Perinatal lethality of microtubule-associated protein 1B-deficient mice expressing alternative isoforms of the protein at low levels

Elevated levels of FMRP-target MAP1B impair human and mouse neuronal development and mouse social behaviors via autophagy pathway

Fragile X messenger ribonucleoprotein 1 protein (FMRP) binds many mRNA targets in the brain. The contribution of these targets to fragile X syndrome (FXS) and related autism spectrum disorder (ASD) remains unclear. Here, we show that FMRP deficiency leads to elevated microtubule-associated protein 1B (MAP1B) in developing human and non-human primate cortical neurons. Targeted MAP1B gene activation in healthy human neurons or MAP1B gene triplication in ASD patient-derived neurons inhibit morphological and physiological maturation. Activation of Map1b in adult male mouse prefrontal cortex excitatory neurons impairs social behaviors. We show that elevated MAP1B sequesters components of autophagy and reduces autophagosome formation. Both MAP1B knockdown and autophagy activation rescue deficits of both ASD and FXS patients' neurons and FMRP-deficient neurons in ex vivo human brain tissue. Our study demonstrates conserved FMRP regulation of MAP1B in primate neurons and establishes a causal link between MAP1B elevation and deficits of FXS and ASD.

Multi-omics profiling identifies a deregulated FUS-MAP1B axis in ALS/FTD-associated UBQLN2 mutants

Analysis of ALS patient-derived and engineered cells revealed that mutant UBQLN2 increases mRNA and protein of MAP1B which is mediated by dephosphorylation of FUS within its RNA-binding domain.

Ubiquilin-2 (UBQLN2) is a ubiquitin-binding protein that shuttles ubiquitinated proteins to proteasomal and autophagic degradation. UBQLN2 mutations are genetically linked to the neurodegenerative disorders amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). However, it remains elusive how UBQLN2 mutations cause ALS/FTD. Here, we systematically examined proteomic and transcriptomic changes in patient-derived lymphoblasts and CRISPR/Cas9–engineered HeLa cells carrying ALS/FTD UBQLN2 mutations. This analysis revealed a strong up-regulation of the microtubule-associated protein 1B (MAP1B) which was also observed in UBQLN2 knockout cells and primary rodent neurons depleted of UBQLN2, suggesting that a UBQLN2 loss-of-function mechanism is responsible for the elevated MAP1B levels. Consistent with MAP1B’s role in microtubule binding, we detected an increase in total and acetylated tubulin. Furthermore, we uncovered that UBQLN2 mutations result in decreased phosphorylation of MAP1B and of the ALS/FTD–linked fused in sarcoma (FUS) protein at S439 which is critical for regulating FUS-RNA binding and MAP1B protein abundance. Together, our findings point to a deregulated UBQLN2-FUS-MAP1B axis that may link protein homeostasis, RNA metabolism, and cytoskeleton dynamics, three molecular pathomechanisms of ALS/FTD.

MAP1B mutations cause intellectual disability and extensive white matter deficit

Discovery of coding variants in genes that confer risk of neurodevelopmental disorders is an important step towards understanding the pathophysiology of these disorders. Whole-genome sequencing of 31,463 Icelanders uncovers a frameshift variant (E712KfsTer10) in microtubule-associated protein 1B (MAP1B) that associates with ID/low IQ in a large pedigree (genome-wide corrected P = 0.022). Additional stop-gain variants in MAP1B (E1032Ter and R1664Ter) validate the association with ID and IQ. Carriers have 24% less white matter (WM) volume (β = −2.1SD, P = 5.1 × 10⁻⁸), 47% less corpus callosum (CC) volume (β = −2.4SD, P = 5.5 × 10⁻¹⁰) and lower brain-wide fractional anisotropy (P = 6.7 × 10⁻⁴). In summary, we show that loss of MAP1B function affects general cognitive ability through a profound, brain-wide WM deficit with likely disordered or compromised axons.

Intellectual disability (ID) is characterized by an intelligence quotient of below 70 and impaired adaptive skills. Here, analyzing whole genome sequences from 31,463 Icelanders, Walters et al. identify variants in MAP1B associated with ID and extensive brain-wide white matter deficits.

Neuronal Growth Cone Size-Dependent and -Independent Parameters of Microtubule Polymerization

Migration and pathfinding of neuronal growth cones during neurite extension is critically dependent on dynamic microtubules. In this study we sought to determine, which aspects of microtubule polymerization relate to growth cone morphology and migratory characteristics. We conducted a multiscale quantitative microscopy analysis using automated tracking of microtubule plus ends in migrating growth cones of cultured murine dorsal root ganglion (DRG) neurons. Notably, this comprehensive analysis failed to identify any changes in microtubule polymerization parameters that were specifically associated with spontaneous extension vs. retraction of growth cones. This suggests that microtubule dynamicity is a basic mechanism that does not determine the polarity of growth cone response but can be exploited to accommodate diverse growth cone behaviors. At the same time, we found a correlation between growth cone size and basic parameters of microtubule polymerization including the density of growing microtubule plus ends and rate and duration of microtubule growth. A similar correlation was observed in growth cones of neurons lacking the microtubule-associated protein MAP1B. However, MAP1B-null growth cones, which are deficient in growth cone migration and steering, displayed an overall reduction in microtubule dynamicity. Our results highlight the importance of taking growth cone size into account when evaluating the influence on growth cone microtubule dynamics of different substrata, guidance factors or genetic manipulations which all can change growth cone morphology and size. The type of large scale multiparametric analysis performed here can help to separate direct effects that these perturbations might have on microtubule dynamics from indirect effects resulting from perturbation-induced changes in growth cone size.

Rab35 Functions in Axon Elongation Are Regulated by P53-Related Protein Kinase in a Mechanism That Involves Rab35 Protein Degradation and the Microtubule-Associated Protein 1B

Rab35 is a key protein for cargo loading in the recycling endosome. In neuronal immortalized cells, Rab35 promotes neurite differentiation. Here we describe that Rab35 favors axon elongation in rat primary neurons in an activity-dependent manner. In addition, we show that the p53-related protein kinase (PRPK) negatively regulates axonal elongation by reducing Rab35 protein levels through the ubiquitin-proteasome degradation pathway. PRPK-induced Rab35 degradation is regulated by its interaction with microtubule-associated protein 1B (MAP1B), a microtubule stabilizing binding protein essential for axon elongation. Consistently, axon defects found in MAP1B knock-out neurons were reversed by Rab35 overexpression or PRPK inactivation suggesting an epistatic relationship among these proteins. These results define a novel mechanism to support axonal elongation, by which MAP1B prevents PRPK-induced Rab35 degradation. Such a mechanism allows Rab35-mediated axonal elongation and connects the regulation of actin dynamics with membrane trafficking. In addition, our study reveals for the first time that the ubiquitin-proteasome degradation pathway regulates a Rab GTPase.

SIGNIFICANCE STATEMENT

Rab35 is required for axonal outgrowth. We define that its protein levels are negatively regulated by p53-related protein kinase (PRPK). We show that microtubule-associated protein 1B (MAP1B) interacts with PRPK, preventing PRPK-dependent Rab35 proteasome degradation. We demonstrate that Rab35 regulates Cdc42 activity in neurons. This is the first evidence showing that a Rab protein is regulated by degradation dependent on the ubiquitin-proteasome system.

Two transgenic mouse models for β-subunit components of succinate-CoA ligase yielding pleiotropic metabolic alterations

Succinate-CoA ligase (SUCL) is a heterodimer enzyme composed of Suclg1 α-subunit and a substrate-specific Sucla2 or Suclg2 β-subunit yielding ATP or GTP, respectively. In humans, the deficiency of this enzyme leads to encephalomyopathy with or without methylmalonyl aciduria, in addition to resulting in mitochondrial DNA depletion. We generated mice lacking either one Sucla2 or Suclg2 allele. Sucla2 heterozygote mice exhibited tissue- and age-dependent decreases in Sucla2 expression associated with decreases in ATP-forming activity, but rebound increases in cardiac Suclg2 expression and GTP-forming activity. Bioenergetic parameters including substrate-level phosphorylation (SLP) were not different between wild-type and Sucla2 heterozygote mice unless a submaximal pharmacological inhibition of SUCL was concomitantly present. mtDNA contents were moderately decreased, but blood carnitine esters were significantly elevated. Suclg2 heterozygote mice exhibited decreases in Suclg2 expression but no rebound increases in Sucla2 expression or changes in bioenergetic parameters. Surprisingly, deletion of one Suclg2 allele in Sucla2 heterozygote mice still led to a rebound but protracted increase in Suclg2 expression, yielding double heterozygote mice with no alterations in GTP-forming activity or SLP, but more pronounced changes in mtDNA content and blood carnitine esters, and an increase in succinate dehydrogenase activity. We conclude that a partial reduction in Sucla2 elicits rebound increases in Suclg2 expression, which is sufficiently dominant to overcome even a concomitant deletion of one Suclg2 allele, pleiotropically affecting metabolic pathways associated with SUCL. These results as well as the availability of the transgenic mouse colonies will be of value in understanding SUCL deficiency.

MAP1B and NOS1 genes are associated with working memory in youths with attention-deficit/hyperactivity disorder

Diverse efforts have been done to improve the etiologic understanding of mental disorders, such as attention-deficit/hyperactivity disorder (ADHD). It becomes clear that research in mental disorders needs to move beyond descriptive syndromes. Several studies support recent theoretical models implicating working memory (WM) deficits in ADHD complex neuropsychology. The aim of this study was to examine the association between rs2199161 and rs478597 polymorphisms at MAP1B and NOS1 genes with verbal working memory in children and adolescents with ADHD. A total of 253 unrelated ADHD children/adolescents were included. The sample was diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders-4th edition criteria. Digit Span from the Wechsler Intelligence Scale for Children-Third Edition was used to assess verbal WM. The raw scores from both forward and backward conditions of Digit Span were summed and converted into scaled scores according to age. The means of scaled Digit Span were compared according to genotypes by ANOVA. Significant differences in Digit Span scores between MAP1B genotype groups (rs2199161: F = 5.676; p = 0.018) and NOS1 (rs478597: F = 6.833; p = 0.009) genes were detected. For both polymorphisms, the CC genotype carriers showed a worse performance in WM task. Our findings suggest possible roles of NOS1 and MAP1B genes in WM performance in ADHD patients, replicating previous results with NOS1 gene in this cognitive domain in ADHD children.

NOS1 and SNAP25 polymorphisms are associated with Attention-Deficit/Hyperactivity Disorder symptoms in adults but not in children

Several investigations documented that Attention-Deficit/Hyperactivity Disorder (ADHD) is better conceptualized as a dimensional disorder. At the same time, the disorder seems to have different neurobiological underpinnings and phenotypic presentation in children compared to adults. Neurodevelopmental genes could explain, at least partly these differences. The aim of the present study was to examine possible associations between polymorphisms in SNAP25, MAP1B and NOS1 genes and ADHD symptoms in Brazilian samples of children/adolescents and adults with ADHD. The youth sample consisted of 301 patients whereas the adult sample comprises 485 individuals with ADHD. Diagnoses of ADHD and comorbidities were based on the Diagnostic and Statistical Manual of Mental Disorders-4th edition criteria. The Swanson, Nolan and Pelham Scale-Version IV (SNAP-IV) was applied by psychiatrists blinded to genotype. The total SNAP-IV scores were compared between genotypes. Impulsivity SNAP-IV scores were also compared according to NOS1 genotypes. Adult patients homozygous for the C allele at SNAP25 rs8636 showed significantly higher total SNAP-IV scores (F = 11.215; adjusted P-value = 0.004). Impulsivity SNAP-IV scores were also significantly different according to NOS1 rs478597 polymorphisms in adults with ADHD (F = 6.282; adjusted P-value = 0.026). These associations were not observed in children and adolescents with ADHD. These results suggest that SNAP25 and NOS1 genotypes influence ADHD symptoms only in adults with ADHD. Our study corroborates previous evidences for differences in the genetic contribution to adult ADHD compared with childhood ADHD.

Defects in Synaptic Plasticity, Reduced NMDA-Receptor Transport, and Instability of Postsynaptic Density Proteins in Mice Lacking Microtubule-Associated Protein 1A

Microtubule-associated protein 1A (MAP1A) is a member of the major non-motor microtubule-binding proteins. It has been suggested that MAP1A tethers NMDA receptors (NRs) to the cytoskeleton by binding with proteins postsynaptic density (PSD)-93 and PSD-95, although the function of MAP1A in vivo remains elusive. The present study demonstrates that mouse MAP1A plays an essential role in maintaining synaptic plasticity through an analysis of MAP1A knock-out mice. The mice exhibited learning disabilities, which correlated with decreased long-term potentiation and long-term depression in the hippocampal neurons, as well as a concomitant reduction in the extent of NR-dependent EPSCs. Surface expression of NR2A and NR2B subunits also decreased. Enhanced activity-dependent degradation of PSD-93 and reduced transport of NR2A/2B in dendrites was likely responsible for altered receptor function in neurons lacking MAP1A. These data suggest that tethering of NR2A/2B with the cytoskeleton through MAP1A is fundamental for synaptic function.

SIGNIFICANCE STATEMENT

This work is the first report showing the significance of non-motor microtubule-associated protein in maintaining synaptic plasticity thorough a novel mechanism: anchoring of NMDA receptors to cytoskeleton supports transport of NMDA receptors and stabilizes postsynaptic density scaffolds binding to NMDA receptors. Newly generated mutant mice lacking MAP1A exhibited learning disabilities and reduced synaptic plasticity attributable to disruptions of the anchoring machinery.

Repulsive axon guidance by Draxin is mediated by protein Kinase B (Akt), glycogen synthase kinase-3β (GSK-3β) and microtubule-associated protein 1B

Draxin is an important axon guidance cue necessary for the formation of forebrain commissures including the corpus callosum, but the molecular details of draxin signaling are unknown. To unravel how draxin signals are propagated we used murine cortical neurons and genetic and pharmacological approaches. We found that draxin-induced growth cone collapse critically depends on draxin receptors (deleted in colorectal cancer, DCC), inhibition of protein kinase B/Akt, activation of GSK-3β (glycogen synthase kinase-3β) and the presence of microtubule-associated protein MAP1B. This study, for the first time elucidates molecular events in draxin repulsion, links draxin and DCC to MAP1B and identifies a novel MAP1B-depenent GSK-3β pathway essential for chemo-repulsive axon guidance cue signaling.

A presynaptic role of microtubule-associated protein 1/Futsch in Drosophila: regulation of active zone number and neurotransmitter release

Structural microtubule-associated proteins (MAPs), like MAP1, not only control the stability of microtubules, but also interact with postsynaptic proteins in the nervous system. Their presynaptic role has barely been studied. To tackle this question, we used the Drosophila model in which there is only one MAP1 homolog: Futsch, which is expressed at the larval neuromuscular junction, presynaptically only. We show that Futsch regulates neurotransmitter release and active zone density. Importantly, we provide evidence that this role of Futsch is not just the consequence of its microtubule-stabilizing function. Using high-resolution microscopy, we show that Futsch and microtubules are almost systematically present in close proximity to active zones, with Futsch being localized in-between microtubules and active zones. Using proximity ligation assays, we further demonstrate the proximity of Futsch, but not microtubules, to active zone components. Altogether our data are in favor of a model by which Futsch locally stabilizes active zones, by reinforcing their link with the underlying microtubule cytoskeleton.

The MAP1B case: an old MAP that is new again

The functions of microtubule-associated protein 1B (MAP1B) have historically been linked to the development of the nervous system, based on its very early expression in neurons and glial cells. Moreover, mice in which MAP1B is genetically inactivated have been used extensively to show its role in axonal elongation, neuronal migration, and axonal guidance. In the last few years, it has become apparent that MAP1B has other cellular and molecular functions that are not related to its microtubule-stabilizing properties in the embryonic and adult brain. In this review, we present a systematic review of the canonical and novel functions of MAP1B and propose that, in addition to regulating the polymerization of microtubule and actin microfilaments, MAP1B also acts as a signaling protein involved in normal physiology and pathological conditions in the nervous system.

Backbone and partial side chain assignment of the microtubule binding domain of the MAP1B light chain

Microtubule-associated protein 1B (MAP1B) is a classical high molecular mass microtubule-associated protein expressed at high levels in the brain. It confers specific properties to neuronal microtubules and is essential for neuronal differentiation, brain development and synapse maturation. Misexpression of the protein contributes to the development of brain disorders in humans. However, despite numerous reports demonstrating the importance of MAP1B in regulation of the neuronal cytoskeleton during neurite extension and axon guidance, its mechanism of action is still elusive. Here we focus on the intrinsically disordered microtubule binding domain of the light chain of MAP1B. In order to obtain more detailed structural information about this domain we assigned NMR chemical shifts of backbone and aliphatic side chain atoms.

CaV2.2 channel cell surface expression is regulated by the light chain 1 (LC1) of the microtubule-associated protein B (MAP1B) via UBE2L3-mediated ubiquitination and degradation

Microtubule-associated protein B is a cytoskeleton protein consisting of heavy and light (LC) chains that play important roles in the regulation of neuronal morphogenesis and function. LC1 is also well known to interact with diverse ionotropic receptors at postsynapse. Much less is known, however, regarding the role of LC1 at presynaptic level where voltage-gated N-type Ca(2+) channels couple membrane depolarization to neurotransmitter release. Here, we investigated whether LC1 interacts with the N-type channels. Co-localization analysis revealed spatial proximity of the two proteins in hippocampal neurons. The interaction between LC1 and the N-type channel was demonstrated using co-immunoprecipitation experiments and in vitro pull-down assays. Detailed biochemical analysis suggested that the interaction occurs through the N-terminal of LC1 and the C-terminal of the pore-forming CaVα1 subunit of the channels. Patch-clamp studies in HEK-293 cells revealed a significant decrease in N-type currents upon LC1 expression, without apparent changes in kinetics. Recordings performed in the presence of MG132 prevented the actions of LC1 suggesting enhanced channel proteasomal degradation. Interestingly, using the yeast two-hybrid system and immunoprecipitation assays in HEK-293 cells, we revealed an interaction between LC1 and the ubiquitin-conjugating enzyme UBE2L3. Furthermore, we found that the LC1/UBE2L3 complex could interact with the N-type channels, suggesting that LC1 may act as a scaffold protein to increase UBE2L3-mediated channel ubiquitination. Together these results revealed a novel functional coupling between LC1 and the N-type channels.

MAP1B-dependent Rac activation is required for AMPA receptor endocytosis during long-term depression

The microtubule-associated protein 1B (MAP1B) plays critical roles in neurite growth and synapse maturation during brain development. This protein is well expressed in the adult brain. However, its function in mature neurons remains unknown. We have used a genetically modified mouse model and shRNA techniques to assess the role of MAP1B at established synapses, bypassing MAP1B functions during neuronal development. Under these conditions, we found that MAP1B deficiency alters synaptic plasticity by specifically impairing long-term depression (LTD) expression. Interestingly, this is due to a failure to trigger AMPA receptor endocytosis and spine shrinkage during LTD. These defects are accompanied by an impaired targeting of the Rac1 activator Tiam1 at synaptic compartments. Accordingly, LTD and AMPA receptor endocytosis are restored in MAP1B-deficient neurons by providing additional Rac1. Therefore, these results indicate that the MAP1B-Tiam1-Rac1 relay is essential for spine structural plasticity and removal of AMPA receptors from synapses during LTD. This work highlights the importance of MAPs as signalling hubs controlling the actin cytoskeleton and receptor trafficking during plasticity in mature neurons.

MAP1B regulates microtubule dynamics by sequestering EB1/3 in the cytosol of developing neuronal cells

MAP1B, a structural microtubule (MT)-associated protein highly expressed in developing neurons, plays a key role in neurite and axon extension. However, not all molecular mechanisms by which MAP1B controls MT dynamics during these processes have been revealed. Here, we show that MAP1B interacts directly with EB1 and EB3 (EBs), two core 'microtubule plus-end tracking proteins' (+TIPs), and sequesters them in the cytosol of developing neuronal cells. MAP1B overexpression reduces EBs binding to plus-ends, whereas MAP1B downregulation increases binding of EBs to MTs. These alterations in EBs behaviour lead to changes in MT dynamics, in particular overstabilization and looping, in growth cones of MAP1B-deficient neurons. This contributes to growth cone remodelling and a delay in axon outgrowth. Together, our findings define a new and crucial role of MAP1B as a direct regulator of EBs function and MT dynamics during neurite and axon extension. Our data provide a new layer of MT regulation: a classical MAP, which binds to the MT lattice and not to the end, controls effective concentration of core +TIPs thereby regulating MTs at their plus-ends.

The light chain 1 subunit of the microtubule-associated protein 1B (MAP1B) is responsible for Tiam1 binding and Rac1 activation in neuronal cells

Microtubule-associated protein 1B (MAP1B) is a neuronal protein involved in the stabilization of microtubules both in the axon and somatodendritic compartments. Acute, genetic inactivation of MAP1B leads to delayed axonal outgrowth, most likely due to changes in the post-translational modification of tubulin subunits, which enhances microtubule polymerization. Furthermore, MAP1B deficiency is accompanied by abnormal actin microfilament polymerization and dramatic changes in the activity of small GTPases controlling the actin cytoskeleton. In this work, we showed that MAP1B interacts with a guanine exchange factor, termed Tiam1, which specifically activates Rac1. These proteins co-segregated in neurons, and interact in both heterologous expression systems and primary neurons. We dissected the molecular domains involved in the MAP1B-Tiam1 interaction, and demonstrated that pleckstrin homology (PH) domains in Tiam1 are responsible for MAP1B binding. Interestingly, only the light chain 1 (LC1) of MAP1B was able to interact with Tiam1. Moreover, it was able to increase the activity of the small GTPase, Rac1. These results suggest that the interaction between Tiam1 and MAP1B, is produced by the binding of LC1 with PH domains in Tiam1. The formation of such a complex impacts on the activation levels of Rac1 confirming a novel function of MAP1B related with the control of small GTPases. These results also support the idea of cross-talk between cytoskeleton compartments inside neuronal cells.

The light chains of microtubule-associated proteins MAP1A and MAP1B interact with α1-syntrophin in the central and peripheral nervous system

Microtubule-associated proteins of the MAP1 family (MAP1A, MAP1B, and MAP1S) share, among other features, a highly conserved COOH-terminal domain approximately 125 amino acids in length. We conducted a yeast 2-hybrid screen to search for proteins interacting with this domain and identified α1-syntrophin, a member of a multigene family of adapter proteins involved in signal transduction. We further demonstrate that the interaction between the conserved COOH-terminal 125-amino acid domain (which is located in the light chains of MAP1A, MAP1B, and MAP1S) and α1-syntrophin is direct and occurs through the pleckstrin homology domain 2 (PH2) and the postsynaptic density protein 95/disk large/zonula occludens-1 protein homology domain (PDZ) of α1-syntrophin. We confirmed the interaction of MAP1B and α1-syntrophin by co-localization of the two proteins in transfected cells and by co-immunoprecipitation experiments from mouse brain. In addition, we show that MAP1B and α1-syntrophin partially co-localize in Schwann cells of the murine sciatic nerve during postnatal development and in the adult. However, intracellular localization of α1-syntrophin and other Schwann cell proteins such as ezrin and dystrophin-related protein 2 (DRP2) and the localization of the axonal node of Ranvier-associated protein Caspr1/paranodin were not affected in MAP1B null mice. Our findings add to a growing body of evidence that classical MAPs are likely to be involved in signal transduction not only by directly modulating microtubule function, but also through their interaction with signal transduction proteins.

Microtubule-associated protein 1B (MAP1B) is required for dendritic spine development and synaptic maturation

Microtubule-associated protein 1B (MAP1B) is prominently expressed during early stages of neuronal development, and it has been implicated in axonal growth and guidance. MAP1B expression is also found in the adult brain in areas of significant synaptic plasticity. Here, we demonstrate that MAP1B is present in dendritic spines, and we describe a decrease in the density of mature dendritic spines in neurons of MAP1B-deficient mice that was accompanied by an increase in the number of immature filopodia-like protrusions. Although these neurons exhibited normal passive membrane properties and action potential firing, AMPA receptor-mediated synaptic currents were significantly diminished. Moreover, we observed a significant decrease in Rac1 activity and an increase in RhoA activity in the post-synaptic densities of adult MAP1B(+/-) mice when compared with wild type controls. MAP1B(+/-) fractions also exhibited a decrease in phosphorylated cofilin. Taken together, these results indicate a new and important role for MAP1B in the formation and maturation of dendritic spines, possibly through the regulation of the actin cytoskeleton. This activity of MAP1B could contribute to the regulation of synaptic activity and plasticity in the adult brain.

MAP1B regulates axonal development by modulating Rho-GTPase Rac1 activity

This article shows a novel function for the MAP1B protein, related to the control of actin dynamics through interaction with Tiam1.

Cultured neurons obtained from MAP1B-deficient mice have a delay in axon outgrowth and a reduced rate of axonal elongation compared with neurons from wild-type mice. Here we show that MAP1B deficiency results in a significant decrease in Rac1 and cdc42 activity and a significant increase in Rho activity. We found that MAP1B interacted with Tiam1, a guanosine nucleotide exchange factor for Rac1. The decrease in Rac1/cdc42 activity was paralleled by decreases in the phosphorylation of the downstream effectors of these proteins, such as LIMK-1 and cofilin. The expression of a constitutively active form of Rac1, cdc42, or Tiam1 rescued the axon growth defect of MAP1B-deficient neurons. Taken together, these observations define a new and crucial function of MAP1B that we show to be required for efficient cross-talk between microtubules and the actin cytoskeleton during neuronal polarization.

MAP1B binds to the NMDA receptor subunit NR3A and affects NR3A protein concentrations

Incorporation of the N-methyl-d-aspartate receptor (NMDAR) subunit NR3A into functional NMDARs results in reduced channel conductance and Ca(2+) permeability. To further investigate the function of NR3A, we have set out to characterize its intracellular binding partners. Here, we report a novel protein interaction between NR3A and microtubule associated-protein (MAP) 1B, which both are localized to dendritic shafts and filopodia. NR3A protein levels were increased in MAP1B deficient (-/-) mice, with a corresponding decrease in NR1 levels, but the fraction of filopodia immunoreactive for NR3A was equal in cells from -/- and wild type (WT) mice. NR3A has previously been shown to interact with another member of the MAP1 family, MAP1S. We showed that MAP1S binds to microtubules in a similar manner as MAP1B, and suggest that MAP1S and MAP1B both are involved in regulating trafficking of NR3A-containing NMDAR.

Kidins220/ARMS modulates the activity of microtubule-regulating proteins and controls neuronal polarity and development

In order for neurons to perform their function, they must establish a highly polarized morphology characterized, in most of the cases, by a single axon and multiple dendrites. Herein we find that the evolutionarily conserved protein Kidins220 (kinase D-interacting substrate of 220-kDa), also known as ARMS (ankyrin repeat-rich membrane spanning), a downstream effector of protein kinase D and neurotrophin and ephrin receptors, regulates the establishment of neuronal polarity and development of dendrites. Kidins220/ARMS gain and loss of function experiments render severe phenotypic changes in the processes extended by hippocampal neurons in culture. Although Kidins220/ARMS early overexpression hinders neuronal development, its down-regulation by RNA interference results in the appearance of multiple longer axon-like extensions as well as aberrant dendritic arbors. We also find that Kidins220/ARMS interacts with tubulin and microtubule-regulating molecules whose role in neuronal morphogenesis is well established (microtubule-associated proteins 1b, 1a, and 2 and two members of the stathmin family). Importantly, neurons where Kidins220/ARMS has been knocked down register changes in the phosphorylation activity of MAP1b and stathmins. Altogether, our results indicate that Kidins220/ARMS is a key modulator of the activity of microtubule-regulating proteins known to actively regulate neuronal morphogenesis and suggest a mechanism by which it contributes to control neuronal development.

The microtubule network and neuronal morphogenesis: Dynamic and coordinated orchestration through multiple players

Nervous system function and plasticity rely on the complex architecture of neuronal networks elaborated during development, when neurons acquire their specific and complex shape. During neuronal morphogenesis, the formation and outgrowth of functionally and structurally distinct axons and dendrites require a coordinated and dynamic reorganization of the microtubule cytoskeleton involving numerous regulators. While most of these factors act directly on microtubules to stabilize them or promote their assembly, depolymerization or fragmentation, others are now emerging as essential regulators of neuronal differentiation by controlling tubulin availability and modulating microtubule dynamics. In this review, we recapitulate how the microtubule network is actively regulated during the successive phases of neuronal morphogenesis, and what are the specific roles of the various microtubule-regulating proteins in that process. We then describe the specific signaling pathways and inter-regulations that coordinate the different activities of these proteins to sustain neuronal development in response to environmental cues.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

Role of chromodomain helicase DNA-binding protein 2 in DNA damage response signaling and tumorigenesis

The chromodomain helicase DNA-binding proteins (CHDs) are known to affect transcription through their ability to remodel chromatin and modulate histone deacetylation. In an effort to understand the functional role of the CHD2 in mammals, we have generated a Chd2 mutant mouse model. Remarkably, the Chd2 protein appears to play a critical role in the development, hematopoiesis and tumor suppression. The Chd2 heterozygous mutant mice exhibit increased extramedullary hematopoiesis and susceptibility to lymphomas. At the cellular level, Chd2 mutants are defective in hematopoietic stem cell differentiation, accumulate higher levels of the chromatin-associated DNA damage response mediator, gamma H2AX, and exhibit an aberrant DNA damage response after X-ray irradiation. Our data suggest a direct role for the chromatin remodeling protein in DNA damage signaling and genome stability maintenance.

Association analysis of schizophrenia on 18 genes involved in neuronal migration: MDGA1 as a new susceptibility gene

Several lines of evidence support the theory of schizophrenia (SZ) being a neurodevelopmental disorder. The structural, cytoarchitectural and functional brain abnormalities reported in patients with SZ, might be due to aberrant neuronal migration, since the final position of neurons affects neuronal function, morphology, and formation of synaptic connections. We have investigated the putative association between SZ and gene variants engaged in the neuronal migration process, by performing an association study on 839 cases and 1,473 controls of Scandinavian origin. Using a gene-wide approach, tagSNPs in 18 candidate genes have been genotyped, with gene products involved in the neuron-to-glial cell adhesion, interactions with the DISC1 protein and/or rearrangements of the cytoskeleton. Of the 289 markers tested, 19 markers located in genes MDGA1, RELN, ITGA3, DLX1, SPARCL1, and ASTN1, attained nominal significant P-values (P < 0.05) in either a genotypic or allelic association test. All of these genes, except transcription factor DLX1, are involved in the adhesion between neurons and radial glial cells. Eight markers obtained nominal significance in both tests, and were located in intronic or 3'UTR regions of adhesion molecule MDGA1 and previously reported SZ candidate RELN. The most significant result was attained for MDGA1 SNP rs9462341 (unadjusted association results: genotypic P = 0.00095; allelic P = 0.010). Several haplotypes within MDGA1, RELN, ITGA3, and ENAH were nominally significant. Further studies in independent samples are needed, including upcoming genome wide association study results, but our data suggest that MDGA1 is a new SZ susceptibility gene, and that altered neuronal migration is involved in SZ pathology.

S-nitrosylation of microtubule-associated protein 1B mediates nitric-oxide-induced axon retraction

Treatment of cultured vertebrate neurons with nitric oxide leads to growth-cone collapse, axon retraction and the reconfiguration of axonal microtubules. We show that the light chain of microtubule-associated protein (MAP) 1B is a substrate for S-nitrosylation in vivo, in cultured cells and in vitro. S-nitrosylation occurs at Cys 2457 in the COOH terminus. Nitrosylation of MAP1B leads to enhanced interaction with microtubules and correlates with the inhibition of neuroblastoma cell differentiation. We further show, in dorsal root ganglion neurons, that MAP1B is necessary for neuronal nitric oxide synthase control of growth-cone size, growth-cone collapse and axon retraction. These results reveal an S-nitrosylation-dependent signal-transduction pathway that is involved in regulation of the axonal cytoskeleton and identify MAP1B as a major component of this pathway. We propose that MAP1B acts by inhibiting a microtubule- and dynein-based mechanism that normally prevents axon retraction.

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

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

COUP-TFI is required for the formation of commissural projections in the forebrain by regulating axonal growth

The transcription factor COUP-TFI (NR2F1), an orphan member of the nuclear receptor superfamily, is an important regulator of neurogenesis, cellular differentiation and cell migration. In the forebrain, COUP-TFI controls the connectivity between thalamus and cortex and neuronal tangential migration in the basal telencephalon. Here, we show that COUP-TFI is required for proper axonal growth and guidance of all major forebrain commissures. Fibres of the corpus callosum, the hippocampal commissure and the anterior commissure project aberrantly and fail to cross the midline in COUP-TFI null mutants. Moreover, hippocampal neurons lacking COUP-TFI have a defect in neurite outgrowth and show an abnormal axonal morphology. To search for downstream effectors, we used microarray analysis and showed that, in the absence of COUP-TFI, expression of various cytoskeleton molecules involved in neuronal morphogenesis is affected. Diminished protein levels of the microtubule-associated protein MAP1B and increased levels of the GTP-binding protein RND2 were confirmed in the developing cortex in vivo and in primary hippocampal neurons in vitro. Therefore, based on morphological studies, gene expression profiling and primary cultured neurons, the present data uncover a previously unappreciated intrinsic role for COUP-TFI in axonal growth in vivo and supply one of the premises for COUP-TFI coordination of neuronal morphogenesis in the developing forebrain.

The MAP1 family of microtubule-associated proteins

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice and stabilize microtubules.

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice. The founding members, MAP1A and MAP1B, are predominantly expressed in neurons, where they are thought to be important in the formation and development of axons and dendrites. Mammalian genomes usually contain three family members, MAP1A, MAP1B and a shorter, more recently identified gene called MAP1S. By contrast, only one family member, Futsch, is found in Drosophila. After their initial expression, the MAP1A and MAP1B polypeptides are cleaved into light and heavy chains, which are then assembled into mature complexes together with the separately encoded light chain 3 subunit (LC3). Both MAP1A and MAP1B are well known for their microtubule-stabilizing activity, but MAP1 proteins can also interact with other cellular components, including filamentous actin and signaling proteins. Furthermore, the activity of MAP1A and MAP1B is controlled by upstream signaling mechanisms, including the MAP kinase and glycogen synthase kinase-3 β pathways.

Heterotypic complex formation between subunits of microtubule-associated proteins 1A and 1B is due to interaction of conserved domains

The microtubule-associated proteins MAP1A and MAP1B are related but distinct multi-subunit protein complexes that consist of heavy and light chains. The predominant forms of these complexes are homotypic, i.e. they consist of a MAP1A heavy chain associated with MAP1A light chains or a MAP1B heavy chain associated with MAP1B light chains, respectively. In addition, MAP1A and MAP1B can exchange subunits and form heterotypic complexes consisting of a MAP1A heavy chain associated with MAP1B light chains which might play a role in a transition period of neuronal differentiation. Here we extend previous findings by confirming that heterotypic MAP1B heavy chain-MAP1A light chain complexes also exist in the developing murine brain. We show that these complexes form through interaction of homologous domains conserved in heavy and light chains of MAP1A and MAP1B. Likewise, conserved domains of the MAP1A and MAP1B light chains account for formation of light chain heterodimers. By yeast 2-hybrid analysis we located the light chain binding domain on the heavy chain to amino acids 211-508, thereby defining a new functional subdomain.

Role of MAP1B in axonal retrograde transport of mitochondria

The MAPs (microtubule-associated proteins) MAP1B and tau are well known for binding to microtubules and stabilizing these structures. An additional role for MAPs has emerged recently where they appear to participate in the regulation of transport of cargos on the microtubules found in axons. In this role, tau has been associated with the regulation of anterograde axonal transport. We now report that MAP1B is associated with the regulation of retrograde axonal transport of mitochondria. This finding potentially provides precise control of axonal transport by MAPs at several levels: controlling the anterograde or retrograde direction of transport depending on the type of MAP involved, controlling the speed of transport and controlling the stability of the microtubule tracks upon which transport occurs.

Impaired hippocampal long-term potentiation in microtubule-associated protein 1B-deficient mice

Microtubule-associated protein (MAP)1B-heterozygous (MAP1B+/-) mice are deficient in the expression of MAP1B in the hippocampus, cerebellum, and olfactory cortex. Although MAP1B+/- mice showed half the normal levels of MAP1B protein, they had no measurable amounts of phosphorylated MAP1B. High-frequency theta burst stimulation of Schaffer collateral-CA1 axons in hippocampal slices from MAP1B+/- mice elicited long-term potentiation (LTP) that decayed rapidly to baseline, in contrast to the non-decremental LTP exhibited by age-matched wild-type slices. A separate group of MAP1B+/- and wild-type slices was examined for a longer time course of 3 hr post-tetanus in response to multiple high-frequency stimulus trains that induced saturated LTP. MAP1B+/- slices showed marked reductions in both immediate post-tetanic potentiation and LTP that decayed much more rapidly than that in wild-type slices. The induction of LTP was associated with a rapid dephosphorylation of MAP1B within 5-15 min post-tetanus, suggesting that the normal expression of MAP1B and conversion to a dephosphorylated state may be a cellular mediator of cytoskeletal alterations necessary for long-term activity-dependent synaptic plasticity.

Mice deficient in microtubule-associated protein MAP1B show a distinct behavioral phenotype and altered retina function

We investigated mice deficient for the microtubule-associated protein MAP1B, a cytoskeletal element highly expressed in the developing nervous system, for altered performance in behavior, learning, and memory. Using the multiple T-maze, the open field and the Morris water maze we found that mice homozygous for a deletion of the MAP1B gene demonstrate impaired locomotor activity most likely correlated to a lack of physical endurance in general. In contrast, there were no significant differences in cognitive function and memory retention. In addition, we performed electroretinography and observed a reduction of the a-wave amplitude in response to single flash, white light stimulation. Taken together, these data provide further evidence for an important role of MAP1B in synaptic neurotransmission.

Binding of microtubule-associated protein 1B to LIS1 affects the interaction between dynein and LIS1

For neuronal migration to occur, the cell must undergo morphological changes that require modifications of the cytoskeleton. Several different MAPs (microtubule-associated proteins) or actin-binding proteins are proposed to be involved in the migration of neurons. Therefore we have specifically analysed how two members of the MAP family, MAP1B and LIS1 (lissencephaly-related protein 1), interact with one another and participate in neuronal migration. Our results indicate that, in hippocampal neurons, MAP1B and LIS1 co-localize, associate and interact with each another. The interaction between these two MAPs is regulated by the phosphorylation of MAP1B. Furthermore, this interaction interferes with the association between LIS1 and the microtubule-dependent molecular motor, dynein. Clearly, the differential binding of these cytoskeletal proteins could regulate the functions attributed to the LIS1-dynein complex, including those related to extension of the neural processes necessary for neuronal migration.

The gene trap resource: a treasure trove for hemopoiesis research

The laboratory mouse is an invaluable tool for functional gene discovery because of its genetic malleability and a biological similarity to human systems that facilitates identification of human models of disease. A number of mutagenic technologies are being used to elucidate gene function in the mouse. Gene trapping is an insertional mutagenesis strategy that is being undertaken by multiple research groups, both academic and private, in an effort to introduce mutations across the mouse genome. Large-scale, publicly funded gene trap programs have been initiated in several countries with the International Gene Trap Consortium coordinating certain efforts and resources. We outline the methodology of mammalian gene trapping and how it can be used to identify genes expressed in both primitive and definitive blood cells and to discover hemopoietic regulator genes. Mouse mutants with hematopoietic phenotypes derived using gene trapping are described. The efforts of the large-scale gene trapping consortia have now led to the availability of libraries of mutagenized ES cell clones. The identity of the trapped locus in each of these clones can be identified by sequence-based searching via the world wide web. This resource provides an extraordinary tool for all researchers wishing to use mouse genetics to understand gene function.

End binding protein-1 (EB1) complements microtubule-associated protein-1B during axonogenesis

The different strains of microtubule-associated protein (MAP)-1B-deficient mice that have been generated appear to express different phenotypes. This variability could be the consequence of the distinct genetic backgrounds of the animals used to generate these lines. Certain proteins might be able to complement the deficit of MAP1B function in these mice. Therefore, we examined whether the concentrations of potential compensatory proteins varied among these mutant strains. In this way, we identified significant differences in the amounts of the microtubule-associated EB1 protein between two of these strains. Furthermore, in transfection studies, we demonstrated that the overexpression of end binding protein-1 (EB1) could facilitate axonogenesis in MAP1B-/- cells in which EB1 is normally weakly expressed. Thus, we suggest that EB1 could complement MAP1B function during neural development.

Acute inactivation of MAP1b in growing sympathetic neurons destabilizes axonal microtubules

Microtubule-associated-protein 1b (MAP1b) is abundant in neurons actively extending axons. MAP1b is present on microtubules throughout growing axons, but is preferentially concentrated on microtubule polymer in the distal axon and growth cone. Although MAP1b has been implicated in axon growth and pathfinding, its specific functions are not well understood. Biochemical and transfection studies suggest that MAP1b has microtubule-stabilizing activity, but recent studies with neurons genetically deficient in MAP1b have not confirmed this. We have explored MAP1b functions in growing sympathetic neurons using an acute inactivation approach. Neurons without axons were injected with polyclonal MAP1b antibodies and then stimulated to extend axons. Injected cells were compared to controls in terms of axon growth behavior and several properties of axonal microtubules. The injected antibodies rapidly and quantitatively sequestered MAP1b in the cell body, making it unavailable to perform its normal functions. This immunodepletion of MAP1b had no statistically significant effect on axon growth, the amount of microtubule polymer in the axon, and the relative tyrosinated tubulin content of this polymer, and this was true in sympathetic neurons from rat, wild type mice, and tau knockout mice. Thus, robust axon growth can occur in the absence of MAP1b alone or both MAP1b and tau. However, immunodepletion of MAP1b significantly increased the sensitivity of microtubules in the distal axon and growth cone to nocodazole-induced depolymerization. These results indicate that MAP1b has microtubule-stabilizing activity in growing axons. This stabilizing activity may be required for some axonal functions, but it is not necessary for axon growth.

Disruption of the MAP1B-related protein FUTSCH leads to changes in the neuronal cytoskeleton, axonal transport defects, and progressive neurodegeneration in Drosophila

The elaboration of neuronal axons and dendrites is dependent on a functional cytoskeleton. Cytoskeletal components have been shown to play a major role in the maintenance of the nervous system through adulthood, and changes in neurofilaments and microtubule-associated proteins (MAPs) have been linked to a variety of neurodegenerative diseases. Here we show that Futsch, the fly homolog of MAP1B, is involved in progressive neurodegeneration. Although Futsch is widely expressed throughout the CNS, degeneration in futsch(olk) primarily occurs in the olfactory system and mushroom bodies. Consistent with the predicted function of Futsch, we find abnormalities in the microtubule network and defects in axonal transport. Degeneration in the adult brain is preceded by learning deficits, revealing a neuronal dysfunction before detectable levels of cell death. Futsch is negatively regulated by the Drosophila Fragile X mental retardation gene, and a mutation in this gene delays the onset of neurodegeneration in futsch(olk). A similar effect is obtained by expression of either fly or bovine tau, suggesting a certain degree of functional redundancy of MAPs. The futsch(olk) mutants exhibit several characteristics of human neurodegenerative diseases, providing an opportunity to study the role of MAPs in progressive neurodegeneration within an experimentally accessible, in vivo model system.

A role of MAP1B in Reelin-dependent neuronal migration

The signaling cascades governing neuronal migration are believed to link extracellular signals to cytoskeletal components. MAP1B is a neuron-specific microtubule-associated protein implicated in the control of the dynamic stability of microtubules and in the cross-talk between microtubules and actin filaments. Here we show that Reelin can induce mode I MAP1B phosphorylation, both in vivo and in vitro, through gsk3 and cdk5 activation. Additionally, mDab1 participates in the signaling cascade responsible for mode I MAP1B phosphorylation. Conversely, MAP1B-deficient mice display an abnormal structuring of the nervous system, especially in brain laminated areas, indicating a failure in neuronal migration. Therefore, we propose that Reelin can induce post-translational modifications on MAP1B that could correlate with its function in neuronal migration.

Microtubule-associated protein 1S, a short and ubiquitously expressed member of the microtubule-associated protein 1 family

The related high molecular mass microtubule-associated proteins (MAPs) MAP1A and MAP1B are predominantly expressed in the nervous system and are involved in axon guidance and synaptic function. MAP1B is implicated in fragile X mental retardation, giant axonal neuropathy, and ataxia type 1. We report the functional characterization of a novel member of the microtubule-associated protein 1 family, which we termed MAP1S (corresponding to sequence data bank entries for VCY2IP1 and C19ORF5). MAP1S contains the three hallmark domains of the microtubule-associated protein 1 family but hardly any additional sequences. It decorates neuronal microtubules and copurifies with tubulin from brain. MAP1S is synthesized as a precursor protein that is partially cleaved into heavy and light chains in a tissue-specific manner. Heavy and light chains interact to form the MAP1S complex. The light chain binds, bundles, and stabilizes microtubules and binds to actin. The heavy chain appears to regulate light chain activity. In contrast to MAP1A and MAP1B, MAP1S is expressed in a wide range of tissues in addition to neurons and represents the non-neuronal counterpart of this cytolinker family.

MAP1B is required for Netrin 1 signaling in neuronal migration and axonal guidance

BACKGROUND

The signaling cascades governing neuronal migration and axonal guidance link extracellular signals to cytoskeletal components. MAP1B is a neuron-specific microtubule-associated protein implicated in the crosstalk between microtubules and actin filaments.

RESULTS

Here we show that Netrin 1 regulates, both in vivo and in vitro, mode I MAP1B phosphorylation, which controls MAP1B activity, in a signaling pathway that depends essentially on the kinases GSK3 and CDK5. We also show that map1B-deficient neurons from the lower rhombic lip and other brain regions have reduced chemoattractive responses to Netrin 1 in vitro. Furthermore, map1B mutant mice have severe abnormalities, similar to those described in netrin 1-deficient mice, in axonal tracts and in the pontine nuclei.

CONCLUSIONS

These data indicate that MAP1B phosphorylation is controlled by Netrin 1 and that the lack of MAP1B impairs Netrin 1-mediated chemoattraction in vitro and in vivo. Thus, MAP1B may be a downstream effector in the Netrin 1-signaling pathway.

Targeted mutagenesis of the murine IRP1 and IRP2 genes reveals context-dependent RNA processing differences in vivo

We report the targeted mutagenesis of the murine iron regulatory protein (IRP)-1 and IRP2 genes, respectively, with a classical gene trap construct. Insertion of the targeting cassette into the second intron of either gene by homologous recombination interrupts their open reading frames near the N termini. Mice that are homozygous for the correctly modified IRP1 or IRP2 alleles, respectively, display a strong reduction (90%, IRP1(-/-)) or nondetectable levels (IRP2(-/-)) of the targeted proteins. Interestingly, the pre-mRNAs transcribed from the identical targeting cassettes are processed differently within the two different contexts. Detailed analysis of the respective products identifies the choice of alternative splice and 3' end processing sites in the same tissues in vivo. We discuss the implications for the understanding of RNA processing and for targeting strategies for functional genomics in the mouse.

High molecular weight microtubule-associated protein 2 over-expression in neuronal migration disorders caused by N-methyl-D-aspartate receptor activation in hamsters

To evaluate the neuronal cytoarchitectural changes in neuronal migration disorders, the immunohistochemical expression of microtubule-associated proteins (MAPs) was analyzed using the experimental model induced by ibotenate in newborn hamsters. The cortical lesions observed after intracerebral ibotenate injections strongly resembled the following neuronal migration disorders: (1) microgyria; (2) focal subcortical heterotopia; (3) focal subependymal heterotopia; and (4) leptomeningeal glioneuronal heterotopia. Microgyria and leptomeningeal glioneuronal heterotopia had MAP2 (HM-2: high and low-molecular-weight forms of MAP2) immunoreactive dendritic processes or neuronal elements. The high molecular weight isoform of MAP2 (AP-20), which is more characteristic of mature neurons, showed enhanced expression in neurons of focal subcortical or subependymal heterotopia, although MAP1B (AA6: early form of MAP) immunoreactive elements were not detected in these heterotopic areas. We conclude that high molecular weight isoform of MAP2 is closely associated with cytoarchitectural repair and remodeling of neuronal processes, resulting in neuronal heterotopia after NMDA receptor activation.

Microtubule-associated protein 1B function during normal development, regeneration, and pathological conditions in the nervous system

Microtubule-associated protein 1B is the first MAP to be expressed during the development of the nervous system. Several different approaches have revealed that MAP1B function is associated with microtubule and actin microfilament polymerization and dynamics. In recent years, the generation of molecular models to inactivate MAP1B function in invertebrates and mammals has sparked some controversy about the real role of MAP1B. Despite discrepancies between some studies, it is clear that MAP1B plays a principal role in the development of the nervous system. In this article, we summarize the evidence for MAP1B function in a wide variety of cellular processes implicated in the proper construction of the nervous system. We also discuss the role of MAP1B in pathological processes.

A cAMP-activated pathway, including PKA and PI3K, regulates neuronal differentiation

Neuronal differentiation is a complex process in which many different signalling pathways may be involved. An increase in the intracellular levels of cyclic AMP (cAMP) has been shown to induce neuronal differentiation and also to cooperate with NGF to induce PC12 neurite outgrowth in a Ras-dependent manner. However, the neuritogenic activities associated with cAMP are still not well understood. The purpose of this study was to investigate the potential neuritogenic activities mediated by cAMP. For this purpose, we used the human neuroblastoma cell line SH-SY5Y. These neuroblastoma cells respond to cAMP by forming neurite-like extensions. We tried to identify some essential pathways involved in the cAMP-induced neurite elongation of these cells. Our results indicated that PKA is transiently activated in this elongation model. When we blocked PKA activity, elongation did not take place. Similarly, PI3K also plays an essential role because when we blocked this kinase activity, there was no neurite elongation. Indeed, over-expression of the p110-catalytic subunit or an activating form of the p85-regulatory subunit (p65) is able to induce some degree of neurite extension. Moreover, our results showed that when elongation is initiated, PI3K is still essential for maintenance of the neuronal morphology, whereas PKA or MAPK (ERKs or p38) activation does not appear to be necessary during this process.

NGF activates the phosphorylation of MAP1B by GSK3beta through the TrkA receptor and not the p75(NTR) receptor

We have recently shown that nerve growth factor (NGF) induces the phosphorylation of the microtubule-associated protein 1B (MAP1B) by activating the serine/threonine kinase glycogen synthase kinase 3beta (GSK3beta) in a spatio-temporal pattern in PC12 cells that correlates tightly with neurite growth. PC12 cells express two types of membrane receptor for NGF: TrkA receptors and p75NTR receptors, and it was not clear from our studies which receptor was responsible. We show here that brain-derived neurotrophic factor, which activates p75NTR but not TrkA receptors, does not stimulate GSK3beta phosphorylation of MAP1B in PC12 cells. Similarly, NGF fails to activate GSK3beta phosphorylation of MAP1B in PC12 cells that lack TrkA receptors but express p75NTR receptors (PC12 nnr). Chick ciliary ganglion neurons in culture lack TrkA receptors but express p75NTR and also fail to show NGF-dependent GSK3beta phosphorylation of MAP1B, whereas in rat superior cervical ganglion neurons in culture, NGF activation of TrkA receptors elicits GSK3beta phosphorylation of MAP1B. Finally, inhibition of TrkA receptor tyrosine kinase activity in PC12 cells and superior cervical ganglion neurons with K252a potently and dose-dependently inhibits neurite elongation while concomitantly blocking GSK3beta phosphorylation of MAP1B. These results suggest that the activation of GSK3beta by NGF is mediated through the TrkA tyrosine kinase receptor and not through p75NTR receptors.