Prevention of vincristine-induced peripheral neuropathy by genetic deletion of SARM1 in mice

Permeant fluorescent probes visualize the activation of SARM1 and uncover an anti-neurodegenerative drug candidate

SARM1 regulates axonal degeneration through its NAD-metabolizing activity and is a drug target for neurodegenerative disorders. We designed and synthesized fluorescent conjugates of styryl derivative with pyridine to serve as substrates of SARM1, which exhibited large red shifts after conversion. With the conjugates, SARM1 activation was visualized in live cells following elevation of endogenous NMN or treatment with a cell-permeant NMN-analog. In neurons, imaging documented mouse SARM1 activation preceded vincristine-induced axonal degeneration by hours. Library screening identified a derivative of nisoldipine (NSDP) as a covalent inhibitor of SARM1 that reacted with the cysteines, especially Cys311 in its ARM domain and blocked its NMN-activation, protecting axons from degeneration. The Cryo-EM structure showed that SARM1 was locked into an inactive conformation by the inhibitor, uncovering a potential neuroprotective mechanism of dihydropyridines.

Structural and Mechanistic Regulation of the Pro-degenerative NAD Hydrolase SARM1

The NADase SARM1 is a central switch in injury-activated axon degeneration, an early hallmark of many neurological diseases. Here, we present cryo-electron microscopy (cryo-EM) structures of autoinhibited (3.3 Å) and active SARM1 (6.8 Å) and provide mechanistic insight into the tight regulation of SARM1's function by the local metabolic environment. Although both states retain an octameric core, the defining feature of the autoinhibited state is a lock between the autoinhibitory Armadillo/HEAT motif (ARM) and catalytic Toll/interleukin-1 receptor (TIR) domains, which traps SARM1 in an inactive state. Mutations that break this lock activate SARM1, resulting in catastrophic neuronal death. Notably, the mutants cannot be further activated by the endogenous activator nicotinamide mononucleotide (NMN), and active SARM1 is product inhibited by Nicotinamide (NAM), highlighting SARM1's functional dependence on key metabolites in the NAD salvage pathway. Our studies provide a molecular understanding of SARM1's transition from an autoinhibited to an injury-activated state and lay the foundation for future SARM1-based therapies to treat axonopathies.

Sarm1-mediated neurodegeneration within the enteric nervous system protects against local inflammation of the colon

Axonal degeneration is one of the key features of neurodegenerative disorders. In the canonical view, axonal degeneration destructs neural connections and promotes detrimental disease defects. Here, we assessed the enteric nervous system (ENS) of the mouse, non-human primate, and human by advanced 3D imaging. We observed the profound neurodegeneration of catecholaminergic axons in human colons with ulcerative colitis, and similarly, in mouse colons during acute dextran sulfate sodium-induced colitis. However, we unexpectedly revealed that blockage of such axonal degeneration by the Sarm1 deletion in mice exacerbated the colitis condition. In contrast, pharmacologic ablation or chemogenetic inhibition of catecholaminergic axons suppressed the colon inflammation. We further showed that the catecholaminergic neurotransmitter norepinephrine exerted a pro-inflammatory function by enhancing the expression of IL-17 cytokines. Together, this study demonstrated that Sarm1-mediated neurodegeneration within the ENS mitigated local inflammation of the colon, uncovering a previously-unrecognized beneficial role of axonal degeneration in this disease context.

SARM1 is a metabolic sensor activated by an increased NMN/NAD+ ratio to trigger axon degeneration

Axon degeneration is a central pathological feature of many neurodegenerative diseases. Sterile alpha and Toll/interleukin-1 receptor motif-containing 1 (SARM1) is a nicotinamide adenine dinucleotide (NAD+)-cleaving enzyme whose activation triggers axon destruction. Loss of the biosynthetic enzyme NMNAT2, which converts nicotinamide mononucleotide (NMN) to NAD+, activates SARM1 via an unknown mechanism. Using structural, biochemical, biophysical, and cellular assays, we demonstrate that SARM1 is activated by an increase in the ratio of NMN to NAD+ and show that both metabolites compete for binding to the auto-inhibitory N-terminal armadillo repeat (ARM) domain of SARM1. We report structures of the SARM1 ARM domain bound to NMN and of the homo-octameric SARM1 complex in the absence of ligands. We show that NMN influences the structure of SARM1 and demonstrate via mutagenesis that NMN binding is required for injury-induced SARM1 activation and axon destruction. Hence, SARM1 is a metabolic sensor responding to an increased NMN/NAD+ ratio by cleaving residual NAD+, thereby inducing feedforward metabolic catastrophe and axonal demise.

SARM1 acts downstream of neuroinflammatory and necroptotic signaling to induce axon degeneration

Neuroinflammation and necroptosis are major contributors to neurodegenerative disease, and axon dysfunction and degeneration is often an initiating event. SARM1 is the central executioner of pathological axon degeneration. Here, we demonstrate functional and mechanistic links among these three pro-degenerative processes. In a neuroinflammatory model of glaucoma, TNF-α induces SARM1-dependent axon degeneration, oligodendrocyte loss, and subsequent retinal ganglion cell death. TNF-α also triggers SARM1-dependent axon degeneration in sensory neurons via a noncanonical necroptotic signaling mechanism. MLKL is the final executioner of canonical necroptosis; however, in axonal necroptosis, MLKL does not directly trigger degeneration. Instead, MLKL induces loss of the axon survival factors NMNAT2 and STMN2 to activate SARM1 NADase activity, which leads to calcium influx and axon degeneration. Hence, these findings define a specialized form of axonal necroptosis. The demonstration that neuroinflammatory signals and necroptosis can act locally in the axon to stimulate SARM1-dependent axon degeneration identifies a therapeutically targetable mechanism by which neuroinflammation can stimulate axon loss in neurodegenerative disease.

Small extracellular vesicles ameliorate peripheral neuropathy and enhance chemotherapy of oxaliplatin on ovarian cancer

There are no effective treatments for chemotherapy induced peripheral neuropathy (CIPN). Small extracellular vesicles (sEVs) facilitate intercellular communication and mediate nerve function and tumour progression. We found that the treatment of mice bearing ovarian tumour with sEVs derived from cerebral endothelial cells (CEC-sEVs) in combination with a chemo-drug, oxaliplatin, robustly reduced oxaliplatin-induced CIPN by decreasing oxaliplatin-damaged myelination and nerve fibres of the sciatic nerve and significantly amplified chemotherapy of oxaliplatin by reducing tumour size. The combination therapy substantially increased a set of sEV cargo-enriched miRNAs, but significantly reduced oxaliplatin-increased proteins in the sciatic nerve and tumour tissues. Bioinformatics analysis revealed the altered miRNAs and proteins formed two distinct networks that regulate neuropathy and tumour growth, respectively. Intravenously administered CEC-sEVs were internalized by axons of the sciatic nerve and cancer cells. Reduction of CEC-sEV cargo miRNAs abolished the effects of CEC-sEVs on oxaliplatin-inhibited axonal growth and on amplification of the anti-cancer effect in ovarian cancer cells, suggesting that alterations in the networks of miRNAs and proteins in recipient cells contribute to the therapeutic effect of CEC-sEVs on CIPN. Together, the present study demonstrates that CEC-sEVs suppressed CIPN and enhanced chemotherapy of oxaliplatin in the mouse bearing ovarian tumour.

Neuroinflammation in Prion Disease

Neuroinflammation, typically manifest as microglial activation and astrogliosis accompanied by transcriptomic alterations, represents a common hallmark of various neurodegenerative conditions including prion diseases. Microglia play an overall neuroprotective role in prion disease, whereas reactive astrocytes with aberrant phenotypes propagate prions and contribute to prion-induced neurodegeneration. The existence of heterogeneous subpopulations and dual functions of microglia and astrocytes in prion disease make them potential targets for therapeutic intervention. A variety of neuroinflammation-related molecules are involved in prion pathogenesis. Therapeutics targeting neuroinflammation represents a novel approach to combat prion disease. Deciphering neuroinflammation in prion disease will deepen our understanding of pathogenesis of other neurodegenerative disorders.

Emerging Pharmacological and Non-Pharmacological Therapeutics for Prevention and Treatment of Chemotherapy-Induced Peripheral Neuropathy

Simple Summary

Chemotherapy-induced peripheral neuropathy (CIPN) is a common and persistent complication of commonly used chemotherapy drugs. This article provides an overview of emerging therapeutics for the prevention and treatment of CIPN and focuses on pharmacological strategies that are derived from novel mechanistic insights and have the potential to be translated into clinically beneficial approaches. It is our contention to call for fostering collaboration between basic and clinical researchers to improve the development of effective strategies.

Abstract

Chemotherapy-induced peripheral neuropathy (CIPN) is a common adverse event of several first-line chemotherapeutic agents, including platinum compounds, taxanes, vinca alkaloids, thalidomide, and bortezomib, which negatively affects the quality of life and clinical outcome. Given the dearth of effective established agents for preventing or treating CIPN, and the increasing number of cancer survivors, there is an urgent need for the identification and development of new, effective intervention strategies that can prevent or mitigate this debilitating side effect. Prior failures in the development of effective interventions have been due, at least in part, to a lack of mechanistic understanding of CIPN and problems in translating this mechanistic understanding into testable hypotheses in rationally-designed clinical trials. Recent progress has been made, however, in the pathogenesis of CIPN and has provided new targets and pathways for the development of emerging therapeutics that can be explored clinically to improve the management of this debilitating toxicity. This review focuses on the emerging therapeutics for the prevention and treatment of CIPN, including pharmacological and non-pharmacological strategies, and calls for fostering collaboration between basic and clinical researchers to improve the development of effective strategies.

SARM1 is required in human derived sensory neurons for injury-induced and neurotoxic axon degeneration

Axonal degeneration contributes to the pathogenesis of many neurodegenerative disorders, motivating efforts to dissect the mechanism of pathological axon loss in order to develop therapies for axonal preservation. SARM1 is a particularly attractive therapeutic target, as it is an inducible NAD⁺ cleaving enzyme that is required for axon loss in multiple mouse models of traumatic and degenerative neurological disease. However, it is essential to establish whether SARM1 triggers axon degeneration in human neurons before proceeding with the development of SARM1-directed therapeutics. Here we combine genome engineering with the production of human stem cell-derived neurons to test the role of human SARM1 in traumatic and neurotoxic axon degeneration. We have generated two independent SARM1 knockout human iPSC lines that do not express SARM1 protein upon differentiation into neurons. We have developed a modified sensory neuron differentiation protocol that generates human sensory neurons with high yield and purity. We find that SARM1 is required for axon degeneration in response to both physical trauma and in a cellular model of chemotherapy-induced peripheral neuropathy. Finally, we identify cADPR as a biomarker of SARM1 enzyme activity in both healthy and injured human sensory neurons. These findings are consistent with prior molecular and cellular studies in mouse neurons, and highlight the therapeutic potential of SARM1 inhibition for the prevention and treatment of human neurological disease.

Phosphorylation at S548 as a Functional Switch of Sterile Alpha and TIR Motif-Containing 1 in Cerebral Ischemia/Reperfusion Injury in Rats

Sterile alpha and Toll/interleukin-1 receptor motif-containing 1 (SARM1) is a pro-degenerative molecule in Wallerian degeneration, which is mainly expressed in brain/neurons and colocalized with mitochondria and microtubules. The aim of this study was to determine the role of SARM1 in cerebral ischemia/reperfusion (I/R) injury and the underlying mechanisms. In vivo, a middle cerebral artery occlusion/reperfusion (MCAO/R) model in adult male Sprague Dawley rats (250-300 g) was established, and in vitro, cultured primary neurons were subjected to oxygen-glucose deprivation/reoxygenation (OGD/R) to imitate I/R injury. Overexpression lentiviruses encoding wild-type SARM1 and SARM1 with serine 548 alanine mutation (S548A) were constructed and administered to rats by intra-penumbral injection. First, the potential role of SARM1 in cerebral I/R injury was confirmed by the increased protein levels of SARM1 within penumbra tissue, especially in neurons. Second, there was an increase in the phosphorylation ratio of p-SARM1(S548)/SARM1 at 2 h after MCAO/R. Third, on the basis of site-specific mutagenesis, we identified S548 as a key site for SARM1 phosphorylation in I/R conditions. Fourth, SARM1 (S548A) overexpression reduced infarct size, neuronal death, and neurobehavioral dysfunction, while wild-type SARM1 overexpression had the opposite effects. Finally, we found that SARM1 phosphorylation at the S548 site switched SARM1 function from promoting mitochondrial transport to inhibiting mitochondrial transport along axons after I/R injury. Restriction of SARM1 phosphorylation at S548 may be a promising intervention target for I/R injury; thus, exogenous administration of SARM1 (S548A) may be a novel strategy for improving neurological outcomes.

Vincristine- and bortezomib-induced neuropathies - from bedside to bench and back

Vincristine and bortezomib are effective chemotherapeutics widely used to treat hematological cancers. Vincristine blocks tubulin polymerization, whereas bortezomib is a proteasome inhibitor. Despite different mechanisms of action, the main non-hematological side effect of both is peripheral neuropathy that can last long after treatment has ended and cause permanent disability. Many different cellular and animal models of various aspects of vincristine and bortezomib-induced neuropathies have been generated to investigate underlying molecular mechanisms and serve as platforms to develop new therapeutics. These models revealed that bortezomib induces several transcriptional programs in dorsal root ganglia that result in the activation of different neuroinflammatory pathways and secondary central sensitization. In contrast, vincristine has direct toxic effects on the axon, which are accompanied by changes similar to those observed after nerve cut. Axon degeneration following both vincristine and bortezomib is mediated by a phylogenetically ancient, genetically encoded axon destruction program that leads to the activation of the Toll-like receptor adaptor SARM1 (sterile alpha and TIR motif containing protein 1) and local decrease of nicotinamide dinucleotide (NAD+). Here, I describe current in vitro and in vivo models of vincristine- and bortezomib induced neuropathies, present discoveries resulting from these models in the context of clinical findings and discuss how increased understanding of molecular mechanisms underlying different aspects of neuropathies can be translated to effective treatments to prevent, attenuate or reverse vincristine- and bortezomib-induced neuropathies. Such treatments could improve the quality of life of patients both during and after cancer therapy and, accordingly, have enormous societal impact.

Protection against oxaliplatin-induced mechanical and thermal hypersensitivity in Sarm1-/- mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side effect of cancer treatment, often associated with degeneration of sensory axons or their terminal regions. Presence of the slow Wallerian degeneration protein (WLD^S), or genetic deletion of sterile alpha and TIR motif containing protein 1 (SARM1), which strongly protect axons from degeneration after injury or axonal transport block, alleviate pain in several CIPN models. However, oxaliplatin can cause an acute pain response, suggesting a different mechanism of pain generation. Here, we tested whether the presence of WLD^S or absence of SARM1 protects against acute oxaliplatin-induced pain in mice after a single oxaliplatin injection. In BL/6 and Wld^S mice, oxaliplatin induced significant mechanical and cold hypersensitivities which were absent in Sarm1^-/- mice. Despite the presence of hypersensitivity there was no significant loss of intraepidermal nerve fibers (IENFs) in the footpads of any mice after oxaliplatin treatment, suggesting that early stages of pain hypersensitivity could be independent of axon degeneration. To identify other changes that could underlie the pain response, RNA sequencing was carried out in DRGs from treated and control mice of each genotype. Sarm1^-/- mice had fewer gene expression changes than either BL/6 or Wld^S mice. This is consistent with the pain measurements in demonstrating that Sarm1^-/- DRGs remain relatively unchanged after oxaliplatin treatment, unlike those in BL/6 and Wld^S mice. Changes in levels of four transcripts - Alas2, Hba-a1, Hba-a2, and Tfrc - correlated with oxaliplatin-induced pain, or absence thereof, across the three genotypes. Our findings suggest that targeting SARM1 could be a viable therapeutic approach to prevent oxaliplatin-induced acute neuropathic pain.

The neuroprotective effect of oxytocin on vincristine-induced neurotoxicity in mice

Vincristine (VCR) is commonly used to treat a variety of hematological malignancies and solid tumors in pediatric and adult patients. However, peripheral neuropathy is a dose-limiting side effect that leaves some patients with functional disability and long-term pain. Oxytocin (OT) has demonstrated analgesic and anti-inflammatory properties, but there is no evidence regarding its effects on VCR-induced neurotoxicity. Therefore, we evaluated the potential protective effects of OT on VCR-induced neurotoxicity. In vitro, VCR (0.005 ∼ 0.1 μmol/l) and OT (10^-8 ∼ 10^-5 mol/l) were added into cultured primary dorsal root ganglion (DRG) neurons of mice. The length of neurites was counted by using immunofluorescence. In vivo, neurotoxicity was induced in mice by administration of VCR (0.1 mg/kg, intraperitoneal injection for 14 days) with or without pretreatment of OT (0.1 mg/kg or 1 mg/kg). Atosiban, an OT receptor (OTR) antagonist and OTR knockout (KO) mice were used for evaluating effects of OTR. Mechanical hyperalgesia was measured by using von Frey filaments. Histology of plantar skin, sciatic nerve and DRG was observed by using transmission electron microscopy (TEM) and hematoxylin-eosin (HE) staining. Results indicated that OT alleviated VCR-induced neurite damage in cultured primary DRG neurons in vitro. In vivo, OT ameliorated VCR-induced hyperalgesia. Histologically, OT attenuated the VCR-induced damages of nerve endings, myelin sheaths and Schwann cells in sciatic nerve and DRG. These effects were antagonized by atosiban. In addition, OTR knockout mice exhibited more severe hyperalgesia than wild-type mice. Globally, these results indicated that OT may have neuroprotective effects on vincristine-induced neurotoxicity in mice.

Small Molecule SARM1 Inhibitors Recapitulate the SARM1-/- Phenotype and Allow Recovery of a Metastable Pool of Axons Fated to Degenerate

Axonal degeneration is responsible for disease progression and accumulation of disability in many neurodegenerative conditions. The axonal degenerative process can generate a metastable pool of damaged axons that remain structurally and functionally viable but fated to degenerate in the absence of external intervention. SARM1, an NADase that depletes axonal energy stores upon activation, is the central driver of an evolutionarily conserved program of axonal degeneration. We identify a potent and selective small molecule isoquinoline inhibitor of SARM1 NADase that recapitulates the SARM1^-/- phenotype and protects axons from degeneration induced by axotomy or mitochondrial dysfunction. SARM1 inhibition post-mitochondrial injury with rotenone allows recovery and rescues axons that already entered the metastable state. We conclude that SARM1 inhibition with small molecules has the potential to treat axonopathies of the central and peripheral nervous systems by preventing axonal degeneration and by allowing functional recovery of a metastable pool of damaged, but viable, axons.

Sodium channel Nav1.6 in sensory neurons contributes to vincristine-induced allodynia

Vincristine, a widely used chemotherapeutic agent, produces painful peripheral neuropathy. The underlying mechanisms are not well understood. In this study, we investigated whether voltage-gated sodium channels are involved in the development of vincristine-induced neuropathy. We established a mouse model in which repeated systemic vincristine treatment results in the development of significant mechanical allodynia. Histological examinations did not reveal major structural changes at proximal sciatic nerve branches or distal toe nerve fascicles at the vincristine dose used in this study. Immunohistochemical studies and in vivo two-photon imaging confirmed that there is no significant change in density or morphology of intra-epidermal nerve terminals throughout the course of vincristine treatment. These observations suggest that nerve degeneration is not a prerequisite of vincristine-induced mechanical allodynia in this model. We also provided the first detailed characterization of tetrodotoxin-sensitive (TTX-S) and resistant (TTX-R) sodium currents in dorsal root ganglion neurons following vincristine treatment. Accompanying the behavioural hyperalgesia phenotype, voltage-clamp recordings of small and medium dorsal root ganglion neurons from vincristine-treated animals revealed a significant upregulation of TTX-S Na+ current in medium but not small neurons. The increase in TTX-S Na+ current density is likely mediated by Nav1.6, because in the absence of Nav1.6 channels, vincristine failed to alter TTX-S Na+ current density in medium dorsal root ganglion neurons and, importantly, mechanical allodynia was significantly attenuated in conditional Nav1.6 knockout mice. Our data show that TTX-S sodium channel Nav1.6 is involved in the functional changes of dorsal root ganglion neurons following vincristine treatment and it contributes to the maintenance of vincristine-induced mechanical allodynia.

Cisplatin induced neurotoxicity is mediated by Sarm1 and calpain activation

Cisplatin is a commonly used chemotherapy agent with significant dose-limiting neurotoxicity resulting in peripheral neuropathy. Although it is postulated that formation of DNA-platinum adducts is responsible for both its cytotoxicity in cancer cells and side effects in neurons, downstream mechanisms that lead to distal axonal degeneration are unknown. Here we show that activation of calpains is required for both neurotoxicity and formation of DNA-platinum adduct formation in neurons but not in cancer cells. Furthermore, we show that neurotoxicity of cisplatin requires activation of Sarm1, a key regulator of Wallerian degeneration, as mice lacking the Sarm1 gene do not develop peripheral neuropathy as evaluated by both behavioral or pathological measures. These findings indicate that Sarm1 and/or specific calpain inhibitors could be developed to prevent cisplatin induced peripheral neuropathy.

An overview of ongoing clinical trials assessing pharmacological therapeutic strategies to manage chemotherapy-induced peripheral neuropathy, based on preclinical studies in rodent models

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect induced by a variety of chemotherapeutic agents. Symptoms are mainly sensory: pain, tingling, numbness, and temperature sensitivity. They may require the tapering of chemotherapy regimens or even their cessation; thus, the prevention/treatment of CIPN is critical to increase effectiveness of cancer treatment. However, CIPN management is mainly based on conventional neuropathic pain treatments, with poor clinical efficacy. Therefore, significant effort is made to identify new pharmacological targets to prevent/treat CIPN. Animal modeling is a key component in predicting human response to drugs and in understanding the pathophysiological mechanisms underlying CIPN. In fact, studies performed in rodents highlighted several pharmacological targets to treat/prevent CIPN. This review provides updated information about ongoing clinical trials testing drugs for the management of CIPN and presents some of their proof-of-concept studies conducted in rodent models. The presented drugs target oxidative stress, renin-angiotensin system, glutamatergic neurotransmission, sphingolipid metabolism, neuronal uptake transporters, nicotinamide adenine dinucleotide metabolism, endocannabinoid system, transient receptor potential channels, and serotoninergic receptors. As some clinical trials focus on the effect of the drugs on pain, others evaluate their efficacy by assessing general neuropathy. Moreover, based on studies conducted in rodent models, it remains unclear if some of the tested drugs act in an antinociceptive fashion or have neuroprotective properties. Thus, further investigations are needed to understand their mechanism of action, as well as a global standardization of the methods used to assess efficacy of new therapeutic strategies in the treatment of CIPN.

Structural basis for SARM1 inhibition and activation under energetic stress

SARM1, an executor of axonal degeneration, displays NADase activity that depletes the key cellular metabolite, NAD+, in response to nerve injury. The basis of SARM1 inhibition and its activation under stress conditions are still unknown. Here, we present cryo-EM maps of SARM1 at 2.9 and 2.7 Å resolutions. These indicate that SARM1 homo-octamer avoids premature activation by assuming a packed conformation, with ordered inner and peripheral rings, that prevents dimerization and activation of the catalytic domains. This inactive conformation is stabilized by binding of SARM1's own substrate NAD+ in an allosteric location, away from the catalytic sites. This model was validated by mutagenesis of the allosteric site, which led to constitutively active SARM1. We propose that the reduction of cellular NAD+ concentration contributes to the disassembly of SARM1's peripheral ring, which allows formation of active NADase domain dimers, thereby further depleting NAD+ to cause an energetic catastrophe and cell death.

Effect of lubiprostone on vinca alkaloid-induced constipation in patients with hematological malignancies: a propensity score-matched analysis

Patients receiving vinca alkaloids for hematological malignancies frequently experience constipation that is unresponsive to laxatives. Research on treatment of vinca alkaloid-induced constipation is limited. This study aimed to determine whether the chloride channel activator lubiprostone ameliorates vinca alkaloid-induced constipation in patients with hematological malignancies. In this retrospective cohort study, vinca alkaloid-induced constipation (grade ≥ 3 using the Common Terminology Criteria for Adverse Events) was investigated in patients treated for hematological malignancies between July 2014 and June 2019 who had already been prescribed osmotic laxatives and additionally received either a stimulant laxative or lubiprostone. Univariate and multivariate analyses were performed to identify the risk factors for persistent constipation after introduction of the second laxative. A propensity score model was used to match 67 patients taking a stimulant laxative and 67 treated with lubiprostone, and the occurrence of intractable constipation was compared between groups. Overall, 203 patients were included, among whom 50 (25%) had constipation. On multivariate analysis, body mass index, opioid use, and addition of lubiprostone were independently associated with constipation. Patients treated with lubiprostone were significantly less likely to experience intractable constipation than did those treated with stimulant laxatives (10% vs. 34%, P = 0.002). Moreover, post-constipation diarrhea was significantly less frequent among patients treated with lubiprostone (42% vs. 63%, P = 0.024). Lubiprostone was more effective than stimulant laxatives at treating vinca alkaloid-induced intractable constipation in patients with hematological malignancies, and its use could enable safe vinca alkaloid chemotherapy.

Aripiprazole prevents from development of vincristine-induced neuropathic nociception by limiting neural NOS overexpression and NF-kB hyperactivation

PURPOSE

Increased nitric oxide (NO) synthesis and NF-kB activation have been shown as critical players in the pathophysiology of vincristine-induced peripheral neuropathy. Consistently, neural nitric oxide synthase (nNOS) inhibitors alleviated the neuropathic pain. Previous studies demonstrated that aripiprazole is capable of modulating NO synthesis and also has been reported its modulatory effect on NF-kB activity.

METHODS

Aripiprazole was administered daily to the male Wistar rats at the same time with establishing neuropathic model by I.P. injection of vincristine every 2 days, over 2 weeks. Efficacy of aripiprazole in suppressing the development of neuropathy was evaluated by assessing changes in body weight, mechanical threshold, withdrawal latency, sciatic nerve conduction velocity (SNCV), and compound motor action potential (CMAP) characteristics. Expression of nNOS and NF-kB activation were evaluated by western blotting RESULTS: Rats receiving aripiprazole during neuropathy establishment period demonstrated a normal weight gain pattern, a significantly higher mechanical withdrawal threshold, and SNCV compared to vincristine-treated group. Furthermore, the amplitude and area of CMAP were significantly higher in aripiprazole group. Western blotting demonstrated a significantly reduced expression of nNOS and NF-kB activation in dorsal root ganglia of aripiprazole co-treated rats.

CONCLUSION

In conclusion, aripiprazole effectively prevents from vincristine-induced neuropathy by limiting nNOS overexpression and NF-kB hyperactivation.

Emerging pharmacological strategies for the management of chemotherapy-induced peripheral neurotoxicity (CIPN), based on novel CIPN mechanisms

INTRODUCTION

Based on results of available clinical trials, the treatment and prevention of chemotherapy-induced peripheral neurotoxicity (CIPN) largely remains an unmet clinical need. However, new approaches have emerged in the last few years, attempting to modify the natural history of acute and late CIPN effects through a better knowledge of the pathogenic process on the molecular level.

AREAS COVERED

Clinical results of recently published (last 5 years) or ongoing emerging therapeutic/preventive pharmacological approaches based on novel CIPN mechanisms have been identified from Pubmed and ClinicalTrials.gov. Results are reviewed and discussed, in order to assess the trend of new clinical studies but also to infer the role novel approaches may have in the future.

EXPERT OPINION

The large heterogeneity of disease-causing mechanisms prevents researchers from identifying a reliable approach to effectively and safely treat or prevent CIPN. Understanding of novel pathophysiologic processes is leading the way to novel therapies, which, through targeting the sphingosine 1-phosphate receptor or pharmacologically inhibiting axonal degeneration might achieve in the future both treatment and prevention of CIPN. Toward this end, a multi-targeting approach, combining drugs to target different CIPN pathomechanisms seems to be a rational approach that warrants testing.

The SARM1 axon degeneration pathway: control of the NAD+ metabolome regulates axon survival in health and disease

Axons are essential for nervous system function and axonal pathology is a common hallmark of many neurodegenerative diseases. Over a century and a half after the original description of Wallerian axon degeneration, advances over the past five years have heralded the emergence of a comprehensive, mechanistic model of an endogenous axon degenerative process that can be activated by both injury and disease. Axonal integrity is maintained by the opposing actions of the survival factors NMNAT2 and STMN2 and pro-degenerative molecules DLK and SARM1. The balance between axon survival and self-destruction is intimately tied to axonal NAD+ metabolism. These mechanistic insights may enable axon-protective therapies for a variety of human neurodegenerative diseases including peripheral neuropathy, traumatic brain injury and potentially ALS and Parkinson's.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

Identification of the first noncompetitive SARM1 inhibitors

Sterile Alpha and Toll Interleukin Receptor Motif-containing protein 1 (SARM1) is a key therapeutic target for diseases that exhibit Wallerian-like degeneration; Wallerian degeneration is characterized by degeneration of the axon distal to the site of injury. These diseases include traumatic brain injury, peripheral neuropathy, and neurodegenerative diseases. SARM1 promotes neurodegeneration by catalyzing the hydrolysis of NAD⁺ to form a mixture of ADPR and cADPR. Notably, SARM1 knockdown prevents degeneration, indicating that SARM1 inhibitors will likely be efficacious in treating these diseases. Consistent with this hypothesis is the observation that NAD⁺ supplementation is axoprotective. To identify compounds that block the NAD⁺ hydrolase activity of SARM1, we developed and performed a high-throughput screen (HTS). This HTS assay exploits an NAD⁺ analog, etheno-NAD⁺ (ENAD) that fluoresces upon cleavage of the nicotinamide moiety. From this screen, we identified berberine chloride and zinc chloride as the first noncompetitive inhibitors of SARM1. Though modest in potency, the noncompetitive mode of inhibition, suggests the presence of an allosteric binding pocket on SARM1 that can be targeted for future therapeutic development. Additionally, zinc inhibition and site-directed mutagenesis reveals that cysteines 629 and 635 are critical for SARM1 catalysis, highlighting these sites for the design of inhibitors targeting SARM1.

Sarm1 knockout protects against early but not late axonal degeneration in experimental allergic encephalomyelitis

Programmed axonal degeneration, also known as Wallerian degeneration, occurs in immune-mediated central nervous system (CNS) inflammatory disorders such as multiple sclerosis and the animal model experimental allergic encephalomyelitis (EAE). Sterile alpha and TIR domain containing protein 1 (SARM1) functions to promote programmed axonal degeneration. To test the hypothesis that loss of SARM1 will reduce axonal degeneration in immune-mediated CNS inflammatory disorders, the course and pathology of EAE was compared in Sarm1 knockout mice and wild type littermates. The clinical course of EAE was similar in Sarm1 knockout and wild type. Analysis of EAE in mice expressing neuronal yellow fluorescent protein (YFP) showed significantly less axonal degeneration in Sarm1 knockout mice compared to wild type littermates at 14 days post-induction of EAE. At 21 days post-induction, however, difference in axonal degeneration was not significant. At 42 days post-induction, Sarm1 knockout mice were indistinguishable from wild type with respect to markers of axonal injury, and were similar with respect to axonal density in the lumbar cords. There was no significant change in peripheral immune activation or CNS inflammatory cell infiltration associated with EAE in Sarm1 knockout mice. In conclusion, Sarm1 deletion delayed axonal degeneration early in the course of CNS inflammation, but did not confer long-term protection from axonal degeneration in an animal model of immune-mediated CNS inflammation.

Axon Degeneration: Which Method to Choose?

Axons are diverse. They have different lengths, different branching patterns, and different biological roles. Methods to study axon degeneration are also diverse. The result is a bewildering range of experimental systems in which to study mechanisms of axon degeneration, and it is difficult to extrapolate from one neuron type and one method to another. The purpose of this chapter is to help readers to do this and to choose the methods most appropriate for answering their particular research question.

Assessing Axonal Degeneration in Embryonic Dorsal Root Ganglion Neurons In Vitro

The molecular players regulating the axon degeneration pathway have been identified using in vitro experimental models. Here, we describe an in vitro assay to assess the axonal fragmentation induced by mechanical injury to axons in cultured mouse embryonic dorsal root ganglion (DRG) neurons. DRG neurons are pseudounipolar and therefore suitable for an assay of axonal degeneration after injury. In addition, the time course of the axonal fragmentation is stereotyped, enabling the identification of reagents that either expedite or impede the degeneration process. With an image-based quantification method, the in vitro degeneration assay can be utilized as a platform supporting high-throughput screens for pharmacological or genetic reagents delaying axon degeneration.

Characterizing the Causal Pathway for Genetic Variants Associated with Neurological Phenotypes Using Human Brain-Derived Proteome Data

Leveraging high-dimensional molecular datasets can help us develop mechanistic insight into associations between genetic variants and complex traits. In this study, we integrated human proteome data derived from brain tissue to evaluate whether targeted proteins putatively mediate the effects of genetic variants on seven neurological phenotypes (Alzheimer disease, amyotrophic lateral sclerosis, depression, insomnia, intelligence, neuroticism, and schizophrenia). Applying the principles of Mendelian randomization (MR) systematically across the genome highlighted 43 effects between genetically predicted proteins derived from the dorsolateral prefrontal cortex and these outcomes. Furthermore, genetic colocalization provided evidence that the same causal variant at 12 of these loci was responsible for variation in both protein and neurological phenotype. This included genes such as DCC, which encodes the netrin-1 receptor and has an important role in the development of the nervous system (p = 4.29 × 10-11 with neuroticism), as well as SARM1, which has been previously implicated in axonal degeneration (p = 1.76 × 10-08 with amyotrophic lateral sclerosis). We additionally conducted a phenome-wide MR study for each of these 12 genes to assess potential pleiotropic effects on 700 complex traits and diseases. Our findings suggest that genes such as SNX32, which was initially associated with increased risk of Alzheimer disease, may potentially influence other complex traits in the opposite direction. In contrast, genes such as CTSH (which was also associated with Alzheimer disease) and SARM1 may make worthwhile therapeutic targets because they did not have genetically predicted effects on any of the other phenotypes after correcting for multiple testing.

Nicotinamide Mononucleotide Administration Prevents Experimental Diabetes-Induced Cognitive Impairment and Loss of Hippocampal Neurons

Diabetes predisposes to cognitive decline leading to dementia and is associated with decreased brain NAD⁺ levels. This has triggered an intense interest in boosting nicotinamide adenine dinucleotide (NAD⁺) levels to prevent dementia. We tested if the administration of the precursor of NAD⁺, nicotinamide mononucleotide (NMN), can prevent diabetes-induced memory deficits. Diabetes was induced in Sprague-Dawley rats by the administration of streptozotocin (STZ). After 3 months of diabetes, hippocampal NAD⁺ levels were decreased (p = 0.011). In vivo localized high-resolution proton magnetic resonance spectroscopy (MRS) of the hippocampus showed an increase in the levels of glucose (p < 0.001), glutamate (p < 0.001), gamma aminobutyric acid (p = 0.018), myo-inositol (p = 0.018), and taurine (p < 0.001) and decreased levels of N-acetyl aspartate (p = 0.002) and glutathione (p < 0.001). There was a significant decrease in hippocampal CA1 neuronal volume (p < 0.001) and neuronal number (p < 0.001) in the Diabetic rats. Diabetic rats showed hippocampal related memory deficits. Intraperitoneal NMN (100 mg/kg) was given after induction and confirmation of diabetes and was provided on alternate days for 3 months. NMN increased brain NAD⁺ levels, normalized the levels of glutamate, taurine, N-acetyl aspartate (NAA), and glutathione. NMN-treatment prevented the loss of CA1 neurons and rescued the memory deficits despite having no significant effect on hyperglycemic or lipidemic control. In hippocampal protein extracts from Diabetic rats, SIRT1 and PGC-1α protein levels were decreased, and acetylation of proteins increased. NMN treatment prevented the diabetes-induced decrease in both SIRT1 and PGC-1α and promoted deacetylation of proteins. Our results indicate that NMN increased brain NAD⁺, activated the SIRT1 pathway, preserved mitochondrial oxidative phosphorylation (OXPHOS) function, prevented neuronal loss, and preserved cognition in Diabetic rats.

Chemotherapy-induced peripheral neuropathy: part 1-current state of knowledge and perspectives for pharmacotherapy

Background

Despite the increasing knowledge of the etiology of neuropathic pain, this type of chronic pain is resistant to available analgesics in approximately 50% of patients and therefore is continuously a subject of considerable interest for physiologists, neurologists, medicinal chemists, pharmacologists and others searching for more effective treatment options for this debilitating condition.

Materials and methods

The present review article is the first of the two articles focused on chemotherapy-induced peripheral neuropathy (CIPN).

Results

CIPN is regarded as one of the most common drug-induced neuropathies and is highly pharmacoresistant. The lack of efficacious pharmacological methods for treating CIPN and preventing its development makes CIPN-related neuropathic pain a serious therapeutic gap in current medicine and pharmacotherapy. In this paper, the most recent advances in the field of studies on CIPN caused by platinum compounds (namely oxaliplatin and cisplatin), taxanes, vinca alkaloids and bortezomib are summarized.

Conclusions

The prevalence of CIPN, potential causes, risk factors, symptoms and molecular mechanisms underlying this pharmacoresistant condition are discussed.

Graphic abstract

SARM1 deficiency promotes rod and cone photoreceptor cell survival in a model of retinal degeneration

This study reports that activation of SARM1 causes destruction of NAD pools in photoreceptor cells of the retina and identifies a role for SARM1-dependent photoreceptor cell death in retinal degeneration.

Retinal degeneration is the leading cause of incurable blindness worldwide and is characterised by progressive loss of light-sensing photoreceptors in the neural retina. SARM1 is known for its role in axonal degeneration, but a role for SARM1 in photoreceptor cell degeneration has not been reported. SARM1 is known to mediate neuronal cell degeneration through depletion of essential metabolite NAD and induction of energy crisis. Here, we demonstrate that SARM1 is expressed in photoreceptors, and using retinal tissue explant, we confirm that activation of SARM1 causes destruction of NAD pools in the photoreceptor layer. Through generation of rho^−/−sarm1^−/− double knockout mice, we demonstrate that genetic deletion of SARM1 promotes both rod and cone photoreceptor cell survival in the rhodopsin knockout (rho^−/−) mouse model of photoreceptor degeneration. Finally, we demonstrate that SARM1 deficiency preserves cone visual function in the surviving photoreceptors when assayed by electroretinography. Overall, our data indicate that endogenous SARM1 has the capacity to consume NAD in photoreceptor cells and identifies a previously unappreciated role for SARM1-dependent cell death in photoreceptor cell degeneration.

Axon degeneration: mechanistic insights lead to therapeutic opportunities for the prevention and treatment of peripheral neuropathy

Peripheral neuropathy is the most common neurodegenerative disease affecting hundreds of millions of patients worldwide and is an important cause of chronic pain. Typical peripheral neuropathies are characterized by dysesthesias including numbness, crawling skin, a sensation of "pins and needles," and burning and stabbing pain. In addition, peripheral neuropathy can affect the motor and autonomic systems leading to symptoms such as weakness, constipation, and dysregulation of blood pressure. Peripheral neuropathies can be either hereditary or acquired and are a common consequence of diabetes and treatment with chemotherapy agents. Many neuropathies are due to degeneration of long axons; however, the mechanisms driving axon loss were unknown, and so no therapies are available to preserve vulnerable axons and prevent the development of peripheral neuropathy. With the recent identification of SARM1 as an injury-activated NADase enzyme that triggers axon degeneration, there is now a coherent picture emerging for the mechanism of axonal self-destruction. Here, we will present evidence that inhibiting the SARM1 pathway can prevent the development of peripheral neuropathy, describe the emerging mechanistic understanding of the axon degeneration program, and discuss how these mechanistic insights may be translated to the clinic for the prevention and treatment of peripheral neuropathy and other neurodegenerative disorders.

Programmed axon degeneration: from mouse to mechanism to medicine

Wallerian degeneration is a widespread mechanism of programmed axon degeneration. In the three decades since the discovery of the Wallerian degeneration slow (Wld^S) mouse, research has generated extensive knowledge of the molecular mechanisms underlying Wallerian degeneration, demonstrated its involvement in non-injury disorders and found multiple ways to block it. Recent developments have included: the detection of NMNAT2 mutations that implicate Wallerian degeneration in rare human diseases; the capacity for lifelong rescue of a lethal condition related to Wallerian degeneration in mice; the discovery of 'druggable' enzymes, including SARM1 and MYCBP2 (also known as PHR1), in Wallerian pathways; and the elucidation of protein structures to drive further understanding of the underlying mechanisms and drug development. Additionally, new data have indicated the potential of these advances to alleviate a number of common disorders, including chemotherapy-induced and diabetic peripheral neuropathies, traumatic brain injury, and amyotrophic lateral sclerosis.

Mitoquinone alleviates vincristine-induced neuropathic pain through inhibiting oxidative stress and apoptosis via the improvement of mitochondrial dysfunction

Chemotherapy drugs such as vincristine (Vin) could cause neuropathic pain. However, it is still lack of ideal therapeutic strategy to treat it. Mitochondrial dysfunction has been involved in the pathogenesis of neuropathic pain. The mitochondrial-targeted antioxidant, mitoquinone (MitoQ), is able to modify mitochondrial signaling, showing beneficial effects on various diseases. In the study, we investigated whether MitoQ could regulate Vin-induced neuropathic pain, and the underlying molecular mechanisms. The results showed that MitoQ significantly improved Vin-induced pain hypersensitivity and glial activation in mice. In addition, Vin resulted in severe oxidative stress in spinal cord tissues of mice, which were inhibited by MitoQ treatment through improving Nrf2 (NF-E2-related factor 2) expression in nuclear. Also, MitoQ treatment dose-dependently reduced the expression of pro-inflammatory cytokines, indicating its anti-inflammatory effects. Importantly, Vin stimulation contributed to mitochondrial fission, as evidenced by the increased expression of phosphorylated Drp1 (dynamin related protein 1) and Fis (mitochondrial fission protein 1), whereas mitochondrial fussion was inhibited. However, these effects were notably abrogated by MitoQ, subsequently improving mitochondrial dysfunction. Moreover, neuron death evoked by Vin was significantly rescued by MitoQ treatment. We also observed significantly reduced expression of cleaved Caspase-3 and Bax expression in spinal cord of MitoQ-treated mice with Vin stimulation. In contrast, anti-apoptotic factor Bcl-2 protein levels decreased by Vin were restored by MitoQ. The process of Cyto-c release from mitochondria triggered by Vin was effectively inhibited in mice treated with MitoQ. These in vivo results were further verified in the primary neurons using the in vitro and ex vivo experiments. Furthermore, MitoQ treatment alleviated axonal degeneration and mitochondria dysfunction induced by Vin. Thus, mitoquinone could alleviate vincristine-induced neuropathic pain by inhibiting oxidative stress and apoptosis via the improvement of mitochondrial dysfunction.

Cytotoxic Immunity in Peripheral Nerve Injury and Pain

Cytotoxicity and consequent cell death pathways are a critical component of the immune response to infection, disease or injury. While numerous examples of inflammation causing neuronal sensitization and pain have been described, there is a growing appreciation of the role of cytotoxic immunity in response to painful nerve injury. In this review we highlight the functions of cytotoxic immune effector cells, focusing in particular on natural killer (NK) cells, and describe the consequent action of these cells in the injured nerve as well as other chronic pain conditions and peripheral neuropathies. We describe how targeted delivery of cytotoxic factors via the immune synapse operates alongside Wallerian degeneration to allow local axon degeneration in the absence of cell death and is well-placed to support the restoration of homeostasis within the nerve. We also summarize the evidence for the expression of endogenous ligands and receptors on injured nerve targets and infiltrating immune cells that facilitate direct neuro-immune interactions, as well as modulation of the surrounding immune milieu. A number of chronic pain and peripheral neuropathies appear comorbid with a loss of function of cellular cytotoxicity suggesting such mechanisms may actually help to resolve neuropathic pain. Thus while the immune response to peripheral nerve injury is a major driver of maladaptive pain, it is simultaneously capable of directing resolution of injury in part through the pathways of cellular cytotoxicity. Our growing knowledge in tuning immune function away from inflammation toward recovery from nerve injury therefore holds promise for interventions aimed at preventing the transition from acute to chronic pain.

cADPR is a gene dosage-sensitive biomarker of SARM1 activity in healthy, compromised, and degenerating axons

SARM1 is the central executioner of pathological axon degeneration, promoting axonal demise in response to axotomy, traumatic brain injury, and neurotoxic chemotherapeutics that induce peripheral neuropathy. SARM1 is an injury-activated NAD⁺ cleavage enzyme, and this NADase activity is required for the pro-degenerative function of SARM1. At present, SARM1 function is assayed by either analysis of axonal loss, which is far downstream of SARM1 enzymatic activity, or via NAD⁺ levels, which are regulated by many competing pathways. Here we explored the utility of measuring cADPR, a product of SARM1-dependent cleavage of NAD⁺, as an in cell and in vivo biomarker of SARM1 enzymatic activity. We find that SARM1 is a major producer of cADPR in cultured dorsal root ganglion (DRG) neurons, sciatic nerve, and brain, demonstrating that SARM1 has basal activity in the absence of injury. Following injury, there is a dramatic SARM1-dependent increase in the levels of axonal cADPR that precedes morphological axon degeneration. In vivo, there is also a rapid and large injury-stimulated increase in cADPR in sciatic and optic nerves. The increase in cADPR after injury is proportional to SARM1 gene dosage, suggesting that SARM1 activity is the prime regulator of cADPR levels. The role of cADPR as an important calcium mobilizing agent prompted exploration of its functional contribution to axon degeneration. We used multiple bacterial and mammalian engineered enzymes to manipulate cADPR levels in neurons but found no changes in the time course of axonal degeneration, suggesting that cADPR is unlikely to be an important contributor to the degenerative mechanism. Using cADPR as a SARM1 biomarker, we find that SARM1 can be partially activated by a diverse array of mitochondrial toxins administered at doses that do not induce axon degeneration. Hence, the subcritical activation of SARM1 induced by mitochondrial dysfunction may contribute to the axonal vulnerability common to many neurodegenerative diseases. Finally, we assay levels of both nerve cADPR and plasma neurofilament light chain (NfL) following nerve injury in vivo, and demonstrate that both biomarkers are excellent readouts of SARM1 activity, with cADPR reporting the early molecular changes in the nerve and NfL reporting subsequent axonal breakdown. The identification and characterization of cADPR as a SARM1 biomarker will help identify neurodegenerative diseases in which SARM1 contributes to axonal loss and expedite target validation studies of SARM1-directed therapeutics.

Initial Kinetic Characterization of Sterile Alpha and Toll/Interleukin Receptor Motif-Containing Protein 1

Sterile alpha and toll/interleukin receptor (TIR) motif-containing protein 1 (SARM1) plays a pivotal role in triggering the neurodegenerative processes that underlie peripheral neuropathies, traumatic brain injury, and neurodegenerative diseases. Importantly, SARM1 knockdown or knockout prevents degeneration, thereby demonstrating that SARM1 is a promising therapeutic target. Recently, SARM1 was shown to promote neurodegeneration via its ability to hydrolyze NAD⁺, forming nicotinamide and ADP ribose (ADPR). Herein, we describe the initial kinetic characterization of full-length SARM1, as well as the truncated constructs corresponding to the SAM^1-2TIR and TIR domains, highlighting the distinct challenges that have complicated efforts to characterize this enzyme. Moreover, we show that bacterially expressed full-length SARM1 (k_cat/K_M = 6000 ± 2000 M^-1 s^-1) is at least as active as the TIR domain alone (k_cat/K_M = 1500 ± 300 M^-1 s^-1). Finally, we show that the SARM1 hydrolyzes NAD⁺ via an ordered uni-bi reaction in which nicotinamide is released prior to ADPR.

Systemic loss of Sarm1 protects Schwann cells from chemotoxicity by delaying axon degeneration

Protecting the nervous system from chronic effects of physical and chemical stress is a pressing clinical challenge. The obligate pro-degenerative protein Sarm1 is essential for Wallerian axon degeneration. Thus, blocking Sarm1 function is emerging as a promising neuroprotective strategy with therapeutic relevance. Yet, the conditions that will most benefit from inhibiting Sarm1 remain undefined. Here we combine genome engineering, pharmacology and high-resolution intravital videmicroscopy in zebrafish to show that genetic elimination of Sarm1 increases Schwann-cell resistance to toxicity by diverse chemotherapeutic agents after axonal injury. Synthetic degradation of Sarm1-deficient axons reversed this effect, suggesting that glioprotection is a non-autonomous effect of delayed axon degeneration. Moreover, loss of Sarm1 does not affect macrophage recruitment to nerve-wound microenvironment, injury resolution, or neural-circuit repair. These findings anticipate that interventions aimed at inhibiting Sarm1 can counter heightened glial vulnerability to chemical stressors and may be an effective strategy to reduce chronic consequences of neurotrauma.

Levo-corydalmine Attenuates Vincristine-Induced Neuropathic Pain in Mice by Upregulating the Nrf2/HO-1/CO Pathway to Inhibit Connexin 43 Expression

Antimicrotubulin chemotherapeutic agents, including plant-derived vincaalkaloids such as vincristine, can cause peripheral neuropathic pain. Exogenously activated heme oxygenase 1 (HO-1) is a potential therapy for chemotherapy-induced neuroinflammation. In this study, we investigated a role for Nrf2/HO-1/CO in mediating vincristine-induced neuroinflammation by inhibiting connexin 43 (Cx43) production in the spinal cord following the intrathecal application of the HO-1 inducer protoporphyrin IX cobalt chloride (CoPP) or inhibitor protoporphyrin IX zinc (ZnPP), and we analyzed the underlying mechanisms by which levo-corydalmine (l-CDL, a tetrahydroprotoberberine) attenuates vincristine-induced pain. Treatment with levo-corydalmine or oxycodone hydrochloride (a semisynthetic opioid analgesic, used as a positive control) attenuated vincristine-induced persistent pain hypersensitivity and degeneration of the sciatic nerve. In addition, the increased prevalence of atypical mitochondria induced by vincristine was ameliorated by l-CDL in both A-fibers and C-fibers. Next, we evaluated whether nuclear factor E2-related factor 2 (Nrf2), an upstream activator of HO-1, directly bound to the HO-1 promoter sequence and degraded heme to produce carbon monoxide (CO) following stimulation with vincristine. Notably, l-CDL dose-dependently increased HO-1/CO expression by activating Nrf2 to inhibit Cx43 expression in both the spinal cord and in cultured astrocytes stimulated with TNF-α, corresponding to decreased Cx43-mediated hemichannel. Furthermore, l-CDL had no effect on Cx43 following the silencing of the HO-1 gene. Taken together, our findings reveal a novel mechanism by which Nrf2/HO-1/CO mediates Cx43 expression in vincristine-induced neuropathic pain. In addition, the present findings suggest that l-CDL likely protects against nerve damage and attenuates vincristine-induced neuroinflammation by upregulating Nrf2/HO-1/CO to inhibit Cx43 expression.

Overexpression of Sirtuin 1 protein in neurons prevents and reverses experimental diabetic neuropathy

In diabetic neuropathy, there is activation of axonal and sensory neuronal degeneration pathways leading to distal axonopathy. The nicotinamide-adenine dinucleotide (NAD+)-dependent deacetylase enzyme, Sirtuin 1 (SIRT1), can prevent activation of these pathways and promote axonal regeneration. In this study, we tested whether increased expression of SIRT1 protein in sensory neurons prevents and reverses experimental diabetic neuropathy induced by a high fat diet (HFD). We generated a transgenic mouse that is inducible and overexpresses SIRT1 protein in neurons (nSIRT1OE Tg). Higher levels of SIRT1 protein were localized to cortical and hippocampal neuronal nuclei in the brain and in nuclei and cytoplasm of small to medium sized neurons in dorsal root ganglia. Wild-type and nSIRT1OE Tg mice were fed with either control diet (6.2% fat) or a HFD (36% fat) for 2 months. HFD-fed wild-type mice developed neuropathy as determined by abnormal motor and sensory nerve conduction velocity, mechanical allodynia, and loss of intraepidermal nerve fibres. In contrast, nSIRT1OE prevented a HFD-induced neuropathy despite the animals remaining hyperglycaemic. To test if nSIRT1OE would reverse HFD-induced neuropathy, nSIRT1OE was activated after mice developed peripheral neuropathy on a HFD. Two months after nSIRT1OE, we observed reversal of neuropathy and an increase in intraepidermal nerve fibre. Cultured adult dorsal root ganglion neurons from nSIRT1OE mice, maintained at high (30 mM) total glucose, showed higher basal and maximal respiratory capacity when compared to adult dorsal root ganglion neurons from wild-type mice. In dorsal root ganglion protein extracts from nSIRT1OE mice, the NAD+-consuming enzyme PARP1 was deactivated and the major deacetylated protein was identified to be an E3 protein ligase, NEDD4-1, a protein required for axonal growth, regeneration and proteostasis in neurodegenerative diseases. Our results indicate that nSIRT1OE prevents and reverses neuropathy. Increased mitochondrial respiratory capacity and NEDD4 activation was associated with increased axonal growth driven by neuronal overexpression of SIRT1. Therapies that regulate NAD+ and thereby target sirtuins may be beneficial in human diabetic sensory polyneuropathy.

Disrupting insulin signaling in Schwann cells impairs myelination and induces a sensory neuropathy

Although diabetic mice have been studied for decades, little is known about the cell type specific contributions to diabetic neuropathy (DN). Schwann cells (SCs) myelinate and provide trophic support to peripheral nervous system axons. Altered SC metabolism leads to myelin defects, which can be seen both in inherited and DNs. How SC metabolism is altered in DN is not fully understood, but it is clear that insulin resistance underlies impaired lipid metabolism in many cell types throughout the body via the phosphoinositide 3-kinase/protein kinase b (PKB)/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway. Here, we created an insulin resistant SC by deleting both insulin receptor (INSR) and insulin-like growth factor receptor 1 (IGF1R), to determine the role of this signaling pathway in development and response to injury in order to understand SC defects in DN. We found that myelin is thinner throughout development and adulthood in INSR/IGF1R Schwann cell specific knock out mice. The nerves of these mutant mice had reduced expression of key genes that mediate fatty acid and cholesterol synthesis due to reduced mTOR-sterol regulatory element-binding protein signaling. In adulthood, these mice show sensory neuropathy phenotypes reminiscent of diabetic mice. Altogether, these data suggest that SCs may play an important role in DN and targeting their metabolism could lead to new therapies for DN.

Emergence of SARM1 as a Potential Therapeutic Target for Wallerian-type Diseases

Wallerian degeneration is a neuronal death pathway that is triggered in response to injury or disease. Death was thought to occur passively until the discovery of a mouse strain, i.e., Wallerian degeneration slow (WLD^S), which was resistant to degeneration. Given that the WLD^S mouse encodes a gain-of-function fusion protein, its relevance to human disease was limited. The later discovery that SARM1 (sterile alpha and toll/interleukin receptor [TIR] motif-containing protein 1) promotes Wallerian degeneration suggested the existence of a pathway that might be targeted therapeutically. More recently, SARM1 was found to execute degeneration by hydrolyzing NAD⁺. Notably, SARM1 knockdown or knockout prevents neuron degeneration in response to a range of insults that lead to peripheral neuropathy, traumatic brain injury, and neurodegenerative disease. Here, we discuss the role of SARM1 in Wallerian degeneration and the opportunities to target this enzyme therapeutically.

DLK Activation Synergizes with Mitochondrial Dysfunction to Downregulate Axon Survival Factors and Promote SARM1-Dependent Axon Degeneration

Axon degeneration is a prominent component of many neurological disorders. Identifying cellular pathways that contribute to axon vulnerability may identify new therapeutic strategies for maintenance of neural circuits. Dual leucine zipper kinase (DLK) is an axonal stress response MAP3K that is chronically activated in several neurodegenerative diseases. Activated DLK transmits an axon injury signal to the neuronal cell body to provoke transcriptional adaptations. However, the consequence of enhanced DLK signaling to axon vulnerability is unknown. We find that stimulating DLK activity predisposes axons to SARM1-dependent degeneration. Activating DLK reduces levels of the axon survival factors NMNAT2 and SCG10, accelerating their loss from severed axons. Moreover, mitochondrial dysfunction independently decreases the levels of NMNAT2 and SCG10 in axons, and in conjunction with DLK activation, leads to a dramatic loss of axonal NMNAT2 and SCG10 and evokes spontaneous axon degeneration. Hence, enhanced DLK activity reduces axon survival factor abundance and renders axons more susceptible to trauma and metabolic insult.

Sarm1 Gene Deficiency Attenuates Diabetic Peripheral Neuropathy in Mice

Diabetic peripheral neuropathy (DPN) is the most common complication in both type 1 and type 2 diabetes, but any treatment toward the development of DPN is not yet available. Axon degeneration is an early feature of many peripheral neuropathies, including DPN. Delay of axon degeneration has beneficial effects on various neurodegenerative diseases, but its effect on DPN is yet to be elucidated. Deficiency of Sarm1 significantly attenuates axon degeneration in several models, but the effect of Sarm1 deficiency on DPN is still unclear. In this study, we show that Sarm1 knockout mice exhibit normal glucose metabolism and pain sensitivity, and deletion of the Sarm1 gene alleviates hypoalgesia in streptozotocin-induced diabetic mice. Moreover, Sarm1 gene deficiency attenuates intraepidermal nerve fiber loss in footpad skin; alleviates axon degeneration, the change of g-ratio in sciatic nerves, and NAD⁺ decrease; and relieves axonal outgrowth retardation of dorsal root ganglia from diabetic mice. In addition, Sarm1 gene deficiency markedly diminishes the changes of gene expression profile induced by streptozotocin in the sciatic nerve, especially some abundant genes involved in neurodegenerative diseases. These findings demonstrate that Sarm1 gene deficiency attenuates DPN in mice and suggest that slowing down axon degeneration is a potential promising strategy to combat DPN.

Sarm1 deletion suppresses TDP-43-linked motor neuron degeneration and cortical spine loss

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative condition that primarily affects the motor system and shares many features with frontotemporal dementia (FTD). Evidence suggests that ALS is a 'dying-back' disease, with peripheral denervation and axonal degeneration occurring before loss of motor neuron cell bodies. Distal to a nerve injury, a similar pattern of axonal degeneration can be seen, which is mediated by an active axon destruction mechanism called Wallerian degeneration. Sterile alpha and TIR motif-containing 1 (Sarm1) is a key gene in the Wallerian pathway and its deletion provides long-term protection against both Wallerian degeneration and Wallerian-like, non-injury induced axonopathy, a retrograde degenerative process that occurs in many neurodegenerative diseases where axonal transport is impaired. Here, we explored whether Sarm1 signalling could be a therapeutic target for ALS by deleting Sarm1 from a mouse model of ALS-FTD, a TDP-43Q331K, YFP-H double transgenic mouse. Sarm1 deletion attenuated motor axon degeneration and neuromuscular junction denervation. Motor neuron cell bodies were also significantly protected. Deletion of Sarm1 also attenuated loss of layer V pyramidal neuronal dendritic spines in the primary motor cortex. Structural MRI identified the entorhinal cortex as the most significantly atrophic region, and histological studies confirmed a greater loss of neurons in the entorhinal cortex than in the motor cortex, suggesting a prominent FTD-like pattern of neurodegeneration in this transgenic mouse model. Despite the reduction in neuronal degeneration, Sarm1 deletion did not attenuate age-related behavioural deficits caused by TDP-43Q331K. However, Sarm1 deletion was associated with a significant increase in the viability of male TDP-43Q331K mice, suggesting a detrimental role of Wallerian-like pathways in the earliest stages of TDP-43Q331K-mediated neurodegeneration. Collectively, these results indicate that anti-SARM1 strategies have therapeutic potential in ALS-FTD.

Vinca alkaloids, thalidomide and eribulin-induced peripheral neurotoxicity: From pathogenesis to treatment

Vinca alkaloids, thalidomide, and eribulin are widely used to treat patients with childhood acute lymphoblastic leukemia (ALL), adults affected by multiple myeloma and locally invasive or metastatic breast cancer, respectively. However, soon after their introduction into clinical practice, chemotherapy-induced peripheral neurotoxicity (CIPN) emerged as their main non-hematological and among dose-limiting adverse events. It is generally perceived that vinca alkaloids and the antiangiogenic agent thalidomide are more neurotoxic, compared to eribulin. The exposure to these chemotherapeutic agents is associated with an axonal, length-dependent, sensory polyneuropathy of mild to moderate severity, whereas it is considered that the peripheral nerve damage, unless severe, usually resolves soon after treatment discontinuation. Advanced age, high initial and prolonged dosing, coadministration of other neurotoxic chemotherapeutic agents and pre-existing neuropathy are the common risk factors. Pharmacogenetic biomarkers might be used to define patients at increased susceptibility of CIPN. Currently, there is no established therapy for CIPN prevention or treatment; symptomatic treatment for neuropathic pain and dose reduction or withdrawal in severe cases is considered, at the cost of reduced cancer therapeutic efficacy. This review critically examines the pathogenesis, epidemiology, risk factors (both clinical and pharmacogenetic), clinical phenotype and management of CIPN as a result of exposure to vinca alkaloids, thalidomide and its analogue lenalidomide as also eribulin.