Axonal transport deficit in a KIF5A( -/- ) mouse model

Altered molecular and cellular mechanisms in KIF5A-associated neurodegenerative or neurodevelopmental disorders

Mutations targeting distinct domains of the neuron-specific kinesin KIF5A associate with different neurodegenerative/neurodevelopmental disorders, but the molecular bases of this clinical heterogeneity are unknown. We characterised five key mutants covering the whole spectrum of KIF5A-related phenotypes: spastic paraplegia (SPG, R17Q and R280C), Charcot-Marie-Tooth disease (CMT, R864*), amyotrophic lateral sclerosis (ALS, N999Vfs*40), and neonatal intractable myoclonus (NEIMY, C975Vfs*73) KIF5A mutants. CMT-R864*-KIF5A and ALS-N999Vfs*40-KIF5A showed impaired autoinhibition and peripheral localisation accompanied by altered mitochondrial distribution, suggesting transport competence disruption. ALS-N999Vfs*40-KIF5A formed SQSTM1/p62-positive inclusions sequestering WT-KIF5A, indicating a gain of toxic function. SPG-R17Q-KIF5A and ALS-N999Vfs*40-KIF5A evidenced a shorter half-life compared to WT-KIF5A, and proteasomal blockage determined their accumulation into detergent-insoluble inclusions. Interestingly, SPG-R280C-KIF5A and ALS-N999Vfs*40-KIF5A both competed for degradation with proteasomal substrates. Finally, NEIMY-C975Vfs*73-KIF5A displayed a similar, but more severe aberrant behaviour compared to ALS-N999Vfs*40-KIF5A; these two mutants share an abnormal tail but cause disorders on the opposite end of KIF5A-linked phenotypic spectrum. Thus, our observations support the pathogenicity of novel KIF5A mutants, highlight abnormalities of recurrent variants, and demonstrate that both unique and shared mechanisms underpin KIF5A-related diseases.

KIF5A regulates axonal repair and time-dependent axonal transport of SFPQ granules and mitochondria in human motor neurons

Mutations in the microtubule binding motor protein, kinesin family member 5A (KIF5A), cause the fatal motor neuron disease, Amyotrophic Lateral Sclerosis. While KIF5 family members transport a variety of cargos along axons, it is still unclear which cargos are affected by KIF5A mutations. We generated KIF5A null mutant human motor neurons to investigate the impact of KIF5A loss on the transport of various cargoes and its effect on motor neuron function at two different timepoints in vitro. The absence of KIF5A resulted in reduced neurite complexity in young motor neurons (DIV14) and significant defects in axonal regeneration capacity at all developmental stages. KIF5A loss did not affect neurofilament transport but resulted in decreased mitochondria motility and anterograde speed at DIV42. More prominently, KIF5A depletion strongly reduced anterograde transport of SFPQ-associated RNA granules in DIV42 motor neuron axons. We conclude that KIF5A most prominently functions in human motor neurons to promote axonal regrowth after injury as well as to anterogradely transport mitochondria and, to a larger extent, SFPQ-associated RNA granules in a time-dependent manner.

ALS-associated C21ORF2 variant disrupts DNA damage repair, mitochondrial metabolism, neuronal excitability and NEK1 levels in human motor neurons

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease leading to motor neuron loss. Currently mutations in > 40 genes have been linked to ALS, but the contribution of many genes and genetic mutations to the ALS pathogenic process remains poorly understood. Therefore, we first performed comparative interactome analyses of five recently discovered ALS-associated proteins (C21ORF2, KIF5A, NEK1, TBK1, and TUBA4A) which highlighted many novel binding partners, and both unique and shared interactors. The analysis further identified C21ORF2 as a strongly connected protein. The role of C21ORF2 in neurons and in the nervous system, and of ALS-associated C21ORF2 variants is largely unknown. Therefore, we combined human iPSC-derived motor neurons with other models and different molecular cell biological approaches to characterize the potential pathogenic effects of C21ORF2 mutations in ALS. First, our data show C21ORF2 expression in ALS-relevant mouse and human neurons, such as spinal and cortical motor neurons. Further, the prominent ALS-associated variant C21ORF2-V58L caused increased apoptosis in mouse neurons and movement defects in zebrafish embryos. iPSC-derived motor neurons from C21ORF2-V58L-ALS patients, but not isogenic controls, show increased apoptosis, and changes in DNA damage response, mitochondria and neuronal excitability. In addition, C21ORF2-V58L induced post-transcriptional downregulation of NEK1, an ALS-associated protein implicated in apoptosis and DDR. In all, our study defines the pathogenic molecular and cellular effects of ALS-associated C21ORF2 mutations and implicates impaired post-transcriptional regulation of NEK1 downstream of mutant C21ORF72 in ALS.

The ALS-associated KIF5A P986L variant is not pathogenic for Drosophila motoneurons

Amyotrophic lateral sclerosis (ALS) is a devastating paralytic disorder caused by the death of motoneurons. Several mutations in the KIF5A gene have been identified in patients with ALS. Some mutations affect the splicing sites of exon 27 leading to its deletion (Δ27 mutation). KIF5A Δ27 is aggregation-prone and pathogenic for motoneurons due to a toxic gain of function. Another mutation found to be enriched in ALS patients is a proline/leucine substitution at position 986 (P986L mutation). Bioinformatic analyses strongly suggest that this variant is benign. Our study aims to conduct functional studies in Drosophila to classify the KIF5A P986L variant. When expressed in motoneurons, KIF5A P986L does not modify the morphology of larval NMJ or the synaptic transmission. In addition, KIF5A P986L is uniformly distributed in axons and does not disturb mitochondria distribution. Locomotion at larval and adult stages is not affected by KIF5A P986L. Finally, both KIF5A WT and P986L expression in adult motoneurons extend median lifespan compared to control flies. Altogether, our data show that the KIF5A P986L variant is not pathogenic for motoneurons and may represent a hypomorphic allele, although it is not causative for ALS.

Static magnetic field-modulated mesenchymal stem cell-derived mitochondria-containing microvesicles for enhanced intervertebral disc degeneration therapy

Intervertebral disc degeneration (IVDD) is characterized by the senescence and declining vitality of nucleus pulposus cells (NPCs), often driven by mitochondrial dysfunction. This study elucidates that mesenchymal stem cells (MSCs) play a crucial role in attenuating NPC senescence by secreting mitochondria-containing microvesicles (mitoMVs). Moreover, it demonstrates that static magnetic fields (SMF) enhance the secretion of mitoMVs by MSCs. By distinguishing mitoMV generation from exosomes, this study shifts focus to understanding the molecular mechanisms of SMF intervention, emphasizing cargo transport and plasma membrane budding processes, with RNA sequencing indicating the potential involvement of the microtubule-based transport protein Kif5b. The study further confirms the interaction between Rab22a and Kif5b, revealing Rab22a's role in sorting mitoMVs into microvesicles (MVs) and potentially mediating subsequent plasma membrane budding. Subsequent construction of a gelatin methacrylate (GelMA) hydrogel delivery system further addresses the challenges of in vivo application and verifies the substantial potential of mitoMVs in delaying IVDD. This research not only sheds light on the molecular intricacies of SMF-enhanced mitoMV secretion but also provides innovative perspectives for future IVDD therapeutic strategies.

ALS-Associated KIF5A Mutation Causes Locomotor Deficits Associated with Cytoplasmic Inclusions, Alterations of Neuromuscular Junctions, and Motor Neuron Loss

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motor neurons. Recently, genome-wide association studies identified KIF5A as a new ALS-causing gene. KIF5A encodes a protein of the kinesin-1 family, allowing the anterograde transport of cargos along the microtubule rails in neurons. In ALS patients, mutations in the KIF5A gene induce exon 27 skipping, resulting in a mutated protein with a new C-terminal region (KIF5A Δ27). To understand how KIF5A Δ27 underpins the disease, we developed an ALS-associated KIF5A Drosophila model. When selectively expressed in motor neurons, KIF5A Δ27 alters larval locomotion as well as morphology and synaptic transmission at neuromuscular junctions in both males and females. We show that the distribution of mitochondria and synaptic vesicles is profoundly disturbed by KIF5A Δ27 expression. That is consistent with the numerous KIF5A Δ27-containing inclusions observed in motor neuron soma and axons. Moreover, KIF5A Δ27 expression leads to motor neuron death and reduces life expectancy. Our in vivo model reveals that a toxic gain of function underlies the pathogenicity of ALS-linked KIF5A mutant.SIGNIFICANCE STATEMENT Understanding how a mutation identified in patients with amyotrophic lateral sclerosis (ALS) causes the disease and the loss of motor neurons is crucial to fight against this disease. To this end, we have created a Drosophila model based on the motor neuron expression of the KIF5A mutant gene, recently identified in ALS patients. KIF5A encodes a kinesin that allows the anterograde transport of cargos. This model recapitulates the main features of ALS, including alterations of locomotion, synaptic neurotransmission, and morphology at neuromuscular junctions, as well as motor neuron death. KIF5A mutant is found in cytoplasmic inclusions, and its pathogenicity is because of a toxic gain of function.

Mitochondrial transport in neurons and evidence for its involvement in acute neurological disorders

Ensuring mitochondrial quality is essential for maintaining neuronal homeostasis, and mitochondrial transport plays a vital role in mitochondrial quality control. In this review, we first provide an overview of neuronal mitochondrial transport, followed by a detailed description of the various motors and adaptors associated with the anterograde and retrograde transport of mitochondria. Subsequently, we review the modest evidence involving mitochondrial transport mechanisms that has surfaced in acute neurological disorders, including traumatic brain injury, spinal cord injury, spontaneous intracerebral hemorrhage, and ischemic stroke. An in-depth study of this area will help deepen our understanding of the mechanisms underlying the development of various acute neurological disorders and ultimately improve therapeutic options.

Role of microtubule actin crosslinking factor 1 (MACF1) in bipolar disorder pathophysiology and potential in lithium therapeutic mechanism

Bipolar affective disorder (BPAD) are life-long disorders that account for significant morbidity in afflicted patients. The etiology of BPAD is complex, combining genetic and environmental factors to increase the risk of disease. Genetic studies have pointed toward cytoskeletal dysfunction as a potential molecular mechanism through which BPAD may arise and have implicated proteins that regulate the cytoskeleton as risk factors. Microtubule actin crosslinking factor 1 (MACF1) is a giant cytoskeletal crosslinking protein that can coordinate the different aspects of the mammalian cytoskeleton with a wide variety of actions. In this review, we seek to highlight the functions of MACF1 in the nervous system and the molecular mechanisms leading to BPAD pathogenesis. We also offer a brief perspective on MACF1 and the role it may be playing in lithium's mechanism of action in treating BPAD.

Impaired motor unit recovery and maintenance in a knock-in mouse model of ALS-associated Kif5a variant

Kinesin family member 5A (KIF5A) is an essential, neuron-specific microtubule-associated motor protein responsible for the anterograde axonal transport of various cellular cargos. Loss of function variants in the N-terminal, microtubule-binding domain are associated with hereditary spastic paraplegia and hereditary motor neuropathy. These variants result in a loss of the ability of the mutant protein to process along microtubules. Contrastingly, gain of function splice-site variants in the C-terminal, cargo-binding domain of KIF5A are associated with amyotrophic lateral sclerosis (ALS), a neurodegenerative disease involving death of upper and lower motor neurons, ultimately leading to degradation of the motor unit (MU; an alpha motor neuron and all the myofibers it innervates) and death. These ALS-associated variants result in loss of autoinhibition, increased procession of the mutant protein along microtubules, and altered cargo binding. To study the molecular and cellular consequences of ALS-associated variants in vivo, we introduced the murine homolog of an ALS-associated KIF5A variant into C57BL/6 mice using CRISPR-Cas9 gene editing which produced mutant Kif5a mRNA and protein in neuronal tissues of heterozygous (Kif5a+/c.3005+1G>A; HET) and homozygous (Kif5ac.3005+1G>A/c.3005+1G>A; HOM) mice. HET and HOM mice appeared normal in behavioral and electrophysiological (compound muscle action potential [CMAP] and MU number estimation [MUNE]) outcome measures at one year of age. When subjected to sciatic nerve injury, HET and HOM mice have delayed and incomplete recovery of the MUNE compared to wildtype (WT) mice suggesting an impairment in MU repair. Moreover, aged mutant Kif5a mice (aged two years) had reduced MUNE independent of injury, and exacerbation of the delayed and incomplete recovery after injury compared to aged WT mice. These data suggest that ALS-associated variants may result in an impairment of the MU to respond to biological challenges such as injury and aging, leading to a failure of MU repair and maintenance. In this report, we present the behavioral, electrophysiological and pathological characterization of mice harboring an ALS-associated Kif5a variant to understand the functional consequences of KIF5A C-terminal variants in vivo.

Kif5a Regulates Mitochondrial Transport in Developing Retinal Ganglion Cells In Vitro

Purpose

Axon transport of organelles and neurotrophic factors is necessary for maintaining cellular function and survival of retinal ganglion cells (RGCs). However, it is not clear how trafficking of mitochondria, essential for RGC growth and maturation, changes during RGC development. The purpose of this study was to understand the dynamics and regulation of mitochondrial transport during RGC maturation using acutely purified RGCs as a model system.

Methods

Primary RGCs were immunopanned from rats of either sex during three stages of development. MitoTracker dye and live-cell imaging were used to quantify mitochondrial motility. Analysis of single-cell RNA sequencing was used to identify Kinesin family member 5A (Kif5a) as a relevant motor candidate for mitochondrial transport. Kif5a expression was manipulated with either short hairpin RNA (shRNA) or exogenous expression adeno-associated virus viral vectors.

Results

Anterograde and retrograde mitochondrial trafficking and motility decreased through RGC development. Similarly, the expression of Kif5a, a motor protein that transports mitochondria, also decreased during development. Kif5a knockdown decreased anterograde mitochondrial transport, while Kif5a expression increased general mitochondrial motility and anterograde mitochondrial transport.

Conclusions

Our results suggested that Kif5a directly regulates mitochondrial axonal transport in developing RGCs. Future work exploring the role of Kif5a in vivo in RGCs is indicated.

Ataxia and spastic paraplegia in mitochondrial disease

Degenerative ataxias and hereditary spastic paraplegias (HSPs) form a continuous, often overlapping disease spectrum sharing not only phenotypic features and underlying genes, but also cellular pathways and disease mechanisms. Mitochondrial metabolism presents a major molecular theme underlying both multiple ataxias and HSPs, thus indicating a heightened vulnerability of Purkinje cells, spinocerebellar tracts, and motor neurons to mitochondrial dysfunction, which is of particular interest for translational approaches. Mitochondrial dysfunction might be the primary (upstream) or secondary (downstream) result of a genetic defect, with underlying genetic defects in nuclear-encoded genes being much more frequent than in mtDNA genes in both, ataxias and HSPs. Here, we outline the substantial number of ataxias, spastic ataxias and HSPs caused by mutated genes implicated in (primary or secondary) mitochondrial dysfunction, highlighting several key "mitochondrial" ataxias and HSPs which are of particular interest for their frequency, pathogenesis and translational opportunities. We then showcase prototypic mitochondrial mechanisms by which disruption of these ataxia and HSP genes contributes to Purkinje cells or corticospinal neuron dysfunction, thus elucidating hypotheses on Purkinje cells and corticospinal neuron vulnerability to mitochondrial dysfunction.

Dysregulated expression and distribution of Kif5α in neurites of wobbler motor neurons

Impaired axonal transport has been observed in patients with amyotrophic lateral sclerosis (ALS) and in animal models, suggesting that transport proteins likely play a critical role in the pathological mechanism of ALS. Dysregulation of Kinesin-family-member 5α (Kif5α), a neuron-specific isoform of heavy chain kinesin family, has been described in several neurological disorders, in humans and animal models, including ALS. In this study, we determined Kif5α expression by gene sequencing, quantitative reverse transcription-polymerase chain reaction, and western blot assay in the cervical spinal cord of wobbler mice and immunofluorescence staining in dissociated cultures of the ventral horn. Further, we observed the distribution of Kif5α and mitochondria along motor neuronal branches by confocal imaging. Our results showed that Kif5α expression was greatly dysregulated in wobbler mice, which resulted in altered distribution of Kif5α along motor neuronal branches with an abnormal mitochondrial distribution. Thus, our results indicate that dysregulation of Kif5 and therefore abnormal transport in motor neuronal branches in this ALS model could be causative for several pathological findings at the cellular level, like misallocation of cytoskeletal proteins or organelles like mitochondria.

Axonal transport deficits in the pathogenesis of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a chronic and prevalent metabolic disease that gravely endangers human health and seriously affects the quality of life of hyperglycemic patients. More seriously, it can lead to amputation and neuropathic pain, imposing a severe financial burden on patients and the healthcare system. Even with strict glycemic control or pancreas transplantation, peripheral nerve damage is difficult to reverse. Most current treatment options for DPN can only treat the symptoms but not the underlying mechanism. Patients with long-term diabetes mellitus (DM) develop axonal transport dysfunction, which could be an important factor in causing or exacerbating DPN. This review explores the underlying mechanisms that may be related to axonal transport impairment and cytoskeletal changes caused by DM, and the relevance of the latter with the occurrence and progression of DPN, including nerve fiber loss, diminished nerve conduction velocity, and impaired nerve regeneration, and also predicts possible therapeutic strategies. Understanding the mechanisms of diabetic neuronal injury is essential to prevent the deterioration of DPN and to develop new therapeutic strategies. Timely and effective improvement of axonal transport impairment is particularly critical for the treatment of peripheral neuropathies.

Abnormal course of the corticospinal tracts in KIF5C-related encephalopathy

KIF5C encodes a protein belonging to the kinesin family and involved in cellular transport. Variants in KIF5C were first associated a decade ago with microcephaly and malformations of cortical development, with a phenotypic spectrum ranging from polymicrogyria to pachygyria. Currently, eight patients have been reported so far. Here we describe a new paediatric patient carrying the recurrent p.(Glu237Lys) KIF5C variant associated with a distinctive neuroradiological pattern of abnormal posterior course of the corticospinal tract at the level of the pons with a thickened anterior component of the transverse pontine fibers. This finding is likely related to altered axonal guidance and requires further evidence in other patients with KIF5C-related disorder.

Axon Biology in ALS: Mechanisms of Axon Degeneration and Prospects for Therapy

This review addresses the longstanding debate over whether amyotrophic lateral sclerosis (ALS) is a 'dying back' or 'dying forward' disorder in the light of new gene identifications and the increased understanding of mechanisms of action for previously identified ALS genes. While the topological pattern of pathology in animal models, and more anecdotally in patients is indeed 'dying back', this review discusses how this fits with the fact that many of the major initiating events are thought to occur within the soma. It also discusses how widely varying ALS risk factors, including some impacting axons directly, may combine to drive a common pathway involving TAR DNA binding protein 43 (TDP-43) and neuromuscular junction (NMJ) denervation. The emerging association between sterile alpha and TIR motif-containing 1 (SARM1), a protein so far mostly associated with axon degeneration, and sporadic ALS is another major theme. The strengths and limitations of the current evidence supporting an association are considered, along with ways in which SARM1 could become activated in ALS. The final section addresses SARM1-based therapies along with the prospects for targeting other axonal steps in ALS pathogenesis.

Recent Updates on the Genetics of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) primarily affect the motor and frontotemporal areas of the brain, respectively. These disorders share clinical, genetic, and pathological similarities, and approximately 10-15% of ALS-FTD cases are considered to be multisystemic. ALS-FTD overlaps have been linked to families carrying an expansion in the intron of C9orf72 along with inclusions of TDP-43 in the brain. Other overlapping genes (VCP, FUS, SQSTM1, TBK1, CHCHD10) are also involved in similar functions that include RNA processing, autophagy, proteasome response, protein aggregation, and intracellular trafficking. Recent advances in genome sequencing have identified new genes that are involved in these disorders (TBK1, CCNF, GLT8D1, KIF5A, NEK1, C21orf2, TBP, CTSF, MFSD8, DNAJC7). Additional risk factors and modifiers have been also identified in genome-wide association studies and array-based studies. However, the newly identified genes show higher disease frequencies in combination with known genes that are implicated in pathogenesis, thus indicating probable digenetic/polygenic inheritance models, along with epistatic interactions. Studies suggest that these genes play a pleiotropic effect on ALS-FTD and other diseases such as Alzheimer's disease, Ataxia, and Parkinsonism. Besides, there have been numerous improvements in the genotype-phenotype correlations as well as clinical trials on stem cell and gene-based therapies. This review discusses the possible genetic models of ALS and FTD, the latest therapeutics, and signaling pathways involved in ALS-FTD.

Heroin Addiction Induces Axonal Transport Dysfunction in the Brain Detected by In Vivo MRI

Heroin is a highly addictive drug that causes axonal damage. Here, manganese-enhanced magnetic resonance imaging (MEMRI) was used to dynamically monitor axonal transport at different stages of heroin addiction. Rat models of heroin addiction (HA) and prolonged heroin addiction (PHA) were established by injecting rats with heroin at different stages. Heroin-induced learning and memory deficits were evaluated in the Morris water maze (MWM), and MEMRI was used to dynamically evaluate axonal transport in the olfactory pathway. The expression of proteins related to axonal structure and function was also assessed by Western blotting. Transmission electron microscopy (TEM) was used to observe ultrastructural changes, and protein levels of neurofilament heavy chain (NF-H) were analyzed by immunofluorescence staining. HA rats, especially PHA rats, exhibited worse spatial learning and memory than control rats. Compared with HA rats and control rats, PHA rats exhibited significantly longer escape latencies, significantly fewer platform-location crossings, and significantly more time in the target quadrant during the MWM test. Mn²⁺ transport was accelerated in HA rats. PHA rats exhibited severely reduced Mn²⁺ transport, and the axonal transport rate (ATR) was significantly lower in these rats than in control rats (P < 0.001). The levels of cytoplasmic dynein and kinesin-1 were significantly decreased in the PHA group than in the control group (P < 0.001); additionally, the levels of energy-related proteins, including cytochrome c oxidase (COX) IV and ATP synthase subunit beta (ATPB), were lower in the PHA group (P < 0.001). The brains of heroin-exposed rats displayed an abnormal ultrastructure, with neuronal apoptosis and mitochondrial dysfunction. Heroin exposure decreased the expression of NF-H, as indicated by significantly reduced staining intensities in tissues from HA and PHA rats (P < 0.05). MEMRI detected axonal transport dysfunction caused by long-term repeated exposure to heroin. The main causes of axonal transport impairment may be decreases in the levels of motor proteins and mitochondrial dysfunction. This study shows that MEMRI is a potential tool for visualizing axonal transport in individuals with drug addictions, providing a new way to evaluate addictive encephalopathy.

Tau phosphorylation and PAD exposure in regulation of axonal growth

Introduction: Tau is a microtubule associated phosphoprotein found principally in neurons. Prevailing dogma continues to define microtubule stabilization as the major function of tau in vivo, despite several lines of evidence suggesting this is not the case. Most importantly, tau null mice have deficits in axonal outgrowth and neuronal migration while still possessing an extensive microtubule network. Instead, mounting evidence suggests that tau may have a major function in the regulation of fast axonal transport (FAT) through activation of neuronal signaling pathways. Previous studies identified a phosphatase activating domain (PAD) at the tau N-terminal that is normally sequestered, but is constitutively exposed in tauopathies. When exposed, the PAD activates a signaling cascade involving PP1 and GSK3β which affects cellular functions including release of cargo from kinesin. Furthermore, we discovered that PAD exposure can be regulated by a single phosphorylation at T205. Exposure of the PAD is an early event in multiple tauopathies and a major contributing factor to neurodegeneration associated with tau hyperphosphorylation. However, effects of tau PAD exposure on anterograde FAT raised the interesting possibility that this pathway may be a mechanism for physiological regulation of cargo delivery through site-specific phosphorylation of tau and transient activation of PP1 and GSK3β. Significantly, there is already evidence of local control of PP1 and GSK3β at sites which require cargo delivery. Methods: To investigate this hypothesis, first we evaluated cellular localization of tau PAD exposure, pT205 tau phosphorylation, and active GSK3β in primary hippocampal neurons during development. Second, we analyzed the axonal outgrowth of tau knockout neurons following transfection with full length hTau40-WT, hTau40-ΔPAD, or hTau40-T205A. Results and Discussion: The results presented here suggest that transient activation of a PP1-GSK3β signaling pathway through locally regulated PAD exposure is a mechanism for cargo delivery, and thereby important for neurite outgrowth of developing neurons.

Current Knowledge of Endolysosomal and Autophagy Defects in Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of neurological disorders involving the degeneration of motor neurons. Due to their clinical and genetic heterogeneity, finding common effective therapeutics is difficult. Therefore, a better understanding of the common pathological mechanisms is necessary. The role of several HSP genes/proteins is linked to the endolysosomal and autophagic pathways, suggesting a functional convergence. Furthermore, impairment of these pathways is particularly interesting since it has been linked to other neurodegenerative diseases, which would suggest that the nervous system is particularly sensitive to the disruption of the endolysosomal and autophagic systems. In this review, we will summarize the involvement of HSP proteins in the endolysosomal and autophagic pathways in order to clarify their functioning and decipher some of the pathological mechanisms leading to HSP.

Motor proteins at the mitochondria-cytoskeleton interface

Mitochondria are multifunctional organelles that not only produce energy for the cell, but are also important for cell signalling, apoptosis and many biosynthetic pathways. In most cell types, they form highly dynamic networks that are constantly remodelled through fission and fusion events, repositioned by motor-dependent transport and degraded when they become dysfunctional. Motor proteins and their tracks are key regulators of mitochondrial homeostasis, and in this Review, we discuss the diverse functions of the three classes of motor proteins associated with mitochondria - the actin-based myosins, as well as the microtubule-based kinesins and dynein. In addition, Miro and TRAK proteins act as adaptors that link kinesin-1 and dynein, as well as myosin of class XIX (MYO19), to mitochondria and coordinate microtubule- and actin-based motor activities. Here, we highlight the roles of motor proteins and motor-linked track dynamics in the transporting and docking of mitochondria, and emphasize their adaptations in specialized cells. Finally, we discuss how motor-cargo complexes mediate changes in mitochondrial morphology through fission and fusion, and how they modulate the turnover of damaged organelles via quality control pathways, such as mitophagy. Understanding the importance of motor proteins for mitochondrial homeostasis will help to elucidate the molecular basis of a number of human diseases.

KIF5A-dependent axonal transport deficiency disrupts autophagic flux in trimethyltin chloride-induced neurotoxicity

Trimethyltin chloride (TMT) is widely used as a constituent of fungicides and plastic stabilizers in the industrial and agricultural fields, and is generally acknowledged to have potent neurotoxicity, especially in the hippocampus; however, the mechanism of induction of neurotoxicity by TMT remains elusive. Herein, we exposed Neuro-2a cells to different concentrations of TMT (2, 4, and 8 μM) for 24 h. Proteomic analysis, coupled with bioinformatics analysis, revealed the important role of macroautophagy/autophagy-lysosome machinery in TMT-induced neurotoxicity. Further analysis indicated significant impairment of autophagic flux by TMT via suppressed lysosomal function, such as by inhibiting lysosomal proteolysis and changing the lysosomal pH, thereby contributing to defects in autophagic clearance and subsequently leading to nerve cell death. Mechanistically, molecular interaction networks of Ingenuity Pathway Analysis identified a downregulated molecule, KIF5A (kinesin family member 5A), as a key target in TMT-impaired autophagic flux. TMT decreased KIF5A protein expression, disrupted the interaction between KIF5A and lysosome, and impaired lysosomal axonal transport. Moreover, Kif5a overexpression restored axonal transport, increased lysosomal dysfunction, and antagonized TMT-induced neurotoxicity in vitro. Importantly, in TMT-administered mice with seizure symptoms and histomorphological injury in the hippocampus, TMT inhibited KIF5A expression in the hippocampus. Gene transfer of Kif5a enhanced autophagic clearance in the hippocampus and alleviated TMT-induced neurotoxicity in vivo. Our results are the first to demonstrate KIF5A-dependent axonal transport deficiency to cause autophagic flux impairment via disturbance of lysosomal function in TMT-induced neurotoxicity; manipulation of KIF5A may be a therapeutic approach for antagonizing TMT-induced neurotoxicity.Abbreviations: 3-MA: 3-methyladenine; AAV: adeno-associated virus; ACTB: actin beta; AGC: automatic gain control; ATG: autophagy-related; ATP6V0D1: ATPase H+ transporting lysosomal V0 subunit D1; ATP6V1E1: ATPase H+ transporting lysosomal V1 subunit E1; CA: cornu ammonis; CQ: chloroquine; CTSB: cathepsin B; CTSD: cathepsin D; DCTN1: dynactin subunit 1; DG: dentate gyrus; DYNLL1: dynein light chain LC8-type 1; FBS: fetal bovine serum; GABARAP: GABA type A receptor-associated protein; GABARAPL1: GABA type A receptor associated protein like 1; GABARAPL2: GABA type A receptor associated protein like 2; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; IPA: Ingenuity Pathway Analysis; KEGG: Kyoto Encyclopedia of Genes and Genomes; KIF5A: kinesin family member 5A; LAMP: lysosomal-associated membrane protein; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; NBR1: NBR1 autophagy cargo receptor; OPTN: optineurin; PBS: phosphate-buffered saline; PFA: paraformaldehyde; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PRM: parallel reaction monitoring; siRNA: small interfering RNA; SQSTM1/p62: sequestosome 1; SYP: synaptophysin; TAX1BP1: Tax1 binding protein 1; TMT: trimethyltin chloride; TUB: tubulin.

Normal levels of KIF5 but reduced KLC1 levels in both Alzheimer disease and Alzheimer disease in Down syndrome: evidence suggesting defects in anterograde transport

BACKGROUND

Impaired axonal transport may contribute to the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD) and Down syndrome (DS). Axonal transport is a complex process in which specific motor proteins move cargoes to and from neuronal cell bodies and their processes. Inconsistent reports point to the changes in AD in the levels of the classical anterograde motor protein kinesin family member 5 (KIF5) and the primary neuronal KIF regulator kinesin light chain 1 (KLC1), raising the possibility that anterograde transport is compromised in AD.

METHODS AND MATERIALS

To address inconsistencies and determine if the shared pathologies in AD and elderly DS subjects with dementia (AD in DS; AD-DS) extend to the changes in KIF5 and KLC1, we measured the levels of all the three KIF5 family members and KLC1 in the AD and AD-DS frontal cortex and AD temporal cortex and cerebellum in samples taken with a short postmortem interval. To support future studies to explore the cell biological basis for any changes detected, we also examined the levels of these proteins in the brains of young and aged adult mice in the Dp (16)1Yey/+ (Dp16) mouse model of DS and J20 mouse model of AD.

RESULTS

There were no changes in comparison with controls in KIF5 family members in either the AD or AD-DS samples when normalized to either β-actin or glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Interestingly, however, samples from control brains as well as from AD and AD-DS demonstrated strong positive correlations between the levels of KIF5 family members, suggesting positive co-regulated expression. Importantly, while earlier reports pointed to a negative correlation between the levels of the amyloid precursor protein (APP) and KIF5A levels, we found the opposite to be true in AD-DS; this was especially striking given triplication of the APP gene, with increased APP protein levels. AD and control samples showed positive correlations between fl-hAPP and KIF5 members, but they were less consistent. In contrast to the findings for KIF5, the levels of KLC1 were downregulated in the frontal cortex of both AD and AD-DS brains; interestingly, this change was not seen in the AD temporal cortex or cerebellum. As postmortem interval has a negative effect on the levels of KLC1, but not KIF5 members, we analyzed a subset of samples with a very short postmortem interval (PMI) (≤ 6 h), a PMI that was not significantly correlated with the levels of KLC1 in either AD or AD-DS samples; we confirmed the presence of a statistically significant reduction of KLC1 in AD and AD-DS brains as compared with control brains. Studies comparing Dp16 to its euploid control recapitulated human studies in demonstrating no change in KIF5 levels and a positive correlation between the levels of KIF5 family members. J20 mice also showed normal KIF5 levels. However, unlike the AD and AD-DS frontal cortex, KLC1 levels were not reduced in the brains of Dp16 or J20 mice.

CONCLUSION

These data point to significant reductions in KLC1 in AD and AD-DS. In so doing, they raise the possibility of compromised KLC1-mediated axonal transport in these conditions, a posit that can now be pursued in model systems in which KLC1 expression is reduced.

CRMP/UNC-33 organizes microtubule bundles for KIF5-mediated mitochondrial distribution to axon

Neurons are highly specialized cells with polarized cellular processes and subcellular domains. As vital organelles for neuronal functions, mitochondria are distributed by microtubule-based transport systems. Although the essential components of mitochondrial transport including motors and cargo adaptors are identified, it is less clear how mitochondrial distribution among somato-dendritic and axonal compartment is regulated. Here, we systematically study mitochondrial motors, including four kinesins, KIF5, KIF17, KIF1, KLP-6, and dynein, and transport regulators in C. elegans PVD neurons. Among all these motors, we found that mitochondrial export from soma to neurites is mainly mediated by KIF5/UNC-116. Interestingly, UNC-116 is especially important for axonal mitochondria, while dynein removes mitochondria from all plus-end dendrites and the axon. We surprisingly found one mitochondrial transport regulator for minus-end dendritic compartment, TRAK-1, and two mitochondrial transport regulators for axonal compartment, CRMP/UNC-33 and JIP3/UNC-16. While JIP3/UNC-16 suppresses axonal mitochondria, CRMP/UNC-33 is critical for axonal mitochondria; nearly no axonal mitochondria present in unc-33 mutants. We showed that UNC-33 is essential for organizing the population of UNC-116-associated microtubule bundles, which are tracks for mitochondrial trafficking. Disarrangement of these tracks impedes mitochondrial transport to the axon. In summary, we identified a compartment-specific transport regulation of mitochondria by UNC-33 through organizing microtubule tracks for different kinesin motors other than microtubule polarity.

Lysosome Function and Dysfunction in Hereditary Spastic Paraplegias

Hereditary Spastic Paraplegias (HSPs) are a genetically diverse group of inherited neurological diseases with over 80 associated gene loci. Over the last decade, research into mechanisms underlying HSPs has led to an emerging interest in lysosome dysfunction. In this review, we highlight the different classes of HSPs that have been linked to lysosome defects: (1) a subset of complex HSPs where mutations in lysosomal genes are causally linked to the diseases, (2) other complex HSPs where mutation in genes encoding membrane trafficking adaptors lead to lysosomal defects, and (3) a subset of HSPs where mutations affect genes encoding proteins whose function is primarily linked to a different cellular component or organelle such as microtubule severing and Endoplasmic Reticulum-shaping, while also altering to lysosomes. Interestingly, aberrant axonal lysosomes, associated with the latter two subsets of HSPs, are a key feature observed in other neurodegenerative diseases such as Alzheimer's disease. We discuss how altered lysosome function and trafficking may be a critical contributor to HSP pathology and highlight the need for examining these features in the cortico-spinal motor neurons of HSP mutant models.

The Neglected Genes of ALS: Cytoskeletal Dynamics Impact Synaptic Degeneration in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that selectively affects motor neurons (MNs) of the cortex, brainstem, and spinal cord. Several genes have been linked to both familial (fALS) and sporadic (sALS) cases of ALS. Among all the ALS-related genes, a group of genes known to directly affect cytoskeletal dynamics (ALS2, DCTN1, PFN1, KIF5A, NF-L, NF-H, PRPH, SPAST, and TUBA4A) is of high importance for MN health and survival, considering that MNs are large polarized cells with axons that can reach up to 1 m in length. In particular, cytoskeletal dynamics facilitate the transport of organelles and molecules across the long axonal distances within the cell, playing a key role in synapse maintenance. The majority of ALS-related genes affecting cytoskeletal dynamics were identified within the past two decades, making it a new area to explore for ALS. The purpose of this review is to provide insights into ALS-associated cytoskeletal genes and outline how recent studies have pointed towards novel pathways that might be impacted in ALS. Further studies making use of extensive analysis models to look for true hits, the newest technologies such as CRIPSR/Cas9, human induced pluripotent stem cells (iPSCs) and axon sequencing, as well as the development of more transgenic animal models could potentially help to: differentiate the variants that truly act as a primary cause of the disease from the ones that act as risk factors or disease modifiers, identify potential interactions between two or more ALS-related genes in disease onset and progression and increase our understanding of the molecular mechanisms leading to cytoskeletal defects. Altogether, this information will give us a hint on the real contribution of the cytoskeletal ALS-related genes during this lethal disease.

Molecular and Cellular Mechanisms Affected in ALS

Amyotrophic lateral sclerosis (ALS) is a terminal late-onset condition characterized by the loss of upper and lower motor neurons. Mutations in more than 30 genes are associated to the disease, but these explain only ~20% of cases. The molecular functions of these genes implicate a wide range of cellular processes in ALS pathology, a cohesive understanding of which may provide clues to common molecular mechanisms across both familial (inherited) and sporadic cases and could be key to the development of effective therapeutic approaches. Here, the different pathways that have been investigated in ALS are summarized, discussing in detail: mitochondrial dysfunction, oxidative stress, axonal transport dysregulation, glutamate excitotoxicity, endosomal and vesicular transport impairment, impaired protein homeostasis, and aberrant RNA metabolism. This review considers the mechanistic roles of ALS-associated genes in pathology, viewed through the prism of shared molecular pathways.

Disruption of genes associated with Charcot-Marie-Tooth type 2 lead to common behavioural, cellular and molecular defects in Caenorhabditis elegans

Charcot-Marie-Tooth (CMT) disease is an inherited peripheral motor and sensory neuropathy. The disease is divided into demyelinating (CMT1) and axonal (CMT2) neuropathies, and although we have gained molecular information into the details of CMT1 pathology, much less is known about CMT2. Due to its clinical and genetic heterogeneity, coupled with a lack of animal models, common underlying mechanisms remain elusive. In order to gain an understanding of the normal function of genes associated with CMT2, and to draw direct comparisons between them, we have studied the behavioural, cellular and molecular consequences of mutating nine different genes in the nematode Caenorhabditis elegans (lin-41/TRIM2, dyn-1/DNM2, unc-116/KIF5A, fzo-1/MFN2, osm-9/TRPV4, cua-1/ATP7A, hsp-25/HSPB1, hint-1/HINT1, nep-2/MME). We show that C. elegans defective for these genes display debilitated movement in crawling and swimming assays. Severe morphological defects in cholinergic motors neurons are also evident in two of the mutants (dyn-1 and unc-116). Furthermore, we establish methods for quantifying muscle morphology and use these to demonstrate that loss of muscle structure occurs in the majority of mutants studied. Finally, using electrophysiological recordings of neuromuscular junction (NMJ) activity, we uncover reductions in spontaneous postsynaptic current frequency in lin-41, dyn-1, unc-116 and fzo-1 mutants. By comparing the consequences of mutating numerous CMT2-related genes, this study reveals common deficits in muscle structure and function, as well as NMJ signalling when these genes are disrupted.

Role of dynein-dynactin complex, kinesins, motor adaptors, and their phosphorylation in dendritogenesis

One of the characteristic features of different classes of neurons that is vital for their proper functioning within neuronal networks is the shape of their dendritic arbors. To properly develop dendritic trees, neurons need to accurately control the intracellular transport of various cellular cargo (e.g., mRNA, proteins, and organelles). Microtubules and motor proteins (e.g., dynein and kinesins) that move along microtubule tracks play an essential role in cargo sorting and transport to the most distal ends of neurons. Equally important are motor adaptors, which may affect motor activity and specify cargo that is transported by the motor. Such transport undergoes very dynamic fine-tuning in response to changes in the extracellular environment and synaptic transmission. Such regulation is achieved by the phosphorylation of motors, motor adaptors, and cargo, among other mechanisms. This review focuses on the contribution of the dynein-dynactin complex, kinesins, their adaptors, and the phosphorylation of these proteins in the formation of dendritic trees by maturing neurons. We primarily review the effects of the motor activity of these proteins in dendrites on dendritogenesis. We also discuss less anticipated mechanisms that contribute to dendrite growth, such as dynein-driven axonal transport and non-motor functions of kinesins.

Complexity of Generating Mouse Models to Study the Upper Motor Neurons: Let Us Shift Focus from Mice to Neurons

Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input. Therefore, both the cortex and the spinal cord are involved. The cortex has special importance for motor neuron diseases, in which initiation and modulation of voluntary movement is affected. Amyotrophic lateral sclerosis (ALS) is defined by the progressive degeneration of both the upper and lower motor neurons, whereas hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS) are characterized mainly by the loss of upper motor neurons. In an effort to reveal the cellular and molecular basis of neuronal degeneration, numerous model systems are generated, and mouse models are no exception. However, there are many different levels of complexities that need to be considered when developing mouse models. Here, we focus our attention to the upper motor neurons, which are one of the most challenging neuron populations to study. Since mice and human differ greatly at a species level, but the cells/neurons in mice and human share many common aspects of cell biology, we offer a solution by focusing our attention to the affected neurons to reveal the complexities of diseases at a cellular level and to improve translational efforts.

Altered CSF levels of monoamines in hereditary spastic paraparesis 10: A case series

Objective

To perform a comprehensive clinical characterization and biochemical CSF profile analyses in 2 Swedish families with hereditary spastic paraparesis (HSP) 10 (SPG10) caused by 2 different mutations in the neuronal kinesin heavy chain gene (KIF5A).

Methods

Structured clinical assessment, genetic studies, and neuroradiologic and electrophysiological evaluations were performed in 4 patients from 2 families with SPG10. Additional CSF analysis was conducted in 3 patients with regard to levels of neurodegenerative markers and monoamine metabolism.

Results

All patients exhibited a complex form of HSP with a mild to moderate concurrent axonal polyneuropathy. The heterozygous missense mutations c.767A>G and c.967C>T in KIF5A were found. Wide intrafamilial phenotype variability was evident in both families. CSF analysis demonstrated a mild elevation of neurofilament light (NFL) chain in the patient with longest disease duration. Unexpectedly, all patients exhibited increased levels of the dopamine metabolite, homovanillic acid, whereas decreased levels of the noradrenergic metabolite, 3-methoxy-4-hydroxyphenylglycol, were found in 2 of 3 patients.

Conclusions

We report on CSF abnormalities in SPG10, demonstrating that NFL elevation is not a mandatory finding but may appear after long-standing disease. Impaired transportation of synaptic proteins may be a possible explanation for the increased dopaminergic turnover and noradrenergic deficiency identified. The reasons for these selective abnormalities, unrelated to obvious clinical features, remain to be explained. Our findings need further confirmation in larger cohorts of patients harboring KIF5A mutations.

Amyloid beta-mediated KIF5A deficiency disrupts anterograde axonal mitochondrial movement

Mitochondria are crucial organelles for neurophysiology and brain mitochondrial defects constitute a characteristic of Alzheimer's disease (AD). Impaired axonal mitochondrial traffic, especially the anterograde axonal mitochondrial transport is a pronouncing mitochondrial defect that underlies synaptic failure in AD-related conditions. However, the detailed molecular mechanisms of such axonal mitochondrial abnormality have not been fully understood. KIF5A is a key isoform of kinesin-1, which is a key molecular machinery in facilitating anterograde axonal mitochondrial transport. In this study, we have determined a downregulation of KIF5A in postmortem AD temporal lobes. Further experiments on amyloid beta (Aβ)-treated primary neuron culture and 5 × FAD mice suggest a close association of Aβ toxicity and KIF5A loss. Downregulation of KIF5A mimics Aβ-induced axonal mitochondrial transport deficits, indicating a potential role of KIF5A deficiency in AD-relevant axonal mitochondrial traffic abnormalities. Importantly, the restoration of KIF5A corrects Aβ-induced impairments in axonal mitochondrial transport, especially the anterograde traffic, with little or no impact on retrograde axonal mitochondrial motility. Our findings suggest a novel KIF5A-associated mechanism conferring Aβ toxicity to axonal mitochondrial deficits. Furthermore, the results implicate a potential therapeutic avenue by protecting KIF5A function for the treatment of AD.

A Novel Mutation in the Stalk Domain of KIF5A Causes a Slowly Progressive Atypical Motor Syndrome

KIF5A encodes the heavy chain A of kinesin; A motor protein involved in motility functions within neuron. Mutations in the KIF5A N-terminal motor domain are known to cause SPG10; An autosomal dominant hereditary spastic paraplegia (HSP), as well as rare Charcot-Marie-Tooth disease 2 (CMT2) cases. Recently C-terminal cargo-binding tail domain mutations have been associated with an amyotrophic lateral sclerosis (ALS) phenotype. Here we describe a subject presenting with an atypical slowly progressive motor syndrome evolving over a period of 4 years; Characterized by walking difficulties; Muscle hypotrophy mainly involving upper limbs and pyramidal signs confined to the lower limbs. Electromyography demonstrated chronic neurogenic damage and active denervation while electroneurography showed slowly worsening axonal damage. We identified the novel heterozygote variant c.2341A>G in the exon 21 of the KIF5A gene resulting in the amino acid change p.Lys781Glu. The residue Lys781 is located within the terminal region of the stalk domain and is highly evolutionary conserved. Our findings confirm that mutations in KIF5A cause ALS-like phenotypes. However, the stalk domain mutation described here appears to result in an “intermediate” slowly progressive phenotype having aspects resembling ALS as well as HSP and axonal neuropathy. We suggest that KIF5A gene should be considered as a candidate gene in all atypical progressive motor syndromes.

Impairment of Axonal Transport in Diabetes: Focus on the Putative Mechanisms Underlying Peripheral and Central Neuropathies

Diabetes mellitus is a chronic disease with numerous complications that severely impact on the quality of life of patients. Different neuropathies may arise as complications associated with the nervous system, both peripherally and at the central level. The mechanisms behind these neuronal complications are far from being clarified, but axonal transport impairment, a vital process for neuronal physiology, has been described in the context of experimental diabetes. Alterations in neuronal cytoskeleton and motor proteins, deficits in ATP supply or neuroinflammation, as processes that disturb the effective transport of cargoes along the axon, were reported as putative causes of axonal impairment, ultimately leading to axonal degeneration. The main goal of the present review is to reunite the main studies in the literature exploring diabetes-induced alterations likely involved in axonal transport deficits, and call the attention for the uttermost importance of further exploring the field. Understanding the mechanisms underlying neuronal deficits in diabetes is crucial for the development of new therapeutic strategies to prevent neuronal degeneration in diabetes and related neuropathies.

Altered distribution of ATG9A and accumulation of axonal aggregates in neurons from a mouse model of AP-4 deficiency syndrome

The hereditary spastic paraplegias (HSP) are a clinically and genetically heterogeneous group of disorders characterized by progressive lower limb spasticity. Mutations in subunits of the heterotetrameric (ε-β4-μ4-σ4) adaptor protein 4 (AP-4) complex cause an autosomal recessive form of complicated HSP referred to as "AP-4 deficiency syndrome". In addition to lower limb spasticity, this syndrome features intellectual disability, microcephaly, seizures, thin corpus callosum and upper limb spasticity. The pathogenetic mechanism, however, remains poorly understood. Here we report the characterization of a knockout (KO) mouse for the AP4E1 gene encoding the ε subunit of AP-4. We find that AP-4 ε KO mice exhibit a range of neurological phenotypes, including hindlimb clasping, decreased motor coordination and weak grip strength. In addition, AP-4 ε KO mice display a thin corpus callosum and axonal swellings in various areas of the brain and spinal cord. Immunohistochemical analyses show that the transmembrane autophagy-related protein 9A (ATG9A) is more concentrated in the trans-Golgi network (TGN) and depleted from the peripheral cytoplasm both in skin fibroblasts from patients with mutations in the μ4 subunit of AP-4 and in various neuronal types in AP-4 ε KO mice. ATG9A mislocalization is associated with increased tendency to accumulate mutant huntingtin (HTT) aggregates in the axons of AP-4 ε KO neurons. These findings indicate that the AP-4 ε KO mouse is a suitable animal model for AP-4 deficiency syndrome, and that defective mobilization of ATG9A from the TGN and impaired autophagic degradation of protein aggregates might contribute to neuroaxonal dystrophy in this disorder.

Genome-wide Analyses Identify KIF5A as a Novel ALS Gene

To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

Regulation of long-distance transport of mitochondria along microtubules

Mitochondria are cellular organelles of crucial importance, playing roles in cellular life and death. In certain cell types, such as neurons, mitochondria must travel long distances so as to meet metabolic demands of the cell. Mitochondrial movement is essentially microtubule (MT) based and is executed by two main motor proteins, Dynein and Kinesin. The organization of the cellular MT network and the identity of motors dictate mitochondrial transport. Tight coupling between MTs, motors, and the mitochondria is needed for the organelle precise localization. Two adaptor proteins are involved directly in mitochondria-motor coupling, namely Milton known also as TRAK, which is the motor adaptor, and Miro, which is the mitochondrial protein. Here, we discuss the active mitochondria transport process, as well as motor-mitochondria coupling in the context of MT organization in different cell types. We focus on mitochondrial trafficking in different cell types, specifically neurons, migrating cells, and polarized epithelial cells.

Advances in understanding the role of disease-associated proteins in spinal muscular atrophy

INTRODUCTION

Spinal muscular atrophy (SMA) is a neurodegenerative disorder characterized by alpha motor neuron loss in the spinal cord due to reduced survival motor neuron (SMN) protein level. While the genetic basis of SMA is well described, the specific molecular pathway underlying SMA is still not fully understood. Areas covered: This review discusses the recent advancements in understanding the molecular pathways in SMA using different omics approaches and genetic modifiers identified in both vertebrate and invertebrate systems. The findings that are summarized in this article were deduced from original articles and reviews with a particular focus on the latest advancements in the field. Expert commentary: The identification of genetic modifiers such as PLS3 and NCALD in humans or of SMA modulators such as Elavl4 (HuD), Copa, Uba1, Mapk10 (Jnk3), Nrxn2 and Tmem41b (Stasimon) in various SMA animal models improved our knowledge of impaired cellular pathways in SMA. Inspiration from modifier genes and their functions in motor neuron and neuromuscular junctions may open a new avenue for future SMA combinatorial therapies.

KIF1A mediates axonal transport of BACE1 and identification of independently moving cargoes in living SCG neurons

Neurons rely heavily on axonal transport to deliver materials from the sites of synthesis to the axon terminals over distances that can be many centimetres long. KIF1A is the neuron-specific kinesin with the fastest reported anterograde motor activity. Previous studies have shown that KIF1A transports a subset of synaptic proteins, neurofilaments and dense-core vesicles. Using two-colour live imaging, we showed that beta-secretase 1 (BACE1)-mCherry moves together with KIF1A-GFP in both the anterograde and retrograde directions in superior cervical ganglions (SCG) neurons. We confirmed that KIF1A is functionally required for BACE1 transport by using KIF1A siRNA and a KIF1A mutant construct (KIF1A-T312M) to impair its motor activity. We further identified several cargoes that have little or no co-migration with KIF1A-GFP and also move independently from BACE1-mCherry. Together, these findings support a primary role for KIF1A in the anterograde transport of BACE1 and suggest that axonally transported cargoes are sorted into different classes of carrier vesicles in the cell body and are transported by cargo-specific motor proteins through the axon.

KIF5A mutations cause an infantile onset phenotype including severe myoclonus with evidence of mitochondrial dysfunction

Missense mutations in kinesin family member 5A (KIF5A) cause spastic paraplegia 10. We report on 2 patients with de novo stop-loss frameshift variants in KIF5A resulting in a novel phenotype that includes severe infantile onset myoclonus, hypotonia, optic nerve abnormalities, dysphagia, apnea, and early developmental arrest. We propose that alteration and elongation of the carboxy-terminus of the protein has a dominant-negative effect, causing mitochondrial dysfunction in the setting of an abnormal kinesin "motor." These results highlight the role of expanded testing and whole-exome sequencing in critically ill infants and emphasize the importance of accurate test interpretation. Ann Neurol 2016;80:633-637.

Role of kinesin-1-based microtubule sliding in Drosophila nervous system development

The plus-end microtubule (MT) motor kinesin-1 is essential for normal development, with key roles in the nervous system. Kinesin-1 drives axonal transport of membrane cargoes to fulfill the metabolic needs of neurons and maintain synapses. We have previously demonstrated that kinesin-1, in addition to its well-established role in organelle transport, can drive MT-MT sliding by transporting "cargo" MTs along "track" MTs, resulting in dramatic cell shape changes. The mechanism and physiological relevance of this MT sliding are unclear. In addition to its motor domain, kinesin-1 contains a second MT-binding site, located at the C terminus of the heavy chain. Here, we mutated this C-terminal MT-binding site such that the ability of kinesin-1 to slide MTs is significantly compromised, whereas cargo transport is unaffected. We introduced this mutation into the genomic locus of kinesin-1 heavy chain (KHC), generating the Khc(mutA) allele. Khc(mutA) neurons displayed significant MT sliding defects while maintaining normal transport of many cargoes. Using this mutant, we demonstrated that MT sliding is required for axon and dendrite outgrowth in vivo. Consistent with these results, Khc(mutA) flies displayed severe locomotion and viability defects. To test the role of MT sliding further, we engineered a chimeric motor that actively slides MTs but cannot transport organelles. Activation of MT sliding in Khc(mutA) neurons using this chimeric motor rescued axon outgrowth in cultured neurons and in vivo, firmly establishing the role of sliding in axon outgrowth. These results demonstrate that MT sliding by kinesin-1 is an essential biological phenomenon required for neuronal morphogenesis and normal nervous system development.

Mitochondrial traffic jams in Alzheimer's disease - pinpointing the roadblocks

The vigorous axonal transport of mitochondria, which serves to distribute these organelles in a dynamic and non-uniform fashion, is crucial to fulfill neuronal energetic requirements allowing the maintenance of neurons structure and function. Particularly, axonal transport of mitochondria and their spatial distribution among the synapses are directly correlated with synaptic activity and integrity. Despite the basis of Alzheimer's disease (AD) remains enigmatic, axonal pathology and synaptic dysfunction occur prior the occurrence of amyloid-β (Aβ) deposition and tau aggregation, the two classical hallmarks of this devastating neurodegenerative disease. Importantly, the early stages of AD are marked by defects on axonal transport of mitochondria as denoted by the abnormal accumulation of mitochondria within large swellings along dystrophic and degenerating neuritis. Within this scenario, this review is devoted to identify the molecular "roadblocks" underlying the abnormal axonal transport of mitochondria and consequent synaptic "starvation" and neuronal degeneration in AD. Understanding the molecular nature of defective mitochondrial transport may provide a new avenue to counteract AD pathology.

A novel mutation in motor domain of KIF5A associated with an HSP/axonal neuropathy phenotype

SPG10 is an autosomal dominant hereditary spastic paraplegia (HSP) caused by mutations in the gene KIF5A encoding the heavy chain of kinesin, a motor protein implied in motility functions within cells. Most of the KIF5A mutations are clustered in 2 areas of motor domain of the protein, the switch regions I and II, that are necessary for microtubules interaction. The set of mutations in KIF5A described so far account for a spectrum of clinical heterogeneity ranging from pure HSP to isolated peripheral nerve involvement (Charcot-Marie-Tooth phenotype) or complicated HSP phenotypes. We here describe a patient presenting with progressive walking difficulties and burning dysesthesias, numbness, and pain at distal segments of the 4 limbs. Neurological examination revealed severe spastic gait and vibratory and proprioception sensory reduction in the lower limbs. Motor and sensory nerve conduction studies disclosed axonal damage of peripheral nerves at lower limbs. We identified the novel variant c.967C>T in the KIF5A gene resulting in the R323W change, which is located at the C-terminus of the motor domain of the KIF5A protein, just upstream the linker region but out of the switch regions. Our findings confirm that the "mixed" central-peripheral involvement is the most frequent clinical picture related to KIF5A motor domain mutations and that motor domain "in toto," even outside of the switch regions, is a hot spot for pathogenic mutations. We stress the concept that detection of a peripheral axonal neuropathy in an autosomal dominant HSP patient should be regarded as an important diagnostic tool and should guide clinicians to seek, first of all, KIF5A mutations.

Deletion of a kinesin I motor unmasks a mechanism of homeostatic branching control by neurotrophin-3

Development and function of highly polarized cells such as neurons depend on microtubule-associated intracellular transport, but little is known about contributions of specific molecular motors to the establishment of synaptic connections. In this study, we investigated the function of the Kinesin I heavy chain Kif5aa during retinotectal circuit formation in zebrafish. Targeted disruption of Kif5aa does not affect retinal ganglion cell differentiation, and retinal axons reach their topographically correct targets in the tectum, albeit with a delay. In vivo dynamic imaging showed that anterograde transport of mitochondria is impaired, as is synaptic transmission. Strikingly, disruption of presynaptic activity elicits upregulation of Neurotrophin-3 (Ntf3) in postsynaptic tectal cells. This in turn promotes exuberant branching of retinal axons by signaling through the TrkC receptor (Ntrk3). Thus, our study has uncovered an activity-dependent, retrograde signaling pathway that homeostatically controls axonal branching.

Delving into the complexity of hereditary spastic paraplegias: how unexpected phenotypes and inheritance modes are revolutionizing their nosology

Hereditary spastic paraplegias (HSP) are rare neurodegenerative diseases sharing the degeneration of the corticospinal tracts as the main pathological characteristic. They are considered one of the most heterogeneous neurological disorders. All modes of inheritance have been described for the 84 different loci and 67 known causative genes implicated up to now. Recent advances in molecular genetics have revealed clinico-genetic heterogeneity of these disorders including their clinical and genetic overlap with other diseases of the nervous system. The systematic analysis of a large set of genes, including exome sequencing, is unmasking unusual phenotypes or inheritance modes associated with mutations in HSP genes and related genes involved in various neurological diseases. A new nosology may emerge after integration and understanding of these new data to replace the current classification. Collectively, functions of the known genes implicate the disturbance of intracellular membrane dynamics and trafficking as the consequence of alterations of cytoskeletal dynamics, lipid metabolism and organelle structures, which represent in fact a relatively small number of cellular processes that could help to find common curative approaches, which are still lacking.

Regulation of mitochondrial transport in neurons

Mitochondria are cellular power plants that supply ATP to power various biological activities essential for neuronal growth, survival, and function. Due to unique morphological features, neurons face exceptional challenges to maintain ATP and Ca(2+) homeostasis. Neurons require specialized mechanisms distributing mitochondria to distal areas where energy and Ca(2+) buffering are in high demand, such as synapses and axonal branches. These distal compartments also undergo development- and activity-dependent remodeling, thereby altering mitochondrial trafficking and distribution. Mitochondria move bi-directionally, pause briefly, and move again, frequently changing direction. In mature neurons, only one-third of axonal mitochondria are motile. Stationary mitochondria serve as local energy sources and buffer intracellular Ca(2+). The balance between motile and stationary mitochondria responds quickly to changes in axonal and synaptic physiology. Furthermore, neurons are postmitotic cells surviving for the lifetime of the organism; thus, mitochondria need to be removed when they become aged or dysfunction. Mitochondria also alter their motility under stress conditions or when their integrity is impaired. Therefore, regulation of mitochondrial transport is essential to meet altered metabolic requirements and to remove aged and damaged mitochondria or replenish healthy ones to distal terminals. Defects in mitochondrial transport and altered distribution are implicated in the pathogenesis of several major neurological disorders. Thus, research into the mechanisms regulating mitochondrial motility is an important emerging frontier in neurobiology. This short review provides an updated overview on motor-adaptor machineries that drive and regulate mitochondrial transport and docking receptors that anchor axonal mitochondria in response to the changes in synaptic activity, metabolic requirement, and altered mitochondrial integrity. The review focuses on microtubule (MT)-based mitochondrial trafficking and anchoring. Additional insight from different perspectives can be found in other in-depth reviews.

Unique function of Kinesin Kif5A in localization of mitochondria in axons

Mutations in Kinesin proteins (Kifs) are linked to various neurological diseases, but the specific and redundant functions of the vertebrate Kifs are incompletely understood. For example, Kif5A, but not other Kinesin-1 heavy-chain family members, is implicated in Charcot-Marie-Tooth disease (CMT) and Hereditary Spastic Paraplegia (HSP), but the mechanism of its involvement in the progressive axonal degeneration characteristic of these diseases is not well understood. We report that zebrafish kif5Aa mutants exhibit hyperexcitability, peripheral polyneuropathy, and axonal degeneration reminiscent of CMT and HSP. Strikingly, although kif5 genes are thought to act largely redundantly in other contexts, and zebrafish peripheral neurons express five kif5 genes, kif5Aa mutant peripheral sensory axons lack mitochondria and degenerate. We show that this Kif5Aa-specific function is cell autonomous and is mediated by its C-terminal tail, as only Kif5Aa and chimeric motors containing the Kif5Aa C-tail can rescue deficits. Finally, concurrent loss of the kinesin-3, kif1b, or its adaptor kbp, exacerbates axonal degeneration via a nonmitochondrial cargo common to Kif5Aa. Our results shed light on Kinesin complexity and reveal determinants of specific Kif5A functions in mitochondrial transport, adaptor binding, and axonal maintenance.