Current and Future Developments in Physiology

In recent decades, intense efforts have been made to understand the cellular and molecular mechanisms controlling β-cell development. This process is well coordinated and consists of multiple steps. Many studies have tried to identify (i) molecular signals governing the proliferation of progenitors and (ii) their differentiation into mature pancreatic β-cells. A number of laboratories have focused on the role of transcription factors, and well constructed experiments have contributed to defining a hierarchy, highlighting the importance of each transcription factor in the interconnected network. Moreover, studies over the last 10 years have shown that the pancreatic mesenchymal cells, which are in contact with progenitors, influence pancreas organogenesis. Recent work has also indicated that the intra-uterine milieu influences gene expression and endocrine development. Indeed, nutrients, locally expressed growth factors and even the partial pressure of oxygen also control pancreas development. In a more applied setting, these understandings may improve our knowledge on the different forms of diabetes and, importantly, allow us to mimic a similar developmental process in vitro. This is because the precise understanding of each step in vivo seems to be necessary for designing protocols to generate β-cells from embryonic stem (ES) cells or induced pluripotent stem cells (iPS). These stem cellderived β-cells should, in theory, provide new sources of insulin-secreting cells for transplantation into diabetic patients. A description of the recent advances in the field will be presented and illustrated in this chapter.

Increased age confers a greater risk for the development of type 2 diabetes (T2D), and also has significant consequences for β-cell growth and regeneration. Pancreatic insulin-producing β-cells are long-lived, and exhibit very little turnover in adult life. The severe decline in β-cell proliferation contributes to a decreased capacity for β-cell regeneration with age. β-cell regeneration is dependent on mitogenic signals, receptor and downstream signal transduction, cell cycle progression, and epigenetic regulation of gene expression, all of which are significantly affected by increasing age. Studies suggest that circulating growth factors and their receptors are decreased with age, along with important intracellular signaling molecules, such as Pdx-1 and FoxM1. Cell cycle progression is inhibited by an increased expression of cell cycle inhibitors and a reduction in cell cycle kinase complexes (Cyclin/Cdks). Moreover, decreased expression of epigenetic silencers, such as polycomb group proteins, results in derepression of the cell cycle inhibitor p16, and a significant reduction in β-cell proliferation. Collectively, these age-induced changes present obstacles for the design of β-cell regenerative therapies for diabetes; however, some reports suggest that even very old β-cells can re-enter cell cycle. Future studies will further define the effects of aging on β-cell proliferation and elucidate new drug targets for diabetes therapy.

Regulation of pancreatic β-cell mass is an essential matter to understand pathophysiology of diabetes. Physiological and pathological changes of β-cell mass associated with aging, obesity and diabetes have been reported for over a century. However, the degree of compensation or alteration significantly varies among literature. The difficulty in studying the human pancreas is its large size and uneven distribution of β-cells/islets. Whole pancreas analysis has revealed intra-individual (regional) and inter-individual heterogeneity in β-cell mass, which hampers accurate quantification. Furthermore, physical β-cell loss is not the only contributing factor, but “dysfunctional” β-cells may be involved in insulin deficiency as well. Development of a practical stereological approach to quantify β-cell mass to overcome intra-individual and inter-individual heterogeneity would provide a standardized methodology in the field. Identification of marker(s) for quantifying dysfunctional β-cells that synthesize insulin but are deficient in insulin secretion should lead to a better understanding of β-cell pathophysiology.

The gestational environment can have profound effects on the future health of the offspring, including a greater risk of type 2 diabetes and of cardiovascular diseases. Whilst the function of numerous tissues that can impact on future metabolism are altered by an adverse fetal environment, including the hypothalamic control of appetite and the release of glucocorticoids, hepatic function, and the insulin sensitive tissues such as skeletal muscle and adipose, some of the most definitive data concerns changes in the phenotype and function of the pancreatic β-cells. A number of animal models of intrauterine growth restriction (IUGR) have been utilized to study the longterm effects on the offspring, such as a reduced maternal calorie intake, a reduced protein content of the diet, uterine vessel occlusion, and nicotine administration. Changes to the pancreatic β-cells are remarkably similar and include a reduced tissue mass, lower rate of proliferation, increased developmental apoptosis, less plasticity following damage postnatally, higher sensitivity to cytotoxic cytokines, and reduced glucose-stimulated insulin release. These changes persist into adulthood and result in impaired glucose tolerance, Similar changes are also seen in offspring from pregnancies complicated by maternal diabetes. The mechanisms responsible for the altered β-cells function include changes to the mTOR signaling pathway, epigenetic changes altering the expression of key genes involved with β-cell growth and insulin synthesis, and changes in the rate of telomere shortening resulting in premature cellular aging. These pathways may also be influenced by environmental toxins during pregnancy. Nutritional intervention by micronutrient supplementation of the mother, or treatment of the newborn with peptide hormones trophic for the β-cells can reverse the pancreatic phenotype and reduce the risk of adult metabolic disease.

The development of the structure and function of the endocrine pancreas is known to be influenced by altered nutritional experience during the fetal period. Nutritional modifications in the suckling period are also recognized as contributing factors to developmental programming of the endocrine pancreas. In this chapter we describe the malprogramming of rat pancreatic islet structure and β cell functions in response to an increased intake of carbohydrate-derived calories in a milk formula (HC) during the suckling period. Alterations in β cell function of HC rat pups result in the development of hyperinsulinemia due to β cell plasticity in the immediate postnatal period. These modifications include: altered islet architecture and increased insulinproducing mass, increased insulin secretion capacity with a leftward shift in glucosestimulated insulin secretion, insulin secretion in the absence of glucose and/or Ca2+, increased gene transcription of several genes crucial for β cell development and function, and increased parasympathetic input, as well as malprogramming of orexigenic circuitry in the hypothalamus. Interestingly, these alterations in β cell function are maintained even after weaning of HC rats on a standard rodent chow, resulting in adult-onset obesity due to development of hyperphagia. It is possible that early introduction of carbohydrate-rich infant supplemental foods could contribute to modified β cell functions in infants which could, in turn, over a longer period predispose to the development of childhood obesity and/or adult-onset obesity and its associated metabolic complications including type 2 diabetes.

The regulation of pancreatic β-cell function and mass is critical to the maintenance of euglycemia. β-cells integrate numerous signals from the host to secrete appropriate amounts of insulin and maintain tight control of blood glucose levels. Together with glucose; nutrients, amino acids, hormones, and metabolic by-products contribute to this physiologic response. Within the islet microenvironment, where β- cells reside, there exists a network of interacting pathways that contribute to insulin secretion and regulation of β-cell mass. While factors within these pathways are often sourced from digestive processes and peripheral tissues, intra-islet-derived factors are also important components in the ability of -βcells to accurately integrate metabolic demands with β-cell function. In recent years, many biologic factors have been found to have previously unappreciated autocrine and paracrine roles within the islet. Moreover, differences have been described between signaling within rodent and human islets that are important for informing our understanding of autocrine/paracrine signaling between species. In this review, we highlight these new findings and future directions for this field of study.

Pregnancy is a physiological condition associated with β-cell mass expansion occurring in response to increased insulin demand. If the insulin resistance is not compensated by proper augmented insulin production gestational diabetes will occur. As reviewed herein, pregnancy induced hormonal changes have occupied scientists since the beginning of the last century where important discoveries of the hormonal regulation of metabolism during pregnancy have been accomplished. Of the multiple hormonal and metabolic changes the somatolactogenic hormones, placental lactogens (PL) and placental growth hormone (GH-V) are the most described and are found to have dual roles by induction of insulin resistance and promotion of β-cell function and expansion. More recently, the direct effects on isolated pancreatic islets and the influence of signaling pathways involved in the adaptation of β-cell growth and function during pregnancy have been elucidated. This has identified contributions of a number of known peptide hormones and growth factors (EGF, NGF, HGF, IGFs, GLP-1) and steroid hormones (progesterone, estrogens, glucocorticoids). In addition, glucokinase has been found to be essential for the both proliferation and glucose stimulated insulin secretion during pregnancy. Some transcriptional activators and repressors (FoxM1, HNF4α, Myc, Bcl6, Men1) have been implicated in β-cell growth and survival, but also systemic factors like betatrophin, serotonin and osteoprotegerin have been reported to stimulate β-cell proliferation during pregnancy. Gene expression studies and proteomics of islets from pregnant rodent have furthermore revealed upregulation of a number of genes (e.g. cyclophilin B, stathmins, dlk-1, trefoil factor-3 and several others) that may influence β-cell growth and function during pregnancy although the mechanisms driving these changes are not yet known. Similarly, circulating factors in serum from pregnant women have been identified. Among the stimulating factors are peptide fragments of alpha-1 antitrypsin, kininogen-1, apolipoprotein-1, fibrinogen alpha chain and angiotensinogen. An intriguing question remains about the origin of the increased β-cell mass in pregnancy. In humans, studies have primarily reported an increase in the number of small islets, suggesting that neogenesis as the primary driver of β-cell mass expansion in human. In rodents, however, β-cell replication is believed to be the primary mechanism, although increased expression the neogenesis marker, neurogenin-3, has also been reported in pancreas of pregnant rodents. Interestingly, recent studies have suggested that the apparent loss of β-cells occurring during development of diabetes may be due to dedifferentiation rather than cell death, suggesting contributions from mechanisms going beyond neogenesis and replication. In summary, gestational diabetes (GDM) is associated with lack of appropriate adaptation of the β-cells that may be due to a reduced pre-pregnancy β-cell mass, lack of stimulating hormones and growth factors or appearance of β cytotoxic metabolites or factors. This chapter reviews the existing knowledge of multiple factors and put forward new mechanisms of pregnancy induced β-cell mass expansion, which are not yet completely understood.

Diabetes is a chronic autoimmune disease, causing the destruction of the insulin-producing β-cells of the pancreatic islet and leading to glycemic dysregulation. Exogenous insulin administration and glucose testing moderately rectifies hyperglycemia, but does not provide adequate fine tuning necessary for complete prevention of hypoglycemia acutely, nor micro- and macro-vascular complications in the long-term. Islet transplants have shown great promise for this dynamic glucose regulation, but a shortage of cadaveric-sourced cells, and lifelong immune suppression requirements vastly restrict this technique from being widely available to patients with the disease. Therefore alternative sources of insulin-producing cells are needed. In this chapter, the role of stem cell biology in the current context of diabetes therapy is discussed, including an assessment of human embryonic and human induced pluripotent stem cells for the restoration of β-cell mass. Additionally, the existence of putative resident stem cells, and possible fluidity in lineage fate determination within endocrine pancreas- related cell types is examined.

The International Diabetes Federation estimates 382 million people are currently living with diabetes mellitus worldwide; and with increasing rates of obesity in an aging population this number is predicted to increase to 592 million by 2035. The inability to ameliorate the causes of diabetes has motivated researchers to develop novel approaches aimed at providing curative therapies to replace current symptomatic management using exogenous insulin. Accordingly, postnatal or adult stem cell transplantation has recently emerged as a promising therapeutic strategy following reports detailing the stimulation of islet regeneration in preclinical and early clinical studies. Postnatal bone marrow (BM) and umbilical cord blood (UCB) sources contain progenitor cells of hematopoietic, endothelial, and mesenchymal lineages; and each have demonstrated islet regenerative functions in animal models of diabetes. In the context of this chapter, we summarize accumulating evidence from preclinical and clinical studies describing transplantation of these specific postnatal lineages to stimulate the regeneration of endogenous insulin secreting β-cells, and how these stem cells may be used to provide paracrine support alongside the transplantation of allogeneic islets.