Evidence for the role of MAP1B in axon formation

Exploring novel and potent glycogen synthase kinase-3β inhibitors through systematic drug designing approach

Significant implications of glycogen synthase kinase-3β (GSK-3β) have been reported in various neuronal disorders and malignant cancers. GSK-3β modulates diverse protein targets through phosphorylation, and its aberrant activity leads to neurological complications as well as tumour onset. Therefore, inhibiting GSK-3β activity through active-site fitting molecules may offer a favourable strategy for intercepting these disorders. This comprehensive study used multiple assays in tandem in order to explore the most potent GSK-3β inhibitor. Following structural similarity screening, 135 molecular docking and 135 standard MM-GBSA experiments were performed using AZD1080, a known inhibitor as standard. Among the 32 molecules demonstrating a stronger binding affinity than reference, only the two most potent molecules were chosen and their binding free energy was compared to AZD1080 using the Desmond trajectory clustering and eventual MM-GBSA. Additionally, the interaction status of these molecules and AZD1080 with GSK-3β was explored post-molecular dynamics. The stability of the strongest molecule (most potent) was evaluated in the active site of the above-mentioned kinase keeping its apo-form as reference. Notably, the e-Pharmacophores mapping was performed to address the level of complementarity of the most potent molecule and AZD1080 with the functional site of GSK-3β. Using various techniques, we identified the molecule with PubChem CID: 11167509 as the strongest molecule for obstructing GSK-3β, which may serve as a promising therapeutic after the meticulous evaluation on diverse models.

Tubulin posttranslational modifications through the lens of new technologies

The Tubulin Code revolutionizes our understanding of microtubule dynamics and functions, proposing a nuanced system governed by tubulin isotypes, posttranslational modifications (PTMs) and microtubule-associated proteins (MAPs). Tubulin isotypes, diverse across species, contribute structural complexity, and are thought to influence microtubule functions. PTMs encode dynamic information on microtubules, which are read by several microtubule interacting proteins and impact on cellular processes. Here we discuss recent technological and methodological advances, such as in genome engineering, live cell imaging, expansion microscopy, and cryo-electron microscopy that reveal new elements and levels of complexity of the tubulin code, including new modifying enzymes and nanopatterns of PTMs on individual microtubules. The Tubulin Code's exploration holds transformative potential, guiding therapeutic strategies and illuminating connections to diseases like cancer and neurodegenerative disorders, underscoring its relevance in decoding fundamental cellular language.

Microtubule-binding protein MAP1B regulates interstitial axon branching of cortical neurons via the tubulin tyrosination cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial (or collateral) axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs). This method allows for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. These data suggest a cell-autonomous molecular regulation of cortical neuron axon morphology, in which GSK3β can release a MAP1B-mediated brake on interstitial axon branching upstream of the posttranslational tubulin code.

FMRP regulates tangential neuronal migration via MAP1B

The Fragile X Syndrome (FXS) represents the most prevalent form of inherited intellectual disability and is the first monogenic cause of Autism Spectrum Disorder. FXS results from the absence of the RNA-binding protein FMRP (Fragile X Messenger Ribonucleoprotein). Neuronal migration is an essential step of brain development allowing displacement of neurons from their germinal niches to their final integration site. The precise role of FMRP in neuronal migration remains largely unexplored. Using live imaging of postnatal Rostral Migratory Stream (RMS) neurons in Fmr1-null mice, we observed that the absence of FMRP leads to delayed neuronal migration and altered trajectory, associated with defects of centrosomal movement. RNA-interference-induced knockdown of Fmr1 shows that these migratory defects are cell-autonomous. Notably, the primary FMRP mRNA target implicated in these migratory defects is MAP1B (Microtubule-Associated Protein 1B). Knocking-down MAP1B expression effectively rescued most of the observed migratory defects. Finally, we elucidate the molecular mechanisms at play by demonstrating that the absence of FMRP induces defects in the cage of microtubules surrounding the nucleus of migrating neurons, which is rescued by MAP1B knockdown. Our findings reveal a novel neurodevelopmental role for FMRP in collaboration with MAP1B, jointly orchestrating neuronal migration by influencing the microtubular cytoskeleton.

Inhibition of microtubule detyrosination by parthenolide facilitates functional CNS axon regeneration

Injured axons in the central nervous system (CNS) usually fail to regenerate, causing permanent disabilities. However, the knockdown of Pten knockout or treatment of neurons with hyper-IL-6 (hIL-6) transforms neurons into a regenerative state, allowing them to regenerate axons in the injured optic nerve and spinal cord. Transneuronal delivery of hIL-6 to the injured brain stem neurons enables functional recovery after severe spinal cord injury. Here we demonstrate that the beneficial hIL-6 and Pten knockout effects on axon growth are limited by the induction of tubulin detyrosination in axonal growth cones. Hence, cotreatment with parthenolide, a compound blocking microtubule detyrosination, synergistically accelerates neurite growth of cultured murine CNS neurons and primary RGCs isolated from adult human eyes. Systemic application of the prodrug dimethylamino-parthenolide (DMAPT) facilitates axon regeneration in the injured optic nerve and spinal cord. Moreover, combinatorial treatment further improves hIL-6-induced axon regeneration and locomotor recovery after severe SCI. Thus, DMAPT facilitates functional CNS regeneration and reduces the limiting effects of pro-regenerative treatments, making it a promising drug candidate for treating CNS injuries.

MAP1B Regulates Cortical Neuron Interstitial Axon Branching Through the Tubulin Tyrosination Cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs), allowing for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. We propose that MAP1B functions as a brake on axon branching that can be released by GSK3β activation, regulating the tubulin code and thereby playing an integral role in sculpting cortical neuron axon morphology.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Microtubule Cytoskeletal Network Alterations in a Transgenic Model of Tuberous Sclerosis Complex: Relevance to Autism Spectrum Disorders

Tuberous sclerosis complex (TSC) is a rare genetic multisystem disorder caused by loss-of-function mutations in the tumour suppressors TSC1/TSC2, both of which are negative regulators of the mammalian target of rapamycin (mTOR) kinase. Importantly, mTOR hyperactivity seems to be linked with the pathobiology of autism spectrum disorders (ASD). Recent studies suggest the potential involvement of microtubule (MT) network dysfunction in the neuropathology of "mTORopathies", including ASD. Cytoskeletal reorganization could be responsible for neuroplasticity disturbances in ASD individuals. Thus, the aim of this work was to study the effect of Tsc2 haploinsufficiency on the cytoskeletal pathology and disturbances in the proteostasis of the key cytoskeletal proteins in the brain of a TSC mouse model of ASD. Western-blot analysis indicated significant brain-structure-dependent abnormalities in the microtubule-associated protein Tau (MAP-Tau), and reduced MAP1B and neurofilament light (NF-L) protein level in 2-month-old male B6;129S4-Tsc2tm1Djk/J mice. Alongside, pathological irregularities in the ultrastructure of both MT and neurofilament (NFL) networks as well as swelling of the nerve endings were demonstrated. These changes in the level of key cytoskeletal proteins in the brain of the autistic-like TSC mice suggest the possible molecular mechanisms responsible for neuroplasticity alterations in the ASD brain.

Deciphering the Tubulin Language: Molecular Determinants and Readout Mechanisms of the Tubulin Code in Neurons

Microtubules (MTs) are dynamic components of the cell cytoskeleton involved in several cellular functions, such as structural support, migration and intracellular trafficking. Despite their high similarity, MTs have functional heterogeneity that is generated by the incorporation into the MT lattice of different tubulin gene products and by their post-translational modifications (PTMs). Such regulations, besides modulating the tubulin composition of MTs, create on their surface a "biochemical code" that is translated, through the action of protein effectors, into specific MT-based functions. This code, known as "tubulin code", plays an important role in neuronal cells, whose highly specialized morphologies and activities depend on the correct functioning of the MT cytoskeleton and on its interplay with a myriad of MT-interacting proteins. In recent years, a growing number of mutations in genes encoding for tubulins, MT-interacting proteins and enzymes that post-translationally modify MTs, which are the main players of the tubulin code, have been linked to neurodegenerative processes or abnormalities in neural migration, differentiation and connectivity. Nevertheless, the exact molecular mechanisms through which the cell writes and, downstream, MT-interacting proteins decipher the tubulin code are still largely uncharted. The purpose of this review is to describe the molecular determinants and the readout mechanisms of the tubulin code, and briefly elucidate how they coordinate MT behavior during critical neuronal events, such as neuron migration, maturation and axonal transport.

Alterations in Cerebellar Microtubule Cytoskeletal Network in a ValproicAcid-Induced Rat Model of Autism Spectrum Disorders

Autism spectrum disorders (ASD) are neurodevelopmental diseases characterised by deficits in social communication, restricted interests, and repetitive behaviours. The growing body of evidence points to a role for cerebellar changes in ASD pathology. Some of the findings suggest that not only motor problems but also social deficits, repetitive behaviours, and mental inflexibility associated with ASD are connected with damage to the cerebellum. However, the understanding of this brain structure's functions in ASD pathology needs future investigations. Therefore, in this study, we generated a rodent model of ASD through a single prenatal administration of valproic acid (VPA) into pregnant rats, followed by cerebellar morphological studies of the offspring, focusing on the alterations of key cytoskeletal elements. The expression (Western blot) of α/β-tubulin and the major neuronal MT-associated proteins (MAP) such as MAP-Tau and MAP1B, MAP2, MAP6 (STOP) along with actin-crosslinking αII-spectrin and neurofilament light polypeptide (NF-L) was investigated. We found that maternal exposure to VPA induces a significant decrease in the protein levels of α/β-tubulin, MAP-Tau, MAP1B, MAP2, and αII-spectrin. Moreover, excessive MAP-Tau phosphorylation at (Ser396) along with key Tau-kinases activation was indicated. Immunohistochemical staining showed chromatolysis in the cerebellum of autistic-like rats and loss of Purkinje cells shedding light on one of the possible molecular mechanisms underpinning neuroplasticity alterations in the ASD brain.

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.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

Maternal oxytocin administration modulates gene expression in the brains of perinatal mice

OBJECTIVES

Oxytocin (OXT) is widely used to facilitate labor. However, little is known about the effects of perinatal OXT exposure on the developing brain. We investigated the effects of maternal OXT administration on gene expression in perinatal mouse brains.

METHODS

Pregnant C57BL/6 mice were treated with saline or OXT at term (n=6-7/group). Dams and pups were euthanized on gestational day (GD) 18.5 after delivery by C-section. Another set of dams was treated with saline or OXT (n=6-7/group) and allowed to deliver naturally; pups were euthanized on postnatal day 9 (PND9). Perinatal/neonatal brain gene expression was determined using Illumina BeadChip Arrays and real time quantitative PCR. Differential gene expression analyses were performed. In addition, the effect of OXT on neurite outgrowth was assessed using PC12 cells.

RESULTS

Distinct and sex-specific gene expression patterns were identified in offspring brains following maternal OXT administration at term. The microarray data showed that female GD18.5 brains exhibited more differential changes in gene expression compared to male GD18.5 brains. Specifically, Cnot4 and Frmd4a were significantly reduced by OXT exposure in male and female GD18.5 brains, whereas Mtap1b, Srsf11, and Syn2 were significantly reduced only in female GD18.5 brains. No significant microarray differences were observed in PND9 brains. By quantitative PCR, OXT exposure reduced Oxtr expression in female and male brains on GD18.5 and PND9, respectively. PC12 cell differentiation assays revealed that OXT induced neurite outgrowth.

CONCLUSIONS

Prenatal OXT exposure induces sex-specific differential regulation of several nervous system-related genes and pathways with important neural functions in perinatal brains.

Quantitative Phosphoproteomics Reveals Extensive Protein Phosphorylation Dysregulation in the Cerebral Cortex of Huntington's Disease Mice Prior to Onset of Symptoms

Protein phosphorylation plays a role in many important cellular functions such as cellular plasticity, gene expression, and intracellular trafficking. All of these are dysregulated in Huntington's disease (HD), a devastating neurodegenerative disorder caused by an expanded CAG repeat in exon 1 of the huntingtin gene. However, no studies have yet found protein phosphorylation differences in preclinical HD mouse models. Our current study investigated changes occurring in the cortical phosphoproteome of 8-week-old (prior to motor deficits) and 20-week-old (fully symptomatic) R6/1 transgenic HD mice. When comparing 8-week-old HD mice with their wild-type (WT) littermates, we found 660 peptides differentially phosphorylated, which were mapped to 227 phosphoproteins. These proteins were mainly involved in synaptogenesis, cytoskeleton organization, axon development, and nervous system development. Tau protein, found hyperphosphorylated at multiple sites in early symptomatic HD mice, also appeared as a main upstream regulator for the changes observed. Surprisingly, we found fewer changes in the phosphorylation profile of HD mice at the fully symptomatic stage, with 29 peptides differentially phosphorylated compared to WT mice, mapped to 25 phosphoproteins. These proteins were involved in cAMP signaling, dendrite development, and microtubule binding. Furthermore, huntingtin protein appeared as an upstream regulator for the changes observed at the fully symptomatic stage, suggesting impacts on kinases and phosphatases that extend beyond the mutated polyglutamine tract. In summary, our findings show that the most extensive changes in the phosphorylation machinery appear at an early presymptomatic stage in HD pathogenesis and might constitute a new target for the development of treatments.

Bridging the Gap: The Importance of TUBA1A α-Tubulin in Forming Midline Commissures

Developing neurons undergo dramatic morphological changes to appropriately migrate and extend axons to make synaptic connections. The microtubule cytoskeleton, made of α/β-tubulin dimers, drives neurite outgrowth, promotes neuronal growth cone responses, and facilitates intracellular transport of critical cargoes during neurodevelopment. TUBA1A constitutes the majority of α-tubulin in the developing brain and mutations to TUBA1A in humans cause severe brain malformations accompanied by varying neurological defects, collectively termed tubulinopathies. Studies of TUBA1A function in mammalian cells have been limited by the presence of multiple genes encoding highly similar tubulin proteins, which leads to α-tubulin antibody promiscuity and makes genetic manipulation challenging. Here, we test mutant tubulin levels and assembly activity and analyze the impact of TUBA1A reduction on growth cone composition, neurite extension, and commissural axon architecture during brain development. We present a novel tagging method for studying and manipulating TUBA1A in cells without impairing tubulin function. Using this tool, we show that a TUBA1A loss-of-function mutation TUBA1A N102D (TUBA1A ND ), reduces TUBA1A protein levels and prevents incorporation of TUBA1A into microtubule polymers. Reduced Tuba1a α-tubulin in heterozygous Tuba1a ND/+ mice leads to grossly normal brain formation except a significant impact on axon extension and impaired formation of forebrain commissures. Neurons with reduced Tuba1a as a result of the Tuba1a ND mutation exhibit slower neuron outgrowth compared to controls. Neurons deficient in Tuba1a failed to localize microtubule associated protein-1b (Map1b) to the developing growth cone, likely impacting stabilization of microtubules. Overall, we show that reduced Tuba1a is sufficient to support neuronal migration and cortex development but not commissure formation, and provide mechanistic insight as to how TUBA1A tunes microtubule function to support neurodevelopment.

Protocol to study starvation-induced autophagy in developing rat neurons

Autophagy is being involved in an increasing number of cellular pathways. It now appears that autophagy stimulation and inhibition have complex effects in neurons. Here, we present a simple yet powerful protocol to induce autophagy in primary neurons in culture by partial nutrient deprivation, in neurons with or without transfection of plasmids encoding the Longin domain of VAMP7 or a nanobody directed against VAMP7. Although limited to cells in culture, this protocol can facilitate the study of autophagy in neurons.

For complete details on the use and execution of this protocol, please refer to Wojnacki et al. (2020).

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Primary neuron hippocampal culture for morphological analysis after starvation

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In vitro neuronal starvation by culture media dilution

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Fixation and immuno staining for morphological analysis

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Image acquisition and determination of axonal length after 24 h starvation

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

Cytoskeletal regulation guides neuronal trafficking to effectively supply the synapse

The development and proper function of the brain requires the formation of highly complex neuronal circuitry. These circuits are shaped from synaptic connections between neurons and must be maintained over a lifetime. The formation and continued maintenance of synapses requires accurate trafficking of presynaptic and postsynaptic components along the axon and dendrite, respectively, necessitating deliberate and specialized delivery strategies to replenish essential synaptic components. Maintenance of synaptic transmission also requires readily accessible energy stores, produced in part by localized mitochondria, that are tightly regulated with activity level. In this review, we focus on recent developments in our understanding of the cytoskeletal environment of axons and dendrites, examining how local regulation of cytoskeletal dynamics and organelle trafficking promotes synapse-specific delivery and plasticity. These new insights shed light on the complex and coordinated role that cytoskeletal elements play in establishing and maintaining neuronal circuitry.

Identification of SNPs Associated with Stress Response Traits within High Stress and Low Stress Lines of Japanese Quail

Mitigation of stress is of great importance in poultry production, as chronic stress can affect the efficiency of production traits. Selective breeding with a focus on stress responses can be used to combat the effects of stress. To better understand the genetic mechanisms driving differences in stress responses of a selectively bred population of Japanese quail, we performed genomic resequencing on 24 birds from High Stress (HS) and Low Stress (LS) lines of Japanese quail using Illumina HiSeq 2 × 150 bp paired end read technology in order to analyze Single Nucleotide Polymorphisms (SNPs) within the genome of each line. SNPs are common mutations that can lead to genotypic and phenotypic variations in animals. Following alignment of the sequencing data to the quail genome, 6,364,907 SNPs were found across both lines of quail. 10,364 of these SNPs occurred in coding regions, from which 2886 unique, non-synonymous SNPs with a SNP% ≥ 0.90 and a read depth ≥ 10 were identified. Using Ingenuity Pathway Analysis, we identified genes affected by SNPs in pathways tied to immune responses, DNA repair, and neurological signaling. Our findings support the idea that the SNPs found within HS and LS lines of quail could direct the observed changes in phenotype.

New insights on epigenetic mechanisms supporting axonal development: histone marks and miRNAs

Mechanisms supporting axon growth and the establishment of neuronal polarity have remained largely disconnected from their genetic and epigenetic fundamentals. Recently, post-transcriptional modifications of histones involved in chromatin folding and transcription, and microRNAs controlling translation have emerged as regulators of axonal specification, growth, and guidance. In this article, we review novel evidence supporting the concept that epigenetic mechanisms work at both transcriptional and post-transcriptional levels to shape axons. We also discuss the role of splicing on axonal growth, as one of the most (if not the most) powerful post-transcriptional mechanism to diversify genetic information. Overall, we think exploring the gap between epigenetics and axonal growth raises new questions and perspectives to the development of axons in physiological and pathological contexts.

Protocol for Evaluating Neuronal Polarity in Murine Models

The establishment of polarity is crucial for the physiology and wiring of neurons. Therefore, monitoring the axo-dendritic specification allows the mechanisms and signals associated with development, growth, and disease to be explored. Here, we describe major and minor steps to study polarity acquisition, using primary cultures of hippocampal neurons isolated from embryonic rat hippocampi, for in vitro monitoring. Furthermore, we use in utero electroporated, GFP-expressing embryonic mouse brains for visualizing cortical neuron migration and polarization in situ. Some underreported after-protocol steps are also included.

For complete details on the use and execution of this protocol, please refer to Wilson et al. (2020).

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Dissection, isolation, and digestion of embryonic (E18.5) rat hippocampi

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Culturing isolated hippocampal neurons and monitoring polarity acquisition in vitro

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In utero electroporation of embryonic (E15.5) mouse brains with GFP plasmids

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Visualization of migration and polarization of E17.5–E18.5 cortical neurons in situ

Redox modifications in synaptic components as biomarkers of cognitive status, in brain aging and disease

Aging is a natural process that includes several changes that gradually make organisms degenerate and die. Harman's theory proposes that aging is a consequence of the progressive accumulation of oxidative modifications mediated by reactive oxygen/nitrogen species, which plays an essential role in the development and progression of many neurodegenerative diseases. This review will focus on how abnormal redox modifications induced by age impair the functionality of neuronal redox-sensitive proteins involved in axonal elongation and guidance, synaptic plasticity, and intercellular communication. We will discuss post-transcriptional regulation of gene expression by microRNAs as a mechanism that controls the neuronal redox state. Finally, we will discuss how some brain-permeant antioxidants from the diet have a beneficial effect on cognition. Taken together, the evidence revised here indicates that oxidative-driven modifications of specific proteins and changes in microRNA expression may be useful biomarkers for aging and neurodegenerative diseases. Also, some specific antioxidant therapies have undoubtedly beneficial neuroprotective effects when administered in the correct doses, in the ideal formulation combination, and during the appropriate therapeutic window. The use of some antioxidants is, therefore, still poorly explored for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Microtubule-Associated Proteins: Structuring the Cytoskeleton

Microtubule-associated proteins (MAPs) were initially discovered as proteins that bind to and stabilize microtubules. Today, an ever-growing number of MAPs reveals a more complex picture of these proteins as organizers of the microtubule cytoskeleton that have a large variety of functions. MAPs enable microtubules to participate in a plethora of cellular processes such as the assembly of mitotic and meiotic spindles, neuronal development, and the formation of the ciliary axoneme. Although some subgroups of MAPs have been exhaustively characterized, a strikingly large number of MAPs remain barely characterized other than their interactions with microtubules. We provide a comprehensive view on the currently known MAPs in mammals. We discuss their molecular mechanisms and functions, as well as their physiological role and links to pathologies.

De novo and inherited private variants in MAP1B in periventricular nodular heterotopia

Periventricular nodular heterotopia (PVNH) is a malformation of cortical development commonly associated with epilepsy. We exome sequenced 202 individuals with sporadic PVNH to identify novel genetic risk loci. We first performed a trio-based analysis and identified 219 de novo variants. Although no novel genes were implicated in this initial analysis, PVNH cases were found overall to have a significant excess of nonsynonymous de novo variants in intolerant genes (p = 3.27x10-7), suggesting a role for rare new alleles in genes yet to be associated with the condition. Using a gene-level collapsing analysis comparing cases and controls, we identified a genome-wide significant signal driven by four ultra-rare loss-of-function heterozygous variants in MAP1B, including one de novo variant. In at least one instance, the MAP1B variant was inherited from a parent with previously undiagnosed PVNH. The PVNH was frontally predominant and associated with perisylvian polymicrogyria. These results implicate MAP1B in PVNH. More broadly, our findings suggest that detrimental mutations likely arising in immediately preceding generations with incomplete penetrance may also be responsible for some apparently sporadic diseases.

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.

MAP1B Light Chain Modulates Synaptic Transmission via AMPA Receptor Intracellular Trapping

The regulated transport of AMPA-type glutamate receptors (AMPARs) to the synaptic membrane is a key mechanism to determine the strength of excitatory synaptic transmission in the brain. In this work, we uncovered a new role for the microtubule-associated protein MAP1B in modulating access of AMPARs to the postsynaptic membrane. Using mice and rats of either sex, we show that MAP1B light chain (LC) accumulates in the somatodendritic compartment of hippocampal neurons, where it forms immobile complexes on microtubules that limit vesicular transport. These complexes restrict AMPAR dendritic mobility, leading to the intracellular trapping of receptors and impairing their access to the dendritic surface and spines. Accordingly, increasing MAP1B-LC expression depresses AMPAR-mediated synaptic transmission. This effect is specific for the GluA2 subunit of the AMPAR and requires glutamate receptor interacting protein 1 (GRIP1) interaction with MAP1B-LC. Therefore, MAP1B-LC represents an alternative link between GRIP1-AMPARs and microtubules that does not result in productive transport, but rather limits AMPAR availability for synaptic insertion, with a direct impact on synaptic transmission.SIGNIFICANCE STATEMENT The ability of neurons to modify their synaptic connections, known as synaptic plasticity, is accepted as the cellular basis for learning and memory. One mechanism for synaptic plasticity is the regulated addition and removal of AMPA-type glutamate receptors (AMPARs) at excitatory synapses. In this study, we found that a microtubule-associated protein, MAP1B light chain (MAP1B-LC), participates in this process. MAP1B-LC forms immobile complexes along dendrites. These complexes limit intracellular vesicular trafficking and trap AMPARs inside the dendritic shaft. In this manner, MAP1B restricts the access of AMPARs to dendritic spines and the postsynaptic membrane, contributing to downregulating synaptic transmission.

p39 Is Responsible for Increasing Cdk5 Activity during Postnatal Neuron Differentiation and Governs Neuronal Network Formation and Epileptic Responses

Two distinct protein cofactors, p35 and p39, independently activate Cyclin-dependent kinase 5 (Cdk5), which plays diverse roles in normal brain function and the pathogenesis of many neurological diseases. The initial discovery that loss of p35 impairs neuronal migration in the embryonic brain prompted intensive research exploring the function of p35-dependent Cdk5 activity. In contrast, p39 expression is restricted to the postnatal brain and its function remains poorly understood. Despite the robustly increased Cdk5 activity during neuronal differentiation, which activator is responsible for enhancing Cdk5 activation and how the two distinct activators direct Cdk5 signaling to govern neuronal network formation and function still remains elusive. Here we report that p39, but not p35, is selectively upregulated by histone acetylation-mediated transcription, which underlies the robust increase of Cdk5 activity during rat and mouse neuronal differentiation. The loss of p39 attenuates overall Cdk5 activity in neurons and preferentially affects phosphorylation of specific Cdk5 targets, leading to aberrant axonal growth and impaired dendritic spine and synapse formation. In adult mouse brains, p39 deficiency results in dysregulation of p35 and Cdk5 targets in synapses. Moreover, in contrast to the proepileptic phenotype caused by the lack of p35, p39 loss leads to deficits in maintaining seizure activity and induction of immediate early genes that control hippocampal excitability. Together, our studies demonstrate essential roles of p39 in neuronal network development and function. Furthermore, our data support a model in which Cdk5 activators play nonoverlapping and even opposing roles to govern balanced Cdk5 signaling in the postnatal brain.

SIGNIFICANCE STATEMENT

Neuronal network development requires tightly regulated activation of Cyclin-dependent kinase 5 (Cdk5) by two distinct cofactors, p35 and p39. Despite the well-known p35-dependent Cdk5 function, why postnatal neurons express abundant p39 in addition to p35 remained unknown for decades. In this study, we discovered that selective upregulation of p39 is the underlying mechanism that accommodates the increased functional requirement of Cdk5 activation during neuronal differentiation. In addition, we demonstrated that p39 selectively directs Cdk5 to phosphorylate protein substrates essential for axonal development, dendritic spine formation, and synaptogenesis. Moreover, our studies suggest opposing roles of p39 and p35 in synaptic Cdk5 function and epileptic responses, arguing that cooperation between Cdk5 activators maintains balanced Cdk5 signing, which is crucial for postnatal brain function.

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.

Neuronal polarization: From spatiotemporal signaling to cytoskeletal dynamics

Neuronal polarization establishes distinct molecular structures to generate a single axon and multiple dendrites. Studies over the past years indicate that this efficient separation is brought about by a network of feedback loops. Axonal growth seems to play a major role in fueling those feedback loops and thereby stabilizing neuronal polarity. Indeed, various effectors involved in feedback loops are pivotal for axonal growth by ultimately acting on the actin and microtubule cytoskeleton. These effectors have key roles in interconnecting actin and microtubule dynamics - a mechanism crucial to commanding the growth of axons. We propose a model connecting signaling with cytoskeletal dynamics and neurite growth to better describe the underlying processes involved in neuronal polarization. We will discuss the current views on feedback loops and highlight the current limits of our understanding.

Microtubule-regulating proteins and cAMP-dependent signaling in neuroblastoma differentiation

Neurons are highly differentiated cells responsible for the conduction and transmission of information in the nervous system. The proper function of a neuron relies on the compartmentalization of their intracellular domains. Differentiated neuroblastoma cells have been extensively used to study and understand the physiology and cell biology of neuronal cells. Here, we show that differentiation of N1E-115 neuroblastoma cells is more pronounced upon exposure of a chemical analog of cyclic AMP (cAMP), db-cAMP. We next analysed the expression of key microtubule-regulating proteins in differentiated cells and the expression and activation of key cAMP players such as EPAC, PKA and AKAP79/150. Most of the microtubule-promoting factors were up regulated during differentiation of N1E-115 cells, while microtubule-destabilizing proteins were down regulated. We observed an increase in tubulin post-translational modifications related to microtubule stability. As expected, db-cAMP increased PKA- and EPAC-dependent signalling. Consistently, pharmacological modulation of EPAC activity instructed cell differentiation, number of neurites, and neurite length in N1E-115 cells. Moreover, disruption of the PKA-AKAP interaction reduced these morphometric parameters. Interestingly, PKA and EPAC act synergistically to induce neuronal differentiation in N1E-115. Altogether these results show that the changes observed in the differentiation of N1E-115 cells proceed by regulating several microtubule-stabilizing factors, and the acquisition of a neuronal phenotype is a process involving concerted although independent functions of EPAC and PKA.

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Microtubule-associated protein 1B (MAP1B)-deficient neurons show structural presynaptic deficiencies in vitro and altered presynaptic physiology

Microtubule-associated protein 1B (MAP1B) is expressed predominantly during the early stages of development of the nervous system, where it regulates processes such as axonal guidance and elongation. Nevertheless, MAP1B expression in the brain persists in adult stages, where it participates in the regulation of the structure and physiology of dendritic spines in glutamatergic synapses. Moreover, MAP1B expression is also found in presynaptic synaptosomal preparations. In this work, we describe a presynaptic phenotype in mature neurons derived from MAP1B knockout (MAP1B KO) mice. Mature neurons express MAP1B, and its deficiency does not alter the expression levels of a subgroup of other synaptic proteins. MAP1B KO neurons display a decrease in the density of presynaptic and postsynaptic terminals, which involves a reduction in the density of synaptic contacts, and an increased proportion of orphan presynaptic terminals. Accordingly, MAP1B KO neurons present altered synaptic vesicle fusion events, as shown by FM4-64 release assay, and a decrease in the density of both synaptic vesicles and dense core vesicles at presynaptic terminals. Finally, an increased proportion of excitatory immature symmetrical synaptic contacts in MAP1B KO neurons was detected. Altogether these results suggest a novel role for MAP1B in presynaptic structure and physiology regulation in vitro.

Cellular and molecular mechanisms regulating neuronal growth by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) and its receptors TrkB and p75 regulate dendritic and axonal growth during development and maintenance of the mature nervous system; however, the cellular and molecular mechanisms underlying this process are not fully understood. In recent years, several advances have shed new light on the processes behind the regulation of BDNF-mediated structural plasticity including control of neuronal transcription, local translation of proteins, and regulation of cytoskeleton and membrane dynamics. In this review, we summarize recent advances in the field of BDNF signaling in neurons to induce neuronal growth. © 2016 Wiley Periodicals, Inc.

The cytoskeleton as a novel therapeutic target for old neurodegenerative disorders

Cytoskeleton defects, including alterations in microtubule stability, in axonal transport as well as in actin dynamics, have been characterized in several unrelated neurodegenerative conditions. These observations suggest that defects of cytoskeleton organization may be a common feature contributing to neurodegeneration. In line with this hypothesis, drugs targeting the cytoskeleton are currently being tested in animal models and in human clinical trials, showing promising effects. Drugs that modulate microtubule stability, inhibitors of posttranslational modifications of cytoskeletal components, specifically compounds affecting the levels of tubulin acetylation, and compounds targeting signaling molecules which regulate cytoskeleton dynamics, constitute the mostly addressed therapeutic interventions aiming at preventing cytoskeleton damage in neurodegenerative disorders. In this review, we will discuss in a critical perspective the current knowledge on cytoskeleton damage pathways as well as therapeutic strategies designed to revert cytoskeleton-related defects mainly focusing on the following neurodegenerative disorders: Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, Amyotrophic Lateral Sclerosis and Charcot-Marie-Tooth Disease.

Dendrite and Axon Specific Geometrical Transformation in Neurite Development

We propose a model of neurite growth to explain the differences in dendrite and axon specific neurite development. The model implements basic molecular kinetics, e.g., building protein synthesis and transport to the growth cone, and includes explicit dependence of the building kinetics on the geometry of the neurite. The basic assumption was that the radius of the neurite decreases with length. We found that the neurite dynamics crucially depended on the relationship between the rate of active transport and the rate of morphological changes. If these rates were in the balance, then the neurite displayed axon specific development with a constant elongation speed. For dendrite specific growth, the maximal length was rapidly saturated by degradation of building protein structures or limited by proximal part expansion reaching the characteristic cell size.

Microtubule-associated protein 1b is required for shaping the neural tube

BACKGROUND

Shaping of the neural tube, the precursor of the brain and spinal cord, involves narrowing and elongation of the neural tissue, concomitantly with other morphogenetic changes that contribue to this process. In zebrafish, medial displacement of neural cells (neural convergence or NC), which drives the infolding and narrowing of the neural ectoderm, is mediated by polarized migration and cell elongation towards the dorsal midline. Failure to undergo proper NC results in severe neural tube defects, yet the molecular underpinnings of this process remain poorly understood.

RESULTS

We investigated here the role of the microtubule (MT) cytoskeleton in mediating NC in zebrafish embryos using the MT destabilizing and hyperstabilizing drugs nocodazole and paclitaxel respectively. We found that MTs undergo major changes in organization and stability during neurulation and are required for the timely completion of NC by promoting cell elongation and polarity. We next examined the role of Microtubule-associated protein 1B (Map1b), previously shown to promote MT dynamicity in axons. map1b is expressed earlier than previously reported, in the developing neural tube and underlying mesoderm. Loss of Map1b function using morpholinos (MOs) or δMap1b (encoding a truncated Map1b protein product) resulted in delayed NC and duplication of the neural tube, a defect associated with impaired NC. We observed a loss of stable MTs in these embryos that is likely to contribute to the NC defect. Lastly, we found that Map1b mediates cell elongation in a cell autonomous manner and polarized protrusive activity, two cell behaviors that underlie NC and are MT-dependent.

CONCLUSIONS

Together, these data highlight the importance of MTs in the early morphogenetic movements that shape the neural tube and reveal a novel role for the MT regulator Map1b in mediating cell elongation and polarized cell movement in neural progenitor cells.

Proteomic analysis of the palmitoyl protein thioesterase 1 interactome in SH-SY5Y human neuroblastoma cells

Neuronal ceroid lipofuscinoses (NCL) are a group of inherited progressive childhood disorders, characterized by early accumulation of autofluorescent storage material in lysosomes of neurons or other cells. Clinical symptoms of NCL include: progressive loss of vision, mental and motor deterioration, epileptic seizures and premature death. CLN1 disease (MIM#256730) is caused by mutations in the CLN1 gene, which encodes palmitoyl protein thioesterase 1 (PPT1). In this study, we utilised single step affinity purification coupled to mass spectrometry (AP-MS) to unravel the in vivo substrates of human PPT1 in the brain neuronal cells. Protein complexes were isolated from human PPT1 expressing SH-SY5Y stable cells, subjected to filter-aided sample preparation (FASP) and analysed on a Q Exactive Hybrid Quadrupole-Orbitrap mass spectrometer. A total of 23 PPT1 interacting partners (IP) were identified from label free quantitation of the MS data by SAINT platform. Three of the identified PPT1 IP, namely CRMP1, DBH, and MAP1B are predicted to be palmitoylated. Our proteomic analysis confirmed previously suggested roles of PPT1 in axon guidance and lipid metabolism, yet implicates the enzyme in novel roles including: involvement in neuronal migration and dopamine receptor mediated signalling pathway.

BIOLOGICAL SIGNIFICANCE

The significance of this work lies in the unravelling of putative in vivo substrates of human CLN1 or PPT1 in brain neuronal cells. Moreover, the PPT1 IP implicate the enzyme in novel roles including: involvement in neuronal migration and dopamine receptor mediated signalling pathway.

The role of microtubule-associated protein 1B in axonal growth and neuronal migration in the central nervous system

In this review, we discuss the role of microtubule-associated protein 1B (MAP1B) and its phosphorylation in axonal development and regeneration in the central nervous system. MAP1B exhibits similar functions during axonal development and regeneration. MAP1B and phosphorylated MAP1B in neurons and axons maintain a dynamic balance between cytoskeletal components, and regulate the stability and interaction of microtubules and actin to promote axonal growth, neural connectivity and regeneration in the central nervous system.

Sustained GSK3 activity markedly facilitates nerve regeneration

Promotion of axonal growth of injured DRG neurons improves the functional recovery associated with peripheral nerve regeneration. Both isoforms of glycogen synthase kinase 3 (GSK3; α and β) are phosphorylated and inactivated via phosphatidylinositide 3-kinase (PI3K)/AKT signalling upon sciatic nerve crush (SNC). However, the role of GSK3 phosphorylation in this context is highly controversial. Here we use knock-in mice expressing GSK3 isoforms resistant to inhibitory PI3K/AKT phosphorylation, and unexpectedly find markedly accelerated axon growth of DRG neurons in culture and in vivo after SNC compared with controls. Moreover, this enhanced regeneration strikingly accelerates functional recovery after SNC. These effects are GSK3 activity dependent and associated with elevated MAP1B phosphorylation. Altogether, our data suggest that PI3K/AKT-mediated inhibitory phosphorylation of GSK3 limits the regenerative outcome after peripheral nerve injury. Therefore, suppression of this internal 'regenerative break' may potentially provide a new perspective for the clinical treatment of nerve injuries.

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.

Microtubule dynamics in axon guidance

Precise modulation of the cytoskeleton is involved in a variety of cellular processes including cell division, migration, polarity, and adhesion. In developing post-mitotic neurons, extracellular guidance cues not only trigger signaling cascades that act at a distance to indirectly regulate microtubule distribution, and assembly and disassembly in the growth cone, but also directly modulate microtubule stability and dynamics through coupling of guidance receptors with microtubules to control growth-cone turning. Microtubule-associated proteins including classical microtubule-associated proteins and microtubule plus-end tracking proteins are required for modulating microtubule dynamics to influence growth-cone steering. Multiple key signaling components, such as calcium, small GTPases, glycogen synthase kinase-3β, and c-Jun N-terminal kinase, link upstream signal cascades to microtubule stability and dynamics in the growth cone to control axon outgrowth and projection. Understanding the functions and regulation of microtubule dynamics in the growth cone provides new insights into the molecular mechanisms of axon guidance.

Nanomechanical property maps of breast cancer cells as determined by multiharmonic atomic force microscopy reveal Syk-dependent changes in microtubule stability mediated by MAP1B

The Syk protein-tyrosine kinase, a well-characterized modulator of immune recognition receptor signaling, also plays important, but poorly characterized, roles in tumor progression, acting as an inhibitor of cellular motility and metastasis in highly invasive cancer cells. Multiharmonic atomic force microscopy (AFM) was used to map nanomechanical properties of live MDA-MB-231 breast cancer cells either lacking or expressing Syk. The expression of Syk dramatically altered the cellular topography, reduced cell height, increased elasticity, increased viscosity, and allowed visualization of a more substantial microtubule network. The microtubules of Syk-expressing cells were more stable to nocodazole-induced depolymerization and were more highly acetylated than those of Syk-deficient cells. Silencing of MAP1B, a major substrate for Syk in MDA-MB-231 cells, attenuated Syk-dependent microtubule stability and reversed much of the effect of Syk on cellular topography, stiffness, and viscosity. This study illustrates the use of multiharmonic AFM both to quantitatively map the local nanomechanical properties of living cells and to identify the underlying mechanisms by which these properties are modulated by signal transduction machinery.

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.

Rho GTPases at the crossroad of signaling networks in mammals: impact of Rho-GTPases on microtubule organization and dynamics

Microtubule (MT) organization and dynamics downstream of external cues is crucial for maintaining cellular architecture and the generation of cell asymmetries. In interphase cells RhoA, Rac, and Cdc42, conspicuous members of the family of small Rho GTPases, have major roles in modulating MT stability, and hence polarized cell behaviors. However, MTs are not mere targets of Rho GTPases, but also serve as signaling platforms coupling MT dynamics to Rho GTPase activation in a variety of cellular conditions. In this article, we review some of the key studies describing the reciprocal relationship between small Rho-GTPases and MTs during migration and polarization.

Modulation of mGluR-dependent MAP1B translation and AMPA receptor endocytosis by microRNA miR-146a-5p

The translation of dendritic microtubule-associated protein 1B (MAP1B) is exaggerated upon group I mGluR activation leading to AMPA receptor (AMPAR) endocytosis and consequent long-term depression. However, the mechanisms of regulation of MAP1B protein synthesis in the mature dendrites remain unclear. Here we have identified miR-146a-5p that targets the 3' UTR of MAP1B mRNA and represses its translation. Inhibition of the endogenous miR-146a-5p in mouse cultured hippocampal neurons triggers an increase of the dendritic MAP1B protein as well as the internalization of AMPARs, resulting in a decline in synaptic transmission. Conversely, enforced expression of miR-146a-5p inhibits MAP1B translation and attenuates group I mGluR-induced AMPAR endocytosis. Moreover, siRNA-mediated knockdown of MAP1B recovers the impairment of synaptic transmission caused by inhibition of miR-146a-5p. These results reveal that miR-146a-5p modulates the synaptic plasticity via repression of MAP1B protein synthesis.

RNAi mediated Tiam1 gene knockdown inhibits invasion of retinoblastoma

T lymphoma invasion and metastasis protein (Tiam1) is up-regulated in variety of cancers and its expression level is related to metastatic potential of the type of cancer. Earlier, Tiam1 was shown to be overexpressed in retinoblastoma (RB) and we hypothesized that it was involved in invasiveness of RB. This was tested by silencing Tiam1 in RB cell lines (Y79 and Weri-Rb1) using siRNA pool, targeting different regions of Tiam1 mRNA. The cDNA microarray of Tiam1 silenced cells showed gene regulations altered by Tiam1 were predominantly on the actin cytoskeleton interacting proteins, apoptotic initiators and tumorogenic potential targets. The silenced phenotype resulted in decreased growth and increased apoptosis with non-invasive characteristics. Transfection of full length and N-terminal truncated construct (C1199) clearly revealed membrane localization of Tiam1 and not in the case of C580 construct. F-actin staining showed the interaction of Tiam1 with actin in the membrane edges that leads to ruffling, and also imparts varying invasive potential to the cell. The results obtained from our study show for the first time that Tiam1 modulates the cell invasion, mediated by actin cytoskeleton remodeling in RB.

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.

Actin filaments and microtubules in dendritic spines

Dendritic spines are small protrusions emerging from their parent dendrites, and their morphological changes are involved in synaptic plasticity. These tiny structures are composed of thousands of different proteins belonging to several subfamilies such as membrane receptors, scaffold proteins, signal transduction proteins, and cytoskeletal proteins. Actin filaments in dendritic spines consist of double helix of actin protomers decorated with drebrin and ADF/cofilin, and the balance of the two is closely related to the actin dynamics, which may govern morphological and functional synaptic plasticity. During development, the accumulation of drebrin-binding type actin filaments is one of the initial events occurring at the nascent excitatory postsynaptic site, and plays a pivotal role in spine formation as well as small GTPases. It has been recently reported that microtubules transiently appear in dendritic spines in correlation with synaptic activity. Interestingly, it is suggested that microtubule dynamics might couple with actin dynamics. In this review, we will summarize the contribution of both actin filaments and microtubules to the formation and regulation of dendritic spines, and further discuss the role of cytoskeletal deregulation in neurological disorders.