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

Amygdala stimulation promotes recovery of behavioral performance in a spatial memory task and increases GAP-43 and MAP-2 in the hippocampus and prefrontal cortex of male rats

The relationships between affective and cognitive processes are an important issue of present neuroscience. The amygdala, the hippocampus and the prefrontal cortex appear as main players in these mechanisms. We have shown that post-training electrical stimulation of the basolateral amygdala (BLA) speeds the acquisition of a motor skill, and produces a recovery in behavioral performance related to spatial memory in fimbria-fornix (FF) lesioned animals. BLA electrical stimulation rises bdnf RNA expression, BDNF protein levels, and arc RNA expression in the hippocampus. In the present paper we have measured the levels of one presynaptic protein (GAP-43) and one postsynaptic protein (MAP-2) both involved in synaptogenesis to assess whether structural neuroplastic mechanisms are involved in the memory enhancing effects of BLA stimulation. A single train of BLA stimulation produced in healthy animals an increase in the levels of GAP-43 and MAP-2 that lasted days in the hippocampus and the prefrontal cortex. In FF-lesioned rats, daily post-training stimulation of the BLA ameliorates the memory deficit of the animals and induces an increase in the level of both proteins. These results support the hypothesis that the effects of amygdala stimulation on memory recovery are sustained by an enhanced formation of new synapses.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

Early changes in Huntington's disease patient brains involve alterations in cytoskeletal and synaptic elements

Huntington's disease (HD) is caused by a polyglutamine repeat expansion in the N-terminus of the huntingtin protein. Huntingtin is normally present in the cytoplasm where it may interact with structural and synaptic elements. The mechanism of HD pathogenesis remains unknown but studies indicate a toxic gain-of-function possibly through aberrant protein interactions. To investigate whether early degenerative changes in HD involve alterations of cytoskeletal and vesicular components, we examined early cellular changes in the frontal cortex of HD presymptomatic (PS), early pathological grade (grade 1) and late-stage (grade 3 and 4) patients as compared to age-matched controls. Morphologic analysis using silver impregnation revealed a progressive decrease in neuronal fiber density and organization in pyramidal cell layers beginning in presymptomatic HD cases. Immunocytochemical analyses for the cytoskeletal markers alpha -tubulin, microtubule-associated protein 2, and phosphorylated neurofilament demonstrated a concomitant loss of staining in early grade cases. Immunoblotting for synaptic proteins revealed a reduction in complexin 2, which was marked in some grade 1 HD cases and significantly reduced in all late stage cases. Interestingly, we demonstrate that two synaptic proteins, dynamin and PACSIN 1, which were unchanged by immunoblotting, showed a striking loss by immunocytochemistry beginning in early stage HD tissue suggesting abnormal distribution of these proteins. We propose that mutant huntingtin affects proteins involved in synaptic function and cytoskeletal integrity before symptoms develop which may influence early disease onset and/or progression.

Signaling mechanisms underlying reversible, activity-dependent dendrite formation

Neuronal activity and neurotrophins play a central role in the formation, maintenance, and plasticity of dendritic arbors. Here, we show that neuronal activity, mediated by electrical stimulation, KCl depolarization, or cholinergic receptor activation, promotes reversible dendrite formation in sympathetic neurons and that this effect is enhanced by NGF. Activity-dependent dendrite formation is accompanied by increased association of HMW MAP2 with microtubules and increased microtubule stability. Inhibition of either CaMKII or the MEK-ERK pathway, both of which phosphorylate MAP2, inhibits dendrite formation, but inhibition of both pathways simultaneously is required for dendrites to retract. These data indicate that neuronal activity signals via CamKII and the ERKs to regulate MAP2:microtubule interactions and hence reversible dendrite stability, and to provide a mechanism whereby activity and neurotrophins converge intracellularly to dynamically regulate dendritic morphology.

Microtubule-associated protein 2 phosphorylation is decreased in the human epileptic temporal lobe cortex

Microtubule-associated protein 2 (MAP2) is an abundant component of the neuronal cytoskeleton whose function is related to the outgrowth and stability of neuronal processes, to synaptic plasticity and neuronal cell death. We have sought to study whether abnormal patterns of neuronal activity which are characteristic of epileptic patients are associated to alterations of MAP2 phosphorylation. An antibody (305) that selectively recognizes a phosphorylated epitope in a proline-rich region of the MAP2 molecule has been used to analyze neocortical biopsy samples from temporal lobe epileptic patients, whose electrocorticogram activity had been previously monitored. Immunoblot analysis showed that samples with greater spiking activity displayed significantly diminished MAP2 phosphorylation. Immunocytochemical analysis revealed the occurrence of discrete areas in the neocortex with highly decreased or no immunostaining for antibody 305, which showed a clear, although non-significant, tendency to appear more frequently in areas with greater spiking activity. To further support an association between epileptiform activity and MAP2 dephosphorylation an experimental model of epileptiform activity in cultures of rat hippocampal neurons was used. Neurons were cultured during 15 days in the presence of kynurenic acid, an antagonist of glutamate receptors. At this time, kynurenic acid was removed from the culture medium and neurons developed seizure-like activity. Using antibody 305, we found a decrease of MAP2 phosphorylation that was already visible after 15 min of kynurenic acid withdrawal. We therefore propose that MAP2 phosphorylation is decreased in the neocortex of epileptic patients and that this decrease is a likely consequence of seizure activity. Also, MAP2 dephosphorylation may lead to alterations of the neuronal cytoskeleton and eventually to neuronal damage and loss, which is typical of epileptic patients.

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 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.

Experience-dependent modifications in MAP2 phosphorylation in rat olfactory bulb

Microtubule-associated protein 2 (MAP2) is a neuron-specific cytoskeletal protein, enriched in dendrites and cell bodies, that helps determine dendritic shape. MAP2 regulates microtubule stability in a phosphorylation-dependent manner. The present study used immunocytochemistry with phosphoepitope-specific and phosphorylation state-independent antibodies to examine experience-dependent changes in MAP2 expression during postnatal development of the olfactory bulb. Our results demonstrate that immunoreactivity reflecting total MAP2 expression reaches a maximal level by postnatal day 20 (P20). The degree of staining for phosphoindependent forms of MAP2 is relatively unaffected by blocking odorant passage to one half the nasal epithelium via unilateral naris closure, a manipulation that attenuates physiological activity in the bulb. However, olfactory restriction from P1 dramatically reduces immunoreactivity for antibody AP18, which recognizes MAP2 only when phosphorylated on Ser136. Quantification of staining in the granule cell layer indicates that the greatest difference (64%) between control and experimental bulbs occurs after occlusion from P1 to P30 compared with animals deprived from P1 to P10 or P1 to P20. The shift in MAP2 phosphorylation occurs even when deprivation is delayed until P30, after the sensitive period for experience-dependent changes in bulb volume. Thus, the degree of the phosphorylation shift depends on the duration but not the time of onset of naris closure. Because staining for phosphorylation-independent forms of MAP2 is unchanged by naris closure, the total amount of the protein per unit area is probably not significantly altered. However, the large reductions of AP18-immunoreactivity in the bulb after olfactory restriction suggest that there is an activity-dependent stimulation of MAP2 phosphorylation.

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.

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.

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.

Protein kinase C in the rat cerebral cortex during spreading depression

Activity and distribution of protein kinase C (PKC) in the rat cerebral cortex was correlated with the development of spreading depression. When the 'waves' of the slow potential shift, induced by topical application of concentrated KCl solutions, were allowed to spread over the cerebral cortex for 10 min, both cytosolic and particulate fractions of the enzyme were increased to 169% and 143%, respectively, of the control values obtained from the contralateral, relatively intact hemicortex. When the enzyme activities were correlated with development of a single slow potential shift, it appeared that in the cortical area fully depolarized (under the maximum of negativity), the respective values were 175% and 157%. One min after recovery of the single wave of spreading depression both cytosolic and particulate fractions continued to rise up to 218% and 239%, respectively. During 5 min of recovery both the cytosolic and particulate fractions fell to 71% and 57%, respectively, of control levels. Even at 10 min the cytosolic enzyme was still decreased to 80%. At 20 min no difference between control and experimental values was found (soluble, 111%; particulate, 96%). The results are discussed in the context of data obtained in a few studies dealing with depolarization-induced changes of PKC in vitro.

Postsynaptic mechanisms for bidirectional control of MAP2 phosphorylation by glutamate receptors

Many activity-dependent changes in synaptic efficacy occur through elevations in postsynaptic calcium triggered by glutamate receptor activation. Here, the postsynaptic, neuron-specific microtubule-associated protein MAP2 is identified as a target of bidirectional calcium-dependent signaling pathways activated by glutamate. Glutamate produced a biphasic change in MAP2: a rapid, transient increase in phosphorylation mediated by metabotropic receptors and attenuated by inhibitors of calcium/calmodulin-dependent protein kinases and protein kinase C, followed by a persistent dephosphorylation of MAP2 mediated by NMDA receptors and activation of the calcium/calmodulin-dependent protein phosphatase 2B (calcineurin). Thus, a single transmembrane signal, glutamate, and the increased intracellular calcium it evokes can have opposing actions on a postsynaptic target phosphoprotein. The phosphorylation state of MAP2 determines its interaction with microtubules and actin filaments, suggesting that glutamatergic regulation of MAP2 phosphorylation may transduce neural activity into modifications in dendritic structure.

N-methyl-D-aspartate and nitric oxide regulate the expression of calcium/calmodulin-dependent kinase II in the hippocampal dentate gyrus

Injection of small volumes of N-methyl-D-aspartate (NMDA) or Sin-1 molsidomine (a nitric oxide releasing agent) onto the dendrites of granule cells in the hippocampal dentate gyrus leads to changes in the level of expression of a number of genes. There is a fall in prodynorphin mRNA levels with a corresponding increase in proenkephalin mRNA levels. Similar changes in opioid gene expression occur following the induction of long-term potentiation (LTP). We report here that at short time periods (1-6 h) after injections of NMDA or sin-1 molsidomine, there is an increase in the levels of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKII alpha), consistent with a report of elevated CaMKII alpha mRNA in postsynaptic neurons in the CA1 region of the hippocampus following LTP induction [54]. However, we also report that 24 h after injection of NMDA or sin-1, there is a dramatic decrease in CaMKII alpha mRNA levels in the vicinity of the injection. This effect is specific for CaMKII alpha mRNA, in that many other mRNA species are not affected, and occurs in the dendritic population of CaMKII alpha mRNA as well as in the pool of mRNA in the granule cell bodies. The effect is blocked by an inhibitor of cGMP-dependent protein kinase. The biphasic regulation of CaMKII alpha mRNA may be of considerable functional importance for the long-term response of granule cells to local stimulation of NMDA receptors or NO release.

Modulation of the phosphorylation state of tau in situ: the roles of calcium and cyclic AMP

Alterations in situ in the phosphorylation state of the microtubule-associated protein tau were examined in response to increasing intracellular levels of Ca2+ through N-methyl-D-aspartate (NMDA)-receptor activation, or activating cyclic AMP (cAMP)-dependent protein kinase (cAMP-PK), in rat cerebral-cortical slices. Increasing intracellular concentrations of Ca2+ by treatment of the brain slices with the glutamate analogue NMDA in depolarizing conditions (55 mM KCl) resulted in dephosphorylation of tau. Addition of KCl+NMDA to the slices resulted in a 40% decrease in 32P incorporation into tau, whereas addition of KCl or NMDA alone had no effect on tau phosphorylation. The KCl+NMDA-induced dephosphorylation of tau was blocked by the non-competitive NMDA-receptor antagonist MK801. Determine the involvement of the Ca2+/calmodulin-dependent phosphatase, calcineurin, in the KCl+NMDA-induced dephosphorylation of tau, slices were pretreated with the calcineurin inhibitor Cyclosporin A. Pretreatment of the rat brain slices with Cyclosporin A completely abolished the dephosphorylation of tau induced by the addition of KCl+NMDA. The dephosphorylation of tau in situ was site-selective, as indicated by the loss of 32P label from only a few select peptides. Activation of cAMP-PK by stimulating adenylate cyclase in rat cerebral-cortical slices with forskolin resulted in a 73% increase over control levels in 32P incorporation into immunoprecipitated tau. Two-dimensional phosphopeptide mapping revealed that most of the sites on tau phosphorylated in brain slices in response to increased cAMP levels were the same as those phosphorylated on isolated tau by purified cAMP-PK. Although the state of tau phosphorylation is certainly regulated by many protein phosphatases and kinases in vivo, to our knowledge this study provides the first direct evidence of a specific protein phosphatase and kinase that modulate the phosphorylation state of tau in situ.

Variations in in vivo phosphorylation at the proline-rich domain of the microtubule-associated protein 2 (MAP2) during rat brain development

Microtubule-associated protein 2 (MAP2) is an in vitro substrate for MAP kinase. Part of the phosphorylation occurs at the C-terminal microtubule-binding domain of the molecule which contains a cluster of putative consensus sites for MAP kinase on a proline-rich region. A peptide with the sequence RTPGTPG-TPSY, located at this region of the molecule, is efficiently phosphorylated by MAP kinase in vitro. An antibody (972) raised against this non-phosphorylated peptide has been used to test for in vivo phosphorylation at the proline-rich domain of the MAP2 molecule. The reaction of purified MAP2 with antibody 972 diminishes after in vitro phosphorylation by MAP kinase and is enhanced after in vitro dephosphorylation by alkaline phosphatase. A fraction of brain MAP2 isolated by iron-chelation affinity chromatography appears to be phosphorylated in vivo at the site recognized by antibody 972. There is some variation in the phosphorylation of MAP2 at the proline-rich region throughout rat brain development. MAP2C is more highly phosphorylated in the developing rat brain, whereas high-molecular-mass MAP2 is more extensively phosphorylated in the adult rat brain.