Both adult and juvenile tau microtubule-associated proteins are axon specific in the developing and adult rat cerebellum

Wherefore Art Tau? Functional importance of site-specific tau phosphorylation in diverse subcellular domains

Tau has canonically been considered as an axonal protein, but studies have observed tau localization in other subcellular domains of neurons. This relocated tau has been identified in both physiological and pathological conditions, and it is often labeled mislocalized. Furthermore, these forms of tau are referred to as "hyperphosphorylated" without specifying the phosphosites involved. On the contrary, we speculate that tau may have multiple physiological functions in various locations regulated via specific phosphorylation sites, although this picture is obscured by a lack of comprehensive phosphosite analysis. Here, we examine findings in the literature on the subcellular location of tau and potential roles tau has in those regions. We intentionally focus on the site-specific phosphorylated patterns involved in governing these properties, which are not well elucidated. To facilitate understanding of these events, we have begun establishing a comprehensive map of tau phosphorylation signatures. Such efforts may clarify tau's diverse physiological functions beyond the axon as well as promote development of novel therapeutic strategies directed against distinct tau subpopulations.

Identification of Tau protein as a novel marker for maturation and pathological changes of oligodendrocytes

Microtubule-associated protein Tau is primarily expressed in axons of neurons, but also in Olig2-positive oligodendrocytes in adult rodent and monkey brains. In this study, we sought to determine at what cell stage Tau becomes expressed in the oligodendrocyte lineage. We performed immunostaining of adult mouse brain sections using well-known markers of oligodendrocyte lineage and found that Tau is expressed in mature oligodendrocytes, but not in oligodendrocyte progenitors and immature pre-oligodendrocytes. We also investigated Tau expression in developing mouse brain. Surprisingly, Tau expression occurred after the peak of myelination and even exceeded GSTπ expression, which has been considered as a marker of myelinating oligodendrocytes. These results suggest Tau as a novel marker of oligodendrocyte maturation. We then investigated whether Tau is important for oligodendrocyte development and/or myelination and how Tau changes in demyelination. First, we found no changes in myelination and oligodendrocyte markers in Tau knockout mice, suggesting that Tau is dispensable. Next, we analyzed the proteolipid protein 1 transgenic model of Pelizaeus-Merzbacher disease, which is a rare leukodystrophy. In hemizygous transgenic mice, the number of Tau-positive cells were significantly increased as compared with wild type mice. These cells were also positive for Olig2, CC1, and GSTπ, but not PDGFRα and GPR17. In stark contrast, the expression level of Tau, as well as GSTπ, was dramatically decreased in the cuprizone-induced model of multiple sclerosis. Taken together, we propose Tau as a new marker of oligodendrocyte lineage and for investigating demyelination lesions.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Structural study of the recognition mechanism of tau antibody Tau2r3 with the key sequence (VQIINK) in tau aggregation

One of the histopathological features of Alzheimer's disease (AD) is higher order neurofibrillary tangles formed by abnormally aggregated tau protein. The sequence ²⁷⁵VQIINK²⁸⁰ in the microtubule-binding domain of tau plays a key role in tau aggregation. Therefore, an aggregation inhibitor targeting the VQIINK region in tau may be an effective therapeutic agent for AD. We have previously shown that the Fab domain (Fab2r3) of a tau antibody that recognizes the VQIINK sequence can inhibit tau aggregation, and we have determined the tertiary structure of the Fab2r3-VQIINK complex. In this report, we determined the tertiary structure of apo Fab2r3 and analyzed differences in the structures of apo Fab2r3 and Fab2r3-VQIINK to examine the ligand recognition mechanism of Fab2r3. In comparison with the Fab2r3-VQIINK structure, there were large differences in the arrangement of the constant and variable domains in apo Fab2r3. Remarkable structural changes were especially observed in the H3 and L3 loop regions of the complementarity determining regions (CDRs) in apo Fab2r3 and the Fab2r3-VQIINK complex. These structural differences in CDRs suggest that formation of hydrophobic pockets suitable for the antigen is important for antigen recognition by tau antibodies.

Crystal structure of the human tau PHF core domain VQIINK complexed with the Fab domain of monoclonal antibody Tau2r3

Neurofibrillary tangles formed by abnormally aggregated tau protein are a histopathological feature of tauopathies. A tau aggregation inhibitor is a potential therapeutic agent for tauopathies. In this study, we prepared a monoclonal antibody for tau, monoclonal antibody to tau protein (Tau2r3), using as epitope the 272 GGKVQIINKKLD283 peptide in the microtubule-binding domain of tau, the key region mediating tau aggregation. We show that Tau2r3 clearly inhibits tau aggregation. To analyze the inhibition mechanism of Tau2r3, we solved the crystal structure of the Fab domain of Tau2r3 (Fab2r3) in complex with the VQIINK peptide. In the Fab2r3-VQIINK structure, the second and sixth polar residues and the fourth hydrophobic residue of VQIINK are crucial for binding to Fab2r3. The structural data for the Fab2r3-VQIINK complex could contribute to the design of new tau aggregation inhibitors.

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.

The toxin MPTP generates similar cognitive and locomotor deficits in hTau and tau knock-out mice

Parkinson's disease (PD) is characterized by motor deficits, although cognitive disturbances are frequent and have been noted early in the disease. The main pathological characteristics of PD are the loss of dopaminergic neurons and the presence of aggregated α-synuclein in Lewy bodies of surviving cells. Studies have also documented the presence of other proteins within Lewy bodies, particularly tau, a microtubule-associated protein implicated in a wide range of neurodegenerative diseases, including Alzheimer's disease (AD). In AD, tau pathology correlates with cognitive dysfunction, and tau mutations have been reported to lead to dementia associated with parkinsonism. However, the role of tau in PD pathogenesis remains unclear. To address this question, we induced parkinsonism by injecting the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in hTau mice, a mouse model of tauopathy expressing human tau, and a mouse model knock-out for tau (TKO). We found that although MPTP impaired locomotion (gait analysis) and cognition (Barnes maze), there were no discernable differences between hTau and TKO mice. MPTP also induced a slight but significant increase in tau phosphorylation (Thr205) in the hippocampus of hTau mice, as well as a significant decrease in the soluble and insoluble tau fractions that correlated with the loss of dopaminergic neurons in the brainstem. Overall, our findings suggest that, although MPTP can induce an increase in tau phosphorylation at specific epitopes, tau does not seem to causally contribute to cognitive and locomotor deficits induced by this toxin.

High-Molecular-Weight Paired Helical Filaments from Alzheimer Brain Induces Seeding of Wild-Type Mouse Tau into an Argyrophilic 4R Tau Pathology in Vivo

In Alzheimer disease, the development of tau pathology follows neuroanatomically connected pathways, suggesting that abnormal tau species might recruit normal tau by passage from cell to cell. Herein, we analyzed the effect of stereotaxic brain injection of human Alzheimer high-molecular-weight paired helical filaments (PHFs) in the dentate gyrus of wild-type and mutant tau THY-Tau22 mice. After 3 months of incubation, wild-type and THY-Tau22 mice developed an atrophy of the dentate gyrus and a tau pathology characterized by Gallyas and tau-positive grain-like inclusions into granule cells that extended in the hippocampal hilus and eventually away into the alveus, and the fimbria. Gallyas-positive neuropil threads and oligodendroglial coiled bodies were also observed. These tau inclusions were composed only of mouse tau, and were immunoreactive with antibodies to 4R tau, phosphotau, misfolded tau, ubiquitin, and p62. Although local hyperphosphorylation of tau was increased in the dentate gyrus in THY-Tau22 mice, the development of neurofibrillary tangles made of mutant human tau was not accelerated in the hippocampus, indicating that wild-type human PHFs were inefficient in seeding tau aggregates made of G272V/P301S mutant human tau. Our results indicate thus that injection of human wild-type Alzheimer PHF seeded aggregation of wild-type murine tau into an argyrophilic 4R tau pathology, and constitutes an interesting model independent of expression of a mutant tau protein.

The Tau/A152T mutation, a risk factor for frontotemporal-spectrum disorders, leads to NR2B receptor-mediated excitotoxicity

We report on a novel transgenic mouse model expressing human full-length Tau with the Tau mutation A152T (hTau(AT)), a risk factor for FTD-spectrum disorders including PSP and CBD Brain neurons reveal pathological Tau conformation, hyperphosphorylation, mis-sorting, aggregation, neuronal degeneration, and progressive loss, most prominently in area CA3 of the hippocampus. The mossy fiber pathway shows enhanced basal synaptic transmission without changes in short- or long-term plasticity. In organotypic hippocampal slices, extracellular glutamate increases early above control levels, followed by a rise in neurotoxicity. These changes are normalized by inhibiting neurotransmitter release or by blocking voltage-gated sodium channels. CA3 neurons show elevated intracellular calcium during rest and after activity induction which is sensitive to NR2B antagonizing drugs, demonstrating a pivotal role of extrasynaptic NMDA receptors. Slices show pronounced epileptiform activity and axonal sprouting of mossy fibers. Excitotoxic neuronal death is ameliorated by ceftriaxone, which stimulates astrocytic glutamate uptake via the transporter EAAT2/GLT1. In summary, hTau(AT) causes excitotoxicity mediated by NR2B-containing NMDA receptors due to enhanced extracellular glutamate.

Navigating the cell: how motors overcome roadblocks and traffic jams to efficiently transport cargo

Intracellular transport plays an essential role in maintaining the organization of cells. The importance of long-range, bi-directional transport is evidenced by the fact that its failure goes hand in hand with several diseases including neurodegenerative diseases such as Alzheimer's and Amyotrophic Lateral Sclerosis. The nanoscale cellular transport machinery consisting of cytoskeletal tracks and motor-proteins is responsible for effectively delivering important materials to specific locations inside the cell. Motor-proteins manage to overcome several challenges in the crowded cellular environment to achieve well-coordinated and effective transport. In recent years, thanks to state-of-the-art single molecule biophysical tools, we have started to gain insights into the cellular traffic rules. This perspective summarizes the challenges that motors face in navigating the complex cytoskeleton and the lessons learned about transport in crowded environments from both bottom-up in vitro studies as well as top-down in vivo studies.

Tau immunization: a cautionary tale?

The amyloid β (Aβ)-protein and microtubule-associated protein, tau, are the major components of the amyloid plaques and neurofibrillary tangles that typify Alzheimer's disease (AD) pathology. As such both Aβ and tau have long been proposed as therapeutic targets. Immunotherapy, particularly targeting Aβ, is currently the most advanced clinical strategy for treating AD. However, several Aβ-directed clinical trials have failed, and there is concern that targeting this protein may not be useful. In contrast, there is a growing optimism that tau immunotherapy may prove more efficacious. Here, for the first time, we studied the effects of chronic administration of an anti-tau monoclonal antibody (5E2) in amyloid precursor protein transgenic mice. For our animal model, we chose the J20 mouse line because prior studies had shown that the cognitive deficits in these mice require expression of tau. Despite the fact that 5E2 was present and active in the brains of immunized mice and that this antibody appeared to engage with extracellular tau, 5E2-treatment did not recover age-dependent spatial reference memory deficits. These results indicate that the memory impairment evident in J20 mice is unlikely to be mediated by a form of extracellular tau recognized by 5E2. In addition to the lack of positive effect of anti-tau immunotherapy, we also documented a significant increase in mortality among J20 mice that received 5E2. Because both the J20 mice used here and tau transgenic mice used in prior tau immunotherapy trials are imperfect models of AD our results recommend extensive preclinical testing of anti-tau antibody-based therapies using multiple mouse models and a variety of different anti-tau antibodies.

Biochemical Distribution of Tau Protein in Synaptosomal Fraction of Transgenic Mice Expressing Human P301L Tau

Alzheimer’s disease is a progressive dementia that is characterized by a loss of recent memory. Evidence has accumulated to support the hypothesis that synapses are critical storage sites for memory. However, it is still uncertain whether tau protein is involved in associative memory storage and whether tau is distributed in mature brain synapses. To address this question, we examined the synaptosomal distribution of tau protein in both JNPL3 transgenic mice expressing human P301L tau and non-transgenic littermates. The JNPL3 mouse line is known as one of the mouse models of human tauopathy that develop motor and behavioral deficits with intracellular tau aggregates in the spinal cord and brainstem. The phenotype of disease progression is highly dependent on strain background. In this study, we confirmed that male JNPL3 transgenic mice with C57BL/6J strain background showed neither any sign of motor deficits nor accumulation of hyperphosphorylated tau in the sarkosyl-insoluble fraction until 18 months of age. Subcellular fractionation analysis showed that both mouse tau and human P301L tau were present in the synaptosomal fraction. Those tau proteins were less-phosphorylated than tau in the cytosolic fraction. Human P301L tau was preferentially distributed in the synaptosomal fraction while mouse endogenous tau was more distributed in the cytosolic fraction. Interestingly, a human-specific tau band with phosphorylation at Ser199 and Ser396 was observed in the synaptosomal fraction of JNPL3 mice. This tau was not identical to either tau species in cytosolic fraction or a prominent hyperphosphorylated 64 kDa tau species that was altered to tau pathology. These results suggest that exogenous human P301L tau induces synaptosomal distribution of tau protein with a certain phosphorylation. Regulating the synaptosomal tau level might be a potential target for a therapeutic intervention directed at preventing neurodegeneration.

Regulatable transgenic mouse models of Alzheimer disease: onset, reversibility and spreading of Tau pathology

Accumulation of amyloidogenic proteins such as Tau is a hallmark of neurodegenerative diseases including Alzheimer disease and fronto-temporal dementias. To link Tau pathology to cognitive impairments and defects in synaptic plasticity, we created four inducible Tau transgenic mouse models with expression of pro- and anti-aggregant variants of either full-length human Tau (hTau40/ΔK280 and hTau40/ΔK280/PP) or the truncated Tau repeat domain (Tau(RD)/ΔK280 and Tau(RD)/ΔK280/PP). Here we review the histopathological features caused by pro-aggregant Tau, and correlate them with behavioral deficits and impairments in synaptic transmission. Both pro-aggregant Tau variants cause Alzheimer-like features, including synapse loss, mis-localization of Tau into the somatodendritic compartment, conformational changes and hyperphosphorylation. However, there is a clear difference in the extent of Tau aggregation and neurotoxicity. While pro-aggregant full-length hTau40/ΔK280 leads to a 'pre-tangle' pathology, the repeat domain Tau(RD)/ΔK280 causes massive formation of neurofibrillary tangles and neuronal loss in the hippocampus. However, both Tau variants cause co-aggregation of human and mouse Tau and similar functional impairments. Thus, earlier Tau pathological stages and not necessarily neurofibrillary tangles are critical for the development of cognitive malfunctions. Most importantly, memory and synapses recover after switching off expression of pro-aggregant Tau. The rescue of functional impairments correlates with the rescue of most Tau pathological changes and most strikingly the recovery of synapses. This implies that tauopathies as such are reversible, provided that amyloidogenic Tau is removed. Therefore, our Tau transgenic mice may serve as model systems for in vivo validation of therapeutic strategies and drug candidates with regard to cognition and synaptic function.

Neural progenitors derived from human induced pluripotent stem cells survive and differentiate upon transplantation into a rat model of amyotrophic lateral sclerosis

Human induced pluripotent stem cells (iPSCs) offer hope for personalized regenerative cell therapy in amyotrophic lateral sclerosis (ALS). We analyzed the fate of human iPSC-derived neural progenitors transplanted into the spinal cord of wild-type and transgenic rats carrying a human mutated SOD1(G93A) gene. The aim was to follow survival and differentiation of human neural progenitors until day 60 post-transplantation in two different in vivo environments, one being ALS-like. iPSC-derived neural progenitors efficiently engrafted in the adult spinal cord and survived at high numbers. Different neural progenitor, astroglial, and neuronal markers indicated that, over time, the transplanted nestin-positive cells differentiated into cells displaying a neuronal phenotype in both wild-type and transgenic SOD1 rats. Although a transient microglial phenotype was detected at day 15, astroglial staining was negative in engrafted cells from day 1 to day 60. At day 30, differentiation toward a neuronal phenotype was identified, which was further established at day 60 by the expression of the neuronal marker MAP2. A specification process into motoneuron-like structures was evidenced in the ventral horns in both wild-type and SOD1 rats. Our results demonstrate proof-of-principle of survival and differentiation of human iPSC-derived neural progenitors in in vivo ALS environment, offering perspectives for the use of iPSC-based therapy in ALS.

Identification of feline panleukopenia virus proteins expressed in Purkinje cell nuclei of cats with cerebellar hypoplasia

Parvoviruses depend on initiation of host cell division for their replication. Undefined parvoviral proteins have been detected in Purkinje cells of the cerebellum after experimental feline panleukopenia virus (FPV) infection of neonatal kittens and in naturally occurring cases of feline cerebellar hypoplasia. In this study, a parvoviral protein in the nucleus of Purkinje cells of kittens with cerebellar hypoplasia was shown by immunoprecipitation to be the FPV viral capsid protein VP2. In PCR-confirmed, FPV-associated feline cerebellar hypoplasia, expression of the FPV VP2 protein was demonstrated by immunohistochemistry in Purkinje cell nuclei in 4/10 cases and expression of the FPV non-structural protein NS1 was demonstrated in Purkinje cell nuclei in 5/10 cases. Increased nuclear ERK1 expression was observed in several Purkinje cells in 1/10 kittens. No expression of the G1 and S mitotic phase marker proliferating cell nuclear antigen (PCNA) was evident in Purkinje cell nuclei. These results support the hypothesis that FPV is able to proceed far into its replication cycle in post-mitotic Purkinje cells.

Cognitive defects are reversible in inducible mice expressing pro-aggregant full-length human Tau

Neurofibrillary lesions of abnormal Tau are hallmarks of Alzheimer disease and frontotemporal dementias. Our regulatable (Tet-OFF) mouse models of tauopathy express variants of human full-length Tau in the forebrain (CaMKIIα promoter) either with mutation ΔK280 (pro-aggregant) or ΔK280/I277P/I308P (anti-aggregant). Co-expression of luciferase enables in vivo quantification of gene expression by bioluminescence imaging. Pro-aggregant mice develop synapse loss and Tau-pathology including missorting, phosphorylation and early pretangle formation, whereas anti-aggregant mice do not. We correlated hippocampal Tau pathology with learning/memory performance and synaptic plasticity. Pro-aggregant mice at 16 months of gene expression exhibited severe cognitive deficits in Morris water maze and in passive-avoidance paradigms, whereas anti-aggregant mice were comparable to controls. Cognitive impairment of pro-aggregant mice was accompanied by loss of hippocampal LTP in CA1 and CA3 areas and by a reduction of synaptic proteins and dendritic spines, although no neuronal loss was observed. Remarkably, memory and LTP recovered when pro-aggregant Tau was switched-OFF for ~4 months, Tau phosphorylation and missorting were reversed, and synapses recovered. Moreover, soluble and insoluble pro-aggregant hTau40 disappeared, while insoluble mouse Tau was still present. This study links early Tau pathology without neurofibrillary tangles and neuronal death to cognitive decline and synaptic dysfunction. It demonstrates that Tau-induced impairments are reversible after switching-OFF pro-aggregant Tau. Therefore, our mouse model may mimic an early phase of AD when the hippocampus does not yet suffer from irreversible cell death but cognitive deficits are already striking. It offers potential to evaluate drugs with regard to learning and memory performance.

Chronic restraint stress alters the expression and distribution of phosphorylated tau and MAP2 in cortex and hippocampus of rat brain

Microtubule-associated proteins (MAPs) play a critical role in maintaining normal cytoskeletal architecture and functions. In the present study, we aim to explore the effects of the emotional stressor, chronic restraint stress, on the expression levels and localization of tau and MAP2. We found that after chronic restraint stress, soluble hyperphosphorylated tau was greatly increased, whereas MAP2 was decreased. Moreover, immunohistochemistry analysis demonstrated that phosphorylated tau and MAP2 displayed the similar subcellular distribution pattern after chronic restraint stress. Robust hyperphosphorylated tau immunolabeling was observed both in cortex and hippocampus of stressed animals and mainly located to perikaryal/dendritic elements. After stress, the MAP2 was mainly distributed in the perikaryal compartments, discontinuous dendrites and neuropil. Moreover, the distribution pattern of MAP2 in hippocampus significantly changed. Immunofluorescence double labeling indicated that hyperphosphorylated tau increased in the regions where there displayed an decrease of MAP2. These results suggest that the involvement of repeated restraint stress may not only induce phosphorylation state of tau and distribution of phosphorylated tau, but also alter the content and neuronal distribution of MAP2. Tau and MAP2 are most important MAPs for neuronal cells, the subcellular distribution change of them might be link to functional change of neurons after emotional stress.

Dynamic changes of phosphorylated tau in mouse hippocampus after cold water stress

The abnormal hyperphosphorylation of tau protein in brain is attributed to a number of neurodegenerative diseases such as Alzheimer disease. It has been reported that cold water stress (CWS) could cause rapid reversible tau phosphorylation in brain. To explore the possible long-tem effects of CWS on tau phosphorylation, we employed the immunoblot and immunohistochemical methods to analyze the phosphorylation of tau in the hippocampus of mice subjected to CWS. Results showed that CWS stimulation caused not only an early phase reversible tau phosphorylation, but also a later phase tau phosphorylation after 6h. The distribution pattern of phosphorylated tau (P-tau) in the later phase was different to that of early phase. At 1h after CWS, defined as early phase, P-tau was strikingly located in the mossy fibers and nerve terminals at the molecular layer of dentate gray (DG), whereas at 12h, defined as later phase, P-tau was dominantly located in the somatodendritic compartments of neurons in DG and CA3/CA1 regions, but obviously decreased in the mossy fibers and nerve terminals of molecular layer. These findings demonstrate that CWS leads to prominent changes of tau phosphorylation and P-tau localization in the hippocampus in a time dependent manner.

Characterisation of cytoskeletal abnormalities in mice transgenic for wild-type human tau and familial Alzheimer's disease mutants of APP and presenilin-1

To study the role of Abeta amyloid deposits in the generation of cytoskeletal lesions, we have generated a transgenic mouse line coexpressing in the same neurons a wild-type human tau isoform (0N3R), a mutant form of APP (751SL) and a mutant form of PS1 (M146L). These mice developed early cerebral extracellular deposits of Abeta, starting at 2.5 months. A somatodendritic neuronal accumulation of transgenic tau protein was observed in tau only and in tau/PS1/APP transgenic mice, including in neurons adjacent to Abeta deposits. The phosphorylation status of this somatodendritic tau was similar in the two transgenic lines. The Abeta deposits were surrounded by a neuritic reaction composed of axonal dystrophic processes, immunoreactive for many phosphotau epitopes and for the human tau transgenic protein. Ultrastructural observation showed in these dystrophic neurites a disorganisation of the microtubule and the neurofilament network but animals that were observed up to 18 months of age did not develop neurofibrillary tangles. These results indicate that overexpression of mutant PS1, mutant APP and of wild-type human tau were not sufficient per se to drive the formation of neurofibrillary tangles in a transgenic model. The Abeta deposits, however, were associated to marked changes in cytoskeletal organisation and in tau phosphorylation in adjacent dystrophic neurites.

Olivopontocerebellar atrophy in two adult cats, sporadic cases or new genetic entity

Two otherwise healthy adult cats were presented with progressive cerebellar signs of different severity. Owners requested euthanasia. Necropsy disclosed whole cerebellum and pontine atrophy, with a severity paralleling the neurologic dysfunction. We used cell type-specific immunolabelings to characterize the lesions. The severity of the cerebellar cortex atrophy followed a general gradient from the midvermis toward the hemispheres and a local gradient from the depth of the folia toward their tip. Along these gradients, Purkinje cells were the first to disappear, followed by basket, Golgi, and stellate cells, and eventually by granule cells. Bergmann glia cells and unipolar brush cells were preserved. Pontine nuclei and the olivary complex were also severely depopulated. Neurons in the cerebellar nuclei, vestibular nuclei, and other cerebellar system-associated structures were preserved, as well as substantia nigra. Olivopontocerebellar atrophy (OPCA) in a domestic animal species was rarely reported. Some features allow tentative linking to either familial or sporadic OPCA of humans. However, the ordered disappearance of all cortical neuronal types has never been described before. Either this entity is cat specific or it might pinpoint the need for increased knowledge about differential gene expression depending on genetic background, i.e., among different species. It also would open prospects about gene product interactions within neurons.

Functional activation of cerebral metabolism in mice with mutated thyroid hormone nuclear receptors

Neonatal hypothyroidism impairs structural maturation in the brain and results in diminished electrical activities and energy metabolism. We recently found that glucose utilization (CMR(glc)) is markedly depressed throughout the brain in mice with targeted mutations in thyroid hormone receptor alpha1 (TR alpha 1), but not TR beta. Previous studies had shown that CMR(glc) increases linearly with spike frequency in the afferent pathways to synapse-rich regions in neuropil, but not in neuronal cell bodies. To determine whether the decreased CMR(glc) in mutant TR alpha 1(PV/+) mice reflected lesser synaptic density or reduced functional activity in existing synapses, we stimulated vibrissae unilaterally and measured CMR(glc) bilaterally in four stations of the whisker-to-barrel cortex pathway. Baseline CMR(glc) (unstimulated side) was markedly lower in all four stations in the TR alpha 1(PV/+) mutants than in wild-type controls, even though Northern blot and immunohistochemical examinations showed normal Na(+),K(+)-adenosine triphosphatase expression and neuronal differentiation. Despite the lower baseline CMR(glc), however, vibrissal stimulation evoked percent increases in CMR(glc) in the TR alpha 1(PV/+) mutants that were as great as those in wild-type mice. These results indicate that in the TR alpha 1(PV/+) mutants there it is a reduction in synaptic density that is responsible for the decrease in CMR(glc), but functionality of existing synapses is retained.

Tau and axonopathy in neurodegenerative disorders

The microtubule (MT)-associated protein (MAP) tau in neurons has been implicated as a significant factor in the axonal growth, development of neuronal polarity, and the maintenance of MT dynamics. Tau is localized to the axon, and is known to promote MT assembly and to stabilize axonal MTs. These functions of tau are primarily regulated by the activities of protein kinases and phosphatases. In Alzheimer's disease and other neurodegenerative disorders, abundant filamentous tau inclusions are found to be major neuropathological characteristics of these diseases. Both somato-dendritic and axonal tau lesions appear to be closely associated with axonal disruption. Furthermore, recent discoveries of pathogenic mutations on the tau gene suggest that abnormalities of tau alone are causative of neurodegeneration. Finally, analyses of transgenic mice that express human tau proteins have enabled in vivo quantitative assessments of axonal functions and have provided information about mechanistic relationships between pathological alteration of tau and axonal degeneration.

Interferon gamma induces retrograde dendritic retraction and inhibits synapse formation

The expression of interferon gamma (IFNgamma) increases after neural injury, and it is sustained in chronic inflammatory conditions such as multiple sclerosis and infection with human immunodeficiency virus. To understand how exposure to this proinflammatory cytokine might affect neural function, we examined its effects on cultures of neurons derived from the central and peripheral nervous systems. IFNgamma inhibits initial dendritic outgrowth in cultures of embryonic rat sympathetic and hippocampal neurons, and this inhibitory effect on process growth is associated with a decrease in the rate of synapse formation. In addition, in older cultures of sympathetic neurons, IFNgamma also selectively induces retraction of existing dendrites, ultimately leading to an 88% decrease in the size of the arbor. Dendritic retraction induced by IFNgamma represents a specific cellular response because it occurs without affecting axonal outgrowth or cell survival, and it is not observed with tumor necrosis factor alpha or other inflammatory cytokines. IFNgamma-induced dendritic retraction is associated with the phosphorylation and nuclear translocation of signal transducer and activator of transcription 1 (STAT1), and expression of a dominant-negative STAT1 construct attenuates the inhibitory effect of IFNgamma. Moreover, retrograde dendritic retraction is observed when distal axons are selectively exposed to IFNgamma. These data imply that IFNgamma-mediated STAT1 activation induces both dendritic atrophy and synaptic loss and that this occurs both at the sites of IFNgamma release and at remote loci. Regressive actions of IFNgamma on dendrites may contribute to the neuropathology of inflammatory diseases.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Insulin-like growth factor-1 and insulin mediate transient site-selective increases in tau phosphorylation in primary cortical neurons

The modulation of tau phosphorylation and localization in response to insulin-like growth factor-1 or insulin was examined in primary cultures of rat cortical neurons. Insulin and insulin-like growth factor-1 treatment resulted in a rapid and transient increase in tau phosphorylation at specific epitopes. These effects were completely inhibited by lithium, revealing that the insulin and insulin-like growth factor-1 induced changes in tau phosphorylation were mediated by glycogen synthase kinase-3beta. In addition, the increase in tau phosphorylation directly correlated with a transient dissociation of tau from the cytoskeleton, indicating that insulin and insulin-like growth factor-1 treatment resulted in a change in tau localization. Using immunocytochemistry, it was also demonstrated that treatment of neurons with insulin-like growth factor-1 for 3 min resulted in a redistribution of tau to the growth cone and the distal segment of the axons. Further, insulin-like growth factor-1 treatment resulted in an increased immunoreactivity with the phospho-dependent antibody AT8 in the same areas of the axons. Thus, the phosphorylation state and distribution of tau can be modulated by insulin and insulin-like growth factor-1 signaling pathways involving glycogen synthase kinase-3beta. We propose that by transiently increasing tau phosphorylation, insulin and insulin-like growth factor-1 may contribute to the reorganization of the cytoskeleton necessary for the development and growth of the neurites.

Phosphorylation of tau alters its association with the plasma membrane

1. The potential functions of the microtubule-associated protein tau have been expanded by the recent demonstration of its interaction with the plasma membrane. Since the association of tau with microtubules is regulated by phosphorylation, herein we examine whether or not the association of tau with the plasma membrane is also regulated by phosphorylation. 2. A range of tau isoforms migrating from 46 to 64 kDa was associated with crude particulate fractions derived from SH-SY-5Y human neuroblastoma cells, and were retained during the initial stages of plasma membrane purification. During the extensive washing utilized in purification of the plasma membrane, portions of each of these isoforms were depleted from the resultant purified membrane. Immunoblot analysis with phospho-dependent and -independent antibodies revealed selective depletion of phospho isoforms during membrane washing. This effect was more pronounced for the slowest-migrating (64-kDa) tau isoform. 3. This putative influence of phosphorylation on the association of tau with the plasma membrane was further probed by transfection of SH-SY-5Y human neuroblastoma cells with a tau construct that could associate with the plasma membrane but not with microtubules. Treatment with phorbol ester or calcium ionophore, both of which increased phospho-tau levels within the cytosol and plasma membrane, was accompanied by the dissociation of this tau construct from the membrane. 4. These data indicate that phosphorylation regulates the association with the plasma membrane. Dissociation from the membrane by phosphorylation may place tau at risk for hyperphosphorylation and ultimate PHF formation in a manner previously considered for tau dissociated from microtubules.

Nucleolar localization of the microtubule-associated protein tau in neuroblastomas using sense and anti-sense transfection strategies

The Tau-1 monoclonal antibody was localized to the nucleolus of interphase cells and the nucleolar organizing regions (NORs) of acrocentric chromosomes in cultured human cells. Putative nucleolar and NOR tau was found in CG neuroblastoma cells which contain nucleolar tau and little cytoplasmic tau. To further establish the presence of tau in the nucleolus of these cells, sense and anti-sense transfection strategies were used. CG neuroblastoma cells were transfected with tau sense cDNA and immunostained with Tau-1. Cytoplasmic Tau-1 staining was greatly increased in CG cells which contain very little endogenous cytoplasmic tau. Nucleolar Tau-1 staining was also increased in certain CG cells indicating an increase in nucleolar tau in a subset of transfected cells. CG cells were also transfected with tau anti-sense cDNA which abolished Tau-1 staining in the nucleolus. These results contribute to a growing body of evidence defining tau as a multifunctional protein found in both the cytoplasm and nucleoli of primate cells.

Increased dendritic MAP2 expression in the hippocampus in schizophrenia

Microtubule associated proteins (MAPs) are central to the development of normal neuronal cytoarchitecture and have been reported to be altered in schizophrenia. In 12 schizophrenic (DSM-III-R criteria) and 12 control hippocampi, we estimated the MAP2 immunoreactive dendritic length using antibodies that recognize total MAP2 (MAP2-T), and a non-phosphorylated form of MAP2 (MAP2-NP). Within the corona ammonis (CA) subregions, and the subiculum, we estimated, for each antibody, the length of the immunoreactive dendritic arborisation using a stereological length estimation technique based on Bouffon's Needle principle and image analysis computer software. Controlling for the confounding effects of age and post-mortem delay, we have found an elevation in overall MAP2-NP immunoreactive dendritic length among schizophrenic subjects in the CA3 (F=5.9, p=0.03), CA2 (F=6.5, p=0.02), CA1 (F=8.3, p=0.01) and subicular (F=9.5, p=0.008) hippocampal subregions. Similar analyses of MAP2-T immunoreactive dendritic length demonstrated significant elevations in the CA1 (F=8.3, p=0.02), CA4 (F=4.9, p=0.04) and subicular (F=7.4, p=0.01) regions. The findings of this quantitative study of increased MAP2 immunoreactive dendritic arborisation in schizophrenia are most likely to reflect either an altered dendritic arborisation or a generalised increase in levels of MAP2 with the hippocampal pyramidal neurons. These findings add to the growing literature indicating the presence of synaptodendritic abnormalities in schizophrenia.

Neuronal morphology, axonal integrity, and axonal regeneration in situ are regulated by cytoskeletal phosphorylation in identified lamprey central neurons

The CNS of the sea lamprey (Petromyzon marinus) contains giant, individually identifiable neurons that can be microinjected intracellularly in the living animal. We have used the unique accessibility of this system to investigate the role played by serine/threonine kinases and phosphatases in regulating cytoskeletal stability in identified reticulospinal neurons (ABCs) in situ. Injection of broad spectrum kinase and phosphatase inhibitors induce marked changes in ABC gross morphology and in the extent and morphology of sprouts induced by axotomy. The kinase inhibitor K-252a causes regenerating sprouts to be longer and narrower than those seen in control preparations, and significantly reduces the diameters of axon stumps; this latter effect is similar to the effect of microinjecting anti neurofilament (NF) antibodies. By contrast, the phosphatase inhibitor okadaic acid (OA) causes the selective disruption of axonal integrity, blocking axonal regeneration and causing axon stump retraction in axotomized ABCs. The microtubule (MT) disrupting drug colchicine has an effect similar but less marked than OA on ABC axonal morphology. Both OA and colchicine also induce the formation of large somatodendritic swellings in axotomized (but not intact) ABCs by 1-3 weeks post-injection. Immunocytochemical analyses indicate that both colchicine and OA treatments result in the destabilization of MTs and the phosphorylation of NFs, while OA induces the accumulation of phosphorylated tau protein in some dendritic swellings. Control injections of inactive substances have none of these effects. These results suggest that OA does not have its primary effect on NF assembly at the doses used, but may block axonal regeneration by inducing a prolonged disruption of axonal MTs, possibly via an indirect mechanism involving the hyperphosphorylation of tau and other MAPs. K-252a, on the other hand, may interfere with NF assembly and sidearm phosphorylation, thereby reducing NF transport into both axon stumps and sprouts and in turn reducing sprout diameter. The implications of these results for the respective roles of MTs, MAPs, and NFs in axonal regeneration in the vertebrate CNS are discussed.

Ephrin-dependent growth and pruning of hippocampal axons

Neuronal connections are arranged topographically such that the spatial organization of neurons is preserved by their termini in the targets. During the development of topographic projections, axons initially explore areas much wider than the final targets, and mistargeted axons are pruned later. The molecules regulating these processes are not known. We report here that the ligands of the Eph family tyrosine kinase receptors may regulate both the initial outgrowth and the subsequent pruning of axons. In the presence of ephrins, the outgrowth and branching of the receptor-positive hippocampal axons are enhanced. However, these axons are induced later to degenerate. These observations suggest that the ephrins and their receptors may regulate topographic map formation by stimulating axonal arborization and by pruning mistargeted axons.

Transgenic expression of the shortest human tau affects its compartmentalization and its phosphorylation as in the pretangle stage of Alzheimer's disease

We have generated transgenic mice expressing the shortest human tau protein, the microtubule-associated protein that composes paired helical filaments in Alzheimer's disease. Transgenic tau transcripts and proteins were strongly expressed in neurons in the developing and adult brain. In contrast to the endogenous tau that progressively disappeared from neuronal cell bodies during development, the human transgenic tau remained abundant in cell bodies and dendrites of a subset of neurons in the adult. This somatodendritic transgenic tau was immunoreactive with antibodies to tau phosphorylated on Thr181 and Thr231 and with the conformation-dependent Alz50 antibody. A few astrocytes expressing the transgenic tau were strongly immunoreactive with antibodies to additional tau phosphorylation sites, ie, at Ser262/ 356 and Ser396/404. All of these phosphorylation sites have been identified in paired helical filaments-tau proteins. In electron microscopy, the transgenic tau was detected into microtubules in axons and in dendrites but not in cell bodies. Neurofibrillary tangles were not detected in transgenic animals examined up to the age of 19 months. These results indicate that transgenic manipulation of tau expression and intracellular targeting is sufficient per se to affect tau compartmentalization, phosphorylation, and conformation partly as it is observed at the pretangle stage in Alzheimer's disease.

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.

Neurofibrillary tangles and Alzheimer's disease

The neuropathological diagnosis of Alzheimer's disease relies on the presence of both neurofibrillary tangles and senile plaques. The number of neurofibrillary tangles is tightly linked to the degree of dementia, suggesting that the formation of neurofibrillary tangles more directly correlates with neuronal dysfunction. The regional pattern of areas affected by neurofibrillary tangles formation during the course of the disease is relatively stereotyped. Neurofibrillary tangles are composed of highly phosphorylated forms of the microtubule-associated protein tau. Phosphorylated tau proteins accumulate early in neurons, even before formation of neurofibrillary tangles, suggesting that an imbalance between the activities of protein kinases and phosphatases acting on tau is an early phenomenon. The latter might be related to changes in signalling through transduction cascades, since many of the protein kinases generating phosphorylated tau species participate in signalling pathways. The accumulation of neurofibrillary tangles and phosphorylated tau species is associated with disturbances of the microtubule network and, as a consequence of the latter, of axoplasmic flows. The mechanistic relationship between the formation of neurofibrillary tangles and senile plaques is still little understood and in vivo formation of neurofibrillary tangles in experimental models has not yet been achieved. Future animal models, e.g. transgenic animals expressing combined key human proteins, will hopefully reproduce faithfully all the major cellular lesions of the disease.

Protein phosphatase 1 is targeted to microtubules by the microtubule-associated protein Tau

Phosphorylation has been implicated in the regulation of microtubule (MT) stability and function by controlling the interactions between MTs and MT-associated proteins. We found previously that protein phosphatase inhibitors selectively break down stable MTs, suggesting that protein phosphatases may be involved in regulating MT stability. To identify the protein phosphatases involved, we examined purified calf brain MTs and found a protein phosphatase activity that copurified with MTs to constant stoichiometry. Western blot analysis and inhibitor profiles demonstrated that the MT-associated phosphatase was a type 1 protein phosphatase (PP1), which we named PP1MT. Recombinant PP1 catalytic subunit (PP1c) did not bind to MTs, whereas PP1MT did bind, suggesting the presence of proteins that target PP1 to MTs. By Sepharose CL-6B chromatography, the phosphatase activity of PP1MT eluted as a large protein complex of approximately 400 kDa. High salt (2 M NaCl) treatment followed by CL-6B chromatography dissociated PP1MT into PP1c and the MT-targeting subunit(s). The MT-targeting subunit was shown to be the MT-associated protein tau by PP1 blot overlays and other assays. Also, recombinant tau reconstituted the binding of PP1c to MTs. These results identify PP1 as the first tau binding protein and suggest that tau is a novel PP1-targeting subunit.

Mitotic phosphoepitopes precede paired helical filaments in Alzheimer's disease

We have shown previously that the TG-3 and MPM-2 antibodies recognize phosphoepitopes common to mitosis and degenerating neurons of Alzheimer's disease(AD) brain. Here, we have evaluated their occurrence in human brain biopsy tissue, and confirm that they are absent in mature neurons of adult brain, but reappear during neurodegeneration in AD. The TG-3 epitope appears ahead of the MPM-2 epitope and is distributed throughout the neuronal soma. Tau is the major TG-3 antigen in AD brain. The initial localization of MPM-2 immunoreactivity in primary dendrites, it's robust occurrence in granulovacuolar bodies, and the increased immunoreactivity with 300-350-kDa proteins, suggest MAPI B as a candidate MPM-2 antigen in AD. Production of mitotic phosphepitopes in more than one type of human neurodegenerative lesion implicates mitotic kinases as common mediators of neuronal death. Because mitotic phosphoepitopes appear before paired helical filaments, it is suggested that mitotic kinase activation triggers neurofibrillary tangle formation. Future studies will need to focus on factors influencing mitotic kinase activity, a point with potential for early diagnosis and disease abrogation.

Microtubule-associated proteins (MAPs) in the peripheral nervous system during development and regeneration

In this article, we have described the structure and distribution of the various variants of the microtubule-associated proteins (MAPs), tau, MAP2, MAP1A, and MAP1B, that are expressed in the dorsal root ganglion (DRG) and spinal cord during development and regeneration. We have summarized the data on their gene structure and compared the sequence of the major transcripts encoding these MAPs that are expressed in the brain, the spinal cord, and the DRG. Finally, we have surveyed the studies that used a variety of experimental approaches (e.g., antisense inhibition, transgenic knockouts, and expression in neuronal and nonneuronal cells) to understand the functional significance of MAPs heterogeneity and differences observed between the central nervous system (CNS) and the peripheral nervous system (PNS) both during development and regeneration.

Human tau becomes phosphorylated and forms filamentous deposits when overexpressed in lamprey central neurons in situ

Microinjection of plasmids encoding human tau (htau) protein into identified lamprey reticulospinal neurons (anterior bulbar cells, or ABCs) in situ induces chronic htau expression. htau protein is transported to both the axon and dendrites of expressing ABCs by mechanisms that require the C-terminal domain of htau protein but do not require directed htau mRNA transport. htau becomes phosphorylated at the PHF-1 (Ser-396/404) and TAU-1/AT8 (Ser-199/202) epitopes throughout ABCs with heavy htau accumulations; many such cells also exhibit degenerative changes, which include the development of extracellular htau deposits. Finally, expression of htau protein fused to green fluorescent protein induced the somatodendritic accumulation of filaments containing htau when examined by immunoelectron microscopy. These results suggest that chronic expression of htau in lamprey ABCs may be useful for studying cellular mechanisms underlying tau hyperphosphorylation and filament formation in vertebrate central neurons in situ.

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.

A spatial gradient of tau protein phosphorylation in nascent axons

Mechanisms underlying axonogenesis remain obscure. Although a large number of proteins eventually become polarized to the axonal domain, in no case does protein compartmentalization occur before or simultaneous with the earliest morphological expression of axonal properties. How then might initially unpolarized proteins, such as the microtubule-associated protein tau, play a role in the microdifferentiation of axons? We hypothesized that tau function could be locally regulated by phosphorylation during the period of axonogenesis. To test this hypothesis, we mapped relative levels of tau phosphorylation within developing cultured hippocampal neurons. This was accomplished using calibrated immunofluorescence ratio measurements employing phosphorylation state-dependent and state-independent antibodies. Tau in the nascent axon is more highly dephosphorylated at the site recognized by the tau-1 antibody than tau in the somatodendritic compartment. The change in phosphorylation state from soma to axon takes the form of a smooth proximo-distal gradient, with tau in the soma, immature dendrites and proximal axon approximately 80% phosphorylated at the tau-1 site, and that in the axonal growth cone only 20% phosphorylated. The existence of real spatial differences in tau phosphorylation state was confirmed by in situ phosphatase and kinase treatment. Pervanadate, a tyrosine phosphatase inhibitor, induced rapid tau dephosphorylation within live cells, effectively abolishing the phosphorylation gradient. Thus, the gradient is dynamic and potentially regulatable by upstream signals involving tyrosine phosphorylation. Phosphorylation gradients are likely to be present on many neuronal proteins in addition to tau, and their modulation by transmembrane signals could direct the establishment of polarity.

The ganglionic eminence may be an intermediate target for corticofugal and thalamocortical axons

In the nervous system of many species, growing axons associate transiently with cellular groupings along their path. Whether this mechanism applies to the development of corticothalamic and thalamocortical projections is unknown. Using carbocyanine dyes, we studied the early growth of both corticofugal and thalamocortical fibers in hamster embryos. At embryonic day 11.5 (E11.5), corticofugal fibers invade the lateral ganglionic eminence (LGE), and thalamocortical fibers invade the medial ganglionic eminence (MGE). At this age, both sets of fibers are not yet in contact with each other. At the same time, neurons in each subdivision of the GE grow toward the cortex and thalamus. During the next 24 hr, corticofugal and thalamocortical fibers remain within the confines of the GE, where they course at different radial levels and bear large and complex growth cones. In the LGE, corticofugal fibers are often found in close association with cells that are likely to be neuronal. Starting on E13.5, both early projections from the GE decrease, and corticothalamic and thalamocortical fibers invade their definitive target regions. To test whether the GE specifically orients the growth and trajectories of cortical fibers even in the absence of the reciprocal thalamic projection, we cocultured explants of cortex and GE from either hamster or mouse embryos. These experiments showed that the GE, but not other tested brain regions, is able specifically to orient the growth of cortical axons. We therefore suggest that the GE may be an intermediate target in the pathfinding of axons between the cortex and the thalamus.

New forms of HMW MAP2 are preferentially expressed in the spinal cord

The high molecular weight forms of microtubule-associated protein 2 (MAP2a and b) play a central role in the specification of dendrites. RT-PCR amplification of a portion of the N-terminal and middle MAP2b domains of rat spinal cord cDNAs allowed identification of new variants containing both exon 8 (246 bp) and a new exon, 7A (237 bp), located at the beginning of the middle MAP2b region. The brain and the spinal cord express transcripts containing exon 8, whereas exon 7A alone or exons 7A+8 were detected, whatever the developmental stage, only in the spinal cord.

Neurodegenerative changes including altered tau phosphorylation and neurofilament immunoreactivity in mice transgenic for the serine/threonine kinase Mos

Transgenic mice expressing the oncogenic protein-serine/threonine kinase Mos at high levels in the brain display progressive neuronal degeneration and gliosis. Gliosis developed in parallel with the onset of postnatal transgene expression and led to a dramatic increase in the number of astrocytes positive for GFAP, vimentin, and possibly tau. Interestingly, vimentin is normally expressed only in immature or neoplastic astrocytes, but appears to be induced to high levels in Mos-transgenic, mature astrocytes. Mos can activate mitogen activated protein kinase (MAPK) and MAPK has been implicated in Alzheimer-type tau phosphorylation. In the Mos-transgenic brain we found increased levels of phosphorylation at one epitope on tau containing serines 199 and 202 (numbering according to human tau), a pattern similar but not identical to that found in Alzheimer's disease. In addition, Mos-transgenic mice express a novel neurofilament-related protein that might be a proteolytic neurofilament heavy chain degradation product. These results suggest that activation of protein phosphorylation in neurons can result in changes in cytoskeletal proteins that might contribute to neuronal degeneration.

Selective stabilization of tau in axons and microtubule-associated protein 2C in cell bodies and dendrites contributes to polarized localization of cytoskeletal proteins in mature neurons

In mature neurons, tau is abundant in axons, whereas microtubule-associated protein 2 (MAP2) and MAP2C are specifically localized in dendrites. Known mechanisms involved in the compartmentalization of these cytoskeletal proteins include the differential localization of mRNA (MAP2 mRNA in dendrites, MAP2C mRNA in cell body, and Tau mRNA in proximal axon revealed by in situ hybridization) (Garner, C.C., R.P. Tucker, and A. Matus. 1988. Nature (Lond.). 336:674-677; Litman, P., J. Barg, L. Rindzooski, and I. Ginzburg. 1993. Neuron. 10:627-638), suppressed transit of MAP2 into axons (revealed by cDNA transfection into neurons) (Kanai, Y., and N. Hirokawa. 1995. Neuron. 14:421-432), and differential turnover of MAP2 in axons vs dendrites (Okabe, S., and N. Hirokawa. 1989. Proc. Natl. Acad. Sci. USA. 86:4127-4131). To investigate whether differential turnover of MAPs contributes to localization of other major MAPs in general, we microinjected biotinylated tau, MAP2C, or MAP2 into mature spinal cord neurons in culture (approximately 3 wk) and then analyzed their fates by antibiotin immunocytochemistry. Initially, each was detected in axons and dendrites, although tau persisted only in axons, whereas MAP2C and MAP2 were restricted to cell bodies and dendrites. Injected MAP2C and MAP2 bound to dendritic microtubules more firmly than to microtubules in axons, while injected tau bound to axonal microtubules more firmly than to microtubules in dendrites. Thus, beyond contributions from mRNA localization and selective axonal transport, compartmentalization of each of the three major MAPs occurs through local differential turnover.

Interaction of tau with the neural plasma membrane mediated by tau's amino-terminal projection domain

The neuronal microtubule-associated protein tau is required for the development of cell polarity in cultured neurons. Using PC12 cells that stably express tau and tau amino-terminal fragments, we report that tau interacts with the neural plasma membrane through its amino-terminal projection domain. In differentiated PC12 transfectants, tau is found in growth cone-like structures in a nonmicrotubule-dependent manner. In hippocampal neurons, tau is differentially extracted by detergent and enriched in the growth cone and the distal axon when membrane is left intact. In PC12 transfectants, overexpression of tau's amino-terminal fragment, but not of full-length tau, suppresses NGF-induced process formation. Our data suggest that tau's amino-terminal projection domain has an important role in neuritic development and establishes tau as a mediator of microtubule-plasma membrane interactions.

Functional implications for the microtubule-associated protein tau: localization in oligodendrocytes

We present evidence that the microtubule-associated protein tau is present in oligodendrocytes (OLGs), the central nervous system cells that make myelin. By showing that tau is distributed in a pattern similar to that of myelin basic protein, our results suggest a possible involvement of tau in some aspect of myelination. Tau protein has been identified in OLGs in situ and in vitro. In interfascicular OLGs, tau localization, revealed by monoclonal antibody Tau-5, was confined to the cell somata. However, in cultured ovine OLGs with an exuberant network of processes, tau was detected in cell somata, cellular processes, and membrane expansions at the tips of these processes. Moreover, in such cultures, tau appeared localized adjacent to or coincident with myelin basic protein in membrane expansions along and at the ends of the cellular processes. The presence of tau mRNA was documented using fluorescence in situ hybridization. The distribution of the tau mRNA was similar to that of the tau protein. Western blot analysis of cultured OLGs showed the presence of many tau isoforms. Together, these results demonstrate that tau is a genuine oligodendrocyte protein and pave the way for determining its functional role in these cells.

Changes in the microtubule proteins in the developing and transected spinal cords of the bullfrog tadpole: induction of microtubule-associated protein 2c and enhanced levels of Tau and tubulin in regenerating central axons

The distribution of tubulin, microtubule-associated protein 2 and Tau in the spinal cords of bullfrog tadpoles during development and after transection was studied. alpha-Tubulin or beta-tubulin immunoreactivity was present in the axons, neuronal perikarya and dendrites, as revealed by immunocytochemistry. The axonal staining intensity of the tubulins in the tadpoles was significantly stronger than that in the adult bullfrog. Microtubule-associated protein 2 immunoreactivity was localized largely to dendrites and expanded from distal to proximal dendrites with time; a high-molecular-weight microtubule-associated protein 2 was seen on the immunoblots of cord homogenates throughout development Tau1 stained mainly the axons. Two-dimensional gel immunoblotting disclosed that the tadpole contained a greater number of isoforms of Tau than the frog. Complete transection of the spinal cords of stage IV tadpoles was followed by regeneration of the damaged cord region. The levels of tubulin and Tau immunoreactivity in the regenerating axons of the ventral fasciculi were generally increased. Strikingly, microtubule-associated protein 2 immunoreactivity appeared in the regenerating axons and the chromatolytic cell bodies of axotomized motor neurons, paralleling the induction of microtubule-associated protein 2c in the regenerating cord segment shown by immunoblotting. The chromatolytic cell bodies were also markedly labeled by Tau1, whereas the high-molecular-weight microtubule-associated protein 2 diminished on the immunoblots, in accordance with the reduced level of staining for the dendrites. It is apparent that the changes in the cytoskeletal proteins in the regenerating axons mostly recapitulated their developmental patterns. Moreover, the data indicate a close relationship between tubulin and microtubule-associated proteins in axonal growth as well as providing evidence for similar molecular mechanisms underlying successful regeneration for central and peripheral axons.

Heterogeneity of tau protein and mRNA expression during the development of cerebellar granule cell neurons in vitro

Tau microtubule-associated proteins constitute a group of developmentally regulated neuronal proteins which promote microtubule polymerization and stabilization and hence have important implications during neuronal morphogenesis. We have examined the expression of tau mRNA and protein levels during the differentiation of cerebellar granule neurons over a period of 3 weeks in vitro. Oligonucleotide probes directed towards either immature or mature forms of tau mRNA were detected by in situ hybridization. Such experiments demonstrated that the time interval between 1 and 4 days in vitro represents a developmental epoch in the regulation of tau mRNA whereby the dominant immature tau messages were gradually replaced by mature mRNAs. Analysis of the profile of the various tau isoforms showed further developmental regulation with the transient rise in immature tau variants followed by the appearance of mature isoforms in older cultures. The increase in tau heterogeneity during granule neuron differentiation was enhanced by and could be attributed to intensive post-translational phosphorylation. Dephosphorylation of cell cultures demonstrated that the majority of tau was phosphorylated and that such a modification had profound affects on the localization of tau within developing neurons by immunocytochemistry. This study describes the profile of tau protein and mRNA levels expressed by differentiating cerebellar granule neurons in vitro and clearly demonstrates that tau is developmentally regulated and that important changes in tau expression occur at a time when processes are consolidating their first contacts.