The Phr1 ubiquitin ligase promotes injury-induced axon self-destruction

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in Caenorhabditis elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signalling effector that controls diverse cellular functions.

Cell death classification: A new insight based on molecular mechanisms

Cells tend to disintegrate themselves or are forced to undergo such destructive processes in critical circumstances. This complex cellular function necessitates various mechanisms and molecular pathways in order to be executed. The very nature of cell death is essentially important and vital for maintaining homeostasis, thus any type of disturbing occurrence might lead to different sorts of diseases and dysfunctions. Cell death has various modalities and yet, every now and then, a new type of this elegant procedure gets to be discovered. The diversity of cell death compels the need for a universal organizing system in order to facilitate further studies, therapeutic strategies and the invention of new methods of research. Considering all that, we attempted to review most of the known cell death mechanisms and sort them all into one arranging system that operates under a simple but subtle decision-making (If \ Else) order as a sorting algorithm, in which it decides to place and sort an input data (a type of cell death) into its proper set, then a subset and finally a group of cell death. By proposing this algorithm, the authors hope it may solve the problems regarding newer and/or undiscovered types of cell death and facilitate research and therapeutic applications of cell death.

The Molecular Mechanisms Involved in Axonal Degeneration and Retrograde Retinal Ganglion Cell Death

Axonal degeneration is a pathologic change common to multiple retinopathies and optic neuropathies. Various pathologic factors, such as mechanical injury, inflammation, and ischemia, can damage retinal ganglion cell (RGC) somas and axons, eventually triggering axonal degeneration and RGC death. The molecular mechanisms of somal and axonal degeneration are distinct but also overlap, and axonal degeneration can result in retrograde somal degeneration. While the mitogen-activated protein kinase pathway acts as a central node in RGC axon degeneration, several newly discovered molecules, such as sterile alpha and Toll/interleukin-1 receptor motif-containing protein 1 and nicotinamide mononucleotide adenylyltransferase 2, also play a critical role in this pathological process following different types of injury. Therefore, we summarize the types of injury that cause RGC axon degeneration and retrograde RGC death and important underlying molecular mechanisms, providing a reference for the identification of targets for protecting axons and RGCs.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in C. elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signaling effector that controls diverse cellular functions.

A nerve-wracking buzz: lessons from Drosophila models of peripheral neuropathy and axon degeneration

The degeneration of axons and their terminals occurs following traumatic, toxic, or genetically-induced insults. Common molecular mechanisms unite these disparate triggers to execute a conserved nerve degeneration cascade. In this review, we will discuss how models of peripheral nerve injury and neuropathy in Drosophila have led the way in advancing molecular understanding of axon degeneration and nerve injury pathways. Both neuron-intrinsic as well as glial responses to injury will be highlighted. Finally, we will offer perspective on what additional questions should be answered to advance these discoveries toward clinical interventions for patients with neuropathy.

SARM1-Dependent Axon Degeneration: Nucleotide Signaling, Neurodegenerative Disorders, Toxicity, and Therapeutic Opportunities

Axons are an essential component of the nervous system, and axon degeneration is an early feature of many neurodegenerative disorders. The NAD+ metabolome plays an essential role in regulating axonal integrity. Axonal levels of NAD+ and its precursor NMN are controlled in large part by the NAD+ synthesizing survival factor NMNAT2 and the pro-neurodegenerative NADase SARM1, whose activation triggers axon destruction. SARM1 has emerged as a promising axon-specific target for therapeutic intervention, and its function, regulation, structure, and role in neurodegenerative diseases have been extensively characterized in recent years. In this review, we first introduce the key molecular players involved in the SARM1-dependent axon degeneration program. Next, we summarize recent major advances in our understanding of how SARM1 is kept inactive in healthy neurons and how it becomes activated in injured or diseased neurons, which has involved important insights from structural biology. Finally, we discuss the role of SARM1 in neurodegenerative disorders and environmental neurotoxicity and its potential as a therapeutic target.

Glycolytic System in Axons Supplement Decreased ATP Levels after Axotomy of the Peripheral Nerve

Wallerian degeneration (WD) occurs in the early stages of numerous neurologic disorders, and clarifying WD pathology is crucial for the advancement of neurologic therapies. ATP is acknowledged as one of the key pathologic substances in WD. The ATP-related pathologic pathways that regulate WD have been defined. The elevation of ATP levels in axon contributes to delay WD and protects axons. However, ATP is necessary for the active processes to proceed WD, given that WD is stringently managed by auto-destruction programs. But little is known about the bioenergetics during WD. In this study, we made sciatic nerve transection models for GO-ATeam2 knock-in rats and mice. We presented the spatiotemporal ATP distribution in the injured axons with in vivo ATP imaging systems, and investigated the metabolic source of ATP in the distal nerve stump. A gradual decrease in ATP levels was observed before the progression of WD. In addition, the glycolytic system and monocarboxylate transporters (MCTs) were activated in Schwann cells following axotomy. Interestingly, in axons, we found the activation of glycolytic system and the inactivation of the tricarboxylic acid (TCA) cycle. Glycolytic inhibitors, 2-deoxyglucose (2-DG) and MCT inhibitors, a-cyano-4-hydroxycinnamic acid (4-CIN) decreased ATP and enhanced WD progression, whereas mitochondrial pyruvate carrier (MPC) inhibitors (MSDC-0160) did not change. Finally, ethyl pyruvate (EP) increased ATP levels and delayed WD. Together, our findings suggest that glycolytic system, both in Schwann cells and axons, is the main source of maintaining ATP levels in the distal nerve stump.

TDP-43 dysregulation and neuromuscular junction disruption in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disease characterized by upper and lower motor neuron (MN) loss with a signature feature of cytoplasmic aggregates containing TDP-43, which are detected in nearly all patients. Mutations in the gene that encodes TDP-43 (TARBDP) are known to result in both familial and sporadic ALS. In ALS, disruption of neuromuscular junctions (NMJs) constitutes a critical event in disease pathogenesis, leading to denervation atrophy, motor impairments and disability. Morphological defects and impaired synaptic transmission at NMJs have been reported in several TDP-43 animal models and in vitro, linking TDP-43 dysregulation to the loss of NMJ integrity in ALS. Through the lens of the dying-back and dying-forward hypotheses of ALS, this review discusses the roles of TDP-43 related to synaptic function, with a focus on the potential molecular mechanisms occurring within MNs, skeletal muscles and glial cells that may contribute to NMJ disruption in ALS.

NAD+ metabolism in peripheral neuropathic pain

Nicotinamide adenine dinucleotide (NAD⁺) is an omnipresent metabolite that participates in redox reactions. Multiple NAD⁺-consuming enzymes are implicated in numerous biological processes, including transcription, signaling, and cell survival. Multiple pieces of evidence have demonstrated that NAD⁺-consuming enzymes, including poly(ADP-ribose) polymerases (PARPs), sirtuins (SIRTs), and sterile alpha and TIR motif-containing 1 (SARM1), play major roles in peripheral neuropathic pain of various etiologies. These NAD⁺ consumers primarily participate in peripheral neuropathic pain via mechanisms such as mitochondrial dysfunction, oxidative stress, and inflammation. Furthermore, NAD⁺ synthase and nicotinamide phosphoribosyltransferase (NAMPT) have recently been found to contribute to the regulation of pain. Here, we review the evidence indicating the involvement of NAD⁺ metabolism in the pathological mechanisms of peripheral neuropathic pain. Advanced understanding of the molecular and cellular mechanisms associated with NAD⁺ in peripheral neuropathic pain will facilitate the development of novel treatment options for diverse types of peripheral neuropathic pain.

Autophagy protein ULK1 interacts with and regulates SARM1 during axonal injury

Autophagy is a cellular catabolic pathway generally thought to be neuroprotective. However, autophagy and in particular its upstream regulator, the ULK1 kinase, can also promote axonal degeneration. We examined the role and the mechanisms of autophagy in axonal degeneration using a mouse model of contusive spinal cord injury (SCI). Consistent with activation of autophagy during axonal degeneration following SCI, autophagosome marker LC3, ULK1 kinase, and ULK1 target, phospho-ATG13, accumulated in the axonal bulbs and injured axons. SARM1, a TIR NADase with a pivotal role in axonal degeneration, colocalized with ULK1 within 1 h after SCI, suggesting possible interaction between autophagy and SARM1-mediated axonal degeneration. In our in vitro experiments, inhibition of autophagy, including Ulk1 knockdown and ULK1 inhibitor, attenuated neurite fragmentation and reduced accumulation of SARM1 puncta in neurites of primary cortical neurons subjected to glutamate excitotoxicity. Immunoprecipitation data demonstrated that ULK1 physically interacted with SARM1 in vitro and in vivo and that SAM domains of SARM1 were necessary for ULK1-SARM1 complex formation. Consistent with a role in regulation of axonal degeneration, in primary cortical neurons ULK1-SARM1 interaction increased upon neurite damage. Supporting a role for autophagy and ULK1 in regulation of SARM1 in axonal degeneration in vivo, axonal ULK1 activation and accumulation of SARM1 were both decreased after SCI in Becn1^+/- autophagy hypomorph mice compared to wild-type (WT) controls. These findings suggest a regulatory crosstalk between autophagy and axonal degeneration pathways, which is mediated through ULK1-SARM1 interaction and contributes to the ability of SARM1 to accumulate in injured axons.

An atypical ubiquitin ligase at the heart of neural development and programmed axon degeneration

The degeneration of nerve fibres following injury was first described by Augustus Waller over 170 years ago. Initially assumed to be a passive process, it is now evident that axons respond to insult via regulated cellular signaling events resulting in their programmed degeneration. Pro-survival and pro-degenerative factors have been identified and their regulatory mechanisms are beginning to emerge. The ubiquitin system has been implicated in the pro-degenerative process and a key component is the ubiquitin E3 ligase MYCBP2 (also known as PHR1). Ubiquitin E3 ligases are tasked with the transfer of the small protein modifier ubiquitin to substrates and consist of hundreds of members. They can be classified as single subunit systems or as multi-subunit complexes. Their catalytic domains can also be assigned to three general architectures. Hints that MYCBP2 might not conform to these established formats came to light and it is now clear from biochemical and structural studies that MYCBP2 is indeed an outlier in terms of its modus operandi. Furthermore, the unconventional way in which MYCBP2 transfers ubiquitin to substrates has been linked to neurodevelopmental and pro-degenerative function. Herein, we will summarize these research developments relating to the unusual features of MYCBP2 and postulate therapeutic strategies that prevent Wallerian degeneration. These have exciting potential for providing relief from pathological neuropathies and neurodegenerative diseases.

Mitochondrial Localization of SARM1 in Acrylamide Intoxication Induces Mitophagy and Limits Neuropathy

Sterile α and toll/interleukin 1 receptor motif-containing protein 1 (SARM1) is the defining molecule and central executioner of programmed axon death, also known as Wallerian degeneration. SARM1 has a mitochondrial targeting sequence, and it can bind to and stabilize PTEN-induced putative kinase 1 (PINK1) for mitophagy induction, but the deletion of the mitochondrial localization sequence is found to disrupt the mitochondrial localization of SARM1 in neurons without altering its ability to promote axon degeneration after axotomy. The biological significance of SARM1 mitochondrial localization remains elusive. In this study, we observed that the pro-degeneration factor, SARM1, was upregulated in acrylamide (ACR) neuropathy, a slow, Wallerian-like, programmed axonal death process. The upregulated SARM1 accumulated on mitochondria, interfered with mitochondrial dynamics, and activated PINK1-mediated mitophagy. Importantly, rapamycin (RAPA) intervention eliminated mitochondrial accumulation of SARM1 and partly attenuated ACR neuropathy. Thus, mitochondrial localization of SARM1 may contribute to its clearance through the SARM1-PINK1 mitophagy pathway, which inhibits axonal degeneration through a negative feedback loop. The mitochondrial localization of SARM1 complements the coordinated activity of the pro-survival factor, nicotinamide mononucleotide adenyltransferase 2 (NMNAT2), and SARM1 and is part of the self-limiting molecular mechanisms underpinning programmed axon death in ACR neuropathy. Mitophagy clearance of SARM1 is complementary to the coordinated activity of NMNAT2 and SARM1 in ACR neuropathy.

Multifaceted roles of SARM1 in axon degeneration and signaling

Axons are considered to be particularly vulnerable components of the nervous system; impairments to a neuron’s axon leads to an effective silencing of a neuron’s ability to communicate with other cells. Nervous systems have therefore evolved plasticity mechanisms for adapting to axonal damage. These include acute mechanisms that promote the degeneration and clearance of damaged axons and, in some cases, the initiation of new axonal growth and synapse formation to rebuild lost connections. Here we review how these diverse processes are influenced by the therapeutically targetable enzyme SARM1. SARM1 catalyzes the breakdown of NAD+, which, when unmitigated, can lead to rundown of this essential metabolite and axonal degeneration. SARM1’s enzymatic activity also triggers the activation of downstream signaling pathways, which manifest numerous functions for SARM1 in development, innate immunity and responses to injury. Here we will consider the multiple intersections between SARM1 and the injury signaling pathways that coordinate cellular adaptations to nervous system damage.

Of axons that struggle to make ends meet: Linking axonal bioenergetic failure to programmed axon degeneration

Axons are the long, fragile, and energy-hungry projections of neurons that are challenging to sustain. Together with their associated glia, they form the bulk of the neuronal network. Pathological axon degeneration (pAxD) is a driver of irreversible neurological disability in a host of neurodegenerative conditions. Halting pAxD is therefore an attractive therapeutic strategy. Here we review recent work demonstrating that pAxD is regulated by an auto-destruction program that revolves around axonal bioenergetics. We then focus on the emerging concept that axonal and glial energy metabolism are intertwined. We anticipate that these discoveries will encourage the pursuit of new treatment strategies for neurodegeneration.

Emerging evidence for compromised axonal bioenergetics and axoglial metabolic coupling as drivers of neurodegeneration

Impaired bioenergetic capacity of the nervous system is thought to contribute to the pathogenesis of many neurodegenerative diseases (NDD). Since neuronal synapses are believed to be the major energy consumers in the nervous system, synaptic derangements resulting from energy deficits have been suggested to play a central role for the development of many of these disorders. However, long axons constitute the largest compartment of the neuronal network, require large amounts of energy, are metabolically and structurally highly vulnerable, and undergo early injurious stresses in many NDD. These stresses likely impose additional energy demands for continuous adaptations and repair processes, and may eventually overwhelm axonal maintenance mechanisms. Indeed, pathological axon degeneration (pAxD) is now recognized as an etiological focus in a wide array of NDD associated with bioenergetic abnormalities. In this paper I first discuss the recognition that a simple experimental model for pAxD is regulated by an auto-destruction program that exhausts distressed axons energetically. Provision of the energy substrate pyruvate robustly counteracts this axonal breakdown. Importantly, energy decline in axons is not only a consequence but also an initiator of this program. This opens the intriguing possibility that axon dysfunction and pAxD can be suppressed by preemptively energizing distressed axons. Second, I focus on the emerging concept that axons communicate energetically with their flanking glia. This axoglial metabolic coupling can help offset the axonal energy decline that activates the pAxD program but also jeopardize axon integrity as a result of perturbed glial metabolism. Third, I present compelling evidence that abnormal axonal energetics and compromised axoglial metabolic coupling accompany the activation of the pAxD auto-destruction pathway in models of glaucoma, a widespread neurodegenerative condition with pathogenic overlap to other common NDD. In conclusion, I propose a novel conceptual framework suggesting that therapeutic interventions focused on bioenergetic support of the nervous system should also address axons and their metabolic interactions with glia.

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.

NMNAT2 is downregulated in glaucomatous RGCs, and RGC-specific gene therapy rescues neurodegeneration and visual function

The lack of neuroprotective treatments for retinal ganglion cells (RGCs) and optic nerve (ON) is a central challenge for glaucoma management. Emerging evidence suggests that redox factor NAD+ decline is a hallmark of aging and neurodegenerative diseases. Supplementation with NAD+ precursors and overexpression of NMNAT1, the key enzyme in the NAD+ biosynthetic process, have significant neuroprotective effects. We first profile the translatomes of RGCs in naive mice and mice with silicone oil-induced ocular hypertension (SOHU)/glaucoma by RiboTag mRNA sequencing. Intriguingly, only NMNAT2, but not NMNAT1 or NMNAT3, is significantly decreased in SOHU glaucomatous RGCs, which we confirm by in situ hybridization. We next demonstrate that AAV2 intravitreal injection-mediated overexpression of long half-life NMNAT2 mutant driven by RGC-specific mouse γ-synuclein (mSncg) promoter restores decreased NAD+ levels in glaucomatous RGCs and ONs. Moreover, this RGC-specific gene therapy strategy delivers significant neuroprotection of both RGC soma and axon and preservation of visual function in the traumatic ON crush model and the SOHU glaucoma model. Collectively, our studies suggest that the weakening of NMNAT2 expression in glaucomatous RGCs contributes to a deleterious NAD+ decline, and that modulating RGC-intrinsic NMNAT2 levels by AAV2-mSncg vector is a promising gene therapy for glaucomatous neurodegeneration.

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.

Ubiquitin ligase activity inhibits Cdk5 to control axon termination

The Cdk5 kinase plays prominent roles in nervous system development, plasticity, behavior and disease. It also has important, non-neuronal functions in cancer, the immune system and insulin secretion. At present, we do not fully understand negative regulatory mechanisms that restrict Cdk5. Here, we use Caenorhabditis elegans to show that CDK-5 is inhibited by the RPM-1/FSN-1 ubiquitin ligase complex. This atypical RING ubiquitin ligase is conserved from C. elegans through mammals. Our finding originated from unbiased, in vivo affinity purification proteomics, which identified CDK-5 as a putative RPM-1 substrate. CRISPR-based, native biochemistry showed that CDK-5 interacts with the RPM-1/FSN-1 ubiquitin ligase complex. A CRISPR engineered RPM-1 substrate ‘trap’ enriched CDK-5 binding, which was mediated by the FSN-1 substrate recognition module. To test the functional genetic relationship between the RPM-1/FSN-1 ubiquitin ligase complex and CDK-5, we evaluated axon termination in mechanosensory neurons and motor neurons. Our results indicate that RPM-1/FSN-1 ubiquitin ligase activity restricts CDK-5 to control axon termination. Collectively, these proteomic, biochemical and genetic results increase our understanding of mechanisms that restrain Cdk5 in the nervous system.

Cdk5 is an atypical cyclin dependent kinase and an important player in nervous system development, plasticity, and disease. Decades of research has focused on understanding how Cdk5 is activated. In contrast, we know much less about the genetic and molecular mechanisms that restrict Cdk5 activity. Here, we examined how Cdk5 is inhibited in the nervous system using the model organism C. elegans. Our results indicate that the RPM-1/FSN-1 E3 ubiquitin ligase complex inhibits Cdk5 to control termination of axon growth. Our finding that ubiquitin ligase activity restricts Cdk5 in the nervous system in vivo now opens up the interesting possibility that ubiquitin ligase activity might regulate Cdk5 in other cellular contexts and disease settings.

SARM1 can be a potential therapeutic target for spinal cord injury

Injury to the spinal cord is devastating. Studies have implicated Wallerian degeneration as the main cause of axonal destruction in the wake of spinal cord injury. Therefore, the suppression of Wallerian degeneration could be beneficial for spinal cord injury treatment. Sterile alpha and armadillo motif-containing protein 1 (SARM1) is a key modulator of Wallerian degeneration, and its impediment can improve spinal cord injury to a significant degree. In this report, we analyze the various signaling domains of SARM1, the recent findings on Wallerian degeneration and its relation to axonal insults, as well as its connection to SARM1, the mitogen-activated protein kinase (MAPK) signaling, and the survival factor, nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2). We then elaborate on the possible role of SARM1 in spinal cord injury and explicate how its obstruction could potentially alleviate the injury.

FK506-binding protein-like and FK506-binding protein 8 regulate dual leucine zipper kinase degradation and neuronal responses to axon injury

The dual leucine zipper kinase (DLK) is a key regulator of axon regeneration and degeneration in response to neuronal injury; however, regulatory mechanisms of the DLK function via its interacting proteins are largely unknown. To better understand the molecular mechanism of DLK function, we performed yeast two-hybrid screening analysis and identified FK506-binding protein-like (FKBPL, also known as WAF-1/CIP1 stabilizing protein 39) as a DLK-binding protein. FKBPL binds to the kinase domain of DLK and inhibits its kinase activity. In addition, FKBPL induces DLK protein degradation through ubiquitin-dependent pathways. We further assessed other members in the FKBP protein family and found that FK506-binding protein 8 (FKBP8) also induced DLK degradation. We identified the lysine 271 residue in the kinase domain as a major site of DLK ubiquitination and SUMO3 conjugation and was thus responsible for regulating FKBP8-mediated proteasomal degradation that was inhibited by the substitution of the lysine 271 to arginine. FKBP8-mediated degradation of DLK is mediated by autophagy pathway because knockdown of Atg5 inhibited DLK destabilization. We show that in vivo overexpression of FKBP8 delayed the progression of axon degeneration and suppressed neuronal death after axotomy in sciatic and optic nerves. Taken together, this study identified FKBPL and FKBP8 as novel DLK-interacting proteins that regulate DLK stability via the ubiquitin-proteasome and lysosomal protein degradation pathways.

Phagocytosis and self-destruction break down dendrites of Drosophila sensory neurons at distinct steps of Wallerian degeneration

After injury, severed dendrites and axons expose the "eat-me" signal phosphatidylserine (PS) on their surface while they break down. The degeneration of injured axons is controlled by a conserved Wallerian degeneration (WD) pathway, which is thought to activate neurite self-destruction through Sarm-mediated nicotinamide adenine dinucleotide (NAD+) depletion. While neurite PS exposure is known to be affected by genetic manipulations of NAD+, how the WD pathway coordinates both neurite PS exposure and self-destruction and whether PS-induced phagocytosis contributes to neurite breakdown in vivo remain unknown. Here, we show that in Drosophila sensory dendrites, PS exposure and self-destruction are two sequential steps of WD resulting from Sarm activation. Surprisingly, phagocytosis is the main driver of dendrite degeneration induced by both genetic NAD+ disruptions and injury. However, unlike neuronal Nmnat loss, which triggers PS exposure only and results in phagocytosis-dependent dendrite degeneration, injury activates both PS exposure and self-destruction as two redundant means of dendrite degeneration. Furthermore, the axon-death factor Axed is only partially required for self-destruction of injured dendrites, acting in parallel with PS-induced phagocytosis. Lastly, injured dendrites exhibit a unique rhythmic calcium-flashing that correlates with WD. Therefore, both NAD+-related general mechanisms and dendrite-specific programs govern PS exposure and self-destruction in injury-induced dendrite degeneration in vivo.

The NAD+ precursor NMN activates dSarm to trigger axon degeneration in Drosophila

Axon degeneration contributes to the disruption of neuronal circuit function in diseased and injured nervous systems. Severed axons degenerate following the activation of an evolutionarily conserved signaling pathway, which culminates in the activation of SARM1 in mammals to execute the pathological depletion of the metabolite NAD⁺. SARM1 NADase activity is activated by the NAD⁺ precursor nicotinamide mononucleotide (NMN). In mammals, keeping NMN levels low potently preserves axons after injury. However, it remains unclear whether NMN is also a key mediator of axon degeneration and dSarm activation in flies. Here, we demonstrate that lowering NMN levels in Drosophila through the expression of a newly generated prokaryotic NMN-Deamidase (NMN-D) preserves severed axons for months and keeps them circuit-integrated for weeks. NMN-D alters the NAD⁺ metabolic flux by lowering NMN, while NAD⁺ remains unchanged in vivo. Increased NMN synthesis by the expression of mouse nicotinamide phosphoribosyltransferase (mNAMPT) leads to faster axon degeneration after injury. We also show that NMN-induced activation of dSarm mediates axon degeneration in vivo. Finally, NMN-D delays neurodegeneration caused by loss of the sole NMN-consuming and NAD⁺-synthesizing enzyme dNmnat. Our results reveal a critical role for NMN in neurodegeneration in the fly, which extends beyond axonal injury. The potent neuroprotection by reducing NMN levels is similar to the interference with other essential mediators of axon degeneration in Drosophila.

The key role of the NAD biosynthetic enzyme nicotinamide mononucleotide adenylyltransferase in regulating cell functions

The enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT) catalyzes a reaction central to all known NAD biosynthetic routes. In mammals, three isoforms with distinct molecular and catalytic properties, different subcellular and tissue distribution have been characterized. Each isoform is essential for cell survival, with a critical role in modulating NAD levels in a compartment‐specific manner. Each isoform supplies NAD to specific NAD‐dependent enzymes, thus regulating their activity with impact on several biological processes, including DNA repair, proteostasis, cell differentiation, and neuronal maintenance. The nuclear NMNAT1 and the cytoplasmic NMNAT2 are also emerging as relevant targets in specific types of cancers and NMNAT2 has a key role in the activation of antineoplastic compounds. This review recapitulates the biochemical properties of the three isoforms and focuses on recent advances on their protective function, involvement in human diseases and role as druggable targets.

Regulation of Mitogen-Activated Protein Kinase Signaling Pathways by the Ubiquitin-Proteasome System and Its Pharmacological Potential

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved signaling pathways that play essential roles in transducing extracellular environmental signals into diverse cellular responses to maintain homeostasis. These pathways are classically organized into an architecture of three sequentially acting protein kinases: a MAPK kinase kinase that phosphorylates and activates a MAPK kinase, which in turn phosphorylates and activates the effector MAPK. The activity of MAPKs is tightly regulated by phosphorylation of their activation loop, which can be modulated by positive and negative feedback mechanisms to control the amplitude and duration of the signal. The signaling outcomes of MAPK pathways are further regulated by interactions of MAPKs with scaffolding and regulatory proteins. Accumulating evidence indicates that, in addition to these mechanisms, MAPK signaling is commonly regulated by ubiquitin-proteasome system (UPS)-mediated control of the stability and abundance of MAPK pathway components. Notably, the biologic activity of some MAPKs appears to be regulated mainly at the level of protein turnover. Recent studies have started to explore the potential of targeted protein degradation as a powerful strategy to investigate the biologic functions of individual MAPK pathway components and as a new therapeutic approach to overcome resistance to current small-molecule kinase inhibitors. Here, we comprehensively review the mechanisms, physiologic importance, and pharmacological potential of UPS-mediated protein degradation in the control of MAPK signaling. SIGNIFICANCE STATEMENT: Accumulating evidence highlights the importance of targeted protein degradation by the ubiquitin-proteasome system in regulating and fine-tuning the signaling output of mitogen-activated protein kinase (MAPK) pathways. Manipulating protein levels of MAPK cascade components may provide a novel approach for the development of selective pharmacological tools and therapeutics.

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.

The Drama of Wallerian Degeneration: The Cast, Crew, and Script

Significant advances have been made in recent years in identifying the genetic components of Wallerian degeneration, the process that brings the progressive destruction and removal of injured axons. It has now been accepted that Wallerian degeneration is an active and dynamic cellular process that is well regulated at molecular and cellular levels. In this review, we describe our current understanding of Wallerian degeneration, focusing on the molecular players and mechanisms that mediate the injury response, activate the degenerative program, transduce the death signal, execute the destruction order, and finally, clear away the debris. By highlighting the starring roles and sketching out the molecular script of Wallerian degeneration, we hope to provide a useful framework to understand Wallerian and Wallerian-like degeneration and to lay a foundation for developing new therapeutic strategies to treat axon degeneration in neural injury as well as in neurodegenerative disease. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Axon morphogenesis and maintenance require an evolutionary conserved safeguard function of Wnk kinases antagonizing Sarm and Axed

The molecular and cellular mechanisms underlying complex axon morphogenesis are still poorly understood. We report a novel, evolutionary conserved function for the Drosophila Wnk kinase (dWnk) and its mammalian orthologs, WNK1 and 2, in axon branching. We uncover that dWnk, together with the neuroprotective factor Nmnat, antagonizes the axon-destabilizing factors D-Sarm and Axundead (Axed) during axon branch growth, revealing a developmental function for these proteins. Overexpression of D-Sarm or Axed results in axon branching defects, which can be blocked by overexpression of dWnk or Nmnat. Surprisingly, Wnk kinases are also required for axon maintenance of adult Drosophila and mouse cortical pyramidal neurons. Requirement of Wnk for axon maintenance is independent of its developmental function. Inactivation of dWnk or mouse Wnk1/2 in mature neurons leads to axon degeneration in the adult brain. Therefore, Wnk kinases are novel signaling components that provide a safeguard function in both developing and adult axons.

A Novel NAD Signaling Mechanism in Axon Degeneration and its Relationship to Innate Immunity

Axon degeneration represents a pathological feature of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease where axons die before the neuronal soma, and axonopathies, such as Charcot-Marie-Tooth disease and hereditary spastic paraplegia. Over the last two decades, it has slowly emerged that a central signaling pathway forms the basis of this process in many circumstances. This is an axonal NAD-related signaling mechanism mainly regulated by the two key proteins with opposing roles: the NAD-synthesizing enzyme NMNAT2, and SARM1, a protein with NADase and related activities. The crosstalk between the axon survival factor NMNAT2 and pro-degenerative factor SARM1 has been extensively characterized and plays an essential role in maintaining the axon integrity. This pathway can be activated in necroptosis and in genetic, toxic or metabolic disorders, physical injury and neuroinflammation, all leading to axon pathology. SARM1 is also known to be involved in regulating innate immunity, potentially linking axon degeneration to the response to pathogens and intercellular signaling. Understanding this NAD-related signaling mechanism enhances our understanding of the process of axon degeneration and enables a path to the development of drugs for a wide range of neurodegenerative diseases.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

Injury-Induced Inhibition of Bystander Neurons Requires dSarm and Signaling from Glia

Nervous system injury and disease have broad effects on the functional connectivity of the nervous system, but how injury signals are spread across neural circuits remains unclear. We explored how axotomy changes the physiology of severed axons and adjacent uninjured "bystander" neurons in a simple in vivo nerve preparation. Within hours after injury, we observed suppression of axon transport in all axons, whether injured or not, and decreased mechano- and chemosensory signal transduction in uninjured bystander neurons. Unexpectedly, we found the axon death molecule dSarm, but not its NAD+ hydrolase activity, was required cell autonomously for these early changes in neuronal cell biology in bystander neurons, as were the voltage-gated calcium channel Cacophony (Cac) and the mitogen-activated protein kinase (MAPK) signaling cascade. Bystander neurons functionally recovered at later time points, while severed axons degenerated via α/Armadillo/Toll-interleukin receptor homology domain (dSarm)/Axundead signaling, and independently of Cac/MAPK. Interestingly, suppression of bystander neuron function required Draper/MEGF10 signaling in glia, indicating glial cells spread injury signals and actively suppress bystander neuron function. Our work identifies a new role for dSarm and glia in suppression of bystander neuron function after injury and defines two genetically and temporally separable phases of dSarm signaling in the injured nervous system.

Regulation of degenerative spheroids after injury

Neuronal injury leads to rapid, programmed disintegration of axons distal to the site of lesion. Much like other forms of axon degeneration (e.g. developmental pruning, toxic insult from neurodegenerative disorder), Wallerian degeneration associated with injury is preceded by spheroid formation along axons. The mechanisms by which injury leads to formation of spheroids and whether these spheroids have a functional role in degeneration remain elusive. Here, using neonatal mouse primary sympathetic neurons, we investigate the roles of players previously implicated in the progression of Wallerian degeneration in injury-induced spheroid formation. We find that intra-axonal calcium flux is accompanied by actin-Rho dependent growth of calcium rich axonal spheroids that eventually rupture, releasing material to the extracellular space prior to catastrophic axon degeneration. Importantly, after injury, Sarm1-/- and DR6-/-, but not Wlds (excess NAD+) neurons, are capable of forming spheroids that eventually rupture, releasing their contents to the extracellular space to promote degeneration. Supplementation of exogenous NAD+ or expressing WLDs suppresses Rho-dependent spheroid formation and degeneration in response to injury. Moreover, injured or trophically deprived Sarm1-/- and DR6-/-, but not Wlds neurons, are resistant to degeneration induced by conditioned media collected from wild-type axons after spheroid rupture. Taken together, these findings place Rho-actin and NAD+ upstream of spheroid formation and may suggest that other mediators of degeneration, such as DR6 and SARM1, mediate post-spheroid rupture events that lead to catastrophic axon disassembly.

A glycolytic shift in Schwann cells supports injured axons

Axon degeneration is a hallmark of many neurodegenerative disorders. The current assumption is that the decision of injured axons to degenerate is cell-autonomously regulated. Here we show that Schwann cells (SCs), the glia of the peripheral nervous system, protect injured axons by virtue of a dramatic glycolytic upregulation that arises in SCs as an inherent adaptation to axon injury. This glycolytic response, paired with enhanced axon-glia metabolic coupling, supports the survival of axons. The glycolytic shift in SCs is largely driven by the metabolic signaling hub, mammalian target of rapamycin complex 1, and the downstream transcription factors hypoxia-inducible factor 1-alpha and c-Myc, which together promote glycolytic gene expression. The manipulation of glial glycolytic activity through this pathway enabled us to accelerate or delay the degeneration of perturbed axons in acute and subacute rodent axon degeneration models. Thus, we demonstrate a non-cell-autonomous metabolic mechanism that controls the fate of injured axons.

Structural basis for RING-Cys-Relay E3 ligase activity and its role in axon integrity

MYCBP2 is a ubiquitin (Ub) E3 ligase (E3) that is essential for neurodevelopment and regulates axon maintenance. MYCBP2 transfers Ub to non-lysine substrates via a newly discovered RING-Cys-Relay (RCR) mechanism where Ub is relayed from an upstream cysteine to a downstream substrate esterification site. The molecular bases for E2-E3 Ub transfer and Ub relay are unknown. Whether these activities are linked to the neural phenotypes is also unclear. We describe the crystal structure of a covalently trapped E2-Ub:MYCBP2 transfer intermediate revealing key structural rearrangements upon E2-E3 Ub transfer and Ub relay. Our data suggest that transfer to the dynamic upstream cysteine, whilst mitigating lysine activity, requires a closed-like E2-Ub conjugate with tempered reactivity, and Ub relay is facilitated by a helix-coil transition. Furthermore, neurodevelopmental defects and delayed injury-induced degeneration in RCR-defective knock-in mice suggest its requirement, and that of substrate esterification activity, for normal neural development and programmed axon degeneration.

Loss of highwire Protects Against the Deleterious Effects of Traumatic Brain Injury in Drosophila Melanogaster

Traumatic brain injury is a major global cause of death and disability. Axonal injury is a major underlying mechanism of TBI and could represent a major therapeutic target. We provide evidence that targeting the axonal death pathway known as Wallerian degeneration improves outcome in a Drosophila Melanogaster model of high impact trauma. This cell-autonomous neurodegenerative pathway is initiated following axon injury, and in Drosophila, involves activity of the E3 ubiquitin ligase highwire. We demonstrate that a loss-of-function mutation in the highwire gene rescues deleterious effects of a traumatic injury, including-improved functional outcomes, lifespan, survival of dopaminergic neurons, and retention of synaptic proteins. This data suggests that highwire represents a potential therapeutic target in traumatic injury.

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.

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.

Mitochondrial impairment activates the Wallerian pathway through depletion of NMNAT2 leading to SARM1-dependent axon degeneration

Wallerian degeneration of physically injured axons involves a well-defined molecular pathway linking loss of axonal survival factor NMNAT2 to activation of pro-degenerative protein SARM1. Manipulating the pathway through these proteins led to the identification of non-axotomy insults causing axon degeneration by a Wallerian-like mechanism, including several involving mitochondrial impairment. Mitochondrial dysfunction is heavily implicated in Parkinson’s disease, Charcot-Marie-Tooth disease, hereditary spastic paraplegia and other axonal disorders. However, whether and how mitochondrial impairment activates Wallerian degeneration has remained unclear. Here, we show that disruption of mitochondrial membrane potential leads to axonal NMNAT2 depletion in mouse sympathetic neurons, increasing the substrate-to-product ratio (NMN/NAD) of this NAD-synthesising enzyme, a metabolic fingerprint of Wallerian degeneration. The mechanism appears to involve both impaired NMNAT2 synthesis and reduced axonal transport. Expression of WLD^S and Sarm1 deletion both protect axons after mitochondrial uncoupling. Blocking the pathway also confers neuroprotection and increases the lifespan of flies with Pink1 loss-of-function mutation, which causes severe mitochondrial defects. These data indicate that mitochondrial impairment replicates all the major steps of Wallerian degeneration, placing it upstream of NMNAT2 loss, with the potential to contribute to axon pathology in mitochondrial disorders.

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.

Autophagy is inhibited by ubiquitin ligase activity in the nervous system

Autophagy is an intracellular catabolic process prominent in starvation, aging and disease. Neuronal autophagy is particularly important, as it affects the development and function of the nervous system, and is heavily implicated in neurodegenerative disease. Nonetheless, how autophagy is regulated in neurons remains poorly understood. Using an unbiased proteomics approach, we demonstrate that the primary initiator of autophagy, the UNC-51/ULK kinase, is negatively regulated by the ubiquitin ligase RPM-1. RPM-1 ubiquitin ligase activity restricts UNC-51 and autophagosome formation within specific axonal compartments, and exerts effects broadly across the nervous system. By restraining UNC-51 activity, RPM-1 inhibits autophagosome formation to affect axon termination, synapse maintenance and behavioral habituation. These results demonstrate how UNC-51 and autophagy are regulated subcellularly in axons, and unveils a mechanism for restricting initiation of autophagy across the nervous system. Our findings have important implications beyond nervous system development, given growing links between altered autophagy regulation and neurodegenerative diseases.

Degeneration of Injured Axons and Dendrites Requires Restraint of a Protective JNK Signaling Pathway by the Transmembrane Protein Raw

The degeneration of injured axons involves a self-destruction pathway whose components and mechanism are not fully understood. Here, we report a new regulator of axonal resilience. The transmembrane protein Raw is cell autonomously required for the degeneration of injured axons, dendrites, and synapses in Drosophila melanogaster In both male and female raw hypomorphic mutant or knock-down larvae, the degeneration of injured axons, dendrites, and synapses from motoneurons and sensory neurons is strongly inhibited. This protection is insensitive to reduction in the levels of the NAD+ synthesis enzyme Nmnat (nicotinamide mononucleotide adenylyl transferase), but requires the c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinase and the transcription factors Fos and Jun (AP-1). Although these factors were previously known to function in axonal injury signaling and regeneration, Raw's function can be genetically separated from other axonal injury responses: Raw does not modulate JNK-dependent axonal injury signaling and regenerative responses, but instead restrains a protective pathway that inhibits the degeneration of axons, dendrites, and synapses. Although protection in raw mutants requires JNK, Fos, and Jun, JNK also promotes axonal degeneration. These findings suggest the existence of multiple independent pathways that share modulation by JNK, Fos, and Jun that influence how axons respond to stress and injury.SIGNIFICANCE STATEMENT Axonal degeneration is a major feature of neuropathies and nerve injuries and occurs via a cell autonomous self-destruction pathway whose mechanism is poorly understood. This study reports the identification of a new regulator of axonal degeneration: the transmembrane protein Raw. Raw regulates a cell autonomous nuclear signaling pathway whose yet unknown downstream effectors protect injured axons, dendrites, and synapses from degenerating. These findings imply that the susceptibility of axons to degeneration is strongly regulated in neurons. Future understanding of the cellular pathway regulated by Raw, which engages the c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinase and Fos and Jun transcription factors, may suggest new strategies to increase the resiliency of axons in debilitating neuropathies.

Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.

Axon death signalling in Wallerian degeneration among species and in disease

Axon loss is a shared feature of nervous systems being challenged in neurological disease, by chemotherapy or mechanical force. Axons take up the vast majority of the neuronal volume, thus numerous axonal intrinsic and glial extrinsic support mechanisms have evolved to promote lifelong axonal survival. Impaired support leads to axon degeneration, yet underlying intrinsic signalling cascades actively promoting the disassembly of axons remain poorly understood in any context, making the development to attenuate axon degeneration challenging. Wallerian degeneration serves as a simple model to study how axons undergo injury-induced axon degeneration (axon death). Severed axons actively execute their own destruction through an evolutionarily conserved axon death signalling cascade. This pathway is also activated in the absence of injury in diseased and challenged nervous systems. Gaining insights into mechanisms underlying axon death signalling could therefore help to define targets to block axon loss. Herein, we summarize features of axon death at the molecular and subcellular level. Recently identified and characterized mediators of axon death signalling are comprehensively discussed in detail, and commonalities and differences across species highlighted. We conclude with a summary of engaged axon death signalling in humans and animal models of neurological conditions. Thus, gaining mechanistic insights into axon death signalling broadens our understanding beyond a simple injury model. It harbours the potential to define targets for therapeutic intervention in a broad range of human axonopathies.

Sarm1 deletion reduces axon damage, demyelination, and white matter atrophy after experimental traumatic brain injury

Traumatic brain injury (TBI) often damages axons in white matter tracts and causes corpus callosum (CC) atrophy in chronic TBI patients. Injured axons encounter irreversible damage if transected, or alternatively may maintain continuity and subsequently either recover or degenerate. Secondary mechanisms can cause further axon damage, myelin pathology, and neuroinflammation. Molecular mechanisms regulating the progression of white matter pathology indicate potential therapeutic targets. SARM1 is essential for execution of the conserved axon death pathway. We examined white matter pathology following mild TBI with CC traumatic axonal injury in mice with Sarm1 gene deletion (Sarm1-/-). High resolution ultrastructural analysis at 3 days post-TBI revealed dramatically reduced axon damage in Sarm1-/- mice, as compared to Sarm1+/+ wild-type controls. Sarm1 deletion produced larger axons with thinner myelin, and attenuated TBI induced demyelination, i.e. myelin loss along apparently intact axons. At 6 weeks post-TBI, Sarm1-/- mice had less demyelination and thinner myelin than Sarm1+/+ mice, but axonal protection was no longer observed. We next used Thy1-YFP crosses to assess Sarm1 involvement in white matter neurodegeneration and neuroinflammation at 8 weeks post-TBI, when significant CC atrophy indicates chronic pathology. Thy1-YFP expression demonstrated continued CC axon damage yet absence of overt cortical pathology. Importantly, significant CC atrophy in Thy1-YFP/Sarm1+/+ mice was associated with reduced neurofilament immunolabeling of axons. Both effects were attenuated in Thy1-YFP/Sarm1-/- mice. Surprisingly, Thy1-YFP/Sarm1-/- mice had increased CC astrogliosis. This study demonstrates that Sarm1 inactivation reduces demyelination, and white matter atrophy after TBI, while the post-injury stage impacts when axon protection is effective.

Axon injury signaling and compartmentalized injury response in glaucoma

Axonal degeneration is an active, highly controlled process that contributes to beneficial processes, such as developmental pruning, but also to neurodegeneration. In glaucoma, ocular hypertension leads to vision loss by killing the output neurons of the retina, the retinal ganglion cells (RGCs). Multiple processes have been proposed to contribute to and/or mediate axonal injury in glaucoma, including: neuroinflammation, loss of neurotrophic factors, dysregulation of the neurovascular unit, and disruption of the axonal cytoskeleton. While the inciting injury to RGCs in glaucoma is complex and potentially heterogeneous, axonal injury is ultimately thought to be the key insult that drives glaucomatous neurodegeneration. Glaucomatous neurodegeneration is a complex process, with multiple molecular signals contributing to RGC somal loss and axonal degeneration. Furthermore, the propagation of the axonal injury signal is complex, with injury triggering programs of degeneration in both the somal and axonal compartment. Further complicating this process is the involvement of multiple cell types that are known to participate in the process of axonal and neuronal degeneration after glaucomatous injury. Here, we review the axonal signaling that occurs after injury and the molecular signaling programs currently known to be important for somal and axonal degeneration after glaucoma-relevant axonal injuries.

The ubiquitin ligase PHR promotes directional regrowth of spinal zebrafish axons

To reconnect with their synaptic targets, severed axons need to regrow robustly and directionally along the pre-lesional trajectory. While mechanisms directing axonal regrowth are poorly understood, several proteins direct developmental axon outgrowth, including the ubiquitin ligase PHR (Mycbp2). Invertebrate PHR also limits regrowth of injured axons, whereas its role in vertebrate axonal regrowth remains elusive. Here we took advantage of the high regrowth capacity of spinal zebrafish axons and observed robust and directional regrowth following laser transection of spinal Mauthner axons. We found that PHR directs regrowing axons along the pre-lesional trajectory and across the transection site. At the transection site, initial regrowth of wild-type axons was multidirectional. Over time, misdirected sprouts were corrected in a PHR-dependent manner. Ablation of cyfip2, known to promote F-actin-polymerization and pharmacological inhibition of JNK reduced misdirected regrowth of PHR-deficient axons, suggesting that PHR controls directional Mauthner axonal regrowth through cyfip2- and JNK-dependent pathways.

Mechanisms of injury-induced axon degeneration

Injury-induced axon degeneration in model organisms and cell culture has emerged as an area of growing interest due to its experimental tractability and to the promise of identifying conserved mechanisms that mediate axon loss in human disease. Injury-induced axon degeneration is also observed within the well-studied process of Wallerian degeneration, a complex phenomenon triggered by axon injury to peripheral nerves in mammals. Recent studies have led to the identification of key molecular components of injury-induced axon degeneration. Axon survival factors, such as NMNAT2, act to protect injured axons from degeneration. By contrast, factors such as SARM1, MAPK, and PHR1 act to promote degeneration. The coordinated activity of these factors determines axon fate after injury. Since axon loss is an early feature of neurodegenerative diseases, it is possible that understanding the molecular mechanism of injury-induced degeneration will lead to new treatments for axon loss in neurodegenerative disease. Here, we discuss the critical pathways for injury-induced axon degeneration across species with an emphasis on their interactions in an integrated signaling network.

Die in pieces: How Drosophila sheds light on neurite degeneration and clearance

Dendrites and axons are delicate neuronal membrane extensions that undergo degeneration after physical injuries. In neurodegenerative diseases, they often degenerate prior to neuronal death. Understanding the mechanisms of neurite degeneration has been an intense focus of neurobiology research in the last two decades. As a result, many discoveries have been made in the molecular pathways that lead to neurite degeneration and the cell-cell interactions responsible for the subsequent clearance of neuronal debris. Drosophila melanogaster has served as a prime in vivo model system for identifying and characterizing the key molecular players in neurite degeneration, thanks to its genetic tractability and easy access to its nervous system. The knowledge learned in the fly provided targets and fuel for studies in other model systems that have further enhanced our understanding of neurodegeneration. In this review, we will introduce the experimental systems developed in Drosophila to investigate injury-induced neurite degeneration, and then discuss the biological pathways that drive degeneration. We will also cover what is known about the mechanisms of how phagocytes recognize and clear degenerating neurites, and how recent findings in this area enhance our understanding of neurodegenerative disease pathology.