Targeting cholesterol trafficking to mitigate axonal degeneration in hereditary spastic paraplegia

Axonal degeneration underlies many debilitating diseases including hereditary spastic paraplegia(HSP),a genetically and clinically diverse group of disorders characterized by spasticity and weakness of the lower extremities.HSP is one significant cause of chronic neurodisability due to the lack of effective treatments and a wide range of onset ages from early childhood to 70 years.

A lead role for a“secondary”axonal injury response

Stress signaling following axon injury stimulates a transcriptional program for regeneration that might be exploited to promote central nervous system repair.However,this stress response drives neuronal apoptosis in non-regenerative environments.This duality presents a quandary for the development of therapeutic interventions:manipulating stress signaling to enhance recovery of damaged neurons risks accelerating neurodegeneration or restricting regenerative potential.This dichotomy is well illustrated by the fates of retinal ganglion cells(RGCs)following optic nerve crush.In this central nervous system injury model,disruption of a stress-activated MAP kinase(MAPK)cascade blocks the extensive apoptosis of RGCs that occurs in wild-type mice(Watkins et al.,2013;Welsbie et al.,2017).

Mechanisms behind elevated serum levels of plasminogen activator inhibitor-1 in frontotemporal lobar degeneration

Frontotemporal lobar degeneration(FTLD)is a form of progressive dementia characterized by degeneration of the frontal and temporal lobes of the brain.This pathology involves a series of cognitive,behavioral,and neurological symptoms that influence personality,decision-making ability,and language.

Polyethylene glycol has immunoprotective effects on sciatic allografts, but behavioral recovery and graft tolerance require neurorrhaphy and axonal fusion

Behavioral recovery using(viable)peripheral nerve allografts to repair ablation-type(segmental-loss)peripheral nerve injuries is delayed or poor due to slow and inaccurate axonal regeneration.Furthermore,such peripheral nerve allografts undergo immunological rejection by the host immune system.In contrast,peripheral nerve injuries repaired by polyethylene glycol fusion of peripheral nerve allografts exhibit excellent behavioral recovery within weeks,reduced immune responses,and many axons do not undergo Wallerian degeneration.The relative contribution of neurorrhaphy and polyethylene glycol-fusion of axons versus the effects of polyethylene glycol per se was unknown prior to this study.We hypothesized that polyethylene glycol might have some immune-protective effects,but polyethylene glycol-fusion was necessary to prevent Wallerian degeneration and functional/behavioral recovery.We examined how polyethylene glycol solutions per se affect functional and behavioral recovery and peripheral nerve allograft morphological and immunological responses in the absence of polyethylene glycol-induced axonal fusion.Ablation-type sciatic nerve injuries in outbred Sprague–Dawley rats were repaired according to a modified protocol using the same solutions as polyethylene glycol-fused peripheral nerve allografts,but peripheral nerve allografts were loose-sutured(loose-sutured polyethylene glycol)with an intentional gap of 1–2 mm to prevent fusion by polyethylene glycol of peripheral nerve allograft axons with host axons.Similar to negative control peripheral nerve allografts not treated by polyethylene glycol and in contrast to polyethylene glycol-fused peripheral nerve allografts,animals with loose-sutured polyethylene glycol peripheral nerve allografts exhibited Wallerian degeneration for all axons and myelin degeneration by 7 days postoperatively and did not recover sciatic-mediated behavioral functions by 42 days postoperatively.Other morphological signs of rejection,such as collapsed Schwann cell basal lamina tubes,were absent in polyethylene glycol-fused peripheral nerve allografts but commonly observed in negative control and loose-sutured polyethylene glycol peripheral nerve allografts at 21 days postoperatively.Loose-sutured polyethylene glycol peripheral nerve allografts had more pro-inflammatory and less anti-inflammatory macrophages than negative control peripheral nerve allografts.While T cell counts were similarly high in loose-sutured-polyethylene glycol and negative control peripheral nerve allografts,loose-sutured polyethylene glycol peripheral nerve allografts expressed some cytokines/chemokines important for T cell activation at much lower levels at 14 days postoperatively.MHCI expression was elevated in loose-sutured polyethylene glycol peripheral nerve allografts,but MHCII expression was modestly lower compared to negative control at 21 days postoperatively.We conclude that,while polyethylene glycol per se reduces some immune responses of peripheral nerve allografts,successful polyethylene glycol-fusion repair of some axons is necessary to prevent Wallerian degeneration of those axons and immune rejection of peripheral nerve allografts,and produce recovery of sensory/motor functions and voluntary behaviors.Translation of polyethylene glycol-fusion technologies would produce a paradigm shift from the current clinical practice of waiting days to months to repair ablation peripheral nerve injuries.

Inhibition of the NLRP3 inflammasome attenuates spiral ganglion neuron degeneration in aminoglycoside-induced hearing loss

Aminoglycosides are a widely used class of antibacterials renowned for their effectiveness and broad antimicrobial spectrum.However,their use leads to irreversible hearing damage by causing apoptosis of hair cells as their direct target.In addition,the hearing damage caused by aminoglycosides involves damage of spiral ganglion neurons upon exposure.To investigate the mechanisms underlying spiral ganglion neuron degeneration induced by aminoglycosides,we used a C57BL/6J mouse model treated with kanamycin.We found that the mice exhibited auditory deficits following the acute loss of outer hair cells.Spiral ganglion neurons displayed hallmarks of pyroptosis and exhibited progressive degeneration over time.Transcriptomic profiling of these neurons showed significant upregulation of genes associated with inflammation and immune response,particularly those related to the NLRP3 inflammasome.Activation of the canonical pyroptotic pathway in spiral ganglion neurons was observed,accompanied by infiltration of macrophages and the release of proinflammatory cytokines.Pharmacological intervention targeting NLRP3 using Mcc950 and genetic intervention using NLRP3 knockout ameliorated spiral ganglion neuron degeneration in the injury model.These findings suggest that NLRP3 inflammasome-mediated pyroptosis plays a role in aminoglycoside-induced spiral ganglion neuron degeneration.Inhibition of this pathway may offer a potential therapeutic strategy for treating sensorineural hearing loss by reducing spiral ganglion neuron degeneration.

The gut-eye axis:from brain neurodegenerative diseases to age-related macular degeneration

Age-related macular degeneration is a serious neurodegenerative disease of the retina that significantly impacts vision.Unfortunately,the specific pathogenesis remains unclear,and effective early treatment options are consequently lacking.The microbiome is defined as a large ecosystem of microorganisms living within and coexisting with a host.The intestinal microbiome undergoes dynamic changes owing to age,diet,genetics,and other factors.Such dysregulation of the intestinal flora can disrupt the microecological balance,resulting in immunological and metabolic dysfunction in the host,and affecting the development of many diseases.In recent decades,significant evidence has indicated that the intestinal flora also influences systems outside of the digestive tract,including the brain.Indeed,several studies have demonstrated the critical role of the gut-brain axis in the development of brain neurodegenerative diseases,including Alzheimer’s disease and Parkinson’s disease.Similarly,the role of the“gut-eye axis”has been confirmed to play a role in the pathogenesis of many ocular disorders.Moreover,age-related macular degeneration and many brain neurodegenerative diseases have been shown to share several risk factors and to exhibit comparable etiologies.As such,the intestinal flora may play an important role in age-related macular degeneration.Given the above context,the present review aims to clarify the gut-brain and gut-eye connections,assess the effect of intestinal flora and metabolites on age-related macular degeneration,and identify potential diagnostic markers and therapeutic strategies.Currently,direct research on the role of intestinal flora in age-related macular degeneration is still relatively limited,while studies focusing solely on intestinal flora are insufficient to fully elucidate its functional role in age-related macular degeneration.Organ-on-a-chip technology has shown promise in clarifying the gut-eye interactions,while integrating analysis of the intestinal flora with research on metabolites through metabolomics and other techniques is crucial for understanding their potential mechanisms.

Brain-derived neurotrophic factor signaling in the neuromuscular junction during developmental axonal competition and synapse elimination

During the development of the nervous system,there is an overproduction of neurons and synapses.Hebbian competition between neighboring nerve endings and synapses performing different activity levels leads to their elimination or strengthening.We have extensively studied the involvement of the brain-derived neurotrophic factor-Tropomyosin-related kinase B receptor neurotrophic retrograde pathway,at the neuromuscular junction,in the axonal development and synapse elimination process versus the synapse consolidation.The purpose of this review is to describe the neurotrophic influence on developmental synapse elimination,in relation to other molecular pathways that we and others have found to regulate this process.In particular,we summarize our published results based on transmitter release analysis and axonal counts to show the different involvement of the presynaptic acetylcholine muscarinic autoreceptors,coupled to downstream serine-threonine protein kinases A and C(PKA and PKC)and voltage-gated calcium channels,at different nerve endings in developmental competition.The dynamic changes that occur simultaneously in several nerve terminals and synapses converge across a postsynaptic site,influence each other,and require careful studies to individualize the mechanisms of specific endings.We describe an activity-dependent balance(related to the extent of transmitter release)between the presynaptic muscarinic subtypes and the neurotrophin-mediated TrkB/p75NTR pathways that can influence the timing and fate of the competitive interactions between the different axon terminals.The downstream displacement of the PKA/PKC activity ratio to lower values,both in competing nerve terminals and at postsynaptic sites,plays a relevant role in controlling the elimination of supernumerary synapses.Finally,calcium entry through L-and P/Q-subtypes of voltage-gated calcium channels(both channels are present,together with the N-type channel in developing nerve terminals)contributes to reduce transmitter release and promote withdrawal of the most unfavorable nerve terminals during elimination(the weakest in acetylcholine release and those that have already become silent).The main findings contribute to a better understanding of punishment-rewarding interactions between nerve endings during development.Identifying the molecular targets and signaling pathways that allow synapse consolidation or withdrawal of synapses in different situations is important for potential therapies in neurodegenerative diseases.

Subretinal fibrosis secondary to neovascular age-related macular degeneration:mechanisms and potential therapeutic targets

Subretinal fibrosis is the end-stage sequelae of neovascular age-related macular degeneration.It causes local damage to photoreceptors,retinal pigment epithelium,and choroidal vessels,which leads to permanent central vision loss of patients with neovascular age-related macular degeneration.The pathogenesis of subretinal fibrosis is complex,and the underlying mechanisms are largely unknown.Therefore,there are no effective treatment options.A thorough understanding of the pathogenesis of subretinal fibrosis and its related mechanisms is important to elucidate its complications and explore potential treatments.The current article reviews several aspects of subretinal fibrosis,including the current understanding on the relationship between neovascular age-related macular degeneration and subretinal fibrosis;multimodal imaging techniques for subretinal fibrosis;animal models for studying subretinal fibrosis;cellular and non-cellular constituents of subretinal fibrosis;pathophysiological mechanisms involved in subretinal fibrosis,such as aging,infiltration of macrophages,different sources of mesenchymal transition to myofibroblast,and activation of complement system and immune cells;and several key molecules and signaling pathways participating in the pathogenesis of subretinal fibrosis,such as vascular endothelial growth factor,connective tissue growth factor,fibroblast growth factor 2,platelet-derived growth factor and platelet-derived growth factor receptor-β,transforming growth factor-βsignaling pathway,Wnt signaling pathway,and the axis of heat shock protein 70-Toll-like receptors 2/4-interleukin-10.This review will improve the understanding of the pathogenesis of subretinal fibrosis,allow the discovery of molecular targets,and explore potential treatments for the management of subretinal fibrosis.

Müller cells are activated in response to retinal outer nuclear layer degeneration in rats subjected to simulated weightlessness conditions

A microgravity environment has been shown to cause ocular damage and affect visual acuity,but the underlying mechanisms remain unclear.Therefore,we established an animal model of weightlessness via tail suspension to examine the pathological changes and molecular mechanisms of retinal damage under microgravity.After 4 weeks of tail suspension,there were no notable alterations in retinal function and morphology,while after 8 weeks of tail suspension,significant reductions in retinal function were observed,and the outer nuclear layer was thinner,with abundant apoptotic cells.To investigate the mechanism underlying the degenerative changes that occurred in the outer nuclear layer of the retina,proteomics was used to analyze differentially expressed proteins in rat retinas after 8 weeks of tail suspension.The results showed that the expression levels of fibroblast growth factor 2(also known as basic fibroblast growth factor)and glial fibrillary acidic protein,which are closely related to Müller cell activation,were significantly upregulated.In addition,Müller cell regeneration and Müller cell gliosis were observed after 4 and 8 weeks,respectively,of simulated weightlessness.These findings indicate that Müller cells play an important regulatory role in retinal outer nuclear layer degeneration during weightlessness.

Chemotactic signaling and beyond: link between interleukin-16 and axonal degeneration in multiple sclerosis

Multiple sclerosis（MS）is a progressive inflammatory,and chronic demyelinating,neurodegenerative disease of central nervous system（CNS）.Autoimmune responses to myelin and other CNS antigens mediated by cluster of differentiation 4（CD4＋）T cells are critical for initiation and progression of disease.Migration of autoimmune T cells from the peripheral lymph organs into CNS parenchyma leads to inflammation,demyelination and damage of axonal cytoskeleton, which manifest in decreased impulse conduction velocity of motor and sensory nerves. Myelin and axonal pathology causes motor, sensory and autonomic nerve dysfunction, including optic nerve damage leading to double or distorted vision; paresis and paralysis of extremities, painful sensations, and bladder sphincter dysfunction, manifested as bladder incontinence. Gray matter pa- thology in cortical and subcortical regions, including cerebellum and hippocampus underlies cognitive and behavioral dysfunction consisting of memory deficits, depression, and ataxic gait and tremor.

Collapsin response mediator protein-2 plays a major protective role in acute axonal degeneration

Axonal degeneration is a key pathological feature in many neurological diseases. It often leads to persistent deficits due to the inability of axons to regenerate in the central nervous system. Therefore therapeutic approaches should optimally both attenuate axonal degeneration and foster axonal regeneration. Compelling evidence suggests that collapsin response mediator protein-2（CRMP2） might be a molecular target fulfilling these requirements. In this mini-review, we give a compact overview of the known functions of CRMP2 and its molecular interactors in neurite outgrowth and in neurodegenerative conditions. Moreover, we discuss in detail our recent findings on the role of CRMP2 in acute axonal degeneration in the optic nerve. We found that the calcium influx induced by the lesion activates the protease calpain which cleaves CRMP2, leading to impairment of axonal transport. Both calpain inhibition and CRMP2 overexpression effectively protected the proximal axons against acute axonal degeneration. Taken together, CRMP2 is further characterized as a central molecular player in acute axonal degeneration and thus evolves as a promising therapeutic target to both counteract axonal degeneration and foster axonal regeneration in neurodegenerative and neurotraumatic diseases.

Diffusion tensor imaging as a tool to detect presymptomatic axonal degeneration in a preclinical spinal cord model of amyotrophic lateral sclerosis

The G93A-SOD1 mice model and MRI diffusion as a preclinical tool to study amyotrophic lateral sclerosis （ALS）： ALS is a progressive neurological disease characterized primarily by the development of limb paralysis, which eventually leads to lack of control on muscles under voluntary control and death within 3–5 years. Genetic heterogeneity and environmental factors play a critical role in the rate of disease progression and patients display faster declines once the symptoms have manifested. Since its original discovery, ALS has been associated with pathological alterations in motor neurons located in the spinal cord （SC）, where neuronal loss by a mutation in the protein superoxide dismutase in parenthesis （mSOD1） and impairment in axonal connectivity, have been linked to early functional impairments. In addition,mechanisms of neuroinflammation, apoptosis, necroptosis and autophagy have been also implicated in the development of this disease. Among different animal models developed to study ALS, the transgenic G93A-SOD1 mouse has become recognized as a benchmark model for preclinical screening of ALS therapies. Furthermore, the progressive alterations in the locomotor phenotype expressed in this model closely resemble the progressive lower limb dysfunction of ALS patients. Among other imaging tools, MR diffusion tensor imaging （DTI） has emerged as a crucial, noninvasive and real time neuroimaging tool to gather information in ALS. One of the current concerns with the use of DTI is the lack of biological validation of the microstructural information given by this technique. Although clinical studies using DTI can provide a remarkable insight on the targets of neurodegeneration and disease course,they lack histological correlations. To address these shortcomings, preclinical models can be designed to validate the microstructural information unveiled by this particular MRI technique. Thus, the scope of this review is to describe how MRI diffusion and optical microscopy evaluate axonal structural changes at early stages of the disease in a preclinical model of ALS.

Early cyclosporin A treatment retards axonal degeneration in an experimental peripheral nerve injection injury model

Injury to peripheral nerves during injections of therapeutic agents such as penicillin G potas-sium is common in developing countries. It has been shown that cyclosporin A, a powerful immunosuppressive agent, can retard Wallerian degeneration after peripheral nerve crush injury. However, few studies are reported on the effects of cyclosporin A on peripheral nerve drug in-jection injury. This study aimed to assess the time-dependent efifcacy of cyclosporine-A as an immunosuppressant therapy in an experimental rat nerve injection injury model established by penicillin G potassium injection. The rats were randomly divided into three groups based on the length of time after nerve injury induced by cyclosporine-A administration （30 minutes, 8 or 24 hours）. The compound muscle action potentials were recorded pre-injury, early post-injury （within 1 hour） and 4 weeks after injury and compared statistically. Tissue samples were taken from each animal for histological analysis. Compared to the control group, a significant im-provement of the compound muscle action potential amplitude value was observed only when cyclosporine-A was administered within 30 minutes of the injection injury （P 〈 0.05）; at 8 or 24 hours after cyclosporine-A administration, compound muscle action potential amplitude was not changed compared with the control group. Thus, early immunosuppressant drug therapy may be a good alternative neuroprotective therapy option in experimental nerve injection injury induced by penicillin G potassium injection.

Compartmentalized necroptosis activation in excitotoxicityinduced axonal degeneration: a novel mechanism implicated in neurodegenerative disease pathology

Excitotoxicity and neuronal cell death: Glutamate is the main excitatory neurotransmitter of the central nervous system and functionally involved in most brain activities, including brain development, synaptic plasticity, learning and memory. Exc让atory synaptic transmission is primarily mediated by ligana-gated ion channels, including a-amino-3-hydroxy-5-methylisoxazoIe-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA) and kainate receptors. Activation of glutamate receptors, particularly NMDA receptors, usually leads to calcium influx, whicn can act as a second messenger for several processes to mediate synaptic activity and brain function. Nevertheless, excessive release of glutamate neurotransmitter may produce intracellular calcium overload, leading to a cascade of events mediating cytoskeleton damage accompanied w让h reactive oxygen species (ROS) generation, mitochondrial dysfunction and ultimately neuronal cell death. These toxic effects of glutamate are known as excitotoxicity. Neuronal excitotoxicity has been linked to several acute and chronic brain diseases, such as stroke/ischemia, epilepsy and a range of neurodegenerative disorders, including Alzheimer's disease (AD), Huntingtons disease, amyotrophic lateral sclerosis (ALS) and Parkinsons disease (PD), contributing to the neuronal lost in different brain regions. Unfortunately, treating nervous system disorders with general glutamate receptor blockers has been associated with undesirable side effects, becoming increasingly necessary to unravel downstream effectors in the excitotoxicity-dependent cell death pathway in order to develop novel therapeutic strategies.

SARM1 participates in axonal degeneration and mitochondrial dysfunction in prion disease

Prion disease represents a group of fatal neurogenerative diseases in humans and animals that are associated with energy loss,axonal degeneration,and mitochondrial dysfunction.Axonal degeneration is an early hallmark of neurodegeneration and is triggered by SARM1.We found that depletion or dysfunctional mutation of SARM1 protected against NAD+loss,axonal degeneration,and mitochondrial functional disorder induced by the neurotoxic peptide PrP106-126.NAD+supplementation rescued prion-triggered axonal degeneration and mitochondrial dysfunction and SARM1 overexpression suppressed this protective effect.NAD+supplementation in PrP106-126-incubated N2a cells,SARM1 depletion,and SARM1 dysfunctional mutation each blocked neuronal apoptosis and increased cell survival.Our results indicate that the axonal degeneration and mitochondrial dysfunction triggered by PrP^(106-126) are partially dependent on SARM1 NADase activity.This pathway has potential as a therapeutic target in the early stages of prion disease.

Exploratory use of ultrasound to determine whether demyelination following carpal tunnel syndrome co-exists with axonal degeneration

Carpal tunnel syndrome （CTS） accompanied by secondary axonal degeneration cannot be clearly dis- criminated using the current cross-validated ultrasound severity classification system. This study aimed at exploring cut-off values of ultrasound parameters, including wrist cross-sectional area （W-CSA）, wrist perimeter （W-P）, ratio of cross-sectional area （R-CSA） and perimeter （R-P）, changes of CSA and P from wrist to one third distal forearm （△CSA＆AP） for differentiation. Seventy-three patients （13 male and 60 female） were assigned into group A （demyelination only, n = 40） and group B （demyelination with secondary axonal degeneration, n = 33） based on the outcomes of nerve conduction studies （NCS）. Receiver Operative Characteristics （ROC） curves were plotted to obtain sensitivity, specificity, and accuracy of cut- off values for all the ultrasound parameters. The overall identified cut-off values （W-CSA 12.0 mm2, W-P 16.27 mm, R-CSA 1.85, R-P 1.48, △CSA 6.98 mm2, △P 5.77 mm） had good sensitivity （77.1-88.6%）, fair specificity （40-62.2%） and fair-to-good accuracy （0.676-0.758）. There were also significant differences in demographics （age and severity gradation, P 〈 0.001）, NCS findings （wrist motor latency and conduction velocity, P 〈 0.0001; wrist motor amplitude, P 〈 0.05; distal sensory latency, P 〈 0.05; sensory amplitude, P 〈 0.001） and ultrasound measurements （W-CSA, W-P, R-CSA, R-P, △CSA＆△P, P 〈 0.05） between groups. These findings suggest that ultrasound can be potentially used to differentiate demyelinating CTS with sec- ondary axonal degeneration and provide better treatment guidance.

Potential utility of aldose reductase-deficient Schwann cells IKARS1 for the study of axonal degeneration and regeneration

Diabetic peripheral neuropathy（DPN）is one of the most common and intractable complications of diabetes mellitus.Its irritating symptoms,such as paresthesia,hyperalgesia and allodynia,can be causes of insomnia and depression;whereas its progression to more advanced stages can result in serious consequences,such as lower limb amputations and lethal arrhythmias.

Neutrophil peptide 1 accelerates the clearance of degenerative axons during Wallerian degeneration by activating macrophages after peripheral nerve crush injury

Macrophages play an important role in peripheral nerve regeneration,but the specific mechanism of regeneration is still unclear.Our preliminary findings indicated that neutrophil peptide 1 is an innate immune peptide closely involved in peripheral nerve regeneration.However,the mechanism by which neutrophil peptide 1 enhances nerve regeneration remains unclear.This study was designed to investigate the relationship between neutrophil peptide 1 and macrophages in vivo and in vitro in peripheral nerve crush injury.The functions of RAW 264.7 cells we re elucidated by Cell Counting Kit-8 assay,flow cytometry,migration assays,phagocytosis assays,immunohistochemistry and enzyme-linked immunosorbent assay.Axonal debris phagocytosis was observed using the CUBIC(Clear,Unobstructed Brain/Body Imaging Cocktails and Computational analysis)optical clearing technique during Wallerian degeneration.Macrophage inflammatory factor expression in different polarization states was detected using a protein chip.The results showed that neutrophil peptide 1 promoted the prolife ration,migration and phagocytosis of macrophages,and CD206 expression on the surfa ce of macrophages,indicating M2 polarization.The axonal debris clearance rate during Wallerian degeneration was enhanced after neutrophil peptide 1 intervention.Neutrophil peptide 1 also downregulated inflammatory factors interleukin-1α,-6,-12,and tumor necrosis factor-αin invo and in vitro.Thus,the results suggest that neutrophil peptide 1 activates macrophages and accelerates Wallerian degeneration,which may be one mechanism by which neutrophil peptide 1 enhances peripheral nerve regeneration.

Approaches to quantify axonal morphology for the analysis of axonal degeneration

Morphological hallmarks of axonal degeneration(AxD):Axons transmit signals from one neuron to another and a re crucial for the proper communication in the nervous system.Therefore,the disintegration of axons,a process named AxD,has detrimental consequences and plays a key role in many neurological diseases.

Non-cell autonomous role of astrocytes in axonal degeneration of cortical projection neurons in hereditary spastic paraplegias

Impai red axonal deve lopment and degeneration underlie many debilitating diseases,including hereditary spastic paraplegia(HSP).HSPs are a heterogeneous group of neurogenetic disorders characterized by axonopathy of cortical motor neurons(Fink,2006;Blackstone et al.,2010).The axonal degeneration of these cortical projection neurons(PNs)in HSP patients disrupts the signals from brain to spinal motor neurons,leading to muscle weakness and spasticity.Since the discovery of the first HSP gene(SPAST)in 1999,over 80 distinct genetic loci associated with HSP have been identified.How the mutations of these divergent genes specifically result in axonal degeneration of cortical PNs remains largely unclear,which contributes to the lack of effective treatment to ameliorate,stop,or reverse axonal defects in HSPs.