Diabetic neuropathy.

The global epidemic of prediabetes and diabetes has led to a corresponding epidemic of complications of these disorders. The most prevalent complication is neuropathy, of which distal symmetric polyneuropathy (for the purpose of this Primer, referred to as diabetic neuropathy) is very common. Diabetic neuropathy is a loss of sensory function beginning distally in the lower extremities that is also characterized by pain and substantial morbidity. Over time, at least 50% of individuals with diabetes develop diabetic neuropathy. Glucose control effectively halts the progression of diabetic neuropathy in patients with type 1 diabetes mellitus, but the effects are more modest in those with type 2 diabetes mellitus. These findings have led to new efforts to understand the aetiology of diabetic neuropathy, along with new 2017 recommendations on approaches to prevent and treat this disorder that are specific for each type of diabetes. In parallel, new guidelines for the treatment of painful diabetic neuropathy using distinct classes of drugs, with an emphasis on avoiding opioid use, have been issued. Although our understanding of the complexities of diabetic neuropathy has substantially evolved over the past decade, the distinct mechanisms underlying neuropathy in type 1 and type 2 diabetes remains unknown. Future discoveries on disease pathogenesis will be crucial to successfully address all aspects of diabetic neuropathy, from prevention to treatment.

Physical activity and dietary interventions in diabetic neuropathy: a systematic review.

PURPOSE: Diabetic neuropathy is a common and disabling disorder, and there are currently no proven effective disease-modifying treatments. Physical activity and dietary interventions in patients with diabetes and diabetic neuropathy have multiple beneficial effects and are generally low risk, which makes lifestyle interventions an attractive treatment option. We reviewed the literature on the effects of physical activity and dietary interventions on length-dependent peripheral neuropathy and cardiac autonomic neuropathy in diabetes. METHODS: The electronic database PubMed was systematically searched for original human and mouse model studies examining the effect of either dietary or physical activity interventions in subjects with diabetes, prediabetes, or metabolic syndrome. RESULTS: Twenty studies are included in this review. Fourteen studies were human studies and six were in mice. Studies were generally small with few controlled trials, and there are no widely agreed upon outcome measures. CONCLUSIONS: Recent research indicates that dietary interventions are effective in modifying diabetic neuropathy in animal models, and there are promising data that they may also ameliorate diabetic neuropathy in humans. It has been known for some time that lifestyle interventions can prevent the development of diabetic neuropathy in type 2 diabetes mellitus subjects. However, there is emerging evidence that lifestyle interventions are effective in individuals with established diabetic neuropathy. In addition to the observed clinical value of lifestyle interventions, there is emerging evidence of effects on biochemical pathways that improve muscle function and affect other organ systems, including the peripheral nerve. However, data from randomized controlled trials are needed.

Potential roles of PINK1 for increased PGC-1α-mediated mitochondrial fatty acid oxidation and their associations with Alzheimer disease and diabetes.

Down-regulation of PINK1 and PGC-1α proteins is implicated in both mitochondrial dysfunction and oxidative stress potentially linking metabolic abnormality and neurodegeneration. Here, we report that PGC-1α and PINK1 expression is markedly decreased in Alzheimer disease (AD) and diabetic brains. We observed a significant down-regulation of PGC-1α and PINK1 protein expression in H2O2-treated cells but not in those cells treated with N-acetyl cysteine. The protein levels of two key enzymes of the mitochondrial ß-oxidation machinery, acyl-coenzyme A dehydrogenase, very long chain (ACADVL) and mitochondrial trifunctional enzyme subunit α are significantly decreased in AD and diabetic brains. Moreover, we observed a positive relationship between ACADVL and 64 kDa PINK1 protein levels in AD and diabetic brains. Overexpression of PGC-1α decreases lipid-droplet accumulation and increases mitochondrial fatty acid oxidation; down-regulation of PINK1 abolishes these effects. Together, these results provide new insights into potential cooperative roles of PINK1 and PGC-1α in mitochondrial fatty acid oxidation, suggesting possible regulatory roles for mitochondrial function in the pathogenesis of AD and diabetes.

Identification of novel targets for PGC-1alpha and histone deacetylase inhibitors in neuroblastoma cells.

Recent evidence suggests that the transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator 1alpha (PGC-1alpha) is involved in the pathology of Huntington's Disease (HD). While animals lacking PGC-1alpha express lower levels of genes involved in antioxidant defense and oxidative phosphorylation in the brain, little is known about other targets for PGC-1alpha in neuronal cells and whether there are ways to pharmacologically target PGC-1alpha in neurons. Here, PGC-1alpha overexpression in SH-SY5Y neuroblastoma cells upregulated expression of genes involved in mitochondrial function, glucose transport, fatty acid metabolism, and synaptic function. Overexpression also decreased vulnerability to hydrogen peroxide-induced cell death and caspase 3 activation. Treatment of cells with the histone deacetylase inhibitors (HDACi's) trichostatin A and valproic acid upregulated PGC-1alpha and glucose transporter 4 (GLUT4). These results suggest that PGC-1alpha regulates multiple pathways in neurons and that HDACi's may be good candidates to target PGC-1alpha and GLUT4 in HD and other neurological disorders.

Regulation of PGC-1alpha and PGC-1alpha-responsive genes with forskolin-induced Schwann cell differentiation.

Recent evidence indicates that mitochondrial homeostasis is critical for myelination and maintenance of peripheral nerve function. Mice lacking the metabolic transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator 1alpha (PGC-1alpha) show reductions in expression of myelin-related proteins and exhibit myelin-associated lesions, so we identified PGC-1alpha target genes in Schwann cells (SCs) in vitro to determine potential roles for PGC-1alpha in glia and tested whether PGC-1alpha was sufficient for SC differentiation and myelination. Forskolin-induced differentiation was associated with an upregulation of PGC-1alpha mRNA and protein, and while overexpression of PGC-1alpha upregulated genes such as manganese superoxide dismutase and estrogen-related receptor alpha, it was not sufficient for induction of differentiation. Both PGC-1alpha overexpression and forskolin exposure caused an increase in the mitochondrial fusion-related protein mitofusin 1. These studies suggest that PGC-1alpha might be a potential target to promote mitochondrial stability during differentiation and myelination.

Transforming growth factor-beta induces cellular injury in experimental diabetic neuropathy.

The mechanism/s leading to diabetic neuropathy are complex. Transforming growth factor-beta1 (TGF-beta1) has been associated with diabetic nephropathy and retinopathy but not neuropathy. In this study, changes in TGF-beta isoforms were examined in vivo and in vitro. Two groups of animals, streptozotocin diabetic with neuropathy and non-diabetic controls were examined at 4 weeks (n=10/group) and 12 weeks (n=8/group). In diabetic DRG using quantitative real-time PCR (QRT-PCR), TGF-beta1 and TGF-beta2 mRNA, but not TGF-beta3, was increased at 4 and 12 weeks. In sciatic nerve TGF-beta3 mRNA was primarily increased. Immunohistochemistry (DRG) and immunoblotting (sciatic nerve) showed similar differential protein expression. In sciatic nerve TGF-beta formed homo- and hetero-dimers, of which beta(2)/beta(3), beta(1)/beta(1), and beta(1)/beta(3) were significantly increased, while that of the TGF-beta(2)/beta(2) homodimer was decreased, in diabetic compared to non-diabetic rats. In vitro, pretreatment of embryonic DRG with TGF-beta neutralizing antibody prevents the increase in total TGF-beta protein observed with high glucose using immunoblotting. In high glucose conditions, combination with TGF-beta2>beta1 increases the percent of cleaved caspase-3 compared to high glucose alone and TGF-beta neutralizing antibody inhibits this increase. Furthermore, consistent with the findings in diabetic DRG and nerve, TGF-beta isoforms applied directly in vitro reduce neurite outgrowth, and this effect is partially reversed by TGF-beta neutralizing antibody. These findings implicate upregulation of TGF-beta in experimental diabetic peripheral neuropathy and indicate a novel mechanism of cellular injury related to elevated glucose levels. In combination, these findings indicate a potential new target for treatment of diabetic peripheral neuropathy.

Metabotropic glutamate receptor 3 protects neurons from glucose-induced oxidative injury by increasing intracellular glutathione concentration.

High glucose concentrations cause oxidative injury and programmed cell death in neurons, and can lead to diabetic neuropathy. Activating the type 3 metabotropic glutamate receptor (mGluR3) prevents glucose-induced oxidative injury in dorsal root ganglion neurons co-cultured with Schwann cells. To determine the mechanisms of protection, studies were performed in rat dorsal root ganglion neuron-Schwann cell co-cultures. The mGluR3 agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate prevented glucose-induced inner mitochondrial membrane depolarization, reactive oxygen species accumulation, and programmed cell death, and increased glutathione (GSH) concentration in co-cultured neurons and Schwann cells, but not in neurons cultured without Schwann cells. Protection was diminished in neurons treated with the GSH synthesis inhibitor l-buthionine-sulfoximine, suggesting that mGluR-mediated protection requires GSH synthesis. GSH precursors and the GSH precursor GSH-ethyl ester also protected neurons from glucose-induced injury, indicating that GSH synthesis in Schwann cells, and transport of reaction precursors to neurons, may underlie mGluR-mediated neuroprotection. These results support the conclusions that activating glial mGluR3 protects neurons from glucose-induced oxidative injury by increasing free radical scavenging and stabilizing mitochondrial function, through increased GSH antioxidant defense.

Advances in understanding drug-induced neuropathies.

Many commonly used medications have neurotoxic adverse effects; the most common of these is peripheral neuropathy. Neuropathy can be a dose-limiting adverse effect for many medications used in life-threatening conditions, such as malignancy and HIV-related disease. Epidemiological evidence supports previous case reports of HMG-CoA reductase inhibitors (or 'statins') causing an axonal sensorimotor neuropathy or a purely small-fibre neuropathy in some patients. The neuropathy improves when the medication is withdrawn. Despite the association between HMG-CoA reductase inhibitors and neuropathy, the risk is low compared with the significant vascular protective benefits. Oxaliplatin, a new platinum chemotherapy agent designed to have fewer adverse effects than other such agents, has been shown to cause a transient initial dysaesthesia in addition to an axonal polyneuropathy. Thalidomide, an old therapy currently being utilised for new therapeutic indications (e.g. treatment of haematological malignancies), is associated with a painful, axonal sensorimotor neuropathy that does not improve on withdrawal of the drug. Nucleoside reverse transcriptase inhibitors are important components of highly active antiretroviral therapy, but are associated with a sensory neuropathy that is likely to be due to a direct effect of these drugs on mitochondrial DNA replication. New research demonstrates that lactate levels may help discriminate between neuropathy caused by nucleoside analogues and HIV-induced neuropathy. Understanding the mechanism of drug-induced neuropathy has led to advances in preventing this disabling condition.

Uncoupling proteins prevent glucose-induced neuronal oxidative stress and programmed cell death.

The central role of mitochondria in most pathways leading to programmed cell death (PCD) has focused our investigations into the mechanisms of glucose-induced neuronal degeneration. It has been postulated that hyperglycemic neuronal injury results from mitochondria membrane hyperpolarization and reactive oxygen species formation. The present study not only provides further evidence to support our model of glucose-induced PCD but also demonstrates a potent ability for uncoupling proteins (UCPs) to prevent this process. Dorsal root ganglion (DRG) neurons were screened for UCP expression by Western blotting and immunocytochemistry. The abilities of individual UCPs to prevent hyperglycemic PCD were assessed by adenovirus-mediated overexpression of UCP1 and UCP3. Interestingly, UCP3 is expressed not only in muscle, but also in DRG neurons under control conditions. UCP3 expression is rapidly downregulated by hyperglycemia in diabetic rats and by high glucose in cultured neurons. Overexpression of UCPs prevents glucose-induced transient mitochondrial membrane hyperpolarization, reactive oxygen species formation, and induction of PCD. The loss of UCP3 in DRG neurons may represent a significant contributing factor in glucose-induced injury. Furthermore, the ability to prevent UCP3 downregulation or to reproduce the uncoupling response in DRG neurons constitutes promising novel approaches to avert diabetic complications such as neuropathy.

Metabotropic glutamate receptor regulation of neuronal cell death.

The metabotropic glutamate receptors (mGluRs) are a family of glutamate-sensitive receptors that regulate neuronal function separately from the ionotropic glutamate receptors. By coupling to guanosine nucleotide-binding proteins (G proteins), mGluRs are able to regulate neuronal injury and survival, likely through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrial regulated programmed cell death (PCD). The physiological relevance of this system is supported by evidence that mGluRs are associated with cell survival in several central nervous system neurodegenerative diseases. Evidence is presented that mGluRs are also able to prevent PCD in the peripheral nervous system, and that this may provide a novel mechanism for treatment of diabetic neuropathy. In dorsal root ganglion (DRG) neurons, a high glucose load increases generation of reactive oxygen species (ROS), destabilizes the inner mitochondrial membrane potential (Deltapsi(M)), induces cytochrome c release from the mitochondrial intermembrane space, and induces downstream activation of caspases. In high-glucose conditions, the group II metabotropic glutamate agonist N-acetylaspartylglutamate (NAAG) blocks caspase activation and is completely reversed by the mGluR3 antagonist (S)-alpha-ethylglutamic acid (EGLU). Furthermore, the direct mGluR3 agonist (2R,4R)-4-aminopyrrolidine-2, 4-dicarboxylate (APDC) prevents induction of ROS. Together these findings are consistent with an emerging concept that mGluRs may protect against cellular injury by regulating oxidative stress in the neuron. More complete understanding of the complex PCD regulatory pathways mediated by mGluRs will provide new therapeutic approaches for the treatment of a wide variety of neurodegenerative diseases.

High glucose-induced oxidative stress and mitochondrial dysfunction in neurons.

The current study examines the association between glucose induction of reactive oxygen species (ROS), mitochondrial (Mt) depolarization, and programmed cell death in primary neurons. In primary dorsal root ganglion (DRG) neurons, 45 mM glucose rapidly induces a peak rise in ROS corresponding to a 50% increase in mean Mt size at 6 h (P<0.001). This is coupled with loss of regulation of the Mt membrane potential (Mt membrane hyperpolarization, followed by depolarization, MMD), partial depletion of ATP, and activation of caspase-3 and -9. Glucose-induced activation of ROS, MMD, and caspase-3 and -9 activation is inhibited by myxothiazole and thenoyltrifluoroacetone (P<0.001), which inhibit specific components of the Mt electron transfer chain. Similarly, MMD and caspase-3 activation are inhibited by 100 microM bongkrekic acid (an inhibitor of the adenosine nucleotide translocase ANT). These results indicate that mild increases in glucose induce ROS and Mt swelling that precedes neuronal apoptosis. Glucotoxicity is blocked by inhibiting ROS induction, MMD, or caspase cleavage by specific inhibitors of electron transfer, or by stabilizing the ANT.

Physiological characterization of neuropathy in Fabry's disease.

Fabry's disease is commonly associated with a painful, debilitating neuropathy. Characterization of the physiological abnormalities is an important step in evaluating response to specific therapies. Twenty-two patients with Fabry's disease, and with relatively preserved renal function, underwent conventional and near-nerve conduction studies, electromyography, sympathetic skin responses, and quantitative sensory testing (QST). Nerve conduction studies were mostly normal except for an increased frequency of median nerve entrapment at the wrist in 6 (27%) patients. Sympathetic skin responses were preserved in 19 of 20 (95%) of the patients. The QST showed increased or immeasurable cold and warm detection thresholds in patients, significantly different from controls (n = 28) in the hand (P < 0.001, P = 0.04, respectively) and foot (P < 0.001 for both). Cold thresholds were more often abnormal than were warm thresholds. Vibration thresholds were normal in the feet and, in some patients, elevated in the hand only, probably due to frequent median nerve entrapment at the wrist. Our findings suggest that the neuropathy of Fabry's disease is characterized by an increased prevalence of median nerve entrapment at the wrist and by thermal afferent fiber dysfunction in a length-dependent fashion, with greater impairment of cold than warm sensation.

Recent advances in drug-induced neuropathies.

PURPOSE OF REVIEW: Peripheral neuropathy is a common neurotoxic effect of medications. When medications are used to treat life-threatening illnesses, balancing the toxic effects of peripheral neuropathy with the therapeutic benefits of the drug can be difficult. This article examines recent research into the cellular mechanisms associated with neuropathy after treatment with medications to treat cancer, and HIV, and to prevent transplant rejection. RECENT FINDINGS: Cisplatin and suramin induce a length, dose, and time-dependent axonal sensorimotor polyneuropathy. Cisplatin and suramin both result in apoptosis in dorsal root ganglion neurons that may partially explain the neuropathy that develops with treatment. In contrast, nerve growth factor prevents initiation of the programmed cell death associated with cisplatin neurotoxicity. Suramin causes accumulation of lamellar inclusion bodies in dorsal root ganglion neurons related to dose of administration and severity of the neuropathy. Nucleoside reverse transcriptase inhibitors affect mitochondrial function and lead to depletion of the nerve's mitochondrial DNA and inhibition of DNA polymerase. These effects on the mitochondrion may be related to the polyneuropathy that develops in these patients. In contrast to the axonal neuropathies, tacrolimus and rarely suramin can result in a demyelinating neuropathy that may mimic Guillain-Barré syndrome or chronic inflammatory demyelinating polyneuropathy. Many of these neuropathies can be reversed by early recognition of the symptoms or by using sensitive electrophysiological testing. In certain instances, specific therapies may ameliorate the neuropathy. Glutamine may reduce paclitaxel-induced toxicity, while some patients with tacrolimus or suramin-induced demyelinating neuropathy may respond to intravenous immunoglobulin or plasmapheresis. SUMMARY: Improved understanding of neurotoxic mechanisms in the peripheral nervous system associated with chemotherapeutic and anti-HIV medications, coupled with early improved diagnosis, promises to help limit neurotoxicity associated with these medications.