Anthony M.P. Montgomery

Recognition of the Neural Chemoattractant Netrin-1 by Integrins α6β4 and α3β1 Regulates Epithelial Cell Adhesion and Migration

Netrins, axon guidance cues in the CNS, have also been detected in epithelial tissues. In this study, using the embryonic pancreas as a model system, we show that Netrin-1 is expressed in a discrete population of epithelial cells, localizes to basal membranes, and specifically associates with elements of the extracellular matrix. We demonstrate that alpha6beta4 integrin mediates pancreatic epithelial cell adhesion to Netrin-1, whereas recruitment of alpha6beta4 and alpha3beta1 regulate the migration of CK19+/PDX1+ putative pancreatic progenitors on Netrin-1. These results provide evidence for the activation of epithelial cell adhesion and migration by a neural chemoattractant, and identify Netrin-1/integrin interactions as adhesive/guidance cues for epithelial cells.

Regulation of Human β-Cell Adhesion, Motility, and Insulin Secretion by Collagen IV and Its Receptor α1β1

Collagens have been shown to influence the survival and function of cultured β-cells; however, the utilization and function of individual collagen receptors in β-cells is largely unknown. The integrin superfamily contains up to five collagen receptors, but we have determined that α1β1 is the primary receptor utilized by both fetal and adult β-cells. Cultured β-cells adhered to and migrated on collagen type IV (Col-IV), and these responses were mediated almost exclusively by α1β1. The migration of cultured β-cells to Col-IV significantly exceeded that to other matrix components suggesting that this substrate is of unique importance for β-cell motility. The interaction of α1β1 with Col-IV also resulted in significant insulin secretion at basal glucose concentrations. A subset of β-cells in developing islets was confirmed to express α1β1, and this expression co-localized with Col-IV in the basal membranes of juxtaposed endothelial cells. Our findings indicate that α1β1 and Col-IV contribute to β-cell functions known to be important for islet morphogenesis and glucose homeostasis.

A Novel Approach for the Derivation of Putative Primordial Germ Cells and Sertoli Cells from Human Embryonic Stem Cells

Using human embryonic stem cells (hESCs), we describe a novel method for the rapid derivation and enrichment of cells that are comparable to primordial germ cells (PGCs) and Sertoli cells. The methodology described is based on modest changes to the growth conditions commonly used to expand hESCs and does not require genetic manipulation or complex three‐dimensional culture. Remarkably, we have determined that simply reducing the size of cultured ESC colonies and manipulating the number of feeding cycles, results in the rapid emergence of cells that are comparable to migratory PGCs. Importantly, these cells can be monitored and purified on the basis of the expression of the chemokine receptor CXCR4. Under more stringent differentiating conditions these cells mature and upregulate the expression of specific germ cell markers. Importantly, this process is accompanied by the development of Sertoli‐like support cells. Such cells normally provide trophic support and immunoprotection to developing germ cells and may have significant clinical utility in the prevention of graft rejection. The putative Sertoli‐germ cell cocultures generated in this study may ultimately be developed to study and manipulate interactions and processes involved in human gametogenesis. STEM CELLS 2009;27:68–77

Impact of Defined Matrix Interactions on Insulin Production by Cultured Human β-Cells: Effect on Insulin Content, Secretion, and Gene Transcription

The impact of extracellular matrix on insulin production needs to be understood both to optimize the derivation of functional β-cells for transplantation and to understand mechanisms controlling islet neogenesis and glucose homeostasis. In this study, we present evidence that adhesion to some common matrix constituents has a profound impact on the transcription, secretion, and storage of insulin by human β-cells. The integrin-dependent adhesion of fetal β-cells to both collagen IV and vitronectin induces significant glucose-independent insulin secretion and a substantial reciprocal decline in insulin content. Collagen IV, but not vitronectin, induces comparable responses in adult β-cells. Inhibition of extracellular signal–regulated kinase activation abrogates matrix-induced insulin secretion and effectively preserves the insulin content of adherent β-cells. Using real-time PCR, we demonstrate that adhesion of both fetal and adult β-cells to collagen IV and vitronectin also results in the marked suppression of insulin gene transcription. Based on these findings, we contend that integrin-dependent adhesion and signaling in response to certain matrices can have a significant negative impact on insulin production by primary human β-cells. Such responses were not found to be associated with cell death but may precede β-cell dedifferentiation.

Endothelium-derived Netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1.

Background Netrins have been extensively studied in the developing central nervous system as pathfinding guidance cues, and more recently in non-neural tissues where they mediate cell adhesion, migration and differentiation. Netrin-4, a distant relative of Netrins 1–3, has been proposed to affect cell fate determination in developing epithelia, though receptors mediating these functions have yet to be identified. Methodology/Principal Findings Using human embryonic pancreatic cells as a model of developing epithelium, here we report that Netrin-4 is abundantly expressed in vascular endothelial cells and pancreatic ductal cells, and supports epithelial cell adhesion through integrins α2β1and α3β1. Interestingly, we find that Netrin-4 recognition by embryonic pancreatic cells through integrins α2β1 and α3β1 promotes insulin and glucagon gene expression. In addition, full genome microarray analysis revealed that fetal pancreatic cell adhesion to Netrin-4 causes a prominent down-regulation of cyclins and up-regulation of negative regulators of the cell cycle. Consistent with these results, a number of other genes whose activities have been linked to developmental decisions and/or cellular differentiation are up-regulated. Conclusions/Significance Given the recognized function of blood vessels in epithelial tissue morphogenesis, our results provide a mechanism by which endothelial-derived Netrin-4 may function as a pro-differentiation cue for adjacent developing pancreatic cell populations expressing adhesion receptors α2β1 and α3β1 integrins.

The human neural cell adhesion molecule L1 functions as a costimulatory molecule in T cell activation

L1 is a neural cell adhesion molecule (CAM) known to be important for normal neurological development. Despite being described as a neural CAM, we have documented L1 expression by antigen‐presenting cells of myelomonocytic origin. Here we demonstrate that L1 can function as a costimulatory molecule in T cell activation. A monoclonal antibody that abrogates L1‐L1 homophilic binding significantly reduced mixed leukocyte responses initiated by allogeneic L1+ dendritic cells. Autologous T cell activation in response to phytohemagglutinin was also inhibited by blockade of L1. In accordance with these results, transfection of human L1 into a murine myeloma cell line significantly increased the capacity of these cells to stimulate xenogeneic T cell responses. As a costimulatory ligand L1 could represent a novel target for immunotherapeutic intervention and may act as an important intermediary in neuroimmunological processes and disease.

Inhibitors of Src and Focal Adhesion Kinase Promote Endocrine Specification IMPACT ON THE DERIVATION OF β-CELLS FROM HUMAN PLURIPOTENT STEM CELLS

Background: Signaling mechanisms regulating the endocrine specification of stem cell derivatives. Results: Inhibiting Src family kinase (SFK) and focal adhesion kinase (FAK) activity promotes endocrine commitment. Conclusion: Select inhibitors of SFK/FAK signaling increase the derivation of β-cells. Significance: Enhanced derivation of insulin-producing β-cells for the cell-based therapy of diabetes. Stepwise approaches for the derivation of β-cells from human embryonic stem cells have been described. However, low levels of endocrine specification limit the final yield of insulin-producing β-cells. In this study, we show that the pyrrolo-pyrimidine Src family kinase (SFK) inhibitor PP2 effectively promotes the endocrine specification of human embryonic stem cell derivatives based on its capacity to induce the expression of proendocrine transcription factors (NGN3, NEUROD1, NKX2.2, and PAX4) and to significantly increase the final yield of insulin-positive cells. We further demonstrate that PP2 inhibits the activation of focal adhesion kinase (FAK), and selective inhibition of this kinase is also sufficient to induce early endocrine commitment based on increased expression of NGN3, NEUROD1, and NKX2.2. Additional studies using dominant negative constructs and isolated human fetal pancreata suggest that c-Src is at least partially responsible for inhibiting early endocrine specification. Mechanistically, we propose that inhibition of SFK/FAK signaling can promote endocrine specification by limiting activation of the TGFβR/Smad2/3 pathway. Moreover, we show that inhibition of SFK/FAK signaling suppresses cell growth, increases the expression of the β-cell-associated cyclin-dependent kinase inhibitor p57kip2, and simultaneously suppresses the expression of Id1 and Id2. This study has important implications for the derivation of β-cells for the cell-based therapy of diabetes and sheds new light on the signaling events that regulate early endocrine specification.

Impact of integrin–matrix interaction and signaling on insulin gene expression and the mesenchymal transition of human β-cells†

A critical shortage of donor pancreata currently prevents the development of a universal cell‐based therapy for type I diabetes. The ex vivo expansion of insulin‐producing β‐cells offers a potential solution but is problematic due to the inherent tendency of these cells to transition into mesenchymal‐like cells that are devoid of function. Here, we demonstrate for the first time that exposure to elements of the extracellular matrix (ECM) directly potentiates the mesenchymal transition of cultured fetal β‐cells and causes associated declines in insulin gene expression. Individual ECM constituents varied in their ability to induce such responses, with collagen‐IV (C‐IV) and fibronectin inducing strong responses, whereas laminin‐1 had no significant effect. Mesenchymal transition and concomitant losses in insulin gene expression observed on C‐IV were found to be dependent on β1‐integrin ligation and were augmented in the presence of hepatocyte growth factor. Importantly, selective inhibition of c‐Src, c‐Jun N‐terminal kinase (JNK), and extracellular signal‐regulated kinase (ERK) prior to exposure to C‐IV prevented mesenchymal transition and effectively preserved insulin expression. Fetal β‐cells undergoing mesenchymal transition were found to acquire α1β1 expression, and ligation of this integrin then promotes declines in insulin gene expression and a marked increase in β‐cell motility. Inhibition of Src‐, ERK‐, or JNK‐dependent signaling combined with the selective regulation of matrix exposure may ultimately facilitate the development of more effective β‐cell expansion protocols. J. Cell. Physiol. 224:101–111, 2010

The epithelial-to-mesenchymal transition of human pancreatic β-cells: inductive mechanisms and implications for the cell-based therapy of type I diabetes.

As a therapy for type I diabetes, islet transplantation provides clear benefits in terms of increased insulin-independence and a reduced risk of hypoglycemia. However, a critical shortage of donor pancreata means that few can benefit from this approach. The ex vivo expansion of human β-cells prior to transplantation could ameliorate this problem, however, attempts to grow large numbers of β-cells that retain their native phenotype have thus far failed. Recent lineage tracing studies suggest that this problem is due to the inherent tendency of cultured human β-cells to undergo a process reminiscent of epithelial-to-mesenchymal transition (EMT). EMT describes a highly complex process that culminates in a loss of epithelial cell polarity, severance of intercellular adhesive junctions and the acquisition of a highly motile mesenchymal phenotype. Interestingly, recent evidence suggests that a transient EMT-like process may also contribute to the delamination of endocrine progenitors and subsequent islet neogenesis. The inherent susceptibility of cultured human β-cells to EMT, and the potential involvement of this process during islet neogenesis, raises important questions as to how this process is triggered and subsequently regulated. The primary purpose of this review is to describe those factors, pathways or processes that are complicit in inducing or regulating the mesenchymal transition of human β-cells. This includes addressing the role of the extracellular matrix, the contribution of select signaling pathways, and the regulatory function of microRNAs. We propose that manipulation of these cues and pathways offers the greatest potential for restoring β-cell function after ex vivo expansion.