SORL1 is a receptor for tau that promotes tau seeding.

Sortilin-related receptor 1 (SORL1) is an intracellular sorting receptor genetically implicated in Alzheimer's disease (AD) that impacts amyloid precursor protein trafficking. The objective of these studies was to test the hypothesis that SORL1 binds tau, modulates its cellular trafficking and impacts the aggregation of cytoplasmic tau induced by pathological forms of tau. Using surface plasmon resonance measurements, we observed high-affinity binding of tau to SORL1 and the vacuolar protein sorting 10 domain of SORL1. Interestingly, unlike LDL receptor-related protein 1, SORL1 binds tau at both pH 7.4 and pH 5.5, revealing its ability to bind tau at endosomal pH. Immunofluorescence studies confirmed that exogenously added tau colocalized with SORL1 in H4 neuroglioma cells, while overexpression of SORL1 in LDL receptor-related protein 1-deficient Chinese hamster ovary (CHO) cells resulted in a marked increase in the internalization of tau, indicating that SORL1 can bind and mediate the internalization of monomeric forms of tau. We further demonstrated that SORL1 mediates tau seeding when tau RD P301S FRET biosensor cells expressing SORL1 were incubated with high molecular weight forms of tau isolated from the brains of patients with AD. Seeding in H4 neuroglioma cells is significantly reduced when SORL1 is knocked down with siRNA. Finally, we demonstrate that the N1358S mutant of SORL1 significantly increases tau seeding when compared to WT SORL1, identifying for the first time a potential mechanism that connects this specific SORL1 mutation to Alzheimer's disease. Together, these studies identify SORL1 as a receptor that contributes to trafficking and seeding of pathogenic tau.

Modulation of intracortical circuits in primary motor cortex during automatic action tendencies.

Humans display automatic action tendencies toward emotional stimuli, showing faster automatic behavior (i.e., approaching a positive stimulus and avoiding a negative stimulus) than regulated behavior (i.e., avoiding a positive stimulus and approaching a negative stimulus). Previous studies have shown that the primary motor cortex is involved in the processing of automatic actions, with higher motor evoked potential amplitudes during automatic behavior elicited by single-pulse transcranial magnetic stimulation. However, it is unknown how intracortical circuits are involved with automatic action tendencies. Here, we measured short-interval intracortical inhibition and intracortical facilitation within the primary motor cortex by using paired-pulse transcranial magnetic stimulation protocols during a manikin task, which has been widely used to explore approaching and avoiding behavior. Results showed that intracortical facilitation was stronger during automatic behavior than during regulated behavior. Moreover, there was a significant negative correlation between reaction times and intracortical facilitation effect during automatic behavior: individuals with short reaction times had stronger faciliatory activity, as shown by higher intracortical facilitation. By contrast, no significant difference was found for short-interval intracortical inhibition between automatic behavior and regulated behavior. The results indicated that the intracortical facilitation circuit, mediated by excitatory glutamatergic neurons, in the primary motor cortex, plays an important role in mediating automatic action tendencies. This finding further supports the link between emotional perception and the action system.

Divergent Molecular Pathways for Toxicity of Selected Mutant C9ORF72-derived Dipeptide Repeats.

Expansion of a hexanucleotide repeat in a noncoding region of the C9ORF72 gene is responsible for a significant fraction of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) cases, but identifying specific toxic gene products and mechanisms has been difficult. Pathogenesis was proposed to involve the production of toxic RNA species and/or accumulation of toxic dipeptide repeats (DPRs), but distinguishing between these mechanisms has been challenging. In this study, we first use complementary model systems for analyzing pathogenesis in adult-onset neurodegenerative diseases to characterize the pathogenicity of DPRs produced by Repeat Associated Non-ATG (RAN) translation of C9ORF72 in specific cellular compartments: isolated axoplasm and giant synapse from the squid. Results showed selective axonal and presynaptic toxicity of GP-DPRs, independent of associated RNA. These effects involved downstream ASK1 signaling pathways that affect fast axonal transport and synaptic function, a pathogenic mechanism shared with other mutant proteins associated with familial ALS, like SOD1 and FUS. These pathways are sufficient to produce the "dying-back" axonopathy seen in ALS. However, other mutant genes (e.g., SOD1) that activate this mechanism rarely produce FTD. When parallel studies in primary motor neurons from rats were conducted, an additional pathogenic mechanism was revealed. The GR- and PR-DPRs, which had no effect on axonal transport or synaptic transmission, were found to disrupt the nuclei of transfected neurons, leading to "dying-forward" neuropathy. All C9-DRP-mediated toxic effects observed here are independent of whether the corresponding mRNAs contained hexanucleotide repeats or alternative codons. These studies establish the divergent toxicity of C9-DPRs that cause neurodegeneration in ALS and FTD, suggesting that these two independent pathogenic mechanisms may contribute to disease heterogeneity and/or synergize on disease progression in C9ORF72 patients with both ALS and FTD symptoms.

Axon-Autonomous Effects of the Amyloid Precursor Protein Intracellular Domain (AICD) on Kinase Signaling and Fast Axonal Transport.

The amyloid precursor protein (APP) is a key molecular component of Alzheimer's disease (AD) pathogenesis. Proteolytic APP processing generates various cleavage products, including extracellular amyloid beta (Aß) and the cytoplasmic APP intracellular domain (AICD). Although the role of AICD in the activation of kinase signaling pathways is well established in the context of full-length APP, little is known about intracellular effects of the AICD fragment, particularly within discrete neuronal compartments. Deficits in fast axonal transport (FAT) and axonopathy documented in AD-affected neurons prompted us to evaluate potential axon-autonomous effects of the AICD fragment for the first time. Vesicle motility assays using the isolated squid axoplasm preparation revealed inhibition of FAT by AICD. Biochemical experiments linked this effect to aberrant activation of selected axonal kinases and heightened phosphorylation of the anterograde motor protein conventional kinesin, consistent with precedents showing phosphorylation-dependent regulation of motors proteins powering FAT. Pharmacological inhibitors of these kinases alleviated the AICD inhibitory effect on FAT. Deletion experiments indicated this effect requires a sequence encompassing the NPTY motif in AICD and interacting axonal proteins containing a phosphotyrosine-binding domain. Collectively, these results provide a proof of principle for axon-specific effects of AICD, further suggesting a potential mechanistic framework linking alterations in APP processing, FAT deficits, and axonal pathology in AD.

Toxic effects of mutant huntingtin in axons are mediated by its proline-rich domain.

Huntington's disease (HD) results from expansion of a polyglutamine tract (polyQ) in mutant huntingtin (mHTT) protein, but mechanisms underlying polyQ expansion-mediated toxic gain-of-mHTT function remain elusive. Here, deletion and antibody-based experiments revealed that a proline-rich domain (PRD) adjacent to the polyQ tract is necessary for mutant huntingtin (mHTT) to inhibit fast axonal transport and promote axonal pathology in cultured mammalian neurons. Further, polypeptides corresponding to subregions of the PRD sufficed to elicit the toxic effect on fast axonal transport, which was mediated by JNK kinases and involved PRD binding to one or more SH3-domain containing proteins. Collectively, these data suggested a mechanism whereby polyQ tract expansion in mHTT promotes aberrant PRD exposure and interactions of this domain with SH3 domain-containing proteins including some involved in activation of JNK kinases. In support, biochemical and immunohistochemical experiments linked aberrant PRD exposure to increased JNK activation in striatal tissues of the zQ175 mouse model and from post-mortem HD patients. Collectively, these findings support a critical role of PRD on mHTT toxicity, suggesting a novel framework for the potential development of therapies aimed to halt or reduce axonal pathology in HD.

HUWE1 Amplifies Ubiquitin Modifications to Broadly Stimulate Clearance of Proteins and Aggregates.

Selective breakdown of proteins and aggregates is crucial for maintaining normal cellular activities and is involved in the pathogenesis of diverse diseases. How the cell recognizes and tags these targets in different structural states for degradation by the proteasome and autophagy pathways has not been well understood. Here, we discovered that a HECT-family ubiquitin ligase HUWE1 is broadly required for the efficient degradation of soluble factors and for the clearance of protein aggregates/condensates. Underlying this capacity of HUWE1 is a novel Ubiquitin-Directed ubiquitin Ligase (UDL) activity which recognizes both soluble substrates and aggregates that carry a high density of ubiquitin chains and rapidly expand the ubiquitin modifications on these targets. Ubiquitin signal amplification by HUWE1 recruits the ubiquitin-dependent segregase p97/VCP to process these targets for subsequent degradation or clearance. HUWE1 controls the cytotoxicity of protein aggregates, mediates Targeted Protein Degradation and regulates cell-cycle transitions with its UDL activity.

Approach-avoidance behavior and motor-specific modulation towards smoking-related cues in smokers.

AIMS: By performing three transcranial magnetic stimulation (TMS) experiments, we measured the motor-specific modulatory mechanisms in the primary motor cortex (M1) at both the intercortical and intracortical levels when smokers actively approach or avoid smoking-related cues. DESIGN, SETTING AND PARTICIPANTS: For all experiments, the design was group (smokers versus non-smokers) × action (approach versus avoidance) × image type (neutral versus smoking-related). The study was conducted at the Shanghai University of Sport, CHN, TMS Laboratory. For experiment 1, 30 non-smokers and 30 smokers; for experiment 2, 16 non-smokers and 16 smokers; for experiment 3, 16 non-smokers and 16 smokers. MEASUREMENTS: For all experiments, the reaction times were measured using the smoking stimulus-response compatibility task. While performing the task, single-pulse TMS was applied to the M1 in experiment 1 to measure the excitability of the corticospinal pathways, and paired-pulse TMS was applied to the M1 in experiments 2 and 3 to measure the activity of intracortical facilitation (ICF) and short-interval intracortical inhibition (SICI) circuits, respectively. FINDINGS: Smokers had faster responses when approaching smoking-related cues (F1,58 = 36.660, P < 0.001, η p 2 = 0.387), accompanied by higher excitability of the corticospinal pathways (F1,58 = 10.980, P = 0.002, η p 2 = 0.159) and ICF circuits (F1,30 = 22.187, P < 0.001, η p 2 = 0.425), while stronger SICI effects were observed when they avoided these cues (F1,30 = 10.672, P = 0.003, η p 2 = 0.262). CONCLUSIONS: Smokers appear to have shorter reaction times, higher motor-evoked potentials and stronger intracortical facilitation effects when performing approach responses to smoking-related cues and longer reaction times, a lower primary motor cortex descending pathway excitability and a stronger short-interval intracortical inhibition effect when avoiding them.

Involvement of the primary motor cortex in the early processing stage of the affective stimulus-response compatibility effect in a manikin task.

Compatible (positive approaching and negative avoiding) and incompatible (positive avoiding and negative approaching) behavior are of great significance for biological adaptation and survival. Previous research has found that reaction times of compatible behavior are shorter than the incompatible behavior, which is termed the stimulus-response compatibility (SRC) effect. However, the underlying neurophysiological mechanisms of the SRC effect applied to affective stimuli is still unclear. Here, we investigated preparatory activities in both the left and right primary motor cortex (M1) before the execution of an approaching-avoiding behavior using the right index finger in a manikin task based on self-identity. The results showed significantly shorter reaction times for compatible than incompatible behavior. Most importantly, motor-evoked potential (MEP) amplitudes from left M1 stimulation were significantly higher during compatible behavior than incompatible behavior at 150 and 200 ms after stimulus presentation, whereas the reversed was observed for right M1 stimulation with lower MEP amplitude in compatible compared to incompatible behavior at 150 ms. The current findings revealed the compatibility effect at both behavioral and neurophysiological levels, indicating that the affective SRC effect occurs early in the motor cortices during stimulus processing, and MEP modulation at this early processing stage could be a physiological marker of the affective SRC effect.

Structure, Folding and Stability of Nucleoside Diphosphate Kinases.

Nucleoside diphosphate kinases (NDPK) are oligomeric proteins involved in the synthesis of nucleoside triphosphates. Their tridimensional structure has been solved by X-ray crystallography and shows that individual subunits present a conserved ferredoxin fold of about 140 residues in prokaryotes, archaea, eukaryotes and viruses. Monomers are functionally independent from each other inside NDPK complexes and the nucleoside kinase catalytic mechanism involves transient phosphorylation of the conserved catalytic histidine. To be active, monomers must assemble into conserved head to tail dimers, which further assemble into hexamers or tetramers. The interfaces between these oligomeric states are very different but, surprisingly, the assembly structure barely affects the catalytic efficiency of the enzyme. While it has been shown that assembly into hexamers induces full formation of the catalytic site and stabilizes the complex, it is unclear why assembly into tetramers is required for function. Several additional activities have been revealed for NDPK, especially in metastasis spreading, cytoskeleton dynamics, DNA binding and membrane remodeling. However, we still lack the high resolution structural data of NDPK in complex with different partners, which is necessary for deciphering the mechanism of these diverse functions. In this review we discuss advances in the structure, folding and stability of NDPKs.

Synaptic Actions of Amyotrophic Lateral Sclerosis-Associated G85R-SOD1 in the Squid Giant Synapse.

Altered synaptic function is thought to play a role in many neurodegenerative diseases, but little is known about the underlying mechanisms for synaptic dysfunction. The squid giant synapse (SGS) is a classical model for studying synaptic electrophysiology and ultrastructure, as well as molecular mechanisms of neurotransmission. Here, we conduct a multidisciplinary study of synaptic actions of misfolded human G85R-SOD1 causing familial amyotrophic lateral sclerosis (ALS). G85R-SOD1, but not WT-SOD1, inhibited synaptic transmission, altered presynaptic ultrastructure, and reduced both the size of the readily releasable pool (RRP) of synaptic vesicles and mobility from the reserved pool (RP) to the RRP. Unexpectedly, intermittent high-frequency stimulation (iHFS) blocked inhibitory effects of G85R-SOD1 on synaptic transmission, suggesting aberrant Ca2+ signaling may underlie G85R-SOD1 toxicity. Ratiometric Ca2+ imaging showed significantly increased presynaptic Ca2+ induced by G85R-SOD1 that preceded synaptic dysfunction. Chelating Ca2+ using EGTA prevented synaptic inhibition by G85R-SOD1, confirming the role of aberrant Ca2+ in mediating G85R-SOD1 toxicity. These results extended earlier findings in mammalian motor neurons and advanced our understanding by providing possible molecular mechanisms and therapeutic targets for synaptic dysfunctions in ALS as well as a unique model for further studies.

A dynamic view of the proteomic landscape during differentiation of ReNcell VM cells, an immortalized human neural progenitor line.

The immortalized human ReNcell VM cell line represents a reproducible and easy-to-propagate cell culture system for studying the differentiation of neural progenitors. To better characterize the starting line and its subsequent differentiation, we assessed protein and phospho-protein levels and cell morphology over a 15-day period during which ReNcell progenitors differentiated into neurons, astrocytes and oligodendrocytes. Five of the resulting datasets measured protein levels or states of phosphorylation based on tandem-mass-tag (TMT) mass spectrometry and four datasets characterized cellular phenotypes using high-content microscopy. Proteomic analysis revealed reproducible changes in pathways responsible for cytoskeletal rearrangement, cell phase transitions, neuronal migration, glial differentiation, neurotrophic signalling and extracellular matrix regulation. Proteomic and imaging data revealed accelerated differentiation in cells treated with the poly-selective CDK and GSK3 inhibitor kenpaullone or the HMG-CoA reductase inhibitor mevastatin, both of which have previously been reported to promote neural differentiation. These data provide in-depth information on the ReNcell progenitor state and on neural differentiation in the presence and absence of drugs, setting the stage for functional studies.

The effect of Baduanjin qigong combined with CBT on physical fitness and psychological health of elderly housebound.

BACKGROUND: To investigate the effectiveness of Baduanjin qigong combined with cognitive-behavior therapy (CBT) on the physical fitness and psychological health of elderly housebound. MATERIALS AND METHODS: The 120 elderly housebound were randomly divided into 3 intervention groups: Baduanjin training, Baduanjin training combined with CBT, and CBT. The interventions were conducted by means of home visits over 6 months. Spirometry, SF-36 health survey of quality of life, and Lawton and Brody Instrumental Activities of Daily Living Scale (IADL) were used to collect physical health data, and self-evaluation of overall health status, self-evaluation of loneliness, and short-form geriatric depression scale (GDS-15) were used to collect mental health data at baseline, 3 months, and 6 months after intervention. Data was analyzed by repeated measures analysis of variance (rANOVA) and chi-squared test (χ test). RESULTS: Forced vital capacity (FVC), maximum voluntary ventilation (MVV), quality of life (QOL), and self-reported health status were significantly increased (P < .05) in the group receiving joint Baduanjin and CBT intervention at 3 months and 6 months, as compared to the Baduanjin only group or the CBT only group. Activities of daily living (ADL), self-evaluated loneliness, and level of depression were significantly lowered (P < .05) in the group receiving joint Baduanjin and CBT intervention at 3 months and 6 months, as compared to the Baduanjin only group or the CBT only group. CONCLUSIONS: Physical and psychological statuses of elderly housebound were significantly improved by Baduanjin training combined with CBT. The effect of the combined intervention exceeded that of CBT or Baduanjin alone.

Site occupancy calibration of taxane pharmacology in live cells and tissues.

Drug receptor site occupancy is a central pharmacology parameter that quantitatively relates the biochemistry of drug binding to the biology of drug action. Taxanes and epothilones bind to overlapping sites in microtubules (MTs) and stabilize them. They are used to treat cancer and are under investigation for neurodegeneration. In cells, they cause concentration-dependent inhibition of MT dynamics and perturbation of mitosis, but the degree of site occupancy required to trigger different effects has not been measured. We report a live cell assay for taxane-site occupancy, and relationships between site occupancy and biological effects across four drugs and two cell lines. By normalizing to site occupancy, we were able to quantitatively compare drug activities and cell sensitivities independent of differences in drug affinity and uptake/efflux kinetics. Across all drugs and cells tested, we found that inhibition of MT dynamics, postmitotic micronucleation, and mitotic arrest required successively higher site occupancy. We also found interesting differences between cells and drugs, for example, insensitivity of the spindle assembly checkpoint to site occupancy. By extending our assay to a mouse xenograft tumor model, we estimated the initial site occupancy required for paclitaxel to completely prevent tumor growth as 80%. The most important cellular action of taxanes for cancer treatment may be formation of micronuclei, which occurs over a broad range of site occupancies.

Copper-Binding Small Molecule Induces Oxidative Stress and Cell-Cycle Arrest in Glioblastoma-Patient-Derived Cells.

Transition metals are essential, but deregulation of their metabolism causes toxicity. Here, we report that the compound NSC319726 binds copper to induce oxidative stress and arrest glioblastoma-patient-derived cells at picomolar concentrations. Pharmacogenomic analysis suggested that NSC319726 and 65 other structural analogs exhibit lethality through metal binding. Although NSC319726 has been reported to function as a zinc ionophore, we report here that this compound binds to copper to arrest cell growth. We generated and validated pharmacogenomic predictions: copper toxicity was substantially inhibited by hypoxia, through an hypoxia-inducible-factor-1α-dependent pathway; copper-bound NSC319726 induced the generation of reactive oxygen species and depletion of deoxyribosyl purines, resulting in cell-cycle arrest. These results suggest that metal-induced DNA damage may be a consequence of exposure to some xenobiotics, therapeutic agents, as well as other causes of copper dysregulation, and reveal a potent mechanism for targeting glioblastomas.

Prion protein inhibits fast axonal transport through a mechanism involving casein kinase 2.

Prion diseases include a number of progressive neuropathies involving conformational changes in cellular prion protein (PrPc) that may be fatal sporadic, familial or infectious. Pathological evidence indicated that neurons affected in prion diseases follow a dying-back pattern of degeneration. However, specific cellular processes affected by PrPc that explain such a pattern have not yet been identified. Results from cell biological and pharmacological experiments in isolated squid axoplasm and primary cultured neurons reveal inhibition of fast axonal transport (FAT) as a novel toxic effect elicited by PrPc. Pharmacological, biochemical and cell biological experiments further indicate this toxic effect involves casein kinase 2 (CK2) activation, providing a molecular basis for the toxic effect of PrPc on FAT. CK2 was found to phosphorylate and inhibit light chain subunits of the major motor protein conventional kinesin. Collectively, these findings suggest CK2 as a novel therapeutic target to prevent the gradual loss of neuronal connectivity that characterizes prion diseases.

Mutant spastin proteins promote deficits in axonal transport through an isoform-specific mechanism involving casein kinase 2 activation.

Mutations of various genes cause hereditary spastic paraplegia (HSP), a neurological disease involving dying-back degeneration of upper motor neurons. From these, mutations in the SPAST gene encoding the microtubule-severing protein spastin account for most HSP cases. Cumulative genetic and experimental evidence suggests that alterations in various intracellular trafficking events, including fast axonal transport (FAT), may contribute to HSP pathogenesis. However, the mechanisms linking SPAST mutations to such deficits remain largely unknown. Experiments presented here using isolated squid axoplasm reveal inhibition of FAT as a common toxic effect elicited by spastin proteins with different HSP mutations, independent of microtubule-binding or severing activity. Mutant spastin proteins produce this toxic effect only when presented as the tissue-specific M1 isoform, not when presented as the ubiquitously-expressed shorter M87 isoform. Biochemical and pharmacological experiments further indicate that the toxic effects of mutant M1 spastins on FAT involve casein kinase 2 (CK2) activation. In mammalian cells, expression of mutant M1 spastins, but not their mutant M87 counterparts, promotes abnormalities in the distribution of intracellular organelles that are correctable by pharmacological CK2 inhibition. Collectively, these results demonstrate isoform-specific toxic effects of mutant M1 spastin on FAT, and identify CK2 as a critical mediator of these effects.

ALS-linked FUS exerts a gain of toxic function involving aberrant p38 MAPK activation.

Mutations in Fused in Sarcoma/Translocated in Liposarcoma (FUS) cause familial forms of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by progressive axonal degeneration mainly affecting motor neurons. Evidence from transgenic mouse models suggests mutant forms of FUS exert an unknown gain-of-toxic function in motor neurons, but mechanisms underlying this effect remain unknown. Towards this end, we studied the effect of wild type FUS (FUS WT) and three ALS-linked variants (G230C, R521G and R495X) on fast axonal transport (FAT), a cellular process critical for appropriate maintenance of axonal connectivity. All ALS-FUS variants impaired anterograde and retrograde FAT in squid axoplasm, whereas FUS WT had no effect. Misfolding of mutant FUS is implicated in this process, as the molecular chaperone Hsp110 mitigated these toxic effects. Interestingly, mutant FUS-induced impairment of FAT in squid axoplasm and of axonal outgrowth in mammalian primary motor neurons involved aberrant activation of the p38 MAPK pathway, as also reported for ALS-linked forms of Cu, Zn superoxide dismutase (SOD1). Accordingly, increased levels of active p38 MAPK were detected in post-mortem human ALS-FUS brain tissues. These data provide evidence for a novel gain-of-toxic function for ALS-linked FUS involving p38 MAPK activation.

Optical magnetic detection of single-neuron action potentials using quantum defects in diamond.

Magnetic fields from neuronal action potentials (APs) pass largely unperturbed through biological tissue, allowing magnetic measurements of AP dynamics to be performed extracellularly or even outside intact organisms. To date, however, magnetic techniques for sensing neuronal activity have either operated at the macroscale with coarse spatial and/or temporal resolution-e.g., magnetic resonance imaging methods and magnetoencephalography-or been restricted to biophysics studies of excised neurons probed with cryogenic or bulky detectors that do not provide single-neuron spatial resolution and are not scalable to functional networks or intact organisms. Here, we show that AP magnetic sensing can be realized with both single-neuron sensitivity and intact organism applicability using optically probed nitrogen-vacancy (NV) quantum defects in diamond, operated under ambient conditions and with the NV diamond sensor in close proximity (∼10 µm) to the biological sample. We demonstrate this method for excised single neurons from marine worm and squid, and then exterior to intact, optically opaque marine worms for extended periods and with no observed adverse effect on the animal. NV diamond magnetometry is noninvasive and label-free and does not cause photodamage. The method provides precise measurement of AP waveforms from individual neurons, as well as magnetic field correlates of the AP conduction velocity, and directly determines the AP propagation direction through the inherent sensitivity of NVs to the associated AP magnetic field vector.

HIV Glycoprotein Gp120 Impairs Fast Axonal Transport by Activating Tak1 Signaling Pathways.

Sensory neuropathies are the most common neurological complication of HIV. Of these, distal sensory polyneuropathy (DSP) is directly caused by HIV infection and characterized by length-dependent axonal degeneration of dorsal root ganglion (DRG) neurons. Mechanisms for axonal degeneration in DSP remain unclear, but recent experiments revealed that the HIV glycoprotein gp120 is internalized and localized within axons of DRG neurons. Based on these findings, we investigated whether intra-axonal gp120 might impair fast axonal transport (FAT), a cellular process critical for appropriate maintenance of the axonal compartment. Significantly, we found that gp120 severely impaired both anterograde and retrograde FAT. Providing a mechanistic basis for these effects, pharmacological experiments revealed an involvement of various phosphotransferases in this toxic effect, including members of mitogen-activated protein kinase pathways (Tak-1, p38, and c-Jun N-terminal Kinase (JNK)), inhibitor of kappa-B-kinase 2 (IKK2), and PP1. Biochemical experiments and axonal outgrowth assays in cell lines and primary cultures extended these findings. Impairments in neurite outgrowth in DRG neurons by gp120 were rescued using a Tak-1 inhibitor, implicating a Tak-1 mitogen-activated protein kinase pathway in gp120 neurotoxicity. Taken together, these observations indicate that kinase-based impairments in FAT represent a novel mechanism underlying gp120 neurotoxicity consistent with the dying-back degeneration seen in DSP. Targeting gp120-based impairments in FAT with specific kinase inhibitors might provide a novel therapeutic strategy to prevent axonal degeneration in DSP.