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  • br Introduction Throughout early prophase I meiotic cells

    2024-06-14


    Introduction Throughout early prophase I, meiotic cells intentionally generate DSBs as part of a stringently controlled process to initiate homologous recombination (HR). HR is a critical DSB repair pathway ppy that ultimately permits the exchange of genetic information between homologous chromosomes (homologues) and which is essential for reductional chromosome segregation during the first meiotic nuclear division. Saccharomyces cerevisiae, within which much of the molecular detail of HR has been elucidated, employs the meiosis-specific and evolutionarily conserved type-II topoisomerase-like enzyme, Spo11, to generate DSBs [2]. DSB formation is accomplished via a Spo11-catalysed transesterification reaction that generates covalent protein-DNA intermediates—whereby Spo11 monomers remain linked to the newly created 5′-termini. Spo11 and DSB formation additionally require the concerted efforts of nine other factors including Mre11, Rad50, Xrs2/Nbs1, Sae2, Rec114, Mer2 and Mei4. Mre11, Rad50, Xrs2 and Sae2 coordinate removal of Spo11-intermediates via endonucleolytic cleavage, releasing Spo11-oligonucleotides. Subsequent exonucleolytic resection generates 3′ ssDNA tails—the prime substrates for HR [3]. These ssDNA tails, aided by specialised recombinases, invade complementary regions on homologues generating inter-homologue interactions. In many organisms, inter-homologue interactions drive the physical synapsis of the involved chromosomes via formation of a proteinaceous “zipper” designated the synaptonemal complex (SC) [4]. Given that DNA damage resides at the heart of meiosis, it is unsurprising to find that ATM/ATR and respective orthologues feature prominently in the meiotic landscape. To date, ATM/ATR and their downstream effectors have been implicated in a wide range of meiotic events including promotion of HR at various steps, repair-template choice and DSB repair, control of crossover formation and distribution, synapsis checkpoints and homolog pairing, meiotic chromosomal segregation, X-chromosome inactivation and sex body formation. Such work, reviewed extensively in [5], has revealed that not only have traditional ATM/ATR ppy surveillance mechanisms been co-opted for meiosis but that the kinases are additionally involved in uniquely meiotic processes and have evolved meiosis-specific targets. Interestingly, while the number of DSBs typically formed per meiotic cycle differs between species, such differences do not significantly scale with genome size [6], [7], [8], [9], [10], [11]. Moreover, DSB frequency is maintained at a moderate level despite an apparent excess of Spo11 protein [12], hinting at strict regulatory control. This phenomenon, termed DSB homeostasis [13], [14], is proposed to maintain levels of DSBs within genetically-encoded ranges in order to prevent the deleterious effects associated with too few or too many DSBs [10], [15], [16]. Several recent developments have strongly implicated ATM/ATR-dependent systems as being integral to DSB homeostasis and the regulation of DSB formation. In this review we will thus explore the mechanisms underpinning these two inter-related aspects of ATM/ATR meiotic function and provide an overall framework for meiotic DSB homeostasis.
    Meiotic checkpoints and signalling Akin to mitotic cycles, checkpoint mechanisms exist within meiosis. The pachytene-checkpoint, operating during prophase I, surveys the status of DSB-repair and homolog-synapsis in order to arrest cells until such processes are completed [5], [17]. Given that premature anaphase I entry proves lethal, this checkpoint is of critical importance [18]. Transmission of pachytene checkpoint signals primarily depends upon the ssDNA-sensing ATR system comprising ATR, the RAD9–RAD1–HUS1 (9–1–1) clamp complex and the RAD17 clamp-loader; respectively designated Mec1, Rad17, Mec3, Ddc1 and Rad24 in S. cerevisiae[18], [19], [20]. A central target of the checkpoint in S. cerevisiae is Ndt80, a meiosis-specific transcription factor responsible for exit from pachytene into anaphase via the induction of key genes involved in cell cycle progression and Holliday-junction resolution [21], [22], [23]. Checkpoint signals inhibit Ndt80 via suppression of its hyper-phosphorylation—a modification required for its transcription factor activity—ultimately arresting cells within prophase I [24] [Fig. 1]. In addition to this, a mitotic-like replication checkpoint also appears to be active in pre-meiotic S-phase within S. cerevisiae and requires Mec1-signalling [25]. ATM (Tel1 in S. cerevisiae) primarily signals via CHK2 and is recruited to DSB ends via the Mre11–Rad50–Xrs1/Nbs1 complex (MRX/N), whose Mre11 subunit exhibits direct DSB-binding activity [1]. The pachytene checkpoint also relies upon ATM signalling, albeit to a lesser extent [5]. A meiosis-specific paralogue of CHK2, known as Mek1, has been identified within S. cerevisiae[26], [27]. Both Mec1 and Tel1 signals feed into Mek1 activation during meiosis, integrating multiple stimuli into a common target [27], [28], [29]. Ablation of Mek1 activity reduces the viability of S. cerevisiae spores (the haploid products of yeast meiosis) suggesting that Mek1 is a major effector of Mec1/Tel1 meiotic activity [26], [30]. In some lab strains of S. cerevisiae, prophase arrest is also regulated by the evolutionarily conserved hexameric ATPase, Pch2 (TRIP13 in mammals). Pch2/TRIP13 appears to act by promoting the remodelling of the HORMA domain-containing meiotic chromosome component Hop1 (HORMAD1/2 in mammals), which are targets of the ATM/ATR response, thereby aiding prophase arrest in response to defects in chromosome pairing and synapsis [29], [31]. Despite sharing a subset of downstream effectors and targeting identical motifs (SQ/TQ sites) [1], ATM and ATR appear to possess distinct roles during meiosis as explored in the following sections.