Gary L. Pittenger

Efforts to cure diabetes are now focused on restoring a physiologically-regulated population of insulinproducing cells to the patient. A number of animal models of β cell regeneration have been employed to study the mechanisms of the process. Islet neogenesis, the regeneration of pancreatic islets from pancreatic stem cells, is arguably the least fraught with barriers to widespread use as a therapy for diabetes. These animal models have led to the description of the reg family of proteins that appear to be related to islet regeneration. Islet neogenesis-associated protein (INGAP) is an initiator of islet neogenesis in animal models and a peptide sequence from INGAP carries the biological activity. INGAP peptide has been shown to stimulate an increase in β cell mass in mice, rats, hamsters and dogs. INGAP is also found in the pancreas in human pathological states involving islet neogenesis. The peptide has been tested in human clinical trials, with success being reported. The evidence points to INGAP as a major factor in stimulating islet neogenesis, and, therefore, may play a significant therapeutic role in diabetes.

Objectives: Diabetes is a serious health problem. It has been proposed that islet neogenesis from pancreatic progenitor cells may restore insulin secretion in diabetic mammals. Islet neogenesis- associated protein (INGAP) stimulates islet neogenesis; therefore, we hypothesized that it would stimulate islet neogenesis in dogs. Methods: Forty nondiabetic beagle dogs were randomly divided into 4 groups. Group 1 received daily intramuscular injections of vehicle, whereas the other 3 groups received daily INGAP^104-118 injections of 0.5, 1.5, or 10 mg/kg. After 30 days, pancreatic tissues were collected, and RNA and histological sections were analyzed. Results: In dogs treated with 1.5 mg/kg INGAP^104-118, there was a significant (P < 0.001) increase in the percentage of insulin-positive cells (P < 0.001) and insulin gene expression. There was a trend to increased insulin-positive cells and gene expression with treatments of 0.5 and 10 mg/kg peptide. Protein gene product 9.5-positive cells were increased with treatment. Conclusions: These results indicate that INGAP stimulates cells in the pancreatic duct epithelium of healthy dogs (putative islet progenitor cells) to develop along a neuroendocrine pathway and form new islets in response to INGAP peptide. The INGAP might be an effective therapy for diabetes.

OBJECTIVE—Small-fiber neuropathies may be symptomatic yet escape detection by standard tests. We hypothesized that morphologic changes in intraepidermal nerves would correlate with clinical measures of small-fiber neuropathy. RESEARCH DESIGN AND METHODS—We studied 25 diabetic and 23 nondiabetic patients with neuropathy defined by signs, symptoms, and quantitative testing and 20 control subjects. Skin biopsies were obtained from forearm, thigh, proximal leg, and distal leg, and nerves identified using immunofluorescence with antibody to protein gene product (PGP) 9.5. RESULTS—Mean dendritic length (MDL) (P < 0.01) and intraepidermal nerve fiber density (IENF) (P < 0.001) progressively decreased from proximal to distal sites only in patients with neuropathy. There was a significant reduction in IENF when comparing control subjects and patient groups in the distal leg (P < 0.001). MDL was significantly decreased in the thigh (P < 0.005) and in the proximal (P < 0.01) and distal (P < 0.002) leg in patients compared with control subjects. IENF was not significantly altered in diabetic patients of 5 years’ duration. MDL showed a linear decrease with increasing duration of diabetes. Distal leg IENF showed significant negative correlations with warm (P < 0.02) and cold (P < 0.05) thermal threshold, heat pain (P < 0.05), pressure sense (P < 0.05), and neurological disability score total sensory (P < 0.03) and total neuropathy (P < 0.03) values. CONCLUSIONS—IENF was not significantly altered in these patients at <5 years’ duration of diabetes, but fell significantly after 5 years of diabetes. MDL exhibited a linear loss with time, suggesting a different mechanism of change. MDL and IENF together may prove a useful end point in therapeutic trials for neuropathy.

Neuropathy is one of the most debilitating complications of both type 1 and type 2 diabetes, with estimates of prevalence between 50–90% depending on the means of detection. Diabetic neuropathies are heterogeneous and there is variable involvement of large myelinated fibers and small, thinly myelinated fibers. Many of the neuronal abnormalities in diabetes can be duplicated by experimental depletion of specific neurotrophic factors, their receptors or their binding proteins. In experimental models of diabetes there is a reduction in the availability of these growth factors, which may be a consequence of metabolic abnormalities, or may be independent of glycemic control. These neurotrophic factors are required for the maintenance of the neurons, the ability to resist apoptosis and regenerative capacity. The best studied of the neurotrophic factors is nerve growth factor (NGF) and the related members of the neurotrophin family of peptides. There is increasing evidence that there is a deficiency of NGF in diabetes, as well as the dependent neuropeptides substance P (SP) and calcitonin gene‐related peptide (CGRP) that may also contribute to the clinical symptoms resulting from small fiber dysfunction. Similarly, NT3 appears to be important for large fiber and IGFs for autonomic neuropathy. Whether the observed growth factor deficiencies are due to decreased synthesis, or functional, e.g. an inability to bind to their receptor, and/or abnormalities in nerve transport and processing, remains to be established. Although early studies in humans on the role of neurotrophic factors as a therapy for diabetic neuropathy have been unsuccessful, newer agents and the possibilities uncovered by further studies should fuel clinical trials for several generations. It seems reasonable to anticipate that neurotrophic factor therapy, specifically targeted at different nerve fiber populations, might enter the therapeutic armamentarium.

OBJECTIVE: Neuropathy is the most common complication of diabetes, and toxic serum factors may contribute to its genesis. RESEARCH DESIGN AND METHODS: We assessed neurotoxicity in the serum of 39 diabetic patients and correlated it with clinical measures of somatic and autonomic nerve fiber damage. Sera were applied to N1E-115 and VSC4.1 neuroblastoma cells in vitro as models of sensory/autonomic (S/A) and motor neurons, respectively. Neurotoxicity was measured as either complete or near-complete cell death (highly toxic), inhibited cell growth (moderately toxic), or normal cell proliferation (nontoxic) compared with pooled human serum controls during culture over 4 days. RESULTS: There was an inverse correlation between neurotoxicity and vibration perception threshold (P < 0.01). Age (P < 0.02), duration of diabetes (P < 0.02), and HbA1c (P < 0.03) correlated with neurotoxicity, suggesting that glycation may contribute to cytotoxicity in this model. S/A neurotoxicity occurred more frequently in the sera of patients with type 1 (19 of 25) than type 2 (5 of 14) diabetes (P < 0.02). None of the sera from either type 1 or type 2 diabetic patients displayed neurotoxicity on VSC4.1 cells, whereas sera from patients with motor neuropathy were highly toxic. CONCLUSIONS: These studies indicate that there is a relationship between the specific nerve fiber dysfunction in the patient and the type of neuronal cell killed, not only for diabetic neuropathy but also for known forms of autoimmune neuropathies. Such toxic factors may contribute to diabetic neuropathy by acting in concert with hyperglycemia to damage sensory/autonomic neurons.