Association of microtubule-associated protein 2 (MAP 2) with microtubules and intermediate filaments in cultured brain cells

The classification of MAP 2 as a microtubule-associated protein is based on its affinity for microtubules in vitro and its filamentous distribution in cultured cells. We sought to determine whether MAP 2 is also able to bind in situ to organelles other than microtubules. For this purpose, primary cultures of rat brain cells were stained for immunofluorescence microscopy with a rabbit anti-MAP 2 antibody prepared in our laboratory, as well as with antibodies to vimentin, an intermediate filament protein, and to tubulin, the major subunit of microtubules. MAP 2 was present on cytoplasmic fibers in neurons and in a subpopulation of the flat cells present in the cultures. Our observations were concentrated on the flat cells because of their suitability for high-resolution immunofluorescence microscopy. Double antibody staining revealed co-localization of MAP 2 with both tubulin and vimentin in the flat cells. Pretreatment of the cultures with vinblastine resulted in the redistribution of MAP 2 into perinuclear cables that contained vimentin. Tubulin paracrystals were not stained by anti-MAP 2. In cells extracted with digitonin, the normal fibrillar distribution of MAP 2 was resistant to several treatments (PIPES buffer plus 10 mM Ca++, phosphate buffer at pH 7 or 9) that induced depolymerization of microtubules, but not intermediate filaments. Staining of the primary brain cells was not observed with preimmune serum nor with immune serum adsorbed prior to use with pure MAP 2. We detected MAP 2 on intermediate filaments not only with anti-MAP 2 serum, but also with affinity purified anti-MAP 2 and with a monoclonal anti-MAP 2 prepared in another laboratory. We conclude from these experiments that material recognized by anti-MAP 2 antibodies associates with both microtubules and intermediate filaments. We propose that one function of MAP 2 is to cross-link the two types of cellular filaments.     

The pool of map kinase associated with microtubules is small but constitutively active.

Mitogen-activated protein kinase (MAPK) is activated by many kinds of stimuli and plays an important role in integrating signal transduction cascades. MAPK is present abundantly in brain, where we have studied its association with microtubules. Immunofluorescence of primary hippocampal neurons revealed that MAPK staining co-localized with microtubules and biochemical analyses showed that MAPK co-purified with microtubules. Approximately 4% of MAPK in cytosolic extracts was associated with microtubules, where it was associated with both tubulin and microtubule-associated proteins (MAPs) fractions. Further fractionation of MAPs suggested that a portion of MAPK is associated with MAP2. An association with MAP2 was also demonstrated by co-immunoprecipitation and in vitro binding experiments. A similar association was shown for the juvenile MAP2 isoform, MAP2C. The pool of MAPK associated with microtubules had a higher activity relative to the nonassociated pool in both brain and proliferating PC12 cells. Although MAPK was activated by nerve growth factor in PC12 cells, the activity of microtubule-associated MAPK did not further increase. These results raise the possibility that microtubule-associated MAPK operates through constitutive phosphorylation activity to regulate microtubule function in neurons.     

Identification of a 100 kD protein associated with microtubules, intermediate filaments and coated vesicles in cultured cells

We have obtained several hybridoma clones producing antibodies to microtubule-associated proteins (MAPs) from bovine brain. Interaction of one of these antibodies, named RN 17, with cultured cells was studied by indirect immunofluorescence and immunoelectron microscopy. RN 17 antibody recognized both high molecular weight (HMW) MAPs, MAP 1 and MAP 2, in immunoblotting reaction with brain microtubules. In lysates of cultured cells, it bound to a protein doublet with a molecular weight of 100 kD. By immunofluorescence microscopy we showed that RN 17 antibody stained cytoplasmic fibrils, mitotic spindles and small particles in the cytoplasm of various cultured cells. The cytoplasmic fibrils were identified as both microtubules and intermediate filaments by double fluorescence microscopy and by their response to colcemid and 0.6 M KCl. This identification was confirmed by immunoelectron microscopy which also showed that the particles stained by RN 17 antibody are coated vesicles. Thus, cultured non-neural cells may contain a novel protein that binds to microtubules, intermediate filaments, and coated vesicles.     

Structure and composition of the cytoskeleton of nucleated erythrocytes I. The presence of microtubule-associated protein 2 in the marginal band

The marginal band (MB) of nucleated erythrocytes is composed of a bundle of microtubules that encircles the cell immediately beneath the plasma membrane. When cells are lysed in buffer containing Triton X-100 the MB microtubules remain intact, and the nucleus remains suspended at the cell center by a filamentous network called the trans-MB material that connects the nucleus to the peripheral MB. When these lysed cells are prepared for indirect immunofluorescence by use of an antibody to chick brain microtubule-associated protein 2 (MAP 2), intense staining of the MB results; no staining is evident in the areas occupied by the nucleus or the trans-MB material. Controls demonstrate that the staining is specific, because no staining occurs with fluorescent goat antirabbit serum alone or when nonimmune serum is used as the first antibody. Furthermore, the fluorescence of the MB is not affected by pretreatment of the immune serum with purified tubulin, but staining is prevented by pretreatment of the immune serum with purified MAP 2. To determine which protein component of the MB was responsible for the positive immunofluorescence results, 125I-protein A staining was used after the protein components of the isolated cytoskeleton had been resolved by SDS-polyacrylamide gels. Controls showed that the antiserum could react on SDS gels with MAP 2 from purified chick brain microtubules. The results with the cytoskeletal proteins demonstrated that the antiserum reacted only with a high molecular weight protein having a molecular weight similar, but not identical, to that of chick brain MAP 2. Thus, it is concluded that a protein with antigenic characteristics similar to those of chick brain MAP 2 is a component of the MB. The results are discussed in terms of the possible function of MAP 2 in the MB.     

Packing volume of sedimented microtubules: regulation and potential relationship to an intracellular matrix

To determine the contribution of microtubules to a hypothetical intracellular matrix, we have analyzed the space occupied by microtubules in vitro. Taxol-stabilized microtubules assembled from purified (three-times-cycled) bovine brain microtubule protein were pelleted by centrifugation under standardized conditions. The specific volume of the pellet, defined as the microliter volume per milligram protein, was 22.4. As suggested by others, this volume was strongly dependent on microtubule-associated proteins (MAPs), as shown by quantitation of the effects of purified MAP supplementation on specific volume. The specific volumes of microtubule pellets stripped of MAPs by high salt or chymotryptic digestion approached the mathematically optimal (least occupied space) and increased 14-fold with the highest MAP concentrations employed. Packing was also dependent on pH. Specific volumes comparable to those of MAP-depleted microtubules were attainable at pH's from 5.5 to 6.0, and specific volumes more than doubled at pH 7.5. MAP content was unaffected by pH. We present a theoretical analysis that suggests that as microtubules are centrifuged the mixture behaves as a liquid crystal. With packing, the mixture undergoes an isotropic-nematic phase transition in which the microtubules become oriented principally as parallel rods, mimicking their orientation in vivo. From the known concentration of microtubules in vivo, it can be inferred from our measurements that in some cells a large fraction, perhaps 40-50% of the cytosolic volume, is occupied by microtubules that form a mechanically irreducible space. Further theoretical analysis employing Ogston's formulation of the penetrability of fibrous networks suggests that the space between microtubules (in contrast to the extracellular matrix) imposes little barrier to the diffusion of macromolecules. A microtubule array thus achieves mechanical stability without affecting transport by diffusion. The space can accommodate other fibrous networks that could then affect transport, and, as we show, the space itself may be regulated by MAP content and intracellular pH.     

Microtubule-associated protein 2 within axons of spinal motor neurons: associations with microtubules and neurofilaments in normal and beta,beta'-iminodipropionitrile-treated axons

We have examined the distribution of microtubule-associated protein 2 (MAP2) in the lumbar segment of spinal cord, ventral and dorsal roots, and dorsal root ganglia of control and beta,beta'-iminodipropionitrile- treated rats. The peroxidase-antiperoxidase technique was used for light and electron microscopic immunohistochemical studies with two monoclonal antibodies directed against different epitopes of Chinese hamster brain MAP2, designated AP9 and AP13. MAP2 immunoreactivity was present in axons of spinal motor neurons, but was not detected in axons of white matter tracts of spinal cord and in the majority of axons of the dorsal root. A gradient of staining intensity among dendrites, cell bodies, and axons of spinal motor neurons was present, with dendrites staining most intensely and axons the least. While dendrites and cell bodies of all neurons in the spinal cord were intensely positive, neurons of the dorsal root ganglia were variably stained. The axons of labeled dorsal root ganglion cells were intensely labeled up to their bifurcation; beyond this point, while only occasional central processes in dorsal roots were weakly stained, the majority of peripheral processes in spinal nerves were positive. beta,beta'- Iminodipropionitrile produced segregation of microtubules and membranous organelles from neurofilaments in the peripheral nervous system portion and accumulation of neurofilaments in the central nervous system portion of spinal motor axons. While both anti-MAP2 hybridoma antibodies co-localized with microtubules in the central nervous system portion, only one co-localized with microtubules in the peripheral nervous system portion of spinal motor axons, while the other antibody co-localized with neurofilaments and did not stain the central region of the axon which contained microtubules. These findings suggest that (a) MAP2 is present in axons of spinal motor neurons, albeit in a lower concentration or in a different form than is present in dendrites, and (b) the MAP2 in axons interacts with both microtubules and neurofilaments.     

Bovine platelets contain a 280 kDa microtubule-associated protein antigenically related to brain MAP 2.

Resting bovine platelets contain a microtubule coil which reorganizes into linear arrays upon thrombin activation. Microtubule arrays in both resting and activated platelets are extensively cross-linked. In an effort to determine the proteins responsible for this cross-linking, we have developed a method to isolate taxol-stabilized microtubule coils directly from platelet-rich plasma. Negatively stained coils are still cross-linked, and fine filamentous projections are seen between adjacent microtubules. Critical-point-dried rotary shadowed replicas of these coils most clearly demonstrate the projections radiating from individual microtubules as well as along the microtubule coil. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis of isolated coils shows many microtubule-associated proteins (MAPs) present in addition to tubulin. One of these proteins, a 280 kDa MAP, cross-reacts with an antibody to bovine brain MAP 2 by immunoblot analysis. Immunofluorescence localization of this protein with both monoclonal and polyclonal antibodies demonstrates that it is associated with the microtubule coil in resting platelets and with the linear microtubule array present after thrombin activation. Immunoelectron microscopic localization demonstrates that projections from individual microtubules are labeled by the antibodies. We suggest that this MAP, along with several other potential MAPs, is responsible for the cross-linking and stability of bovine platelet microtubules.     

PC12 cells express juvenile microtubule-associated proteins during nerve growth factor-induced neurite outgrowth

Microtubule-associated proteins (MAPs) are believed to play an important role in regulating the growth of neuronal processes. The nerve growth factor-induced differentiation of PC12 pheochromocytoma cells is a widely used tissue culture model for studying this mechanism. We have found that contrary to previous suggestions, the major MAPs of adult brain, MAP1 and MAP2, are minor components of PC12 cells. Instead two novel MAPs characteristic of developing brain, MAP3 and MAP5, are present and increase more than 10-fold after nerve growth factor treatment; the timing of these increases coinciding with the bundling of microtubules and neurite outgrowth. Immunocytochemical staining showed that MAP3 and MAP5 are initially distributed throughout the cytoplasm. Subsequently MAP5 becomes associated with microtubules in both neurites and growth cones but MAP3 distribution remained diffuse. Thus MAP3 and MAP5, which are characteristic of developing neurons in the juvenile brain, are also induced in PC12 cells during neurite outgrowth in culture. In contrast MAP1, which is characteristic of mature neurons, does not increase during PC12 cell differentiation. These results provide evidence that one set of MAPs is expressed during neurite outgrowth and a different set during the maintenance of neuronal form. It also appears that the PC12 system is an appropriate model for studying the active neurite growth phase of neuronal differentiation but not for neuronal maturation.     

Developmental regulation of two microtubule-associated proteins (MAP2 and MAP5) in the embryonic avian retina

Previous studies with the mammalian brain have shown that the expression of a number of neuronal microtubule-associated proteins (MAPs) is developmentally regulated. For example, the low-molecular-weight form of MAP2 (MAP2c) is abundant in neonatal rat brains and is less abundant in adults. Similarly, MAP5 levels decrease during postnatal development. Using monoclonal antibodies, we have followed the time of first appearance, cellular distribution, and molecular form of MAP2 and MAP5 during the morphogenesis of the quail retina. MAP2 first appears in ganglion cell bodies and in the axons of the optic fibre layer (OFL) at embryonic day 4 (E4). Anti-MAP2 staining remains restricted to these sites until E10, when staining appears in the inner plexiform layer (IPL). At E14, one day before hatching, anti-MAP2 staining is found in three broad laminae in the IPL, as well as in photosensitive cells. MAP5 is present in ganglion cell axons from the onset of neurite elongation at E3 and is limited to the OFL until E10. The intensity of anti-MAP5 staining in the OFL and optic nerve decreases after E7, which corresponds with a decrease in the number of actively growing ganglion cell axons. By E14, anti-MAP5 stains five layers in the IPL that correspond with layers of amacrine cell process arborizations. Western blots of E10 brain microtubule proteins show that MAP2 is represented by both a 260 x 10(3) Mr protein and a 60-65 x 10(3) Mr protein; the latter is much more abundant. Anti-MAP5 recognizes a 320 x 10(3) Mr brain microtubule protein in both the quail and the rat. We conclude that the cellular distribution, developmental regulation and molecular forms of MAP2 and MAP5 are similar in the rat and quail, suggesting that these molecules have conserved and hence fundamental roles in the growth and differentiation of neuronal processes.     

Immunocytochemical localization of microtubule-associated protein 1 in rat cerebellum using monoclonal antibodies

Immunohistochemical staining with monoclonal antibodies showed that microtubule-associated protein 1 (MAP1) has a restricted cellular distribution in the rat cerebellum. Anti-MAP1 staining was found only in neurons, where it was much stronger in dendrites than in axons. There were striking variations in the apparent concentration of MAP1 in different classes of neurons. Purkinje cells were the most strongly labeled, while granule cell neurons gave a faint, threshold-level reaction with the antibody. The reaction of Golgi neurons was intermediate between these two extremes. Equivalent results were obtained using two different methods of tissue preparation. Thus MAP1 appears to be a neuron-specific protein that is highly concentrated in dendrites and occurs at markedly different levels in different types of neurons. These observations provide further indications of heterogeneity among brain microtubules.     

Widespread distribution of the major polypeptide component of MAP 1 (microtubule-associated protein 1) in the nervous system

We prepared a monoclonal antibody to microtubule-associated protein 1 (MAP 1), one of the two major high molecular weight MAP found in microtubules isolated from brain tissue. We found that MAP 1 can be resolved by SDS PAGE into three electrophoretic bands, which we have designated MAP 1A, MAP 1B, and MAP 1C in order of increasing electrophoretic mobility. Our antibody recognized exclusively MAP 1A, the most abundant and largest MAP 1 polypeptide. To determine the distribution of MAP 1A in nervous system tissues and cells, we examined tissue sections from rat brain and spinal cord, as well as primary cultures of newborn rat brain by immunofluorescence microscopy. Anti-MAP 1A stained white matter and gray matter regions, while a polyclonal anti-MAP 2 antibody previously prepared in this laboratory stained only gray matter. This confirmed our earlier biochemical results, which indicated that MAP 1 is more uniformly distributed in brain tissue than MAP 2 (Vallee, R.B., 1982, J. Cell Biol., 92:435-442). To determine the identity of cells and cellular processes immunoreactive with anti-MAP 1A, we examined a variety of brain and spinal cord regions. Fibrous staining of white matter by anti-MAP 1A was generally observed. This was due in part to immunoreactivity of axons, as judged by examination of axonal fiber tracts in the cerebral cortex and of large myelinated axons in the spinal cord and in spinal nerve roots. Cells with the morphology of oligodendrocytes were brightly labeled in white matter. Intense staining of Purkinje cell dendrites in the cerebellar cortex and of the apical dendrites of pyramidal cells in the cerebral cortex was observed. By double-labeling with antibodies to MAP 1A and MAP 2, the presence of both MAP in identical dendrites and neuronal perikarya was found. In primary brain cell cultures anti-MAP 2 stained predominantly cells of neuronal morphology. In contrast, anti-MAP 1A stained nearly all cells. Included among these were neurons, oligodendrocytes and astrocytes as determined by double-labeling with anti-MAP 1A in combination with antibody to MAP 2, myelin basic protein or glial fibrillary acidic protein, respectively. These results indicate that in contrast to MAP 2, which is specifically enriched in dendrites and perikarya of neurons, MAP 1A is widely distributed in the nervous system.     

MAP 4: occurrence in mouse tissues

A polyclonal antiserum to a microtubule-associated protein (MAP) from mouse neuroblastoma cells (MAP 4) was used to examine the distribution of this protein in mouse tissues. Immunoblots of neuroblastoma cell microtubule protein preparations demonstrated that the antiserum reacted with a triplet of proteins at 215,000-240,000 mol wt. Antibodies affinity purified from any of the bands showed cross- reaction with the other bands, indicating these polypeptides were all immunologically related. Antibodies specific to MAP 4 decorated microtubules in cultured murine cells fixed with glutaraldehyde, and diffuse staining was seen following treatment of cells with nocodazole. The antiserum reacted with MAP 4 in extracts of brain, heart, liver, and lung from adult mouse; the triplet in brain was more closely spaced than in the other tissues or neuroblastoma cells. In kidney, spleen, and stomach, only a single band (band 4) was labeled; this band was immunologically related to the triplet and was also present in all tissues positive for the triplet. Skeletal muscle, sperm, and peripheral blood contained no reactive polypeptides. After taxol- induced polymerization, the MAP 4 triplet was preferentially associated with the microtubule pellet whereas band 4 remained in the supernatant. These data indicate that there is tissue specificity in the distribution of MAP 4, and that some tissues contain a polypeptide related to MAP 4 (band 4) that does not bind to microtubules in vitro.     

Localization of kinesin in cultured cells

Kinesin was isolated from bovine brain and used to elicit polyclonal antibodies in rabbits. The specificities of the resulting antibodies were evaluated by immunoblotting. Antibodies purified from these sera by their affinity for brain kinesin react with a polypeptide of approximately 120 kD in extracts from bovine brain, PtK1 cells, and mouse neuroblastoma cells. They bind to a pair of polypeptides of approximately 120 kD present in crude kinesin prepared from Xenopus eggs and with a single polypeptide of approximately 115 kD in extracts from Drosophila embryos. Antibodies raised against kinesin prepared from fruit fly embryos (by W. M. Saxton, Indiana University, Bloomington, IN) and from neural tissues of the squid (by M. P. Sheetz, Washington University, St. Louis, MO) cross react with the mammalian, the fly, and the frog polypeptides. Kinesin antigen was localized in cultured cells by indirect immunofluorescence. PtK1 cells in interphase showed dim background staining of cytoplasmic membranous components and bright staining of a small, fibrous, juxtanuclear structure. Double staining with antibodies to microtubules showed that the fibrous object was usually located near the centrosome. On the basis of shape, size, and location, we identify the kinesin-positive structure as a primary cilium. PtK1 cells in mitosis are stained at their poles during all stages of division. The structure stained is approximately spherical, but wisps of faint fluorescence also extend into the body of the spindle. Antibodies to squid or fruit fly kinesin produce identical patterns in PtK1 cells. Controls with preimmune and preabsorbed sera show that the centrosome staining is not due simply to the common tendency of rabbit antisera to stain this structure. Similar centrosome and spindle pole staining was visible when antibodies to bovine brain or squid kinesin were applied to the A6 cell line (kidney epithelial cells from Xenopus laevis). Some possible functions of kinesin localized at the spindle poles are discussed.     

Widespread cellular distribution of MAP-1A (microtubule-associated protein 1A) in the mitotic spindle and on interphase microtubules

In the accompanying paper (Bloom, G.S., T.A. Schoenfeld, and R.B. Vallee, 1983, J. Cell Biol. 98:320-330), we reported that microtubule-associated protein 1 (MAP 1) from brain comprises multiple protein species, and that the principal component, MAP 1A, can be detected in both neuronal and glial cells by immunofluorescence microscopy using a monoclonal antibody. In the present study, we sought to determine the cellular and subcellular distribution of MAP 1A in commonly used cultured cell systems. For this purpose we used immunofluorescence microscopy and immunoblot analysis with anti-MAP 1A to examine 18 types of mammalian cell cultures. MAP 1A was detected in every culture system examined. Included among these were cells of mouse, rat, Chinese hamster, Syrian hamster, Potoroo (marsupial), and human origin derived from a broad variety of tissues and organs. Anti-MAP 1A consistently labeled mitotic spindles and stained cytoplasmic fibers during interphase in most of the cultures. These fibers were identified as microtubules by co-localization with tubulin in double-labeling experiments, by their disappearance in response to colchicine or vinblastine, and by their reorganization in response to taxol. The anti-MAP 1A stained microtubules in a punctate manner, raising the possibility that MAP 1A is located along microtubules at discrete foci that might represent sites of interaction between microtubules and other organelles. Verification that MAP 1A was, indeed, the reactive material in immunofluorescence microscopy was obtained from immunoblots. Anti-MAP 1A stained a band at the position of MAP 1A in all cultures examined. These results establish that MAP 1A, a major MAP from brain, is widely distributed among cultured mammalian cells both within and outside of the nervous system.     

The novel microtubule-associated protein MAP3 contributes to the in vitro assembly of brain microtubules

MAP3 is a novel microtubule-associated protein found in brain and a variety of other tissues (Huber, G., Alaimo-Beuret, D., and Matus, A. (1985) J. Cell Biol. 100, 496-507). In this study, monoclonal antibodies were used to assess its influence on the polymerization of brain tubulin. When added to unpolymerized brain microtubules, anti-MAP3 IgG produced a dose-related inhibition of subsequent assembly. Under the same circumstances, nonimmune mouse IgG did not influence either the rate or the extent of tubulin polymerization. We also used immobilized antibodies to deplete brain MAPs selectively in either MAP3 or MAP1. MAP3-depleted MAPs showed a reproducible decrease in activity compared to control preparations that had been exposed to immobilized nonimmune IgG. MAP1-depleted MAPs did not differ significantly in performance from the nonimmune treated controls. We conclude that MAP3 contributes to the net assembly of brain microtubules observed in vitro. This may be particularly relevant in neonatal animals where brain MAP3 is more abundant than in the adult.     

Green algal microtubule-associated protein with a molecular weight of 90 kDa which bundles microtubules

Summary Cytoplasmic streaming in the freshwater, coencytic green alga,Dichotomosiphon tuberosus, is regulated by light. Conspicuous changes are observed in the number of microtubules cross-linked together in bundles when the cytoplasmic streaming is modulated by light. In an attempt to identify the cross-linker, we stainedD. tuberosus cells with antibodies specific for several different microtubules-associated proteins (MAPs) from vertebrates. Antibodies raised against bovine adrenal 190 kDa MAP stained the algal cells, and the pattern of staining was quite similar to that obtained with tubulin-specific antibodies. Examination by immunoelectron microscopy revealed that the antibodies specific for the 190 kDa microtubule-associated protein (MAP) were located along the microtubules. Western blotting demonstrated that the antibodies crossreacted with a peptide fromD. tuberosus with a molecular weight of about 90 kDa. This peptide was heat-stable, a property shared by the bovine 190 kDa MAP. Moreover, this 90 kDa peptide, crossreacted with antibodies raised against a synthetic peptide, identical to the tubulin-binding domain found in the 190 kDa MAP and in a tau protein. Partially purified 90 kDa protein fromD. tuberosus has the ability to bundle microtubules when mixed with a tubulin fraction fromD. tuberosus, in the presence of taxol. These results suggest that the 90 kDa protein fromD. tuberosus is a MAP that bundles microtubules.Abbreviations APMSF (p-amidinophenyl) methanesulfonyl fluoride - BSA bovine serum albumin - CBB Coomassie Brilliant Blue R - DEAE diethylaminoethyl - DMSO dimethyl sulfoxide - DOC deoxycholic acid - DTT dithiothreitol - EDTA ethylenediaminetetraacetic acid - EGTA ethyleneglycol-bis-(-aminoethyl ether)-N,N,N,N-tetraacetic acid - FITC fluorescein isothiocyanate - MAP microtubule-associated protein - MES 2[N-morpholino] ethanesulfonic acid - PBS phosphate-buffered saline - PIPES piperazine-N,N-bis[2-ethane-sulfonic acid] - TLCK N-p-tosyl-lysine chloromethyl ketone     

Developmental regulation of cardiac MAP4 protein expression

It has been shown that the level of expression of microtubule-associated protein 4 (MAP4) mRNAs changes throughout neonatal heart development [Chapin SJ, et al. 1995. Biochemistry 34:2289]. In the present study, both immunofluorescence and western blotting methods were used to monitor MAP4 protein expression levels in the developing heart. By both methods, it was shown that the levels of total MAP4 protein were maximal during the first postnatal week, and then declined progressively to adulthood. In addition, four major electrophoretic species that reacted with MAP4-specific antibodies (called bands 1-4) were observed in all heart tissue samples. Three of the four bands decreased in abundance throughout postnatal development, but at different rates. The fourth band remained relatively constant in abundance with increasing postnatal age. To determine if phosphorylation events might contribute to this heterogeneity, western blotting experiments using phospho-specific antibodies and phosphatase digestion of extract samples were performed. No phosphorylation-specific antibody staining was observed and no significant changes were demonstrated in the bands after phosphatase treatment, implying that the observed complexity was due mainly to alternative start site or differential isoform expression. Finally, it was discovered that cardiomyocyte MAP4 associated with drug- and cold-stable microtubules in early neonatal myocytes. Thus, the complex regulation of MAP4 protein expression may play a key role in the functional differentiation of myocyte microtubules during heart development.     

Interaction of microtubule-associated protein-2 and p63: a new link between microtubules and rough endoplasmic reticulum membranes in neurons

Neurons are polarized cells presenting two distinct compartments, dendrites and an axon. Dendrites can be distinguished from the axon by the presence of rough endoplasmic reticulum (RER). The mechanism by which the structure and distribution of the RER is maintained in these cells is poorly understood. In the present study, we investigated the role of the dendritic microtubule-associated protein-2 (MAP2) in the RER membrane positioning by comparing their distribution in brain subcellular fractions and in primary hippocampal cells and by examining the MAP2-microtubule interaction with RER membranes in vitro. Subcellular fractionation of rat brain revealed a high MAP2 content in a subfraction enriched with the endoplasmic reticulum markers ribophorin and p63. Electron microscope morphometry confirmed the enrichment of this subfraction with RER membranes. In cultured hippocampal neurons, MAP2 and p63 were found to concomitantly compartmentalize to the dendritic processes during neuronal differentiation. Protein blot overlays using purified MAP2c protein revealed its interaction with p63, and immunoprecipitation experiments performed in HeLa cells showed that this interaction involves the projection domain of MAP2. In an in vitro reconstitution assay, MAP2-containing microtubules were observed to bind to RER membranes in contrast to microtubules containing tau, the axonal MAP. This binding of MAP2c microtubules was reduced when an anti-p63 antibody was added to the assay. The present results suggest that MAP2 is involved in the association of RER membranes with microtubules and thereby could participate in the differential distribution of RER membranes within a neuron.     

Identification of the 190 kD microtubule-associated protein in cultured fibroblasts and its association with interphase and mitotic microtubules

We previously investigated the biochemical characteristics of microtubule-associated proteins (MAPs) of the adrenal medulla and adrenal cortex and found that they contain a new kind of MAP with a molecular weight of 190,000 (190 kD MAP) as a major species (Kotani, S., H. Murofushi, S. Maekawa, C. Sato, and H. Sakai. Eur. J. Biochem. 156, 23-29, 1986). We now have used an affinity purified anti-(190 kD MAP) antibody and show by indirect immunofluorescent microscopy the association of this MAP with microtubules in situ in TIG-3 cells (human embryonic lung fibroblasts). The 190 kD MAP was present along the interphase and mitotic microtubules, and there was no marked difference between the staining pattern with anti-tubulin and that with anti-(190 kD MAP) antibodies, evidence that the localization of 190 kD MAP is not restricted to the subset of microtubules. We also isolated MAPs from TIG-3 cells and identified their 190 kD MAP as a major heat-stable component. Several other unidentified polypeptides were recovered in the MAP fraction specifically.     

Cytoskeletal architecture and immunocytochemical localization of microtubule-associated proteins in regions of axons associated with rapid axonal transport: the beta,beta''-iminodipropionitrile-intoxicated axon as a model system

Axons from rats treated with the neurotoxic agent beta,beta'-iminodipropionitrile (IDPN) were examined by quick-freeze, deep-etch electron microscopy. Microtubules formed bundles in the central region of the axons, whereas neurofilaments were segregated to the periphery. Most membrane-bounded organelles, presumably including those involved in rapid axonal transport, were associated with the microtubule domain. The high resolution provided by quick-freeze, deep-etch electron microscopy revealed that the microtubules were coated with an extensive network of fine strands that served both to cross-link the microtubules and to interconnect them with the membrane-bounded organelles. The strands were decorated with granular materials and were irregular in dimension. They appeared either singly or as an extensive anastomosing network in fresh axons. The microtubule-associated strands were observed in fresh, saponin-extracted, or aldehyde-fixed tissue. To explore further the identity of the microtubule-associated strands, microtubules purified from brain tissue and containing the high molecular weight microtubule-associated proteins MAP 1 and MAP 2 were examined by quick-freeze, deep-etch electron microscopy. The purified microtubules were connected by a network of strands quite similar in appearance to those observed in the IDPN axons. Control microtubule preparations consisting only of tubulin and lacking the MAPs were devoid of associated strands. To learn which of the MAPs were present in the microtubule bundles in the axon, sections of axons from IDPN-treated rats were examined by immunofluorescence microscopy using antibodies to MAP 1A, MAP 1B, MAP 2, and tubulin. Anti-MAP 2 staining was only marginally detectable in the IDPN-treated axons, consistent with earlier observations. Anti-MAP 1A and anti-MAP 1B brightly stained the IDPN-treated axons, with the staining exclusively limited to the microtubule domains. Furthermore, thin section-immunoelectron microscopy using colloidal gold-labeled second antibodies revealed that both anti-MAP 1A and anti-MAP 1B stained fuzzy filamentous structures between microtubules. In view of earlier work indicating that rapid transport is associated with the microtubule domain in the IDPN-treated axon, it now appears that MAP 1A and MAP 1B may play a role in this process. We believe that MAP 1A and MAP 1B are major components of the microtubule-associated fibrillar matrix in the axon.