A comparative study of the expression patterns of Fign family members in zebrafish embryonic development

Downregulation of Fidgetin-Like 2 Increases Microglial Function: The Relationship Between Microtubules, Morphology, and Activity

The microtubule cytoskeleton regulates microglial morphology, motility, and effector functions. The microtubule-severing enzyme, fidgetin-like 2 (FL2), negatively regulates cell motility and nerve regeneration, making it a promising therapeutic target for central nervous system injury. Microglia perform important functions in response to inflammation and injury, but how FL2 affects microglia is unclear. In this study, we investigated the role of FL2 in microglial morphology and injury responses in vitro. We first determined that the pro-inflammatory stimulus, lipopolysaccharide (LPS), induced a dose- and time-dependent reduction in FL2 expression associated with reduced microglial ramification. We then administered nanoparticle-encapuslated FL2 siRNA to knockdown FL2 and assess microglial functions compared to negative control siRNA and vehicle controls. Time-lapse live-cell microscopy showed that FL2 knockdown increased the velocity of microglial motility. After incubation with fluorescently labeled IgG-opsonized beads, FL2 knockdown increased phagocytosis. Microglia were exposed to low-dose LPS after nanoparticle treatment to model injury-induced cytokine secretion. FL2 knockdown enhanced LPS-induced cytokine secretion of IL-1α, IL-1β, and TNFα. These results identify FL2 as a regulator of microglial morphology and suggest that FL2 can be targeted to increase or accelerate microglial injury responses.

Evolution and functional divergence of the Fidgetin family

The Fidgetin (FIGN) family, which comprises FIGN, Fidgetin-like 1 (FIGNL1), and Fidgetin-like 2 (FIGNL2), is a vital group of microtubule-severing proteins. These proteins feature a conserved AAA+ domain essential for ATPase activity and a hexameric assembly. This review provides an in-depth analysis of the evolution and functional divergence of the FIGN family members, highlighting their role in the dynamic organization of the cytoskeleton. We further explore their broader biological functions across various species, systems, and subcellular localization. Although the FIGN family is conserved, each member exhibits unique structural characteristics and functions that reflect their evolutionary adaptations. FIGNL1 is found across animal species, while FIGNL2 is specific to vertebrates, thereby indicating its more recent evolutionary origin. Moreover, synteny analysis has revealed that FIGN is located in a more conserved genomic region compared to FIGNL2, which has undergone substantial evolutionary changes. The expression patterns of the FIGN members also vary across organisms and tissues. For example, FIGNL2 shows a notably reduced expression in the mammalian nervous system compared to that in lower vertebrates. The FIGN family members have distinct roles in microtubule severing, cell division, and DNA repair. Specifically, FIGN is involved in cell division and neuronal regeneration, FIGNL1 in axonal growth and DNA repair, and FIGNL2 in cell migration and vascular development. Their involvement in these processes underscores their role as potential biomarkers for certain cancers as well as therapeutic targets for diseases affecting the nervous system and cardiovascular development. All these evolutionary insights and functional distinctions of the FIGN family offer a comprehensive framework for understanding cytoskeletal regulation and its implications in health and disease.

The fidgetin family: Shaking things up among the microtubule-severing enzymes

The microtubule cytoskeleton is required for several crucial cellular processes, including chromosome segregation, cell polarity and orientation, and intracellular transport. These functions rely on microtubule stability and dynamics, which are regulated by microtubule-binding proteins (MTBPs). One such type of regulator is the microtubule-severing enzymes (MSEs), which are ATPases Associated with Diverse Cellular Activities (AAA+ ATPases). The most recently identified family are the fidgetins, which contain three members: fidgetin, fidgetin-like 1 (FL1), and fidgetin-like 2 (FL2). Of the three known MSE families, the fidgetins have the most diverse range of functions in the cell, spanning mitosis/meiosis, development, cell migration, DNA repair, and neuronal function. Furthermore, they offer intriguing novel therapeutic targets for cancer, cardiovascular disease, and wound healing. In the two decades since their first report, there has been great progress in our understanding of the fidgetins; however, there is still much left unknown about this unusual family. This review aims to consolidate the present body of knowledge of the fidgetin family of MSEs and to inspire deeper exploration into the fidgetins and the MSEs as a whole.

SASH1 contributes to glial cell migration in the early development of the central nervous system

SAM and SH3 domain-containing 1 (SASH1), a member of the SLy protein family, is a tumor suppressor gene that has been studied for its association with various cancers. SASH1 is highly expressed in the mammalian central nervous system, particularly in glial cells, and is expressed in the central nervous system during zebrafish embryo development. However, SASH1's role in brain development has rarely been investigated. In this study, Morpholino oligonucleotides (MO) were used to down-regulate sash1a expression in zebrafish to observe morphological changes in the brain. Three transgenic zebrafish lines, Tg(gfap:eGFP), Tg(hb9:eGFP), and Tg(coro1a:eGFP) were selected to observe changes in glial cells, neurons, and immune cells after sash1a knockdown. Our results showed that the number of microglia residing in the developmental brain was reduced, whereas the axonal growth of caudal primary motor neurons was unaffected by sash1a downregulation. And more significantly, the gfap + glia presented abnormal arrangements and disordered orientations in sash1a morphants. The similar phenotype was verified in the mutation induced by the injection of cas9 mRNA and sash1a sgRNA. We further performed behavioral experiments in zebrafish larvae that had been injected with sash1a MO at one-cell stage, and found them exhibiting abnormal behavior trajectories. Moreover, injecting the human SASH1 mRNA rescued these phenomena in sash1a MO zebrafish. In summary, our study revealed that the downregulation of SASH1 leads to malformations in the embryonic brain and disorganization of glial cell marshalling, suggesting that SASH1 plays an important role in the migration of glial cells during embryonic brain development.

mRNA Localization and Local Translation of the Microtubule Severing Enzyme, Fidgetin-Like 2, in Polarization, Migration and Outgrowth

Cell motility requires strict spatiotemporal control of protein expression. During cell migration, mRNA localization and local translation in subcellular areas like the leading edge and protrusions are particularly advantageous for regulating the reorganization of the cytoskeleton. Fidgetin-Like 2 (FL2), a microtubule severing enzyme (MSE) that restricts migration and outgrowth, localizes to the leading edge of protrusions where it severs dynamic microtubules. FL2 is primarily expressed during development but in adulthood, is spatially upregulated at the leading edge minutes after injury. Here, we show mRNA localization and local translation in protrusions of polarized cells are responsible for FL2 leading edge expression after injury. The data suggests that the RNA binding protein IMP1 is involved in the translational regulation and stabilization of FL2 mRNA, in competition with the miRNA let-7. These data exemplify the role of local translation in microtubule network reorganization during migration and elucidate an unexplored MSE protein localization mechanism.

Fidgetin-like 2 negatively regulates axonal growth and can be targeted to promote functional nerve regeneration

The microtubule (MT) cytoskeleton plays a critical role in axon growth and guidance. Here, we identify the MT-severing enzyme fidgetin-like 2 (FL2) as a negative regulator of axon regeneration and a therapeutic target for promoting nerve regeneration after injury. Genetic knockout of FL2 in cultured adult dorsal root ganglion neurons resulted in longer axons and attenuated growth cone retraction in response to inhibitory molecules. Given the axonal growth-promoting effects of FL2 depletion in vitro, we tested whether FL2 could be targeted to promote regeneration in a rodent model of cavernous nerve (CN) injury. The CNs are parasympathetic nerves that regulate blood flow to the penis, which are commonly damaged during radical prostatectomy (RP), resulting in erectile dysfunction (ED). Application of FL2-siRNA after CN injury significantly enhanced functional nerve recovery. Remarkably, following bilateral nerve transection, visible and functional nerve regeneration was observed in 7 out of 8 animals treated with FL2-siRNA, while no control-treated animals exhibited regeneration. These studies identify FL2 as a promising therapeutic target for enhancing regeneration after peripheral nerve injury and for mitigating neurogenic ED after RP - a condition for which, at present, only poor treatment options exist.

Microtubule Severing Protein Fignl2 Contributes to Endothelial and Neuronal Branching in Zebrafish Development

Previously, fidgetin (fign) and its family members fidgetin-like 1 (fignl1) and fidgetin-like 2 (fignl2) were found to be highly expressed during zebrafish brain development, suggesting their functions in the nervous system. In this study, we report the effects of loss-of-function of these genes on development. We designed and identified single-guide RNAs targeted to generate fign, fignl1, and fignl2 mutants and then observed the overall morphological and behavioral changes. Our findings showed that while fign and fignl1 null mutants displayed no significant defects, fignl2 null zebrafish mutants displayed pericardial edema, reduced heart rate, and smaller eyes; fignl2 null mutants responded to the light-darkness shift with a lower swimming velocity. fignl2 mRNAs were identified in vascular endothelial cells by in situ hybridization and re-analysis of an online dataset of single-cell RNAseq results. Finally, we used morpholino oligonucleotides to confirm that fignl2 knockdown resulted in severe heart edema, which was caused by abnormal vascular branching. The zebrafish fignl2 morphants also showed longer axonal length and more branches of caudal primary neurons. Taken together, we summarize that Fignl2 functions on cellular branches in endothelial cells and neurons. This study reported for the first time that the microtubule-severing protein Fignl2 contributes to cell branching during development.