Distinct Roles of Meiosis-Specific Cohesin Complexes in Mammalian Spermatogenesis

摘要

Mammalian meiocytes feature four meiosis-specific cohesin proteins in addition to ubiquitous ones, but the roles of the individual cohesin complexes are incompletely understood. To decipher the functions of the two meiosis-specific kleisins, REC8 or RAD21L, together with the only meiosis-specific SMC protein SMC1β, we generated Smc1 β~( -/- ) Rec8 ~( -/- )and Smc1β ~( -/- ) Rad21L ~( -/- )mouse mutants. Analysis of spermatocyte chromosomes revealed that besides SMC1β complexes, SMC1α/RAD21 and to a small extent SMC1α/REC8 contribute to chromosome axis length. Removal of SMC1β and RAD21L almost completely abolishes all chromosome axes. The sex chromosomes do not pair in single or double mutants, and autosomal synapsis is impaired in all mutants. Super resolution microscopy revealed synapsis-associated SYCP1 aberrantly deposited between sister chromatids and on single chromatids in Smc1β ~( -/- ) Rad21L ~( -/- )cells. All mutants show telomere length reduction and structural disruptions, while wild-type telomeres feature a circular TRF2 structure reminiscent of t-loops. There is no loss of centromeric cohesion in both double mutants at leptonema/early zygonema, indicating that, at least in the mutant backgrounds, an SMC1α/RAD21 complex provides centromeric cohesion at this early stage. Thus, in early prophase I the most prominent roles of the meiosis-specific cohesins are in axis-related features such as axis length, synapsis and telomere integrity rather than centromeric cohesion. Author Summary Unlike somatic cells, which feature two different cohesin complexes, in spermatocytes at least six distinct cohesin complexes form, whose concerted functions are little understood. This study focuses on three meiosis-specific cohesins. Meiosis features specific chromosome structures and dynamics, and we revealed individual contributions of meiotic cohesin complexes to chromosome axes length, centromeric cohesion, telomere integrity and synapsis. The only meiosis-specific SMC protein, SMC1β, was removed leaving only complexes based on the universal SMC1α. In addition to SMC1β, either one of the two meiosis-specific kleisins REC8 or RAD21L, proteins that close the cohesin ring-like structure, were eliminated. “Double-knockout” mutants were compared to the single “knockouts” and wild-type. Telomeres and chromosome synapsis are impaired to different degrees in all mutants. In early prophase I prominent roles of meiosis-specific cohesins are in axis length and synapsis rather than centromeric cohesion. Removal of SMC1β and RAD21L almost completely abolishes all chromosome axes. Centromeric cohesion is initially provided by SMC1α complex(es). Later in meiosis, SMC1β ensures centromeric cohesion, suggesting functional replacement of SMC1α. Thus, different cohesin complexes in spermatocytes contribute distinctly to different structures and processes in these cells, but there is also some functional redundancy.