Parkinsonism and impaired axonal transport in a mouse model of frontotemporal dementia

Tau pathology induces intraneuronal cholesterol accumulation

Epidemiologic and experimental data suggest the involvement of cholesterol metabolism in the development and progression of Alzheimer disease and Niemann-Pick type C disease, but not of frontotemporal dementias. In these 3 neurodegenerative diseases, however, protein tau hyperphosphorylation and aggregation into neurofibrillary tangles are observed. To elucidate the relationship between cholesterol and tau, we compared sterol levels of neurons burdened with neurofibrillary tangles with those of their unaffected neighbors using semiquantitative filipin fluorescence microscopy in mice expressing P301L mutant human tau (a well-described model of FTDP-17) and in P301L transgenic mice lacking apolipoprotein E (the major cholesterol transporter in the brain). Cellular unesterified cholesterol was higher in neurons affected by tau pathology irrespective of apolipoprotein E deficiency. This argues for an impact of tau pathology on cellular cholesterol homeostasis. We suggest that there is a bidirectional mode of action: Disturbances in cellular cholesterol metabolism may promote tau pathology, but tau pathology may also alter neuronal cholesterol homeostasis; once it is established, a vicious cycle may promote neurofibrillary tangle formation.

Alpha-synuclein and tau: teammates in neurodegeneration?

The accumulation of α-synuclein aggregates is the hallmark of Parkinson’s disease, and more generally of synucleinopathies. The accumulation of tau aggregates however is classically found in the brains of patients with dementia, and this type of neuropathological feature specifically defines the tauopathies. Nevertheless, in numerous cases α-synuclein positive inclusions are also described in tauopathies and vice versa, suggesting a co-existence or crosstalk of these proteinopathies. Interestingly, α-synuclein and tau share striking common characteristics suggesting that they may work in concord. Tau and α-synuclein are both partially unfolded proteins that can form toxic oligomers and abnormal intracellular aggregates under pathological conditions. Furthermore, mutations in either are responsible for severe dominant familial neurodegeneration. Moreover, tau and α-synuclein appear to promote the fibrillization and solubility of each other in vitro and in vivo. This suggests that interactions between tau and α-synuclein form a deleterious feed-forward loop essential for the development and spreading of neurodegeneration. Here, we review the recent literature with respect to elucidating the possible links between α-synuclein and tau.

Connecting the dots between tau dysfunction and neurodegeneration

Tauopathies are devastating and ultimately fatal neurodegenerative diseases, which are histopathologically defined by insoluble filamentous deposits of abnormally phosphorylated tau protein within neurons and glia. Identifying the causes of abnormal tau phosphorylation and subsequent aggregation has been the focus of much research, and is currently a major target for the development of therapeutic interventions for tauopathies, including Alzheimer's disease (AD). Much has recently been learned about the sequence of events that lead from tau dysfunction to neuronal death. This review focuses on the cascade of events that are catalyzed by pathological tau, and highlights current and potential therapeutic strategies to target this pathway.

Neuronal vesicular trafficking and release in age-related cognitive impairment

Aging is a common major risk factor for many neurological disorders resulting in cognitive impairment and neurodegeneration including Parkinson's and Alzheimer's diseases. Novel results from the fields of molecular neuroscience and aging research provide evidence for a link between decline of various cognitive, executive functions and changes in neuronal mechanisms of intracellular trafficking and regulated vesicle release processes in the aging nervous system. In this Perspective, we review these recent findings and formulate a hypothesis on how cargo delivery to the synapses and the release of neurotrophic factors may be involved in maintaining learning and memory capabilities during healthy aging and present examples on how defects of those disrupt normal cognition. We provide an overview of emerging new concepts and approaches that will significantly advance our understanding of the aging brain and pathophysiology of dementia. This knowledge will be instrumental in defining drug targets and designing novel therapeutic strategies.

New Treatment for Alzheimer's Disease, Kamikihito, Reverses Amyloid-β-Induced Progression of Tau Phosphorylation and Axonal Atrophy

Aims. We previously reported that kamikihito (KKT), a traditional Japanese medicine, improved memory impairment and reversed the degeneration of axons in the 5XFAD mouse model of Alzheimer's disease (AD). However, the mechanism underlying the effects of KKT remained unknown. The aim of the present study was to investigate the mechanism by which KKT reverses the progression of axonal degeneration. Methods. Primary cultured cortical neurons were treated with amyloid beta (A β ) fragment comprising amino acid residues (25-35) (10  μ M) in an in vitro AD model. KKT (10  μ g/mL) was administered to the cells before or after A β treatment. The effects of KKT on A β -induced tau phosphorylation, axonal atrophy, and protein phosphatase 2A (PP2A) activity were investigated. We also performed an in vivo assay in which KKT (500 mg/kg/day) was administered to 5XFAD mice once a day for 15 days. Cerebral cortex homogenates were used to measure PP2A activity. Results. KKT improved A β -induced tau phosphorylation and axonal atrophy after they had already progressed. In addition, KKT increased PP2A activity in vitro and in vivo. Conclusions. KKT reversed the progression of A β -induced axonal degeneration. KKT reversed axonal degeneration at least in part through its role as an exogenous PP2A stimulator.

Tau pathology is present in vivo and develops in vitro in sensory neurons from human P301S tau transgenic mice: a system for screening drugs against tauopathies

Intracellular tau aggregates are the neuropathological hallmark of several neurodegenerative diseases, including Alzheimer's disease, progressive supranuclear palsy, and cases of frontotemporal dementia, but the link between these aggregates and neurodegeneration remains unclear. Neuronal models recapitulating the main features of tau pathology are necessary to investigate the molecular mechanisms of tau malfunction, but current models show little and inconsistent spontaneous tau aggregation. We show that dorsal root ganglion (DRG) neurons in transgenic mice expressing human P301S tau (P301S-htau) develop tau pathology similar to that found in brain and spinal cord and a significant reduction in mechanosensation occurs before detectable fibrillar tau formation. DRG neuronal cultures established from adult P301S-htau mice at different ages retained the pattern of aberrant tau found in vivo. Moreover, htau became progressively hyperphosphorylated over 2 months in vitro beginning with nonsymptomatic neurons, while hyperphosphorylated P301S-htau-positive neurons from 5-month-old mice cultured for 2 months died preferentially. P301S-htau-positive neurons grew aberrant axons, including spheroids, typically found in human tauopathies. Neurons cultured at advanced stages of tau pathology showed a 60% decrease in the fraction of moving mitochondria. SEG28019, a novel O-GlcNAcase inhibitor, reduced steady-state pSer396/pSer404 phosphorylation over 7 weeks in a significant proportion of DRG neurons showing for the first time the possible beneficial effect of prolonged dosing of O-GlcNAcase inhibitor in vitro. Our system is unique in that fibrillar tau forms without external manipulation and provides an important new tool for understanding the mechanisms of tau dysfunction and for screening of compounds for treatment of tauopathies.

Early-onset cognitive deficits and axonal transport dysfunction in P301S mutant tau transgenic mice

Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) are neurodegenerative "tauopathies" characterized by hyperphosphorylated tau accumulation and neurofibrillary tangles. The P301S mutation of tau, a causal mutation of a familial type of FTLD, is believed to be involved in neurodegenerative progression. We developed a transgenic mouse, named TPR50, harboring human P301S tau. Tau phosphorylation in the hippocampus of TPR50 mice increased with age, particularly at S202/T205. Insolubilization and intracellular accumulation of tau were detected in the hippocampus by 9 months of age. Expression of calbindin was significantly reduced in 6- and 9-month-old TPR50 mice but not in 3-month-old mice. TPR50 mice demonstrated cognitive dysfunction at 5 months. At this age or earlier, although no intracellular tau accumulation was observed in the hippocampus, abnormally increased microtubule (MT)-related proteins and MT hyperdynamics in the hippocampus, and impaired axonal transport in the septo-hippocampal pathway were already observed. Therefore, cognitive dysfunction in TPR50 mice may result from early MT dysfunction and impaired axonal transport rather than accumulation of insoluble tau and neurodegeneration. TPR50 mice are a valuable new model to study progression of tauopathies at both the behavioral and neurocellular levels and may also prove useful for testing new therapies for neurodegenerative diseases.

Photobiomodulation with near infrared light mitigates Alzheimer's disease-related pathology in cerebral cortex - evidence from two transgenic mouse models

INTRODUCTION

Previous work has demonstrated the efficacy of irradiating tissue with red to infrared light in mitigating cerebral pathology and degeneration in animal models of stroke, traumatic brain injury, parkinsonism and Alzheimer's disease (AD). Using mouse models, we explored the neuroprotective effect of near infrared light (NIr) treatment, delivered at an age when substantial pathology is already present in the cerebral cortex.

METHODS

We studied two mouse models with AD-related pathologies: the K369I tau transgenic model (K3), engineered to develop neurofibrillary tangles, and the APPswe/PSEN1dE9 transgenic model (APP/PS1), engineered to develop amyloid plaques. Mice were treated with NIr 20 times over a four-week period and histochemistry was used to quantify AD-related pathological hallmarks and other markers of cell damage in the neocortex and hippocampus.

RESULTS

In the K3 mice, NIr treatment was associated with a reduction in hyperphosphorylated tau, neurofibrillary tangles and oxidative stress markers (4-hydroxynonenal and 8-hydroxy-2'-deoxyguanosine) to near wildtype levels in the neocortex and hippocampus, and with a restoration of expression of the mitochondrial marker cytochrome c oxidase in surviving neurons. In the APP/PS1 mice, NIr treatment was associated with a reduction in the size and number of amyloid-β plaques in the neocortex and hippocampus.

CONCLUSIONS

Our results, in two transgenic mouse models, suggest that NIr may have potential as an effective, minimally-invasive intervention for mitigating, and even reversing, progressive cerebral degenerations.

Profiling murine tau with 0N, 1N and 2N isoform-specific antibodies in brain and peripheral organs reveals distinct subcellular localization, with the 1N isoform being enriched in the nucleus

In the adult murine brain, the microtubule-associated protein tau exists as three major isoforms, which have four microtubule-binding repeats (4R), with either no (0N), one (1N) or two (2N) amino-terminal inserts. The human brain expresses three additional isoforms with three microtubule-binding repeats (3R) each. However, little is known about the role of the amino-terminal inserts and how the 0N, 1N and 2N tau species differ. In order to investigate this, we generated a series of isoform-specific antibodies and performed a profiling by Western blotting and immunohistochemical analyses using wild-type mice in three age groups: two months, two weeks and postnatal day 0 (P0). This revealed that the brain is the only organ to express tau at significant levels, with 0N4R being the predominant isoform in the two month-old adult. Subcellular fractionation of the brain showed that the 1N isoform is over-represented in the soluble nuclear fraction. This is in agreement with the immunohistochemical analysis as the 1N isoform strongly localizes to the neuronal nucleus, although it is also found in cell bodies and dendrites, but not axons. The 0N isoform is mainly found in cell bodies and axons, whereas nuclei and dendrites are only slightly stained with the 0N antibody. The 2N isoform is highly expressed in axons and in cell bodies, with a detectable expression in dendrites and a very slight expression in nuclei. The 2N isoform that was undetectable at P0, in adult brain was mainly found localized to cell bodies and dendrites. Together these findings reveal significant differences between the three murine tau isoforms that are likely to reflect different neuronal functions.

The Beta-amyloid protein of Alzheimer's disease: communication breakdown by modifying the neuronal cytoskeleton

Alzheimer's disease (AD) is one of the most prevalent severe neurological disorders afflicting our aged population. Cognitive decline, a major symptom exhibited by AD patients, is associated with neuritic dystrophy, a degenerative growth state of neurites. The molecular mechanisms governing neuritic dystrophy remain unclear. Mounting evidence indicates that the AD-causative agent, β-amyloid protein (Aβ), induces neuritic dystrophy. Indeed, neuritic dystrophy is commonly found decorating Aβ-rich amyloid plaques (APs) in the AD brain. Furthermore, disruption and degeneration of the neuronal microtubule system in neurons forming dystrophic neurites may occur as a consequence of Aβ-mediated downstream signaling. This review defines potential molecular pathways, which may be modulated subsequent to Aβ-dependent interactions with the neuronal membrane as a consequence of increasing amyloid burden in the brain.

Notch signalling in adult neurons: a potential target for microtubule stabilization

Cytoskeletal dysfunction has been proposed during the last decade as one of the main mechanisms involved in the aetiology of several neurodegenerative diseases. Microtubules are basic elements of the cytoskeleton and the dysregulation of microtubule stability has been demonstrated to be causative for axonal transport impairment, synaptic contact degeneration, impaired neuronal function leading finally to neuronal loss. Several pathways are implicated in the microtubule assembly/disassembly process. Emerging evidence is focusing on Notch as a microtubule dynamics regulator. We demonstrated that activation of Notch signalling results in increased microtubule stability and changes in axonal morphology and branching. By contrast, Notch inhibition leads to an increase in cytoskeleton plasticity with intense neurite remodelling. Until now, several microtubule-binding compounds have been tested and the results have provided proof of concept that microtubule-binding agents or compounds with the ability to stabilize microtubules may have therapeutic potential for the treatment of Alzheimer's disease and other neurodegenerative diseases. In this review, based on its key role in cytoskeletal dynamics modulation, we propose Notch as a new potential target for microtubule stabilization.

Inducible, tightly regulated and non-leaky neuronal gene expression in mice

The Tetracycline (Tet)-controlled inducible system is the most widely used reversible system for transgene expression in mice with over 500 lines created to date. Although this system has been optimized over the years, it still has limitations such as residual transgene expression when turned off, referred to as leakiness. Here, we present a series of new Tet-OFF transgenic mice based on the second generation tetracycline-responsive transactivator system. The tTA-Advanced (tTA2(S)) is expressed under control of the neuron-specific Thy1.2 promoter (Thy-OFF), to regulate expression in the mouse brain. In addition, we generated a lacZ reporter line, utilizing the P tight Tet-responsive promoter (P(tight)-lacZ), to test our system. Two Thy-OFF transgenic lines displaying two distinct patterns of expression were selected. Oral doxycycline treatment of Thy-OFF/P tight-lacZ mice demonstrated tight transgene regulation with no leak expression. These new Thy-OFF mice are valuable for studies in a broad range of neurodegenerative diseases such as Alzheimer's disease and related forms of dementia, where control of transgene expression is critical to understanding mechanisms underlying the disease. Furthermore, P tight-lacZ reporter mice may be widely applicable.

March separate, strike together--role of phosphorylated TAU in mitochondrial dysfunction in Alzheimer's disease

The energy demand and calcium buffering requirements of the brain are met by the high number of mitochondria in neurons and in these, especially at the synapses. Mitochondria are the major producer of reactive oxygen species (ROS); at the same time, they are damaged by ROS that are induced by abnormal protein aggregates that characterize human neurodegenerative diseases such as Alzheimer's disease (AD). Because synaptic mitochondria are long-lived, any damage exerted by these aggregates impacts severely on neuronal function. Here we review how increased TAU, a defining feature of AD and related tauopathies, impairs mitochondrial function by following the principle: 'March separate, strike together!' In the presence of amyloid-β, TAU's toxicity is augmented suggesting synergistic pathomechanisms. In order to restore mitochondrial functions in neurodegeneration as a means of therapeutic intervention it will be important to integrate the various aspects of dysfunction and get a handle on targeting distinct cell types and subcellular compartments.

The impact of near-infrared light on dopaminergic cell survival in a transgenic mouse model of parkinsonism

We have examined whether near-infrared light (NIr) treatment mitigates oxidative stress and increased expression of hyperphosphorylated tau in a tau transgenic mouse strain (K3) that has a progressive degeneration of dopaminergic cells in the substantia nigra pars compacta (SNc). The brains of wild-type (WT), untreated K3 and NIr-treated K3 mice, aged five months (thus after the onset of parkinsonian signs and neuropathology), were labelled immunohistochemically for the oxidative stress markers 4-hydroxynonenal (4-HNE) and 8-hydroxy-2'-deoxyguanosine (8-OHDG), hyperphosphorylated tau (using the AT8 antibody) and tyrosine hydroxylase (TH). The average intensity and area of 4-HNE, 8-OHDG and AT8 immunoreactivity were measured using the MetaMorph software and TH⁺ cell number was estimated using stereology. Our results showed immunoreactivity for 4-HNE, 8-OHDG and AT8 within the SNc was increased in K3 mice compared to WT, and that this increase was mitigated by NIr. Results further showed that TH⁺ cell number was lower in K3 mice than in WT, and that this loss was mitigated by NIr. In summary, NIr treatment reduced the oxidative stress caused by the tau transgene in the SNc of K3 mice and saved SNc cells from degeneration. Our results, when taken together with those in other models, strengthen the notion that NIr treatment saves dopaminergic cells in the parkinsonian condition.

The importance of tau phosphorylation for neurodegenerative diseases

Fibrillar deposits of highly phosphorylated tau are a key pathological feature of several neurodegenerative tauopathies including Alzheimer's disease (AD) and some frontotemporal dementias. Increasing evidence suggests that the presence of these end-stage neurofibrillary lesions do not cause neuronal loss, but rather that alterations to soluble tau proteins induce neurodegeneration. In particular, aberrant tau phosphorylation is acknowledged to be a key disease process, influencing tau structure, distribution, and function in neurons. Although typically described as a cytosolic protein that associates with microtubules and regulates axonal transport, several additional functions of tau have recently been demonstrated, including roles in DNA stabilization, and synaptic function. Most recently, studies examining the trans-synaptic spread of tau pathology in disease models have suggested a potential role for extracellular tau in cell signaling pathways intrinsic to neurodegeneration. Here we review the evidence showing that tau phosphorylation plays a key role in neurodegenerative tauopathies. We also comment on the tractability of altering phosphorylation-dependent tau functions for therapeutic intervention in AD and related disorders.

Therapeutic strategies for tau mediated neurodegeneration

Based on the amyloid hypothesis, controlling β-amyloid protein (Aβ) accumulation is supposed to suppress downstream pathological events, tau accumulation, neurodegeneration and cognitive decline. However, in recent clinical trials, Aβ removal or reducing Aβ production has shown limited efficacy. Moreover, while active immunisation with Aβ resulted in the clearance of Aβ, it did not prevent tau pathology or neurodegeneration. This prompts the concern that it might be too late to employ Aβ targeting therapies once tau mediated neurodegeneration has occurred. Therefore, it is timely and very important to develop tau directed therapies. The pathomechanisms of tau mediated neurodegeneration are unclear but hyperphosphorylation, oligomerisation, fibrillisation and propagation of tau pathology have been proposed as the likely pathological processes that induce loss of function or gain of toxic function of tau, causing neurodegeneration. Here we review the strategies for tau directed treatments based on recent progress in research on tau and our understanding of the pathomechanisms of tau mediated neurodegeneration.

What Renders TAU Toxic

TAU is a microtubule-associated protein that under pathological conditions such as Alzheimer's disease (AD) forms insoluble, filamentous aggregates. When 20 years after TAU's discovery the first TAU transgenic mouse models were established, one declared goal that was achieved was the modeling of authentic TAU aggregate formation in the form of neurofibrillary tangles. However, as we review here, it has become increasingly clear that TAU causes damage much before these filamentous aggregates develop. In fact, because TAU is a scaffolding protein, increased levels and an altered subcellular localization (due to an increased insolubility and impaired clearance) result in the interaction of TAU with cellular proteins with which it would otherwise either not interact or do so to a lesser degree, thereby impairing their physiological functions. We specifically discuss the non-axonal localization of TAU, the role phosphorylation has in TAU toxicity and how TAU impairs mitochondrial functions. A major emphasis is on what we have learned from the four available TAU knock-out models in mice, and the knock-out of the TAU/MAP2 homolog PTL-1 in worms. It has been proposed that in human pathological conditions such as AD, a rare toxic TAU species exists which needs to be specifically removed to abrogate TAU's toxicity and restore neuronal functions. However, what is toxic in one context may not be in another, and simply reducing, but not fully abolishing TAU levels may be sufficient to abrogate TAU toxicity.

Why size matters - balancing mitochondrial dynamics in Alzheimer's disease

Once perceived as solitary structures, mitochondria are now recognized as highly dynamic, interconnected organelles. The tight control of their fusion and fission, a process termed 'mitochondrial dynamics', is crucial for neurons, given their unique architecture and special energy and calcium-buffering requirements at the synapse. Interestingly, in Alzheimer's disease (AD), a condition initiated at the synapse, mitochondrial dynamics are severely impaired. Of the two proteins implicated in AD pathogenesis, amyloid-β (Aβ) and TAU, only the impact of Aβ on mitochondrial dynamics has been studied in detail. We highlight recent findings that TAU exerts a determinative effect in the regulation of mitochondrial dynamics, and therefore neuronal function. In this process, the GTPase DRP1 has emerged as a key target of both Aβ and TAU.

PTL-1 regulates neuronal integrity and lifespan in C. elegans

Protein with tau-like repeats (PTL-1) is the sole Caenorhabditis elegans homolog of tau and MAP2, which are members of the mammalian family of microtubule-associated proteins (MAPs). In mammalian neurons, tau and MAP2 are segregated, with tau being mainly localised to the axon and MAP2 mainly to the dendrite. In particular, tau plays a crucial role in pathology, as elevated levels lead to the formation of tau aggregates in many neurodegenerative conditions including Alzheimer's disease. We used PTL-1 in C. elegans to model the biological functions of a tau-like protein without the complication of functional redundancy that is observed among the mammalian MAPs. Our findings indicate that PTL-1 is important for the maintenance of neuronal health as animals age, as well as in the regulation of whole organism lifespan. In addition, gene dosage of PTL-1 is crucial because variations from wild-type levels are detrimental. We also observed that human tau is unable to robustly compensate for loss of PTL-1, although phenotypes observed in tau transgenic worms are dependent on the presence of endogenous PTL-1. Our data suggest that some of the effects of tau pathology result from the loss of physiological tau function and not solely from a toxic gain-of-function due to accumulation of tau.

Progress in frontotemporal dementia research

Frontotemporal dementia (FTD) is the second most common type of presenile dementia and is the most common form of dementia with the onset before 60 years of age. Its typical symptoms include behavioral disorders, affective symptoms, and language disorders. The FTD is a genetically and pathologically heterogeneous degenerative disorder. Animal models have provided more insights into the pathogenic mechanisms. There are currently no medications that are specifically approved for the treatment of FTD by the Food and Drug Administration. In this article, we review the recent advances in the molecular pathogenesis, pathology, animal models, and therapy for FTD. Better understanding of the pathogenesis and the use of animal models will help develop novel therapeutic strategies and provide new targets for FTD treatment.

Loss of prohibitin membrane scaffolds impairs mitochondrial architecture and leads to tau hyperphosphorylation and neurodegeneration

Fusion and fission of mitochondria maintain the functional integrity of mitochondria and protect against neurodegeneration, but how mitochondrial dysfunctions trigger neuronal loss remains ill-defined. Prohibitins form large ring complexes in the inner membrane that are composed of PHB1 and PHB2 subunits and are thought to function as membrane scaffolds. In Caenorhabditis elegans, prohibitin genes affect aging by moderating fat metabolism and energy production. Knockdown experiments in mammalian cells link the function of prohibitins to membrane fusion, as they were found to stabilize the dynamin-like GTPase OPA1 (optic atrophy 1), which mediates mitochondrial inner membrane fusion and cristae morphogenesis. Mutations in OPA1 are associated with dominant optic atrophy characterized by the progressive loss of retinal ganglion cells, highlighting the importance of OPA1 function in neurons. Here, we show that neuron-specific inactivation of Phb2 in the mouse forebrain causes extensive neurodegeneration associated with behavioral impairments and cognitive deficiencies. We observe early onset tau hyperphosphorylation and filament formation in the hippocampus, demonstrating a direct link between mitochondrial defects and tau pathology. Loss of PHB2 impairs the stability of OPA1, affects mitochondrial ultrastructure, and induces the perinuclear clustering of mitochondria in hippocampal neurons. A destabilization of the mitochondrial genome and respiratory deficiencies manifest in aged neurons only, while the appearance of mitochondrial morphology defects correlates with tau hyperphosphorylation in the absence of PHB2. These results establish an essential role of prohibitin complexes for neuronal survival in vivo and demonstrate that OPA1 stability, mitochondrial fusion, and the maintenance of the mitochondrial genome in neurons depend on these scaffolding proteins. Moreover, our findings establish prohibitin-deficient mice as a novel genetic model for tau pathologies caused by a dysfunction of mitochondria and raise the possibility that tau pathologies are associated with other neurodegenerative disorders caused by deficiencies in mitochondrial dynamics.

Mitochondria are the major site of cellular ATP production and are essential for the survival of neurons. High ATP levels are required to sustain neuronal activities and axonal transport of macromolecules and organelles. The functional integrity of mitochondria depends on fusion and fission of their membranes, which maintain a dynamic mitochondrial network in cells. Interference with these processes causes neurodegenerative disorders that are characterized by axonal degeneration of distinct neurons. However, how an impaired fusion affects mitochondrial activities and neuronal survival remains poorly understood. Here, we have addressed this question by analyzing forebrain-specific knockout mice lacking prohibitins. Prohibitin complexes form membrane scaffolds in the inner membrane, which we now show are required for mitochondrial fusion, ultrastructure, and genome stability in neurons. Loss of prohibitins triggers extensive neurodegeneration associated with behavioral and cognitive deficiencies. Surprisingly, we observe hyperphosphorylation and filament formation of the microtubule-associated protein tau, reminiscent of a large group of neurodegenerative disorders termed tauopathies. Our findings, therefore, not only provide new insight into how defects in mitochondrial fusion affect neuronal survival, but also point to an intimate relationship of deficiencies in mitochondrial dynamics and tau pathologies.

Tau promotes neurodegeneration via DRP1 mislocalization in vivo

Mitochondrial abnormalities have been documented in Alzheimer's disease and related neurodegenerative disorders, but the causal relationship between mitochondrial changes and neurodegeneration, and the specific mechanisms promoting mitochondrial dysfunction, are unclear. Here, we find that expression of human tau results in elongation of mitochondria in both Drosophila and mouse neurons. Elongation is accompanied by mitochondrial dysfunction and cell cycle-mediated cell death, which can be rescued in vivo by genetically restoring the proper balance of mitochondrial fission and fusion. We have previously demonstrated that stabilization of actin by tau is critical for neurotoxicity of the protein. Here, we demonstrate a conserved role for actin and myosin in regulating mitochondrial fission and show that excess actin stabilization inhibits association of the fission protein DRP1 with mitochondria, leading to mitochondrial elongation and subsequent neurotoxicity. Our results thus identify actin-mediated disruption of mitochondrial dynamics as a direct mechanism of tau toxicity in neurons in vivo.

Tau-targeted treatment strategies in Alzheimer's disease

With populations ageing worldwide, the need for treating and preventing diseases associated with high age is pertinent. Alzheimer's disease (AD) is reaching epidemic proportions, yet the currently available therapies are limited to a symptomatic relief, without halting the degenerative process that characterizes the AD brain. As in AD cholinergic neurons are lost at high numbers, the initial strategies were limited to the development of acetylcholinesterase inhibitors, and more recently the NMDA receptor antagonist memantine, in counteracting excitotoxicity. With the identification of the protein tau in intracellular neurofibrillary tangles and of the peptide amyloid-β (Aβ) in extracellular amyloid plaques in the AD brain, and a better understanding of their role in disease, newer strategies are emerging, which aim at either preventing their formation and deposition or at accelerating their clearance. Interestingly, what is well established to combat viral diseases in peripheral organs - vaccination - seems to work for the brain as well. Accordingly, immunization strategies targeting Aβ show efficacy in mice and to some degree also in humans. Even more surprising is the finding in mice that immunization strategies targeting tau, a protein that forms aggregates in nerve cells, ameliorates the tau-associated pathology. We are reviewing the literature and discuss what can be expected regarding the translation into clinical practice and how the findings can be extended to other neurodegenerative diseases with protein aggregation in brain.

Non-invasive, in vivo monitoring of neuronal transport impairment in a mouse model of tauopathy using MEMRI

The impairment of axonal transport by overexpression or hyperphosphorylation of tau is well documented for in vitro conditions; however, only a few studies on this phenomenon have been conducted in vivo, using invasive procedures, and with contradictory results. Here we used the non-invasive, Manganese-Enhanced Magnetic Resonance Imaging technique (MEMRI), to study for the first time a pure model of tauopathy, the JNPL3 transgenic mouse line, which overexpresses a mutated (P301L) form of the human tau protein. We show progressive impairment in neuronal transport as tauopathy advances. These findings are further supported by a significant correlation between the severity of the impairment in neuronal transport assessed by MEMRI, and the degree of abnormal tau assessed by histology. Unlike conventional techniques that focus on axonal transport measurement, MEMRI can provide a global analysis of neuronal transport, i.e. from dendrites to axons and at the macroscopic scale of fiber tracts. Neuronal transport impairment has been shown to be a key pathogenic process in Alzheimer's disease and numerous other neurodegenerative disorders. Hence, MEMRI provides a promising set of functional biomarkers to be used during preclinical trials to facilitate the selection of new drugs aimed at restoring neuronal transport in neurodegenerative diseases.

New insights into behaviour using mouse ENU mutagenesis

Identifying genes involved in behavioural disorders in man is a challenge as the cause is often multigenic and the phenotype is modulated by environmental cues. Mouse mutants are a valuable tool for identifying novel pathways underlying specific neurological phenotypes and exploring the influence both genetic and non-genetic factors. Many human variants causing behavioural disorders are not gene deletions but changes in levels of expression or activity of a gene product; consequently, large-scale mouse ENU mutagenesis has the advantage over the study of null mutants in that it generates a range of point mutations that frequently mirror the subtlety and heterogeneity of human genetic lesions. ENU mutants have provided novel and clinically relevant functional information on genes that influence many aspects of mammalian behaviour, from neuropsychiatric endophenotypes to circadian rhythms. This review will highlight some of the most important findings that have been made using this method in several key areas of neurological disease research.

Alzheimer's disease models and functional genomics-How many needles are there in the haystack?

Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) are complex human brain disorders that affect an increasing number of people worldwide. With the identification first of the proteins that aggregate in AD and FTLD brains and subsequently of pathogenic gene mutations that cause their formation in the familial cases, the foundation was laid for the generation of animal models. These recapitulate essential aspects of the human conditions; expression of mutant forms of the amyloid-β protein-encoding APP gene in mice reproduces amyloid-β (Aβ) plaque formation in AD, while that of mutant forms of the tau-encoding microtubule-associated protein tau (MAPT) gene reproduces tau-containing neurofibrillary tangle formation, a lesion that is also prevalent in FTLD-Tau. The mouse models have been complemented by those in lower species such as C. elegans or Drosophila, highlighting the crucial role for Aβ and tau in human neurodegenerative disease. In this review, we will introduce selected AD/FTLD models and discuss how they were instrumental, by identifying deregulated mRNAs, miRNAs and proteins, in dissecting pathogenic mechanisms in neurodegenerative disease. We will discuss some recent examples, which includes miRNA species that are specifically deregulated by Aβ, mitochondrial proteins that are targets of both Aβ and tau, and the nuclear splicing factor SFPQ that accumulates in the cytoplasm in a tau-dependent manner. These examples illustrate how a functional genomics approach followed by a careful validation in experimental models and human tissue leads to a deeper understanding of the pathogenesis of AD and FTLD and ultimately, may help in finding a cure.

Lessons from tau-deficient mice

Both Alzheimer's disease (AD) and frontotemporal dementia (FTD) are characterized by the deposition of hyperphosphorylated forms of the microtubule-associated protein tau in neurons and/or glia. This unifying pathology led to the umbrella term “tauopathies” for these conditions, also emphasizing the central role of tau in AD and FTD. Generation of transgenic mouse models expressing human tau in the brain has contributed to the understanding of the pathomechanistic role of tau in disease. To reveal the physiological functions of tau in vivo, several knockout mouse strains with deletion of the tau-encoding MAPT gene have been established over the past decade, using different gene targeting constructs. Surprisingly, when initially introduced tau knockout mice presented with no overt phenotype or malformations. The number of publications using tau knockout mice has recently markedly increased, and both behavioural changes and motor deficits have been identified in aged mice of certain strains. Moreover, tau knockout mice have been instrumental in identifying novel functions of tau, both in cultured neurons and in vivo. Importantly, tau knockout mice have significantly contributed to the understanding of the pathophysiological interplay between Aβ and tau in AD. Here, we review the literature that involves tau knockout mice to summarize what we have learned so far from depleting tau in vivo.

Inhibition of tau aggregation in a novel Caenorhabditis elegans model of tauopathy mitigates proteotoxicity

Increased Tau protein amyloidogenicity has been causatively implicated in several neurodegenerative diseases, collectively called tauopathies. In pathological conditions, Tau becomes hyperphosphorylated and forms intracellular aggregates. The deletion of K280, which is a mutation that commonly appears in patients with frontotemporal dementia with Parkinsonism linked to chromosome 17, enhances Tau aggregation propensity (pro-aggregation). In contrast, introduction of the I277P and I308P mutations prevents β-sheet formation and subsequent aggregation (anti-aggregation). In this study, we created a tauopathy model by expressing pro- or anti-aggregant Tau species in the nervous system of Caenorhabditis elegans. Animals expressing the highly amyloidogenic Tau species showed accelerated Tau aggregation and pathology manifested by severely impaired motility and evident neuronal dysfunction. In addition, we observed that the axonal transport of mitochondria was perturbed in these animals. Control animals expressing the anti-aggregant combination had rather mild phenotype. We subsequently tested several Tau aggregation inhibitor compounds and observed a mitigation of Tau proteotoxicity. In particular, a novel compound that crosses the blood-brain barrier of mammals proved effective in ameliorating the motility as well as delaying the accumulation of neuronal defects. Our study establishes a new C. elegans model of Tau aggregation-mediated toxicity and supports the emerging notion that inhibiting the nucleation of Tau aggregation can be neuroprotective.

Reduced axonal transport and increased excitotoxic retinal ganglion cell degeneration in mice transgenic for human mutant P301S tau

The effects of tau hyperphosphorylation and aggregation on axonal transport were investigated in the optic nerve of mice transgenic for human mutant P301S tau. Transport was examined using cholera toxin B tracing. Retrograde transport was reduced in transgenic mice at 3 and 5 months of age, when compared to C57/Bl6 control mice. Anterograde axonal transport was also reduced in 3-month-old transgenic mice. Mild excitotoxic injury of retinal ganglion cells resulted in greater nerve cell loss in retinas from 3- and 5-month old P301S transgenic mice, when compared to controls. In conjunction with the detection of abnormal tau in the optic nerve in human and experimental glaucoma, the present findings suggest that tau hyperphosphorylation and aggregation may constitute targets for neuroprotective therapies in glaucoma as well as tauopathies.

A neuroprotective role for the DNA damage checkpoint in tauopathy

ATM and p53, effectors of the DNA damage checkpoint, are generally considered pro-apoptotic in neurons. We show that DNA damage and checkpoint activation occurs in postmitotic neurons in animal models of tauopathy, neurodegenerative disorders that include Alzheimer's disease. Surprisingly, checkpoint attenuation potently increases neurodegeneration through aberrant cell cycle re-entry of postmitotic neurons. These data suggest an unexpected neuroprotective role for the DNA damage checkpoint in tauopathies.

Tau deficiency induces parkinsonism with dementia by impairing APP-mediated iron export

The microtubule-associated protein tau has risk alleles for both Alzheimer's disease and Parkinson's disease and mutations that cause brain degenerative diseases termed tauopathies. Aggregated tau forms neurofibrillary tangles in these pathologies, but little is certain about the function of tau or its mode of involvement in pathogenesis. Neuronal iron accumulation has been observed pathologically in the cortex in Alzheimer's disease, the substantia nigra (SN) in Parkinson's disease and various brain regions in the tauopathies. Here we report that tau-knockout mice develop age-dependent brain atrophy, iron accumulation and SN neuronal loss, with concomitant cognitive deficits and parkinsonism. These changes are prevented by oral treatment with a moderate iron chelator, clioquinol. Amyloid precursor protein (APP) ferroxidase activity couples with surface ferroportin to export iron, but its activity is inhibited in Alzheimer's disease, thereby causing neuronal iron accumulation. In primary neuronal culture, we found loss of tau also causes iron retention, by decreasing surface trafficking of APP. Soluble tau levels fall in affected brain regions in Alzheimer's disease and tauopathies, and we found a similar decrease of soluble tau in the SN in both Parkinson's disease and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model. These data suggest that the loss of soluble tau could contribute to toxic neuronal iron accumulation in Alzheimer's disease, Parkinson's disease and tauopathies, and that it can be rescued pharmacologically.

Cargo distributions differentiate pathological axonal transport impairments

Axonal transport is an essential process in neurons, analogous to shipping goods, by which energetic and cellular building supplies are carried downstream (anterogradely) and wastes are carried upstream (retrogradely) by molecular motors, which act as cargo porters. Impairments in axonal transport have been linked to devastating and often lethal neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis, Huntington's, and Alzheimer's. Axonal transport impairment types include a decrease in available motors for cargo transport (motor depletion), the presence of defective or non-functional motors (motor dilution), and the presence of increased or larger cargos (protein aggregation). An impediment to potential treatment identification has been the inability to determine what type(s) of axonal transport impairment candidates that could be present in a given disease. In this study, we utilize a computational model and common axonal transport experimental metrics to reveal the axonal transport impairment general characteristics or "signatures" that result from three general defect types of motor depletion, motor dilution, and protein aggregation. Our results not only provide a means to discern these general impairments types, they also reveal key dynamic and emergent features of axonal transport, which potentially underlie multiple impairment types. The identified characteristics, as well as the analytical method, can be used to help elucidate the axonal transport impairments observed in experimental and clinical data. For example, using the model-predicted defect signatures, we identify the defect candidates, which are most likely to be responsible for the axonal transport impairments in the G93A SOD1 mouse model of ALS.

Biochemical isolation of insoluble tau in transgenic mouse models of tauopathies

Tau is a highly soluble microtubule-associated protein (MAP) that is abundant in the central nervous system and expressed mainly in neuronal axons. Intracellular aggregates of insoluble tau protein are present in a group of neurodegenerative diseases called tauopathies, which include Alzheimer's disease. Numerous transgenic mouse models of tauopathies have been produced in the last decade, and analysis of insoluble tau in these animals has provided a powerful tool to understand the development of tau pathology. In this short chapter, we aim at reviewing the two main isolation methods, sarkosyl and formic acid extraction (and their variations), used for the biochemical isolation of insoluble tau in transgenic mouse models of tauopathy, and discuss their advantages and drawbacks.

PATHOLOGIES OF AXONAL TRANSPORT IN NEURODEGENERATIVE DISEASES

Gene products such as organelles, proteins and RNAs are actively transported to synaptic terminals for the remodeling of pre-existing neuronal connections and formation of new ones. Proteins described as molecular motors mediate this transport and utilize specialized cytoskeletal proteins that function as molecular tracks for the motor based transport of cargos. Molecular motors such as kinesins and dynein's move along microtubule tracks formed by tubulins whereas myosin motors utilize tracks formed by actin. Deficits in active transport of gene products have been implicated in a number of neurological disorders. We describe such disorders collectively as "transportopathies". Here we review current knowledge of critical components of active transport and their relevance to neurodegenerative diseases.

Tau and tauopathies

Tauopathies are age-related neurodegenerative diseases that are characterized by the presence of aggregates of abnormally phosphorylated tau. As tau was originally discovered as a microtubule-associated protein, it has been hypothesized that neurodegeneration results from a loss of the ability of tau to associate with microtubules. However, tau has been found to have other functions aside from the promotion and stabilization of microtubule assembly. It is conceivable that such functions may be affected by the abnormal phosphorylation of tau and might have consequences for neuronal function or viability. This chapter provides an overview of tau structure, functions, and its involvement in neurodegenerative diseases.

ENU mutagenesis screen to establish motor phenotypes in wild-type mice and modifiers of a pre-existing motor phenotype in tau mutant mice

Modifier screening is a powerful genetic tool. While not widely used in the vertebrate system, we applied these tools to transgenic mouse strains that recapitulate key aspects of Alzheimer's disease (AD), such as tau-expressing mice. These are characterized by a robust pathology including both motor and memory impairment. The phenotype can be modulated by ENU mutagenesis, which results in novel mutant mouse strains and allows identifying the underlying gene/mutation. Here we discuss this strategy in detail. We firstly obtained pedigrees that modify the tau-related motor phenotype, with mapping ongoing. We further obtained transgene-independent motor pedigrees: (i) hyperactive, circling ENU 37 mice with a causal mutation in the Tbx1 gene—the complete knock-out of Tbx1 models DiGeorge Syndrome; (ii) ENU12/301 mice that show sudden jerky movements and tremor constantly; they have a causal mutation in the Kcnq1 gene, modelling aspects of the Romano-Ward and Jervell and Lange-Nielsen syndromes; and (iii) ENU16/069 mice with tremor and hypermetric gait that have a causal mutation in the Mpz (Myelin Protein Zero) gene, modelling Charcot-Marie-Tooth disease type 1 (CMT1B). Together, we provide evidence for a real potential of an ENU mutagenesis to dissect motor functions in wild-type and tau mutant mice.

Loss of dopaminoreceptive neuron causes L-dopa resistant parkinsonism in tauopathy

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a family of inherited dementias caused by tauopathy. A mutation in exon 10 of the tau gene, N279K, causes a particular kindred of FTDP-17, which is predominant for parkinsonism. The disease initially presents as L-dopa resistant parkinsonism which then rapidly progresses. The final pathological features reveal disappearing dopamine (DA) neurons, but the causes remain poorly understood. We previously established a transgenic mouse with human N279K mutant tau as a model for FTDP-17, which showed cognitive dysfunctions caused by the mutant. Here we analyze L-dopa resistant parkinsonism by several behavioral tests, and focus on the distributions and accumulations of the mutant tau in the DA system by immunohistochemistry and Western blot. Interestingly, dopaminoreceptive (DAr) neurons in the striatum showed neurofibrils degeneration and apoptosis through caspase-3 activation by mutant tau accumulation. The DAr neuron loss in the caudoputamen, the target of the nigrostriatal system occurred before DA neuron loss in young symptomatic mice. Residual DA neurons in the mouse functioned in DA transportation, whereas dysregulation of intracellular DA compartmentalization implied an excess level of DA caused by DAr neuron loss. In the final stages, both DAr and DA neurons decreased equally, unlike Parkinson's disease. Therefore, DAr neurons were fundamentally vulnerable to the mutation indicating a critical role for the L-dopa resistant parkinsonism in tauopathy.

Tau-targeted immunization impedes progression of neurofibrillary histopathology in aged P301L tau transgenic mice

In Alzheimer's disease (AD) brains, the microtubule-associated protein tau and amyloid-β (Aβ) deposit as intracellular neurofibrillary tangles (NFTs) and extracellular plaques, respectively. Tau deposits are furthermore found in a significant number of frontotemporal dementia cases. These diseases are characterized by progressive neurodegeneration, the loss of intellectual capabilities and behavioral changes. Unfortunately, the currently available therapies are limited to symptomatic relief. While active immunization against Aβ has shown efficacy in both various AD mouse models and patients with AD, immunization against pathogenic tau has only recently been shown to prevent pathology in young tau transgenic mice. However, if translated to humans, diagnosis and treatment would be routinely done when symptoms are overt, meaning that the histopathological changes have already progressed. Therefore, we used active immunization to target pathogenic tau in 4, 8, and 18 months-old P301L tau transgenic pR5 mice that have an onset of NFT pathology at 6 months of age. In all age groups, NFT pathology was significantly reduced in treated compared to control pR5 mice. Similarly, phosphorylation of tau at pathological sites was reduced. In addition, increased astrocytosis was found in the oldest treated group. Taken together, our data suggests that tau-targeted immunization slows the progression of NFT pathology in mice, with practical implications for human patients.

The toxicity of tau in Alzheimer disease: turnover, targets and potential therapeutics

It has been almost 25 years since the initial discovery that tau was the primary component of the neurofibrillary tangles (NFTs) in Alzheimer disease (AD) brain. Although AD is defined by both β-amyloid (Aβ) pathology (Aβ plaques) and tau pathology (NFTs), whether or not tau played a critical role in disease pathogenesis was a subject of discussion for many years. However, given the increasing evidence that pathological forms of tau can compromise neuronal function and that tau is likely an important mediator of Aβ toxicity, there is a growing awareness that tau is a central player in AD pathogenesis. In this review we begin with a brief history of tau, then provide an overview of pathological forms of tau, followed by a discussion of the differential degradation of tau by either the proteasome or autophagy and possible mechanisms by which pathological forms of tau may exert their toxicity. We conclude by discussing possible avenues for therapeutic intervention based on these emerging themes of tau's role in AD.

Cytoplasmic accumulation and aggregation of TDP-43 upon proteasome inhibition in cultured neurons

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are characterized by intraneuronal deposition of the nuclear TAR DNA-binding protein 43 (TDP-43) caused by unknown mechanisms. Here, we studied TDP-43 in primary neurons under different stress conditions and found that only proteasome inhibition by MG-132 or lactacystin could induce significant cytoplasmic accumulation of TDP-43, a histopathological hallmark in disease. This cytoplasmic accumulation was accompanied by phosphorylation, ubiquitination and aggregation of TDP-43, recapitulating major features of disease. Proteasome inhibition produced similar effects in both hippocampal and cortical neurons, as well as in immortalized motor neurons. To determine the contribution of TDP-43 to cell death, we reduced TDP-43 expression using small interfering RNA (siRNA), and found that reduced levels of TDP-43 dose-dependently rendered neurons more vulnerable to MG-132. Taken together, our data suggests a role for the proteasome in subcellular localization of TDP-43, and possibly in disease.

Loss of mitochondrial complex I activity potentiates dopamine neuron death induced by microtubule dysfunction in a Parkinson's disease model

The combination of microtubule depolymerization and the accumulation of cytosolic dopamine and reactive oxygen species selectively affects survival of dopaminergic neurons.

Mitochondrial complex I dysfunction is regarded as underlying dopamine neuron death in Parkinson’s disease models. However, inactivation of the Ndufs4 gene, which compromises complex I activity, does not affect the survival of dopamine neurons in culture or in the substantia nigra pars compacta of 5-wk-old mice. Treatment with piericidin A, a complex I inhibitor, does not induce selective dopamine neuron death in either Ndufs4^+/+ or Ndufs4^−/− mesencephalic cultures. In contrast, rotenone, another complex I inhibitor, causes selective toxicity to dopamine neurons, and Ndufs4 inactivation potentiates this toxicity. We identify microtubule depolymerization and the accumulation of cytosolic dopamine and reactive oxygen species as alternative mechanisms underlying rotenone-induced dopamine neuron death. Enhanced rotenone toxicity to dopamine neurons from Ndufs4 knockout mice may involve enhanced dopamine synthesis caused by the accumulation of nicotinamide adenine dinucleotide reduced. Our results suggest that the combination of disrupting microtubule dynamics and inhibiting complex I, either by mutations or exposure to toxicants, may be a risk factor for Parkinson’s disease.

Modes of Aβ toxicity in Alzheimer's disease

Alzheimer’s disease (AD) is reaching epidemic proportions, yet a cure is not yet available. While the genetic causes of the rare familial inherited forms of AD are understood, the causes of the sporadic forms of the disease are not. Histopathologically, these two forms of AD are indistinguishable: they are characterized by amyloid-β (Aβ) peptide-containing amyloid plaques and tau-containing neurofibrillary tangles. In this review we compare AD to frontotemporal dementia (FTD), a subset of which is characterized by tau deposition in the absence of overt plaques. A host of transgenic animal AD models have been established through the expression of human proteins with pathogenic mutations previously identified in familial AD and FTD. Determining how these mutant proteins cause disease in vivo should contribute to an understanding of the causes of the more frequent sporadic forms. We discuss the insight transgenic animal models have provided into Aβ and tau toxicity, also with regards to mitochondrial function and the crucial role tau plays in mediating Aβ toxicity. We also discuss the role of miRNAs in mediating the toxic effects of the Aβ peptide.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.