Single-molecule imaging and mechanisms of crossover regulation on meiotic chromosomes

PROJECT SUMMARY/ABSTRACT Sexually reproducing organisms use meiosis to distribute one copy of each homologous chromosome pair to developing gametes. This relies on the formation of physical linkages between chromosomes known as crossovers, and crossover errors are a leading cause of infertility and conditions such as Down syndrome. Accordingly, while crossovers appear at random genomic positions in each meiosis, they are tightly regulated to ensure each chromosome pair receives at least one crossover and that crossovers beyond the first are well- separated along the length of the chromosomes. Yet while these phenomena were first observed over a century ago, the mechanism by which crossover locations are coordinated along chromosomes remains hotly debated. To investigate this longstanding question, our lab recently pioneered the use of single-molecule imaging to track the motion of proteins known to regulate crossover formation. We found that these proteins are recruited by and move along a nanoscale protein assembly between the chromosomes known as the synaptonemal complex, allowing them to coordinate their activity at sites millions of base pairs apart. Our current research program works to understand how dynamics in this liquid-like compartment create emergent patterns of crossovers. In our first major research direction, we will study the proteins required for crossovers to form. One leading theory of crossover patterning proposes that the distribution of crossovers along chromosomes emerges from competition between sites to recruit enough of these proteins to form crossovers. To explore this possibility, we will evaluate how quickly and how far different proteins can move along chromosomes, how proteins bind to potential crossover sites, and what regulates these dynamics. Using our precise measurements of diffusion and binding kinetics, we will use computer modeling to understand how these dynamics collectively generate tightly regulated crossover numbers and spacing, and how mutations in these proteins disrupt regulation. In our second major research direction, we will study the dynamic properties of the synaptonemal complex itself. As a liquid-like assembly, small genetic perturbations or changes in environmental conditions can alter the material properties of the synaptonemal complex and drastically impair fertility. We will study how structural changes and shifts in temperature alter the function of the synaptonemal complex, as well as how the synaptonemal complex in turn is patterned and internally organized by the formation of crossovers. Altogether, our research program will uncover fundamental mechanisms of how crossovers are regulated and how molecular activity along chromosomes is spatially patterned. The insights provided from this research are critical for understanding how infertility and birth defects arise in humans and how liquid-like compartments organize the nucleus, with implications for related diseases such as errors in DNA repair in cancer.