Microtubule formation and neurite growth in cerebellar macroneurons which develop in vitro: evidence for the involvement of the microtubule-associated proteins, MAP-1a, HMW-MAP2 and Tau

Advances in the detection of Alzheimer's disease-use of cerebrospinal fluid biomarkers

The diagnosis of Alzheimer's disease (AD) is still made by excluding other disorders with a similar clinical picture. In addition, an analysis of symptoms and signs, blood analyses and brain imaging are the major ingredients of the clinical diagnostic work-up. However, the sensitivity of a clinical diagnosis using these instruments is unsatisfactory and disease markers with high sensitivity and specificity for AD would be a welcome supplement. Ideally, such markers should reflect the pathophysiological mechanisms of AD, that is, according to the currently predominant hypothesis mismetabolism of beta-amyloid and neurofibrillary degeneration. Among several, we have focused on three candidates that have been suggested to fulfil the requirements for biomarkers of AD: beta-amyloid42 (Abeta42), total tau (T-tau) and tau phosphorylated at various epitopes (P-tau). The cerebrospinal fluid (CSF) levels of these proteins reflect the metabolism of these proteins in the central nervous system. Only published articles using established ELISA methods for the quantification of these markers in CSF and preferably also presenting sensitivity and specificity figures have been included in this review. The number of patients included in the different studies varies; some having included only a few patients. Furthermore, diagnostic criteria vary and clinicopathological studies are scarce. However, there are some large studies, including even minor studies, and most have found reduced CSF levels of Abeta42 and increased CSF levels of T-tau in AD. The sensitivity and specificity of these measures are high for separation of AD patients from controls, but their specificity against other dementias is moderate. It increases if P-tau is added. An increasing number of studies suggest that supplementary use of these CSF markers, preferably in combination, adds to the accuracy of an AD diagnosis.

MAPK signal transduction pathway mediates agrin effects on neurite elongation in cultured hippocampal neurons

We have previously shown that agrin regulates the rates of axonal and dendritic elongation by modulating the expression of microtubule-associated proteins in cultured hippocampal neurons. However, the mechanisms by which agrin-induced signals are propagated to the nucleus where they can lead to the phosphorylation, and hence the activation, of transcription factors, are not known. In the present study, we identified downstream elements that play essential roles in the agrin-signaling pathway in developing central neurons. Our results indicate that agrin induces the combined activation of the extracellular signal-regulated kinases (ERK1/ERK2) and p38 in central neurons. In addition, they showed that PD98059 and SB202190, synthetic inhibitors of ERK1/ERK2 and p38 respectively, prevented the changes in the rate of neurite elongation induced by agrin in cultured hippocampal neurons. Collectively, these results suggest that agrin might modulate the expression of neuron-specific genes involved in neurite elongation by inducing CREB phosphorylation through the activation of the MAPK signal transduction pathway in cultured hippocampal neurons.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.

Novel activities of pro-IGF-I E peptides: regulation of morphological differentiation and anchorage-independent growth in human neuroblastoma cells

Insulin-like growth factor-I (IGF-I) is translated as a pre-pro-peptide that is posttranslationally processed to its mature form by proteolytic removal of the signal peptide and the E-domain peptide. Contrary to the mature human (h) IGF-I, the recombinant rtEa4 -peptide significantly reduced the anchorage-independent cell growth in human neuroblastoma cells (SK-N-F1), shown by colony formation assay in vitro. Significant inhibition of colony formation is also observed in SK-N-F1 cells stably transfected with a bicistronic expression construct encoding a secretory form of the rtEa4 peptide. Furthermore, treatment with the recombinant rtEa4 peptide, but not the mature hIGF-I, resulted in morphological differentiation of SK-N-F1 cells characterized by long neurite outgrowth. Similar morphological differentiation is also observed in SK-N-F1 cell clones stably transfected with the rtEa4 peptide expression construct. A spectrum of biological activities similar to those of rtEa4 peptide is also observed in the synthetic hEb peptide, but not-the hEa peptide. Results of further characterization reveal that neurites induced by rtEa4 or hEb peptide contain neuronal-specific MAP-2, Tau, and neurofilament (NF-160), accompanied by an increased expression of the neuronal marker gene neuropeptide tyrosine (NPY). Furthermore, effects of signal transduction inhibitors are indicative of the involvement of MAP-kinase PI-3-kinase cascades. The activation of ERK-1/-2 is markedly increased in response to rtEa-4 or hEb peptide stimulation, further indicating the involvement of MAPK signaling cascade. These unique biological activities exhibited by the rtEa4 or hEb peptide suggest that E peptide of the pro-IGF-I may play distinct roles in regulating cell growth and differentiation in neuroblastoma cells.

Regulation of neurotransmitter release by synapsin III

Synapsin III is the most recently identified member of the synapsin family, a group of synaptic vesicle proteins that play essential roles in neurotransmitter release and neurite outgrowth. Here, through the generation and analysis of synapsin III knock-out mice, we demonstrate that synapsin III regulates neurotransmitter release in a manner that is distinct from that of synapsin I or synapsin II. In mice lacking synapsin III, the size of the recycling pool of synaptic vesicles was increased, and synaptic depression was reduced. The number of vesicles that fuse per action potential was similar between synapsin III knock-out and wild-type mice, and there was no change in the quantal content of EPSCs; however, IPSCs were greatly reduced in synapsin III-deficient neurons. The density and distribution of synaptic vesicles in presynaptic terminals did not appear to be different in synapsin III knock-out mice in comparison to wild-type littermates. In addition to the changes in neurotransmitter release, we observed a specific delay in axon outgrowth in cultured hippocampal neurons from synapsin III knock-out mice. Our data indicate that synapsin III plays unique roles both in early axon outgrowth and in the regulation of synaptic vesicle trafficking.

Tau is essential to beta -amyloid-induced neurotoxicity

Senile plaques and neurofibrillary tangles, the two hallmark lesions of Alzheimer's disease, are the results of the pathological deposition of proteins normally present throughout the brain. Senile plaques are extracellular deposits of fibrillar beta-amyloid peptide (Abeta); neurofibrillary tangles represent intracellular bundles of self-assembled hyperphosphorylated tau proteins. Although these two lesions are often present in the same brain areas, a mechanistic link between them has yet to be established. In the present study, we analyzed whether tau plays a key role in fibrillar Abeta-induced neurite degeneration in central neurons. Cultured hippocampal neurons obtained from wild-type, tau knockout, and human tau transgenic mice were treated with fibrillar Abeta. Morphological analysis indicated that neurons expressing either mouse or human tau proteins degenerated in the presence of Abeta. On the other hand, tau-depleted neurons showed no signs of degeneration in the presence of Abeta. These results provide direct evidence supporting a key role for tau in the mechanisms leading to Abeta-induced neurodegeneration in the central nervous system. In addition, the analysis of the composition of the cytoskeleton of tau-depleted neurons suggested that the formation of more dynamic microtubules might confer resistance to Abeta-mediated neurodegeneration.

Agrin differentially regulates the rates of axonal and dendritic elongation in cultured hippocampal neurons

In the present study, we examined the role of agrin in axonal and dendritic elongation in central neurons. Dissociated hippocampal neurons were grown in the presence of either recombinant agrin or antisense oligonucleotides designed to block agrin expression. Our results indicate that agrin differentially regulates axonal and dendritic growth. Recombinant agrin decreased the rate of elongation of main axons but induced the formation of axonal branches. On the other hand, agrin induced both dendritic elongation and dendritic branching. Conversely, cultured hippocampal neurons depleted of agrin extended longer, nonbranched axons and shorter dendrites when compared with controls. These changes in the rates of neurite elongation and branching were paralleled by changes in the composition of the cytoskeleton. In the presence of agrin, there was an upregulation of the expression of microtubule-associated proteins MAP1B, MAP2, and tau. In contrast, a downregulation of the expression of these MAPs was detected in agrin-depleted cells. Taken collectively, these results suggest an important role for agrin as a trigger of the transcription of neuro-specific genes involved in neurite elongation and branching in central neurons.

Neu differentiation factor regulates tau protein and mRNA in cultured neonatal oligodendrocytes

Axonal signals activate myelinogenesis via regulation of the extent to which oligodendrocyte (OLG) processes wrap around the axon. The cytoskeleton in OLG processes is actively involved in myelination and is a putative target for axonal regulation of myelination. The axon-associated neuregulins may regulate the cytoskeleton extensions in OLG processes. Here, we report that the neuregulin neu differentiation factor (NDF) increases the expression of tau mRNA and tau protein in OLGs. Treatment of neonatal OLGs with alpha-NDF or beta-NDF resulted in dramatic increases in the length of OLG processes, which appeared either as singular unbranched extensions or as a network of extensively branched processes. By immunoblot analysis with tau-1 mAb, which recognizes the dephosphorylated form of the tau proteins, neonatal OLGs treated with alpha-NDF or beta-NDF, had an increase in tau protein levels. The increase of tau levels in beta-NDF-treated cells is much greater than the twofold increase present in alpha-NDF-treated cells. By immunoblot analysis with the phosphorylation-insensitive tau-5 mAb, beta-NDF-treated cells had a twofold increase in tau. Immunoblot analysis suggest that alpha-NDF and beta-NDF promote a twofold increase in the tau protein levels in OLG, with the beta-factor also promoting a tau dephosphorylation. Using promoters spanning the amino-terminal region of tau, we found that OLGs treated with alpha-NDF or beta-NDF contained approximately twofold more tau mRNA than untreated cells. However, there was no qualitative difference between control and NDF-treated cells in the pattern of tau mRNA isoforms expressed. A model is proposed in which the axonal NDF-induced regulation of tau expression in OLGs may be part of the mechanism by which the axon regulates myelination.

Evidence for the role of MAP1B in axon formation

Cultured neurons obtained from a hypomorphous MAP1B mutant mouse line display a selective and significant inhibition of axon formation that reflects a delay in axon outgrowth and a reduced rate of elongation. This phenomenon is paralleled by decreased microtubule formation and dynamics, which is dramatic at the distal axonal segment, as well as in growth cones, where the more recently assembled microtubule polymer normally predominates. These neurons also have aberrant growth cone formation and increased actin-based protrusive activity. Taken together, this study provides direct evidence showing that by promoting microtubule dynamics and regulating cytoskeletal organization MAP1B has a crucial role in axon formation.

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.

Exogenous BDNF, NT-3 and NT-4 differentially regulate neurite outgrowth in cultured hippocampal neurons

Multiple growth factors contribute to the differentiation of dendritic and axonal processes by a neuron. Cultured hippocampal cells elaborate dendritic and axonal processes following well-defined steps. We used this culture system to determine the specific effects of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) on dendritic and axonal differentiation in hippocampal pyramidal neurons. We demonstrated that each of these neurotrophins exert distinct effects on neurite outgrowth. Both BDNF and NT-3 had positive effects on the outgrowth of undifferentiated neurites, called minor neurites, and on the axonal process of hippocampal pyramidal neurons. However, the effect of NT-3 was more important than that of BDNF. On the other hand, NT-4 did not enhance axonal outgrowth but had only an effect on the outgrowth of minor neurites. Since cytoskeletal proteins play crucial roles in promoting neurite outgrowth, we examined the protein levels of some of these proteins that are associated with neurite outgrowth: beta-actin, gamma-actin, alpha-tubulin, MAP2 and tau. Surprisingly, we did not detect any change in their protein levels. Taken together, our results show that BDNF, NT-3 and NT-4 exert distinct effects on the neuritic compartments of hippocampal neurons.

Sex steroids and the brain: lessons from animal studies

Gonadal steroid hormones have multiple effects throughout development on steroid responsive tissues in the brain. The belief that the cellular morphology of the adult brain cannot be modulated or that the synaptic connectivity is "hard-wired" is being rapidly refuted by abundant and growing evidence. Indeed, the brain is capable of undergoing many morphological changes throughout life and gonadal steroids play an important role in many of these processes. Gonadal steroids are implicated in the development of sexually dimorphic structures in the brain, in the control of physiological behaviors and functions and the brain's response to physiological or harmful substances. The effect of sex steroids on neuroprotection and neuroregeneration is an important and expanding area of investigation. Astroglia are targets for estrogen and testosterone and are apparently involved in the actions of sex steroids on the central nervous system. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes and the extent to which neuronal membranes are covered by astroglial processes. These changes are linked to modifications in the number of synaptic inputs to neurons and suggest that astrocytes may participate in the genesis of gonadal steroid-induced sex differences in synaptic connectivity and synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in the regenerative and neuroprotective effects of sex steroids since astroglial activation after brain injury or after peripheral nerve axotomy is regulated by sex hormones.

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.

Synapsin III: developmental expression, subcellular localization, and role in axon formation

We have investigated the developmental expression and subcellular localization of synapsin III, the newest member of the synapsin family, in cultured mouse hippocampal neurons. Our results indicate that synapsin III is expressed early during development, with levels peaking 7 d after plating and declining thereafter. Synapsin III is highly concentrated in growth cones. Using specific antisense oligonucleotides, we have also examined the effect of depleting synapsin III on neurite elongation and synaptogenesis. When synapsin III was suppressed immediately after plating, hippocampal neurons extended minor processes but failed to differentiate one of them as the axon. The suppression of synapsin III after axonal elongation did not affect the time course of synapse formation. The results indicate that synapsin III has a developmental time course, a subcellular localization, and a developmental function very different from those of synapsin I and synapsin II.

Synapse formation proceeds independently of dendritic elongation in cultured hippocampal neurons

In central neurons, dendritic differentiation begins well after axonal elongation and is accompanied by the compartmentation of the microtubule-associated protein 2 (MAP2) in the somatodendritic domain. Whether MAP2 plays a role in the morphological and functional maturation of dendrites remains an open question and is the focus of this study. Cultured hippocampal neurons depleted of MAP2 by means of antisense oligonucleotides failed to elongate their dendrites. On the other hand, MAP2-depleted neurons were capable of receiving synapses within the same time course as their control counterparts. However, both the number of synapses per cell and the synaptic density were markedly reduced in neurons in which dendritic elongation has been impaired. Taken collectively, these results suggest that the expression of MAP2 is required for the morphological differentiation of dendrites. Dendritic elongation, however, is not a prerequisite for synapse formation in cultured hippocampal neurons.

Human tau filaments induce microtubule and synapse loss in an in vivo model of neurofibrillary degenerative disease

The intracellular accumulation of tau protein and its aggregation into filamentous deposits is the intracellular hallmark of neurofibrillary degenerative diseases such as Alzheimer's Disease and familial tauopathies in which tau is now thought to play a critical pathogenic role. Until very recently, the lack of a cellular model in which human tau filaments can be experimentally generated has prevented direct investigation of the causes and consequences of tau filament formation in vivo. In this study, we show that human tau filaments formed in lamprey central neurons (ABCs) that chronically overexpress human tau resemble the ‘straight filaments’ seen in Alzheimer's Disease and other neurofibrillary conditions, and are distinguishable from neurofilaments by their ultrastructure, distribution and intracellular behavior. We also show that tau filament formation in ABCs is associated with a distinctive pattern of dendritic degeneration that closely resembles the cytopathology of human neurofibrillary degenerative disease. This pattern includes localized cytoskeletal disruption and aggregation of membranous organelles, distal dendritic beading, and the progressive loss of dendritic microtubules and synapses. These results suggest that tau filament formation may be responsible for many key cytopathological features of neurofibrillary degeneration, possibly via the loss of microtubule based intracellular transport.

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.

Tau protein function in axonal formation

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

Differential association of tau with subsets of microtubules containing posttranslationally-modified tubulin variants in neuroblastoma cells

Neuronal cells display different subsets of dynamic microtubules. In axons and extending neurites, this intrinsic dynamics is modulated by the microtubule-associated protein tau. Moreover, posttranslational modifications of tubulin, namely acetylation, tyrosination or glutamylation are directly involved in determining the stability of neuronal microtubules. Studies were carried out to analyze the interaction patterns of tau with subsets of microtubules in N2A neuroblastoma cells, which can differentiate in the presence of dibutyryl cAMP. Double labeling studies showed a differential pattern of tau association with microtubules containing acetylated and tyrosinated tubulin. Furthermore, studies using depolymerizing drugs revealed a selectivity in the association of tau with microtubular polymers and microfilaments, within the organization of the neuronal cytoskeleton. In order to study the association of specific tau isoforms with microtubules containing modified tubulin variants, immunoprecipitation studies were carried out. The coimmunoprecipitation data indicated a selective binding of specific tau isoforms to either modified tubulin variant. To assess the hypothesis on the roles of tau isoforms in the stabilization of microtubules containing modified tubulins, the association of those variants with tau isoforms was analyzed in overlay experiments. A preferential binding of acetylated tubulin from undifferentiated N2A cell extracts, to at least one slow-migrating tau isoform was revealed. However, acetylated tubulin from N2A cells containing long neurites displayed a preferential association with two isoforms of tau. On the other hand, tyrosinated tubulin from N2A extracts bound to the entire set of neuronal tau isoforms. These studies, along with the tau association with microtubules with different stability, indicate that tau segregates into subsets of microtubules in the axonal processes. The studies also suggest that these interactions may respond to a functional versatility of these polymers in differentiating neurons.

PD98059 prevents neurite degeneration induced by fibrillar beta-amyloid in mature hippocampal neurons

How senile plaques and neurofibrillary tangles are linked represents a major gap in our understanding of the pathophysiology of Alzheimer's disease (AD). We have previously shown that the addition of fibrillar beta-amyloid (Abeta) to mature hippocampal neurons results in progressive neuritic degeneration accompanied by the enhanced phosphorylation of adult tau isoforms. In the present study, we sought to obtain more direct evidence of the signal transduction pathway(s) activated by fibrillar Abeta leading to tau phosphorylation and the generation of dystrophic neurites. Our results indicated that fibrillar Abeta induced the progressive and sustained activation of the mitogen-activated protein kinase (MAPK) in mature hippocampal neurons. On the other hand, the specific inhibition of the MAPK signal transduction pathway by means of PD98059, a MAPK kinase (MEK) specific inhibitor, prevented the phosphorylation of tau (at Ser199/Ser202) induced by fibrillar Abeta. In addition, the inhibition of MAPK activation partially prevented neurite degeneration. Taken collectively, our results suggest that the sustained activation of the MAPK signal transduction pathway induced by fibrillar Abeta may lead to the abnormal phosphorylation of tau and the neuritic degeneration observed in AD.

Abnormal synapse formation in agrin-depleted hippocampal neurons

Agrin, a 200 kDa extracellular matrix protein, participates in the maturation of the postsynaptic target at the neuromuscular junction. Although agrin has also been detected in central neurons, little is known about its role in the formation of their synapses. In the present study, the pattern of expression, localization and function of agrin in developing hippocampal neurons were analyzed. The results indicate that an increase in agrin protein levels precedes synaptogenesis in cultured hippocampal neurons. This increase in agrin expression is accompanied by its extracellular deposition along the distal third of the axon. To investigate whether agrin plays a role during synapse formation, its expression in cultured hippocampal neurons was suppressed by means of antisense oligonucleotide treatment. The suppression of agrin expression results in the impairment of dendritic development and the formation of fewer synapses than in non-treated or sense-treated neurons. Moreover, this decreased synaptic density is accompanied by a selective inhibition of the clustering of GABA receptors. These results lead to the conclusion that agrin may be an important regulator of the maturation of dendrites and synaptogenesis in central neurons.

Developmental expression and localization of glycogen synthase kinase-3beta in rat brain

Glycogen synthase kinase (GSK)-3beta is a protein kinase in the wingless/wnt pathway and as such is involved in the regulation of growth and development of the neural tissue in Drosophila and in vertebrates. This enzyme is also abundantly expressed in the mammal adult brain, where it might play a role in the regulation of several substrates. The expression and the neuroanatomical distribution of GSK-3beta immunoreactivity in the rat brain from embryonic up to adult stages has been studied. GSK-3beta was expressed in the developing brain with the highest expression observed from 18 days of embryonic life up to 10 days of postnatal life. Its expression decreased thereafter and was lowest in the adult. GSK-3beta was strongly expressed in developing neurons but only weakly expressed in layers containing neuroblasts. In the adult and during development, GSK-3beta was detected in the pericarya and proximal part of dendrites. In the embryo, an intense GSK-3beta immunoreactivity was also observed in axonal tracts. This axonal immunoreactivity had markedly decreased by 10 days of postnatal life and was absent at 20 days of postnatal life and in the adult. No GSK-3beta immunoreactivity was detected in astrocytes. The GSK-3beta immunoreactivity was found in most brain regions, although significant local variations of GSK-3beta expression were observed. The developmental evolution of GSK-3beta compartmentalization in neurons parallels that of phosphorylated tau, a protein considered to be a physiological substrate for the kinase.

Regulatory roles of microtubule-associated proteins in neuronal morphogenesis. Involvement of the extracellular matrix

As a result of recent investigations, the cytoskeleton can be viewed as a cytoplasmic system of interconnected filaments with three major integrative levels: self-assembling macromolecules, filamentous polymers, e.g., microtubules, intermediate filaments and actin filaments, and supramolecular structures formed by bundles of these filaments or networks resulting from cross-bridges between these major cytoskeletal polymers. The organization of this biological structure appears to be sensitive to fine spatially and temporally dependent regulatory signals. In differentiating neurons, regulation of cytoskeleton organization is particularly relevant, and the microtubule-associated protein (MAP) tau appears to play roles in the extension of large neuritic processes and axons as well as in the stabilization of microtubular polymers along these processes. Within this context, tau is directly involved in defining neuronal polarity as well as in the generation of neuronal growth cones. There is increasing evidence that elements of the extracellular matrix contribute to the control of cytoskeleton organization in differentiating neurons, and that these regulations could be mediated by changes in MAP activity. In this brief review, we discuss the possible roles of tau in mediating the effects of extracellular matrix components on the internal cytoskeletal arrays and its organization in growing neurons.

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

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

beta-Actin is confined to structures having high capacity of remodelling in developing and adult rat cerebellum

Neurons undergo complex morphological changes during differentiation and in cases of plasticity. A major determinant of cell morphology is the actin cytoskeleton, which in neurons is comprised of two actin isoforms, non-muscle gamma- and beta-actin. To better understand their respective roles during differentiation and plasticity, their cellular and subcellular localization was examined in developing and adult cerebellar cortex. It was observed that gamma-actin is expressed at a constant level throughout development, while the level of beta-actin expression rapidly decreases with age. At the light microscopic level, gamma-actin staining is ubiquitous and the only developmental change observed is a relative reduction of its concentration in cell bodies and white matter. In contrast, beta-actin staining almost completely disappears from the cytoplasm of cell bodies, primary dendrites and axons. In young cerebellar cultures, gamma-actin is found in the cell body, neurites and growth cones, while beta-actin is mainly found in growth cones, as previously reported in other primary neuronal culture systems [Kaech et al. (1997), J. Neuroscience, 17, 9565-9572; Bassell et al., (1998), J. Neuroscience, 18, 251-265]. Electron microscopy of post-embedding immunogold-labelled tissue confirms the widespread distribution of gamma-actin, and also reveals an increased concentration of gamma-actin in dendritic spines in the adult. During development, beta-actin accumulation is observed in actively growing structures, e.g., growth cones, filopodia, cell bodies and axonal tracts. In the adult cerebellar cortex, beta-actin is preferentially found in dendritic spines, structures which are known to retain their capacity for morphological modifications in the adult brain. This differential subcellular localization and developmental regulation of the two actin isoforms point to their different roles in neurons.

Differential interactions of MAP2, tau and MAP5 during axogenesis in culture

The subcellular interactions of the neuronal microtubule-associated proteins tau, MAP2 and MAP5 were determined in cultured rat hippocampal neurons using differential detergent extraction and laser scanning microscopy. Axon development correlated with a transition from a MAP2-actin filament interaction to a MAP2-microtubule association and binding of tau to the distal axon. Tau and MAP2 binding specifically increased in the axon and the minor neurites, respectively. No compartment-specific association of MAP5 was observed. Tau binding preceded the accumulation of microtubules at the distal axon which represented a characteristic event during axogenesis. The data provide evidence for a role of MAP2 in regulating microfilament-microtubule interactions during neurite formation and of tau in organizing microtubules at the distal axon.

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.

Making sense of the multiple MAP-2 transcripts and their role in the neuron

Microtubule-associated protein-2 (MAP-2) is a family of heat-stable, phosphoproteins expressed predominantly in the cell body and dendrites of neurons. Three major MAP-2 isoforms, (MAP-2a, MAP-2b, MAP-2c) are differentially expressed during the development of the nervous system and have an important role in microtubule dynamics. Several MAP-2 cDNA clones that correspond to the major MAP-2 transcripts and additional, novel MAP-2 transcripts expressed in the CNS and PNS have been characterized. The transcripts result from the alternative splicing of a single MAP-2 gene consisting of 20 exons. Studies are now being directed toward understanding the role of the multiple MAP-2 forms that contain novel exons in the nervous system. The expression, localization, and possible functions of the newly identified spliced forms are the focus of this review.

Direct visualization and characterization of stable microtubules from the neurites of cultured dorsal root ganglion cells

We tested the stability of microtubules (MTs) in the neurites of cultured dorsal root ganglion cells by dissolving the cytoplasmic membrane with detergent and exposing them to defined extracellular medium under observation with a video-enhanced differential interference contrast (DIC) microscope. Smooth cytoplasmic filaments visualized after membrane removal were suggested to be MTs by the preservation of all of the filaments in the presence but not in the absence of taxol. They were further confirmed to be MTs by specific immunostaining with anti-tubulin antibody. A significant number of MTs in the established neurites of 6-day-old cultures remained longer than 10 min after membrane removal while MTs in the Schwann cell processes or in the distal regions of the growth cone-bearing neurites of 3-day-old cultures disappeared within 2 min. A population of very stable MTs persisting longer than 30 min was also found specifically in the 6-day-old cultures. Association with other structures or bundling seemed to stabilize the MTs to some degree. The most stable MTs, however, were not associated with some structure along the length but were mainly anchored at points, suggesting that specific point attachments may be another important mechanism operating in MT stabilization. The present method is thus capable of directly demonstrating the unusual stability of neuritic MTs, and provides a new system for further investigation on the mechanism of stabilization.

The structure of divalent cation-induced aggregates of PIP2 and their alteration by gelsolin and tau

Phosphatidylinositol bisphosphate (PIP2) serves as a precursor for diacylglycerol and inositol trisphosphate in signal transduction cascades and regulates the activities of several actin binding proteins that influence the organization of the actin cytoskeleton. Molecules of PIP2 form 6-nm diameter micelles in water, but aggregate into larger, multilamellar structures in physiological concentrations of divalent cations. Electron microscopic analysis of these aggregates reveals that they are clusters of striated filaments, suggesting that PIP2 aggregates form stacks of discoid micelles rather than multilamellar vesicles or inverted hexagonal arrays as previously inferred from indirect observations. The distance between striations within the filaments varies from 4.2 to 5.4 nm and the diameter of the filaments depends on the dehydrated ionic radius of the divalent cation, with average diameters of 19, 12, and 10 nm for filaments formed by Mg2+, Ca2+, and Ba2+, respectively. The structure of the divalent cation-induced aggregates can be altered by PIP2 binding proteins. Gelsolin and the microtubule associated protein tau both affect the formation of aggregates, indicating that tau acts as a PIP2 binding protein in a manner similar to gelsolin. In contrast, another PIP2 binding protein, profilin, does not modify the aggregates.

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

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

Selective phosphorylation of adult tau isoforms in mature hippocampal neurons exposed to fibrillar A beta

How senile plaques and neurofibrillary tangles are linked represents a major gap in our understanding of the pathophysiology of Alzheimer's disease. We characterized a hippocampal neuronal culture system in which tau undergoes maturation in vivo; rat neurons maintained in culture for more than 3 weeks replicated the splicing and phosphorylation changes that tau undergoes upon maturation in situ. Using this model system, we induced an Alzheimer-like neuritic dystrophy following the application of fibrillar beta-amyloid. The dystrophy consisted of focal distortions and swellings within the neurites and an altered phosphorylation of the adult tau isoforms. Fibrillar beta-amyloid induced the concomitant activation of MAP kinase and GSK3 beta. The aberrant activation of several signaling pathways may lead to the abnormal phosphorylation of tau and neuritic degeneration.

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.

Expression and polarization of VGF in developing hippocampal neurons

VGF is an NGF-inducible protein whose expression closely correlates with PC12 cell neurite outgrowth. Immunolocalization of VGF in PC12 cells by electron microscopy revealed labeling of a subpopulation of large dense core vesicles. To determine whether VGF expression may be involved in neural and axonal differentiation, we examined VGF immunoreactivity in developing cultured hippocampal neurons. Most VGF appeared to be in vesicles which were present in neurons from the earliest developmental stages and were particularly abundant in a small number of neurons, most of which were GABAergic. VGF became restricted to axons after dendrites had begun to mature, a stage more closely associated with synaptogenesis than axonal polarization. Colocalization studies suggest that VGF-containing and synaptophysin-containing vesicles are distinct and differentially distributed.

Interaction of insulin-like growth factor-I and estradiol signaling pathways on hypothalamic neuronal differentiation

Neurotrophic effects of estradiol and insulin-like growth factor-I were assessed in primary cultures from fetal rat hypothalamus. Cultured neurons were immunostained with an antibody for the microtubule-associated protein-2. While both estradiol and insulin-like growth factor-I increased the number of microtubule-associated protein-2-immunoreactive neurons and the extension of immunoreactive processes, the effect of these two factors was not additive. The estradiol-induced increases in neuronal numbers and extension of neuronal processes were blocked by either the estrogen receptor antagonist ICI 182,780 or by an anti-sense oligonucleotide to the estrogen receptor. Furthermore, incubation of the cultures with an anti-sense oligonucleotide directed against the insulin-like growth factor-I messenger RNA also blocked the effect of estradiol. In turn, the effects of insulin-like growth factor-I were blocked by the estrogen receptor antagonist ICI 182,780 and by the anti-sense oligonucleotide to the estrogen receptor. These findings suggest that estradiol-induced activation of the estrogen receptor in developing hypothalamic cells requires the presence of insulin-like growth factor-I, and that both estradiol and insulin-like growth factor-I use the estrogen receptor as a mediator of their trophic effects on hypothalamic neurons.

Tau is enriched on dynamic microtubules in the distal region of growing axons

It is widely held that tau determines the stability of microtubules in growing axons, although direct evidence supporting this hypothesis is lacking. Previous studies have shown that the microtubule polymer in the distal axon and growth cone is the most dynamic of growing axons; it turns over more rapidly and is more sensitive to microtubule depolymerizing drugs than the polymer situated proximally. We reasoned that if the stability of axonal microtubules is directly related to their content of tau, then the polymer in the distal axon should have less tau than the polymer in the proximal axon. We tested this proposition by measuring the relative tau content of microtubule along growing axons of cultured sympathetic neurons immunostained for tau and tubulin. Our results show that the tau content of microtubules varies along the axon, but in the opposite way predicted. Specifically, the relative tau content of microtubules increases progressively along the axon to reach a peak near the growth cone that is severalfold greater than that observed proximally. Thus, tau is most enriched on the most dynamic polymer of the axon. We also show that the gradient in tau content of microtubules does not generate corresponding gradients in the extent of tubulin assembly or in the sensitivity of axonal microtubules to nocodazole. On the basis of these findings, we propose that tau in growing axons has functions other than promoting microtubule assembly and stability and the key sites for these functions are the distal axon and growth cone.

Accelerated neuronal differentiation induced by p53 suppression

p53, a tumor suppressor gene product, has been implicated in the control of cell growth and malignant transformation in different cell types. Here we studied the role of p53 in normal central nervous system development. We show that p53 is expressed in neuroblasts and is down regulated when migrating neurons reach their destination. The suppression of p53 either by the addition of antisense oligonucleotides to culture medium or by the culture of neurons from p53-/- mice accelerated their differentiation. This effect is accompanied by an early induction of MAP1b and a premature dephosphorylation of tau. p53 suppression also reduced levels of p21. Taken collectively these results suggest that the expression of p53 in neuroblasts might prevent neuronal terminal differentiation.

MAP-1B/TAU functional redundancy during laminin-enhanced axonal growth

When cultured cerebellar macroneurons develop attached to a laminin-containing substrate or after the acute addition of laminin to the tissue culture medium, there is an acceleration in the rate and extent of axonal elongation. Furthermore, laminin is capable of inducing axonal formation and microtubule stabilization in neurons arrested at stage II of neuritic development by tau suppression (Caceres and Kosik, 1990; Caceres et al., 1991). Laminin-enhanced or induced axonal extension is paralleled by a selective and dramatic incorporation of phosphorylated MAP-1b into axonal microtubules. Axonal formation in neurons growing in the presence of laminin is prevented by treatment of the cultures with a mixture of MAP-1b and tau antisense oligonucleotides, but not by the single suppression of any one of these MAPs. However, suppression of MAP-1b, but not of tau, greatly reduces the increase in the rate and extent of axonal elongation induced by laminin. No such effects are elicited by MAP-1b antisense oligonucleotides in neurons growing in the absence of laminin, e.g. polylysine alone, where most of the MAP-1b present in the cells is dephosphorylated and not associated with the cytoskeleton. Taken collectively, these data suggest that, with regard to axonal elongation, MAP-1b and tau can be functionally substituted, and that extracellular matrix molecules, such as laminin, affect axonal extension by promoting the in vivo utilization of MAP-1b.

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.

Post-translational modifications of tubulin suggest that dynamic microtubules are present in sensory cells and stable microtubules are present in supporting cells of the mammalian cochlea

Post-translational modifications to tubulin in the sensory and supporting cells of the cochlea were studied using antibodies specific to the tyrosinated, detyrosinated, acetylated and polyglutamylated isoforms. In the sensory cells, microtubules which label intensely with antibodies to tyrosinated tubulin are found in networks within the cytoplasm. Microtubules which label with antibodies to detyrosinated tubulin and polyglutamylated tubulin, but not acetylated tubulin, form a small component of the microtubules found in the cytoplasm only in the region below the cuticular plate. Microtubules in the supporting cells (inner and outer pillar cells and Deiters cells) are arranged in bundles and contain little tyrosinated tubulin. They are composed instead of predominantly post-translationally modified isoforms which include detyrosinated, acetylated and polyglutamylated tubulin. The findings suggest that microtubules in the sensory cells form dynamic structures, since microtubules that undergo cyclic polymerization and depolymerization predominantly contain tubulin that has not yet had its carboxy-terminal tyrosine residue removed. The presence of microtubules in the supporting cells in which the tubulin has been polymerized into microtubules long enough to be post-translationally modified, provides evidence that these microtubules are stable, long-lived and could contribute to the structural support of the sensory organ of Corti.

Testis/brain RNA-binding protein attaches translationally repressed and transported mRNAs to microtubules

We have previously identified a testicular phosphoprotein that binds to highly conserved sequences (Y and H elements) in the 3' untranslated regions (UTRs) of testicular mRNAs and suppresses in vitro translation of mRNA constructs that contain these sequences. This protein, testis/brain RNA-binding protein (TB-RBP) also is abundant in brain and binds to brain mRNAs whose 3' UTRs contain similar sequences. Here we show that TB-RBP binds specific mRNAs to microtubules (MTs) in vitro. When TB-RBP is added to MTs reassembled from either crude brain extracts or from purified tubulin, most of the TB-RBP binds to MTs. The association of TB-RBP with MTs requires the assembly of MTs and is diminished by colcemid, cytochalasin D, and high levels of salt. Transcripts from the 3' UTRs of three mRNAs that contain the conserved sequence elements (transcripts for protamine 2, tau protein, and myelin basic protein) are linked by TB-RBP to MTs, whereas transcripts that lack the conserved sequences do not bind TB-RBP. We conclude that TB-RBP serves as an attachment protein for the MT association of specific mRNAs. Considering its ability to arrest translation in vitro, we propose that TB-RBP functions in the storage and transportation of mRNAs to specific intracellular sites where they are translated.

Suppression of synapsin II inhibits the formation and maintenance of synapses in hippocampal culture

Numerous synaptic proteins, including several integral membrane proteins, have been assigned roles in synaptic vesicle fusion with or retrieval from the presynaptic plasma membrane. In contrast, the synapsins, neuron-specific phosphoproteins associated with the cytoplasmic surface of synaptic vesicles, appear to play a much broader role, being involved in the regulation of neurotransmitter release and in the organization of the nerve terminal. Here we have administered antisense synapsin II oligonucleotides to dissociated hippocampal neurons, either before the onset of synaptogenesis or 1 week after the onset of synaptogenesis. In both cases, synapsin II was no longer detectable within 24-48 h of treatment. After 5 days of treatment, cultures were analyzed for the presence of synapses by synapsin I and synaptophysin antibody labeling and by electron microscopy. Cultures in which synapsin II was suppressed after axon elongation, but before synapse formation, did not develop synapses. Cultures in which synapsin II was suppressed after the development of synapses lost most of their synapses. Remarkably, with the removal of the antisense oligonucleotides, neurons and their synaptic connections recovered. These studies lead us to conclude that synapsin II is involved in the formation and maintenance of synapses in hippocampal neurons.

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.

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.

cis-acting signals and trans-acting proteins are involved in tau mRNA targeting into neurites of differentiating neuronal cells

Tau microtubule-associated protein is a neuron specific protein found primarily in axons and is developmentally regulated. The function of tau is in stabilization of microtubules, which is important in establishing and maintaining neuronal morphology. Axonal localization of tau involves a multistep process which is studied in differentiating primary neuronal culture. The initial step involves sorting and subcellular localization of its encoding mRNA into the proximal portion of the axon. Using the transfection assay into neuronal cells, we have demonstrated that sequences located in the 3'-untranslated region include a cis-acting signal which is involved in tau mRNA targeting. In addition, using ultraviolet cross-linking assay, two RNA-binding proteins of 43 and 38 kDa were identified, that exhibit specific binding to a minimal sequence of 91 nucleotides located within the same functional region, which is involved in targeting. The 43 and 38-kDa RNA-binding proteins are present in cytoplasmic extracts, prepared from neuronal cells, and in isolated microtubule preparations. Our results support a novel model in which cis-acting signals, together with RNA-binding proteins are involved in the targeting of tau mRNA, that may ultimately lead to its axonal localization.

Identification of tau protein regions required for process formation in PC12 cells

Tau is a neuronal microtubule-associated protein that is required for the development and maintenance of neuronal cell polarity. It promotes microtubule assembly in vitro and we have recently reported that a specific tau region, which spans amino acid residues 154–172 of human fetal tau, is not required for growth of existing microtubules, but is required for nucleation of new microtubules. These residues also confer stronger microtubule binding activity in 3T3 cells. The aim of this study was to investigate the functional organization of tau in relation to its role in promoting process formation in a neuronal model system. We transfected undifferentiated PC12 cells with vectors expressing tau fragments and treated the expressing cells with cytochalasin B to allow process extension. We found that deletion of amino acid residues 154–172 greatly reduced the percentage of transfected cells bearing processes compared to that of cells transfected with full-length tau or with an amino-terminally deleted tau fragment containing residues 154–172. These differences do not appear to result from a quantitative difference in protein expression, as shown by immunoblot analysis of transfected cells. We also observed that while the presence of tau fragments increases acetylation of microtubules, the pattern of acetylation in cells transfected with the fragment missing residues 154–172 is less extensive, suggesting that it does not result in the same level of stabilization as the longer tau fragments. Taxol promoted process outgrowth in cells treated with cytochalasin and restored process outgrowth to cells transfected with the tau fragment lacking this activity. Therefore, process formation involves primarily the stabilization and nucleation of microtubules. We conclude that the residues necessary for conferring microtubule nucleation activity of tau in vitro are important for process formation in vivo. It is likely that these residues influence the binding affinity and therefore the stabilization activity of tau.

Microtubules are involved in the localization of tau mRNA in primary neuronal cell cultures

Subcellular localization of neuronal mRNAs contributes to the development of identifiable microdomains. In differentiated neurons, tau mRNA is localized in the cell body and the proximal portion of the axon, and MAP2 mRNA is localized in the cell body and dendrites, whereas tubulin mRNA is restricted to the cell body. To investigate the mechanism(s) leading to segregation of mictrotubule-associated protein mRNA, we examined the role of the cytoskeleton in this process. Detergent extraction of primary neuronal cells in culture followed by in situ hybridization analysis demonstrated that tau mRNA remains bound to cytoskeleton of the treated cells. In addition, biochemical fractionation showed that tau and MAP2 mRNAs are preferentially associated with the fraction of assembled microtubules. In contrast, mRNAs restricted to the neuronal cell body, such as those of tubulin, the 68 kDa neurofilament, and mouse GAPDH, are preferentially found in the supernatant. Using cytoskeletal inhibitors, we demonstrate that tau mRNA is associated with the microtubule system, and not with the actin filaments, thus supporting the hypothesis that the mechanism of mRNA localization is a multistep pathway in which the microtubules play a crucial role.