MAP2 is localized to the dendrites of hippocampal neurons which develop in culture

Immunohistochemical demonstration of neuron-specific enolase and microtubule-associated protein 2 in reactive astrocytes after injury in the adult forebrain

Transformation of normal resting astrocytes to reactive astrocytes in the adult brain after injury has been well documented. Using double immunofluorescent labeling methods, we report that astrocytes in both the ischemically damaged and the retrogradely/anterogradely degenerating forebrain nuclei express not only the glial cell markers glial fibrillary acidic protein and vimentin, but also the neuronal markers neuron-specific enolase and microtubule-associated protein 2. Since these neuronal markers are expressed in glial precursor cells, these results suggest that one of the characteristic responses of astrocytes in the adult brain after injury may be re-expression of fetal trait(s) of early differentiating glial cells/neurons.

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.

Association of poly(A) mRNA with microtubules in cultured neurons

The structural basis for the synthesis of specific proteins within distinct intraneuronal compartments is unknown. We studied the distribution of poly(A) mRNA within cultured cerebrocortical neurons using high resolution in situ hybridization to identify cytoskeletal components that may anchor mRNA. After 1 day in culture, poly(A) mRNA was distributed throughout all of the initial neurites, including the axon-like process. At 4 days in culture, poly(A) mRNA was distributed throughout the cell body and dendritic processes, but confined to the proximal segment of the axon. Poly(A) mRNA was bound to the cytoskeleton as demonstrated by resistance to detergent extraction. Perturbation of microtubules with colchicine resulted in a major reduction of dendritic poly(A) mRNA; however, this distribution was unaffected by cytochalasin. Ultrastructural in situ hybridization revealed that poly(A) mRNA and associated ribosomes were excluded from tightly bundled microtubules.

Peripherin expression in hippocampal neurons induced by muscle soluble factor(s)

Previous studies have shown that neuronal cells in culture can switch neurotransmitters when grown in the presence of different target cells. To examine whether this plasticity extends to structural proteins, we cocultured hippocampal neurons and pituitary-derived neuroendocrine (AtT20) cells with astrocytes, kidney epithelial cells, or skeletal muscle cells. As a marker of phenotypic change we used the cytoskeletal protein peripherin, a type III intermediate filament (IF) subunit which is not expressed in hippocampal neurons and AtT20 cells. We show here that soluble factor(s) secreted specifically from skeletal muscle cells can induce the expression and de novo assembly of peripherin in a subset of post-mitotic neurons. We further demonstrate that one of these factors is the Leukemia Inhibitory Factor/Cholinergic Neuronal Differentiation Factor. The environmentally regulated expression of peripherin implies a remarkable degree of plasticity in the cytoskeletal organization of postmitotic CNS cells and provides a noninvasive model system to examine the de novo assembly of IF proteins under in vivo conditions.

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)

Protein transport to the dendritic plasma membrane of cultured neurons is regulated by rab8p

In the companion paper (Huber, L. A., S. W. Pimplikar, R. G. Parton, H. Virta, M. Zerial, and K. Simons. J. Cell Biol. 123:35-45) we reported that the small GTPase rab8p is involved in transport from the TGN to the basolateral plasma membrane in epithelia. In the present work we investigated the localization and function of rab8p in polarized hippocampal neurons. By immunofluorescence microscopy we found that rab8p localized preferentially in the somatodendritic domain, and was excluded from the axon. Double-labeling immunofluorescence showed that some of the rab8p co-localized in the dendrites with the Semliki Forest Virus glycoprotein E2 (SFV-E2). An antisense oligonucleotide approach was used to investigate the role of rab8p in dendritic transport of newly synthesized viral glycoproteins. Antisense oligonucleotides corresponding to the initiation region of the rab8 coding sequence were added to the cultured neurons for four days. This treatment resulted in a significant decrease in cellular levels of rab8p and transport of SFV-E2 from the cell body to the dendrites was significantly reduced. However, no effect was observed on axonal transport of influenza HA. From these results we conclude that rab8p is involved in transport of proteins to the dendritic surface in neurons.

Transcytosis of the polymeric immunoglobulin receptor in cultured hippocampal neurons

BACKGROUND

A wide variety of proteins are transported across epithelial cells by vesicular carriers. This process, transcytosis, is used to generate cell surface polarity and to transport macromolecules between the luminal and serosal sides of the epithelial layer. The polymeric immunoglobulin receptor is a well-characterized transcytotic molecule in epithelia. It binds to its ligand, polymeric immunoglobulin, at the basolateral surface, and the receptor-ligand complex is transcytosed to the apical surface, where the ligand is released. Our previous studies have shown that hippocampal neurons may employ mechanisms similar to those of epithelial cells to sort proteins to two plasma membrane domains. The machinery used for axonal delivery recognizes proteins that are targeted apically in epithelia, whereas basolaterally destined proteins are delivered to the dendrites. It has not been clear, however, whether transcytosis occurs in neurons.

RESULTS

We report expression of the polymeric immunoglobulin receptor in cultured hippocampal neurons, using a Semliki Forest Virus expression system, and show by immunofluorescence microscopy that the newly synthesized receptor is targeted from the Golgi complex predominantly to the dendrites - only about 20% of the infected neurons display axonal immunofluorescence. Addition of ligand leads to significant redistribution of the receptor to the axons, shown by an approximately three-fold increase in axonal immunoreactivity with the anti-receptor antibodies.

CONCLUSIONS

Our results suggest that a transcytotic route, analogous to that in epithelia, exists in neurons, where it transports proteins from the somatodendritic to the axonal domain. Cultured neurons expressing the polymeric immunoglobulin receptor offer an experimental system that should be useful for further characterization of this novel neuronal pathway at the molecular and functional level.

Localization of the multiple calmodulin messenger RNAs in differentiated PC12 cells

Calmodulin, a ubiquitous calcium-binding protein which is involved in many biological processes, including cell proliferation and differentiation, has been shown to be encoded by three genes from which five calmodulin messenger RNAs are transcribed. In our previous studies, using the PC12 pheochromocytoma cell line as a model system for neuronal differentiation, all five calmodulin messenger RNAs were found to be present, and treatment with both nerve growth factor and dibutyryl cyclic AMP, which induce neurite outgrowth in these cells, increased the level of calmodulin and differentially increased the levels of the various calmodulin messenger RNAs. In an attempt to uncover the nature of the differential increase in the calmodulin messenger RNAs during neuronal differentiation, we examined here the subcellular distribution of the individual calmodulin messenger RNAs in PC12 cells treated with nerve growth factor and dibutyryl cyclic AMP by in situ hybridization cytochemistry, using radiolabeled oligodeoxynucleotide probes. Using an oligodeoxynucleotide probe which detects all of the calmodulin transcripts, the calmodulin messenger RNAs were found to be distributed throughout the cell bodies of differentiated PC12 cells; significant amounts of calmodulin messenger RNAs were also found in most neurites (approximately 70% of the total number). Using specific probes for the calmodulin messenger RNAs derived from each calmodulin gene, distinct patterns of localization of the different calmodulin messenger RNAs were revealed. The messenger RNAs from calmodulin genes I and II were readily detected in all cell bodies and in about one-half of the neurites. In contrast, a weak signal for the messenger RNAs from calmodulin gene III was associated with cell bodies, while no significant signal was found in neurites. A population distribution analysis of the labeling of individual PC12 cell bodies, as determined by counting autoradiographic grains, revealed differences in the relative abundance of each group of messenger RNAs derived from each of the three calmodulin genes. The order of relative abundance of the messenger RNAs in cell bodies was found to be: calmodulin gene II messenger RNA > calmodulin gene I messenger RNAs >> calmodulin gene III messenger RNAs. An analysis of the labeling density along neurites indicated a similar density of neuritic messenger RNAs from calmodulin gene I and calmodulin gene II, whereas there was no significant signal for the messenger RNAs from calmodulin gene III.(ABSTRACT TRUNCATED AT 400 WORDS)

High external potassium induces an increase in the phosphorylation of the cytoskeletal protein MAP2 in rat hippocampal slices

Depolarization induced in rat hippocampal slices by a high concentration of extracellular K+ leads to an increase in the phosphorylation of microtubule-associated protein MAP2. The comparison of the major phosphopeptides derived from in situ and in vitro phosphorylated MAP2 suggests the implication of calcium-dependent protein kinases, including calcium/calmodulin-dependent protein kinase type II and protein kinase C, in the up-phosphorylation of MAP2. In particular, a peptide containing the tubulin-binding domain of the MAP2 molecule may be phosphorylated by protein kinase C. As the association of MAP2 with the cytoskeleton may be regulated by phosphorylation, we suggest that changes in the phosphorylation level of MAP2 might be involved in synaptic remodelling in hippocampal neurons.

Expression of heterologous proteins in cultured rat hippocampal neurons using the Semliki Forest virus vector

The Semliki Forest virus expression vector (Liljeström and Garoff: Bio/Technology 9:1356-1361, 1991) was tested in cultured rat hippocampal neurons using two Madin-Darby canine kidney (MDCK) cell membrane-associated proteins as reporters: rab8, a small GTPase involved in post-Golgi vesicle transport, and VIP21, an integral membrane protein of caveolae, trans-Golgi network, and post-Golgi vesicles. Expression of the c-myc epitope-tagged proteins was visualized by immunofluorescence microscopy. The proteins were first detected in neurons after 3-4 hr infection by the recombinant viruses. The infection efficiency on neurons was high: after 6 hr infection at a multiplicity of one, 50-60% of the cells expressed the reporter proteins. The neurons tolerated the infection well up to 8 hr. Their polarized organization was not disturbed, as judged from morphology and from distribution of the dendritic MAP2 and axonal synaptophysin marker proteins. The Semliki Forest virus vector thus seems suitable for short-term expression of proteins in cultured neurons.

NILE/L1 and NCAM-polysialic acid expression on growing axons of isolated neurons

The neuron adhesion molecules NILE/L1 and NCAM may be involved in axonal guidance and cell recognition. To investigate all exposed membrane domains of single neurons, something which has not previously been done for any adhesion molecule, we used digitally processed scanning electron microscopy with a high-energy backscatter electron detector. This allowed a quantitative analysis of immunogold staining densities on all surfaces of isolated rat hippocampal neurons in culture to study NILE/L1 and NCAM expression independent of potentially inductive innervation. During early stages of neuritic extension, all growth cones showed similar NILE/L1 expression, but as soon as a single process extended farther than the others (by 20 hours), this putative axon and its growth cone generally showed a stronger level of NILE/L1 immunogold labeling than the other neurites. This is the earliest evidence of plasma membrane differentiation between axons and dendrites. With further neuritic growth, the relative NILE/L1 expression on axons and their growth cones continued to increase. In contrast to some earlier reports, NILE/L1 was expressed on axonal growth cones growing on both polylysine-coated glass and astrocyte substrates. Strong immunostaining for NCAM-related polysialic acid (PSA) was found on axonal growth cones and filopodia, suggesting that the homophilic adhesive action of NCAM may be reduced during axonal growth. PSA showed greater labeling on distal axons than on other areas of the neuron, indicating a variable NCAM-mediated adhesion on different regions of the same cell. Neither NILE/L1, NCAM, nor PSA appeared to show regional differences in axons fasciculating or defasciculating on themselves. A strong intercellular heterogeneity of NILE/L1, NCAM, and PSA expression levels on neurons in the same culture dish was found, suggesting that subsets of cells from the hippocampus may express biologically relevant differences in adhesion molecules compared to neighboring neurons. In light of the growing body of evidence pointing to the multifaceted array of homophilic and heterophilic binding interactions that NILE/L1 and NCAM may exhibit, and the functional importance of molecular densities, the quantitative data here support the hypothesis that sufficient cellular and subcellular heterogeneity exists for these molecules to be involved in some aspects of axonal guidance.

N-methyl-D-aspartate stimulates the dephosphorylation of the microtubule-associated protein 2 and potentiates excitatory synaptic pathways in the rat hippocampus

We have studied the effect of brief (50-150 s) applications of N-methyl-D-aspartate (10-100 microM) on the phosphorylated state of the microtubule-associated protein 2 in slices of rat hippocampus. Following a similar experimental protocol we also studied the pattern of excitatory postsynaptic potentials intracellularly recorded in CA1 pyramidal cells elicited by stimulation of the Schaffer collateral-commissural pathway. N-Methyl-D-aspartate treatment produced a marked and specific dephosphorylation of the cytoskeletal microtubule-associated protein 2, which was not due to enhanced proteolytic activity. Dephosphorylation of the microtubule-associated protein 2 affects mainly the tubulin-binding domain of the molecule and seems to be a consequence of the activation of the Ca2+/calmodulin-dependent phosphatase calcineurin, as it is partially inhibited by calmidazolium but not by okadaic acid. A few minutes after N-methyl-D-aspartate treatment we observed a 23 +/- 17% increase in the amplitude of the monosynaptic excitatory postsynaptic potential recorded in the cells and the appearance of a large polysynaptic excitatory postsynaptic potential. Both effects lasted for several tens of minutes. The late polysynaptic potential was not observed when the CA3 and CA1 subfields were surgically separated. Our results indicate that the N-methyl-D-aspartate receptor activation leads to the dephosphorylation of the microtubule-associated protein 2 via a Ca2+/calmodulin phosphatase, probably calcineurine. This may, in turn, participate in the potentiation of synaptic efficacy.

The low molecular weight form of microtubule-associated protein 2 is transported into both axons and dendrites

In the developing brain microtubule-associated protein MAP2 occurs as both a high molecular weight form, MAP2b, which is present only in dendrites, and a low molecular weight form, MAP2c, which is also present in axons. Because the MAP2c amino acid sequence is entirely contained within that of MAP2b it is not possible to raise a MAP2c-specific antibody, so that it has been impossible to determine whether MAP2c is present in dendrites along with MAP2b. To answer this question we have generated a MAP2c cDNA clone tagged with a 10 amino acid epitope from human c-myc. This additional sequence does not alter either the binding of MAP2c to microtubules or its effects on microtubules in non-neuronal cells. When expressed in cultured primary neurons by transfection, the myc tag allowed the distribution of MAP2c to be determined independently of endogenous MAP2 protein by immunostaining with an anti-myc antibody. This showed that MAP2c is present in all processes, indicating that it can enter all kinds of processes and is stable in their cytoplasm. The results further suggest that the selective association of high molecular weight MAP2 with dendrites depends on a mechanism that prevents either its entrance or survival in the axonal compartment.

Acrylamide-induced depletion of microtubule-associated proteins (MAP1 and MAP2) in the rat extrapyramidal system

Acrylamide, an occupational neurotoxicant, reduced MAP1 and MAP2 distribution in different regions of rat brain. Different components of the extrapyramidal system (caudate-putamen, globus pallidus, substantia nigra and red nucleus) revealed differential distribution of MAP1 and MAP2 in acrylamide-treated animals. Rats were treated with acrylamide (estimated mean dose: 15 mg/kg/day) for 2 weeks and MAP1 and MAP2 were localized according to Sternberger's peroxidase-anti-peroxidase technique. MAP1 labelled neuronal perikarya and dendrites almost with a similar intensity, but MAP2 immunostaining was more intense in dendrites than neuronal perikarya. Acrylamide caused a near-total loss of MAP1 and MAP2 immunoreactivity in caudate-putamen. Other components of the extrapyramidal system were relatively less affected by acrylamide. These results indicate that caudate-putamen is more susceptible to the action of acrylamide than other components of the extrapyramidal system studied. The depletion of MAP1 and MAP2 immunoreactivity by acrylamide appears to be an early biochemical event preceding peripheral neuropathy. The loss of MAPs immunoreactivity occurs first in dendrites and proceeds toward the perikarya. This study indicates that acrylamide not only causes axonal damage but may also induce dendritic degeneration.

Steric inhibition of cytoplasmic dynein and kinesin motility by MAP2

Using several in vitro motility assays, we found that motility driven by the microtubule (MT) motors, kinesin and cytoplasmic dynein, could be inhibited by MAP2 but not by tau protein or the MT-binding proteolytic fragment of MAP2. In MT gliding assays, even the presence of one MAP2 molecule per sixty-nine tubulin dimers caused an inhibition of about 75% of MT motility at low concentrations of both motors. The percent inhibition of motility decreased with increasing concentration of either motor, suggesting that the inhibition was the result of competition for access to the MT surface. The decrease in the number of moving MTs with MAP2 was correlated with an increase in the frequency of release of moving MTs from the motor-coated glass. In assays of in vitro vesicular organelle motility and formation of ER networks, the presence of MAP2 inhibited small vesicle movements and to a lesser extent ER network formation. To determine if competition for specific sites on the MT or coating of the MT surface inhibited motility, we used tau protein and the chymotryptic MT-binding fragments of MAP2 to coat MTs. No inhibition was observed and there was even an increase in the number of attached and moving MTs in the gliding assay with tau-coated MTs. Because MAP2, tau and the chymotryptic MT-binding fragments of MAP2 bind to the same domain on tubulin, masking of the MT surface sites does not appear responsible for the inhibition of motility by MAP2. Rather, we suggest that the sidearm of MAP2 interfered with the interaction of motors with MTs and caused a dramatic increase in the rate of MT release. In vivo, MAP2 could play a major role in the generation of cellular polarity even at substoichiometric levels by inhibiting transport on microtubules in specific domains of the cytoplasm.

Induction of neuron-specific tropomyosin mRNAs by nerve growth factor is dependent on morphological differentiation

We have examined the expression of brain-specific tropomyosins during neuronal differentiation. Both TmBr-1 and TmBr-3 were shown to be neuron specific. TmBr-1 and TmBr-3 mRNA levels increased during the most active phase of neurite outgrowth in the developing rat cerebellum. In PC12 cells stimulated by nerve growth factor (NGF) to differentiate to the neuronal phenotype, TmBr-1 and TmBr-3 levels increased with an increasing degree of morphological differentiation. Induction of TmBr-1 and TmBr-3 expression only occurred under conditions where PC12 cells were permitted to extend neurites. NGF was unable to maintain levels of TmBr-1 and TmBr-3 with the loss of neuronal phenotype by resuspension of differentiated PC12 cells. The unique cellular expression and regulation in vivo and in vitro of TmBr-1 and TmBr-3 strongly suggests a critical role of these tropomyosins in neuronal microfilament function. The findings reveal that the induction and maintenance of the neuronal tropomyosins is dependent on morphological differentiation and the maintenance of the neuronal phenotype.

Hippocampal growth cone responses to focally applied electric fields

A wide variety of cell types respond to electric fields in culture. Despite evidence for electric fields existing in the mammalian embryo, there are few studies testing the effects electric fields exert on neurons from the mammalian central nervous system (CNS). The present study demonstrates orientation responses to focally applied electric fields of embryonic rat hippocampal neurons isolated in culture. The most striking result from this study is that different growth cones of the same neuron can show differential responsiveness to focally applied electric fields: growth cones on the short, straight processes that are destined to become dendrites, oriented toward the cathode, whereas growth cones on the longest process, the presumptive axon, did not orient. The present experiments bring a significant increase in resolution to the study of neuronal growth cone orientation by applied electric fields: a novel examination of the early events leading to orientation. Growth cones on dendrites displayed a spectrum of orientation responses: directed lamellipodial extension, directed filopodial extension and/or reorientation, cytoplasmic swelling of existing filopodia, consolidation of filopodia, and rapid elongation of the entire process. Individual growth cones displayed only one or two of these responses. Additionally, not all growth cones on these short processes sustained their initial orientation response: 35% adapted within 6 min.

Laminin selectively enhances axonal growth and accelerates the development of polarity by hippocampal neurons in culture

We have examined the effects of laminin on the morphological development of embryonic rat hippocampal neurons maintained in tissue culture. Forty-eight hours after plating, neurons grown on a polylysine-coated substrate had become polarized, typically having one long axon and 4 or 5 minor processes. Adsorption of laminin to the substrate did not cause changes in the number of axons extended by hippocampal neurons but did cause significant increases in the length of the axonal plexus and in axonal branching. In contrast to its effects on axons, laminin did not influence the number, length, or branching of the minor processes that eventually become dendrites or the morphology of definite dendrites as assessed after 7 days in culture. In addition to selectively enhancing axonal growth, laminin greatly increased the rate of polarization of hippocampal neurons such that most became polarized within 18 h. Analysis of the time course of laminin's effects revealed that the acceleration of polarization was not associated with a change in the time of initial process formation, but rather with a selective stimulation of the growth of the longest process at all times from the 12th through the 48th h in vitro. These data suggest that even though the basic shape of hippocampal neurons may be intrinsically programmed, critical aspects of their morphological development may be modulated by extracellular matrix molecules such as laminin.

Expression of inherent neuronal shape characteristics after transient sensitivity to epigenetic factors

We investigated effects of different substrates and culture media on the early morphological differentiation of rat neocortical neurons in culture. In particular, we examined the effects of homotypic astrocytes, the adhesive glycoprotein laminin and the polycationic substrate poly-L-lysine, as well as diffusible astrocyte-derived conditioned medium factors and serum on (1) soma area, (2) total neuron area and (3) primary neurite number. To assess variations in morphological reactions of neurons with a defined neurotransmitter phenotype, we analyzed the differentiation of GABAergic neurons. The morphology of young neocortical neurons was dramatically affected by both substrate and culture medium. Replacement of the astrocytic monolayer or the astrocyte-conditioned medium by other substrates or non-conditioned medium, respectively, was accompanied by (1) spreading and flattening of neuronal somata, (2) a marked decrease in total neuron area and (3) an increase in the number of primary neurites. The various morphological parameters studied exhibited different sensitivities to changes of these external factors. Moreover, the influences of epigenetic factors on the generation of primary neurites depended on the transmitter phenotype of the neuron. The induced morphological alterations were transient. At the end of the first week in culture, the surviving neurons underwent substantial remodeling of their morphology leading to an expression of in vivo shape characteristics. These observations suggest that despite an early, transient sensitivity to environmental influences, the neuronal differentiation with respect to the morphological parameters studied in culture is to a large degree determined by intrinsic factors.

Implication of brain cdc2 and MAP2 kinases in the phosphorylation of tau protein in Alzheimer's disease

Brain tau protein is phosphorylated in vitro by cdc2 and MAP2 kinases, obtained through immunoaffinity purification from rat brain extracts. The phosphorylation sites are located on the tau molecule both upstream and downstream of the tubulin-binding motifs. A synthetic peptide comprising residues 194-213 of the tau sequence, which contains the epitope recognized by the monoclonal antibody tau-1, is also efficiently phosphorylated in vitro by cdc2 and MAP2 kinases. Phosphorylation of this peptide markedly reduces its interaction with the antibody tau-1, as it has been described for tau protein in Alzheimer's disease. Both cdc2 and MAP2 kinases are present in brain extracts obtained from Alzheimer's disease patients. Interestingly, the level of cdc2 kinase may be increased in patient brains as compared with non-demented controls. These results suggest a role for cdc2 and MAP2 kinases in phosphorylating tau protein at the tau-1 epitope in Alzheimer's disease.

Changes of chemoarchitectural organization of the rat spinal cord following ventral and dorsal root transection

Time-related changes in the distribution of chemical messengers in the rat spinal cord following the transection of dorsal and ventral roots were observed by using immunohistochemistry for the following antigens: microtubule-associated protein 2 (MAP2), calcitonin gene-related peptide (CGRP), substance P (SP), galanin (Gal), Met-enkephalin (Enk), neuropeptide Y (NPY), and serotonin (5-HT). To investigate dendrocytoarchitectonic organizational changes, morphometric analyses were performed on both the gray and the white matter of tissue samples stained with MAP2 antiserum. A significant reduction in the area of gray matter on the lesioned side was seen from 1 to 24 weeks postoperation, and progressive changes in the shape of the gray matter were also observed. CGRP-immunoreactive fibers were reduced in number in the posterior horn after root transection, except in the lateral part of lamina I. In contrast, CGRP immunoreactivity in the anterior horn cells of the ipsilateral side was increased early after transection, but later it progressively decreased. Root transection also caused significant reduction in the number of SP-immunoreactive fibers in the posterior horn, but no changes were seen in the anterior horn. Gal immunoreactivity was also affected by root transection, and it changed in a similar way to CGRP immunoreactivity. 5-HT-immunoreactive fibers were increased in the posterior horn after transection, and later decreased. In the anterior horn, there were no changes in the intensity or distribution pattern of 5-HT-immunoreactive nerve fibers following root transection. Enk and NPY immunoreactivity in the anterior and posterior horns was not affected by root transection up to 24 weeks postoperative. These results show that spinal root transection caused significant changes in the chemoarchitectural organization of nerve fibers containing certain types of chemical messengers, such as CGRP, SP, Gal, and 5-HT, in addition to altering dendritic geometry in the spinal cord.

Immortal rat hippocampal cell lines exhibit neuronal and glial lineages and neurotrophin gene expression

Clonal cell lines of rat embryonic hippocampal origin have been developed by using retroviral transduction of temperature-sensitive simian virus 40 large tumor antigens. The cell lines undergo morphological differentiation at the nonpermissive temperature and in response to differentiating agents. Immunocytochemical analysis indicates that various lines are derived from progenitors of neuronal, glial, and bipotential lineages. Selected neuronal lines differentiate in response to diffusible factors released by primary glia, and one line of glial lineage supports the maturation of primary neurons in culture. Selected cell lines exhibit different patterns of neurotrophin gene expression that change after differentiation. In some lines, the relative levels of neurotrophin 3 and brain-derived neurotrophic factor message expression may reflect the developmental or regional differential expression seen for these genes in the hippocampus in situ. These hippocampal cell lines, which express markers indicative of commitment to neuronal or glial lineages, are valuable for studies of development and plasticity in these lineages, as well as for studies of the regulation of neural trophic interactions.

Suppression of kinesin expression in cultured hippocampal neurons using antisense oligonucleotides

Kinesin, a microtubule-based force-generating molecule, is thought to translocate organelles along microtubules. To examine the function of kinesin in neurons, we sought to suppress kinesin heavy chain (KHC) expression in cultured hippocampal neurons using antisense oligonucleotides and study the phenotype of these KHC "null" cells. Two different antisense oligonucleotides complementary to the KHC sequence reduced the protein levels of the heavy chain by greater than 95% within 24 h after application and produced identical phenotypes. After inhibition of KHC expression for 24 or 48 h, neurons extended an array of neurites often with one neurite longer than the others; however, the length of all these neurites was significantly reduced. Inhibition of KHC expression also altered the distribution of GAP-43 and synapsin I, two proteins thought to be transported in association with membranous organelles. These proteins, which are normally localized at the tips of growing neurites, were confined to the cell body in antisense-treated cells. Treatment of the cells with the corresponding sense oligonucleotides affected neither the distribution of GAP-43 and synapsin I, nor the length of neurites. A full recovery of neurite length occurred after removal of the antisense oligonucleotides from the medium. These data indicate that KHC plays a role in the anterograde translocation of vesicles containing GAP-43 and synapsin I. A deficiency in vesicle delivery may also explain the inhibition of neurite outgrowth. Despite the inhibition of KHC and the failure of GAP-43 and synapsin I to move out of the cell body, hippocampal neurons can extend processes and acquire as asymmetric morphology.

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.

Calpain-mediated proteolysis of microtubule associated proteins MAP1B and MAP2 in developing brain

Microtubule associated proteins MAP1B and MAP2 are important components of the neuronal cytoskeleton. During early development of the brain, MAP1B (340 kDa) is present as two isoforms that differ in their level of phosphorylation, while MAP2 is expressed as a single high molecular weight isoform (MAP2B, 280 kDa) and a low molecular weight form (MAP2C, 70 kDa). In this study we examined and compared the sensitivities of MAP1B and MAP2, obtained from MT preparations and brain homogenates of young rats, to degradation by calcium-activated neutral protease, calpain II. We found that in MAPs prepared from microtubules the two isoforms of MAP1B had comparable sensitivity to calpain-mediated proteolysis. Similarly, the high and low molecular weight forms of MAP2 were equally sensitive to digestion by calpain. However, although both MAPs were very susceptible to calpain-mediated proteolysis, MAP1B was more resistant to degradation by calpain than MAP2. Furthermore, the endogenous degradation of MAPs in neonate brain homogenates was calcium-dependent and inhibited by leupeptin, and the pattern of degradation products for MAP1B and MAP2 was similar to that of calpain-mediated proteolysis. These data suggest that calpain can play a role in the regulation of MAPs levels during brain development, in relation to normal neuronal differentiation and disorders associated with neurodegeneration.

Neuronal polarity: targeting of microtubule components into axons and dendrites

The functional polarity of nerve cells depends on the outgrowth of both axons and dendrites. These processes, which were distinguished by morphological and physiological criteria, have been shown in recent years to differ in molecular composition, including their cytoskeleton. The asymmetric distribution of cytoskeletal elements and, particularly, the segregation of microtubule-associated proteins by their differential transport, may play an important role in the assembly of distinct microtubules in the two neuronal domains. An additional mechanism to achieve this subcellular localization is the transport of specific mRNAs to allow the local synthesis of specific proteins close to their functional site. This may endow the cell with a rapid mechanism for the regulation of synthesis under special conditions, which may be important during neuronal development and plasticity.

Polarized sorting of glypiated proteins in hippocampal neurons

Our recent studies suggested that neurons and epithelial cells sort viral glycoproteins in a similar manner. The apical influenza virus haemagglutinin was preferentially delivered to the axon of hippocampal neurons in culture, whereas the basolateral vesicular stomatitis virus glycoprotein was sorted to the dendrites. To investigate whether other membrane proteins showed similar sorting in neurons and epithelial cells, we have analysed the localization of a glypiated (glycosylphosphatidylinositol anchored) protein, Thy-1, in hippocampal neurons in culture. In MDCK and other epithelial cells, endogenous glycosylphosphatidylinositol (GPI)-anchored proteins, as well as mutated exogenous proteins containing the GPI-attachment signal, undergo preferential delivery to the apical surface. This polarized sorting of GPI-anchored proteins has been proposed to occur by the same mechanisms as the sorting of glycolipids to the apical surface. We report here that the neuronal GPI-protein Thy-1 is present in hippocampal neurons in culture and is exclusively located on the axonal surface. This finding further strengthens our hypothesis that the mechanisms of sorting of surface components may be similar in neurons and epithelial cells.

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.

Activation of NMDA receptors induces rapid dephosphorylation of the cytoskeletal protein MAP2

Hippocampal slices were preincubated with 32P-orthophosphate and used to study the effect of glutamate analogs on protein phosphorylation. NMDA induced a rapid, 70% decrease in the phosphorylation of the microtubule-associated protein MAP2, with no change in the total amount of MAP2. Both competitive and noncompetitive NMDA antagonists blocked the effect of NMDA, but a glutamate antagonist acting at non-NMDA receptors did not. Kainate and quisqualate were less potent than NMDA in stimulating dephosphorylation of MAP2. Other forebrain regions (necortex, striatum, and olfactory bulb) also showed dephosphorylation of MAP2 in response to NMDA. These and other results suggest that NMDA receptor activation induces the dephosphorylation of MAP2 by stimulating a protein phosphatase, possibly the calcium/calmodulin-dependent protein phosphatase calcineurin. Moreover, they indicate that alteration in the properties of a microtubule-associated protein may account for some of the effects of glutamate on postsynaptic neurons.

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.

Synthesis, axonal transport, and turnover of the high molecular weight microtubule-associated protein MAP 1A in mouse retinal ganglion cells: tubulin and MAP 1A display distinct transport kinetics

Microtubule-associated proteins (MAPs) in neurons establish functional associations with microtubules, sometimes at considerable distances from their site of synthesis. In this study we identified MAP 1A in mouse retinal ganglion cells and characterized for the first time its in vivo dynamics in relation to axonally transported tubulin. A soluble 340-kD polypeptide was strongly radiolabeled in ganglion cells after intravitreal injection of [35S]methionine or [3H]proline. This polypeptide was identified as MAP 1A on the basis of its co-migration on SDS gels with MAP 1A from brain microtubules; its co-assembly with microtubules in the presence of taxol or during cycles of assembly-disassembly; and its cross-reaction with well-characterized antibodies against MAP 1A in immunoblotting and immunoprecipitation assays. Glial cells of the optic nerve synthesized considerably less MAP 1A than neurons. The axoplasmic transport of MAP 1A differed from that of tubulin. Using two separate methods, we observed that MAP 1A advanced along optic axons at a rate of 1.0-1.2 mm/d, a rate typical of the Group IV (SCb) phase of transport, while tubulin moved 0.1-0.2 mm/d, a group V (SCa) transport rate. At least 13% of the newly synthesized MAP 1A entering optic axons was incorporated uniformly along axons into stationary axonal structures. The half-residence time of stationary MAP 1A in axons (55-60 d) was 4.6 times longer than that of MAP 1A moving in Group IV, indicating that at least 44% of the total MAP 1A in axons is stationary. These results demonstrate that cytoskeletal proteins that become functionally associated with each other in axons may be delivered to these sites at different transport rates. Stable associations between axonal constituents moving at different velocities could develop when these elements leave the transport vector and incorporate into the stationary cytoskeleton.

An immunohistochemical study of the primitive and maturing elements of human cerebral medulloepitheliomas

Four examples of human cerebral medulloepithelioma were studied immunohistochemically with a panel of antibodies and antisera to neuronal and glial proteins. The tumors, in addition to primitive medullary epithelium, contained areas of neuroblastic, ganglionic, astrocytic, ependymoblastic and ependymal differentiation, and in one tumor, areas resembling polar spongioblastoma. Tumor cells throughout the primitive medullary epithelium displayed focal immunoreactivity for vimentin, glial fibrillary acidic (GFA) protein and for the neuron-associated class III beta-tubulin isotype. Neuroblasts showed immunoreactivity for the class III beta-tubulin isotype, microtubule-associated protein 2 and neuron-specific enolase. Immunoreactivity for neurofilament epitopes and synaptophysin was detected in areas of ganglionic differentiation and coincided with the demonstration of neurofibrils in Bielschowsky's silver impregnations. Vimentin was the only marker detected in ependymoblastic and ependymal rosettes or in areas of polar spongioblastoma, as well as in mesenchymal cells. The results indicate that, even in very primitive neoplastic neuroepithelium, immunocytochemical evidence of early commitment of some of the cells to a neuronal or glial lineage can be demonstrated. The neuron-associated class III beta-tubulin isotype appears to be one of the earliest markers indicative of neuronal differentiation in normal and neoplastic primitive neuroepithelium.

A novel heat-stable 205 kDa microtubule-associated protein is involved in the neural development of the rat brain

A novel microtubule associated protein (MAP) was identified with a monoclonal antibody (mAb) among rat brain microtubule (MT) proteins. This MAP, which is found in a heat-stable MAP fraction, is co-polymerized with tubulin from rat brain homogenates both in cycled and in taxol-driven assembly purifications. When the heat-stable fraction of MT proteins and purified tubulin are assembled in vitro without taxol, 205 kDa MAP is identified in the MT pellet. Immunofluorescent studies using the mAb against this MAP on sections of a rat spinal cord, hippocampus, retina and cerebellar cortex, showed that 205 kDa MAP was localized in not only neural cell bodies, dendrites, and axons, but also in glial cells. This 205 kDa MAP is clearly distinct from previously reported MAPs as to molecular mass, heat stability, immunoreactivity, distribution among tissues or within a neuron, and the developmental aspect of the postnatal cerebellar cortex. As for development, the 205 kDa MAP has already been seen to appear at postnatal day 3. Throughout the postnatal development of the cerebellar cortex, 205 kDa MAP is expressed in developing Purkinje cells, with the staining reaction being particularly intense in the more superficially positioned parallel fiber profiles. This points out the possible important functions of 205 kDa MAP in the morphogenesis of parallel fiber axons.

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.

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.

Regulation of microtubule-associated protein 2 (MAP2) mRNA expression during rat brain development

The expression of MAP2 during rat brain development was studied by using specific antibodies and cDNA probes. MAP2 cDNAs were isolated from a rat brain lambda gt11 library, and their identity was confirmed by the reactivity of their fusion proteins with several independent monoclonal antibodies that recognize MAP2. Northern blot analyses of the RNA prepared from whole brains, cerebral cortex, hypothalamus, brain stem, olfactory bulbs, and cerebellum showed that the levels of MAP2 mRNA increase during the initial phase of development, reach a maximum between postnatal weeks 2 and 3, and then decrease in the adult. The time course and the kinetics of this change varied between different brain regions and appeared to reflect the pattern of morphological changes in these regions. RNA blots were also analyzed with beta-tubulin and beta-actin cDNA probes to ensure the quality and the quantity of the RNA. The levels of MAP2 mRNA and protein showed similar changes during the initial part of brain development and suggested a transcriptional control. However, while MAP2 protein levels remained high throughout development, MAP2 mRNA levels decreased in adulthood. We suggest that the increased stability of the MAP2 molecule may be a contributing factor in the developmental regulation of steady-state levels of MAP2.

Microtubule-associated protein 1B: molecular structure, localization, and phosphorylation-dependent expression in developing neurons

Two monoclonal antibodies, 5E6 and 1B6, were raised against microtubule-associated protein 1B (MAP1B), a major component of the neuronal cytoskeleton. 5E6 recognized the entire MAP1B population, while 1B6 detected only phosphorylated forms. Affinity-purified MAP1B appeared as a long, filamentous molecule (186 +/- 38 nm) with a small spherical portion at one end, forming long cross-bridges between microtubules in vitro. These results, together with in vivo data from immunogold methods, demonstrate that MAP1B is a component of cross-bridges between microtubules in neurons. By immunohistochemical analysis, phosphorylated forms were shown to exist mainly in axons, whereas unphosphorylated forms were limited to cell bodies and dendrites. Phosphorylated MAP1B was quite abundant in developing axons, suggesting its essential role in axonal elongation.

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.

Rapid turnover of microtubule-associated protein MAP2 in the axon revealed by microinjection of biotinylated MAP2 into cultured neurons

We studied the mechanism of compartmentation of microtubule-associated protein 2 (MAP2) in the dendrites and cell bodies by using microinjection of biotin-labeled MAP2 into mature spinal cord neurons in culture. MAP2 molecules microinjected into the nerve cell body were distributed not only throughout the cytoplasm of the cell body and dendrites, but also in the axon as far as a few millimeters from the cell body within 24 hr after injection. However, when injected cells were incubated for more than 3 days, the amount of biotin-labeled MAP2 in the axon decreased remarkably compared with that in the dendrites. This indicates that there is no sorting mechanism in the cell body for the transport of MAP2 selectively into the dendrites but that the turnover rate of MAP2 in the axons differs from that in the dendrites. To further characterize the mechanism of MAP2 compartmentation, we performed immunoelectron microscopy of injected cells and detergent extraction of microinjected cells prior to immunocytochemistry with anti-biotin. The results strongly suggest that a large part of axonal MAP2 is not associated with cytoskeleton and that this weak association of MAP2 favors selective loss of MAP2 from the axon.

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.

Excitatory and inhibitory neurotransmitters in the generation and degeneration of hippocampal neuroarchitecture

The possibility that excitatory and inhibitory inputs to neurons can affect the generation and degeneration of neuroarchitecture was examined in hippocampal pyramidal neurons in isolated cell culture. Dendritic outgrowth and cell survival were directly monitored in neurons exposed to: the excitatory neurotransmitter glutamate, the inhibitory transmitter GABA, anticonvulsants or combinations of these agents. Glutamate caused a graded series of changes in pyramidal neuron cytoarchitecture: a selective inhibition in dendritic outgrowth and dendritic pruning was observed with subtoxic levels of glutamate while cell death was induced by higher levels. Low levels of GABA alone or in combination with diazepam, carbamazepine, phenobarbital or phenytoin were without effect on dendrite outgrowth while higher levels caused moderate reductions in outgrowth. Neither GABA nor the anticonvulsants affected cell survival. GABA plus diazepam, phenobarbital, carbamazepine and phenytoin each significantly reduced the dendritic regression and cell death normally caused by glutamate. Elevation of extracellular K+ to 50 mM caused dendritic regression and 100 mM K+ caused cell death; these effects were greatly reduced by GABA and anticonvulsants. The calcium channel blocker Co2+ prevented the dendritic regression and cell death caused by both glutamate and K+ indicating that calcium influx was required for the neuroarchitectural responses. Taken together, these results demonstrate that neurotransmitters and neuromodulatory drugs can have direct and interactive effects on both neurite outgrowth and cell survival. Such neurotransmitter actions may play roles in both the formation and degeneration of the neuronal circuits in which they participate in information coding.

Characteristics of microtubules at the different stages of neuronal differentiation and maturation

The developing nervous system has proved to be a very powerful tool to analyze how MT are involved in basic biological processes such as cell proliferation, cell migration, cell shaping, and transport. A better knowledge of the basic events occurring during neurogenesis also affords us the possibility of establishing the basis of experiments and trying to solve unanswered and important questions. Despite the considerable value of cell culture, we need to use more discrete regions of the developing brain in situ in order to analyze the MT and their modifications into cells developing their "natural" environment. One major problem remains the question of the mode of assembly and disassembly, that is, the behavior of MT in living cells. Dynamic instability and/or treadmilling are accurate interpretations of the dynamics of MT at least in vitro or in cell culture, but we do need more information on what happens in situ and in vitro. One of the main tasks of cell biologists is to devise satisfactory tests to approach this fundamental question. In this view, pharmacological manipulation of embryos treated in whole-embryo culture systems might be a possible way. Microtubules are ubiquitous cell components. However, the extensive heterogeneity of MAP and tubulin in the CNS confers on the neurons a wide range of capabilities of assembly of these proteins and suggests that the neuron has a unique potential of a relation between MT composition and cell function. We have seen that each major event during neurogenesis is related to a specific series of modifications of the MT components. It remains to be determined if there is a causal or just a correlative relationship between the appearance of specific isotypes and the occurrence of specific events and/or functions. We have also to determine the exact spatial and temporal relations among the different isotypes of MT proteins, tubulin, and MAP. Is there a close correspondence between a tubulin and a MAP isotype? Can the appearance of one isotype of tubulin influence the appearance and the assembly of a specific MAP, or vice versa? Recent results obtained with the Tyr- and Glu-MT shed light on these questions and suggest a whole series of possibilities for cells to modulate the structure, behavior, and function of MT in specific domains of the neuron or in specific regions of the brain, by only a minute modification of the molecule of tubulin. Microtubule protein heterogeneity raises also a number of questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective localization of messenger RNA for cytoskeletal protein MAP2 in dendrites

For nerve cells to develop their highly polarized form, appropriate structural molecules must be targeted to either axons or dendrites. This could be achieved by the synthesis of structural proteins in the cell body and their sorting to either axons or dendrites by specific transport mechanisms. For dendrites, an alternative possibility is that proteins could be synthesized locally in the dendritic cytoplasm. This is an attractive idea because it would allow regulation of the production of structural molecules in response to local demand during dendritic development. The feasibility of dendritic protein synthesis is suggested both by the existence of dendritic polyribosomes and by the recent demonstration that newly synthesized RNA is transported into the dendrites of neurons differentiating in culture. However, to date there has been no demonstration of the selective synthesis of an identified dendrite-specific protein in the dendritic cytoplasm. Here, we use in situ hybridization with specific complementary DNA probes to show that messenger RNA for the dendrite-specific microtubule-associated protein MAP2 (refs 3-5) is present in dendrites in the developing brain. By contrast the mRNA for tubulin, a protein present in both axons and dendrites is located exclusively in neuronal cell bodies.

Morphological differentiation of embryonic rat sympathetic neurons in tissue culture. I. Conditions under which neurons form axons but not dendrites

We have examined the morphology of fetal rat sympathetic neurons grown in serum-free medium in the absence of nonneuronal cells. Because cell density can affect phenotypic expression in vitro, the morphological analysis was subdivided into the study of isolated neurons (neurons whose somata were at least 150 micron from their nearest neighbor) and of more highly aggregated neurons. When isolated neurons were injected with intracellular markers, it was found that most (79%) had a single process emanating from their somata and that this unipolar state persisted for at least 8 weeks in vitro. The processes of unipolar sympathetic neurons had the appearance of axons in that they were thin and long, had a constant diameter, and were relatively unbranched. Cytochemical methods revealed that such processes had other axonal characteristics: (1) they were more reactive with a monoclonal antibody against phosphorylated forms of the M and H neurofilament subunits than with an antibody to nonphosphorylated forms of these proteins; (2) they also reacted with antibodies to the tau microtubule-associated protein and to the phosphorylated forms of the H neurofilament subunit; and (3) they contained only small amounts of RNA as determined by [3H]uridine autoradiography. These data indicate that neurons which normally form dendrites in vivo need not express this capacity in vitro and that axonal and dendritic growth can be dissociated under some conditions in culture. While most isolated neurons were unipolar, neurons in regions of high neuronal cell density were usually multipolar. In addition to axons, multipolar neurons had processes with some of the characteristics expected of rudimentary dendrites: they ended locally (usually within 100 micron), were often highly branched, and reacted with an antibody to nonphosphorylated forms of the M and H neurofilament subunits. The effects of density were most prominent when neurons were within aggregates in which the somata were in close apposition. Density-dependent changes in morphology were less frequently observed when neuronal somata were separated by greater distances (30-100 micron). These data indicate that the morphology of sympathetic neurons is subject to environmental regulation and that neuron-neuron interactions can promote the extension of rudimentary dendrites in vitro.

Microtubule-associated protein 2 (MAP 2) immunoreactivity in human fetal neocortex

We used a monoclonal antibody to study the immunocytochemical distribution of microtubule-associated protein 2 (MAP 2) in human fetal neocortex between the ages of 16 and 22 weeks gestation. The staining pattern was lamina-specific. Neuronal somata and dendrites in all cortical layers and in the intermediate zone were labelled. Cajal-Retzius cells of layer I, large pyramidal neurons in the inner cortical plate and neurons in the subplate were most strongly immunoreactive. Separate from the underlying cortical plate a thin sheet of small neurons in the inner marginal zone was highlighted by MAP 2 immunoreactivity. The morphologic diversity, density and regional distribution of the interstitial neurons in the subplate was emphasized by MAP 2 staining. In general, the intensity of MAP 2 immunoreactivity in cell somata and dendrites correlated with the degree of neuronal differentiation but the pattern of intracellular staining also varied as a function of laminar position, and presumably cell type.

The distribution of microtubule-associated protein 2 changes when dendritic growth is induced in rat sympathetic neurons in vitro

We have examined the distribution of microtubule-associated protein 2 in embryonic rat sympathetic neurons grown under culture conditions that alter morphological development. Cultures were established in serum-free medium. After 8 days some were transferred to a serum-containing medium, which promotes dendritic development. Sister cultures were maintained in serum-free medium, which inhibits dendritic growth but permits normal axonal development. After growth for 2-6 weeks in serum-containing medium, sympathetic neurons were multipolar, with short, tapering dendrites and long, thin axons. Intense immunoreactivity for microtubule-associated protein 2 was observed in the somata and dendrites of all neurons, but there was little or no staining of the network of axonal processes that ran between cell somata. When the morphology of individual cells was assessed by injection of fluorescent dye before immunostaining, we found that staining for microtubule-associated protein 2 extended to the distal tips of the dendrites while the axon was essentially unstained, even in its proximal portion. Neurons from sister cultures that were not exposed to serum were usually unipolar, having only an axon. Under these conditions microtubule-associated protein 2 was also expressed, but its distribution was altered: intense immunostaining for microtubule-associated protein 2 was present in axons as well as somata. Staining in axons could sometimes be traced for several millimeters, but, since unstained segments of axons were also common, microtubule-associated protein 2 probably was not present throughout the entire axonal arborization. These results show that the expression of microtubule-associated protein 2 is not of itself sufficient to induce the formation of dendrites. Despite the association of microtubule-associated protein 2 with the axonal cytoskeleton, the light microscopic morphology of the axons was not obviously altered.

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

Using a monoclonal antibody against the microtubule-associated protein tau we compared the distribution and the biochemical maturation of this protein in hippocampal pyramidal neurons in the rat in tau and in culture. In tissue sections from mature animals tau was localized heterogeneously within neurons. It was concentrated in axons; dendrites and somata showed little or no staining. In hippocampal cultures ranging from 12 h to 4 weeks in vitro tau was present in neurons but not in glial cells, as it is in situ. Within cultured neurons, however, tau was not compartmentalized but was present throughout the dendrites, axons and somata. Immunoblotting experiments showed that the biochemical maturation of tau that occurs in situ also failed to occur in culture. The young form of tau persisted, and the adult forms did not develop. In contrast the biochemical maturation and the compartmentalization of microtubule-associated protein 2 occurred normally in hippocampal cultures. These results show that the biochemical maturation and the intraneuronal compartmentalization of these two microtubule-associated proteins are independently controlled. Despite the non-restricted distribution of tau in hippocampal neurons in culture, and despite the presence of only the immature isoform which has a lessened stimulatory effect on microtubule polymerization, axons and dendrites appear to grow normally and to exhibit appropriate functional properties.

An immunocytochemical analysis of the ontogeny of the microtubule-associated proteins MAP-2 and Tau in the nervous system of the rat

The developmental distribution patterns of beta-tubulin and the microtubule-associated proteins, MAP-2 and Tau, were studied by immunocytochemistry with monoclonal antibodies. The analysis of the in situ distribution of these proteins in embryonic brain tissue revealed intense immunoreactivity for beta-tubulin in proliferative and migrating neuroblasts. On the contrary, no immunoreactivity for MAP-2 or Tau was detected in this neuroepithelium; specific immunostaining for these MAPs was only present in those neuroblasts which have reached their final destination within a developing brain area, and have initiated terminal differentiation, i.e. the sprouting of axons and dendrites. During the initial stages of neuritic outgrowth both MAPs were detected in the somatodendritic compartment of developing brain neurons; Tau was also present in axons. While the distribution of MAP-2 remained essentially the same throughout development, Tau was progressively lost from cell bodies and dendrites. This pattern of compartmentation was observed in pyramidal neurons of the cerebral cortex and hippocampus, as well as in cells of other brain regions (e.g. thalamus, hypothalamus, cerebral amygdala and tectum). It was not detected in cerebellar Purkinje cells which compartmentalize Tau to axons from the outset of neuritic differentiation, and in neurons of the Gasser ganglion which transiently express MAP-2 in axons. The expression and distribution of these MAPs was also analyzed in embryonic cerebellar and hippocampal pyramidal neurons grown in culture. Both MAPs were found in these cells as soon as 6 h after plating; they were also present in all of the neurites, axons and dendrites, that these cells extend after development in vitro for several days. With subsequence development (more than 4 days in vitro) MAP-2 was lost from axons, while Tau remained homogeneously distributed in both types of neurites. Taken collectively, the present results indicate that the development of the compartmentalized distribution of MAP-2 and Tau follows a complex pattern which is specific for each of these MAPs, and which varies as a function of the neuron type and the conditions under which the cell develops. In addition, the complex variations in the distribution of both MAPs during in situ and in vitro development make it unlikely that these proteins have a role in determining the fate of a neurite as an axon or a dendrite.