The role of kinetochore dynein in checkpoint silencing is restricted to disassembly of the corona

During mitosis, kinetochore-microtubule attachments are monitored by a molecular surveillance system known as the spindle assembly checkpoint. The prevailing model posits that dynein evicts checkpoint proteins (e.g., Mad1, Mad2) from stably attached kinetochores by transporting them away from kinetochores, thus contributing to checkpoint silencing. However, the mechanism by which dynein performs this function, and its precise role in checkpoint silencing remain unresolved. Here, we find that dynein's role in checkpoint silencing is restricted to evicting checkpoint effectors from the fibrous corona, and not the outer kinetochore. Dynein evicts these molecules from the corona in a manner that does not require stable, end-on microtubule attachments. Thus, by disassembling the corona through indiscriminate microtubule encounters, dynein primes the checkpoint signaling apparatus so it can respond to stable end-on microtubule attachments and permit cells to progress through mitosis. Accordingly, we find that dynein function in checkpoint silencing becomes largely dispensable in cells in which checkpoint effectors are excluded from the corona.

Production and purification of recombinant monoclonal antibodies from human cells based on a primary sequence

There are challenges to using commercially available antibodies generated in animals, including concerns with reproducibility, high costs, and ethical issues. Here, we present a protocol for generating and purifying recombinant antibodies from human HEK293 suspension culture cells from a primary sequence. We describe the steps to generate antibody heavy and light chain plasmids, followed by transfection of the plasmids into cells and purification of antibodies. This protocol can produce high-yield recombinant monoclonal antibodies at a relatively low cost. For complete details on the use and execution of this protocol, please refer to DeLuca et al. (2021).¹.

Generation and diversification of recombinant monoclonal antibodies

Antibodies are indispensable tools used for a large number of applications in both foundational and translational bioscience research; however, there are drawbacks to using traditional antibodies generated in animals. These include a lack of standardization leading to problems with reproducibility, high costs of antibodies purchased from commercial sources, and ethical concerns regarding the large number of animals used to generate antibodies. To address these issues, we have developed practical methodologies and tools for generating low-cost, high-yield preparations of recombinant monoclonal antibodies and antibody fragments directed to protein epitopes from primary sequences. We describe these methods here, as well as approaches to diversify monoclonal antibodies, including customization of antibody species specificity, generation of genetically encoded small antibody fragments, and conversion of single chain antibody fragments (e.g. scFv) into full-length, bivalent antibodies. This study focuses on antibodies directed to epitopes important for mitosis and kinetochore function; however, the methods and reagents described here are applicable to antibodies and antibody fragments for use in any field.

Aurora B kinase is recruited to multiple discrete kinetochore and centromere regions in human cells

Aurora B kinase has a critical role in regulating attachments between kinetochores and spindle microtubules during mitosis. Early in mitosis, kinase activity at kinetochores is high to promote attachment turnover, and in later mitosis, activity decreases to ensure attachment stabilization. Aurora B localizes prominently to inner centromeres, and a population of the kinase is also detected at kinetochores. How Aurora B is recruited to and evicted from these regions to regulate kinetochore-microtubule attachments remains unclear. Here, we identified and investigated discrete populations of Aurora B at the centromere/kinetochore region. An inner centromere pool is recruited by Haspin phosphorylation of histone H3, and a kinetochore-proximal outer centromere pool is recruited by Bub1 phosphorylation of histone H2A. Finally, a third pool resides ~20 nm outside of the inner kinetochore protein CENP-C in early mitosis and does not require either the Bub1/pH2A/Sgo1 or Haspin/pH3 pathway for localization or activity. Our results suggest that distinct molecular pathways are responsible for Aurora B recruitment to centromeres and kinetochores.

The Hec1/Ndc80 tail domain is required for force generation at kinetochores, but is dispensable for kinetochore-microtubule attachment formation and Ska complex recruitment

The conserved kinetochore-associated NDC80 complex (composed of Hec1/Ndc80, Nuf2, Spc24, and Spc25) has well-documented roles in mitosis including 1) connecting mitotic chromosomes to spindle microtubules to establish force-transducing kinetochore–microtubule attachments and 2) regulating the binding strength between kinetochores and microtubules such that correct attachments are stabilized and erroneous attachments are released. Although the NDC80 complex plays a central role in forming and regulating attachments to microtubules, additional factors support these processes as well, including the spindle and kinetochore-associated (Ska) complex. Multiple lines of evidence suggest that Ska complexes strengthen attachments by increasing the ability of NDC80 complexes to bind microtubules, especially to depolymerizing microtubule plus ends, but how this is accomplished remains unclear. Using cell-based and in vitro assays, we demonstrate that the Hec1 tail domain is dispensable for Ska complex recruitment to kinetochores and for generation of kinetochore–microtubule attachments in human cells. We further demonstrate that Hec1 tail phosphorylation regulates kinetochore–microtubule attachment stability independently of the Ska complex. Finally, we map the location of the Ska complex in cells to a region near the coiled-coil domain of the NDC80 complex and demonstrate that this region is required for Ska complex recruitment to the NDC80 complex-–microtubule interface.

Use of Confocal Microscopy to Evaluate Equine Zygote Development After Sperm Injection of Oocytes Matured In Vivo or In Vitro

Confocal microscopy was used to image stages of equine zygote development, at timed intervals, after intracytoplasmic sperm injection (ICSI) of oocytes that were matured in vivo or in vitro. After fixation for 4, 6, 8, 12, or 16 h after ICSI, zygotes were incubated with α/β tubulin antibodies and human anticentromere antibody (CREST/ACA), washed, incubated in secondary antibodies, conjugated to either Alexa 488 or Alexa 647, and incubated with 561-Phalloidin and Hoechst 33258. An Olympus IX81 spinning disk confocal microscope was used for imaging. Data were analyzed using χ 2 and Fisher's exact tests. Minor differences in developmental phases were observed for oocytes matured in vivo or in vitro. Oocytes formed pronuclei earlier when matured in vivo (67% at 6 h and 80% at 8 h) than in vitro (13% at 6 and 8 h); 80% of oocytes matured in vitro formed pronuclei by 12 h. More (p=0.04) zygotes had atypical phenotypes, indicative of a failure of normal zygote development, when oocyte maturation occurred in vitro versus in vivo (30 and 11%, respectively). Some potential zygotes from oocytes matured in vivo had normal phenotypes, although development appeared to be delayed or arrested. Confocal microscopy provided a feasible method to assess equine zygote development using limited samples.

Aurora A kinase phosphorylates Hec1 to regulate metaphase kinetochore-microtubule dynamics

Precise regulation of kinetochore-microtubule attachments is essential for successful chromosome segregation. Central to this regulation is Aurora B kinase, which phosphorylates kinetochore substrates to promote microtubule turnover. A critical target of Aurora B is the N-terminal "tail" domain of Hec1, which is a component of the NDC80 complex, a force-transducing link between kinetochores and microtubules. Although Aurora B is regarded as the "master regulator" of kinetochore-microtubule attachment, other mitotic kinases likely contribute to Hec1 phosphorylation. In this study, we demonstrate that Aurora A kinase regulates kinetochore-microtubule dynamics of metaphase chromosomes, and we identify Hec1 S69, a previously uncharacterized phosphorylation target site in the Hec1 tail, as a critical Aurora A substrate for this regulation. Additionally, we demonstrate that Aurora A kinase associates with inner centromere protein (INCENP) during mitosis and that INCENP is competent to drive accumulation of the kinase to the centromere region of mitotic chromosomes. These findings reveal that both Aurora A and B contribute to kinetochore-microtubule attachment dynamics, and they uncover an unexpected role for Aurora A in late mitosis.

Cytoskeletal alterations associated with donor age and culture interval for equine oocytes and potential zygotes that failed to cleave after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) is an established method to fertilise equine oocytes, but not all oocytes cleave after ICSI. The aims of the present study were to examine cytoskeleton patterns in oocytes after aging in vitro for 0, 24 or 48h (Experiment 1) and in potential zygotes that failed to cleave after ICSI of oocytes from donors of different ages (Experiment 2). Cytoplasmic multiasters were observed after oocyte aging for 48h (P<0.01). A similar increase in multiasters was observed with an increased interval after ICSI for young mares (9-13 years) but not old (20-25 years) mares. Actin vesicles were observed more frequently in sperm-injected oocytes from old than young mares. In the present study, multiasters appeared to be associated with cell aging, whereas actin vesicles were associated with aging of the oocyte donor.

Real-time quantification of single RNA translation dynamics in living cells

Although messenger RNA (mRNA) translation is a fundamental biological process, it has never been imaged in real time in vivo with single-molecule precision. To achieve this, we developed nascent chain tracking (NCT), a technique that uses multi-epitope tags and antibody-based fluorescent probes to quantify protein synthesis dynamics at the single-mRNA level. NCT reveals an elongation rate of ~10 amino acids per second, with initiation occurring stochastically every ~30 seconds. Polysomes contain ~1 ribosome every 200 to 900 nucleotides and are globular rather than elongated in shape. By developing multicolor probes, we showed that most polysomes act independently; however, a small fraction (~5%) form complexes in which two distinct mRNAs can be translated simultaneously. The sensitivity and versatility of NCT make it a powerful new tool for quantifying mRNA translation kinetics.

Measuring Kinetochore-Microtubule Attachment Stability in Cultured Cells

Duplicated sister chromatids connect to the mitotic spindle through kinetochores, large proteinaceous structures built at sites of centromeric heterochromatin. Kinetochores are responsible for harnessing the forces generated by microtubule polymerization and depolymerization to power chromosome movements. The fidelity of chromosome segregation relies on proper kinetochore function, as precise regulation of the attachment between kinetochores and microtubules is essential to prevent mitotic errors, which are linked to the initiation and progression of cancer and the formation of birth defects (Godek et al., Nat Rev Mol Cell Biol 16(1):57-64, 2014; Ricke and van Deursen, Semin Cell Dev Biol 22(6):559-565, 2011; Holland and Cleveland, EMBO Rep 13(6):501-514, 2012). Here we describe assays to quantitatively measure kinetochore-microtubule attachment stability in cultured cells.

Accurate phosphoregulation of kinetochore-microtubule affinity requires unconstrained molecular interactions

Accurate regulation of kinetochore–microtubule affinity is driven by incremental phosphorylation of an NDC80 molecular “lawn,” in which NDC80–microtubule bonds reorganize dynamically in response to the number and stability of microtubule attachments.

Accurate chromosome segregation relies on dynamic interactions between microtubules (MTs) and the NDC80 complex, a major kinetochore MT-binding component. Phosphorylation at multiple residues of its Hec1 subunit may tune kinetochore–MT binding affinity for diverse mitotic functions, but molecular details of such phosphoregulation remain elusive. Using quantitative analyses of mitotic progression in mammalian cells, we show that Hec1 phosphorylation provides graded control of kinetochore–MT affinity. In contrast, modeling the kinetochore interface with repetitive MT binding sites predicts a switchlike response. To reconcile these findings, we hypothesize that interactions between NDC80 complexes and MTs are not constrained, i.e., the NDC80 complexes can alternate their binding between adjacent kinetochore MTs. Experiments using cells with phosphomimetic Hec1 mutants corroborate predictions of such a model but not of the repetitive sites model. We propose that accurate regulation of kinetochore–MT affinity is driven by incremental phosphorylation of an NDC80 molecular “lawn,” in which the NDC80–MT bonds reorganize dynamically in response to the number and stability of MT attachments.

KNL1 facilitates phosphorylation of outer kinetochore proteins by promoting Aurora B kinase activity

KNL1 is essential for efficient kinetochore–microtubule attachment dynamics during mitosis, in part through promotion of Bub1-mediated targeting of Aurora B to the kinetochore.

Aurora B kinase phosphorylates kinetochore proteins during early mitosis, increasing kinetochore–microtubule (MT) turnover and preventing premature stabilization of kinetochore–MT attachments. Phosphorylation of kinetochore proteins during late mitosis is low, promoting attachment stabilization, which is required for anaphase onset. The kinetochore protein KNL1 recruits Aurora B–counteracting phosphatases and the Aurora B–targeting factor Bub1, yet the consequences of KNL1 depletion on Aurora B phospho-regulation remain unknown. Here, we demonstrate that the KNL1 N terminus is essential for Aurora B activity at kinetochores. This region of KNL1 is also required for Bub1 kinase activity at kinetochores, suggesting that KNL1 promotes Aurora B activity through Bub1-mediated Aurora B targeting. However, ectopic targeting of Aurora B to kinetochores does not fully rescue Aurora B activity in KNL1-depleted cells, suggesting KNL1 influences Aurora B activity through an additional pathway. Our findings establish KNL1 as a requirement for Aurora B activity at kinetochores and for wild-type kinetochore–MT attachment dynamics.

Temporal changes in Hec1 phosphorylation control kinetochore-microtubule attachment stability during mitosis

Precise control of the attachment strength between kinetochores and spindle microtubules is essential to preserve genomic stability. Aurora B kinase has been implicated in regulating the stability of kinetochore-microtubule attachments but its relevant kinetochore targets in cells remain unclear. Here, we identify multiple serine residues within the N-terminus of the kinetochore protein Hec1 that are phosphorylated in an Aurora-B-kinase-dependent manner during mitosis. On all identified target sites, Hec1 phosphorylation at kinetochores is high in early mitosis and decreases significantly as chromosomes bi-orient. Furthermore, once dephosphorylated, Hec1 is not highly rephosphorylated in response to loss of kinetochore-microtubule attachment or tension. We find that a subpopulation of Aurora B kinase remains localized at the outer kinetochore even upon Hec1 dephosphorylation, suggesting that Hec1 phosphorylation by Aurora B might not be regulated wholly by spatial positioning of the kinase. Our results define a role for Hec1 phosphorylation in kinetochore-microtubule destabilization and error correction in early mitosis and for Hec1 dephosphorylation in maintaining stable attachments in late mitosis.