Microfilament-associated growth cone component depends upon Tau for its intracellular localization

Tau Protein as Therapeutic Target for Cancer? Focus on Glioblastoma

Despite being extensively studied for several decades, the microtubule-associated protein Tau has not finished revealing its secrets. For long, Tau has been known for its ability to promote microtubule assembly. A less known feature of Tau is its capability to bind to cancer-related protein kinases, suggesting a possible role of Tau in modulating microtubule-independent cellular pathways that are associated with oncogenesis. With the intention of finding new therapeutic targets for cancer, it appears essential to examine the interaction of Tau with these kinases and their consequences. This review aims at collecting the literature data supporting the relationship between Tau and cancer with a particular focus on glioblastoma tumors in which the pathological significance of Tau remains largely unexplored. We will first treat this subject from a mechanistic point of view showing the pivotal role of Tau in oncogenic processes. Then, we will discuss the involvement of Tau in dysregulating critical pathways in glioblastoma. Finally, we will outline promising strategies to target Tau protein for the therapy of glioblastoma.

Tau: A Signaling Hub Protein

Over four decades ago, in vitro experiments showed that tau protein interacts with and stabilizes microtubules in a phosphorylation-dependent manner. This observation fueled the widespread hypotheses that these properties extend to living neurons and that reduced stability of microtubules represents a major disease-driving event induced by pathological forms of tau in Alzheimer’s disease and other tauopathies. Accordingly, most research efforts to date have addressed this protein as a substrate, focusing on evaluating how specific mutations, phosphorylation, and other post-translational modifications impact its microtubule-binding and stabilizing properties. In contrast, fewer efforts were made to illuminate potential mechanisms linking physiological and disease-related forms of tau to the normal and pathological regulation of kinases and phosphatases. Here, we discuss published work indicating that, through interactions with various kinases and phosphatases, tau may normally act as a scaffolding protein to regulate phosphorylation-based signaling pathways. Expanding on this concept, we also review experimental evidence linking disease-related tau species to the misregulation of these pathways. Collectively, the available evidence supports the participation of tau in multiple cellular processes sustaining neuronal and glial function through various mechanisms involving the scaffolding and regulation of selected kinases and phosphatases at discrete subcellular compartments. The notion that the repertoire of tau functions includes a role as a signaling hub should widen our interpretation of experimental results and increase our understanding of tau biology in normal and disease conditions.

Cellular Biology of Tau Diversity and Pathogenic Conformers

Tau accumulation is a prominent feature in a variety of neurodegenerative disorders and remarkable effort has been expended working out the biochemistry and cell biology of this cytoplasmic protein. Tau's wayward properties may derive from germline mutations in the case of frontotemporal lobar degeneration (FTLD-MAPT) but may also be prompted by less understood cues—perhaps environmental or from molecular damage as a consequence of chronological aging—in the case of idiopathic tauopathies. Tau properties are undoubtedly affected by its covalent structure and in this respect tau protein is not only subject to changes in length produced by alternative splicing and endoproteolysis, but different types of posttranslational modifications that affect different amino acid residues. Another layer of complexity concerns alternate conformations—“conformers”—of the same covalent structures; in vivo conformers can encompass soluble oligomeric species, ramified fibrillar structures evident by light and electron microscopy and other forms of the protein that have undergone liquid-liquid phase separation to make demixed liquid droplets. Biological concepts based upon conformers have been charted previously for templated replication mechanisms for prion proteins built of the PrP polypeptide; these are now providing useful explanations to feature tau pathobiology, including how this protein accumulates within cells and how it can exhibit predictable patterns of spread across different neuroanatomical regions of an affected brain. In sum, the documented, intrinsic heterogeneity of tau forms and conformers now begins to speak to a fundamental basis for diversity in clinical presentation of tauopathy sub-types. In terms of interventions, emphasis upon subclinical events may be worthwhile, noting that irrevocable cell loss and ramified protein assemblies feature at end-stage tauopathy, whereas earlier events may offer better opportunities for diverting pathogenic processes. Nonetheless, the complexity of tau sub-types, which may be present even within intermediate disease stages, likely mitigates against one-size-fits-all therapeutic strategies and may require a suite of interventions. We consider the extent to which animal models of tauopathy can be reasonably enrolled in the campaign to produce such interventions and to slow the otherwise inexorable march of disease progression.

Tau Modulates VGluT1 Expression

Tau displacement from microtubules is the first step in the onset of tauopathies and is followed by toxic protein aggregation. However, other non-canonical functions of Tau might have a role in these pathologies. Here, we demonstrate that a small amount of Tau localizes in the nuclear compartment and accumulates in both the soluble and chromatin-bound fractions. We show that favoring Tau nuclear translocation and accumulation, by Tau overexpression or detachment from MTs, increases the expression of VGluT1, a disease-relevant gene directly involved in glutamatergic synaptic transmission. Remarkably, the P301L mutation, related to frontotemporal dementia FTDP-17, impairs this mechanism leading to a loss of function. Altogether, our results provide the demonstration of a direct physiological role of Tau on gene expression. Alterations of this mechanism may be at the basis of the onset of neurodegeneration.

The Neurotoxic Role of Extracellular Tau Protein

Tauopathies are a class of neurodegenerative diseases associated with the microtubule-associated protein tau, with Alzheimer’s disease (AD) being the most prevalent related disorder. Neurofibrillary tangles (NFTs) are one of the neuropathological hallmarks present in the brains of AD patients. Because NFTs are aberrant intracellular inclusions formed by hyperphosphorylated tau, it was initially proposed that phosphorylated and/or aggregated intracellular tau protein was causative of neuronal death. However, recent studies suggest a toxic role for non-phosphorylated and non-aggregated tau when it is located in the brain extracellular space. In this work, we will discuss the neurotoxic role of extracellular tau as well its involvement in the spreading of tau pathologies.

Haplotype-specific MAPT exon 3 expression regulated by common intronic polymorphisms associated with Parkinsonian disorders

BACKGROUND

Genome wide association studies have identified microtubule associated protein tau (MAPT) H1 haplotype single nucleotide polymorphisms (SNPs) as leading common risk variants for Parkinson's disease, progressive supranuclear palsy and corticobasal degeneration. The MAPT risk variants fall within a large 1.8 Mb region of high linkage disequilibrium, making it difficult to discern the functionally important risk variants. Here, we leverage the strong haplotype-specific expression of MAPT exon 3 to investigate the functionality of SNPs that fall within this H1 haplotype region of linkage disequilibrium.

METHODS

In this study, we dissect the molecular mechanisms by which haplotype-specific SNPs confer allele-specific effects on the alternative splicing of MAPT exon 3. Firstly, we use haplotype-hybrid whole-locus genomic MAPT vectors studies to identify functional SNPs. Next, we characterise the RNA-protein interactions at two loci by mass spectrometry. Lastly, we knockdown candidate splice factors to determine their effect on MAPT exon 3 using a novel allele-specific qPCR assay.

RESULTS

Using whole-locus genomic DNA expression vectors to express MAPT haplotype variants, we demonstrate that rs17651213 regulates exon 3 inclusion in a haplotype-specific manner. We further investigated the functionality of this region using RNA-electrophoretic mobility shift assays to show differential RNA-protein complex formation at the H1 and H2 sequence variants of SNP rs17651213 and rs1800547 and subsequently identified candidate trans-acting splicing factors interacting with these functional SNPs sequences by RNA-protein pull-down experiment and mass spectrometry. Finally, gene knockdown of candidate splice factors identified by mass spectrometry demonstrate a role for hnRNP F and hnRNP Q in the haplotype-specific regulation of exon 3 inclusion.

CONCLUSIONS

We identified common splice factors hnRNP F and hnRNP Q regulating the haplotype-specific splicing of MAPT exon 3 through intronic variants rs1800547 and rs17651213. This work demonstrates an integrated approach to characterise the functionality of risk variants in large regions of linkage disequilibrium.

Selective suppression of the α isoform of p38 MAPK rescues late-stage tau pathology

BACKGROUND

Hyperphosphorylation and aggregation of tau protein are the pathological hallmarks of Alzheimer's disease and related tauopathies. We previously demonstrated that the microglial activation induces tau hyperphosphorylation and cognitive impairment via activation of p38 mitogen-activated protein kinase (p38 MAPK) in the hTau mouse model of tauopathy that was deficient for microglial fractalkine receptor CX3CR1.

METHOD

We report an isoform-selective, brain-permeable, and orally bioavailable small molecule inhibitor of p38α MAPK (MW181) and its effects on tau phosphorylation in vitro and in hTau mice.

RESULTS

First, pretreatment of mouse primary cortical neurons with MW181 completely blocked inflammation-induced p38α MAPK activation and AT8 (pS199/pS202) site tau phosphorylation, with the maximum effect peaking at 60-90 min after stimulation. Second, treatment of old (~20 months of age) hTau mice with MW181 (1 mg/kg body weight; 14 days via oral gavage) significantly reduced p38α MAPK activation compared with vehicle-administered hTau mice. This also resulted in a significant reduction in AT180 (pT231) site tau phosphorylation and Sarkosyl-insoluble tau aggregates. Third, MW181 treatment significantly increased synaptophysin protein expression and resulted in improved working memory. Fourth, MW181 administration reduced phosphorylated MAPK-activated protein kinase 2 (pMK2) and phosphorylated activating transcription factor 2 (pATF2), which are known substrates of p38α MAPK. Finally, MW181 reduced the expression of interferon-γ and interleukin-1β.

CONCLUSIONS

Taken together, these studies support p38α MAPK as a valid therapeutic target for the treatment of tauopathies.

Nuclear Tau and Its Potential Role in Alzheimer's Disease

Tau protein, found in both neuronal and non-neuronal cells, forms aggregates in neurons that constitutes one of the hallmarks of Alzheimer’s disease (AD). For nearly four decades, research efforts have focused more on tau’s role in physiology and pathology in the context of the microtubules, even though, for over three decades, tau has been localised in the nucleus and the nucleolus. Its nuclear and nucleolar localisation had stimulated many questions regarding its role in these compartments. Data from cell culture, mouse brain, and the human brain suggests that nuclear tau could be essential for genome defense against cellular distress. However, its nature of translocation to the nucleus, its nuclear conformation and interaction with the DNA and other nuclear proteins highly suggest it could play multiple roles in the nucleus. To find efficient tau-based therapies, there is a need to understand more about the functional relevance of the varied cellular distribution of tau, identify whether specific tau transcripts or isoforms could predict tau’s localisation and function and how they are altered in diseases like AD. Here, we explore the cellular distribution of tau, its nuclear localisation and function and its possible involvement in neurodegeneration.

Tau-mediated synaptic damage in Alzheimer's disease

Synapses are the principal sites for chemical communication between neurons and are essential for performing the dynamic functions of the brain. In Alzheimer’s disease and related tauopathies, synapses are exposed to disease modified protein tau, which may cause the loss of synaptic contacts that culminate in dementia. In recent decades, structural, transcriptomic and proteomic studies suggest that Alzheimer’s disease represents a synaptic disorder. Tau neurofibrillary pathology and synaptic loss correlate well with cognitive impairment in these disorders. Moreover, regional distribution and the load of neurofibrillary lesions parallel the distribution of the synaptic loss. Several transgenic models of tauopathy expressing various forms of tau protein exhibit structural synaptic deficits. The pathological tau proteins cause the dysregulation of synaptic proteome and lead to the functional abnormalities of synaptic transmission. A large body of evidence suggests that tau protein plays a key role in the synaptic impairment of human tauopathies.

Microtubule-associated proteins as direct crosslinkers of actin filaments and microtubules

The cytoskeletal polymers--actin, microtubules, and intermediate filaments--are interlinked by coordinated protein interactions to form a complex three-dimensional cytoskeletal network. Association of actin filaments with microtubules is important for various cellular processes such as cell division, migration, vesicle and organelle transport, and axonal growth. Several proteins including signaling molecules, motor proteins, and proteins directly or indirectly associated with microtubules and actin are involved in bridging the cytoskeletal components. Microtubule-associated proteins (MAPs) belonging to the MAP1, 2, 4 family and Tau proteins have been identified as key players that directly crosslink the two cytoskeletons. This review summarizes the current understanding of the interactions of these MAPs with actin filaments and their role in forming the actin-microtubule network and further discusses how the in vitro reconstitution assays can be used to study the dynamics of coordinated networks. Understanding the mechanisms by which actin and microtubules interact is key to decipher cancer, wound healing, and neuronal regeneration.

The role of extracellular Tau in the spreading of neurofibrillary pathology

The microtubule-associated protein (MAP) tau plays a critical role in the pathogenesis of Alzheimer’s disease (AD) and several related disorders collectively known as tauopathies. Development of tau pathology is associated with progressive neuronal loss and cognitive decline. In the brains of AD patients, tau pathology spreads following an anatomically defined pattern. Mounting evidence strongly suggests that accumulation of abnormal tau is mediated through spreading of seeds of the protein from cell to cell and point at the involvement of extracellular tau species as the main agent in the interneuronal propagation of neurofibrillary lesions and spreading of tau toxicity throughout different brain regions in these disorders. That would support the concept that pathology initiates in a very small part of the brain many years before becoming symptomatic, spreading progressively to the whole brain within 10–20 years. Understanding the precise molecular mechanism underlying tau propagation is crucial for the development of therapeutics for this devastating disorder. In this work, we will discuss recent research on the role of extracellular tau in the spreading of tau pathology, through synaptic and non-synaptic mechanisms.

Proteins recruited to exosomes by tau overexpression implicate novel cellular mechanisms linking tau secretion with Alzheimer's disease

Tau misprocessing to form aggregates and other toxic species has emerged as a major feature in our developing understanding of the etiology and pathogenesis of Alzheimer's disease (AD). The significance of tau misprocessing in AD has been further emphasized by recent studies showing that tau can be secreted from neurons via exosomes and may itself be an important agent in the spreading of neurofibrillary lesions within the brain. Tau secretion occurs most readily under disease-associated conditions in cellular models, suggesting that cellular changes responsible for secretion, possibly including tau oligomerization, could play a key role in the propagation of neurofibrillary lesions in neurodegenerative disease. Here we show that overexpression of 4R0N human tau in neuroblastoma cells recruits mitochondrial and axonogenesis-associated proteins relevant to neurodegeneration into the exosomal secretion pathway via distinct mechanisms. The recruitment of mitochondrial proteins appears to be linked to autophagy disruption (exophagy) in multiple neurodegenerative conditions but has few known direct links to AD and tau. By contrast, the involvement of synaptic plasticity and axonogenesis markers is highly specific to both tau and AD and may be relevant to the reactivation of developmental programs involving tau in AD and the recently demonstrated ability of secreted tau to establish tissue distribution gradients in CNS neuropil. We also found a highly significant correlation between genes that are significantly downregulated in multiple forms of AD and proteins that have been recruited to exosomes by tau, which we interpret as strong evidence for the central involvement of tau secretion in AD cytopathogenesis. Our results suggest that multiple cellular mechanisms may link tau secretion to both toxicity and neurofibrillary lesion spreading in AD and other tauopathies.

Tangles, Toxicity, and Tau Secretion in AD - New Approaches to a Vexing Problem

When the microtubule (MT)-associated protein tau is not bound to axonal MTs, it becomes hyperphosphorylated and vulnerable to proteolytic cleavage and other changes typically seen in the hallmark tau deposits (neurofibrillary tangles) of tau-associated neurodegenerative diseases (tauopathies). Neurofibrillary tangle formation is preceded by tau oligomerization and accompanied by covalent crosslinking and cytotoxicity, making tangle cytopathogenesis a natural central focus of studies directed at understanding the role of tau in neurodegenerative disease. Recent studies suggest that the formation of tau oligomers may be more closely related to tau neurotoxicity than the presence of the tangles themselves. It has also become increasingly clear that tau pathobiology involves a wide variety of other cellular abnormalities including a disruption of autophagy, vesicle trafficking mechanisms, axoplasmic transport, neuronal polarity, and even the secretion of tau, which is normally a cytosolic protein, to the extracellular space. In this review, we discuss tau misprocessing, toxicity and secretion in the context of normal tau functions in developing and mature neurons. We also compare tau cytopathology to that of other aggregation-prone proteins involved in neurodegeneration (alpha synuclein, prion protein, and APP). Finally, we consider potential mechanisms of intra- and interneuronal tau lesion spreading, an area of particular recent interest.

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.

Therapeutic targets in Alzheimer's disease and related tauopathies

Alzheimer's disease is a progressive neurodegenerative disease that is characterized histopathologically by the presence of plaques, mainly composed of Abeta amyloid and the tangles, mainly composed of hyperphosphorylated tau. To date, there is no treatment that can reverse the disease, and all the current therapeutics is directed to cope with the symptoms of the disease. Here we describe the efforts dedicated to attack the plaques and, in more detail, the process of neurofibrillary degeneration, linked to the presence of the hyperphosphorylated microtubule associated protein tau. We have identified the different putative targets for therapeutics and the current knowledge on them.

Expression and silencing of the microtubule-associated protein Tau in breast cancer cells

The microtubule-associated protein Tau has been reported to be a predictive factor for clinical response to taxanes in metastatic breast cancer. We generated a panel of eight taxane-resistant variants from four human breast cancer cell lines (MCF-7, T-47D, MDA-MB-231, and BT-549). Four variants had higher levels of Tau compared with their T-47D and MDA-MB-231 parental cells. Using isoform-specific primers, we found that Tau 0N, 1N, 2N, 3R, and 4R isoforms are overexpressed in the resistant variants, as is Tau exon 6 but not exons 4A or 8. To determine whether Tau overexpression produces resistance to taxanes, we derived three independent T-47D clones stably overexpressing Tau 3R and 4R isoforms. Tau overexpression did not result in taxane resistance compared with parental cells transfected with vector alone. We then knocked down Tau expression in three cell lines that expressed Tau constitutively (MCF-7 and ZR-75-1 breast cancer cells, and OVCAR-3 ovarian cancer cells). Lentivirus-mediated silencing of Tau expression in MCF-7 and OVCAR-3 cells did not result in increased taxane sensitivity compared with luciferase short hairpin RNA-infected cells and uninfected parental cells. Transient silencing using Tau-specific small interfering RNAs also did not alter taxane sensitivity relative to nontargeting controls in both MCF-7 and ZR-75-1 cells. These results show that neither overexpression nor depletion of Tau modulates cellular sensitivity to taxanes. Although Tau overexpression has been reported to be a predictive marker of taxane resistance, it is not likely to be a direct mechanism of taxane resistance in breast cancer.

Tau impacts on growth-factor-stimulated actin remodeling

The microtubule-associated protein tau interacts with the SH3 domain of non-receptor Src family protein tyrosine kinases. A potential consequence of the SH3 interaction is the upregulation of tyrosine kinase activity. Here we investigated the activation of Src or Fyn by tau, both in vitro and in vivo. Tau increased the kinase activity in in vitro assays and in transfected COS7 cells. In platelet-derived growth factor (PDGF)-stimulated fibroblasts, tau appeared to prime Src for activation following PDGF stimulation, as reflected by changes in Src-mediated actin rearrangements. In addition, while fibroblasts normally recovered actin stress fibers by 5-7 hours after PDGF stimulation, tau-expressing cells showed sustained actin breakdown. Microtubule association by tau was not required for the observed changes in actin morphology. Inhibition of Src kinases or a mutant deficient in Src interaction reduced the effects, implicating Src family protein tyrosine kinases as a mediator of the effects of tau on actin rearrangements. Our results provide evidence that the interaction of tau with Src upregulates tyrosine kinase activity and that this interaction allows tau to impact on growth-factor-induced actin remodeling.

Abnormal bundling and accumulation of F-actin mediates tau-induced neuronal degeneration in vivo

Hyperphosphorylated forms of the microtubule-associated protein (MAP) tau accumulate in Alzheimer's disease and related tauopathies and are thought to have an important role in neurodegeneration. However, the mechanisms through which phosphorylated tau induces neurodegeneration have remained elusive. Here, we show that tau-induced neurodegeneration is associated with accumulation of filamentous actin (F-actin) and the formation of actin-rich rods in Drosophila and mouse models of tauopathy. Importantly, modulating F-actin levels genetically leads to dramatic modification of tau-induced neurodegeneration. The ability of tau to interact with F-actin in vivo and in vitro provides a molecular mechanism for the observed phenotypes. Finally, we show that the Alzheimer's disease-linked human beta-amyloid protein (Abeta) synergistically enhances the ability of wild-type tau to promote alterations in the actin cytoskeleton and neurodegeneration. These findings raise the possibility that a direct interaction between tau and actin may be a critical mediator of tau-induced neurotoxicity in Alzheimer's disease and related disorders.

Tau associates with actin in differentiating PC12 cells

The microtubule-associated protein tau may be involved in cell morphogenesis and axonal maintenance. In addition to microtubules, tau has been shown to interact with actin in vitro. In the present study interaction of tau and actin was investigated in PC12 cells. No interaction between tau and actin was observed without NGF treatment. Under NGF stimulation, tau distributed at ends of cellular extensions, where it associated with actin in a microtubule-independent manner. F-actin disruption revealed that relocalization and assembly of F-actin at the ends of cellular extensions were necessary for NGF-induced tau reorganization and association with actin. A truncated tau-GFP (tau(1-186)-GFP, N-terminal of tau) did not associate with actin. However, tau23(174-352)-GFP (carboxyl-terminal of Tau23) did associate with actin and the requirement for NGF was lost. Nevertheless, NGF boosted tau23(174-352)-GFP interaction with actin and promoted colocalization at the ends of cellular extensions. This suggests that the C-terminal of tau is required for associating with actin and the tau N-terminal may play a regulatory role in this process. A possible role for tau-actin interaction in neurite outgrowth is postulated.

Misregulation of tau alternative splicing in neurodegeneration and dementia

Tau is a microtubule-associated protein that fulfills several functions critical for neuronal formation and health. Tau discharges its functions by producing multiple isoforms via intricately regulated alternative splicing. These isoforms modulate tau function in normal brain by altering the domains of the protein, thereby influencing its conformation and post-translational modifications and hence its affinity for microtubules and other ligands. Disturbances in tau expression result in disruption of the neuronal cytoskeleton and formation of pathological tau structures (neurofibrillary tangles) found in brains of dementia sufferers. More specifically, aberrations in tau splicing regulation directly cause several neurodegenerative diseases that lead to dementia. This review briefly presents our cumulative knowledge of tau splicing regulation in connection with the alterations in tau splicing seen in neurodegeneration.

Cytoskeletal mechanisms of neuronal morphogenesis

The cytoskeleton is the major intracellular structure that determines the morphology of a neuron. Thus, mechanisms that ensure a precisely regulated assembly of cytoskeletal elements in time and space have an important role in the development from a morphologically simple neuronal precursor cell to a complex polarized neuron that can establish contacts to several hundreds of other cells. Here, cytoskeletal mechanisms that underlie the formation of neurites, directed elongation and stabilization of neuronal processes are summarized. It has become evident that different cytoskeletal elements are highly crosslinked with each other by several classes of specific linker proteins. Of these, microtubule-associated proteins (MAPs) appear to have an important role in connecting the microtubule skeleton to other cytoskeletal filaments and plasma membrane components during neuronal morphogenesis. Future experiments will have to elucidate the function and the regulation of the neuronal cytoskeleton in an authentic nervous system environment during development. Recent approaches are discussed at the end of this article.

Tau isoforms which contain the domain encoded by exon 6 and their role in neurite elongation

The regulation of tau protein expression during different stages of cellular differentiation and development as well as its functional role in morphogenesis, neurofibrillary tangle formation, and neurodegeneration have been topics of extensive study but have not been completely clarified yet. Tau undergoes complex regulated splicing in the mammalian nervous system. Our previous study with tau exon 6 demonstrated that it shows a splicing regulation profile which is distinct from that of the other tau exons as well as a unique expression pattern which is spatially and temporally regulated. In this study, we investigated the expression, localization, and effects of tau isoforms which contain exon 6 in neuroblastoma cells which stably overexpress them. We found that expression of one particular combination of tau exons (the longest adult isoform plus the domain of exon 6) significantly inhibits neurite elongation.

Tau gene alternative splicing: expression patterns, regulation and modulation of function in normal brain and neurodegenerative diseases

Organization of cytoskeletal elements is critical for cellular migration and maintenance of morphology. Tau protein, which binds to and organizes microtubules, is instrumental in forming and maintaining the neuronal axon. Disturbances in tau expression result in disruption of the neuronal cytoskeleton and formation of pathological tau structures (neurofibrillary tangles, NFTs) found in brains of dementia sufferers. Null tau mice, although viable, exhibit developmental and cognitive defects and transgenic mice which overexpress tau develop severe neuropathies. The neuron-specific tau transcript produces multiple isoforms by intricately regulated alternative splicing. These isoforms modulate tau function in normal brain. Moreover, aberrations in tau splicing regulation directly cause several neurodegenerative diseases. Thus, tau splicing regulation is vital to neuronal health and correct brain function. This review briefly presents our cumulative knowledge of tau splicing-cis elements and trans factors which influence it at the RNA level, its effect on the structure and roles of the tau protein and its repercussions on neuronal morphology and neurodegeneration.

Tau and src family tyrosine kinases

The interaction between tau and src family non-receptor tyrosine kinases represents a new function for tau. Mediated by the proline-rich region of tau and the SH3 domain of fyn or src, this interaction has the potential to confer novel cellular activities for tau in the growth cone and in the membrane. The subsequent finding that tau is tyrosine phosphorylated has led to the observation that tau in neurofibrillary tangles is tyrosine phosphorylated. Therefore, a role for tyrosine kinases such as fyn in neuropathogenesis is predicted.

Transcriptional regulation of the mouse microtubule-associated protein tau

The microtubule-associated protein (MAP) tau is found primarily in neurons and errors in its regulation are associated with Alzheimer's disease and other neurodegenerative disorders. Tau expression is transcriptionally regulated and tissue-specific. In this study, starting with a approximately 7500-bp fragment from the mouse tau gene, which includes tau exon -1, we define regions preferentially conferring tissue-specific expression. Furthermore, gel shift assays indicate that transcriptional regulators SP-1 and AP-2 are important for basal expression but not necessary for neuron-specific expression of the tau transcript.

Tau exon 10, whose missplicing causes frontotemporal dementia, is regulated by an intricate interplay of cis elements and trans factors

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. In humans, exon 10 of the gene is an alternatively spliced cassette which is adult-specific and which codes for a microtubule binding domain. Mutations that affect splicing of exon 10 have been shown to cause inherited frontotemporal dementia (FTDP). In this study, we reconstituted naturally occurring exon 10 FTDP mutants and classified their effects on its splicing. We also carried out a comprehensive survey of the influence of splicing regulators on exon 10 inclusion and tentatively identified the site of action for several of these factors. Lastly, we identified the domains of regulators SWAP and hnRNPG, which are required for regulation of exon 10 splicing.

Modulation of the membrane-binding domain of tau protein: splicing regulation of exon 2

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. The N-terminal domain of the protein interacts with the axonal membrane, and is modulated by regulated inclusion of exons 2 and 3. These two tau exons are alternatively spliced cassettes, in which exon 3 never appears independently of exon 2. Previous work with tau minigene constructs indicated that exon 2 resembles a constitutive exon. In this study, we show that exon 2 is regulated by a combination of exonic and intronic enhancers and silencers. Furthermore, we demonstrate that known splicing regulators affect the ratio of exon 2 isoforms. Lastly, we tentatively pinpoint the site of action of several splicing factors which regulate tau exon 2.

Protein kinase MARK/PAR-1 is required for neurite outgrowth and establishment of neuronal polarity

Protein kinases of the microtubule affinity-regulating kinase (MARK) family were originally discovered because of their ability to phosphorylate certain sites in tau protein (KXGS motifs in the repeat domain). This type of phosphorylation is enhanced in abnormal tau from Alzheimer brain tissue and causes the detachment of tau from microtubules. MARK-related kinases (PAR-1 and KIN1) occur in various organisms and are involved in establishing and maintaining cell polarity. Herein, we report the ability of MARK2 to affect the differentiation and outgrowth of cell processes from neuroblastoma and other cell models. MARK2 phosphorylates tau protein at the KXGS motifs; this results in the detachment of tau from microtubules and their destabilization. The formation of neurites in N2a cells is blocked if MARK2 is inactivated, either by transfecting a dominant negative mutant, or by MARK2 inhibitors such as hymenialdisine. Alternatively, neurites are blocked if the target KXGS motifs on tau are rendered nonphosphorylatable by point mutations. The results suggest that MARK2 contributes to the plasticity of microtubules needed for neuronal polarity and the growth of neurites.

Modulation of the membrane-binding projection domain of tau protein: splicing regulation of exon 3

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. The N-terminal domain of the protein interacts with the axonal membrane, and is modulated by differential inclusion of exons 2 and 3. These two tau exons are alternatively spliced cassettes, in which exon 3 never appears independently of exon 2. Previous work with tau minigene constructs indicated that exon 3 is intrinsically suboptimal and its primary regulator is a weak branch point. In this study, we confirm the role of the weak branch point in the regulation of exon 3 but also show that the exon is additionally regulated by a combination of exonic enhancers and silencers. Furthermore, we demonstrate that known splicing regulators affect the ratio of exon 3 isoforms, Lastly, we tentatively pinpoint the site of action of several splicing factors which regulate tau exon 3.

Regulation and differential expression of tau mRNA isoforms as oligodendrocytes mature in vivo: implications for myelination

Oligodendrocytes and neurons derive from the same cell type but develop distinct morphologic and functional properties as they mature in vivo. Both cells express tau protein, a developmentally regulated protein in the central nervous system. The regulation of tau has been investigated extensively in neurons but not in oligodendrocytes, so we studied regulation of tau in oligodendrocytes in vivo. The amino-derived tau isoforms consist of isoforms with zero (A0), one (A1), or two (A2) inserts. We examined the developmental regulation of tau mRNA isoforms at the amino domain by comparing tau expression in oligodendrocytes (OLGs) isolated from 1- and 20-day-old rat brain and in age-matched cortex, which abounds in neurons. In the rat brain, myelination peaks at 20 days. By using semiquantitative RT-PCR, we found that OLGs and cortex from 1-day-old rat brain largely had amino-derived tau isoforms with no insert, whereas OLGs from 20-day-old rat brain had similar levels of amino-derived tau isoforms with no insert or with one insert. We also found that 20-day-old OLGs had twofold more tau mRNA levels than younger OLGs. In contrast to OLGs from 20-day-old rat brain, age-matched cortex had comparable levels of A0, A1, and A2 tau amino-derived isoforms. Further, younger and older OLGs had a reciprocal pattern of expression of both carboxy-derived tau mRNA isoforms with either three (3R) or four (4R) repeats. In contrast, younger and older cortex expressed either 3R or 4R tau. This study showed an upregulation of tau mRNA and cell-specific tau mRNA isoform expression in OLGs forming myelin.

The splicing determinants of a regulated exon in the axonal MAP tau reside within the exon and in its upstream intron

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. Exon 6 of the gene is an alternatively spliced cassette whose expression profile is distinct from that of the other tau regulated exons, implying the utilization of distinct regulatory factors. Previous work had established the use of cryptic splice sites within exon 6 and the influence of flanking exons on the ratio of exon 6 variants. The present work shows that, in addition to the previously identified participants, the ratio of exon 6 isoforms is affected by: (1) suppression of the cryptic sites, (2) deletions of the upstream intron, and (3) the splicing regulators PTB and U2AF, both of which act on the branch point/polypyrimidine tract region. These results strongly suggest that factors binding immediately upstream of exon 6 are involved in regulation of this exon. They also lead to the conclusion that splicing of exon 6 is primarily governed by multiple branch points.

Neuronal response to physical injury and its relationship to the pathology of Alzheimer's disease

1. Central nerve cells undergo a stereotyped regenerative response following physical injury. 2. This reaction involves adaptive changes within the axon and cell body of origin, directed at sprouting and synaptogenesis. 3. Intimately associated with the regenerative response are specific alterations to cytoskeletal proteins, including the neurofilament (NF) triplet. 4. The morphological and neurochemical alterations to NF within axons following injury are reminiscent of plaque-associated dystrophic neurites (DN) in early Alzheimer's disease (AD). 5. Associated changes in perikaryal NF resemble Alzheimer neurofibrillary tangle pathology, while growth-associated sprouting markers are localized to the abnormal neurites of AD. 6. The present review postulates that beta-amyloid plaques in AD cause physical damage to local nerve cell processes and it is the chronic stimulation of the stereotyped response to injury that results in the end-stage pathology and neurodegeneration associated with AD.

Tau is required for neurite outgrowth and growth cone motility of chick sensory neurons

The role of the microtubule-associated protein (MAP) tau in axon growth remains controversial. Antisense experiments have suggested that tau is required for axon outgrowth, whereas genetic knockout and immunodepletion studies have suggested that tau plays no role in this process. To investigate the role of tau in both neurite outgrowth and growth cone motility, we have used a different approach, the chromophore-assisted laser inactivation (CALI) of tau in chick dorsal root ganglion (DRG) neurons in culture. This approach generates an acute loss of tau function that is not subject to compensation by other MAPs. Inactivation of tau in whole DRG neurons (including cell body and neurites) reduced neurite number and length. Inactivation of tau within regions of growth cones using micro-scale CALI caused a decrease in neurite extension rate by approximately 2-fold. Surprisingly, it also caused a approximately 20% decrease in the lamellipodial size within the inactivation region, whereas the filopodial motility was not affected. These results suggest that tau is required in neurite outgrowth and that tau also functions in lamellipodial motility at the growth cone leading edge.

Interaction of tau with the neural membrane cortex is regulated by phosphorylation at sites that are modified in paired helical filaments

The axonal microtubule-associated phosphoprotein tau interacts with neural plasma membrane (PM) components during neuronal development (Brandt, R., Léger, J., and Lee, G. (1995) J. Cell Biol. 131, 1327-1340). To analyze the mechanism and potential regulation of tau's PM association, a method was developed to isolate PM-associated tau using microsphere separation of surface-biotinylated cells. We show that tau's PM association requires an intact membrane cortex and that PM-associated tau and cytosolic tau are differentially phosphorylated at sites detected by several Alzheimer's disease (AD) diagnostic antibodies (Ser(199)/Ser(202), Thr(231), and Ser(396)/Ser(404)). In polar neurons, the association of endogenous tau phosphoisoforms with the membrane cortex correlates with an enrichment in the axonal compartment. To test for a direct effect of AD-specific tau modifications in determining tau's interactions, a phosphomutant that simulates an AD-like hyperphosphorylation of tau was produced by site-directed mutagenesis of Ser/Thr residues to negatively charged amino acids (Glu). These mutations completely abolish tau's association with the membrane cortex; however, the construct retains its capability to bind to microtubules. The data suggest that a loss of tau's association with the membrane cortex as a result of phosphorylation at sites that are modified during disease contributes to somatodendritic tau accumulation, axonal microtubule disintegration, and neuronal death characteristic for AD.

Complex regulation of tau exon 10, whose missplicing causes frontotemporal dementia

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. Exon 10 of the gene is an alternatively spliced cassette that is adult-specific and that codes for a microtubule binding domain. Recently, mutations that affect splicing of exon 10 have been shown to cause inherited frontotemporal dementia (FTDP). In this study, we establish the endogenous expression patterns of exon 10 in human tissue; by reconstituting naturally occurring FTDP mutants in the homologous context of exon 10, we show that the cis determinants of exon 10 splicing regulation include an exonic silencer within the exon, its 5' splice site, and the relative affinities of its flanking exons to it. By cotransfections in vivo, we demonstrate that several splicing regulators affect the ratio of tau isoforms by inhibiting exon 10 inclusion.

Tau protein function in axonal formation

Tau protein is a predominantly neuronal microtubule-associated protein that is enriched in axons and is capable of promoting microtubule assembly and stabilization. In the present article we review some of the key experiments directed to obtain insights about tau protein function in developing neurons. Aspects related to whether or not tau has essential, unique, or complementary functions during axonal formation are discussed.

Differential cytoskeletal changes during growth cone collapse in response to hSema III and thrombin

Growth cones are known as the site of action of many factors that influence neurite growth behavior. To assess how different collapsing agents influence the growth cone cytoskeleton, we used recombinant human Semaphorin III (hSema III) and the serine protease thrombin. Embryonic chick dorsal root ganglion neurons showed a dramatic depolymerization of actin filaments within 5 min upon hSema III exposure and virtually no influence on microtubules (MT). Only at later time points (20-30 min) was the polymerization/depolymerization rate of MT significantly affected. Thrombin induced a morphologically and kinetically similar growth cone collapse. Moreover, thrombin induced an early and selective depolymerization of dynamic MT, accompanied by the formation of loops of stable MT bundles. Selective changes in the phosphorylation pattern of tau were associated with microtubule assembly in thrombin-induced responses. Our data provide evidence that different signal transduction pathways lead to distinct changes of the growth cone cytoskeleton.

Neurochemical diversity of dystrophic neurites in the early and late stages of Alzheimer's disease

We examined the neurochemical and morphological diversity of abnormal neurites associated with beta-amyloid plaque formation in the early and late stages of Alzheimer's disease. Preclinical Alzheimer's disease was characterised by the presence of abnormal neurites containing either neurofilament or chromogranin A immunoreactivity. All clustered dystrophic neurites in these cases were associated with beta-amyloid plaques. Neurofilament immunoreactive dystrophic neurites in preclinical Alzheimer's disease could be further subclassified into bulb- and ring-like structures, and these abnormal neurites contained both phosphorylated and dephosphorylated neurofilament epitopes. Dystrophic neurites in Alzheimer's disease could be subdivided into predominantly neurofilament, tau, or chromogranin A immunolabeled forms. Some neurofilament immunoreactive neurites had a core region labeled for tau. The neurofilaments of the dystrophic neurites in Alzheimer's disease had the same complement of phosphorylation- and dephosphorylation-dependent epitopes as observed in preclinical cases. Therefore, an abnormal accumulation of variably phosphorylated neurofilaments represent the earliest cytoskeletal alteration associated with dystrophic neurite formation. Furthermore, these data indicate that dystrophic neurites may "mature" through neurofilament-abundant forms to the neurites containing the profoundly altered filaments labeled for tau. The precise morphological and neurochemical changes associated with dystrophic neurite formation suggests that beta-amyloid plaques are causing physical damage to surrounding axons. The resultant axonal sprouting and profound cytoskeletal alterations would follow the chronic stimulation of the stereotypical reaction to such physical trauma.

Insulin transiently increases tau phosphorylation: involvement of glycogen synthase kinase-3beta and Fyn tyrosine kinase

The modulation of tau phosphorylation in response to insulin was examined in human neuroblastoma SH-SY5Y cells. Insulin treatment resulted in a transient increase in tau phosphorylation followed by a decrease in tau phosphorylation that correlated directly with a sequential activation and deactivation of glycogen synthase kinase-3beta (GSK-3beta). The insulin-induced increase in tau phosphorylation and concurrent activation of GSK-3beta was rapid (<2 min) and transient, and was associated with increased tyrosine phosphorylation of GSK-3beta. The increase in GSK-3beta tyrosine phosphorylation corresponded directly to an increase in the association of Fyn tyrosine kinase with GSK-3beta, and Fyn immunoprecipitated from cells treated with insulin for 1 min phosphorylated GSK-3beta to a significantly greater extent than Fyn immunoprecipitated from control cells. Subsequent to the increase in GSK-3beta activation and tau phosphorylation, treatment of cells with insulin for 60 min resulted in a dephosphorylation of tau and a decrease in GSK-3beta activity. Thus, insulin rapidly and transiently activated GSK-3beta and modulated tau phosphorylation, alterations that may contribute to neuronal plasticity.

Evidence for the participation of the neuron-specific CDK5 activator P35 during laminin-enhanced axonal growth

Cultures of cerebellar macroneurons were used to study the pattern of expression, subcellular localization, and function of the neuronal cdk5 activator p35 during laminin-enhanced axonal growth. The results obtained indicate that laminin, an extracellular matrix molecule capable of selectively stimulating axonal extension and promoting MAP1B phosphorylation at a proline-directed protein kinase epitope, selectively stimulates p35 expression, increases its association with the subcortical cytoskeleton, and accelerates its redistribution to the axonal growth cones. Besides, suppression of p35, but not of a highly related isoform designated as p39, by antisense oligonucleotide treatment selectively reduces cdk5 activity, laminin-enhanced axonal elongation, and MAP1b phosphorylation. Taken collectively, the present results suggest that cdk5/p35 may serve as an important regulatory linker between environmental signals (e.g., laminin) and constituents of the intracellular machinery (e.g., MAP1B) involved in axonal elongation.

Acute inactivation of tau has no effect on dynamics of microtubules in growing axons of cultured sympathetic neurons

Tau is a developmentally regulated microtubule (MT)-associated protein in neurons that has been implicated in neuronal morphogenesis. On the basis of test tube studies, tau has been proposed to function in axon growth by stabilizing MTs and thereby promoting MT assembly. We have tested this hypothesis by examining the effects of acute inactivation of tau on axonal MTs. Tau was inactivated by microinjecting purified antibodies against recombinant tau into neurons before they extended axons. The injected antibodies quantitatively precipitated tau into aggregates in the soma. With these conditions the neurons elaborate normal-appearing axons, and MTs extend throughout the axons and into the growth cones, but the axons and their MTs are depleted of tau. The immunodepletion of tau had no detectable effect on several parameters of the dynamics of axonal MTs. Depletion of tau also was not accompanied by a reorganization of other major MT-associated proteins or actin filaments in these neurons. Thus, neurons effectively depleted of tau can extend axons that resemble those of control cells, and the axons contain normal-appearing MT arrays with normal dynamic behavior. These observations are exactly the opposite of those expected on the basis of the hypothesis that the stability of axonal MTs is a direct function of their content of tau, indicating that tau in growing axons of cultured sympathetic neurons is not specialized to promote microtubule assembly and stability.

Tau interacts with src-family non-receptor tyrosine kinases

Tau and other microtubule-associated proteins promote the assembly and stabilization of neuronal microtubules. While each microtubule-associated protein has distinct properties, their in vivo roles remain largely unknown. Tau is important in neurite outgrowth and axonal development. Recently, we showed that the amino-terminal region of tau, which is not involved in microtubule interactions, is important in NGF induced neurite outgrowth in PC12 cells. Here we report that a proline rich sequence in the amino terminus of tau interacts with the SH3 domains of fyn and src non-receptor tyrosine kinases. Tau and fyn were co-immunoprecipitated from human neuroblastoma cells and co-localization of tau and fyn was visualized in co-transfected NIH3T3 cells. Co-transfection of tau and fyn also resulted in an alteration in NIH3T3 cell morphology, consistent with an in vivo interaction. Fyn-dependent tyrosine phosphorylation of tau occurred in transfected cells and tyrosine phosphorylated tau was identified in human neuroblastoma cells as well. Our data suggest that tau is involved in signal transduction pathways. An interaction between tau and fyn may serve as a mechanism by which extracellular signals influence the spatial distribution of microtubules. The tyrosine phosphorylation of tau by fyn may also have a role in neuropathogenesis, as fyn is upregulated in Alzheimer's disease.

Suppression of radixin and moesin alters growth cone morphology, motility, and process formation in primary cultured neurons

In this study we have examined the cellular functions of ERM proteins in developing neurons. The results obtained indicate that there is a high degree of spatial and temporal correlation between the expression and subcellular localization of radixin and moesin with the morphological development of neuritic growth cones. More importantly, we show that double suppression of radixin and moesin, but not of ezrin-radixin or ezrin-moesin, results in reduction of growth cone size, disappearance of radial striations, retraction of the growth cone lamellipodial veil, and disorganization of actin filaments that invade the central region of growth cones where they colocalize with microtubules. Neuritic tips from radixin-moesin suppressed neurons displayed high filopodial protrusive activity; however, its rate of advance is 8-10 times slower than the one of growth cones from control neurons. Radixin-moesin suppressed neurons have short neurites and failed to develop an axon-like neurite, a phenomenon that appears to be directly linked with the alterations in growth cone structure and motility. Taken collectively, our data suggest that by regulating key aspects of growth cone development and maintenance, radixin and moesin modulate neurite formation and the development of neuronal polarity.

Expression and distribution of IGF-1 receptors containing a beta-subunit variant (betagc) in developing neurons

Betagc is a beta-subunit variant of the insulin-like growth factor-1 (IGF-1) receptor highly enriched in growth cone membranes prepared by subcellular fractionation of fetal rat brain (). The present study is focused on the expression and on the cellular and subcellular distribution of betagc in developing neurons and differentiating PC12 cells. In the developing cerebral cortex and, at least at early stages, in cultured primary neurons, betagc expression was found to be correlated with neurite outgrowth. In PC12 cells betagc expression was nerve growth factor (NGF)-dependent and also paralleled neurite outgrowth. In contrast, beta-subunits of the insulin receptor and/or of other IGF-1 receptors ("betaP5"; detected with antibody AbP5) were downregulated as betagc expression increased. Immunofluorescence studies confirmed the enrichment of betagc at growth cones and demonstrated morphologically its spatial separation from betaP5, which is confined to the perikaryon. At the growth cone, betagc colocalizes and associates in a proximal region with microtubules, but it seems independent of the more peripheral microfilaments. Some betagc immunoreactivity is detected in the perinuclear region of PC12 cells, most likely the Golgi complex and its vicinity. betagc seems to emerge from the periphery of this structure in an apparently vesicular compartment distinct from that carrying synaptophysin to the growth cones. The facts that (1) betagc expression is correlated closely with neurite outgrowth, that (2) it is regulated in PC12 cells by a neurotrophin, NGF, and that (3) betagc is concentrated in the proximal growth cone region raise new questions regarding a possible role of IGF-1 receptors containing betagc in the regulation of neurite growth.

Analysis of the expression, distribution and function of cyclin dependent kinase 5 (cdk5) in developing cerebellar macroneurons

Cultures of cerebellar macroneurons were used to study the expression, activity, subcellular localization, and function of cdk5 during neuronal morphogenesis. The results obtained indicate that in non-polarized neurons cdk5 is restricted to the cell body but as soon as polarity is established it becomes highly concentrated at the distal tip of growing axons where it associates with microtubules and the subcortical cytoskeleton. In addition, we show that laminin, an extracellular matrix molecule capable of stimulating axonal extension and promoting MAP1b phosphorylation (DiTella et al., 1996), accelerates the redistribution of cdk5 to the axonal tip and dramatically increases its activity. Finally, our results indicate that cdk5 suppression by antisense oligonucleotide treatment selectively reduces axonal elongation and decreases the phosphorylation status of MAP1b, as well as its binding to microtubules. Taken collectively, our observations suggest that cdk5 may serve as an important regulatory linker between environmental signals (e.g. laminin) and constituents of the intracellular machinery (e.g. MAP1b) involved in axonal formation.

Microtubule-microfilament synergy in the cytoskeleton

This review describes examples of structural and functional synergy of the microtubule and actin filament cytoskeleton. An analysis of basal body (centriole)-associated fibrillar networks includes studies of ciliated epithelium, neurosensory epithelium, centrosomes, and ciliated protozoa. Microtubule and actin filament interactions in cell division and development are illustrated by centrosome motility, cleavage furrow positioning, centriole migration, nuclear migration, dynamics in the phragmoplast, growth cone motility, syncytial organization, and ring canals. Model systems currently used for studies on organelle transport are described in relation to mitochondrial transport in axons and vesicular transport in polarized epithelium. Evidence that both anterograde and retrograde motors are associated with one organelle is also discussed. The final section reviews proteins that bind both microtubules and actin filaments and are possible regulators of microtubule-microfilament interactions. Regulatory roles for posttranslational modifications, microtubule and microfilament dynamics, and multisubunit complexes are considered.

Microtubule reorganization is obligatory for growth cone turning

To examine the role of microtubules in growth cone turning, we have compared the microtubule organization in growth cones advancing on uniform laminin substrates with their organization in growth cones turning at a laminin-tenascin border. The majority (82%) of growth cones on laminin had a symmetrical microtubule organization, in which the microtubules entering the growth cone splay out toward the periphery of the growth cone. Growth cones at tenascin borders had symmetrically arranged microtubules in only 34% of cases, whereas in the majority of cases the microtubules were displaced toward one-half of the growth cone, presumably stabilizing in the direction of the turn along the tenascin border. These results suggest that reorganization of microtubules could underlie growth cone turning. Further evidence for the involvement of microtubule rearrangement in growth cone turning was provided by experiments in which growth cones approached tenascin borders in the presence of nanomolar concentrations of the microtubule stabilizing compound, Taxol. Taxol altered the organization of microtubules in growth cones growing on laminin by restricting their distribution to the proximal regions of the growth cone and increasing their bundling. Taxol did not stop growth cone advance on laminin. When growing in the presence of Taxol, growth cones at tenascin borders were not able to turn and grow along the laminin-tenascin border, and consequently stopped at the border. Growth cones were arrested at borders for as long as Taxol was present (up to 6 h) without showing any signs of drug toxicity. These effects of Taxol were reversible. Together, these results suggest that microtubule reorganization in growth cones is a necessary event in growth cone turning.

Tau binds to the distal axon early in development of polarity in a microtubule- and microfilament-dependent manner

Microtubule-associated protein tau is localized to the axon in situ and has been implicated in the development of neuronal polarity. Here we report that tau is extracted differentially in cultured hippocampal neurons yielding an axon-specific localization under conditions that keep the integrity of the plasma membrane. The amount of bound tau increases toward the distal axon and is highest at the transition from the axonal shaft to the growth cone. This distribution is significantly different from the distribution of axonal microtubules that are most concentrated at the proximal axon. Distal binding of tau to one process appears early in development of polarity in culture and correlates with the onset of axon formation (day 2 in culture). Binding to the distal axon requires intact microtubules and microfilaments. Distal tau binding does not stabilize microtubules selectively against drug-induced disassembly, because colchicine-induced microtubule depolymerization is highest distally. We conclude that binding of tau to the distal axon follows a complex mechanism, is an early event in the development of polarity, and reproduces the axon-specific localization of tau in situ.