Caenorhabditis elegans Histone Methyltransferase MET-2 Shields the Male X Chromosome from Checkpoint Machinery and Mediates Meiotic Sex Chromosome Inactivation

Meiosis is a specialized form of cellular division that results in the precise halving of the genome to produce gametes for sexual reproduction. Checkpoints function during meiosis to detect errors and subsequently to activate a signaling cascade that prevents the formation of aneuploid gametes. Indeed, asynapsis of a homologous chromosome pair elicits a checkpoint response that can in turn trigger germline apoptosis. In a heterogametic germ line, however, sex chromosomes proceed through meiosis with unsynapsed regions and are not recognized by checkpoint machinery. We conducted a directed RNAi screen in Caenorhabditis elegans to identify regulatory factors that prevent recognition of heteromorphic sex chromosomes as unpaired and uncovered a role for the SET domain histone H3 lysine 9 histone methyltransferase (HMTase) MET-2 and two additional HMTases in shielding the male X from checkpoint machinery. We found that MET-2 also mediates the transcriptional silencing program of meiotic sex chromosome inactivation (MSCI) but not meiotic silencing of unsynapsed chromatin (MSUC), suggesting that these processes are distinct. Further, MSCI and checkpoint shielding can be uncoupled, as double-strand breaks targeted to an unpaired, transcriptionally silenced extra-chromosomal array induce checkpoint activation in germ lines depleted for met-2. In summary, our data uncover a mechanism by which repressive chromatin architecture enables checkpoint proteins to distinguish between the partnerless male X chromosome and asynapsed chromosomes thereby shielding the lone X from inappropriate activation of an apoptotic program

DNA repair, recombination, and damage signaling

DNA must be accurately copied and propagated from one cell division to the next, and from one generation to the next. To ensure the faithful transmission of the genome, a plethora of distinct as well as overlapping DNA repair and recombination pathways have evolved. These pathways repair a large variety of lesions, including alterations to single nucleotides and DNA single and double-strand breaks, that are generated as a consequence of normal cellular function or by external DNA damaging agents. In addition to the proteins that mediate DNA repair, checkpoint pathways have also evolved to monitor the genome and coordinate the action of various repair pathways. Checkpoints facilitate repair by mediating a transient cell cycle arrest, or through initiation of cell suicide if DNA damage has overwhelmed repair capacity. In this chapter, we describe the attributes of Caenorhabditis elegans that facilitate analyses of DNA repair, recombination, and checkpoint signaling in the context of a whole animal. We review the current knowledge of C. elegans DNA repair, recombination, and DNA damage response pathways, and their role during development, growth, and in the germ line. We also discuss how the analysis of mutational signatures in C. elegans is helping to inform cancer mutational signatures in humans

Dual role for phosphoinositides in regulation of yeast and mammalian phospholipase D enzymes

Phospholipase D (PLD) generates lipid signals that coordinate membrane trafficking with cellular signaling. PLD activity in vitro and in vivo is dependent on phosphoinositides with a vicinal 4,5-phosphate pair. Yeast and mammalian PLDs contain an NH2-terminal pleckstrin homology (PH) domain that has been speculated to specify both subcellular localization and regulation of PLD activity through interaction with phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2). We report that mutation of the PH domains of yeast and mammalian PLD enzymes generates catalytically active PI(4,5)P2-regulated enzymes with impaired biological functions. Disruption of the PH domain of mammalian PLD2 results in relocalization of the protein from the PI(4,5)P2-containing plasma membrane to endosomes. As a result of this mislocalization, mutations within the PH domain render the protein unresponsive to activation in vivo. Furthermore, the integrity of the PH domain is vital for yeast PLD function in both meiosis and secretion. Binding of PLD2 to model membranes is enhanced by acidic phospholipids. Studies with PLD2-derived peptides suggest that this binding involves a previously identified polybasic motif that mediates activation of the enzyme by PI(4,5)P2. By comparison, the PLD2 PH domain binds PI(4,5)P2 with lower affinity but sufficient selectivity to function in concert with the polybasic motif to target the protein to PI(4,5)P2-rich membranes. Phosphoinositides therefore have a dual role in PLD regulation: membrane targeting mediated by the PH domain and stimulation of catalysis mediated by the polybasic motif

BRCA1 and BRCA2 Tumor Suppressor Function in Meiosis.

Meiosis is a specialized cell cycle that results in the production of haploid gametes for sexual reproduction. During meiosis, homologous chromosomes are connected by chiasmata, the physical manifestation of crossovers. Crossovers are formed by the repair of intentionally induced double strand breaks by homologous recombination and facilitate chromosome alignment on the meiotic spindle and proper chromosome segregation. While it is well established that the tumor suppressors BRCA1 and BRCA2 function in DNA repair and homologous recombination in somatic cells, the functions of BRCA1 and BRCA2 in meiosis have received less attention. Recent studies in both mice and the nematode Caenorhabditis elegans have provided insight into the roles of these tumor suppressors in a number of meiotic processes, revealing both conserved and organism-specific functions. BRCA1 forms an E3 ubiquitin ligase as a heterodimer with BARD1 and appears to have regulatory roles in a number of key meiotic processes. BRCA2 is a very large protein that plays an intimate role in homologous recombination. As women with no indication of cancer but carrying BRCA mutations show decreased ovarian reserve and accumulated oocyte DNA damage, studies in these systems may provide insight into why BRCA mutations impact reproductive success in addition to their established roles in cancer

End3p-Mediated Endocytosis Is Required for Spore Wall Formation in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, vesicles transported to the vicinity of spindle pole bodies are fused to each other to generate bilayered prospore membranes (PSMs). PSMs encapsulate the haploid nuclei that arise from the meiotic divisions and serve as platforms for spore wall deposition. Membrane trafficking plays an important role in supplying vesicles for these processes. The endocytosis-deficient mutant, end3Δ, sporulated poorly and the spores produced lost resistance to ether vapor, suggesting that END3-mediated endocytosis is important for sporulation. End3p-GFP localized to cell and spore peripheries in vegetative and sporulating cells and colocalized with actin structures. Correspondingly, the actin cytoskeleton appeared aberrant during sporulation in end3Δ. Analysis of meiosis in end3Δ mutants revealed that the meiotic divisions occurred with wild-type kinetics. Furthermore, PSMs were assembled normally. However, the levels of proteins required for spore wall synthesis and components of the spore wall layers at spores were reduced, indicating that end3Δ mutants are defective in spore wall synthesis. Thus, END3-mediated endocytosis is important for spore wall formation. Additionally, cytological analyses suggest that trafficking between the plasma membrane and PSMs is important earlier during sporulation

Sorting Signals within the Saccharomyces cerevisiae Sporulation-Specific Dityrosine Transporter, Dtr1p, C Terminus Promote Golgi-to-Prospore Membrane Transport▿

During sporulation in Saccharomyces cerevisiae, the dityrosine transporter Dtr1p, which is required for formation of the outermost layer of the spore wall, is specifically expressed and transported to the prospore membrane, a novel double-lipid-bilayer membrane. Dtr1p consists of 572 amino acids with predicted N- and C-terminal cytoplasmic extensions and 12 transmembrane domains. Dtr1p missing the largest internal cytoplasmic loop was trapped in the endoplasmic reticulum in both mitotically dividing cells and cells induced to sporulate. Deletion of the carboxyl 15 amino acids, but not the N-terminal extension of Dtr1p, resulted in a protein that failed to localize to the prospore membrane and was instead observed in cytoplasmic puncta. The puncta colocalized with a cis-Golgi marker, suggesting that Dtr1p missing the last 15 amino acids was trapped in an early Golgi compartment. Deletion of the C-terminal 10 amino acids resulted in a protein that localized to the prospore membrane with a delay and accumulated in cytoplasmic puncta that partially colocalized with a trans-Golgi marker. Both full-length Dtr1p and Dtr1p missing the last 10 amino acids expressed in vegetative cells localized to the plasma membrane and vacuoles, while Dtr1p deleted for the carboxyl-terminal 15 amino acids was observed only at vacuoles, suggesting that transport to the prospore membrane is mediated by distinct signals from those that specify plasma membrane localization. Transfer-of-function experiments revealed that both the carboxyl transmembrane domain and the C-terminal tail are important for Golgi complex-to-prospore membrane transport

The spindle assembly checkpoint: More than just keeping track of the spindle.

Genome stability is essential for cell proliferation and survival. Consequently, genome integrity is monitored by two major checkpoints, the DNA damage response (DDR) and the spindle assembly checkpoint (SAC). The DDR monitors DNA lesions in G1, S, and G2 stages of the cell cycle and the SAC ensures proper chromosome segregation in M phase. There have been extensive studies characterizing the roles of these checkpoints in response to the processes for which they are named; however, emerging evidence suggests significant crosstalk between the checkpoints. Here we review recent findings demonstrating overlapping roles for the SAC and DDR in metaphase, and in response to DNA damage throughout the cell cycle

Phosphatidylinositol-4,5-Bisphosphate and Phospholipase D-Generated Phosphatidic Acid Specify SNARE-Mediated Vesicle Fusion for Prospore Membrane Formation▿

The soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family of proteins is required for eukaryotic intracellular membrane fusions. Vesicle fusion for formation of the prospore membrane (PSM), a membrane compartment that forms de novo during yeast sporulation, requires SNARE function, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], and the activity of the phospholipase D (PLD) Spo14p, which generates phosphatidic acid (PA). The SNARE syntaxin Sso1p is essential for PSM production while the functionally redundant homolog in vegetative growth, Sso2p, is not. We demonstrate that Sso1p and Sso2p bind similarly in vitro to PA or phosphoinositide-containing liposomes and that the conserved SNARE (H3) domain largely mediates PA-binding. Both green fluorescent protein-Sso fusion proteins localize to the developing PSM in wild-type cells and to the spindle pole body in spo14Δ cells induced to sporulate. However, the autoregulatory region of Sso1p binds PI(4,5)P2-containing liposomes in vitro with a greater ability than the equivalent region of Sso2p. Overexpression of the phosphatidylinositol-4-phosphate 5-kinase MSS4 in sso1Δ cells induced to sporulate stimulates PSM production; PLD activity is not increased under these conditions, indicating that PI(4,5)P2 has roles in addition to stimulating PLD in PSM formation. These data suggest that PLD-generated PA and PI(4,5)P2 collaborate at multiple levels to promote SNARE-mediated fusion for PSM formation