GO:0051013 microtubule severing: Mechanism, Genes, Functions

Microtubule severing (GO:0051013) is a fundamental biological process that regulates microtubule dynamics by breaking microtubules into smaller segments. This process is mediated by a family of ATP-dependent enzymes known as microtubule-severing enzymes, which include katanin, spastin, fidgetin, and FIGNL1. Unlike depolymerizing kinesins that act at microtubule ends, severing enzymes internalize within the filament lattice and remove tubulin dimers to create new ends, thereby rapidly remodeling the cytoskeleton. This unique mechanism allows cells to control microtubule organization and dynamics with high spatial and temporal precision. Microtubule severing is critical for numerous cellular functions, including mitotic spindle assembly, cytokinesis, neuronal development, and ciliogenesis. In neurons, severing enzymes generate microtubule fragments that are transported to growing axons and dendrites, contributing to neuronal morphogenesis and plasticity. During cell division, severing activity is required for proper chromosome segregation and abscission. Given its broad impact on cell physiology, microtubule severing is an active area of research, with implications for understanding developmental disorders, neurodegeneration, and cancer. Researchers studying microtubule severing employ a variety of experimental approaches, from in vitro reconstitution assays to live-cell imaging and genetic manipulation. The advent of CRISPR-Cas9 genome editing has enabled precise knockout, point mutation, and knock-in models to dissect the roles of specific severing enzymes in health and disease. This article provides a comprehensive overview of the molecular mechanism, key genes, regulation, disease associations, and research methods for GO:0051013, serving as a resource for scientists and AI-driven discovery.

Microtubule severing is the process in which a microtubule is broken down into smaller segments. Severing enzymes remove dimers from the middle of the filament to create new ends, unlike depolymerizing kinesins that use ATP to uncap microtubules at their ends.

Microtubule severing is essential for cellular processes that require rapid cytoskeletal reorganization, such as cell division, neuronal development, and ciliary assembly. Dysregulation of severing enzymes is associated with a range of human diseases, including hereditary spastic paraplegia, cancer, and neurodevelopmental disorders. Understanding the molecular mechanisms and regulation of microtubule severing provides insights into fundamental cell biology and offers potential therapeutic targets for these conditions.

The following genes encode microtubule-severing enzymes and associated proteins that are central to GO:0051013.

Microtubule severing is regulated at multiple levels, including enzyme expression, post-translational modifications, and interactions with adaptor proteins. Phosphorylation of spastin by cyclin-dependent kinases modulates its activity during the cell cycle. Polyglutamylation and acetylation of tubulin act as molecular switches that enhance or inhibit severing. Additionally, the ESCRT-III complex recruits spastin to specific membrane sites during abscission, ensuring spatial control of severing. These regulatory mechanisms allow cells to fine-tune microtubule severing in response to developmental and environmental cues.

Mutations in SPAST, which encodes spastin, are the most common cause of hereditary spastic paraplegia (HSP), a neurodegenerative disorder characterized by progressive spasticity and weakness of the lower limbs. These mutations often impair spastin's microtubule-severing activity, leading to abnormal microtubule dynamics in neurons. Similarly, mutations in KATNA1 and KATNB1 have been linked to neurodevelopmental disorders with cortical malformations. Dysfunctional severing contributes to axonal degeneration and impaired neuronal transport, highlighting the importance of microtubule severing in neuronal health.

Altered expression of microtubule-severing enzymes is observed in various cancers, where they can promote or inhibit tumorigenesis depending on context. For example, spastin overexpression is associated with poor prognosis in some cancers, and its depletion leads to mitotic defects and reduced cell proliferation. FIGNL1 is involved in DNA repair and its loss sensitizes cancer cells to DNA-damaging agents. Targeting severing enzymes is being explored as a therapeutic strategy in oncology.

Microtubule severing plays a role in ciliogenesis, and defects in severing enzymes can lead to ciliopathies. Katanin and spastin are required for the disassembly of cilia and the regulation of ciliary length. Mutations in KATNB1 cause a ciliopathy-like phenotype with microcephaly and skeletal abnormalities. These findings underscore the importance of severing in developmental processes and organ function.

Live-cell imaging using fluorescently labeled tubulin or microtubule-binding proteins allows real-time visualization of microtubule severing events. Techniques such as spinning-disk confocal microscopy and total internal reflection fluorescence (TIRF) microscopy enable high-resolution tracking of severing in vitro and in cells. These methods are used to quantify severing frequency, enzyme localization, and the effects of mutations.

In vitro reconstitution assays with purified severing enzymes and microtubules provide mechanistic insights into severing activity. These assays often use TIRF microscopy to observe individual microtubules and measure the rate of severing, ATP dependence, and the effect of tubulin modifications. They are essential for dissecting the biochemical properties of severing enzymes.

CRISPR-Cas9 technology enables precise knockout, point mutation, and knock-in of severing enzyme genes in cell lines and model organisms. Knockout models reveal loss-of-function phenotypes, while point mutations mimic disease-associated alleles. Knock-in of tagged enzymes allows for localization and interaction studies. These approaches are crucial for linking specific genes to microtubule severing functions.

Affinity purification coupled with mass spectrometry identifies proteins that interact with severing enzymes, revealing regulatory complexes. Proximity labeling techniques such as BioID can map the interactome of spastin and katanin in living cells. These methods help uncover novel regulators and substrates of microtubule severing.

Researchers studying microtubule severing-related genes often need to determine whether a candidate gene is causally involved in the process, and how specific mutations affect enzyme function. EDITGENE provides a comprehensive suite of CRISPR-based services to accelerate this discovery, from gene knockout to precise point mutations and library screening.

Microtubule severing (GO:0051013) is a vital biological process that regulates cytoskeletal dynamics through the action of ATP-dependent severing enzymes. Its roles in cell division, neuronal development, and ciliary function make it a key area of research, with direct implications for hereditary spastic paraplegia, cancer, and developmental disorders. Advances in CRISPR genome editing and imaging technologies continue to unravel the molecular mechanisms and regulatory networks of severing enzymes. EDITGENE offers comprehensive CRISPR services to support researchers in exploring the functions and disease relevance of microtubule severing genes.