Protein Phosphatase 4 Promotes Chromosome Pairing and Synapsis, and Contributes to Maintaining Crossover Competence with Increasing Age

Abstract

Prior to the meiotic divisions, dynamic chromosome reorganizations including pairing, synapsis, and recombination of maternal and paternal chromosome pairs must occur in a highly regulated fashion during meiotic prophase. How chromosomes identify each other's homology and exclusively pair and synapse with their homologous partners, while rejecting illegitimate synapsis with non-homologous chromosomes, remains obscure. In addition, how the levels of recombination initiation and crossover formation are regulated so that sufficient, but not deleterious, levels of DNA breaks are made and processed into crossovers is not understood well. We show that in Caenorhabditis elegans, the highly conserved Serine/Threonine protein phosphatase PP4 homolog, PPH-4.1, is required independently to carry out four separate functions involving meiotic chromosome dynamics: (1) synapsis-independent chromosome pairing, (2) restriction of synapsis to homologous chromosomes, (3) programmed DNA double-strand break initiation, and (4) crossover formation. Using quantitative imaging of mutant strains, including super-resolution (3D-SIM) microscopy of chromosomes and the synaptonemal complex, we show that independently-arising defects in each of these processes in the absence of PPH-4.1 activity ultimately lead to meiotic nondisjunction and embryonic lethality. Interestingly, we find that defects in double-strand break initiation and crossover formation, but not pairing or synapsis, become even more severe in the germlines of older mutant animals, indicating an increased dependence on PPH-4.1 with increasing maternal age. Our results demonstrate that PPH-4.1 plays multiple, independent roles in meiotic prophase chromosome dynamics and maintaining meiotic competence in aging germlines. PP4's high degree of conservation suggests it may be a universal regulator of meiotic prophase chromosome dynamics.

Meiosis

“…Mutations in the gene encoding the serine/threonine phosphatase PPH-4.1 cause defective autosomal chromosome pairing and synapsis between non-homologous chromosomes. The relevant targets for PPH-4.1 are still unknown (Sato-Carlton et al, 2014: PMID 25340746). In C. elegans , homologous recombination is not required for establishment of homolog alignment since spo-11 mutants, in which meiotic DSBs are not formed, are proficient to pair and synapse with their homologs (Dernburg et al, 1998: PMID 9708740).…”

Sex and Mitonuclear Adaptation in Experimental Caenorhabditis elegans Populations

“…Many of the same SNPs mapped to GO terms found to be significantly enriched: two biogenesis and organizational categories along with protein modification, and phosphoprotein phosphatase ( Figure S7 and File S4). For instance, pph-4.1, encoding a protein phosphatase subunit necessary for proper centrosome functioning and meiotic chromosome dynamics (Sumiyoshi et al 2002;Sato-Carlton et al 2014), acquired an intronic mutation that appears in the gas-1-centric interactome ( Figure S6B), where it interacts with a histone deacetylase gene, hda-1 (Table S7), and maps to all four significantly enriched GO terms ( Figure S7). Several RC line SNPs were annotated as being involved in fertility-related processes (File S4).…”

Phosphorylation of the synaptonemal complex protein SYP-1 promotes meiotic chromosome segregation

“…The number of DAPI bodies counted for H3T3ph is 151 for the wild type and 111 for syp-1(12A) mutants. Quantifications of the length of SUN-1 Ser8ph staining as well as the TZ (defined by clustered nuclei without resolvable chromatids), and of RAD-51 foci were performed, as described in (Sato-Carlton et al, 2014). For quantification of RAD-51 foci per nucleus, the nuclei on the coverslip-proximal side of four gonads were scored for each genotype: the numbers of nuclei scored for zone 1-7 are as follows: for wt, 142, 163, 173, 146, 127, 111, 92; for syp-1(10A), 210, 241, 172, 129, 142, 104, 104; for syp-1(12A), 154, 144, 86, 75, 83, 79, 74.…”