Jake A. Kushner

Haematopoietic stem cells do not asymmetrically segregate chromosomes or retain BrdU

Stem cells are proposed to segregate chromosomes asymmetrically during self-renewing divisions so that older (‘immortal’) DNA strands are retained in daughter stem cells whereas newly synthesized strands segregate to differentiating cells. Stem cells are also proposed to retain DNA labels, such as 5-bromo-2-deoxyuridine (BrdU), either because they segregate chromosomes asymmetrically or because they divide slowly. However, the purity of stem cells among BrdU-label-retaining cells has not been documented in any tissue, and the ‘immortal strand hypothesis’ has not been tested in a system with definitive stem cell markers. Here we tested these hypotheses in haematopoietic stem cells (HSCs), which can be highly purified using well characterized markers. We administered BrdU to newborn mice, mice treated with cyclophosphamide and granulocyte colony-stimulating factor, and normal adult mice for 4 to 10 days, followed by 70 days without BrdU. In each case, less than 6% of HSCs retained BrdU and less than 0.5% of all BrdU-retaining haematopoietic cells were HSCs, revealing that BrdU has poor specificity and poor sensitivity as an HSC marker. Sequential administration of 5-chloro-2-deoxyuridine and 5-iodo-2-deoxyuridine indicated that all HSCs segregate their chromosomes randomly. Division of individual HSCs in culture revealed no asymmetric segregation of the label. Thus, HSCs cannot be identified on the basis of BrdU-label retention and do not retain older DNA strands during division, indicating that these are not general properties of stem cells.

Cyclins D2 and D1 Are Essential for Postnatal Pancreatic β-Cell Growth

ABSTRACT Regulation of adult β-cell mass in pancreatic islets is essential to preserve sufficient insulin secretion in order to appropriately regulate glucose homeostasis. In many tissues mitogens influence development by stimulating D-type cyclins (D1, D2, or D3) and activating cyclin-dependent kinases (CDK4 or CDK6), which results in progression through the G1 phase of the cell cycle. Here we show that cyclins D2 and D1 are essential for normal postnatal islet growth. In adult murine islets basal cyclin D2 mRNA expression was easily detected, while cyclin D1 was expressed at lower levels and cyclin D3 was nearly undetectable. Prenatal islet development occurred normally in cyclin D2− / − or cyclin D1 +/ − D2 − / − mice. However, β-cell proliferation, adult mass, and glucose tolerance were decreased in adult cyclin D2 − / − mice, causing glucose intolerance that progressed to diabetes by 12 months of age. Although cyclin D1 +/ − mice never developed diabetes, life-threatening diabetes developed in 3-month-old cyclin D1 − /+ D2 − / − mice as β-cell mass decreased after birth. Thus, cyclins D2 and D1 were essential for β-cell expansion in adult mice. Strategies to tightly regulate D-type cyclin activity in β cells could prevent or cure diabetes.

Effects of dietary glycaemic index on adiposity, glucose homoeostasis, and plasma lipids in animals

BACKGROUND Clinical studies suggest a role for dietary glycaemic index (GI) in bodyweight regulation and diabetes risk. However, partly because manipulation of GI can produce changes in potentially confounding dietary factors such as fibre content, palatability, and energy density, its relevance to human health remains controversial. This study examined the independent effects of GI in animals. METHODS Partially pancreatectomised male Sprague-Dawley rats were given diets with identical nutrients, except for the type of starch: high-GI (n=11) or low-GI (n=10). The animals were fed in a controlled way to maintain the same mean bodyweight in the two groups for 18 weeks. Further experiments examined the effects of GI in rats in a cross-over design and C57BL/6J mice in a parallel design. FINDINGS Despite having similar mean bodyweight (547.9 [SE 13.4] vs 549.2 [15.2] g), rats given high-GI food had more body fat (97.8 [13.6] vs 57.3 [7.2] g; p=0.0152) and less lean body mass (450.1 [9.6] vs 491.9 [11.7] g; p=0.0120) than those given low-GI food. The high-GI group also had greater increases over time in the areas under the curve for blood glucose and plasma insulin after oral glucose, lower plasma adiponectin concentrations, higher plasma triglyceride concentrations, and severe disruption of islet-cell architecture. Mice on the high-GI diet had almost twice the body fat of those on the low-GI diet after 9 weeks. INTERPRETATION These findings provide a mechanistic basis for interpretation of studies of GI in human beings. RELEVANCE TO PRACTICE The term GI describes how a food, meal, or diet affects blood sugar during the postprandial period. GI as an independent factor can cause obesity and increase risks of diabetes and heart disease in animals. Use of low-GI diets in prevention and treatment of human disease merits thorough examination.

Dysregulation of insulin receptor substrate 2 in β cells and brain causes obesity and diabetes

The molecular link between obesity and beta cell failure that causes diabetes is difficult to establish. Here we show that a conditional knockout of insulin receptor substrate 2 (Irs2) in mouse pancreas beta cells and parts of the brain--including the hypothalamus--increased appetite, lean and fat body mass, linear growth, and insulin resistance that progressed to diabetes. Diabetes resolved when the mice were between 6 and 10 months of age: functional beta cells expressing Irs2 repopulated the pancreas, restoring sufficient beta cell function to compensate for insulin resistance in the obese mice. Thus, Irs2 signaling promotes regeneration of adult beta cells and central control of nutrient homeostasis, which can prevent obesity and diabetes in mice.

Adaptive β-Cell Proliferation Is Severely Restricted With Advanced Age

OBJECTIVE Regeneration of the insulin-secreting β-cells is a fundamental research goal that could benefit patients with either type 1 or type 2 diabetes. β-Cell proliferation can be acutely stimulated by a variety of stimuli in young rodents. However, it is unknown whether this adaptive β-cell regeneration capacity is retained into old age. RESEARCH DESIGN AND METHODS We assessed adaptive β-cell proliferation capacity in adult mice across a wide range of ages with a variety of stimuli: partial pancreatectomy, low-dose administration of the β-cell toxin streptozotocin, and exendin-4, a glucagon-like peptide 1 (GLP-1) agonist. β-Cell proliferation was measured by administration of 5-bromo-2′-deoxyuridine (BrdU) in the drinking water. RESULTS Basal β-cell proliferation was severely decreased with advanced age. Partial pancreatectomy greatly stimulated β-cell proliferation in young mice but failed to increase β-cell replication in old mice. Streptozotocin stimulated β-cell replication in young mice but had little effect in old mice. Moreover, administration of GLP-1 agonist exendin-4 stimulated β-cell proliferation in young but not in old mice. Surprisingly, adaptive β-cell proliferation capacity was minimal after 12 months of age, which is early middle age for the adult mouse life span. CONCLUSIONS Adaptive β-cell proliferation is severely restricted with advanced age in mice, whether stimulated by partial pancreatectomy, low-dose streptozotocin, or exendin-4. Thus, β-cells in middle-aged mice appear to be largely postmitotic. Young rodents may not faithfully model the regenerative capacity of β-cells in mature adult mice.

Pdx1 restores β cell function in Irs2 knockout mice

The homeodomain transcription factor Pdx1 is required for pancreas development, including the differentiation and function of beta cells. Mutations in Pdx1 or upstream hepatocyte nuclear factors cause autosomal forms of early-onset diabetes (maturity-onset diabetes of the young [MODY]). In mice, the Irs2 branch of the insulin/Igf signaling system mediates peripheral insulin action and pancreatic beta cell growth and function. To investigate whether beta cell failure in Irs2(-/-) mice might be related to dysfunction of MODY-related transcription factors, we measured the expression of Pdx1 in islets from young Irs2(-/-) mice. Before the onset of diabetes, Pdx1 was reduced in islets from Irs2(-/-) mice, whereas it was expressed normally in islets from wild-type or Irs1(-/-) mice, which do not develop diabetes. Whereas male Irs2(-/-)Pdx1(+/+) mice developed diabetes between 8 and 10 weeks of age, haploinsufficiency for Pdx1 caused diabetes in newborn Irs2(-/-) mice. By contrast, transgenic expression of Pdx1 restored beta cell mass and function in Irs2(-/-) mice and promoted glucose tolerance throughout life, as these mice survived for at least 20 months without diabetes. Our results suggest that dysregulation of Pdx1 might represent a common link between ordinary type 2 diabetes and MODY.

Effects of Autoimmunity and Immune Therapy on β-Cell Turnover in Type 1 Diabetes

β-Cell mass can expand in response to demand: during pregnancy, in the setting of insulin resistance, or after pancreatectomy. It is not known whether similar β-cell hyperplasia occurs following immune therapy of autoimmune diabetes, but the clinical remission soon after diagnosis and the results of recent immune therapy studies suggest that β-cell recovery is possible. We studied changes in β-cell replication, mass, and apoptosis in NOD mice during progression to overt diabetes and following immune therapy with anti-CD3 monoclonal antibodies (mAbs) or immune regulatory T-cells (Tregs). β-Cell replication increases in pre-diabetic mice, after adoptive transfer of diabetes with increasing islet inflammation but before an increase in blood glucose concentration or a significant decrease in β-cell mass. The pathogenic cells are responsible for increasing β-cell replication because replication was reduced during diabetes remission induced by anti-CD3 mAb or Tregs. β-Cell replication stimulated by the initial inflammatory infiltrate results in increased production of new β-cells after immune therapy and increased β-cell area, but the majority of this increased β-cell area represents regranulated β-cells rather than newly produced cells. We conclude that β-cell replication is closely linked to the islet inflammatory process. A significant proportion of degranulated β-cells remain, at the time of diagnosis of diabetes, that can recover after metabolic correction of hyperglycemia. Correction of the β-cell loss in type 1 diabetes will, therefore, require strategies that target both the immunologic and cellular mechanisms that destroy and maintain β-cell mass.

miR-211 Is a Prosurvival MicroRNA that Regulates chop Expression in a PERK-Dependent Manner

MicroRNAs typically function at the level of posttranscriptional gene silencing within the cytoplasm; however, increasing evidence suggests that they may also function in nuclear, Argonaut-containing complexes, to directly repress target gene transcription. We have investigated the role of microRNAs in mediating endoplasmic reticulum (ER) stress responses. ER stress triggers the activation of three signaling molecules: Ire-1α/β, PERK, and ATF6, whose function is to facilitate adaption to the ensuing stress. We demonstrate that PERK induces miR-211, which in turn attenuates stress-dependent expression of the proapoptotic transcription factor chop/gadd153. MiR-211 directly targets the proximal chop/gadd153 promoter, where it increases histone methylation and represses chop expression. Maximal chop accumulation ultimately correlates with miR-211 downregulation. Our data suggest a model in which PERK-dependent miR-211 induction prevents premature chop accumulation and thereby provides a window of opportunity for the cell to re-establish homeostasis prior to apoptotic commitment.

β-Cell Mass and Type 1 Diabetes: Going, Going, Gone?

OBJECTIVE— β-Cell regeneration is a fundamental but elusive goal for type 1 diabetes research. Our objective is to review newer human and animal studies of β-cell destruction and regeneration and consider the implications for treatment of type 1 diabetes. RESEARCH DESIGN AND METHODS— Recent human and animal studies of β-cell destruction and regeneration in type 1 diabetes are reviewed. RESULTS— The loss of β-cells that characterizes type 1 diabetes reflects the net effects of destruction and regeneration. These processes have been examined in the nonobese diabetic (NOD) mouse; uncertainty remains about β-cell dynamics in humans. Islet inflammation stimulates β-cell replication that produces new insulin-positive cells. The regenerative process may tide the loss of overall β-cell function, but it also may enhance the autoimmune attack on β-cells by providing new epitopes. The highest rates of β-cell replication are at the time of diagnosis of diabetes in NOD mice, and if autoimmunity and islet inflammation are arrested, new β-cells are formed. However, the majority of β-cells after treatment with immune modulators such as anti-CD3 monoclonal antibody, and most likely during the “honeymoon” in human disease, are recovered β-cells that had been degranulated but present at the time of diagnosis of diabetes. CONCLUSIONS— Residual β-cells play a significant role for the design of therapeutic trials: they not only may respond to combination therapies that include stimulants of metabolic function but are also the potential source of new β-cells.

The role of aging upon β cell turnover

Preservation and regeneration of β cell endocrine function is a long-sought goal in diabetes research. Defective insulin secretion from β cells underlies both type 1 and type 2 diabetes, thus fueling considerable interest in molecules capable of rebuilding β cell secretion capacity. Though early work in rodents suggested that regeneration might be possible, recent studies have revealed that aging powerfully restricts cell cycle entry of β cells, which may limit regeneration capacity. Consequently, aging has emerged as an enigmatic challenge that might limit β cell regeneration therapies. This Review summarizes recent data regarding the role of aging in β cell regeneration and proposes models explaining these phenomena.