Based on cytological observations the existence of a mechani

Based on cytological observations, the existence of a mechanism linking meiotic chromosome dynamics with cell-cycle progression has been inferred in C. elegans. Polarized nuclear organization is greatly extended in mutants that disrupt synapsis (Colaiácovo et al., 2003, MacQueen et al., 2002), hinting that cell-cycle progression is delayed. Molecular markers of early prophase, including SUN-1 Radotinib and chromosomal localization of DSB-1/2, also persist longer in mutants that are proficient for synapsis but fail to establish crossovers (Rosu et al., 2013, Stamper et al., 2013, Woglar et al., 2013), implying that meiotic recombination is also subject to surveillance. However, it has been unclear whether these responses occur through the same feedback circuit because the molecular basis for this regulation has not been elucidated.
Results
Discussion Here we establish a key kinase cascade that acts at PCs to promote homolog pairing and synapsis. CHK-2 phosphorylates the zinc finger proteins that specify PCs, which, in turn, primes their recruitment of PLK-2 (Figure 7D). By recruiting these two kinases, PCs serve as signaling hubs that mediate early meiotic chromosome dynamics. Although most Chk2 family kinases function downstream of ATM/ATR in the DNA damage response (Matsuoka et al., 1998), C. elegans CHK-2 does not require DSBs for its initial activation and lacks clusters of SQ/TQ sites that define ATM/ATR targets. Instead, CHK-2 is essential for DSB formation and acts as a master regulator that governs pairing, synapsis, and recombination during meiotic prophase (MacQueen and Villeneuve, 2001). This rewiring of regulatory circuitry may have accompanied the emergence of homolog pairing mechanisms that function independently of meiotic recombination in C. elegans (Dernburg et al., 1998). We demonstrate that CHK-2′s kinase activity normally declines when all chromosomes have accomplished synapsis and crossover, but is prolonged in mutants that disrupt synapsis or crossover formation. Adding to recent evidence for feedback regulation of crossover formation (Rosu et al., 2013, Stamper et al., 2013, Woglar et al., 2013), we now show that two distinct pathways controlled by CHK-2 (synapsis and meiotic recombination) both feed back to regulate CHK-2 activity (Figure 7D). This common circuitry delays meiotic progression and extends the temporal window for active pairing, synapsis, and DSB formation. This mechanism meets the original definition of a cell-cycle checkpoint (Hartwell and Weinert, 1989) in that it makes meiotic progression contingent on the formation of a crossover on each homolog pair. Our finding that mutations in the meiotic HORMA domain proteins fail to extend CHK-2 activity despite severe defects in pairing, synapsis, and crossover formation highlights the central and conserved role of these proteins in checkpoint control. Although CHK-2 is primarily detected at PCs, it clearly acts elsewhere within the nucleus. Our evidence demonstrates that the meiotic checkpoint is fully functional even in the absence of PC activity and that CHK-2 feedback regulation is, rather, a nucleus-wide response. This directly refutes the recent proposal that HIM-8 is required for feedback in early meiotic prophase (Silva et al., 2014). This confusion arose because the previous study did not directly monitor CHK-2 activity but, instead, the recruitment of PLK-2 to PCs, which requires not only CHK-2 activity but also the zinc finger proteins, specifically HIM-8, under conditions where the ZIMs do not remain phosphorylated.