Meiotic cohesins mediate initial loading of HORMAD1 to the chromosomes and coordinate SC formation during meiotic prophase

Abstract

During meiotic prophase, sister chromatids are organized into axial element (AE), which underlies the structural framework for the meiotic events such as meiotic recombination and homolog synapsis. HORMA domain-containing proteins (HORMADs) localize along AE and play critical roles in the regulation of those meiotic events. Organization of AE is attributed to two groups of proteins: meiotic cohesins REC8 and RAD21L; and AE components SYCP2 and SYCP3. It has been elusive how these chromosome structural proteins contribute to the chromatin loading of HORMADs prior to AE formation. Here we newly generated Sycp2 null mice and showed that initial chromatin loading of HORMAD1 was mediated by meiotic cohesins prior to AE formation. HORMAD1 interacted not only with the AE components SYCP2 and SYCP3 but also with meiotic cohesins. Notably, HORMAD1 interacted with meiotic cohesins even in Sycp2-KO, and localized along cohesin axial cores independently of the AE components SYCP2 and SYCP3. Hormad1/Rad21L-double knockout (dKO) showed more severe defects in the formation of synaptonemal complex (SC) compared to Hor-mad1-KO or Rad21L-KO. Intriguingly, Hormad1/Rec8-dKO but not Hormad1/Rad21L-dKO showed precocious separation of sister chromatid axis. These findings suggest that meiotic cohesins REC8 and RAD21L mediate chromatin loading and the mode of action of HOR-MAD1 for synapsis during early meiotic prophase.

Sycp1 Is Not Required for Subtelomeric DNA Double-Strand Breaks but Is Required for Homologous Alignment in Zebrafish Spermatocytes

“…IHO1 was first identified as a HORMAD1-interacting protein that promotes DSBs in mice ( Stanzione et al, 2016 ). Although axis localization of IHO1 is largely lost in Hormad1 mutant mice, a recent study indicated that HORMAD1 is not the sole factor required for axis recruitment of IHO1 ( Fujiwara et al, 2020 ). Furthermore, another mouse study reported that DSB formation at the pseudoautosomal region and several chromosome ends are associated with a particular type of minisatellite repeat and HORMAD1-independent accumulation of DSB-promoting proteins, such as REC114, MEI4, IHO1 and MEI1 ( Acquaviva et al, 2020 ).…”

Chromosome Organization in Early Meiotic Prophase

“…In mouse male meiosis, where most of the observations were made, REC8 is detected early before the premeiotic S phase, when it starts to form foci, visible by immunostaining ( Eijpe et al, 2003 ). Shortly afterward, in premeiotic S phase, the RAD21L-cohesin complex also starts forming foci all over the nucleus ( Lee and Hirano, 2011 ; Ishiguro et al, 2014 ; Fujiwara et al, 2020 ). After premeiotic S phase, REC8 cohesin and RAD21L cohesin complexes show differences in localization on meiotic chromosomes.…”

“…In vivo , immunofluorescence and proximity ligation assays demonstrated that HORMAD1 appears as distinct foci at preleptotene, when it mostly co-localizes with REC8 and RAD21L cohesins. Super-resolution microscopy showed that RAD21L tends to colocalize more frequently with HORMAD1 than REC8 ( Fujiwara et al, 2020 ). At leptotene, both HORMAD proteins form short stretches that overlap with RAD21L, REC8 and the axial element proteins SYCP3 and SYCP2 (see below) ( Wojtasz et al, 2009 ; Shin et al, 2010 ; Fujiwara et al, 2020 ).…”

“…Super-resolution microscopy showed that RAD21L tends to colocalize more frequently with HORMAD1 than REC8 ( Fujiwara et al, 2020 ). At leptotene, both HORMAD proteins form short stretches that overlap with RAD21L, REC8 and the axial element proteins SYCP3 and SYCP2 (see below) ( Wojtasz et al, 2009 ; Shin et al, 2010 ; Fujiwara et al, 2020 ). At this stage, the chromosomal localization of HORMAD1 is slightly reduced in the absence of SYCP2, suggesting that this protein might be involved in HORMAD1 stabilization on chromosomes ( Fujiwara et al, 2020 ).…”

“…Immunostaining of spermatocyte spreads showed that SYCP3 first appears at preleptotene/early leptotene, where it forms small stretches of axial-like structures that colocalize with REC8 and RAD21L cohesins, HORMADs and SYCP2 (see below). The association of SYCP3 with axial elements might be stabilized by SYCP2, because in its absence SYCP3 signal is reduced ( Fujiwara et al, 2020 ). At this early stage, most centromere regions also co-localize with SYCP3 ( Bisig et al, 2012 ).…”