MAP1B Regulates Axonal Development by Modulating Rho-GTPase Rac1 Activity (original) (raw)
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Evidence for the Role of MAP1B in Axon Formation
2001
Cultured neurons obtained from a hypomorphous MAP1B mutant mouse line display a selective and significant inhibition of axon formation that reflects a delay in axon outgrowth and a reduced rate of elongation. This phenomenon is paralleled by decreased microtubule formation and dynamics, which is dramatic at the distal axonal segment, as well as in growth cones, where the more recently assembled microtubule polymer normally predominates. These neurons also have aberrant growth cone formation and increased actin-based protrusive activity. Taken together, this study provides direct evidence showing that by promoting microtubule dynamics and regulating cytoskeletal organization MAP1B has a crucial role in axon formation.
Rac-GTPases Regulate Microtubule Stability and Axon Growth of Cortical GABAergic Interneurons
Cerebral Cortex, 2014
Cortical interneurons are characterized by extraordinary functional and morphological diversity. Although tremendous progress has been made in uncovering molecular and cellular mechanisms implicated in interneuron generation and function, several questions still remain open. Rho-GTPases have been implicated as intracellular mediators of numerous developmental processes such as cytoskeleton organization, vesicle trafficking, transcription, cell cycle progression, and apoptosis. Specifically in cortical interneurons, we have recently shown a cell-autonomous and stage-specific requirement for Rac1 activity within proliferating interneuron precursors. Conditional ablation of Rac1 in the medial ganglionic eminence leads to a 50% reduction of GABAergic interneurons in the postnatal cortex. Here we examine the additional role of Rac3 by analyzing Rac1/ Rac3 double-mutant mice. We show that in the absence of both Rac proteins, the embryonic migration of medial ganglionic eminencederived interneurons is further impaired. Postnatally, double-mutant mice display a dramatic loss of cortical interneurons. In addition, Rac1/Rac3-deficient interneurons show gross cytoskeletal defects in vitro, with the length of their leading processes significantly reduced and a clear multipolar morphology. We propose that in the absence of Rac1/Rac3, cortical interneurons fail to migrate tangentially towards the pallium due to defects in actin and microtubule cytoskeletal dynamics.
PLoS ONE, 2012
Microtubule-associated protein 1B (MAP1B) is a neuronal protein involved in the stabilization of microtubules both in the axon and somatodendritic compartments. Acute, genetic inactivation of MAP1B leads to delayed axonal outgrowth, most likely due to changes in the post-translational modification of tubulin subunits, which enhances microtubule polymerization. Furthermore, MAP1B deficiency is accompanied by abnormal actin microfilament polymerization and dramatic changes in the activity of small GTPases controlling the actin cytoskeleton. In this work, we showed that MAP1B interacts with a guanine exchange factor, termed Tiam1, which specifically activates Rac1. These proteins co-segregated in neurons, and interact in both heterologous expression systems and primary neurons. We dissected the molecular domains involved in the MAP1B-Tiam1 interaction, and demonstrated that pleckstrin homology (PH) domains in Tiam1 are responsible for MAP1B binding. Interestingly, only the light chain 1 (LC1) of MAP1B was able to interact with Tiam1. Moreover, it was able to increase the activity of the small GTPase, Rac1. These results suggest that the interaction between Tiam1 and MAP1B, is produced by the binding of LC1 with PH domains in Tiam1. The formation of such a complex impacts on the activation levels of Rac1 confirming a novel function of MAP1B related with the control of small GTPases. These results also support the idea of cross-talk between cytoskeleton compartments inside neuronal cells.
Scientific Reports
The small-GTPase Rac1 is a key molecular regulator linking extracellular signals to actin cytoskeleton dynamics. Loss-of-function mutations in RAC1 and other genes of the Rac signaling pathway have been implicated in the pathogenesis of Intellectual Disability (ID). The Rac1 activity is negatively controlled by GAP proteins, however the effect of Rac1 hyperactivity on neuronal networking in vivo has been poorly studied. ArhGAP15 is a Rac-specific negative regulator, expressed in the main subtypes of pyramidal cortical neurons. In the absence of ArhGAP15, cortical pyramidal neurons show defective neuritogenesis, delayed axonal elongation, reduced dendritic branching, both in vitro and in vivo. These phenotypes are associated with altered actin dynamics at the growth cone due to increased activity of the PAK-LIMK pathway and hyperphosphorylation of ADF/cofilin. These results can be explained by shootin1 hypo-phosphorylation and uncoupling with the adhesion system. Functionally, ArhGAP15 −/− mice exhibit decreased synaptic density, altered electroencephalographic rhythms and cognitive deficits. These data suggest that both hypo-and hyperactivation of the Rac pathway due to mutations in Rac1 regulators can result in conditions of ID, and that a tight regulation of Rac1 activity is required to attain the full complexity of the cortical networks. Intellectual Disability (ID) is a neurodevelopmental disorder characterized by significant impairments in both intellectual functioning and in adaptive skills. ID patients show defects in network connectivity and altered excitation/inhibition balance of cerebral cortex and hippocampus, and these alterations may result in abnormal information processing 1. Mutations of RAC1 and of genes involved in the Rac-GTPase pathway such as PAK3, ARHGEF9 and ARHGEF6 (also known as αPIX) have been identified in patients with ID 1-5. Collectively, the genetic variants identified in ID patients are all of the loss-of-function type, implying that a hypoactive main Rac pathway is the pathogenic cause of these conditions 1,2 , although the identification of the endophenotype underlying ID is far from being clarified. Rac-GTPases are key molecular switches that link extracellular signals to actin cytoskeleton dynamics. They cycle between an inactive GDP-bound form and an active GTP-bound state, a binary switch that is tightly regulated by several guanine nucleotide exchange factors (GEFs), by GTPase-activating proteins (GAPs), and by guanine nucleotide dissociation inhibitors (GDIs) 6. Over the past several years, it has become clear that Rac-GTPases and their regulators control the dynamics of actin cytoskeleton rearrangements, inducing stabilization and
Involvement of Rho-family GTPases in axon branching
Small GTPases, 2014
Development of the nervous system requires efficient extension and guidance of axons and dendrites culminating in synapse formation. Axonal growth and navigation during embryogenesis are controlled by extracellular cues. Many of the same extracellular signals also regulate axonal branching. The emergence of collateral branches from the axon augments the complexity of nervous system innervation and provides an additional mechanism for target selection. Rho-family GTPases play an important role in regulating intracellular cytoskeletal and signaling pathways that facilitate axonal morphological changes. RhoA/G and Rac1 GTPase functions are complex and they can induce or inhibit branch formation, depending on neuronal type, cell context or signaling mechanisms. Evidence of a role of Cdc42 in axon branching is mostly lacking. In contrast, Rac3 has thus far been implicated in the regulation of axon branching. Future analysis of the upstream regulators and downstream effectors mediating th...
The role of small GTPases in neuronal morphogenesis and polarity
Cytoskeleton, 2012
The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.
Rho family GTPases: key players in neuronal development, neuronal survival, and neurodegeneration
Frontiers in cellular neuroscience, 2014
The Rho family of GTPases belongs to the Ras superfamily of low molecular weight (∼21 kDa) guanine nucleotide binding proteins. The most extensively studied members are RhoA, Rac1, and Cdc42. In the last few decades, studies have demonstrated that Rho family GTPases are important regulatory molecules that link surface receptors to the organization of the actin and microtubule cytoskeletons. Indeed, Rho GTPases mediate many diverse critical cellular processes, such as gene transcription, cell-cell adhesion, and cell cycle progression. However, Rho GTPases also play an essential role in regulating neuronal morphology. In particular, Rho GTPases regulate dendritic arborization, spine morphogenesis, growth cone development, and axon guidance. In addition, more recent efforts have underscored an important function for Rho GTPases in regulating neuronal survival and death. Interestingly, Rho GTPases can exert either a pro-survival or pro-death signal in neurons depending upon both the cel...
Evidence for the Involvement of Tiam1 in Axon Formation
2001
In cultured neurons, axon formation is preceded by the appearance in one of the multiple neurites of a large growth cone containing a labile actin network and abundant dynamic microtubules. The invasion-inducing T-lymphoma and metastasis 1 (Tiam1) protein that functions as a guanosine nucleotide exchange factor for Rac1 localizes to this neurite and its growth cone, where it associates with microtubules. Neurons overexpressing Tiam1 extend several axon-like neurites, whereas suppression of Tiam1 prevents axon formation, with most of the cells failing to undergo changes in growth cone size and in cytoskeletal organization typical of prospective axons. Cytochalasin D reverts this effect leading to multiple axon formation and penetration of microtubules within neuritic tips devoid of actin filaments. Taken together, these results suggest that by regulating growth cone actin organization and allowing microtubule invasion within selected growth cones, Tiam1 promotes axon formation and hence participates in neuronal polarization.
Journal of Biological Chemistry, 2011
Background: Polarity-regulating kinase PAR1b is also involved in regulation of the actin cytoskeleton. Results: PAR1b inhibits RhoA activator GEF-H1 by inducing phosphorylation on S885 and S959. Conclusion: PAR1b controls RhoA activity through phosphorylation-dependent regulation of GEF-H1. Significance: PAR1b coordinates the microtubule-and actin-cytoskeletal systems in cell regulation. Partitioning-defective 1b (PAR1b), also known as microtubule affinity-regulating kinase 2 (MARK2), is a member of evolutionally conserved PAR1/MARK serine/threonine kinase family, which plays a key role in the establishment and maintenance of cell polarity at least partly by phosphorylating microtubule-associated proteins (MAPs) that regulate microtubule stability. PAR1b has also been reported to influence actin cytoskeletal organization, raising the possibility that PAR1b functionally interacts with the Rho family of small GTPases, central regulators of the actin cytoskeletal system. Consistent with this notion, PAR1 was recently found to be physically associated with a RhoA-specific guanine nucleotide exchange factor H1 (GEF-H1). This observation suggests a functional link between PAR1b and GEF-H1. Here we show that PAR1b induces phosphorylation of GEF-H1 on serine 885 and serine 959. We also show that PAR1b-induced serine 885/serine 959 phosphorylation inhibits RhoA-specific GEF activity of GEF-H1. As a consequence, GEF-H1 phosphorylated on both of the serine residues loses the ability to stimulate RhoA and thereby fails to induce RhoA-dependent stress fiber formation. These findings indicate that PAR1b not only regulates microtubule stability through phosphorylation of MAPs but also influences actin stress fiber formation by inducing GEF-H1 phosphorylation. The dual function of PAR1b in the microtubule-based cytoskeletal system and the actin-based cytoskeletal system in the coordinated regulation of cell polarity, cell morphology, and cell movement. Partitioning-defective 1 (PAR1) 2 is a serine/threonine kinase that is highly conserved from yeast to mammals. PAR1 was originally isolated as one of the partitioning defective genes in Caenorhabditis elegans (1-3). In mammals, PAR1 was first identified as a microtubule affinity-regulating kinase (MARK) that phosphorylates microtubule-associated proteins (MAPs) and thereby destabilizes microtubules (4, 5). Mammalian PAR1/MARK comprises four isoforms, PAR1a/MARK3, PAR1b/MARK2, PAR1c/MARK1, and PAR1d/MARK4. As in the case of C. elegans and Drosophila, the mammalian PAR1 isoforms, especially PAR1b, act as key regulators for the development and maintenance of cell polarity in various cell systems (6, 7). During epithelial polarization, PAR1b, a major PAR1 isoform, specifically localizes to the baso-lateral membrane, whereas the atypical protein kinase C (aPKC)/PAR3/PAR6 complex localizes specifically to the apical membrane. The asymmetric distribution of these two kinases, PAR1b and aPKC/PAR3/PAR6 complex, ensures the apical-basal polarization of epithelial cells (7-9). Phosphorylation-dependent regulation of microtubule stability by PAR1 is thought to underlie the asymmetric distribution of molecules that mediates the establishment and maintenance of epithelial cell polarity (4, 5, 7, 8). Actin filaments are key structures of the cytoskeleton in cells (10). The actin cytoskeleton is involved in many cellular processes, including cell integrity, cell mobility, cell shape, cell division, and contraction. Actin filaments consist of polarized ATP-bound globular actin monomers. These filamentous actin (F-actin) molecules can associate into bundles or networks through actin cross-linking molecules (11). The best characterized F-actin bundles and networks are filopodia, lamellipodia, and actin stress fibers. Formation of the different actin bundles and networks is regulated by signal transduction pathways that depend on Rho family GTPases, most notably RhoA, Rac, and Cdc42 (12-15). Constitutively activated Rac and Cdc42 induce surface protrusion (lamellipodia) with membrane ruffling and * This work was supported by Grants-in-Aid for the Scientific Research on Innovative Area from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
Rho family GTPases and neuronal growth cone remodelling: relationship between …
Molecular and cellular …, 1997
Rho family GTPases have been assigned important roles in the formation of actin-based morphologies in nonneuronal cells. Here we show that microinjection of Cdc42Hs and Rac1 promoted formation of filopodia and lamellipodia in N1E-115 neuroblastoma growth cones and along neurites. These actin-containing structures were also induced by injection of Clostridium botulinum C3 exoenzyme, which abolishes RhoA-mediated functions such as neurite retraction. The C3 response was inhibited by coinjection with the dominant negative mutant Cdc42Hs T17N , while the Cdc42Hs response could be competed by coinjection with RhoA. We also demonstrate that the neurotransmitter acetylcholine (ACh) can induce filopodia and lamellipodia on neuroblastoma growth cones via muscarinic ACh receptor activation, but only when applied in a concentration gradient. ACh-induced formation of filopodia and lamellipodia was inhibited by preinjection with the dominant negative mutants Cdc42Hs T17N and Rac1 T17N , respectively. Lysophosphatidic acid (LPA)-induced neurite retraction, which is mediated by RhoA, was inhibited by ACh, while C3 exoenzyme-mediated neurite outgrowth was inhibited by injection with Cdc42Hs T17N or Rac1 T17N. Together these results suggest that there is competition between the ACh-and LPA-induced morphological pathways mediated by Cdc42Hs and/or Rac1 and by RhoA, leading to either neurite development or collapse.