In the search of potential human islet stem cells: is tetranectin showing us the way?

The adipocyte-enriched secretory protein tetranectin exacerbates type 2 diabetes by inhibiting insulin secretion from β cells

Pancreatic β cell failure is a hallmark of diabetes. However, the causes of β cell failure remain incomplete. Here, we report the identification of tetranectin (TN), an adipose tissue-enriched secretory molecule, as a negative regulator of insulin secretion in β cells in diabetes. TN expression is stimulated by high glucose in adipocytes via the p38 MAPK/TXNIP/thioredoxin/OCT4 signaling pathway, and elevated serum TN levels are associated with diabetes. TN treatment greatly exacerbates hyperglycemia in mice and suppresses glucose-stimulated insulin secretion in islets. Conversely, knockout of TN or neutralization of TN function notably improves insulin secretion and glucose tolerance in high-fat diet-fed mice. Mechanistically, TN binds with high selectivity to β cells and inhibits insulin secretion by blocking L-type Ca²⁺ channels. Our study uncovers an adipocyte-β cell cross-talk that contributes to β cell dysfunction in diabetes and suggests that neutralization of TN levels may provide a new treatment strategy for type 2 diabetes.

Progress in islet transplantation in patients with type 1 diabetes mellitus

More than 500 patients with type 1 diabetes mellitus have now received islet transplants at over 50 institutions worldwide in the past 5 years. Rates of insulin independence at 1 year with current protocols are impressive. However, inexorable decay of islet function over time indicates that there are many opportunities for improvement. Improved control of glycosylated hemoglobin and reduced risk of recurrent hypoglycemia are seen as important benefits of islet transplantation, irrespective of the status regarding insulin independence. For the use of islet transplantation to expand it is essential that the donor-to-recipient ratio be reliably reduced to 1 : 1. Enormous opportunities lie ahead for the development of successful living donor islet transplantation, single donor protocols, improved engraftment, islet proliferation in vitro and in the recipient, alternative islet sources, and novel tolerizing drugs. With these emerging opportunities, islet transplantation may expand to include more patients with type 1 diabetes, including children, and will not be restricted to the most unstable forms of the disease, as it is today.

Molecular and cellular key players in human islet transplantation

Human islet transplantation could represent an attractive alternative to insulin injections for the treatment of diabetes type 1. However, such an approach requires a better understanding of the molecular and cellular switches controlling β-cell function in general as well as after transplantation into the liver. Although much research has been done into the suitability of stem or progenitor cells to generate a limitless supply of human β-cells, a reproducible and efficient protocol for the differentiation of such cells into stably insulin-secreting β-cells suitable for transplantation has yet to be reported. Fueled by recent findings showing that mature β-cells are able to regenerate, many efforts have been undertaken to expand this cell pool. Unfortunately, also these approaches had problems to yield sufficiently differentiated human islet cells. The aim of this review is to summarize recent findings describing some of the molecular and cellular key players of islet biology. A more complete understanding of their orchestration and the use of new methods such as real time confocal imaging for the assessment of islet quality may yield the necessary advancements for more successful human islet transplantation.

Dickkopf-3 is expressed in a subset of adult human pancreatic beta cells

The Dickkopf (Dkk) gene family of secretory modulators of canonical Wnt/beta catenin signals is involved in the control of stem cell proliferation, homeostasis and differentiation. Bioinformatic data on dkk-1/3 gene expression, indicating high expression levels in the human pancreas, led us to analyze these two proteins in adult human pancreatic tissue. Dkk-1/3 mRNA levels and protein distribution were analyzed in isolated human islets vs. the exocrine/ductal pancreatic cells and in paraffin sections of adult human pancreata. Using real time PCR only lowest amounts of dkk-1 mRNA were detectable in the endocrine fractions. Immunohistochemistry did not reveal any Dkk-1 protein in adult human pancreatic tissue. Interestingly, Dkk-3 mRNA and protein were clearly present in adult human pancreatic islets. Messenger RNA levels for Dkk-3 were significantly higher in isolated islets as compared to the exocrine/ductal fraction. Co-staining with an antibody against insulin identified the beta cells of the pancreas as the Dkk-3-positive cells. Notably, only a subset of beta cells contained Dkk-3. As shown by western blot analysis Dkk-3 seems to be proteolytically processed in beta cells. To our knowledge, this is the first study describing a molecule with which the pool of pancreatic beta cells can be further subdivided. Future studies will show whether this sub-classification of beta cells translates into functional differences.

Current status of pancreatic islet transplantation

DM (diabetes mellitus) is a metabolic disorder of either absolute or relative insulin deficiency. Optimized insulin injections remain the mainstay life-sustaining therapy for patients with T1DM (Type I DM) in 2006; however, a small subset of patients with T1DM (approx. 10%) are exquisitely sensitive to insulin and lack counter-regulatory measures, putting them at higher risk of neuroglycopenia. One alternative strategy to injected insulin therapy is pancreatic islet transplantation. Islet transplantation came of age when Paul E. Lacy successfully reversed chemical diabetes in rodent models in 1972. In a landmark study published in 2000, Shapiro et al. [A. M. Shapiro, J. R. Lakey, E. A. Ryan, G. S. Korbutt, E. Toth, G. L. Warnock, N. M. Kneteman and R. V. Rajotte (2000) N. Engl. J. Med. 343, 230-238] reported seven consecutive patients treated with islet transplants under the Edmonton protocol, all of whom maintained insulin independence out to 1 year. Substantial progress has occurred in aspects of pancreas procurement, transportation (using the oxygenated two-layer method) and in islet isolation (with controlled enzymatic perfusion and subsequent digestion in the Ricordi chamber). Clinical protocols to optimize islet survival and function post-transplantation improved dramatically with the introduction of the Edmonton protocol, but it is clear that this approach still has potential limitations. Newer pharmacotherapies and interventions designed to promote islet survival, prevent apoptosis, to promote islet growth and to protect islets in the long run from immunological injury are rapidly approaching clinical trials, and it seems likely that clinical outcomes of islet transplantation will continue to improve at the current exponential pace.

“Real-Time” Assessment of Human Islet Preparations With Confocal Live Cell Imaging

Pancreatic islet cell transplantation is a promising approach to restoring normoglycemia in diabetic patients. The outcome of islet transplantation is uncertain for two reasons: The quality of isolated islets is still poorly defined and the functional potential of transplanted islets is difficult to predict. Therefore, one of the primary challenges in islet transplantation is to identify and understand the changes taking place in islets after isolation and culture. Description of such changes in living islet cells offers insights not achievable by use of fixed-cell techniques. Three fluorescent dyes, dichlorodihydrofluorescein diacetate, tetramethylrhodamine methyl ester perchlorate (TMRM), and fluorescent wheat germ agglutinin, were used to assess either overall oxidative stress, time-dependent mitochondrial membrane potentials, or localization of oligosaccharides. Confocal microscopy was performed with a microlens-enhanced Nipkow disk-based confocal system mounted on an inverse microscope. We were able to show differences in the amount of oligosaccharides on the cell surface between endocrine and exocrine cells in freshly isolated human islet preparations. The study of the mitochondrial membrane potential via TMRM proved to be useful to early identification of damaged or stressed cells. Thus a combination of fluorescent dyes as subcellular markers, with a powerful live confocal imaging system may be of great value to isolation and culture.