Differential distribution of tau proteins in developing cat cerebellum

Biomarkers of injury to neural tissue in veterinary medicine

There are numerous biomarkers of central and peripheral nervous system damage described in human and veterinary medicine. Many of these are already used as tools in the diagnosis of human neurological disorders, and many are investigated in regard to their use in small and large animal veterinary medicine. The following review presents the current knowledge about the application of cell-type (glial fibrillary acidic protein, neurofilament subunit NF-H, myelin basic protein) and central nervous system specific proteins (S100B, neuron specific enolase, tau protein, alpha II spectrin, ubiquitin carboxy-terminal hydrolase L1, creatine kinase BB) present in the cerebrospinal fluid and/or serum of animals in the diagnosis of central or peripheral nervous system damage in veterinary medicine.

The cellular distribution and Ser262 phosphorylation of tau protein are regulated by BDNF in vitro

The brain-enriched microtubule-associated protein tau, a critical regulator of cytoskeletal dynamics, forms insoluble aggregates in a number of neurodegenerative diseases termed tauopathies, including Alzheimer's disease (AD). Hyperphosphorylation of tau protein is an important mechanism for aggregation, so many studies on the pathogenesis of AD and other tauopathies have focused on regulation of tau phosphorylation by kinases and phosphatases. Less studied are mechanisms of tau transcriptional and post-transcriptional regulation by extracellular signals such as BDNF and how such changes alter neuronal function. Previously, we reported that tau is required for morphological plasticity induced by BDNF. Here, we further explore tau modification during BDNF-induced changes in neuronal cell morphology. In undifferentiated SH-SY5Y cells lacking neurites, tau formed a sphere within the soma as revealed by immunocytochemistry. In contrast, tau was enriched in the neurites and sparse in the soma of SH-SY5Y cells induced to differentiate by retinoic acid (RA). Treatment with RA also increased total tau protein levels but decreased expression of tau phosphorylated at Ser262 as determined by Western blot. Both effects were further enhanced by subsequent BDNF treatment. Upregulation of tau protein and downregulation of p-Ser262 tau were correlated with total neurite length (R = .94 and R = −.98, respectively). When primary E18 hippocampal neurons were treated with nocodazole, a blocker of microtubule polymerization, nascent neurites were lost and tau shifted to the soma. This process of retrograde tau movement away from neurites was reversed by BDNF. These results indicate that tau is redistributed to neurites and dephosphorylated during RA- and BDNF-mediated differentiation.

Anesthesia and tau pathology

Alzheimer's disease (AD) is the most common form of dementia and remains a growing worldwide health problem. As life expectancy continues to increase, the number of AD patients presenting for surgery and anesthesia will steadily rise. The etiology of sporadic AD is thought to be multifactorial, with environmental, biological and genetic factors interacting together to influence AD pathogenesis. Recent reports suggest that general anesthetics may be such a factor and may contribute to the development and exacerbation of this neurodegenerative disorder. Intra-neuronal neurofibrillary tangles (NFT), composed of hyperphosphorylated and aggregated tau protein are one of the main neuropathological hallmarks of AD. Tau pathology is important in AD as it correlates very well with cognitive dysfunction. Lately, several studies have begun to elucidate the mechanisms by which anesthetic exposure might affect the phosphorylation, aggregation and function of this microtubule-associated protein. Here, we specifically review the literature detailing the impact of anesthetic administration on aberrant tau hyperphosphorylation as well as the subsequent development of neurofibrillary pathology and degeneration.

Phosphorylation of neurofilament subunit NF-M is regulated by activation of NMDA receptors and modulates cytoskeleton stability and neuronal shape

The cytoskeleton is essential for the structural organization of neurons and is influenced during development by excitatory stimuli such as activation of glutamate receptors. In particular, NMDA receptors are known to modulate the function of several cytoskeletal proteins and to influence cell morphology, but the underlying molecular and cellular mechanisms remain unclear. Here, we characterized the neurofilament subunit NF-M in cultures of developing mouse cortical neurons chronically exposed to NMDA receptor antagonists. Western blots analysis showed that treatment of cortical neurons with MK801 or AP5 shifted the size of NF-M towards higher molecular weights. Dephosphorylation assay revealed that this increased size of NF-M observed after chronic exposure to NMDA receptor antagonists was due to phosphorylation. Neurons treated with cyclosporin, an inhibitor of the Ca(2+)-dependent phosphatase calcineurin, also showed increased levels of phosphorylated NF-M. Moreover, analysis of neurofilament stability revealed that the phosphorylation of NF-M, resulting from NMDA receptor inhibition, enhanced the solubility of NF-M. Finally, cortical neurons cultured in the presence of the NMDA receptor antagonists MK801 and AP5 grew longer neurites. Together, these data indicate that a blockade of NMDA receptors during development of cortical neurons increases the phosphorylation state and the solubility of NF-M, thereby favoring neurite outgrowth. This also underlines that dynamics of the neurofilament and microtubule cytoskeleton is fundamental for growth processes.

Developmental changes in the neuronal protein composition: A study by high resolution 2D-gel electrophoresis

Cerebellar granular neurons were grown in culture up to 21 days and the protein compositions of undifferentiated (day 1), partially differentiated (day 7) and fully differentiated (day 21) neurons were analyzed by high-resolution 2D-gel electrophoresis. During neuronal differentiation there were not only increase in the amount of several known proteins, viz. actin, tubulin (both alpha and beta subunits), myosin (heavy and light chains), but very interesting changes were also observed in the expressions of different subunits and isoforms of those proteins. Furthermore, both in the acidic (pI 4.0-4.5) and alkaline (pI 7.0-8.5) regions interesting up and down regulations of several unidentified proteins were observed during the neuronal differentiation. These results indicated that there were several unidentified proteins that might be very valuable targets for studying regulation of neuronal differentiation. Research is going on for further characterization of those proteins using recently developed proteomics technology.

Development of an assay to screen for inhibitors of tau phosphorylation by cdk5

A high-throughput assay for tau phosphorylation by cdk5/p25 is described. Full-length recombinant tau was used as a substrate in the presence of saturating adenosine triphosphate (ATP). Using PHF-1, an antibody directed specifically against 2 tau phosphorylation epitopes (serine 396 and serine 404), an enzyme-linked immunosorbent assay (ELISA)-based colorimetric assay was formatted in 384-well plates. The assay was validated by measuring kinetic parameters for cdk5/p25 catalysis and known inhibitors. Rate constants for the site-specific phosphorylations at the PHF-1 epitopes were determined and suggested preferential phosphorylation at these sites. The performance of this assay in a high-throughput format was demonstrated and used to identify inhibitors of tau phosphorylation at specific epitopes phosphorylated by cdk5/p25.

Modeling tauopathy: a range of complementary approaches

The large group of neurodegenerative diseases which feature abnormal metabolism and accumulation of tau protein (tauopathies) characteristically produce a multiplicity of cellular and systemic abnormalities in human patients. Understanding the complex pathogenetic mechanisms by which abnormalities in tau lead to systemic neurofibrillary degenerative disease requires the construction and use of model experimental systems in which the behavior of human tau can be analyzed under controlled conditions. In this paper, we survey the ways in which in vitro, cellular and whole-animal models of human tauopathy are being used to add to our knowledge of the pathogenetic mechanisms underlying these conditions. In particular, we focus on the complementary advantages and limitations of various approaches to constructing tauopathy models presently in use with respect to those of murine transgenic tauopathy models.

Changed conformation of mutant Tau-P301L underlies the moribund tauopathy, absent in progressive, nonlethal axonopathy of Tau-4R/2N transgenic mice

Protein tau-3R/4R isoform ratio and phosphorylation regulates binding to microtubules and, when disturbed by aging or mutations, results in diverse tauopathies and in neurodegeneration. The underlying mechanisms were studied here in three transgenic mouse strains with identical genetic background, all expressing the tau-4R/2N isoform driven specifically in neurons by the thy1 gene promoter. Two strains, expressing human tau-4R/2N or mutant tau-4R/2N-P301L at similar, moderate levels, developed very different phenotypes. Tau-4R/2N mice became motor-impaired already around age 6-8 weeks, accompanied by axonopathy (dilatations, spheroids), but no tau aggregates, and surviving normally. In contrast, tau-P301L mice developed neurofibrillary tangles from age 6 months, without axonal dilatations and, despite only minor motor problems, all succumbing before the age of 13 months. The third strain, obtained by tau knock-out/knock-in (tau-KOKI), expressed normal levels of wild-type human tau-4R/2N replacing all mouse tau isoforms. Tau-KOKI mice survived normally with minor motor problems late in life and without any obvious pathology. Biochemically, a fraction of neuronal tau in aging tau-P301L mice was hyperphosphorylated concomitant with conformational changes and aggregation, but overall, tau-4R/2N was actually more phosphorylated than tau-P301L. Significantly, tau with changed conformation and with hyperphosphorylation colocalized in the same neurons in aging tau-P301L mice. Taken together, we conclude that excessive binding of tau-4R/2N as opposed to reduced binding of tau-P301L to microtubules is responsible for the development of axonopathy and tauopathy, respectively, in tau-4R/2N and tau-P301L mice and that the conformational change of tau-P301L is a major determinant in triggering the tauopathy.

Tau phosphorylation by cyclin-dependent kinase 5/p39 during brain development reduces its affinity for microtubules

The microtubule-associated protein tau is a developmentally regulated neuronal phosphoprotein. The phosphorylation of tau reduces its ability to bind and stabilize axonal microtubules during axonal growth. Although tau is phosphorylated by cyclin-dependent kinase 5 (Cdk5) in vitro, its in vivo roles remain unclear. Here, we show that tau is phosphorylated by Cdk5/p39 during brain development, resulting in a reduction of its affinity for microtubules. The activity of Cdk5 is tightly regulated by association with its neuronal activators, p35 or p39. The p35 and p39 expression levels were investigated in the developing mouse brain; the p39 expression level was higher in embryonic hind brain and spinal cord and in postnatal cerebral cortex, whereas that of p35 was most prominent in cerebral cortex at earlier stages of development. The ability of Cdk5 to phosphorylate tau was higher when in association with p39 than in association with p35. Tau phosphorylation at Ser-202 and Thr-205 was decreased in Cdk5-/- mouse brain but not in p35-/- mouse brain, suggesting that Cdk5/p39 is responsible for the in vivo phosphorylation of tau at these sites. Our data suggest that tau phosphorylation by Cdk5 may provide the neuronal microtubules with dynamic properties in a region-specific and developmentally regulated manner.

Niemann-Pick disease type C yields possible clue for why cerebellar neurons do not form neurofibrillary tangles

It is unknown why cerebellar neurons resist neurofibrillary tangle (NFT) formation. In Niemann-Pick disease Type C (NPC), NFT-mediated neurodegeneration occurs throughout brain, but the cerebellum degenerates conspicuously without NFT. To understand why, we have studied markers of NFT pathogenesis in cerebellum from 17 NPC cases, all having abundant NFT in forebrain. Remarkably, we found that NPC cerebella display several early markers of NFT formation, i.e., hyperphosphorylated tau and an array of cell cycle regulators, suggesting that cerebellar neurons in NPC undergo similar modifications as other neurons that develop NFT. However, cerebellar neurons are deficient in tau, the building block of NFT, and this may be one reason for their inability to form NFT. Even without NFT, cerebellar neurodegeneration may be triggered by the inappropriate activation of the cell cycle cdc2 kinase, and the npc-1 murine model provides an opportunity to test this hypothesis.

Tau blocks traffic of organelles, neurofilaments, and APP vesicles in neurons and enhances oxidative stress

We studied the effect of microtubule-associated tau protein on trafficking of vesicles and organelles in primary cortical neurons, retinal ganglion cells, and neuroblastoma cells. Tau inhibits kinesin-dependent transport of peroxisomes, neurofilaments, and Golgi-derived vesicles into neurites. Loss of peroxisomes makes cells vulnerable to oxidative stress and leads to degeneration. In particular, tau inhibits transport of amyloid precursor protein (APP) into axons and dendrites, causing its accumulation in the cell body. APP tagged with yellow fluorescent protein and transfected by adenovirus associates with vesicles moving rapidly forward in the axon (approximately 80%) and slowly back (approximately 20%). Both movements are strongly inhibited by cotransfection with fluorescently tagged tau (cyan fluorescent protein-tau) as seen by two-color confocal microscopy. The data suggests a linkage between tau and APP trafficking, which may be significant in Alzheimer's disease.

Estrogen receptor beta immunoreactivity in differentiating cells of the developing rat cerebellum

Estrogen receptors (ER) play a significant role in the development of some regions of the mammalian brain. Recently, ER-beta (ERbeta) mRNA and protein were shown to be expressed in the rat cerebellum. In the present study, the ontogeny of ERbeta protein expression was examined in the rat cerebellum during postnatal development. Western blot analysis indicated that a single ERbeta-like immunoreactive species of approximately 55 kDa was present in protein lysates prepared from the cerebella of female and male Sprague-Dawley rat pups. Immunocytochemical analysis of cerebellar sections from the midline vermis revealed that during development, the expression of ERbeta varied with age and cell-type, but not sex. In the developing cerebellum, highest levels of ERbeta-immunoreactivity (IR) were detected in neurons during neurite growth, and in some glia during migration. Throughout the first postnatal week, ERbeta-IR was localized to differentiating granule cells in the external germinal layer and to migrating glia. Differentiating granule cells expressed detectable levels of ERbeta throughout development. In Purkinje cells, ERbeta-IR was first detected on postnatal day 6 (P6), with peak intensities of immunostaining coinciding with the initiation of axonal and dendritic growth that occurs between P7 and P8. Expression of ERbeta-IR remained high during maturation of Purkinje cell dendrites, and then decreased to a lower level maintained in the adult. From the third postnatal week, ERbeta-IR was also detected in the later developing Golgi, stellate, and basket neurons. These results suggest that ERbeta may play a role in growth-related mechanisms during differentiation of cerebellar neurons and glia.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

The Golgi apparatus and the centrosome are localized to the sites of newly emerging axons in cerebellar granule neurons in vitro

Cultured cerebellar granule neurons develop their characteristic axonal and dendritic morphologies in a series of discrete temporal steps highly similar to those observed in situ, initially extending a single process, followed by the extension of a second process from the opposite pole of the cell, both of which develop into axons to generate a bipolar morphology. A mature morphology is attained following the outgrowth of multiple, short dendrites [Powell et al., 1997: J. Neurobiol. 32:223-236]. To determine the relationship between the localization of the Golgi apparatus, the site of microtubule nucleation (the centrosome), and the sites of initial and secondary axonal extension, the intracellular positioning of the Golgi and centrosome was observed during the differentiation of postnatal mouse granule neurons in vitro. The Golgi was labeled using the fluorescent lipid analogue, C5-DMB-Ceramide, or by indirect immunofluorescence using antibodies against the Golgi resident protein, alpha-mannosidase II. At 1-2 days in vitro (DIV), the Golgi was positioned at the base of the initial process in 99% of unipolar cells observed. By 3 DIV, many cells began the transition to a bipolar morphology by extending a short neurite from the pole of the cell opposite to the initial process. The Golgi was observed at this site of secondary outgrowth in 92% of these "transitional" cells, suggesting that the Golgi was repositioned from the base of the initial process to the site of secondary neurite outgrowth. As the second process elongated and the cells proceeded to the bipolar stage of development, or at later stages when distinct axonal and somatodendritic domains had been established, the Golgi was not consistently positioned at the base of either axons or dendrites, and was most often found at sites on the plasma membrane from which no processes originated. To determine the location of the centrosome in relation to the Golgi during development, granule neurons were labeled with antibodies against gamma-tubulin and optically sectioned using confocal microscopy. The centrosome was consistently co-localized with the Golgi during all stages of differentiation, and also appeared to be repositioned to the base of the newly emerging axon during the transition from a unipolar to a bipolar morphology. These findings indicate that during the early stages of granule cell axonal outgrowth, the Golgi-centrosome is positioned at the base of the initial axon and is then repositioned to the base of the newly emerging secondary axon. Such an intracellular reorientation of these organelles may be important in maintaining the characteristic developmental pattern of granule neurons by establishing the polarized microtubule network and the directed flow of membranous vesicles required for initial axonal elaboration.

Developmental regulation and PKC dependence of Alzheimer's-type tau phosphorylations in cultured fetal rat hippocampal neurons

Attempts to describe a mechanism of neurofibrillary tangle formation often focus on site specific phosphorylations of tau protein. These have typically been described in both Alzheimer's disease and developing brains. Therefore, study of the developmental regulation of Alzheimer epitope tau phosphorylations may help explain their persistence or recurrence during Alzheimer's disease. Using fetal rat hippocampal cultures, we report a spatial and temporal expression of tau phosphorylation during neuronal differentiation. We have examined phosphorylation at the epitopes recognized by monoclonal antibodies, PHF-1 and Tau 1. Tau was highly phosphorylated at the PHF-1 epitope at all culture ages examined using both immunohistochemical staining and Western blots. Tau was heavily phosphorylated at the Tau 1 epitope only in older cultures. The populations of tau recognized by the two antibodies also exhibited different solubilities, suggesting different microtubule binding behaviors: tau phosphorylated at PHF-1 was retained in axons following solubilization whereas Tau 1 immunoreactive tau was not retained in any cell compartment. Finally, in this culture system, maintenance of phosphorylation at the PHF-1 epitope, but not the Tau 1 epitope, required protein kinase C activity. These results indicate unique regulatory mechanisms and roles for each of these phosphorylated tau epitopes.

Identification of in vitro phosphorylation sites in the growth cone protein SCG10. Effect Of phosphorylation site mutants on microtubule-destabilizing activity

SCG10 is a neuron-specific, membrane-associated protein that is highly concentrated in growth cones of developing neurons. Previous studies have suggested that it is a regulator of microtubule dynamics and that it may influence microtubule polymerization in growth cones. Here, we demonstrate that in vivo, SCG10 exists in both phosphorylated and unphosphorylated forms. By two-dimensional gel electrophoresis, two phosphoisoforms were detected in neonatal rat brain. Using in vitro phosphorylated recombinant protein, four phosphorylation sites were identified in the SCG10 sequence. Ser-50 and Ser-97 were the target sites for protein kinase A, Ser-62 and Ser-73 for mitogen-activated protein kinase and Ser-73 for cyclin-dependent kinase. We also show that overexpression of SCG10 induces a disruption of the microtubule network in COS-7 cells. By expressing different phosphorylation site mutants, we have dissected the roles of the individual phosphorylation sites in regulating its microtubule-destabilizing activity. We show that nonphosphorylatable mutants have increased activity, whereas mutants in which phosphorylation is mimicked by serine-to-aspartate substitutions have decreased activity. These data suggest that the microtubule-destabilizing activity of SCG10 is regulated by phosphorylation, and that SCG10 may link signal transduction of growth or guidance cues involving serine/threonine protein kinases to alterations of microtubule dynamics in the growth cone.

Normal and pathological Tau proteins as factors for microtubule assembly

Tau proteins are microtubule-associated proteins. They regulate the dynamics of the microtubule network, especially involved in the axonal transport and neuronal plasticity. Tau proteins belong to a family of developmentally regulated isoforms generated by alternative splicing and phosphorylation. This generates several Tau variants that interact with tubulin and other proteins. Therefore, Tau proteins are influenced by many physiological regulations. Tau proteins are also powerful markers of the neuronal physiological state. Their degree of phosphorylation is a good marker of cell integrity. It is heavily disturbed in numerous neurodegenerative disorders, leading to a collapse of the microtubule network and the presence of intraneuronal lesions resulting from Tau aggregation. However, different biochemical and immunological patterns of pathological Tau proteins found among neurodegenerative disorders are useful markers for the understanding of the role of Tau protein isoforms and the diagnosis of these pathological conditions.

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.

Differential expression and modification of neurofilament triplet proteins during cat cerebellar development

Neurofilament (NF) proteins consist of three subunits of different molecular weights defined as NF-H, NF-M, and NF-L. They are typical structures of the neuronal cytoskeleton. Their immunocytochemical distribution during postnatal development of cat cerebellum was studied with several monoclonal and polyclonal antibodies against phosphorylated or unmodified sites. Expression and distribution of the triplet neurofilament proteins changed with maturation. Afferent mossy and climbing fibers in the medullary layer contained NF-M and NF-L already at birth, whereas NF-H appeared later. Within the first three postnatal weeks, all three subunits appeared in mossy and climbing fibers in the internal granular and molecular layers and in the axons of Purkinje cells. Axons of local circuit neurons such as basket cells expressed these proteins at the end of the first month, whereas parallel fibers expressed them last, at the beginning of the third postnatal month. Differential localization was especially observed for NF-H. Depending on phosphorylation, NF-H proteins were found in different axon types in climbing, mossy, and basket fibers or additionally in parallel fibers. A nonphosphorylated NF-H subunit was exclusively located in some Purkinje cells at early developmental stages and in some smaller interneurons later. A novel finding is the presence of a phosphorylation site in the NF-H subunit that is localized in dendrites of Purkinje cells but not in axons. Expression and phosphorylation of the NF-H subunit, especially, is cell-type specific and possibly involved in the adult-type stabilization of the axonal and dendritic cytoskeleton.

Widespread appearance of Alz-50 immunoreactive neurons in the human brain with cerebral infarction

BACKGROUND AND PURPOSE

Tau-like immunoreactivity is known to develop in neurons under some experimental conditions simulating ischemia. The purpose of this study is to investigate the expression of tau-like immunoreactivity in the human brain after ischemic insult.

METHODS

A series of autopsied human brains with or without ischemic lesion were investigated with immunohistochemistry (Alz-50, anti-tau, and anti-ubiquitin) and with silver-staining methods (Gallyas and Bodian methods).

RESULTS

Punctate immunoreactivity to Alz-50 was visualized in the cytoplasm not only of the neurons in and around the ischemic lesion but also of the neurons free from classic ischemic changes around the necrosis. Some of the neurons around the ischemic lesion were stained by the Gallyas method. Immunostaining with anti-tau and anti-ubiquitin antibodies and the conventional Bodian method failed to visualize these neurons.

CONCLUSIONS

The widespread appearance of Alz-50 immunoreactive neurons during the ischemic process signifies that tau-related proteins may be related to ischemic necrosis, but the lack of neurofibrillary tangles morphologically distinguishes ischemic development of tau-related proteins from the neurofibrillary degeneration in Alzheimer's disease.

Developmental expression of tau proteins in the chicken and rat brain: rapid down-regulation of a paired helical filament epitope in the rat cerebral cortex coincides with the transition from immature to adult tau isoforms

The monoclonal antibodies TAU-1 and AT8 are directed at human microtubule-associated protein tau epitopes that contain a dephosphorylated and phosphorylated Ser202, respectively, while AT180 and AT270 are anti-tau monoclonals with epitopes that require phosphorylated Thr181 and Thr231, respectively. We used these antibodies to study the developmental profiles of tau proteins in rat cerebral cortex and chicken optic lobes. In tau extracts from perinatal rat cerebral cortex. AT8 recognized one major protein band of approximately 50 kDa that peaks on postnatal day 6 and declines rapidly to lower levels at day 12. At later stages, the AT8 epitope was expressed by several adult tau isoforms that were, however, stained only very faintly in highly enriched samples. Two additional tau epitopes recognized by AT180 and AT270 were found to be expressed by one or two protein bands up to about postnatal day 19 and then declined. Unlike the AT8 epitope, in the mature brain these epitopes were stained strongly in enriched samples, where they were expressed by a greater number of adult isoforms. Between embryonic day 19 and postnatal day 12, TAU-1 was found to recognize one major protein band of approximately 50 kDa which migrated slightly faster than the AT8-binding band. At postnatal day 19 and all older stages (including adult cortex), at least three additional TAU-1-binding isoforms with higher apparent molecular weights were present. Hence, the transition from one immature to several adult TAU-1-binding tau isoforms between postnatal day 12 and 19 in rat cerebral cortex coincides with the phase of rapid down-regulation of the AT8 epitope. As in the rat cerebrum, in chicken optic lobes there is a developmental decrease of AT8-binding proteins which is paralleled by striking changes in the electrophoretic pattern of tau isoforms recognized by TAU-1. In both rat cerebral cortex and chicken optic lobes, the period of maximal expression of AT8-binding tau is morphologically characterized by intense axonal growth and beginning synaptogenesis, whereas its subsequent rapid down-regulation and the appearance of novel TAU-1-binding isoforms correlates with synaptic maturation, the onset of spontaneous electrical activity and the beginning of myelination.

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.

The microtubule cytoskeleton and the development of neuronal polarity

The concept that axons and dendrites represent a fundamental polarization of the nerve cell has been borne out by numerous morphological, functional, and molecular studies. How does polarity arise during development? We and others have focused on the role of the microtubule cytoskeleton because microtubules (a) are essential components of axons and dendrites; (b) possess an inherent polarity at the molecular level; (c) are regulated by interactions with microtubule associated proteins (MAPs), some of which have polarized distributions in mature neurons. Here we review data on the initial acquisition of polarity as observed in neuronal culture and roles for microtubules and MAPs in this morphogenetic event. We present data clarifying some previously conflicting results on tau localization during the establishment of polarity and provide new evidence that phosphorylation of tau is spatially regulated during the development of polarity in culture. Elucidation of mechanisms locally regulating tau phosphorylation during normal neuronal development may provide clues to the significance of its abnormal phosphorylation in Alzheimer's disease.