Matefin/SUN-1 Phosphorylation Is Part of a Surveillance Mechanism to Coordinate Chromosome Synapsis and Recombination with Meiotic Progression and Chromosome Movement

Abstract

Faithful chromosome segregation during meiosis I depends on the establishment of a crossover between homologous chromosomes. This requires induction of DNA double-strand breaks (DSBs), alignment of homologs, homolog association by synapsis, and repair of DSBs via homologous recombination. The success of these events requires coordination between chromosomal events and meiotic progression. The conserved SUN/KASH nuclear envelope bridge establishes transient linkages between chromosome ends and cytoskeletal forces during meiosis. In Caenorhabditis elegans, this bridge is essential for bringing homologs together and preventing nonhomologous synapsis. Chromosome movement takes place during synapsis and recombination. Concomitant with the onset of chromosome movement, SUN-1 clusters at chromosome ends associated with the nuclear envelope, and it is phosphorylated in a chk-2- and plk-2-dependent manner. Identification of all SUN-1 phosphomodifications at its nuclear N terminus allowed us to address their role in prophase I. Failures in recombination and synapsis led to persistent phosphorylations, which are required to elicit a delay in progression. Unfinished meiotic tasks elicited sustained recruitment of PLK-2 to chromosome ends in a SUN-1 phosphorylation–dependent manner that is required for continued chromosome movement and characteristic of a zygotene arrest. Furthermore, SUN-1 phosphorylation supported efficient synapsis. We propose that signals emanating from a failure to successfully finish meiotic tasks are integrated at the nuclear periphery to regulate chromosome end–led movement and meiotic progression. The single unsynapsed X chromosome in male meiosis is precluded from inducing a progression delay, and we found it was devoid of a population of phosphorylated SUN-1. This suggests that SUN-1 phosphorylation is critical to delaying meiosis in response to perturbed synapsis. SUN-1 may be an integral part of a checkpoint system to monitor establishment of the obligate crossover, inducible only in leptotene/zygotene. Unrepaired DSBs and unsynapsed chromosomes maintain this checkpoint, but a crossover intermediate is necessary to shut it down.

DNA Helicase HIM-6/BLM Both Promotes MutSγ-Dependent Crossovers and Antagonizes MutSγ-Independent Interhomolog Associations During Caenorhabditis elegans Meiosis

“…In both the wild-type and him-6 mutant, phosphorylation of nuclear envelope protein SUN-1 (SUN-1 S8Pi) and association of the DSB-promoting protein DSB-2 with chromatin show similar dynamics: They are detected in germ-cell nuclei at the onset of meiotic prophase and then decline and disappear from most nuclei during mid-pachytene. As SUN-1 S8Pi and DSB-2 persist until late pachytene in multiple mutants that fail to make crossover-eligible recombination intermediates Woglar et al 2013) (see B), this finding is consistent with the conclusion that him-6 mutants are proficient for generating crossover intermediates. Scale bar, 10 mm.…”

“…Numbers of nuclei scored: him-17, n = 95; him-6, n = 83. Woglar et al 2013), immunofluorescence analysis using these markers indicates that the timing of meiotic prophase progression in him-6 mutant germ lines is similar to that in wild-type controls. This finding further supports our inference that him-6 mutants are proficient for multiple steps in recombination leading up to and including formation of COspecific recombination intermediates, and thus the HIM-6/ BLM protein is dispensable for these steps.…”

“…Markers of meiotic progression appear normal in him-6 mutants: Several recent studies have provided evidence for a checkpoint-like negative feedback network that operates during C. elegans meiosis to couple multiple aspects of meiotic prophase progression to the formation of crossovereligible recombination intermediates (Rosu et al 2011;Rosu et al 2013;Stamper et al 2013;Woglar et al 2013). These Figure 3 Timely transition in pachytene progression in the him-6 mutant.…”

“…Numbers of germ lines scored: wild type, 18; him-6, 21; cosa-1, 20; cosa-1; him-6, 17. studies suggested that germ cells have a capacity to sense whether CO-eligible recombination intermediates have formed on all six chromosome pairs; fulfillment of this requirement enables nuclei to shut down early processes and progress in a timely manner to the late-pachytene stage, when pro-CO factors become concentrated at a single CO site per homolog pair. Conversely, lack of CO-eligible intermediates on one or more homolog pairs delays multiple aspects of prophase progression, which can be detected cytologically as prolonged persistence of several early prophase markers, including association of meiotic DSBpromoting protein DSB-2 on chromatin and phosphorylation of nuclear envelope protein SUN-1 (SUN-1 S8Pi) Woglar et al 2013).…”

Manipulation of Karyotype inCaenorhabditis elegansReveals Multiple Inputs Driving Pairwise Chromosome Synapsis During Meiosis

“…Several recent studies have converged on the conclusion that formation and repair of DSBs during meiosis are governed by a checkpoint-like negative feedback network in which germ cells monitor the formation of recombination intermediates that have the capacity to become interhomolog crossovers ("CO-eligible intermediates") and couple detection of such intermediates to progression through meiotic prophase (Rosu et al 2011Stamper et al 2013;Woglar et al 2013). If sufficient intermediates have formed to guarantee a CO for each homolog pair, the cell will be permitted to undergo a major transition affecting multiple distinct aspects of the meiotic program, including cessation of competence for DSB formation and shutting down of access to the homolog as a template for DSB repair; if the condition is not met, this transition will be delayed.…”