DP) reduced the hormone-stimulated mitotic activity in neonatal rat β-cells from 2.9 ± 0.4-fold to 1.3 ± 0.3-fold. The hormone-induced expression of α6β1 integrin was shown to be mediated via STAT5 as a dominant negative (DN) mutant prevented and a constitutive active (CA) mutant augmented the hGH-stimulated expression. The DP was found to inhibit hGH-induced transactivation of the PRL receptor promoter 1A and reduce the hGH-induced phosphorylation of STAT5.

CONCLUSION

These results show that integrin-α6 in β-cells is upregulated by lactogenic hormones and is required but not sufficient for the expansion of the β-cell mass in pregnancy in the rat, which may have implications for the understanding and treatment of gestational diabetes mellitus.

Beta-cell β1 integrin deficiency affects in utero development of islet growth and vascularization

The β1 integrin subunit contributes to pancreatic beta cell growth and function through communication with the extracellular matrix (ECM). The effects of in vitro and in vivo β1 integrin knockout have been extensively studied in mature islets, yet no study to date has examined how the loss of β1 integrin during specific stages of pancreatic development impacts beta cell maturation. Beta-cell-specific tamoxifen-inducible Cre recombinase (MIP-CreERT) mice were crossed with mice containing floxed Itgb1 (β1 integrin) to create an inducible mouse model (MIPβ1KO) at the second transition stage (e13.5) of pancreas development. By e19.5-20.5, the expression of beta-cell β1 integrin in fetal MIPβ1KO mice was significantly reduced and these mice displayed decreased beta cell mass, density and proliferation. Morphologically, fetal MIPβ1KO pancreata exhibited reduced islet vascularization and nascent endocrine cells in the ductal region. In addition, decreased ERK phosphorylation was observed in fetal MIPβ1KO pancreata. The expression of transcription factors needed for beta-cell development was unchanged in fetal MIPβ1KO pancreata. The findings from this study demonstrate that β1 integrin signaling is required during a transition-specific window in the developing beta-cell to maintain islet mass and vascularization.

SPARC is required for the maintenance of glucose homeostasis and insulin secretion in mice

Obesity, metabolic syndrome, and type 2 diabetes, three strongly interrelated diseases, are associated to increased morbidity and mortality worldwide. The pathogenesis of obesity-associated disorders is still under study. Secreted protein acidic and rich in cysteine (SPARC) is a matricellular glycoprotein expressed in many cell types including adipocytes, parenchymal, and non-parenchymal hepatic cells and pancreatic cells. Studies have demonstrated that SPARC inhibits adipogenesis and promotes insulin resistance; in addition, circulating SPARC levels were positively correlated with body mass index in obese individuals. Therefore, SPARC is being proposed as a key factor in the pathogenesis of obesity-associated disorders. The aim of this study is to elucidate the role of SPARC in glucose homeostasis. We show here that SPARC null (SPARC−/−) mice displayed an abnormal insulin-regulated glucose metabolism. SPARC−/− mice presented an increased adipose tissue deposition and an impaired glucose homeostasis as animals aged. In addition, the absence of SPARC worsens high-fat diet-induced diabetes in mice. Interestingly, although SPARC−/− mice on high-fat diet were sensitive to insulin they showed an impaired insulin secretion capacity. Of note, the expression of glucose transporter 2 in islets of SPARC−/− mice was dramatically reduced. The present study provides the first evidence that deleted SPARC expression causes diabetes in mice. Thus, SPARC deficient mice constitute a valuable model for studies concerning obesity and its related metabolic complications, including diabetes.

INGAP-PP effects on β-cell mass and function are related to its positive effect on islet angiogenesis and VEGFA production

Our aim was to determine whether islet angiogenesis and VEGFA production/release participate in the mechanism by which INGAP-PP enhances β-cell function and mass. We used two models: a) in vivo (normal rats injected with INGAP-PP for 10 days) and b) in vitro (normal islets cultured for 4 days with INGAP-PP, VEGFA, Rapamycin, and the specific VEGF-Receptor inhibitor, SU5416). INGAP-PP administration enhanced insulin secretion, β-cell mass, islet vascularization, and angiogenesis without affecting glucose homeostasis. Normal islets cultured with INGAP-PP and VEGFA increased insulin and VEGFA secretion while apoptosis decreased. INGAP-PP-induced effects were prevented by both Rapamycin and SU5416. INGAP-PP effects on β-cell mass and function were significantly associated with a positive effect on islet angiogenesis and VEGFA production/release. VEGF-A possibly potentiates INGAP-PP effect through mTORC pathway.

Extracellular matrix molecules and their potential contribution to the function of transplanted pancreatic islets

Extracellular matrix (ECM) molecules are responsible for structural and biochemical support, as well as for regulation of molecular signalling and tissue repair in many organ structures, including the pancreas. In pancreatic islets, collagen type IV and VI, and laminins are the most abundant molecules, but other ECM molecules are also present. The ECM interacts with specific combinations of integrin α/β heterodimers on islet cells and guides many cellular processes. More specifically, some ECM molecules are involved in beta cell survival, function and insulin production, while others can fine tune the susceptibility of islet cells to cytokines. Further, some ECM induce release of growth factors to facilitate tissue repair. During enzymatic isolation of islets for transplantation, the ECM is damaged, impacting islet function. However, restoration of the ECM in human islets (for example by adding ECM to the interior of immunoprotective capsules) has been shown to enhance islet function. Here, we provide current insight into the role of ECM molecules in islet function and discuss the clinical potential of ECM manipulation to enhance pancreatic islet function and survival.

Adhesion characteristics of porcine pancreatic islets and exocrine tissue to coating materials

Since the report of the Edmonton protocol in 2000, islet transplantation has been implemented worldwide, and xenotransplantation using porcine islets has also been reported. In addition, many basic experiments using pancreatic islets and exocrine tissue after isolation have been reported. Recently, exocrine cells have been found to be essential for inducing the differentiation of pancreatic islets. Therefore, the importance of the culture conditions for pancreatic tissue when conducting experiments using pancreatic tissue is also increasing. In this study, we focused on the coat material and examined the adhesive properties of porcine pancreatic islets and exocrine tissue after isolation. Porcine islet isolation was performed, and isolated islets (purity ≥95%) and exocrine tissue (purity ≥99%) were used to achieve adhesion to several extracellular matrixes, fibronectin, collagen type I, collagen type IV, laminin I, fibrinogen, and bovine serum albumin (BSA). DMEM with 0.5% FBS was used as the assay medium. For exocrine tissue, the adhesion was promoted in fibronectin, collagen type I, laminin I, and fibrinogen. The adhesive ability to fibronectin was more than twice that to BSA, while the adhesive ability to collagen type I, laminin I, and fibrinogen was less than twice that to BSA. For islets, the adhesive ability to fibronectin was weaker than that of exocrine tissue. Furthermore, the adhesion effect in fibronectin was obtained within 30 minutes and in medium containing little serum for both islets and exocrine tissues. These data suggest that fibronectin may be useful for the adhesion of pancreatic tissue.

Signals in the pancreatic islet microenvironment influence β-cell proliferation

The progressive loss of pancreatic β-cell mass that occurs in both type 1 and type 2 diabetes is a primary factor driving efforts to identify strategies for effectively increasing, enhancing or restoring β-cell mass. While factors that seem to influence β-cell proliferation in specific contexts have been described, reliable stimulation of human β-cell proliferation has remained a challenge. Importantly, β-cells exist in the context of a complex, integrated pancreatic islet microenvironment where they interact with other endocrine cells, vascular endothelial cells, extracellular matrix, neuronal projections and islet macrophages. This review highlights different components of the pancreatic microenvironment, and reviews what is known about how signaling that occurs between β-cells and these other components influences β-cell proliferation. Future efforts to further define the role of the pancreatic islet microenvironment on β-cell proliferation may lead to the development of successful approaches to increase or restore β-cell mass in diabetes.

β1-Integrin Deletion From the Lens Activates Cellular Stress Responses Leading to Apoptosis and Fibrosis

Purpose

Previous research showed that the absence of β1-integrin from the mouse lens after embryonic day (E) 13.5 (β1MLR10) leads to the perinatal apoptosis of lens epithelial cells (LECs) resulting in severe microphthalmia. This study focuses on elucidating the molecular connections between β1-integrin deletion and this phenotype.

Methods

RNA sequencing was performed to identify differentially regulated genes (DRGs) in β1MLR10 lenses at E15.5. By using bioinformatics analysis and literature searching, Egr1 (early growth response 1) was selected for further study. The activation status of certain signaling pathways (focal adhesion kinase [FAK]/Erk, TGF-β, and Akt signaling) was studied via Western blot and immunohistochemistry. Mice lacking both β1-integrin and Egr1 genes from the lenses were created (β1MLR10/Egr1-/-) to study their relationship.

Results

RNA sequencing identified 120 DRGs that include candidates involved in the cellular stress response, fibrosis, and/or apoptosis. Egr1 was investigated in detail, as it mediates cellular stress responses in various cell types, and is recognized as an upstream regulator of numerous other β1MLR10 lens DRGs. In β1MLR10 mice, Egr1 levels are elevated shortly after β1-integrin loss from the lens. Further, pErk1/2 and pAkt are elevated in β1MLR10 LECs, thus providing the potential signaling mechanism that causes Egr1 upregulation in the mutant. Indeed, deletion of Egr1 from β1MLR10 lenses partially rescues the microphthalmia phenotype.

Conclusions

β1-integrin regulates the appropriate levels of Erk1/2 and Akt phosphorylation in LECs, whereas its deficiency results in the overexpression of Egr1, culminating in reduced cell survival. These findings provide insight into the molecular mechanism underlying the microphthalmia observed in β1MLR10 mice.

Critical role of β1 integrin in postnatal beta-cell function and expansion

β1 integrin is essential for pancreatic beta-cell development and maintenance in rodents and humans. However, the effects of a temporal beta-cell specific β1 integrin knockout on adult islet function are unknown. We utilized a mouse insulin 1 promoter driven tamoxifen-inducible Cre-recombinase β1 integrin knockout mouse model (MIPβ1KO) to investigate β1 integrin function in adult pancreatic beta-cells. Adult male MIPβ1KO mice were significantly glucose intolerant due to impaired glucose-stimulated insulin secretion in vivo and ex vivo at 8 weeks post-tamoxifen. The expression of Insulin and Pancreatic and duodenal homeobox-1 mRNA was significantly reduced in MIPβ1KO islets, along with reductions in insulin exocytotic proteins. Morphological analyses demonstrated that beta-cell mass, islet density, and the number of large-sized islets was significantly reduced in male MIPβ1KO mice. Significant reductions in the phosphorylation of signaling molecules focal adhesion kinase, extracellular signal-regulated kinases 1 and 2, and v-Akt murine thymoma viral oncogene were observed in male MIPβ1KO islets when compared to controls. MIPβ1KO islets displayed a significant increase in protein levels of the apoptotic marker cleaved-Poly (ADP-ribose) polymerase and a reduction of the cell cycle marker cyclin D1. Female MIPβ1KO mice did not develop glucose intolerance or reduced beta-cell mass until 16 weeks post-tamoxifen. Glucose intolerance remained in both genders of aged MIPβ1KO mice. This data demonstrates that β1 integrin is required for the maintenance of glucose homeostasis through postnatal beta-cell function and expansion.

Intra-islet endothelial cell and β-cell crosstalk: Implication for islet cell transplantation

The intra-islet microvasculature is a critical interface between the blood and islet endocrine cells governing a number of cellular and pathophysiological processes associated with the pancreatic tissue. A growing body of evidence indicates a strong functional and physical interdependency of β-cells with endothelial cells (ECs), the building blocks of islet microvasculature. Intra-islet ECs, actively regulate vascular permeability and appear to play a role in fine-tuning blood glucose sensing and regulation. These cells also tend to behave as “guardians”, controlling the expression and movement of a number of important immune mediators, thereby strongly contributing to the physiology of islets. This review will focus on the molecular signalling and crosstalk between the intra-islet ECs and β-cells and how their relationship can be a potential target for intervention strategies in islet pathology and islet transplantation.

Role and impact of the extracellular matrix on integrin-mediated pancreatic β-cell functions

Understanding the organisation and role of the extracellular matrix (ECM) in islets of Langerhans is critical for maintaining pancreatic β-cells, and to recognise and revert the physiopathology of diabetes. Indeed, integrin-mediated adhesion signalling in response to the pancreatic ECM plays crucial roles in β-cell survival and insulin secretion, two major functions, which are affected in diabetes. Here, we would like to present an update on the major components of the pancreatic ECM, their role during integrin-mediated cell-matrix adhesions and how they are affected during diabetes. To treat diabetes, a promising approach consists in replacing β-cells by transplantation. However, efficiency is low, because β-cells suffer of anoikis, due to enzymatic digestion of the pancreatic ECM, which affects the survival of insulin-secreting β-cells. The strategy of adding ECM components during transplantation, to reproduce the pancreatic microenvironment, is a challenging task, as many of the regulatory mechanisms that control ECM deposition and turnover are not sufficiently understood. A better comprehension of the impact of the ECM on the adhesion and integrin-dependent signalling in β-cells is primordial to improve the healthy state of islets to prevent the onset of diabetes as well as for enhancing the efficiency of the islet transplantation therapy.

The biomechanical properties of an epithelial tissue determine the location of its vasculature

An important question is how growing tissues establish a blood vessel network. Here we study vascular network formation in pancreatic islets, endocrine tissues derived from pancreatic epithelium. We find that depletion of integrin-linked kinase (ILK) in the pancreatic epithelial cells of mice results in glucose intolerance due to a loss of the intra-islet vasculature. In turn, blood vessels accumulate at the islet periphery. Neither alterations in endothelial cell proliferation, apoptosis, morphology, Vegfa expression and VEGF-A secretion nor ‘empty sleeves' of vascular basement membrane are found. Instead, biophysical experiments reveal that the biomechanical properties of pancreatic islet cells, such as their actomyosin-mediated cortex tension and adhesive forces to endothelial cells, are significantly changed. These results suggest that a sorting event is driving the segregation of endothelial and epithelial cells and indicate that the epithelial biomechanical properties determine whether the blood vasculature invades or envelops a growing epithelial tissue.

Vasculature is denser in soft than in stiff tissues. Kragl et al. suggest a mechanistic link between biomechanical tissue properties and vascularization by showing that integrin-linked kinase reduces the contractile forces of the cell cortex in endocrine pancreatic cells, facilitating their adhesion to blood vessels and enabling pancreatic islet vascularization.

A holistic approach to dissecting SPARC family protein complexity reveals FSTL-1 as an inhibitor of pancreatic cancer cell growth

SPARC is a matricellular protein that is involved in both pancreatic cancer and diabetes. It belongs to a wider family of proteins that share structural and functional similarities. Relatively little is known about this extended family, but evidence of regulatory interactions suggests the importance of a holistic approach to their study. We show that Hevin, SPOCKs, and SMOCs are strongly expressed within islets, ducts, and blood vessels, suggesting important roles for these proteins in the normal pancreas, while FSTL-1 expression is localised to the stromal compartment reminiscent of SPARC. In direct contrast to SPARC, however, FSTL-1 expression is reduced in pancreatic cancer. Consistent with this, FSTL-1 inhibited pancreatic cancer cell proliferation. The complexity of SPARC family proteins is further revealed by the detection of multiple cell-type specific isoforms that arise due to a combination of post-translational modification and alternative splicing. Identification of splice variants lacking a signal peptide suggests the existence of novel intracellular isoforms. This study underlines the importance of addressing the complexity of the SPARC family and provides a new framework to explain their controversial and contradictory effects. We also demonstrate for the first time that FSTL-1 suppresses pancreatic cancer cell growth.

Maintaining human fetal pancreatic stellate cell function and proliferation require β1 integrin and collagen I matrix interactions

Pancreatic stellate cells (PaSCs) are cells that are located around the acinar, ductal, and vasculature tissue of the rodent and human pancreas, and are responsible for regulating extracellular matrix (ECM) turnover and maintaining the architecture of pancreatic tissue. This study examines the contributions of integrin receptor signaling in human PaSC function and survival. Human PaSCs were isolated from pancreata collected during the 2nd trimester of pregnancy and identified by expression of stellate cell markers, ECM proteins and associated growth factors. Multiple integrins are found in isolated human PaSCs, with high levels of β1, α3 and α5. Cell adhesion and migration assays demonstrated that human PaSCs favour collagen I matrix, which enhanced PaSC proliferation and increased TGFβ1, CTGF and α3β1 integrin. Significant activation of FAK/ERK and AKT signaling pathways, and up-regulation of cyclin D1 protein levels, were observed within PaSCs cultured on collagen I matrix. Blocking β1 integrin significantly decreased PaSC adhesion, migration and proliferation, further complementing the aforementioned findings. This study demonstrates that interaction of β1 integrin with collagen I is required for the proliferation and function of human fetal PaSCs, which may contribute to the biomedical engineering of the ECM microenvironment needed for the efficient regulation of pancreatic development.

In vitro and in vivo evaluation of the effect of nano-sized collagen molecules and nicotinamide on mesenchymal stem cell differentiation

Advances and improvements in mesenchymal stromal/stem cells (MSCs) and cell replacement therapies have been promising approaches to treat diabetes mellitus (DM) since their potent capacities for differentiation into various functional cells match the demands of tissue repair and regeneration.

Ghrelin Protects against Dexamethasone-Induced INS-1 Cell Apoptosis via ERK and p38MAPK Signaling

Glucocorticoid excess induces apoptosis of islet cells, which may result in diabetes. In this study, we investigated the protective effect of ghrelin on dexamethasone-induced INS-1 cell apoptosis. Our data showed that ghrelin (0.1 μM) inhibited dexamethasone-induced (0.1 μM) apoptosis of INS-1 cells and facilitated cell proliferation. Moreover, ghrelin upregulated Bcl-2 expression, downregulated Bax expression, and decreased caspase-3 activity. The protective effect of ghrelin against dexamethasone-induced INS-1 cell apoptosis was mediated via growth hormone secretagogue receptor 1a. Further studies revealed that ghrelin increased ERK activation and decreased p38MAPK expression after dexamethasone treatment. Ghrelin-mediated protection of dexamethasone-induced apoptosis of INS-1 cells was attenuated using the ERK inhibitor U0126 (10 μM), and cell viability increased using the p38MAPK inhibitor SB203580 (10 μM). In conclusion, ghrelin could protect against dexamethasone-induced INS-1 cell apoptosis, at least partially via GHS-R1a and the signaling pathway of ERK and p38MAPK.

Antiaging Gene Klotho Attenuates Pancreatic β-Cell Apoptosis in Type 1 Diabetes

Apoptosis is the major cause of death of insulin-producing β-cells in type 1 diabetes mellitus (T1DM). Klotho is a recently discovered antiaging gene. We found that the Klotho gene is expressed in pancreatic β-cells. Interestingly, halplodeficiency of Klotho (KL(+/-)) exacerbated streptozotocin (STZ)-induced diabetes (a model of T1DM), including hyperglycemia, glucose intolerance, diminished islet insulin storage, and increased apoptotic β-cells. Conversely, in vivo β-cell-specific expression of mouse Klotho gene (mKL) attenuated β-cell apoptosis and prevented STZ-induced diabetes. mKL promoted cell adhesion to collagen IV, increased FAK and Akt phosphorylation, and inhibited caspase 3 cleavage in cultured MIN6 β-cells. mKL abolished STZ- and TNFα-induced inhibition of FAK and Akt phosphorylation, caspase 3 cleavage, and β-cell apoptosis. These promoting effects of Klotho can be abolished by blocking integrin β1. Therefore, these cell-based studies indicated that Klotho protected β-cells by inhibiting β-cell apoptosis through activation of the integrin β1-FAK/Akt pathway, leading to inhibition of caspase 3 cleavage. In an autoimmune T1DM model (NOD), we showed that in vivo β-cell-specific expression of mKL improved glucose tolerance, attenuated β-cell apoptosis, enhanced insulin storage in β-cells, and increased plasma insulin levels. The beneficial effect of Klotho gene delivery is likely due to attenuation of T-cell infiltration in pancreatic islets in NOD mice. Overall, our results demonstrate for the first time that Klotho protected β-cells in T1DM via attenuating apoptosis.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

Myt3 Mediates Laminin-V/Integrin-β1-Induced Islet-Cell Migration via Tgfbi

Myt3 is a prosurvival factor in pancreatic islets; however, its role in islet-cell development is not known. Here, we demonstrate that myelin transcription factor 3 (Myt3) is expressed in migrating islet cells in the developing and neonatal pancreas and thus sought to determine whether Myt3 plays a role in this process. Using an ex vivo model of islet-cell migration, we demonstrate that Myt3 suppression significantly inhibits laminin-V/integrin-β1-dependent α- and β-cell migration onto 804G, and impaired 804G-induced F-actin and E-cadherin redistribution. Exposure of islets to proinflammatory cytokines, which suppress Myt3 expression, had a similar effect, whereas Myt3 overexpression partially rescued the migratory ability of the islet cells. We show that loss of islet-cell migration, due to Myt3 suppression or cytokine exposure, is independent of effects on islet-cell survival or proliferation. Myt3 suppression also had no effect on glucose-induced calcium influx, F-actin remodeling or insulin secretion by β-cells. RNA-sequencing (RNA-seq) analysis of transduced islets showed that Myt3 suppression results in the up-regulation of Tgfbi, a secreted diabetogenic factor thought to impair cellular adhesion. Exposure of islets to exogenous transforming growth factor β-induced (Tgfbi) impaired islet-cell migration similar to Myt3 suppression. Taken together, these data suggest a model by which cytokine-induced Myt3 suppression leads to Tgfbi de-repression and subsequently to impaired islet-cell migration, revealing a novel role for Myt3 in regulating islet-cell migration.

Fibrin supports human fetal islet-epithelial cell differentiation via p70(s6k) and promotes vascular formation during transplantation

The human fetal pancreas expresses a variety of extracellular matrix (ECM) binding receptors known as integrins. A provisional ECM protein found in blood clots that can bind to integrin receptors and promote β cell function and survival is fibrin. However, its role in support of human fetal pancreatic cells is unknown. We investigated how fibrin promotes human fetal pancreatic cell differentiation in vitro and in vivo. Human fetal pancreata were collected from 15 to 21 weeks of gestation and collagenase digested. Cells were then plated on tissue-culture polystyrene, or with 2D or 3D fibrin gels up to 2 weeks, or subcutaneously transplanted in 3D fibrin gels. The human fetal pancreas contained rich ECM proteins and expressed integrin αVβ3. Fibrin-cultured human fetal pancreatic cells had significantly increased expression of PDX-1, glucagon, insulin, and VEGF-A, along with increased integrin αVβ3 and phosphorylated FAK and p70(s6k). Fibrin-cultured cells treated with rapamycin, the mTOR pathway inhibitor, had significantly decreased phospho-p70(s6k) and PDX-1 expression. Transplanting fibrin-mixed cells into nude mice improved vascularization compared with collagen controls. These results suggest that fibrin supports islet cell differentiation via p70(s6k) and promotes vascularization in human fetal islet-epithelial clusters in vivo.

Human β-cell proliferation and intracellular signaling: part 3

This is the third in a series of Perspectives on intracellular signaling pathways coupled to proliferation in pancreatic β-cells. We contrast the large knowledge base in rodent β-cells with the more limited human database. With the increasing incidence of type 1 diabetes and the recognition that type 2 diabetes is also due in part to a deficiency of functioning β-cells, there is great urgency to identify therapeutic approaches to expand human β-cell numbers. Therapeutic approaches might include stem cell differentiation, transdifferentiation, or expansion of cadaver islets or residual endogenous β-cells. In these Perspectives, we focus on β-cell proliferation. Past Perspectives reviewed fundamental cell cycle regulation and its upstream regulation by insulin/IGF signaling via phosphatidylinositol-3 kinase/mammalian target of rapamycin signaling, glucose, glycogen synthase kinase-3 and liver kinase B1, protein kinase Cζ, calcium-calcineurin-nuclear factor of activated T cells, epidermal growth factor/platelet-derived growth factor family members, Wnt/β-catenin, leptin, and estrogen and progesterone. Here, we emphasize Janus kinase/signal transducers and activators of transcription, Ras/Raf/extracellular signal-related kinase, cadherins and integrins, G-protein-coupled receptors, and transforming growth factor β signaling. We hope these three Perspectives will serve to introduce these pathways to new researchers and will encourage additional investigators to focus on understanding how to harness key intracellular signaling pathways for therapeutic human β-cell regeneration for diabetes.

Fibrin, a scaffold material for islet transplantation and pancreatic endocrine tissue engineering

Fibrin is derived from fibrinogen during injury to produce a blood clot and thus promote wound repair. Composed of different domains, including Arg-Gly-Asp amino acid motifs, fibrin is used extensively as a hydrogel and sealant in the clinic. By binding to cell surface receptors like integrins and acting as a supportive 3D scaffold, fibrin has been useful in promoting cell differentiation, proliferation, function, and survival. In particular, fibrin has been able to maintain islet cell architecture, promote beta cell insulin secretion, and islet angiogenesis, as well as inducing a protective effect against cell death. During islet transplantation, fibrin improved neovascularization and islet function. These improvements resulted in reduced number of transplanted islets necessary to reverse diabetes. Therefore, fibrin, as a biocompatible and biodegradable scaffold, should be considered during subcutaneous islet transplantation and beta cell expansion in vitro to ensure maintenance of islet cell function, proliferation, and survival to develop effective cell-based therapies for the treatment of diabetes.

The endocrine role between β cells and intra-islet endothelial cells

The islets of Langerhans is the endocrine function region of pancreas, which exist in five cell types. The majority of endocrine cells are insulin-secreting β cells, mixed up with glucagon-secreting α-cells. The islets of Langerhans are highly vascularized, and the capillary network around the islet is about five times denser than that in the exocrine tissues. It guarantees endocrine cells adequately contact with the capillary networks. Above mentioned is the basis of deep study the interaction between β cells and capillary. Increasing number of studies contribute to the consensus that endothelial cells have positive effects in the islet microenvironment. Endothelial cells can act as endocrine cells which release many active substances, such as hepatocyte growth factors (HGF), thrombospondin-1(TSP-1), laminins, and collagens by means of different molecule pathways, inducing β cells differentiation, proliferation, survivor, and insulin release next to the vessels. Apart from the effect of endothelial cells on β cells by paracrine fashion, the islets can utilize VEGF-A, angiopoietin-1 and insulin signaling to increase the interaction with endothelial cells. As the endocrine role of endothelial cells to β cells, it may be a novel target to stimulate β cells regeneration, promote vascularization post islet transplantation strategy in the treatment of diabetes mellitus.

Endothelial progenitor cells enhance islet engraftment, influence β-cell function, and modulate islet connexin 36 expression

The success of pancreatic islet transplantation is limited by delayed engraftment and suboptimal function in the longer term. Endothelial progenitor cells (EPCs) represent a potential cellular therapy that may improve the engraftment of transplanted pancreatic islets. In addition, EPCs may directly affect the function of pancreatic β-cells. The objective of this study was to examine the ability of EPCs to enhance pancreatic islet transplantation in a murine syngeneic marginal mass transplant model and to examine the mechanisms through which this occurs. We found that cotransplanted EPCs improved the cure rate and initial glycemic control of transplanted islets. Gene expression data indicate that EPCs, or their soluble products, modulate the expression of the β-cell surface molecule connexin 36 and affect glucose-stimulated insulin release in vitro. In conclusion, EPCs are a promising candidate for improving outcomes in islet transplantation, and their mechanisms of action warrant further study.

Fibrin improves beta (INS-1) cell function, proliferation and survival through integrin αvβ3

Extracellular matrix (ECM)-integrin stimulation can promote beta cell differentiation, proliferation and function. However, beta cells lose their insulin secretion function in response to glucose stimulation, and senesce when cultured with ECM proteins for a long time. Fibrin is a provisional ECM protein that is capable of maintaining beta cell function, yet the mechanisms by which this occurs is unknown. The present study examined how fibrin interacts with integrin receptors to promote beta cell cluster formation, proliferation and function. The rat insulinoma cell line, INS-1, was cultured on tissue-culture polystyrene, or with 2-D or 3-D fibrin gels for up to 4 weeks. Cells cultured with fibrin formed islet-like clusters and showed direct contacts with fibrin determined by scanning electron microscopy. Fibrin-cultured INS-1 cells also had significantly increased glucose-stimulated insulin secretion. A significant increase in integrin αvβ3 protein and phosphorylated FAK, Erk1/2 and Akt levels was observed in fibrin-cultured INS-1 cells, which was associated with significantly increased cell proliferation and decreased cell apoptosis. Integrin αvβ3 blockade affected INS-1 cell spreading on fibrin gels, and resulted in significantly decreased FAK phosphorylation and increased cleaved caspase-3 levels. These results show that fibrin promotes beta cell function, proliferation and survival via integrin αvβ3 interactions.

β1 integrin is a crucial regulator of pancreatic β-cell expansion

Development of the endocrine compartment of the pancreas, as represented by the islets of Langerhans, occurs through a series of highly regulated events encompassing branching of the pancreatic epithelium, delamination and differentiation of islet progenitors from ductal domains, followed by expansion and three-dimensional organization into islet clusters. Cellular interactions with the extracellular matrix (ECM) mediated by receptors of the integrin family are postulated to regulate key functions in these processes. Yet, specific events regulated by these receptors in the developing pancreas remain unknown. Here, we show that ablation of the β1 integrin gene in developing pancreatic β-cells reduces their ability to expand during embryonic life, during the first week of postnatal life, and thereafter. Mice lacking β1 integrin in insulin-producing cells exhibit a dramatic reduction of the number of β-cells to only ∼18% of wild-type levels. Despite the significant reduction in β-cell mass, these mutant mice are not diabetic. A thorough phenotypic analysis of β-cells lacking β1 integrin revealed a normal expression repertoire of β-cell markers, normal architectural organization within islet clusters, and a normal ultrastructure. Global gene expression analysis revealed that ablation of this ECM receptor in β-cells inhibits the expression of genes regulating cell cycle progression. Collectively, our results demonstrate that β1 integrin receptors function as crucial positive regulators of β-cell expansion.

β1 integrin-extracellular matrix interactions are essential for maintaining exocrine pancreas architecture and function

Integrin receptors are responsible for integrating extracellular matrix signals inside the cell. The most prominent integrin receptor, β1 integrin, has a role in cell function, survival and differentiation. Recently, we demonstrated a profound in vivo role of β1 integrin expression in the pancreas on glucose homeostasis and islet function. Here, we extend these results by examining the role of β1 integrin in exocrine pancreatic structure and function. Adult C57Bl/6 mice hemizygous for a collagen type Iα2 (Col1a2) promoter-controlled tamoxifen-inducible Cre recombinase gene and homozygous for loxP-β1 integrin were injected with tamoxifen or corn oil to generate mice deleted or not for β1 integrin. Pancreata derived from these male mice were analyzed by quantitative reverse transcriptase-polymerase chain reaction, western blot and immunofluorescence. Our results showed that β1 integrin-deficient mice displayed a significant decrease in pancreas weight with a significant reduction of amylase, regenerating islet-derived protein II and carboxypeptidase-A expression (P<0.05-0.01). Compared with control pancreata, β1 integrin-deficient pancreata showed reduced mRNA expression of extracellular matrix (collagen type Iα2, fibronectin and laminin) genes (P<0.05), detached acini clusters and lost focal adhesion structure. Moreover, β1 integrin-deficient pancreatic acinar cells displayed decreased proliferation (P<0.05) and increased apoptosis (P<0.001). Apoptosis was reduced to that of controls when isolated exocrine clusters were cultured in media supplemented with extracellular matrix proteins. Taken together, these results implicate β1 integrin as an essential component for maintaining exocrine pancreatic structure and function.

Integrin β1 is required for dermal homeostasis

Integrins are heterodimers that mediate cell attachment to the extracellular matrix. Previously, we used fibroblast-specific knockout mice to show that the integrin β1 subunit was required in vivo for dermal fibrogenesis and cutaneous tissue repair. Here, we show that integrin β1 expression by fibroblasts is required for dermal homeostasis; at 8 weeks after deletion of integrin β1, the dermis is significantly thinner, expressing less collagen and displaying reduced generation of reactive oxygen species (ROS). Cultured integrin β1-deficient fibroblasts show reduced rac1 activation and ROS generation. Overexpressing rac1 in integrin β1-deficient fibroblasts restored ROS generation; adding hydrogen peroxide to integrin β1-deficient fibroblasts restored Col1a2 (collagen, type I, α2) and α-smooth muscle actin (α-SMA) mRNA expression and α-SMA protein expression and stress fiber formation. Thus integrin β1 expression by fibroblasts is required for the maintenance of dermal connective tissue via a rac/ROS pathway.

Critical role of c-Kit in beta cell function: increased insulin secretion and protection against diabetes in a mouse model

AIMS/HYPOTHESIS

The receptor tyrosine kinase, c-Kit, and its ligand, stem cell factor, control a variety of cellular processes, including pancreatic beta cell survival and differentiation as revealed in c-Kit ( Wv ) mice, which have a point mutation in the c-Kit allele leading to loss of kinase activity and develop diabetes. The present study further investigated the intrinsic role of c-Kit in beta cells, especially the underlying mechanisms that influence beta cell function.

METHODS

We generated a novel transgenic mouse model with c-KIT overexpression specifically in beta cells (c-KitβTg) to further examine the physiological and functional roles of c-Kit in beta cells. Isolated islets from these mice were used to investigate the underlying molecular pathway of c-Kit in beta cells. We also characterised the ability of c-Kit to protect animals from high-fat-diet-induced diabetes, as well as to rescue c-Kit ( Wv ) mice from early onset of diabetes.

RESULTS

c-KitβTg mice exhibited improved beta cell function, with significantly improved insulin secretion, and increased beta cell mass and proliferation in response to high-fat-diet-induced diabetes. c-KitβTg islets exhibited upregulation of: (1) insulin receptor and IRSs; (2) Akt and glycogen synthase kinase 3β phosphorylation; and (3) transcription factors important for islet function. c-KIT overexpression in beta cells also rescued diabetes observed in c-Kit ( Wv ) mice.

CONCLUSIONS/INTERPRETATION

These findings demonstrate that c-Kit plays a direct protective role in beta cells, by regulating glucose metabolism and beta cell function. c-Kit may therefore represent a novel target for treating diabetes.

In vivo role of focal adhesion kinase in regulating pancreatic β-cell mass and function through insulin signaling, actin dynamics, and granule trafficking

Focal adhesion kinase (FAK) acts as an adaptor at the focal contacts serving as a junction between the extracellular matrix and actin cytoskeleton. Actin dynamics is known as a determinant step in insulin secretion. Additionally, FAK has been shown to regulate insulin signaling. To investigate the essential physiological role of FAK in pancreatic β-cells in vivo, we generated a transgenic mouse model using rat insulin promoter (RIP)-driven Cre-loxP recombination system to specifically delete FAK in pancreatic β-cells. These RIPcre(+)fak(fl/fl) mice exhibited glucose intolerance without changes in insulin sensitivity. Reduced β-cell viability and proliferation resulting in decreased β-cell mass was observed in these mice, which was associated with attenuated insulin/Akt (also known as protein kinase B) and extracellular signal-related kinase 1/2 signaling and increased caspase 3 activation. FAK-deficient β-cells exhibited impaired insulin secretion with normal glucose sensing and preserved Ca(2+) influx in response to glucose, but a reduced number of docked insulin granules and insulin exocytosis were found, which was associated with a decrease in focal proteins, paxillin and talin, and an impairment in actin depolymerization. This study is the first to show in vivo that FAK is critical for pancreatic β-cell viability and function through regulation in insulin signaling, actin dynamics, and granule trafficking.

Diabetes imaging-quantitative assessment of islets of Langerhans distribution in murine pancreas using extended-focus optical coherence microscopy

Diabetes is characterized by hyperglycemia that can result from the loss of pancreatic insulin secreting β-cells in the islets of Langerhans. We analyzed ex vivo the entire gastric and duodenal lobes of a murine pancreas using extended-focus Optical Coherence Microscopy (xfOCM). To identify and quantify the islets of Langerhans observed in xfOCM tomograms we implemented an active contour algorithm based on the level set method. We show that xfOCM reveals a three-dimensional islet distribution consistent with Optical Projection Tomography, albeit with a higher resolution that also enables the detection of the smallest islets (≤ 8000 μm(3)). Although this category of the smallest islets represents only a negligible volume compared to the total β-cell volume, a recent study suggests that these islets, located at the periphery, are the first to be destroyed when type I diabetes develops. Our results underline the capability of xfOCM to contribute to the understanding of the development of diabetes, especially when considering islet volume distribution instead of the total β-cell volume only.