Polyploidy in the pancreas of the normal and diabetic mutant mouse

Pancreatic β-cell heterogeneity in health and diabetes: classes, sources, and subtypes

Pancreatic β-cells perform glucose-stimulated insulin secretion, a process at the center of type 2 diabetes etiology. Efforts to understand how β-cells behave in healthy and stressful conditions have revealed a wide degree of morphological, functional, and transcriptional heterogeneity. Sources of heterogeneity include β-cell topography, developmental origin, maturation state, and stress response. Advances in sequencing and imaging technologies have led to the identification of β-cell subtypes, which play distinct roles in the islet niche. This review examines β-cell heterogeneity from morphological, functional, and transcriptional perspectives, and considers the relevance of topography, maturation, development, and stress response. It also discusses how these factors have been used to identify β-cell subtypes, and how heterogeneity is impacted by diabetes. We examine open questions in the field and discuss recent technological innovations that could advance understanding of β-cell heterogeneity in health and disease.

Residual Diploidy in Polyploid Tissues: A Cellular State with Enhanced Proliferative Capacity for Tissue Regeneration?

A major objective of modern biomedical research aims to promote tissue self-regeneration after injury, obviating the need for whole organ transplantation and avoiding mortality due to organ failure. Identifying the population of cells capable of regeneration, alongside understanding the molecular mechanisms that activate that population to re-enter the cell cycle, are two important steps to advancing the field of endogenous tissue regeneration toward the clinic. In recent years, an emerging trend has been observed, whereby polyploidy of relevant parenchymal cells, arising from alternative cell cycles as part of a normal developmental process, is linked to restricted proliferative capacity of those cells. An accompanying hypothesis, therefore, is that a residual subpopulation of diploid parenchymal cells retains proliferative competence and is the major driver for any detected postnatal cell turnover. In this perspective review, we examine the emerging literature on residual diploid parenchymal cells and the possible link of this population to endogenous tissue regeneration, in the context of both heart and liver. We speculate on additional cell types that may play a similar role in their respective tissues and discuss outstanding questions for the field.

Atypical E2f functions are critical for pancreas polyploidization

The presence of polyploid cells in the endocrine and exocrine pancreas has been reported for four decades. In rodents, pancreatic polyploidization is initiated after weaning and the number of polyploid cells increases with age. Surprisingly the molecular regulators and biological functions of polyploidization in the pancreas are still unknown. We discovered that atypical E2f activity is essential for polyploidization in the pancreas, using an inducible Cre/LoxP approach in new-born mice to delete ubiquitously the atypical E2f transcription factors, E2f7 and E2f8. In contrast to its critical role in embryonic survival, conditional deletion of both of both atypical E2fs in newborn mice had no impact on postnatal survival and mice lived until old age. However, deficiency of E2f7 or E2f8 alone was sufficient to suppress polyploidization in the pancreas and associated with only a minor decrease in blood serum levels of glucose, insulin, amylase and lipase under 4 hours starvation condition compared to wildtype littermates. In mice with fewer pancreatic polyploid cells that were fed ad libitum, no major impact on hormones or enzymes levels was observed. In summary, we identified atypical E2fs to be essential for polyploidization in the pancreas and discovered that postnatal induced loss of both atypical E2fs in many organs is compatible with life until old age.

The diabetes-prone NZO/Hl strain. Proliferation capacity of beta cells in hyperinsulinemia and hyperglycemia

New Zealand Obese (NZO) male mice develop a polygenic juvenile-onset obesity and maturity onset hyperinsulinemia. Approximately 50% transit to chronic hyperglycemia. Here we report on the proliferation of beta cells in relation to both the individual's metabolic status and structural parameters of the endocrine pancreas. Proliferating beta cells were quantified in pancreas sections by immunoenzymatic double staining of Ki-67 protein, as a marker for proliferating cells, and endocrine non-beta cells in order to distinguish them from beta cells. In normoglycemic NZO/Hl males Ki-67 labelling indices (IKi-67) of beta cells varied between 0.14 and 1.5%, and correlated significantly with both serum insulin levels and beta cell size. There was no correlation with the glycemic status. In diabetic males, beta cell size was increased. IKi-67 varied between 1 and 3%. The data suggest that the secretory activity of beta cells triggered by glucose, entailed changes in both beta cell hypertrophy and proliferation. As shown by morphometric measurements, beta cell expansion in diabetic mice was limited, in spite of high IKi-67 values. This suggested increased death rates of beta cells.

No evidence for significant transdifferentiation of bone marrow into pancreatic beta-cells in vivo

Several recent studies have suggested that the adult bone marrow harbors cells that can differentiate into tissues from all three germ layers. Other reports have contradicted these findings or attributed them to cell fusion. In this study, we investigated whether bone marrow-derived cells contribute to the renewal of adult pancreatic endocrine cells, in particular insulin-producing beta-cells, in vivo. To address this issue, we studied mice transplanted with green fluorescent protein (GFP)-positive, sex-mismatched bone marrow. We also extended our studies to pancreatic injury models (partial pancreatectomy and streptozotocin administration). All animals showed stable full donor chimerism in the peripheral blood and microscopic analysis at 4-6 weeks and 3 months after transplantation, indicating that the GFP(+) and Y chromosome-positive donor bone marrow contributed substantially to blood, lymphatic, and interstitial cells in the pancreas. However, after examining >100,000 beta-cells, we found only 2 beta-cells positive for GFP, both of which were in control animals without pancreatic injury. Thus our study results did not support the concept that bone marrow contributes significantly to adult pancreatic beta-cell renewal.

Polyploidy in islets of normal and diabetic humans

This study was designed as a pilot project to determine whether the increased polyploidization of pancreatic B cells in diabetic mice reported from our laboratory is also characteristic of human diabetes. Nuclei of hematoxylin and eosin stained islets cells were traced by camera lucida and their volumes determined by semiautomatic particle size analysis. Previous studies have confirmed that nuclear volume can be used as an index of polyploidy, since, in mouse and human islets, the nuclear DNA content is directly proportional to the nuclear volume. Five insulin independent and three insulin dependent human diabetic patients and their age and sex matched controls were studied. The percentages of polyploid nuclei in insulin independent diabetic islets were elevated significantly over those in control subjects in four of five cases; the exception was an 85 year old male who was diagnosed as a diabetic only two months prior to death. Of the three insulin dependent diabetics, one, whose islets appeared otherwise normal, had a significantly greater percentage of polyploid nuclei than the controls; the other two, whose islets were markedly hyalinized, exhibited percentages of polyploid nuclei within normal ranges. The implications of relative percentages of polyploid nuclei in normal and diabetic islets and the possible relationship with previous studies of genetically diabetic mice are discussed.

Long-term culture of pancreatic islet cells with special reference to the beta-cell function

Islet cells of adult rat pancreas, dissociated with EDTA-Dispase, were cultivated in Microtest wells for over 2 months. In our cultures, islet cells were free-floating and cohered with each other to reorganize histotypic aggregates resembling the nondissociated islets. Morphologically, excellent preservation of islet cells in the aggregates was confirmed during the culture using both light and electron microscopy. The function of islet beta-cells, as demonstrated by the synthesis and release of insulin, also was retained throughout the culture period. Islet beta-cells cultured for long periods exhibited better response to the short-term stimulation of theophylline than to a high concentration of glucose, as observed in the islets of fetal or newborn rat pancreas.

The ultrastructure of polyploid B-cells in the islets of normal mice

Light and electron microscopic studies of diploid, tetraploid and octaploid B-cells in the islets of normal C57BL/KsJ mice revealed that polyploid cells were characterized by a wider range of granulated states than diploid B-cells. The maximum granule densities were similar for polyploid and diploid cells; however, some polyploid cells were almost devoid of granules, while the least granulated diploid cells contained intermediate granule densities. The tetraploid cell also appeared to be characterized by an increased mitochondrial stage which suggests compensation for the greater degree of degranulation. These observations were confirmed by morphometric analysis. Two interpretations of the apparent polyploidy are discussed; that polyploid B-cells may be more responsive to insulin releasing stimuli than diploid B-cells and that tetraploid cells may only be diploid cells in the G2 phase of the mitotic cycle.