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

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.

Exocrine-Endocrine Crosstalk: The Influence of Pancreatic Cellular Communications on Organ Growth, Function and Disease

Diabetes mellitus, a disease that affects nearly 536.6 million people worldwide, is characterized by the death or dysfunction of insulin-producing beta cells of the pancreas. The beta cells are found within the islets of Langerhans, which are composed of multiple hormone-producing endocrine cells including the alpha (glucagon), delta (somatostatin), PP (pancreatic polypeptide), and epsilon (ghrelin) cells. There is direct evidence that physical and paracrine interactions between the cells in the islet facilitate and support beta cell function. However, communication between endocrine and exocrine cells in the pancreas may also directly impact beta cell growth and function. Herein we review literature that contributes to the view that "crosstalk" between neighboring cells within the pancreas influences beta cell growth and function and the maintenance of beta cell health.

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.

Single-Cell Transcriptomics Links Loss of Human Pancreatic β-Cell Identity to ER Stress

The maintenance of pancreatic islet architecture is crucial for proper β-cell function. We previously reported that disruption of human islet integrity could result in altered β-cell identity. Here we combine β-cell lineage tracing and single-cell transcriptomics to investigate the mechanisms underlying this process in primary human islet cells. Using drug-induced ER stress and cytoskeleton modification models, we demonstrate that altering the islet structure triggers an unfolding protein response that causes the downregulation of β-cell maturity genes. Collectively, our findings illustrate the close relationship between endoplasmic reticulum homeostasis and β-cell phenotype, and strengthen the concept of altered β-cell identity as a mechanism underlying the loss of functional β-cell mass.

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.

Coordinated interactions between endothelial cells and macrophages in the islet microenvironment promote β cell regeneration

Endogenous β cell regeneration could alleviate diabetes, but proliferative stimuli within the islet microenvironment are incompletely understood. We previously found that β cell recovery following hypervascularization-induced β cell loss involves interactions with endothelial cells (ECs) and macrophages (MΦs). Here we show that proliferative ECs modulate MΦ infiltration and phenotype during β cell loss, and recruited MΦs are essential for β cell recovery. Furthermore, VEGFR2 inactivation in quiescent ECs accelerates islet vascular regression during β cell recovery and leads to increased β cell proliferation without changes in MΦ phenotype or number. Transcriptome analysis of β cells, ECs, and MΦs reveals that β cell proliferation coincides with elevated expression of extracellular matrix remodeling molecules and growth factors likely driving activation of proliferative signaling pathways in β cells. Collectively, these findings suggest a new β cell regeneration paradigm whereby coordinated interactions between intra-islet MΦs, ECs, and extracellular matrix mediate β cell self-renewal.

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.

Postnatal knockout of beta cell insulin receptor impaired insulin secretion in male mice exposed to high-fat diet stress

The presence of insulin receptor (IR) on insulin-secreting beta cells suggests an autocrine regulatory role for insulin in its own signalling. Congenital beta cell-specific IR knockout (βIRKO) mouse studies have demonstrated the development of age-dependent glucose intolerance. We investigated the role of beta cell IR signalling specifically during postnatal life following undisturbed prenatal pancreatic development and maturation. We utilized a tamoxifen-inducible mouse insulin 1 promoter (MIP) driven Cre recombinase IR knockout mouse model (MIP-βIRKO) to achieve partial knockout of IR in islets and determine the functional role of beta cell IR in adult mice fed a control normal diet (ND) or 60% high-fat diet (HFD). At 24 weeks of age, MIP-βIRKO ND mice maintained glucose tolerance, insulin release, and unchanged beta cell mass when compared to control ND mice. In contrast, 24-week-old MIP-βIRKO mice demonstrated significant glucose intolerance and lower insulin release after 18 weeks of HFD feeding. A reduction in beta cell soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein expression, phosphorylated Akt^S473 and P70S6K1^T389, and glucose transporter 2 (GLUT2) expression were also identified in MIP-βIRKO HFD islets. Overall, the postnatal knockout of beta cell IR in HFD-fed mice resulted in decreased expression of beta cell glucose-sensing and exocytotic proteins and a reduction in intracellular signalling. These findings highlight that IR expression in the adult islet is required to maintain beta cell function under hyperglycemic stress.

Extracellular Matrix-Associated Factors Play Critical Roles in Regulating Pancreatic β-Cell Proliferation and Survival

This review describes formation of the islet basement membrane and the function of extracellular matrix (ECM) components in β-cell proliferation and survival. Implications for islet transplantation are discussed. The insulin-producing β-cell is key for maintaining glucose homeostasis. The islet microenvironment greatly influences β-cell survival and proliferation. Within the islet, β-cells contact the ECM, which is deposited primarily by intraislet endothelial cells, and this interaction has been shown to modulate proliferation and survival. ECM-localized growth factors, such as vascular endothelial growth factor and cellular communication network 2, signal through specific receptors and integrins on the β-cell surface. Further understanding of how the ECM functions to influence β-cell proliferation and survival will provide targets for enhancing functional β-cell mass for the treatment of diabetes.

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

AIMS/HYPOTHESIS

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

METHODS

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

RESULTS

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

CONCLUSIONS/INTERPRETATION

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

Enhanced differentiation of human pluripotent stem cells into pancreatic progenitors co-expressing PDX1 and NKX6.1

Background

Pancreatic progenitors (PPs) co-expressing the two transcription factors (TFs) PDX1 and NKX6.1 are recognized as the indispensable precursors of functional pancreatic β cells. Here, we aimed to establish an efficient protocol for maximizing generation of PDX1⁺/NKX6.1⁺ PPs from human pluripotent stem cells (hPSCs).

Methods

In order to enhance the PDX1⁺/NKX6.1⁺ population, we manipulated in vitro culture conditions during differentiation by dissociating densely formed endodermal cells and re-plating them at different densities. These dissociated cells were subjected to an augmented duration of retinoid and fibroblast growth factor (FGF)10 signaling to induce higher PDX1 and NKX6.1 expression.

Results

Our optimized protocol dramatically increased the expression of NKX6.1, leading to an increase in the proportion of PDX1⁺/NKX6.1⁺ progenitors (~90%) in monolayer, higher than the previously published protocols, as well as upregulated key TFs controlling pancreatic development. The improved efficiency of pancreatic differentiation was complemented by an inhibited hepatic specification and an increased proliferation of NKX6.1⁺ cells. Interestingly, we were able to enrich a novel PDX1^–/NKX6.1⁺ population by manipulating the re-plating density; these oriented themselves in three-dimensional clusters. Further differentiation validated the ability of our PDX1⁺/NKX6.1⁺ progenitors to generate NGN3⁺ endocrine progenitors.

Conclusions

We provide a novel technique that facilitates appropriate cellular rearrangement in monolayer culture to yield a high proportion of PDX1⁺/NKX6.1⁺ PPs with an elevated self-replicating capacity, thereby aiding scalable production of functional β cells from hPSCs in vitro. Our innovative method also enriches a novel NKX6.1⁺/PDX1^– population, with characteristics of proposed endocrine precursors, allowing further studies on deciphering routes to β-cell development.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0759-z) contains supplementary material, which is available to authorized users.