Pathology and pathogenetic mechanisms of diabetic neuropathy

Oxidative Stress and Mitochondrial Dysfunction Associated with Peripheral Neuropathy in Type 1 Diabetes

Significance: This review highlights the many intracellular processes generating reactive oxygen species (ROS) in the peripheral nervous system in the context of type 1 diabetes. The major sources of superoxide and hydrogen peroxide (H₂O₂) are described, and scavenging systems are explained. Important roles of ROS in regulating normal redox signaling and in a disease setting, such as diabetes, contributing to oxidative stress and cellular damage are outlined. The primary focus is the role of hyperglycemia in driving elevated ROS production and oxidative stress contributing to neurodegeneration in diabetic neuropathy (within the dorsal root ganglia [DRG] and peripheral nerve). Recent Advances: Contributors to ROS production under high intracellular glucose concentration such as mitochondria and the polyol pathway are discussed. The primarily damaging impact of ROS on multiple pathways including mitochondrial function, endoplasmic reticulum (ER) stress, autophagy, and epigenetic signaling is covered. Critical Issues: There is a strong focus on mechanisms of diabetes-induced mitochondrial dysfunction and how this may drive ROS production (in particular superoxide). The mitochondrial sites of superoxide/H₂O₂ production via mitochondrial metabolism and aerobic respiration are reviewed. Future Directions: Areas for future development are highlighted, including the need to clarify diabetes-induced changes in autophagy and ER function in neurons and Schwann cells. In addition, more clarity is needed regarding the sources of ROS production at mitochondrial sites under high glucose concentration (and lack of insulin signaling). New areas of study should be introduced to investigate the role of ROS, nuclear lamina function, and epigenetic signaling under diabetic conditions in peripheral nerve.

Epigenetic basis of diabetic vasculopathy

Type 2 diabetes mellitus (T2DM) causes peripheral vascular disease because of which several blood-borne factors, including vital nutrients fail to reach the affected tissue. Tissue epigenome is sensitive to chronic hyperglycemia and is known to cause pathogenesis of micro- and macrovascular complications. These vascular complications of T2DM may perpetuate the onset of organ dysfunction. The burden of diabetes is primarily because of a wide range of complications of which nonhealing diabetic ulcers represent a major component. Thus, it is imperative that current research help recognize more effective methods for the diagnosis and management of early vascular injuries. This review addresses the significance of epigenetic processes such as DNA methylation and histone modifications in the evolution of macrovascular and microvascular complications of T2DM.

Inflammatory biomarkers as a part of diagnosis in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN), a chronic neurological complication of diabetes mellitus, remains scantily addressed area of research. Many lacunae in the temporal sequence between cause and effect of DPN still remain unfilled and therefore treatment of DPN remains unsatisfactory. This is largely due to the conventional glucocentric focus to resolve the problem. This focus over hyperglycemia should be shifted to consider, chronic low grade inflammation as the major determinant in DPN. Rapidly emerging evidences from recent studies suggest that chronic low grade inflammation leads to the activation of innate immune system response, loss of insulin signaling and insulin resistance, endoplasmic reticulum stress, mitochondrial stress, leading to production of kinases like protein kinase C, mitogen activated protein kinase and jun-N-terminal kinase, pro-inflammatory cytokines and inter leukins-1b, 2, 6 and 8, tumour necrosis factor-alpha and other chemokines, leading to DPN. These biomarkers can be early predictors of DPN and therefore should be the focus of work testing their clinical utility to identify high-risk individuals as well as perhaps to target interventions. In this paper, we would like to review all the aspects of DPN, laying greater emphasis on inflammatory biomarkers as a tool for early diagnosis of DPN and the possible research approaches to address it satisfactorily.

Aldose Reductase and the Polyol Pathway in Schwann Cells: Old and New Problems

Aldose reductase (AR) is a member of the reduced nicotinamide adenosine dinucleotide phosphate (NADPH)-dependent aldo-keto reductase superfamily. It is also the rate-limiting enzyme of the polyol pathway, catalyzing the conversion of glucose to sorbitol, which is subsequently converted to fructose by sorbitol dehydrogenase. AR is highly expressed by Schwann cells in the peripheral nervous system (PNS). The excess glucose flux through AR of the polyol pathway under hyperglycemic conditions has been suggested to play a critical role in the development and progression of diabetic peripheral neuropathy (DPN). Despite the intensive basic and clinical studies over the past four decades, the significance of AR over-activation as the pathogenic mechanism of DPN remains to be elucidated. Moreover, the expected efficacy of some AR inhibitors in patients with DPN has been unsatisfactory, which prompted us to further investigate and review the understanding of the physiological and pathological roles of AR in the PNS. Particularly, the investigation of AR and the polyol pathway using immortalized Schwann cells established from normal and AR-deficient mice could shed light on the causal relationship between the metabolic abnormalities of Schwann cells and discordance of axon-Schwann cell interplay in DPN, and led to the development of better therapeutic strategies against DPN.

Exaggerated cardiovascular responses to muscle contraction and tendon stretch in UCD type-2 diabetes mellitus rats

Patients with type-2 diabetes mellitus (T2DM) have exaggerated sympathetic activity and blood pressure responses to exercise. However, the underlying mechanisms for these responses, as well as how these responses change throughout disease progression, are not completely understood. For this study, we examined the effect of the progression of T2DM on the exercise pressor reflex, a critical neurocardiovascular mechanism that functions to increase sympathetic activity and blood pressure during exercise. We also aimed to examine the effect of T2DM on reflexive cardiovascular responses to static contraction, as well as those responses to tendon stretch when an exaggerated exercise pressor reflex was present. We evoked the exercise pressor reflex and mechanoreflex by statically contracting the hindlimb muscles and stretching the Achilles tendon, respectively, for 30 s. We then compared pressor and cardioaccelerator responses in unanesthetized, decerebrated University of California Davis (UCD)-T2DM rats at 21 and 31 wk following the onset of T2DM to responses in healthy nondiabetic rats. We found that the pressor response to static contraction was greater in the 31-wk T2DM [change in mean arterial pressure (∆MAP) = 39 ± 5 mmHg] but not in the 21-wk T2DM (∆MAP = 24 ± 5 mmHg) rats compared with nondiabetic rats (∆MAP = 18 ± 2 mmHg; P < 0.05). Similarly, the pressor and the cardioaccelerator responses to tendon stretch were significantly greater in the 31-wk T2DM rats [∆MAP = 69 ± 6 mmHg; change in heart rate (∆HR) = 28 ± 4 beats/min] compared with nondiabetic rats (∆MAP = 14 ± 2 mmHg; ∆HR = 5 ± 3 beats/min; P < 0.05). These findings suggest that the exercise pressor reflex changes as T2DM progresses and that a sensitized mechanoreflex may play a role in exaggerating these cardiovascular responses.NEW & NOTEWORTHY This is the first study to provide evidence that as type-2 diabetes mellitus (T2DM) progresses, the exercise pressor reflex becomes exaggerated, an effect that may be due to a sensitized mechanoreflex. Moreover, these findings provide compelling evidence suggesting that impairments in the reflexive control of circulation contribute to exaggerated blood pressure responses to exercise in T2DM.

Extended Duration of Hyperglycemia Result in Human-Like Corneal Nerve Lesions in Mice With Alloxan- and Streptozotocin-Induced Type 1 Diabetes

Purpose

Previous experimental studies assessing corneal nerves as a measure of the severity of diabetic peripheral neuropathy have yielded discordant results; this may have been due to the effect of the short duration of the induced diabetes. We investigated whether increases in the duration of hyperglycemia result in the development of corneal lesions in a mouse model of alloxan (AL)- or streptozotocin (STZ)-induced type 1 diabetes. We further determined whether corneal nerve fiber density, intraepidermal nerve fiber density (IENFD), and sural nerve morphology can be used as morphologic markers of diabetic peripheral neuropathy in rodent models.

Methods

A total of 30 female ICR mice were divided into three groups: those with STZ-induced (STZ group) and AL-induced (AL group) diabetes, and a control group. Hyperglycemia was maintained in diabetic mice for 35 weeks. Animals were euthanized at 41 weeks of age.

Results

Subbasal nerve plexus density (SBNPD) and terminal epithelial nerve density (TEND) in the cornea, as well as IENFD, were significantly lower, and mean sural nerve axon sizes were smaller in mice in the STZ and AL groups than in the control group. There were significant correlations between IENFD and SBNPD, and between IENFD and TEND.

Conclusions

These results indicate that the TEND and SBNTD of the cornea may be useful morphologic markers for diabetic peripheral neuropathy.

Polyphenols of marine red macroalga Symphyocladia latiuscula ameliorate diabetic peripheral neuropathy in experimental animals

Aims

Chronic hyperglycaemia activates the polyol pathway of glucose metabolism thereby stimulating the activation aldose reductase enzyme that in turn initiates a cascade of deleterious events, eventually, leading to nerve damage or neuropathy. Marine macroalgae and their isolated chemical constituents have been found to possess potential antidiabetic activity and have proved beneficial in the treatment of diabetes. In this study the neuroprotective effect of polyphenols isolated from the red macroalga Symphyocladia latiuscula was evaluated in experimental diabetic peripheral neuropathy.

Main methods

The polyphenolic fraction from Symphyocladia latiuscula was isolated. Diabetic peripheral neuropathy (DPN) was induced in animals by intraperitoneal injection of streptozotocin (45 mg/kg, b. w) and maintained for 6 weeks followed by treatment with SLPP or epalrestat. Nerve Conduction Velocity (NCV) and Compound Muscle Action Potential (CMAP) were measured using a non-invasive method followed by muscular grip strength test. Sciatic nerve aldose reductase activity, sorbitol accumulation, Na⁺K⁺-ATPase activity, production of pro-inflammatory cytokines and expression of AR and PKC were assessed.

Key findings

The Symphyocladia latiuscula polyphenols (SLPP) were found to inhibit aldose reductase activity as well as their expression in diabetic animals thereby improving the NCV, CMAP and muscle grip strength. Improvements in the sciatic nerve Na⁺K⁺-ATPase activity and intraneural accumulation of sorbitol, an index of aldose reductase overactivity, were evident with SLPP treatment. The production of pro-inflammatory cytokines (IL-6, IL-1β and TNF-α) and expression of protein kinase C (PKC) were also diminished.

Significance

The data suggest that the polyphenols of Symphyocladia latiuscula have neuroprotective potential against experimental DPN.

Association of left ventricular myocardial dysfunction with diabetic polyneuropathy

BACKGROUND

The pathogenesis of left ventricular (LV) dysfunction in diabetes has been attracting attention. It has been reported that LV longitudinal systolic myocardial function determined by speckle tracking echocardiography (STE) is associated with diabetic polyneuropathy (DPN). However, the relationship between the severity of peripheral neuropathy and LV myocardial dysfunction is unknown. This study examined the relationship between the severity of DPN and echocardiographic parameters as well as clinical features.

METHODS

The subjects were 166 patients (57 ± 14 years old) with diabetes who had a normal LV ejection fraction (≥ 55%). To assess LV longitudinal systolic function, global longitudinal strain (GLS) was calculated by two-dimensional STE as the average peak strain of 18 LV segments in three standard apical views. A nerve conduction study (NCS) was performed in each subject to assess the severity of neuropathy based on the NCS Baba Classification (Grade 0: no apparent abnormalities-Grade IV: abolition). Three nerves in the lower extremity were examined: tibial nerve (F-wave latency, motor nerve conduction velocity, and amplitude), sural nerve (sensory conduction velocity and amplitude), and peroneal nerve (motor nerve conduction velocity and amplitude).

RESULTS

Of the 166 subjects, 112 subjects (67.5%) were confirmed to have DPN, and all the subjects were divided into two groups according to the presence/absence of DPN. When multivariate analysis was performed using significant factors from univariate logistic regression analysis as explanatory variables, GLS was found to be an independent determinant of DPN (odds ratio: 0.55, p < 0.001). In multivariate analysis of NCS data, F-wave latency was the most important determinant of DPN (odds ratio: 1.43, p < 0.001). There was a significant negative correlation between F-wave latency and GLS (r = - 0.43, p < 0.001). Regarding the relation between GLS and the severity of DPN, GLS was significantly lower in patients with Grade I or higher DPN than in patients without DPN, but showed no significant difference between the grades of neuropathy. In addition, GLS was significantly lower when 2-3 lower extremity nerves were affected by DPN than in patients without DPN.

CONCLUSION

Patients with diabetes may already have subclinical LV myocardial dysfunction when DPN is Grade I. Assessment of LV longitudinal systolic function by GLS may be important in diabetic patients with DPN.

Chronic hyperglycemia mediated physiological alteration and metabolic distortion leads to organ dysfunction, infection, cancer progression and other pathophysiological consequences: An update on glucose toxicity

Chronic exposure of glucose rich environment creates several physiological and pathophysiological changes. There are several pathways by which hyperglycemia exacerbate its toxic effect on cells, tissues and organ systems. Hyperglycemia can induce oxidative stress, upsurge polyol pathway, activate protein kinase C (PKC), enhance hexosamine biosynthetic pathway (HBP), promote the formation of advanced glycation end-products (AGEs) and finally alters gene expressions. Prolonged hyperglycemic condition leads to severe diabetic condition by damaging the pancreatic β-cell and inducing insulin resistance. Numerous complications have been associated with diabetes, thus it has become a major health issue in the 21st century and has received serious attention. Dysregulation in the cardiovascular and reproductive systems along with nephropathy, retinopathy, neuropathy, diabetic foot ulcer may arise in the advanced stages of diabetes. High glucose level also encourages proliferation of cancer cells, development of osteoarthritis and potentiates a suitable environment for infections. This review culminates how elevated glucose level carries out its toxicity in cells, metabolic distortion along with organ dysfunction and elucidates the complications associated with chronic hyperglycemia.

Temporal changes in the exercise pressor reflex in type 1 diabetic rats

Previous studies have shown that diabetic peripheral neuropathy affects both unmyelinated and myelinated afferents, similar to those evoking the exercise pressor reflex. However, the effect of type 1 diabetes (T1DM) on this reflex is not known. We examined, in decerebrate male and female T1DM [streptozotocin (STZ)] and healthy control (CTL) rats, pressor and cardioaccelerator responses to isometric contraction of the hindlimb muscles during the early and late stages of the disease. STZ (50 mg/kg) was injected to induce diabetes, and experiments were conducted at 1, 3, and 6 wk after injection. On the day of the experiment, we statically contracted the hindlimb muscles by stimulating the sciatic nerve and measured changes in mean arterial pressure and heart rate. We found that the pressor but not cardioaccelerator response was exaggerated in STZ rats at 1 wk (STZ: 21 ± 3 mmHg, n = 10, and CTL: 14 ± 2 mmHg, n = 10, P < 0.05) and at 3 wk (STZ: 26 ± 5 mmHg, n = 10, and CTL: 17 ± 3 mmHg, n = 11, P < 0.05) after injection. However, at 6 wk, and only in male rats, both the pressor (STZ: 13 ± 3 mmHg, n = 12, and CTL: 17 ± 3 mmHg, n = 13, P < 0.05) and cardioaccelerator responses (STZ: 7 ± 3 beats/min, n = 12, and CTL: 10 ± 3 beats/min, n = 13, P < 0.05) to contraction were significantly attenuated in STZ rats compared with CTL rats. These data indicate that T1DM exaggerates the exercise pressor reflex during the early stages of the disease in both male and female rats. Conversely, T1DM attenuates this reflex in the late stage of the disease in male but not female rats.

NEW & NOTEWORTHY This is the first study to provide evidence that the pressor and cardioaccelerator responses to skeletal muscle contraction vary depending on the duration of type 1 diabetes.

Is Stem Cell Transplantation Ready for Prime Time in Diabetic Polyneuropathy?

Diabetic polyneuropathy (DPN) is the most common complication that emerges early in patients who have diabetes. Curative treatment for overt or symptomatic DPN has not been established, requiring much effort to explore new modalities. Thus, the use of various kinds of stem cells as a potential therapeutic option for DPN is of particular interest. The beneficial effects were proposed to be attributed to either cytokine released from transplanted stem cells or the differentiation of stem cells to substitute the damaged peripheral nerve. Furthermore, based on the concept that humoral factors secreted from stem cells play a pivotal role in tissue regeneration, the utilization of conditioned medium derived from the stem cell culture serves as a novel tool for regenerative therapy. However, many questions have not been yet answered to determine whether stem cell therapy is essential in clinical application of DPN. In this report, we review the current status of preclinical studies on stem cell therapy for DPN and discuss future prospects.

MicroRNA 146a locally mediates distal axonal growth of dorsal root ganglia neurons under high glucose and sildenafil conditions

Axonal loss contributes to induction of diabetic peripheral neuropathy. Sildenafil, a phosphodiesterase type 5 inhibitor, ameliorates neurological dysfunction in diabetic peripheral neuropathy. However, the direct effect of high glucose and sildenafil on axonal growth has not been extensively investigated. Using rat primary dorsal root ganglia (DRG) neurons cultured in a microfluidic chamber, we investigated the effect of axonal application of high glucose and sildenafil on distal axonal growth. We found that axonal, but not cell body, application of high glucose locally inhibited distal axonal growth. However, axonal application of sildenafil overcame high glucose-reduced axonal growth. Quantitative real-time RT-PCR (qRT-PCR) and Western blot analysis of distal axonal samples revealed that high glucose reduced axonal miR-146a levels and substantially increased miR-146a target genes, IRAK1 and TRAF6 in the axon. In contrast, sildenafil significantly reversed high glucose-reduced miR-146a levels and high glucose-increased IRAK1 and TRAF6. Gain- and loss-of function of miR-146a in DRG neurons revealed that miR-146a mediated the local effect of high glucose on the distal axonal growth. These in vitro data provide new insights into molecular mechanisms of diabetic peripheral neuropathy.

Pyruvate Dehydrogenase Kinase-mediated Glycolytic Metabolic Shift in the Dorsal Root Ganglion Drives Painful Diabetic Neuropathy

The dorsal root ganglion (DRG) is a highly vulnerable site in diabetic neuropathy. Under diabetic conditions, the DRG is subjected to tissue ischemia or lower ambient oxygen tension that leads to aberrant metabolic functions. Metabolic dysfunctions have been documented to play a crucial role in the pathogenesis of diverse pain hypersensitivities. However, the contribution of diabetes-induced metabolic dysfunctions in the DRG to the pathogenesis of painful diabetic neuropathy remains ill-explored. In this study, we report that pyruvate dehydrogenase kinases (PDK2 and PDK4), key regulatory enzymes in glucose metabolism, mediate glycolytic metabolic shift in the DRG leading to painful diabetic neuropathy. Streptozotocin-induced diabetes substantially enhanced the expression and activity of the PDKs in the DRG, and the genetic ablation of Pdk2 and Pdk4 attenuated the hyperglycemia-induced pain hypersensitivity. Mechanistically, Pdk2/4 deficiency inhibited the diabetes-induced lactate surge, expression of pain-related ion channels, activation of satellite glial cells, and infiltration of macrophages in the DRG, in addition to reducing central sensitization and neuroinflammation hallmarks in the spinal cord, which probably accounts for the attenuated pain hypersensitivity. Pdk2/4-deficient mice were partly resistant to the diabetes-induced loss of peripheral nerve structure and function. Furthermore, in the experiments using DRG neuron cultures, lactic acid treatment enhanced the expression of the ion channels and compromised cell viability. Finally, the pharmacological inhibition of DRG PDKs or lactic acid production substantially attenuated diabetes-induced pain hypersensitivity. Taken together, PDK2/4 induction and the subsequent lactate surge induce the metabolic shift in the diabetic DRG, thereby contributing to the pathogenesis of painful diabetic neuropathy.

Mitochondrial dysfunction in diabetic neuropathy: a series of unfortunate metabolic events

Diabetic neuropathy is a dying back neurodegenerative disease of the peripheral nervous system where mitochondrial dysfunction has been implicated as an etiological factor. Diabetes (type 1 or type 2) invokes an elevation of intracellular glucose concentration simultaneously with impaired growth factor support by insulin, and this dual alteration triggers a maladaptation in metabolism of adult sensory neurons. The energy sensing pathway comprising the AMP-activated protein kinase (AMPK)/sirtuin (SIRT)/peroxisome proliferator-activated receptor-γ coactivator α (PGC-1α) signaling axis is the target of these damaging changes in nutrient levels, e.g., induction of nutrient stress, and loss of insulin-dependent growth factor support and instigates an aberrant metabolic phenotype characterized by a suppression of mitochondrial oxidative phosphorylation and shift to anaerobic glycolysis. There is discussion of how this loss of mitochondrial function and transition to overreliance on glycolysis contributes to the diminishment of collateral sprouting and axon regeneration in diabetic neuropathy in the context of the highly energy-consuming nerve growth cone.

Effect of deoxycorticosterone acetate-salt-induced hypertension on diabetic peripheral neuropathy in alloxan-induced diabetic WBN/Kob rats

The relationship between hypertension and diabetic peripheral neuropathy (DPN) has recently been reported in clinical research, but it remains unclear whether hypertension is a risk factor for DPN. To investigate the effects of hypertension on DPN, we analyzed morphological features of peripheral nerves in diabetic rats with hypertension. Male WBN/Kob rats were divided into 2 groups: alloxan-induced diabetic rats with deoxycorticosterone acetate-salt (DOCA-salt) treatment (ADN group) and nondiabetic rats with DOCA-salt treatment (DN group). Sciatic, tibial (motor) and sural (sensory) nerves were subjected to qualitative and quantitative histomorphological analysis. Systolic blood pressure in the two groups exhibited a higher value (>140 mmHg), but there was no significant difference between the two groups. Endoneurial blood vessels in both groups presented endothelial hypertrophy and narrowing of the vascular lumen. Electron microscopically, duplication of basal lamina surrounding the endothelium and pericyte of the endoneurial vessels was observed, and this lesion appeared to be more frequent and severe in the ADN group than the DN group. Many nerve fibers of the ADN and DN groups showed an almost normal appearance, whereas morphometrical analysis of the tibial nerve showed a significant shift to smaller fiber and myelin sizes in the ADN group compared with DN group. In sural nerve, the fiber and axon-size significantly shifted to a smaller size in ADN group compared with the DN group. These results suggest that combined diabetes and hypertension could induce mild peripheral nerve lesions with vascular changes.

Effects of rosmarinic acid on an experimental model of painful diabetic neuropathy in rats

CONTEXT

Diabetic neuropathic (DN) pain is one of the diabetes complications. Rosmarinic acid (RA), a natural phenol antioxidant, shows some biological activities, including anti-inflammatory, analgesic, and anti-diabetic effects.

OBJECTIVES

We investigated the efficacy of RA administration (10 and 30 mg/kg) on streptozotocin (STZ)-induced neuropathy in rats.

MATERIAL AND METHODS

The animals received saline or RA (10 and 30 mg/kg, p.o.; once daily) for 8 weeks. DN was evaluated by the tail flick (TF) method, formalin test, and tactile allodynia. At the end, all rats were weighed and underwent plasma glucose measurement.

RESULTS

There was an increase in licking time during both formalin test phases in diabetic animals (138.5 ± 10.7 and 448.7 ± 2.6 s) that was decreased by RA10 mg/kg (103.5 ± 7.5 and 284.4 ± 19 s) and RA 30 mg/kg (81.8 ± 11 and 192.7 ± 14 s). RA 30 mg/kg caused anti-nociception during the early phase in treated controls (52.1 ± 6 s) than untreated controls (99.4 ± 5.9 s). The TF latency in diabetics (2.9 ± 0.1 s) was increased in RA10 and 30 mg/kg treated diabetics (5.3 ± 0.4 and 6 ± 0.86 s). The paw withdrawal threshold (PWT) of the diabetics (3.6 ± 0.7 g) was increased after RA 10 and 30 mg/kg (13.8 ± 0.3 and 14 ± 0.4 g) treatment. RA did not induce a significant change in body weight and plasma glucose of rats.

CONCLUSION

RA showed efficacy in amelioration of some aspects of DN. Therefore, RA makes a good candidate for DN treatment in clinical studies.

Calcium signalling in sensory neurones and peripheral glia in the context of diabetic neuropathies

Peripheral sensory nervous system is comprised of neurones with their axons and neuroglia that includes satellite glial cells in sensory ganglia, myelinating, non-myelinating and perisynaptic Schwann cells. Pathogenesis of peripheral diabetic polyneuropathies is associated with aberrant function of both neurones and glia. Deregulated Ca(2+) homoeostasis and aberrant Ca(2+) signalling in neuronal and glial elements contributes to many forms of neuropathology and is fundamental to neurodegenerative diseases. In diabetes both neurones and glia experience metabolic stress and mitochondrial dysfunction which lead to deregulation of Ca(2+) homeostasis and Ca(2+) signalling, which in their turn lead to pathological cellular reactions contributing to development of diabetic neuropathies. Molecular cascades responsible for Ca(2+) homeostasis and signalling, therefore, can be regarded as potential therapeutic targets.

Mechanism of diabetic neuropathy: Where are we now and where to go?

Neuropathy is the most common complication of diabetes. As a consequence of longstanding hyperglycemia, a downstream metabolic cascade leads to peripheral nerve injury through an increased flux of the polyol pathway, enhanced advanced glycation end‐products formation, excessive release of cytokines, activation of protein kinase C and exaggerated oxidative stress, as well as other confounding factors. Although these metabolic aberrations are deemed as the main stream for the pathogenesis of diabetic microvascular complications, organ‐specific histological and biochemical characteristics constitute distinct mechanistic processes of neuropathy different from retinopathy or nephropathy. Extremely long axons originating in the small neuronal body are vulnerable on the most distal side as a result of malnutritional axonal support or environmental insults. Sparse vascular supply with impaired autoregulation is likely to cause hypoxic damage in the nerve. Such dual influences exerted by long‐term hyperglycemia are critical for peripheral nerve damage, resulting in distal‐predominant nerve fiber degeneration. More recently, cellular factors derived from the bone marrow also appear to have a strong impact on the development of peripheral nerve pathology. As evident from such complicated processes, inhibition of single metabolic factors might not be sufficient for the treatment of neuropathy, but a combination of several inhibitors might be a promising approach to overcome this serious disorder. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00070.x, 2010)

Neuropathy induced by exogenously administered advanced glycation end-products in rats

Aims/Introduction:  Advanced glycation end‐products (AGE) have been implicated in the development of diabetic neuropathy. It still remains unknown, however, how AGE cause functional and structural changes of the peripheral nerve in diabetes. To explore the role of AGE in diabetic neuropathy, we examined the peripheral nerve by injecting AGE into normal Wistar rats.

Materials and Methods:  Young, normal male Wistar rats were injected intraperitoneally (i.p.) daily for 12 weeks with purified AGE prepared by incubating D‐glucose with bovine serum albumin (BSA). A control group received BSA alone. A group of rats given AGE were co‐treated with aminoguanidine (50 mg/kg/day, i.p.). Peripheral nerve function and structure, as well as nerve Na⁺,K⁺‐ATPase activity, were examined in these rats. Immunohistochemical expressions of 8‐hydroxy‐2′‐deoxyguanosine (8OHdG) and nuclear factor‐κB (NF‐κB)p65 were also examined.

Results:  Serum AGE levels were increased two to threefold in the AGE‐treated group compared with those in the BSA‐treated control group. AGE‐treated rats showed a marked slowing of motor nerve conduction velocity (MNCV) and decreased nerve Na⁺,K⁺‐ATPase activity compared with those in the BSA‐treated group. These changes were accompanied by intensified expressions of 8OHdG and NF‐κBp65 in endothelial cells and Schwann cells. Aminoguanidine treatment corrected MNCV delay, Na⁺,K⁺‐ATPase activity, and suppressed the expression of 8OHdG and NF‐κB, despite there being no influence on serum AGE levels.

Conclusions:  The results suggest that an elevated concentration of blood AGE might be one of the contributing factors to the development of neuropathic changes in diabetes.

Experimental diabetes in neonatal mice induces early peripheral sensorimotor neuropathy

Animal models of diabetes do not reach the severity of human diabetic neuropathy but relatively mild neurophysiological deficits and minor morphometric changes. The lack of degenerative neuropathy in diabetic rodent models seems to be a consequence of the shorter length of the axons or the shorter animal life span. Diabetes-induced demyelination needs many weeks or even months before it can be evident by morphometrical analysis. In mice myelination of the peripheral nervous system starts at the prenatal period and it is complete several days after birth. Here we induced experimental diabetes to neonatal mice and we evaluated its effect on the peripheral nerve 4 and 8 weeks after diabetes induction. Neurophysiological values showed a decline in sensory nerve conduction velocity at both time-points. Morphometrical analysis of the tibial nerve demonstrated a decrease in the number of myelinated fibers, fiber size and myelin thickness at both time-points studied. Moreover, aldose reductase and poly(ADP-ribose) polymerase activities were increased even if the amount of the enzyme was not affected. Thus, type 1 diabetes in newborn mice induces early peripheral neuropathy and may be a good model to assay pharmacological or gene therapy strategies to treat diabetic neuropathy.

Renal nerve ultrastructural alterations in short term and long term experimental diabetes

BACKGROUND

Despite the evidence that renal hemodynamics is impaired in experimental diabetes, associated with glomeruli structural alterations, renal nerves were not yet investigated in experimental models of diabetes and the contribution of nerve alterations to the diabetic nephropathy remains to be investigated. We aimed to determine if ultrastructural morphometric parameters of the renal nerves are affected by short term and/or long term experimental diabetes and if insulin treatment reverses these alterations. Left renal nerves were evaluated 15 days or 12 weeks (N = 10 in each group) after induction of diabetes, with a single injection of streptozotocin (STZ). Control rats (N = 10 in each group) were injected with vehicle (citrate buffer). Treated animals (N = 10 in each group) received a single subcutaneous injection of insulin on a daily basis. Arterial pressure, together with the renal nerves activity, was recorded 15 days (short-term) or 12 weeks (long-term) after STZ injection. After the recordings, the renal nerves were dissected, prepared for light and transmission electron microscopy, and fascicle and fibers morphometry were carried out with computer software.

RESULTS

The major diabetic alteration on the renal nerves was a small myelinated fibers loss since their number was smaller on chronic diabetic animals, the average morphometric parameters of the myelinated fibers were larger on chronic diabetic animals and distribution histograms of fiber diameter was significantly shifted to the right on chronic diabetic animals. These alterations began early, after 15 days of diabetes induction, associated with a severe mitochondrial damage, and were not prevented by conventional insulin treatment.

CONCLUSIONS

The experimental diabetes, induced by a single intravenous injection of STZ, in adult male Wistar rats, caused small fiber loss in the renal nerves, probably due to the early mitochondrial damage. Conventional treatment with insulin was able to correct the weight gain and metabolic changes in diabetic animals, without, however, correcting and / or preventing damage to the thin fibers caused by STZ-induced diabetes. The kidney innervation is impaired in this diabetic model suggesting that alterations of the renal nerves may play a role in the development of the diabetic nephropathy.

Exploring a new therapy for diabetic polyneuropathy - the application of stem cell transplantation

Diabetic polyneuropathy (DPN) is the most common complication that emerges early in diabetic patients. Intervention with strict blood glucose control or treatment with aldose reductase inhibitor is reported to be effective in early stages of DPN. Curative treatment for overt or symptomatic DPN, however, has not been established, thus requiring much effort to explore a new therapy. Recent preclinical studies on the use of gene or cell therapy have provided promising results in the treatment of DPN. Of particular interest, induced pluripotent stem cells are introduced. In these studies, restoration of DPN was proposed to be attributed to either neurotrophic factors released from transplanted stem cells or differentiation of stem cells to substitute the damaged peripheral nerve. There are still several problems, however, that remain to be overcome, such as perturbed function, fragility, or limited survival of transplanted cells in diabetes milieu and risk for malignant transformation of transplanted cells. Questions, which cell is the most appropriate as the source for cell therapy, or which site is the best for transplantation to obtain the most effective results, remain to be answered. In this communication, we overview the current status of preclinical studies on the cell therapy for DPN and discuss the future prospect.

Normalization of NF-κB activity in dorsal root ganglia neurons cultured from diabetic rats reverses neuropathy-linked markers of cellular pathology

AIMS/HYPOTHESIS

Dorsal root ganglia (DRG) sensory neurons cultured from 3 to 5 month streptozotocin (STZ)-induced diabetic rats exhibit structural and biochemical changes seen in peripheral nerve fibers in vivo, including axonal swellings, oxidative damage, reduced axonal sprouting, and decreased NF-κB activity. NF-κB is a transcription factor required by DRG neurons for survival and plasticity, and regulates transcription of antioxidant proteins (e.g. MnSOD). We hypothesized that the diabetes-induced decrease in NF-κB activity in DRG contributes to pathological phenomena observed in cultured DRG neurons from diabetic rats.

METHODS

NF-κB localization was assessed in intact DRG and neuron cultures using immunostaining. NF-κB activity was manipulated in sensory neuron cultures derived from age-matched normal or 3-5 month STZ-diabetic rats using pharmacological means and lentiviral expression of shRNA. The impact of diabetes and altered NF-κB activity on neuronal phenotype involved analysis of neurite outgrowth, neurite morphology, oxidative stress (lipid peroxidation) and expression of MnSOD.

RESULTS

STZ-induced diabetes caused a significant decrease in nuclear localization of NF-κB subunits p50 and c-rel, but no change in p65 in intact DRG. Inhibition of NF-κB in normal neuron cultures significantly increased axonal swellings and oxidative stress, and reduced both neurite outgrowth and expression of MnSOD. These phenomena mimicked markers of pathology in cultured DRG neurons from diabetic rats. Enhancement of NF-κB activity in cultured diabetic DRG neurons ameliorated the sub-optimal neurite outgrowth and MnSOD levels triggered by diabetes. Exogenous insulin enhanced nuclear localization of p50 and c-rel but not p65 in diabetic neuronal cultures.

CONCLUSION/INTERPRETATION

The diabetes-induced decrease of nuclear localization of NF-κB subunits p50 and c-rel in DRG contributes to development of in vitro markers of peripheral neuropathy, possibly through impaired mitochondrial ROS scavenging by deficient MnSOD.

Ciliary neurotrophic factor activates NF-κB to enhance mitochondrial bioenergetics and prevent neuropathy in sensory neurons of streptozotocin-induced diabetic rodents

Diabetes causes mitochondrial dysfunction in sensory neurons that may contribute to peripheral neuropathy. Ciliary neurotrophic factor (CNTF) promotes sensory neuron survival and axon regeneration and prevents axonal dwindling, nerve conduction deficits and thermal hypoalgesia in diabetic rats. In this study, we tested the hypothesis that CNTF protects sensory neuron function during diabetes through normalization of impaired mitochondrial bioenergetics. In addition, we investigated whether the NF-κB signal transduction pathway was mobilized by CNTF. Neurite outgrowth of sensory neurons derived from streptozotocin (STZ)-induced diabetic rats was reduced compared to neurons from control rats and exposure to CNTF for 24 h enhanced neurite outgrowth. CNTF also activated NF-κB, as assessed by Western blotting for the NF-κB p50 subunit and reporter assays for NF-κB promoter activity. Conversely, blockade of NF-κB signaling using SN50 peptide inhibited CNTF-mediated neurite outgrowth. Studies in mice with STZ-induced diabetes demonstrated that systemic therapy with CNTF prevented functional indices of peripheral neuropathy along with deficiencies in dorsal root ganglion (DRG) NF-κB p50 expression and DNA binding activity. DRG neurons derived from STZ-diabetic mice also exhibited deficiencies in maximal oxygen consumption rate and associated spare respiratory capacity that were corrected by exposure to CNTF for 24 h in an NF-κB-dependent manner. We propose that the ability of CNTF to enhance axon regeneration and protect peripheral nerve from structural and functional indices of diabetic peripheral neuropathy is associated with targeting of mitochondrial function, in part via NF-κB activation, and improvement of cellular bioenergetics.

Diabetic peripheral neuropathy in Spontaneously Diabetic Torii-Lepr(fa) (SDT fatty) rats

Spontaneously Diabetic Torii (SDT) rat is a hereditary model of diabetes. Although the SDT rat shows severe diabetic complications, the onset of hyperglycemia is late. SDT fatty rat, established by introducing the fa allele of the Zucker fatty rat to SDT rat, develops diabetes much faster than SDT rat. In the present study, diabetic peripheral neuropathy (DPN) was evaluated to show the further usefulness of this animal model. Motor nerve conduction velocity (MNCV) was delayed, and the number of sural nerve fibers was decreased in SDT fatty rat. Treatment of pioglitazone lowered blood glucose level and prevented delay of MNCV in SDT fatty rats. SDT fatty rat is a useful animal model for studies of DPN in type 2 diabetes.

Impaired adenosine monophosphate-activated protein kinase signalling in dorsal root ganglia neurons is linked to mitochondrial dysfunction and peripheral neuropathy in diabetes

Mitochondrial dysfunction occurs in sensory neurons and may contribute to distal axonopathy in animal models of diabetic neuropathy. The adenosine monophosphate-activated protein kinase and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signalling axis senses the metabolic demands of cells and regulates mitochondrial function. Studies in muscle, liver and cardiac tissues have shown that the activity of adenosine monophosphate-activated protein kinase and PGC-1α is decreased under hyperglycaemia. In this study, we tested the hypothesis that deficits in adenosine monophosphate-activated protein kinase/PGC-1α signalling in sensory neurons underlie impaired axonal plasticity, suboptimal mitochondrial function and development of neuropathy in rodent models of type 1 and type 2 diabetes. Phosphorylation and expression of adenosine monophosphate-activated protein kinase/PGC-1α and mitochondrial respiratory chain complex proteins were downregulated in dorsal root ganglia of both streptozotocin-diabetic rats and db/db mice. Adenoviral-mediated manipulation of endogenous adenosine monophosphate-activated protein kinase activity using mutant proteins modulated neurotrophin-directed neurite outgrowth in cultures of sensory neurons derived from adult rats. Addition of resveratrol to cultures of sensory neurons derived from rats after 3-5 months of streptozotocin-induced diabetes, significantly elevated adenosine monophosphate-activated protein kinase levels, enhanced neurite outgrowth and normalized mitochondrial inner membrane polarization in axons. The bioenergetics profile (maximal oxygen consumption rate, coupling efficiency, respiratory control ratio and spare respiratory capacity) was aberrant in cultured sensory neurons from streptozotocin-diabetic rats and was corrected by resveratrol treatment. Finally, resveratrol treatment for the last 2 months of a 5-month period of diabetes reversed thermal hypoalgesia and attenuated foot skin intraepidermal nerve fibre loss and reduced myelinated fibre mean axonal calibre in streptozotocin-diabetic rats. These data suggest that the development of distal axonopathy in diabetic neuropathy is linked to nutrient excess and mitochondrial dysfunction via defective signalling of the adenosine monophosphate-activated protein kinase/PGC-1α pathway.

The role of aberrant mitochondrial bioenergetics in diabetic neuropathy

Diabetic neuropathy is a neurological complication of diabetes that causes significant morbidity and, because of the obesity-driven rise in incidence of type 2 diabetes, is becoming a major international health problem. Mitochondrial phenotype is abnormal in sensory neurons in diabetes and may contribute to the etiology of diabetic neuropathy where a distal dying-back neurodegenerative process is a key component contributing to fiber loss. This review summarizes the major features of mitochondrial dysfunction in neurons and Schwann cells in human diabetic patients and in experimental animal models (primarily exhibiting type 1 diabetes). This article attempts to relate these findings to the development of critical neuropathological hallmarks of the disease. Recent work reveals that hyperglycemia in diabetes triggers nutrient excess in neurons that, in turn, mediates a phenotypic change in mitochondrial biology through alteration of the AMP-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signaling axis. This vital energy sensing metabolic pathway modulates mitochondrial function, biogenesis and regeneration. The bioenergetic phenotype of mitochondria in diabetic neurons is aberrant due to deleterious alterations in expression and activity of respiratory chain components as a direct consequence of abnormal AMPK/PGC-1α signaling. Utilization of innovative respirometry equipment to analyze mitochondrial function of cultured adult sensory neurons from diabetic rodents shows that the outcome for cellular bioenergetics is a reduced adaptability to fluctuations in ATP demand. The diabetes-induced maladaptive process is hypothesized to result in exhaustion of the ATP supply in the distal nerve compartment and induction of nerve fiber dissolution. The role of mitochondrial dysfunction in the etiology of diabetic neuropathy is compared with other types of neuropathy with a distal dying-back pathology such as Friedreich ataxia, Charcot-Marie-Tooth disease type 2 and human immunodeficiency virus-associated distal-symmetric neuropathy.

Comparative study of peripheral neuropathy and nerve regeneration in NOD and ICR diabetic mice

The non-obese diabetic (NOD) mouse was suggested as an adequate model for diabetic autonomic neuropathy. We evaluated sensory-motor neuropathy and nerve regeneration following sciatic nerve crush in NOD males rendered diabetic by multiple low doses of streptozotocin, in comparison with similarly treated Institute for Cancer Research (ICR) mice, a widely used model for type I diabetes. Neurophysiological values for both strains showed a decline in motor and sensory nerve conduction velocity at 7 and 8 weeks after induction of diabetes in the intact hindlimb. However, amplitudes of compound muscle and sensory action potentials (CMAPs and CNAPs) were significantly reduced in NOD but not in ICR diabetic mice. Morphometrical analysis showed myelinated fiber loss in highly hyperglycemic NOD mice, but no significant changes in fiber size. There was a reduction of intraepidermal nerve fibers, more pronounced in NOD than in ICR diabetic mice. Interestingly, aldose reductase and poly(ADP-ribose) polymerase (PARP) activities were increased already at 1 week of hyperglycemia, persisting until the end of the experiment in both strains. Muscle and nerve reinnervation was delayed in diabetic mice following sciatic nerve crush, being more marked in NOD mice. Thus, diabetes of mid-duration induces more severe peripheral neuropathy and slower nerve regeneration in NOD than in ICR mice.

Sensory neurons derived from diabetic rats have diminished internal Ca2+ stores linked to impaired re-uptake by the endoplasmic reticulum

Distal symmetrical sensory neuropathy in diabetes involves the dying back of axons, and the pathology equates with axonal dystrophy generated under conditions of aberrant Ca2+ signalling. Previous work has described abnormalities in Ca2+ homoeostasis in sensory and dorsal horn neurons acutely isolated from diabetic rodents. We extended this work by testing the hypothesis that sensory neurons exposed to long-term Type 1 diabetes in vivo would exhibit abnormal axonal Ca2+ homoeostasis and focused on the role of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). DRG (dorsal root ganglia) sensory neurons from age-matched normal and 3-5-month-old STZ (streptozotocin)-diabetic rats (an experimental model of Type 1 diabetes) were cultured. At 1-2 days in vitro an array of parameters were measured to investigate Ca2+ homoeostasis including (i) axonal levels of intracellular Ca2+, (ii) Ca2+ uptake by the ER (endoplasmic reticulum), (iii) assessment of Ca2+ signalling following a long-term thapsigargin-induced blockade of SERCA and (iv) determination of expression of ER mass and stress markers using immunocytochemistry and Western blotting. KCl- and caffeine-induced Ca2+ transients in axons were 2-fold lower in cultures of diabetic neurons compared with normal neurons indicative of reduced ER calcium loading. The rate of uptake of Ca2+ into the ER was reduced by 2-fold (P<0.05) in diabetic neurons, while markers for ER mass and ER stress were unchanged. Abnormalities in Ca2+ homoeostasis in diabetic neurons could be mimicked via long-term inhibition of SERCA in normal neurons. In summary, axons of neurons from diabetic rats exhibited aberrant Ca2+ homoeostasis possibly triggered by sub-optimal SERCA activity that could contribute to the distal axonopathy observed in diabetes.

História familiar positiva de diabetes altera a resposta cronotrópica ao exercício agudo

O objetivo do presente estudo foi verificar os efeitos de uma sessão de exercício aeróbio (SEA) no perfil metabólico e hemodinâmico em filhos de diabéticos do tipo II. Jovens saudáveis do sexo masculino foram divididos em dois grupos: filhos de diabéticos (FD, n = 7) e filhos de normoglicêmicos (FN, n = 7). Foram realizadas medidas metabólicas (triglicérides e glicemia) e cardiovasculares (pressão arterial, PA, e frequência cardíaca, FC) em repouso, durante e após a SEA. A SEA foi realizada em esteira ergométrica com duração de 30 minutos e intensidade progressiva. Os grupos apresentaram valores semelhantes em relação ao triglicérides, PA e FC em repouso. A glicemia de jejum do grupo FD foi maior em relação ao grupo FN (99 ± 2 versus 89 ± 2mg/dL). O grupo FD apresentou maior resposta cronotrópica no início da SEA quando comparado ao grupo FN (86 ± 4 versus 125 ± 8bpm), mas a FC foi semelhante entre os grupos nas demais intensidades. No quinto minuto de recuperação da SEA a FC foi maior no grupo FD comparado ao grupo FN (FN: 88 ± 3 versus FD: 97 ± 4bpm). A PA foi semelhante entre os grupos durante e após a SEA. Os resultados demonstram que filhos de diabéticos jovens já apresentam alterações metabólicas em repouso e resposta exacerbada da FC no início e nos primeiros minutos de recuperação da SEA, sugerindo um maior risco cardiovascular nessa população.

Cilostazol: a pilot study on safety and clinical efficacy in neuropathies of diabetes mellitus type 2 (ASCEND)

BACKGROUND

Diabetic polyneuropathy may have vascular and metabolic components in its pathophysiologic mechanism. Cilostazol, aside from its antiplatelet and vasodilatory properties, may increase nerve blood flow and potentially improve neuropathy.

OBJECTIVE

To assess the efficacy and safety of cilostazol in diabetic polyneuropathy.

METHODS

Forty-seven diabetic patients were randomized into placebo, low-dose (100 mg/d), and high-dose (200 mg/d) cilostazol groups. Primary efficacy parameter was a change in neuropathy symptom scores and secondary efficacy parameter was a change in walking speed from baseline to week 12. Safety parameters were changes in nerve conduction studies as well as reporting of adverse events.

RESULTS/CONCLUSION

Despite significant improvement in the neuropathy symptom scores in the overall motor and sensory categories of the 3 arms of the study from baseline to week 12, no significant differences were found among the groups, indicating nonsuperiority of cilostazol in regard to improvement of neuropathy symptoms over the short study span. However, cilostazol, at low dose, was effective in improving walking speed from baseline to week 12, implying an improved blood flow. No significant worsening nor improvement in motor and sensory nerve conduction parameters were observed, comparing the 3 study arms from baseline to weeks 4, 12, and 16, supporting cilostazol's safety. Overall, the adverse events of the 3 study arms did not significantly differ, and neither were there serious adverse events reported, also signifying safety and tolerability in our Filipino cohort of patients with neuropathy in diabetes mellitus treated with cilostazol.

Pharmacological evaluation of rat dorsal root ganglion neurons as an in vitro model for diabetic neuropathy

BACKGROUND

Diabetic neuropathy is a complication of diabetes mellitus that develops in about 50% of people with diabetes. Despite its widespread occurrence and devastating effects, this complication is still not fully understood, and there is no treatment available to prevent its development.

METHODS

In this study, immunocytochemistry for activating transcription factor 3, a marker for cell injury, was used to investigate the stress response in dorsal root ganglion neurons in both in vitro and ex vivo models of diabetic neuropathy.

RESULTS

Our findings showed increased activating transcription factor 3 expression in hyperglycemic culture conditions and in dorsal root ganglion neurons isolated from diabetic rats. Glial cell line-derived neurotrophic factor, a substance with known neuroprotective properties, was able to reduce diabetes mellitus-induced neuronal stress in vitro, while gabapentin and carbamazepine, currently used to treat neuropathic pain, showed only limited effects.

CONCLUSION

Growth factors may have a therapeutic benefit as neurotrophic agents in the treatment of diabetic peripheral neuropathy, but gabapentin and carbamazepine have no direct protective effect on sensory neurons. This research also indicates that immunocytochemistry for activating transcription factor 3 is a valuable tool for evaluation of pharmacological substances in dorsal root ganglion cultures.

The involvement of aldose reductase in alterations to neurotrophin receptors and neuronal cytoskeletal protein mRNA levels in the dorsal root ganglion of streptozotocin-induced diabetic rats

Dorsal root ganglia (DRG) are recognized as one of the organs which are damaged in peripheral sensory diabetic neuropathy. In an experimental animal model, the alteration of the mRNA expression level of neurotrophins, their receptors and neuronal cytoskeletal protein have been reported. In this study, we examined whether these changes are improved by treatment with the aldose reductase inhibitor, zenarestat, in early-stage diabetic neuropathy of streptozotocin (STZ)-induced diabetic rats. Two weeks after the induction of diabetes mellitus by STZ treatment, zenarestat or a vehicle were given orally for two weeks. After the zenarestat treatment, the mRNA expression levels of neurotrophin receptors and neuronal cytoskeletal proteins in dorsal root ganglia were determined with a real-time polymerase chain reaction (PCR) method. Compared with the expression level of normal rats, a significant increase in Trk-C and Talpha1 alpha-tubulin and a decrease in neurofilament H mRNA expression level were observed in the DRG of STZ rats, while there were no significant changes in Trk-A, Trk-B, p75, neurofilament L, neurofilament M and betaIII tubulin mRNA expression. Zenarestat treatment significantly ameliorated the abnormal increase in Trk-C mRNA expression level. These data suggest that hyperactivation of the polyol pathway induces a deficit in neurotropism on peripheral sensory diabetic neuropathy.

Auditory brainstem evoked responses in insulin-dependent (ID) and non-insulin-dependent (NID) diabetic subjects with normal hearing

Hearing impairment has been reported to be one of the late complications of diabetes mellitus (DM), and the frequency varies. Previous data suggest that auditory brainstem potentials deteriorate long before the hearing impairment appears in patients with DM. Delay in neural conductance along the auditory pathway due to DM was assessed by means of auditory brainstem response (ABR) in 43 patients with normal hearing in a controlled study. Patients were classified according to age, presence of neuropathy. metabolic control, and duration and type of DM. ABR recordings revealed that absolute latencies of waves I, III and V were prolonged significantly in the diabetic group when compared to the control group (p < 0.05). When two diabetic groups (insulin-dependent and non-insulin-dependent) were compared with each other, the difference between the latency of wave I and the inter-peak latencies of I-III, III-V and I-V was not significant (p > 0.05). However, the difference between the latencies of waves III and V in the two diabetic groups was statistically significant. The duration of diabetes, blood glucose level and age were not associated with prolonged ABR latencies (p > 0.05). Prolongation of latency of ABR in patients with DM should alert us to possible damage to the auditory nerve, and close follow-up is needed in these patients.

Development of selective axonopathy in adult sensory neurons isolated from diabetic rats: role of glucose-induced oxidative stress

OBJECTIVE

Reactive oxygen species (ROS) are pro-oxidant factors in distal neurodegeneration in diabetes. We tested the hypothesis that sensory neurons exposed to type 1 diabetes would exhibit enhanced ROS and oxidative stress and determined whether this stress was associated with abnormal axon outgrowth.

RESEARCH DESIGN AND METHODS

Lumbar dorsal root ganglia sensory neurons from normal or 3- to 5-month streptozotocin (STZ)-diabetic rats were cultured with 10 or 25-50 mmol/l glucose. Cell survival and axon outgrowth were assessed. ROS were analyzed using confocal microscopy. Immunofluorescent staining detected expression of manganese superoxide dismutase (MnSOD) and adducts of 4-hydroxy-2-nonenal (4-HNE), and MitoFluor Green dye detected mitochondria.

RESULTS

Dorsal root ganglion neurons from normal rats exposed to 25-50 mmol/l glucose did not exhibit oxidative stress or cell death. Cultures from diabetic rats exhibited a twofold (P < 0.001) elevation of ROS in axons after 24 h in 25 mmol/l glucose compared with 10 mmol/l glucose or mannitol. Perikarya exhibited no change in ROS levels. Axonal outgrowth was reduced by approximately twofold (P < 0.001) in diabetic cultures compared with control, as was expression of MnSOD. The antioxidant N-acetyl-cysteine (1 mmol/l) lowered axonal ROS levels, normalized aberrant axonal structure, and prevented deficits in axonal outgrowth in diabetic neurons (P < 0.05).

CONCLUSIONS

Dorsal root ganglia neurons with a history of diabetes expressed low MnSOD and high ROS in axons. Oxidative stress was initiated by high glucose concentration in neurons with an STZ-induced diabetic phenotype. Induction of ROS was associated with impaired axonal outgrowth and aberrant dystrophic structures that may precede or predispose the axon to degeneration and dissolution in human diabetic neuropathy.

Serum VEGF increases in diabetic polyneuropathy, particularly in the neurologically active symptomatic stage

AIM

To identify the relationship between vascular endothelial growth factor (VEGF) and diabetic polyneuropathy (DPN).

METHODS

Two hundred and twenty diabetic patients participated, 113 with DPN and 107 without DPN. All patients were also classified according to the four stages of DPN (no neuropathy: stage 0; asymptomatic neuropathy: stage 1; symptomatic neuropathy: stage 2; disabling neuropathy: stage 3). Serum VEGF concentration was measured using an enzyme-linked immunosorbent assay (ELISA) and levels between the patients with and without DPN and also between the different stages of DPN, were compared.

RESULTS

The mean serum VEGF level in all patients was 264.6 +/- 218.8 pg/ml. The mean serum VEGF level was higher in patients with DPN (310.1 +/- 224.3 pg/ml) than in the patients without DPN (216.5 +/- 204.0 pg/ml, P = 0.0014). Serum VEGF was higher in the 'symptomatic' stage (stage 2, 364.8 +/- 225.9 pg/ml) in comparison with the 'asymptomatic' (stage 1, 256.7 +/- 224.4 pg/ml, P = 0.015) and 'disabling' (stage 3, 180.3 +/- 109.4 pg/ml, P = 0.042) stages. The mean serum VEGF level in patients with diabetic retinopathy (261.1 +/- 210.6 pg/ml) and in patients with diabetic nephropathy (241.5 +/- 185.7 pg/ml) was not increased.

CONCLUSIONS

The serum VEGF level is increased in patients with DPN, particularly in patients in the neurologically active 'symptomatic' stage.

Thirteen-month inhibition of aldose reductase by zenarestat prevents morphological abnormalities in the dorsal root ganglia of streptozotocin-induced diabetic rats

The dorsal root ganglia (DRG) have been identified as the target tissue in diabetic somatosensory neuropathy. It has been reported that, in the chronically diabetic state, DRG sensory neurons may undergo morphological changes. In this study, we examined the effect of zenarestat, an aldose reductase inhibitor, on the morphological derangement of the DRG and the sural nerve of streptozotocin-induced diabetic rats (STZ rats) over a 13-month period. The cell area of the DRG in STZ rats was smaller than that in normal rats. A decrease in fiber size was apparent in the sural nerve of the STZ rats, and the fiber density was greater. These morphological changes were reversed in zenarestat-treated STZ rats. The data suggest that, in peripheral sensory diabetic neuropathy, hyperactivation of the polyol pathway induces abnormalities not only in peripheral nerve fiber, but also in the DRG, which is an aggregate of primary sensory afferent cell bodies.

Efficacy and safety of pregabalin 600 mg/d for treating painful diabetic peripheral neuropathy: a double-blind placebo-controlled trial

Background

Recent consensus guidelines recommend pregabalin as a first-tier treatment for painful diabetic peripheral neuropathy (DPN). We evaluated the efficacy of pregabalin 600 mg/d (300 mg dosed BID) versus placebo for relieving DPN-associated neuropathic pain, and assessed its safety using objective measures of nerve conduction (NC).

Methods

In this randomized, double-blind, placebo-controlled trial, the primary efficacy measure was endpoint mean pain score (MPS) from daily pain diaries (11-point scale). NC velocity and sensory and motor amplitudes were assessed at baseline, endpoint, and end of follow-up (2 weeks post-treatment). At each timepoint, the median-motor, median-sensory, ulnar-sensory, and peroneal-motor nerves were evaluated. Secondary efficacy measures included weekly MPS and proportion of responders (patients achieving ≥50% reduction in MPS from baseline to endpoint). After 1-weeks' dosage escalation, pregabalin-treated patients received 300 mg BID for 12 weeks.

Results

Eighty-two patients received pregabalin and 85 placebo. Mean durations were 10 years for diabetes and ~5 years for painful DPN. Pregabalin-treated patients had lower MPS than controls (mean difference, -1.28; p <.001). For all four nerves, 95% CIs for median differences in amplitude and velocity from baseline to endpoint and baseline to follow-up included 0 (ie, no significant difference vs. placebo). Significant pain improvement among pregabalin-treated patients was evident at week 1 and sustained at every weekly timepoint. More pregabalin-treated patients (49%) than controls (23%) were responders (p <.001).

Conclusion

Pregabalin 600 mg/d (300 mg BID) effectively reduced pain, was well tolerated, and had no statistically significant or clinically meaningful effect on NC in patients with painful DPN.

Trial registration

ClinicalTrials.gov NCT00159679

Short-term intensive glycemic control improves vibratory sensation in type 2 diabetes

Strict long-term glycemic control has been reported to prevent or improve diabetic peripheral neuropathy, but the effects of short-term glycemic control have not been clarified in patients with type 2 diabetes. To investigate reversibility of impaired vibratory sensation by short-term glycemic control, we used the TM31 liminometer and C64 tuning fork methods to measure peripheral neuropathy. Thirty-one type 2 diabetes patients with poor glycemic control (HbA1c: 10.8+/-0.4%, mean+/-S.E.M., range from 7.9% to 16.2%) were administered strict glycemic control. Vibratory sensation before and after short-term glycemic control was evaluated, and the metabolic profile including plasma glucose, HbA1c, total cholesterol, HDL cholesterol, triglyceride, and free fatty acid (FFA) was measured. After 20.0+/-2.1 days of strict glycemic control, vibratory sensation improved significantly in both upper and lower extremities, assessed by TM31 liminometer and C64 tuning fork. Along with the improved glycemic control, lipid metabolism (total cholesterol, triglyceride and FFA) was significantly improved. Thus, short-term intensive glycemic control can improve vibratory sensation, metabolic changes in glucose and lipid metabolism being the factors responsible for improved of peripheral nerve function.

Correction of protein kinase C activity and macrophage migration in peripheral nerve by pioglitazone, peroxisome proliferator activated-gamma-ligand, in insulin-deficient diabetic rats

Pioglitazone, one of thiazolidinediones, a peroxisome proliferator-activated receptor (PPAR)-gamma ligand, is known to have beneficial effects on macrovascular complications in diabetes, but the effect on diabetic neuropathy is not well addressed. We demonstrated the expression of PPAR-gamma in Schwann cells and vascular walls in peripheral nerve and then evaluated the effect of pioglitazone treatment for 12 weeks (10 mg/kg/day, orally) on neuropathy in streptozotocin-diabetic rats. At end, pioglitazone treatment improved nerve conduction delay in diabetic rats without affecting the expression of PPAR-gamma. Diabetic rats showed suppressed protein kinase C (PKC) activity of endoneurial membrane fraction with decreased expression of PKC-alpha. These alterations were normalized in the treated group. Enhanced expression of phosphorylated extracellular signal-regulated kinase detected in diabetic rats was inhibited by the treatment. Increased numbers of macrophages positive for ED-1 and 8-hydroxydeoxyguanosine-positive Schwann cells in diabetic rats were also corrected by the treatment. Pioglitazone lowered blood lipid levels of diabetic rats, but blood glucose and nerve sorbitol levels were not affected by the treatment. In conclusion, our study showed that pioglitazone was beneficial for experimental diabetic neuropathy via correction of impaired PKC pathway and proinflammatory process, independent of polyol pathway.

Pathology and pathogenetic mechanisms of diabetic neuropathy: correlation with clinical signs and symptoms

Drastic increase in diabetic patients poses serious problems in the care of neuropathy so that there needs to explore the pathogenesis and to establish the effective treatment. Recent clinical and basic studies revealed characteristic pathophysiology of diabetic neuropathy and some clue to the direction of the treatment. The pathology of diabetic neuropathy is characterized by progressive nerve fiber loss that gives rise to positive and negative clinical signs and symptoms such as pain, paresthesia and loss of sensation. The nerve fiber loss takes the form of pan-modal pattern with proximo-distal gradient. Endoneurial microangiopathic change is also a constant feature of peripheral nerve pathology and negatively correlates with nerve fiber density. The vascular change and distal nerve fiber loss of small caliber, in particular, at the site of epidermis, commence even in subjects with impaired glucose tolerance and precede loss of nerve fibers in the nerve trunk of lower extremities. Pathogenetic mechanisms underlying the progressive nerve fiber loss seem to be multifactorial, including polyol pathway, glycation, reactive oxygen species, and altered protein kinase C activity. Clinical trials based on this background confirmed that fundamental treatment is in fact beneficial for the prevention and halting of this intractable disorder.

Parasympathetic dysfunction is associated with baroreflex and chemoreflex impairment in streptozotocin-induced diabetes in rats

This study explored physiological mechanisms of diabetic dysfunction in baroreceptors and chemoreceptors-mediated hemodynamic responses, and cholinergic neurotransmission in 30-day diabetic rats (n = 14) and controls (n = 14). Basal hemodynamic data and vagal response to electrical stimulation and methacholine injection were also evaluated. Muscarinic receptors were characterized using a radioligand receptor binding assay ([3H]N methylscopolamine). Experimental diabetes (50 mg/kg of STZ, i.v.) decreased systolic, diastolic, and mean arterial pressure and basal heart rate. Heart rate (HR) responses to vagal electrical stimulation (16, 32, and 64 Hz) were 15%, 11%, and 14% higher in diabetics vs non-diabetics, as were HR responses to methacholine injection (-130+/-24, -172+/-18, -206+/-15 bpm vs. -48+/-15, -116+/-12, -151+/-18 bpm, P < 0.05). Muscarinic receptor density was higher (267.4+/-11 vs 193.5+/-22 fmol/mg/prot, P < 0.05) in the atria of diabetic rats than in those of controls; the affinity was similar between groups. Diabetes-induced reduction of reflex responses to baro- (reflex bradycardia: -3.4+/-0.3 and -2.7+/-0.2 bpm/mm Hg; reflex tachycardia: -1.6+/-0.1 and -1.4+/-0.07 bpm/mm Hg, in control and diabetics, P < 0.05) and chemoreceptor stimulation, enhancement of HR responsiveness to cardiac vagal electrical stimulation and methacholine stimulation, plus an increase in the number of atrial muscarinic receptors indicates reduced parasympathetic activity, which is probably derived from central nervous system derangement.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

Oral magnesium administration prevents thermal hyperalgesia induced by diabetes in rats

BACKGROUND

Peripheral neuropathy is a common complication of diabetes mellitus. It has been shown that hyperglycemia may contribute to its development but the exact pathophysiology underlying this complication is not fully understood. Since oral magnesium supplementation can normalize hyperglycemia induced by diabetes in rats, this study was designed to examine the effect of oral magnesium administration on thermal hyperalgesia in streptozocin-induced diabetic rats.

MATERIAL AND METHODS

Twenty-four male adult wistar rats were divided equally into control, magnesium-treated control, diabetic and magnesium-treated diabetic groups. In magnesium-treated diabetic rats, magnesium sulfate (10g/l) was added into the drinking water once diabetes was established (10 days after STZ injection) and continued for 8 weeks. Mg-treated control animals received magnesium sulfate in the same dose and over the same time period. The other two groups; control and diabetic animals, only received tap water. At the end of the 8 weeks, thermal pain threshold was assessed by tail flick test and magnesium and glucose plasma levels were measured in all groups.

RESULT

A significant decrease (p<0.001) in thermal pain threshold and plasma magnesium levels and an increase in plasma glucose levels (p<0.001) were seen in diabetic rats 8 weeks after diabetes induction. After 8 weeks of oral magnesium, thermal hyperalgesia was normalized and plasma magnesium and glucose levels were restored towards normal.

CONCLUSION

It is concluded that oral magnesium administration given at the time of diabetes induction may be able to restore thermal hyperalgesia, magnesium deficiency and hyperglycemia and in diabetic rats.

Interleukin-6 attenuates the development of experimental diabetes-related neuropathy

Neuropathy is the most severe and the least understood complication of diabetes. We investigated the potential neuroprotective effect of IL-6 therapy in an experimental model of diabetic neuropathy. A single i.v. injection of streptozotocin (STZ, 55 mg/kg) was used to induce experimental diabetes in adult males. IL-6 (1, 10 or 30 microg/kg) was administrated either intraperitoneally on a daily basis or subcutaneously (s.c.) on a daily, on a three times or one time per week basis, starting at day 10 post-STZ. A decrease in sensory nerve conduction velocity (SNCV), indicative of neuropathy, is seen in STZ rats as early as day 10 post-STZ, a time at which blood glycaemia is already maximal. At later time points, this electrophysiological impairment became severe and clinically apparent by affecting tail flick latency. Motor dysfunction defined by a significant increase in compound muscle action potential (CMAP) latency was also recorded. At the completion of the study (day 40 post-STZ), histological examination revealed significant axonopathy and myelin loss, along with an increase in the proportion of fibers with abnormal appearance in sciatic nerves of STZ rats. These changes were not observed in non-diabetic rats and were significantly prevented by IL-6 treatment. The optimal dose appeared to be 10 microg/kg s.c. three injections per week, which showed a better effect in most of the parameters studied than 4-methylcatechol, a NGF-like neuroprotective compound. Once weekly and three times weekly administrations of IL-6 were as effective as daily treatment. Taken together, these results support the potential neuroprotective actions of IL-6. The fact that the half-life of IL-6 is only approximately 5 h while weekly dosing was neuroprotective strongly suggests activation by IL-6 of effector molecule(s) with longer duration of action.

Diabetic neuropathies: features and mechanisms

Diabetic neuropathies include both focal neuropathies and diffuse polyneuropathy. Polyneuropathy, the most common of the diabetic neuropathies excluding focal entrapment, has not yet been explained by a single disease mechanism despite intensive investigation. A number of abnormalities appear to cascade into a 'vicious cycle' of progressive microvascular disease associated with motor, sensory and autonomic fiber loss. These abnormalities include excessive polyol (sugar alcohol) flux through the aldose reductase pathway, functional and structural alterations of nerve microvessels, nerve and ganglia hypoxia, oxidative stress, nonspecific glycosylation of axon and microvessel proteins, and impairment in the elaboration of trophic factors critical for peripheral nerves and their ganglia. While an initiating role for nerve ischemia in the development of polyneuropathy has been proposed, the evidence for it can be questioned. The role of sensory and autonomic ganglia in the development of polyneuropathy has had relatively less attention despite the possibility that they may be vulnerable to a variety of insults, particularly neurotrophin deficiency. Superimposed on the deficits of polyneuropathy is the failure of diabetic nerves to regenerate as effectively as nondiabetics. Polyneuropathy has not yet yielded to specific forms of treatment but a variety of new trials addressing plausible hypotheses have been initiated. This review will summarize some of the clinical, pathological and experimental work applied toward understanding human diabetic neuropathy and will emphasize ideas on pathogenesis.

Effects of the Ca2+ antagonist nimodipine on functional deficits in the peripheral and central nervous system of streptozotocin-diabetic rats

Diabetes mellitus can lead to functional and structural deficits in both the peripheral and central nervous system. The pathogenesis of these deficits is multifactorial, probably involving, among others, microvascular dysfunction and alterations in intracellular calcium homeostasis. The present study examined the effects of treatment with the Ca2+ antagonist nimodipine (20 mg/kg, intraperitoneal injection, every 48 h) on functional deficits in the peripheral and central nervous system in streptozotocin-diabetic rats. In a prevention experiment, treatment was initiated immediately after diabetes induction and continued for 10 weeks. In a reversal experiment, treatment was initiated 16 weeks after diabetes induction and continued for 12 weeks. Sciatic nerve motor and sensory conduction velocity, brainstem auditory-evoked potentials, and visual-evoked potentials were measured in control, untreated, and nimodipine-treated diabetic rats. In addition, long-term potentiation, a form of synaptic plasticity used as a model for learning and memory at the cellular level, was examined in hippocampal slices. Nimodipine treatment partially prevented deficits in nerve conduction velocity and hippocampal long-term potentiation in diabetic rats. However, nimodipine intervention treatment was unable to reverse established deficits in nerve conduction velocity, evoked potential latencies, or long-term potentiation. It is concluded that nimodipine can partially prevent early functional deficits in the peripheral and central nervous system of streptozotocin-diabetic rats but is unable to reverse late deficits.

Diabetic complications in a new animal model (TSOD mouse) of spontaneous NIDDM with obesity

The TSOD mouse has been established as an inbred strain with spontaneous development of diabetes mellitus as the first clinical signs of diabetes. Polydipsia and polyuria are observed at about 2 months old only in male mice, after which hyperglycemia and hyperinsulinemia are detected. Following these symptoms obesity gradually develops until about 12 months old. In histopathological examination of the pancreas, severe hypertrophy of pancreatic islets was observed due to proliferation and swelling of B cells. In the kidney, thickening of the basement membrane in glomeruli and an increase of the mesangial area were observed at 18 months old. Motor neuropathy in TSOD mice began to appear at 14 months old and most male mice at 17 months old showed weakness of front and hind paws caused by neuron degeneration in the peripheral nerve. In sensory neuropathy, the threshold in the tail pressure test decreased significantly at 12 months old. Light microscopic and electron microscopic examination of sciatic nerves showed a decrease in the density of nerve fibers by the endoneural fibrosis and loss of these fibers. Degenerative changes of myelinated fibers, separation of myelin sheaths with intralamellar edema and remyelination were frequently observed. In the severely affected nerve fibers, the lamellar structure was completely destroyed and macrophages migrated around the myelin sheath or invaded the intramyelin space. Considering these findings similar to non-insulin dependent diabetes mellitus (NIDDM) in humans, the TSOD mouse should be a useful model for the pathogenic study of diabetic complications, especially of peripheral neuropathy.