Zhen-guo Li

Hippocampal neuronal apoptosis in type 1 diabetes

Duration-related cognitive impairment is an increasingly recognized complication of type 1 diabetes. To explore potential underlying mechanisms, we examined hippocampal abnormalities in the spontaneously type 1 diabetic BB/W rat. As a functional assay of cognition, the Morris water maze test showed significantly prolonged latencies in 8-month diabetic rats not present at 2 months of diabetes. These abnormalities were associated with DNA fragmentation, positive TUNEL staining, elevated Bax/Bcl-x(L) ratio, increased caspase 3 activities and decreased neuronal densities in diabetic hippocampi. These changes were not caused by hypoglycemic episodes or reduced weight in diabetic animals. To explore potential mechanisms responsible for the apoptosis, we examined the expression of the IGF system. Western blotting and in situ hybridization revealed significant reductions in the expression of IGF-I, IGF-II, IGF-IR and IR preceding (2 months) and accompanying (8 months) the functional cognitive impairments and the apoptotic neuronal loss in hippocampus. These data suggest that a duration-related apoptosis-induced neuronal loss occurs in type 1 diabetes associated with cognitive impairment. The data also suggest that this is at least in part related to impaired insulin and/or IGF activities.

C-peptide prevents and improves chronic Type I diabetic polyneuropathy in the BB/Wor rat.

Aims/hypothesis. Insulin and C-peptide exert neuroprotective effects and are deficient in Type I (insulin-dependent) diabetes mellitus but not in Type II (non-insulin-dependent) diabetes mellitus. These studies were designed to test the preventive and interventional effects of C-peptide replacement on diabetic polyneuropathy in the Type I diabetic BB/Wor rat. Methods. Diabetic BB/Wor rats were replaced with rat C-peptide from onset of diabetes and between 5 and 8 months of diabetes. They were examined at 2 and 8 months and compared to non-C-peptide replaced BB/Wor rats, Type II diabetic (non-C-peptide deficient) BB/Z rats and non-diabetic control rats. Animals were monitored as to hyperglycaemia and nerve conduction velocity (NCV). Acute changes such as neural Na+/K+-ATPase and paranodal swelling were examined at 2 months, morphometric and teased fiber analyses were done at 8 months. Results. C-peptide replacement for 2 months in Type I diabetic rats prevented the acute NCV defect by 59 % (p < 0.005), the neural Na+/K+-ATPase defect by 55 % (p < 0.001) and acute paranodal swelling by 61 % (p < 0.001). Eight months of C-peptide replacement prevented the chronic nerve conduction defect by 71 % (p < 0.001) and totally prevented axoglial dysjunction (p < 0.001) and paranodal demyelination (p < 0.001). C-peptide treatment from 5 to 8 months showed a 13 % (p < 0.05) improvement in NCV, a 33 % (p < 0.05) improvement in axoglial dysjunction, normalization (p < 0.001) of paranodal demyelination, repair of axonal degeneration (p < 0.01), and a fourfold (p < 0.001) increase in nerve fibre regeneration. Conclusion/interpretation. C-peptide replacement of Type I BB/Wor-rats partially prevents acute and chronic metabolic, functional and structural changes that separate Type I diabetic polyneuropathy from its Type II counterpart suggesting that C-peptide deficiency plays a pathogenetic role in Type I diabetic polyneuropathy. [Diabetologia (2001) 44: 889–897]

Molecular basis for the insulinomimetic effects of C-peptide

Abstract.Aims/hypothesis: C-peptide, released by the beta-cells of pancreatic islets, elicits salutary responses in Type I (insulin-dependent) diabetes mellitus but the molecular mechanisms behind these effects are not known. We assessed whether synthetic rat C-peptide stimulates insulin-like cellular effects in a classic insulin target tissue. Methods: To clarify the molecular mechanisms involved in several insulinomimetic actions, we investigated the effect of C-peptide on the insulin signalling pathway in rat skeletal muscle cells. We used L6 myoblasts and myocytes to measure the effects of C-peptide or insulin or both on glycogen synthesis and amino acid uptake. We also studied the effects of C-peptide on insulin receptor autophosphorylation, its tyrosine kinase activity, phosphorylation of IRS-1, PI 3-kinase, Akt, p90Rsk, MAPK, and GSK3 in these cells. Results: In L6 cells, physiological concentrations of C-peptide (0.3–3 nmol/l) significantly activated insulin receptor tyrosine kinase, IRS-1 tyrosine phosphorylation, PI 3-kinase activity, MAPK phosphorylation, p90Rsk, and GSK3 phosphorylation. A scrambled C-peptide sequence – the control – showed no effects. Wortmannin blocked C-peptide-induced glycogen synthesis while pertussis toxin had no effect. Only submaximal insulin concentrations (up to 10 nmol/l) combined with submaximal C-peptide concentrations led to additive effects. Conclusion/interpretation: C-peptide added to the maximal insulin dose (100 nmol/l) did not increase the effect of insulin alone. We thus conclude that the same signalling elements are used by both ligands. However, the lack of Akt activation by C-peptide and the bell-shaped dose response induced by C-peptide indicate that C-peptide has some effects by another distinct mechanism. We speculate that C-peptide could modulate the metabolic effects of insulin by enhancing them at low hormone concentrations and dampening them at high hormone concentrations. [Diabetologia (2001) 44: 1247–1257]

C-peptide enhances insulin-mediated cell growth and protection against high glucose- induced apoptosis in SH-SY5Y cells

We have previously reported that C‐peptide exerts preventive and therapeutic effects on diabetic neuropathy in type 1 diabetic BB/Wor‐rats and that it prevents duration‐dependent hippocampal apoptosis in the same animal model. In the present study, we examined human neuroblastoma SH‐SY5Y cells to examine whether C‐peptide stimulates cell proliferation/neurite outgrowth and whether it has antiapoptotic effects.

The role of impaired insulin/IGF action in primary diabetic encephalopathy

We have previously shown that hippocampal neuronal apoptosis accompanied by impaired cognitive functions occurs in type 1 diabetic BB/Wor rats. To differentiate the contribution by insulin deficiency vs. that by hyperglycemia on neuronal apoptosis, we examined the activities of various apoptotic pathways in hippocampi from type 1 diabetic BB/Wor rats (hyperglycemic and insulinopenic) and type 2 diabetic BBZDR/Wor rats (hyperglycemic and hyperinsulinemic). DNA fragmentation was demonstrated by LM-PCR in type 1 diabetic BB/Wor rats, but was not detectable in duration- and hyperglycemia-matched type 2 BBZDR/Wor rats. Of various apoptotic pathways, Fas activations, 8-OHdG expression, and caspase-12 were demonstrated in type 1 diabetic BB/Wor rats only. In contrast, perturbations of the IGF and NGF systems and PARP activation were demonstrated in type 1 and to a lesser extent in type 2 diabetes. Expressions of Bax and active caspase-3 were significantly increased in type 1, but not in type 2, diabetic rats. These data suggest a lesser apoptogenic stress in type 2 vs. type 1 diabetes. These differences translated into a more profound neuronal loss in the hippocampus of type 1 rats. The results demonstrate that caspase-dependent apoptotic activities dominate in type 1 diabetes, whereas PARP-mediated caspase-independent apoptotic stress is present in both type 1 and type 2 diabetes. The findings suggest that insulin deficiency plays a compounding role to that of hyperglycemia in neuronal apoptosis underpinning primary diabetic encephalopathy.

C-Peptide Prevents Hippocampal Apoptosis in Type 1 Diabetes

To explore mechanisms underlying central nervous system (CNS) complications in diabetes, we examined hippocampal neuronal apoptosis and loss, and the effect of C-peptide replacement in type 1 diabetic BB/W rats. Apoptosis was demonstrated after 8 months of diabetes, by DNA fragmentation, increased number of apoptotic cells, and an elevated ratio of Bax/Bcl-xL, accompanied by reduced neuronal density in the hippocampus. No apoptotic activity was detected and neuronal density was unchanged in 2-month diabetic hippocampus, whereas insulin-like growth factor (IGF) activities were impaired. In type 1 diabetic BB/W rats replaced with C-peptide, no TdT-mediated dUTP nick-end labeling (TUNEL)- positive cells were shown and DNA laddering was not evident in hippocampus at either 2 or 8 months. C-peptide administration prevented the preceding perturbation of IGF expression and reduced the elevated ratio of Bax/Bcl-xL. Our data suggest that type 1 diabetes causes a duration-dependent programmed cell death of the hippocampus, which is partially prevented by C-peptide.

C-peptide and Central Nervous System Complications in Diabetes

Substantial evidence collected from clinical data and experimental studies has indicated that CNS is not spared from diabetes complications. Impairments in CNS function are well documented in both type 1 and type 2 diabetic patients as well as in various animal models of diabetes, in terms of alterations in cognition, neuropsychology, neurobehavior, electrophysiology, structure, neurochemistry and apoptotic activities. These data suggest that primary diabetic encephalopathy exists as a definable diabetic complication. The mechanisms underlying this CNS complication are not clear. Experimental studies have suggested that neuronal apoptosis may play an important role in neuronal loss and impaired cognitive function. In diabetes multiple factors are responsible for neuronal apoptosis, such as a perturbed IGF system, hyperglycemia and the aging process itself. Recent data suggest that insulin/C-peptide deficiency may exert an eminent role. Administration of C-peptide partially corrects the perturbed IGF system in the brain and prevents neuronal apoptosis in hippocampus of type 1 diabetes. In neuroblastoma SH-SY5Y cells C-peptide provides a dose-dependent stimulation on cell proliferation and an anti-apoptotic effect as well. These studies provide a basis for administration of C-peptide as a potentially effective therapy for type 1 diabetes.

Diabetes and Alzheimer's Disease - Is There a Connection?

It has been known for some time that diabetes may be associated with impaired cognitive function. During the last decade, epidemiological data have emerged suggesting a linkage between diabetes, particularly type 2 diabetes, and Alzheimers disease (AD). There is evidence to suggest that impaired activities of neurotrophic factors such as insulin, IGF-1 and NGF, which occur in both diabetes and AD, may provide a mechanistic link between the two disorders. An additional probable factor that has been less evaluated to date is hypercholesterolemia, a common accompaniment to type 2 diabetes. Increased cholesterol availability is believed to play a crucial role in the abnormal metabolism of amyloid precursor protein leading to accumulation of amyloid-beta. Impaired insulin signaling in particular appears to be involved in hyperphosphorylation of the tau protein, which constitutes neurofibrillary tangles in AD. The linkage between abnormal amyloid metabolism and phosphor-tau is likely to be provided by the activation of caspases both by increased amyloid-beta and by impaired insulin signaling. Although the details of many of these components still await evaluation, it appears clear that commonalities exist in the underlying pathogenesis of diabetes and Alzheimers disease. In this review we provide a brief update on linkages between these two diverse but common disorders.

The Insulin-Like Growth Factor System and Neurological Complications in Diabetes

The IGF system plays vital roles in neuronal development, metabolism, regeneration and survival. It consists of IGF-I, IGF-II, insulin, IGF-I-receptor, and those of IGF-II and insulin as well as IGF-binding proteins. In the last decades it has become clear that perturbations of the IGF system play important roles in the pathogenesis of diabetic neurological complications. In the peripheral nervous system IGF-I, insulin, and C-peptide particularly in type 1 diabetes participate in the development of axonal degenerative changes and contributes to impaired regenerative capacities. These abnormalities of the IGF system appear to be less pronounced in type 2 diabetes, which may in part account for the relatively milder neurological complications in this type of diabetes. The members of the IGF system also provide anti-apoptotic effects on both peripheral and central nervous system neurons. Furthermore, both insulin and C-peptide and probably IGF-I possess gene regulatory capacities on myelin constituents and axonal cytoskeletal proteins. Therefore, replenishment of various members of the IGF system provides a reasonable rational for prevention and treatment of diabetic neurological complications.