Phosphoregulation of HORMA domain protein HIM-3 promotes asymmetric synaptonemal complex disassembly in meiotic prophase in Caenorhabditis elegans.

In the two cell divisions of meiosis, diploid genomes are reduced into complementary haploid sets through the discrete, two-step removal of chromosome cohesion, a task carried out in most eukaryotes by protecting cohesion at the centromere until the second division. In eukaryotes without defined centromeres, however, alternative strategies have been innovated. The best-understood of these is found in the nematode Caenorhabditis elegans: after the single off-center crossover divides the chromosome into two segments, or arms, several chromosome-associated proteins or post-translational modifications become specifically partitioned to either the shorter or longer arm, where they promote the correct timing of cohesion loss through as-yet unknown mechanisms. Here, we investigate the meiotic axis HORMA-domain protein HIM-3 and show that it becomes phosphorylated at its C-terminus, within the conserved "closure motif" region bound by the related HORMA-domain proteins HTP-1 and HTP-2. Binding of HTP-2 is abrogated by phosphorylation of the closure motif in in vitro assays, strongly suggesting that in vivo phosphorylation of HIM-3 likely modulates the hierarchical structure of the chromosome axis. Phosphorylation of HIM-3 only occurs on synapsed chromosomes, and similarly to other previously-described phosphorylated proteins of the synaptonemal complex, becomes restricted to the short arm after designation of crossover sites. Regulation of HIM-3 phosphorylation status is required for timely disassembly of synaptonemal complex central elements from the long arm, and is also required for proper timing of HTP-1 and HTP-2 dissociation from the short arm. Phosphorylation of HIM-3 thus plays a role in establishing the identity of short and long arms, thereby contributing to the robustness of the two-step chromosome segregation.

Intestine-to-Germline Transmission of Epigenetic Information Intergenerationally Ensures Systemic Stress Resistance in C. elegans.

Changes in epigenetic states affect organismal homeostasis, including stress resistance. However, the mechanisms coordinating epigenetic states and systemic stress resistance remain largely unknown. Here, we identify the intestine-to-germline communication of epigenetic states, which intergenerationally enhances stress resistance in C. elegans. The alterations in epigenetic states by deficiency of the histone H3K4me3 modifier ASH-2 in the intestine or germline increase organismal stress resistance, which is abrogated by knockdown of the H3K4 demethylase RBR-2. Remarkably, the increase in stress resistance induced by ASH-2 deficiency in the intestine is abrogated by RBR-2 knockdown in the germline, suggesting the intestine-to-germline transmission of epigenetic information. This communication from intestine to germline in the parental generation increases stress resistance in the next generation. Moreover, the intertissue communication is mediated partly by transcriptional regulation of F08F1.3. These results reveal that intertissue communication of epigenetic information provides mechanisms for intergenerational regulation of systemic stress resistance.

The meiotic phosphatase GSP-2/PP1 promotes germline immortality and small RNA-mediated genome silencing.

Germ cell immortality, or transgenerational maintenance of the germ line, could be promoted by mechanisms that could occur in either mitotic or meiotic germ cells. Here we report for the first time that the GSP-2 PP1/Glc7 phosphatase promotes germ cell immortality. Small RNA-induced genome silencing is known to promote germ cell immortality, and we identified a separation-of-function allele of C. elegans gsp-2 that is compromised for germ cell immortality and is also defective for small RNA-induced genome silencing and meiotic but not mitotic chromosome segregation. Previous work has shown that GSP-2 is recruited to meiotic chromosomes by LAB-1, which also promoted germ cell immortality. At the generation of sterility, gsp-2 and lab-1 mutant adults displayed germline degeneration, univalents, histone methylation and histone phosphorylation defects in oocytes, phenotypes that mirror those observed in sterile small RNA-mediated genome silencing mutants. Our data suggest that a meiosis-specific function of GSP-2 ties small RNA-mediated silencing of the epigenome to germ cell immortality. We also show that transgenerational epigenomic silencing at hemizygous genetic elements requires the GSP-2 phosphatase, suggesting a functional link to small RNAs. Given that LAB-1 localizes to the interface between homologous chromosomes during pachytene, we hypothesize that small localized discontinuities at this interface could promote genomic silencing in a manner that depends on small RNAs and the GSP-2 phosphatase.

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

Chromosomes that have undergone crossing over in meiotic prophase must maintain sister chromatid cohesion somewhere along their length between the first and second meiotic divisions. Although many eukaryotes use the centromere as a site to maintain cohesion, the holocentric organism Caenorhabditis elegans instead creates two chromosome domains of unequal length termed the short arm and long arm, which become the first and second site of cohesion loss at meiosis I and II. The mechanisms that confer distinct functions to the short and long arm domains remain poorly understood. Here, we show that phosphorylation of the synaptonemal complex protein SYP-1 is required to create these domains. Once crossover sites are designated, phosphorylated SYP-1 and PLK-2 become cooperatively confined to short arms and guide phosphorylated histone H3 and the chromosomal passenger complex to the site of meiosis I cohesion loss. Our results show that PLK-2 and phosphorylated SYP-1 ensure creation of the short arm subdomain, promoting disjunction of chromosomes in meiosis I.

5-Hydroxymethylcytosine Marks Sites of DNA Damage and Promotes Genome Stability.

5-hydroxymethylcytosine (5hmC) is a DNA base created during active DNA demethylation by the recently discovered TET enzymes. 5hmC has essential roles in gene expression and differentiation. Here, we demonstrate that 5hmC also localizes to sites of DNA damage and repair. 5hmC accumulates at damage foci induced by aphidicolin and microirradiation and colocalizes with major DNA damage response proteins 53BP1 and γH2AX, revealing 5hmC as an epigenetic marker of DNA damage. Deficiency for the TET enzymes eliminates damage-induced 5hmC accumulation and elicits chromosome segregation defects in response to replication stress. Our results indicate that the TET enzymes and 5hmC play essential roles in ensuring genome integrity.