The Wld(S) mutation delays anterograde, but not retrograde, axonal degeneration of the dopaminergic nigro-striatal pathway in vivo

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.

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.

Mitochondrial dysfunction as a trigger of programmed axon death

Mitochondrial failure has long been associated with programmed axon death (Wallerian degeneration, WD), a widespread and potentially preventable mechanism of axon degeneration. While early findings in axotomised axons indicated that mitochondria are involved during the execution steps of this pathway, recent studies suggest that in addition, mitochondrial dysfunction can initiate programmed axon death without physical injury. As mitochondrial dysfunction is associated with disorders involving early axon loss, including Parkinson's disease, peripheral neuropathies, and multiple sclerosis, the findings that programmed axon death is activated by mitochondrial impairment could indicate the involvement of druggable mechanisms whose disruption may protect axons in such diseases. Here, we review the latest developments linking mitochondrial dysfunction to programmed axon death and discuss their implications for injury and disease.

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.

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.

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.

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.

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.

Retrograde Axonal Degeneration in Parkinson Disease

In spite of tremendous research efforts we have not yet achieved two of our principal therapeutic goals in the treatment of Parkinson’s disease (PD), to prevent its onward progression and to provide restoration of systems that have already been damaged by the time of diagnosis. There are many possible reasons for our inability to make progress. One possibility is that our efforts thus far may not have been directed towards the appropriate cellular compartments. Up until now research has been largely focused on the loss of neurons in the disease. Thus, neuroprotection approaches have been largely aimed at blocking mechanisms that lead to destruction of the neuronal cell body. Attempts to provide neurorestoration have been almost entirely focused on replacement of neurons. We herein review the evidence that the axonal component of diseased neuronal systems merit more of our attention. Evidence from imaging studies, from postmortem neurochemical studies, and from genetic animal models suggests that the axons of the dopaminergic system are involved predominantly and early in PD. Since the mechanisms of axonal destruction are distinct from those of neuron cell body degeneration, a focus on axonal neurobiology will offer new opportunities for preventing their degeneration. At present these mechanisms remain largely obscure. However, defining them is likely to offer new opportunities for neuroprotection. In relation to neurorestoration, while it has been classically believed that neurons of the adult central nervous system are incapable of new axon growth, recent evidence shows that this is not true for the dopaminergic projection. In conclusion, the neurobiology of axons is likely to offer many new approaches to protective and restorative therapeutics.

Pioglitazone Attenuates Neuroinflammation and Promotes Dopaminergic Neuronal Survival in the Nigrostriatal System of Rats after Diffuse Brain Injury

Increasing evidence suggests that traumatic brain injury (TBI) may raise the risk of developing late-onset Parkinson's disease (PD). Recently, the peroxisome proliferation-activated receptor gamma (PPARγ) agonist pioglitazone has been demonstrated to be neuroprotective in animal models of neurodegeneration. The present study investigates the vulnerability of the nigrostriatal system after TBI, and intervention with pioglitazone treatment. Adult male Sprague-Dawley rats were subjected to sham or moderate midline fluid percussion brain injury (mFPI), followed by an intraperitoneal injection of 10 mg/kg pioglitazone or vehicle beginning 30 min after the injury and subsequently every 24 h for 5 days. Following injury, pro-inflammatory cytokines and chemokine were acutely increased in the striatum and substantia nigra within 6 h. Dopaminergic axonal damage and microglial activation were revealed using immunohistochemistry in the medial forebrain bundle at 1 day post-injury. Microglial activation identified by Iba1 and OX-6 immunostaining was persistently increased in the substantia nigra pars compacta 7 to 28 days post-injury. Further, brain injury induced significant dopaminergic neuronal loss, which was quantified by tyrosine hydroxylase immunostaining and retrograde fluorescent tracer fluorogold labeling in the nigra at 28 days. Loss of neurons was accompanied by increased extracellular dopamine (DA) turnover in the striatum, indicating enhanced dopaminergic activity in functional compensation after nigrostriatal damage. Strikingly, pioglitazone treatment greatly attenuated microglial activation and improved dopaminergic neuronal survival in the nigrostriatal system, which may promote locomotor recovery. These results suggest that interventions that attenuate secondary inflammation could be a feasible therapeutic treatment to improve outcome after TBI.

The Parkinsonian mimetic, 6-OHDA, impairs axonal transport in dopaminergic axons

6-hydroxydopamine (6-OHDA) is one of the most commonly used toxins for modeling degeneration of dopaminergic (DA) neurons in Parkinson's disease. 6-OHDA also causes axonal degeneration, a process that appears to precede the death of DA neurons. To understand the processes involved in 6-OHDA-mediated axonal degeneration, a microdevice designed to isolate axons fluidically from cell bodies was used in conjunction with green fluorescent protein (GFP)-labeled DA neurons. Results showed that 6-OHDA quickly induced mitochondrial transport dysfunction in both DA and non-DA axons. This appeared to be a general effect on transport function since 6-OHDA also disrupted transport of synaptophysin-tagged vesicles. The effects of 6-OHDA on mitochondrial transport were blocked by the addition of the SOD1-mimetic, Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP), as well as the anti-oxidant N-acetyl-cysteine (NAC) suggesting that free radical species played a role in this process. Temporally, microtubule disruption and autophagy occurred after transport dysfunction yet before DA cell death following 6-OHDA treatment. The results from the study suggest that ROS-mediated transport dysfunction occurs early and plays a significant role in inducing axonal degeneration in response to 6-OHDA treatment.

Axon degeneration and PGC-1α-mediated protection in a zebrafish model of α-synuclein toxicity

α-synuclein (aSyn) expression is implicated in neurodegenerative processes, including Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). In animal models of these diseases, axon pathology often precedes cell death, raising the question of whether aSyn has compartment-specific toxic effects that could require early and/or independent therapeutic intervention. The relevance of axonal pathology to degeneration can only be addressed through longitudinal, in vivo monitoring of different neuronal compartments. With current imaging methods, dopaminergic neurons do not readily lend themselves to such a task in any vertebrate system. We therefore expressed human wild-type aSyn in zebrafish peripheral sensory neurons, which project elaborate superficial axons that can be continuously imaged in vivo. Axonal outgrowth was normal in these neurons but, by 2 days post-fertilization (dpf), many aSyn-expressing axons became dystrophic, with focal varicosities or diffuse beading. Approximately 20% of aSyn-expressing cells died by 3 dpf. Time-lapse imaging revealed that focal axonal swelling, but not overt fragmentation, usually preceded cell death. Co-expressing aSyn with a mitochondrial reporter revealed deficits in mitochondrial transport and morphology even when axons appeared overtly normal. The axon-protective protein Wallerian degeneration slow (WldS) delayed axon degeneration but not cell death caused by aSyn. By contrast, the transcriptional coactivator PGC-1α, which has roles in the regulation of mitochondrial biogenesis and reactive-oxygen-species detoxification, abrogated aSyn toxicity in both the axon and the cell body. The rapid onset of axonal pathology in this system, and the relatively moderate degree of cell death, provide a new model for the study of aSyn toxicity and protection. Moreover, the accessibility of peripheral sensory axons will allow effects of aSyn to be studied in different neuronal compartments and might have utility in screening for novel disease-modifying compounds.

Retrograde and Wallerian axonal degeneration occur synchronously after retinal ganglion cell axotomy

Axonal injury and degeneration are pivotal pathological events in diseases of the nervous system. In the past decade, it has been recognized that the process of axonal degeneration is distinct from somal degeneration and that axoprotective strategies may be distinct from those that protect the soma. Preserving the cell body via neuroprotection cannot improve function if the axon is damaged, because the soma is still disconnected from its target. Therefore, understanding the mechanisms of axonal degeneration is critical for developing new therapeutic interventions for axonal disease treatment. We combined in vivo imaging with a multilaser confocal scanning laser ophthalmoscope and in vivo axotomy with a diode-pumped solid-state laser to assess the time course of Wallerian and retrograde degeneration of unmyelinated retinal ganglion cell axons in living rats for 4 weeks after intraretinal axotomy. Laser injury resulted in reproducible axon loss both distal and proximal to the site of injury. Longitudinal polarization-sensitive imaging of axons demonstrated that Wallerian and retrograde degeneration occurred synchronously. Neurofilament immunostaining of retinal whole-mounts confirmed axonal loss and demonstrated sparing of adjacent axons to the axotomy site. In vivo fluorescent imaging of axonal transport and photobleaching of labeled axons demonstrated that the laser axotomy model did not affect adjacent axon function. These results are consistent with a shared mechanism for Wallerian and retrograde degeneration.

Axonal degeneration as a therapeutic target in the CNS

Degeneration of the axon is an important step in the pathomechanism of traumatic, inflammatory and degenerative neurological diseases. Increasing evidence suggests that axonal degeneration occurs early in the course of these diseases and therefore represents a promising target for future therapeutic strategies. We review the evidence for axonal destruction from pathological findings and animal models with particular emphasis on neurodegenerative and neurotraumatic disorders. We discuss the basic morphological and temporal modalities of axonal degeneration (acute, chronic and focal axonal degeneration and Wallerian degeneration). Based on the mechanistic concepts, we then delineate in detail the major molecular mechanisms that underlie the degenerative cascade, such as calcium influx, axonal transport, protein aggregation and autophagy. We finally concentrate on putative therapeutic targets based on the mechanistic prerequisites.

Axon degeneration in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative disease of the basal ganglia. Like other adult-onset neurodegenerative disorders, it is without a treatment that forestalls its chronic progression. Efforts to develop disease-modifying therapies to date have largely focused on the prevention of degeneration of the neuron soma, with the tacit assumption that such approaches will forestall axon degeneration as well. We herein propose that future efforts to develop neuroprotection for PD may benefit from a shift in focus to the distinct mechanisms that underlie axon degeneration. We review evidence from human post-mortem studies, functional neuroimaging, genetic causes of the disease and neurotoxin models that axon degeneration may be the earliest feature of the disease, and it may therefore be the most appropriate target for early intervention. In addition, we present evidence that the molecular mechanisms of degeneration of axons are separate and distinct from those of neuron soma. Progress is being made in understanding these mechanisms, and they provide possible new targets for therapeutic intervention. We also suggest that the potential for axon re-growth in the adult central nervous system has perhaps been underestimated, and it offers new avenues for neurorestoration. In conclusion, we propose that a new focus on the neurobiology of axons, their molecular pathways of degeneration and growth, will offer novel opportunities for neuroprotection and restoration in the treatment of PD and other neurodegenerative diseases.

Optic nerve disease and axon pathophysiology

Optic neuropathy is the most common cause of irreversible blindness worldwide. Although the most common optic neuropathy is glaucoma, there are also many other optic neuropathies, for example, those associated with multiple sclerosis, giant cell arteritis, ischemia, and many other diseases. In almost all cases, the pathogenesis involves injury to the retinal ganglion cell axon, with consequent somal and axonal degeneration. This chapter reviews the clinical and pathophysiological properties associated with three of the most common optic neuropathies, as well as recent findings in understanding axonal degeneration. It concludes with a status report on therapies for optic nerve disease, including axoprotection, an approach being studied that has the goal of maintaining axonal integrity and function after injury.

Akt suppresses retrograde degeneration of dopaminergic axons by inhibition of macroautophagy

Axon degeneration is a hallmark of neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Such degeneration is not a passive event but rather an active process mediated by mechanisms that are distinct from the canonical pathways of programmed cell death that mediate destruction of the cell soma. Little is known of the diverse mechanisms involved, particularly those of retrograde axon degeneration. We have previously observed in living animal models of degeneration in the nigrostriatal projection that a constitutively active form of the kinase, myristoylated Akt (Myr-Akt), demonstrates an ability to suppress programmed cell death and preserve the soma of dopamine neurons. Here, we show in both neurotoxin and physical injury (axotomy) models that Myr-Akt is also able to preserve dopaminergic axons due to suppression of acute retrograde axon degeneration. This cellular phenotype is associated with increased mammalian target of rapamycin (mTor) activity and can be recapitulated by a constitutively active form of the small GTPase Rheb, an upstream activator of mTor. Axon degeneration in these models is accompanied by the occurrence of macroautophagy, which is suppressed by Myr-Akt. Conditional deletion of the essential autophagy mediator Atg7 in adult mice also achieves striking axon protection in these acute models of retrograde degeneration. The protection afforded by both Myr-Akt and Atg7 deletion is robust and lasting, because it is still observed as protection of both axons and dopaminergic striatal innervation weeks after injury. We conclude that acute retrograde axon degeneration is regulated by Akt/Rheb/mTor signaling pathways.

Dopaminergic pathway reconstruction by Akt/Rheb-induced axon regeneration

OBJECTIVE

A prevailing concept in neuroscience has been that the adult mammalian central nervous system is incapable of restorative axon regeneration. Recent evidence, however, has suggested that reactivation of intrinsic cellular programs regulated by protein kinase B (Akt)/mammalian target of rapamycin (mTor) signaling may restore this ability.

METHODS

To assess this possibility in the brain, we have examined the ability of adenoassociated virus (AAV)-mediated transduction of dopaminergic neurons of the substantia nigra (SN) with constitutively active forms of the kinase Akt and the GTPase Ras homolog enriched in brain (Rheb) to induce regrowth of axons after they have been destroyed by neurotoxin lesion.

RESULTS

Both constitutively active myristoylated Akt and hRheb(S16H) induce regrowth of axons from dopaminergic neurons to their target, the striatum. Histological analysis demonstrates that these new axons achieve morphologically accurate reinnervation. In addition, functional reintegration into target circuitry is achieved, as indicated by partial behavioral recovery.

INTERPRETATION

We conclude that regrowth of axons within the adult nigrostriatal projection, a system that is prominently affected in Parkinson's disease, can be achieved by activation of Akt/mTor signaling in surviving endogenous mesencephalic dopaminergic neurons by viral vector transduction.

Targeting NMNAT1 to axons and synapses transforms its neuroprotective potency in vivo

Axon and synapse degeneration are common components of many neurodegenerative diseases, and their rescue is essential for effective neuroprotection. The chimeric Wallerian degeneration slow protein (Wld(S)) protects axons dose dependently, but its mechanism is still elusive. We recently showed that Wld(S) acts at a non-nuclear location and is present in axons. This and other recent reports support a model in which Wld(S) protects by extranuclear redistribution of its nuclear NMNAT1 portion. However, it remains unclear whether cytoplasmic NMNAT1 acts locally in axons and synapses or at a non-nuclear site within cell bodies. The potency of axon protection by non-nuclear NMNAT1 relative to Wld(S) also needs to be established in vivo. Because the N-terminal portion of Wld(S) (N70) localized to axons, we hypothesized that it mediates the trafficking of the NMNAT1 portion. To test this, we substituted N70 with an axonal targeting peptide derived from amyloid precursor protein, and fused this to NMNAT1 with disrupted nuclear targeting. In transgenic mice, this transformed NMNAT1 from a molecule unable to inhibit Wallerian degeneration, even at high expression levels, into a protein more potent than Wld(S), able to preserve injured axons for several weeks at undetectable expression levels. Preventing NMNAT1 axonal delivery abolished its protective effect. Axonally targeted NMNAT1 localized to vesicular structures, colocalizing with extranuclear Wld(S), and was cotransported at least partially with mitochondria. We conclude that axonal targeting of NMNAT activity is both necessary and sufficient to delay Wallerian degeneration, and that promoting axonal and synaptic delivery greatly enhances the effectiveness.

Intracellular signalling pathways in dopamine cell death and axonal degeneration

The pathways of programmed cell death (PCD) are now understood in extraordinary detail at the molecular level. Although much evidence suggests that they are likely to play a role in Parkinson's disease (PD), the precise nature of that role remains unknown. Two pathways of cell death that are especially well characterized are cyclin-dependent kinase 5-mediated phosphorylation of myocyte enhancer factor 2 and the mitogen-activated protein kinase signalling cascade. Although blockade of these pathways in animals has achieved a truly remarkable degree of neuroprotection of the neuron cell soma, it has not achieved protection of axons. Thus, there is a need to explore beyond the canonical pathways of PCD and investigate mechanisms of axon destruction. We also need to move beyond the narrow classic concept that the mechanisms of PCD are activated exclusively 'downstream', following cellular injury. Studies in the genetics of PD suggest that in some forms of the disease, activation may be an early 'upstream' event. Additionally, recent observations suggest that cell death in some contexts may not be initiated by injury, but instead by a failure of intrinsic cell survival signalling. These new points of view offer new opportunities for molecular targeting.