WldS mice are resistant to paclitaxel (taxol) neuropathy

Elevated intracellular Ca2+ functions downstream of mitodysfunction to induce Wallerian-like degeneration and necroptosis in organophosphorus-induced delayed neuropathy

Neurotoxic organophosphorus compounds can induce a type of delayed neuropathy in humans and sensitive animals, known as organophosphorus-induced delayed neuropathy (OPIDN). OPIDN is characterized by axonal degeneration akin to Wallerian-like degeneration, which is thought to be caused by increased intra-axonal Ca2+ concentrations. This study was designed to investigate that deregulated cytosolic Ca2+ may function downstream of mitodysfunction in activating Wallerian-like degeneration and necroptosis in OPIDN. Adult hens were administrated a single dosage of 750 mg/kg tri-ortho-cresyl phosphate (TOCP), and then sacrificed at 1 day, 5 day, 10 day and 21 day post-exposure, respectively. Sciatic nerves and spinal cords were examined for pathological changes and proteins expression related to Wallerian-like degeneration and necroptosis. In vitro experiments using differentiated neuro-2a (N2a) cells were conducted to investigate the relationship among mitochondrial dysfunction, Ca2+ influx, axonal degeneration, and necroptosis. The cells were co-administered with the Ca2+-chelator BAPTA-AM, the TRPA1 channel inhibitor HC030031, the RIPK1 inhibitor Necrostatin-1, and the mitochondrial-targeted antioxidant MitoQ along with TOCP. Results demonstrated an increase in cytosolic calcium concentration and key proteins associated with Wallerian degeneration and necroptosis in both in vivo and in vitro models after TOCP exposure. Moreover, co-administration with BATPA-AM or HC030031 significantly attenuated the loss of NMNAT2 and STMN2 in N2a cells, as well as the upregulation of SARM1, RIPK1 and p-MLKL. In contrast, Necrostatin-1 treatment only inhibited the TOCP-induced elevation of p-MLKL. Notably, pharmacological protection of mitochondrial function with MitoQ effectively alleviated the increase in intracellular Ca2+ following TOCP and mitigated axonal degeneration and necroptosis in N2a cells, supporting mitochondrial dysfunction as an upstream event of the intracellular Ca2+ imbalance and neuronal damage in OPIDN. These findings suggest that mitochondrial dysfunction post-TOCP intoxication leads to an elevated intracellular Ca2+ concentration, which plays a pivotal role in the initiation and development of OPIDN through inducing SARM1-mediated axonal degeneration and activating the necroptotic signaling pathway.

NAD+, Axonal Maintenance, and Neurological Disease

Significance: The remarkable geometry of the axon exposes it to unique challenges for survival and maintenance. Axonal degeneration is a feature of peripheral neuropathies, glaucoma, and traumatic brain injury, and an early event in neurodegenerative diseases. Since the discovery of Wallerian degeneration (WD), a molecular program that hijacks nicotinamide adenine dinucleotide (NAD+) metabolism for axonal self-destruction, the complex roles of NAD+ in axonal viability and disease have become research priority. Recent Advances: The discoveries of the protective Wallerian degeneration slow (WldS) and of sterile alpha and TIR motif containing 1 (SARM1) activation as the main instructive signal for WD have shed new light on the regulatory role of NAD+ in axonal degeneration in a growing number of neurological diseases. SARM1 has been characterized as a NAD+ hydrolase and sensor of NAD+ metabolism. The discovery of regulators of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) proteostasis in axons, the allosteric regulation of SARM1 by NAD+ and NMN, and the existence of clinically relevant windows of action of these signals has opened new opportunities for therapeutic interventions, including SARM1 inhibitors and modulators of NAD+ metabolism. Critical Issues: Events upstream and downstream of SARM1 remain unclear. Furthermore, manipulating NAD+ metabolism, an overdetermined process crucial in cell survival, for preventing the degeneration of the injured axon may be difficult and potentially toxic. Future Directions: There is a need for clarification of the distinct roles of NAD+ metabolism in axonal maintenance as contrasted to WD. There is also a need to better understand the role of NAD+ metabolism in axonal endangerment in neuropathies, diseases of the white matter, and the early stages of neurodegenerative diseases of the central nervous system. Antioxid. Redox Signal. 39, 1167-1184.

Axonal degeneration in chemotherapy-induced peripheral neurotoxicity: clinical and experimental evidence

Multiple pathological mechanisms are involved in the development of chemotherapy-induced peripheral neurotoxicity (CIPN). Recent work has provided insights into the molecular mechanisms underlying chemotherapy-induced axonal degeneration. This review integrates evidence from preclinical and clinical work on the onset, progression and outcome of axonal degeneration in CIPN. We review likely triggers of axonal degeneration in CIPN and highlight evidence of molecular pathways involved in axonal degeneration and their relevance to CIPN, including SARM1-mediated axon degeneration pathway. We identify potential clinical markers of axonal dysfunction to provide early identification of toxicity as well as present potential treatment strategies to intervene in axonal degeneration pathways. A greater understanding of axonal degeneration processes in CIPN will provide important information regarding the development and progression of axonal dysfunction more broadly and will hopefully assist in the development of successful interventions for CIPN and other neurodegenerative disorders.

Towards a mechanistic understanding of axon transport and endocytic changes underlying paclitaxel-induced peripheral neuropathy

Paclitaxel is a common chemotherapeutic agent widely used to treat solid cancer. However, it frequently causes peripheral sensory neuropathy, resulting in sensory abnormalities and pain in patients receiving treatment for cancer. As one of the most widely used chemotherapeutics, many preclinical studies on paclitaxel-induced peripheral neuropathy (PIPN) have been performed. Yet, there remain no effective options for treatment or prevention. Due to paclitaxel's ability to bind to and stabilize microtubules, a change in microtubule dynamics and subsequent disruptions in axonal transport has been predicted as a major underlying cause of paclitaxel-induced toxicity. However, the systemic understanding of PIPN mechanisms is largely incomplete, and various phenotypes have not been directly attributed to microtubule-related effects. This review aims to provide an overview of the literature involving paclitaxel-induced alteration in microtubule dynamics, axonal transport, and endocytic changes. It also aims to provide insights into how the microtubule-mediated hypothesis may relate to various phenotypes reported in PIPN studies.

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

Cellular Pathogenesis of Chemotherapy-Induced Peripheral Neuropathy: Insights From Drosophila and Human-Engineered Skin Models

Chemotherapy-induced peripheral neuropathy (CIPN) is a highly prevalent and complex condition arising from chemotherapy cancer treatments. Currently, there are no treatment or prevention options in the clinic. CIPN accompanies pain-related sensory functions starting from the hands and feet. Studies focusing on neurons in vitro and in vivo models significantly advanced our understanding of CIPN pathological mechanisms. However, given the direct toxicity shown in both neurons and non-neuronal cells, effective in vivo or in vitro models that allow the investigation of neurons in their local environment are required. No single model can provide a complete solution for the required investigation, therefore, utilizing a multi-model approach would allow complementary advantages of different models and robustly validate findings before further translation. This review aims first to summarize approaches and insights from CIPN in vivo models utilizing small model organisms. We will focus on Drosophila melanogaster CIPN models that are genetically amenable and accessible to study neuronal interactions with the local environment in vivo. Second, we will discuss how these findings could be tested in physiologically relevant vertebrate models. We will focus on in vitro approaches using human cells and summarize the current understanding of engineering approaches that may allow the investigation of pathological changes in neurons and the skin environment.

Upregulation of neuronal progranulin mediates the antinociceptive effect of trimetazidine in paclitaxel-induced peripheral neuropathy: Role of ERK1/2 signaling

Neuronal progranulin (PGRN) overexpression is an endogenous adaptive pain defense following nerve injury. It allows the survival of injured neurons to block enhanced nociceptive responses. Trimetazidine (TMZ) is widely used by cardiac patients as an anti-anginal drug, reflecting its anti-ischemic property. TMZ promotes axonal regeneration of sciatic nerves after crush injury. This study explored the interplay between PGRN and extracellular signal-regulated kinases (ERK1/2) to address mechanisms underlying neuropathic pain alleviation following paclitaxel (PTX) administration. Rats were given four injections of PTX (2 mg/kg, i.p.) every other day. Two days after the last dose, rats received TMZ (25 mg/kg) with or without the ERK inhibitor, PD98059, daily for 21 days. TMZ preserved the integrity of myelinated nerve fibers, as evidenced by an obvious reduction in axonal damage biomarkers. Accordingly, it alleviated PTX-evoked thermal, cold, and mechanical hyperalgesia/allodynia. TMZ also promoted ERK1/2 phosphorylation with a profound upsurge in PGRN content. These effects were associated with a substantial increase in Notch1 receptor gene expression and a prominent anti-inflammatory effect with a marked increase in mRNA expression of secretory leukocyte protease inhibitor. Further, TMZ decreased oxidative stress and caspase-3 activity in the sciatic nerve. Conversely, co-administration of PD98059 completely abolished these beneficial effects. Thus, the robust antinociceptive effect of TMZ is largely attributed to upregulating PGRN and Notch1 receptors via ERK1/2 activation.

Discovery of small-molecule activators of nicotinamide phosphoribosyltransferase (NAMPT) and their preclinical neuroprotective activity

The decline of nicotinamide adenine dinucleotide (NAD) occurs in a variety of human pathologies including neurodegeneration. NAD-boosting agents can provide neuroprotective benefits. Here, we report the discovery and development of a class of potent activators (NATs) of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in the NAD salvage pathway. We obtained the crystal structure of NAMPT in complex with the NAT, which defined the allosteric action of NAT near the enzyme active site. The optimization of NAT further revealed the critical role of K189 residue in boosting NAMPT activity. NATs effectively increased intracellular levels of NAD and induced subsequent metabolic and transcriptional reprogramming. Importantly, NATs exhibited strong neuroprotective efficacy in a mouse model of chemotherapy-induced peripheral neuropathy (CIPN) without any overt toxicity. These findings demonstrate the potential of NATs in the treatment of neurodegenerative diseases or conditions associated with NAD level decline.

Ubiquitin Proteasome System and Microtubules Are Master Regulators of Central and Peripheral Nervous System Axon Degeneration

Axonal degeneration is an active process that differs from neuronal death, and it is the hallmark of many disorders affecting the central and peripheral nervous system. Starting from the analyses of Wallerian degeneration, the simplest experimental model, here we describe how the long projecting neuronal populations affected in Parkinson’s disease and chemotherapy-induced peripheral neuropathies share commonalities in the mechanisms and molecular players driving the earliest phase of axon degeneration. Indeed, both dopaminergic and sensory neurons are particularly susceptible to alterations of microtubules and axonal transport as well as to dysfunctions of the ubiquitin proteasome system and protein quality control. Finally, we report an updated review on current knowledge of key molecules able to modulate these targets, blocking the on-going axonal degeneration and inducing neuronal regeneration. These molecules might represent good candidates for disease-modifying treatment, which might expand the window of intervention improving patients’ quality of life.

NAD+-dependent mechanism of pathological axon degeneration

Pathological axon degeneration is broadly observed in neurodegenerative diseases. This unique process of axonal pathology could directly interfere with the normal functions of neurocircuitries and contribute to the onset of clinical symptoms in patients. It has been increasingly recognized that functional preservation of axonal structures is an indispensable part of therapeutic strategies for treating neurological disorders. In the past decades, the research field has witnessed significant breakthroughs in understanding the stereotyped self-destruction of axons upon neurodegenerative insults, which is distinct from all the known types of programmed cell death. In particular, the novel NAD+-dependent mechanism involving the WLDs, NMNAT2, and SARM1 proteins has emerged. This review summarizes the landmark discoveries elucidating the molecular pathway of pathological axon degeneration and highlights the evolving concept that neurodegeneration would be intrinsically linked to NAD+ and energy metabolism.

SARM1 knockout does not rescue neuromuscular phenotypes in a Charcot-Marie-Tooth disease Type 1A mouse model

Charcot-Marie-Tooth disease Type 1A (CMT1A) is caused by duplication of the PMP22 gene and is the most common inherited peripheral neuropathy. Although CMT1A is a dysmyelinating peripheral neuropathy, secondary axon degeneration has been suggested to drive functional deficits in patients. Given that SARM1 knockout is a potent inhibitor of the programmed axon degeneration pathway, we asked whether SARM1 knockout rescues neuromuscular phenotypes in CMT1A model (C3-PMP) mice. CMT1A mice were bred with SARM1 knockout mice to generate CMT1A/SARM1^-/- mice. A series of behavioral assays were employed to evaluate motor and sensorimotor function. Electrophysiological and histological studies of the tibial branch of the sciatic nerve were performed. Additionally, gastrocnemius and soleus muscle morphology were evaluated histologically. Although clear behavioral and electrophysiological deficits were observed in CMT1A model mice, genetic deletion of SARM1 conferred no significant improvement. Nerve morphometry revealed predominantly myelin deficits in CMT1A model mice and SARM1 knockout yielded no improvement in all nerve morphometry measures. Similarly, muscle morphometry deficits in CMT1A model mice were not improved by SARM1 knockout. Our findings demonstrate that programmed axon degeneration pathway inhibition does not provide therapeutic benefit in C3-PMP CMT1A model mice. Our results indicate that the clinical phenotypes observed in CMT1A mice are likely caused primarily by prolonged dysmyelination, motivate further investigation into mechanisms of dysmyelination in these mice and necessitate the development of improved CMT1A rodent models that recapitulate the secondary axon degeneration observed in patients.

Inhibition of dual leucine zipper kinase prevents chemotherapy-induced peripheral neuropathy and cognitive impairments

ABSTRACT

Chemotherapy-induced peripheral neuropathy (CIPN) and chemotherapy-induced cognitive impairments (CICI) are common, often severe neurotoxic side effects of cancer treatment that greatly reduce quality of life of cancer patients and survivors. Currently, there are no Food and Drug Administration-approved agents for the prevention or curative treatment of CIPN or CICI. The dual leucine zipper kinase (DLK) is a key mediator of axonal degeneration that is localized to axons and coordinates the neuronal response to injury. We developed a novel brain-penetrant DLK inhibitor, IACS'8287, which demonstrates potent and highly selective inhibition of DLK in vitro and in vivo. Coadministration of IACS'8287 with the platinum derivative cisplatin prevents mechanical allodynia, loss of intraepidermal nerve fibers in the hind paws, cognitive deficits, and impairments in brain connectivity in mice, all without interfering with the antitumor activity of cisplatin. The protective effects of IACS'8287 are associated with preservation of mitochondrial function in dorsal root ganglion neurons and in brain synaptosomes. In addition, RNA sequencing analysis of dorsal root ganglia reveals modulation of genes involved in neuronal activity and markers for immune cell infiltration by DLK inhibition. These data indicate that CIPN and CICI require DLK signaling in mice, and DLK inhibitors could become an attractive treatment in the clinic when coadministered with cisplatin, and potentially other chemotherapeutic agents, to prevent neurotoxicities as a result of cancer treatment.

Lessons from Injury: How Nerve Injury Studies Reveal Basic Biological Mechanisms and Therapeutic Opportunities for Peripheral Nerve Diseases

Since Waller and Cajal in the nineteenth and early twentieth centuries, laboratory traumatic peripheral nerve injury studies have provided great insight into cellular and molecular mechanisms governing axon degeneration and the responses of Schwann cells, the major glial cell type of peripheral nerves. It is now evident that pathways underlying injury-induced axon degeneration and the Schwann cell injury-specific state, the repair Schwann cell, are relevant to many inherited and acquired disorders of peripheral nerves. This review provides a timely update on the molecular understanding of axon degeneration and formation of the repair Schwann cell. We discuss how nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha TIR motif containing protein 1 (SARM1) are required for axon survival and degeneration, respectively, how transcription factor c-JUN is essential for the Schwann cell response to nerve injury and what each tells us about disease mechanisms and potential therapies. Human genetic association with NMNAT2 and SARM1 strongly suggests aberrant activation of programmed axon death in polyneuropathies and motor neuron disorders, respectively, and animal studies suggest wider involvement including in chemotherapy-induced and diabetic neuropathies. In repair Schwann cells, cJUN is aberrantly expressed in a wide variety of human acquired and inherited neuropathies. Animal models suggest it limits axon loss in both genetic and traumatic neuropathies, whereas in contrast, Schwann cell secreted Neuregulin-1 type 1 drives onion bulb pathology in CMT1A. Finally, we discuss opportunities for drug-based and gene therapies to prevent axon loss or manipulate the repair Schwann cell state to treat acquired and inherited neuropathies and neuronopathies.

Development of EQ-6, a Novel Analogue of Ethoxyquin to Prevent Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a common and often dose-limiting side effect of many cancer drugs. Because the onset of neuronal injury is known, it is an ideal clinical target to develop neuroprotective strategies. Several years ago, we had identified ethoxyquin as a potent neuroprotective drug against CIPN through a phenotypic drug screening and demonstrated a novel mechanism of action, inhibition of chaperone domain of heat shock protein 90. To improve its drug-like properties we synthesized a novel analogue of ethoxyquin and named it EQ-6 (6-(5-amino)-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline hydrochloride). Here we show that EQ-6 prevents axon degeneration in primary dorsal root ganglion neurons in vitro, and this axon protection is associated with preserved levels of nicotinamide adenine dinucleotide, a key metabolite in programmed axon degeneration pathway. We also found that EQ-6 prevents loss of epidermal nerve fibers in a mouse model of CIPN induced by paclitaxel and that doses of EQ-6 that provide neuroprotection are associated with reduced tissue levels of SF3B2, a potential biomarker of target engagement. Furthermore, we show that EQ-6 is safe in vitro and in mice with daily administration for a month. We found that oral bioavailability is about 10%, partly due to rapid metabolism in liver, but EQ-6 appears to be concentrated in neural tissues. Given these findings, we propose EQ-6 as a first-in-class drug to prevent CIPN.

Pharmacological SARM1 inhibition protects axon structure and function in paclitaxel-induced peripheral neuropathy

Axonal degeneration is an early and ongoing event that causes disability and disease progression in many neurodegenerative disorders of the peripheral and central nervous systems. Chemotherapy-induced peripheral neuropathy (CIPN) is a major cause of morbidity and the main cause of dose reductions and discontinuations in cancer treatment. Preclinical evidence indicates that activation of the Wallerian-like degeneration pathway driven by sterile alpha and TIR motif containing 1 (SARM1) is responsible for axonopathy in CIPN. SARM1 is the central driver of an evolutionarily conserved programme of axonal degeneration downstream of chemical, inflammatory, mechanical or metabolic insults to the axon. SARM1 contains an intrinsic NADase enzymatic activity essential for its pro-degenerative functions, making it a compelling therapeutic target to treat neurodegeneration characterized by axonopathies of the peripheral and central nervous systems. Small molecule SARM1 inhibitors have the potential to prevent axonal degeneration in peripheral and central axonopathies and to provide a transformational disease-modifying treatment for these disorders.

Using a biochemical assay for SARM1 NADase we identified a novel series of potent and selective irreversible isothiazole inhibitors of SARM1 enzymatic activity that protected rodent and human axons in vitro. In sciatic nerve axotomy, we observed that these irreversible SARM1 inhibitors decreased a rise in nerve cADPR and plasma neurofilament light chain released from injured sciatic nerves in vivo. In a mouse paclitaxel model of CIPN we determined that Sarm1 knockout mice prevented loss of axonal function, assessed by sensory nerve action potential amplitudes of the tail nerve, in a gene-dosage-dependent manner. In that CIPN model, the irreversible SARM1 inhibitors prevented loss of intraepidermal nerve fibres induced by paclitaxel and provided partial protection of axonal function assessed by sensory nerve action potential amplitude and mechanical allodynia.

Genetic diversity of axon degenerative mechanisms in models of Parkinson's disease

Parkinson's disease (PD) is the most common form of neurodegenerative movement disorder, associated with profound loss of dopaminergic neurons from the basal ganglia. Though loss of dopaminergic neuron cell bodies from the substantia nigra pars compacta is a well-studied feature, atrophy and loss of their axons within the nigrostriatal tract is also emerging as an early event in disease progression. Genes that drive the Wallerian degeneration, like Sterile alpha and toll/interleukin-1 receptor motif containing (Sarm1), are excellent candidates for driving this axon degeneration, given similarities in the morphology of axon degeneration after axotomy and in PD. In the present study we assessed whether Sarm1 contributes to loss of dopaminergic projections in mouse models of PD. In Sarm1 deficient mice, we observed a significant delay in the degeneration of severed dopaminergic axons distal to a 6-OHDA lesion of the medial forebrain bundle (MFB) in the nigrostriatal tract, and an accompanying rescue of morphological, biochemical and behavioural phenotypes. However, we observed no difference compared to controls when striatal terminals were lesioned with 6-OHDA to induce a dying back form of neurodegeneration. Likewise, when PD phenotypes were induced using AAV-induced alpha-synuclein overexpression, we observed similar modest loss of dopaminergic terminals in Sarm1 knockouts and controls. Our data argues that axon degeneration after MFB lesion is Sarm1-dependent, but that other models for PD do not require Sarm1, or that Sarm1 acts with other redundant genetic pathways. This work adds to a growing body of evidence indicating Sarm1 contributes to some, but not all types of neurodegeneration, and supports the notion that while axon degeneration in many context appears morphologically similar, a diversity of axon degeneration programs exist.

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.

Drug Repositioning for the Prevention and Treatment of Chemotherapy-Induced Peripheral Neuropathy: A Mechanism- and Screening-Based Strategy

Chemotherapy-induced peripheral neuropathy (CIPN) is a severe adverse effect observed in most patients treated with neurotoxic anti-cancer drugs. Currently, there are no therapeutic options available for the prevention of CIPN. Furthermore, few drugs are recommended for the treatment of existing neuropathies because the mechanisms of CIPN remain unclear. Each chemotherapeutic drug induces neuropathy by distinct mechanisms, and thus we need to understand the characteristics of CIPN specific to individual drugs. Here, we review the known pathogenic mechanisms of oxaliplatin- and paclitaxel-induced CIPN, highlighting recent findings. Cancer chemotherapy is performed in a planned manner; therefore, preventive strategies can be planned for CIPN. Drug repositioning studies, which identify the unexpected actions of already approved drugs, have increased in recent years. We have also focused on drug repositioning studies, especially for prevention, because they should be rapidly translated to patients suffering from CIPN.

Microtubule Dysfunction: A Common Feature of Neurodegenerative Diseases

Neurons are particularly susceptible to microtubule (MT) defects and deregulation of the MT cytoskeleton is considered to be a common insult during the pathogenesis of neurodegenerative disorders. Evidence that dysfunctions in the MT system have a direct role in neurodegeneration comes from findings that several forms of neurodegenerative diseases are associated with changes in genes encoding tubulins, the structural units of MTs, MT-associated proteins (MAPs), or additional factors such as MT modifying enzymes which modulating tubulin post-translational modifications (PTMs) regulate MT functions and dynamics. Efforts to use MT-targeting therapeutic agents for the treatment of neurodegenerative diseases are underway. Many of these agents have provided several benefits when tested on both in vitro and in vivo neurodegenerative model systems. Currently, the most frequently addressed therapeutic interventions include drugs that modulate MT stability or that target tubulin PTMs, such as tubulin acetylation. The purpose of this review is to provide an update on the relevance of MT dysfunctions to the process of neurodegeneration and briefly discuss advances in the use of MT-targeting drugs for the treatment of neurodegenerative disorders.

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.

Wallerian degeneration as a therapeutic target in traumatic brain injury

PURPOSE OF REVIEW

Diffuse or traumatic axonal injury is one of the principal pathologies encountered in traumatic brain injury (TBI) and the resulting axonal loss, disconnection, and brain atrophy contribute significantly to clinical morbidity and disability. The seminal discovery of the slow Wallerian degeneration mice (Wld) in which transected axons do not degenerate but survive and function independently for weeks has transformed concepts on axonal biology and raised hopes that axonopathies may be amenable to specific therapeutic interventions. Here we review mechanisms of axonal degeneration and also describe how these mechanisms may inform biological therapies of traumatic axonopathy in the context of TBI.

RECENT FINDINGS

In the last decade, SARM1 [sterile a and Toll/interleukin-1 receptor (TIR) motif containing 1] and the DLK (dual leucine zipper bearing kinase) and LZK (leucine zipper kinase) MAPK (mitogen-activated protein kinases) cascade have been established as the key drivers of Wallerian degeneration, a complex program of axonal self-destruction which is activated by a wide range of injurious insults, including insults that may otherwise leave axons structurally robust and potentially salvageable. Detailed studies on animal models and postmortem human brains indicate that this type of partial disruption is the main initial pathology in traumatic axonopathy. At the same time, the molecular dissection of Wallerian degeneration has revealed that the decision that commits axons to degeneration is temporally separated from the time of injury, a window that allows potentially effective pharmacological interventions.

SUMMARY

Molecular signals initiating and triggering Wallerian degeneration appear to be playing an important role in traumatic axonopathy and recent advances in understanding their nature and significance is opening up new therapeutic opportunities for TBI.

Blockade of IL-6 signaling prevents paclitaxel-induced neuropathy in C57Bl/6 mice

The microtubule-stabilizing agent paclitaxel frequently leads to chemotherapy-induced peripheral neuropathy (CIN), which further increases the burden of disease and often necessitates treatment limitations. The pathophysiology of CIN appears to involve both “upstream” effects including altered intracellular calcium signaling and activation of calcium dependent proteases such as calpain as well as subsequent “downstream” neuro-inflammatory reactions with cytokine release and macrophage infiltration of dorsal root ganglia. In this study, we aimed to investigate whether these processes are linked by the pro-inflammatory cytokine interleukin-6 (IL-6). We observed that paclitaxel exposure induced IL-6 synthesis in cultured sensory neurons from postnatal Wistar rats, which could be prevented by co-treatment with a calpain inhibitor. This suggests a calcium dependent process. We demonstrate that adult C57BL/6 mice deficient in IL-6 are protected from developing functional and histological changes of paclitaxel-induced neuropathy. Furthermore, pretreatment with an IL-6-neutralizing antibody resulted in the prevention of paclitaxel-induced neuropathy in C57BL/6 mice. Electrophysiological data from our preclinical model was adequately reflected by measurements of patients undergoing paclitaxel therapy for ovarian cancer. In this cohort, measured Il-6 levels correlated with the severity of neuropathy. Our findings demonstrate that IL-6 plays a pivotal role in the pathophysiology of paclitaxel-induced neuropathy per se and that pharmacological or genetic interference with this signaling pathway prevents the development of this potentially debilitating adverse effect. These findings provide a rationale for a clinical trial with IL-6 neutralizing antibodies to prevent dose-limiting neurotoxic adverse effects of paclitaxel chemotherapy.

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.

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.

Targeting the programmed axon degeneration pathway as a potential therapeutic for Charcot-Marie-Tooth disease

The programmed axon degeneration pathway has emerged as an important process contributing to the pathogenesis of several neurological diseases. The most crucial events in this pathway include activation of the central executioner SARM1 and NAD⁺ depletion, which leads to an energetic failure and ultimately axon destruction. Given the prevalence of this pathway, it is not surprising that inhibitory therapies are currently being developed in order to treat multiple neurological diseases with the same therapy. Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of neurological diseases that may also benefit from this therapeutic approach. To evaluate the appropriateness of this strategy, the contribution of the programmed axon degeneration pathway to the pathogenesis of different CMT subtypes is being actively investigated. The subtypes CMT1A, CMT1B and CMT2D are the first to have been examined. Based on the results from these studies and advances in developing therapies to block the programmed axon degeneration pathway, promising therapeutics for CMT are now on the horizon.

Altered mitochondrial bioenergetics are responsible for the delay in Wallerian degeneration observed in neonatal mice

Neurodegenerative and neuromuscular disorders can manifest throughout the lifespan of an individual, from infant to elderly individuals. Axonal and synaptic degeneration are early and critical elements of nearly all human neurodegenerative diseases and neural injury, however the molecular mechanisms which regulate this process are yet to be fully elucidated. Furthermore, how the molecular mechanisms governing degeneration are impacted by the age of the individual is poorly understood. Interestingly, in mice which are under 3 weeks of age, the degeneration of axons and synapses following hypoxic or traumatic injury is significantly slower. This process, known as Wallerian degeneration (WD), is a molecularly and morphologically distinct subtype of neurodegeneration by which axons and synapses undergo distinct fragmentation and death following a range of stimuli. In this study, we first use an ex-vivo model of axon injury to confirm the significant delay in WD in neonatal mice. We apply tandem mass-tagging quantitative proteomics to profile both nerve and muscle between P12 and P24 inclusive. Application of unbiased in silico workflows to relevant protein identifications highlights a steady elevation in oxidative phosphorylation cascades corresponding to the accelerated degeneration rate. We demonstrate that inhibition of Complex I prevents the axotomy-induced rise in reactive oxygen species and protects axons following injury. Furthermore, we reveal that pharmacological activation of oxidative phosphorylation significantly accelerates degeneration at the neuromuscular junction in neonatal mice. In summary, we reveal dramatic changes in the neuromuscular proteome during post-natal maturation of the neuromuscular system, and demonstrate that endogenous dynamics in mitochondrial bioenergetics during this time window have a functional impact upon regulating the stability of the neuromuscular system.

Implications of NAD metabolism in pathophysiology and therapeutics for neurodegenerative diseases

Nicotinamide adenine dinucleotide (NAD) is an essential coenzyme that mediates various redox reactions. Particularly, mitochondrial NAD plays a critical role in energy production pathways, including the tricarboxylic acid (TCA) cycle, fatty acid oxidation, and oxidative phosphorylation. NAD also serves as a substrate for ADP-ribosylation and deacetylation by poly(ADP-ribose) polymerases (PARPs) and sirtuins, respectively. Thus, NAD regulates energy metabolism, DNA damage repair, gene expression, and stress response. Numerous studies have demonstrated the involvement of NAD metabolism in neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and retinal degenerative diseases. Mitochondrial dysfunction is considered crucial pathogenesis for neurodegenerative diseases such as AD and PD. Maintaining appropriate NAD levels is important for mitochondrial function. Indeed, decreased NAD levels are observed in AD and PD, and supplementation of NAD precursors ameliorates disease phenotypes by activating mitochondrial functions. NAD metabolism also plays an important role in axonal degeneration, a characteristic feature of peripheral neuropathy and neurodegenerative diseases. In addition, dysregulated NAD metabolism is implicated in retinal degenerative diseases such as glaucoma and Leber congenital amaurosis, and NAD metabolism is considered a therapeutic target for these diseases. In this review, we summarize the involvement of NAD metabolism in axon degeneration and various neurodegenerative diseases and discuss perspectives of nutritional intervention using NAD precursors.

Axon Degeneration: Too Much NMN Is Actually Bad?

The mechanism of axon degeneration is incompletely understood. A recent study demonstrates that transgenic expression of bacterial nicotinamide adenine mononucleotide (NMN) in zebrafish and mice, which decreases NMN levels by converting it to NaMN, protects against axon degeneration.

Loss of Sarm1 does not suppress motor neuron degeneration in the SOD1G93A mouse model of amyotrophic lateral sclerosis

Axon degeneration occurs in all neurodegenerative diseases, but the molecular pathways regulating axon destruction during neurodegeneration are poorly understood. Sterile Alpha and TIR Motif Containing 1 (Sarm1) is an essential component of the prodegenerative pathway driving axon degeneration after axotomy and represents an appealing target for therapeutic intervention in neurological conditions involving axon loss. Amyotrophic lateral sclerosis (ALS) is characterized by rapid, progressive motor neuron degeneration and muscle atrophy, causing paralysis and death. Patient tissue and animal models of ALS show destruction of upper and lower motor neuron cell bodies and loss of their associated axons. Here, we investigate whether loss of Sarm1 can mitigate motor neuron degeneration in the SOD1^G93A mouse model of ALS. We found no change in survival, behavioral, electrophysiogical or histopathological outcomes in SOD1^G93A mice null for Sarm1. Blocking Sarm1-mediated axon destruction alone is therefore not sufficient to suppress SOD1^G93A-induced neurodegeneration. Our data suggest the molecular pathways driving axon loss in ALS may be Sarm1-independent or involve genetic pathways that act in a redundant fashion with Sarm1.

Clinical and preclinical perspectives on Chemotherapy-Induced Peripheral Neuropathy (CIPN): a narrative review

This review provides an update on the current clinical and preclinical understanding of chemotherapy induced peripheral neuropathy (CIPN). The overview of the clinical syndrome includes a review of its assessment, diagnosis and treatment. CIPN is caused by several widely-used chemotherapeutics including paclitaxel, oxaliplatin, bortezomib. Severe CIPN may require dose reduction, or cessation, of chemotherapy, impacting on patient survival. While CIPN often resolves after chemotherapy, around 30% of patients will have persistent problems, impacting on function and quality of life. Early assessment and diagnosis is important, and we discuss tools developed for this purpose. There are no effective strategies to prevent CIPN, with limited evidence of effective drugs for treating established CIPN. Duloxetine has moderate evidence, with extrapolation from other neuropathic pain states generally being used to direct treatment options for CIPN. The preclinical perspective includes a discussion on the development of clinically-relevant rodent models of CIPN and some of the potentially modifiable mechanisms that have been identified using these models. We focus on the role of mitochondrial dysfunction, oxidative stress, immune cells and changes in ion channels from summary of the latest literature in these areas. Many causal mechanisms of CIPN occur simultaneously and/or can reinforce each other. Thus, combination therapies may well be required for most effective management. More effective treatment of CIPN will require closer links between oncology and pain management clinical teams to ensure CIPN patients are effectively monitored. Furthermore, continued close collaboration between clinical and preclinical research will facilitate the development of novel treatments for CIPN.

Benefits of Enhancing Nicotinamide Adenine Dinucleotide Levels in Damaged or Diseased Nerve Cells

Three unbiased lines of research have commonly pointed to the benefits of enhanced levels of nicotinamide adenine dinucleotide (NAD⁺) to diseased or damaged neurons. Mice carrying a triplication of the gene encoding the culminating enzyme in NAD⁺ salvage from nicotinamide, NMNAT, are protected from a variety of insults to axons. Protection from Wallerian degeneration of axons is also observed in flies and mice bearing inactivating mutations in the SARM1 gene. Functional studies of the SARM1 gene product have revealed the presence of an enzymatic activity directed toward the hydrolysis of NAD⁺ Finally, an unbiased drug screen performed in living mice led to the discovery of a neuroprotective chemical designated P7C3. Biochemical studies of the P7C3 chemical show that it can enhance recovery of NAD⁺ from nicotinamide by activating NAMPT, the first enzyme in the salvage pathway. In combination, these three unrelated research endeavors offer evidence of the benefits of enhanced NAD⁺ levels to damaged neurons.

Gene therapy targeting SARM1 blocks pathological axon degeneration in mice

Axonal degeneration (AxD) following nerve injury, chemotherapy, and in several neurological disorders is an active process driven by SARM1, an injury-activated NADase. Axons of SARM1-null mice exhibit greatly delayed AxD after transection and in models of neurological disease, suggesting that inhibiting SARM1 is a promising strategy to reduce pathological AxD. Unfortunately, no drugs exist to target SARM1. We, therefore, developed SARM1 dominant-negatives that potently block AxD in cellular models of axotomy and neuropathy. To assess efficacy in vivo, we used adeno-associated virus-mediated expression of the most potent SARM1 dominant-negative and nerve transection as a model of severe AxD. While axons of vehicle-treated mice degenerate rapidly, axons of mice expressing SARM1 dominant-negative can remain intact for >10 d after transection, similar to the protection observed in SARM1-null mice. We thus developed a novel in vivo gene therapeutic to block pathological axon degeneration by inhibiting SARM1, an approach that may be applied clinically to treat manifold neurodegenerative diseases characterized by axon loss.

SARM: From immune regulator to cell executioner

SARM is the fifth and most conserved member of the Toll/Il-1 Receptor (TIR) adaptor family. However, unlike the other TIR adaptors, MyD88, Mal, TRIF and TRAM, SARM does not participate in transducing signals downstream of TLRs. By contrast SARM inhibits TLR signalling by interacting with the adaptors TRIF and MyD88. In addition, SARM also has positive roles in innate immunity by activating specific transcriptional programs following immune challenge. SARM has a pivotal role in activating different forms of cell death following cellular stress and viral infection. Many of these functions of mammalian SARM are also reflected in SARM orthologues in lower organisms such as C. elegans and Drosophila. SARM expression is particularly enriched in neurons of the CNS and SARM has a critical role in neuronal death and in axon degeneration. Recent fascinating molecular insights have been revealed as to the molecular mechanism of SARM mediated axon degeneration. SARM has been shown to deplete NAD+ by possessing intrinsic NADase activity in the TIR domain of the protein. This activity can be activated experimentally by forced dimerization of the TIR domain. It is thought that this activity of SARM is normally switched off by the axo-protective activities of NMNAT2 which maintain low levels of the NAD+ precursor NMN. Therefore, there is now great excitement in the field of SARM research as targeting this enzymatic activity of SARM may lead to the development of new therapies for neurodegenerative diseases such as multiple sclerosis and motor neuron disease.

Clinical electrophysiology of axonal polyneuropathies

Axonal neuropathies encompass a wide range of acquired and inherited disorders with electrophysiologic characteristics that arise from the unique neurophysiology of the axon. Accurate interpretation of nerve conduction studies and electromyography requires an in-depth understanding of the pathophysiology of the axon. Here we review the unique neurophysiologic properties of the axon and how they relate to clinical electrodiagnostic features. We review the length-dependent Wallerian or "dying-back" processes as well as the emerging body of literature from acquired axonal neuropathies that highlights the importance of axonal disease at the nodes of Ranvier. Neurophysiologic features of individual inherited and acquired axonal diseases, including primary nerve disease as well as systemic immune mediated, metabolic, and toxic diseases involving the peripheral nerve, are reviewed. This comprehensive review of electrodiagnostic findings coupled with the current understanding of pathophysiology will aid the clinician in the evaluation of axonal polyneuropathies.

Metabolic aspects of neuronal degeneration: From a NAD+ point of view

Cellular metabolism maintains the life of cells, allowing energy production required for building cellular constituents and maintaining homeostasis under constantly changing external environments. Neuronal cells maintain their structure and function for the entire life of organisms and the loss of neurons, with limited neurogenesis in adults, directly causes loss of complexity in the neuronal networks. The nervous system organizes the neurons by placing cell bodies containing nuclei of similar types of neurons in discrete regions. Accordingly, axons must travel great distances to connect different types of neurons and peripheral organs. The enormous surface area of neurons makes them high-energy demanding to keep their membrane potential. Distal axon survival is dependent on axonal transport that is another energy demanding process. All of these factors make metabolic stress a potential risk factor for neuronal death and neuronal degeneration often associated with metabolic diseases. This review discusses recent findings on metabolic dysregulations under neuronal degeneration and pathways protecting neurons in these conditions.

Nmnat mitigates sensory dysfunction in a Drosophila model of paclitaxel-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is the major dose-limiting side effect of many commonly used chemotherapeutic agents, including paclitaxel. Currently, there are no neuroprotective or effective symptomatic treatments for CIPN. Lack of understanding of the in vivo mechanisms of CIPN has greatly impeded the identification of therapeutic targets. Here, we optimized a model of paclitaxel-induced peripheral neuropathy using Drosophila larvae that recapitulates aspects of chemotherapy-induced sensory dysfunction. We showed that nociceptive sensitivity is associated with disrupted organization of microtubule-associated MAP1B/Futsch and aberrant stabilization of peripheral sensory dendrites. These findings establish a robust and amenable model for studying peripheral mechanisms of CIPN. Using this model, we uncovered a critical role for nicotinamide mononucleotide adenylyltransferase (Nmnat) in maintaining the integrity and function of peripheral sensory neurons and uncovered Nmnat's therapeutic potential against diverse sensory symptoms of CIPN.

Summary: Neurotoxic side effects of chemotherapy are poorly understood. Here, the authors optimize a Drosophila model of paclitaxel-induced sensory dysfunction, which is then used to explore the neuroprotective capacity of Nmnat.

Deletion of Sarm1 gene is neuroprotective in two models of peripheral neuropathy

Distal axon degeneration seen in many peripheral neuropathies is likely to share common molecular mechanisms with Wallerian degeneration. Although several studies in mouse models of peripheral neuropathy showed prevention of axon degeneration in the slow Wallerian degeneration (Wlds) mouse, the role of a recently identified player in Wallerian degeneration, Sarm1, has not been explored extensively. In this study, we show that mice lacking the Sarm1 gene are resistant to distal axonal degeneration in a model of chemotherapy induced peripheral neuropathy caused by paclitaxel and a model of high fat diet induced putative metabolic neuropathy. This study extends the role of Sarm1 to axon degeneration seen in peripheral neuropathies and identifies it as a likely target for therapeutic development.

Heparanase attenuates axon degeneration following sciatic nerve transection

Axon degeneration underlies many nervous system diseases; therefore understanding the regulatory signalling pathways is fundamental to identifying potential therapeutics. Previously, we demonstrated heparan sulphates (HS) as a potentially new target for promoting CNS repair. HS modulate cell signalling by both acting as cofactors in the formation of ligand-receptor complexes and in sequestering ligands in the extracellular matrix. The enzyme heparanase (Hpse) negatively regulates these processes by cleaving HS and releasing the attached proteins, thereby attenuating their ligand-receptor interaction. To explore a comparative role for HS in PNS axon injury/repair we data mined published microarrays from distal sciatic nerve injury. We identified Hpse as a previously unexplored candidate, being up-regulated following injury. We confirmed these results and demonstrated inhibition of Hpse led to an acceleration of axonal degeneration, accompanied by an increase in β-catenin. Inhibition of β-catenin and the addition of Heparinase I both attenuated axonal degeneration. Furthermore the inhibition of Hpse positively regulates transcription of genes associated with peripheral neuropathies and Schwann cell de-differentiation. Thus, we propose Hpse participates in the regulation of the Schwann cell injury response and axo-glia support, in part via the regulation of Schwann cell de-differentiation and is a potential therapeutic that warrants further investigation.

Identification of fluocinolone acetonide to prevent paclitaxel-induced peripheral neuropathy

Paclitaxel (PTX) is among the most commonly used cancer drugs that cause chemotherapy-induced peripheral neuropathy (CIPN), a debilitating and serious dose-limiting side effect. Currently, no drugs exist to prevent CIPN, and symptomatic therapy is often ineffective. In order to identify therapeutic candidates to prevent axonal degeneration induced by PTX, we carried out a phenotypic drug screening using primary rodent dorsal root ganglion sensory neurons. We identified fluocinolone acetonide as a neuroprotective compound and verified it through secondary screens. Furthermore, we showed its efficacy in a mouse model of PTX-induced peripheral neuropathy and confirmed with four different cancer cell lines that fluocinolone acetonide does not interfere with PTX's antitumor activity. Our study identifies fluocinolone acetonide as a potential therapy to prevent CIPN caused by PTX.

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.

Pharmacological augmentation of nicotinamide phosphoribosyltransferase (NAMPT) protects against paclitaxel-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) arises from collateral damage to peripheral afferent sensory neurons by anticancer pharmacotherapy, leading to debilitating neuropathic pain. No effective treatment for CIPN exists, short of dose-reduction which worsens cancer prognosis. Here, we report that stimulation of nicotinamide phosphoribosyltransferase (NAMPT) produced robust neuroprotection in an aggressive CIPN model utilizing the frontline anticancer drug, paclitaxel (PTX). Daily treatment of rats with the first-in-class NAMPT stimulator, P7C3-A20, prevented behavioral and histologic indicators of peripheral neuropathy, stimulated tissue NAD recovery, improved general health, and abolished attrition produced by a near maximum-tolerated dose of PTX. Inhibition of NAMPT blocked P7C3-A20-mediated neuroprotection, whereas supplementation with the NAMPT substrate, nicotinamide, potentiated a subthreshold dose of P7C3-A20 to full efficacy. Importantly, P7C3-A20 blocked PTX-induced allodynia in tumored mice without reducing antitumoral efficacy. These findings identify enhancement of NAMPT activity as a promising new therapeutic strategy to protect against anticancer drug-induced peripheral neurotoxicity.

Paclitaxel Reduces Axonal Bclw to Initiate IP3R1-Dependent Axon Degeneration

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating side effect of many cancer treatments. The hallmark of CIPN is degeneration of long axons required for transmission of sensory information; axonal degeneration causes impaired tactile sensation and persistent pain. Currently the molecular mechanisms of CIPN are not understood, and there are no available treatments. Here we show that the chemotherapeutic agent paclitaxel triggers CIPN by altering IP₃ receptor phosphorylation and intracellular calcium flux, and activating calcium-dependent calpain proteases. Concomitantly paclitaxel impairs axonal trafficking of RNA-granules and reduces synthesis of Bclw (bcl2l2), a Bcl2 family member that binds IP₃R1 and restrains axon degeneration. Surprisingly, Bclw or a stapled peptide corresponding to the Bclw BH4 domain interact with axonal IP₃R1 and prevent paclitaxel-induced degeneration, while Bcl2 and Bclx_L cannot do so. Together these data identify a Bclw-IP₃R1-dependent cascade that causes axon degeneration and suggest that Bclw-mimetics could provide effective therapy to prevent CIPN.

Lost in elimination: mechanisms of axonal loss

Axonal loss is an important process both dur­ing development and diseases of the ner­vous system. While the molecular mecha­nisms that mediate axonal loss are largely elusive, modern imaging technology affords an increasingly clear view of the cellular processes that allow nerve cells to shed individiual axon branches or even dismantle entire parts of their axonal projections. The present review discusses the characteristics of post-traumatic Wallerian degeneration, the process of axonal loss currently best understood. Subsequently, the properties of a number of recently discovered axonal loss phenome­na are described. These phenomena explain some of the axonal loss that occurs locally after axon transection, during neuro-inflammatory insults, and as part of normal neurode­velopment.

Epigenetic Regulation of Axon Regeneration after Neural Injury

When peripheral axons are damaged, neuronal injury signaling pathways induce transcriptional changes that support axon regeneration and consequent functional recovery. The recent development of bioinformatics techniques has allowed for the identification of many of the regeneration-associated genes that are regulated by neural injury, yet it remains unclear how global changes in transcriptome are coordinated. In this article, we review recent studies on the epigenetic mechanisms orchestrating changes in gene expression in response to nerve injury. We highlight the importance of epigenetic mechanisms in discriminating efficient axon regeneration in the peripheral nervous system and very limited axon regrowth in the central nervous system and discuss the therapeutic potential of targeting epigenetic regulators to improve neural recovery.

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

Peripheral polyneuropathy is a common and dose-limiting side effect of many important chemotherapeutic agents. Most such neuropathies are characterized by early axonal degeneration, yet therapies that inhibit this axonal destruction process do not currently exist. Recently, we and others discovered that genetic deletion of SARM1 (sterile alpha and TIR motif containing protein 1) dramatically protects axons from degeneration after axotomy in mice. This finding fuels hope that inhibition of SARM1 or its downstream components can be used therapeutically in patients threatened by axonal loss. However, axon loss in most neuropathies, including chemotherapy-induced peripheral neuropathy, is the result of subacute/chronic processes that may be regulated differently than the acute, one time insult of axotomy. Here we evaluate if genetic deletion of SARM1 decreases axonal degeneration in a mouse model of neuropathy induced by the chemotherapeutic agent vincristine. In wild-type mice, 4 weeks of twice-weekly intraperitoneal injections of 1.5 mg/kg vincristine cause pronounced mechanical and heat hyperalgesia, a significant decrease in tail compound nerve action potential amplitude, loss of intraepidermal nerve fibres and significant degeneration of myelinated axons in both the distal sural nerve and nerves of the toe. Neither the proximal sural nerve nor the motor tibial nerve exhibit axon loss. These findings are consistent with the development of a distal, sensory predominant axonal polyneuropathy that mimics vincristine-induced peripheral polyneuropathy in humans. Using the same regimen of vincristine treatment in SARM1 knockout mice, the development of mechanical and heat hyperalgesia is blocked and the loss in tail compound nerve action potential amplitude is prevented. Moreover, SARM1 knockout mice do not lose unmyelinated fibres in the skin or myelinated axons in the sural nerve and toe after vincristine. Hence, genetic deletion of SARM1 blocks the development of vincristine-induced peripheral polyneuropathy in mice. Our results reveal that subacute/chronic axon loss induced by vincristine occurs via a SARM1 mediated axonal destruction pathway, and that blocking this pathway prevents the development of vincristine-induced peripheral polyneuropathy. These findings, in conjunction with previous studies with axotomy and traumatic brain injury, establish SARM1 as the central determinant of a fundamental axonal degeneration pathway that is activated by diverse insults. We suggest that targeting SARM1 or its downstream effectors may be a viable therapeutic option to prevent vincristine-induced peripheral polyneuropathy and possibly other peripheral polyneuropathies.

Neurotoxic mechanisms of paclitaxel are local to the distal axon and independent of transport defects

Chemotherapy-induced peripheral neuropathy (CIPN) is a dose-limiting side effect of paclitaxel and other chemotherapeutic agents. Paclitaxel binds and stabilizes microtubules, but the cellular mechanisms that underlie paclitaxel's neurotoxic effects are not well understood. We therefore used primary cultures of adult murine dorsal root ganglion neurons, the cell type affected in patients, to examine leading hypotheses to explain paclitaxel neurotoxicity. We address the role of microtubule hyperstabilization and its downstream effects. Paclitaxel administered at 10-50nM for 1-3days induced retraction bulbs at the tips of axons and arrested axon growth without triggering axon fragmentation or cell death. By correlating the toxic effects and microtubule stabilizing activity of structurally different microtubule stabilizing compounds, we confirmed that microtubule hyperstabilization, rather than an off-target effect, is the likely primary cause of paclitaxel neurotoxicity. We examined potential downstream consequences of microtubule hyperstabilization and found that changes in levels of tubulin posttranslational modifications, although present after paclitaxel exposure, are not implicated in the paclitaxel neurotoxicity we observed in the cultures. Additionally, defects in axonal transport were not implicated as an early, causative mechanism of paclitaxel's toxic effects on dorsal root ganglion neurons. By using microfluidic chambers to selectively treat different parts of the axon with paclitaxel, we found that the distal axon was primarily vulnerable to paclitaxel, indicating that paclitaxel acts directly on the distal axon to induce degenerative effects. Together, our findings point to local effects of microtubule hyperstabilization on the distal-most portion of the axon as an early mediator of paclitaxel neurotoxicity. Because sensory neurons have a unique and ongoing requirement for distal growth in order to reinnervate the epidermis as it turns over, we propose that the ability of paclitaxel to arrest their growth accounts for the selective vulnerability of sensory neurons to paclitaxel neurotoxicity.