Radford, SJ, McKim KS.  2016.  Techniques for Imaging Prometaphase and Metaphase of Meiosis I in Fixed Drosophila Oocytes.. Journal of visualized experiments : JoVE. (116) Abstract
Chromosome segregation in human oocytes is error prone, resulting in aneuploidy, which is the leading genetic cause of miscarriage and birth defects. The study of chromosome behavior in oocytes from model organisms holds much promise to uncover the molecular basis of the susceptibility of human oocytes to aneuploidy. Drosophila melanogaster is amenable to genetic manipulation, with over 100 years of research, community, and technique development. Visualizing chromosome behavior and spindle assembly in Drosophila oocytes has particular challenges, however, due primarily to the presence of membranes surrounding the oocyte that are impenetrable to antibodies. We describe here protocols for the collection, preparation, and imaging of meiosis I spindle assembly and chromosome behavior in Drosophila oocytes, which allow the molecular dissection of chromosome segregation in this important model organism.
Radford, SJ, Nguyen AL, Schindler K, McKim KS.  2016.  The chromosomal basis of meiotic acentrosomal spindle assembly and function in oocytes.. Chromosoma. Abstract
Several aspects of meiosis are impacted by the absence of centrosomes in oocytes. Here, we review four aspects of meiosis I that are significantly affected by the absence of centrosomes in oocyte spindles. One, microtubules tend to assemble around the chromosomes. Two, the organization of these microtubules into a bipolar spindle is directed by the chromosomes. Three, chromosome bi-orientation and attachment to microtubules from the correct pole require modification of the mechanisms used in mitotic cells. Four, chromosome movement to the poles at anaphase cannot rely on polar anchoring of spindle microtubules by centrosomes. Overall, the chromosomes are more active participants during acentrosomal spindle assembly in oocytes, compared to mitotic and male meiotic divisions where centrosomes are present. The chromosomes are endowed with information that can direct the meiotic divisions and dictate their own behavior in oocytes. Processes beyond those known from mitosis appear to be required for their bi-orientation at meiosis I. As mitosis occurs without centrosomes in many systems other than oocytes, including all plants, the concepts discussed here may not be limited to oocytes. The study of meiosis in oocytes has revealed mechanisms that are operating in mitosis and will probably continue to do so.
Gyuricza, MR, Manheimer KB, Apte V, Krishnan B, Joyce EF, McKee BD, McKim KS.  2016.  Dynamic and Stable Cohesins Regulate Synaptonemal Complex Assembly and Chromosome Segregation.. Current biology : CB. 26(13):1688-1698. Abstract
Assembly of the synaptonemal complex (SC) in Drosophila depends on two independent pathways defined by the chromosome axis proteins C(2)M and ORD. Because C(2)M encodes a Kleisin-like protein and ORD is required for sister-chromatid cohesion, we tested the hypothesis that these two SC assembly pathways depend on two cohesin complexes. Through single- and double-mutant analysis to study the mitotic cohesion proteins Stromalin (SA) and Nipped-B (SCC2) in meiosis, we provide evidence that there are at least two meiosis-specific cohesin complexes. One complex depends on C(2)M, SA, and Nipped-B. Despite the presence of mitotic cohesins SA and Nipped-B, this pathway has only a minor role in meiotic sister-centromere cohesion and is primarily required for homolog interactions. C(2)M is continuously incorporated into pachytene chromosomes even though SC assembly is complete. In contrast, the second complex, which depends on meiosis-specific proteins SOLO, SUNN, and ORD is required for sister-chromatid cohesion, localizes to the centromeres and is not incorporated during prophase. Our results show that the two cohesin complexes have unique functions and are regulated differently. Multiple cohesin complexes may provide the diversity of activities required by the meiotic cell. For example, a dynamic complex may allow the chromosomes to regulate meiotic recombination, and a stable complex may be required for sister-chromatid cohesion.
Das, A, Shah SJ, Fan B, Paik D, DiSanto DJ, Hinman A M, Cesario JM, Battaglia RA, Demos N, McKim KS.  2016.  Spindle Assembly and Chromosome Segregation Requires Central Spindle Proteins in Drosophila Oocytes.. Genetics. 202(1):61-75. Abstract
Oocytes segregate chromosomes in the absence of centrosomes. In this situation, the chromosomes direct spindle assembly. It is still unclear in this system which factors are required for homologous chromosome bi-orientation and spindle assembly. The Drosophila kinesin-6 protein Subito, although nonessential for mitotic spindle assembly, is required to organize a bipolar meiotic spindle and chromosome bi-orientation in oocytes. Along with the chromosomal passenger complex (CPC), Subito is an important part of the metaphase I central spindle. In this study we have conducted genetic screens to identify genes that interact with subito or the CPC component Incenp. In addition, the meiotic mutant phenotype for some of the genes identified in these screens were characterized. We show, in part through the use of a heat-shock-inducible system, that the Centralspindlin component RacGAP50C and downstream regulators of cytokinesis Rho1, Sticky, and RhoGEF2 are required for homologous chromosome bi-orientation in metaphase I oocytes. This suggests a novel function for proteins normally involved in mitotic cell division in the regulation of microtubule-chromosome interactions. We also show that the kinetochore protein, Polo kinase, is required for maintaining chromosome alignment and spindle organization in metaphase I oocytes. In combination our results support a model where the meiotic central spindle and associated proteins are essential for acentrosomal chromosome segregation.
Radford, SJ, Go A MM, McKim KS.  2016.  Cooperation Between Kinesin Motors Promotes Spindle Symmetry and Chromosome Organization in Oocytes.. Genetics. Abstract
The oocyte spindle in most animal species is assembled in the absence of the microtubule-organizing centers called centrosomes. Without the organization provided by centrosomes, acentrosomal meiotic spindle organization may rely heavily on the bundling of microtubules by kinesin motor proteins. Indeed, the minus-end directed kinesin-14 NCD and the plus-end directed kinesin-6 Subito are known to be required for oocyte spindle organization in Drosophila melanogaster How multiple microtubule-bundling kinesins interact to produce a functional acentrosomal spindle is not known. In addition, there have been few studies on the meiotic function of one of the most important microtubule-bundlers in mitotic cells, the kinesin-5 KLP61F. We have found that the kinesin-5 KLP61F is required for spindle and centromere symmetry in oocytes. The asymmetry observed in the absence of KLP61F depends on NCD, the kinesin-12 KLP54D, and the microcephaly protein ASP. In contrast, KLP61F and Subito work together in maintaining a bipolar spindle. We propose that the prominent central spindle, stabilized by Subito, provides the framework for the coordination of multiple microtubule-bundling activities. The activities of several proteins, including NCD, KLP54D, and ASP, generate asymmetries within the acentrosomal spindle, while KLP61F and Subito balance these forces resulting in the capacity to accurately segregate chromosomes.
Głuszek, AA, Cullen FC, Li W, Battaglia RA, Radford SJ, Costa MF, McKim KS, Goshima G, Ohkura H.  2015.  The microtubule catastrophe promoter Sentin delays stable kinetochore-microtubule attachment in oocytes.. The Journal of cell biology. 211(6):1113-20. Abstract
The critical step in meiosis is to attach homologous chromosomes to the opposite poles. In mouse oocytes, stable microtubule end-on attachments to kinetochores are not established until hours after spindle assembly, and phosphorylation of kinetochore proteins by Aurora B/C is responsible for the delay. Here we demonstrated that microtubule ends are actively prevented from stable attachment to kinetochores until well after spindle formation in Drosophila melanogaster oocytes. We identified the microtubule catastrophe-promoting complex Sentin-EB1 as a major factor responsible for this delay. Without this activity, microtubule ends precociously form robust attachments to kinetochores in oocytes, leading to a high proportion of homologous kinetochores stably attached to the same pole. Therefore, regulation of microtubule ends provides an alternative novel mechanism to delay stable kinetochore-microtubule attachment in oocytes.
Radford, SJ, Hoang TL, Gluszek AA, Ohkura H, McKim KS.  2015.  Lateral and End-On Kinetochore Attachments Are Coordinated to Achieve Bi-orientation in Drosophila Oocytes. PLoS Genetics. 11(10):e1005605.Website
Mathieu, J, Cauvin C, Moch C, Radford SJ, Sampaio P, Perdigoto CN, Schweisguth F, Bardin AJ, Sunkel CE, McKim K et al..  2013.  Aurora B and cyclin B have opposite effects on the timing of cytokinesis abscission in Drosophila germ cells and in vertebrate somatic cells. Dev Cell. 26(3):250-65.Website
Radford, SJ, Jang JK, McKim KS.  2012.  The Chromosomal Passenger Complex is required for Meiotic Acentrosomal Spindle Assembly and Chromosome Bi-orientation. Genetics. 192:417-429. AbstractWebsite
During meiosis in the females of many species, spindle assembly occurs in the absence of the microtubule-organizing centers called centrosomes. In the absence of centrosomes, the nature of the chromosome-based signal that recruits microtubules to promote spindle assembly as well as how spindle bipolarity is established and the chromosomes orient correctly towards the poles is not known. To address these questions, we focused on the chromosomal passenger complex (CPC). We have found that the CPC localizes in a ring around the meiotic chromosomes that is aligned with the axis of the spindle at all stages. Using new methods which dramatically increase the effectiveness of RNAi in the germline, we show that the CPC interacts with Drosophila oocyte chromosomes and is required for the assembly of spindle microtubules. Furthermore, chromosome bi-orientation and the localization of the central spindle kinesin-6 protein Subito, which is required for spindle bipolarity, depend on the CPC components Aurora B and Incenp. Based on these data we propose that the ring of CPC around the chromosomes regulates multiple aspects of meiotic cell division including spindle assembly, the establishment of bipolarity, the recruitment of important spindle organization factors, and the bi-orientation of homologous chromosomes.
Radford, SJ, Harrison AM, McKim KS.  2012.  Microtubule-depolymerizing Kinesin KLP10A Restricts the Length of the Acentrosomal Meiotic Spindle in Drosophila Females. Genetics. 192:431-440. AbstractWebsite
During cell division, a bipolar array of microtubules forms the spindle through which the forces required for chromosome segregation are transmitted. Interestingly, the spindle as a whole is stable enough to support these forces even though it is composed of dynamic microtubules, which are constantly undergoing periods of growth and shrinkage. Indeed, the regulation of microtubule dynamics is essential to the integrity and function of the spindle. We show here that a member of an important class of microtubule-depolymerizing kinesins, KLP10A, is required for the proper organization of the acentrosomal meiotic spindle in Drosophila melanogaster oocytes. In the absence of KLP10A, microtubule length is not controlled, resulting in extraordinarily long and disorganized spindles. In addition, the interactions between chromosomes and spindle microtubules are disturbed and can result in the loss of contact. These results indicate that the regulation of microtubule dynamics through KLP10A plays a critical role in restricting the length and maintaining bipolarity of the acentrosomal meiotic spindle and in promoting the contacts that the chromosomes make with microtubules required for meiosis I segregation.
Joyce, EF, Paul A, Chen KE, McKim KS.  2012.  Multiple Barriers to Non-homologous DNA End Joining During Meiosis in Drosophila. Genetics. 191:739-46. AbstractWebsite
Repair of meiotic double-strand breaks (DSBs) uses the homolog and recombination to yield crossovers while alternative pathways such as nonhomologous end-joining (NHEJ) are suppressed. Our results indicate that NHEJ is blocked at two steps of DSB repair during meiotic prophase: first by the activity of the MCM-like protein MEI-218 that is required for crossover formation and, second, by Rad51-related proteins SPN-B (XRCC3) and SPN-D (RAD51C) that physically interact and promote homologous recombination. We further show that the MCM-like proteins also promote the activity of the DSB repair checkpoint pathway, indicating an early requirement for these proteins in DSB processing. We propose that when a meiotic DSB is formed in the absence of both MEI-218 and SPN-B or SPN-D, a DSB substrate is generated that can enter the NHEJ repair pathway. Indeed, due to its high error rate, multiple barriers may have evolved to prevent NHEJ activity during meiosis.
Joyce, EF, Pedersen M, Tiong S, White-Brown SK, Paul A, Campbell SD, McKim KS.  2011.  Drosophila ATM and ATR have distinct activities in the regulation of meiotic DNA damage and repair. J Cell Biol. 195:359-67. AbstractWebsite
Ataxia telangiectasia-mutated (ATM) and ataxia telangiectasia-related (ATR) kinases are conserved regulators of cellular responses to double strand breaks (DSBs). During meiosis, however, the functions of these kinases in DSB repair and the deoxyribonucleic acid (DNA) damage checkpoint are unclear. In this paper, we show that ATM and ATR have unique roles in the repair of meiotic DSBs in Drosophila melanogaster. ATR mutant analysis indicated that it is required for checkpoint activity, whereas ATM may not be. Both kinases phosphorylate H2AV (gamma-H2AV), and, using this as a reporter for ATM/ATR activity, we found that the DSB repair response is surprisingly dynamic at the site of DNA damage. gamma-H2AV is continuously exchanged, requiring new phosphorylation at the break site until repair is completed. However, most surprising is that the number of gamma-H2AV foci is dramatically increased in the absence of ATM, but not ATR, suggesting that the number of DSBs is increased. Thus, we conclude that ATM is primarily required for the meiotic DSB repair response, which includes functions in DNA damage repair and negative feedback control over the level of programmed DSBs during meiosis.
Tanneti, NS, Landy K, Joyce EF, McKim KS.  2011.  A Pathway for Synapsis Initiation during Zygotene in Drosophila Oocytes. Curr Biol. 21:1852-7. AbstractWebsite
Formation of the synaptonemal complex (SC), or synapsis, between homologs in meiosis is essential for crossing over and chromosome segregation [1-4]. How SC assembly initiates is poorly understood but may have a critical role in ensuring synapsis between homologs and regulating double-strand break (DSB) and crossover formation. We investigated the genetic requirements for synapsis in Drosophila and found that there are three temporally and genetically distinct stages of synapsis initiation. In "early zygotene" oocytes, synapsis is only observed at the centromeres. We also found that nonhomologous centromeres are clustered during this process. In "mid-zygotene" oocytes, SC initiates at several euchromatic sites. The centromeric and first euchromatic SC initiation sites depend on the cohesion protein ORD. In "late zygotene" oocytes, SC initiates at many more sites that depend on the Kleisin-like protein C(2)M. Surprisingly, late zygotene synapsis initiation events are independent of the earlier mid-zygotene events, whereas both mid and late synapsis initiation events depend on the cohesin subunits SMC1 and SMC3. We propose that the enrichment of cohesion proteins at specific sites promotes homolog interactions and the initiation of euchromatic SC assembly independent of DSBs. Furthermore, the early euchromatic SC initiation events at mid-zygotene may be required for DSBs to be repaired as crossovers.
Cesario, J, McKim KS.  2011.  RanGTP is required for meiotic spindle organization and the initiation of embryonic development in Drosophila. J Cell Sci. 124:3797-810. AbstractWebsite
RanGTP is important for chromosome-dependent spindle assembly in Xenopus extracts. Here we report on experiments to determine the role of the Ran pathway on microtubule dynamics in Drosophila oocytes and embryos. Females expressing a dominant-negative form of Ran have fertility defects, suggesting that RanGTP is required for normal fertility. This is not, however, because of a defect in acentrosomal meiotic spindle assembly. Therefore, RanGTP does not appear to be essential or sufficient for the formation of the acentrosomal spindle. Instead, the most important function of the Ran pathway in spindle assembly appears to be in the tapering of microtubules at the spindle poles, which might be through regulation of proteins such as TACC and the HURP homolog, Mars. One consequence of this spindle organization defect is an increase in the nondisjunction of achiasmate chromosomes. However, the meiotic defects are not severe enough to cause the decreased fertility. Reductions in fertility occur because RanGTP has a role in microtubule assembly that is not directly nucleated by the chromosomes. This includes microtubules nucleated from the sperm aster, which are required for pronuclear fusion. We propose that following nuclear envelope breakdown, RanGTP is released from the nucleus and creates a cytoplasm that is activated for assembling microtubules, which is important for processes such as pronuclear fusion. Around the chromosomes, however, RanGTP might be redundant with other factors such as the chromosome passenger complex.
McKim, KS, Joyce EF, Jang JK.  2009.  Cytological analysis of meiosis in fixed Drosophila ovaries. Methods Mol Biol. 558:197-216. AbstractWebsite
Methods are described to analyze two different parts of the Drosophila ovary, which correspond to early stages (pachytene) and late stages (metaphase I and beyond) of meiosis. In addition to taking into account morphology, the techniques differ by fixation conditions and the method to isolate the tissue. Most of these methods are whole mounts, which preserve the three-dimensional structure.
Wu, C, Singaram V, McKim KS.  2008.  mei-38 is required for chromosome segregation during meiosis in Drosophila females. Genetics. 180:61-72.Website
Colombie, N, Cullen CF, Brittle AL, Jang JK, Earnshaw WC, Carmena M, McKim K, Ohkura H.  2008.  Dual roles of Incenp crucial to the assembly of the acentrosomal metaphase spindle in female meiosis. Development. 135:3239-46.Website
McKim, KS.  2007.  Meiotic pairing: a place to hook up. Curr Biol. 17:R165-8.Website
Doubilet, S, McKim KS.  2007.  Spindle assembly in the oocytes of mouse and Drosophila--similar solutions to a problem. Chromosome Res. 15:681-96.Website