The expression and distribution of the microtubule-associated proteins tau and microtubule-associated protein 2 in hippocampal neurons in the rat in situ and in cell culture

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.

Immunolocalization of GLUT1 and GLUT3 glucose transporters in primary cultured neurons and glia

Immunofluorescence analysis was used to study the cellular localization of glucose transporters 1 and 3 (GLUT1 and GLUT3) in primary rat neuronal and glial cultures. In primary cultured cerebellar granule neurons and cortical neurons, GLUT3 was detected in a pattern consistent with a generalized cell surface distribution. GLUT3 distribution corresponded most closely with the neural cell adhesion molecule (NCAM), and showed overlapping but distinct distributions compared to synaptophysin, microtubule-associated protein 2 (MAP2), neurofilament protein, and growth-associated protein (GAP43). Culture of neurons in the presence of glia did not alter the cellular localization of GLUT3. GLUT1 was detectable in primary cerebellar granule neurons both at the cell surface and in the cytoplasm, and appeared decreased in neurons cocultured with glia. GLUT1, but not GLUT3, was detected in glial fibrillary acidic protein (GFAP)-positive astrocytes present in mixed neuronal-glial cultures derived from cerebellum and cerebral cortex, as well as in cortical astrocyte cultures. GLUT1, but not GLUT3, was also detected in microglia and oligodendrocytes present in these cultures. This study indicates a generalized cell surface expression of the glucose transporters expressed in neurons and glia, rather than selective targeting to different cellular domains or subcellular locations.

Transport of dendritic microtubules establishes their nonuniform polarity orientation

The immature processes that give rise to both axons and dendrites contain microtubules (MTs) that are uniformly oriented with their plus-ends distal to the cell body, and this pattern is preserved in the developing axon. In contrast, developing dendrites gradually acquire nonuniform MT polarity orientation due to the addition of a subpopulation of oppositely oriented MTs (Baas, P. W., M. M. Black, and G. A. Banker. 1989. J. Cell Biol. 109:3085-3094). In theory, these minus-end-distal MTs could be locally nucleated and assembled within the dendrite itself, or could be transported into the dendrite after their nucleation within the cell body. To distinguish between these possibilities, we exposed cultured hippocampal neurons to nanomolar levels of vinblastine after one of the immature processes had developed into the axon but before the others had become dendrites. At these levels, vinblastine acts as a kinetic stabilizer of MTs, inhibiting further assembly while not substantially depolymerizing existing MTs. This treatment did not abolish dendritic differentiation, which occurred in timely fashion over the next two to three days. The resulting dendrites were flatter and shorter than controls, but were identifiable by their ultrastructure, chemical composition, and thickened tapering morphology. The growth of these dendrites was accompanied by a diminution of MTs from the cell body, indicating a net transfer of MTs from one compartment into the other. During this time, minus-end-distal microtubules arose in the experimental dendrites, indicating that new MT assembly is not required for the acquisition of nonuniform MT polarity orientation in the dendrite. Minus-end-distal microtubules predominated in the more proximal region of experimental dendrites, indicating that most of the MTs at this stage of development are transported into the dendrite with their minus-ends leading. These observations indicate that transport of MTs from the cell body is an essential feature of dendritic development, and that this transport establishes the nonuniform polarity orientation of MTs in the dendrite.

Behaviour of small inhibitory interneurons in early postnatal mouse cerebellar microexplant cultures: a video time-lapse analysis

The aim of this work was to investigate how the environment of the neuropil determines the positioning and differentiation of neurons that are postsynaptic to them. We investigated how stellate and basket cells, the small inhibitory interneurons of the cerebellar cortex, find their perpendicular orientation to the direction of fasciculated granule cell axons. Cultures of early postnatal mouse cerebellar microexplants showing this cellular behaviour in vitro were analysed by video time-lapse cinematography and evaluated by morphometry. The small interneurons were first detectable when they migrated, intermingled with granule cells, away from the explant along the radial fascicles of granule cell neurites. During migration some cells suddenly changed their orientation by extending neurites in perpendicular orientation to the radial fascicles. These cells were all GABA-immunoreactive and expressed the cytoskeletal markers tau in the thin axon-like process and MAP2 in the thicker dendrite-like arborizations at the opposite pole of the cell body. After having translocated in perpendicular orientation, these neurons were again able to turn back to move along the radial neurite bundles to another position. Furthermore, while in perpendicular orientation, the processes of these cells repelled each other upon contact of their growth cones, leading to equal spacing between the cell bodies with time in culture.

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.

Cloning of AL-1, a ligand for an Eph-related tyrosine kinase receptor involved in axon bundle formation

REK7 is an Eph-related tyrosine kinase receptor expressed exclusively in the nervous system, predominantly in hippocampus and cortex. A soluble REK7-IgG fusion protein, produced to analyze the biological role of REK7, prevents axon bundling in cocultures of cortical neurons with astrocytes, a model of late stage nervous system development and differentiation. Using REK7-IgG as an affinity reagent, we purified and cloned a novel REK7 ligand called AL-1, a GPI-linked protein homologous to other members of an emerging ligand family. Membrane attachment of AL-1 appears necessary for receptor activation, since REK7 on cortical neurons is efficiently activated by transfected cells expressing GPI-linked AL-1, but not by soluble AL-1. Consistent with this, soluble AL-1 blocks axon bundling. Our findings, together with the observation that both molecules are expressed in the brain, suggest a role in the formation of neuronal pathways, a crucial feature of nervous system development and regeneration.

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.

Synaptogenesis and distribution of presynaptic axonal varicosities in low density primary cultures of neocortex: an immunocytochemical study utilizing synaptic vesicle-specific antibodies, and an electrophysiological examination utilizing whole cell recording

Low-density primary cultures of neocortical neurons were utilized to examine: (i) early interactions of growing neurites with morphological characteristics of axons with other neuronal elements, and (ii) the distribution of presynaptic axonal varicosities closely apposed to MAP-2 immunoreactive, putatively postsynaptic, dendrites. At the light microscopical level axonal varicosities, presumably presynaptic terminals, were identified using immunocytochemistry incorporating antibodies specific for the synaptic vesicle antigens synaptophysin and synapsin. The presence of synaptophysin- and synapsin-immunoreactive swellings along axonal processes was first detected at 5 days post-plating and was also apparent in axons growing in isolation. At 5-7 days in vitro, immunolabelled axonal varicosities in close apposition to putative postsynaptic dendrites (MAP-2 immunoreactive) dendrites were detected. Electrophysiologically active synaptic contacts can also readily be detected at this stage. After 3 weeks in vitro presynaptic contacts do appear to be distributed heterogeneously along postsynaptic dendrites of many neurons in culture. As the culture matures a higher number of presynaptic profiles can be seen along dendrites, with a centrifugal distribution, e.g. a higher density of presynaptic axonal terminals in close apposition to more distal regions of larger dendrites, putatively considered to be apical dendrites of pyramidal-like neurons. In our cultures, the overall increase in the density and the pattern of distribution of presynaptic axon terminals immunoreactive for synaptic vesicle antigens closely apposed to putative post-synaptic structures mimics the general postnatal increase of synaptic density in the neocortex in vivo. Thus, low density primary cultures of neocortical neurons offer a valuable system to explore and manipulate (i) the molecular and cellular basis of neocortical synaptogenesis, and (ii) the pharmacology of neocortical synaptic transmission.

Expression and distribution of phosphorylated MAP1B in growing axons of cultured hippocampal neurons

Microtubule associated proteins (MAPs) interact with tubulin to modulate neurite stability and growth during development. The phosphorylated form of one of these MAPs, MAP1B (MAP1B-P) is hypothesized to be of particular importance for the regulation of neurite outgrowth. To investigate the mechanisms by which MAP1B and MAP1B-P contribute to this regulation, we used a new antibody against an isoform of MAP1B-P to determine its pattern of expression during neuronal development in vitro. We examined cultured hippocampal neurons because these provide a well-established system to evaluate the development of axons and dendrites. MAP1B, MAP1B-P and MAP2 colocalized to the cell bodies and minor processes during the first 24 hours of culture, but MAP1B-P also extended well into the growth cones. As neurite outgrowth and differentiation proceeded, MAP1B and MAP1B-P became localized to the cell bodies and axons, and MAP2 to the cell bodies and dendrites. After 3 days, MAP1B-P declined in the cell body and was segregated to the distal axon; MAP1B remained in the cell body, but was also concentrated in the distal axon. Over 5-9 days in culture, MAP1B-P levels decreased and became undetectable; MAP1B levels decreased later (19-23 days). MAP2 levels, however, remained high through the entire culture period in cell bodies and dendrites. These results are consistent with the hypothesis that MAP1B-P plays an important role in the initiation and elongation of axons by regulating the dynamics of microtubules near the growth cone: MAP1B-P expression is greatest during the period of active neurite extension, is particularly prominent in growth cones where axon outgrowth is most active, and decreases along with the decline in active axon extension.

An increase in phosphorylation of microtubule-associated protein 2 accompanies dendrite extension during the differentiation of cultured hippocampal neurones

Hippocampal neurones, from embryonic rats, were cultured for different times and the extension of dendrite-like processes was analysed morphologically and by immunofluorescence, using microtubule-associated protein 2 (MAP2) as a marker. Simultaneously, the changes in phosphorylation in MAP2 were analyzed and a correlation between dendrite sprouting and an increase in MAP2 phosphorylation was found. Phospho-MAP2 was cleaved by Staphylococcus aureus V8 protease limited proteolysis and its phosphopeptide pattern was compared to that obtained with two protein kinases (calcium/calmodulin-dependent kinase and protein kinase C) in vitro. An involvement of calcium/calmodulin-dependent protein kinase in the phosphorylation of MAP2, occurring simultaneously with dendrite extension during neuronal differentiation in vitro, is suggested.

Distribution of the phosphorylated microtubule-associated protein tau in developing cortical neurons

During brain development, the microtubule-associated protein tau presents a transient state of high phosphorylation. We have investigated the developmental distribution of the phosphorylated fetal-type tau in the developing rat cortex and in cultures of embryonic cortical neurons, using antibodies which react with tau in a phosphorylation-dependent manner. The phosphorylated fetal-type tau was present in the developing cortex at 20 days but not at 18 days of embryonic life and was not detected before four to five days in neuronal culture. The cyclin-dependent kinase p34cdc2 was expressed only in germinal layers in the embryonic brain and was not co-localized with phosphorylated tau. After 10 days of postnatal life, the phosphorylated tau progressively disappeared from cortical neurons, disappearing first from the deepest cortical layers where neurons are ontogenetically the oldest. Phosphorylated tau was found in axons and dendrites of cortical neurons at all developmental stages whereas unphosphorylated tau tended to disappear from dendrites during development. The timing of appearance of phosphorylated tau in the cortex, by comparison with the expression of other developmental markers, indicates that phosphorylated tau is present at a high level only during the period of intense neuritic outgrowth and that it disappears during the period of neurite stabilization and synaptogenesis, concomitantly to the expression of adult tau isoforms. In control cultures and in cultures treated with colchicine, the phosphorylated tau was not associated to cold-stable and to colchicine-resistant microtubules. These in vivo results suggest that the high expression of phosphorylated tau species is correlated with the presence of a dynamic microtubule network during a period of high plasticity in the developing brain.

Polarized distribution of the trans-Golgi network marker TGN38 during the in vitro development of neocortical neurons: effects of nocodazole and brefeldin A

Neurons are polarized secretory cells whose cytoplasm and plasma membrane are polarized to form two compartments: dendrites and axons. In mature, fully polarized neurons, the microtubule-associated protein Map2 is targeted to dendrites, while tau is mainly restricted to axons. However, the intraneuronal distribution of secretory pathway organelles, such as the endoplasmic reticulum and the Golgi complex, which give rise to all constitutive, regulated and lysosome vesicles, is poorly understood. Thus, to investigate the distribution of the trans-Golgi network during the development and maturation of rat neocortical neurons in vitro, we have utilized an antibody recognizing a 38 kDa trans-Golgi network-specific protein, TGN38, and immunofluorescence microscopy. Before neurons have established polarity. TGN38 immunoreactivity outlines several vesicles dispersed throughout the cell body cytoplasm; these converge close to a major Map2-immunopositive process during the establishment of neuronal polarity, and later merge into a single structure located at the base of a thick Map2-immunopositive process, approximately 18 h after plating. At this stage TGN38 immunoreactivity is located within 45 degrees of the major Map2-immunoreactive process in 54% of neurons, while in only 6% of cells it is located at the opposite pole. After 3 days in vitro, during the segregation of microtubule-associated proteins to either dendrites or axons. TGN38 immunoreactivity clusters continue to be located close to a major dendrite, and in some neurons these clusters begin to enter a major Map2-immunoreactive process. At 10 days in vitro TGN38 immunoreactivity extends into a major dendrite for 5-30 microns in many neurons. Thus, the distribution of TGN38 immunoreactivity becomes polarized, being localized within a single, usually the major, neocortical dendrite. Our results also show that the morphological appearance of TGN38-immunoreactive structures is microtubule-dependent, since nocodazole treatment of polarized neurons induces scattering of TGN38-immunoreactive vesicles throughout the cell body's cytoplasm. Treatment with brefeldin A induces scattering of small TGN38-immunoreactive vesicles throughout the neuronal cytoplasm and processes, a different response to that observed in non-neuronal cells.

Localization of differentially phosphorylated isoforms of microtubule-associated protein 1B in cultured rat hippocampal neurons

The development and plasticity of axons and dendrites in mammalian neurons may depend on the presence and phosphorylation state of cytoskeletal proteins, including certain microtubule-associated proteins. One of these proteins, microtubule-associated protein 1B, is modified by different protein kinases, which give rise to two major types of phosphorylated isoforms. The distribution of these isoforms in cultured hippocampal neurons has been studied using antibodies to specific phosphorylation-sensitive epitopes. Mode I-phosphorylated MAP1B is largely restricted to developing axonal processes, particularly at their distal regions including their growth cones where no mode I-dephosphorylated MAP1B is present. Axonal maturation is accompanied by dephosphorylation of MAP1B at mode I sites. Thus, mode I-phosphorylated MAP1B may be a marker for axonal growth. In contrast, mode II-phosphorylated MAP1B is abundant in the axonal and somatodendritic compartments, and no increased dephosphorylation occurs during maturation. These results are compatible with a role for the mode I phosphorylation of MAP1B (which might be catalysed by proline-directed protein kinases) in supporting a rapid axonal-specific growth mechanism and a more general role for the mode II phosphorylation of MAP1B (which seems to be catalysed by casein kinase II) in controlling axonal and dendritic growth and remodeling.

Selective labeling of embryonic neurons cultured on astrocyte monolayers with 5(6)-carboxyfluorescein diacetate (CFDA)

A method for selectively labeling cultured neurons using the vital dye, 5(6)-carboxyfluorescein diacetate (CFDA), is described. This non-fluorescent membrane-permeant dye is cleaved by cytosolic esterases into the fluorescent anion, 5(6)-carboxyfluorescein (CF). Both astrocytes and neurons exhibit brilliant fluorochromasia within minutes of CFDA loading. However, following a brief rinse in buffered saline in the absence of CFDA, the astrocytes rapidly lose their cellular fluorescence while the neurons retain the dye for several hours. The fluorochromasia is uniformly distributed throughout the soma and processes which greatly facilitates the morphological identification of viable neurons. In addition, this protocol can be used to conveniently quantify neuronal survival in assays of the activities of neurotrophic or neurotoxic substances.

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

We report here a novel intracellular localization and function of Tau proteins in cultured cerebellar neurons. Immunofluorescence staining of detergent-extracted cytoskeletons with antibodies specific for Tau proteins revealed intense labeling of growth cone microtubules. Besides, suppression of Tau by antisense oligonucleotide treatment results in the complete disappearance of antigen 13H9, a specific growth cone component with properties of microfilament- and microtubule-associated protein [Goslin et al., 1989: J. Cell Biol. 109:1621-1631], from its normal intracellular location. This phenomenon is unique to neurite-bearing cells, is not associated with the disappearance of microtubules from growth cones, and is not reversed by taxol, a microtubule-stabilizing agent. In addition, Tau-suppressed neurons display a significant reduction in growth cone area and fillopodial number; on the contrary, fillopodial length increases significantly. The alterations in growth cone morphology are accompanied by considerable changes in the phalloidin staining of assembled actin. Taken together, the present results suggest that in developing neurons Tau proteins participate in mediating interactions between elements of the growth cone cytoskeleton important for maintaining the normal structural organization of this neuritic domain.

Glutamate neurotoxicity in mesencephalic dopaminergic neurons in culture

The neurotoxic effect of glutamate in cultured mouse mesencephalic dopaminergic neurons was investigated. Neuron-rich cell cultures were prepared from 13-14-day-old fetal mouse ventral mesencephalic tissue. Cultures were exposed to glutamate for 10 min and evaluated for glutamate neurotoxicity (GNT) 18-24 hr later by tyrosine hydroxylase (TH) immunostaining, microtubule associated protein-2 (MAP2) immunostaining, and radiolabeled dopamine uptake assay. In glutamate-exposed cultures, the number of TH-positive neurons and the level of dopamine uptake were reduced to 40% (35-45%) and 50% (47-52%), respectively, of control cultures. The number of MAP2-positive neurons was also reduced to 47%, indicating that the GNT was not restricted or selective to dopaminergic neurons. It is concluded that GNT was mediated by the N-methyl-D-aspartic acid (NMDA) receptor from the following observations: 1) GNT was completely blocked by MK-801, an NMDA receptor antagonist; 2) NMDA itself was as toxic as glutamate; 3) 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate (AMPA/KA) receptor, did not block GNT; 4) kainate did not show neurotoxicity at a low concentration; and 5) two modulators of the NMDA receptor, 7-chlorokynurenic acid and magnesium, were effective in blocking GNT. Protective effects of phorbol myristate acetate, a tumor promoter, and gangliosides (GM1 and GT1b) on GNT were also demonstrated. Possible interactions between GNT and several protein kinase cascades were also investigated. Forskolin, an activator of adenyl cyclase and protein kinase A, showed some protective effect on GNT. But okadaic acid, an inhibitor of phosphatases, and genistein, a tyrosine kinase inhibitor, did not show any protective effect. These results suggest that 1) glutamate is capable of causing neuronal death in the substantia nigra; 2) GNT on dopaminergic neurons is mainly mediated by the NMDA receptor under the conditions of our study; 3) protein kinase C translocation is a key mechanism of GNT; and 4) there is an interplay of a signal transduction system in the pathomechanism of GNT.

The segregation and expression of glutamate receptor subunits in cultured hippocampal neurons

The distribution and expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-selective glutamate receptor subunits (GluR1-4) were studied in cultured hippocampal neurons using antibodies generated against peptides corresponding to the C-termini of GluR1, GluR2/3 and GluR4, and with a set of oligonucleotide probes designed complementary to specific pan, flip and flop GluR1-4 messenger RNA sequences. GluR1-4 subunit proteins were localized in fixed hippocampal neurons (2 h to three weeks after plating) by immunocytochemistry with light and electron microscopy. At early stages in culture, moderate staining with antibodies to GluR1 and GluR2/3 and very light staining with antibody to GluR4 was observed in cell bodies and proximal portions of all neurites of some neurons. Upon establishment of identified axons and dendrites by seven days in culture, staining was intense with specific antibodies to GluR1 and GluR2/3 and light with anti-GluR4 antibody in cell bodies and dendrites. Little or no staining was observed in axons. Cells at seven days in culture exhibited a variety of morphologies. However, we could not assign a pattern of staining to a particular type. As the cultures matured over two and three weeks, staining was limited to the somatodendritic compartment. The intensity of glutamate receptor subunit staining increased and the extent of staining proceeded to the distal extreme of many dendrites. Moreover, antibodies to GluR1-4 subunits were co-localized in neurons. Immunocytochemistry on living neurons did not result in any significant labeling, suggesting that the epitope is either not expressed on the surface of the neurons, or is present, but inaccessible to the antibody. Electron microscopy demonstrated receptor localization similar to that found in brain, with staining of postsynaptic membrane and density, dendritic cytoplasm and cell body, but not within the synaptic cleft. We examined the possible role of "cellular compartmentation" in the pattern of glutamate receptor expression in hippocampal neurons. Compartmentalization studies of the subcellular distribution of messenger RNAs encoding GluR1-4 subunits was determined in mature cultures by in situ hybridization. Significant silver grain appearance was restricted to the cell body, indicating that the synthesis of glutamate receptor subunits is limited largely to the neuronal cell body. The expression of microtubule-associated protein 2 was studied in parallel. Microtubule-associated protein 2 expression appeared 6 h after plating, while glutamate receptor subunit expression was present at 2 h. This indicates that microtubule-associated protein 2 does not regulate the initial distribution of glutamate receptor subunits into neurites.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of cytoskeletal and neuronal markers in micromass cultures of rat embryonic midbrain cells

Micromass cultures of rat embryonic midbrain cells were characterized with regard to the immunolocalization of neuronal and cytoskeletal markers. Cells taken from gestational day-12 embryos and cultured for 5 days in vitro comprise at least two morphologically distinct cells types: fibroblast-like cells and neurons. Antibodies to the following markers yielded preferential staining of neuronal cells: A2B5 (GQ ganglioside), gamma-aminobutyric acid (GABA), microtubule-associated protein 2 (MAP2), MAP5, neuron-specific enolase (NSE), neural cell adhesion molecule (N-CAM), and tau. Antibodies to beta-tubulin, c-neu, MAP1, and neurofilament (NF-H) stained both neuronal and fibroblast-like cells. Antibodies to glial fibrillary acidic protein (GFAP) and vimentin failed to immunoreact with any cells in day-5 CNS cultures. SDS-PAGE and Western analysis were employed to determine the specificity of the antibodies and determine the electrophoretic profiles of the markers. We conclude that the pattern of neuronal differentiation in CNS micromass cultures exhibits certain similarities to that observed in vivo. In addition, certain markers identified in this study may be of potential utility as (1) biomarkers of chemically-induced developmental neurotoxicity, and (2) indicators of differential toxicity toward the diverse cell types that comprise the mammalian central nervous system.

Process formation in Sf9 cells induced by the expression of a microtubule-associated protein 2C-like construct

To understand the roles of various microtubule-associated proteins (MAPs) in the development of axons and dendrites, we have expressed individual neuronal MAPs in normally rounded Sf9 host cells. We previously reported that expression of tau protein in these cells results in the elaboration of long processes containing dense bundles of microtubules (MTs). These bundles generally terminate in the hillock region of the cell body, and almost all of the MTs within the bundles are oriented with their plus ends distal to the cell body. Here we report the expression of a construct that approximates the MAP2C sequence and also induces the elaboration of processes with dense bundles of predominantly plus-end-distal MTs. Whereas tau generally results in a single process, there is a significantly greater tendency for the MAP2C-like construct to induce multiple processes. In contrast to the tau processes, the MT bundle in these processes extends far into the cell body. This latter observation suggests that MAP2C and tau have different effects on MT assembly and/or transport events in the cell. Although both of these MAPs can organize MTs that are competent to participate in process formation, the detailed organization of MTs induced by each of the two constructs is distinctive, and these differences may be relevant to axonal and dendritic differentiation.

Time lapse study of neurite growth in hypothalamic dissociated neurons in culture: sex differences and estrogen effects

Cultures of dissociated hypothalamic cells taken from rat fetuses of 19 days of gestation were studied using time-lapse recording and sequential microphotography from 1 to 5 days in vitro (DIV) and at 7 and 21 DIV. Cultures were seeded with cells taken from fetuses grouped by sex or sexually mixed; experimental cultures were raised in medium containing 17-beta-estradiol 100 nM (E2). Cells were plated on poly-D-lysine-coated coverslips at a culture density of approximately 4,000 cells/cm2. Immunocytochemistry of cell cultures was performed using a Tau monoclonal antibody (clone Tau-1 PC1C6) and a monoclonal antibody against MAP-2 (clone AP-20). Cells started to produce lamellipodia and neuritic processes approximately 4 hr after plating. Forty-eight hours later a few neurons had defined their morphological polarity by the differentiation of an axon-like process that grows faster than the others; at 5 DIV almost all neurons had defined their axons. At this time, monoclonal antibody against MAP-2 clearly stained soma and dendrites, but not axons. Tau immunoreactivity (lots CCA101 and CCA101N from Boeringher Mannheim) was differentially distributed, with a clear predominance in axon and soma. Results on the morphometric analysis of control and E2 treated neurons provide direct evidence for the existence of sex related differences in the neurite outgrowth response of hypothalamic neurons, since cultured neurons taken from female fetuses differentiated axons later and had fewer primary neurites and shorter dendrites than neurons taken from male fetuses or sexually mixed cultures. Also, it was demonstrated in living neurons that E2 effectively enhances outgrowth and elongation in axons. The frequency distribution curves of axonal length for control and E2 treated cultures was unimodal, suggesting that the effect of E2 was a uniform increase in the axonal length of all neurons. The structural differences between neurons from both sexes and the changes induced by E2 may contribute to explain the differences in brain function found between the sexes.

Expression and phosphorylation of a three-repeat isoform of tau in transfected non-neuronal cells

The neuronal microtubule-associated protein, tau, is expressed as a set of isoforms containing either three or four tandemly repeated 31-amino-acid motifs in the C-terminal half of the molecule that can bind to microtubules. Three-repeat forms are the only ones expressed early in development. A single three-repeat isoform of tau has been stably expressed in non-neuronal cells which do not express endogenous tau. Chinese hamster ovary (CHO) cells were transfected with a full-length cDNA coding for the foetal form of human tau cloned downstream of the simian virus 40 (SV40) promoter, and a cell line constitutively expressing tau, CHO[pSVtau3], was isolated. Double-label immunofluorescence microscopy reveals that tau co-localizes with the microtubular network of normal or taxol-treated CHO[pSVtau3] cells, without inducing any dramatic change in cell morphology. Tau is expressed in CHO[pSVtau3] cells as three bands in SDS/PAGE recognized by antibodies to tau, the slow-migrating tau species being the most abundant. Tau also appears as three bands in a heat-stable fraction from CHO[pSVtau3] cells, but a single band of enhanced immunoreactivity is detected following treatment of this fraction with alkaline phosphatase. This single band co-migrates with the fast-migrating band of untreated fractions or whole-cell extracts. In conclusion, a three-repeat isoform of tau is capable of binding to microtubules in transfected non-neuronal cells; furthermore, in this system, the protein is phosphorylated in at least two different states inducing a reduced electrophoretic mobility.

Microtubule-associated protein tau is required for axonal neurite elaboration by neuroblastoma cells

NB2a/d1 neuroblastoma cells constitutively express multiple isoforms of the microtubule-associated protein tau and incorporate this protein into the axonal neurites elaborated during serum deprivation. To examine whether or not tau played an essential role in axonal outgrowth, cells cultured in serum-free medium were treated at 24 h intervals with antisense- and sense-oriented cDNA oligonucleotides (25 or 36 mers that span or are upstream of tau initiation codon) and were simultaneously serum deprived. Oligonucleotide uptake was confirmed by determination of intracellular levels of radiolabeled oligonucleotides. Treatment for 48 h with tau antisense oligonucleotides reversibly inhibited the expression of tau and the number of neurite-bearing cells compared with treatment with sense oligonucleotides. By contrast, tubulin expression was not affected. When cells were treated with antisense oligonucleotide simultaneously with serum deprivation, the initial outgrowth of neurites was unaffected, but continued neurite elongation was prevented. By contrast, neurite outgrowth at 4 h was inhibited when cells were pretreated with tau antisense 24 h before serum deprivation. Furthermore, intracellular delivery of anti-tau antiserum prevented neurite outgrowth and, in cells that had previously been deprived of serum for 24 h, induced retraction of existing neurites. These findings indicate that both the initiation and the continued outgrowth of neurites are dependent on tau and that pre-existing cytoplasmic pools of tau can mediate initial neuritogenesis.

Effects of microtubule stabilization and destabilization on tau immunoreactivity in cultured hippocampal neurons

Tau immunoreactivity is altered in neurofibrillary tangles (NFT) and degenerating neurites in Alzheimer's disease (AD). In addition, cytoskeletal proteins including tau are excessively phosphorylated in AD. Previous data indicated that calcium influx can cause antigenic changes in tau in cultured rat hippocampal and human cortical neurons similar to those seen in NFT. The present study used cultured hippocampal neurons to test the hypothesis that disruption of microtubules is a key event leading to altered antigenic properties of tau that result from calcium influx. As previously reported, we found that glutamate (100-500 microM) and calcium ionophore A23187 (0.5-1 microM) elevated intraneuronal calcium levels and caused a reduction in microtubules, a marked increase in staining with Alz-50 and 5E2, and a decrease in tau-1 immunoreactivity. The microtubule-disrupting agent colchicine (1 microM) caused increased immunoreactivity of neurons towards tau antibodies Alz-50 and 5E2, and these effects of colchicine occurred in the absence of an increase in intracellular calcium levels. The microtubule-stabilizing drug taxol (100 nM) reduced neuronal immunoreactivity towards Alz-50 and 5E2 in untreated cultures and in cultures exposed to glutamate or A23187. Western blot analysis indicated that A23187 caused a reduction in tau levels which was partially prevented by taxol, suggesting that tau associated with microtubules is less susceptible to calcium-mediated degradation. Acid phosphatase treatment increased neuronal immunoreactivity towards tau-1 and reduced immunoreactivity towards Alz-50. The calcium-induced alterations in tau immunoreactivity were, and the colchicine-induced alterations were not, affected by acid phosphatase treatment. Taken together, the data indicate that microtubule depolymerization can cause antigenic changes in tau similar to those seen in NFT independently of an increase in intraneuronal calcium levels. Stabilization of microtubules prevented the antigenic changes in tau suggesting that microtubules affect the availability and/or properties of epitopes on tau that are recognized by antibodies that stain NFT.

A light and electron microscopic study of intracellularly HRP-labeled lumbar motoneurons after intramedullary axotomy in the adult cat

In contrast to many other neurons in the central nervous system, spinal motoneurons in adult cats have been shown to regenerate their axons after an axotomy accomplished within the CNS compartment. This regenerative capacity may be the result of extrinsic influences, or intrinsic properties of the motoneurons themselves, or interactions between extrinsic and intrinsic factors. As part of the effort to establish circumstances of importance for this central regeneration, a detailed analysis of the morphology of lumbar motoneurons was performed 3-11 weeks following a ventral funiculus axotomy. Fourteen large neurons considered to be intramedullarly axotomized alpha motoneurons were labeled intracellularly with horseradish peroxidase. Twelve out of the fourteen analyzed neurons had an axonlike regenerating process. These twelve neurons could, in turn, be separated into two groups, based on the proximity of the axonal lesion and the proximal morphology of the regenerating process. Thus, after a comparatively proximal axotomy, new axons were produced, originating either from the cell soma or from a distal dendritic branch. After a more distal axotomy, but still intramedullarly, it seemed as if the proximal part of the original axon always persisted and subsequently regenerated. Analysis of the relation between the cell soma diameter and the diameter and number of its stem dendrites revealed that dendrites become thinner and also decrease in number after an intramedullary axotomy. In this way, it may be calculated that the total dendritic surface area of lesioned motoneurons will decrease by approximately half. In four neurons, most dendrites had an abnormal appearance in the light microscope with increasing diameter of distal branches. Ultrastructural analysis revealed that such dendrites were surrounded by myelin sheaths. Small filopodia in close relation to axon terminals were found to emerge from the cell membrane of the lesioned motoneurons. Their function may be to establish contact with presynaptic elements and then retract them to the cell membrane. We interpret the morphological changes of the motoneurons as signs of a large capacity for axonal regeneration, even after axotomy in the central nervous system.

Development and maintenance of the neuronal cytoskeleton in aggregated cell cultures of fetal rat telencephalon and influence of elevated K+ concentrations

Serum-free aggregating cell cultures of fetal rat telencephalon were examined by biochemical and immunocytochemical methods for their development-dependent expression of several cytoskeletal proteins, including the heavy- and medium-sized neurofilament subunits (H-NF and M-NF, respectively); brain spectrin; synapsin I; beta-tubulin; and the microtubule-associated proteins (MAPs) 1, 2, and 5 and tau protein. It was found that with time in culture the levels of most of these cytoskeletal proteins increased greatly, with the exceptions of the particular beta-tubulin form studied, which remained unchanged, and MAP 5, which greatly decreased. Among the neurofilament proteins, expression of M-NF preceded that of H-NF, with the latter being detectable only after approximately 3 weeks in culture. Furthermore, MAP 2 and tau protein showed a development-dependent change in expression from the juvenile toward the adult form. The comparison of these developmental changes in cytoskeletal protein levels with those observed in rat brain tissue revealed that protein expression in aggregate cultures is nearly identical to that in vivo during maturation of the neuronal cytoskeleton. Aggregate cultures deprived of glial cells, i.e., neuron-enriched cultures prepared by treating early cultures with the antimitotic drug cytosine arabinoside, exhibited pronounced deficits in M-NF, H-NF, MAP 2, MAP 1, synapsin I, and brain spectrin, with increased levels of a 145-kDa brain spectrin breakdown product. These adverse effects of glial cell deprivation could be reversed by the maintenance of neuron-enriched cultures at elevated concentrations of KCl (30 mM). This chronic treatment had to be started at an early developmental stage to be effective, a finding suggesting that sustained depolarization by KCl is able to enhance the developmental expression and maturation of the neuronal cytoskeleton.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

Processes induced by tau expression in Sf9 cells have an axon-like microtubule organization

We have indirectly analyzed the role of tau in generating the highly organized microtubule (MT) array of the axon. Axons contain MT arrays of uniform polarity orientation, plus ends distal to the cell body (Heidemann, S. R., J. M. Landers, and M. A. Hamborg. 1981. J. Cell Biol. 91:661-673). Surprisingly, these MTs do not radiate from a single discrete nucleating structure in the cell body (Sharp, G. A., K. Weber, and M. Osborn. 1982. Eur. J. Cell Biol. 29: 97-103), but rather stop and start at multiple sites along the length of the axon (Bray, D., and M. B. Bunge. 1981. J. Neurocytol. 10:589-605). When Sf9 ovarian cells are induced to express high levels of tau protein, they develop cellular processes which are similar in appearance to axons and which contain dense arrays of MTs (Knops, J., K. S. Kosik, G. Lee, J. D. Pardee, L. Cohen-Gould, and L. McConlogue. 1991. J. Cell Biol. 114:725-734). We have analyzed the organization of MTs within these arrays, and determined it to be similar, but not identical, to the organization of MTs within the axon. The caliber, MT number, and MT density vary significantly from process to process, but on average are manyfold higher in the tau-induced processes than typically found in axons. Greater than 89% of the MTs in the processes are oriented with their plus ends distal to the cell body, and this proportion is even higher in the processes that are most similar to axons with regard to caliber, MT number, and MT density. Similar to the situation in the axon, MTs are discontinuous along the length of the tau-induced processes, and do not emanate from any observable nucleating structure in the cell body. We have also identified bundles of MTs throughout the cell bodies of the Sf9 cells induced to express tau. Similar to the MT arrays in the processes, these MT bundles are not visibly associated with any other cytological structures that might regulate their polarity orientation. Nevertheless, these bundles consist of MTs most (greater than 82%) of which have the same polarity orientation. Collectively, these results suggest that tau may play a fundamental role in generating MT organization in the axon. In particular, a key property of tau may be to bundle MTs preferentially with the same polarity orientation.

Proteolysis of microtubule-associated protein 2 and tubulin by cathepsin D

The in vitro degradation of microtubule-associated protein 2 (MAP-2) and tubulin by the lysosomal aspartyl endopeptidase cathepsin D was studied. MAP-2 was very sensitive to cathepsin D-induced hydrolysis in a relatively broad, acidic pH range (3.0-5.0). However, at a pH value of 5.5, cathepsin D-mediated hydrolysis of MAP-2 was significantly reduced and at pH 6.0 only a small amount of MAP-2 was degraded at 60 min. Interestingly, the two electrophoretic forms of MAP-2 showed different sensitivities to cathepsin D-induced degradation, with MAP-2b being significantly more resistant to hydrolysis than MAP-2a. To our knowledge, this is the first clear demonstration that MAP-2 is a substrate in vitro for cathepsin D. In contrast to MAP-2, tubulin was relatively resistant to cathepsin D-induced hydrolysis. At pH 3.5 and an enzyme-to-substrate ratio of 1: 20, only 35% of the tubulin was degraded by cathepsin D at 60 min. The cathepsin D-mediated hydrolysis of tubulin was optimal only at pH 4.5. These results demonstrate that MAP-2 and tubulin are unequally susceptible to degradation by cathepsin D. These data also imply a potential for rapid degradation of MAP-2 in vivo by cathepsin D either in lysosomes or perhaps autophagic vacuoles of the neuron.

Immunofluorescent characterization of retinal ganglion cell neurites cultured on substrates coated with antibodies against Thy-1

Neurite outgrowth from rat retinal ganglion cells is enhanced if they are cultured on a substrate coated with antibodies against Thy-1. We show that the ganglion cell neurites that grow on antibodies against Thy-1 display immunoreactivity for microtubule-associated protein 2 and tau, cytoskeletal proteins normally localized to the somatodendritic and axonal compartments, respectively. This result suggests that these neurites are incompletely differentiated and have axonal and dendritic features. We also show that the antibodies against Thy-1 used to coat the substrate bind specifically to ganglion cell membranes within 1 day of culture.

Role of the growth-associated protein B-50/GAP-43 in neuronal plasticity

The neuronal phosphoprotein B-50/GAP-43 has been implicated in neuritogenesis during developmental stages of the nervous system and in regenerative processes and neuronal plasticity in the adult. The protein appears to be a member of a family of acidic substrates of protein kinase C (PKC) that bind calmodulin at low calcium concentrations. Two of these substrates, B-50 and neurogranin, share the primary sequence coding for the phospho- and calmodulin-binding sites and might exert similar functions in axonal and dendritic processes, respectively. In the adult brain, B-50 is exclusively located at the presynaptic membrane. During neuritogenesis in cell culture, the protein is translocated to the growth cones, i.e., into the filopodia. In view of many positive correlations between B-50 expression and neurite outgrowth and the specific localization of B-50, a role in growth cone function has been proposed. Its phosphorylation state may regulate the local intracellular free calmodulin and calcium concentrations or vice versa. Both views link the B-50 protein to processes of signal transduction and transmitter release.

Inhibition of MAP2 expression affects both morphological and cell division phenotypes of neuronal differentiation

Expression of the differentiated neuronal phenotype is typically manifest in several properties: distinct morphologies and organizations of the underlying cytoskeleton; appearance of specific macromolecules; and cessation of cell division. All of these properties are induced in undifferentiated embryonal carcinoma cells exposed to retinoic acid. We show here that the mRNA and protein for the microtubule component MAP2 is also induced by retinoic acid. Stable transfectants of undifferentiated cells, constitutively expressing MAP2 antisense RNA, show significantly reduced levels of MAP2 antisense RNA, show significantly reduced levels of MAP2 protein upon induction compared with controls. These cells do express other neuronal markers, but they do not undergo normal morphological differentiation nor do they withdraw from the cell cycle. The results suggest that MAP2 expression may be necessary for both neurite extension and cessation of cell division.

Early in vitro genesis and differentiation of axons and dendrites by hippocampal neurons analyzed quantitatively with neurofilament-H and microtubule-associated protein 2 antibodies

Differentiating neurons initially extend neurites that are the precursors of axons and dendrites. The temporal pattern of neurite outgrowth has been studied extensively, but mostly qualitative analyses have been used to study this phenomenon. We have examined neurite outgrowth of hippocampal neurons in primary cultures using a polyclonal antibody against microtubule-associated protein 2 (MAP2) and a novel monoclonal antibody against the phosphorylated form of high neurofilament subunit (NF-H). These antibodies serve as markers for dendrites and axons, respectively. The neurite staining patterns were quantified during the first 10 days in culture and the analysis revealed that primary processes undergo three phases of differentiation: (i) in the first 24 h, the majority of primary neurites express MAP2 only and a small percentage express both MAP2 and NF-H; (ii) between 24 and 48 h, NF-H expression increases and it is coexpressed with MAP2 in many neurites as they begin to lengthen; and (iii) between 48 h and 4 days, MAP2 and NF-H protein expression occurs in separate populations of neurites. While most of the earliest forming primary neurites appear to be dendritic (MAP2 only), the coexpression of dendritic and axonal protein markers in a group of early forming processes suggests that these neurites may not be predetermined to become a dendrite or an axon. Our data also indicate that NF-H is detectable early in primary neurite development and that, based on in vivo localization and morphology of cultured neurites, the phosphorylated form of NF-H is concentrated in axons.

The temporal expression of amyloid precursor protein mRNA in vitro in dissociated hippocampal neuron cultures

The subunit protein of the neurofibrillary tangle, the core protein of the neuritic plaque, and the amyloid of the cerebrovasculature in Alzheimer disease and normal aging is a unique 42-amino acid protein (amyloid beta-protein), suggesting a common origin for these pathological entities. However, the expression of the amyloid precursor protein mRNA (APP mRNA) from which the amyloid beta-protein is derived varies between specific neuronal populations. To determine the conditions under which neuronal synthesis of amyloid beta-protein might contribute to the formation of these structures, we have studied the temporal pattern of APP mRNA expression in developing fetal rabbit hippocampal neurons in vitro. Using in situ hybridization with a biotinylated riboprobe transcribed from a cDNA which includes the region encoding the amyloid beta-protein, we have observed a developmentally specific pattern of APP mRNA hybridization during neuronal maturation in vitro.

Segregation of viral structural proteins in cultured neurons of rat spinal ganglia and cord

Cultured spinal ganglion and spinal cord neurons were used to examine the intraneuronal distribution of five structural proteins of Sendai virus by immunohistochemistry. In spinal ganglion cells the internal, cytosolic viral proteins (the nucleocapsid, polymerase and matrix proteins) were confined to the perikarya, while the envelope glycoproteins (the haemagglutinin-neuraminidase and fusion proteins) also appeared in the axon-like processes. All five proteins occurred in the dendrite-like processes of spinal cord neurons. In both types of neuron the cytosolic viral proteins showed a pattern of distribution similar to that observed for the microtubule-associated protein MAP2. The segregated occurrence of the viral envelope and cytosolic proteins in axons may prevent virus assembly in axons and limit long-distance spread of paramyxoviruses in the nervous system.

Polarized sorting of viral glycoproteins to the axon and dendrites of hippocampal neurons in culture

Cultured hippocampal neurons were infected with a temperature-sensitive mutant of vesicular stomatitis virus (VSV) and a wild-type strain of the avian influenza fowl plague virus (FPV). The intracellular distribution of viral glycoproteins was monitored by immunofluorescence microscopy. In mature, fully polarized neurons the VSV glycoprotein (a basolateral protein in epithelial MDCK cells) moved from the Golgi complex to the dendritic domain, whereas the hemagglutinin protein of FPV (an apically sorted protein in MDCK cells) was targeted preferentially, but not exclusively, to the axon. The VSV glycoprotein appeared in clusters on the dendritic surface, while the hemagglutinin was distributed uniformly along the axonal membrane. Based on the finding that the same viral glycoproteins are sorted in a polarized fashion in both neuronal and epithelial cells, we propose that the molecular mechanisms of surface protein sorting share common features in the two cell types.

Changes in microtubule polarity orientation during the development of hippocampal neurons in culture

Microtubules in the dendrites of cultured hippocampal neurons are of nonuniform polarity orientation. About half of the microtubules have their plus ends oriented distal to the cell body, and the other half have their minus ends distal; in contrast, microtubules in the axon are of uniform polarity orientation, all having their plus ends distal (Baas, P.W., J.S. Deitch, M. M. Black, and G. A. Banker. 1988. Proc. Natl. Acad. Sci. USA. 85:8335-8339). Here we describe the developmental changes that give rise to the distinct microtubule patterns of axons and dendrites. Cultured hippocampal neurons initially extend several short processes, any one of which can apparently become the axon (Dotti, C. G., and G. A. Banker. 1987. Nature [Lond.]. 330:477-479). A few days after the axon has begun its rapid growth, the remaining processes differentiate into dendrites (Dotti, C. G., C. A. Sullivan, and G. A. Banker. 1988. J. Neurosci. 8:1454-1468). The polarity orientation of the microtubules in all of the initial processes is uniform, with plus ends distal to the cell body, even through most of these processes will become dendrites. This uniform microtubule polarity orientation is maintained in the axon at all stages of its growth. The polarity orientation of the microtubules in the other processes remains uniform until they begin to grow and acquire the morphological characteristics of dendrites. It is during this period that microtubules with minus ends distal to the cell body first appear in these processes. The proportion of minus end-distal microtubules gradually increases until, by 7 d in culture, about equal numbers of dendritic microtubules are oriented in each direction. Thus, the establishment of regional differences in microtubule polarity orientation occurs after the initial polarization of the neuron and is temporally correlated with the differentiation of the dendrites.

Monoclonal antibody markers for amphibian oligodendrocytes and neurons

Few immunocytochemical probes have been developed for cold-blooded vertebrates, thus hampering analyses of cellular processes in these species. Those developed from mammalian and avian tissue often fail either to react or to show similar specificities in poikilotherms. Therefore, we have begun raising monoclonal antibodies (mabs) in mice against frog and tadpole brain tissue. The following analyses of two of these mabs suggest that these antibodies represent specific probes for frog axons and oligodendrocytes. Mab Olig recognizes all the myelinated axon tracts of the mature frog brain and spinal cord, as well as the tracts of the developing tadpole CNS once they have become myelinated. Axons cut in cross section show characteristic o-shaped staining around individual axons when processed with this antibody. Particularly easy to visualize in the tadpole are immunoreactive cell bodies and processes, seen in continuity with the myelin sheath. Occasionally, in this developing tissue, cells with highly branched processes characteristic of immature oligodendrocytes are observed. No other cells or processes within the brain or spinal cord react with this antibody. Mab Linc stains numerous filaments in all axonal projections. Occasionally, a thin rim of filamentous staining is observed in cell somata, but many regions rich in neuronal somata or dendrites are unreactive to this antibody. This in vivo staining pattern suggests that the Linc antigen is differentially distributed within neurons and exhibits a high concentration in axons. Linc immunoreactivity is robust in the processes of a subpopulation of dissociated tectal cells in culture. These Linc-positive cells are characterized as neurons on morphological criteria. Also, intense Linc immunoreactivity is present in the bundles of retinal axons that extend from retinal explants. Olig immunoreactivity, however, has not been detected in tectal cultures or retinal explants. Improved staining following Triton X-100 treatment of tissue sections suggests that neither of the mabs recognizes lipid antigens and that both are probably localized within the cell cytoplasm. Only the Linc mab reacts on Western blots of denatured brain protein. Linc consistently recognizes two Triton X-100-insoluble proteins with apparent molecular weights of 56 and 58 kD. The epitopes recognized by the Olig and Linc mabs have been surveyed in terms of their resistance to optic nerve crush and their consequent value in studies requiring such procedures. Possible homologies to known cell-type-specific molecules are discussed.

Microtubular reorganization and dendritic growth response in Alzheimer's disease

Cytoskeletal disruption is a key pathological feature of Alzheimer's disease (AD). We used refined immunocytochemical techniques to define the range of abnormalities affecting the microtubule system in AD hippocampus. Minimal tau and tubulin immunoreactivity was granular and accumulated in otherwise normal neuronal perikarya. As tau-reactive neurofibrillary tangles formed, granular tau and tubulin staining diminished, and ubiquitin reactivity developed. In regions of high neurofibrillary tangle density, microtubule-associated protein 2 (MAP2) histochemical features of remaining nontangled neurons included apical dendritic degeneration with proliferation of basal dendrites. In addition to perisomatic dendritic proliferation, there was massive sprouting of tau-immunoreactive distal dystrophic neurites. Sprouting proximal dendrites and dystrophic neurites often demonstrated growth-cone-like lamellipodia and filopodia. Degeneration of the perisomatic proliferating dendrites was characterized by the accumulation of fibrillar tau immunoreactivity. The colocalization of MAP2 and tau in growth structures recapitulated their codistribution in developing neurites. The data suggest that extensive plasticity and growth response occur in tandem with neuronal degeneration in AD, and that reorganization of the cytoskeletal microtubule system may underlie these proliferative changes.

The expression of acetylated microtubules during axonal and dendritic growth in cerebellar macroneurons which develop in vitro

The distribution of acetylated microtubules in cerebellar macroneurons which develop in culture was studied with 6-11B-1, a monoclonal antibody specific for acetylated alpha-tubulin. In these neurons 6-11B-1 helps to define a subset of stable, colchicine-resistant, microtubules that during early neuronal development are exclusively localized in the neuron's axon. On the other hand, in mature neurons acetylated microtubules display a widespread distribution being localized in the axon and thick dendritic trunks, a phenomenon correlated with an increase in the levels of acetylated alpha-tubulin and colchicine-resistant microtubules. Taken collectively, these observations suggest that the acetylation of alpha-tubulin is important for differentiating microtubules during neurite growth: in young neurons this post-translational modification may contribute to determine a selective stabilization of microtubules accompanying axonal differentiation.

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

The relationship between the expression of microtubule-associated proteins (MAPs) and microtubule formation was studied in embryonic cerebellar macroneurons maintained in culture. The results obtained suggest that in these neurons high molecular weight-MAP2 (HMW-MAP2) acts as a promoter of tubulin assembly since its induction and pattern of distribution are highly correlated with the increase in microtubule mass which parallels axonal and dendritic growth; MAP-1a may have a similar role but restricted to the assembly of dendritic microtubules. On the other hand, Tau expression and accumulation follows a time course identical to that of the induction of stable microtubules; besides, at all stages of neurite differentiation and growth this protein seems to be preferentially associated with this subset of microtubules as opposed to the other MAPs, observations which suggest an important role for this protein in determining microtubule stability during axonal and dendritic elongation. Finally, the present results show that environmental stimuli are capable of regulating the expression of these MAPs; the induction of each of them varies as a function of the type of signal. Thus, while diffusable substances are able to dramatically induce HMW-MAP2, MAP-1a and Tau inductions depend on cell substrate attachment and/or cell-cell interactions.

An immunohistochemical characterization of the primitive and maturing neuroepithelial components in the OTT-6050 transplantable mouse teratoma

The neuroepithelial component of the OTT-6050 mouse teratoma has previously been characterized as an experimental system for the study of differentiation and cytologic maturation in embryonal tumours of the human central nervous system. A number of transplantable tumours composed of primitive stem cells and of a neuroepithelial component displaying a spectrum of differentiation were previously produced by centrifugal elutriation of the dissociated OTT-6050 teratoma. These tumours have provided a reproducible cell population that has permitted the study of both the early and later stages of neoplastic neurocytogenesis. The purpose of the present study was to detect, by immunohistochemistry, the earliest stages of neurocytogenesis in these tumours as shown by the expression of neuron-associated microtubule proteins. This was correlated to the appearance and localization of other markers associated with neuronal and glial differentiation. The primitive neuroepithelial structures resembling neural tubes (medulloepithelial rosettes) contained single or small groups of cells which reacted with the monoclonal antibody TUJ1, specific for the neuron-associated class III beta-tubulin isotype. Immature neuroblasts and maturing polar neurons also showed immunoreactivity with TUJ1, whereas reactivity for microtubule-associated protein 2 (MAP2), tau, the 200 kilodalton isoform of neurofilament protein, neuron-specific enolase and synaptophysin was primarily seen in maturing neurons. By comparison, both medulloepithelial and ependymoblastic rosettes, neuroblasts and glial cells were immunopositive with monoclonal antibody TU27, which defines an antigenic site shared by most mammalian beta-tubulin isotypes. Astroglia were reactive with antisera to glial fibrillary acidic and S-100 proteins, but not with monoclonal antibody (MAb) TUJ1, or with MAbs to the other neuron-associated cytoskeletal proteins, MAP2, tau and the 200 kilodalton subunit of neurofilament protein. Our findings suggest that (1) expression of the class III beta-tubulin isotype is an early event during neoplastic neurocytogenesis, (2) this isotype is subsequently preserved in maturing neuronal populations, and (3) it is not present at detectable levels in stem cells or glial cells. The observation that morphologically undifferentiated neuroepithelial cells express a neuron-associated beta-tubulin isotype signifies the value of examining tubulin isotype expression in the characterization of normal and neoplastic neuroepithelial differentiation.

Tau in situ hybridization in normal and Alzheimer brain: localization in the somatodendritic compartment

Tau messenger RNA in situ hybridization in human postmortem brain revealed that neurons are the predominant cell type labeled. The probe used includes the nucleotide sequence coding for the amino acids recognized by the well-characterized tau monoclonal antibodies 5E2 and tau 1. The distribution of the tau RNA is abundant throughout the neuronal somata and into the proximal parts of dendrites of pyramidal cells in the cerebral cortex and the hippocampus. The distal extent to which silver grains could be visualized in the pyramidal cell dendrite was comparable to that seen with a probe to ribosomal RNA. In contrast to the tau probe the ribosomal probe also labeled glial cells. Sections hybridized with the tau probe and then double-labeled with thioflavine S revealed that neurons containing neurofibrillary tangles continue to synthesize tau protein.

Distribution of phosphate-independent MAP2 epitopes revealed with monoclonal antibodies in microwave-denatured human nervous system tissues

In contrast with results obtained in experimental animals, antibodies to microtubule associated protein-2 (MAP2) preferentially label abnormal structures in human nervous system tissue samples, but the normal sites at which MAP2 is expressed are not well-defined. To determine the distribution of MAP2 in the human central (CNS) and peripheral (PNS) nervous systems, we prepared monoclonal antibodies (MAbs) specific to MAP2, and compared the localization of this MAP in postmortem bovine and human tissues as well as in several human neural cell lines that express either neurofilament (NF) or glial filament (GF) proteins. Eight MAbs specific for phosphate-independent epitopes in bovine and human MAP2 were obtained, and those that performed well in tissues produced immunoreactivity confined to the somatodendritic domain of neurons in bovine and human CNS and PNS tissues. Other neural cells (e.g. astrocytes) did not express MAP2 immunoreactivity using these MAbs. Postmortem delays of less than 24 h prior to tissue denaturation did not affect the distribution of MAP2 immunoreactivity. However, microwave denaturation of these tissues preserved MAP2 immunoreactivity better than fixation with Bouin's solution or formalin. Microwave treatment also improved the immunoreactivity of several MAbs for NF and GF proteins. Finally, MAP2 was not detected in human neural cell lines that express NF (2) or GF (1) proteins. We conclude that microwave denaturation provides an effective means to preserve the immunoreactivity of normal human neuronal cytoskeletal proteins, and that this method of tissue denaturation allows the normal distribution of MAP2 to be defined in postmortem samples of human CNS and PNS tissues.

Localization and mobility of omega-conotoxin-sensitive Ca2+ channels in hippocampal CA1 neurons

Voltage-dependent Ca2+ channels (VDCCs) are modulators of synaptic plasticity, oscillatory behavior, and rhythmic firing in brain regions such as the hippocampus. The distribution and lateral mobility of VDCCs on CA1 hippocampal neurons have been determined with biologically active fluorescent and biotinylated derivatives of the selective probe omega-conotoxin in conjunction with circular dityndallism, digital fluorescence imaging, and photobleach recovery microscopy. On noninnervated cell bodies, VDCCs were found to be organized in multiple clusters, whereas after innervation the VDCCs were concentrated and immobilized at synaptic contact sites. On dendrites, VDCC distribution was punctate and was interrupted by extensive bare regions or abruptly terminated. More than 85% of the dendritic VDCCs were found to be immobile by fluorescence photobleach recovery. Thus, before synaptic contact, specific mechanisms target, segregate, and immobilize VDCCs to neuronal cell bodies and to specialized dendritic sites. Regulation of this distribution may be critical in determining the firing activity and integrative properties of hippocampal CA1 neurons.

Experimental observations on the development of polarity by hippocampal neurons in culture

In culture, hippocampal neurons develop a polarized form, with a single axon and several dendrites. Transecting the axons of hippocampal neurons early in development can cause an alteration of polarity; a process that would have become a dendrite instead becomes the axon (Dotti, C. G., and G. A. Banker. 1987. Nature (Lond.). 330:254-256). To investigate this phenomenon more systematically, we transected axons at varying lengths. The greater the distance of the transection from the soma, the greater the probability for regrowth of the original axon. However, it was not the absolute length of the axonal stump that determined the response to transection, but rather its length relative to the lengths of the cell's other processes. If one process was greater than 10 microns longer than the others, it invariably became the axon regardless of its identity before transection. Conversely, when a cell's processes were nearly equal in length, it was impossible to predict which would become the axon. In these cases, axonal outgrowth began only after a long latency. During this interval, the processes appeared to be in dynamic equilibrium, some growing for short distances while others retracted. When one process exceeded the others by a critical length, it rapidly elongated to become the axon. The establishment of neuronal polarity during normal development may similarly involve an interaction among processes whose identities have not yet been determined. When, by chance, one exceeds the others by a critical length, it becomes specified as the axon.