The expression of phosphorylated and non-phosphorylated forms of MAP5 in the amphibian CNS

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

Distribution of the phosphorylated form of microtubule associated protein 1B in the fish visual system during optic nerve regeneration

Microtubule associated proteins are a heterogeneous group of proteins that have been implicated in regulating microtubule stability. They play an important role in the organisation of the neuronal cytoskeleton during neurite outgrowth, plasticity and regeneration. The fish visual system presents a considerable degree of plasticity. Thus, the retina grows continually throughout life and the optic nerve regenerates after crush. In the present study, we compared the distribution of the microtubule associated protein 1B in its phosphorylated form (MAP1B-phos) in the normal adult fish visual system with that observed during optic nerve regeneration after adult optic nerve crush using a specific monoclonal antibody mAb-150. Expression of MAP1B-phos was observed in some ganglion cell somata and in developing, growing axons within the control optic nerve. Few immunoreactive terminals were seen in the control optic tectum. After optic nerve crush, we found additional MAP1B-phos expression in regenerating axons throughout the visual system. Our results demonstrate that MAP1B-phos is present in growing and regenerating axons of fish retinal ganglion cells, which suggests that the phosphorylated form of MAP1B may play an important role in developmental and regeneration processes within the fish central nervous system.

Expression of SCG10 and stathmin proteins in the rat olfactory system during development and axonal regeneration

The membrane-associated protein SCG10 is expressed specifically by neuronal cells. Recent experiments have suggested that it promotes neurite outgrowth by increasing microtubule dynamics in growth cones. SCG10 is related to the ubiquitous but neuron-enriched cytosolic protein stathmin. To better understand the role played by SCG10 and stathmin in vivo, we have analyzed the expression and localization of these proteins in both the olfactory epithelium and the olfactory bulb in developing and adult rats, as well as in adult bulbectomized rats. The olfactory epithelium is exceptional in that olfactory receptor neurons constantly regenerate and reinnervate the olfactory bulb throughout animal life-span. SCG10 and stathmin expression in the olfactory receptor neurons was found to be regulated during embryonic and postnatal development and to correlate with neuronal maturation. Whereas SCG10 expression was restricted to immature olfactory receptor neurons (GAP-43-positive, olfactory marker protein-negative), stathmin was also expressed by the basal cells. In the olfactory bulb of postnatal and adult rats, a moderate to strong SCG10 immunoreactivity was present in the olfactory nerve layer, whereas no labeling was detected in the glomerular layer. Olfactory glomeruli also showed no apparent immunoreactivity for several cytoskeletal proteins such as tubulin and microtubule-associated proteins. In unilaterally bulbectomized rats, SCG10 and stathmin were seen to be up-regulated in the regenerating olfactory epithelium at postsurgery stages corresponding to olfactory axon regeneration. Our data strongly suggest that, in vivo, both SCG10 and stathmin may play a role in axonal outgrowth during ontogenesis as well as during axonal regeneration.

The phosphorylated isoform of microtubule associated protein 1B (MAP1B) is expressed in the visual system of the tench (Tinca tinca, L) during optic nerve regeneration

By using Western blot analysis and immunohistochemistry we have demonstrated that microtubule associated protein 1B (MAP1B)-phos is present in growing and regenerating axons of retinal ganglion cells of fish (Tinca tinca, L). We have found that the levels of MAP1B-phos substantially increase in regenerating optic nerves. Our observations suggest that MAP1 B-phos plays an important role in regeneration processes in the central nervous system (CNS) of the fish. These results are compared in the present paper with that found in the regenerating peripheral nervous system (PNS) of mammals.

The neurofilament antibody RT97 recognises a developmentally regulated phosphorylation epitope on microtubule-associated protein 1B

Microtubules are important for the growth and maintenance of stable neuronal processes and their organisation is controlled partly by microtubule-associated proteins (MAPs). MAP 1B is the first MAP to be expressed in neurons and plays an important role in neurite outgrowth. MAP 1B is phosphorylated at multiple sites and it is believed that the function of the protein is regulated by its phosphorylation state. We have shown that the monoclonal antibody (mAb) RT97, which recognises phosphorylated epitopes on neurofilament proteins, fetal tau, and on Alzheimer's paired helical filament-tau, also recognises a developmentally regulated phosphorylation epitope on MAP 1B. In the rat cerebellum, Western blot analysis shows that mAb RT97 recognises the upper band of the MAP 1B doublet and that the amount of this epitope peaks very early postnatally and decreases with increasing age so that it is absent in the adult, despite the continued expression of MAP 1B in the adult. We confirmed that mAb RT97 binds to MAP 1B by showing that it recognises MAP 1B immunoprecipitated from postnatal rat cerebellum using polyclonal antibodies to recombinant MAP 1B proteins. We established that the RT97 epitope on MAP 1B is phosphorylated by showing that antibody binding was abolished by alkaline phosphatase treatment of immunoblots. Epitope mapping experiments suggest that the mAb RT97 site on MAP 1B is near the N-terminus of the molecule. Despite our immunoblotting data, immunostaining of sections of postnatal rat cerebellum with mAb RT97 shows a staining pattern typical of neurofilaments with no apparent staining of MAP 1B. For instance, basket cell axons and axons in the granule cell layer and white matter stained, whereas parallel fibres did not. These results suggest that the MAP 1B epitope is masked or lost under the immunocytochemical conditions in which the cerebellar sections are prepared. The upper band of the MAP 1B doublet is believed to be predominantly phosphorylated by proline-directed protein kinases (PDPKs). PDPKs are also good candidates for phosphorylating neurofilament proteins and tau and therefore we postulate that the sites recognised by RT97 on these neuronal cytoskeletal proteins may be phosphorylated by similar kinases. Important goals are to determine the precise location of the RT97 epitope on MAP 1B and the kinase responsible.

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

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

Differential expression of microtubule-associated protein 1B phosphorylated isoforms in the adult rat nervous system

Phosphorylated microtubule-associated protein 1B isoforms are thought to be involved in the plastic events taking place in neurons during development. However, little is known about their expression and possible role in the mature nervous system. To gain insight into the mechanisms underlying neuronal plasticity in the adult, we studied the pattern of expression of three microtubule-associated protein 1B isoforms in the entire adult rat nervous system. Accordingly, we performed western blots and immunohistochemistries using the antibodies 125, 150 and 531, which specifically recognize phosphorylated and unphosphorylated microtubule-associated protein 1B epitopes. Two electrophoretically distinct microtubule-associated protein 1B isoforms, slow-migrating and fast-migrating, were detected with the antibodies. The pattern of expression of these isoforms in the adult rat nervous system was region specific. Phosphorylated slow-migrating microtubule-associated protein 1B was expressed at all cellular compartments of primary sensory neurons in the central and peripheral nervous systems. In addition to primary sensory axons, slow-migrating microtubule-associated protein 1B was encountered at some other axons within the central nervous system. We discuss the correlation between slow-migrating microtubule-associated protein 1B axonal content and the regenerative potential of neurons. Phosphorylated fast-migrating microtubule-associated protein 1B was exclusively found in central nervous system dendrites where synaptic plasticity with morphological changes occurs in the adult. Unphosphorylated fast-migrating microtubule-associated protein 1B was the only isoform present in the bodies and dendrites of all motor neurons, and in peripheral and central nervous system glial cells of myelinated tracts with slow-migrating microtubule-associated protein 1B-containing axons. In summary, this report describes the pattern of expression of microtubule-associated protein 1B isoforms in the entire adult rat nervous system. In addition, it provides some information about the possible functional implications of phosphorylated microtubule-associated protein 1B isoforms in the adult.

Expression of a developmentally regulated, phosphorylated isoform of microtubule-associated protein 1B in sprouting and regenerating axons in vitro

We have developed a novel culture system for studying axonal regeneration. Short lengths of spinal nerves with their attached dorsal root ganglia were removed from adult mice, explanted into Matrigel and maintained in serum-free medium for up to eight days. Profuse outgrowth of unfasciculated, naked axons occurred within 6 h from the cut ends of the peripheral nerve, dorsal roots and eventually from the ganglion itself, and continued to grow throughout the observation period. Some axons were entirely smooth, whilst others showed prominent varicosities. The former stained with antibody RT97, a marker for large-calibre, myelinated axons, whilst the latter stained with antibodies to calcitonin gene-related peptide, predominantly a marker for unmyelinated and small-diameter myelinated sensory axons. All axons stained with a monoclonal antibody (150) that recognizes a developmentally regulated phosphorylated isoform of the microtubule-associated protein 1B [Gordon-Weeks P. R. et al. (1993) Eur. J. Neurosci. 5, 1302-1311]. Monoclonal antibody 150 staining was observed along the entire length of all axons growing out of the explant; the proximal regions of these axons within the explant itself did not stain. The staining extended to the growth cones, which had elaborate morphologies. Other antibodies (e.g. to growth-associated protein 43) labelled axons within the nerve, as well as those growing in Matrigel. In preparations where the peripheral nerve had been crushed half-way along its length at the time of explantation, monoclonal antibody 150 staining was absent from axons in the nerve proximal to the crush, but present in axons which had regenerated within the nerve distal to the crush. The results indicate that re-expression during axonal regeneration of the phosphorylated isoform of microtubule-associated protein 1B recognized by monoclonal antibody 150 is restricted to the newly formed lengths of regenerated axons. The correlation between its expression and axonal growth during development and regeneration suggests that it may play a role in axonal extension. Our observations also demonstrate the usefulness of these explant cultures for studying axonal regeneration.

Transgenic expression of embryonic MAP2 in adult mouse brain: implications for neuronal polarization

The major neuronal microtubule-associated protein MAP2 is selectively localized in dendrites, where its expression is under strong developmental regulation. To learn more about its potential effects on neuronal morphogenesis and its sorting within the neuronal cytoplasm, we have raised transgenic mice that express high levels of the embryonic form, MAP2c, in the adult brain. One transgenic line expressed higher levels of MAP2c than endogenous adult MAP2. This had no detectable effect on either the arrangement or morphology of neurons, suggesting that although MAP2c is necessary for neuronal morphogenesis it is not involved in its regulation. Like endogenous adult MAP2, transgenic MAP2c was present in dendrites but not axons, indicating that the signal responsible for its cytoplasmic sorting is contained within the 1.5 kb of its coding sequence. In situ hybridization with specific probes showed that transgenic MAP2c mRNA was limited to cell bodies. Thus, the dendritic localization of MAP2c protein cannot be the result of previous transport of its mRNA but must depend on a signal associated with the protein itself. Furthermore, because the amino acid sequence of MAP2c is present in all forms of MAP2, this signal is also contained within adult high-M(r) MAP2 protein. This raises the possibility that, rather than the conventional scheme of mRNA sorting preceding protein localization, the transport of adult MAP2 mRNA into dendrites could depend on it being part of a translation complex in which the targeting signal is on the nascent protein.

Compartmentation of alpha-internexin and neurofilament triplet proteins in cultured hippocampal neurons

Intermediate filaments comprise an integral part of the neuronal cytoskeleton. However, little is known about their function, and there remains some uncertainty about their precise subcellular localization. We examined the timing of expression and distribution of alpha-internexin, neurofilament triplet proteins and peripherin using immunocytochemistry in cultured hippocampal neurons. alpha-Internexin immunostaining was present in all neurons at all developmental stages. Immunostaining appeared as long filaments in axons and short fragments in dendrites which extended into dendritic spines. The presence of alpha-internexin in dendritic spines was confirmed in situ by electron microscopy of rat hippocampal tissue sections and suggests that this intermediate filament may serve as a link between cytoskeletal elements in dendritic shafts and spines. In culture, immunostaining using antibodies against individual triplet protein subunits indicated that light (NF-L) and middle (NF-M) subunits were first expressed in cells shortly after the initiation of axonal outgrowth. Expression of the heavy (NF-H) subunit occurred a few days later. Although timing and localization of expression did not correlate with the initiation of axonal or dendritic processes, it was coincident with periods of rapid outgrowth. Triplet proteins were more abundant in axons and appeared to be incorporated into lengthier filaments than in dendrites. Highly phosphorylated NFH/M immunoreactivity was polarized to axons after 6 days in culture. The distribution of one NF-H epitope was restricted to GABAergic neurons in mature cultures, suggesting a cell-type specific modification. Peripherin was not detectable at any time in hippocampal cultures. Our results show that intermediate filaments are integral components of the neuronal cytoskeleton of cultured hippocampal neurons throughout development. Furthermore, the localization of alpha-internexin suggests that it may be involved in the formation or maintenance of dendritic spines.

An analysis of an axonal gradient of phosphorylated MAP 1B in cultured rat sensory neurons

The present study investigated the cellular distribution of a developmentally regulated phosphorylated form of MAP 1B recognized by monoclonal antibody (mAb) 150 in cultures of dorsal root ganglia. The cell soma and the whole axon, when it first appears, are labelled, but longer axons label with a proximodistal gradient, such that the cell soma and proximal axon become unlabelled, whilst the distal axon and growth cone label strongly. Double-labelling experiments with mAb 150 and a polyclonal antibody (N1-15) that recognizes all forms of MAP 1B demonstrated that MAP 1B is distributed along the entire length of axons with gradients, so the gradient of phosphorylated MAP 1B is not due to a loss or absence of MAP 1B from the proximal axon. The proportion of axons from 20 h cultures that were labelled with a mAb 150 gradient was at least 80% and this proportion was independent of the nerve growth factor concentration of the culture medium. Analysis of axons ranging in length from 100 to 700 microm and labelled with a gradient showed that the unlabelled proximal portions of axons increased in length more slowly than the labelled distal axon. Axons labelled along their entire length accounted for no more than 19% of th axonal population and analysis of these showed them to be frequently < 400 microm long. After simultaneously fixing and detergent-extracting cultures this proportion rose significantly to 93%, suggesting that in the proximal axon the mAb 150 epitope is masked by some factor(s) that is removed by detergent extraction. The possibility that mAb 150 could not access the epitope in the proximal axon was discounted because another IgM, mAb 125, which recognizes a different phosphorylation epitope on MAP 1B, labelled the proximal axon of conventionally fixed cultures. In growth cones of fixed and extracted neurons examined by immunofluorescence, the mAb 150 labelling strongly colocalized to bundled microtubules in the distal axon shaft and the C-domain. In the P-domain, mAb 150 staining was weaker and more widely distributed than the microtubules. Immunogold electron microscopy confirmed that antibody N1-15 and mAb 150 strongly labelled the bundled microtubules in the C-domain and also showed that individual microtubules in the P-domain, some of which lie alongside actin filament bundles of filopodia, were labelled lightly and discontinuously with both antibodies. This suggests that the phosphorylated isoform of MAP 1B recognized by mAb 150 may be microtubules and actin filaments in the P-domain.

Differential phosphorylation of MAP1b during postnatal development of the cat brain

Microtubule-associated protein 1b, previously also referred to as microtubule-associated protein 5 or microtubule-associated protein 1x, is a major component of the juvenile cytoskeleton, and is essential during the early differentiation of neurons. It is required for axonal growth and its function is influenced by phosphorylation. The distribution of microtubule-associated protein 1b in kitten cerebellum and cortex during postnatal development was studied with two monoclonal antibodies. Hybridoma clone AA6 detected a non-phosphorylated site, while clone 125 detected a site phosphorylated by casein-kinase II. On blots, both monoclonal antibodies stained the same two proteins of similar molecular weights, also referred to as microtubule-associated protein 5a and 5b. Antibody 125 detected a phosphorylated epitope on both microtubule-associated protein 1b forms; dephosphorylation by alkaline phosphatase abolished the immunological detection. During development of cat cortex and cerebellum, AA6 stained the perikarya and dendrites of neurons during their early differentiation, and especially labelled newly generated axons. The staining decreased during development, and axonal staining was reduced in adult tissue. In contrast to previous reports which demonstrated that antibodies against phosphorylated microtubule-associated protein 1b label exclusively axons, antibody 125 also localized microtubule-associated protein 1b in cell bodies and dendrites, even in adulthood. Some nuclear staining was observed, indicating that a phosphorylated form of microtubule-associated protein 1b may participate in nuclear function. These results demonstrate that microtubule-associated protein 1b is subject to CK2-type phosphorylation throughout neuronal maturation and suggest that phosphorylation of microtubule-associated protein 1b may participate in juvenile and mature-type microtubule functions throughout development.

Distribution and expression of developmentally regulated phosphorylation epitopes on MAP 1B and neurofilament proteins in the developing rat spinal cord

The distribution and expression of developmentally regulated phosphorylation epitopes on the microtubule-associated protein 1B and on neurofilament proteins recognized by monoclonal antibody (mAb) 150 and mAb SMI-31 was investigated in the developing rat spinal cord. In the embryonic day 11 spinal cord, mAb 150 stained the first axons to appear, whereas mAb SMI-31 staining did not appear until embryonic day 12. At the start of axonogenesis, mAb 150 stained neuronal cell bodies and axons whereas at later times only the distal axon was stained, this is the first demonstration in vivo of a mAb 150 axonal gradient similar to that seen previously in vitro (Mansfield et al., 1991). During the postnatal period, axonal staining by mAb 150 dramatically declined so that by the third postnatal week, only the corticospinal tract, which contains axons that are still growing, was labelled. There was no evidence of dendritic staining except of adult primary motoneurons. In contrast, mAb SMI-31 staining of axons was not present as a gradient. Instead, mAb SMI-31 staining increased progressively throughout this period, persisted into adulthood and was shown by immunoblotting to be related to the increased phosphorylation of the medium and heavy neurofilament proteins. Axonal staining by mAb 150 re-appears in a sub-population of the SMI-31-labelled myelinated axons in the adult spinal cord and PNS and in the perikarya and dendrites of primary motoneurons, where it probably recognizes a phosphorylation epitope on heavy neurofilament proteins. This late appearing epitope has some similarities to that recognized by mAb SMI-31 on neurofilaments, but it is not identical. These cross-reactivities of mAbs that recognize phosphorylation epitopes on otherwise unrelated proteins dictate caution in interpreting immunohistochemical data. It may now be necessary in some cases to re-appraise published studies using these two antibodies.

Regenerating sciatic nerve axons contain the adult rather than the embryonic pattern of microtubule associated proteins

Microtubule associated proteins play a central role in the control of axon growth. We have used immunohistochemical techniques to establish which microtubule-associated proteins are present in the rat hindlimb spinal cord, dorsal root ganglia and peripheral nerves during axonal growth during embryogenesis, in adulthood, and during regeneration of crushed sciatic nerves. During embryogenesis microtubule-associated protein-1b and tau are present in all neurons and axons, microtubule-associated protein-2 is present in neurons but not in axons, and there is no microtubule-associated protein-1a. In adults, microtubule-associated protein-1a and microtubule-associated protein-1b are present in all sciatic nerve axons and in motor and dorsal root ganglion neurons. Tau, in its adult form, is present in many fine probably sensory axons, but not in most larger axons, and in motor and sensory neurons. Microtubule-associated protein-2 is present only in neurons. During regeneration the pattern of microtubule-associated protein expression retains the adult pattern. All regenerating axons contain microtubule-associated protein-1a and microtubule-associated protein-1b, none contain microtubule-associated protein-2, and a subset of fine axons contain tau. There is no detectable change in microtubule-associated protein expression by motoneurons. While axons are clearly able to regenerate without either microtubule-associated protein-2 or tau, tau containing axons appear to regenerate faster than those which lack it. It is possible that the failure of neurons to recapitulate the embryonic pattern of microtubule-associated protein expression during regeneration could be a reason why regenerative axon growth is slower and less vigorous than axon growth in embryos.

A phosphorylation epitope on MAP 1B that is transiently expressed in growing axons in the developing rat nervous system

We have isolated a monoclonal antibody (150) that recognizes a phosphorylation epitope on the microtubule-associated protein (MAP) 1B. Immunoblot analysis of the developing rat central nervous system shows that monoclonal antibody 150 is directed against a protein of approximately 325 kDa (MAP 1B) that copolymerizes with microtubules through successive cycles of temperature-dependent assembly and disassembly. Furthermore, immunoprecipitated MAP 1B contains the epitope recognized by monoclonal antibody 150. Removal of phosphate from blotted proteins using alkaline phosphatase abolishes the binding of monoclonal antibody 150 to MAP 1B, indicating that the epitope is phosphorylated. In the developing rat nervous system, immunohistochemistry with monoclonal antibody 150 shows that the phosphorylation epitope on MAP 1B is transiently expressed in growing axons but not in dendrites. For instance, in the neonatal rat cerebellum, the parallel fibres of granule cells are stained only during elongation and not after synaptogenesis. The monoclonal antibody 150 epitope is also transiently expressed in radial glial fibres and in certain cell nuclei. All immunostaining of sections with monoclonal antibody 150 was completely abolished by alkaline phosphatase treatment. These observations and previous ones made by us in cell culture (Mansfield et al., J. Neurocytol., 20, 654-666, 1991) suggest that the phosphorylation epitope on MAP 1B recognized by monoclonal antibody 150, which has not been previously detected in vivo, may be important in axonogenesis.

Organization of microtubules in axonal growth cones: a role for microtubule-associated protein MAP 1B

Neuronal growth cones guide growing axons and dendrites (neurites) through developing embryos by detecting extrinsic guidance cues and transducing the signal into changes in motile behaviour. In this brief review, the role of the growth cone cytoskeleton in these events, in particular the microtubules, is discussed. Microtubules in the neurite are mainly bundled into fascicles whereas on entering the growth cone they diverge from each other and traverse the central (C)-domain of the growth cone. Occasionally, individual microtubules extend as far as the peripheral (P)-domain and may even enter filopodia. Microtubules in the growth cone are probably dynamically unstable, exchanging dimer with a large pool of soluble tubulin. It is proposed that the 'capture' of dynamically unstable microtubules by filopodial actin filament bundles is a crucial step underlying directed growth. Localised assembly of microtubules at the growth cone, rather than at the cell body followed by transport of polymer to the growth cone, may facilitate the delivery of material to specific regions of the growth cone and hence allow vectorial growth. Bundling of microtubules and capture of microtubules by filopodia both imply roles for microtubule-associated proteins (MAPs). Several microtubule-associated proteins are present within growth cones, including MAP 1B, MAP2 and tau. Recent experiments point toward a phosphorylated form of MAP 1B as an important component in neurite elongation and in particular in the bundling of microtubules in the growth cone.

Reorganisation of the microtubular cytoskeleton by embryonic microtubule-associated protein 2 (MAP2c)

Microtubule-associated protein 2c (MAP2c) is one of a set of embryonic MAP forms that are expressed during neuronal differentiation in the developing nervous system. We have investigated its mode of action by expressing recombinant protein in non-neuronal cell lines using cell cDNA transfection techniques. At every level of expression, all the MAP2c was bound to cellular microtubules. At low MAP2c levels, the microtubules retained their normal arrangement, radiating from the centrosomal microtubule-organising centre (MTOC) but at higher levels an increasing proportion of microtubules occurred independently of the MTOC. In most cells, radially oriented microtubules still attached to the MTOC co-existed with detached microtubules, suggesting that the primary effect of MAP2 is to increase the probability that tubulin polymerisation will occur independently of the MTOC. The MTOC-independent microtubules formed bundles whose distribution depended on their length in relation to the diameter of the transfected cell. Short bundles were attached to the cell cortex at one end and followed a straight course through the cytoplasm, whereas longer bundles followed a curved path around the periphery of the cell. By comparing these patterns to those produced by two chemical agents that stabilise microtubules, taxol and dimethyl sulphoxide, we conclude that effects of MAP2c arise from two sources. It stabilises microtubules without providing assembly initiation sites and as a result produces relatively few, long microtubule bundles. These bend only when they encounter the restraining influence of the cortical cytoskeleton of the cell, indicating that MAP2c also imparts stiffness to them. By conferring these properties of stability and stiffness to neuronal microtubules MAP2c contributes to supporting the structure of developing neurites.

Phosphorylation of vinculin in human platelets spreading on a solid surface

Vinculin is a cytoskeletal protein believed to be involved in linking microfilaments to the cell membrane. It is a substrate for the Ca(2+)- and phospholipid-dependent protein kinase C. We show here that when human platelets attach and spread on a solid surface, the alpha isoforms of vinculin become phosphorylated at serine and/or threonine residues. Phosphorylation is dependent on adhesion to a surface, since suspended, unattached platelets can produce filopodia but no phosphorylation of vinculin. Phosphorylation is also dependent on actin polymerization, as it does not occur when platelets had been pretreated with cytochalasin B. Most likely, protein kinase C is responsible for the phosphorylation of vinculin, since phosphorylation also occurs when platelets are treated with a phorbol ester, which activates protein kinase C, and is blocked by treatment with a staurosporine derivative which inhibits this enzyme. These results suggest that phosphorylation plays a role in anchoring vinculin at sites of microfilament-membrane interaction.

Differential distribution of two microtubule-associated proteins, MAP2 and MAP5, during chick dorsal root ganglion development in situ and in culture

Microtubule-associated proteins (MAPs) are essential components necessary for the early growth process of axons and dendrites, and for the structural organization within cells. Both MAP2 and MAP5 are involved in these events, MAP2 occupying a role predominantly in dendrites, and MAP5 being involved in both axonal and dendritic growth. In the chick dorsal root ganglia, pseudo-unipolar sensory neurons have a T-shaped axon and are devoid of any dendrites. Therefore, they offer an ideal model to study the differential expression of MAPs during DRG development, specifically during axonal growth. In this study we have analyzed the expression and localization of MAP2 and MAP5 isoforms during chick dorsal root ganglia development in vivo, and in cell culture. In DRG, both MAPs appeared as early as E5. MAP2 consists of the 3 isoforms MAP2a, b and c. On blots, no MAP2a could be found at any stage. MAP2b increased between E6 and E10 and thereafter diminished slowly in concentration, while MAP2c was found between stages E6 and E10 in DRG. By immunocytochemistry, MAP2 isoforms were mainly located in the neuronal perikarya and in the proximal portion of axons, but could not be localized to distal axonal segments, nor in sciatic nerve at any developmental stage. On blots, MAP5 was present in two isoforms, MAP5a and MAP5b. The concentration of MAP5a was highest at E6 and then decreased to a low level at E18. In contrast, MAP5b increased between E6 and E10, and rapidly decreased after E14. Only MAP5a was present in sciatic nerve up to E14. Immunocytochemistry revealed that MAP5 was localized mainly in axons, although neuronal perikarya exhibited a faint immunostaining. Strong staining of axons was observed between E10 and E14, at a time coincidental to a period of intense axonal outgrowth. After E14 immunolabeling of MAP5 decreased abruptly. In DRG culture, MAP2 was found exclusively in the neuronal perikarya and the most proximal neurite segment. In contrast, MAP5 was detected in the neuronal cell bodies and all along their neurites. In conclusion, MAP2 seems involved in the early establishment of the cytoarchitecture of cell bodies and the proximal axon segment of somatosensory neurons, while MAP5 is clearly related to axonal growth.

The postnatal spatial and temporal development of corticospinal projections in cats

Orthograde labeling and immunocytochemical techniques were used to study the postnatal spatial and temporal development of corticospinal projections in cats. Findings from the orthograde labeling studies indicate that there are three major phases in the spatial development of corticospinal projections: an early period (1-10 postnatal days) when cortical axons grow into the spinal gray from the white matter; an intermediate period (2-5 postnatal weeks) where corticospinal axons develop terminal arborizations in a rostral to caudal, medial to lateral and intermediate gray to dorsal and ventral horn sequence; and, a late period (6-7 postnatal weeks) during which some corticospinal projections are eliminated. The time period over which cortical axons grow into the spinal cord was determined immunocytochemically using a monoclonal antibody against a microtubule associated protein (MAP 1B) present in growing axons. The corticospinal tracts were strongly immunoreactive for MAP 1B during the first three postnatal weeks. MAP 1B immunostaining of these tracts started to decline in the fourth postnatal week and was completely absent by five weeks of age. These findings indicate that the postnatal development of corticospinal projections is spatially and temporally protracted in cats.

Intrinsic neuronal determinants of regeneration

Axon growth and axon regeneration are co-operative processes; the speed and extent of axon growth are influenced both by the properties of the environment surrounding the axon growth cone, and the properties of the neuron itself. In recent years, the environmental influences on axon growth have received most of the attention directed towards this area of research, but the properties of the neurons themselves are likely to be just as important. Within both adults and embryos there are differences in the growth potential of different neuronal types, and there is also evidence for an overall decrease in the vigour of axon growth with neuronal age.

The distribution and phosphorylation of the microtubule-associated protein MAP 1B in growth cones

Primary cultures of dissociated embryonic day 18 rat cerebral cortices were labelled by immunofluorescence with antibodies directed either against phosphorylated and non-phosphorylated MAP 1B (antibody 81) or against phosphorylated MAP 1B (antibody 150). Both antibodies stain cortical neurons, including their neurites and growth cones, in early (18 h) cultures, whereas only antibody 81 stained glial cells. By 4 days in culture, phosphorylated MAP 1B is largely restricted to axonal processes and growth cones, where it is often distributed in a gradient that is highest distally. In axonal processes and growth cones after 18 h and 4 days in culture, the phosphorylated form of MAP 1B is present both in a soluble form and bound to microtubules. Growth cones isolated from postnatal day 5 rat forebrain were labelled in vitro with 32P-orthophosphate and detergent soluble and insoluble (cytoskeleton) fractions prepared. SDS-PAGE analysis revealed several major phosphoproteins in isolated growth cone cytoskeletons, including MAP 1B. Phosphorylated MAP 1B was also present in the detergent soluble fraction of growth cones. Immunoblotting and immunoprecipitation with MAP 1B antibodies confirmed the identification of MAP 1B and that the protein is phosphorylated in growth cones. These data show that MAP 1B, in particular the phosphorylated isoform, is present in growth cones and suggest that phosphorylation of MAP 1B may play an important role in neurite elongation.

MAP5 in cultured hippocampal neurons: expression diminishes with time and growth cones are not immunostained

A monoclonal antibody was used to determine both the expression of the microtubule-associated protein MAP5 in cultured foetal rat hippocampal neurons as a function of culture age and the cellular distribution of the protein. When cultures at days 2 and 3 were examined by fluorescence microscopy, MAP5 immunostaining was localized intensely in neuronal cell bodies and neurites but not in growth cones. Extensive labelling of axons was seen at days 4 and 5. MAP5 staining was still prominent in neurons after 16 days in culture, and neurites at this time had grown over astrocytes but had completely avoided islands of non-astrocytic cells. MAP5 immunostaining was almost undetectable in cells that had been in culture for 20 days. The decreasing expression of MAP5 in cultured neurons as a function of time parallels that previously shown for MAP5 in intact neonatal rat brain. The effect of elevated temperature on MAP5 expression was also examined. Neurons grown for 9 days at 40 degrees C showed the same cellular distribution of MAP5 as cells grown at 37 degrees C. In particular, growth cones were again negative for MAP5 immunostaining. The absence of MAP5 in growth cones appears consistent with the fact that these structures contain labile microtubules. MAP5 has been shown to be a component of microtubule crossbridges and its absence might thus be expected to contribute to microtubule lability.

Growth cones: the mechanism of neurite advance

Growth cones are the highly motile structures found at the tips of growing axons and dendrites (neurites), which extend from neurones, during the development of the nervous system. They function both as detectors and transducers of extrinsic guidance cues and as regions where the neurite assembly, advance cannot occur. Assembly of the neurite cytoskeleton in growing neurites chiefly involves microtubule assembly at the growth cone. Some of the factors that may influence microtubule assembly in growth cones are becoming apparent and include post-translational modification of tubulin itself and microtubule associated proteins, particularly tau and MAP1B.

Non-motor microtubule-associated proteins

Cloning of primary sequences has generated information on the structures of the non-motor microtubule-associated proteins and their relationship to one another. Questions about how classes of microtubule-associated proteins interact are starting to be addressed in vitro and, in vivo, tests of function are being pursued using a variety of cellular and molecular biological strategies.

The roles of microtubule-associated proteins in brain morphogenesis: a review

Microtubule-associated proteins (MAPs) are a diverse family of cytoskeletal proteins that copurify with tubulin in vitro. Recently a number of novel approaches have been used to learn more about the functions of MAPs during brain development, including: localization of MAPs and their mRNA in the developing brain, comparisons of MAPs between species to learn potential fundamental characteristics, biochemical analysis of changes in MAPs in process-bearing cell lines, and sequence analysis of MAP cDNAs and cDNA transfection studies. Taken together, these data allow us to assign roles to MAPs which are abundant in the developing brain, and to develop models for future studies. Four MAPs are particularly abundant in the developing brain: MAP1B, the high and low-molecular weight forms of MAP2, and juvenile tau. MAP1B is the only MAP to be found consistently in extending processes in both the developing and adult brain, making it a likely regulator of neurite outgrowth. High-molecular weight MAP2 and tau crosslink microtubules in dendrites and axons, respectively. Low-molecular weight MAP2 may be able to regulate MAP2-mediated crosslinking to make processes more labile during development and in adult brain regions where synaptogenesis is active. Tau-mediated crosslinking may be regulated by temporal regulation of the expression of tau forms with different binding affinities to tubulin. High-molecular weight MAP2 is sequestered into dendrites by the selective transport of its mRNA. This allows rapid and local regulation of MAP2 synthesis.