Ensuring correct meiotic chromosome behaviour: mechanisms of synaptonemal complex stabilization and extension

• Applicant: Professor Dr. Attila Tóth
• Subject Area: Cell Biology ; General Genetics and Functional Genome Biology
• Term: since 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 236843383

In meiosis, the correct segregation of chromosomes requires that homologous copies of each chromosome (homologs) form physical links through DNA recombination. Meiotic recombination is initiated by programmed formation of DNA double-strand breaks (DSBs) along proteinaceous chromosome axes. These DSBs result in single-stranded DNA ends that invade the DNA of homologs, forming recombination intermediates that constitute discrete links between homologs. A poorly understood mechanism extends these separate inter-homolog connections into continuous juxtaposition of homolog axes, which manifests as synaptonemal complexes (SCs) along the entire lengths of chromosomes. In turn, SCs enable correct DSB repair and generation of chromosomal crossovers by recombination. Given the importance of SCs for germline genome integrity, we aim to uncover how SC completion is achieved through controlled SC extension to non-recombining axis segments. Our prior work uncovered important reciprocal control between the chromosome axis protein HORMAD1 and the SC. HORMAD1 promotes SC formation in three ways: enhancing DSB formation, controlling DSB repair, and an enigmatic DSB-independent mechanism. Importantly, the SC triggers a remodeling of HORMAD1 conformation, resulting in HORMAD1 depletion from synapsed axes, which shows both synergy and antagonism in the HORMAD1-SC relationship. To understand these relationships, we tagged HORMAD1 with an N-terminal HA tag in mice (Hormad1HA/HA). This HA tagging prevents SC-triggered HORMAD1 remodeling and HORMAD1 loss from SC, thereby allowing us to test the role of HORMAD1-SC antagonism in vivo. Hormad1HA/HA mice show sterility and defective synapsis. Based on the nature of the SC defects, we hypothesize that SC-triggered remodeling of HORMAD1 is mechanistically required for SC stabilization and SC extension into non-recombining axis segments. This function may be crucial in evolution. Sequence divergence of homologs (which characterizes outbred populations and hybrids) hinders inter-homolog recombination and synapsis, leading to reproductive isolation between emerging species. We hypothesize that, by enabling SC stabilization/extension, HORMAD1 remodeling counters the SC-destabilizing effect of homolog divergence, which enhances reproductive fitness and slows speciation in wild populations. In our project, we will test these hypotheses and uncover how SC extension is controlled. To this end, we will investigate HORMAD1 protein-interactions, and analyze the phenotypic consequences of HA-HORMAD1 expression in (1) diverse meiosis-mutants that allow separation of distinct HORMAD1 functions and (2) hybrids featuring distinct levels of heterozygosity (to test if HORMAD1 remodeling enhances the tolerance of SC to divergence in homologs). Given the novelty of Hormad1HA/HA phenotypes the project is expected to reveal crucial insights into the mechanisms of genetic inheritance with implications for human reproductive health.

• DFG Programme: Research Grants