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

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.
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.