Emmanuel Nsamba

All Publications

• Differential contributions of beta-tubulin isotypes to acentrosomal oocyte meiosis inC. elegans. bioRxiv : the preprint server for biology Nsamba, E. T., Villeneuve, A. M. 2025

  Abstract

  Oocyte meiotic spindles must achieve bipolarity and segregate chromosomes in the absence of centrosomes. Here we use high-resolution immunofluorescence microscopy and live imaging to investigate the differential contributions of beta-tubulin isotypes (TBB-1 and TBB-2) to assembly and function of acentrosomal spindles in Caenorhabditis elegans oocytes. By combining strains with altered beta-tubulin isotype composition with mutations affecting microtubule-crosslinking motor KLP-18 and/or mutations affecting katanin-mediated microtubule severing, we show that TBB-1 and TBB-2 make distinct contributions to promoting spindle bipolarity. Further, by measuring multiple spindle features in wild-type and beta-tubulin isotype substitution strains, we reveal contributions of isotype composition to spindle morphology, kinetics of anaphase chromosome separation and maintenance of spindle structural integrity under stress. Together, our data support a model in which beta-tubulin isotype composition helps to maintain a balance between microtubule crosslinking and severing activities during oocyte meiosis. We further propose that this balance is crucial for establishing spindle bipolarity, maintaining spindle structures, and modulating the dynamics of chromosome separation.

  View details for DOI 10.64898/2025.12.03.692175

  View details for PubMedID 41542501

• Active maintenance of meiosis-specific chromosome structures inC. elegansby the deubiquitinase DUO-1. bioRxiv : the preprint server for biology Strand, L. G., Choi, C. P., McCoy, S., Nsamba, E. T., Silva, N., Villeneuve, A. M. 2025

  Abstract

  Meiotic prophase is characterized by a dynamic program in which germ cells undergo a complex series of associations and dissociations of protein complexes that drive assembly, remodeling, and disassembly of meiosis-specific chromosome structures and dramatic changes in chromosome compaction. Failure to properly coordinate these processes can result in improper chromosome segregation, producing aneuploid gametes and inviable zygotes. Here, we investigate the roles of C. elegans DUO-1, an ortholog of mammalian ubiquitin-specific proteases USP26 and USP29, in mediating these dynamic chromosomal events during meiotic prophase. Cytological analyses of duo-1 null mutants indicate that loss of DUO-1 function leads to impaired assembly of synaptonemal complexes (SCs), loss of integrity of meiotic chromosome axes, ineffective homolog pairing, premature separation of sister chromatids, and late-prophase chromosome decompaction. Further, SC instability in duo-1 mutants correlates with depletion of REC-8 cohesin complexes and is accompanied by massive accumulation of early DSB repair intermediates. By using a dual-AID-tagged allele to deplete DUO-1 during meiotic development, we demonstrate that DUO-1 is continually required throughout meiotic prophase progression, to promote proper axis/SC assembly in early prophase, to maintain axis/SC stability during the late pachytene stage, and to promote/maintain chromosome compaction at the end of meiotic prophase. Together, our data reveal that meiotic chromosome structure and meiosis-specific chromosome architecture require active maintenance throughout meiotic prophase, and that this maintenance is necessary for successful meiosis.

  View details for DOI 10.1101/2025.09.11.675685

  View details for PubMedID 41542524

• Tubulin isotypes contribute opposing properties to balance anaphase spindle morphogenesis JOURNAL OF CELL BIOLOGY Nsamba, E. T., Bera, A., Todi, V., Savoy, L., Gupta, R. M., Gupta Jr, M. L. 2025; 224 (9)

  Abstract

  Faithful chromosome segregation requires proper function of the mitotic spindle, which is built from, and depends on, the coordinated regulation of many microtubules and the activities of molecular motors and MAPs. In addition, microtubules themselves are assembled from multiple variants, or isotypes of α- and β-tubulin, yet whether they mediate the activities of motors and MAPs required for proper spindle function remains poorly understood. Here, we use budding yeast to reveal that α-tubulin isotypes regulate opposing outward- and inward-directed forces in the spindle midzone that facilitate optimal spindle elongation and length control. Moreover, we show that the isotypes mediate balanced spindle forces by differentially localizing the antagonistic force generators Cin8 (kinesin-5) and Kar3 (kinesin-14) to interpolar microtubules. Our results reveal new roles for tubulin isotypes in orchestrating motor and MAP activities and provide insights into how forces in the spindle are properly calibrated to ensure proper mitotic spindle morphogenesis.

  View details for DOI 10.1083/jcb.202301115

  View details for Web of Science ID 001539694900001

  View details for PubMedID 40711471

• Tubulin isotypes - functional insights from model organisms. Journal of cell science Nsamba, E. T., Gupta, M. L. 2022; 135 (9)

  Abstract

  The microtubule cytoskeleton is assembled from the alpha- and beta-tubulin subunits of the canonical tubulin heterodimer, which polymerizes into microtubules, and a small number of other family members, such as gamma-tubulin, with specialized functions. Overall, microtubule function involves the collective action of multiple alpha- and beta-tubulin isotypes. However, despite 40 years of awareness that most eukaryotes harbor multiple tubulin isotypes, their role in the microtubule cytoskeleton has remained relatively unclear. Various model organisms offer specific advantages for gaining insight into the role of tubulin isotypes. Whereas simple unicellular organisms such as yeast provide experimental tractability that can facilitate deeper access to mechanistic details, more complex organisms, such as the fruit fly, nematode and mouse, can be used to discern potential specialized functions of tissue- and structure-specific isotypes. Here, we review the role of alpha- and beta-tubulin isotypes in microtubule function and in associated tubulinopathies with an emphasis on the advances gained using model organisms. Overall, we argue that studying tubulin isotypes in a range of organisms can reveal the fundamental mechanisms by which they mediate microtubule function. It will also provide valuable perspectives on how these mechanisms underlie the functional and biological diversity of the cytoskeleton.

  View details for DOI 10.1242/jcs.259539

  View details for PubMedID 35522156

• A new partial loss of function allele of rad-54.L. microPublication biology Akerib, C. C., Yokoo, R., Nsamba, E. T., Strand, L. G., Yamaya, K., Villeneuve, A. M. 2022; 2022

  Abstract

  RAD-54.L is required for the repair of meiotic double-strand DNA breaks (DSBs), playing an essential role in promoting removal of recombinase RAD-51 and normal completion of meiotic recombination. Failure to complete meiotic DSB repair leads to 100% lethality of embryos produced by rad-54.L null mutant mothers. Here we report a new partial loss of function allele, rad-54.L(me139) , that may prove useful for investigating meiotic mechanisms by providing a sensitized genetic background that reduces but does not eliminate the essential functions of RAD-54.L.

  View details for DOI 10.17912/micropub.biology.000637

  View details for PubMedID 36247323

• Tubulin isotypes optimize distinct spindle positioning mechanisms during yeast mitosis. The Journal of cell biology Nsamba, E. T., Bera, A., Costanzo, M., Boone, C., Gupta, M. L. 2021; 220 (12)

  Abstract

  Microtubules are dynamic cytoskeleton filaments that are essential for a wide range of cellular processes. They are polymerized from tubulin, a heterodimer of alpha- and beta-subunits. Most eukaryotic organisms express multiple isotypes of alpha- and beta-tubulin, yet their functional relevance in any organism remains largely obscure. The two alpha-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.

  View details for DOI 10.1083/jcb.202010155

  View details for PubMedID 34739032