Structural basis of meiotic chromosome organization by the synaptonemal complex.

Lead Research Organisation: University of Cambridge
Department Name: Biochemistry

Abstract

Sperm and egg cells are required for the generation of new human life. They are produced through meiosis, a process of cell division in which genetic information is reshuffled to enhance diversity. Upon fertilisation, the DNA contained within sperm and egg is combined to provide the full complement of genetic information that defines a unique human being. While the process of meiosis appears conceptually simple, it is in fact a complicated process that can frequently go wrong, resulting in either sperm or egg with the incorrect number of chromosomes, or in no germ cell being produced at all, causing miscarriage and infertility. It may also result in children being born with genetic diseases such as Down's syndrome.

At the heart of meiosis is the process of genetic exchange between pairs of matching chromosomes. In order to be able to align with each other and exchange genetic material, each pair of matching chromosomes needs to be organised in a specific three-dimensional shape which is unique to meiosis. Previous research has shown that a large protein structure known as the synaptonemal complex (SC) is essential for this, by working as a molecular scaffold that holds chromosomes together in the correct way for the exchange of DNA to take place. Although the importance of the SC in meiosis has been known for a long time, we still know little regarding how the SC is constructed and how it functions.

We plan to discover how the SC carries out its crucial function in meiosis by studying its complex molecular structure. Our specific aim is to work out exactly how the SC determines the shape of each matching chromosome, folding its DNA into the required three-dimensional structure necessary for the steps of pairing, genetic exchange and eventual separation. We will achieve this by producing a molecular picture at atomic resolution of the protein building blocks of the SC, which will show how they interact with the DNA and with each other, in order to build the correct molecular framework for genetic exchange to occur.

Our research into the molecular structure of the SC will help us understand its important role in meiosis and to explain how defects in its assembly can lead to infertility, miscarriage and genetic disease. In addition, our work will improve our knowledge of the different ways in which human cells handle large chromosomal DNA molecules. This information is important as incorrect sorting of our chromosomes during cellular proliferation, caused by defective packaging of the DNA, can lead to unwanted alterations in the genome and to disease such as cancer.

Technical Summary

Sexually reproducing organisms rely on the process of meiosis for the generation of genetic diversity across generations. At the physical and functional centre of meiosis is the synaptonemal complex (SC), an evolutionarily conserved proteinaceous ultrastructure that mediates synapsis and genetic exchange, by holding homologous chromosomes in close and orderly juxtaposition during DNA recombination. Correct SC assembly is essential for proper recombination and segregation of homologous chromosomes and therefore ultimately for successful completion of meiosis, upon which human fertility depends.

So far, our knowledge of SC function in meiosis has been based largely on genetic and cellular studies. However, lack of detailed knowledge about the molecular structure of the protein components of the SC and their mechanism of assembly restricts severely our current understanding of SC function in meiosis. In 2010, we initiated a research program aimed at providing a structural basis of SC function through the biochemical reconstitution and biophysical analysis of SC proteins (Davies et al, Open Biol, 2012; Syrjanen et al, eLife, 2014).

The current proposal builds on our most recent work, to focus on the role played by SC proteins SYCP2 and SYCP3 in defining the 3D-architecture of the meiotic chromosome, which is essential for correct homologue pairing and crossover. To achieve our aim, we have designed a multi-disciplinary research plan that includes biochemical, structural, biophysical and cellular experiments, at bulk and single-particle level. Our work will advance our understanding of the molecular mechanisms underpinning genetic recombination in meiosis and ultimately human fertility. Furthermore, it will illustrate molecular principles of DNA organisation that will be relevant to our understanding of chromosome structure and dynamics and of pathologies resulting from aneuploidy and incorrect chromosome segregation.

Planned Impact

The synaptonemal complex (SC) is essential for the completion of meiosis and hence fertility. However, despite its discovery over 50 years ago, we still lack a basic molecular understanding of its structure, assembly and functional role in meiosis. The successful completion of the experiments in this proposal will move our understanding of the SC to the molecular level, leading to the following academic, economic and societal impacts:

Academic impacts
The principal academic impacts of this work come from structural models of the SC and the identification of critical mechanisms controlling SC assembly and function in meiosis. Molecular models and testable mechanisms of SC function will be invaluable to to elucidate the critical role of the SC in vivo. In addition, the communities of interested researchers will benefit from the provision of biochemical reagents, including recombinant protein fragments to inhibit SC assembly in cell culture, and recombinant SC proteins and complexes that will advance the biochemical analysis of meiotic recombination. Our work will further benefit biotechnology companies that may develop small-molecule inhibitors of SC assembly or meiosis, of commercial value in basic science, medical, agricultural and livestock industries. The large number of basic science and commercial beneficiaries within this country means that our work will have direct impact on basic science and biotechnology in the UK.

Economic and societal impacts - Medical and agricultural
The most immediate medical impact of our research will come from molecular models of the SC, which will enable defects in SC assembly to be predicted from genetic analysis. This will allow diagnosis of the molecular cause of infertility or recurrent miscarriage, and rational assessment of the risk of aneuploidy in pregnancy. Accurate diagnosis will better guide the choice of assisted reproduction method, tailoring it to individual couples and assisting the NHS in allocating resources to cases for which treatments are most likely to succeed. Fertility is an important concern of a large majority of the UK population, so such work is likely to have widespread impact on health and well-being, providing substantial economic benefits and with significant ability to attract R&D investment.
The methods developed for human fertility will likely first impact on the agricultural industry. Novel methods for controlling the fertility of livestock may increase the number of healthy animals born, reducing animal suffering, ensuring our food security and enhancing profitability. The latter is particularly applicable in the case of prize animals and crops, meaning that this would be likely to attract significant R&D investment.

Economic and societal impacts - Diversity and evolution
A precise understanding of genetic inheritance, through molecular understanding of crossover formation, will help us provide an accurate account of how the current wealth of genetic diversity has been attained. The explanation of our genetic history and the origin of diversity within our species is of particular interest to the general public and will raise awareness of the societal benefits stemming from our research.

Publications

10 25 50
 
Description Preparation of C. elegans germ nuclei for analysis of the synaptonemal complex (SC) structure by cryo-electron tomography 
Organisation Harvard University
Department Department of Genetics
Country United States 
Sector Academic/University 
PI Contribution The aim of this collaboration is prepare suitable material for the analysis of the SC structure in worm germ cells by electron tomography. We will receive samples from the laboratory of Prof. Monica Colaiacovo, a cell biologist with world-leading expertise in the study of meiosis and the SC, for tomographic analysis. Depending on the nature of the samples, they will be analysed using the EM facility at the Department of Material Science in Cambridge, in our newly-established cryoEM facility at the Department of Biochemistry or at the eBic facility of the Diamond Light Source.
Collaborator Contribution Prof. Colaiacovo will send us samples consisting of plastic-embedded sections of C. elegans strains with arrested meiosis at the pachytene stage of meiotic prophase I. She is also developing metods for the isolation of samples enriched in germ cells arrested at the appropriate meiotic stage, to facilitate the analysis of the SC structure by cryo-electron tomography, using either vitreous sections or Focused Ion Beam milling.
Impact The collaboration started last year and has not produced any complete outcome yet.
Start Year 2017
 
Description Single-molecule analysis of SC proteins of the lateral element and their interaction with DNA 
Organisation Free University of Amsterdam
Country Netherlands 
Sector Academic/University 
PI Contribution We expressed and purified human recombinant SYCP3, and labelled it at a specific amino acid site with the fluorescent dye Alexa 555. The purpose was to study the DNA binding behaviour of SYCP3 using the state of the art single-molecule setup developed by Profs. Erwin Peterman and Gijs Wuite at the LaserLab of the VU , Amsterdam, which combines microfluidics, optical tweezers and confocal fluorescence.
Collaborator Contribution Our collaborators Profs. Erwin Peterman and Gijs Wuite are world leaders in the study of protein - DNA interactions using single-molecule approaches. As part of the collaboration, they have offered training, expertise and access to their state-of-art instrumentation at the LaserLab of the VU Amsterdam. The data deriving from a series of experiments about the interaction of SYCP3 with DNA have provided important new insight into the role of SYCP3 in lateral element assembly and meiosis. The findings have been recently published (Syrjanen et al, eLife, 2017).
Impact The findings obtained based on our collaborative work have been recently published (Syrjanen et al, eLife, 2017).
Start Year 2015