Effects of experimental diabetes on axonal and Schwann cell changes in sciatic nerve isografts

Targeting Neuroimmune Interactions in Diabetic Neuropathy with Nanomedicine

Significance: Diabetes is a major source of neuropathy and neuropathic pain that is set to continue growing in prevalence. Diabetic peripheral neuropathy (DPN) and pain associated with diabetes are not adequately managed by current treatment regimens. Perhaps the greatest difficulty in treating DPN is the complex pathophysiology, which involves aspects of metabolic disruption and neurotrophic deficits, along with neuroimmune interactions. There is, therefore, an urgent need to pursue novel therapeutic options targeting the key cellular and molecular players. Recent Advances: To that end, cellular targeting becomes an increasingly compelling drug delivery option as our knowledge of neuroimmune interactions continues to mount. These nanomedicine-based approaches afford a potentially unparalleled specificity and longevity of drug targeting, using novel or established compounds, all while minimizing off-target effects. Critical Issues: The DPN therapeutics directly targeted at the nervous system make up the bulk of currently available treatment options. However, there are significant opportunities based on the targeting of non-neuronal cells and neuroimmune interactions in DPN. Future Directions: Nanomedicine-based agents represent an exciting opportunity for the treatment of DPN with the goals of improving the efficacy and safety profile of analgesia, as well as restoring peripheral neuroregenerative capacity. Antioxid. Redox Signal. 36, 122-143.

Myelination, axonal loss and Schwann cell characteristics in axonal polyneuropathy compared to controls

Introduction

Polyneuropathy is a debilitating condition characterized by distal sensory and motor deficits. Schwann cell dysfunction and axonal loss are integral factors in pathophysiology and disease progression of polyneuropathy.

Aims

The aim of this study was the assessment of Schwann cell characteristics, nerve fibers and myelination parameters in polyneuropathy patients compared to controls.

Methods

Nerve tissue was obtained from polyneuropathy patients (n = 10) undergoing diagnostic sural nerve biopsies. Biopsies of healthy peripheral nerves (n = 5) were harvested during elective sural nerve grafting for chronic peripheral nerve lesions. Exclusion criteria for the healthy control group were recent neurological trauma, diabetes, neurological and cardiovascular disease, as well as active malignancies and cytotoxic medication within the last 12 months. The over-all architecture of nerve sections and myelination parameters were histomorphometrically analyzed. Immunofluorescent imaging was used to evaluate Schwann cell phenotypes, senescence markers and myelination parameters.

Results

Histomorphometric analysis of nerve biopsies showed significant axonal loss in polyneuropathy patients compared to controls, which was in accordance with the neuropathological findings. Immunofluorescent staining of Schwann cells and myelin basic protein indicated a significant impairment of myelination and lower Schwann cell counts compared to controls. Phenotypic alterations and increased numbers of non-myelinating p75-positive Schwann cells were found in polyneuropathy patients.

Discussion

This study provided quantitative data of axonal loss, reduced myelination and Schwann cell dysfunction of polyneuropathy patients compared to neurologically healthy controls. Phenotypic alterations of Schwann cells were similar to those seen after peripheral nerve injury, highlighting the clinical relevance of Schwann cell dysfunction.

Acidic fibroblast growth factor attenuates type 2 diabetes-induced demyelination via suppressing oxidative stress damage

Prolonged type 2 diabetes mellitus (T2DM) produces a common complication, peripheral neuropathy, which is accompanied by nerve fiber disorder, axon atrophy, and demyelination. Growing evidence has characterized the beneficial effects of acidic fibroblast growth factor (aFGF) and shown that it relieves hyperglycemia, increases insulin sensitivity, and ameliorates neuropathic impairment. However, there is scarce evidence on the role of aFGF on remodeling of aberrant myelin under hyperglycemia condition. Presently, we observed that the expression of aFGF was rapidly decreased in a db/db T2DM mouse model. Administration of exogenous aFGF was sufficient to block acute demyelination and nerve fiber disorganization. Furthermore, this strong anti-demyelinating effect was most likely dominated by an aFGF-mediated increase of Schwann cell (SC) proliferation and migration as well as suppression of its apoptosis. Mechanistically, the beneficial biological effects of aFGF on SC behavior and abnormal myelin morphology were likely due to the inhibition of hyperglycemia-induced oxidative stress activation, which was most likely activated by kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid-derived-like 2 (Nrf2) signaling. Thus, this evidence indicates that aFGF is a promising protective agent for relieving myelin pathology through countering oxidative stress signaling cascades under diabetic conditions.

Taurine protects against myelin damage of sciatic nerve in diabetic peripheral neuropathy rats by controlling apoptosis of schwann cells via NGF/Akt/GSK3β pathway

Diabetic peripheral neuropathy is a common complications of Type 2 Diabetes and its main pathological feature is myelin sheath damage of peripheral nerve that was induced by Schwann cells (SCs) apoptosis. Increasing evidence suggested that taurine might play a role in improving DPN because of its ability to prevent SCs apoptosis. In this study, we explore the effect of taurine on preventing SCs apoptosis and its underlying mechanism. Sprague Dawley rats were treated with streptozotocin to establish the diabetes model. Rats were randomly divided into control, diabetes, taurine treatment (as giving 0.5%, 1% and 2% taurine in drinking water) groups. RSC96 cell (a rat SCs line) was used for intervention experiments in vitro. Results showed that taurine significantly corrected morphology of damaged myelin sheath and inhibited SCs apoptosis in sciatic nerve of diabetic rats. Moreover, taurine prevented apoptosis of RSC96 cells exposed to high glucose. Mechanistically, taurine up-regulated NGF expression and phosphorylation levels of Akt and GSK3β, while, blocking activation of NGF and phosphorylation of Akt and GSK3β increased apoptosis of high glucose-exposed RSC96 cells with taurine supplement. These results revealed taurine improved the myelin sheath damage of sciatic nerve in diabetic rats by controlling SCs apoptosis via NGF/Akt/GSK3β signaling pathways, which provides some clues that taurine might be effective and feasible candidate for the treatment of DPN.

1,25-(OH)2D3 protects Schwann cells against advanced glycation end products-induced apoptosis through PKA-NF-κB pathway

AIMS

To explore the effect and mechanism of 1, 25-(OH)₂D₃ on Schwann cell apoptosis induced by advanced glycation end products.

MAIN METHODS

Schwann cells, isolated from rodent sciatic nerve were incubated with AGE-modified bovine serum albumin(AGE) to mimic diabetic conditions and 1,25-(OH)₂D₃ was used as protector. Cell apoptosis was detected by PI/Annexin-V staining, caspase 3 activity assay and western blotting for caspase 3 and PARP. The activation of protein kinase A (PKA) and nuclear factor kappa-B (NF-κB) was evaluated by western blot. Immunofluorescent staining was used for intercellular location of NF-κB. Cytokine secretion was evaluated by enzyme-linked immunosorbent assay.

KEY FINDINGS

Schwann cell apoptosis accelerated after incubating with AGE. However, if combining 1,25-(OH)₂D₃ with AGE, apoptosis decreased significantly. 1,25-(OH)₂D₃ enhanced PKA activity, but inhibited AGE-induced nuclear translocation of NF-κB. Furthermore, PKA activator (8-bromoadenoside cyclic adenoside monophosphate, 8-Br-cAMP) or NF-κB inhibitor (caffeic acid phenethyl ester, CAPE) could reduce the apoptosis, decreased cleaved caspase 3 and cleaved PARP, suggesting the involvement of PKA and NF-κB pathways in the protection of 1,25-(OH)₂D₃ on Schwann cells. Moreover, 8-Br-cAMP and CAPE could inhibit AGE-induced secretion of interleukin(IL)-1β, prostaglandin E2(PEG2) and cyclooxygenase 2(COX2). Interestingly, 8-Br-cAMP decreased phospho-NF-κB and inhibited nucleus translocation of NF-κB. It hinted at the regulation of PKA to NF-κB. Finally, a pre-treatment of H-89 (an inhibitor of PKA) could block the protection of 1,25-(OH)₂D₃ on cell apoptosis. In conclusion, 1,25-(OH)₂D₃ could protect Schwann cell against AGE-induced apoptosis through PKA/NF-κB pathway.

SIGNIFICANCE

These findings provide experimental rationales for using vitamin D for diabetic neuropathy.

Salvianolic acid B inhibits the development of diabetic peripheral neuropathy by suppressing autophagy and apoptosis

OBJECTIVES

The aim of this study was to evaluate the neuroprotective effects of SalB on high glucose (HG)-induced excessive autophagy and apoptosis in vitro.

METHODS

The proliferation and apoptosis of RSC96 cells were determined using the MTT assay and flow cytometry, respectively. Western blot analysis was performed to examine the expression of autophagy and apoptosis-related proteins. RT-PCR and flow cytometry were manipulated to examine the level of Bcl-2. The signals of autophagy markers were detected using immunofluorescence methods.

KEY FINDINGS

We found that HG significantly reduced RSC96 cell's proliferation and induced apoptosis. What's more, HG increased the level of autophagy and apoptosis-related proteins. However, these effects were reversed by SalB. In addition, we also found that 3-MA decreased the expression of LC3A/B and Beclin1, while the JNK inhibitor SP600125 reduced the levels of phosphorylated JNK, LC3A/B and Beclin1.

CONCLUSIONS

High glucose not only induced apoptosis but also caused autophagic cell death by activating the JNK pathway. These effects prevented by SalB in an opposite manner.

Injury-induced gp130 cytokine signaling in peripheral ganglia is reduced in diabetes mellitus

Neuropathy is a major diabetic complication. While the mechanism of this neuropathy is not well understood, it is believed to result in part from deficient nerve regeneration. Work from our laboratory established that gp130 family of cytokines are induced in animals after axonal injury and are involved in the induction of regeneration-associated genes (RAGs) and in the conditioning lesion response. Here, we examine whether a reduction of cytokine signaling occurs in diabetes. Streptozotocin (STZ) was used to destroy pancreatic β cells, leading to chronic hyperglycemia. Mice were injected with either low doses of STZ (5×60mg/kg) or a single high dose (1×200mg/kg) and examined after three or one month, respectively. Both low and high dose STZ treatment resulted in sustained hyperglycemia and functional deficits associated with the presence of both sensory and autonomic neuropathy. Diabetic mice displayed significantly reduced intraepidermal nerve fiber density and sudomotor function. Furthermore, low and high dose diabetic mice showed significantly reduced tactile touch sensation measured with Von Frey monofilaments. To look at the regenerative and injury-induced responses in diabetic mice, neurons in both superior cervical ganglia (SCG) and the 4th and 5th lumbar dorsal root ganglia (DRG) were unilaterally axotomized. Both high and low dose diabetic mice displayed significantly less axonal regeneration in the sciatic nerve, when measured in vivo, 48h after crush injury. Significantly reduced induction of two gp130 cytokines, leukemia inhibitory factor and interleukin-6, occurred in diabetic animals in SCG 6h after injury compared to controls. Injury-induced expression of interleukin-6 was also found to be significantly reduced in the DRG at 6h after injury in low and high dose diabetic mice. These effects were accompanied by reduced phosphorylation of signal transducer and activator of transcription 3 (STAT3), a downstream effector of the gp130 signaling pathway. We also found decreased induction of several gp130-dependent RAGs, including galanin and vasoactive intestinal peptide. Together, these data suggest a novel mechanism for the decreased response of diabetic sympathetic and sensory neurons to injury.

Epalrestat Upregulates Heme Oxygenase-1, Superoxide Dismutase, and Catalase in Cells of the Nervous System

Heme oxygenase (HO)-1 has potent antioxidant and anti-inflammatory functions. Recent studies have shown that the upregulation of HO-1 is beneficial to counteract neuroinflammation, making HO-1 a new therapeutic target for neurological diseases. We have reported that epalrestat (EPS), which is currently used for the treatment of diabetic neuropathy, increases HO-1 levels through the activation of nuclear factor erythroid 2-related factor 2 (Nrf2) in bovine aortic endothelial cells. In this study, we tested the hypothesis that EPS upregulates HO-1 via Nrf2 activation in the component cells of the nervous system, by using rat Schwann cells and human SH-SY5Y cells. Treatment of Schwann cells with EPS at near-plasma concentration led to a dramatic increase in HO-1 levels. Nrf2 knockdown by small interfering RNA (siRNA) suppressed the EPS-induced HO-1 expression. EPS did not promote the intracellular accumulation of free ferrous ion and reactive oxygen species, by increasing ferritin via Nrf2 during HO-1 induction. Moreover, EPS stimulated the expression of superoxide dismutase 1 and catalase, which also are Nrf2 target gene products. It also markedly increased HO-1 levels in SH-SY5Y cells through the activation of Nrf2. We demonstrated for the first time that EPS upregulates HO-1, superoxide dismutase, and catalase by activating Nrf2. We suggest that EPS has the potential to prevent several neurological diseases.

Protective Effect of Puerarin Against Oxidative Stress Injury of Neural Cells and Related Mechanisms

Background

Parkinson’s disease (PD) is manifested as degeneration of dopaminergic neurons in substantia nigra compacta. The mitochondrial dysfunction induced by oxidative stress is believed to a major cause of PD. Puerarin has been widely applied due to its estrogen nature and anti-oxidative function. This study thus investigated the protective role of puerarin against oxidative stress injury on PC12 neural cells, in addition to related mechanisms.

Material/Methods

PC12 cells were pre-treated with gradient concentrations of puerarin, followed by the induction of 0.5 mM H₂O₂. MTT assay was used to detect cell viability. Enzyme-linked immunosorbent assay (ELISA) was employed to detect intracellular level of superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione (GSH). Cell apoptosis was determined by Annexin-V/7-AAD double labelling. Reactive oxidative species (ROS) and lactate dehydrogenase (LDH) activities were then measured. Cellular levels of caspase-3 and caspase-9 were also determined.

Results

The pre-treatment using puerarin significantly reversed H₂O₂-induced oxidative stress injury, as it can increase proliferation, SOD and GSH activities, decrease MDA activity, suppress apoptosis of PC12 cells, and decrease ROS and LDH production (p<0.05 in all cases). Further assays showed depressed up-regulation of caspase-3 and caspase-9 after puerarin pretreatment.

Conclusions

Puerarin pretreatment can decrease activity of caspase-3 and caspase-9 activity in PC12 cells, thus protecting cells from oxidative injury.

Lentivirus mediated over expression of CGRP inhibited oxidative stress in Schwann cell line

CGRP is reported to be implicated in the process of diabetes and neuronal disease. However, the role and underlying mechanism of CGRP involved in diabetic neuropathy is unknown. Schwann cells play a central role in diabetic neuropathy, therefore the protective effect of CGRP on Schwann cells exposed to high glucose is determined. In the present study, full-length CGRP cDNA was isolated and then transferred to gateway adapted lentivirus expression vector by LR recombination reaction. Afterwards, the CGRP bearing recombinant virus was prepared in 293 FT cells and used to infect Schwann cells. The viability and superoxide anions of Schwann cells were evaluated following stimulation with high glucose, and levels of SOD, MDA and NOX1 were assessed. The results suggested that CGRP expression was up-regulated following lentivirus transfection. Lenti-CGRP increased cell viability in high glucose, but the effect was transient. Further lenti-CGRP protected against oxidative stress in Schwann cells triggered by high glucose and lenti-CGRP was effective in increasing SOD and decreasing MDA level. Meanwhile, the increased level of NOX1 caused by high glucose was reversed by lenti-CGRP overexpression. We therefore, suggest that lenti-CGRP may play a role in inhibiting oxidative stress in Schwann cell lines following hyperglycemic stimulation.

Puerarin prevents high glucose-induced apoptosis of Schwann cells by inhibiting oxidative stress

Oxidative stress may be the unifying factor for the injury caused by hyperglycemia in diabetic peripheral neuropathy. Puerarin is the major isoflavonoid derived from Radix puerariae and has been shown to be effective in increasing superoxide dismutase activity. This study sought to investigate the neuroprotective effect of puerarin on high glucose-induced oxidative stress and Schwann cell apoptosis in vitro. Intracellular reactive oxygen radicals and mitochondrial transmembrane potential were detected by flow cytometry analysis. Apoptosis was confirmed by TUNEL and oxidative stress was monitored using an enzyme-linked immunosorbent assay for the DNA marker 8-hydroxy-2-deoxyguanosine. The expression levels of bax and bcl-2 were analyzed by quantitative real-time reverse transcriptase-PCR, while protein expression of cleaved caspase-3 and -9 were analyzed by means of western blotting. Results suggested that puerarin treatment inhibited high glucose-induced oxidative stress, mitochondrial depolarization and apoptosis in a dose-dependent manner. Furthermore, puerarin treatment downregulated Bax expression, upregulated bcl-2 expression and attenuated the activation of caspase-3 and -9. Overall, our results indicated that puerarin antagonized high glucose-induced oxidative stress and apoptosis in Schwann cells.

Erythropoietin attenuates oxidative stress and apoptosis in Schwann cells isolated from streptozotocin-induced diabetic rats

OBJECTIVES

High glucose-evoked oxidative stress and apoptosis within Schwann cells (SCs) are mechanisms facilitating the procession of diabetic peripheral neuropathy (DPN). Although erythropoietin (EPO) was demonstrated to have neuroprotective effects in neurodegenerative diseases, the effects of EPO on glucose-evoked oxidative stress and apoptosis of SCs remain unknown.

METHODS

Primary cultured SCs isolated from streptozotocin (STZ)-induced diabetic peripheral neuropathic rats and normal control rats were exposed to high or normal glucose condition with or without EPO incubation for 72 h. Cell viability, apoptotic rate, cellular reactive oxygen species (ROS) level, total glutathione (GSH) level, EPO mRNA and erythropoietin receptor (EPOR) mRNA levels were assayed.

KEY FINDINGS

SCs from diabetic rats showed a lower cell viability and a higher apoptotic rate. High glucose culture condition elevated ROS level and diminished total GSH level of SCs. EPO improved cell viability and decreased cell apoptotic rate of SCs. EPO also elevated total GSH level and decreased intracellular ROS level. SCs from diabetic rats exhibited higher EPO mRNA and EPOR mRNA levels than SCs from normal control rats.

CONCLUSIONS

The data of this study offered fresh viewpoints for interpreting the pathogenesis of DPN and novel pharmacological principles implicit in the therapeutic effect of EPO.

Diabetic Schwann cells suffer from nerve growth factor and neurotrophin-3 underproduction and poor associability with axons

Schwann cells (SCs) are integral to peripheral nerve biology, contributing to saltatory conduction along axons, nerve and axon development, and axonal regeneration. SCs also provide a microenvironment favoring neural regeneration partially due to production of several neurotrophic factors. Dysfunction of SCs may also play an important role in the pathogenesis of peripheral nerve diseases such as diabetic peripheral neuropathy where hyperglycemia is often considered pathogenic. In order to study the impact of diabetes mellitus (DM) upon the regenerative capacity of adult SCs, we investigated the differential production of the neurotrophic factors nerve growth factor (NGF) and neurotrophin-3 (NT3) by SCs harvested from the sciatic nerves of murine models of type 1 DM (streptozotocin treated C57BL/6J mice) and type 2 DM (LepR(-/-) or db/db mice) or non-diabetic cohorts. In vitro, SCs from diabetic and control mice were maintained under similar hyperglycemic and euglycemic conditions respectively. Mature SCs from diabetic mice produced lower levels of NGF and NT3 under hyperglycemic conditions when compared to SCs in euglycemia. In addition, SCs from both DM and non-DM mice appear to be incapable of insulin production, but responded to exogenous insulin with greater proliferation and heightened myelination potentiation. Moreover, SCs from diabetic animals showed poorer association with co-cultured axons. Hyperglycemia had significant impact upon SCs, potentially contributing to the pathogenesis of diabetic peripheral neuropathy.

The adjuvant effect of hypertension upon diabetic peripheral neuropathy in experimental type 2 diabetes

Type 2 diabetes (DM) is the most common cause of peripheral neuropathy in the Western world. A comorbidity, hypertension, has been speculated to contribute to initiation or worsening of diabetic peripheral neuropathy. We studied adult rat models using genetic strains with DM (Zucker Diabetic Fat rats)±hypertension (HTN (ZSF-1 rats)) to investigate the relative contributions of DM and HTN and the potential for additive effects of HTN upon existing DM for the development of peripheral neuropathy. Long duration sensorimotor behavioral and electrophysiological testing was complemented by histological and molecular methods. Only DM led to tactile and thermal hyperalgesia and affected motor nerve electrophysiology. Although DM led to marked loss of sensory amplitudes and to sensory conduction slowing, a mild additive effect from HTN contributed after 6months of DM with worsening of slowing of sensory nerve conduction velocities, but without effect upon sensory amplitudes. At the sensory dominant sural nerve, mild (<10%) but greater degrees of myelin thinning were noted with DM and HTN combined, suggesting a mild additive effect. Matrix metalloproteinase (MMP) expression was increased only at the sural nerve in the presence of HTN with co-localization to Schwann cells and myelin. The effects of DM and HTN upon peripheral nerve are dissimilar, with HTN contributing to MMP upregulation at the sites of myelin thinning at sensory nerve fibers, potentially worsening comorbid DM. Together, our results indicate that HTN has a mild additive contribution to diabetic peripheral neuropathy at sensory peripheral nerve fibers manifesting with the loss of myelin thickness.

Protective effects of Salvianolic acid B on Schwann cells apoptosis induced by high glucose

Diabetic peripheral neuropathy (DPN) is one of the most common and debilitating microvascular complications of diabetes, and there is no effective therapy for the prevention or treatment of DPN. Oxidative stress triggers several pathways of injury and may be the unifying factor of hyperglycemia. The aim of this study was to investigate protective effect of Salvianolic acid B (Sal B) on the high glucose (HG)-induced oxidative stress-induced mitochondrial pathway activation and Schwann cells (SCs) apoptosis in vitro. We found that Sal B inhibited the HG-induced oxidative stress by reducing ROS and 8-hydroxy-2-deoxy Guanosine (8-OHdG) production, and mitochondrial depolarization and apoptosis in SCs in a dose-dependent manner. Furthermore, Sal B down-regulated the HG-mediated Bax expression and AIF nuclear translocation and the release of cytochrome c, but up-regulated the HG-induced BcL-2 expression in SCs. In addition, Sal B attenuated the HG-induced activation of caspase 3 and 9 and minimized the cleavage of PARP in SCs. Our results indicated that Sal B antagonized the HG-induced oxidative stress, activation of the mitochondrial pathway and apoptosis in SCs.

4-Methlycatechol prevents NGF/p75(NTR)-mediated apoptosis via NGF/TrkA system in pancreatic β cells

In this study, it was aimed to investigate whether 4-methylcatechol (4-MC) could serve as an autocrine antiapoptotic agent by increasing nerve growth factor (NGF) in β cells of hyperglycemic rats. Rats were divided into four groups: the first group was given citrate buffer and saline, the second group was administered 4-MC, the third group received streptozotocin (STZ), and the fourth group was given both 4-MC and STZ. 4-MC (10 μg/kg) was administered by daily intraperitoneal injection for 10 days before the animals were rendered hyperglycemic by administration of STZ (75 mg/kg). With 4-MC pretreatment on hyperglycemic rats the following results were noted: (i) Increase in plasma glucose, β cell apoptosis and caspase-8 activation was prevented. (ii) Reduction of NGF+ and tyrosine receptor kinase A (TrkA)+ β cell number was blocked. (iii) p75 neurotrophin receptor (p75(NTR))+ β cell number was increased. These data suggest that 4-MC might exert its antiapoptotic actions through NGF/TrkA system which may block NGF/p75(NTR) activation in pancreatic β cells of hyperglycemic rats.

Exendin-4 exerts its effects through the NGF/p75NTR system in diabetic mouse pancreas

Glucagon-like peptide-1 (GLP-1) ameliorates the symptoms of diabetes through stimulation of insulin secretion. We have investigated the possible components of cellular mechanism triggered by exendin-4, a potent GLP-1 receptor agonist, in streptozotocin (STZ) induced diabetic mice pancreas. BALB/c male mice were divided into four groups for this investigation. The first group was given citrate buffer only, the second group was administered exendin-4 alone, the third group received STZ, and the fourth group was given both STZ and exendin-4. Exendin-4 (3 microg/kg) was administered by daily subcutaneous injection for 30 days after the animals were rendered diabetic by administration of STZ (200 mg/kg). With exendin-4 treatment on diabetic mice, the following results were noted: (i) exendin-4 suppressed the increase in plasma glucose and inhibited somatostatin expression induced by STZ, (ii) reduction of insulin prevalence was inhibited, while expression of p75 neurotrophin receptor (p75NTR), pancreatic nerve growth factor (NGF), and NGF-positive islet cell prevalence increased, (iii) there were no alterations in the severity of proliferated cell nuclear antigen positive or apoptotic beta cells in pancreatic islets, and (iv) pancreatic catalase, glutathione peroxidase, and superoxide dismutase activities significantly increased. In conclusion, these data suggest that exendin-4 might exert its actions through the NGF/p75NTR system and decrease somatostatin expression.

Effect of panaxydol on hypoxia-induced cell death and expression and secretion of neurotrophic factors (NTFs) in hypoxic primary cultured Schwann cells

It has been shown that panaxydol (PND) can mimic the neurotrophic effect of nerve growth factor (NGF) normally secreted by Schwann cells (SC) and protect neurons against injury. To evaluate the effect of PND on hypoxia-induced SC death and expression and secretion of neurotrophic factors (NGF and brain derived neurotrophic factor (BDNF)), hypoxic SCs were cultured in vitro and then treated with PND (0-20 microM). The MTT (3(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide) assay, immunocytochemistry, ELISA and RT-PCR were employed to examine the effects. We found that hypoxia resulted in a significant decrease in SCs viability (MTT: 64+/-4.7% of control group) and nearly a 3.3-fold increase of intracellular level of active caspase-3. PND (5-20 microM) treatment significantly rescued the SCs from hypoxia-induced injury (85+/-8.2%; 92+/-8.6%; 87+/-7.3%) and reduced caspase-3 activity with the maximal effect occurred at 10 microM (P<0.01), reducing to about 1.6-fold of control level. Furthermore, PND treatment also enhanced NGF and BDNF mRNA levels in hypoxic SCs and promoted protein expression and secretion. BDNF mRNA in hypoxic SCs was restored to about 90% of normal level and NGF mRNA was elevated to 1.4-fold of control after 10 microM PND treatment. These observations showed that PND protects primary cultured SCs against hypoxia-induced injury and enhances NTF-associated activities.

The effect of non-enzymatic glycation of extracellular matrix proteins on axonal regeneration in vitro

Non-enzymatic glycation of peripheral nerve extracellular matrix (ECM) may contribute to the development of diabetic distal sensory neuropathy (DNP). We investigated the relative importance of glycation of collagen types I and IV, laminin and fibronectin in DNP-related impairment in peripheral nerve regeneration. Dorsal root ganglia (DRGs) from young adult mice were embedded in collagen type I modified by 10% substitution with normal or glycated forms of the proteins and incubated for 3 days. Outgrowth of axons and migration of cells into the ECM were quantified. Mean length of growing axons was significantly reduced by glycation of laminin and collagen type IV. The sum of lengths of all axons from each DRG was greatly reduced with glycated laminin, collagen types IV and I. Glycation of fibronectin had no effect on axonal growth. The number of migrating cells was not affected by glycation. We conclude that non-enzymatic glycation of laminin and collagen types IV and I (in decreasing order) impairs peripheral nerve regeneration in vitro.

Impaired peripheral nerve regeneration in diabetes mellitus

Diabetes mellitus impairs peripheral nerve regeneration. Regenerative failure likely exacerbates deficits from polyneuropathy or focal neuropathies in patients who might otherwise exhibit spontaneous improvement. Some focal neuropathies, like carpal tunnel syndrome, are common, yet render ongoing disability because of their delayed recovery. Why diabetic nerves fail to regenerate is an interesting question to consider because several mechanisms likely contribute. In this review, we examine a number of these causes. These causes include microangiopathy or disease of small blood vessels, failure to provide proper metabolic support for repair, defects in the entry and actions of inflammatory cells within the injury milieu, less robust support of axons by their Schwann cells, and lack of a full repertoire of trophic factors. A number of the mechanisms that generate neuropathy in the first place also likely contribute to failed regenerative programs, but how they do so is not clear.

Neurotrophin-3 null mutant mice display a postnatal motor neuropathy

This paper examines early postnatal development of the neuromuscular system in mice with a null mutation in the gene for neurotrophin-3. We report that alpha-motoneurons at first develop substantially normally, despite a known 15% deficit in their somal size [Woolley et al. (1999)Neurosci. Lett., 272, 107-110.] and the absence of proprioceptive input [Ernfors et al. (1994)Cell, 77, 503-512]. At birth, motor axons have extended into the muscle, forming normal-looking neuromuscular junctions with focal accumulations of acetylcholine receptors. Detailed ultrastructural analysis does however, reveal subtle abnormalities at this time, particularly a decrease in the extent of occupancy of the postsynaptic site by nerve terminals, and a small but significant deficit in myofibre number. After the relative normality of this early neuromuscular development, there then occurs a catastrophic postnatal loss of motor nerve terminals, resulting in complete denervation of hindlimb muscles by P7. In systematic semi-serial samples through the entire muscle endplate zones, no neuromuscular junctions can be found. Intramuscular axons are fragmented, as shown by both electron microscopic observations and neurofilament immunohistochemistry, and alpha-bungarotoxin detection of acetylcholine receptors indicates dispersal of the junctional accumulation. At earlier times (postnatal days three and four) the terminal Schwann cells show ultrastructural abnormalities, and preliminary observations suggest marked disturbance of myelination. Based on comparison with other literature, the peripheral nerve degeneration seems unlikely to have arisen as a secondary effect of de-afferentation. We discuss whether the neural degeneration is secondary to the disturbance of Schwann cell function, or due directly to a loss of neurotrophin-3 based support of the motoneuron.

Methylglyoxal induces apoptosis through activation of p38 MAPK in rat Schwann cells

The formation of glucose-derived methylglyoxal (MG), a highly reactive dicarbonyl compound, is accelerated under diabetic conditions. We examined whether MG was capable of inducing apoptosis in Schwann cells (SCs), since recent studies have suggested a potential involvement of apoptotic cell death in the development of diabetic neuropathy. MG induced apoptosis in SCs in a dose-dependent manner, accompanied by a reduction of intracellular glutathione content and activation of the p38 MAPK. Inhibiting the p38 MAPK activation by SB203580 successfully suppressed the MG-induced apoptosis in SCs. Aminoguanidine and N-acetyl-L-cysteine also inhibited the MG-induced p38 MAPK activation and apoptosis along with restoration of the intracellular glutathione content. These results suggest a potential role for MG in SC injury through oxidative stress-mediated p38 MAPK activation under diabetic conditions, and it may serve as a novel insight into therapeutic strategies for diabetic neuropathy.

Role of the Schwann cell in diabetic neuropathy

The relationships among Schwann cells, axons, and the perineurial barrier emphasize the key role Schwann cells play in normal functions of the nerve. Schwann cells are responsible for action potential velocity through insulation of axons, maintenance of axonal caliber, and correct localization of Na+ channels; immunological and funcitonal integrity of the nerve through the perineurial blood-nerve-barrier; and effective nerve regeneration. In diabetic neuropathy, many of these facets of nerve function are defective. Hypoxia, hyerglycemia, and increased oxidative stress contribute directly and indirectly to Schwann cell dysfunction. The results include impaired paranodal barrier function, damaged myelin, reduced antioxidative capacity, and decreased neurotrophic support for axons. This chapter discusses the role of the Schwann cell in the normal or regenerating nerve nad in the altered metabolic conditons of diabetes.