Microtubules in Microorganisms: How Tubulin Isotypes Contribute to Diverse Cytoskeletal Functions.

The cellular functions of the microtubule (MT) cytoskeleton range from relatively simple to amazingly complex. Assembled from tubulin, a heterodimeric protein with α- and ß-tubulin subunits, microtubules are long, hollow cylindrical filaments with inherent polarity. They are intrinsically dynamic polymers that utilize GTP binding by tubulin, and subsequent hydrolysis, to drive spontaneous assembly and disassembly. Early studies indicated that cellular MTs are composed of multiple variants, or isotypes, of α- and ß-tubulins, and that these multi-isotype polymers are further diversified by a range of posttranslational modifications (PTMs) to tubulin. These findings support the multi-tubulin hypothesis whereby individual, or combinations of tubulin isotypes possess unique properties needed to support diverse MT structures and/or cellular processes. Beginning 40 years ago researchers have sought to address this hypothesis, and the role of tubulin isotypes, by exploiting experimentally accessible, genetically tractable and functionally conserved model systems. Among these systems, important insights have been gained from eukaryotic microbial models. In this review, we illustrate how using microorganisms yielded among the earliest evidence that tubulin isotypes harbor distinct properties, as well as recent insights as to how they facilitate specific cellular processes. Ongoing and future research in microorganisms will likely continue to reveal basic mechanisms for how tubulin isotypes facilitate MT functions, along with valuable perspectives on how they mediate the range of conserved and diverse processes observed across eukaryotic microbes.

Tubulin isotypes optimize distinct spindle positioning mechanisms during yeast mitosis.

Microtubules are dynamic cytoskeleton filaments that are essential for a wide range of cellular processes. They are polymerized from tubulin, a heterodimer of α- and ß-subunits. Most eukaryotic organisms express multiple isotypes of α- and ß-tubulin, yet their functional relevance in any organism remains largely obscure. The two α-tubulin isotypes in budding yeast, Tub1 and Tub3, are proposed to be functionally interchangeable, yet their individual functions have not been rigorously interrogated. Here, we develop otherwise isogenic yeast strains expressing single tubulin isotypes at levels comparable to total tubulin in WT cells. Using genome-wide screening, we uncover unique interactions between the isotypes and the two major mitotic spindle positioning mechanisms. We further exploit these cells to demonstrate that Tub1 and Tub3 optimize spindle positioning by differentially recruiting key components of the Dyn1- and Kar9-dependent mechanisms, respectively. Our results provide novel mechanistic insights into how tubulin isotypes allow highly conserved microtubules to function in diverse cellular processes.

Checkpoint Proteins Bub1 and Bub3 Delay Anaphase Onset in Response to Low Tension Independent of Microtubule-Kinetochore Detachment.

The spindle assembly checkpoint (SAC) delays anaphase onset until sister chromosomes are bound to microtubules from opposite spindle poles. Only then can dynamic microtubules produce tension across sister kinetochores. The interdependence of kinetochore attachment and tension has proved challenging to understanding SAC mechanisms. Whether the SAC responds simply to kinetochore attachment or to tension status remains obscure. Unlike higher eukaryotes, budding yeast kinetochores bind only one microtubule, simplifying the relation between attachment and tension. We developed a Taxol-sensitive yeast model to reduce tension in fully assembled spindles. Our results show that low tension on bipolar-attached kinetochores delays anaphase onset, independent of detachment. The delay is transient relative to that imposed by unattached kinetochores. Furthermore, it is mediated by Bub1 and Bub3, but not Mad1, Mad2, and Mad3 (BubR1). Our results demonstrate that reduced tension delays anaphase onset via a signal that is temporally and mechanistically distinct from that produced by unattached kinetochores.

Promise of adeno-associated virus as a gene therapy vector for cardiovascular diseases.

Cardiovascular diseases pose a unique threat to global mortality because it presents as one of the most diverse conglomerations of pathophysiological conditions that can create significant casualty even without straying into its collateral damage. This puts them right beside obesity and cancer in terms of severity. Their pervasive nature and high prevalence prompted biologists to seek newer prophylactic avenues of addressing this global hazard, among which adeno-associated virus (AAV) gene therapy rose to significant prominence. By virtue of its unrivaled clinical safety quotient, AAVs have been used to rectify various subtypes of cardiovascular ailments, beginning from commonly occurring heart failure to vascular diseases. The review focuses on the history of AAV-mediated gene therapy and contemporary breakthroughs in terms of novel innovations in vector engineering to reduce detargeting, immune response, untimely expression, and so on. We have also focused on the molecular world of cardiomyocytes and endothelial cells but interpreted the therapies in a broader context of cardiovascular pathology. The advances made in each mode of intervention as well as the ones that are beyond the scope of AAV gene therapy or has not been approached through AAV gene therapy as of now have been provided in detail to illustrate the bigger picture of where we stand to combat cardiovascular diseases most efficiently.