Dr. Kim S. McKim is investigating Meiosis, a process that is essential for sexual reproduction, and is using these fruit flies as a model organism to help us better understand Human Meiosis.

Research Summary

     Meiosis is the process by which the chromosome number is divided precisely in half. When defects occur in the meiotic process, the oocyte or sperm receives an abnormal number of chromosomes (aneuploidy). Aneuploidy is usually catastrophic and is the leading cause of infertility in women and the cause of disorders such as Down syndrome. In the McKim Lab, we use the model organism Drosophila melanogaster to investigate the mechanisms that promote accurate chromosome segregation in oocytes.

 

     By utilizing the experimental benefits of Drosophila, mutations that disrupt various steps in the meiotic program can be isolated and characterized. Currently, the lab focuses on two of the most important aspects of meiosis: i) the repair of programmed double strand breaks (DSBs) in the DNA into crossovers, and ii) the segregation of homologous chromosomes via interaction with the spindle.

    

     In the past we have taken a classical genetics approach to dissect the different steps of meiotic recombination and chromosome segregation; identifying genes using unbiased screens for recessive mutants causing nondisjunction of the sex chromosomes.  More recently, we are using a variety of reverse genetics approaches such as gene targeting and transposon mobilization to mutate interesting genes identified based on their sequence.  Most exciting is the development of techniques which allow efficient RNAi in the Drosophila germline.  This allows us to knock down and usually eliminate the protein of almost any gene and study its affects on meiosis.  This is particularly important for genes which are essential, since mutations in these genes cause lethality and thus meiosis, which must be studied in adult females, cannot be analyzed.  With these new methods, the RNAi can be expressed only in the female germline, bypassing the requirement for a gene on viability.  Finally, our work heavily depends on high resolution microscopy using immunofluorescence to detect important meiotic proteins.  Antibodies to specific proteins are used in conjunction with high resolution imaging to investigate the structure and behavior of meiotic and mitotic chromosomes.

Recent Publications

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, 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
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.