Coupling of dynamic microtubules to F-actin by Fmn2 regulates chemotaxis of neuronal growth cones

Genome-wide study reveals novel roles for formin-2 in axon regeneration as a microtubule dynamics regulator and therapeutic target for nerve repair

Peripheral nerves regenerate successfully; however, clinical outcome after injury is poor. We demonstrated that low-dose ionizing radiation (LDIR) promoted axon regeneration and function recovery after peripheral nerve injury (PNI). Genome-wide CpG methylation profiling identified LDIR-induced hypermethylation of the Fmn2 promoter, exhibiting injury-induced Fmn2 downregulation in dorsal root ganglia (DRGs). Constitutive knockout or neuronal Fmn2 knockdown accelerated nerve repair and function recovery. Mechanistically, increased microtubule dynamics at growth cones was observed in time-lapse imaging of Fmn2-deficient DRG neurons. Increased HDAC5 phosphorylation and rapid tubulin deacetylation were found in regenerating axons of neuronal Fmn2-knockdown mice after injury. Growth-promoting effect of neuronal Fmn2 knockdown was eliminated by pharmaceutical blockade of HDAC5 or neuronal Hdac5 knockdown, suggesting that Fmn2deletion promotes axon regeneration via microtubule post-translational modification. In silico screening of FDA-approved drugs identified metaxalone, administered either immediately or 24-h post-injury, accelerating function recovery. This work uncovers a novel axon regeneration function of Fmn2 and a small-molecule strategy for PNI.

Effects of three microtubule-associated proteins (MAP2, MAP4, and Tau) on microtubules' physical properties and neurite morphology

The physical properties of cytoskeletal microtubules have a multifaceted effect on the expression of their cellular functions. A superfamily of microtubule-associated proteins, MAP2, MAP4, and tau, promote the polymerization of microtubules, stabilize the formed microtubules, and affect the physical properties of microtubules. Here, we show differences in the effects of these three MAPs on the physical properties of microtubules. When microtubule-binding domain fragments of MAP2, tau, and three MAP4 isoforms were added to microtubules in vitro and observed by fluorescence microscopy, tau-bound microtubules showed a straighter morphology than the microtubules bound by MAP2 and the three MAP4 isoforms. Flexural rigidity was evaluated by the shape of the teardrop pattern formed when microtubules were placed in a hydrodynamic flow, revealing that tau-bound microtubules were the least flexible. When full-length MAPs fused with EGFP were expressed in human neuroblastoma (SH-SY5Y) cells, the microtubules in apical regions of protrusions expressing tau were straighter than in cells expressing MAP2 and MAP4. On the other hand, the protrusions of tau-expressing cells had the fewest branches. These results suggest that the properties of microtubules, which are regulated by MAPs, contribute to the morphogenesis of neurites.

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

The altered expression of cytoskeletal and synaptic remodeling proteins during epilepsy

The cytoskeleton plays an important role in epilepsy; however, the mechanism is unknown. Therefore, this study aimed to reveal the mechanism of cytoskeletal proteins in epilepsy by investigating the expression of cytoskeletal proteins and synaptophysin (SYP) in mice at 0, 3, 6, and 24 h, 3 days, and 7 days in a kainic acid (KA)-induced epileptic model. Our results demonstrated that the expression of F-actin decreased significantly between 3 and 6 h, 6 and 24 h, and 24 h and 3 days (P < 0.05). Meanwhile, the expression of the neurofilament light chain, neurofilament medium chain, and neurofilament heavy chain subunits was significantly decreased (P < 0.001) at 3 h after the KA injection compared to the KA 0 h group, followed by an elevation at 6 h and a further decrease at 24 h compared to at 6 h. SYP expression was significantly decreased between 0 and 3 h as well as between 3 and 6 h (P < 0.05). At 24 h, the level was increased compared to at 6 h and continued to increase at 3 days after the KA injection. Thus, we propose that cytoskeletal proteins may be involved in the pathogenesis of epilepsy.

Silver nanoparticles exposure induces developmental neurotoxicity in hiPSC-derived cerebral organoids

Silver nanoparticles (AgNPs) are used in various research fields. Although the neurotoxicity of AgNPs has been explored in animal models and 2D cell-culture models, including human stem cells, these models cannot accurately mimic the development of the human brain. Therefore, the potential mechanisms of AgNPs-induced developmental neurotoxicity in humans are still largely unclear. In this study, cerebral organoids derived from induced pluripotent stem cells were treated with 0.1 μg/mL or 0.5 μg/mL AgNPs for 7 days. At the low concentration (0.1 μg/mL), AgNPs increased the cell proliferation and inhibited the neural apoptosis in the organoids, but impaired the cilium assembly and elongation, which may perturb the cell cycle and induce abnormal cerebral-organoid growth. Conversely, at the high concentration (0.5 μg/mL), AgNPs significantly inhibited cell proliferation and induced apoptosis in cerebral organoids. High-concentration AgNPs reduced the expression and co-localization of the cytoskeleton proteins F-actin, myosin, and tubulin, thereby perturbing neurite growth. In conclusion, AgNPs exposure induces developmental neurotoxic effects in cerebral organoids and is thus a potential congenital risk factor.

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.

Re-evaluating the actin-dependence of spectraplakin functions during axon growth and maintenance

Axons are the long and slender processes of neurons constituting the biological cables that wire the nervous system. The growth and maintenance of axons require loose microtubule bundles that extend through their entire length. Understanding microtubule regulation is therefore an essential aspect of axon biology. Key regulators of neuronal microtubules are the spectraplakins, a well-conserved family of cytoskeletal cross-linkers that underlie neuropathies in mouse and humans. Spectraplakin deficiency in mouse or Drosophila causes severe decay of microtubule bundles and reduced axon growth. The underlying mechanisms are best understood for Drosophila's spectraplakin Short stop (Shot) and believed to involve cytoskeletal cross-linkage: Shot's binding to microtubules and Eb1 via its C-terminus has been thoroughly investigated, whereas its F-actin interaction via N-terminal calponin homology (CH) domains is little understood. Here, we have gained new understanding by showing that the F-actin interaction must be finely balanced: altering the properties of F-actin networks or deleting/exchanging Shot's CH domains induces changes in Shot function-with a Lifeact-containing Shot variant causing remarkable remodeling of neuronal microtubules. In addition to actin-microtubule (MT) cross-linkage, we find strong indications that Shot executes redundant MT bundle-promoting roles that are F-actin-independent. We argue that these likely involve the neuronal Shot-PH isoform, which is characterized by a large, unexplored central plakin repeat region (PRR) similarly existing also in mammalian spectraplakins.

Molecular Dissection of DAAM Function during Axon Growth in Drosophila Embryonic Neurons

Axonal growth is mediated by coordinated changes of the actin and microtubule (MT) cytoskeleton. Ample evidence suggests that members of the formin protein family are involved in the coordination of these cytoskeletal rearrangements, but the molecular mechanisms of the formin-dependent actin–microtubule crosstalk remains largely elusive. Of the six Drosophila formins, DAAM was shown to play a pivotal role during axonal growth in all stages of nervous system development, while FRL was implicated in axonal development in the adult brain. Here, we aimed to investigate the potentially redundant function of these two formins, and we attempted to clarify which molecular activities are important for axonal growth. We used a combination of genetic analyses, cellular assays and biochemical approaches to demonstrate that the actin-processing activity of DAAM is indispensable for axonal growth in every developmental condition. In addition, we identified a novel MT-binding motif within the FH2 domain of DAAM, which is required for proper growth and guidance of the mushroom body axons, while being dispensable during embryonic axon development. Together, these data suggest that DAAM is the predominant formin during axonal growth in Drosophila, and highlight the contribution of multiple formin-mediated mechanisms in cytoskeleton coordination during axonal growth.

Mechanisms of microtubule organization in differentiated animal cells

Microtubules are polarized cytoskeletal filaments that serve as tracks for intracellular transport and form a scaffold that positions organelles and other cellular components and modulates cell shape and mechanics. In animal cells, the geometry, density and directionality of microtubule networks are major determinants of cellular architecture, polarity and proliferation. In dividing cells, microtubules form bipolar spindles that pull chromosomes apart, whereas in interphase cells, microtubules are organized in a cell type-specific fashion, which strongly correlates with cell physiology. In motile cells, such as fibroblasts and immune cells, microtubules are organized as radial asters, whereas in immotile epithelial and neuronal cells and in muscles, microtubules form parallel or antiparallel arrays and cortical meshworks. Here, we review recent work addressing how the formation of such microtubule networks is driven by the plethora of microtubule regulatory proteins. These include proteins that nucleate or anchor microtubule ends at different cellular structures and those that sever or move microtubules, as well as regulators of microtubule elongation, stability, bundling or modifications. The emerging picture, although still very incomplete, shows a remarkable diversity of cell-specific mechanisms that employ conserved building blocks to adjust microtubule organization in order to facilitate different cellular functions.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Formins in Human Disease

Almost 25 years have passed since a mutation of a formin gene, DIAPH1, was identified as being responsible for a human inherited disorder: a form of sensorineural hearing loss. Since then, our knowledge of the links between formins and disease has deepened considerably. Mutations of DIAPH1 and six other formin genes (DAAM2, DIAPH2, DIAPH3, FMN2, INF2 and FHOD3) have been identified as the genetic cause of a variety of inherited human disorders, including intellectual disability, renal disease, peripheral neuropathy, thrombocytopenia, primary ovarian insufficiency, hearing loss and cardiomyopathy. In addition, alterations in formin genes have been associated with a variety of pathological conditions, including developmental defects affecting the heart, nervous system and kidney, aging-related diseases, and cancer. This review summarizes the most recent discoveries about the involvement of formin alterations in monogenic disorders and other human pathological conditions, especially cancer, with which they have been associated. In vitro results and experiments in modified animal models are discussed. Finally, we outline the directions for future research in this field.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

The Formin Fmn2b Is Required for the Development of an Excitatory Interneuron Module in the Zebrafish Acoustic Startle Circuit

The formin family member Fmn2 is a neuronally enriched cytoskeletal remodeling protein conserved across vertebrates. Recent studies have implicated Fmn2 in neurodevelopmental disorders, including sensory processing dysfunction and intellectual disability in humans. Cellular characterization of Fmn2 in primary neuronal cultures has identified its function in the regulation of cell-substrate adhesion and consequently growth cone translocation. However, the role of Fmn2 in the development of neural circuits in vivo, and its impact on associated behaviors have not been tested. Using automated analysis of behavior and systematic investigation of the associated circuitry, we uncover the role of Fmn2b in zebrafish neural circuit development. As reported in other vertebrates, the zebrafish ortholog of Fmn2 is also enriched in the developing zebrafish nervous system. We find that Fmn2b is required for the development of an excitatory interneuron pathway, the spiral fiber neuron, which is an essential circuit component in the regulation of the Mauthner cell (M-cell)-mediated acoustic startle response. Consistent with the loss of the spiral fiber neurons tracts, high-speed video recording revealed a reduction in the short latency escape events while responsiveness to the stimuli was unaffected. Taken together, this study provides evidence for a circuit-specific requirement of Fmn2b in eliciting an essential behavior in zebrafish. Our findings underscore the importance of Fmn2 in neural development across vertebrate lineages and highlight zebrafish models in understanding neurodevelopmental disorders.