Differential axonal transport of soluble and insoluble tau in the rat sciatic nerve

Regulatory mechanisms for the axonal localization of tau protein in neurons

Tau is a microtubule (MT)-associated protein that is thought to be localized to the axon. However, its precise localization in developing neurons and mechanisms for the axonal localization have not been fully addressed. In this study, we found that the axonal localization of tau in cultured rat hippocampal neurons mainly occur during early neuronal development. Interestingly, transient expression of human tau in very immature neurons, but not in mature neurons, mimicked the developmental localization of endogenous tau to the axon. We therefore were able to establish an experimental model, in which exogenously expressed tau can be properly localized to the axon. Using this model, we obtained a surprising finding that the axonal localization of tau did not require stable MT binding. Tau lacking the MT-binding domain (MTBD) exhibited high diffusivity but localized properly to the axon. In contrast, a dephosphorylation-mimetic mutant of the proline-rich region 2 showed reinforced MT binding and mislocalization. Our results suggest that tight binding to MTs prevents tau from entering the axon and results in mislocalization in the soma and dendrites when expressed in mature neurons. This study therefore provides a novel mechanism independent of MTBD for the axonal localization of tau.

Ectopic Expression Induces Abnormal Somatodendritic Distribution of Tau in the Mouse Brain

Tau is a microtubule (MT)-associated protein that is localized to the axon. In Alzheimer's disease, the distribution of tau undergoes a remarkable alteration, leading to the formation of tau inclusions in the somatodendritic compartment. To investigate how this mislocalization occurs, we recently developed immunohistochemical tools that can separately detect endogenous mouse and exogenous human tau with high sensitivity, which allows us to visualize not only the pathological but also the pre-aggregated tau in mouse brain tissues of both sexes. Using these antibodies, we found that in tau-transgenic mouse brains, exogenous human tau was abundant in dendrites and somata even in the presymptomatic period, whereas the axonal localization of endogenous mouse tau was unaffected. In stark contrast, exogenous tau was properly localized to the axon in human tau knock-in mice. We tracked this difference to the temporal expression patterns of tau. Endogenous mouse tau and exogenous human tau in human tau knock-in mice exhibited high expression levels during the neonatal period and strong suppression into the adulthood. However, human tau in transgenic mice was expressed continuously and at high levels in adult animals. These results indicated the uncontrolled expression of exogenous tau beyond the developmental period as a cause of mislocalization in the transgenic mice. Superresolution microscopic and biochemical analyses also indicated that the interaction between MTs and exogenous tau was impaired only in the tau-transgenic mice, but not in knock-in mice. Thus, the ectopic expression of tau may be critical for its somatodendritic mislocalization, a key step of the tauopathy.SIGNIFICANCE STATEMENT Somatodendritic localization of tau may be an early step leading to the neuronal degeneration in tauopathies. However, the mechanisms of the normal axonal distribution of tau and the mislocalization of pathological tau remain obscure. Our immunohistochemical and biochemical analyses demonstrated that the endogenous mouse tau is transiently expressed in neonatal brains, that exogenous human tau expressed corresponding to such tau expression profile can distribute into the axon, and that the constitutive expression of tau into adulthood (e.g., human tau in transgenic mice) results in abnormal somatodendritic localization. Thus, the expression profile of tau is tightly associated with the localization of tau, and the ectopic expression of tau in matured neurons may be involved in the pathogenesis of tauopathy.

Prion-like properties of disease-relevant proteins in amyotrophic lateral sclerosis

The hallmark of age-related neurodegenerative diseases is the appearance of cellular protein deposits and spreading of this pathology throughout the central nervous system. Growing evidence has shown the involvement and critical role of proteins with prion-like properties in the formation of these characteristic cellular aggregates. Prion-like domains of such proteins with their proposed function in the organization of membraneless organelles are prone for misfolding and promoting further aggregation. Spreading of these toxic aggregates between cells and across tissues can explain the progression of clinical phenotypes and pathology in a stereotypical manner, characteristic for almost every neurodegenerative disease. Here, we want to review the current evidence for the role of prion-like mechanisms in classical neurodegenerative diseases and ALS in particular. We will also discuss an intriguingly central role of the protein TDP-43 in the majority of cases of this devastating disease.

Neurogenetics of slow axonal transport: from cells to animals

Slow axonal transport is a multivariate phenomenon implicated in several neurodegenerative disorders. Recent reports have unraveled the molecular basis of the transport of certain slow component proteins, such as the neurofilament subunits, tubulin, and certain soluble enzymes such as Ca(2+)/calmodulin-dependent protein kinase IIa (CaM kinase IIa), etc., in tissue cultured neurons. In addition, genetic analyses also implicate microtubule-dependent motors and other housekeeping proteins in this process. However, the biological relevance of this phenomenon is not so well understood. Here, the authors have discussed the possibility of adopting neurogenetic analyses in multiple model organisms to correlate molecular level measurements of the slow transport phenomenon to animal behavior, thus facilitating the investigation of its biological efficacy.

Do axonal defects in tau and amyloid precursor protein transgenic animals model axonopathy in Alzheimer's disease?

The subcellular localization of organelles, mRNAs and proteins is particularly challenging in neurons. Owing to their extended morphology, with axons in humans exceeding a meter in length, in addition to which they are not renewed but persist for the entire lifespan, it is no surprise that neurons are highly vulnerable to any perturbation of their sophisticated transport machinery. There is emerging evidence that impaired transport is not only causative for a range of motor disorders, but possibly also for Alzheimer's disease (AD) and related neurodegenerative disorders. Support for this hypothesis comes from transgenic animal models. Overexpression of human tau and amyloid precursor protein (APP) in mice and flies models the key hallmark histopathological characteristics of AD, such as somatodendritic accumulation of phosphorylated forms of tau and beta-amyloid (Abeta) peptide-containing amyloid plaques, as well as axonopathy. The latter has also been demonstrated in mutant mice with altered levels of Alzheimer-associated genes, such as presenilin (PS). In Abeta-producing APP transgenic mice, axonopathy was observed before the onset of plaque formation and tau hyperphosphorylation. In human AD brain, an axonopathy was revealed for early but not late Braak stages. The overall picture is that key players in AD, such as tau, APP and PS, perturb axonal transport early on in AD, causing impaired synaptic plasticity and reducing survival rates. It will be challenging to determine the molecular mechanisms of these different axonopathies, as this might assist in the development of new therapeutic strategies.

Axonal transport of human α-synuclein slows with aging but is not affected by familial Parkinson's disease-linked mutations

Biochemical and genetic abnormalities of alpha-synuclein (alpha-Syn) are implicated in the pathogenesis of Parkinson's disease (PD) and other alpha-synucleinopathies. The abnormal intraneuronal accumulations of alpha-Syn in Lewy bodies (LBs) and Lewy neurites (LNs) have implicated defects in axonal transport of alpha-Syn in the alpha-synucleinopathies. Using human (Hu) alpha-Syn transgenic (Tg) mice, we have examined whether familial PD (FPD)-linked mutations (A30P and A53T) alter axonal transport of Hualpha-Syn. Our studies using peripheral nerves show that Hualpha-Syn and Moalpha-Syn are almost exclusively transported in the slow component (SC) of axonal transport and that the FPD-linked alpha-Syn mutations do not have obvious effects on the axonal transport of alpha-Syn. Moreover, older pre-symptomatic A53T Hualpha-Syn Tg mice do not show gross alterations in the axonal transport of alpha-Syn and other proteins in the SC, indicating that the early stages of alpha-synucleinopathy in A53T alpha-Syn Tg mice are not associated with gross alterations in the slow axonal transport. However, the axonal transport of alpha-Syn slows significantly with aging. Because the rate of axonal transport affects the stability and accumulation of proteins in axons, age-dependent-slowing alpha-Syn is a likely contributor to axonal aggregation of alpha-Syn in alpha-synucleinopathy.

Acute changes in the axonal cytoskeleton after mild stretching of the rat brachial plexus

We have developed an animal model to investigate acute changes in the axonal cytoskeleton caused by a mild stretching of the peripheral nerve in the upper limbs of rats. Rat forelimbs were continuously stretched at 2 N for 1 h. Thereafter, a part of the brachial plexus and median nerve were harvested and processed for electron microscopic analysis. The total number of microtubules in the brachial plexus decreased to 55% of that of the control animals (p<0.05) without change in the number of neurofilaments. No significant changes in microtubules or neurofilaments were observed in the median nerve. By Western blotting analysis, the amount of tau protein in the stretch group significantly decreased in the brachial plexus but not in the median nerve. However, no significant changes in the amount of tubulin protein were observed in either the brachial plexus or median nerve. These results suggest that the microtubules were depolymerized by stretching of the brachial plexus and that the depolymerization may have been mediated by the decrease in the tau protein.

The slow axonal transport of the microtubule-associated protein tau and the transport rates of different isoforms and mutants in cultured neurons

We demonstrate that the microtubule-associated protein tau, in the form of enhanced green fluorescent protein (EGFP) tau, is transported along axons of neurons in culture in the slow component of axonal transport with a speed comparable with that previously measured in vivo. It was demonstrated that the EGFP tag has no effect on transport characteristics, and the methodology enables slow transport rates of individual tau isoforms and tau mutants to be measured. We also expressed EGFP-tagged tau isoforms containing either three or four C-terminal repeats and zero or two N-terminal inserts in cultured neurons. No significant differences were found in the average rate of slow transport of the wild-type tau isoforms, suggesting that the exon 10 C-terminal repeat or the N-terminal inserts do not contain regions that play a significant regulatory role in axonal transport. Similarly, we found that missense mutations in tau have no noticeable effect on the rate of transport; hence their ability to cause neurodegeneration is by another mechanism other than that affecting the overall slow axonal transport of tau.

Tau and transgenic animal models

Advances in genetics and transgenic approaches have a continuous impact on our understanding of Alzheimer's disease (AD) and related disorders, especially as aspects of the histopathology and neurodegeneration can be reproduced in animal models. AD is characterized by extracellular Abeta peptide-containing plaques and neurofibrillary aggregates of hyperphosphorylated isoforms of microtubule-associated protein tau. A causal link between Abeta production, neurodegeneration and dementia has been established with the identification of familial forms of AD which are linked to mutations in the amyloid precursor protein APP, from which the Abeta peptide is derived by proteolysis. No mutations have been identified in the tau gene in AD until today. Tau filament formation, in the absence of Abeta production, is also a feature of several additional neurodegenerative diseases including progressive supranuclear palsy, corticobasal degeneration, Pick's disease, and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in the tau gene which are linked to FTDP-17 established that dysfunction of tau can, as well as Abeta formation, lead to neurodegeneration and dementia. In this review, newly recognized cellular functions of tau, and the neuropathology and clinical syndrome of FTDP-17 will be presented, as well as recent advances that have been achieved in studies of transgenic mice expressing tau and AD-related kinases and phosphatases. These models link neurofibrillary lesion formation to neuronal loss, provide an in vivo model in which therapies can be assessed, and may contribute to determine the relationship between Abeta production and tau pathology.

Axonal transport of microtubule-associated protein 1B (MAP1B) in the sciatic nerve of adult rat: distinct transport rates of different isoforms

Cytoskeletal proteins are axonally transported with slow components a and b (SCa and SCb). In peripheral nerves, the transport velocity of SCa, which includes neurofilaments and tubulin, is 1-2 mm/d, whereas SCb, which includes actin, tubulin, and numerous soluble proteins, moves as a heterogeneous wave at 2-4 mm/d. We have shown that two isoforms of microtubule-associated protein 1B (MAP1B), which can be separated on SDS polyacrylamide gels on the basis of differences in their phosphorylation states (band I and band II), were transported at two different rates. All of band I MAP1B moved as a coherent wave at a velocity of 7-9 mm/d, distinct from slow axonal transport components SCa and SCb. Several other proteins were detected within the component that moved at the velocity of 7-9 mm/d, including the leading wave of tubulin and actin. The properties of this component define a distinct fraction of the slow axonal transport that we suggest to term slow component c (SCc). The relatively fast transport of the phosphorylated MAP1B isoform at 7-9 mm/d may account for the high concentration of phosphorylated MAP1B in the distal end of growing axons. In contrast to band I MAP1B, the transport profile of band II was complex and contained components moving with SCa and SCb and a leading edge at SCc. Thus, MAP1B isoforms in different phosphorylation states move with distinct components of slow axonal transport, possibly because of differences in their abilities to associate with other proteins.

Axonal transport of synucleins is mediated by all rate components

Synucleins are abundant nerve terminal proteins of hitherto unknown function. In diseases with Lewy bodies, human alpha-synuclein concentrates in these lesions in the cell body and mutations in alpha-synuclein lead to heritable Parkinson's disease with Lewy bodies. This indicates that changes in the normal metabolism and axonal transport of alpha-synuclein is perturbed in these diseases. To investigate the normal axonal transport of synucleins we studied the rat visual system by nerve crush operations and metabolic labelling of the retinal ganglion cells followed by immunoprecipitation of nerve segments. We found by immunofluorescence microscopy of the crush-operated nerves that synucleins are transported by fast antero- and retrograde transport and colocalize with synaptophysin and SNAP-25 around the lesion. The metabolic labelling studies demonstrated that synucleins were moved through the nerve with all the rate components, the fast component and the slow components a and b, with component b predominating. Two-dimensional gel electrophoresis revealed that both alpha- and beta-synuclein migrate through the nerve by slow component b in a ratio of 2:1.

Regulated phosphorylation and dephosphorylation of tau protein: effects on microtubule interaction, intracellular trafficking and neurodegeneration

This review attempts to summarize what is known about tau phosphorylation in the context of both normal cellular function and dysfunction. However, conceptions of tau function continue to evolve, and it is likely that the regulation of tau distribution and metabolism is complex. The roles of microtubule-associated kinases and phosphatases have yet to be fully described, but may afford insight into how tau phosphorylation at the distal end of the axon regulates cytoskeletal-membrane interactions. Finally, lipid and glycosaminoglycan modification of tau structure affords yet more complexity for regulation and aggregation. Continued work will help to determine what is causal and what is coincidental in Alzheimer's disease, and may lead to identification of therapeutic targets for halting the progression of paired helical filament formation.