Seth J. Salpeter

Systemic Regulation of the Age-Related Decline of Pancreatic β-Cell Replication

The frequency of pancreatic β-cell replication declines dramatically with age, potentially contributing to the increased risk of type 2 diabetes in old age. Previous studies have shown the involvement of cell-autonomous factors in this phenomenon, particularly the decline of polycomb genes and accumulation of p16/INK4A. Here, we demonstrate that a systemic factor found in the circulation of young mice is able to increase the proliferation rate of old pancreatic β-cells. Old mice parabiosed to young mice have increased β-cell replication compared with unjoined old mice or old mice parabiosed to old mice. In addition, we demonstrate that old β-cells transplanted into young recipients have increased replication rate compared with cells transplanted into old recipients; conversely, young β-cells transplanted into old mice decrease their replication rate compared with young cells transplanted into young recipients. The expression of p16/INK4A mRNA did not change in heterochronic parabiosis, suggesting the involvement of other pathways. We conclude that systemic factors contribute to the replicative decline of old pancreatic β-cells.

Glucose regulates cyclin D2 expression in quiescent and replicating pancreatic β-cells through glycolysis and calcium channels.

Understanding the molecular triggers of pancreatic β-cell proliferation may facilitate the development of regenerative therapies for diabetes. Genetic studies have demonstrated an important role for cyclin D2 in β-cell proliferation and mass homeostasis, but its specific function in β-cell division and mechanism of regulation remain unclear. Here, we report that cyclin D2 is present at high levels in the nucleus of quiescent β-cells in vivo. The major regulator of cyclin D2 expression is glucose, acting via glycolysis and calcium channels in the β-cell to control cyclin D2 mRNA levels. Furthermore, cyclin D2 mRNA is down-regulated during S-G(2)-M phases of each β-cell division, via a mechanism that is also affected by glucose metabolism. Thus, glucose metabolism maintains high levels of nuclear cyclin D2 in quiescent β-cells and modulates the down-regulation of cyclin D2 in replicating β-cells. These data challenge the standard model for regulation of cyclin D2 during the cell division cycle and suggest cyclin D2 as a molecular link between glucose levels and β-cell replication.

Determinants of pancreatic β-cell regeneration

Recent studies have revealed a surprising plasticity of pancreatic β‐cell mass. β‐cell mass is now recognized to increase and decrease in response to physiological demand, for example during pregnancy and in insulin‐resistant states. Moreover, we and others have shown that mice recover spontaneously from diabetes induced by killing of 70–80% of β‐cells, by β‐cell regeneration. The major cellular source for new β‐cells following specific ablation, as well as during normal homeostatic maintenance of adult β‐cells, is proliferation of differentiated β‐cells. More recently, it was shown that one form of severe pancreatic injury, ligation of the main pancreatic duct, activates a population of embryonic‐type endocrine progenitor cells, which can differentiate into new β‐cells. The molecular triggers for enhanced β‐cell proliferation during recovery from diabetes and for activation of embryonic‐type endocrine progenitors remain unknown and represent key challenges for future research. Taken together, recent data suggest that regenerative therapy for diabetes may be a realistic goal.

Short-term overexpression of VEGF-A in mouse beta cells indirectly stimulates their proliferation and protects against diabetes

Aims/hypothesisVascular endothelial growth factor (VEGF) has been recognised by loss-of-function experiments as a pleiotropic factor with importance in embryonic pancreas development and postnatal beta cell function. Chronic, non-conditional overexpression of VEGF-A has a deleterious effect on beta cell development and function. We report, for the first time, a conditional gain-of-function study to evaluate the effect of transient VEGF-A overexpression by adult pancreatic beta cells on islet vasculature and beta cell proliferation and survival, under both normal physiological and injury conditions.MethodsIn a transgenic mouse strain, overexpressing VEGF-A in a doxycycline-inducible and beta cell-specific manner, we evaluated the ability of VEGF-A to affect islet vessel density, beta cell proliferation and protection of the adult beta cell mass from toxin-induced injury.ResultsShort-term VEGF-A overexpression resulted in islet hypervascularisation, increased beta cell proliferation and protection from toxin-mediated beta cell death, and thereby prevented the development of hyperglycaemia. Extended overexpression of VEGF-A led to impaired glucose tolerance, elevated fasting glycaemia and a decreased beta cell mass.Conclusions/interpretationOverexpression of VEGF-A in beta cells time-dependently affects glycometabolic control and beta cell protection and proliferation. These data nourish further studies to examine the role of controlled VEGF delivery in (pre)clinical applications aimed at protecting and/or restoring the injured beta cell mass.

G0-G1 Transition and the Restriction Point in Pancreatic β-Cells In Vivo

Most of our knowledge on cell kinetics stems from in vitro studies of continuously dividing cells. In this study, we determine in vivo cell-cycle parameters of pancreatic β-cells, a largely quiescent population, using drugs that mimic or prevent glucose-induced replication of β-cells in mice. Quiescent β-cells exposed to a mitogenic glucose stimulation require 8 h to enter the G1 phase of the cell cycle, and this time is prolonged in older age. The duration of G1, S, and G2/M is ∼5, 8, and 6 h, respectively. We further provide the first in vivo demonstration of the restriction point at the G0-G1 transition, discovered by Arthur Pardee 40 years ago. The findings may have pharmacodynamic implications in the design of regenerative therapies aimed at increasing β-cell replication and mass in patients with diabetes.

Ready, Set, Cleave: Proteases in Action

Activity-based probes are small molecules that can be used to monitor enzyme activity by covalently binding to specific residues in the active site. In this issue of Chemistry & Biology, Lu and colleagues developed a specific fluorescent activity-based probe that targets the papain-like cysteine bacterial type III effector protease AvrPphB and used it to demonstrate the regulation of the protease secretion and pathogenesis.

Beta-Cell Replication

Patients suffering from type 1 and type 2 diabetes exhibit a decrease in the mass of insulin-producing beta cells. Both the ability to generate and expand large amounts of transplantable beta cells and the capacity to encourage beta-cell proliferation in the patient represent potential cures for the disease. Understanding the basic cell cycle machinery responsible for the replication of pancreatic beta cells is therefore an important challenge in diabetes research today, in hopes that it will provide useful insights into betacell growth and proliferation. Though for many years pancreas biologists believed that adult beta cells emerged from progenitor cells and remained post-mitotic throughout their lifetimes, recent work has demonstrated that adult beta cells are a dynamic and replicating population. In light of this new understanding of pancreatic beta cells, much attention is currently being focused on the regulation of the replication process. However, even as biologists focus on the particular machinery involved in division, a proper understanding can only be obtained in light of beta-cell development, origins, and dynamics. In this review, we present a brief introduction to beta-cell development and origins, followed by a description of beta-cell proliferation machinery. We conclude with a discussion of a possible regulatory model for beta-cell proliferation.