The effects of C-peptide on type 1 diabetic polyneuropathies and encephalopathy in the BB/Wor-rat

Biological Activity of c-Peptide in Microvascular Complications of Type 1 Diabetes-Time for Translational Studies or Back to the Basics?

People with type 1 diabetes have an increased risk of developing microvascular complications, which have a negative impact on the quality of life and reduce life expectancy. Numerous studies in animals with experimental diabetes show that c-peptide supplementation exerts beneficial effects on diabetes-induced damage in peripheral nerves and kidneys. There is substantial evidence that c-peptide counteracts the detrimental changes caused by hyperglycemia at the cellular level, such as decreased activation of endothelial nitric oxide synthase and sodium potassium ATPase, and increase in formation of pro-inflammatory molecules mediated by nuclear factor kappa-light-chain-enhancer of activated B cells: cytokines, chemokines, cell adhesion molecules, vascular endothelial growth factor, and transforming growth factor beta. However, despite positive results from cell and animal studies, no successful c-peptide replacement therapies have been developed so far. Therefore, it is important to improve our understanding of the impact of c-peptide on the pathophysiology of microvascular complications to develop novel c-peptide-based treatments. This article aims to review current knowledge on the impact of c-peptide on diabetic neuro- and nephropathy and to evaluate its potential therapeutic role.

Electrical Conduction System Remodeling in Streptozotocin-Induced Diabetes Mellitus Rat Heart

Cardiovascular complications are common in type 1 diabetes mellitus (TIDM) and there is an increased risk of arrhythmias as a result of dysfunction of the cardiac conduction system (CCS). We have previously shown that, in vivo, there is a decrease in the heart rate and prolongation of the QRS complex in streptozotocin-induced type 1 diabetic rats indicating dysfunction of the CCS. The aim of this study was to investigate the function of the ex vivo CCS and key proteins that are involved in pacemaker mechanisms in TIDM. RR interval, PR interval and QRS complex duration were significantly increased in diabetic rats. The beating rate of the isolated sinoatrial node (SAN) preparation was significantly decreased in diabetic rats. The funny current density and cell capacitance were significantly decreased in diabetic nodal cells. Western blot showed that proteins involved in the function of the CCS were significantly decreased in diabetic rats, namely: HCN4, Ca_v1.3, Ca_v3.1, Cx45, and NCX1 in the SAN; RyR2 and NCX1 in the atrioventricular junction and Cx40, Cx43, Cx45, and RyR2 in the Purkinje network. We conclude that there are complex functional and cellular changes in the CCS in TIDM. The changes in the proteins involved in the function of this electrical system are expected to adversely affect action potential generation and propagation, and these changes are likely to be arrhythmogenic.

Chronic insulinopenia/hyperglycemia decreases cannabinoid CB1 receptor density and impairs glucose uptake in the mouse forebrain

Both endocannabinoids and insulin regulate peripheral and cerebral glucose homeostasis via convergent signaling pathways that are impacted by diabetes. Here we asked how glucose metabolism and important facets of insulin signaling are affected in the forebrain of cannabinoid CB₁ receptor knockout mice (CB₁R-KO) and their wild-type (WT) littermates, seven weeks after the induction of insulinopenia/hyperglycemia (diabetes) with intraperitoneal streptozotocin injection. Sham-injected animals served as control. Diabetes caused milder weight loss in the WT mice compared to the phenotypically ˜11% leaner CB₁R-KO, while hyperglycemia was similar. Resting [³H]deoxyglucose uptake was significantly reduced by ˜20% in acute ex vivo frontocortical and hippocampal slices obtained from both the sham-injected CB₁R-KO and the diabetic WT mice. Surprisingly, the third cohort, the diabetic CB₁R-KO showed no further impairment in glucose uptake, as compared to the sham-injected CB₁R-KO. Depolarization-induced [³H]deoxyglucose uptake was proportional to the respective resting values only in the cortex in all four cohorts. The dissipative metabolism of [¹⁴C]-U-glucose remained largely unaffected in all cohorts of animals. However, diabetes reduced cortical CB₁R density by ˜20%, as assessed by Western blotting. Albeit the changes in insulin signaling did not reflect the glucose uptake profile in each cohort, there were significant interactions between diabetes and genotype. In conclusion, a chronic decrease or lack of CB₁R expression reduces glucose uptake in the mouse brain. Additionally, diabetes failed to cause further impairment in cerebral glucose uptake in the CB₁R-KO. These suggest that diabetic encephalopathy may be in part associated with lower CB₁R expression.

In vivo evidence of reduced nodal and paranodal conductances in type 1 diabetes

OBJECTIVES

Diabetic neuropathy is a debilitating complication of diabetes. Animal models of type 1 diabetes (T1DM) suggest that functional and structural changes, specifically axo-glial dysjunction, may contribute to neuropathy development. The present study sought to examine and characterise early sensory axonal function in T1DM patients in the absence of clinical neuropathy.

METHODS

Thirty patients with T1DM (15M:15F) without neuropathy underwent median nerve sensory and motor axonal excitability studies to examine axonal function. A verified mathematical model of human motor and sensory axons was used to elucidate the underlying causes of observed alterations.

RESULTS

Compared to controls (NC), T1DM patients demonstrated significant axonal excitability abnormalities in sensory and motor axons. These included marked reductions in sensory and motor subexcitability during the recovery cycle (T1DM 7.9 ± 0.4:10.4 ± 0.6%, NC 10.4 ± 0.7:15.4 ± 1.2%, P<0.01) and during hyperpolarizing threshold electrotonus at 10-20 ms (T1DM -75.5 ± 0.8:-69.7 ± 0.8%, NC -78.4 ± 1:-72.7 ± 0.9%, P<0.01). Mathematical modelling demonstrated that these changes were due to reduced nodal Na(+) currents, nodal/paranodal K(+) conductances and Na(+)/K(+) pump dysfunction, consistent with axo-glial dysjunction as outlined in animal models of T1DM.

CONCLUSIONS

The study provided support for the occurrence of early changes in nodal and paranodal conductances in patients with T1DM.

SIGNIFICANCE

These data indicate that axonal excitability techniques may detect early changes in diabetic patients, providing a window of opportunity for prophylactic intervention in T1DM.

Sesamol: a Treatment for Diabetes-Associated Blood-Brain Barrier Dysfunction

Diabetes is a long-standing disease that leads to secondary complications of capillaries such as retinopathy, nephropathy and neuropathy. Emerging evidence suggests that diabetes may also affect the cerebromicrovasculature, the blood-brain barrier (BBB), and lead to changes in the brain that affect cognition and mood. Therefore, it is important to identify natural compounds that may have therapeutic benefit for reducing BBB dysfunction and improve patient quality of life. Preclinical evidence suggests that sesamol, a natural antioxidant in sesame seed oil, could have therapeutic benefit for treating BBB dysfunction during diabetes. Similarly, paroxetine, which shares a methylenedioxy moiety with sesamol shows clinical benefit for treating neuropathic pain associated with diabetes. This review emphasizes BBB dysfunction as a treatable secondary complication associated with diabetes and examines the evidence for the use of natural compounds like sesamol or existing therapies like paroxetine to help restore BBB function.

Neuroprotective properties of aucubin in diabetic rats and diabetic encephalopathy rats

In this study, we determined the neuroprotective effect of aucubin on diabetes and diabetic encephalopathy. With the exception of the control group, all rats received intraperitoneal injections of streptozotocin (STZ; 60 mg/kg) to induce type 1 diabetes mellitus (DM). Aucubin (1, 5, 10 mg/kg ip) was used after induction of DM (immediately) and diabetic encephalopathy (65 days after the induction of diabetes). The diabetic encephalopathy treatment groups were divided into short-term and long-term treatment groups. Treatment responses to all parameters were examined (body weight, plasma glucose, Y-maze error rates and proportion of apoptotic cells). In diabetic rats, aucubin controlled blood glucose levels effectively, prevented complications, and improved the quality of life of diabetic rats. In diabetic encephalopathy, aucubin significantly rescued neurons in the hippocampal CA1 subfield and reduced working errors during behavioral testing. The significant neuroprotective effect of aucubin could be seen not only in the short term (15 days) but also in the long term (45 days), which was a highly encouraging finding. These data suggest that aucubin may be a potential neuroprotective agent.

Chronic Opium Treatment Can Differentially Induce Brain and Liver Cells Apoptosis in Diabetic and Non-diabetic Male and Female Rats

It has been shown that some opium derivatives promote cell death via apoptosis. This study was designed to examine the influence of opium addiction on brain and liver cells apoptosis in male and female diabetic and non-diabetic Wistar rats. This experimental study was performed on normal, opium-addicted, diabetic and diabetic opium-addicted male and female rats. Apoptosis was evaluated by TUNEL and DNA fragmentation assays. Results of this study showed that apoptosis in opium-addicted and diabetic opium-addicted brain and liver cells were significantly higher than the both normal and diabetic rats. In addition, we found that apoptosis in brain cells of opium-addicted and diabetic opium-addicted male rats were significantly higher than opium-addicted and diabetic opium-addicted female, whereas apoptosis in liver cells of opium-addicted and diabetic opium-addicted female rats were significantly higher than opium-addicted and diabetic opium-addicted male. Overall, these results indicate that opium probably plays an important role in brain and liver cells apoptosis, therefore, leading neurotoxicity and hepatotoxicity. These findings also in away possibly means that male brain cells are more susceptible than female and interestingly liver of females are more sensitive than males in induction of apoptosis by opium.

Early transcriptional regulation by C-peptide in freshly isolated rat proximal tubular cells

BACKGROUND

Clinical studies have shown that proinsulin C-peptide exerts renoprotective effects in type 1 diabetes, although the underlying mechanisms are poorly understood. As C-peptide has been shown to induce several intracellular events and to localize to nuclei, we aimed to determine whether gene transcription is affected in proximal tubular kidney cells, and if so, whether the genes with altered transcription include those related to protective mechanisms.

METHODS

The effect of C-peptide incubation (2 h) on gene expression was investigated in freshly isolated proximal tubular cells from streptozotocin-diabetic Sprague-Dawley rats using global gene expression profiling and real-time quantitative polymerase chain reaction. Protein expression was assayed using western blotting. Different bioinformatic strategies were employed.

RESULTS

Gene transcription profiling demonstrated differential transcription of 492 genes (p < 0.01) after 2 h of C-peptide exposure, with the majority of these genes repressed (83%). Real-time quantitative polymerase chain reaction validation supported a trend of several G protein-coupled receptors being activated, and certain transcription factors being repressed. Also, C-peptide repressed the transcription of genes associated with the pathways of circulatory and inflammatory diseases.

CONCLUSION

This study shows that C-peptide exerts early effects on gene transcription in proximal tubular cells. The findings also bring further knowledge to the renoprotective mechanisms of C-peptide in type 1 diabetes, and support a transcriptional activity for C-peptide. It is suggested that C-peptide may play a regulatory role in the gene expression of proximal tubular cells.

C-peptide and diabetic encephalopathy

With the changes of life style, diabetes and its complications have become a major cause of morbidity and mortality. It is reasonable to anticipate a continued rise in the incidence of diabetes and its complications along with the aging of the population, increase in adult obesity rate, and other risk factors. Diabetic encephalopathy is one of the severe microvascular complications of diabetes, characterized by impaired cognitive functions, and electrophysiological, neurochemical, and structural abnormalities. It may involve direct neuronal damage caused by intracellular glucose. However, the pathogenesis of this disease is complex and its diagnosis is not very clear. Previous researches have suggested that chronic metabolic alterations, vascular changes, and neuronal apoptosis may play important roles in neuronal loss and damaged cognitive functions. Multiple factors are responsible for neuronal apoptosis, such as disturbed insulin growth factor (IGF) system, hyperglycemia, and the aging process. Recent data suggest that insulin/C-peptide deficiency may exert a primary and key effect in diabetic encephalopathy. Administration of C-peptide partially improves the condition of the IGF system in the brain and prevents neuronal apoptosis in the hippocampus of diabetic patients. Those findings provide a basis for application of C-peptide as a potentially effective therapy for diabetes and diabetic encephalopathy.

Insulin and IGF-1 receptors, nitrotyrosin and cerebral neuronal deficits in two young patients with diabetic ketoacidosis and fatal brain edema

Gray and white matter structural deficits may accompany type 1 diabetes. Earlier experimental studies have demonstrated neuronal deficits associated with impaired neurotrophic support, inflammation and oxidative stress. In this study we demonstrate in two patients with histories of poorly controlled type 1 diabetes and fatal brain edema of ketoacidosis neuronal deficits associated with a decreased presence of insulin and IGF-1 receptors and accumulation of nitrotyrosin in neurons of affected areas and the choroid plexus. The findings add support to the suggested genesis of T1DM encephalopathy due to compromised neurotrophic protection, oxidative stress, inflammation and neuronal deficits, as demonstrated in T1DM encephalopathy in the BB/Wor-rat.

Anti-inflammatory properties of C-Peptide

C-peptide, historically considered a biologically inactive peptide, has been shown to exert insulin-independent biological effects on a number of cells proving itself as a bioactive peptide with anti-inflammatory properties. Type 1 diabetic patients typically lack C-peptide, and are at increased risk of developing both micro- and macrovascular complications, which account for significant morbidity and mortality in this population. Inflammatory mechanisms play a pivotal role in vascular disease. Inflammation and hyperglycemia are major components in the development of vascular dysfunction in type 1 diabetes. The anti-inflammatory properties of C-peptide discovered to date are at the level of the vascular endothelium, and vascular smooth muscle cells exposed to a variety of insults. Additionally, C-peptide has shown anti-inflammatory properties in models of endotoxic shock and type 1 diabetes-associated encephalopathy. Given the anti-inflammatory properties of C-peptide, one may speculate dual hormone replacement therapy with both insulin and C-peptide in patients with type 1 diabetes may be warranted in the future to decrease morbidity and mortality in this population.