Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

Anterograde Axonal Transport in Neuronal Homeostasis and Disease

Neurons are highly polarized cells with an elongated axon that extends far away from the cell body. To maintain their homeostasis, neurons rely extensively on axonal transport of membranous organelles and other molecular complexes. Axonal transport allows for spatio-temporal activation and modulation of numerous molecular cascades, thus playing a central role in the establishment of neuronal polarity, axonal growth and stabilization, and synapses formation. Anterograde and retrograde axonal transport are supported by various molecular motors, such as kinesins and dynein, and a complex microtubule network. In this review article, we will primarily discuss the molecular mechanisms underlying anterograde axonal transport and its role in neuronal development and maturation, including the establishment of functional synaptic connections. We will then provide an overview of the molecular and cellular perturbations that affect axonal transport and are often associated with axonal degeneration. Lastly, we will relate our current understanding of the role of axonal trafficking concerning anterograde trafficking of mRNA and its involvement in the maintenance of the axonal compartment and disease.

The Rab5-Rab11 Endosomal Pathway is Required for BDNF-Induced CREB Transcriptional Regulation in Hippocampal Neurons

Brain-derived neurotrophic factor (BDNF) is a key regulator of the morphology and connectivity of central neurons. We have previously shown that BDNF/TrkB signaling regulates the activity and mobility of the GTPases Rab5 and Rab11, which in turn determine the postendocytic sorting of signaling TrkB receptors. Moreover, decreased Rab5 or Rab11 activity inhibits BDNF-induced dendritic branching. Whether Rab5 or Rab11 activity is important for local events only or for regulating nuclear signaling and gene expression is unknown. Here, we investigated, in rat hippocampal neuronal cultures derived from embryos of unknown sex, whether BDNF-induced signaling cascades are altered when early and recycling endosomes are disrupted by the expression of dominant-negative mutants of Rab5 and Rab11. The activity of both Rab5 and Rab11 was required for sustained activity of Erk1/2 and nuclear CREB phosphorylation, and increased transcription of a BDNF-dependent program of gene expression containing CRE binding sites, which includes activity-regulated genes such as Arc, Dusp1, c-fos, Egr1, and Egr2, and growth and survival genes such as Atf3 and Gem Based on our results, we propose that early and recycling endosomes provide a platform for the integration of neurotrophic signaling from the plasma membrane to the nucleus in neurons, and that this mechanism is likely to regulate neuronal plasticity and survival.SIGNIFICANCE STATEMENT BDNF is a neurotrophic factor that regulates plastic changes in the brain, including dendritic growth. The cellular and molecular mechanisms underlying this process are not completely understood. Our results uncover the cellular requirements that central neurons possess to integrate the plasma membrane into nuclear signaling in neurons. Our results indicate that the endosomal pathway is required for the signaling cascade initiated by BDNF and its receptors at the plasma membrane to modulate BDNF-dependent gene expression and neuronal dendritic growth mediated by the CREB transcription factor. CREB is a key transcription factor regulating circuit development and learning and memory.

Fluoxetine increases hippocampal neural survival by improving axonal transport in stress-induced model of depression male rats

INTRODUCTION

Axonal transport deficit is a key mechanism involved in neurodegenerative conditions. Fluoxetine, a commonly used antidepressant for treatment of depression, is known to regulate several important structural and neurochemical aspects of hippocampal functions. However, the mechanisms underlying these effects are still poorly understood. This study aimed to investigate the effects of chronic fluoxetine treatment on axonal transport in the hippocampus of rat stress-induced model of depression.

METHODS

We have analyzed the effects of chronic fluoxetine treatment (20 mg/kg/day, 24 days) on immobility behavior (forced swimming test), hippocampal iNOS (inflammatory factor) expression (RT-PCR) as well as hippocampal BDNF, kinesin and dynein expression (RT-PCR) and hippocampal neuronal survival (Nissl staining).

RESULTS

This study provided evidence that fluoxetine could effectively suppress iNOS expression following unpredictable chronic mild stress (P < 0.01), increase hippocampal BDNF (P < 0.01), kinesin (P < 0.05) and dynein (P < 0.01) gene expression, and control neuronal death in CA1 (P < 0.01) and CA3 regions (P < 0.01) of the hippocampus and thereby improve immobility behavior (P < 0.001).

CONCLUSION

Based on the findings of this study, we concluded the neuroprotective effect of fluoxetine may be due to its ability to improve axonal transmission, followed by increased energy supply and neurotrophin concentration and function.

The human motor cortex microcircuit: insights for neurodegenerative disease

The human motor cortex comprises a microcircuit of five interconnected layers with different cell types. In this Review, we use a layer-specific and cell-specific approach to integrate physiological accounts of this motor cortex microcircuit with the pathophysiology of neurodegenerative diseases affecting motor functions. In doing so we can begin to link motor microcircuit pathology to specific disease stages and clinical phenotypes. Based on microcircuit physiology, we can make future predictions of axonal loss and microcircuit dysfunction. With recent advances in high-resolution neuroimaging we can then test these predictions in humans in vivo, providing mechanistic insights into neurodegenerative disease.

Reduced TUBA1A Tubulin Causes Defects in Trafficking and Impaired Adult Motor Behavior

Newly born neurons express high levels of TUBA1A α-tubulin to assemble microtubules for neurite extension and to provide tracks for intracellular transport. In the adult brain, Tuba1a expression decreases dramatically. A mouse that harbors a loss-of-function mutation in the gene encoding TUBA1A (Tuba1a^ND/+) allows us to ask whether TUBA1A is important for the function of mature neurons. α-Tubulin levels are about half of wild type in juvenile Tuba1a^ND/+ brains, but are close to normal in older animals. In postnatal day (P)0 cultured neurons, reduced TUBA1A allows for assembly of less microtubules in axons resulting in more pausing during organelle trafficking. While Tuba1a^ND/+ mouse behavior is indistinguishable from wild-type siblings at weaning, Tuba1a^ND/+ mice develop adult-onset ataxia. Neurons important for motor function in Tuba1a^ND/+ remain indistinguishable from wild-type with respect to morphology and number and display no evidence of axon degeneration. Tuba1a^ND/+ neuromuscular junction (NMJ) synapses are the same size as wild-type before the onset of ataxia, but are reduced in size in older animals. Together, these data indicate that the TUBA1A-rich microtubule tracks that are assembled during development are essential for mature neuron function and maintenance of synapses over time.

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.

Omics Approach to Axonal Dysfunction of Motor Neurons in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an intractable adult-onset neurodegenerative disease that leads to the loss of upper and lower motor neurons (MNs). The long axons of MNs become damaged during the early stages of ALS. Genetic and pathological analyses of ALS patients have revealed dysfunction in the MN axon homeostasis. However, the molecular pathomechanism for the degeneration of axons in ALS has not been fully elucidated. This review provides an overview of the proposed axonal pathomechanisms in ALS, including those involving the neuronal cytoskeleton, cargo transport within axons, axonal energy supply, clearance of junk protein, neuromuscular junctions (NMJs), and aberrant axonal branching. To improve understanding of the global changes in axons, the review summarizes omics analyses of the axonal compartments of neurons in vitro and in vivo, including a motor nerve organoid approach that utilizes microfluidic devices developed by this research group. The review also discusses the relevance of intra-axonal transcription factors frequently identified in these omics analyses. Local axonal translation and the relationship among these pathomechanisms should be pursued further. The development of novel strategies to analyze axon fractions provides a new approach to establishing a detailed understanding of resilience of long MN and MN pathology in ALS.

Tubulin polyglutamylation is a general traffic-control mechanism in hippocampal neurons

Neurons are highly complex cells that heavily rely on intracellular transport to distribute a range of functionally essential cargoes within the cell. Post-translational modifications of tubulin are emerging as mechanisms for regulating microtubule functions, but their impact on neuronal transport is only marginally understood. Here, we have systematically studied the impact of post-translational polyglutamylation on axonal transport. In cultured hippocampal neurons, deletion of a single deglutamylase, CCP1 (also known as AGTPBP1), is sufficient to induce abnormal accumulation of polyglutamylation, i.e. hyperglutamylation. We next investigated how hyperglutamylation affects axonal transport of a range of functionally different neuronal cargoes: mitochondria, lysosomes, LAMP1 endosomes and BDNF vesicles. Strikingly, we found a reduced motility for all these cargoes, suggesting that polyglutamylation could act as a regulator of cargo transport in neurons. This, together with the recent discovery that hyperglutamylation induces neurodegeneration, makes it likely that perturbed neuronal trafficking could be one of the central molecular causes underlying this novel type of degeneration.This article has an associated First Person interview with the first author of the paper.

Knocking Out Multiple Genes in Cultured Primary Neurons to Study Tubulin Posttranslational Modifications

Microtubules, as integral part of the eukaryotic cytoskeleton, exert numerous essential functions in cells. A mechanism to control these diverse functions are the posttranslational modifications of tubulin. Despite being known for decades, relatively little insight into the cellular functions of these modifications has been gained so far. The discovery of tubulin-modifying enzymes and a growing number of available knockout mice now allow working with primary cells from those mouse models to address biological functions and molecular mechanisms behind those modifications. However, a number of those mouse models show either lethality or sterility, making it difficult to impossible to obtain a sufficient number of animals for a systematic study with primary cells. Moreover, many of those modifications are controlled by several redundant enzymes, and it is often necessary to knock out several enzymes in parallel to obtain a significant change in a given tubulin modification. Here we describe a method to generate primary cells with combinatorial knockout genotypes using conditional knockout mice. The conditional alleles are converted into knockout in the cultured primary cells by transduction with a lentivirus encoding cre-recombinase. This approach has allowed us to knock out the two main brain deglutamylases in mouse primary neurons, which leads to strongly increased polyglutamylation in these cells. Our method can be applied to measure different cellular processes, such as axonal transport, for which it can be combined with the expression of different fluorescent reporters to label intracellular proteins. Using a panel of conditional knockout mice, our method can further be applied to study the functions of a variety of tubulin modifications that require simultaneous knockout of multiple genes.

Measuring the Impact of Tubulin Posttranslational Modifications on Axonal Transport

Axonal transport is a process essential for neuronal function and survival that takes place on the cellular highways-the microtubules. It requires three major components: the microtubules that serve as tracks for the transport, the motor proteins that drive the movement, and the transported cargoes with their adaptor proteins. Axonal transport could be controlled by tubulin posttranslational modifications, which by decorating specific microtubule tracks could determine the specificity of cargo delivery inside neurons. However, it appears that the effects of tubulin modifications on transport can be rather subtle, and might thus be easily overlooked depending on which parameter of the transport process is analyzed. Here we propose an analysis paradigm that allows detecting rather subtle alterations in neuronal transport, as induced for instance by accumulation of posttranslational polyglutamylation. Analyzing mitochondria movements in axons, we found that neither the average speed nor the distance traveled were affected by hyperglutamylation, but we detected an about 50% reduction of the overall motility, suggesting that polyglutamylation controls the efficiency of mitochondria transport in axons. Our protocol can readily be expanded to the analysis of the impact of other tubulin modifications on the transport of a range of different neuronal cargoes.

Kinesin light chain-1 serine-460 phosphorylation is altered in Alzheimer's disease and regulates axonal transport and processing of the amyloid precursor protein

Damage to axonal transport is an early pathogenic event in Alzheimer’s disease. The amyloid precursor protein (APP) is a key axonal transport cargo since disruption to APP transport promotes amyloidogenic processing of APP. Moreover, altered APP processing itself disrupts axonal transport. The mechanisms that regulate axonal transport of APP are therefore directly relevant to Alzheimer’s disease pathogenesis. APP is transported anterogradely through axons on kinesin-1 motors and one route for this transport involves calsyntenin-1, a type-1 membrane spanning protein that acts as a direct ligand for kinesin-1 light chains (KLCs). Thus, loss of calsyntenin-1 disrupts APP axonal transport and promotes amyloidogenic processing of APP. Phosphorylation of KLC1 on serine-460 has been shown to reduce anterograde axonal transport of calsyntenin-1 by inhibiting the KLC1-calsyntenin-1 interaction. Here we demonstrate that in Alzheimer’s disease frontal cortex, KLC1 levels are reduced and the relative levels of KLC1 serine-460 phosphorylation are increased; these changes occur relatively early in the disease process. We also show that a KLC1 serine-460 phosphomimetic mutant inhibits axonal transport of APP in both mammalian neurons in culture and in Drosophila neurons in vivo. Finally, we demonstrate that expression of the KLC1 serine-460 phosphomimetic mutant promotes amyloidogenic processing of APP. Together, these results suggest that increased KLC1 serine-460 phosphorylation contributes to Alzheimer’s disease.

Dynamic structural determinants underlie the neurotoxicity of the N-terminal tau 26-44 peptide in Alzheimer's disease and other human tauopathies

The intrinsically disordered tau protein plays a pivotal role in the pathogenesis of Alzheimer's disease (AD) and other human tauopathies. Abnormal post-translational modifications of tau, such as truncation, are causally involved in the onset/development of these neurodegenerative diseases. In this context, the AD-relevant N-terminal fragment mapping between 26 and 44 amino acids of protein (tau26-44) is interesting, being endowed with potent neurotoxic effects in vitro and in vivo. However, the understanding of the mechanism(s) of tau26-44 toxicity is a challenging task because, similarly to the full-length tau, it does not have a unique 3D structure but exists as dynamic ensemble of conformations. Here we use Atomic Force Spectroscopy, Small Angle X-ray Scattering and Molecular Dynamics simulation to gather structural and functional information on the tau26-44. We highlight the presence, the type and the location of its temporary secondary structures and we unveil the occurrence of relevant transient tertiary conformations that could contribute to tau26-44 toxicity. Data are compared with those obtained on the biologically-inactive, reverse-sequence (tau44-26 peptide) which has the same mass, charge, aminoacidic composition as well as the same overall unfolded character of tau26-44.

Herpes Simplex Virus, Alzheimer's Disease and a Possible Role for Rab GTPases

Herpes simplex virus (HSV) is a common pathogen, infecting 85% of adults in the United States. After reaching the nucleus of the long-lived neuron, HSV may enter latency to persist throughout the life span. Re-activation of latent herpesviruses is associated with progressive cognitive impairment and Alzheimer's disease (AD). As an enveloped DNA virus, HSV exploits cellular membrane systems for its life cycle, and thereby comes in contact with the Rab family of GTPases, master regulators of intracellular membrane dynamics. Knock-down and overexpression of specific Rabs reduce HSV production. Disheveled membrane compartments could lead to AD because membrane sorting and trafficking are crucial for synaptic vesicle formation, neuronal survival signaling and Abeta production. Amyloid precursor protein (APP), a transmembrane glycoprotein, is the parent of Abeta, the major component of senile plaques in AD. Up-regulation of APP expression due to HSV is significant since excess APP interferes with Rab5 endocytic trafficking in neurons. Here, we show that purified PC12-cell endosomes transport both anterograde and retrograde when injected into the squid giant axon at rates similar to isolated HSV. Intracellular HSV co-fractionates with these endosomes, contains APP, Rab5 and TrkA, and displays a second membrane. HSV infected PC12 cells up-regulate APP expression. Whether interference with Rabs has a specific effect on HSV or indirectly affects membrane compartment dynamics co-opted by virus needs further study. Ultimately Rabs, their effectors or their membrane-binding partners may serve as handles to reduce the impact of viral re-activation on cognitive function, or even as more general-purpose anti-microbial therapies.

Protein Misfolding, Signaling Abnormalities and Altered Fast Axonal Transport: Implications for Alzheimer and Prion Diseases

Histopathological studies revealed that progressive neuropathies including Alzheimer, and Prion diseases among others, include accumulations of misfolded proteins intracellularly, extracellularly, or both. Experimental evidence suggests that among the accumulated misfolded proteins, small soluble oligomeric conformers represent the most neurotoxic species. Concomitant phenomena shared by different protein misfolding diseases includes alterations in phosphorylation-based signaling pathways synaptic dysfunction, and axonal pathology, but mechanisms linking these pathogenic features to aggregated neuropathogenic proteins remain unknown. Relevant to this issue, results from recent work revealed inhibition of fast axonal transport (AT) as a novel toxic effect elicited by oligomeric forms of amyloid beta and cellular prion protein PrP^C, signature pathological proteins associated with Alzheimer and Prion diseases, respectively. Interestingly, the toxic effect of these oligomers was fully prevented by pharmacological inhibitors of casein kinase 2 (CK2), a remarkable discovery with major implications for the development of pharmacological target-driven therapeutic intervention for Alzheimer and Prion diseases.

Dynactin1 depletion leads to neuromuscular synapse instability and functional abnormalities

BACKGROUND

Dynactin subunit 1 is the largest subunit of the dynactin complex, an activator of the molecular motor protein complex dynein. Reduced levels of DCTN1 mRNA and protein have been found in sporadic amyotrophic lateral sclerosis (ALS) patients, and mutations have been associated with disease, but the role of this protein in disease pathogenesis is still unknown.

METHODS

We characterized a Dynactin1a depletion model in the zebrafish embryo and combined in vivo molecular analysis of primary motor neuron development with live in vivo axonal transport assays in single cells to investigate ALS-related defects. To probe neuromuscular junction (NMJ) function and organization we performed paired motor neuron-muscle electrophysiological recordings and GCaMP calcium imaging in live, intact larvae, and the synapse structure was investigated by electron microscopy.

RESULTS

Here we show that Dynactin1a depletion is sufficient to induce defects in the development of spinal cord motor neurons and in the function of the NMJ. We observe synapse instability, impaired growth of primary motor neurons, and higher failure rates of action potentials at the NMJ. In addition, the embryos display locomotion defects consistent with NMJ dysfunction. Rescue of the observed phenotype by overexpression of wild-type human DCTN1-GFP indicates a cell-autonomous mechanism. Synaptic accumulation of DCTN1-GFP, as well as ultrastructural analysis of NMJ synapses exhibiting wider synaptic clefts, support a local role for Dynactin1a in synaptic function. Furthermore, live in vivo analysis of axonal transport and cytoskeleton dynamics in primary motor neurons show that the phenotype reported here is independent of modulation of these processes.

CONCLUSIONS

Our study reveals a novel role for Dynactin1 in ALS pathogenesis, where it acts cell-autonomously to promote motor neuron synapse stability independently of dynein-mediated axonal transport.

LMTK2 binds to kinesin light chains to mediate anterograde axonal transport of cdk5/p35 and LMTK2 levels are reduced in Alzheimer's disease brains

Cyclin dependent kinase-5 (cdk5)/p35 is a neuronal kinase that regulates key axonal and synaptic functions but the mechanisms by which it is transported to these locations are unknown. Lemur tyrosine kinase-2 (LMTK2) is a binding partner for p35 and here we show that LMTK2 also interacts with kinesin-1 light chains (KLC1/2). Binding to KLC1/2 involves a C-terminal tryptophan/aspartate (WD) motif in LMTK2 and the tetratricopeptide repeat (TPR) domains in KLC1/2, and this interaction facilitates axonal transport of LMTK2. Thus, siRNA loss of KLC1 or mutation of the WD motif disrupts axonal transport of LMTK2. We also show that LMTK2 facilitates the formation of a complex containing KLC1 and p35 and that siRNA loss of LMTK2 disrupts axonal transport of both p35 and cdk5. Finally, we show that LMTK2 levels are reduced in Alzheimer's disease brains. Damage to axonal transport and altered cdk5/p35 are pathogenic features of Alzheimer's disease. Thus, LMTK2 binds to KLC1 to direct axonal transport of p35 and its loss may contribute to Alzheimer's disease.

Association Between Axonopathy and Amyloid Plaques in the Spinal Cord of the Transgenic Mice of Alzheimer's Disease

Axonopathy manifested by axon swellings might constitute one of the earliest pathological features of Alzheimer's disease. It has been proposed that axonopathy might be associated with the origin of Aβ plaques. However, how axonopathy leads to Aβ plaque pathogenesis remains elusive. Our previous studies have shown that Aβ neuropathology (mainly diffuse plaques) selectively occurred in the regions of corticospinal tract (CST) pathway and its innervated region in the spinal cord of TgCRND8 mice. In this study, we investigated the occurrence and progression of axonopathy and the possible implication in Aβ plaque pathogenesis in the spinal cord of TgCRND8 mice. By anterograde labeling of CST system with a neuroanatomical tracer, we found that dilated corticospinal axons started to appear at 7 months, then exhibited an age-dependent increase. These abnormal structures appear before any plaque deposits are visible in the spinal cord of the mice. Importantly, they colocalized with Aβ plaques in either the white matter or gray matter of the spinal cord at later stages, suggesting that these axonal swellings might represent the initial stages of Aβ plaque formation, and could play a role in Aβ plaque pathogenesis. Furthermore, using ultrastructural analysis we demonstrated that intracellular contents in the axonal dystrophies such as various dense vesicles leaked out into the extracellular matrix under a condition of axon swelling rupture in CST pathways of spinal cord. This provided precise structural evidence that how the Aβ leaks out from the axonal dystrophies into extracellular matrix and how an axonal swelling might serve as a nidus of amyloid plaque formation.

Intracellular cargo transport by single-headed kinesin motors

Kinesin motor proteins that drive intracellular transport share an overall architecture of two motor domain-containing subunits that dimerize through a coiled-coil stalk. Dimerization allows kinesins to be processive motors, taking many steps along the microtubule track before detaching. However, whether dimerization is required for intracellular transport remains unknown. Here, we address this issue using a combination of in vitro and cellular assays to directly compare dimeric motors across the kinesin-1, -2, and -3 families to their minimal monomeric forms. Surprisingly, we find that monomeric motors are able to work in teams to drive peroxisome dispersion in cells. However, peroxisome transport requires minimal force output, and we find that most monomeric motors are unable to disperse the Golgi complex, a high-load cargo. Strikingly, monomeric versions of the kinesin-2 family motors KIF3A and KIF3B are able to drive Golgi dispersion in cells, and teams of monomeric KIF3B motors can generate over 8 pN of force in an optical trap. We find that intracellular transport and force output by monomeric motors, but not dimeric motors, are significantly decreased by the addition of longer and more flexible motor-to-cargo linkers. Together, these results suggest that dimerization of kinesin motors is not required for intracellular transport; however, it enables motor-to-motor coordination and high force generation regardless of motor-to-cargo distance. Dimerization of kinesin motors is thus critical for cellular events that require an ability to generate or withstand high forces.

Microtubules and Neurodegeneration: The Tubulin Code Sets the Rules of the Road

Two recent papers demonstrate that the 'tubulin code' - the pattern of chemical modifications of tubulin along a microtubule - is disrupted upon deletion or mutation of an enzyme, called CCP1, that removes one of these modifications. Ablation of CCP1 interferes with mitochondrial transport and causes human neurodegenerative disease, which may be amenable to pharmacological therapies.

Tau and Axonal Transport Misregulation in Tauopathies

Tau is a microtubule-associated protein that is involved in both normal and pathological processes in neurons. Since the discovery and characterization of tau over 40 years ago, our understanding of tau's normal functions and toxic roles in neurodegenerative tauopathies has continued to expand. Fast axonal transport is a critical process for maintaining axons and functioning synapses, critical subcellular compartments underlying neuronal connectivity. Signs of fast axonal transport disruption are pervasive in Alzheimer's disease and other tauopathies and various mechanisms have been proposed for regulation of fast axonal transport by tau. Post-translational modifications of tau including phosphorylation at specific sites, FTDP-17 point mutations, and oligomerization, confer upon tau a toxic effect on fast axonal transport. Consistent with the well-established dependence of axons on fast axonal transport, these disease-related modifications are closely associated temporally and spatially with axonal degeneration in the early disease stages. These factors position tau as a potentially critical factor mediating the disruption of fast axonal transport that precedes synaptic dysfunction and axonal degeneration at later disease stages. In this chapter, we review the evidence that tau affects fast axonal transport and examine several potential mechanisms proposed to underlie this toxicity.

Excessive tubulin polyglutamylation causes neurodegeneration and perturbs neuronal transport

Posttranslational modifications of tubulin are emerging regulators of microtubule functions. We have shown earlier that upregulated polyglutamylation is linked to rapid degeneration of Purkinje cells in mice with a mutation in the deglutamylating enzyme CCP1. How polyglutamylation leads to degeneration, whether it affects multiple neuron types, or which physiological processes it regulates in healthy neurons has remained unknown. Here, we demonstrate that excessive polyglutamylation induces neurodegeneration in a cell-autonomous manner and can occur in many parts of the central nervous system. Degeneration of selected neurons in CCP1-deficient mice can be fully rescued by simultaneous knockout of the counteracting polyglutamylase TTLL1. Excessive polyglutamylation reduces the efficiency of neuronal transport in cultured hippocampal neurons, suggesting that impaired cargo transport plays an important role in the observed degenerative phenotypes. We thus establish polyglutamylation as a cell-autonomous mechanism for neurodegeneration that might be therapeutically accessible through manipulation of the enzymes that control this posttranslational modification.

A stochastic model that explains axonal organelle pileups induced by a reduction of molecular motors

Nerve cells are critically dependent on the transport of intracellular cargoes, which are moved by motor proteins along microtubule tracks. Impairments in this movement are thought to explain the focal accumulations of axonal cargoes and axonal swellings observed in many neurodegenerative diseases. In some cases, these diseases are caused by mutations that impair motor protein function, and genetic depletion of functional molecular motors has been shown to lead to cargo accumulations in axons. The evolution of these accumulations has been compared to the formation of traffic jams on a highway, but this idea remains largely untested. In this paper, we investigated the underlying mechanism of local axonal cargo accumulation induced by a global reduction of functional molecular motors in axons. We hypothesized that (i) a reduction in motor number leads to a reduction in the number of active motors on each cargo which in turn leads to less persistent movement, more frequent stops and thus shorter runs; (ii) as cargoes stop more frequently, they impede the passage of other cargoes, leading to local 'traffic jams'; and (iii) collisions between moving and stopping cargoes can push stopping cargoes further away from their microtubule tracks, preventing them from reattaching and leading to the evolution of local cargo accumulations. We used a lattice-based stochastic model to test whether this mechanism can lead to the cargo accumulation patterns observed in experiments. Simulation results of the model support the hypothesis and identify key questions that must be tested experimentally.

Fluid-based proteomics targeted on pathophysiological processes and pathologies in neurodegenerative diseases

Neurodegenerative dementias constitute a broad group of diseases in which abnormally folded proteins accumulate in specific brain regions and result in tissue reactions that eventually cause neuronal dysfunction and degeneration. Depending on where in the brain this happens, symptoms appear which may be used to classify the disorders on clinical grounds. However, brain changes in neurodegenerative dementias start to accumulate many years prior to symptom onset and there is a poor correlation between the clinical picture and what pathology that is the most likely to cause it. Thus, novel drug candidates having disease-modifying effects that is targeting the underlying pathology and changes the course of the disease needs to be defined using objective biomarker-based measures since the clinical symptoms are often non-specific and overlap between different disorders. Furthermore, the treatment should ideally be initiated as soon as symptoms are evident or when biomarkers confirm an underlying pathology (pre-clinical phase of the disease) to reduce irreversible damage to, for example, neurons, synapses and axons. Clinical trials in the pre-clinical phase bring a greater importance to biomarkers since by definition the clinical effects are difficult or slow to discern in a population that is not yet clinically affected. Here, we discuss neuropathological changes that may underlie neurodegenerative dementias, including how they can be detected and quantified using currently available biofluid-based biomarkers and how more of them could be identified using targeted proteomics approaches. This article is part of the special issue "Proteomics".

VGF nerve growth factor inducible is involved in retinal ganglion cells death induced by optic nerve crush

VGF nerve growth factor inducible (VGF) is a polypeptide that is induced by neurotrophic factors and is involved in neurite growth and neuroprotection. The mRNA of the Vgf gene has been detected in the adult rat retina, however the roles played by VGF in the retina are still undetermined. Thus, the purpose of this study was to determine the effects of VGF on the retinal ganglion cells (RGCs) of mice in the optic nerve crush (ONC) model, rat-derived primary cultured RGCs and human induced pluripotent stem cells (iPSCs)-derived RGCs. The mRNA and protein of Vgf were upregulated after the ONC. Immunostaining showed that the VGF was located in glial cells including Müller glia and astrocytes but not in the retinal neurons and their axons. AQEE-30, a VGF peptide, suppressed the loss of RGCs induced by the ONC, and it increased survival rat-derived RGCs and promoted the outgrowth of neurites of rat and human iPSCs derived RGCs in vitro. These findings indicate that VGF plays important roles in neuronal degeneration and has protective effects against the ONC on RGCs. Thus, VGF should be considered as a treatment of RGCs degeneration.

In vivo evidence for reduced ion channel expression in motor axons of patients with amyotrophic lateral sclerosis

KEY POINTS

The progressive loss of motor units in amyotrophic lateral sclerosis (ALS) is initially compensated for by the reinnervation of denervated muscle fibres by surviving motor axons. A disruption in protein homeostasis is thought to play a critical role in the pathogenesis of ALS. The changes in surviving motor neurons were studied by comparing the nerve excitability properties of moderately and severely affected single motor axons from patients with ALS with those from single motor axons in control subjects. A mathematical model indicated that approximately 99% of the differences between the ALS and control units could be explained by a non-selective reduction in the expression of all ion channels. These changes in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron.

ABSTRACT

Amyotrophic lateral sclerosis (ALS) is characterised by a progressive loss of motor units and the reinnervation of denervated muscle fibres by surviving motor axons. This reinnervation preserves muscle function until symptom onset, when some 60-80% of motor units have been lost. We have studied the changes in surviving motor neurons by comparing the nerve excitability properties of 31 single motor axons from patients with ALS with those from 21 single motor axons in control subjects. ALS motor axons were classified as coming from moderately or severely affected muscles according to the compound muscle action potential amplitude of the parent muscle. Compared with control units, thresholds were increased, and there was reduced inward and outward rectification and greater superexcitability following a conditioning impulse. These abnormalities were greater in axons from severely affected muscles, and were correlated with loss of fine motor skills. A mathematical model indicated that 99.1% of the differences between the moderately affected ALS and control units could be explained by a reduction in the expression of all ion channels. For the severely affected units, modelling required, in addition, an increase in the current leak through and under the myelin sheath. This might be expected if the anchoring proteins responsible for the paranodal seal were reduced. We conclude that changes in axonal excitability identified in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron, a conclusion consistent with recent animal and in vitro human data.

Mast cells and neutrophils mediate peripheral motor pathway degeneration in ALS

Neuroinflammation is a recognized pathogenic mechanism underlying motor neuron degeneration in amyotrophic lateral sclerosis (ALS), but the inflammatory mechanisms influencing peripheral motor axon degeneration remain largely unknown. A recent report showed a pathogenic role for c-Kit-expressing mast cells mediating inflammation and neuromuscular junction denervation in muscles from SOD1G93A rats. Here, we have explored whether mast cells infiltrate skeletal muscles in autopsied muscles from ALS patients. We report that degranulating mast cells were abundant in the quadriceps muscles from ALS subjects but not in controls. Mast cells were associated with myofibers and motor endplates and, remarkably, interacted with neutrophils forming large extracellular traps. Mast cells and neutrophils were also abundant around motor axons in the extensor digitorum longus muscle, sciatic nerve, and ventral roots of symptomatic SOD1G93A rats, indicating that immune cell infiltration extends along the entire peripheral motor pathway. Postparalysis treatment of SOD1G93A rats with the tyrosine kinase inhibitor drug masitinib prevented mast cell and neutrophil infiltration, axonal pathology, secondary demyelination, and the loss of type 2B myofibers, compared with vehicle-treated rats. These findings provide further evidence for a yet unrecognized contribution of immune cells in peripheral motor pathway degeneration that can be therapeutically targeted by tyrosine kinase inhibitors.

The many disguises of the signalling endosome

Neurons are highly complex and polarised cells that must overcome a series of logistic challenges to maintain homeostasis across their morphological domains. A very clear example is the propagation of neurotrophic signalling from distal axons, where target-released neurotrophins bind to their receptors and initiate signalling, towards the cell body, where nuclear and cytosolic responses are integrated. The mechanisms of propagation of neurotrophic signalling have been extensively studied and, eventually, the model of a 'signalling endosome', transporting activated receptors and associated complexes, has emerged. Nevertheless, the exact nature of this organelle remains elusive. In this Review, we examine the evidence for the retrograde transport of neurotrophins and their receptors in endosomes, outline some of their diverse physiological and pathological roles, and discuss the main interactors, morphological features and trafficking destinations of a highly flexible endosomal signalling organelle with multiple molecular signatures.

Mechanisms of protein toxicity in neurodegenerative diseases

Protein toxicity can be defined as all the pathological changes that ensue from accumulation, mis-localization, and/or multimerization of disease-specific proteins. Most neurodegenerative diseases manifest protein toxicity as one of their key pathogenic mechanisms, the details of which remain unclear. By systematically deconstructing the nature of toxic proteins, we aim to elucidate and illuminate some of the key mechanisms of protein toxicity from which therapeutic insights may be drawn. In this review, we focus specifically on protein toxicity from the point of view of various cellular compartments such as the nucleus and the mitochondria. We also discuss the cell-to-cell propagation of toxic disease proteins that complicates the mechanistic understanding of the disease progression as well as the spatiotemporal point at which to therapeutically intervene. Finally, we discuss selective neuronal vulnerability, which still remains largely enigmatic.

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

Chlorpyrifos oxon promotes tubulin aggregation via isopeptide cross-linking between diethoxyphospho-Lys and Glu or Asp: Implications for neurotoxicity

Exposure to organophosphorus toxicants (OP) can have chronic adverse effects that are not explained by inhibition of acetylcholinesterase, the cause of acute OP toxicity. We therefore hypothesized that OP-induced chronic illness is initiated by the formation of organophosphorus adducts on lysine residues in proteins, followed by protein cross-linking and aggregation. Here, Western blots revealed that exposure to the OP chlorpyrifos oxon converted porcine tubulin from its original 55-kDa mass to high-molecular-weight aggregates. Liquid chromatography–tandem MS analysis of trypsin-digested samples identified several diethoxyphospho-lysine residues in the OP-treated tubulin. Using a search approach based on the Batch Tag program, we identified cross-linked peptides and found that these chemically activated lysines reacted with acidic amino acid residues creating γ-glutamyl-ϵ-lysine or aspartyl-ϵ-lysine isopeptide bonds between β- and α-tubulin. Of note, these cross-linked tubulin molecules accounted for the high-molecular-weight aggregates. To the best of our knowledge, this is the first report indicating that chlorpyrifos oxon–exposed tubulin protein forms intermolecular cross-links with other tubulin molecules, resulting in high-molecular-weight protein aggregates. It is tempting to speculate that chronic illness from OP exposure may be explained by a mechanism that starts with OP adduct formation on protein lysines followed by protein cross-linking. We further speculate that OP-modified or cross-linked tubulin can impair axonal transport, reduce neuron connections, and result in neurotoxicity.

Inhibiting p38 MAPK alpha rescues axonal retrograde transport defects in a mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by the degeneration of upper and lower motor neurons. Defects in axonal transport have been observed pre-symptomatically in the SOD1^G93A mouse model of ALS, and have been proposed to play a role in motor neuron degeneration as well as in other pathologies of the nervous system, such as Alzheimer's disease and hereditary neuropathies. In this study, we screen a library of small-molecule kinase inhibitors towards the identification of pharmacological enhancers of the axonal retrograde transport of signalling endosomes, which might be used to normalise the rate of this process in diseased neurons. Inhibitors of p38 mitogen-activated protein kinases (p38 MAPK) were identified in this screen and were found to correct deficits in axonal retrograde transport of signalling endosomes in cultured primary SOD1^G93A motor neurons. In vitro knockdown experiments revealed that the alpha isoform of p38 MAPK (p38 MAPKα) was the sole isoform responsible for SOD1^G93A-induced transport deficits. Furthermore, we found that acute treatment with p38 MAPKα inhibitors restored the physiological rate of axonal retrograde transport in vivo in early symptomatic SOD1^G93A mice. Our findings demonstrate the pathogenic effect of p38 MAPKα on axonal retrograde transport and identify a potential therapeutic strategy for ALS.

Life and Work of Stress Granules and Processing Bodies: New Insights into Their Formation and Function

The dynamic formation of stress granules (SGs), processing bodies (PBs), and related RNA organelles regulates diverse cellular processes, including the coordination of functionally connected messengers, the translational regulation at the synapse, and the control of viruses and retrotransposons. Recent studies have shown that pyruvate kinase and other enzymes localize in SGs and PBs, where they become protected from stress insults. These observations may have implications for enzyme regulation and metabolic control exerted by RNA-based organelles. The formation of these cellular bodies is governed by liquid-liquid phase separation (LLPS) processes, and it needs to be strictly controlled to prevent pathogenic aggregation. The intracellular concentration of key metabolites, such as ATP and sterol derivatives, may influence protein solubility, thus affecting the dynamics of liquid organelles. LLPS in vitro depends on the thermal diffusion of macromolecules, which is limited inside cells, where the condensation and dissolution of membrane-less organelles are helped by energy-driven processes. The active transport by the retrograde motor dynein helps SG assembly, whereas the anterograde motor kinesin mediates SG dissolution; a tug of war between these two molecular motors allows transient SG formation. There is evidence that the efficiency of dynein-mediated transport increases with the number of motor molecules associated with the cargo. The dynein-dependent transport may be influenced by cargo size as larger cargos can load a larger number of motors. We propose a model based on this emergent property of dynein motors, which would be collectively stronger during SG condensation and weaker during SG breakdown, thus allowing kinesin-mediated dispersion.

Methods for Quantitative Analysis of Axonal Cargo Transport

Neurons rely on complex axonal transport mechanisms that mediate the intracellular dynamics of proteins, vesicles, and mitochondria along their high polarized structure. The fast improvement of live imaging techniques of fluorescent cargos allowed the identification of the diverse motion properties of different transported molecules. These properties arise as the result of molecular interactions between many players involved in axonal transport. Motor proteins, microtubule tracks, cargo association, and even axonal viscosity contribute to the proper axonal dynamics of different cargos. The unique properties in each cargo determine their distribution and location that is relevant to ensure neuronal cell activity and survival. This chapter provides a computational-based method for the generation of cargo trajectories and the identification of different motion regimes while cargo moves along axons. Then, the procedure to extract relevant parameters from active, diffusive, and confined motion is provided. These properties will allow a better comprehension of the nature and characteristics of cargo motion in living cells, therefore contributing to understanding the consequences of transport defects that arise during diseases of the nervous system.

Caenorhabditis elegans as a model system for target identification and drug screening against neurodegenerative diseases

Over the past decades, Caenorhabditis elegans (C. elegans) has been widely used as a model system because of its small size, transparent body, short generation time and lifespan (~3 days and 3 weeks, respectively), completely sequenced genome and tractability to genetic manipulation. Protein misfolding and aggregation are key pathological features in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease and Amyotrophic lateral sclerosis. Animal models, including C. elegans, have been extensively used to discover and validate new drugs against neurodegenerative diseases. The well-defined and genetically tractable nervous system of C. elegans offers an effective model to explore basic mechanistic pathways of neurodegenerative diseases. Recent progress in high-throughput drug screening also provides a powerful approach for identifying chemical modulators of biological processes. Here, we summarize the latest progress of using C. elegans as a model system for target identification and drug screening in neurodegenerative diseases.

Compartmentalized Signaling in Neurons: From Cell Biology to Neuroscience

Neurons are the largest known cells, with complex and highly polarized morphologies. As such, neuronal signaling is highly compartmentalized, requiring sophisticated transfer mechanisms to convey and integrate information within and between sub-neuronal compartments. Here, we survey different modes of compartmentalized signaling in neurons, highlighting examples wherein the fundamental cell biological processes of protein synthesis and degradation, membrane trafficking, and organelle transport are employed to enable the encoding and integration of information, locally and globally within a neuron. Comparisons to other cell types indicate that neurons accentuate widely shared mechanisms, providing invaluable models for the compartmentalization and transfer mechanisms required and used by most eukaryotic cells.

Phosphorylation of KLC1 modifies interaction with JIP1 and abolishes the enhanced fast velocity of APP transport by kinesin-1

ETOC: Phosphorylation of KLC1 at Thr466 in kinesin-1 regulates the interaction with APP mediated by JIP1b. Substitution of Glu for Thr466 abolished this interaction and impaired the enhanced fast velocity of APP anterograde transport. This phosphorylation of KLC1 increased in aged brains, suggesting deficient APP transport in neurons after aging.

In neurons, amyloid β-protein precursor (APP) is transported by binding to kinesin-1, mediated by JNK-interacting protein 1b (JIP1b), which generates the enhanced fast velocity (EFV) and efficient high frequency (EHF) of APP anterograde transport. Previously, we showed that EFV requires conventional interaction between the JIP1b C-terminal region and the kinesin light chain 1 (KLC1) tetratricopeptide repeat, whereas EHF requires a novel interaction between the central region of JIP1b and the coiled-coil domain of KLC1. We found that phosphorylatable Thr466 of KLC1 regulates the conventional interaction with JIP1b. Substitution of Glu for Thr466 abolished this interaction and EFV, but did not impair the novel interaction responsible for EHF. Phosphorylation of KLC1 at Thr466 increased in aged brains, and JIP1 binding to kinesin-1 decreased, suggesting that APP transport is impaired by aging. We conclude that phosphorylation of KLC1 at Thr466 regulates the velocity of transport of APP by kinesin-1 by modulating its interaction with JIP1b.

Axonal Degeneration in Tauopathies: Disease Relevance and Underlying Mechanisms

Tauopathies are a diverse group of diseases featuring progressive dying-back neurodegeneration of specific neuronal populations in association with accumulation of abnormal forms of the microtubule-associated protein tau. It is well-established that the clinical symptoms characteristic of tauopathies correlate with deficits in synaptic function and neuritic connectivity early in the course of disease, but mechanisms underlying these critical pathogenic events are not fully understood. Biochemical in vitro evidence fueled the widespread notion that microtubule stabilization represents tau's primary biological role and that the marked atrophy of neurites observed in tauopathies results from loss of microtubule stability. However, this notion contrasts with the mild phenotype associated with tau deletion. Instead, an analysis of cellular hallmarks common to different tauopathies, including aberrant patterns of protein phosphorylation and early degeneration of axons, suggests that alterations in kinase-based signaling pathways and deficits in axonal transport (AT) associated with such alterations contribute to the loss of neuronal connectivity triggered by pathogenic forms of tau. Here, we review a body of literature providing evidence that axonal pathology represents an early and common pathogenic event among human tauopathies. Observations of axonal degeneration in animal models of specific tauopathies are discussed and similarities to human disease highlighted. Finally, we discuss potential mechanistic pathways other than microtubule destabilization by which disease-related forms of tau may promote axonopathy.

The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein better known for its participation in the physiopathology of Alzheimer disease as the source of the beta amyloid fragment. However, the physiological functions of the full length protein and its proteolytic fragments have remained elusive. APP was first described as a cell-surface receptor; nevertheless, increasing evidence highlighted APP as a cell adhesion molecule. In this review, we will focus on the current knowledge of the physiological role of APP as a cell adhesion molecule and its involvement in key events of neuronal development, such as migration, neurite outgrowth, growth cone pathfinding, and synaptogenesis. Finally, since APP is over-expressed in Down syndrome individuals because of the extra copy of chromosome 21, in the last section of the review, we discuss the potential contribution of APP to the neuronal and synaptic defects described in this genetic condition. Read the Editorial Highlight for this article on page 9. Cover Image for this issue: doi. 10.1111/jnc.13817.