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.

Misregulation of mitochondria-lysosome contact dynamics in Charcot-Marie-Tooth Type 2B disease Rab7 mutant sensory peripheral neurons.

Inter-organelle contact sites between mitochondria and lysosomes mediate the crosstalk and bidirectional regulation of their dynamics in health and disease. However, mitochondria-lysosome contact sites and their misregulation have not been investigated in peripheral sensory neurons. Charcot-Marie-Tooth type 2B disease is an autosomal dominant axonal neuropathy affecting peripheral sensory neurons caused by mutations in the GTPase Rab7. Using live super-resolution and confocal time-lapse microscopy, we showed that mitochondria-lysosome contact sites dynamically form in the soma and axons of peripheral sensory neurons. Interestingly, Charcot-Marie-Tooth type 2B mutant Rab7 led to prolonged mitochondria-lysosome contact site tethering preferentially in the axons of peripheral sensory neurons, due to impaired Rab7 GTP hydrolysis-mediated contact site untethering. We further generated a Charcot-Marie-Tooth type 2B mutant Rab7 knock-in mouse model which exhibited prolonged axonal mitochondria-lysosome contact site tethering and defective downstream axonal mitochondrial dynamics due to impaired Rab7 GTP hydrolysis as well as fragmented mitochondria in the axon of the sciatic nerve. Importantly, mutant Rab7 mice further demonstrated preferential sensory behavioral abnormalities and neuropathy, highlighting an important role for mutant Rab7 in driving degeneration of peripheral sensory neurons. Together, this study identifies an important role for mitochondria-lysosome contact sites in the pathogenesis of peripheral neuropathy.

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.

Cutaneous innervation in impaired diabetic wound healing.

Type 2 diabetes is associated with several potential comorbidities, among them impaired wound healing, chronic ulcerations, and the requirement for lower extremity amputation. Disease-associated abnormal cellular responses, infection, immunological and microvascular dysfunction, and peripheral neuropathy are implicated in the pathogenesis of the wound healing impairment and the diabetic foot ulcer. The skin houses a dense network of sensory nerve afferents and nerve-derived modulators, which communicate with epidermal keratinocytes and dermal fibroblasts bidirectionally to effect normal wound healing after trauma. However, the mechanisms through which cutaneous innervation modulates wound healing are poorly understood, especially in humans. Better understanding of these mechanisms may provide the basis for targeted treatments for chronic diabetic wounds. This review provides an overview of wound healing pathophysiology with a focus on neural involvement in normal and diabetic wound healing, as well as future therapeutic perspectives to address the unmet needs of diabetic patients with chronic wounds.

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.

Chemogenetic Inhibition of Pain Neurons in a Mouse Model of Osteoarthritis.

OBJECTIVE: To determine the ability of drugs that activate inhibitory G protein-coupled receptors (GPCRs) expressed in peripheral voltage-gated sodium channel 1.8 (NaV 1.8)-positive sensory neurons to control osteoarthritis (OA)-associated pain. METHODS: We used designer receptors exclusively activated by a designer drug (DREADD) technology, which employs engineered GPCRs to activate or inhibit neurons upon binding the synthetic ligand clozapine N-oxide (CNO). NaV 1.8-Pdi C57BL/6 mice were generated to express the inhibitory DREADD receptor Pdi in NaV 1.8-expressing sensory neurons. Destabilization of the medial meniscus (DMM) surgery was performed in 10-week-old male mice. Four, 8, 12, or 16 weeks after surgery, knee hyperalgesia or hind paw mechanical allodynia was tested. Subsequently, CNO or vehicle was administered, and the effect on pain-related behaviors was measured by a blinded observer. Morphine was used as a control. RESULTS: Immunohistochemistry and electrophysiology confirmed functional expression of the inhibitory DREADD receptor Pdi by NaV 1.8-positive sensory neurons. Acute inhibition of NaV 1.8-expressing neurons in mice treated with CNO reduced knee hyperalgesia 4 weeks after DMM surgery and reduced mechanical allodynia 8 weeks after DMM surgery. Inhibition had no effect on pain-related behaviors 12 and 16 weeks after DMM surgery. Morphine, a drug that activates GPCRs in the peripheral and central nervous systems, was still effective in the later stage of experimental OA. CONCLUSION: Chemogenetic inhibition of NaV 1.8-expressing neurons blocks knee hyperalgesia and mechanical allodynia in early experimental OA, but is no longer efficacious in the later stages. These data indicate that activation of inhibitory GPCRs located solely outside the central nervous system may be ineffective in treating chronic OA pain.

Ganglioside GM3 synthase depletion reverses neuropathic pain and small fiber neuropathy in diet-induced diabetic mice.

BACKGROUND: Small fiber neuropathy is a well-recognized complication of type 2 diabetes and has been shown to be responsible for both neuropathic pain and impaired wound healing. In previous studies, we have demonstrated that ganglioside GM3 depletion by knockdown of GM3 synthase fully reverses impaired wound healing in diabetic mice. However, the role of GM3 in neuropathic pain and small fiber neuropathy in diabetes is unknown. PURPOSE: Determine whether GM3 depletion is able to reverse neuropathic pain and small fibers neuropathy and the mechanism of the reversal. RESULTS: We demonstrate that GM3 synthase knockout and the resultant GM3 depletion rescues the denervation in mouse footpad skin and fully reverses the neuropathic pain in diet-induced obese diabetic mice. In cultured dorsal root ganglia from diet-induced diabetic mice, GM3 depletion protects against increased intracellular calcium influx in vitro. CONCLUSIONS: These studies establish ganglioside GM3 as a new candidate responsible for neuropathic pain and small fiber neuropathy in diabetes. Moreover, these observations indicate that systemic or topically applied interventions aimed at depleting GM3 may improve both the painful neuropathy and the wound healing impairment in diabetes by protecting against nerve end terminal degeneration, providing a disease-modifying approach to this common, currently intractable medical issue.

Genetic and physiological evidence that oligodendrocyte gap junctions contribute to spatial buffering of potassium released during neuronal activity.

Mice lacking the K+ channel Kir4.1 or both connexin32 (Cx32) and Cx47 exhibit myelin-associated vacuoles, raising the possibility that oligodendrocytes, and the connexins they express, contribute to recycling the K+ evolved during neuronal activity. To study this possibility, we first examined the effect of neuronal activity on the appearance of vacuoles in mice lacking both Cx32 and Cx47. The size and number of myelin vacuoles was dramatically increased when axonal activity was increased, by either a natural stimulus (eye opening) or pharmacological treatment. Conversely, myelin vacuoles were dramatically reduced when axonal activity was suppressed. Second, we used genetic complementation to test for a relationship between the function of Kir4.1 and oligodendrocyte connexins. In a Cx32-null background, haploinsufficiency of either Cx47 or Kir4.1 did not affect myelin, but double heterozygotes developed vacuoles, consistent with the idea that oligodendrocyte connexins and Kir4.1 function in a common pathway. Together, these results implicate oligodendrocytes and their connexins as having critical roles in the buffering of K+ released during neuronal activity.

Connexins are critical for normal myelination in the CNS.

Mutations in Cx32, a gap-junction channel-forming protein, result in X-linked Charcot-Marie-Tooth disease, a demyelinating disease of the peripheral nervous system. However, although oligodendrocytes express Cx32, central myelination is unaffected. To explore this discrepancy, we searched for additional oligodendrocyte connexins. We found Cx47, which is expressed specifically in oligodendrocytes, regulated in parallel with myelin genes and partially colocalized with Cx32 in oligodendrocytes. Mice lacking either Cx47 or Cx32 are viable. However, animals lacking both connexins die by postnatal week 6 from profound abnormalities in central myelin, characterized by thin or absent myelin sheaths, vacuolation, enlarged periaxonal collars, oligodendrocyte cell death, and axonal loss. These data provide the first evidence that gap-junction communication is crucial for normal central myelination.