Mitochondrial respiratory chain dysfunction in dorsal root ganglia of streptozotocin-induced diabetic rats and its correction by insulin treatment

Transcriptomic profiling of sciatic nerves and dorsal root ganglia reveals site-specific effects of prediabetic neuropathy

Peripheral neuropathy (PN) is a severe and frequent complication of obesity, prediabetes, and type 2 diabetes characterized by progressive distal-to-proximal peripheral nerve degeneration. However, a comprehensive understanding of the mechanisms underlying PN, and whether these mechanisms change during PN progression, is currently lacking. Here, gene expression data were obtained from distal (sciatic nerve; SCN) and proximal (dorsal root ganglia; DRG) injury sites of a high-fat diet (HFD)-induced mouse model of obesity/prediabetes at early and late disease stages. Self-organizing map and differentially expressed gene analyses followed by pathway enrichment analysis identified genes and pathways altered across disease stage and injury site. Pathways related to immune response, inflammation, and glucose and lipid metabolism were consistently dysregulated with HFD-induced PN, irrespective of injury site. However, regulation of oxidative stress was unique to the SCN while dysregulated Hippo and Notch signaling were only observed in the DRG. The role of the immune system and inflammation in disease progression was supported by an increase in the percentage of immune cells in the SCN with PN progression. Finally, when comparing these data to transcriptomic signatures from human patients with PN, we observed conserved pathways related to metabolic dysregulation across species, highlighting the translational relevance of our mouse data. Our findings demonstrate that PN is associated with distinct site-specific molecular re-programming in the peripheral nervous system, identifying novel, clinically relevant therapeutic targets.

Targeted metabolomics reveals the aberrant energy status in diabetic peripheral neuropathy and the neuroprotective mechanism of traditional Chinese medicine JinMaiTong

Diabetic peripheral neuropathy (DPN) is a common and devastating complication of diabetes, for which effective therapies are currently lacking. Disturbed energy status plays a crucial role in DPN pathogenesis. However, the integrated profile of energy metabolism, especially the central carbohydrate metabolism, remains unclear in DPN. Here, we developed a metabolomics approach by targeting 56 metabolites using high-performance ion chromatography-tandem mass spectrometry (HPIC-MS/MS) to illustrate the integrative characteristics of central carbohydrate metabolism in patients with DPN and streptozotocin-induced DPN rats. Furthermore, JinMaiTong (JMT), a traditional Chinese medicine (TCM) formula, was found to be effective for DPN, improving the peripheral neurological function and alleviating the neuropathology of DPN rats even after demyelination and axonal degeneration. JMT ameliorated DPN by regulating the aberrant energy balance and mitochondrial functions, including excessive glycolysis restoration, tricarboxylic acid cycle improvement, and increased adenosine triphosphate (ATP) generation. Bioenergetic profile was aberrant in cultured rat Schwann cells under high-glucose conditions, which was remarkably corrected by JMT treatment. In-vivo and in-vitro studies revealed that these effects of JMT were mainly attributed to the activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK) and downstream peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). Our results expand the therapeutic framework for DPN and suggest the integrative modulation of energy metabolism using TCMs, such as JMT, as an effective strategy for its treatment.

Vincamine as an agonist of G-protein-coupled receptor 40 effectively ameliorates diabetic peripheral neuropathy in mice

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes, which has yet no curable medication. Neuroinflammation and mitochondrial dysfunction are tightly linked to DPN pathology. G-protein-coupled receptor 40 (GPR40) is predominantly expressed in pancreatic β-cells, but also in spinal dorsal horn and dorsal root ganglion (DRG) neurons, regulating neuropathic pain. We previously have reported that vincamine (Vin), a monoterpenoid indole alkaloid extracted from Madagascar periwinkle, is a GPR40 agonist. In this study, we evaluated the therapeutic potential of Vin in ameliorating the DPN-like pathology in diabetic mice. Both STZ-induced type 1 (T1DM) and db/db type 2 diabetic (T2DM) mice were used to establish late-stage DPN model (DPN mice), which were administered Vin (30 mg·kg-1·d-1, i.p.) for 4 weeks. We showed that Vin administration did not lower blood glucose levels, but significantly ameliorated neurological dysfunctions in DPN mice. Vin administration improved the blood flow velocities and blood perfusion areas of foot pads and sciatic nerve tissues in DPN mice. We demonstrated that Vin administration protected against sciatic nerve myelin sheath injury and ameliorated foot skin intraepidermal nerve fiber (IENF) density impairment in DPN mice. Moreover, Vin suppressed NLRP3 inflammasome activation through either β-Arrestin2 or β-Arrestin2/IκBα/NF-κB signaling, improved mitochondrial dysfunction through CaMKKβ/AMPK/SIRT1/PGC-1α signaling and alleviated oxidative stress through Nrf2 signaling in the sciatic nerve tissues of DPN mice and LPS/ATP-treated RSC96 cells. All the above-mentioned beneficial effects of Vin were abolished by GPR40-specific knockdown in dorsal root ganglia and sciatic nerve tissues. Together, these results support that pharmacological activation of GPR40 as a promising therapeutic strategy for DPN and highlight the potential of Vin in the treatment of this disease.

Peripheral mechanisms of peripheral neuropathic pain

Peripheral neuropathic pain (PNP), neuropathic pain that arises from a damage or disease affecting the peripheral nervous system, is associated with an extremely large disease burden, and there is an increasing and urgent need for new therapies for treating this disorder. In this review we have highlighted therapeutic targets that may be translated into disease modifying therapies for PNP associated with peripheral neuropathy. We have also discussed how genetic studies and novel technologies, such as optogenetics, chemogenetics and single-cell RNA-sequencing, have been increasingly successful in revealing novel mechanisms underlying PNP. Additionally, consideration of the role of non-neuronal cells and communication between the skin and sensory afferents is presented to highlight the potential use of drug treatment that could be applied topically, bypassing drug side effects. We conclude by discussing the current difficulties to the development of effective new therapies and, most importantly, how we might improve the translation of targets for peripheral neuropathic pain identified from studies in animal models to the clinic.

New perspectives in diabetic neuropathy

Diabetes prevalence continues to climb with the aging population. Type 2 diabetes (T2D), which constitutes most cases, is metabolically acquired. Diabetic peripheral neuropathy (DPN), the most common microvascular complication, is length-dependent damage to peripheral nerves. DPN pathogenesis is complex, but, at its core, it can be viewed as a state of impaired metabolism and bioenergetics failure operating against the backdrop of long peripheral nerve axons supported by glia. This unique peripheral nerve anatomy and the injury consequent to T2D underpins the distal-to-proximal symptomatology of DPN. Earlier work focused on the impact of hyperglycemia on nerve damage and bioenergetics failure, but recent evidence additionally implicates contributions from obesity and dyslipidemia. This review will cover peripheral nerve anatomy, bioenergetics, and glia-axon interactions, building the framework for understanding how hyperglycemia and dyslipidemia induce bioenergetics failure in DPN. DPN and painful DPN still lack disease-modifying therapies, and research on novel mechanism-based approaches is also covered.

Anti-Glucotoxicity Effect of Phytoconstituents via Inhibiting MGO-AGEs Formation and Breaking MGO-AGEs

The therapeutic benefits of phytochemicals in the treatment of various illnesses and disorders are well documented. They show significant promise for the discovery and creation of novel medications for treating a variety of human diseases. Numerous phytoconstituents have shown antibiotic, antioxidant, and wound-healing effects in the conventional system. Traditional medicines based on alkaloids, phenolics, tannins, saponins, terpenes, steroids, flavonoids, glycosides, and phytosterols have been in use for a long time and are crucial as alternative treatments. These phytochemical elements are crucial for scavenging free radicals, capturing reactive carbonyl species, changing protein glycation sites, inactivating carbohydrate hydrolases, fighting pathological conditions, and accelerating the healing of wounds. In this review, 221 research papers have been reviewed. This research sought to provide an update on the types and methods of formation of methylglyoxal-advanced glycation end products (MGO-AGEs) and molecular pathways induced by AGEs during the progression of the chronic complications of diabetes and associated diseases as well as to discuss the role of phytoconstituents in MGO scavenging and AGEs breaking. The development and commercialization of functional foods using these natural compounds can provide potential health benefits.

Impact of gut-peripheral nervous system axis on the development of diabetic neuropathy

Diabetes is a chronic metabolic disease caused by a reduction in the production and/or action of insulin, with consequent development of hyperglycemia. Diabetic patients, especially those who develop neuropathy, presented dysbiosis, with an increase in the proportion of pathogenic bacteria and a decrease in the butyrate-producing bacteria. Due to this dysbiosis, diabetic patients presented a weakness of the intestinal permeability barrier and high bacterial product translocation to the bloodstream, in parallel to a high circulating levels of pro-inflammatory cytokines such as TNF-α. In this context, we propose here that dysbiosis-induced increased systemic levels of bacterial products, like lipopolysaccharide (LPS), leads to an increase in the production of pro-inflammatory cytokines, including TNF-α, by Schwann cells and spinal cord of diabetics, being crucial for the development of neuropathy.

Anti-hyperalgesic effects of photobiomodulation therapy (904 nm) on streptozotocin-induced diabetic neuropathy imply MAPK pathway and calcium dynamics modulation

Several recent studies have established the efficacy of photobiomodulation therapy (PBMT) in painful clinical conditions. Diabetic neuropathy (DN) can be related to activating mitogen-activated protein kinases (MAPK), such as p38, in the peripheral nerve. MAPK pathway is activated in response to extracellular stimuli, including interleukins TNF-α and IL-1β. We verified the pain relief potential of PBMT in streptozotocin (STZ)-induced diabetic neuropathic rats and its influence on the MAPK pathway regulation and calcium (Ca²⁺) dynamics. We then observed that PBMT applied to the L4-L5 dorsal root ganglion (DRG) region reduced the intensity of hyperalgesia, decreased TNF-α and IL-1β levels, and p38-MAPK mRNA expression in DRG of diabetic neuropathic rats. DN induced the activation of phosphorylated p38 (p-38) MAPK co-localized with TRPV1⁺ neurons; PBMT partially prevented p-38 activation. DN was related to an increase of p38-MAPK expression due to proinflammatory interleukins, and the PBMT (904 nm) treatment counteracted this condition. Also, the sensitization of DRG neurons by the hyperglycemic condition demonstrated during the Ca²⁺ dynamics was reduced by PBMT, contributing to its anti-hyperalgesic effects.

Nox, Nox, Are You There? The Role of NADPH Oxidases in the Peripheral Nervous System

Significance: Reactive oxygen species (ROS) contribute to multiple aspects of peripheral nervous system (PNS) biology ranging from physiological processes (e.g., axonal outgrowth and regeneration) to pathophysiology (e.g., nerve degeneration). Although ROS are derived from multiple sources, NADPH oxidase (Nox) family members are dedicated to ROS generation. Noxs are expressed in the PNS, and their overexpression is associated with detrimental effects on nerve function and contributes, at least in part, to peripheral neuropathies. Recent Advances: Of the seven members, studies mostly focused on Nox1, Nox2, and Nox4, which are expressed in the PNS in a cell-specific manner. We have also recently identified human Nox5 in sural nerve biopsies. When maintained at homeostatic levels, Noxs regulate several aspects of peripheral nerve health, most notably neurite outgrowth and axonal regeneration following nerve lesion. While Nox2 and Nox4 dysregulation is a major source of oxidative stress in PNS disorders, including neuropathic pain and diabetic peripheral neuropathy, recent evidence also implicates Nox1 and Nox5. Critical Issues: Although there is compelling evidence for a direct role of Noxs on nerve function, little is known about their subcellular localization, intercellular regulation, and interaction. These, together with redox signaling, are considered crucial components of nerve redox status. In addition, the lack of isoform-specific inhibitors limits conclusions about the physiological role of Noxs in the PNS and their therapeutic potential in peripheral neuropathies. Future Directions: Future research using isoform-specific genetic and pharmacological approaches are therefore needed to better understand the significance of Nox enzymes in PNS (patho) physiology. Antioxid. Redox Signal. 37, 613-630.

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.

Antagonism of the Muscarinic Acetylcholine Type 1 Receptor Enhances Mitochondrial Membrane Potential and Expression of Respiratory Chain Components via AMPK in Human Neuroblastoma SH-SY5Y Cells and Primary Neurons

Impairments in mitochondrial physiology play a role in the progression of multiple neurodegenerative conditions, including peripheral neuropathy in diabetes. Blockade of muscarinic acetylcholine type 1 receptor (M₁R) with specific/selective antagonists prevented mitochondrial dysfunction and reversed nerve degeneration in in vitro and in vivo models of peripheral neuropathy. Specifically, in type 1 and type 2 models of diabetes, inhibition of M₁R using pirenzepine or muscarinic toxin 7 (MT7) induced AMP-activated protein kinase (AMPK) activity in dorsal root ganglia (DRG) and prevented sensory abnormalities and distal nerve fiber loss. The human neuroblastoma SH-SY5Y cell line has been extensively used as an in vitro model system to study mechanisms of neurodegeneration in DRG neurons and other neuronal sub-types. Here, we tested the hypothesis that pirenzepine or MT7 enhance AMPK activity and via this pathway augment mitochondrial function in SH-SY5Y cells. M₁R expression was confirmed by utilizing a fluorescent dye, ATTO590-labeled MT7, that exhibits great specificity for this receptor. M₁R antagonist treatment in SH-SY5Y culture increased AMPK phosphorylation and mitochondrial protein expression (OXPHOS). Mitochondrial membrane potential (MMP) was augmented in pirenzepine and MT7 treated cultured SH-SY5Y cells and DRG neurons. Compound C or AMPK-specific siRNA suppressed pirenzepine or MT7-induced elevation of OXPHOS expression and MMP. Moreover, muscarinic antagonists induced hyperpolarization by activating the M-current and, thus, suppressed neuronal excitability. These results reveal that negative regulation of this M₁R-dependent pathway could represent a potential therapeutic target to elevate AMPK activity, enhance mitochondrial function, suppress neuropathic pain, and enhance nerve repair in peripheral neuropathy.

Antidiabetic and neuroprotective effects of a novel repaglinide analog

Repaglinide (RPG) is an oral insulin secretagogue used in the treatment of diabetes. In this study, a new RPG analog was synthesized. Its antidiabetic and neuroprotective effects on dorsal root ganglions (DRG) in streptozotocin (STZ)-induced diabetic rats were examined compared to RPG. To assess the effects of 2-methoxy-4-(2-((3-methyl-1-(2-(piperidin-1-yl)phenyl)butyl)amino)-2-oxoethoxy)benzoic acid (OXR), the impact of OXR on oxidative stress biomarkers, motor function, and the expression of the glutamate dehydrogenase 1 (GLUD1), SLC2A2/glucose transporter 2 (GLUT2), and glucokinase (GCK) genes in STZ-induced diabetic rats were assessed. DRGs were examined histologically using hemotoxylin and eosin staining. Molecular docking was used to investigate the interactions between OXR and the binding site of RPG, the ATP-sensitive potassium (KATP) channel. Following 5 weeks of treatment, OXR significantly increased the level of total antioxidant power, decreased reactive oxygen species, and lipid peroxidation in the DRGs of diabetic rats. OXR restored STZ-induced pathophysiological damages in DRG tissues. Administration of OXR improved motor function of rats with diabetic neuropathy. Administration of 0.5 mg/kg OXR reduced blood glucose while promoting insulin, mainly through upregulation of messenger RNA expression of GLUD1, GLUT2, and GCK in the pancreas. Molecular docking revealed a favorable binding mode of OXR to the KATP channel. In conclusion, OXR has neuroprotective effects in diabetic rats by lowering oxidative stress, lowering blood glucose, and stimulating insulin secretion. We report that 0.5 mg/kg OXR administration was the most effective concentration of the compound in this study. OXR may be a promising target for further research on neuroprotective antidiabetic molecules.

Mitochondrial calcium uniporter deletion prevents painful diabetic neuropathy by restoring mitochondrial morphology and dynamics

ABSTRACT

Painful diabetic neuropathy (PDN) is an intractable complication affecting 25% of diabetic patients. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, resulting in calcium overload, axonal degeneration, and loss of cutaneous innervation. The molecular pathways underlying these effects are unknown. Using high-throughput and deep-proteome profiling, we found that mitochondrial fission proteins were elevated in DRG neurons from mice with PDN induced by a high-fat diet (HFD). In vivo calcium imaging revealed increased calcium signaling in DRG nociceptors from mice with PDN. Furthermore, using electron microscopy, we showed that mitochondria in DRG nociceptors had fragmented morphology as early as 2 weeks after starting HFD, preceding the onset of mechanical allodynia and small-fiber degeneration. Moreover, preventing calcium entry into the mitochondria, by selectively deleting the mitochondrial calcium uniporter from these neurons, restored normal mitochondrial morphology, prevented axonal degeneration, and reversed mechanical allodynia in the HFD mouse model of PDN. These studies suggest a molecular cascade linking neuropathic pain to axonal degeneration in PDN. In particular, nociceptor hyperexcitability and the associated increased intracellular calcium concentrations could lead to excessive calcium entry into mitochondria mediated by the mitochondrial calcium uniporter, resulting in increased calcium-dependent mitochondrial fission and ultimately contributing to small-fiber degeneration and neuropathic pain in PDN. Hence, we propose that targeting calcium entry into nociceptor mitochondria may represent a promising effective and disease-modifying therapeutic approach for this currently intractable and widespread affliction. Moreover, these results are likely to inform studies of other neurodegenerative disease involving similar underlying events.

Tannins in the Treatment of Diabetic Neuropathic Pain: Research Progress and Future Challenges

Diabetes mellitus and its consequences continue to put a significant demand on medical resources across the world. Diabetic neuropathic pain (DNP) is a frequent diabetes mellitus chronic microvascular outcome. Allodynia, hyperalgesia, and aberrant or lack of nerve fibre sensation are all symptoms of DNP. These clinical characteristics will lead to worse quality of life, sleep disruption, depression, and increased mortality. Although the availability of numerous medications that alleviate the symptoms of DNP, the lack of long-term efficacy and unfavourable side effects highlight the urgent need for novel treatment strategies. This review paper systematically analysed the preclinical research on the treatment of DNP using plant phytochemicals that contain only tannins. A total of 10 original articles involved in in-vivo and in-vitro experiments addressing the promising benefits of phytochemical tannins on DNP were examined between 2008 and 2021. The information given implies that these phytochemicals may have relevant pharmacological effects on DNP symptoms through their antihyperalgesic, anti-inflammatory, and antioxidant properties; however, because of the limited sample size and limitations of the studies conducted so far, we were unable to make definitive conclusions. Before tannins may be employed as therapeutic agents for DNP, more study is needed to establish the specific molecular mechanism for all of these activities along the pain pathway and examine the side effects of tannins in the treatment of DNP.

Enhancement of Mitochondrial Function by the Neurogenic Molecule NSI-189 Accompanies Reversal of Peripheral Neuropathy and Memory Impairment in a Rat Model of Type 2 Diabetes

AIMS

Mitochondrial dysfunction contributes to many forms of peripheral and central nervous system degeneration. Therapies that protect mitochondrial number and function have the potential to impact the progression of conditions such as diabetic neuropathy. We therefore assessed indices of mitochondrial function in dorsal root ganglia (DRG) and brain cortex of the Zucker diabetic fatty (ZDF) rat model of type 2 diabetes and tested the therapeutic impact of a neurogenic compound, NSI-189, on both mitochondrial function and indices of peripheral and central neurological dysfunction.

MATERIALS AND METHODS

ZDF rats were maintained for 16 weeks of untreated diabetes before the start of oral treatment with NSI-189 for an additional 16 weeks. Nerve conduction velocity, sensitivity to tactile and thermal stimuli, and behavioral assays of cognitive function were assessed monthly. AMP-activated protein kinase (AMPK) phosphorylation, mitochondrial protein levels, and respiratory complex activities were assessed in the DRG and brain cortex after 16 weeks of treatment with NSI-189.

RESULTS

Treatment with NSI-189 selectively elevated the expression of protein subunits of complexes III and V and activities of respiratory complexes I and IV in the brain cortex, and this was accompanied by amelioration of impaired memory function and plasticity. In the sensory ganglia of ZDF rats, loss of AMPK activity was ameliorated by NSI-189, and this was accompanied by reversal of multiple indices of peripheral neuropathy.

CONCLUSIONS

Efficacy of NSI-189 against dysfunction of the CNS and PNS function in type 2 diabetic rats was accompanied by improvement of mitochondrial function. NSI-189 exhibited actions at different levels of mitochondrial regulation in central and peripheral tissues.

Mitochondria as the target for disease related hormonal dysregulation

Mitochondria play an important role in the synthesis of steroid hormones, including the sex hormone estrogen. Sex-specific regulation of these hormones is important for phenotypic development and downstream, sex-specific activational effects in both brain and behavior. First, mitochondrial contribution to the synthesis of estrogen, followed by a discussion of the signaling interactions between estrogen and the mitochondria will be reviewed. Next, disorders with an established sex difference related to aging, mood, and cognition will be examined. Finally, review of mitochondria as a biomarker of disease and data supporting efforts in targeting mitochondria as a therapeutic target for the amelioration of these disorders will be discussed. Taken together, this review aims to assess the influence of E2 on mitochondrial function within the brain via exploration of E2-ER interactions within neural mitochondria and how they may act to influence the development and presentation of neurodegenerative and neurocognitive diseases with known sex differences.

Neurobiological Opportunities in Diabetic Polyneuropathy

This review highlights a selection of potential translational directions for the treatment of diabetic polyneuropathy (DPN) currently irreversible and without approved interventions beyond pain management. The list does not include all diabetic targets that have been generated over several decades of research but focuses on newer work. The emphasis is firstly on approaches that support the viability and growth of peripheral neurons and their ability to withstand a barrage of diabetic alterations. We include a section describing Schwann cell targets and finally how mitochondrial damage has been a common element in discussing neuropathic damage. Most of the molecules and pathways described here have not yet reached clinical trials, but many trials have been negative to date. Nonetheless, these failures clear the pathway for new thoughts over reversing DPN.

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.

Sex differences in insulin resistance, but not peripheral neuropathy, in a diet-induced prediabetes mouse model

Peripheral neuropathy (PN) is a common complication of prediabetes and diabetes and is an increasing problem worldwide. Existing PN treatments rely solely on glycemic control, which is effective in type 1 but not type 2 diabetes. Sex differences in response to anti-diabetic drugs further complicate the identification of effective PN therapies. Preclinical research has been primarily carried out in males, highlighting the need for increased sex consideration in PN models. We previously reported PN sex dimorphism in obese leptin-deficient ob/ob mice. This genetic model is inherently limited, however, owing to leptin's role in metabolism. Therefore, the current study goal was to examine PN and insulin resistance in male and female C57BL6/J mice fed a high-fat diet (HFD), an established murine model of human prediabetes lacking genetic mutations. HFD mice of both sexes underwent longitudinal phenotyping and exhibited expected metabolic and PN dysfunction compared to standard diet (SD)-fed animals. Hindpaw thermal latencies to heat were shorter in HFD females versus HFD males, as well as SD females versus males. Compared to HFD males, female HFD mice exhibited delayed insulin resistance, yet still developed the same trajectory of nerve conduction deficits and intraepidermal nerve fiber density loss. Subtle differences in adipokine levels were also noted by sex and obesity status. Collectively, our results indicate that although females retain early insulin sensitivity upon HFD challenge, this does not protect them from developing the same degree of PN as their male counterparts. This article has an associated First Person interview with the first author of the paper.

Untangling the effect of insulin action on brain mitochondria and metabolism

The regulation of energy homeostasis is controlled by the brain and, besides requiring high amounts of energy, it relies on functional insulin/insulin-like growth factor (IGF)-1 signalling in the central nervous system. This energy is mainly provided by mitochondria in form of ATP. Thus, there is an intricate interplay between mitochondrial function and insulin/IGF-1 action to enable functional brain signalling and, accordingly, propagate a healthy metabolism. To adapt to different nutritional conditions, the brain is able to sense the current energy status via mitochondrial and insulin signalling-dependent pathways and exerts an appropriate metabolic response. However, regional, cell type and receptor-specific consequences of this interaction occur and are linked to diverse outcomes such as altered nutrient sensing, body weight regulation or even cognitive function. Impairments of this cross-talk can lead to obesity and glucose intolerance and are linked to neurodegenerative diseases, yet they also induce a self-sustainable, dysfunctional 'metabolic triangle' characterised by insulin resistance, mitochondrial dysfunction and inflammation in the brain. The identification of causal factors deteriorating insulin action, mitochondrial function and concomitantly a signature of metabolic stress in the brain is of utter importance to offer novel mechanistic insights into development of the continuously rising prevalence of non-communicable diseases such as type 2 diabetes and neurodegeneration. This review aims to determine the effect of insulin action on brain mitochondrial function and energy metabolism. It precisely outlines the interaction and differences between insulin action, insulin-like growth factor (IGF)-1 signalling and mitochondrial function; distinguishes between causality and association; and reveals its consequences for metabolism and cognition. We hypothesise that an improvement of at least one signalling pathway can overcome the vicious cycle of a self-perpetuating metabolic dysfunction in the brain present in metabolic and neurodegenerative diseases.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Sensory neurons derived from diabetic rats exhibit deficits in functional glycolysis and ATP that are ameliorated by IGF-1

Objective

The distal dying-back of the longest nerve fibres is a hallmark of diabetic neuropathy, and impaired provision of energy in the form of adenosine triphosphate (ATP) may contribute to this neurodegenerative process. We hypothesised that energy supplementation via glycolysis and/or mitochondrial oxidative phosphorylation is compromised in cultured dorsal root ganglion (DRG) sensory neurons from diabetic rodents, thus contributing to axonal degeneration. Functional analysis of glycolysis and mitochondrial respiration and real-time measurement of ATP levels in live cells were our specific means to test this hypothesis.

Methods

DRG neuron cultures from age-matched control or streptozotocin (STZ)-induced type 1 diabetic rats were used for in vitro studies. Three plasmids containing ATP biosensors of varying affinities were transfected into neurons to study endogenous ATP levels in real time. The Seahorse XF analyser was used for glycolysis and mitochondrial respiration measurements.

Results

Fluorescence resonance energy transfer (FRET) efficiency (YFP/CFP ratio) of the ATP biosensors AT1.03 (low affinity) and AT1.03^YEMK (medium affinity) were significantly higher than that measured using the ATP-insensitive construct AT1.03^R122/6K in both cell bodies and neurites of DRG neurons (p < 0.0001). The ATP level was homogenous along the axons but higher in cell bodies in cultured DRG neurons from both control and diabetic rats. Treatment with oligomycin (an ATP synthase inhibitor in mitochondria) decreased the ATP levels in cultured DRG neurons. Likewise, blockade of glycolysis using 2-deoxy-d-glucose (2-DG: a glucose analogue) reduced ATP levels (p < 0.001). Cultured DRG neurons derived from diabetic rats showed a diminishment of ATP levels (p < 0.01), glycolytic capacity, glycolytic reserve and non-glycolytic acidification. Application of insulin-like growth factor-1 (IGF-1) significantly elevated all the above parameters in DRG neurons from diabetic rats. Oligomycin pre-treatment of DRG neurons, to block oxidative phosphorylation, depleted the glycolytic reserve and lowered basal respiration in sensory neurons derived from control and diabetic rats. Depletion was much higher in sensory neurons from diabetic rats compared to control rats. In addition, an acute increase in glucose concentration, in the presence or absence of oligomycin, elevated parameters of glycolysis by 1.5- to 2-fold while having no impact on mitochondrial respiration.

Conclusion

We provide the first functional evidence for decreased glycolytic capacity in DRG neurons derived from type 1 diabetic rats. IGF-1 protected against the loss of ATP supplies in DRG cell bodies and axons in neurons derived from diabetic rats by augmenting various parameters of glycolysis and mitochondrial respiration.

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There is an energy deficit in the form of ATP in cell bodies and axons of sensory neurons derived from diabetic rats.

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Glycolytic capacity and reserve are depleted in sensory neurons from diabetic rats.

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IGF-1 treatment reverses the energy deficit and glycolysis defect in sensory neurons from diabetic rats.

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Glycolysis is a significant source of energy comprising up to 30% of ATP in adult rat sensory neurons.

Early disruption of nerve mitochondrial and myelin lipid homeostasis in obesity-induced diabetes

Diabetic neuropathy is a major complication of diabetes. Current treatment options alleviate pain but do not stop the progression of the disease. At present, there are no approved disease-modifying therapies. Thus, developing more effective therapies remains a major unmet medical need. Seeking to better understand the molecular mechanisms driving peripheral neuropathy, as well as other neurological complications associated with diabetes, we performed spatiotemporal lipidomics, biochemical, ultrastructural, and physiological studies on PNS and CNS tissue from multiple diabetic preclinical models. We unraveled potentially novel molecular fingerprints underlying nerve damage in obesity-induced diabetes, including an early loss of nerve mitochondrial (cardiolipin) and myelin signature (galactosylceramide, sulfatide, and plasmalogen phosphatidylethanolamine) lipids that preceded mitochondrial, myelin, and axonal structural/functional defects; started in the PNS; and progressed to the CNS at advanced diabetic stages. Mechanistically, we provided substantial evidence indicating that these nerve mitochondrial/myelin lipid abnormalities are (surprisingly) not driven by hyperglycemia, dysinsulinemia, or insulin resistance, but rather associate with obesity/hyperlipidemia. Importantly, our findings have major clinical implications as they open the door to novel lipid-based biomarkers to diagnose and distinguish different subtypes of diabetic neuropathy (obese vs. nonobese diabetics), as well as to lipid-lowering therapeutic strategies for treatment of obesity/diabetes-associated neurological complications and for glycemic control.

Acute insulin deprivation results in altered mitochondrial substrate sensitivity conducive to greater fatty acid transport

Type 1 and type 2 diabetes are both tightly associated with impaired glucose control. Although both pathologies stem from different mechanisms, a reduction in insulin action coincides with drastic metabolic dysfunction in skeletal muscle and metabolic inflexibility. However, the underlying explanation for this response remains poorly understood, particularly since it is difficult to distinguish the role of attenuated insulin action from the detrimental effects of reactive lipid accumulation, which impairs mitochondrial function and promotes reactive oxygen species (ROS) emission. We therefore utilized streptozotocin to examine the effects of acute insulin deprivation, in the absence of a high-lipid/nutrient excess environment, on the regulation of mitochondrial substrate sensitivity and ROS emission. The ablation of insulin resulted in reductions in absolute mitochondrial oxidative capacity and ADP-supported respiration and reduced the ability for malonyl-CoA to inhibit carnitine palmitoyltransferase I (CPT-I) and suppress fatty acid-supported respiration. These bioenergetic responses coincided with increased mitochondrial-derived H₂O₂ emission and lipid transporter content, independent of major mitochondrial substrate transporter proteins and enzymes involved in fatty acid oxidation. Together, these data suggest that attenuated/ablated insulin signaling does not affect mitochondrial ADP sensitivity, whereas the increased reliance on fatty acid oxidation in situations where insulin action is reduced may occur as a result of altered regulation of mitochondrial fatty acid transport through CPT-I.

Modulation of Sensory Nerve Function by Insulin: Possible Relevance to Pain, Inflammation and Axon Growth

Insulin, besides its pivotal role in energy metabolism, may also modulate neuronal processes through acting on insulin receptors (InsRs) expressed by neurons of both the central and the peripheral nervous system. Recently, the distribution and functional significance of InsRs localized on a subset of multifunctional primary sensory neurons (PSNs) have been revealed. Systematic investigations into the cellular electrophysiology, neurochemistry and morphological traits of InsR-expressing PSNs indicated complex functional interactions among specific ion channels, proteins and neuropeptides localized in these neurons. Quantitative immunohistochemical studies have revealed disparate localization of the InsRs in somatic and visceral PSNs with a dominance of InsR-positive neurons innervating visceral organs. These findings suggested that visceral spinal PSNs involved in nociceptive and inflammatory processes are more prone to the modulatory effects of insulin than somatic PSNs. Co-localization of the InsR and transient receptor potential vanilloid 1 (TRPV1) receptor with vasoactive neuropeptides calcitonin gene-related peptide and substance P bears of crucial importance in the pathogenesis of inflammatory pathologies affecting visceral organs, such as the pancreas and the urinary bladder. Recent studies have also revealed significant novel aspects of the neurotrophic propensities of insulin with respect to axonal growth, development and regeneration.

Muscarinic Toxin 7 Signals Via Ca2+/Calmodulin-Dependent Protein Kinase Kinase β to Augment Mitochondrial Function and Prevent Neurodegeneration

Mitochondrial dysfunction is implicated in a variety of neurodegenerative diseases of the nervous system. Peroxisome proliferator–activated receptor-γ coactivator-1α (PGC-1α) is a regulator of mitochondrial function in multiple cell types. In sensory neurons, AMP-activated protein kinase (AMPK) augments PGC-1α activity and this pathway is depressed in diabetes leading to mitochondrial dysfunction and neurodegeneration. Antimuscarinic drugs targeting the muscarinic acetylcholine type 1 receptor (M₁R) prevent/reverse neurodegeneration by inducing nerve regeneration in rodent models of diabetes and chemotherapy-induced peripheral neuropathy (CIPN). Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ) is an upstream regulator of AMPK activity. We hypothesized that antimuscarinic drugs modulate CaMKKβ to enhance activity of AMPK, and PGC-1α, increase mitochondrial function and thus protect from neurodegeneration. We used the specific M₁R antagonist muscarinic toxin 7 (MT7) to manipulate muscarinic signaling in the dorsal root ganglia (DRG) neurons of normal rats or rats with streptozotocin-induced diabetes. DRG neurons treated with MT7 (100 nM) or a selective muscarinic antagonist, pirenzepine (1 μM), for 24 h showed increased neurite outgrowth that was blocked by the CaMKK inhibitor STO-609 (1 μM) or short hairpin RNA to CaMKKβ. MT7 enhanced AMPK phosphorylation which was blocked by STO-609 (1 μM). PGC-1α reporter activity was augmented up to 2-fold (p < 0.05) by MT7 and blocked by STO-609. Mitochondrial maximal respiration and spare respiratory capacity were elevated after 3 h of exposure to MT7 (p < 0.05). Diabetes and CIPN induced a significant (p < 0.05) decrease in corneal nerve density which was corrected by topical delivery of MT7. We reveal a novel M₁R-modulated, CaMKKβ-dependent pathway in neurons that represents a therapeutic target to enhance nerve repair in two of the most common forms of peripheral neuropathy.

Mitochondrial Respiration Correlates with Prognostic Markers in Chronic Lymphocytic Leukemia and Is Normalized by Ibrutinib Treatment

Mitochondrial bioenergetics profiling, a measure of oxygen consumption rates, correlates with prognostic markers and can be used to assess response to therapy in chronic lymphocytic leukemia (CLL) cells. In this study, we measured mitochondrial respiration rates in primary CLL cells using respirometry to evaluate mitochondrial function. We found significant increases in mitochondrial respiration rates in CLL versus control B lymphocytes. We also observed amongst CLL patients that advanced age, female sex, zeta-chain-associated protein of 70 kD (ZAP-70⁺), cluster of differentiation 38 (CD38⁺), and elevated β2-microglobulin (β2-M) predicted increased maximal respiration rates. ZAP-70⁺ CLL cells exhibited significantly higher bioenergetics than B lymphocytes or ZAP-70⁻ CLL cells and were more sensitive to the uncoupler, carbonyl cyanide-p-trifluoro-methoxyphenylhydrazone (FCCP). Univariable and multivariable linear regression analysis demonstrated that ZAP-70⁺ predicted increased maximal respiration. ZAP-70⁺ is a surrogate for B cell receptor (BCR) activation and can be targeted by ibrutinib, which is a clinically approved Bruton’s tyrosine kinase (BTK) inhibitor. Therefore, we evaluated the oxygen consumption rates (OCR) of CLL cells and plasma chemokine (C-C motif) ligands 3 and 4 (CCL3/CCL4) levels from ibrutinib-treated patients and demonstrated decreased OCR similar to control B lymphocytes, suggesting that ibrutinib treatment resets the mitochondrial bioenergetics, while diminished CCL3/CCL4 levels indicate the down regulation of the BCR signaling pathway in CLL. Our data support evaluation of mitochondrial respiration as a preclinical tool for the response assessment of CLL cells.

Neuroprotective Effect of Salvianolic Acid A against Diabetic Peripheral Neuropathy through Modulation of Nrf2

Oxidative stress has been recognized as the contributor to diabetic peripheral neuropathy (DPN). Antioxidant strategies have been most widely explored; nevertheless, whether antioxidants alone prevent DPN still remains inconclusive. In the present study, we established an in vitro DPN cell model for drug screening using Schwann RSC96 cells under high glucose (HG) stimulation, and we found that salvianolic acid A (SalA) mitigated HG-induced injury evidenced by cell viability and myelination. Mechanistically, SalA exhibited strong antioxidative effects by inhibiting 1,1-diphenyl-2-picrylhydrazyl (DPPH) and reducing reactive oxygen species (ROS), malondialdehyde (MDA), and oxidized glutathione (GSSG) content, as well as upregulating antioxidative enzyme mRNA expression. In addition, SalA significantly extenuated neuroinflammation with downregulated inflammatory factor mRNA expression. Furthermore, SalA improved the mitochondrial function of HG-injured Schwann cells by scavenging mitochondrial ROS, decreasing mitochondrial membrane potential (MMP), and enhancing ATP production, as well as upregulating oxidative phosphorylation gene expression. More importantly, we identified nuclear factor-E2-related factor 2 (Nrf2) as the upstream regulator which mediated protective effects of SalA on DPN. SalA directly bound to the Kelch domain of Kelch-like ECH-associated protein 1 (Keap1) and thus disrupted the interaction of Nrf2 and Keap1 predicted by LibDock of Discovery Studio. Additionally, SalA significantly inhibited Nrf2 promoter activity and downregulated Nrf2 mRNA expression but without affecting Nrf2 protein expression. Interestingly, SalA upregulated the nuclear Nrf2 expression and promoted Nrf2 nuclear translocation by high content screening assay, which was confirmed to be involved in its antiglucotoxicity effect by the knockdown of Nrf2 in RSC96 cells. In KK-Ay mice, we demonstrated that SalA could effectively improve the abnormal glucose and lipid metabolism and significantly protect against DPN by increasing the mechanical withdrawal threshold and sciatic nerve conduction velocity and restoring the ultrastructural impairment of the injured sciatic nerve induced by diabetes. Hence, SalA protected against DPN by antioxidative stress, attenuating neuroinflammation, and improving mitochondrial function via Nrf2. SalA may be prospective therapeutics for treating DPN.

Diabetes, a Contemporary Risk for Parkinson's Disease: Epidemiological and Cellular Evidences

Diabetes mellitus (DM), a group of diseases characterized by defective glucose metabolism, is the most widespread metabolic disorder affecting over 400 million adults worldwide. This pathological condition has been implicated in the pathogenesis of a number of central encephalopathies and peripheral neuropathies. In further support of this notion, recent epidemiological evidence suggests a link between DM and Parkinson’s disease (PD), with hyperglycemia emerging as one of the culprits in neurodegeneration involving the nigrostriatal pathway, the neuroanatomical substrate of the motor symptoms affecting parkinsonian patients. Indeed, dopaminergic neurons located in the mesencephalic substantia nigra appear to be particularly vulnerable to oxidative stress and degeneration, likely because of their intrinsic susceptibility to mitochondrial dysfunction, which may represent a direct consequence of hyperglycemia and hyperglycemia-induced oxidative stress. Other pathological pathways induced by increased intracellular glucose levels, including the polyol and the hexosamine pathway as well as the formation of advanced glycation end-products, may all play a pivotal role in mediating the detrimental effects of hyperglycemia on nigral dopaminergic neurons. In this review article, we will examine the epidemiological as well as the molecular and cellular clues supporting the potential susceptibility of nigrostriatal dopaminergic neurons to hyperglycemia.

Disorders of mitochondrial dynamics in peripheral neuropathy: Clues from hereditary neuropathy and diabetes

Peripheral neuropathy is a common and debilitating complication of diabetes and prediabetes. Recent clinical studies have identified an association between the development of neuropathy and dyslipidemia in prediabetic and diabetic patients. Despite the prevalence of this complication, studies identifying molecular mechanisms that underlie neuropathy progression in prediabetes or diabetes are limited. However, dysfunctional mitochondrial pathways in hereditary neuropathy provide feasible molecular targets for assessing mitochondrial dysfunction in neuropathy associated with prediabetes or diabetes. Recent studies suggest that elevated levels of dietary saturated fatty acids (SFAs) associated with dyslipidemia impair mitochondrial dynamics in sensory neurons by inducing mitochondrial depolarization, compromising mitochondrial bioenergetics, and impairing axonal mitochondrial transport. This causes lower neuronal ATP and apoptosis. Conversely, monounsaturated fatty acids (MUFAs) restore nerve and sensory mitochondrial function. Understanding the mitochondrial pathways that contribute to neuropathy progression in prediabetes and diabetes may provide therapeutic targets for the treatment of this debilitating complication.

Mitochondriopathy: The unifying concept in distal neuropathies?

This is a new, exciting time for the study of peripheral nerve and its diseases. For many years research in peripheral neuropathies largely involved descriptive analysis, a situation which is now rapidly giving way to hypothesis testing with the development and validation of molecular genetic tools. Although it has been known for some time that many neuropathies target the most distal portions of the longest peripheral nerves, a process variously referred to as central-peripheral distal neuropathy, "dying-back" neuropathy, or "stocking-glove" neuropathy, proposed mechanisms driving axon loss have been generally unproven/untestable. Studies have shown that mitochondrial DNA mutations accumulate in distal axons and a unifying theory of distal neuropathy has been proposed based on underlying mitochondrial aging defects in mitogenesis and, thus, distal axon susceptibility, particularly if axonal transport defects also accompanied them. Increased levels of mtDNA mutations have been described in some painful neuropathies (e.g., HIV) compared to baseline HIV patients and controls. For some time no therapies were available to preserve and prevent the development of peripheral neuropathy and, with a variety of expected pathogenetic mechanisms, complex cocktails of therapeutic agents were envisioned. Although structure and ultrastructure continue to be relevant in the studies of mitochondriopathy-driven neuropathies, more techniques have been added and more complex hypotheses now expand the concept and focus directly on mitochondrial pathology or dysfunction. It is now possible to definitively test possible pathogenetic mechanisms with a variety of new tools and to formulate new and testable hypotheses.

Mitochondrial contributions to neuronal development and function

Mitochondria are critical to tissues and organs characterized by high-energy demands, such as the nervous system. They provide essential energy and metabolites, and maintain Ca2+ balance, which is imperative for proper neuronal function and development. Emerging findings further underline the role of mitochondria in neurons. Technical advances in the last decades made it possible to investigate key mechanisms in neuronal development and the contribution of mitochondria therein. In this article, we discuss the latest findings relevant to the involvement of mitochondria in neuronal development, placing emphasis on mitochondrial metabolism and dynamics. In addition, we survey the role of mitochondrial energy metabolism and Ca2+ homeostasis in proper neuronal function, and the involvement of mitochondria in axon myelination.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Agomelatine pretreatment prevents development of hyperglycemia and hypoinsulinemia in streptozotocin-induced diabetes in mice

The main objective of this study was to investigate potential effectiveness of agomelatine pretreatment in the prevention of diabetes itself and encephalopathy, with a focus on brain tissue oxidative stress and inflammatory processes in streptozotocin (STZ)-induced diabetic mice. Interleukine-1β (IL-1β) and TACR1 (NK1), which is a tachykinine receptor, were used for the investigation of inflammation in the brain regions including raphe nucleus, periaqueductal gyrus (PAG), amygdala, and nucleus accumbens. The effects of agomelatine on total antioxidant capacity were also evaluated. In the in vitro part of the study, the effects of agomelatine on cell viability were investigated in dorsal root ganglion (DRG) neurons. Fasting blood glucose levels were measured 72 h after STZ injection to determine the diabetic condition. Agomelatine pretreatment prevented both hyperglycemia and hypoinsulinemia in STZ-treated mice. When STZ was injected to induce diabetes in mice, neither hyperglycemia nor hypoinsulinemia was developed in agomelatine pretreated mice and 6 weeks after development of diabetes, agomelatine treatment significantly decreased levels of IL-1β mRNA in raphe nucleus and nucleus accumbens. TACR1 mRNA levels were lower in raphe nucleus, PAG, and amygdala of agomelatine-treated diabetic mice. The increase in total antioxidant capacity after agomelatine administration may responsible for its beneficial effect in the prevention of diabetes. We showed that agomelatine reversed high glucose-induced cell viability decreases in DRG neurons. Both the antihyperglycemic and antioxidant effects of agomelatine might have contributed to the DRG neuron viability improvement. In conclusion, agomelatine seems to both prevent development of diabetes and reverse the encephalopathic changes caused by diabetes.

Neuroactive Steroids and Sex-Dimorphic Nervous Damage Induced by Diabetes Mellitus

Diabetes mellitus is a metabolic disease where improper glycaemic control may induce severe complications in different organs. In this review, we will discuss alterations occurring in peripheral and central nervous system of patients with type 1 (i.e., insulin dependent diabetes mellitus,) or type 2 diabetes (i.e., non-insulin dependent diabetes mellitus), as well as related experimental models. A particular focus will be on the role exerted by neuroactive steroids (i.e., important regulators of nervous functions) in the nervous damage induced by diabetes. Indeed, the nervous levels of these molecules are affected by the pathology and, in agreement, their neuroprotective effects have been reported. Interestingly, the sex is another important variable. As discussed, nervous diabetic complications show sex dimorphic features in term of incidence, functional outcomes and neuroactive steroid levels. Therefore, these features represent an interesting background for possible sex-oriented therapies with neuroactive steroids aimed to counteract nervous damage observed in diabetic pathology.

High glucose concentration suppresses a SIRT2 regulated pathway that enhances neurite outgrowth in cultured adult sensory neurons

In peripheral nerve under hyperglycemic conditions high flux of d-glucose through the polyol pathway drives an aberrant redox state contributing to neurodegeneration in diabetic sensorimotor polyneuropathy (DSPN). Sirtuins, including SIRT2, detect the redox state via the NAD⁺/NADH ratio to regulate mitochondrial function via, in part, AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α). In adult dorsal root ganglia (DRG) sensory neurons mitochondrial dysfunction has been proposed as an etiological factor in dying-back neuropathy in diabetes. We tested the hypothesis that a high concentration of d-glucose depleted SIRT2 expression via enhancement of polyol pathway activity. We posited that this would lead to impaired mitochondrial function and suppression of neurite outgrowth in cultured sensory neurons. The use of dominant negative mutants or neurons from SIRT2 knockout (KO) mice to block SIRT2 signaling revealed that neurons derived from control or type 1 diabetic rodents required SIRT2 for optimal neurite outgrowth. Over-expression of WT-SIRT2 elevated neurite outgrowth in normal and diabetic cultures. SIRT2 protein isoforms 2.1 and 2.2 were reduced by 20-30% in DRG of type 1 diabetic mice (p < .05). After 72 h exposure to high d-glucose (25 mM vs 5 mM) cultured sensory neurons showed a significant 2-fold (p < .05) decrease in SIRT2 expression, P-AMPK, levels of respiratory Complexes II/III and respiratory capacity. DRG neurons expressed aldose reductase and the aforementioned deficits were prevented by treatment with aldose reductase inhibitors (lidorestat or sorbinil) or sorbitol dehydrogenase inhibitor (SDI-158). In cultures derived from type 1 diabetic rats treatment with SDI-158 elevated expression of SIRT2, P-AMPK/PGC-1α and neurite outgrowth (p < .05). SIRT2 KO neurons exhibited deficits in the LKB-1/AMPK/PGC-1α pathway and mitochondrial function. In cultured neurons the SIRT2 pathway enhances axonal outgrowth and this signaling axis encompassing activation of AMPK/PGC-1α is impaired in DSPN, in part, due to enhanced polyol pathway activity caused by hyperglycemia.

Reduced mitochondrial reactive oxygen species production in peripheral nerves of mice fed a ketogenic diet

NEW FINDINGS

What is the central question of this study? Do peripheral sensory neurons metabolize fat-based fuel sources, and does a ketogenic diet modify these processes? What is the main finding and its importance We show that peripheral axons from mice fed a ketogenic diet respond to fat-based fuel sources with reduced respiration and H₂ O₂ emission compared with mice fed a control diet. These results add to our understanding of the responses of sensory neurons to neuropathy associated with poor diet, obesity and metabolic syndrome. These findings should be incorporated into current ideas of axonal protection and might identify how dietary interventions may change mitochondrial function in settings of sensory dysfunction.

ABSTRACT

Metabolic syndrome and obesity are increasing epidemics that significantly impact the peripheral nervous system and lead to negative changes in sensation and peripheral nerve function. Research to understand the consequences of diet, obesity and fuel usage in sensory neurons has commonly focused on glucose metabolism. Here, we tested whether mouse sensory neurons and nerves have the capacity to metabolize fat-based fuels (palmitoyl-CoA) and whether these effects are altered by feeding of a ketogenic (90% kcal fat) diet compared with a control diet (14% kcal fat). Male C57Bl/6 mice were placed on the diets for 10 weeks, and after the mice were killed, the dorsal root ganglion (DRG) and sciatic nerve (SN) were placed in an Oroboros oxygraph-2K to examine diet-induced alterations in metabolism (respiration) of palmitoyl-CoA and H₂ O₂ emission (fluorescence). In addition, RNAseq was performed on the DRG of mice fed a control or a ketogenic diet for 12 weeks, and genes associated with mitochondrial respiratory function were analysed. Our results suggest that the sciatic nerves from mice fed a ketogenic diet display reduced O₂ respiration and H₂ O₂ emission when metabolizing palmitoyl-CoA compared with mice fed a control diet. Assessments of changes in mRNA gene expression reveal alterations in genes encoding the NADH dehydrogenase complex and complex IV, which could alter production of reactive oxygen species. These new findings highlight the ability of sensory neurons and axons to oxidize fat-based fuel sources and show that these mechanisms are adaptable to dietary changes.

Depressed mitochondrial function and electron transport Complex II-mediated H2O2 production in the cortex of type 1 diabetic rodents

AIMS

Abnormalities in mitochondrial function under diabetic conditions can lead to deficits in function of cortical neurons and their support cells exhibiting a pivotal role in the pathogenesis of several neurodegenerative disorders, including Alzheimer's disease. We aimed to assess mitochondrial respiration rates and membrane potential or H2O2 generation simultaneously and expression of proteins involved in mitochondrial dynamics, ROS scavenging and AMPK/SIRT/PGC-1α pathway activity in cortex under diabetic conditions.

METHODS

Cortical mitochondria from streptozotocin (STZ)-induced type 1 diabetic rats or mice, and aged-matched controls were used for simultaneous measurements of mitochondrial respiration rates and mitochondrial membrane potential (mtMP) or H2O2 using OROBOROS oxygraph. Measurements of enzymatic activities of respiratory complexes were performed using spectophotometry. Protein levels in cortical mitochondria and homogenates were determined by Western blotting.

RESULTS

Mitochondrial coupled respiration rates and FCCP-induced uncoupled respiration rates were significantly decreased in mitochondria of cortex of STZ-diabetic rats compared to controls. The mtMP in the presence of ADP was significantly depolarized and succinate-dependent respiration rates and H2O2 were significantly diminished in cortical mitochondria of diabetic animals compared to controls, accompanied with reduced expression of CuZn- and Mn-superoxide dismutase. The enzymatic activities of Complex I, II, and IV and protein levels of certain components of Complex I and II, mitofusin 2 (Mfn2), dynamin-related protein 1 (DRP1), P-AMPK, SIRT2 and PGC-1α were significantly diminished in diabetic cortex.

CONCLUSION

Deficits in mitochondrial function, dynamics, and antioxidant capabilities putatively mediated through sub-optimal AMPK/SIRT/PGC-1α signaling, are involved in the development of early sub-clinical neurodegeneration in the cortex under diabetic conditions.

Reducing CXCR4-mediated nociceptor hyperexcitability reverses painful diabetic neuropathy

Painful diabetic neuropathy (PDN) is an intractable complication of diabetes that affects 25% of patients. PDN is characterized by neuropathic pain and small-fiber degeneration, accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability and loss of their axons within the skin. The molecular mechanisms underlying DRG nociceptor hyperexcitability and small-fiber degeneration in PDN are unknown. We hypothesize that chemokine CXCL12/CXCR4 signaling is central to this mechanism, as we have shown that CXCL12/CXCR4 signaling is necessary for the development of mechanical allodynia, a pain hypersensitivity behavior common in PDN. Focusing on DRG neurons expressing the sodium channel Nav1.8, we applied transgenic, electrophysiological, imaging, and chemogenetic techniques to test this hypothesis. In the high-fat diet mouse model of PDN, we were able to prevent and reverse mechanical allodynia and small-fiber degeneration by limiting CXCR4 signaling or neuronal excitability. This study reveals that excitatory CXCR4/CXCL12 signaling in Nav1.8-positive DRG neurons plays a critical role in the pathogenesis of mechanical allodynia and small-fiber degeneration in a mouse model of PDN. Hence, we propose that targeting CXCR4-mediated DRG nociceptor hyperexcitability is a promising therapeutic approach for disease-modifying treatments for this currently intractable and widespread affliction.

Axonal transport in a peripheral diabetic neuropathy model: sex-dimorphic features

Background

Disruption of axonal transport plays a pivotal role in diabetic neuropathy. A sex-dimorphism exists in the incidence and symptomatology of diabetic neuropathy; however, no studies so far have addressed sex differences in axonal motor proteins expression in early diabetes as well as the possible involvement of neuroactive steroids. Interestingly, recent data point to a role for mitochondria in the sexual dimorphism of neurodegenerative diseases. Mitochondria have a fundamental role in axonal transport by producing the motors’ energy source, ATP. Moreover, neuroactive steroids can also regulate mitochondrial function.

Methods

Here, we investigated the impact of short-term diabetes in the peripheral nervous system of male and female rats on key motor proteins important for axonal transport, mitochondrial function, and neuroactive steroids levels.

Results

We show that short-term diabetes alters mRNA levels and axoplasm protein contents of kinesin family member KIF1A, KIF5B, KIF5A and Myosin Va in male but not in female rats. Similarly, the expression of peroxisome proliferator-activated receptor γ co-activator-1α, a subunit of the respiratory chain complex IV, ATP levels and the key regulators of mitochondrial dynamics were affected in males but not in females. Concomitant analysis of neuroactive steroid levels in sciatic nerve showed an alteration of testosterone, dihydrotestosterone, and allopregnanolone in diabetic males, whereas no changes were observed in female rats.

Conclusions

These findings suggest that sex-specific decrease in neuroactive steroid levels in male diabetic animals may cause an alteration in their mitochondrial function that in turn might impact in axonal transport, contributing to the sex difference observed in diabetic neuropathy.

Proanthocyanidin B2 attenuates high-glucose-induced neurotoxicity of dorsal root ganglion neurons through the PI3K/Akt signaling pathway

High glucose affects primary afferent neurons in dorsal root ganglia by inhibiting neurite elongation, causing oxidative stress, and inducing neuronal apoptosis and mitochondrial dysfunction, which finally result in neuronal damage. Proanthocyanidin, a potent antioxidant, has been shown to have neuroprotective effects. Proanthocyanidin B2 is a common dimer of oligomeric proanthocyanidins. To date, no studies have reported the neuroprotective effects of proanthocyanidin B2 against high-glucose-related neurotoxicity in dorsal root ganglion neurons. In this study, 10 µg/mL proanthocyanidin B2 was used to investigate its effect on 45 mM high-glucose-cultured dorsal root ganglion neurons. We observed that challenge with high levels of glucose increased neuronal reactive oxygen species and promoted apoptosis, decreased cell viability, inhibited outgrowth of neurites, and decreased growth-associated protein 43 protein and mRNA levels. Proanthocyanidin B2 administration reversed the neurotoxic effects caused by glucose challenge. Blockage of the phosphatidylinositol 3 kinase/Akt signaling pathway with 10 µM LY294002 eliminated the protective effects of proanthocyanidin B2. Therefore, proanthocyanidin B2 might be a potential novel agent for the treatment of peripheral diabetic neuropathy.

Common and Divergent Mechanisms in Developmental Neuronal Remodeling and Dying Back Neurodegeneration

Cell death is an inherent process that is required for the proper wiring of the nervous system. Studies over the last four decades have shown that, in a parallel developmental pathway, axons and dendrites are eliminated without the death of the neuron. This developmentally regulated 'axonal death' results in neuronal remodeling, which is an essential mechanism to sculpt neuronal networks in both vertebrates and invertebrates. Studies across various organisms have demonstrated that a conserved strategy in the formation of adult neuronal circuitry often involves generating too many connections, most of which are later eliminated with high temporal and spatial resolution. Can neuronal remodeling be regarded as developmentally and spatially regulated neurodegeneration? It has been previously speculated that injury-induced degeneration (Wallerian degeneration) shares some molecular features with 'dying back' neurodegenerative diseases. In this opinion piece, we examine the similarities and differences between the mechanisms regulating neuronal remodeling and those being perturbed in dying back neurodegenerative diseases. We focus primarily on amyotrophic lateral sclerosis and peripheral neuropathies and highlight possible shared pathways and mechanisms. While mechanistic data are only just beginning to emerge, and despite the inherent differences between disease-oriented and developmental processes, we believe that some of the similarities between these developmental and disease-initiated degeneration processes warrant closer collaborations and crosstalk between these different fields.

Galangin, a natural flavonoid reduces mitochondrial oxidative damage in streptozotocin-induced diabetic rats

Objective: We designed this study to observe the effect of galangin on damaged mitochondria in the liver of diabetic rats.

Methods: Male albino Wistar rats were made diabetic by injecting streptozotocin (STZ) intraperitoneally (40 mg kg⁻¹ body weight (BW)). Galangin (8 mg kg⁻¹ BW) or glibenclamide (600 µg kg⁻¹ BW) was given orally daily once for 45 days to both healthy and diabetic rats.

Results: Diabetic rats showed significant (P < 0.05) increase in liver mitochondrial oxidant [Thiobarbituric acid reactive substance (TBARS)] level and a significant decrease in enzymatic [superoxide dismutase (SOD), glutathione peroxidase (GPx)] and non-enzymatic (reduced glutathione (GSH)) antioxidant levels when compared with healthy rats. The mitochondrial enzymes isocitrate dehydrogenase (ICDH), alpha-ketoglutarate dehydrogenase (α-KGDH), succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) and mitochondrial respiratory chain enzymes NADH-dehydrogenase and Cytochrome c-oxidase were decreased significantly (P < 0.05) in diabetic rats when compared with healthy rats. A natural flavonoid galangin administered to hyperglycemia-induced rats resulted in the following findings as compared to hyperglycemia-induced control rats: the oxidant levels decreased significantly (P < 0.05); the enzymatic and non-enzymatic antioxidant levels increased significantly (P < 0.05) and the function of mitochondrial enzymes and the mitochondrial respiratory chain enzymes increased significantly (P < 0.05).

Conclusion: From the results, we conclude that galangin could maintain liver mitochondrial function in diabetic rats.

Insulin prevents aberrant mitochondrial phenotype in sensory neurons of type 1 diabetic rats

Diabetic neuropathy affects approximately 50% of diabetic patients. Down-regulation of mitochondrial gene expression and function has been reported in both human tissues and in dorsal root ganglia (DRG) from animal models of type 1 and type 2 diabetes. We hypothesized that loss of direct insulin signaling in diabetes contributes to loss of mitochondrial function in DRG neurons and to development of neuropathy. Sensory neurons obtained from age-matched adult control or streptozotocin (STZ)-induced type 1 diabetic rats were cultured with or without insulin before determining mitochondrial respiration and expression of mitochondrial respiratory chain and insulin signaling-linked proteins. For in vivo studies age-matched control rats and diabetic rats with or without trace insulin supplementation were maintained for 5months before DRG were analyzed for respiratory chain gene expression and cytochrome c oxidase activity. Insulin (10nM) significantly (P<0.05) increased phosphorylation of Akt and P70S6K by 4-fold and neurite outgrowth by 2-fold in DRG cultures derived from adult control rats. Insulin also augmented the levels of selective mitochondrial respiratory chain proteins and mitochondrial bioenergetics parameters in DRG cultures from control and diabetic rats, with spare respiratory capacity increased by up to 3-fold (P<0.05). Insulin-treated diabetic animals exhibited improved thermal sensitivity in the hind paw and had increased dermal nerve density compared to untreated diabetic rats, despite no effect on blood glucose levels. In DRG of diabetic rats there was suppressed expression of mitochondrial respiratory chain proteins and cytochrome c oxidase activity that was corrected by insulin therapy. Insulin elevates mitochondrial respiratory chain protein expression and function in sensory neurons and this is associated with enhanced neurite outgrowth and protection against indices of neuropathy.

Dorsal Root Ganglia Mitochondrial Biochemical Changes in Non-diabetic and Streptozotocin-Induced Diabetic Mice Fed with a Standard or High-Fat Diet

BACKGROUND

Mitochondrial dysfunction is purported as a contributory mechanism underlying diabetic neuropathy, but a defined role for damaged mitochondria in diabetic nerves remains unclear, particularly in standard diabetes models. Experiments here used a high-fat diet in attempt to exacerbate the severity of diabetes and expedite the time-course in which mitochondrial dysfunction may occur. We hypothesized a high-fat diet in addition to diabetes would increase stress on sensory neurons and worsen mitochondrial dysfunction.

METHODS

Oxidative phosphorylation proteins and proteins associated with mitochondrial function were quantified in lumbar dorsal root ganglia. Comparisons were made between non-diabetic and streptozotocin-induced (STZ) C57Bl/6 mice fed a standard or high-fat diet for 8 weeks.

RESULTS

Complex III subunit Core-2 and voltage dependent anion channel were increased (by 36% and 28% respectively, p<0.05) in diabetic mice compared to nondiabetic mice fed the standard diet. There were no differences among groups in UCP2, PGC-1α, PGC-1β levels or Akt, mTor, or AMPK activation. These data suggest compensatory mitochondrial biogenesis occurs to offset potential mitochondrial dysfunction after 8 weeks of STZ-induced diabetes, but a high-fat diet does not alter these parameters.

CONCLUSION

Our results indicate mitochondrial protein changes early in STZ-induced diabetes. Interestingly, a high-fat diet does not appear to affect mitochondrial proteins in either nondiabetic or STZ- diabetic mice.

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.

VEGF-B promotes recovery of corneal innervations and trophic functions in diabetic mice

Vascular endothelial growth factor (VEGF)-B possesses the capacity of promoting injured peripheral nerve regeneration and restore their sensory and trophic functions. However, the contribution and mechanism of VEGF-B in diabetic peripheral neuropathy remains unclear. In the present study, we investigated the expression and role of VEGF-B in diabetic corneal neuropathy by using type 1 diabetic mice and cultured trigeminal ganglion (TG) neurons. Hyperglycemia attenuated the endogenous expression of VEGF-B in regenerated diabetic corneal epithelium, but not that of VEGF receptors in diabetic TG neurons and axons. Exogenous VEGF-B promoted diabetic corneal nerve fiber regeneration through the reactivation of PI-3K/Akt-GSK3β-mTOR signaling and the attenuation of neuronal mitochondria dysfunction via the VEGF receptor-1 and neuropilin-1. Moreover, VEGF-B improved corneal sensation and epithelial regeneration in both normal and diabetic mice, accompanied with the elevated corneal content of pigment epithelial-derived factor (PEDF). PEDF blockade partially abolished trophic function of VEGF-B in diabetic corneal re-innervation. In conclusion, hyperglycemia suppressed endogenous VEGF-B expression in regenerated corneal epithelium of diabetic mice, while exogenous VEGF-B promoted recovery of corneal innervations and trophic functions through reactivating PI-3K/Akt-GSK-3β-mTOR signaling, attenuating neuronal oxidative stress and elevating PEDF expression.

Lipids: A Suitable Therapeutic Target in Diabetic Neuropathy?

Diabetic polyneuropathy (DPN) encompasses multiple syndromes with a common pathogenesis. Glycemic control shows a limited correlation with DPN, arguing in favor of major involvement of other factors, one of which is alterations of lipid and lipoprotein metabolism. Consistent associations have been found between plasma triglycerides/remnant lipoproteins and the risk of DPN. Studies in cultured nerve tissue or in murine models of diabetes have unveiled mechanisms linking lipid metabolism to DPN. Deficient insulin action increases fatty acids flux to nerve cells, inducing mitochondrial dysfunction, anomalous protein kinase C signaling, and perturbations in the physicochemical properties of the plasma membrane. Oxidized low-density lipoproteins bind to cellular receptors and promote generation of reactive oxygen species, worsening mitochondrial function and altering the electrical properties of neurons. Supplementation with specific fatty acids has led to prevention or reversal of different modalities of DPN in animal models. Post hoc and secondary analyses of clinical trials have found benefits of cholesterol reducing (statins and ezetimibe), triglyceride-reducing (fibrates), or lipid antioxidant (thioctic acid) therapies over the progression and severity of DPN. However, these findings are mostly hypothesis-generating. Randomized trials are warranted in which the impact of intensive plasma lipids normalization on DPN outcomes is specifically evaluated.