α1A and α1C form microtubules to display distinct properties mainly mediated by their C-terminal tails

Distinct roles of Kif6 and Kif9 in mammalian ciliary trafficking and motility

Ciliary beat and intraflagellar transport depend on dynein and kinesin motors. The kinesin-9 family members Kif6 and Kif9 are implicated in motile cilia motilities across protists and mammals. How they function and whether they act redundantly, however, remain unclear. Here, we show that Kif6 and Kif9 play distinct roles in mammals. Kif6 forms puncta that move bidirectionally along axonemes, whereas Kif9 appears to oscillate regionally on the ciliary central apparatus. Consistently, only Kif6 displays microtubule-based motor activity in vitro, and its ciliary localization requires its ATPase activity. Kif6 deficiency in mice disrupts coordinated ciliary beat across ependymal tissues and impairs cerebrospinal fluid flow, resulting in severe hydrocephalus and high mortality. Kif9 deficiency causes mild hydrocephalus without obviously affecting the ciliary beat or the lifespan. Kif6-/- and Kif9-/- males are infertile but exhibit oligozoospermia with poor sperm motility and defective forward motion of sperms, respectively. These results suggest Kif6 as a motor for cargo transport and Kif9 as a central apparatus regulator.

LncRNA TubAR complexes with TUBB4A and TUBA1A to promote microtubule assembly and maintain myelination

A long-standing hypothesis proposes that certain RNA(s) must exhibit structural roles in microtubule assembly. Here, we identify a long noncoding RNA (TubAR) that is highly expressed in cerebellum and forms RNA-protein complex with TUBB4A and TUBA1A, two tubulins clinically linked to cerebellar and myelination defects. TubAR knockdown in mouse cerebellum causes loss of oligodendrocytes and Purkinje cells, demyelination, and decreased locomotor activity. Biochemically, we establish the roles of TubAR in promoting TUBB4A-TUBA1A heterodimer formation and microtubule assembly. Intriguingly, different from the hypomyelination-causing mutations, the non-hypomyelination-causing mutation TUBB4A-R2G confers gain-of-function for an RNA-independent interaction with TUBA1A. Experimental use of R2G/A mutations restores TUBB4A-TUBA1A heterodimer formation, and rescues the neuronal cell death phenotype caused by TubAR knockdown. Together, we uncover TubAR as the long-elusive structural RNA for microtubule assembly and demonstrate how TubAR mediates microtubule assembly specifically from αβ-tubulin heterodimers, which is crucial for maintenance of cerebellar myelination and activity.

The discovery of water-soluble indazole derivatives as potent microtubule polymerization inhibitors

Taking a previously discovered indazole derivative 1 as a lead, systematic structural modifications were performed with an indazole core at the 1- and 6-positions to improve its aqueous solubility. Among the designed indazole derivatives, 6-methylpyridin-3-yl indazole derivative 8l and 1H-indol-4-yl indazole derivative 8m exhibited high potency in the low nanomolar range against A549, Huh-7, and T24 cancer cells, including Taxol-resistant variant cells (A549/Tax). As a hydrochloride salt, 8l exhibited much improved aqueous solubility, and its log P value fell into a favorable range. In mechanistic studies, 8l impeded tubulin polymerization through interacting with the colchicine site, resulting in cell cycle arrest and cellular apoptosis. In addition, compared to lead compound 1, 8l reduced cell migration and led to more potent inhibition of tumor growth in vivo without apparent toxicity. In summary, indazole derivative 8l could work as a potential anticancer agent and deserves further investigation for cancer therapy.

Cryo-EM of α-tubulin isotype-containing microtubules revealed a contracted structure of α4A/β2A microtubules

Microtubules are hollow α/β-tubulin heterodimeric polymers that play critical roles in cells. In vertebrates, both α- and β-tubulins have multiple isotypes encoded by different genes, which are intrinsic factors in regulating microtubule functions. However, the structures of microtubules composed of different tubulin isotypes, especially α-tubulin isotypes, remain largely unknown. Here, we purify recombinant tubulin heterodimers composed of different mouse α-tubulin isotypes, including α1A, α1C and α4A, with the β-tubulin isotype β2A. We further assemble and determine the cryo-electron microscopy (cryo-EM) structures of α1A/β2A, α1C/β2A, and α4A/β2A microtubules. Our structural analysis demonstrates that α4A/β2A microtubules exhibit longitudinal contraction between tubulin interdimers compared with α1A/β2A and α1C/β2A microtubules. Collectively, our findings reveal that α-tubulin isotype composition can tune microtubule structures, and also provide evidence for the "tubulin code" hypothesis.

Unlocking Colchicine's Untapped Potential: A Paradigm Shift in Hepatocellular Carcinoma Prevention

Background: Liver cancer and notably hepatocellular carcinoma (HCC), results in significantly high mortality rates worldwide. Chronic hepatitis and fatty liver, recognized precursors, underscore the imperative need for effective preventive strategies. This study explores colchicine, traditionally acknowledged for its anti-inflammatory properties and investigates its potential in liver cancer prevention. Methods: Utilizing the iHi Data Platform of China Medical University Hospital, Taiwan, this study analyzed two decades of medical data, incorporating 10,353 patients each in the Colchicine and Non-Colchicine cohorts, to investigate the association between colchicine use and liver cancer risk. Results: The study identified that colchicine users exhibited a 19% reduction in liver cancer risk, with a multivariable-adjusted odds ratio of 0.81 after accounting for confounding variables. Additionally, the influence of gender and comorbidities like diabetes mellitus on liver cancer risk was identified, corroborating the existing literature. A notable finding was that the prolonged use of colchicine was associated with improved outcomes, indicating a potential dose-response relationship. Conclusions: This study proposes a potential new role for colchicine in liver cancer prevention, extending beyond its established anti-inflammatory applications. While the findings are promising, further research is essential to validate these results. This research may serve as a foundation for future studies, aiming to further explore colchicine's role via clinical trials and in-depth investigations, potentially impacting preventive strategies for liver cancer.

The distinct initiation sites and processing activities of TTLL4 and TTLL7 in glutamylation of brain tubulin

Mammalian brain tubulins undergo a reversible posttranslational modification-polyglutamylation-which attaches a secondary polyglutamate chain to the primary sequence of proteins. Loss of its erasers can disrupt polyglutamylation homeostasis and cause neurodegeneration. Tubulin tyrosine ligase like 4 (TTLL4) and TTLL7 were known to modify tubulins, both with preference for the β-isoform, but differently contribute to neurodegeneration. However, differences in their biochemical properties and functions remain largely unknown. Here, using an antibody-based method, we characterized the properties of a purified recombinant TTLL4 and confirmed its sole role as an initiator, unlike TTLL7, which both initiates and elongates the side chains. Unexpectedly, TTLL4 produced stronger glutamylation immunosignals for α-isoform than β-isoform in brain tubulins. Contrarily, the recombinant TTLL7 raised comparable glutamylation immunoreactivity for two isoforms. Given the site selectivity of the glutamylation antibody, we analyzed modification sites of two enzymes. Tandem mass spectrometry analysis revealed their incompatible site selectivity on synthetic peptides mimicking carboxyl termini of α1- and β2-tubulins and a recombinant tubulin. Particularly, in the recombinant α1A-tubulin, a novel region was found glutamylated by TTLL4 and TTLL7, that again at distinct sites. These results pinpoint different site specificities between two enzymes. Moreover, TTLL7 exhibits less efficiency to elongate microtubules premodified by TTLL4, suggesting possible regulation of TTLL7 elongation activity by TTLL4-initiated sites. Finally, we showed that kinesin behaves differentially on microtubules modified by two enzymes. This study underpins the different reactivity, site selectivity, and function of TTLL4 and TTLL7 on brain tubulins and sheds light on their distinct role in vivo.

Glutamylation is a negative regulator of microtubule growth

Microtubules are noncovalent polymers built from αβ-tubulin dimers. The disordered C-terminal tubulin tails are functionalized with multiple glutamate chains of variable lengths added and removed by tubulin tyrosine ligases (TTLLs) and carboxypeptidases (CCPs). Glutamylation is abundant on stable microtubule arrays such as in axonemes and axons, and its dysregulation leads to human pathologies. Despite this, the effects of glutamylation on intrinsic microtubule dynamics are unclear. Here we generate tubulin with short and long glutamate chains and show that glutamylation slows the rate of microtubule growth and increases catastrophes as a function of glutamylation levels. This implies that the higher stability of glutamylated microtubules in cells is due to effectors. Interestingly, EB1 is minimally affected by glutamylation and thus can report on the growth rates of both unmodified and glutamylated microtubules. Finally, we show that glutamate removal by CCP1 and 5 is synergistic and occurs preferentially on soluble tubulin, unlike TTLL enzymes that prefer microtubules. This substrate preference establishes an asymmetry whereby once the microtubule depolymerizes, the released tubulin is reset to a less-modified state, while polymerized tubulin accumulates the glutamylation mark. Our work shows that a modification on the disordered tubulin tails can directly affect microtubule dynamics and furthers our understanding of the mechanistic underpinnings of the tubulin code.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

Tubulin TUBB4B Is Involved in Spermatogonia Proliferation and Cell Cycle Processes

Tubb4b (tubulin β-4b chain) is essential for cell growth and development as a microtubule network protein. Previous studies have shown that TUBB4B affects mouse pronucleus migration, but the gene function has yet to be elucidated. To study TUBB4B-related functions in mouse reproductive development, we designed a single sgRNA in chromosome 2 and generated a knockout spermatogonia cell line of the β-tubulin isoform Tubb4b by the CRISPR/Cas9 system. Tubb4b-KO spermatogonia recognized abnormal lysosomal membranes and cell morphology defects. Compared to control mouse spermatogonia, the proliferation rate was significantly slower and cycling stagnated in the G1/0 population. Although spermatogonia lacking TUBB4B have abnormal divisions, they are not lethal. We detected the mRNA levels of the cell-regulating cyclins CyclinsD1, CyclinsE, Cdk2, Cdk4, P21, Skp2 and the cell growth factors C/EBP α, C/EBP β, and G-CSF in the spermatogonia of Tubb4b-KO and found that the expressions of CyclinsD1, Skp2 and cell growth factors were significantly reduced. Further analysis revealed that 675 genes were expressed differently after Tubb4b deletion and were enriched in negative regulation of cell population proliferation (GO:0008285), negative regulation of cell cycle G2/M phase transition (GO:1902750), and positive regulation of cell death (GO: 0010942). We also found that there is a common gene Cdkn1a (P21) in these three GO pathways related to cell proliferation and cell cycle, and both quantitative analysis and transcriptome sequencing results showed that the expression of this gene was up-regulated in Tubb4b knockout cells. This implies that Tubb4b may be involved in the division of spermatogonia with multiple cell cycle regulatory proteins. Overall, these data indicate that Tubb4b has a specific role in regulating spermatogonia proliferation and cell cycle.

Reconstituting Microtubules: A Decades-Long Effort From Building Block Identification to the Generation of Recombinant α/β-Tubulin

Microtubules are cytoskeletal filaments underlying the morphology and functions of all eukaryotic cells. In higher eukaryotes, the basic building blocks of these non-covalent polymers, ɑ- and β-tubulins, are encoded by expanded tubulin family genes (i.e., isotypes) at distinct loci in the genome. While ɑ/β-tubulin heterodimers have been isolated and examined for more than 50 years, how tubulin isotypes contribute to the microtubule organization and functions that support diverse cellular architectures remains a fundamental question. To address this knowledge gap, in vitro reconstitution of microtubules with purified ɑ/β-tubulin proteins has been employed for biochemical and biophysical characterization. These in vitro assays have provided mechanistic insights into the regulation of microtubule dynamics, stability, and interactions with other associated proteins. Here we survey the evolving strategies of generating purified ɑ/β-tubulin heterodimers and highlight the advances in tubulin protein biochemistry that shed light on the roles of tubulin isotypes in determining microtubule structures and properties.