Schwann cell interactions with axons and microvessels in diabetic neuropathy

Exosomes Derived From Schwann Cells Ameliorate Peripheral Neuropathy in Type 2 Diabetic Mice

Schwann cell-derived exosomes communicate with dorsal root ganglia (DRG) neurons. The current study investigated the therapeutic effect of exosomes derived from healthy Schwann cells (SC-Exos) on diabetic peripheral neuropathy (DPN). We found that intravenous administration of SC-Exos to type 2 diabetic db/db mice with peripheral neuropathy remarkably ameliorated DPN by improving sciatic nerve conduction velocity and increasing thermal and mechanical sensitivity. These functional improvements were associated with the augmentation of epidermal nerve fibers and remyelination of sciatic nerves. Quantitative RT-PCR and Western blot analysis of sciatic nerve tissues showed that SC-Exo treatment reversed diabetes-reduced mature form of miRNA (miR)-21, -27a, and -146a and diabetes-increased semaphorin 6A (SEMA6A); Ras homolog gene family, member A (RhoA); phosphatase and tensin homolog (PTEN); and nuclear factor-κB (NF-κB). In vitro data showed that SC-Exos promoted neurite outgrowth of diabetic DRG neurons and migration of Schwann cells challenged by high glucose. Collectively, these novel data provide evidence that SC-Exos have a therapeutic effect on DPN in mice and suggest that SC-Exo modulation of miRs contributes to this therapy.

A Metabolic Insight Into the Neuroprotective Effect of Jin-Mai-Tong (JMT) Decoction on Diabetic Rats With Peripheral Neuropathy Using Untargeted Metabolomics Strategy

Jin-Mai-Tong (JMT) decoction is a traditional Chinese compound prescription for treating diabetic peripheral neuropathy (DPN). The aim of this study is to investigate the neuroprotective effect of JMT decoction on diabetic rats with peripheral neuropathy and to elucidate the potential mechanism based on a metabolomics approach. Sprague-Dawley (SD) rats were randomly divided into four groups: control group, Streptozotocin (STZ) induced model group, JMT low dose (JMT-L) treated group and JMT high dose (JMT-H) treated group. After 12 weeks of treatment, behavioral changes, small fiber loss, and histopathological damages of sciatic nerves were estimated. Serum samples were collected for untargeted metabolomics analysis based on UPLC/QTOF-MS and multivariate statistics. As a result, JMT treatment at two dosages (13.9 and 27.8 g/kg⋅d) evidently improved the mechanical pain threshold (P < 0.05), increased the intraepidermal nerve fiber density (IENFD) and subepidermal nerve fiber density (SNFD) (P < 0.05), and renovated the demyelination and axonal atrophy of sciatic nerves on DPN rats. Furthermore, metabolomics study revealed that the serum metabolic profiles altered significantly among the control group and the STZ-induced model group. A total of 21 metabolites were identified as potential biomarkers related to the therapeutic effect of JMT decoction. Among them, 16 biomarkers were found in both JMT-H and JMT-L treated groups, while the five others were specific to JMT-H group. These metabolites mainly involved in lipid metabolism, tricarboxylic acid (TCA) cycle, amino acid metabolism, and so on. Besides, correlation analysis indicated that both mechanical pain threshold and distal nerve fiber density were negatively correlated with the serum levels of metabolites from lipid metabolism and TCA cycle. In conclusion, the results demonstrated that JMT decoction has an obvious protective effect against DPN, which could be mediated via ameliorating the metabolic disorders in diabetic rats with peripheral neuropathy.

Proteome profile of peripheral myelin in healthy mice and in a neuropathy model

Proteome and transcriptome analyses aim at comprehending the molecular profiles of the brain, its cell-types and subcellular compartments including myelin. Despite the relevance of the peripheral nervous system for normal sensory and motor capabilities, analogous approaches to peripheral nerves and peripheral myelin have fallen behind evolving technical standards. Here we assess the peripheral myelin proteome by gel-free, label-free mass-spectrometry for deep quantitative coverage. Integration with RNA-Sequencing-based developmental mRNA-abundance profiles and neuropathy disease genes illustrates the utility of this resource. Notably, the periaxin-deficient mouse model of the neuropathy Charcot-Marie-Tooth 4F displays a highly pathological myelin proteome profile, exemplified by the discovery of reduced levels of the monocarboxylate transporter MCT1/SLC16A1 as a novel facet of the neuropathology. This work provides the most comprehensive proteome resource thus far to approach development, function and pathology of peripheral myelin, and a straightforward, accurate and sensitive workflow to address myelin diversity in health and disease.

Altered peripheral nerve structure and function in latent autoimmune diabetes in adults

AIM

The present study was undertaken to investigate mechanisms of peripheral nerve dysfunction in latent autoimmune diabetes in adults (LADA).

MATERIALS AND METHODS

Participants with LADA (n = 15) underwent median nerve ultrasonography and nerve excitability to examine axonal structure and function, in comparison to cohorts of type 1 diabetes (n = 15), type 2 diabetes (n = 23) and healthy controls (n = 26). The LADA group was matched for diabetes duration, glycaemic control, and neuropathy severity with the type 1 and type 2 diabetes groups. A validated mathematical model of the human axon was utilized to investigate the pathophysiological basis of nerve dysfunction.

RESULTS

The most severe changes in nerve structure and function were noted in the LADA group. The LADA cohort demonstrated a significant increase in nerve cross-sectional area compared to type 1 participants and controls. Compared to type 1 and 2 diabetes, measures of threshold electrotonus, which assesses nodal and internodal conductances, were significantly worse in LADA in response to both depolarising currents and hyperpolarising currents. In the recovery cycle, participants with LADA had a significant increase in the relative refractory period. Mathematical modelling of excitability recordings indicated the basis of nerve dysfunction in LADA was different to type 1 and 2 diabetes.

CONCLUSIONS

Participants with LADA exhibited more severe changes in nerve function and different underlying pathophysiological mechanisms compared to participants with type 1 or 2 diabetes. Intensive management of risk factors to delay the progression of neuropathy in LADA may be required.

Early sensory neurophysiological changes in prediabetes

AIMS/INTRODUCTION

To elucidate whether axonal changes arise in the prediabetic state and to find a biomarker for early detection of neurophysiological changes.

MATERIALS AND METHODS

We enrolled asymptomatic diabetes patients, as well as prediabetic and normoglycemic individuals to test sensory nerve excitability, and we analyzed those findings and their correlation with clinical profiles.

RESULTS

In nerve excitability tests, superexcitability in the recovery cycle showed increasing changes in the normoglycemic, prediabetes and diabetes cohorts (-19.09 ± 4.56% in normoglycemia, -22.39 ± 3.16% in prediabetes and -23.71 ± 5.15% in diabetes, P = 0.002). Relatively prolonged distal sensory latency was observed in the median nerve (3.12 ± 0.29 ms in normoglycemia, 3.23 ± 0.38 ms in prediabetes and 3.45 ± 0.43 ms in diabetes, P = 0.019). Superexcitability was positively correlated with fasting plasma glucose (r = 0.291, P = 0.009) and glycated hemoglobin (r = 0.331, P = 0.003) in all participants.

CONCLUSIONS

Sensory superexcitability and latencies are the most sensitive parameters for detecting preclinical physiological dysfunction in prediabetes. In addition, changes in favor of superexcitability were positively correlated with glycated hemoglobin for all participants. These results suggest that early axonal changes start in the prediabetic stage, and that the monitoring strategy for polyneuropathy should start as early as prediabetes.

Schwann cells apoptosis is induced by high glucose in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes mellitus that affects approximately half of patients with diabetes. Current treatment regimens cannot treat DPN effectively. Schwann cells (SCs) are very sensitive to glucose concentration and insulin, and closely associated with the occurrence and development of type 1 diabetic mellitus (T1DM) and DPN. Apoptosis of SCs is induced by hyperglycemia and is involved in the pathogenesis of DPN. This review considers the pathological processes of SCs apoptosis under high glucose, which include the following: oxidative stress, inflammatory reactions, endoplasmic reticulum stress, autophagy, nitrification and signaling pathways (PI3K/AKT, ERK, PERK/Nrf2, and Wnt/β-catenin). The clarification of mechanisms underlying SCs apoptosis induced by high glucose will help us to understand and identify more effective strategies for the treatment of T1DM DPN.

Different patterns in age-related morphometric alteration of myelinated fibers and capillaries of the tibial nerve: a longitudinal study in normal rats

Age-related regression of myelinated fibers in peripheral nerves of the lower limbs is strongly influenced by capillaries and results in balance dysfunction and falls. However, the temporal relationships between alteration patterns of myelinated fibers and capillaries have not yet been clarified. This study aimed to investigate age-related morphological and histological changes of both myelinated fibers and capillaries in peripheral nerves to clarify whether myelinated fibers or capillaries change earlier. Seven male Wistar rats each were randomly selected at 20 weeks (young group), 70 weeks (middle group), and 97 weeks (old group) for histological evaluations. The left and right tibial nerves were removed morphologically and histologically to examine myelinated fibers and capillaries. Axon diameter and myelin thickness were almost unaltered in the middle group compared with the young group but were significantly reduced in the old group when compared with the other two groups. However, the capillary diameter and number of microvascular branch points were substantially reduced in the middle group. The current study demonstrates that myelinated fibers of peripheral nerves show signs of regression in elderly rats, whereas capillaries start to reduce in middle-aged animals. In normal aging of the tibial nerve, capillaries may regress before myelinated fibers.

Mesenchymal stromal cell-derived exosomes ameliorate peripheral neuropathy in a mouse model of diabetes

AIMS/HYPOTHESIS

Diabetic peripheral neuropathy (DPN) is one of the major complications of diabetes, which contributes greatly to morbidity and mortality. There is currently no effective treatment for this disease. Exosomes are cell-derived nanovesicles and play an important role in intercellular communications. The present study investigated whether mesenchymal stromal cell (MSC)-derived exosomes improve neurological outcomes of DPN.

METHODS

Exosomes were isolated from the medium of cultured mouse MSCs by ultracentrifugation. Diabetic mice (BKS.Cg-m+/+Lepr^db/J, db/db) at the age of 20 weeks were used as DPN models. Heterozygous mice (db/m) of the same age were used as the control. MSC-exosomes were administered weekly via the tail vein for 8 weeks. Neurological function was evaluated by testing motor and sensory nerve conduction velocities, and thermal and mechanical sensitivity. Morphometric analysis was performed by myelin sheath staining and immunohistochemistry. Macrophage markers and circulating cytokines were measured by western blot and ELISA. MicroRNA (miRNA) array and bioinformatics analyses were performed to examine the exosomal miRNA profile and miRNA putative target genes involved in DPN.

RESULTS

Treatment of DPN with MSC-exosomes markedly decreased the threshold for thermal and mechanical stimuli and increased nerve conduction velocity in diabetic mice. Histopathological analysis showed that MSC-exosomes markedly augmented the density of FITC-dextran perfused blood vessels and increased the number of intraepidermal nerve fibres (IENFs), myelin thickness and axonal diameters of sciatic nerves. Western blot analysis revealed that MSC-exosome treatment decreased and increased M1 and M2 macrophage phenotype markers, respectively. Moreover, MSC-exosomes substantially suppressed proinflammatory cytokines. Bioinformatics analysis revealed that MSC-exosomes contained abundant miRNAs that target the Toll-like receptor (TLR)4/NF-κB signalling pathway.

CONCLUSIONS/INTERPRETATION

MSC-derived exosomes alleviate neurovascular dysfunction and improve functional recovery in mice with DPN by suppression of proinflammatory genes.

Immunomodulation by Schwann cells in disease

Schwann cells are the principal glial cells of the peripheral nervous system which maintain neuronal homeostasis. Schwann cells support peripheral nerve functions and play a critical role in many pathological processes including injury-induced nerve repair, neurodegenerative diseases, infections, neuropathic pain and cancer. Schwann cells are implicated in a wide range of diseases due, in part, to their ability to interact and modulate immune cells. We discuss the accumulating examples of how Schwann cell regulation of the immune system initiates and facilitates the progression of various diseases. Furthermore, we highlight how Schwann cells may orchestrate an immunosuppressive tumor microenvironment by polarizing and modulating the activity of the dendritic cells.

Hypertension and Diabetes Are Associated With Clinical Characteristics in Patients Undergoing Microvascular Decompression for Hemifacial Spasm

OBJECTIVES

The aim of the present study is to investigate effect of hypertension and diabetes on neuroelectrophysiology, outcomes and complications in patients with hemifacial spasm (HFS) treated by microvascular decompression (MVD).

METHODS

From June 2014 to December 2016, 476 consecutive HFS patients who had undergone MVD were divided into 2 groups according to the presence or absence of comorbidities: diabetic group (n = 26) versus non-diabetic group (n = 450), and hypertensive group (n = 141) versus normotensive group (n = 335). Relevant patient data, including preoperative and postoperative neuroelectrophysiology, operative findings, outcome of MVD and complications, were collected and analyzed retrospectively over the 2-year follow-up period. The impact of hypertension and diabetes on the clinical features of HFS patients was investigated by using logistic regression models.

RESULTS

Brainstem auditory evoked potential (BAEP), postoperative prognosis and abnormal muscle response (AMR) were not different between any of the 2 groups. Preoperative positive AMR occurred more frequently in the nondiabetic group than diabetic group [OR = 0.202, P = 0.004], whereas hypertension was not independently predictive for neuroelectrophysiology in patients with HFS. Adjusted multivariate analysis indicated that hypertension was the only clinical factor associated with MVD-related complications [OR = 0.482, P = 0.007] and hearing impairment [OR = 0.28, P = 0.004] after various potential confounders were taken into account, whereas diabetes was not predictive for postoperative complications.

CONCLUSIONS

Diabetes is associated with low positive rate of preoperative AMR, thus weakening the predictive role of AMR for successful MVD. Hypertension may be an independent risk factor for hearing impairment after MVD.

Adult tissue-derived neural crest-like stem cells: Sources, regulatory networks, and translational potential

Neural crest (NC) cells are a multipotent stem cell population that give rise to a diverse array of cell types in the body, including peripheral neurons, Schwann cells (SC), craniofacial cartilage and bone, smooth muscle cells, and melanocytes. NC formation and differentiation into specific lineages takes place in response to a set of highly regulated signaling and transcriptional events within the neural plate border. Premigratory NC cells initially are contained within the dorsal neural tube from which they subsequently emigrate, migrating to often distant sites in the periphery. Following their migration and differentiation, some NC‐like cells persist in adult tissues in a nascent multipotent state, making them potential candidates for autologous cell therapy. This review discusses the gene regulatory network responsible for NC development and maintenance of multipotency. We summarize the genes and signaling pathways that have been implicated in the differentiation of a postmigratory NC into mature myelinating SC. We elaborate on the signals and transcription factors involved in the acquisition of immature SC fate, axonal sorting of unmyelinated neuronal axons, and finally the path toward mature myelinating SC, which envelope axons within myelin sheaths, facilitating electrical signal propagation. The gene regulatory events guiding development of SC in vivo provides insights into means for differentiating NC‐like cells from adult human tissues into functional SC, which have the potential to provide autologous cell sources for the treatment of demyelinating and neurodegenerative disorders.

VEGF-A and cardiac autonomic function in newly diagnosed type 2 diabetes mellitus: A cross-sectional study at a tertiary care center

INTRODUCTION

Cardiac autonomic neuropathy (CAN) is a key complication of type 2 diabetes mellitus (Ty2DM). Vascular endothelial-derived growth factor (VEGF-A) plays a key role in diabetic macrovascular and microvascular complications. It is shown to be elevated in diabetic neuropathy and has the potential to serve as a biomarker in Ty2DM. We evaluated VEGF-A levels and cardiac autonomic function in newly diagnosed Ty2DM patients.

MATERIALS AND METHODS

Forty-four newly diagnosed patients (with symptoms within 1 year from the date of recruitment) were included in the study. Cardiac autonomic function was assessed using heart rate variability (HRV) and Ewing's battery tests. Ewing's scores were computed and tabulated. VEGF-A levels were estimated using enzyme-linked immunosorbent assay (ELISA).

RESULTS

The patients demonstrated normal responses to the reactivity tests. Ewing's scores were 0 (0-0) and 0 (0-0) for sympathetic and parasympathetic parameters, respectively. The autonomic tone was impaired as assessed by HRV parameters. VEGF-A levels were elevated (308.3 ± 167.2 pg/mL) when compared with the previous literature.

DISCUSSION

Impaired tone with normal reactivity was suggestive of early stage of autonomic neuropathy. Elevated VEGF-A levels may be attributed to a protective action of the factor seen in early stages of neuropathy in Ty2DM. Serial VEGF-A estimation in large cohorts of newly diagnosed diabetics may validate it as a biomarker in CAN seen in Ty2DM.

Melatonin Prevents Oxidative Stress-Induced Mitochondrial Dysfunction and Apoptosis in High Glucose-Treated Schwann Cells via Upregulation of Bcl2, NF-κB, mTOR, Wnt Signalling Pathways

Neuropathy is a complication that affects more than 50% of long-standing diabetic patients. One of the causes of diabetes neuropathy (DN) is the apoptosis of Schwann cells due to prolonged exposure to high glucose and build-up of oxidative stress. Melatonin is a hormone that has a known antioxidant property. In this study, we investigated the protective effect of melatonin on high glucose-induced Schwann cells' apoptosis. Our results revealed that high glucose promoted apoptosis via mitochondrial-related oxidative stress and downregulated Bcl-2 family proteins in Schwann cells. In this signalling pathway, Bcl-2, Bcl-XL and Mcl-1 proteins were down-regulated while p-BAD and Puma proteins were up-regulated by high glucose treatment. Besides, we also proved that high glucose promoted apoptosis in Schwann cells through decreasing the p-NF-κB in the NF-κB signalling pathway. Key regulators of mTOR signalling pathway such as p-mTOR, Rictor and Raptor were also down-regulated after high glucose treatment. Additionally, high glucose treatment also decreased the Wnt signalling pathway downstream proteins (Wnt 5a/b, p-Lrp6 and Axin). Our results showed that melatonin treatment significantly inhibited high glucose-induced ROS generation, restored mitochondrial membrane potential and inhibited high glucose-induced apoptosis in Schwann cells. Furthermore, melatonin reversed the alterations of protein expression caused by high glucose treatment. Our results concluded that melatonin alleviates high glucose-induced apoptosis in Schwann cells through mitigating mitochondrial-related oxidative stress and the alterations of Bcl-2, NF-κB, mTOR and Wnt signalling pathways.

Quantitative evaluation of skin disorders in type 1 diabetic mice by in vivo optical imaging

Diabetes can affect the skin structure as well as the cutaneous vascular permeability. However, effective methods to quantitatively evaluate diabetes-induced skin disorders in vivo are still lacking. Here, we visualized the skin by using in vivo two-photon imaging and quantitatively evaluated the collagen morphology. The results indicated that diabetes could cause a significant reduction in the number of collagen fibers and lead to the disorder of skin collage fibers. And, the classic histological analysis also showed diabetes did lead to the change of skin filamentous structure. Additionally, the Evans Blue dye was used as an indicator to evaluate vascular permeability. We in vivo monitored cutaneous microvascular permeability by combining spectral imaging with the skin optical clearing method. This work is very useful for quantitative evaluation of skin disorders based on in vivo optical imaging, which has a great reference value in the clinical diagnosis.

Peripheral Nerve Regeneration Is Independent From Schwann Cell p75NTR Expression

Schwann cell reprogramming and differentiation are crucial prerequisites for neuronal regeneration and re-myelination to occur following injury to peripheral nerves. The neurotrophin receptor p75NTR has been identified as a positive modulator for Schwann cell myelination during development and implicated in promoting nerve regeneration after injury. However, most studies base this conclusion on results obtained from complete p75NTR knockout mouse models and cannot dissect the specific role of p75NTR expressed by Schwann cells. In this present study, a conditional knockout model selectively deleting p75NTR expression in Schwann cells was generated, where p75NTR expression is replaced with that of an mCherry reporter. Silencing of Schwann cell p75NTR expression was confirmed in the sciatic nerve in vivo and in vitro, without altering axonal expression of p75NTR. No difference in sciatic nerve myelination during development or following sciatic nerve crush injury was observed, as determined by quantification of both myelinated and unmyelinated nerve fiber densities, myelinated axonal diameter and myelin thickness. However, the absence of Schwann cell p75NTR reduced motor nerve conduction velocity after crush injury. Our data indicate that the absence of Schwann cell p75NTR expression in vivo is not critical for axonal regrowth or remyelination following sciatic nerve crush injury, but does play a key role in functional recovery. Overall, this represents the first step in redefining the role of p75NTR in the peripheral nervous system, suggesting that the Schwann cell-axon unit functions as a syncytium, with the previous published involvement of p75NTR in remyelination most likely depending on axonal/neuronal p75NTR and/or mutual glial-axonal interactions.

Omega-3 polyunsaturated fatty acids exert anti-oxidant effects through the nuclear factor (erythroid-derived 2)-related factor 2 pathway in immortalized mouse Schwann cells

AIMS/INTRODUCTION

Recent studies advocate that omega-3 polyunsaturated fatty acids (ω-3 PUFAs) have direct anti-oxidative and anti-inflammatory effects in the vasculature; however, the role of ω-3 PUFAs in Schwann cells remains undetermined.

MATERIALS AND METHODS

Immortalized mouse Schwann (IMS32) cells were incubated with the ω-3 PUFAs docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). The messenger ribonucleic acid levels of several anti-oxidant enzymes (heme oxygenase-1 [Ho-1], nicotinamide adenine dinucleotide [phosphate] H quinone oxidoreductase 1, catalase, superoxide dismutase and glutathione peroxidase) were identified using real-time reverse transcription polymerase chain reaction. Ho-1 and nicotinamide adenine dinucleotide [phosphate] H quinone oxidoreductase 1 protein levels were evaluated using Western blotting. Nuclear factor (erythroid-derived 2)-related factor 2 (Nrf2) of the nuclear fraction was also quantified using western blotting. Catalase activity and glutathione content were determined by colorimetric assay kits. Nrf2 promoter-luciferase activity was evaluated by a dual luciferase assay system.

RESULTS

Treatment with tert-butyl hydroperoxide decreased cell viability dose-dependently. DHA or EPA pretreatment significantly alleviated tert-butyl hydroperoxide-induced cytotoxicity. DHA or EPA increased the messenger ribonucleic acid levels of Ho-1, nicotinamide adenine dinucleotide (phosphate) H quinone oxidoreductase 1 and catalase dose-dependently. Ho-1 protein level, catalase activity, Nrf2 promoter-luciferase activity and intracellular glutathione content were significantly increased by DHA and EPA.

CONCLUSIONS

These findings show that DHA and EPA can induce Ho-1 and catalase through Nrf2, thus protecting Schwann cells against oxidative stress. ω-3 PUFAs appear to exert their neuroprotective effect by increasing defense mechanisms against oxidative stress in diabetic neuropathies.

Maltol, a food flavor enhancer, attenuates diabetic peripheral neuropathy in streptozotocin-induced diabetic rats

SCOPE

Maltol (3-hydroxy-2-methy-4-pyrone), a potent antioxidative agent, typically is used to enhance flavor and preserve food. This study evaluated its effects on preventing diabetic peripheral neuropathy (DPN) in streptozotocin (STZ)-induced diabetic rats and explored its mechanisms.

METHODS AND RESULTS

We intraperitoneally injected Sprague-Dawley (SD) rats with STZ (65 mg kg-1, ip) and treated the rats with different doses of maltol after 4 weeks of injection. During treatment, we evaluated motor nerve conduction velocity (MNCV) and thermal and mechanical hyperalgesia and assayed the oxidative stress, Na+-K+-ATPase activity, and apoptosis. Repeated treatment with maltol for 12 weeks significantly improved thermal and mechanical hyperalgesia, increased the MNCV, elevated the Na+-K+-ATPase activity, and ameliorated oxidative stress and apoptosis in STZ-induced diabetic rats. We coincubated RSC96 cells, a Schwann cell line, with maltol and hydrogen peroxide (H2O2, 0.6 mM). Evidently, maltol increased cell viability and inhibited apoptosis after injury by H2O2.

CONCLUSIONS

Maltol was demonstrated to prevent DPN development and may provide a new alternative for the treatment of DPN.

High glucose promotes hepatic fibrosis via miR‑32/MTA3‑mediated epithelial‑to‑mesenchymal transition

Hepatic fibrosis is characterized by the aberrant production and deposition of extracellular matrix (ECM) proteins. Growing evidence indicates that the epithelial‑mesenchymal transition serves a crucial role in the progression of liver fibrogenesis. Although a subset of microRNAs (miRNAs or miRs) has recently been identified as essential regulators of the EMT gene expression, studies of the EMT in hyperglycemic‑induced liver fibrosis are limited. In the current study, it was observed that high glucose‑treated AML12 cells occurred EMT process, and miR‑32 expression was markedly increased in the liver tissue of streptozotocin‑induced diabetic rats and in high glucose‑treated AML12 cells. Additionally, the contribution of the EMT to liver fibrosis by targeting metastasis‑associated gene 3 (MTA3) under hyperglycemic conditions was suppressed by AMO‑32. The results indicated that miR‑32 and MTA3 may be considered as novel drug targets in the prevention and treatment of liver fibrosis under hyperglycemic conditions. These finding improves the understanding of the progression of liver fibrogenesis.

Emerging Role of Schwann Cells in Neuropathic Pain: Receptors, Glial Mediators and Myelination

Neuropathic pain caused by nerve injury or disease remains a major challenge for modern medicine worldwide. Most of the pathogenic mechanisms underlying neuropathic pain are centered on neuronal mechanisms. Accumulating evidence suggests that non-neuronal cells, especially glial cells, also play active roles in the initiation and resolution of pain. The preponderance of evidence has implicated central nervous system (CNS) glial cells, i.e., microglia and astrocytes, in the control of pain. The role of Schwann cells in neuropathic pain remains poorly understood. Schwann cells, which detect nerve injury and provide the first response, play a critical role in the development and maintenance of neuropathic pain. The cells respond to nerve injury by changing their phenotype, proliferating and interacting with nociceptive neurons by releasing glial mediators (growth factors, cytokines, chemokines, and biologically active small molecules). In addition, receptors expressed in active Schwann cells have the potential to regulate different pain conditions. In this review article, we will provide and discuss emerging evidence by integrating recent advances related to Schwann cells and neuropathic pain.

Progressive Increase of Inflammatory CXCR4 and TNF-Alpha in the Dorsal Root Ganglia and Spinal Cord Maintains Peripheral and Central Sensitization to Diabetic Neuropathic Pain in Rats

Diabetic neuropathic pain (DNP) is a common and serious complication of diabetic patients. The pathogenesis of DNP is largely unclear. The proinflammation proteins, CXCR4, and TNF-α play critical roles in the development of pain, while their relative roles in the development of DNP and especially its progression is unknown. We proposed that establishment of diabetic pain models in rodents and evaluating the stability of behavioral tests are necessary approaches to better understand the mechanism of DNP. In this study, Von Frey and Hargreaves Apparatus was used to analyze the behavioral changes of mechanical allodynia and heat hyperalgesia in streptozotocin-induced diabetic rats at different phases of diabetes. Moreover, CXCR4 and TNF-α of spinal cord dorsal and dorsal root ganglia (DRG) were detected by western blotting and immunostaining over time. The values of paw withdrawal threshold (PWT) and paw withdrawal latencies (PWL) were reduced as early as 1 week in diabetic rats and persistently maintained at lower levels during the progression of diabetes as compared to control rats that were concomitant with significant increases of both CXCR4 and TNF-α protein expressions in the DRG at 2 weeks and 5 weeks (the end of the experiments) of diabetes. By contrast, CXCR4 and TNF-α in the spinal cord dorsal horn did not significantly increase at 2 weeks of diabetes while both were significantly upregulated at 5 weeks of diabetes. The results indicate that central sensitization of spinal cord dorsal may result from persistent peripheral sensitization and suggest a potential reference for further treatment of DNP.

Inflammatory Macrophages in the Sciatic Nerves Facilitate Neuropathic Pain Associated with Type 2 Diabetes Mellitus

Despite the requirement for effective medication against neuropathic pain associated with type 2 diabetes mellitus (T2DM), mechanism-based pharmacotherapy has yet to be established. Given that long-lasting neuroinflammation, driven by inflammatory macrophages in the peripheral nerves, plays a pivotal role in intractable pain, it is important to determine whether inflammatory macrophages contribute to neuropathic pain associated with T2DM. To generate an experimental model of T2DM, C57BL/6J mice were fed a high-fat diet (HFD) ad libitum. Compared with control diet feeding, obesity and hyperglycemia were observed after HFD feeding, and the mechanical pain threshold evaluated using the von Frey test was found to be decreased, indicating the development of mechanical allodynia. The expression of mRNA markers for macrophages, inflammatory cytokines, and chemokines were significantly upregulated in the sciatic nerve (SCN) after HFD feeding. Perineural administration of saporin-conjugated anti-Mac1 antibody (Mac1-Sap) improved HFD-induced mechanical allodynia. Moreover, treatment of Mac1-Sap decreased the accumulation of F4/80+ macrophages and the upregulation of inflammatory mediators in the SCN after HFD feeding. Inoculation of lipopolysaccharide-activated peritoneal macrophages in tissue surrounding the SCN elicited mechanical allodynia. Furthermore, pharmacological inhibition of inflammatory macrophages by either perineural or systemic administration of TC-2559 [4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate], a α4β2 nicotinic acetylcholine receptor-selective agonist, relieved HFD-induced mechanical allodynia. Taken together, inflammatory macrophages that accumulate in the SCN mediate the pathophysiology of neuropathic pain associated with T2DM. Inhibitory agents for macrophage-driven neuroinflammation could be potential candidates for novel pharmacotherapy against intractable neuropathic pain.

Leptin receptor deficiency induces early, transient and hyperglycaemia-independent blood-brain barrier dysfunction

Diabetes mellitus (DM) significantly increases susceptibility to central nervous system (CNS) pathologies, including stroke, vascular dementia, cognitive deficits and Alzheimer’s disease. Previous studies (mostly using the streptozotocin model) suggested that blood-brain barrier (BBB) disruption is involved in these conditions. Here, we examined the integrity of brain capillaries and BBB permeability in Lepr^db/db obesity-related diabetic mice. Surprisingly, significant BBB leakage was observed only in young mice at the pre-hyperglycemic stage. Thorough examination of barrier permeability at later diabetic stages showed no evidence for significant BBB leakage during the hyperglycemic state. Electron microscopy imaging of mice with short-term hyperglycaemia supported normal BBB permeability but indicated other stress-related changes in capillary ultrastructure, such as mitochondrial degeneration. Based on our study with this mouse genetic model of obesity-related DM, we suggest that previously reported hyperglycaemia-induced BBB leakage is most likely not the underlying mechanism of DM-related CNS pathologies. Finally we propose that BBB hyper-permeability might be an early and transient phenomenon while stress-related endothelial pathologies do correlate with a short-term diabetic state.

Nanoparticle-microRNA-146a-5p polyplexes ameliorate diabetic peripheral neuropathy by modulating inflammation and apoptosis

Nontoxic and nonimmunogenic nanoparticles play an increasingly important role in the application of pharmaceutical nanocarriers. The pathogenesis of diabetic peripheral neuropathy (DPN) has been extensively studied. However, the role of microRNAs in DPN remains to be clarified. We verified in vitro that miR-146a-5p mimics inhibited the expression of proinflammatory cytokines and apoptosis. Then, we explored the protective effect of nanoparticle-miRNA-146a-5p polyplexes (nano-miR-146a-5p) on DPN rats. We demonstrated that nano-miR-146a-5p improved nerve conduction velocity and alleviated the morphological damage and demyelination of the sciatic nerve of DPN rats. The expression of the inflammatory cytokines, caspase-3, and cleaved caspase-3 in the sciatic nerve was inhibited by nano-miR-146a-5p. Additionally, nano-miR-146a-5p increased the expression of myelin basic protein. These results all indicated that nano-miR-146a-5p had a protective effect on peripheral nerves in the DPN rat model, which may occur through the regulation of the inflammatory response and apoptosis.

Emerging Biomarkers, Tools, and Treatments for Diabetic Polyneuropathy

Diabetic neuropathy, with its major clinical sequels, notably neuropathic pain, foot ulcers, and autonomic dysfunction, is associated with substantial morbidity, increased risk of mortality, and reduced quality of life. Despite its major clinical impact, diabetic neuropathy remains underdiagnosed and undertreated. Moreover, the evidence supporting a benefit for causal treatment is weak at least in patients with type 2 diabetes, and current pharmacotherapy is largely limited to symptomatic treatment options. Thus, a better understanding of the underlying pathophysiology is mandatory for translation into new diagnostic and treatment approaches. Improved knowledge about pathogenic pathways implicated in the development of diabetic neuropathy could lead to novel diagnostic techniques that have the potential of improving the early detection of neuropathy in diabetes and prediabetes to eventually embark on new treatment strategies. In this review, we first provide an overview on the current clinical aspects and illustrate the pathogenetic concepts of (pre)diabetic neuropathy. We then describe the biomarkers emerging from these concepts and novel diagnostic tools and appraise their utility in the early detection and prediction of predominantly distal sensorimotor polyneuropathy. Finally, we discuss the evidence for and limitations of the current and novel therapy options with particular emphasis on lifestyle modification and pathogenesis-derived treatment approaches. Altogether, recent years have brought forth a multitude of emerging biomarkers reflecting different pathogenic pathways such as oxidative stress and inflammation and diagnostic tools for an early detection and prediction of (pre)diabetic neuropathy. Ultimately, these insights should culminate in improving our therapeutic armamentarium against this common and debilitating or even life-threatening condition.

Preventive action of benztropine on platinum-induced peripheral neuropathies and tumor growth

The endogenous cholinergic system plays a key role in neuronal cells, by suppressing neurite outgrowth and myelination and, in some cancer cells, favoring tumor growth. Platinum compounds are widely used as part of first line conventional cancer chemotherapy; their efficacy is however limited by peripheral neuropathy as a major side-effect. In a multiple sclerosis mouse model, benztropine, that also acts as an anti-histamine and a dopamine re-uptake inhibitor, induced the differentiation of oligodendrocytes through M1 and M3 muscarinic receptors and enhanced re-myelination. We have evaluated whether benztropine can increase anti-tumoral efficacy of oxaliplatin, while preventing its neurotoxicity.We showed that benztropine improves acute and chronic clinical symptoms of oxaliplatin-induced peripheral neuropathies in mice. Sensory alterations detected by electrophysiology in oxaliplatin-treated mice were consistent with a decreased nerve conduction velocity and membrane hyperexcitability due to alterations in the density and/or functioning of both sodium and potassium channels, confirmed by action potential analysis from ex-vivo cultures of mouse dorsal root ganglion sensory neurons using whole-cell patch-clamp. These alterations were all prevented by benztropine. In oxaliplatin-treated mice, MBP expression, confocal and electronic microscopy of the sciatic nerves revealed a demyelination and confirmed the alteration of the myelinated axons morphology when compared to animals injected with oxaliplatin plus benztropine. Benztropine also prevented the decrease in neuronal density in the paws of mice injected with oxaliplatin. The neuroprotection conferred by benztropine against chemotherapeutic drugs was associated with a lower expression of inflammatory cytokines and extended to diabetic-induced peripheral neuropathy in mice.Mice receiving benztropine alone presented a lower tumor growth when compared to untreated animals and synergized the anti-tumoral effect of oxaliplatin, a phenomenon explained at least in part by benztropine-induced ROS imbalance in tumor cells.This report shows that blocking muscarinic receptors with benztropine prevents peripheral neuropathies and increases the therapeutic index of oxaliplatin. These results can be rapidly transposable to patients as benztropine is currently indicated in Parkinson's disease in the United States.

Functional and Structural Changes of the Blood-Nerve-Barrier in Diabetic Neuropathy

The incidence of diabetes mellitus is approaching global epidemic proportions and should be considered a major health-care problem of modern societies in the twenty-first century. Diabetic neuropathy is a common chronic complication of diabetes and, although an adequate glycemic control can reduce the frequency of diabetic neuropathy in type 1 diabetes, the majority of type 2 diabetic patients will develop this complication. The underlying cellular and molecular mechanisms are still poorly understood, preventing the development of effective treatment strategies. However, accumulating evidence suggests that breakdown of the blood-nerve barrier (BNB) plays a pivotal pathophysiological role in diabetic neuropathy. In the present review, we highlight the structural and functional significance of the BNB in health and disease, focusing on the pathological molecular events leading to BNB dysfunction in diabetic neuropathy. In addition, we discuss potential molecular targets involved in BNB homeostasis that may pave the way toward novel therapeutic strategies for treating diabetic neuropathy.

The LKB1-AMPK and mTORC1 Metabolic Signaling Networks in Schwann Cells Control Axon Integrity and Myelination: Assembling and upholding nerves by metabolic signaling in Schwann cells

The Liver kinase B1 with its downstream target AMP activated protein kinase (LKB1-AMPK), and the key nutrient sensor mammalian target of rapamycin complex 1 (mTORC1) form two signaling systems that coordinate metabolic and cellular activity with changes in the environment in order to preserve homeostasis. For example, nutritional fluctuations rapidly feed back on these signaling systems and thereby affect cell-specific functions. Recent studies have started to reveal important roles of these strategic metabolic regulators in Schwann cells for the trophic support and myelination of axons. Because aberrant intermediate metabolism along with mitochondrial dysfunction in Schwann cells is mechanistically linked to nerve abnormalities found in acquired and inherited peripheral neuropathies, manipulation of the LKB1-AMPK, and mTORC1 signaling hubs may be a worthwhile therapeutic target to mitigate nerve damage in disease. Here, recent advances in our understanding of LKB1-AMPK and mTORC1 functions in Schwann cells are covered, and future research areas for this key metabolic signaling network are proposed.

Conserved Transcriptional Signatures in Human and Murine Diabetic Peripheral Neuropathy

Diabetic peripheral neuropathy (DPN) is one of the most common complications of diabetes. In this study, we employed a systems biology approach to identify DPN-related transcriptional pathways conserved across human and various murine models. Eight microarray datasets on peripheral nerve samples from murine models of type 1 (streptozotocin-treated) and type 2 (db/db and ob/ob) diabetes of various ages and human subjects with non-progressive and progressive DPN were collected. Differentially expressed genes (DEGs) were identified between non-diabetic and diabetic samples in murine models, and non-progressive and progressive human samples using a unified analysis pipeline. A transcriptional network for each DEG set was constructed based on literature-derived gene-gene interaction information. Seven pairwise human-vs-murine comparisons using a network-comparison program resulted in shared sub-networks including 46 to 396 genes, which were further merged into a single network of 688 genes. Pathway and centrality analyses revealed highly connected genes and pathways including LXR/RXR activation, adipogenesis, glucocorticoid receptor signalling, and multiple cytokine and chemokine pathways. Our systems biology approach identified highly conserved pathways across human and murine models that are likely to play a role in DPN pathogenesis and provide new possible mechanism-based targets for DPN therapy.

Recurrent short-term hypoglycemia and hyperglycemia induce apoptosis and oxidative stress via the ER stress response in immortalized adult mouse Schwann (IMS32) cells

Hypoglycemia and fluctuating high or low glucose conditions are under-appreciated sources of oxidative stress contributing to diabetic neuropathy. We investigated the effects of recurrent short-term hypoglycemia and hyperglycemia, on apoptosis and oxidative stress in Schwann cells. Immortalized adult mouse Schwann (IMS32) cells were exposed to five different glucose treatments over 3 days: 1) normal glucose (NG), 2) constant low glucose (LG), 3) constant high glucose (HG), 4) intermittent low glucose (ILG; 1 h three times per day), 5) intermittent high glucose (IHG; 1 h three times per day). Cell viability was decreased by all treatment variants, in comparison to NG. Thiobarbituric acid reactive substance (TBARS) levels were increased by HG, LG, IHG, and ILG. High glucose (HG and IHG) and low glucose (LG and ILG) increased the expression of cleaved caspase-3 and reduced that of Bcl-2. In addition, endoplasmic reticulum (ER) stress-responsive transcription factor C/EBP homologous protein (CHOP) expression was increased under low and high glucose conditions. Cell death and oxidative stress induced by HG, LG, IHG, and ILG were significantly reduced by 4-phenyl butyric acid (4-PBA), an ER stress inhibitor. These findings indicate that recurrent short-term hypoglycemia and hyperglycemia induce apoptosis and oxidative stress via the ER stress response in Schwann cells.

Angiopoietin-1/Tie2 signaling pathway contributes to the therapeutic effect of thymosin β4 on diabetic peripheral neuropathy

Angiopoietin-1 (Ang1) and its receptor Tie2 regulate vascular function. Our previous study demonstrated that thymosin beta 4 (Tβ4) ameliorates neurological function of diabetic peripheral neuropathy. Mechanisms underlying the therapeutic effect of Tβ4 on diabetic peripheral neuropathy have not been fully investigated. The present in vivo study investigated whether the Ang1/Tie2 signaling pathway is involved in Tβ4-improved neurovascular remodeling in diabetic peripheral neuropathy. Diabetic BKS. Cg-m+/+Lepr^db/J (db/db) mice at age 20 weeks were treated with Tβ4 and neutralizing antibody against mouse Tie2 for 4 consecutive weeks. Neurological functional and neurovascular remodeling were measured. Administration of the neutralizing antibody against Tie2 attenuated the therapeutic effect of Tβ4 on improved diabetic peripheral neuropathy as measured by motor and sensory nerve conduction velocity and thermal hypoesthesia compared to diabetic db/db mice treated with Tβ4 only. Histopathological analysis revealed that the neutralizing antibody against Tie2 abolished Tβ4-increased microvascular density in sciatic nerve and intraepidermal nerve fiber density, which were associated with suppression of Tβ4-upregulated occludin expression and Tβ4-reduced protein levels of nuclear factor-κB (NF-κB) and vascular cell adhesion molecule-1 (VCAM1). Our data provide in vivo evidence that the Ang1/Tie2 pathway contributes to the therapeutic effect of Tβ4 on diabetic peripheral neuropathy.

A new look at painful diabetic neuropathy

The prevalence of diabetes mellitus and its chronic complications continue to increase alarmingly. Consequently, the massive expenditure on diabetic distal symmetrical polyneuropathy (DSPN) and its sequelae, will also likely rise. Up to 50% of patients with diabetes develop DSPN, and about 20% develop neuropathic pain (painful-DSPN). Painful-DSPN can cast a huge burden on sufferers' lives with increased rates of unemployment, mental health disorders and physical co-morbidities. Unfortunately, due to limited understanding of the mechanisms leading to painful-DSPN, current treatments remain inadequate. Recent studies examining the pathophysiology of painful-DSPN have identified maladaptive alterations at the level of both the peripheral and central nervous systems. Additionally, genetic studies have suggested that patients with variants of voltage gated sodium channels may be more at risk of developing neuropathic pain in the presence of a disease trigger such as diabetes. We review the recent advances in genetics, skin biopsy immunohistochemistry and neuro-imaging, which have the potential to further our understanding of the condition, and identify targets for new mechanism based therapies.

Vascular and Neural Complications in Type 2 Diabetic Rats: Improvement by Sacubitril/Valsartan Greater Than Valsartan Alone

Previously, we had shown that a vasopeptidase inhibitor drug containing ACE and neprilysin inhibitors was an effective treatment for diabetic vascular and neural complications. However, side effects prevented further development. This led to the development of sacubitril/valsartan, a drug containing angiotensin II receptor blocker and neprilysin inhibitor that we hypothesized would be an effective treatment for diabetic peripheral neuropathy. Using early and late intervention protocols (4 and 12 weeks posthyperglycemia, respectively), type 2 diabetic rats were treated with valsartan or sacubitril/valsartan for 12 weeks followed by an extensive evaluation of vascular and neural end points. The results demonstrated efficacy of sacubitril/valsartan in improving vascular and neural function was superior to valsartan alone. In the early intervention protocol, sacubitril/valsartan treatment was found to slow progression of these deficits and, with late intervention treatment, was found to stimulate restoration of vascular reactivity, motor and sensory nerve conduction velocities, and sensitivity/regeneration of sensory nerves of the skin and cornea in a rat model of type 2 diabetes. These preclinical studies suggest that sacubitril/valsartan may be an effective treatment for diabetic peripheral neuropathy, but additional studies will be needed to investigate these effects further.

Visualization of skin microvascular dysfunction of type 1 diabetic mice using in vivo skin optical clearing method

To realize visualization of the skin microvascular dysfunction of type 1 diabetic mice, we combined laser speckle contrast imaging and hyperspectral imaging to simultaneously monitor the noradrenaline (NE)-induced responses of vascular blood flow and blood oxygen with the development of diabetes through optical clearing skin window. The main results showed that venous and arterious blood flow decreased without recovery after injection of NE; furthermore, the decrease of arterious blood oxygen induced by NE greatly weakened, especially for 2- and 4-week diabetic mice. This change in vasoconstricting effect of NE was related to the expression of α1-adrenergic receptor. This study demonstrated that skin microvascular function was a potential research biomarker for early warning in the occurrence and development of diabetes. The in vivo skin optical clearing method provides a feasible solution to realize visualization of cutaneous microvessels for monitoring microvascular reactivity under pathological conditions. In addition, visual monitoring of skin microvascular function response has guiding significance for early diagnosis of diabetes and clinical research.

Visualization of skin microvascular dysfunction of type 1 diabetic mice using in vivo skin optical clearing method

To realize visualization of the skin microvascular dysfunction of type 1 diabetic mice, we combined laser speckle contrast imaging and hyperspectral imaging to simultaneously monitor the noradrenaline (NE)-induced responses of vascular blood flow and blood oxygen with the development of diabetes through optical clearing skin window. The main results showed that venous and arterious blood flow decreased without recovery after injection of NE; furthermore, the decrease of arterious blood oxygen induced by NE greatly weakened, especially for 2- and 4-week diabetic mice. This change in vasoconstricting effect of NE was related to the expression of α1-adrenergic receptor. This study demonstrated that skin microvascular function was a potential research biomarker for early warning in the occurrence and development of diabetes. The in vivo skin optical clearing method provides a feasible solution to realize visualization of cutaneous microvessels for monitoring microvascular reactivity under pathological conditions. In addition, visual monitoring of skin microvascular function response has guiding significance for early diagnosis of diabetes and clinical research.

Evaluation of combined radiofrequency and chemical blockade of multi-segmental lumbar sympathetic ganglia in painful diabetic peripheral neuropathy

Background

Painful diabetic peripheral neuropathy (PDPN) is one of the most common complications of diabetes. PDPN seriously affects the quality of life and is difficult to treat; therefore, there is an urgent need for new cost-effective treatment methods for PDPN.

Objective

To investigate the efficacy and safety of radiofrequency thermocoagulation (RF) combined with anhydrous ethanol (AE) chemical blockade of lumbar sympathetic ganglia (LSG) in patients with PDPN using computed tomography (CT).

Study design

Retrospective comparative study.

Setting

Shengjing Hospital of China Medical University.

Methods

Ninety patients diagnosed with PDPN were enrolled in this study. The patients were randomly divided into AE group (A, n=30), RF group (B, n=30), and RF+ AE group (C, n=30). The follow-up included preoperative basic conditions, visual analog scale (VAS), the total remission rate (TRR), skin temperature (ST) and the improvement of numbness and hyperalgesia in the lower extremities, complications, and degree of satisfaction (DOS) before and after surgery.

Results

Postoperative VASs were significantly decreased compared to preoperative VASs in all groups (P<0.05). The VAS in group A began to increase 3 months (3M) after surgery; VAS scores at 3M, 6 months (6M) and 1 year (1Y) were significantly different compared to group B and C (P<0.05); VAS in group B began to increase after 6M; VAS scores at 6M and 1Y were significantly different compared to group C (P<0.05); Moreover, group C maintained relatively long duration of pain relief. TRR in group A, group B and group C at 1Y after operation was 66.7%, 73.3% and 93.3%, respectively; TRR in group C was statistically different compared to groups A and B (P<0.05). Higher ST in the lower extremities was observed after surgery in all groups compared to peroration (P<0.05); nonetheless, the difference was not statistically significant. The numbness and hyperalgesia improved in all three groups after surgery compared to preoperational time, the numbness in group C was significantly higher compared to groups A and B. In addition, no severe complications were observed. At 6M and 1Y after surgery, the degree of satisfaction in patients from group C was significantly higher compared to groups A and B.

Conclusion

Radiofrequency thermocoagulation combined with AE chemical blockade of the LSG was safe and effective. Nevertheless, the details underlying analgesic mechanisms still need to be investigated.

Is autophagy associated with diabetes mellitus and its complications? A review

Diabetes mellitus (DM) is an endocrine disorder. In coming decades it will be one of the leading causes of death globally. The key factors in the pathogenesis of diabetes are cellular injuries and disorders of energy metabolism leading to severe diabetic complications. Recent studies have confirmed that autophagy plays a pivotal role in diabetes and its complications. It has been observed that autophagy regulates the normal function of pancreatic β cells and insulin-target tissues, such as skeletal muscle, liver, and adipose tissue. This review will summarize the regulation of autophagy in diabetes and its complications, and explore how this process would emerge as a potential therapeutic target for diabetes treatment.

Glia-neuron energy metabolism in health and diseases: New insights into the role of nervous system metabolic transporters

The brain is, by weight, only 2% the volume of the body and yet it consumes about 20% of the total glucose, suggesting that the energy requirements of the brain are high and that glucose is the primary energy source for the nervous system. Due to this dependence on glucose, brain physiology critically depends on the tight regulation of glucose transport and its metabolism. Glucose transporters ensure efficient glucose uptake by neural cells and contribute to the physiology and pathology of the nervous system. Despite this, a growing body of evidence demonstrates that for the maintenance of several neuronal functions, lactate, rather than glucose, is the preferred energy metabolite in the nervous system. Monocarboxylate transporters play a crucial role in providing metabolic support to axons by functioning as the principal transporters for lactate in the nervous system. Monocarboxylate transporters are also critical for axonal myelination and regeneration. Most importantly, recent studies have demonstrated the central role of glial cells in brain energy metabolism. A close and regulated metabolic conversation between neurons and both astrocytes and oligodendroglia in the central nervous system, or Schwann cells in the peripheral nervous system, has recently been shown to be an important determinant of the metabolism and function of the nervous system. This article reviews the current understanding of the long existing controversies regarding energy substrate and utilization in the nervous system and discusses the role of metabolic transporters in health and diseases of the nervous system.

Insulin-induced upregulation of lipoprotein lipase in Schwann cells during diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the major complications associated with diabetes. It is characterized by the degeneration of the myelin sheath around axons, referred to as demyelination. Such demyelinations are often caused by reduced lipid component of the myelin sheath. Since, lipoprotein lipase (LPL) provides the lipid for myelin sheath by hydrolysing the triglyceride rich lipoproteins, and also helps in the uptake of lipids by the Schwann cells (SCs) for its utilization, LPL is considered as the important factor in the regeneration of myelin sheath during diabetic neuropathy. Earlier reports from our laboratory have provided the insights of insulin and its receptor in SCs during diabetic neuropathy. In order to evaluate the long term effect of insulin on lipid metabolism during diabetic neuropathy, in this study, we analyzed the expression of LPL in SCs under normal, high glucose and insulin treated conditions. A decrease in the expression of LPL was observed in SCs of high glucose condition and it was reversed upon insulin treatment. Histochemical observations of sciatic nerve of insulin treated neuropathy subjects showed the improved nerve morphology, signifying the importance of insulin in restoring the pathophysiology of diabetic neuropathy.

Peripheral Glial Cells in the Development of Diabetic Neuropathy

The global prevalence of diabetes is rapidly increasing, affecting more than half a billion individuals within the next few years. As diabetes negatively affects several physiological systems, this dramatic increase represents not only impaired quality of life on the individual level but also a huge socioeconomic challenge. One of the physiological consequences affecting up to half of diabetic patients is the progressive deterioration of the peripheral nervous system, resulting in spontaneous pain and eventually loss of sensory function, motor weakness, and organ dysfunctions. Despite intense research on the consequences of hyperglycemia on nerve functions, the biological mechanisms underlying diabetic neuropathy are still largely unknown, and treatment options lacking. Research has mainly focused directly on the neuronal component, presumably from the perspective that this is the functional signal-transmitting unit of the nerve. However, it is noteworthy that each single peripheral sensory neuron is intimately associated with numerous glial cells; the neuronal soma is completely enclosed by satellite glial cells and the length of the longest axons covered by at least 1,000 Schwann cells. The glial cells are vital for the neuron, but very little is still known about these cells in general and especially how they respond to diabetes in terms of altered neuronal support. We will discuss current knowledge of peripheral glial cells and argue that increased research in these cells is imperative for a better understanding of the mechanisms underlying diabetic neuropathy.

IRE1α siRNA relieves endoplasmic reticulum stress-induced apoptosis and alleviates diabetic peripheral neuropathy in vivo and in vitro

Diabetic peripheral neuropathy (DPN) is mainly characterized by demyelination resulted from the apoptosis of the Schwann cell (SCs). Although the exact mechanisms underlying DPN remain unclear, endoplasmic reticulum (ER) stress is strongly implicated in the apoptosis. Under ER stress, activated inositol-requiring kinase 1α (IRE1α) unregulated CHOP, phosphorylated JNK and Caspase-12 to aggravate apoptosis-mediated damage of DPN. Therefore, we tested the hypothesis that inhibition of IRE1α could reduce the ER stress-related apoptosis to relieve DPN. Here, we show that IRE1α siRNA improved the neurological morphology and function of DPN rats and rescued ER stress-related apoptosis in the sciatic nerve. Additionally, RSC96 cells transfected with IRE1α siRNA were used as in vitro model of DPN. It was found that IRE1α siRNA also decreased high glucose-induced apoptosis and inhibited ER stress-related apoptosis in the cells. Altogether, our results suggest that IRE1α should be considered a potential therapeutic agent for DPN.

Tang-Luo-Ning, a Traditional Chinese Medicine, Inhibits Endoplasmic Reticulum Stress-Induced Apoptosis of Schwann Cells under High Glucose Environment

Tang-Luo-Ning (TLN) has a definite effect in the clinical treatment of diabetic peripheral neuropathy (DPN). Schwann cells (SCs) apoptosis induced by endoplasmic reticulum stress (ER stress) is one of the main pathogeneses of DPN. This study investigates whether TLN can inhibit SCs apoptosis by inhibiting ER stress-induced apoptosis. Our previous researches have demonstrated that TLN could increase the expression of ER stress marker protein GRP78 and inhibited the expression of apoptosis marker protein CHOP in ER stress. In this study, the results showed that TLN attenuated apoptosis by decreasing Ca²⁺ level in SCs and maintaining ER morphology. TLN could decrease downstream proteins of CHOP including GADD34 and Ero1α, while it increased P-eIF2α and decreased the upstream proteins of CHOP including P-IRE1α/IRE1α and XBP-1, thereby reducing ER stress-induced apoptosis.

Longitudinal study of neuropathy, microangiopathy, and autophagy in sural nerve: Implications for diabetic neuropathy

OBJECTIVES

The progression and pathophysiology of neuropathy in impaired glucose tolerance (IGT) and type 2 diabetes (T2DM) is poorly understood, especially in relation to autophagy. This study was designed to assess whether the presence of autophagy-related structures was associated with sural nerve fiber pathology, and to investigate if endoneurial capillary pathology could predict the development of T2DM and neuropathy.

PATIENTS AND METHODS

Sural nerve physiology and ultrastructural morphology were studied at baseline and 11 years later in subjects with normal glucose tolerance (NGT), IGT, and T2DM.

RESULTS

Subjects with T2DM had significantly lower sural nerve amplitude compared to subjects with NGT and IGT at baseline. Myelinated and unmyelinated fiber, endoneurial capillary morphology, and the presence and distribution of autophagy structures were comparable between groups at baseline, except for a smaller myelinated axon diameter in subjects with T2DM and IGT compared to NGT. The baseline values of the subjects with NGT and IGT who converted to T2DM 11 years later demonstrated healthy smaller endoneurial capillary and higher g-ratio versus subjects who remained NGT. At follow-up, T2DM showed a reduction in nerve conduction, amplitude, myelinated fiber density, unmyelinated axon diameter, and autophagy structures in myelinated axons. Endothelial cell area and total diffusion barrier was increased versus baseline.

CONCLUSIONS

We conclude that small healthy endoneurial capillary may presage the development of T2DM and neuropathy. Autophagy occurs in human sural nerves and can be affected by T2DM. Further studies are warranted to understand the role of autophagy in diabetic neuropathy.