Caveolin-1 and altered neuregulin signaling contribute to the pathophysiological progression of diabetic peripheral neuropathy

Schwann Cell Protein Kinase RNA-like ER Kinase (PERK) Is Not Necessary for the Development of Diabetic Peripheral Neuropathy but Negates the Efficacy of Cemdomespib Therapy

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes arising in part from glycemic damage to neurons and Schwann cells (SC). While the pathogenic mechanisms of DPN are complex, mitochondrial dysfunction and endoplasmic reticulum (ER) stress contribute to the development of DPN and serve as therapeutic targets for disease modification. Cemdomespib is an orally bioavailable small molecule which alleviates clinical indices of DPN that correlate with improvements in neuronal oxidative stress and mitochondrial bioenergetics. However, the contribution of SC ER stress in the onset of DPN and the therapeutic efficacy of cemdomespib remains unknown. To address this issue, mice expressing a conditional deletion of protein kinase RNA-like ER kinase (PERK) in myelinating SCs (SC-cPERK KO) and control SC-PERKf/f mice were rendered diabetic with streptozotocin. Diabetic SC-PERKf/f and SC-cPERK KO mice developed a similar magnitude of DPN as quantified by the onset of a thermal/mechanical hypoalgesia, decreases in nerve conduction velocity (NCV) and intraepidermal fiber density (iENFD). After 8 weeks of diabetes, daily treatment with 1 mg/kg cemdomespib for an additional 8 weeks significantly improved thermal/mechanical hypoalgesia, NCV, iENFD and decreased markers of ER stress in diabetic SC-PERKf/f mice, but the drug had no effect in diabetic SC-cPERK KO mice. Nrf2 is a PERK substrate and studies using rat SCs subjected to ER stress demonstrated that cemdomespib increased Nrf2 activity. Collectively, these data suggest that activation of SC PERK by diabetes is not necessary for the onset of DPN, but serves as a target in the action of cemdomespib, potentially by increasing Nrf2 activity.

Crumbling Pathogenesis and Biomarkers for Diabetic Peripheral Neuropathy

Background: Diabetic sensorimotor polyneuropathy (DSP) is a common chronic diabetic complication. Traditionally, DSP was once considered irreversible with a typical loss of axon. However, the superimpose of acquired demyelination on axonal loss in DSP patients has been observed, implying that DSP may be preventable or reversible, particularly within a subgroup of patients exhibiting early-stage acquired demyelination, underscoring the critical importance of identifying early prognostic markers. Methods: We systemically review the literature on the roles of biomarkers in predicting DSP and monitoring the progress. The underlying mechanisms of biomarkers were also discussed. Results: The pathogenesis of DSP is multifaceted, with various pathological mechanisms contributing to its development. Key mechanisms include aberrant glucose metabolism and induction of oxidative stress and inflammation. Several pathological processes, such as disrupted glucose metabolism, nerve damage, impaired microcirculation, genetic variants, and microRNA dysregulation, lead to molecular and protein changes that may be detectable in blood and other biological compartments, thus serving as potential biomarkers for DSP progression. However, the utility of a biomarker depends on its predictive accuracy, practicality, and ease of measurement. Conclusions: Most biomarkers for predicting DSP have demonstrated suboptimal predictive value, and many lack established accuracy in forecasting DSP progression. Consequently, the diagnostic utility of any single biomarker remains limited. A comprehensive combination of biomarkers from various categories may hold incredible promise for accurate detection. As artificial intelligence (AI) techniques, especially machine learning, rapidly advance, these technologies may offer significant potential for developing diagnostic platforms to integrate and interpret complex biomarker data for DSP.

Cemdomespib Therapy Slows the Progression of Neuromuscular Weakness and Demyelination in the R75W-Connexin 32 Animal Model of Charcot-Marie-Tooth 1X Disease

Mutations in connexin 32 (Cx32) are a common cause of Charcot-Marie-Tooth 1X (CMT1X) disease, an inherited peripheral neuropathy characterized by progressive neuromuscular weakness and demyelination. There are no approved pharmacologic therapies for CMT1X, and identifying new treatments that slow the onset and severity of neuromuscular decline may aid disease management. Cemdomespib is an orally bioavailable small molecule that improved demyelination and neuromuscular junction (NMJ) morphology in mice lacking Cx32 expression. However, whether a similar efficacy may manifest in models of CMT1X arising from Cx32 mutations that cause the organellar accumulation of the protein was unclear. Additionally, it was unclear whether cemdomespib therapy slowed the rate of demyelination/NMJ degeneration or stabilized nerve and NMJ morphology to levels present at the initiation of drug therapy. To address these issues, 4-month-old R75W-Cx32 mice, which accumulate the mutant Cx32 in golgi, were treated for 0, 10, or 20 weeks with 0 or 3 mg/kg cemdomespib. Grip strength, motor nerve conduction velocity (MNCV), femoral nerve myelination, and NMJ morphology were quantified. Daily drug therapy significantly slowed the decline in grip strength over the course of treatment, while 20 weeks of drug treatment significantly improved MNCV and decreased the g-ratio and the number of thinly myelinated femoral nerve axons. Similarly, 20 weeks of cemdomespib therapy improved the NMJ morphology and the overlap between presynaptic (synaptophysin) and postsynaptic (α-bungarotoxin) markers. These data show that cemdomespib therapy slows the rate of neuromuscular decline and demyelination and may present a disease-modifying approach for patients with gain-of-function Cx32 mutations.

Deciphering the molecular landscape of human peripheral nerves: implications for diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a prevalent complication of diabetes mellitus that is caused by metabolic toxicity to peripheral axons. We aimed to gain deep mechanistic insight into the disease process using bulk and spatial RNA sequencing on tibial and sural nerves recovered from lower leg amputations in a mostly diabetic population. First, our approach comparing mixed sensory and motor tibial and purely sensory sural nerves shows key pathway differences in affected nerves, with distinct immunological features observed in sural nerves. Second, spatial transcriptomics analysis of sural nerves reveals substantial shifts in endothelial and immune cell types associated with severe axonal loss. We also find clear evidence of neuronal gene transcript changes, like PRPH, in nerves with axonal loss suggesting perturbed RNA transport into distal sensory axons. This motivated further investigation into neuronal mRNA localization in peripheral nerve axons generating clear evidence of robust localization of mRNAs such as SCN9A and TRPV1 in human sensory axons. Our work gives new insight into the altered cellular and transcriptomic profiles in human nerves in DPN and highlights the importance of sensory axon mRNA transport as an unappreciated potential contributor to peripheral nerve degeneration.

Pharmacologic Targeting of the C-Terminus of Heat Shock Protein 90 Improves Neuromuscular Function in Animal Models of Charcot Marie Tooth X1 Disease

Charcot-Marie-Tooth X1 (CMTX1) disease is an inherited peripheral neuropathy that arises from loss-of-function mutations in the protein connexin 32 (Cx32). CMTX1 currently lacks a pharmacologic approach toward disease management, and we have previously shown that modulating the expression of molecular chaperones using novologue therapy may provide a viable disease-modifying approach to treat metabolic and demyelinating neuropathies. Cemdomespib is an orally bioavailable novologue that manifests neuroprotective activity by modulating the expression of heat shock protein 70 (Hsp70). We examined if 1 to 5 months of daily cemdomespib therapy may improve neuropathic symptoms in three mouse models of CMTX1 (Cx32 deficient (Cx32def), T55I-Cx32def, and R75W-Cx32 mice). Daily drug therapy significantly improved motor nerve conduction velocity (MNCV) and grip strength in all three models, but the compound muscle action potential was only improved in Cx32def mice. Drug efficacy required Hsp70 as improvements in MNCV, and the grip strength was abrogated in Cx32def × Hsp70 knockout mice. Five months of novologue therapy was associated with improved neuromuscular junction morphology, femoral motor nerve myelination, reduction in foamy macrophages, and a decrease in Schwann cell c-jun levels. To determine if c-jun may be downstream of Hsp70 and necessary for drug efficacy, c-jun expression was specifically deleted in Schwann cells of Cx32def mice. While the deletion of c-jun worsened the neuropathy, cemdomespib therapy remained effective in improving MNCV and grip strength. Our data show that cemdomespib therapy improves CMTX1-linked neuropathy in an Hsp70-dependent but a c-jun-independent manner and without regard to the nature of the underlying Cx32 mutation.

FDA-Approved Kinase Inhibitors in Preclinical and Clinical Trials for Neurological Disorders

Cancers and neurological disorders are two major types of diseases. We previously developed a new concept termed "Aberrant Cell Cycle Diseases" (ACCD), revealing that these two diseases share a common mechanism of aberrant cell cycle re-entry. The aberrant cell cycle re-entry is manifested as kinase/oncogene activation and tumor suppressor inactivation, which are hallmarks of both tumor growth in cancers and neuronal death in neurological disorders. Therefore, some cancer therapies (e.g., kinase inhibition, tumor suppressor elevation) can be leveraged for neurological treatments. The United States Food and Drug Administration (US FDA) has so far approved 74 kinase inhibitors, with numerous other kinase inhibitors in clinical trials, mostly for the treatment of cancers. In contrast, there are dire unmet needs of FDA-approved drugs for neurological treatments, such as Alzheimer's disease (AD), intracerebral hemorrhage (ICH), ischemic stroke (IS), traumatic brain injury (TBI), and others. In this review, we list these 74 FDA-approved kinase-targeted drugs and identify those that have been reported in preclinical and/or clinical trials for neurological disorders, with a purpose of discussing the feasibility and applicability of leveraging these cancer drugs (FDA-approved kinase inhibitors) for neurological treatments.

Mechanistic insight into the role of metformin in Alzheimer's disease

Alzheimer's disease (AD), a type of dementia, is characterized by progressive memory decline and cognition impairment. Despite the considerable body of evidence regarding AD pathophysiology, current therapies merely slow down the disease progression, and a comprehensive therapeutic approach is unavailable. Accordingly, finding an efficient multifunctional remedy is necessary to blunt the increasing rate of AD incidence in the upcoming years. AD shares pathophysiological similarities (e.g., impairment of cognitive functions, insulin sensitivity, and brain glucose metabolism) with noninsulin-dependent diabetes mellitus (NIDDM), which offers the utilization of metformin, a biguanide hypoglycemic agent, as an alternative therapeutic approach in AD therapy. Emerging evidence has revealed the impact of metformin in patients suffering from AD. It has been described that metformin employs multiple mechanisms to improve cognition and memory impairment in pre-clinical AD models, including reduction of hippocampal amyloid-beta (Aβ) plaque and neurofibrillary tangles (NFTs) load, suppression of inflammation, amelioration of mitochondrial dysfunction and oxidative stress, restriction of apoptotic neuronal death, and induction of neurogenesis. This review discusses the pre-clinical evidence, which may shed light on the role of metformin in AD and provide a more comprehensive mechanistic insight for future studies in this area of research.

Novologue Therapy Requires Heat Shock Protein 70 and Thioredoxin-Interacting Protein to Improve Mitochondrial Bioenergetics and Decrease Mitophagy in Diabetic Sensory Neurons

Diabetic peripheral neuropathy (DPN) is a complication of diabetes whose pathophysiology is linked to altered mitochondrial bioenergetics (mtBE). KU-596 is a small molecule neurotherapeutic that reverses symptoms of DPN, improves sensory neuron mtBE, and decreases the pro-oxidant protein, thioredoxin-interacting protein (Txnip) in a heat shock protein 70 (Hsp70)-dependent manner. However, the mechanism by which KU-596 improves mtBE and the role of Txnip in drug efficacy remains unknown. Mitophagy is a quality-control mechanism that selectively targets damaged mitochondria for degradation. The goal of this study was to determine if KU-596 therapy improved DPN, mtBE, and mitophagy in an Hsp70- and Txnip-dependent manner. Mito-QC (MQC) mice express a mitochondrially targeted mCherry-GFP fusion protein that enables visualizing mitophagy. Diabetic MQC, MQC × Hsp70 knockout (KO), and MQC × Txnip KO mice developed sensory and nerve conduction dysfunctions consistent with the onset of DPN. KU-596 therapy improved these measures, and this was dependent on Hsp70 but not Txnip. In MQC mice, diabetes decreased mtBE and increased mitophagy and KU-596 treatment reversed these effects. In contrast, KU-596 was unable to improve mtBE and decrease mitophagy in MQC × Hsp70 and MQC × Txnip KO mice. These data suggest that Txnip is not necessary for the development of the sensory symptoms and mitochondrial dysfunction induced by diabetes. KU-596 therapy may improve mitochondrial tolerance to diabetic stress to decrease mitophagic clearance in an Hsp70- and Txnip-dependent manner.

Characterization of Mechanical Allodynia and Skin Innervation in a Mouse Model of Type-2 Diabetes Induced by Cafeteria-Style Diet and Low-Doses of Streptozotocin

Background: Painful distal symmetrical polyneuropathy (DPN) is a frequent complication of type-2 diabetes mellitus (T2DM) that commonly presents as neuropathic pain and loss of skin nerve fibers. However, there are limited therapies to effectively treat DPN and many of the current animal models of T2DM-induced DPN do not appear to mirror the human disease. Thus, we validated a DPN mouse model induced by a cafeteria-style diet plus low-doses of streptozotocin (STZ).

Methods: Female C57BL/6J mice were fed either standard (STD) diet or obesogenic cafeteria (CAF) diet for 32 weeks, starting at 8 weeks old. Eight weeks after starting diets, CAF or STD mice received either four low-doses of STZ or vehicle. Changes in body weight, blood glucose and insulin levels, as well as oral glucose- and insulin-tolerance tests (OGTT and ITT) were determined. The development of mechanical hypersensitivity of the hindpaws was determined using von Frey filaments. Moreover, the effect of the most common neuropathic pain drugs was evaluated on T2DM-induced mechanical allodynia. Finally, the density of PGP -9.5⁺ (a pan-neuronal marker) axons in the epidermis from the hindpaw glabrous skin was quantified.

Results: At 22–24 weeks after STZ injections, CAF + STZ mice had significantly higher glucose and insulin levels compared to CAF + VEH, STD + STZ, and STD + VEH mice, and developed glucose tolerance and insulin resistance. Skin mechanical sensitivity was detected as early as 12 weeks post-STZ injections and it was significantly attenuated by intraperitoneal acute treatment with amitriptyline, gabapentin, tramadol, duloxetine, or carbamazepine but not by diclofenac. The density of PGP-9.5⁺ nerve fibers was reduced in CAF + STZ mice compared to other groups.

Conclusion: This reverse translational study provides a painful DPN mouse model which may help in developing a better understanding of the factors that generate and maintain neuropathic pain and denervation of skin under T2DM and to identify mechanism-based new treatments.

Acoustic Waves in Axonal Membrane and Caveolins are the New Targets for Pain Treatment with High Frequency Ultrasound

Reciprocal interaction between electrical and mechanical waves observed in axonal membrane during its excitation leads to a paradigm shift in pain research making the uncoupling of electro-mechanical signals an interesting target in pain treatment. This uncoupling can be realized either through direct disturbance of the mechanical surface waves in axonal membrane or through shifting of the thermodynamic state of this membrane far from its phase transition point. Both effects can be effectively realized through application of the very high frequency ultrasound waves. Additional target for application of ultrasound in pain treatment is the caveolin-1, which is abundantly present in Schwann cells as well as in the non-axonal tissues. Both targets demonstrate frequency-dependent reactions, thus making a very high frequency ultrasound a promising treatment modality in pain treatment.

Metformin alleviates high glucose-induced ER stress and inflammation by inhibiting the interaction between caveolin1 and AMPKα in rat astrocytes

Hyperglycemia-induced endoplasmic reticulum (ER) stress and inflammatory response afflict neuropathological diseases (such as epilepsy and Alzheimer's disease). Astrocytes are the critical cells that mediate brain inflammation in this process. Metformin is a kind of hypoglycemic drugs widely used in clinical practice, which has anti-inflammatory and antioxidant effects. However, the biological mechanism of metformin in regulating inflammation and ER stress induced by hyperglycemia remains unclear. Therefore, in this study, rat primary astrocytes were preincubated with metformin and AMPK agonist AICAR for 1 h prior to administration of high glucose (33 mM glucose). Our findings indicated that metformin treatment inhibited the elevated ER stress and inflammation in high glucose-treated astrocytes. Moreover, metformin inhibited the formation of caveolin1/AMPKα complex. Additionally, the effects of AICAR on astrocytes were similar to metformin. In conclusion, metformin reduced high glucose-induced ER stress and inflammation by inhibiting the interaction between caveolin1 and AMPKα, suggesting that the caveolin1/AMPKα complex may be a potential therapeutic target for metformin.

Reduce Muscle Fibrosis through Exercise via NRG1/ErbB2 Modification in Diabetic Rats

Diabetic myopathy refers to the manifestations in the skeletal muscle as a result of altered glucose homeostasis which reflects as fibrosis. Since physical exercise has been indicated a protective strategy for improving glucose metabolism in skeletal muscle, we tested a hypothesis under which the endurance exercise training could reverse the produced skeletal muscle fibrosis by diabetes. Eight-week-old male Wistar rats were randomly assigned into four groups including healthy control (HC), healthy trained (HT), diabetic control (DC), and diabetic trained (DT) groups. Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ; 45 mg/kg). Rats in the HT and DT groups carried out an exercise program on a motorized treadmill for five days a week over six weeks. Skeletal muscle levels of NRG1and ErbB2 were measured by the Western blot method. Exercise training decreased blood glucose levels in the DT group. Induction of diabetes increased skeletal muscle fibrosis in both the fast extensor digitorum longus (EDL) and slow soleus muscles, while endurance training modified it in diabetic trained rats. Moreover, muscle NRG1and ErbB2 levels were increased in diabetic rats, while training modified muscle NRG1and ErbB2 levels in diabetic trained rats. Our study provides novel evidence that endurance training could modify skeletal muscle fibrosis through NRG1/ErbB2 modification in STZ-induced diabetic rats.

Role of Caveolin-1 in Diabetes and Its Complications

It is estimated that in 2017 there were 451 million people with diabetes worldwide. These figures are expected to increase to 693 million by 2045; thus, innovative preventative programs and treatments are a necessity to fight this escalating pandemic disorder. Caveolin-1 (CAV1), an integral membrane protein, is the principal component of caveolae in membranes and is involved in multiple cellular functions such as endocytosis, cholesterol homeostasis, signal transduction, and mechanoprotection. Previous studies demonstrated that CAV1 is critical for insulin receptor-mediated signaling, insulin secretion, and potentially the development of insulin resistance. Here, we summarize the recent progress on the role of CAV1 in diabetes and diabetic complications.

Fluoxetine pretreatment enhances neurogenic, angiogenic and immunomodulatory effects of MSCs on experimentally induced diabetic neuropathy

Being one of the most debilitating complications among diabetic patients, diabetic polyneuropathy (DPN) is a paramount point of continuous research. Stem cell therapies have shown promising results. However, limited cell survival and paracrine activities hinder its transfer from bench to bedside. We designed this study to evaluate fluoxetine-pretreatment technique of mesenchymal stem cells (MSCs) as an approach to enhance their paracrine and immunomodulatory properties in DPN. Effects of fluoxetine treatment of MSCs were tested in vitro. Forty-two adult Wistar male albino rats were utilized, further subdivided into control, diabetic, MSC-treated and fluoxetine-pretreated MSC groups. Sciatic nerve sections were prepared for light and electron microscope examination and immunohistochemical detection of neurofilament (NF) protein. Also, we assessed in vitro survival and paracrine properties of fluoxetine-pretreated MSCs. Real time PCR of BDNF, VEGF, IL-1β, and IL-10 expression in tissue homogenate was performed. Our results showed restoration of normal neuronal histomorphology and ultrastructure, moreover, immunohistochemical expression of anti-neurofilament protein was significantly elevated in MSC-treated groups compared to the diabetic one. Fluoxetine enhanced the MSC survival and their paracrine properties of MSCs in vitro. Furthermore, the fluoxetine-pretreated MSC group revealed a significant elevation of mRNA expression of BDNF (neurotrophic factor) and VEGF (angiogenic factor), denoting ameliorated MSC paracrine properties. Similarly, improved immunomodulatory functions were evident by a significant reduction of interleukin-1β mRNA expression (pro-inflammatory) and a reciprocal significant increase of interleukin-10 (anti-inflammatory). We concluded that fluoxetine-pretreatment of MSCs boosts their survival, paracrine, and immunomodulatory traits and directly influenced neuronal histomorphology. Hence, it presents a promising intervention of diabetic polyneuropathy. Graphical Abstract.

Susceptible and Prognostic Genetic Factors Associated with Diabetic Peripheral Neuropathy: A Comprehensive Literature Review

Type 2 diabetes mellitus (T2D) is a disorder of glucose metabolism. It is a complex process involving the regulation of insulin secretion, insulin sensitivity, gluconeogenesis, and glucose uptake at the cellular level. Diabetic peripheral neuropathy (DPN) is one of the debilitating complications that is present in approximately 50% of diabetic patients. It is the primary cause of diabetes-related hospital admissions and nontraumatic foot amputations. The pathogenesis of diabetic neuropathy is a complex process that involves hyperglycemia-induced oxidative stress and altered polyol metabolism that changes the nerve microvasculature, altered growth factor support, and deregulated lipid metabolism. Recent literature has reported that there are several heterogeneous groups of susceptible genetic loci which clearly contribute to the development of DPN. Several studies have reported that some patients with prediabetes develop neuropathic complications, whereas others demonstrated little evidence of neuropathy even after long-standing diabetes. There is emerging evidence that genetic factors may contribute to the development of DPN. This paper aims to provide an up-to-date review of the susceptible and prognostic genetic factors associated with DPN. An extensive survey of the scientific literature published in PubMed using the search terms "Diabetic peripheral neuropathy/genetics" and "genome-wide association study" was carried out, and the most recent and relevant literature were included in this review.

Association between Caveolin-1 expression and pathophysiological progression of femoral nerves in diabetic foot amputation patients

Objective

To evaluate the pathological changes of femoral nerves and the levels of caveolin-1 in diabetic foot amputation patients with neuropathy, and evaluate the association between caveolin-1 and neuropathy development.

Method

Thirty seven diabetic foot amputation patients were consecutively recruited from inpatients of Tianjin Metabolic Diseases Hospital between Jan 2003 and Nov 2005. Symptoms and signs of neuropathy, and scores of Toronto Clinical Scoring System (TCSS) were recorded. The nerve conduction velocity and HbA1c were measured. Femoral nerves were obtained 2-3 minutes after amputation. HE, Masson staining and transmission electron microscopy were used for pathological observation. Immunohistochemistry was used to observe changes of axons and count of nerve fiber density (NFD) and detect the levels of caveolin-1.

Results

HE, Masson and transmission electron microscopy showed nerve fibers were asymmetrical, the degenerated axons part had stronger staining and typical demyelinating changes. Stepwise regression models showed that HbA1c and NFD were the independent factors of caveolin-1 (F=45.090, p<0.001, R2=0.790) expression, and Caveolin-1, diabetes duration were independent factors of NFD (F=27.911, p<0.001, R2=0.691).

Conclusion

Caveolin-1 may be one of the key factors related to pathophysiological progression of femoral nerves in diabetic foot amputation patients.

Neuregulin 1 improves complex 2-mediated mitochondrial respiration in skeletal muscle of healthy and diabetic mice

It has been reported that neuregulin1 (NRG1) improves glucose tolerance in healthy and diabetic rodents. In vitro studies also suggest that NRG1 regulates myocyte oxidative capacity. To confirm this observation in vivo, we evaluated the effect on mitochondrial function of an 8-week treatment with NRG1 in db/db diabetic mice and C57BL/6JRJ healthy controls. NRG1 treatment improved complex 2-mediated mitochondrial respiration in the gastrocnemius of both control and diabetic mice and increased mitochondrial complex 2 subunit content by 2-fold. This effect was not associated with an increase in mitochondrial biogenesis markers. Enhanced ERBB4 phosphorylation could mediate NRG1 effects on mitochondrial function through signalling pathways, independently of ERK1/2, AKT or AMPK.

Schwann cell interactions with axons and microvessels in diabetic neuropathy

The prevalence of diabetes worldwide is at pandemic levels, with the number of patients increasing by 5% annually. The most common complication of diabetes is peripheral neuropathy, which has a prevalence as high as 50% and is characterized by damage to neurons, Schwann cells and blood vessels within the nerve. The pathogenic mechanisms of diabetic neuropathy remain poorly understood, impeding the development of targeted therapies to treat nerve degeneration and its most disruptive consequences of sensory loss and neuropathic pain. Involvement of Schwann cells has long been proposed, and new research techniques are beginning to unravel a complex interplay between these cells, axons and microvessels that is compromised during the development of diabetic neuropathy. In this Review, we discuss the evolving concept of Schwannopathy as an integral factor in the pathogenesis of diabetic neuropathy, and how disruption of the interactions between Schwann cells, axons and microvessels contribute to the disease.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Metabolic Connection of Inflammatory Pain: Pivotal Role of a Pyruvate Dehydrogenase Kinase-Pyruvate Dehydrogenase-Lactic Acid Axis

Pyruvate dehydrogenase kinases (PDK1-4) are mitochondrial metabolic regulators that serve as decision makers via modulation of pyruvate dehydrogenase (PDH) activity to convert pyruvate either aerobically to acetyl-CoA or anaerobically to lactate. Metabolic dysregulation and inflammatory processes are two sides of the same coin in several pathophysiological conditions. The lactic acid surge associated with the metabolic shift has been implicated in diverse painful states. In this study, we investigated the role of PDK-PDH-lactic acid axis in the pathogenesis of chronic inflammatory pain. Deficiency of Pdk2 and/or Pdk4 in mice attenuated complete Freund's adjuvant (CFA)-induced pain hypersensitivities. Likewise, Pdk2/4 deficiency attenuated the localized lactic acid surge along with hallmarks of peripheral and central inflammation following intraplantar administration of CFA. In vitro studies supported the role of PDK2/4 as promoters of classical proinflammatory activation of macrophages. Moreover, the pharmacological inhibition of PDKs or lactic acid production diminished CFA-induced inflammation and pain hypersensitivities. Thus, a PDK-PDH-lactic acid axis seems to mediate inflammation-driven chronic pain, establishing a connection between metabolism and inflammatory pain.

SIGNIFICANCE STATEMENT

The mitochondrial pyruvate dehydrogenase (PDH) kinases (PDKs) and their substrate PDH orchestrate the conversion of pyruvate either aerobically to acetyl-CoA or anaerobically to lactate. Lactate, the predominant end product of glycolysis, has recently been identified as a signaling molecule for neuron-glia interactions and neuronal plasticity. Pathological metabolic shift and subsequent lactic acid production are thought to play an important role in diverse painful states; however, their contribution to inflammation-driven pain is still to be comprehended. Here, we report that the PDK-PDH-lactic acid axis constitutes a key component of inflammatory pain pathogenesis. Our findings establish an unanticipated link between metabolism and inflammatory pain. This study unlocks a previously ill-explored research avenue for the metabolic control of inflammatory pain pathogenesis.

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.

Modulating Molecular Chaperones Improves Mitochondrial Bioenergetics and Decreases the Inflammatory Transcriptome in Diabetic Sensory Neurons

We have previously demonstrated that modulating molecular chaperones with KU-32, a novobiocin derivative, ameliorates physiologic and bioenergetic deficits of diabetic peripheral neuropathy (DPN). Replacing the coumarin core of KU-32 with a meta-fluorinated biphenyl ring system created KU-596, a novobiocin analogue (novologue) that showed neuroprotective activity in a cell-based assay. The current study sought to determine whether KU-596 offers similar therapeutic potential for treating DPN. Administration of 2-20 mg/kg of KU-596 improved diabetes induced hypoalgesia and sensory neuron bioenergetic deficits in a dose-dependent manner. However, the drug could not improve these neuropathic deficits in diabetic heat shock protein 70 knockout (Hsp70 KO) mice. To gain further insight into the mechanisms by which KU-596 improved DPN, we performed transcriptomic analysis of sensory neuron RNA obtained from diabetic wild-type and Hsp70 KO mice using RNA sequencing. Bioinformatic analysis of the differentially expressed genes indicated that diabetes strongly increased inflammatory pathways and that KU-596 therapy effectively reversed these increases independent of Hsp70. In contrast, the effects of KU-596 on decreasing the expression of genes regulating the production of reactive oxygen species were more Hsp70-dependent. These data indicate that modulation of molecular chaperones by novologue therapy offers an effective approach toward correcting nerve dysfunction in DPN but that normalization of inflammatory pathways alone by novologue therapy seems to be insufficient to reverse sensory deficits associated with insensate DPN.

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.

Signaling pathways bridging fate determination of neural crest cells to glial lineages in the developing peripheral nervous system

Fate determination of neural crest cells is an essential step for the development of different crest cell derivatives. Peripheral glia development is marked by the choice of the neural crest cells to differentiate along glial lineages. The molecular mechanism underlying fate acquisition is poorly understood. However, recent advances have identified different transcription factors and genes required for the complex instructive signaling process that comprise both local environmental and cell intrinsic cues. Among others, at least the roles of Sox10, Notch, and neuregulin 1 have been documented in both in vivo and in vitro models. Cooperative interactions of such factors appear to be necessary for the switch from multipotent neural crest cells to glial lineage precursors in the peripheral nervous system. This review summarizes recent advances in the understanding of fate determination of neural crest cells into different glia subtypes, together with the potential implications in regenerative medicine.

Heat shock protein 70 is necessary to improve mitochondrial bioenergetics and reverse diabetic sensory neuropathy following KU-32 therapy

Impaired neuronal mitochondrial bioenergetics contributes to the pathophysiologic progression of diabetic peripheral neuropathy (DPN) and may be a focal point for disease management. We have demonstrated that modulating heat shock protein (Hsp) 90 and Hsp70 with the small-molecule drug KU-32 ameliorates psychosensory, electrophysiologic, morphologic, and bioenergetic deficits of DPN in animal models of type 1 diabetes. The current study used mouse models of type 1 and type 2 diabetes to determine the relationship of changes in sensory neuron mitochondrial bioenergetics to the onset of and recovery from DPN. The onset of DPN showed a tight temporal correlation with a decrease in mitochondrial bioenergetics in a genetic model of type 2 diabetes. In contrast, sensory hypoalgesia developed 10 weeks before the occurrence of significant declines in sensory neuron mitochondrial bioenergetics in the type 1 model. KU-32 therapy improved mitochondrial bioenergetics in both the type 1 and type 2 models, and this tightly correlated with a decrease in DPN. Mechanistically, improved mitochondrial function following KU-32 therapy required Hsp70, since the drug was ineffective in diabetic Hsp70 knockout mice. Our data indicate that changes in mitochondrial bioenergetics may rapidly contribute to nerve dysfunction in type 2 diabetes, but not type 1 diabetes, and that modulating Hsp70 offers an effective approach toward correcting sensory neuron bioenergetic deficits and DPN in both type 1 and type 2 diabetes.

Traditional chinese medicine tang-luo-ning ameliorates sciatic nerve injuries in streptozotocin-induced diabetic rats

Diabetic peripheral neuropathy (DPN) is a common microvascular complication of diabetes associated with high disability rate and low quality of life. Tang-Luo-Ning (TLN) is an effective traditional Chinese medicine for the treatment of DPN. To illustrate the underlying neural protection mechanisms of TLN, the effect of TLN on electrophysiology and sciatic nerve morphology was investigated in a model of streptozotocin-induced DPN, as well as the underlying mechanism. Sciatic motor nerve conduction velocity and digital sensory nerve conduction velocity were reduced in DPN and were significantly improved by TLN or α-lipoic acid at 10 and 20 weeks after streptozotocin injection. It was demonstrated that TLN intervention for 20 weeks significantly alleviated pathological injury as well as increased the phosphorylation of ErbB2, Erk, Bad (Ser112), and the mRNA expression of neuregulin 1 (Nrg1), GRB2-associated binding protein 1 (Gab1), and mammalian target of rapamycin (Mtor) in injured sciatic nerve. These novel therapeutic properties of TLN to promote Schwann cell survival may offer a promising alternative medicine for the patients to delay the progression of DPN. The underlying mechanism may be that TLN exerts neural protection effect after sciatic nerve injury through Nrg1/ErbB2→Erk/Bad Schwann cell survival signaling pathway.

Differential expression of neuregulin-1 isoforms and downregulation of erbin are associated with Erb B2 receptor activation in diabetic peripheral neuropathy

Background

Aberrant neuron/glia interactions can contribute to a variety of neurodegenerative diseases and we have previously demonstrated that enhanced activation of Erb B2, which is a member of the epidermal growth factor receptor (EGFR) family, can contribute to the development of diabetic peripheral neuropathy (DPN). In peripheral nerves, Erb B receptors are activated by various members of the neuregulin-1 (NRG1) family including NRG1 Type I, NRG1 Type II and NRG1 Type III to regulate Schwann cell (SC) growth, migration, differentiation and dedifferentiation. Alternatively, Erb B2 activity can be negatively regulated by association with the Erb B2-interacting protein, erbin. Since the effect of diabetes on the expression of NRG1 isoforms and erbin in peripheral nerve are unknown, the current study determined whether changes in NRG1 isoforms and erbin may be associated with altered Erb B2 signaling in DPN.

Results

Swiss Webster mice were rendered diabetic with streptozotocin (STZ) and after 12 weeks of diabetes, treated with erlotinib, an inhibitor of Erb B2 activation. Inhibition of Erb B2 signaling partially reversed several pathophysiologic aspects of DPN including a pronounced sensory hypoalgesia, nerve conduction velocity deficits and the decrease in epidermal nerve fiber innervation. We also observed a decrease of NRG1 Type III but an increase of NRG1 Type I level in diabetic sural nerves at early stage of diabetes. With disease progression, we detected reduced erbin expression and enhanced MAPK pathway activity in diabetic mice. Inhibition of Erb B2 receptor suppressed MAPK pathway activity in treated-diabetic sural nerves.

Conclusions

These results support that hyperglycemia may impair NRG1/Erb B2 signaling by disrupting the balance between NRG1 isoforms, decreasing the expression of erbin and correspondingly activating the MAPK pathway. Together, imbalanced NRG1 isoforms and downregulated erbin may contribute to the dysregulation of Erb B2 signaling in the development of DPN.

Induction of heat shock protein 70 (Hsp70) prevents neuregulin-induced demyelination by enhancing the proteasomal clearance of c-Jun

Modulating molecular chaperones is emerging as an attractive approach to treat neurodegenerative diseases associated with protein aggregation, DPN (diabetic peripheral neuropathy) and possibly, demyelinating neuropathies. KU-32 [N-(7-((2R,3R,4S,5R)-3,4-dihydroxy-5-methoxy-6,6-dimethyl-tetrahydro-2H-pyran-2-yloxy)-8-methyl-2-oxo-2H-chromen-3-yl)acetamide] is a small molecule inhibitor of Hsp90 (heat shock protein 90) and reverses sensory deficits associated with myelinated fibre dysfunction in DPN. Additionally, KU-32 prevented the loss of myelinated internodes induced by treating myelinated SC (Schwann cell)-DRG (dorsal root ganglia) sensory neuron co-cultures with NRG1 (neuregulin-1 Type 1). Since KU-32 decreased NRG1-induced demyelination in an Hsp70-dependent manner, the goal of the current study was to clarify how Hsp70 may be mechanistically linked to preventing demyelination. The activation of p42/p44 MAPK (mitogen-activated protein kinase) and induction of the transcription factor c-Jun serve as negative regulators of myelination. NRG1 activated MAPK, induced c-Jun expression and promoted a loss of myelin segments in DRG explants isolated from both WT (wild-type) and Hsp70 KO (knockout) mice. Although KU-32 did not block the activation of MAPK, it blocked c-Jun induction and protected against a loss of myelinated segments in WT mice. In contrast, KU-32 did not prevent the NRG1-dependent induction of c-Jun and loss of myelin segments in explants from Hsp70 KO mice. Overexpression of Hsp70 in myelinated DRG explants prepared from WT or Hsp70 KO mice was sufficient to block the induction of c-Jun and the loss of myelin segments induced by NRG1. Lastly, inhibiting the proteasome prevented KU-32 from decreasing c-Jun levels. Collectively, these data support that Hsp70 induction is sufficient to prevent NRG1-induced demyelination by enhancing the proteasomal degradation of c-Jun.

Effects of high fat feeding on liver gene expression in diabetic goto-kakizaki rats

Effects of high fat diet (HFD) on obesity and, subsequently, on diabetes are highly variable and modulated by genetics in both humans and rodents. In this report, we characterized the response of Goto-Kakizaki (GK) rats, a spontaneous polygenic model for lean diabetes and healthy Wistar-Kyoto (WKY) controls, to high fat feeding from weaning to 20 weeks of age. Animals fed either normal diet or HFD were sacrificed at 4, 8, 12, 16 and 20 weeks of age and a wide array of physiological measurements were made along with gene expression profiling using Affymetrix gene array chips. Mining of the microarray data identified differentially regulated genes (involved in inflammation, metabolism, transcription regulation, and signaling) in diabetic animals, as well as the response of both strains to HFD. Functional annotation suggested that HFD increased inflammatory differences between the two strains. Chronic inflammation driven by heightened innate immune response was identified to be present in GK animals regardless of diet. In addition, compensatory mechanisms by which WKY animals on HFD resisted the development of diabetes were identified, thus illustrating the complexity of diabetes disease progression.

Diabetic peripheral neuropathy: should a chaperone accompany our therapeutic approach?

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes that is associated with axonal atrophy, demyelination, blunted regenerative potential, and loss of peripheral nerve fibers. The development and progression of DPN is due in large part to hyperglycemia but is also affected by insulin deficiency and dyslipidemia. Although numerous biochemical mechanisms contribute to DPN, increased oxidative/nitrosative stress and mitochondrial dysfunction seem intimately associated with nerve dysfunction and diminished regenerative capacity. Despite advances in understanding the etiology of DPN, few approved therapies exist for the pharmacological management of painful or insensate DPN. Therefore, identifying novel therapeutic strategies remains paramount. Because DPN does not develop with either temporal or biochemical uniformity, its therapeutic management may benefit from a multifaceted approach that inhibits pathogenic mechanisms, manages inflammation, and increases cytoprotective responses. Finally, exercise has long been recognized as a part of the therapeutic management of diabetes, and exercise can delay and/or prevent the development of painful DPN. This review presents an overview of existing therapies that target both causal and symptomatic features of DPN and discusses the role of up-regulating cytoprotective pathways via modulating molecular chaperones. Overall, it may be unrealistic to expect that a single pharmacologic entity will suffice to ameliorate the multiple symptoms of human DPN. Thus, combinatorial therapies that target causal mechanisms and enhance endogenous reparative capacity may enhance nerve function and improve regeneration in DPN if they converge to decrease oxidative stress, improve mitochondrial bioenergetics, and increase response to trophic factors.

Modulating molecular chaperones improves sensory fiber recovery and mitochondrial function in diabetic peripheral neuropathy

Quantification of intra-epidermal nerve fibers (iENFs) is an important approach to stage diabetic peripheral neuropathy (DPN) and is a promising clinical endpoint for identifying beneficial therapeutics. Mechanistically, diabetes decreases neuronal mitochondrial function and enhancing mitochondrial respiratory capacity may aid neuronal recovery from glucotoxic insults. We have proposed that modulating the activity and expression of heat shock proteins (Hsp) may be of benefit in treating DPN. KU-32 is a C-terminal Hsp90 inhibitor that improved thermal hypoalgesia in diabetic C57Bl/6 mice but it was not determined if this was associated with an increase in iENF density and mitochondrial function. After 16 weeks of diabetes, Swiss Webster mice showed decreased electrophysiological and psychosensory responses and a >30% loss of iENFs. Treatment of the mice with ten weekly doses of 20mg/kg KU-32 significantly reversed pre-existing deficits in nerve conduction velocity and responses to mechanical and thermal stimuli. KU-32 therapy significantly reversed the pre-existing loss of iENFs despite the identification of a sub-group of drug-treated diabetic mice that showed improved thermal sensitivity but no increase in iENF density. To determine if the improved clinical indices correlated with enhanced mitochondrial activity, sensory neurons were isolated and mitochondrial bioenergetics assessed ex vivo using extracellular flux technology. Diabetes decreased maximal respiratory capacity in sensory neurons and this deficit was improved following KU-32 treatment. In conclusion, KU-32 improved physiological and morphologic markers of degenerative neuropathy and drug efficacy may be related to enhanced mitochondrial bioenergetics in sensory neurons.

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.

Similar pattern of peripheral neuropathy in mouse models of type 1 diabetes and Alzheimer's disease

There is an increasing awareness that diabetes has an impact on the CNS and that diabetes is a risk factor for Alzheimer's disease (AD). Links between AD and diabetes point to impaired insulin signaling as a common mechanism leading to defects in the brain. However, diabetes is predominantly characterized by peripheral, rather than central, neuropathy, and despite the common central mechanisms linking AD and diabetes, little is known about the effect of AD on the peripheral nervous system (PNS). In this study, we compared indexes of peripheral neuropathy and investigated insulin signaling in the sciatic nerve of insulin-deficient mice and amyloid precursor protein (APP) overexpressing transgenic mice. Insulin-deficient and APP transgenic mice displayed similar patterns of peripheral neuropathy with decreased motor nerve conduction velocity, thermal hypoalgesia, and loss of tactile sensitivity. Phosphorylation of the insulin receptor and glycogen synthase kinase 3β (GSK3β) was similarly affected in insulin-deficient and APP transgenic mice despite significantly different blood glucose and plasma insulin levels, and nerve of both models showed accumulation of Aβ-immunoreactive protein. Although diabetes and AD have different primary etiologies, both diseases share many abnormalities in both the brain and the PNS. Our data point to common deficits in the insulin-signaling pathway in both neurodegenerative diseases and support the idea that AD may cause disorders outside the higher CNS.

Phenotypic changes in diabetic neuropathy induced by a high-fat diet in diabetic C57BL/6 mice

Emerging evidence suggests that dyslipidemia is an independent risk factor for diabetic neuropathy (DN) (reviewed by Vincent et al. 2009). To experimentally determine how dyslipidemia alters DN, we quantified neuropathic symptoms in diabetic mice fed a high-fat diet. Streptozotocin-induced diabetic C57BL/6 mice fed a high-fat diet developed dyslipidemia and a painful neuropathy (mechanical allodynia) instead of the insensate neuropathy (mechanical insensitivity) that normally develops in this strain. Nondiabetic mice fed a high-fat diet also developed dyslipidemia and mechanical allodynia. Thermal sensitivity was significantly reduced in diabetic compared to nondiabetic mice, but was not worsened by the high-fat diet. Moreover, diabetic mice fed a high-fat diet had significantly slower sensory and motor nerve conduction velocities compared to nondiabetic mice. Overall, dyslipidemia resulting from a high-fat diet may modify DN phenotypes and/or increase risk for developing DN. These results provide new insight as to how dyslipidemia may alter the development and phenotype of diabetic neuropathy.

Targeting endothelial dysfunction in vascular complications associated with diabetes

Cardiovascular complications associated with diabetes remain a significant health issue in westernized societies. Overwhelming evidence from clinical and laboratory investigations have demonstrated that these cardiovascular complications are initiated by a dysfunctional vascular endothelium. Indeed, endothelial dysfunction is one of the key events that occur during diabetes, leading to the acceleration of cardiovascular mortality and morbidity. In a diabetic milieu, endothelial dysfunction occurs as a result of attenuated production of endothelial derived nitric oxide (EDNO) and augmented levels of reactive oxygen species (ROS). Thus, in this review, we discuss novel therapeutic targets that either upregulate EDNO production or increase antioxidant enzyme capacity in an effort to limit oxidative stress and restore endothelial function. In particular, endogenous signaling molecules that positively modulate EDNO synthesis and mimetics of endogenous antioxidant enzymes will be highlighted. Consequently, manipulation of these unique targets, either alone or in combination, may represent a novel strategy to confer vascular protection, with the ultimate goal of improved outcomes for diabetes-associated vascular complications.

The identification of gene expression profiles associated with progression of human diabetic neuropathy

Diabetic neuropathy is a common complication of diabetes. While multiple pathways are implicated in the pathophysiology of diabetic neuropathy, there are no specific treatments and no means to predict diabetic neuropathy onset or progression. Here, we identify gene expression signatures related to diabetic neuropathy and develop computational classification models of diabetic neuropathy progression. Microarray experiments were performed on 50 samples of human sural nerves collected during a 52-week clinical trial. A series of bioinformatics analyses identified differentially expressed genes and their networks and biological pathways potentially responsible for the progression of diabetic neuropathy. We identified 532 differentially expressed genes between patient samples with progressing or non-progressing diabetic neuropathy, and found these were functionally enriched in pathways involving inflammatory responses and lipid metabolism. A literature-derived co-citation network of the differentially expressed genes revealed gene subnetworks centred on apolipoprotein E, jun, leptin, serpin peptidase inhibitor E type 1 and peroxisome proliferator-activated receptor gamma. The differentially expressed genes were used to classify a test set of patients with regard to diabetic neuropathy progression. Ridge regression models containing 14 differentially expressed genes correctly classified the progression status of 92% of patients (P < 0.001). To our knowledge, this is the first study to identify transcriptional changes associated with diabetic neuropathy progression in human sural nerve biopsies and describe their potential utility in classifying diabetic neuropathy. Our results identifying the unique gene signature of patients with progressive diabetic neuropathy will facilitate the development of new mechanism-based diagnostics and therapies.

PARP inhibition alleviates diabetes-induced systemic oxidative stress and neural tissue 4-hydroxynonenal adduct accumulation: correlation with peripheral nerve function

This study evaluated the role of poly(ADP-ribose) polymerase (PARP) in systemic oxidative stress and 4-hydoxynonenal adduct accumulation in diabetic peripheral neuropathy. Control and streptozotocin-diabetic rats were maintained with or without treatment with the PARP inhibitor, 1,5-isoquinolinediol, 3 mg kg(-1) day(-1), for 10 weeks after an initial 2 weeks. Treatment efficacy was evaluated by poly(ADP-ribosyl)ated protein content in peripheral nerve and spinal cord (Western blot analysis) and dorsal root ganglion neurons and nonneuronal cells (fluorescence immunohistochemistry), as well as by indices of peripheral nerve function. Diabetic rats displayed increased urinary isoprostane and 8-hydroxy-2'-deoxyguanosine excretion (ELISA) and 4-hydroxynonenal adduct accumulation in endothelial and Schwann cells of the peripheral nerve, neurons, astrocytes, and oligodendrocytes of the spinal cord and neurons and glial cells of the dorsal root ganglia (double-label fluorescence immunohistochemistry), as well as motor and sensory nerve conduction velocity deficits, thermal hypoalgesia, and tactile allodynia. PARP inhibition counteracted diabetes-induced systemic oxidative stress and 4-hydroxynonenal adduct accumulation in peripheral nerve and spinal cord (Western blot analysis) and dorsal root ganglion neurons (perikarya, fluorescence immunohistochemistry), which correlated with improvement of large and small nerve fiber function. The findings reveal the important role of PARP activation in systemic oxidative stress and 4-hydroxynonenal adduct accumulation in diabetic peripheral neuropathy.

Baicalein alleviates diabetic peripheral neuropathy through inhibition of oxidative-nitrosative stress and p38 MAPK activation

With the consideration of the multifactorial etiology of diabetic peripheral neuropathy, an ideal drug or drug combination should target at least several key pathogenetic mechanisms. The flavonoid baicalein (5,6,7-trihydroxyflavone) has been reported to counteract sorbitol accumulation, activation of 12/15-lipoxygenase, oxidative-nitrosative stress, inflammation, and impaired signaling in models of chronic disease. This study evaluated baicalein on diabetic peripheral neuropathy. Control and streptozotocin-diabetic C57Bl6/J mice were maintained with or without baicalein treatment (30 mg kg(-1) d(-1), i.p., for 4 weeks after 12 weeks without treatment). Neuropathy was evaluated by sciatic motor and hind-limb digital sensory nerve conduction velocities, thermal algesia (Hargreaves test), tactile response threshold (flexible von Frey filament test), and intraepidermal nerve fiber density (fluorescent immunohistochemistry with confocal microscopy). Sciatic nerve and spinal cord 12/15-lipoxygenase and total and phosphorylated p38 mitogen-activated protein kinase expression and nitrated protein levels were evaluated by Western blot analysis, 12(S)hydroxyeicosatetraenoic acid concentration (a measure of 12/15-lipoxygenase activity) by ELISA, and glucose and sorbitol pathway intermediate concentrations by enzymatic spectrofluorometric assays. Baicalein did not affect diabetic hyperglycemia, and alleviated nerve conduction deficit and small sensory nerve fiber dysfunction, but not intraepidermal nerve fiber loss. It counteracted diabetes-associated p38 mitogen-activated protein kinase phosphorylation, oxidative-nitrosative stress, and 12/15-lipoxygenase overexpression and activation, but not glucose or sorbitol pathway intermediate accumulation. In conclusion, baicalein targets several mechanisms implicated in diabetic peripheral neuropathy. The findings provide rationale for studying hydroxyflavones with an improved pharmacological profile as potential treatments for diabetic neuropathy and other diabetic complications.

Role of 12/15-lipoxygenase in nitrosative stress and peripheral prediabetic and diabetic neuropathies

This study evaluated the role of 12/15-lipoxygenase, which converts arachidonic acid to 12(S)- and 15(S)-hydroxyeicosatetraenoic acids, in nitrosative stress in the peripheral nervous system and peripheral prediabetic and diabetic neuropathies. The experiments were performed in C57BL6/J mice made diabetic with streptozotocin or fed a high-fat diet and in human Schwann cells cultured in 5.5 or 30 mM glucose. 12/15-Lipoxygenase overexpression and activation were present in sciatic nerve and spinal cord of diabetic and high-fat diet-fed mice, as well as in human Schwann cells cultured in high concentrations of D-, but not L-glucose. 12/15-Lipoxygenase inhibition by cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate (8 mg kg(-1) day(-1) sc, for 4 weeks after 12 weeks without treatment) alleviated the accumulation of nitrated proteins in the sciatic nerve and spinal cord, and large and small nerve fiber dysfunction, but not intraepidermal nerve fiber loss. 12/15-Lipoxygenase gene deficiency alleviated nitrosative stress and nerve conduction deficit, but not small sensory fiber neuropathy, in high-fat diet-fed mice. In conclusion, 12/15-lipoxygenase is implicated in nitrosative stress and peripheral neuropathy in mouse models of type 1 and early type 2 diabetes. Its presence in human Schwann cells and upregulation by high glucose suggest a potential involvement in human disease.

Inhibiting heat-shock protein 90 reverses sensory hypoalgesia in diabetic mice

Increasing the expression of Hsp70 (heat-shock protein 70) can inhibit sensory neuron degeneration after axotomy. Since the onset of DPN (diabetic peripheral neuropathy) is associated with the gradual decline of sensory neuron function, we evaluated whether increasing Hsp70 was sufficient to improve several indices of neuronal function. Hsp90 is the master regulator of the heat-shock response and its inhibition can up-regulate Hsp70. KU-32 (N-{7-[(2R,3R,4S,5R)-3,4-dihydroxy-5-methoxy-6,6-dimethyl-tetrahydro-2H-pyran-2-yloxy]-8-methyl-2-oxo-2H-chromen-3-yl}acetamide) was developed as a novel, novobiocin-based, C-terminal inhibitor of Hsp90 whose ability to increase Hsp70 expression is linked to the presence of an acetamide substitution of the prenylated benzamide moiety of novobiocin. KU-32 protected against glucose-induced death of embryonic DRG (dorsal root ganglia) neurons cultured for 3 days in vitro. Similarly, KU-32 significantly decreased neuregulin 1-induced degeneration of myelinated Schwann cell DRG neuron co-cultures prepared from WT (wild-type) mice. This protection was lost if the co-cultures were prepared from Hsp70.1 and Hsp70.3 KO (knockout) mice. KU-32 is readily bioavailable and was administered once a week for 6 weeks at a dose of 20 mg/kg to WT and Hsp70 KO mice that had been rendered diabetic with streptozotocin for 12 weeks. After 12 weeks of diabetes, both WT and Hsp70 KO mice developed deficits in NCV (nerve conduction velocity) and a sensory hypoalgesia. Although KU-32 did not improve glucose levels, HbA1c (glycated haemoglobin) or insulin levels, it reversed the NCV and sensory deficits in WT but not Hsp70 KO mice. These studies provide the first evidence that targeting molecular chaperones reverses the sensory hypoalgesia associated with DPN.

Differential effects of myopathy-associated caveolin-3 mutants on growth factor signaling

Caveolin-3 is an important scaffold protein of cholesterol-rich caveolae. Mutations of caveolin-3 cause hereditary myopathies that comprise remarkably different pathologies. Growth factor signaling plays an important role in muscle physiology; it is influenced by caveolins and cholesterol-rich rafts and might thus be affected by caveolin-3 dysfunction. Prompted by the observation of a marked chronic peripheral neuropathy in a patient suffering from rippling muscle disease due to the R26Q caveolin-3 mutation and because TrkA is expressed by neuronal cells and skeletal muscle fibers, we performed a detailed comparative study on the effect of pathogenic caveolin-3 mutants on the signaling and trafficking of the TrkA nerve growth factor receptor and, for comparison, of the epidermal growth factor receptor. We found that the R26Q mutant slightly and the P28L strongly reduced nerve growth factor signaling in TrkA-transfected cells. Surface biotinylation experiments revealed that the R26Q caveolin-3 mutation markedly reduced the internalization of TrkA, whereas the P28L did not. Moreover, P28L expression led to increased, whereas R26Q expression decreased, epidermal growth factor signaling. Taken together, we found differential effects of the R26Q and P28L caveolin-3 mutants on growth factor signaling. Our findings are of clinical interest because they might help explain the remarkable differences in the degree of muscle lesions caused by caveolin-3 mutations and also the co-occurrence of peripheral neuropathy in the R26Q caveolinopathy case presented.

Emerging role of neuregulin as a modulator of muscle metabolism

Neuregulin was described initially as a neurotrophic factor involved in the formation of the neuromuscular junction in skeletal muscle. However, in recent years, neuregulin has been reported to be a myokine that exerts relevant effects on myogenesis and the regulation of muscle metabolism. In this new context, the rapid and chronic metabolic effects of neuregulin appear to be related to muscle contraction. Indeed, the effects of neuregulin resemble those of exercise, which are accompanied by an improvement in insulin sensitivity. In this review, we challenge the classical role assigned to neuregulin in muscle and propound the emerging concept of its involvement in the regulation of energetic metabolism and insulin responsiveness.