Numerous phosphates of microtubule-associated protein 2 in living rat brain

Phosphoproteomic analysis with a solid-phase capture-release-tag approach

A comprehensive study of global phosphorylation events in biological systems is critical. We report a chemistry-based capture-release-tag method for isolation of complex phospho-Ser/Thr-containing peptides by liquid beta-elimination combined with solid-phase Michael addition. The free thiol groups of 6-(mercapto-acetylamino)-hexanoic acid functionalized resin are used as immobilized Michael donors to capture dehydro-serine/threonine peptides. After an acid-mediated release step, phospho-peptides are labeled with a 6-(2-mercapto-acetylamine)-hexanoic amide tag at phosphorylated sites. We applied this method to analyze the phosphorylation status of microtubule-associated proteins. We find that a CDK5 substrate microtubule-associated protein 2 (MAP2) is phosphorylated on residues that are within a homologous region of Tau. The chemical method corroborates previous results and suggests that Tau and MAP2 may contain a CDK5 phosphorylation motif.

Anesthesia and post-mortem interval profoundly influence the regulatory serine phosphorylation of glycogen synthase kinase-3 in mouse brain

Glycogen synthase kinase-3 (GSK3) is a crucial enzyme contributing to the regulation of neuronal structure, plasticity and survival, is implicated as a contributory factor in prevalent diseases such as Alzheimer's disease and mood disorders and is regulated by a wide range of signaling systems and pharmacological agents. Therefore, factors regulating GSK3 in vivo are currently of much interest. GSK3 is inhibited by phosphorylation of serine-9 or serine-21 in GSK3beta and GSK3alpha, respectively. This study found that accurate measurements of phospho-Ser-GSK3 in brain are confounded by a rapid post-mortem dephosphorylation, with approximately 90% dephosphorylation of both GSK3 isoforms occurring within 2 min post-mortem. Furthermore, three anesthetics, pentobarbital, halothane and chloral hydrate, each caused large in vivo increases in the serine phosphorylation of both GSK3beta and GSK3alpha in several regions of mouse brain. Thus, studies of the phosphorylation state of GSK3 in brain, and perhaps in other tissues, need to take into account post-mortem changes and the effects of anesthetics and there is a direct correlation between anesthesia and high levels of serine-phosphorylated GSK3.

The phosphorylation state of threonine-220, a uniquely phosphatase-sensitive protein kinase A site in microtubule-associated protein MAP2c, regulates microtubule binding and stability

Phosphorylation of microtubule-associated protein 2 (MAP2) has a profound effect on microtubule stability and organization. In this work a consensus protein kinase A (PKA) phosphorylation site, T(220), of juvenile MAP2c is characterized. As confirmed by mass spectrometry, this site can be phosphorylated by PKA but shows less than average reactivity among the 3.5 +/- 0.5 phosphate residues incorporated into the protein. In contrast, T(220) is uniquely sensitive to dephosphorylation: three major Ser/Thr protein phosphatases, in the order of efficiency PP2B > PP2A(c) > PP1(c), remove this phosphate group first. MAP2c specifically dephosphorylated at this site binds and stabilizes microtubules stronger than either fully phosphorylated or nonphosphorylated MAP2c. Phosphorylation of this site also affects proteolytic sensitivity of MAP2c, which might represent a further level of control in this system. Thus, the phosphorylation state of T(220) may be a primary determinant of microtubule function.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Nitric oxide affects the phosphorylation state of microtubule-associated protein 2 (MAP-2) and neurofilament: an immunocytochemical study in the brain of rats and neuronal nitric oxide synthase (nNOS)-knockouts

Alterations in function and specific cellular location of cytoskeletal elements are characterized by changes in their phosphorylation state. On this background we studied immunocytochemically the distribution pattern of neurofilament (NF) in its phosphorylated (P-NF) and nonphosphorylated (NP-NF) form and of microtubule-associated protein-2 (MAP-2) in the rat and mouse brain. Neurons that are strongly positive for neuronal nitric oxide synthase (nNOS)-immunoreactivity (IR) showed, interestingly, neither P-NF- nor MAP-2-IR. In contrast, nNOS-negative neuronal cell elements exhibited an intense IR and specific location for both antigens throughout the brain. As a model we chose the dorsolateral tegmental nucleus (LDT) of knockout (nNOS(-/-)) mice in which the main splice isoform nNOSalpha is lacking, but a low nNOS-activity persists, apparently due to the splice isoforms nNOSbeta and gamma. The principal neurons of such nNOS(-/-)-mice, which are equivalent to the nNOS-containing neurons in the LDT of wild-type mice exhibited a decreased nitrotyrosine-IR and an increased phosphotyrosine-IR if compared to those of wild-type mice. The same neurons failed to show NF-IR and MAP-2-IR, though. When the residual nNOS activity in nNOS(-/-)-mice was inhibited by treatment with N-omega-nitro-L-arginine methyl ester (L-NAME) the principal neurons displayed a moderate MAP-2 and NF-staining. NO and NO-derived oxygen species are suggested to modulate the balance between the activities of kinases and phosphatases, thus changing phosphorylation levels for NF, MAP-2, and, possibly, other proteins in neurons and adjacent cell elements.

Microtubule-associated protein autophosphorylation alters in vitro microtubule dynamic instability

While phosphorylation of high-molecular-weight microtubule-associated proteins (MAPs) alters the assembly properties of microtubules in vitro, virtually nothing is known about the influence of MAP phosphorylation on the time-scale of microtubule polymer length redistribution. The latter has been used as an index of microtubule assembly/disassembly turnover as predicted by the dynamic instability model (Mitchison, T.M. and Kirschner, M.W. (1984) Nature 312, 237-242). We have now determined that under conditions leading to the incorporation of 8-10 mol phosphoryl groups per mol MAP-2 (and about 0.2 mol phosphoryl groups per mol MAP-1 and tau), we can reproducibly observe significant acceleration in the polymer length redistribution process in a manner consistent with greater microtubule dynamic instability. We have also found that MAP phosphorylation resulted in more extensive release of MAPs from microtubules as a function of increasing salt concentration. These results are consistent with a weakening of MAP-microtubule interactions upon phosphorylation.

Antipsychotic treatment induces alterations in dendrite- and spine-associated proteins in dopamine-rich areas of the primate cerebral cortex

BACKGROUND

Mounting evidence indicates that long-term treatment with antipsychotic medications can alter the morphology and connectivity of cellular processes in the cerebral cortex. The cytoskeleton plays an essential role in the maintenance of cellular morphology and is subject to regulation by intracellular pathways associated with neurotransmitter receptors targeted by antipsychotic drugs.

METHODS

We have examined whether chronic treatment with the antipsychotic drug haloperidol interferes with phosphorylation state and tissue levels of a major dendritic cytoskeleton-stabilizing agent, microtubule-associated protein 2 (MAP2), as well as levels of the dendritic spine-associated protein spinophilin and the synaptic vesicle-associated protein synaptophysin in various regions of the cerebral cortex of rhesus monkeys.

RESULTS

Among the cortical areas examined, the prefrontal, orbital, cingulate, motor, and entorhinal cortices displayed significant decreases in levels of spinophilin, and with the exception of the motor cortex, each of these regions also exhibited increases in the phosphorylation of MAP2. No changes were observed in either spinophilin levels or MAP2 phosphorylation in the primary visual cortex. Also, no statistically significant changes were found in tissue levels of MAP2 or synaptophysin in any of the cortical regions examined.

CONCLUSIONS

Our findings demonstrate that long-term haloperidol exposure alters neuronal cytoskeleton- and spine-associated proteins, particularly in dopamine-rich regions of the primate cerebral cortex, many of which have been implicated in the psychopathology of schizophrenia. The ability of haloperidol to regulate cytoskeletal proteins should be considered in evaluating the mechanisms of both its palliative actions and its side effects.

Regulated association of microtubule-associated protein 2 (MAP2) with Src and Grb2: evidence for MAP2 as a scaffolding protein

Microtubule-associated protein 2 (MAP2) and tau, which is involved in Alzheimer's disease, are major cytoskeletal proteins in neurons. These proteins are involved in microtubule assembly and stability. To further characterize MAP2, we took a strategy of identifying potential MAP2 binding partners. The low molecular weight MAP2c protein has 11 PXXP motifs that are conserved across species, and these PXXP motifs could be potential ligands for Src homology 3 (SH3) domains. We tested for MAP2 interaction with SH3 domain-containing proteins. All neuronal MAP2 isoforms bound specifically to the SH3 domains of c-Src and Grb2 in an in vitro glutathione S-transferase-SH3 pull-down assay. Interactions between endogenous proteins were confirmed by co-immunoprecipitation using brain lysate. All three proteins were also found co-expressed in neuronal cell bodies and dendrites. Surprisingly, the SH3 domain-binding site was mapped to the microtubule-binding domain that contains no PXXP motif. Src bound primarily the soluble, non-microtubule-associated MAP2c in vitro. This specific MAP2/SH3 domain interaction was inhibited by phosphorylation of MAP2c by the mitogen-activated protein kinase extracellular signal-regulated kinase 2 but not by protein kinase A. This phosphorylation-regulated association of MAP2 with proteins of intracellular signal transduction pathways suggests a possible link between cellular signaling and neuronal cytoskeleton, with MAP2 perhaps acting as a molecular scaffold upon which cytoskeleton-modifying proteins assemble and dissociate in response to neuronal activity.

GSK3beta-mediated phosphorylation of the microtubule-associated protein 2C (MAP2C) prevents microtubule bundling

A major determinant of neuronal morphology is the cytoskeleton. And one of the main regulatory mechanisms of cytoskeletal proteins is the modification of their phosphorylation state via changes in the relative activities of protein kinases and phosphatases in neurons. In particular, the microtubule-associated protein 2 (MAP2) family of proteins are abundant cytoskeletal components predominantly expressed in neurons and have been found to be substrates for most of protein kinases and phosphatases present in neurons, including glycogen-synthase kinase 3 (GSK3). It has been suggested that changes in GSK3-mediated MAP phosphorylation may modify MT stability and could control neuronal development. We have previously shown that MAP2 is phosphorylated in vitro and in situ by GSK3 at Thr1620 and Thr1623, located in the proline-rich region of MAP2 and recognized by antibody 305. However, the function of the phosphorylation of this site of MAP2 is still unknown. In this study, non-neuronal COS-1 cells have been co-transfected with cDNAs encoding MAP2C and either wild type or mutated GSK3beta to analyze possible effects on microtubule stability and on the association of MAP2 with microtubules. We have found that GSK3beta phosphorylates MAP2C in co-transfected cells. Moreover, this phosphorylation is inhibited by the specific GSK3 inhibitor lithium chloride. Additionally, the formation of microtubule bundles, which is observed after transfection with MAP2C, was decreased when MAP2C was co-transfected with GSK3beta wild type. Microtubule bundles were not observed in cells expressing MAP2C phosphorylated at the site recognized by antibody 305. The absence of microtubule bundles was reverted after treatment of MAP2C/GSK3beta wild type transfected cells with lithium chloride. Highly phosphorylated MAP2C species, which were phosphorylated at the site recognized by antibody 305, appeared in cells co-transfected with MAP2C and GSK3beta wild type. Interestingly, these MAP2C species were enriched in cytoskeleton-unbound protein preparations. These data suggests that GSK3-mediated phosphorylation of MAP2 may modify its binding to microtubules and regulate microtubule stability.

Regulation of phosphorylation of neuronal microtubule-associated proteins MAP1b and MAP2 by protein phosphatase-2A and -2B in rat brain

The function of the neuronal high molecular weight microtubule-associated proteins (MAPs) MAP1b and MAP2 is regulated by the degree of their phosphorylation, which in turn is controlled by the activities of protein kinases and protein phosphatases (PP). To investigate the role of PP in the regulation of the phosphorylation of MAP1b and MAP2, we used okadaic acid and cyclosporin A to selectively inhibit PP2A and PP2B activities, respectively, in metabolically competent rat brain slices. The alteration of the phosphorylation levels of MAP1b and MAP2 was examined by Western blots using several phosphorylation-dependent antibodies to these proteins. The inhibition of PP2A, and to a lesser extent of PP2B, was found to induce an increased phosphorylation of MAP1b and inhibit its microtubule binding activity. Immunocytochemically, a marked increase in neuronal staining in inhibitor-treated tissue was observed with antibodies to the phosphorylated MAP1b. The inhibition of PP2A but not of PP2B also induced phosphorylation of MAP2 at multiple sites and impaired its microtubule binding activity. These results suggest that PP2A might be the major PP that participates in regulation of the phosphorylation of MAP1b and MAP2 and their biological activities.

Effect of serine and tyrosine phosphorylation on retroviral proteinase substrates

Vimentin, a cellular substrate of HIV type 1 (HIV-1) proteinase, contains a protein kinase C (PKC) phosphorylation site at one of its cleavage sites. Peptides representing this site were synthesized in P2 Ser-phosphorylated and nonphosphorylated forms. While the nonphosphorylated peptide was a fairly good substrate of the enzyme, phosphorylation prevented hydrolysis. Phosphorylation of human recombinant vimentin by PKC prevented its processing within the head domain, where the phosphorylation occurred. Oligopeptides representing naturally occurring cleavage sites at the C-terminus of the Rous sarcoma virus integrase were assayed as substrates of the avian proteinase. Unlike the nonphosphorylated peptides, a Ser-phosphorylated peptide was not hydrolyzed by the enzyme at the Ser-Pro bond, suggesting the role of previously established phosphorylation in processing at this site. Ser-phosphorylated and Tyr-phosphorylated forms of model substrates were also tested as substrates of the HIV-1 and the avian retroviral proteinases. In contrast to the moderate effect of P4 Ser phosphorylation, phosphorylation of P1 Tyr prevented substrate hydrolysis by HIV-1 proteinase. Substrate phosphorylation had substantially smaller effects on the hydrolysis by the avian retroviral proteinase. As the active retroviral proteinase as well as various protein kinases are incorporated into mature virions, substrate phosphorylation resulting in attenuation or prevention of proteolytic processing may have important consequences in the regulation of the retroviral life cycle as well as in virus-host cell interactions.

Hippocampal microtubule-associated protein-2 alterations with contextual memory

Using immunohistochemistry and immunoblots, we show that alterations in hippocampal microtubule-associated protein-2 appear to be highly correlated with contextual memory as measured by significantly heightened fear responses. Compared to naive controls, rats trained in a novel context showed significantly increased immunostaining for the high molecular weight microtubule-associated protein-2a/b. This increase was observed 2 weeks after training and it was selective for hippocampal CA1 and CA2 pyramidal cells. Pre-exposure to the training context one month before training altered the hippocampal microtubule-associated protein-2 response; in these animals only the dentate gyrus showed significantly increased microtubule-associated protein-2a/b. Training-related increases in immunohistochemical staining for microtubule-associated protein-2 suggested that there was an increase in overall intact protein, an increase in immunoreactive breakdown products, or changes in protein compartmentalization. Immunoblots of hippocampal homogenates reacted with monoclonal antibodies to microtubule-associated protein-2a/b showed an increased presence of breakdown products in trained animals compared to untrained controls. Additional immunoblot studies demonstrated statistically significant decreases in the levels and/or phosphorylation state of the low molecular weight microtubule-associated protein-2c in the hippocampus of trained animals as compared to that of controls. These alterations in microtubule-associated protein-2 may reflect dendritic remodeling related to contextual memory storage.

Regulation of a site-specific phosphorylation of the microtubule-associated protein 2 during the development of cultured neurons

The phosphorylation state of cytoskeletal proteins, including certain microtubule-associated proteins, may influence the development and plasticity of axons and dendrites in mammalian neuron in response to appropriate extracellular stimuli. In particular, high molecular weight microtubule-associated protein 2, has been implicated in dendrite growth and synaptic plasticity and is thought to be modulated by phosphorylation and dephosphorylation. We have previously determined that threonines 1620/1623 on the microtubule-associated protein 2 molecule become phosphorylated in vivo and are targets for proline-directed protein kinases in vitro. Using the phosphorylated site-specific antibody 305, we now report the decreased phosphorylation state of high molecular weight microtubule-associated protein 2 during the development of cultured cerebellar granule neurons. Phosphorylation of high molecular weight microtubule-associated protein 2 at this site is significantly inhibited by lithium in short-term cultured neurons, which suggests the implication of glycogen synthase kinase-3. In long-term cultured neurons, it is also partially inhibited by PD098059, an inhibitor of extracellular signal-regulated protein kinase activation, which indicates an additional contribution of this kinase to high molecular weight microtubule-associated protein 2 phosphorylation at this stage. Both in short-term and long-term cultured neurons, okadaic acid augments high molecular weight microtubule-associated protein 2 phosphorylation at this site through the inhibition of protein phosphatases 1 and/or 2A. Finally, glutamate receptor activation leads to a dephosphorylation of high molecular weight microtubule-associated protein 2 at this site which can also be effectively prevented by okadaic acid. These results are consistent with the participation of glycogen synthase kinase-3, extracellular signal-regulated protein kinases and protein phosphatases 1 and 2A, in the regulation of microtubule-associated protein 2 phosphorylation within living neurons, which may be modulated by extracellular signals like the neurotransmitter glutamate.

MAP2 expression in developing dendrites of human brainstem auditory neurons

Immunostaining of cytoskeletal elements has proved to be a useful technique for tracing ontogenetic development in the human central auditory system. In the present study, dendritic development in brainstem auditory nuclei (dorsal and ventral cochlear nuclei, medial and lateral superior olivary nuclei, and inferior colliculus) was studied using an antibody to a microtubule-associated protein, MAP2, a molecule which stabilizes dendritic processes by promoting assembly of microtubules. At 21-22 weeks of gestation, cells within the auditory nuclei first demonstrate cytoplasmic MAP2 immunoreactivity, but no dendritic structures have formed. Filamentous background staining at this stage may represent immunoreactivity in astrocytic processes. By the 24th fetal week, somata of auditory neurons are strongly immunostained and have developed short dendritic processes. During the perinatal period, dendrites extend up to 100-120 microm in length but are still sparsely branched and lack terminal formations. By the sixth postnatal month, neurons in all auditory nuclei have acquired dendritic arbors with a mature appearance. Thus MAP2 immunohistochemistry demonstrates that dendrogenesis in human brainstem auditory nuclei begins 16 weeks prior to term birth but does not reach the stage of mature dendritic morphology until several months into the postnatal period. This extended course of development implies a significant period of time during which neuronal activity could influence dendritic structure and function.

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

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

Synaptic metaplasticity and the local charge effect in postsynaptic densities

Synaptic plasticity might be one of the elementary processes that underlies higher brain functions, such as learning and memory. Intriguingly, the capacity of a synapse for plastic changes itself displays marked variation or plasticity. This higher-order plasticity, or metaplasticity, appears to depend on the same macromolecules as plasticity, most notably the NMDA receptor and Ca2+/calmodulin kinase II; yet we do not understand metaplasticity in molecular terms. Metaplasticity has a feedback-inhibition character that confers stability to synaptic patterns, whereas in plasticity, the molecular events implicated tend to have an opposite effect. As a resolution to this difference, we suggest that metaplasticity be considered in a biophysical context. It has been shown that autophosphorylation of Ca2+/calmodulin kinase II in postsynaptic densities generates changes in the local electrostatic potential sufficient to affect the direction of synaptic plasticity. We propose that this finding could help explain both the puzzling abundance of Ca2+/calmodulin kinase II in the postsynaptic density and the metaplasticity of synaptic transmission.

Calcium-stimulated phosphorylation of MAP-2 in pancreatic betaTC3-cells is mediated by Ca2+/calmodulin-dependent kinase II

An understanding of the role of CaM kinase II in the pancreatic beta-cell is dependent on the identification of its cellular targets. One of the best substrates of CaM kinase II in vitro that could function in secretory events is the microtubule-associated protein, MAP-2. By immunoblot analysis, a high molecular weight protein with electrophoretic properties characteristic of MAP-2, was identified in rat insulinoma betaTC3 cells and isolated rat islets. In immunoprecipitation experiments employing alpha-toxin-permeabilized betaTC3 cells, elevation of intracellular Ca2+ or addition of forskolin, an adenylate cyclase activator, induced significant phosphorylation of MAP-2 in situ. The effect of Ca2+ was rapid, concentration-dependent and closely correlated with activation of CaM kinase II under similar experimental conditions. H-89, a specific and potent inhibitor of cAMP-dependent protein kinase (PKA), prevented forskolin-induced MAP-2 phosphorylation but had little effect on MAP-2 phosphorylation stimulated by elevated Ca2+. Phosphopeptide mapping revealed that the phosphorylation pattern observed in situ upon incubation of the betaTC3 cells with increased free Ca2+, was strikingly similar to that generated in vitro by CaM kinase II, most notably with regard to the increased phosphate incorporated into one prominent site. These data provide evidence that MAP-2 is phosphorylated by CaM kinase II in the pancreatic beta-cell in situ, and that this event may provide an important link in the mediation of Ca2+-dependent insulin secretion.

Phosphorylation states of microtubule-associated protein 2 (MAP2) determine the regulatory role of MAP2 in microtubule dynamics

Phosphorylation-dependent regulation of microtubule-stabilizing activities of microtubule-associated protein 2 (MAP2) was examined using optical microscopy. MAP2, purified from mammalian brain, was phosphorylated by either cAMP-dependent protein kinase (PKA) or cyclin B-dependent cdc2 kinase. Using PKA, 15 mol of phosphoryl groups was incorporated per mole of MAP2, but about 70% of the phosphates was distributed to the projection region. Using cdc2 kinase, 7-10 mol of phosphoryl groups was incorporated per mole of MAP2, and more than 60% of the phosphates was distributed to the microtubule-binding region. Both types of phosphorylation similarly reduced binding activity of MAP2 onto microtubules. Direct observation of individual microtubules using dark-field microscopy showed that interconversion between the polymerization phase and the depolymerization phase was repeated in both unphosphorylated and PKA-phosphorylated MAP2. In cdc2 kinase-phosphorylated MAP2, however, the phase transition from depolymerization to polymerization occurred with difficulty, with the result being that the half-life of individual microtubules was as short as in the absence of MAP2. Examination of spontaneous polymerization of microtubules using dark-field microscopy showed that the microtubule-nucleating activity of MAP2 was reduced by PKA-dependent phosphorylation and was completely abolished by cdc2 kinase-dependent phosphorylation. These observations show that cdc2 kinase-dependent phosphorylation inhibits both the microtubule-stabilizing activity and the microtubule-nucleating activity of MAP2, while PKA-dependent phosphorylation affects only the microtubule-nucleating activity of MAP2.

NMDA-glutamate receptors regulate phosphorylation of dendritic cytoskeletal proteins in the hippocampus

Most forms of synaptic potentiation need the activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors which generate changes in dendritic morphology of postsynaptic neurons. Since microtubule proteins have an essential role in dendritic morphology, they may be involved and regulated during the modifications of dendritic morphology associated with synaptic potentiation. The phosphorylation of the microtubule-associated proteins (MAPs) has been analyzed in situ after activation or blockade of NMDA-glutamate receptors in hippocampal slices. The phosphorylation of MAP1B and MAP2 has been studied by using several antibodies raised against phosphorylation-sensitive epitopes. Whereas antibodies 125 and 305 recognize phosphorylated epitopes on MAP1B and MAP2, respectively, Ab 842 recognizes a phosphorylatable sequence on MAP1B only when it is dephosphorylated. NMDA treatment decreased the phosphorylation state of the epitope recognized by the antibody 305 on MAP2 and caused a slight dephosphorylation of MAP1B sequences recognized by Ab 125 and 842. Moreover, exposure to APV (an antagonist of NMDA-glutamate receptors) counteracted the effect of NMDA and induced an increase in the phosphorylation state of these sequences in MAP2. Since phosphorylation regulates the interaction of MAPs with cytoskeleton, the results suggest that the modulation of the phosphorylated state of MAP2 by NMDA-glutamate receptors may be implicated in dendritic plasticity.

ADP-ribosylated actin as part of the actin monomer pool in rat brain

Mono-ADP-ribosylation in mammals is poorly understood. In this study, we found mono-ADP-ribosylated actin in rat brains. Mono-ADP-ribosylated actin by ADP-ribosyltransferase or nonenzymatic reaction was shown at a different position from the unmodified actin in the isoelectrical focusing. High-pressure liquid chromatography utilizing a reverse phase (ODS) column separated ADP-ribosylated actin from unmodified actin. In the two-dimensional gel electrophoreses and high-pressure liquid chromatography, the endogenously ADP-ribosylated actin was detected in the supernatant fraction from the rat brain extract, where a nonpolymerizing actin was present after removal of the polymerizing actin. The concentration of NAD and ADP-ribose, after microwave irradiation, was 220 nmol and 150 nmol/g of rat brain tissue. Actin ADP-ribosylated by purified ADP-ribosyltransferase failed to form actin filaments after the addition of Mg2+. Actin ADP-ribosylated by the nonenzymatic reaction could polymerize with the addition of Mg2+. The enzymatically modified actin could form actin filaments after treatment with ADP-ribosylhydrolase but not after treatment with phosphodiesterase. These results suggest that ADP-ribosylated actin by enzymatic or nonenzymatic reaction is one of the sequestering factors in actin-actin binding and is a part of the actin pool in the rat brain.

Phosphorylation and dephosphorylation in the proline-rich C-terminal domain of microtubule-associated protein 2

The C-terminal domain of microtubule-associated protein 2 (MAP2) contains a proline-rich region and the tubulin-binding domain. We have generated antibodies to follow the phosphorylation state of the proline-rich domain. One of these antibodies (no. 305) has been raised against a synthetic peptide P (sequence RTPGTPGTPSY) phosphorylated at the threonine residues. This sequence is present in the proline-rich region of MAP2 and is phosphorylated in vitro by at least three different proline-directed protein kinases: p42mpk, p34cdc2, and GSK3 (glycogen-synthase kinase 3) alpha/beta. The MAP2 sites phosphorylated by these kinases are different, although all of them phosphorylate the C-terminal domain of MAP2 as determined by Staphylococcus aureus V8 protease mapping. Nonphosphorylated peptide P can be phosphorylated in vitro by all three kinases studied with similar efficiency. In high-molecular-mass MAP2, this sequence is highly phosphorylated in vivo at the late stages of rat development. This motif can be rapidly dephosphorylated in vitro by protein-phosphatase 1 (PP1) and 2A (PP2A) catalytic subunits but not by PP2B.

Dephosphorylated but not phosphorylated microtubule associated protein MAP1B binds to microfilaments

We have reported that purified native MAP1B interacts with microtubules but not with microfilaments [Pedrotti and Islam, Cell Motil. Cytoskel. (1995) 30, 301-309]. However, MAP1B can be phosphorylated at multiple sites by casein kinase 11 (CKII) and proline-directed protein kinases (PDPK) and immunoblotting studies show that purified native MAP1B is phosphorylated at least at two CKII sites and at one PDPK site [Pedrotti et al., Biochemistry (1996) 35, 3016-3023]. We now show that phosphorylation affects the in vitro binding of MAP1B with microfilaments. Native MAP1B does not bind to microfilaments but after treatment with alkaline phosphatase the dephosphorylated MAP1B binds and cosediments with microfilaments. Dephosphorylation kinetics suggest that the PDPK site, but not CKII sites, may negatively regulate the interaction with F-actin. The ability of dephosphorylated MAP1B to crosslink microfilaments was also examined and showed that MAP1B exhibits only a weak crosslinking of F-actin when compared with MAP2.

Mutual protection of microtubule-associated protein 2 (MAP2) and cyclic AMP-dependent protein kinase II against mu-calpain

Phosphorylation by adenosine-3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), but not by Ca(++)-calmodulin-dependent protein kinase II (CaMK II), was shown earlier to protect microtubule-associated protein 2 (MAP2) from cleavage by m-calpain (Johnson and Foley: J Neurosci Res 34: 642-647, 1993). We reinvestigated this phenomenon with the physiologically more relevant mu-calpain and found a qualitatively similar but quantitatively different picture. We further demonstrate that 1) protection is biphasically dependent on the degree of phosphorylation; 2) Ca(++)-phospholipid-dependent protein kinase (PKC) has about the same effect as PKA; 3) the effects of kinases A and C are not additive; and 4) stripping of native MAP2 from its phosphate content (17.8 +/- 2.3 mol/mol) enhances calpainolysis 3.6-fold. A reciprocal effect between kinase A and MAP2 was found: the RII, but not the RI, regulatory subunit of kinase A, which was shown to bind specifically to MAP2, is protected by MAP2 from mu-calpain attack. It is suggested that the specific anchoring of kinase A-II on MAP2 may serve not only kinase targeting in the dendrites, but also protection from calpainolytic degradation.

Site-specific regulation of Alzheimer-like tau phosphorylation in living neurons

The microtubule-associated protein tau is more highly phosphorylated at certain residues in developing brain and in Alzheimer's disease paired helical filaments than in adult brain. We examined the regulation of tau phosphorylation at some of these sites in rat brain using the phosphorylation state-dependent anti-tau antibodies AT8, Tau1, and PHF1. The AT8 and PHF1 antibodies bind to phosphorylated tau, while Tau1 binds to unphosphorylated tau. Levels of tau reactive for AT8 were high only during the first postnatal week, with levels in adult declining to approximately 5% of the levels in neonates. In neonatal forebrain slices, tau became rapidly dephosphorylated at the AT8 and Tau1 sites during incubation at 34 degrees C, but was incompletely dephosphorylated at the PHF1 site. Dephosphorylation at AT8 sites, but not at Tau1 or PHF1 sites, was completely inhibited by 1 microM okadaic acid. Hence the regulation of tau phosphorylation by okadaic acid-sensitive phosphatase(s) was site-specific. Addition of 1 microM okadaic acid after dephosphorylation at the AT8 locus yielded a partial recovery of AT8 immunoreactivity, and incubation with basic fibroblast growth factor increased phosphorylation at the AT8 site in a dose-dependent manner. These results indicate that endogenously active and basic fibroblast growth factor stimulated tau kinases directed toward an Alzheimer's disease-related site were present in the slices. In adult brain slices, the small pool of AT8-reactive tau was remarkably insensitive to dephosphorylation during incubation, and okadaic acid treatment induced only small increases in AT8 immunoreactivity. These results suggest that tau phosphorylation in adult brain is less dynamic than in neonatal brain. These findings indicate that neonatal tau is not only phosphorylated more highly than adult tau, but also more dynamically regulated by protein phosphatases and protein kinases than adult tau. The inability of okadaic acid to induce large increases in tau phosphorylation in adult rat brain slices suggests that a loss of protein phosphatase activity alone would not be sufficient to produce the hyperphosphorylation observed in Alzheimer's disease paired helical filaments. Stimulation of kinase activity by basic fibroblast growth factor is likely to modulate tau function during development, and may contribute to the genesis of hyperphosphorylated tau in Alzheimer's disease.

Characterization of microtubule-associated protein MAP1B: phosphorylation state, light chains, and binding to microtubules

We have recently described a procedure for the purification of microtubule associated protein 1B (MAP1B) from calf brain [Pedrotti, B., & Islam K. (1995) Cell Motil. Cytoskeleton 30, 301-309], and this study further characterizes the purified protein and its interaction with microtubules. We show that purified MAP1B (1) is thermostable; (2) is mainly phosphorylated at the casein kinase II (CKII) sites but only partially phosphorylated at the proline-directed protein kinase (PDPK) sites; (3) both the CKII and PDPK sites can be dephosphorylated by alkaline phosphatase; and (4) dephosphorylation results in an increased mobility on SDS-PAGE gels. The ability of MAP1B to interact with microtubules was also examined and shows that (1) phosphorylated (1B-P), alkaline phosphatase-treated (1B-AP), and heat-treated (1B-P), alkaline phosphatase-treated (1B-AP), and heat-treated (1B-HT) MAP1B bind to taxol-stabilized microtubules; (2) 1 mol of 1B-P, 1B-AP, or 1B-HT each binds about 13-14 tubulin dimers; (3) light chain interaction with MAP1B heavy chain is not affected by AP- or heat-treatment; (4) MAP1B can be displaced from taxol-stabilized microtubules by titration with salt; (5) higher salt concentrations are required to displace 1B-AP compared with 1B-P from taxol-stabilized microtubules; and (6) MAP2 is able to displace both 1B-P and 1B-AP from taxol-stabilized microtubules. The role of phosphorylation in regulating MAP1B interaction with microtubules and light chains is discussed.

Regulation of tau phosphorylation in Alzheimer's disease

Alzheimer's disease (AD) is a heterogeneous dementing disorder of the elderly that is characterized by progressive cognitive impairments and the accumulation of abundant amyloid or senile plaques (SPs) and neurofibrillary tangles (NFTs) as well as the massive loss of neurons in the AD brain. Indeed, a secure diagnosis of AD in patients with a chronic progressive dementia requires evidence of numerous SPs and NFTs in the postmortem brain. Although the deposition of fibrillar amyloid or A beta-peptides in extracellular plaques and the accumulation of tau-rich intraneuronal NFTs are not restricted exclusively to AD, there is a close correlation between the burden of tau-rich neurofibrillary lesions in selected telencephalic regions of the brain and the dementia in AD. Since the formation of neurofibrillary lesions from hyperphosphorylated tau proteins may compromise the function and viability of neurons in the AD brain, this review summarizes recent insights into mechanisms that regulate the phosphorylation state of tau in AD.

Partial purification and characterization of two phosvitin phosphatases from rat brain

The mechanism of dephosphorylation of multiphosphorylated proteins in the brain is not well understood. We have used the multiphosphorylated protein, phosvitin as a model substrate and undertaken the purification and characterization of brain phosphatases that preferentially dephosphorylate multiphosphorylated proteins. Two phosvitin phosphatase activities, termed Phosvitin Phosphatase 1 and 2 (PvP1, PvP2), which show acidic pH optima were resolved from the 33,000g supernatant fraction from rat brain by a procedure employing successive DEAE-cellulose, Sepharose 6B, second DEAE-cellulose and FPLC/Superose 6 chromatography steps. Following FPLC/Superose 6 size exclusion chromatography of PvP1 and PvP2, single peaks of phosvitin phosphatase activities were eluted in the range of 160-220 kDa with acidic pH optima. When FPLC/Sepharose 6 chromatography was performed in the presence of 0.5 M NaCl and 0.1% Triton X-100, low molecular mass protein phosphatase forms were produced in addition to the high-M, activity peak, ranging from 25 to 35 kDa (PvP1) and from 15 to 25 kDa (PvP2). Under these conditions, both high- and low-M, forms of PvP1 and PvP2 exhibited neutral pH optima. Both phosphatases dephosphorylate also (i) phosphorylase a, (ii) the alpha and beta subunits of phosphorylase kinase, and (iii) the microtubule-associated protein tau, phosphorylated by cAMP-dependent protein kinase. The present results suggest that two forms of protein phosphatases, displayed molecular and biochemical characteristics both similar and distinct from type 1 and type 2A protein phosphatases, are present in rat brain.

Developmental regulation of MAP2 variants during neuronal differentiation in vitro

Microtubule-associated protein 2 (MAP2) is a major component of the neuronal cytoskeleton and is known to promote the assembly and stabilization of microtubules, functions which have important implications in neuronal differentiation. MAP2 consists of high molecular weight (HMW) proteins MAP2a, MAP2b and a low molecular weight (LMW) isoform MAP2c which are produced from a single gene by alternative splicing. In this study, we describe the expression of the various MAP2 mRNA isotypes and protein isoforms during the development of rat cerebellar granule cell neurons over a 21-day period in vitro. In situ hybridization was used to detect MAP2 mRNA isotypes which corresponded to HMW- and LMW-MAP2 proteins. The distribution of MAP2 mRNAs in the developing P7 cerebellar cortex was related to the different stages of granule neuron development in situ. During early stages of neuronal differentiation in vitro, high levels of MAP2c mRNA were observed which gradually decreased as development progressed. Throughout the period studied, MAP2ab mRNA concentrations remained low although a small transient rise was noted during the first 14 days in vitro (div). The profile of MAP2 protein variants showed further developmental regulation. The expression of the LMW-MAP2c isoform closely mirrored that of its mRNA whilst HMW-MAP2b protein concentrations rose during the first 10 div and were maintained in older cultures. HMW-MAP2a appeared after 4 div and gradually increased throughout the remainder of the study. Clearly, the outline of HMW-MAP2 protein did not relate to its encoding mRNA and such disparity may be due to the operation of different transcriptional and/or posttranslational mechanisms. Immunocytochemical analyses of MAP2 variants provided further information concerning their localization during neurite outgrowth. These results describe the developmental regulation of MAP2 mRNA and protein variants and that the profile of their expression relates to the formation of processes during the differentiation of granule neurons in vitro.

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

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

Microtubule associated protein MAP1A is an actin-binding and crosslinking protein

High molecular weight microtubule-associated proteins MAP1A and MAP2 form thin projections from microtubule surfaces and have been implicated in crosslinking microtubules and other cytoskeletal components. We have purified native MAP1A from bovine brain and have studied its interaction with G- and F-actin. Using a solid-phase immunoassay we show that MAP1A binds in a dose-dependent manner to both G-actin and F-actin. Addition of MAP1A to F-actin causes gelation of F-actin and SDS-PAGE analysis shows that MAP1A co-sediments with the gelled network, under conditions where F-actin alone does not pellet. The low apparent viscosity of F-actin is markedly increased in the presence of MAP1A, suggesting that MAP1A can crosslink F-actin. Co-incubation experiments indicate that MAP1A and MAP2 may bind to common or overlapping sites on the actin molecule. The widespread distribution of MAP1A and its interaction with microtubules, actin, and intermediate filaments suggests that it may constitute an important determinant of neuronal and non-neuronal cellular morphology.

Probing modifications of the neuronal cytoskeleton

The prominent death of central neurons in Alzheimer's and Parkinson's is reflected by changes in cell shape and by the formation of characteristic cytoskeletal inclusions (neurofibrillary tangles, Lewy bodies). This review focuses on the biology of neurofilaments and microtubule-associated proteins and identifies changes that can occur to these elements from basic and clinical research perspectives. Attention is directed at certain advances in neurobiology that have been especially integral to the identification of epitope domains, protein isoforms, and posttranslational (phosphorylation) events related to the composition, development, and structure of the common cytoskeletal modifications. Recently, a number of experimental strategies have emerged to simulate the aberrant changes in neurodegenerative disorders and gain insight into possible molecular events that contribute to alterations of the cytoskeleton. Descriptions of specific systems used to induce modifications are presented. In particular, unique neural transplantation methods in animals have been used to probe possible molecular and cellular conditions concerned with abnormal cytoskeletal changes in neurons.

Okadaic acid induces early changes in microtubule-associated protein 2 and tau phosphorylation prior to neurodegeneration in cultured cortical neurons

Microtubules and their associated proteins play a prominent role in many physiological and morphological aspects of brain function. Abnormal deposition of the microtubule-associated proteins (MAPs), MAP2 and tau, is a prominent aspect of Alzheimer's disease. MAP2 and tau are heat-stable phosphoproteins subject to high rates of phosphorylation/dephosphorylation. The phosphorylation state of these proteins modulates their affinity for tubulin and thereby affects the structure of the neuronal cytoskeleton. The dinoflagellate toxin okadaic acid is a potent and specific inhibitor of protein phosphatases 1 and 2A. In cultured rat cortical neurons and a human neuroblastoma cell line (MSN), okadaic acid induces increased phosphorylation of MAP2 and tau concomitant with early changes in the neuronal cytoskeleton and ultimately leads to cell death. These results suggest that the diminished rate of MAP2 and tau dephosphorylation affects the stability of the neuronal cytoskeleton. The effect of okadaic acid was not restricted to neurons. Astrocytes stained with antibodies to glial fibrillary acidic protein (GFAP) showed increased GFAP staining and changes in astrocyte morphology from a flat shape to a stellate appearance with long processes.

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.

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.

Calpain-mediated proteolysis of microtubule-associated protein 2 (MAP-2) is inhibited by phosphorylation by cAMP-dependent protein kinase, but not by Ca2+/calmodulin-dependent protein kinase II

The effects of cAMP-dependent protein kinase (cAMP-PK) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylation on the calpain-mediated degradation of microtubule-associated protein 2 (MAP-2) were studied. Both cAMP-PK and CaMKII readily phosphorylated MAP-2. However, cAMP-PK phosphorylated MAP-2 to a significantly greater extent than did CaMKII (4.5 mol 32P/mol MAP-2 and 1.4 mol 32P/mol MAP-2, respectively). Phosphorylation of MAP-2 by cAMP-PK, but not by CaMKII, significantly inhibited the calpain-induced hydrolysis of MAP-2. These results demonstrate that the phosphorylation of sites on the MAP-2 molecule accessible to cAMP-PK, but not to CaMKII, result in increased resistance to calpain proteolysis.

Metal (Fe3+) affinity chromatography: differential adsorption of tau phosphoproteins

Tau is a neuronal cytoskeletal protein consisting of a group of isoforms with apparent molecular masses ranging from 45 to 62 kDa. Tau purified from brain exists in multiple phosphorylated forms and abnormally phosphorylated tau appears to play an important role in the neuropathology of Alzheimer's disease. To separate the differentially phosphorylated populations of tau, a chromatographic technique using ferric ions adsorbed onto iminodiacetic acid substituted Sepharose was developed. Several distinct populations of tau were isolated based on the phosphorylation state. These preparations can be used for further investigation of how each specific phosphorylation state modulates the metabolism and function of tau.

Use of microwaves for rapid fixation of tissues in vivo

Accurate knowledge of the concentration in the central nervous system of neurochemicals undergoing rapid enzymatic destruction or synthesis is sparse because of the difficulty in stopping the rapid reactions while causing only minimal adverse changes in the neurochemistry and structure. Microwave heating can be effectively used to rapidly stop enzyme activity in the central nervous system with minimal adverse changes. This rapid inactivation of the enzymes increases the validity of the sample that is taken for analysis of the concentration of the enzyme's substrate.

The role of microtubule-associated protein 2 (MAP-2) in neuronal growth, plasticity, and degeneration

Microtubule associated protein 2 (MAP-2) historically has been perceived primarily as a static, structural protein, necessary along with other cytoskeletal proteins to maintain neuroarchitecture but somewhat removed from the "mainstream" of neuronal response mechanisms. Quite to the contrary, MAP-2 is exquisitely sensitive to many inputs and recent investigations have revealed dynamic functions for MAP-2 in the growth, differentiation, and plasticity of neurons, with key roles in neuronal responses to growth factors, neurotransmitters, synaptic activity, and neurotoxins. These discoveries indicate that modification and rearrangement of MAP-2 is an early obligatory step in many processes which modify neuronal function.

Cleavage of bovine brain microtubule-associated protein-2 by human immunodeficiency virus proteinase

The high-molecular-weight dendritic cytoskeletal protein known as microtubule-associated protein (MAP)-2 displays the capacity to stimulate tubulin polymerization and to associate with microtubules. Serine proteases cleave MAP-2 into a C-terminal M(r) 28,000-35,000 microtubule-binding fragment and a larger N-terminal M(r) 240,000 projection-arm region. We now show that human immunodeficiency virus (HIV) proteinase also progressively degrades purified MAP-2 in vitro. This proteolysis reaction is characterized by transient accumulation of at least six intermediates, and most abundant of these is an M(r) 72,000 species that retains the ability to associate with taxol-stabilized microtubules. Treatment of this M(r) 72,000 species with thrombin releases the same M(r) 28,000 component as that derived from thrombin action on intact high-molecular-weight MAP-2, indicating that the viral aspartoproteinase action preferentially occurs further toward the N-terminus. The association of the M(r) 72,000 component with microtubules can be disrupted by the presence of a 21-amino acid peptide analogue of the second repeated sequence in the MAP-2 microtubule-binding region. We also studied HIV proteinase action on MAP-2 in the presence of tubulin and other MAPs that recycle with tubulin, and contrary to other published studies we found no effect of such treatment on microtubule self-assembly behavior. Cleavage of isolated MAP-2 by the HIV enzyme at high salt concentrations, followed by desalting and addition of tubulin, also resulted in microtubule assembly, albeit with slightly reduced efficiency.

Isolation and characterization of microtubule-associated protein 2 (MAP2) kinase from rat brain

Microtubule-associated protein 2 (MAP2) kinase has been isolated and characterized from rat brain. The enzyme has an apparent M(r) of approximately 42,000 and its pI is 4.9. MAP2 was the preferred substrate, but it also phosphorylated myelin basic protein (MBP), histone V-S, tubulin and the PC12 protein substrate pp250. The enzyme is distinct from protein kinase C, cAMP-dependent kinase and the calcium/calmodulin-dependent kinases, as specific inhibitors of these kinases did not affect MAP2 phosphorylation. The addition of the relatively non-specific protein kinase inhibitor H7 (20 microM) had a modest inhibitory effect. The enzyme was active in both 5 mM Mn2+ and Mg2+, and displayed Kms for MAP2, MBP, and ATP of 56 nM, 254 nM, and 4 microM, respectively. This enzyme, which represents a low abundance protein in whole brain, is analogous to the MAP2 kinase observed in growth factor-stimulated cell lines.

Seizure-induced protein tyrosine phosphorylation in rat brain regions

Phosphorylation of protein tyrosines is an important modulatory process for cell signaling and other cellular functions. Rat brain regions were examined for altered protein phosphotyrosines, using Western blot analysis and microwave irradiation to limit postmortem alterations, after administration of two convulsants: lithium plus pilocarpine or kainic acid (KA). Most phosphotyrosine proteins were unaltered by these treatments, but there was a large, specific increase in the tyrosine phosphorylation of a 40-Kd protein. This increase was evident in all three regions examined: cerebral cortex, hippocampus, and striatum; it occurred abruptly with onset of generalized status epilepticus (SE) and remained elevated for at least 90 min. Most of the tyrosine phosphorylated 40-Kd protein was in the cytosolic fraction. These results demonstrate a large, specific effect of chemically induced seizures on a single phosphotyrosine protein in rat brain.

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

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

Degradation of microtubule-associated protein 2 and brain spectrin by calpain: a comparative study

The in vitro degradation of microtubule-associated protein 2 (MAP-2) and spectrin by the calcium-dependent neutral protease calpain was studied. Five major results are reported. First, MAP-2 isolated from twice-cycled microtubules (2 X MT MAP-2) was extremely sensitive to calpain-induced hydrolysis. Even at an enzyme-to-substrate ratio (wt/wt) of 1:200, 2 X MT MAP-2 was significantly degraded by calpain. Second, MAP-2 purified from the total brain heat-stable fraction (total MAP-2) was significantly more resistant to calpain-induced hydrolysis compared with 2 X MT MAP-2. Third, MAP-2a and MAP-2b were proteolyzed similarly by calpain, although some relative resistance of MAP-2b was observed. Fourth, the presence of calmodulin significantly increased the extent of calpain-induced hydrolysis of the alpha-subunit of spectrin. Fifth, the two neuronal isoforms of brain spectrin (240/235 and 240/235E, referred to as alpha/beta N and alpha/beta E, respectively) showed different sensitivities to calpain. alpha N-spectrin was significantly more sensitive to calpain-induced degradation compared to alpha E-spectrin. Among other things, these results suggest a role for the calpain-induced degradation of MAP-2, as well as spectrin, in such physiological processes as alterations in synaptic efficacy, dendritic remodeling, and in pathological processes associated with neurodegeneration.