Impaired Mitochondrial Transport Research Articles

Mitochondrial DNA and Electron Transport Chain Protein Levels Are Altered in Peripheral Nerve Tissues from Donors with HIV Sensory Neuropathy: A Pilot Study.

Distal sensory polyneuropathy (DSP) and distal neuropathic pain (DNP) remain significant challenges for older people with HIV (PWH), necessitating enhanced clinical attention. HIV and certain antiretroviral therapies (ARTs) can compromise mitochondrial function and impact mitochondrial DNA (mtDNA) replication, which is linked to DSP in ART-treated PWH. This study investigated mtDNA, mitochondrial fission and fusion proteins, and mitochondrial electron transport chain protein changes in the dorsal root ganglions (DRGs) and sural nerves (SuNs) of 11 autopsied PWH. In antemortem standardized assessments, six had no or one sign of DSP, while five exhibited two or more DSP signs. Digital droplet polymerase chain reaction was used to measure mtDNA quantity and the common deletions in isolated DNA. We found lower mtDNA copy numbers in DSP+ donors. SuNs exhibited a higher proportion of mtDNA common deletion than DRGs in both groups. Mitochondrial electron transport chain (ETC) proteins were altered in the DRGs of DSP+ compared to DSP- donors, particularly Complex I. These findings suggest that reduced mtDNA quantity and increased common deletion abundance may contribute to DSP in PWH, indicating diminished mitochondrial activity in the sensory neurons. Accumulated ETC proteins in the DRG imply impaired mitochondrial transport to the sensory neuron's distal portion. Identifying molecules to safeguard mitochondrial integrity could aid in treating or preventing HIV-associated peripheral neuropathy.

Armcx1 Reduces Neurological Damage Via a Mitochondrial Transport Pathway Involving Miro1 After Traumatic Brain Injury

Armcx1 is a member of the ARMadillo repeat-Containing protein on the X chromosome (ARMCX) family, which is recognized to have evolutionary conserved roles in regulating mitochondrial transport and dynamics. Previous research has shown that Armcx1 is expressed at higher levels in mice after axotomy and in adult retinal ganglion cells after crush injury, and this protein increases neuronal survival and axonal regeneration. However, its role in traumatic brain injury (TBI) is unclear. Therefore, the aim of this study was to assess the expression of Armcx1 after TBI and to explore possible related mechanisms by which Armcx1 is involved in TBI. We used C57BL/6 male mice to model TBI and evaluated the role of Armcx1 in TBI by transfecting mice with Armcx1 small interfering RNA (siRNA) to inhibit Armcx1 expression 24 h before TBI modeling. Western blotting, immunofluorescence, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining, Nissl staining, transmission electron microscopy, adenosine triphosphate (ATP) level measurement, neuronal apoptosis analysis, neurological function scoring and the Morris water maze were performed. The results demonstrated that Armcx1 protein expression was elevated after TBI and that the Armcx1 protein was localized in neurons and astroglial cells in cortical tissue surrounding the injury site. In addition, inhibition of Armcx1 expression further led to impaired mitochondrial transport, abnormal morphology, reduced ATP levels, aggravation of neuronal apoptosis and neurological dysfunction, and decrease Miro1 expression. In conclusion, our findings indicate that Armcx1 may exert neuroprotective effects by ameliorating neurological injury after TBI through a mitochondrial transport pathway involving Miro1.

The Role of Impaired Mitochondrial Transport in the Development of Neurodegenerative Diseases.

The fight against neurodegenerative diseases is one of the key direction of modern medicine. Unfortunately, the difficulties in understanding the factors underlying the development of neurodegeneration hinder the development of breakthrough therapeutics that can stop or at least greatly slow down the progression of these diseases. In this review, it is considered the disruption of mitochondrial transport as one of the pathogenesis factors contributing to neurodegeneration using the examples of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Here, the mechanism of mitochondrial transport under normal conditions and the mechanisms of disturbances for the indicated diseases will be considered.

Miro1 provides neuroprotection via the mitochondrial trafficking pathway in a rat model of traumatic brain injury

The outer mitochondrial membrane protein mitochondrial Rho-GTPase 1 (Miro1) is known to be involved in the regulation of mitochondrial transport required for neuronal protection. Previous reports established that disruption of Miro1-dependent mitochondrial movement could result in nervous system diseases such as Parkinson’s disease and Alzheimer’s disease. This study was designed to explore the expression and mechanisms of Miro1 in secondary brain injury after traumatic brain injury (TBI). A total of 115 male Sprague Dawley rats were used in the weight-drop TBI rat model, and Miro1 in vivo knockdown was performed 24 h before TBI modeling by treatment with Miro1 short-interfering RNA. Real-time polymerase chain reaction, western blot, immunofluorescence, adenosine triphosphate (ATP) level assay, neuronal apoptosis, brain water content measurement, and neurological score analyses were carried out. Our results showed that the mRNA and protein levels of Miro1 were increased after TBI and co-localized with neurons and astrocytes in the peri-injury cortex. Moreover, Miro1 knockdown further exacerbated neuronal apoptosis, brain edema, and neurological deficits at 48 h after TBI, accompanied by impaired mitochondrial transport, reduction of mitochondria number and energy deficiency. Additionally, the apoptosis-related factors Bax upregulation and Bcl-2 downregulation as Miro1 knockdown after TBI implied that antiapoptotic effects on neuroprotection of Miro1, which were verified by the Fluoro-Jade C (FJC) staining and TUNEL staining. In conclusion, these findings suggest that Miro1 probably plays a neuroprotective role against secondary brain injury through the mitochondria trafficking pathway, suggesting that enhancing Miro1 might be a new strategy for the treatment of TBI.

MFN2 Deficiency Impairs Mitochondrial Transport and Downregulates Motor Protein Expression in Human Spinal Motor Neurons.

Charcot-Marie-Tooth (CMT) disease is one of the most common genetically inherited neurological disorders and CMT type 2A (CMT 2A) is caused by dominant mutations in the mitofusin-2 (MFN2) gene. MFN2 is located in the outer mitochondrial membrane and is a mediator of mitochondrial fusion, with an essential role in maintaining normal neuronal functions. Although loss of MFN2 induces axonal neuropathy, the detailed mechanism by which MFN2 deficiency results in axonal degeneration of human spinal motor neurons remains largely unknown. In this study, we generated MFN2-knockdown human embryonic stem cell (hESC) lines using lentivirus expressing MFN2 short hairpin RNA (shRNA). Using these hESC lines, we found that MFN2 loss did not affect spinal motor neuron differentiation from hESCs but resulted in mitochondrial fragmentation and dysfunction as determined by live-cell imaging. Notably, MFN2-knockodwn spinal motor neurons exhibited CMT2A disease-related phenotypes, including extensive perikaryal inclusions of phosphorylated neurofilament heavy chain (pNfH), frequent axonal swellings, and increased pNfH levels in long-term cultures. Importantly, MFN2 deficit impaired anterograde and retrograde mitochondrial transport within axons, and reduced the mRNA and protein levels of kinesin and dynein, indicating the interfered motor protein expression induced by MFN2 deficiency. Our results reveal that MFN2 knockdown induced axonal degeneration of spinal motor neurons and defects in mitochondrial morphology and function. The impaired mitochondrial transport in MFN2-knockdown spinal motor neurons is mediated, at least partially, by the altered motor proteins, providing potential therapeutic targets for rescuing axonal degeneration of spinal motor neurons in CMT2A disease.

Analyzing Mitochondrial Transport and Morphology in Human Induced Pluripotent Stem Cell-Derived Neurons in Hereditary Spastic Paraplegia.

Neurons have intense demands for high energy in order to support their functions. Impaired mitochondrial transport along axons has been observed in human neurons, which may contribute to neurodegeneration in various disease states. Although it is challenging to examine mitochondrial dynamics in live human nerves, such paradigms are critical for studying the role of mitochondria in neurodegeneration. Described here is a protocol for analyzing mitochondrial transport and mitochondrial morphology in forebrain neuron axons derived from human induced pluripotent stem cells (iPSCs). The iPSCs are differentiated into telencephalic glutamatergic neurons using well-established methods. Mitochondria of the neurons are stained with MitoTracker CMXRos, and mitochondrial movement within the axons are captured using a live-cell imaging microscope equipped with an incubator for cell culture. Time-lapse images are analyzed using software with "MultiKymograph", "Bioformat importer", and "Macros" plugins. Kymographs of mitochondrial transport are generated, and average mitochondrial velocity in the anterograde and retrograde directions is read from the kymograph. Regarding mitochondrial morphology analysis, mitochondrial length, area, and aspect ratio are obtained using the ImageJ. In summary, this protocol allows characterization of mitochondrial trafficking along axons and analysis of their morphology to facilitate studies of neurodegenerative diseases.

Protective Effects of ACY-1215 Against Chemotherapy-Related Cognitive Impairment and Brain Damage in Mice.

Chemotherapy-related cognitive impairment (CRCI) is a potential long-term side effect during cancer treatment. There are currently no effective treatments for CRCI. Reduction or inhibition of histone deacetylase 6 (HDAC6) has been considered a possible therapeutic strategy for cognitive deficits. HDAC6 inhibition recently has been shown to reverse chemotherapy-induced peripheral neuropathy effectively. In the present study, we examined the effect of HDAC6 inhibitor ACY-1215 (Ricolinostat) on cisplatin-induced brain damage and cognitive deficits in mice. Our results showed that ACY-1215 ameliorated behavioral deficits and dendritic spine loss and increased synaptic density in cisplatin-treated mice. Mechanistically, HDAC6 inhibitor ACY-1215 enhanced α-tubulin acetylation in the hippocampus of cisplatin-treated mice. Furthermore, ACY-1215 recovered cisplatin-induced impaired mitochondrial transport and mitochondrial dysfunction in the hippocampus. Our results suggest that inhibition of HDAC6 improves established cisplatin-induced cognitive deficits by the restoration of mitochondrial and synaptic impairments. These results offer prospective approaches for CRCI, especially because ACY1215 currently serves as an add-on cancer therapy during clinical trials.

Metabolic regulation and glucose sensitivity of cortical radial glial cells

The primary stem cells of the cerebral cortex are the radial glial cells (RGCs), and disturbances in their operation lead to myriad brain disorders in all mammals from mice to humans. Here, we found in mice that maternal gestational obesity and hyperglycemia can impair the maturation of RGC fibers and delay cortical neurogenesis. To investigate potential mechanisms, we used optogenetic live-imaging approaches in embryonic cortical slices. We found that Ca2+ signaling regulates mitochondrial transport and is crucial for metabolic support in RGC fibers. Cyclic intracellular Ca2+ discharge from localized RGC fiber segments detains passing mitochondria and ensures their proper distribution and enrichment at specific sites such as endfeet. Impairment of mitochondrial function caused an acute loss of Ca2+ signaling, while hyperglycemia decreased Ca2+ activity and impaired mitochondrial transport, leading to degradation of the RGC scaffold. Our findings uncover a physiological mechanism indicating pathways by which gestational metabolic disturbances can interfere with brain development.

Glutamatergic synaptic plasticity and dysfunction in Alzheimer disease: Emerging mechanisms.

Impaired synaptic plasticity and dendritic loss in excitatory glutamatergic synapses are early events in Alzheimer disease (AD). These synaptic abnormalities are triggered by accumulation of soluble fibrillary β-amyloid (Aβ) oligomers, which bind to several postsynaptic and presynaptic partners. Many of the synaptic effects of Aβ oligomers involve NMDA receptors (NMDARs) and type 1 metabotropic glutamate receptor 5 (mGluR5). These receptors mediate use-dependent plasticity of glutamatergic synapses in the hippocampus and other brain regions. Synaptic plasticity includes long-term potentiation (LTP) and long-term depression (LTD) of efficacy of synaptic transmission. Studies both in vitro and in animal models indicate that Aβ oligomers disrupt LTP and promote LTD; these effects are associated with decreased density of dendritic spines. The mechanisms by which Aβ oligomers elicit these synaptic changes are similar to those normally utilized in the CNS during development and learning. In addition, interactions of β oligomers with extrasynaptic NMDARs and mGluR5 promote tau-hyperphosphorylation, leading to impaired mitochondrial transport to synapses and thus disrupting calcium homeostasis and energy-dependent processes. Via activation of glial cells, Aβ triggers complement-mediated pruning of vulnerable synapses. Loss of dendritic spines precedes the accumulation of neuritic plaques and neurofibrillary tangles and is potentially reversible. The multiple potential mechanisms by which Aβ oligomers may trigger dysfunction and loss of glutamatergic synapses provide multiple targets for neuroprotective therapy. There are several reviews focused on these topics.1–11 Only selected aspects of synaptic plasticity and its involvement in AD are reviewed here.

Amyloid β oligomers elicit mitochondrial transport defects and fragmentation in a time-dependent and pathway-specific manner.

Small oligomeric forms of amyloid-β (Aβ) are believed to be the culprit for declined brain functions in AD in part through their impairment of neuronal trafficking and synaptic functions. However, the precise cellular actions of Aβ oligomers and underlying mechanisms in neurons remain to be fully defined. Previous studies have identified mitochondria as a major target of Aβ toxicity contributing to early cognitive decline and memory loss in neurodegenerative diseases including Alzheimer’s disease (AD). In this study, we report that Aβ oligomers acutely elicit distinct effects on the transport and integrity of mitochondria. We found that acute exposure of hippocampal neurons to Aβ oligomers from either synthetic peptides or AD brain homogenates selectively impaired fast transport of mitochondria without affecting the movement of late endosomes and lysosomes. Extended exposure of hipoocampal neurons to Aβ oligomers was found to result in mitochondrial fragmentation. While both mitochondrial effects induced by Aβ oligomers can be abolished by the inhibition of GSK3β, they appear to be independent from each other. Aβ oligomers impaired mitochondrial transport through HDAC6 activation whereas the fragmentation involved the GTPase Drp-1. These results show that Aβ oligomers can acutely disrupt mitochondrial transport and integrity in a time-dependent and pathway-specific manner. These findings thus provide new insights into Aβ-induced mitochondrial defects that may contribute to neuronal dysfunction and AD pathogenesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s13041-016-0261-z) contains supplementary material, which is available to authorized users.

Abnormal mitochondrial transport and morphology as early pathological changes in human models of spinal muscular atrophy.

ABSTRACTSpinal muscular atrophy (SMA), characterized by specific degeneration of spinal motor neurons, is caused by mutations in the survival of motor neuron 1, telomeric (SMN1) gene and subsequent decreased levels of functional SMN. How the deficiency of SMN, a ubiquitously expressed protein, leads to spinal motor neuron-specific degeneration in individuals affected by SMA remains unknown. In this study, we examined the role of SMN in mitochondrial axonal transport and morphology in human motor neurons by generating SMA type 1 patient-specific induced pluripotent stem cells (iPSCs) and differentiating these cells into spinal motor neurons. The initial specification of spinal motor neurons was not affected, but these SMA spinal motor neurons specifically degenerated following long-term culture. Moreover, at an early stage in SMA spinal motor neurons, but not in SMA forebrain neurons, the number of mitochondria, mitochondrial area and mitochondrial transport were significantly reduced in axons. Knocking down of SMN expression led to similar mitochondrial defects in spinal motor neurons derived from human embryonic stem cells, confirming that SMN deficiency results in impaired mitochondrial dynamics. Finally, the application of N-acetylcysteine (NAC) mitigated the impairment in mitochondrial transport and morphology and rescued motor neuron degeneration in SMA long-term cultures. Furthermore, NAC ameliorated the reduction in mitochondrial membrane potential in SMA spinal motor neurons, suggesting that NAC might rescue apoptosis and motor neuron degeneration by improving mitochondrial health. Overall, our data demonstrate that SMN deficiency results in abnormal mitochondrial transport and morphology and a subsequent reduction in mitochondrial health, which are implicated in the specific degeneration of spinal motor neurons in SMA.

Neuroprotective effects of 5-(4-hydroxy-3-dimethoxybenzylidene)-thiazolidinone in MPTP induced Parkinsonism model in mice

Parkinson's disease (PD) is a neurological disorder characterized by degeneration of nigrostriatal dopaminergic (DAergic) system. Present treatment targeting to DAergic system solely ameliorated the symptoms but failed to retard the DAergic neuron degeneration, therefore new therapeutic methods aiming at preventing or delaying the neurodegenerative process are urgently needed. In the present study, we found that 5-(4-hydroxy-3-dimethoxybenzylidene)-2-thioxo-4-thiazolidinone (RD-1), a compound derived from rhodanine, protected DAergicneurons from neurotoxicity of MPTP/MPP+. Firstly, RD-1 significantly improved the locomotor ability in the MPTP mice model, and elevated the tyrosine hydroxylase (TH) positive cell numbers in substantianigra pars compacta (SNpc) and the integrated optical density (IOD) of TH-positive nerve fibers in striatum respectively. Since mitochondrial dysfunction plays an important role in pathogenesis of PD, thereby we investigated the molecular mechanisms of RD-1 against MPTP/MPP+ neurotoxicity, focusing on its effects on the mitochondrial dysfunction. Immunoblotting analysis showed that RD-1 significantly elevated the Parkin and Miro2 expression levels in acute MPTP treated mice, and improved mitochondrial membrane potential and ATP synthesis in MPP+-treated Neuro-2a cells. Moreover, RD-1attenuated impaired mitochondrial transport and vesicle release dysfunction evoked by MPP+ cytotoxicity in cultured primary mesencephalic neurons. Taken together, these results indicate that improving the mitochondrial dysfunction may be a good choice to delay the neurodegenerative progression commonly associated with PD.

Loss of the m-AAA protease subunit AFG3L2 causes mitochondrial transport defects and tau hyperphosphorylation

The m-AAA protease subunit AFG₃L₂ is involved in degradation and processing of substrates in the inner mitochondrial membrane. Mutations in AFG₃L₂ are associated with spinocerebellar ataxia SCA28 in humans and impair axonal development and neuronal survival in mice. The loss of AFG₃L₂ causes fragmentation of the mitochondrial network. However, the pathogenic mechanism of neurodegeneration in the absence of AFG₃L₂ is still unclear. Here, we show that depletion of AFG₃L₂ leads to a specific defect of anterograde transport of mitochondria in murine cortical neurons. We observe similar transport deficiencies upon loss of AFG₃L₂ in OMA1-deficient neurons, indicating that they are not caused by OMA1-mediated degradation of the dynamin-like GTPase OPA1 and inhibition of mitochondrial fusion. Treatment of neurons with antioxidants, such as N-acetylcysteine or vitamin E, or decreasing tau levels in axons restored mitochondrial transport in AFG₃L₂-depleted neurons. Consistently, tau hyperphosphorylation and activation of ERK kinases are detected in mouse neurons postnatally deleted for Afg3l2. We propose that reactive oxygen species signaling leads to cytoskeletal modifications that impair mitochondrial transport in neurons lacking AFG₃L₂.

Embryonic Stem Cell–Based Modeling of Tau Pathology in Human Neurons

Alterations in the microtubule (MT)-associated protein, tau, have emerged as a pivotal phenomenon in several neurodegenerative disorders, including frontotemporal dementia and Alzheimer's disease. Although compelling lines of evidence from various experimental models suggest that hyperphosphorylation and conformational changes of tau can cause its aggregation into filaments, the actual tau species and effective mechanisms that conspire to trigger the degeneration of human neurons remain obscure. Herein, we explored whether human embryonic stem cell-derived neural stem cells can be exploited to study consequences of an overexpression of 2N4R tau (two normal N-terminal and four MT-binding domains; n-tau) versus pseudohyperphosphorylated tau (p-tau) directly in human neurons. Given the involvement of tau in MT integrity and cellular homeostasis, we focused on the effects of both tau variants on subcellular transport and neuronal survival. By using inducible lentiviral overexpression, we show that p-tau, but not n-tau, readily leads to an MC-1-positive protein conformation and impaired mitochondrial transport. Although these alterations do not induce cell death under standard culture conditions, p-tau-expressing neurons cultured under non-redox-protected conditions undergo degeneration with formation of axonal varicosities sequestering transported proteins and progressive neuronal cell death. Our data support a causative link between the phosphorylation and conformational state of tau, microtubuli-based transport, and the vulnerability of human neurons to oxidative stress. They further depict human embryonic stem cell-derived neurons as a useful experimental model for studying tau-associated cellular alterations in an authentic human system.

The rs10492972 KIF1B polymorphism and disease progression in Greek patients with multiple sclerosis

A recent genome-wide association study (GWAS) demonstrated an association of the rs10492972[C] variant of KIF1B with multiple sclerosis (MS) [1]. KIF1B encodes a kinesin superfamily member involved in mitochondrial axonal transport and may influence the ability of oligodendrocytes to remyelinate demyelinated axons [2, 3]. Impaired mitochondrial transport and chronic demyelination have been linked to axonal loss, which is important in the development of irreversible disability in MS [2, 4]. We therefore sought to investigate a possible influence of rs10492972 on disease progression in Greek MS patients. We studied 609 patients with MS [5]. Demographic and clinical data are shown in the table. Expanded disability status scale (EDSS) was measured in remission [6]. A total of 243 patients received disease-modifying therapy. MS severity score (MSSS) was determined in 562 patients (disease duration[1 year) [7]. The control group consisted of 230 healthy subjects (mean age 38.1 ± 10.7 years, 121 women). Patients and controls gave written informed consent for molecular genetic studies. DNA was isolated from peripheral blood using standard NaCl extraction. Genotyping of rs10492972 was performed with a standard PCR protocol (primers: 50 GGATACAAGGGGTGGTGA 30, 50 AAGTTATGTGGCCAGGA 30). MboII restriction of the 112-bp product resulted in Tand C-alleles (T-allele: 67 and 45-bp products; C-allele: 112-bp product). Statistical comparisons between patients and controls and between patients with different genotypes were performed using Chi-square, ANOVA, Mann–Whitney, and Kruskal–Wallis tests, as appropriate. Kaplan–Meier analysis (log-rank test) was used to assess the effect of genotypes on time to sustained disability. Analysis was performed with SPSS software. Genotype distribution in patients and controls was in Hardy–Weinberg equilibrium (p = 0.31 and 0.65). C-allele frequency was 0.31 in both patients and controls. There was no difference in allelic or genotype frequencies between patients and controls (p = 0.99 and 1.0). MSSS and time to EDSS 4.0 (Fig. 1) or 6.0 were not significantly

HDAC1 nuclear export induced by pathological conditions is essential for the onset of axonal damage

Histone deacetylase 1 (HDAC1) is a nuclear enzyme involved in transcriptional repression. We report here that cytosolic HDAC1 is detected in damaged axons in brains of human patients with Multiple Sclerosis and of mice with cuprizone-induced demyelination, ex vivo models of demyelination and in cultured neurons exposed to glutamate and TNF-α. Nuclear export of HDAC1 is mediated by the interaction with the nuclear receptor CRM-1 and leads to impaired mitochondrial transport. The formation of complexes between exported HDAC1 and members of the kinesin family of motor proteins hinders the interaction with cargo molecules thereby inhibiting mitochondrial movement and inducing localized beadings. This effect is prevented by inhibiting HDAC1 nuclear export with leptomycin B, treating neurons with pharmacological inhibitors of HDAC activity or silencing HDAC1 but not other HDAC isoforms. Together these data identify nuclear export of HDAC1 as a critical event for impaired mitochondrial transport in damaged neurons.

Drug Recalls

Alterations in the microtubule (MT)–associated protein, tau, have emerged as a pivotal phenomenon in several neurodegenerative disorders, including frontotemporal dementia and Alzheimer's disease. Although compelling lines of evidence from various experimental models suggest that hyperphosphorylation and conformational changes of tau can cause its aggregation into filaments, the actual tau species and effective mechanisms that conspire to trigger the degeneration of human neurons remain obscure. Herein, we explored whether human embryonic stem cell–derived neural stem cells can be exploited to study consequences of an overexpression of 2N4R tau (two normal N-terminal and four MT-binding domains; n-tau) versus pseudohyperphosphorylated tau (p-tau) directly in human neurons. Given the involvement of tau in MT integrity and cellular homeostasis, we focused on the effects of both tau variants on subcellular transport and neuronal survival. By using inducible lentiviral overexpression, we show that p-tau, but not n-tau, readily leads to an MC-1–positive protein conformation and impaired mitochondrial transport. Although these alterations do not induce cell death under standard culture conditions, p-tau–expressing neurons cultured under non–redox-protected conditions undergo degeneration with formation of axonal varicosities sequestering transported proteins and progressive neuronal cell death. Our data support a causative link between the phosphorylation and conformational state of tau, microtubuli-based transport, and the vulnerability of human neurons to oxidative stress. They further depict human embryonic stem cell–derived neurons as a useful experimental model for studying tau-associated cellular alterations in an authentic human system.