α-Tubulin Acetylation Restricts Axon Overbranching by Dampening Microtubule Plus-End Dynamics in Neurons

Interaction Between Aging-Related Elastin-Derived Peptide (VGVAPG) and Sirtuin 2 and its Impact on Functions of Human Neuron Cells in an In Vitro Model

Elastin is a stable protein present in many tissues, including brain tissues, and is one of the most long-life proteins with a half-life of approximately 70 years. The peptide with a Val-Gly-Val-Ala-Pro-Gly (VGVAPG) amino acid sequence is released during elastin decay, which correlates with aging-related neurodegeneration. A recent study has shown enhanced protein expression of Sirtuin 2 (SIRT2 - one of the redox homeostatic factors) in aged rodent brains, while the correlation between VGVAPG and SIRT2 has never been evaluated so far. Therefore, the study aimed to determine the impact of the VGVAPG hexapeptide on SIRT2 and neuronal functions in differentiated SH-SY5Y cells at the gene and protein expression levels. The present results showed that VGVAPG caused a 52.69% decrease in the level of reactive oxygen species (ROS), as in the case of neurons treated with AGK2 (Sirtuin 2 inhibitor) after 24h and 48h. Furthermore, a decrease in superoxide dismutase (SOD) activity was observed. The SIRT2 gene expression was found to fluctuate after 6h and 24h as a result of the exposure to the VGVAPG peptide. In turn, a decrease in the PPARγ, P53, SOD2, and CAT mRNA expression was shown in VGVAPG-treated cells. Additionally, an increase in the Sirtuin 2 protein expression was recorded after 24h and 48h in the VGVAPG peptide-treated neurons. Last but not least, the decrease in the level of acetylation of α-tubulin after the hexapeptide treatment was correlated with shortening of neurites, which may indicate the destabilization of the microtubule and ROS-independent induction of neurodegeneration.

ARHGEF5 binds Drebrin and affects α-tubulin acetylation to direct neuronal morphogenesis and migration during mouse brain development

Rho guanine nucleotide exchange factors (Rho GEFs) activate Rho GTPases, which act as molecular switches regulating various essential cellular functions. This study investigated the role of ARHGEF5, a Rho GEF known for its involvement in cell migration and invasion processes, in the context of brain development. We found that ARHGEF5 is essential for dendrite development during the early stages of neuronal growth. We also discovered that ARHGEF5 binds to Drebrin E, which is vital for coordinating actin and microtubule dynamics, and facilitates the interaction between Drebrin E and Cyclin-dependent kinase 5, which phosphorylates Drebrin E. Notably, ARHGEF5 deficiency resulted in a decrease in acetylated α-tubulin levels, and the expression of an α-tubulin acetylation mimetic mutant (K40Q) rescued the defects in dendrite development and neuronal migration, suggesting ARHGEF5's role in modulating microtubule stability. Additionally, ARHGEF5 was shown to influence Golgi positioning in the leading processes of migrating cortical neurons during brain development. Our study suggests that ARHGEF5 plays a crucial role in integrating cytoskeletal dynamics with neuronal morphogenesis and migration processes during brain development.

The α-tubulin acetyltransferase ATAT1: structure, cellular functions, and its emerging role in human diseases

The acetylation of α-tubulin on lysine 40 is a well-studied post-translational modification which has been associated with the presence of long-lived stable microtubules that are more resistant to mechanical breakdown. The discovery of α-tubulin acetyltransferase 1 (ATAT1), the enzyme responsible for lysine 40 acetylation on α-tubulin in a wide range of species, including protists, nematodes, and mammals, dates to about a decade ago. However, the role of ATAT1 in different cellular activities and molecular pathways has been only recently disclosed. This review comprehensively summarizes the most recent knowledge on ATAT1 structure and substrate binding and analyses the involvement of ATAT1 in a variety of cellular processes such as cell motility, mitosis, cytoskeletal organization, and intracellular trafficking. Finally, the review highlights ATAT1 emerging roles in human diseases and discusses ATAT1 potential enzymatic and non-enzymatic roles and the current efforts in developing ATAT1 inhibitors.

Post-GWAS functional analyses of CNTNAP5 suggests its role in glaucomatous neurodegeneration

Primary angle closure glaucoma (PACG) affects more than 20 million people worldwide, with an increased prevalence in south-east Asia. In a prior haplotype-based GWAS, we identified a novel CNTNAP5 genic region, significantly associated with PACG. In the current study, we have extended our perception of CNTNAP5 involvement in glaucomatous neurodegeneration in a zebrafish model, through investigating phenotypic consequences pertinent to retinal degeneration upon knockdown of cntnap5 by translation-blocking morpholinos. While cntnap5 knockdown was successfully validated using an antibody, immunofluorescence followed by western blot analyses in cntnap5-morphant (MO) zebrafish revealed increased expression of acetylated tubulin indicative of perturbed cytoarchitecture of retinal layers. Moreover, significant loss of Nissl substance is observed in the neuro-retinal layers of cntnap5-MO zebrafish eye, indicating neurodegeneration. Additionally, in spontaneous movement behavioural analysis, cntnap5-MO zebrafish have a significantly lower average distance traversed in light phase compared to mismatch-controls, whereas no significant difference was observed in the dark phase, corroborating with vision loss in the cntnap5-MO zebrafish. This study provides the first direct functional evidence of a putative role of CNTNAP5 in visual neurodegeneration.

Neural deficits in a mouse model of PACS1 syndrome are corrected with PACS1- or HDAC6-targeting therapy

PACS1 syndrome is a neurodevelopmental disorder (NDD) caused by a recurrent de novo missense mutation in PACS1 (p.Arg203Trp (PACS1R203W)). The mechanism by which PACS1R203W causes PACS1 syndrome is unknown, and no curative treatment is available. Here, we use patient cells and PACS1 syndrome mice to show that PACS1 (or PACS-1) is an HDAC6 effector and that the R203W substitution increases the PACS1/HDAC6 interaction, aberrantly potentiating deacetylase activity. Consequently, PACS1R203W reduces acetylation of α-tubulin and cortactin, causing the Golgi ribbon in hippocampal neurons and patient-derived neural progenitor cells (NPCs) to fragment and overpopulate dendrites, increasing their arborization. The dendrites, however, are beset with varicosities, diminished spine density, and fewer functional synapses, characteristic of NDDs. Treatment of PACS1 syndrome mice or patient NPCs with PACS1- or HDAC6-targeting antisense oligonucleotides, or HDAC6 inhibitors, restores neuronal structure and synaptic transmission in prefrontal cortex, suggesting that targeting PACS1R203W/HDAC6 may be an effective therapy for PACS1 syndrome.

Microtubule acetylation dyshomeostasis in Parkinson's disease

The inter-neuronal communication occurring in extensively branched neuronal cells is achieved primarily through the microtubule (MT)-mediated axonal transport system. This mechanistically regulated system delivers cargos (proteins, mRNAs and organelles such as mitochondria) back and forth from the soma to the synapse. Motor proteins like kinesins and dynein mechanistically regulate polarized anterograde (from the soma to the synapse) and retrograde (from the synapse to the soma) commute of the cargos, respectively. Proficient axonal transport of such cargos is achieved by altering the microtubule stability via post-translational modifications (PTMs) of α- and β-tubulin heterodimers, core components constructing the MTs. Occurring within the lumen of MTs, K40 acetylation of α-tubulin via α-tubulin acetyl transferase and its subsequent deacetylation by HDAC6 and SIRT2 are widely scrutinized PTMs that make the MTs highly flexible, which in turn promotes their lifespan. The movement of various motor proteins, including kinesin-1 (responsible for axonal mitochondrial commute), is enhanced by this PTM, and dyshomeostasis of neuronal MT acetylation has been observed in a variety of neurodegenerative conditions, including Alzheimer's disease and Parkinson's disease (PD). PD is the second most common neurodegenerative condition and is closely associated with impaired MT dynamics and deregulated tubulin acetylation levels. Although the relationship between status of MT acetylation and progression of PD pathogenesis has become a chicken-and-egg question, our review aims to provide insights into the MT-mediated axonal commute of mitochondria and dyshomeostasis of MT acetylation in PD. The enzymatic regulators of MT acetylation along with their synthetic modulators have also been briefly explored. Moving towards a tubulin-based therapy that enhances MT acetylation could serve as a disease-modifying treatment in neurological conditions that lack it.

Rac1 and Rac3 GTPases and TPC2 are required for axonal outgrowth and migration of cortical interneurons

Rho GTPases, among them Rac1 and Rac3, are major transducers of extracellular signals and are involved in multiple cellular processes. In cortical interneurons, the neurons that control the balance between excitation and inhibition of cortical circuits, Rac1 and Rac3 are essential for their development. Ablation of both leads to a severe reduction in the numbers of mature interneurons found in the murine cortex, which is partially due to abnormal cell cycle progression of interneuron precursors and defective formation of growth cones in young neurons. Here, we present new evidence that upon Rac1 and Rac3 ablation, centrosome, Golgi complex and lysosome positioning is significantly perturbed, thus affecting both interneuron migration and axon growth. Moreover, for the first time, we provide evidence of altered expression and localization of the two-pore channel 2 (TPC2) voltage-gated ion channel that mediates Ca2+ release. Pharmacological inhibition of TPC2 negatively affected axonal growth and migration of interneurons. Our data, taken together, suggest that TPC2 contributes to the severe phenotype in axon growth initiation, extension and interneuron migration in the absence of Rac1 and Rac3.

α-Tubulin acetylation at lysine 40 regulates dendritic arborization and larval locomotion by promoting microtubule stability in Drosophila

Posttranslational modification of tubulin increases the dynamic complexity and functional diversity of microtubules. Acetylation of α-tubulin at Lys-40 is a highly conserved posttranslational modification that has been shown to improve the flexibility and resilience of microtubules. Here we studied the in vivo functions of α-tubulin acetylation by knocking-out Atat, the Drosophila α-tubulin acetyltransferase, and by mutating Lys-40 to Arg in α1-tubulin. We found a reduction in the dendritic arborization of larval class I dendritic arborization (da) neurons in both mutants. The dendritic developmental defects in atat mutants could be reversed by enhancing the stability of microtubules either through knocking down the microtubule severing protein Katanin 60 or through overexpressing tubulin-specific chaperone E, suggesting that α-tubulin deacetylation impairsed dendritic morphology by decreasing the stability of microtubules. Using time-lapse recordings, we found that atat and α1-tubulinK40R mutations dramatically increased the number of dendritic protrusions that were likely to be immature dendritic precursors. Finally, we showed that both Atat and α-tubulin acetylation were required in class I da neurons to control larval locomotion. These findings add novel insight into the current knowledge of the role of α-tubulin acetylation in regulating neuronal development and functions.

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.

Tubulin deacetylase NDST3 modulates lysosomal acidification: Implications in neurological diseases

Neurological diseases (NDs), featured by progressive dysfunctions of the nervous system, have become a growing burden for the aging populations. N-Deacetylase and N-sulfotransferase 3 (NDST3) is known to catalyze deacetylation and N-sulfation on disaccharide substrates. Recently, NDST3 is identified as a novel deacetylase for tubulin, and its newly recognized role in modulating microtubule acetylation and lysosomal acidification provides fresh insights into ND therapeutic approaches using NDST3 as a target. Microtubule acetylation and lysosomal acidification have been reported to be critical for activities in neurons, implying that the regulators of these two biological processes, such as the previously known microtubule deacetylases, histone deacetylase 6 (HDAC6) and sirtuin 2 (SIRT2), could play important roles in various NDs. Aberrant NDST3 expression or tubulin acetylation has been observed in an increasing number of NDs, including amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD), schizophrenia and bipolar disorder, Alzheimer's disease (AD), and Parkinson's disease (PD), suggesting that NDST3 is a key player in the pathogenesis of NDs and may serve as a target for development of new treatment of NDs.

A comprehensive mouse brain acetylome-the cellular-specific distribution of acetylated brain proteins

Nε-lysine acetylation is a reversible posttranslational modification (PTM) involved in multiple physiological functions. Genetic and animal studies have documented the critical roles of protein acetylation in brain development, functions, and various neurological disorders. However, the underlying cellular and molecular mechanism are still partially understood. Here, we profiled and characterized the mouse brain acetylome and investigated the cellular distribution of acetylated brain proteins. We identified 1,818 acetylated proteins, including 5,196 acetylation modification sites, using a modified workflow comprising filter-aided sample preparation (FSAP), acetylated peptides enrichment, and MS analysis without pre- or post-fraction. Bioinformatics analysis indicated these acetylated mouse brain proteins were mainly located in the myelin sheath, mitochondrial inner membrane, and synapse, as well as their involvement in multiple neurological disorders. Manual annotation revealed that a set of brain-specific proteins were acetylation-modified. The acetylation of three brain-specific proteins was verified, including neurofilament light polypeptide (NEFL), 2’,3’-cyclic-nucleotide 3’-phosphodiesterase (CNP), and neuromodulin (GAP43). Further immunofluorescence staining illustrated that acetylated proteins were mainly distributed in the nuclei of cortex neurons and axons of hippocampal neurons, sparsely distributed in the nuclei of microglia and astrocytes, and the lack of distribution in both cytoplasm and nuclei of cerebrovascular endothelial cells. Together, this study provided a comprehensive mouse brain acetylome and illustrated the cellular-specific distribution of acetylated proteins in the mouse brain. These data will contribute to understanding and deciphering the molecular and cellular mechanisms of protein acetylation in brain development and neurological disorders. Besides, we proposed some problems that need to be solved in future brain acetylome research.

Crosstalk between acetylation and the tyrosination/detyrosination cycle of α-tubulin in Alzheimer’s disease

Microtubules (MTs) support a variety of neuronal functions, such as maintenance of cell structure, transport, and synaptic plasticity. Neuronal MTs are highly heterogeneous due to several tubulin isotypes and the presence of multiple post-translational modifications, such as detyrosination and acetylation. The tubulin tyrosination/detyrosination cycle is a key player in the maintenance of MT dynamics, as tyrosinated tubulin is associated with more dynamic MTs, while detyrosinated tubulin is linked to longer lived, more stable MTs. Dysfunction of tubulin re-tyrosination was recently correlated to Alzheimer’s disease progression. The implication of tubulin acetylation in Alzheimer’s disease has, however, remained controversial. Here, we demonstrate that tubulin acetylation accumulates in post-mortem brain tissues from Alzheimer’s disease patients and human neurons harboring the Alzheimer’s familial APP-V717I mutation. We further show that tubulin re-tyrosination, which is defective in Alzheimer’s disease, can control acetylated tubulin in primary neurons irrespective of the levels of the enzymes regulating tubulin acetylation, suggesting that reduced MT dynamics associated with impaired tubulin re-tyrosination might contribute to the accumulation of tubulin acetylation that we detected in Alzheimer’s disease.

Synaptic branch stability is mediated by non-enzymatic functions of MEC-17/αTAT1 and ATAT-2

Microtubules are fundamental elements of neuronal structure and function. They are dynamic structures formed from protofilament chains of α- and β-tubulin heterodimers. Acetylation of the lysine 40 (K40) residue of α-tubulin protects microtubules from mechanical stresses by imparting structural elasticity. The enzyme responsible for this acetylation event is MEC-17/αTAT1. Despite its functional importance, however, the consequences of altered MEC-17/αTAT1 levels on neuronal structure and function are incompletely defined. Here we demonstrate that overexpression or loss of MEC-17, or of its functional paralogue ATAT-2, causes a delay in synaptic branch extension, and defective synaptogenesis in the mechanosensory neurons of Caenorhabditis elegans. Strikingly, by adulthood, the synaptic branches in these animals are lost, while the main axon shaft remains mostly intact. We show that MEC-17 and ATAT-2 regulate the stability of the synaptic branches largely independently from their acetyltransferase domains. Genetic analyses reveals novel interactions between both mec-17 and atat-2 with the focal adhesion gene zyx-1/Zyxin, which has previously been implicated in actin remodelling. Together, our results reveal new, acetylation-independent roles for MEC-17 and ATAT-2 in the development and maintenance of neuronal architecture.

Deregulation of microtubule organization and RNA metabolism in Arx models for lissencephaly and developmental epileptic encephalopathy

X-linked lissencephaly with abnormal genitalia (XLAG) and developmental epileptic encephalopathy-1 (DEE1) are caused by mutations in the Aristaless-related homeobox (ARX) gene, which encodes a transcription factor responsible for brain development. It has been unknown whether the phenotypically diverse XLAG and DEE1 phenotypes may converge on shared pathways. To address this question, a label-free quantitative proteomic approach was applied to the neonatal brain of Arx knockout (ArxKO/Y) and knock-in polyalanine (Arx(GCG)7/Y) mice that are respectively models for XLAG and DEE1. Gene ontology and protein-protein interaction analysis revealed that cytoskeleton, protein synthesis and splicing control are deregulated in an allelic-dependent manner. Decreased α-tubulin content was observed both in Arx mice and Arx/alr-1(KO) Caenorhabditis elegans ,and a disorganized neurite network in murine primary neurons was consistent with an allelic-dependent secondary tubulinopathy. As distinct features of Arx(GCG)7/Y mice, we detected eIF4A2 overexpression and translational suppression in cortex and primary neurons. Allelic-dependent differences were also established in alternative splicing (AS) regulated by PUF60 and SAM68. Abnormal AS repertoires in Neurexin-1, a gene encoding multiple pre-synaptic organizers implicated in synaptic remodelling, were detected in Arx/alr-1(KO) animals and in Arx(GCG)7/Y epileptogenic brain areas and depolarized cortical neurons. Consistent with a conserved role of ARX in modulating AS, we propose that the allelic-dependent secondary synaptopathy results from an aberrant Neurexin-1 repertoire. Overall, our data reveal alterations mirroring the overlapping and variant effects caused by null and polyalanine expanded mutations in ARX. The identification of these effects can aid in the design of pathway-guided therapy for ARX endophenotypes and NDDs with overlapping comorbidities.

Macrophage migration inhibitory factor (MIF) acetylation protects neurons from ischemic injury

Ischemia-induced neuronal death leads to serious lifelong neurological deficits in ischemic stroke patients. Histone deacetylase 6 (HDAC6) is a promising target for neuroprotection in many neurological disorders, including ischemic stroke. However, the mechanism by which HDAC6 inhibition protects neurons after ischemic stroke remains unclear. Here, we discovered that genetic ablation or pharmacological inhibition of HDAC6 reduced brain injury after ischemic stroke by increasing macrophage migration inhibitory factor (MIF) acetylation. Mass spectrum analysis and biochemical results revealed that HDAC6 inhibitor or aspirin treatment promoted MIF acetylation on the K78 residue. MIF K78 acetylation suppressed the interaction between MIF and AIF, which impaired MIF translocation to the nucleus in ischemic cortical neurons. Moreover, neuronal DNA fragmentation and neuronal death were impaired in the cortex after ischemia in MIF K78Q mutant mice. Our results indicate that the neuroprotective effect of HDAC6 inhibition and aspirin treatment results from MIF K78 acetylation; thus, MIF K78 acetylation may be a therapeutic target for ischemic stroke and other neurological diseases.

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.

Mllt11 Regulates Migration and Neurite Outgrowth of Cortical Projection Neurons during Development

The formation of connections within the mammalian neocortex is highly regulated by both extracellular guidance mechanisms and intrinsic gene expression programs. There are two types of cortical projection neurons (CPNs): those that project locally and interhemispherically and those that project to subcerebral structures such as the thalamus, hindbrain, and spinal cord. The regulation of cortical projection morphologies is not yet fully understood at the molecular level. Here, we report a role for Mllt11 (Myeloid/lymphoid or mixed-lineage leukemia; translocated to chromosome 11/All1 Fused Gene From Chromosome 1q) in the migration and neurite outgrowth of callosal projection neurons during mouse brain formation. We show that Mllt11 expression is exclusive to developing neurons and is enriched in the developing cortical plate (CP) during the formation of the superficial cortical layers. In cultured primary cortical neurons, Mllt11 is detected in varicosities and growth cones as well as the soma. Using conditional loss-of-function and gain-of-function analysis we show that Mllt11 is required for neuritogenesis and proper migration of upper layer CPNs. Loss of Mllt11 in the superficial cortex of male and female neonates leads to a severe reduction in fibers crossing the corpus callosum (CC), a progressive loss in the maintenance of upper layer projection neuron gene expression, and reduced complexity of dendritic arborization. Proteomic analysis revealed that Mllt11 associates with stabilized microtubules, and Mllt11 loss affected microtubule staining in callosal axons. Taken together, our findings support a role for Mllt11 in promoting the formation of mature upper-layer neuron morphologies and connectivity in the cerebral cortex.SIGNIFICANCE STATEMENT The regulation of cortical projection neuron (CPN) morphologies is an area of active investigation since the time of Cajal. Yet the molecular mechanisms of how the complex dendritic and axonal morphologies of projection neurons are formed remains incompletely understood. Although conditional mutagenesis analysis in the mouse, coupled with overexpression assays in the developing fetal brain, we show that a novel protein called Mllt11 is sufficient and necessary to regulate the dendritic and axonal characteristics of callosal projection neurons in the developing mammalian neocortex. Furthermore, we show that Mllt11 interacts with microtubules, likely accounting for its role in neuritogenesis.

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.

Astrocyte-targeting RNA interference against mutated superoxide dismutase 1 induces motoneuron plasticity and protects fast-fatigable motor units in a mouse model of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS) caused by SOD1 gene mutations, both cell‐autonomous and noncell‐autonomous mechanisms lead to the selective degeneration of motoneurons (MN). Here, we evaluate the therapeutic potential of gene therapy targeting mutated SOD1 in mature astrocytes using mice expressing the mutated SOD1^G93A protein. An AAV‐gfaABC₁D vector encoding an artificial microRNA is used to deliver RNA interference against mutated SOD1 selectively in astrocytes. The treatment leads to the progressive rescue of neuromuscular junction occupancy, to the recovery of the compound muscle action potential in the gastrocnemius muscle, and significantly improves neuromuscular function. In the spinal cord, gene therapy targeting astrocytes protects a small pool of the most vulnerable fast‐fatigable MN until disease end stage. In the gastrocnemius muscle of the treated SOD1^G93A mice, the fast‐twitch type IIB muscle fibers are preserved from atrophy. Axon collateral sprouting is observed together with muscle fiber type grouping indicative of denervation/reinnervation events. The transcriptome profiling of spinal cord MN shows changes in the expression levels of factors regulating the dynamics of microtubules. Gene therapy delivering RNA interference against mutated SOD1 in astrocytes protects fast‐fatigable motor units and thereby improves neuromuscular function in ALS mice.

Molecular Analysis of Axonal Transport Dynamics upon Modulation of Microtubule Acetylation

Axonal transport is used by neurons to distribute mRNAs, proteins, and organelles to their peripheral compartments in order to sustain their structural and functional integrity. Cargoes are transported along the microtubule (MT) network whose post-translational modifications influence transport dynamics. Here, we describe methods to modulate MT acetylation and record its impact on axonal transport in cultured mouse cortical projection neurons as well as in motoneurons of Drosophila melanogaster third-instar larvae. Specifically, we provide a step-by step procedure to reduce the level of MT acetylation and to record and analyze the transport of dye-labeled organelles in projection neuron axons cultured in microfluidic chambers. In addition, we describe the method to record and analyze GFP-tagged mitochondria transport along the motoneuron axons of transgenic Drosophila melanogaster third-instar larvae.

Transthyretin Promotes Axon Growth via Regulation of Microtubule Dynamics and Tubulin Acetylation

Transthyretin (TTR), a plasma and cerebrospinal fluid protein, increases axon growth and organelle transport in sensory neurons. While neurons extend their axons, the microtubule (MT) cytoskeleton is crucial for the segregation of functional compartments and axonal outgrowth. Herein, we investigated whether TTR promotes axon elongation by modulating MT dynamics. We found that TTR KO mice have an intrinsic increase in dynamic MTs and reduced levels of acetylated α-tubulin in peripheral axons. In addition, they failed to modulate MT dynamics in response to sciatic nerve injury, leading to decreased regenerative capacity. Importantly, restoring acetylated α-tubulin levels of TTR KO dorsal root ganglia (DRG) neurons using an HDAC6 inhibitor is sufficient to completely revert defective MT dynamics and neurite outgrowth. In summary, our results reveal a new role for TTR in the modulation of MT dynamics by regulating α-tubulin acetylation via modulation of the acetylase ATAT1, and suggest that this activity underlies TTR neuritogenic function.

Cytoskeleton and Membrane Organization at Axon Branches

Axon branching is a critical process ensuring a high degree of interconnectivity for neural network formation. As branching occurs at sites distant from the soma, it is necessary that axons have a local system to dynamically control and regulate axonal growth. This machinery depends on the orchestration of cellular functions such as cytoskeleton, subcellular transport, energy production, protein- and membrane synthesis that are adapted for branch formation. Compared to the axon shaft, branching sites show a distinct and dynamic arrangement of cytoskeleton components, endoplasmic reticulum and mitochondria. This review discusses the regulation of axon branching in the context of cytoskeleton and membrane remodeling.

Neuronal SETD2 activity links microtubule methylation to an anxiety-like phenotype in mice

Gene discovery efforts in autism spectrum disorder have identified heterozygous defects in chromatin remodeller genes, the 'readers, writers and erasers' of methyl marks on chromatin, as major contributors to this disease. Despite this advance, a convergent aetiology between these defects and aberrant chromatin architecture or gene expression has remained elusive. Recently, data have begun to emerge that chromatin remodellers also function directly on the cytoskeleton. Strongly associated with autism spectrum disorder, the SETD2 histone methyltransferase for example, has now been shown to directly methylate microtubules of the mitotic spindle. However, whether microtubule methylation occurs in post-mitotic cells, for example on the neuronal cytoskeleton, is not known. We found the SETD2 α-tubulin lysine 40 trimethyl mark occurs on microtubules in the brain and in primary neurons in culture, and that the SETD2 C-terminal SRI domain is required for binding and methylation of α-tubulin. A CRISPR knock-in of a pathogenic SRI domain mutation (Setd2SRI) that disables microtubule methylation revealed at least one wild-type allele was required in mice for survival, and while viable, heterozygous Setd2SRI/wtmice exhibited an anxiety-like phenotype. Finally, whereas RNA-sequencing (RNA-seq) and chromatin immunoprecipitation-sequencing (ChIP-seq) showed no concomitant changes in chromatin methylation or gene expression in Setd2SRI/wtmice, primary neurons exhibited structural deficits in axon length and dendritic arborization. These data provide the first demonstration that microtubules of neurons are methylated, and reveals a heterozygous chromatin remodeller defect that specifically disables microtubule methylation is sufficient to drive an autism-associated phenotype.

MEC17-induced α-tubulin acetylation restores mitochondrial transport function and alleviates axonal injury after intracerebral hemorrhage in mice

Injury to long axonal projections is a central pathological feature at the early phase of intracerebral hemorrhage (ICH). It has been reported to contribute to persistent functional disability following ICH. However, the molecular mechanisms that drive axonal degeneration remain unclear. Autologous blood was injected into the striatum to mimic the pathology of ICH. Observed significant swollen axons with characteristic retraction bulbs were found around the striatal hematoma at 24 h after ICH. Electronic microscopic examination revealed highly disorganized microtubule and swollen mitochondria in the retraction bulbs. MEC17 is a specific α-tubulin acetyltransferase, ablation of acetylated α-tubulin in MEC17^-/- mice aggravated axonal injury, axonal transport mitochondria dysfunction, and motor dysfunction. In contrast, treatment with tubastatin A (TubA), which promotes microtubule acetylation, significantly alleviated axonal injury and protected the integrity of the corticospinal tract and fine motor function after ICH. Moreover, results showed that 41% mitochondria were preferentially bundled to the acetylated α-tubulin in identifiable axons and dendrites in primary neurons. This impaired axonal transport of mitochondria in primary neurons of MEC17^-/- mice. Given that opening of mitochondrial permeability transition pore (mPTP) induces mitochondrial dysfunction and impairs ATP supply thereby promoting axonal injury, we enhanced the availability of acetylated α-tubulin using TubA and inhibited mPTP opening with cyclosporin A. The results indicated that this combined treatment synergistically protected corticospinal tract integrity and promoted fine motor control recovery. These findings reveal key intracellular mechanisms that drive axonal degeneration after ICH and highlight the need to target multiple factors and respective regulatory mechanisms as an effective approach to prevent axonal degeneration and motor dysfunction after ICH.

Postnatal development of BAG3 expression in mouse cerebral cortex and hippocampus

The decreased efficiency of autophagic processing in the central nervous system during aging may be a contributing factor in neurodegenerative diseases. BAG3 (Bcl2 associated athanogene 3) is a major member of the BAG family of co-molecular chaperones that mediate selective macroautophagy. Therefore, we analyzed the expression and distribution of BAG3 in the brain at postnatal 0 day (P0), P15, 1-, 2-, 9-, 12-, and 18 month-old C57BL/6 mice, thus covering almost all ages. Except for a significant steep drop in mRNA and protein levels in the cortex and hippocampus soon after birth, there were minimal differences in the expression and distribution of BAG3 among P15, M1, M2, M9, and M12 mice; however, at 18 months, BAG3 expression was significantly higher. Immunohistochemical analyses showed that BAG3 is mainly located in the neuronal cytoplasm and processes in C57BL/6 the cerebral cortex and hippocampus from P0 to M18 postnatal development. These findings indicate that BAG3 might be stable in young and middle-aged mice, but unstable in aged mice.

α-TubK40me3 is required for neuronal polarization and migration by promoting microtubule formation

Tri-methylation on lysine 40 of α-tubulin (α-TubK40me3) is a recently identified post-translational modification involved in mitosis and cytokinesis. However, knowledge about α-TubK40me3 in microtubule function and post-mitotic cells remains largely incomplete. Here, we report that α-TubK40me3 is required for neuronal polarization and migration by promoting microtubule formation. α-TubK40me3 is enriched in mouse cerebral cortex during embryonic day (E)14 to E16. Knockdown of α-tubulin methyltransferase SETD2 at E14 leads to the defects in neuronal migration, which could be restored by overexpressing either a cytoplasm-localized SETD2 truncation or α-TubK40me3-mimicking mutant. Furthermore, α-TubK40me3 is preferably distributed on polymerized microtubules and potently promotes tubulin nucleation. Downregulation of α-TubK40me3 results in reduced microtubule abundance in neurites and disrupts neuronal polarization, which could be rescued by Taxol. Additionally, α-TubK40me3 is increased after losing α-tubulin K40 acetylation (α-TubK40ac) and largely rescues α-TubK40ac function. This study reveals a critical role of α-TubK40me3 in microtubule formation and neuronal development.

Post-translational modifications of tubulins regulate microtubule properties and neural development. Here, the authors report that one such post-translational modification, α-TubK40me3, is required for neuronal polarization and migration by promoting microtubule formation.

5'-UTR SNP of FGF13 causes translational defect and intellectual disability

The congenital intellectual disability (ID)-causing gene mutations remain largely unclear, although many genetic variations might relate to ID. We screened gene mutations in Chinese Han children suffering from severe ID and found a single-nucleotide polymorphism (SNP) in the 5′-untranslated region (5′-UTR) of fibroblast growth factor 13 (FGF13) mRNA (NM_001139500.1:c.-32c>G) shared by three male children. In both HEK293 cells and patient-derived induced pluripotent stem cells, this SNP reduced the translation of FGF13, which stabilizes microtubules in developing neurons. Mice carrying the homologous point mutation in 5′-UTR of Fgf13 showed delayed neuronal migration during cortical development, and weakened learning and memory. Furthermore, this SNP reduced the interaction between FGF13 5′-UTR and polypyrimidine-tract-binding protein 2 (PTBP2), which was required for FGF13 translation in cortical neurons. Thus, this 5′-UTR SNP of FGF13 interferes with the translational process of FGF13 and causes deficits in brain development and cognitive functions.

Microtubule acetylation: A reading key to neural physiology and degeneration

Neurons are the perfect example of cells where microtubules are essential to achieve an extraordinary degree of morphological and functional complexity. Different tubulin isoforms and associated post-translational modifications are the basis to establish the diversity in biochemical and biophysical properties of microtubules including their stability and the control of intracellular transport. Acetylation is one of the key tubulin modifications and it can influence important structural, mechanical and biological traits of the microtubule network. Here, we present the emerging evidence for the essential role of microtubule acetylation in the control of neuronal and glial function in healthy and degenerative conditions. In particular, we discuss the pathogenic role of tubulin acetylation in neurodegenerative disorders and focus on Parkinson's disease. We also provide a critical analysis about the possibility to target tubulin acetylation as a novel therapeutic intervention for neuroprotective strategies.

Steric-Free Bioorthogonal Labeling of Acetylation Substrates Based on a Fluorine-Thiol Displacement Reaction

We have developed a novel bioorthogonal reaction that can selectively displace fluorine substitutions alpha to amide bonds. This fluorine-thiol displacement reaction (FTDR) allows for fluorinated cofactors or precursors to be utilized as chemical reporters, hijacking acetyltransferase-mediated acetylation both in vitro and in live cells, which cannot be achieved with azide- or alkyne-based chemical reporters. The fluoroacetamide labels can be further converted to biotin or fluorophore tags using FTDR, enabling the general detection and imaging of acetyl substrates. This strategy may lead to a steric-free labeling platform for substrate proteins, expanding our chemical toolbox for functional annotation of post-translational modifications in a systematic manner.

Tubulin post-translational modifications control neuronal development and functions

Microtubules (MTs) are an essential component of the neuronal cytoskeleton; they are involved in various aspects of neuron development, maintenance, and functions including polarization, synaptic plasticity, and transport. Neuronal MTs are highly heterogeneous due to the presence of multiple tubulin isotypes and extensive post‐translational modifications (PTMs). These PTMs—most notably detyrosination, acetylation, and polyglutamylation—have emerged as important regulators of the neuronal microtubule cytoskeleton. With this review, we summarize what is currently known about the impact of tubulin PTMs on microtubule dynamics, neuronal differentiation, plasticity, and transport as well as on brain function in normal and pathological conditions, in particular during neuro‐degeneration. The main therapeutic approaches to neuro‐diseases based on the modulation of tubulin PTMs are also summarized. Overall, the review indicates how tubulin PTMs can generate a large number of functionally specialized microtubule sub‐networks, each of which is crucial to specific neuronal features.

CAMSAP1 breaks the homeostatic microtubule network to instruct neuronal polarity

The establishment of axon/dendrite polarity is fundamental for neurons to integrate into functional circuits, and this process is critically dependent on microtubules (MTs). In the early stages of the establishment process, MTs in axons change dramatically with the morphological building of neurons; however, how the MT network changes are triggered is unclear. Here we show that CAMSAP1 plays a decisive role in the neuronal axon identification process by regulating the number of MTs. Neurons lacking CAMSAP1 form a multiple axon phenotype in vitro, while the multipolar-bipolar transition and radial migration are blocked in vivo. We demonstrate that the polarity regulator MARK2 kinase phosphorylates CAMSAP1 and affects its ability to bind to MTs, which in turn changes the protection of MT minus-ends and also triggers asymmetric distribution of MTs. Our results indicate that the polarized MT network in neurons is a decisive factor in establishing axon/dendritic polarity and is initially triggered by polarized signals.

New gene discoveries highlight functional convergence in autism and related neurodevelopmental disorders

Over the last two years, remarkable gene discovery efforts have implicated disruption of pathways involving gene regulatory functions and neuronal processes in autism spectrum disorder (ASD), and more broadly defined neurodevelopmental disorders (NDDs). Functional studies in the developing brain and across cell types demonstrate that the spatiotemporal expression patterns of many of these genes coalesce on subnetworks with distinct developmental trajectories. Here, we review the convergent biological processes derived from gene discovery and functional genomics in ASD and NDD from 2018-2020. We further probe the mechanistic insights that suggest these frequently perturbed pathways are interconnected and, ultimately, converge on specific functional deficits in human neurodevelopment.

Axonal transport: Driving synaptic function

The intracellular transport system in neurons is specialized to an extraordinary degree, enabling the delivery of critical cargo to sites in axons or dendrites that are far removed from the cell center. Vesicles formed in the cell body are actively transported by kinesin motors along axonal microtubules to presynaptic sites that can be located more than a meter away. Both growth factors and degradative vesicles carrying aged organelles or aggregated proteins take the opposite route, driven by dynein motors. Distance is not the only challenge; precise delivery of cargos to sites of need must also be accomplished. For example, localized delivery of presynaptic components to hundreds of thousands of "en passant" synapses distributed along the length of a single axon in some neuronal subtypes provides a layer of complexity that must be successfully navigated to maintain synaptic transmission. We review recent advances in the field of axonal transport, with a focus on conceptual developments, and highlight our growing quantitative understanding of neuronal trafficking and its role in maintaining the synaptic function that underlies higher cognitive processes such as learning and memory.

DIP2B Interacts With α-Tubulin to Regulate Axon Outgrowth

Axonal development is essential to the establishment of neuronal morphology and circuitry, although the mechanisms underlying axonal outgrowth during the early developmental stages remain unclear. Here, we showed that the conserved disco-interacting protein B (DIP2B) which consists of a DMAP1 domain and a crotonobetaine/carnitine CoA ligase (Caic) domain, is highly expressed in the excitatory neurons of the hippocampus. DIP2B knockout led to excessive axonal outgrowth but not polarity at an early developmental stage. Furthermore, the loss of DIP2B inhibited synaptic transmission for both spontaneous and rapid release in cultured hippocampal neurons. Interestingly, DIP2B function during axonal outgrowth requires tubulin acetylation. These findings reveal a new conserved regulator of neuronal morphology and provide a novel intervention mechanism for neurocognitive disorders.

Non-enzymatic Activity of the α-Tubulin Acetyltransferase αTAT Limits Synaptic Bouton Growth in Neurons

Neuronal axons terminate as synaptic boutons that form stable yet plastic connections with their targets. Synaptic bouton development relies on an underlying network of both long-lived and dynamic microtubules that provide structural stability for the boutons while also allowing for their growth and remodeling. However, a molecular-scale mechanism that explains how neurons appropriately balance these two microtubule populations remains a mystery. We hypothesized that α-tubulin acetyltransferase (αTAT), which both stabilizes long-lived microtubules against mechanical stress via acetylation and has been implicated in promoting microtubule dynamics, could play a role in this process. Using the Drosophila neuromuscular junction as a model, we found that non-enzymatic dαTAT activity limits the growth of synaptic boutons by affecting dynamic, but not stable, microtubules. Loss of dαTAT results in the formation of ectopic boutons. These ectopic boutons can be similarly suppressed by resupplying enzyme-inactive dαTAT or by treatment with a low concentration of the microtubule-targeting agent vinblastine, which acts to suppress microtubule dynamics. Biophysical reconstitution experiments revealed that non-enzymatic αTAT1 activity destabilizes dynamic microtubules but does not substantially impact the stability of long-lived microtubules. Further, during microtubule growth, non-enzymatic αTAT1 activity results in increasingly extended tip structures, consistent with an increased rate of acceleration of catastrophe frequency with microtubule age, perhaps via tip structure remodeling. Through these mechanisms, αTAT enriches for stable microtubules at the expense of dynamic ones. We propose that the specific suppression of dynamic microtubules by non-enzymatic αTAT activity regulates the remodeling of microtubule networks during synaptic bouton development.

Decoding epigenetic codes: new frontiers in exploring recovery from spinal cord injury

Spinal cord injury that results in severe neurological disability is often incurable. The poor clinical outcome of spinal cord injury is mainly caused by the failure to reconstruct the injured neural circuits. Several intrinsic and extrinsic determinants contribute to this inability to reconnect. Epigenetic regulation acts as the driving force for multiple pathological and physiological processes in the central nervous system by modulating the expression of certain critical genes. Recent studies have demonstrated that post-SCI alteration of epigenetic landmarks is strongly associated with axon regeneration, glial activation and neurogenesis. These findings not only establish a theoretical foundation for further exploration of spinal cord injury, but also provide new avenues for the clinical treatment of spinal cord injury. This review focuses on the epigenetic regulation in axon regeneration and secondary spinal cord injury. Together, these discoveries are a selection of epigenetic-based prognosis biomarkers and attractive therapeutic targets in the treatment of spinal cord injury.

Histone deacetylase 6 inhibition restores autophagic flux to promote functional recovery after spinal cord injury

After spinal cord injury (SCI), the inhibitory molecules derived from scars at the lesion sites and the limited regenerative capacity of neuronal axons pose difficulties for the recovery after SCI. Remodeling of cytoskeleton structures including microtubule assembly and tubulin post-translational modification are widely accepted to play a crucial role in initiation of growth cone and regrowth of injured axon. Although increasing studies have focused on the association between tubulin acetylation and autophagy due to the role of tubulin acetylation in organelles and substances transport, there are no studies exploring the effect of tubulin acetylation on autophagy after spinal cord injury (SCI). Here, we found that histone deacetylase 6 (HDAC6) was significantly up-regulated after SCI, while inhibition of HDAC6 by Tubastatin A induced functional recovery after SCI. In view of enzyme-dependent and -independent mechanisms of HDAC6 to adjust diverse cellular processes, such as autophagy, the ubiquitin proteasome system and post-translational modification of tubulin, we mainly focused on the significance of HDAC6 in axonal regeneration and autophagy after SCI. Western blotting, Co-immunoprecipitation and immunofluorescence staining were conducted to showed that Tubastatin A treatment in nocodazole-treated cells and mice suffering from SCI prompted acetylation and stabilization of microtubules and thus restored transport function, which may contribute to restored autophagic flux and increased axonal length. Whereas inhibition of degradation of autolysosomes by bafilomycin A1 (Baf-A1) reversed functional recovery caused by Tubastatin A, revealing the association between tubulin acetylation and autophagy, which supports HDAC6 inhibition as a potential target for SCI treatment.

A CCRK and a MAK Kinase Modulate Cilia Branching and Length via Regulation of Axonemal Microtubule Dynamics in Caenorhabditis elegans

The diverse morphologies of primary cilia are tightly regulated as a function of cell type and cellular state. CCRK- and MAK-related kinases have been implicated in ciliary length control in multiple species, although the underlying mechanisms are not fully understood. Here, we show that in C. elegans, DYF-18/CCRK and DYF-5/MAK act in a cascade to generate the highly arborized cilia morphologies of the AWA olfactory neurons. Loss of kinase function results in dramatically elongated AWA cilia that lack branches. Intraflagellar transport (IFT) motor protein localization, but not velocities, in AWA cilia is altered upon loss of dyf-18. We instead find that axonemal microtubules are decorated by the EBP-2 end-binding protein along their lengths and that the tubulin load is increased and tubulin turnover is reduced in AWA cilia of dyf-18 mutants. Moreover, we show that predicted microtubule-destabilizing mutations in two tubulin subunits, as well as mutations in IFT proteins predicted to disrupt tubulin transport, restore cilia branching and suppress AWA cilia elongation in dyf-18 mutants. Loss of dyf-18 is also sufficient to elongate the truncated rod-like unbranched cilia of the ASH nociceptive neurons in animals carrying a microtubule-destabilizing mutation in a tubulin subunit. We suggest that CCRK and MAK activity tunes cilia length and shape in part via modulation of axonemal microtubule stability, suggesting that similar mechanisms may underlie their roles in ciliary length control in other cell types.

Microtubule control of functional architecture in neurons

Neurons are exquisitely polarized cells whose structure and function relies on microtubules. Microtubules in signal-receiving dendrites and signal-sending axons differ in their organization and microtubule-associated proteins. These differences, coupled with microtubule post-translational modifications, combine to locally regulate intracellular transport, morphology, and function. Recent discoveries provide new insight into the regulation of non-centrosomal microtubule arrays in neurons, the relationship between microtubule acetylation and mechanosensation, and the spatial patterning of microtubules that regulates motor activity and cargo delivery in axons and dendrites. Together, these new studies bring us closer to understanding how microtubule function is locally tuned to match the specialized tasks associated with signal reception and transmission.

Recent advances in branching mechanisms underlying neuronal morphogenesis

Proper neuronal wiring is central to all bodily functions, sensory perception, cognition, memory, and learning. Establishment of a functional neuronal circuit is a highly regulated and dynamic process involving axonal and dendritic branching and navigation toward appropriate targets and connection partners. This intricate circuitry includes axo-dendritic synapse formation, synaptic connections formed with effector cells, and extensive dendritic arborization that function to receive and transmit mechanical and chemical sensory inputs. Such complexity is primarily achieved by extensive axonal and dendritic branch formation and pruning. Fundamental to neuronal branching are cytoskeletal dynamics and plasma membrane expansion, both of which are regulated via numerous extracellular and intracellular signaling mechanisms and molecules. This review focuses on recent advances in understanding the biology of neuronal branching.

CAMSAP3 maintains neuronal polarity through regulation of microtubule stability

Each neuron forms a single axon and multiple dendrites, and this configuration is important for wiring the brain. How only a single axon extends from a neuron, however, remains unknown. This study demonstrates that CAMSAP3, a protein that binds the minus-end of microtubules, preferentially localizes along axons in hippocampal neurons. Remarkably, mutations of CAMSAP3 lead to production of multiple axons in these neurons. In attempts to uncover mechanisms underlying this abnormal axon extension, the authors found that CAMSAP3-anchored microtubules escape from acetylation, a process mediated by α-tubulin acetyltransferase-1, and depletion of this enzyme abolishes abnormal axon formation in CAMSAP3 mutants. These findings reveal that CAMSAP3 controls microtubule dynamics, preventing tubulin acetylation; this mechanism is required for single-axon formation.

The molecular mechanisms that guide each neuron to become polarized, forming a single axon and multiple dendrites, remain unknown. Here we show that CAMSAP3 (calmodulin-regulated spectrin-associated protein 3), a protein that regulates the minus-end dynamics of microtubules, plays a key role in maintaining neuronal polarity. In mouse hippocampal neurons, CAMSAP3 was enriched in axons. Although axonal microtubules were generally acetylated, CAMSAP3 was preferentially localized along a less-acetylated fraction of the microtubules. CAMSAP3-mutated neurons often exhibited supernumerary axons, along with an increased number of neurites having nocodazole-resistant/acetylated microtubules compared with wild-type neurons. Analysis using cell lines showed that CAMSAP3 depletion promoted tubulin acetylation, and conversely, mild overexpression of CAMSAP3 inhibited it, suggesting that CAMSAP3 works to retain nonacetylated microtubules. In contrast, CAMSAP2, a protein related to CAMSAP3, was detected along all neurites, and its loss did not affect neuronal polarity, nor did it cause increased tubulin acetylation. Depletion of α-tubulin acetyltransferase-1 (αTAT1), the key enzyme for tubulin acetylation, abolished CAMSAP3 loss-dependent multiple-axon formation. These observations suggest that CAMSAP3 sustains a nonacetylated pool of microtubules in axons, interfering with the action of αTAT1, and this process is important to maintain neuronal polarity.