Meiosis: step-by-step through sporulation

Cross-talk between autophagy and sporulation in Saccharomyces cerevisiae

Unicellular organisms, like yeast, have developed mechanisms to overcome environmental stress conditions like nutrient starvation. Autophagy and sporulation are two such mechanisms employed by yeast cells. Autophagy is a well-conserved, catabolic process that degrades excess and unwanted cytoplasmic materials and provides building blocks during starvation conditions. Thus, autophagy maintains cellular homeostasis at basal conditions and acts as a survival mechanism during stress conditions. Sporulation is an essential process that, like autophagy, is triggered due to stress conditions in yeast. It involves the formation of ascospores that protect the yeast cells during extreme conditions and germinate when the conditions are favorable. Studies show that autophagy is required for the sporulation process in yeast. However, the exact mechanism of action is not clear. Furthermore, several of the core autophagy gene knockouts do not sporulate and at what stage of sporulation they are involved is not clear. Besides, many overlapping proteins function in both sporulation and autophagy and it is unclear how the pathway-specific roles of these proteins are determined. All these observations suggest that the two processes cross-talk. Individually, some key features from both the processes remain to be studied with respect to the source of membrane for autophagosomes, prospore membrane (PSM) formation, and closure of the membranes. Therefore, it becomes crucial to study the cross-talk between autophagy and sporulation. In this review, the cross-talk between the two pathways, the common protein machineries have been discussed.

The histone codes for meiosis

Meiosis is a specialized process that produces haploid gametes from diploid cells by a single round of DNA replication followed by two successive cell divisions. It contains many special events, such as programmed DNA double-strand break (DSB) formation, homologous recombination, crossover formation and resolution. These events are associated with dynamically regulated chromosomal structures, the dynamic transcriptional regulation and chromatin remodeling are mainly modulated by histone modifications, termed 'histone codes'. The purpose of this review is to summarize the histone codes that are required for meiosis during spermatogenesis and oogenesis, involving meiosis resumption, meiotic asymmetric division and other cellular processes. We not only systematically review the functional roles of histone codes in meiosis but also discuss future trends and perspectives in this field.

SPO24 is a transcriptionally dynamic, small ORF-encoding locus required for efficient sporulation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, meiosis and sporulation are highly regulated responses that are driven in part by changes in RNA expression. Alternative mRNA forms with extended 5′ UTRs are atypical in S. cerevisiae, and 5′ extensions with upstream open reading frames (uORFs) are even more unusual. Here we characterize the gene YPR036W-A, now renamed SPO24, which encodes a very small (67-amino-acid) protein. This gene gives rise to two mRNA forms: a shorter form throughout meiosis and a longer, 5′-extended form in mid-late meiosis. The latter form includes a uORF for a 14-amino-acid peptide (Spo24^u14). Deletion of the downstream ORF (dORF) leads to sporulation defects and the appearance of pseudohyphae-like projections. Experiments with luciferase reporters indicate that the uORF does not downregulate dORF translation. The protein encoded by the dORF (Spo24^d67) localizes to the prospore membrane and is differentially phosphorylated during meiosis. Transcription of the 5′-extended mRNA in mid-meiosis depends upon the presence of two middle sporulation elements (MSEs). Removal of the MSEs severely inhibits the mid-meiotic appearance of the 5′-extended mRNA and limits the ability of plasmid-borne SPO24 to rescue the sporulation defect of a spo24Δ mutant, suggesting that the 5′-extended mRNA is functionally important. These results reveal Spo24^d67 as a sporulation-related factor that is encoded by a transcriptionally dynamic, uORF-containing locus.

Meiosis-induced alterations in transcript architecture and noncoding RNA expression in S. cerevisiae

Changes in transcript architecture can have powerful effects on protein expression. Regulation of the transcriptome is often dramatically revealed during dynamic conditions such as development. To examine changes in transcript architecture we analyzed the expression and transcript boundaries of protein-coding and noncoding RNAs over the developmental process of meiosis in Saccharomyces cerevisiae. Custom-designed, high-resolution tiling arrays were used to define the time-resolved transcriptome of cells undergoing meiosis and sporulation. These arrays were specifically designed for the S. cerevisiae strain SK1 that sporulates with high efficiency and synchrony. In addition, new methods were created to define transcript boundaries and to identify dynamic changes in transcript expression and architecture over time. Of 8407 total segments, 699 (8.3%) were identified by our algorithm as regions containing potential transcript architecture changes. Our analyses reveal extensive changes to both the coding and noncoding transcriptome, including altered 5' ends, 3' ends, and splice sites. Additionally, 3910 (46.5%) unannotated expressed segments were identified. Interestingly, subsets of unannotated RNAs are located across from introns (anti-introns) or across from the junction between two genes (anti-intergenic junctions). Many of these unannotated RNAs are abundant and exhibit sporulation-specific changes in expression patterns. All work, including heat maps of the tiling array, annotation for the SK1 strain, and phastCONS conservation analysis, is available at http://groups.molbiosci.northwestern.edu/sontheimer/sk1meiosis.php. Our high-resolution transcriptome analyses reveal that coding and noncoding transcript architectures are exceptionally dynamic in S. cerevisiae and suggest a vast array of novel transcriptional and post-transcriptional control mechanisms that are activated upon meiosis and sporulation.

Identification of a new transcript specifically expressed in mouse spermatocytes: mmrp2

In an effort to examine the molecular basis of gametogenesis, we screened Riken cDNA database and the clone 4930481F22 that is expressed preponderantly in mouse testis was identified. In the course of the research, a new isoform of 4930481F22 clone was found, isolated from mouse testis and sequenced. It only lacks the 7th exon of 4930481F22 transcript. The new isoform only has 837 bp and encodes a putative 28.4 kDa protein. We investigated the expression pattern at the mRNA level by RT-PCR and in situ hybridization in testis. The new isoform was only expressed in the gonad, where it began to be detected at day 8 after birth. In situ hybridization proved that the new isoform mostly expressed in spermatocytes. The structure of the predicted protein and the expression pattern of the mRNA suggest that the new isoform could have an important role in meiosis. We temporarily named it mmrp 2 (Mouse Meiosis Related Protein 2).

DSC1-MCB regulation of meiotic transcription in Schizosaccharomyces pombe

Meiosis is initiated from the G1 phase of the mitotic cell cycle, and consists of pre-meiotic S-phase followed by two successive nuclear divisions. Here we show that control of gene expression during pre-meiotic S-phase in the fission yeast Schizosaccharomyces pombe is mediated by a DNA synthesis control-like transcription factor complex (DSC1), which acts upon M lu1 cell cycle box (MCB) promoter motifs. Several genes, including rec8+, rec11+, cdc18+, and cdc22+, which contain MCB motifs in their promoter regions, are found to be co-ordinately regulated during pre-meiotic S-phase. Both synthetic and native MCB motifs are shown to confer meiotic-specific transcription on a heterologous reporter gene. A DSC1-like transcription factor complex that binds to MCB motifs was also identified in meiotic cells. The effect of mutating and over-expressing individual components of DSC1 (cdc10+, res1+, res2+, rep1+ and rep2+) on the transcription of cdc22+, rec8+ and rec11+ during meiosis was examined. We found that cdc10+, res2+, rep1+ and rep2+ are required for correct meiotic transcription, while res1+ is not required for this process. This work demonstrates a role for MCB motifs and a DSC1-like transcription factor complex in controlling transcription during meiosis in fission yeast, and suggests a mechanism for how this specific expression occurs.

Activity of phosphoforms and truncated versions of Ndt80, a checkpoint-regulated sporulation-specific transcription factor of Saccharomyces cerevisiae

Ndt80 contributes to the highly regulated cascade of sequential gene expression that directs spore formation in Saccharomyces cerevisiae. This DNA-binding transcriptional activator, which is responsible for the expression of a set of middle sporulation-specific genes, is a target of the meiotic recombination checkpoint. Triggering of this checkpoint prevents phosphorylation and accumulation of active Ndt80. In this study we have investigated the requirements for the activation function of Ndt80 by exploring the role of phosphorylation in the regulation of its activity and by examining the effect of C-terminal truncations. Of three phosphoforms of Ndt80 that we resolved, which we refer to as P approximately Ndt80", P approximately Ndt80', and P approximately Ndt80 in order of increasing electrophoretic mobility, the P approximately Ndt80" and P approximately Ndt80' isoforms correlated with active Ndt80. In particular, P approximately Ndt80" was present in lysates from wild-type sporulating cells and in cells that bypassed checkpoint-mediated arrest as a result of mutations in RAD17, SUM1, or SWE1, or overexpression of NDT80. P approximately Ndt80' was the slowest-migrating isoform that accumulated in Delta ime2/Delta ime2 Delta sum1/Delta sum1 cells in sporulation medium and in mitotic cells that ectopically expressed NDT80. Nonphosphorylated Ndt80 and P approximately Ndt80, which had a slightly lower mobility than nonphosphorylated Ndt80 and was the predominant phosphoform present in checkpoint-arrested cells, correlated with inactive Ndt80. These data are consistent with the notion that extensive phosphorylation, but not Ime2-dependent phosphorylation, of Ndt80 is required for its activity. Examination of the effect of increasingly extensive truncation of the C terminal region of Ndt80 revealed that some functions of Ndt80 were more sensitive to a reduction in its activity than others. In particular, we found that a truncated version of Ndt80 that lacked the last 110 residues was able to promote expression of some middle sporulation-specific genes, but could not direct spore formation. Full activity, however, could be restored to this version of Ndt80 by increasing its level of expression.

The histone deacetylase Hda1 from Ustilago maydis is essential for teliospore development

In the corn smut fungus Ustilago maydis, pathogenic development is controlled by the b mating type locus that encodes the two homeodomain proteins bE and bW. A heterodimer of bE and bW controls a large set of genes, either directly by binding to cis regulatory sequences or indirectly via a b-dependent regulatory cascade. It is thought that several of the b-regulated genes contribute to processes involved in pathogenicity. In a screen for components of the b-dependent regulatory cascade we have isolated Hda1, a protein with homology to histone deacetylases of the RPD3 class. Hda1 can substitute for the histone deacetylase RPD3 in Saccharomyces cerevisiae, showing that it functions as a histone deacetylase. Deletion of hda1 results in the expression of several genes that are normally expressed only in the dikaryon, among these are several genes that are now expressed independently from their activation by the bE/bW heterodimer. hda1 mutant strains are capable to infect corn, and the proliferation of dikaryotic hyphae within the plant appears comparable to wild-type strains during initial developmental stages. Upon karyogamy, however, the proliferation to mature teliospores is blocked. The block in sporogenesis in Deltahda1 strains is probably a result of the deregulation of a specific set of genes whose temporal or spatial expression prevent the proper developmental progress.

Identification and characterization of a sphingolipid delta 4-desaturase family

Sphingolipids desaturated at the Delta4-position are important signaling molecules in many eukaryotic organisms, including mammals. In a bioinformatics approach, we now identified a new family of protein sequences from animals, plants, and fungi and characterized these sequences biochemically by expression in Saccharomyces cerevisiae. This resulted in the identification of the enzyme sphingolipid Delta4-desaturase (dihydroceramide desaturase) from Homo sapiens, Mus musculus, Drosophila melanogaster, and Candida albicans, in addition to a bifunctional sphingolipid Delta4-desaturase/C-4-hydroxylase from M. musculus. Among the sequences investigated are the Homo sapiens membrane lipid desaturase, the M. musculus degenerative spermatocyte, and the Drosophila melanogaster degenerative spermatocyte proteins. During spermatogenesis, but not oogenesis of des mutant flies, both cell cycle and spermatid differentiation are specifically blocked at the entry into the first meiotic division, leading to male sterility. This mutant phenotype can be restored to wild-type by complementation with a functional copy of the des gene (Endo, K., Akiyama, T., Kobayashi S., and Okada, M. (1996) Mol. Gen. Genet. 253, 157-165). These results suggest that Delta4-desaturated sphingolipids provide an early signal necessary to trigger the entry into both meiotic and spermatid differentiation pathways during Drosophila spermatogenesis.

Dynamics of metabolism and its interactions with gene expression during sporulation in Saccharomyces cerevisiae

The dynamics of metabolism has been shown to be involved in the triggering of events that are concurrent with sporulation of the budding yeast Saccharomyces cerevisiae. Indeed, quantitative correlations have been demonstrated between sporulation and the rate of carbon substrate or oxygen consumption, and the fluxes through gluconeogenic and glyoxylate cycle pathways. The results suggest that an imbalance between catabolic and anabolic fluxes influences the occurrence of the differentiation process. The hypothesis that the initiation of sporulation is triggered by the accumulation of an intracellular metabolite is confronted with the notion that intermediary metabolism and the expression of genes involved in sporulation interact to trigger the differentiation process. Several pieces of evidence indicate that derepression of the gluconeogenic pathway is crucial for the initiation of sporulation. One of the possible pathways through which glucose repression hampers sporulation might be the repression of gluconeogenesis as well as that of respiratory activity, in turn modulating the expression of IMEL++. The stages defined in the dynamics of sporulating cultures, namely readiness and commitment, are related to metabolic events associated with sporulation. An interpretation in terms of metabolic flux dynamics is given to the reversal of commitment occurring when the normal progression to sporulation is somehow blocked. The quantitative data are here integrated in a model attempting to simulate the dynamics of metabolic as well as cellular events during sporulation. The model is envisaged as a test of the hypothesis that an imbalance between anabolism and catabolism is involved in initiation of the sporulation process. It is proposed that such an imbalance may be a signal for differential gene expression associated with the differentiation pathway.

The only function of Grauzone required for Drosophila oocyte meiosis is transcriptional activation of the cortex gene

The grauzone and cortex genes are required for the completion of meiosis in Drosophila oocytes. The grauzone gene encodes a C2H2-type zinc-finger transcription factor that binds to the cortex promoter and is necessary for high-level activation of cortex transcription. Here we define the region of the cortex promoter to which Grauzone binds and show that the binding occurs through the C-terminal, zinc-finger-rich region of the protein. Mutations in two out of the five grauzone alleles result in single amino acid changes within different zinc-finger motifs. Both of these mutations result in the inability of Grauzone to bind DNA effectively. To determine the mechanism by which Grauzone regulates meiosis, transgenic flies were produced with an extra copy of the cortex gene in homozygous grauzone females. This transgene rescued the meiosis arrest of embryos from these mutants and allowed their complete development, indicating that activation of cortex transcription is the primary role of Grauzone during Drosophila oogenesis. These experiments further define a new transcriptional pathway that controls the meiotic cell cycle in Drosophila oocytes.

Saccharomyces cerevisiae checkpoint genes MEC1, RAD17 and RAD24 are required for normal meiotic recombination partner choice

Checkpoint gene function prevents meiotic progression when recombination is blocked by mutations in the recA homologue DMC1. Bypass of dmc1 arrest by mutation of the DNA damage checkpoint genes MEC1, RAD17, or RAD24 results in a dramatic loss of spore viability, suggesting that these genes play an important role in monitoring the progression of recombination. We show here that the role of mitotic checkpoint genes in meiosis is not limited to maintaining arrest in abnormal meioses; mec1-1, rad24, and rad17 single mutants have additional meiotic defects. All three mutants display Zip1 polycomplexes in two- to threefold more nuclei than observed in wild-type controls, suggesting that synapsis may be aberrant. Additionally, all three mutants exhibit elevated levels of ectopic recombination in a novel physical assay. rad17 mutants also alter the fraction of recombination events that are accompanied by an exchange of flanking markers. Crossovers are associated with up to 90% of recombination events for one pair of alleles in rad17, as compared with 65% in wild type. Meiotic progression is not required to allow ectopic recombination in rad17 mutants, as it still occurs at elevated levels in ndt80 mutants that arrest in prophase regardless of checkpoint signaling. These observations support the suggestion that MEC1, RAD17, and RAD24, in addition to their proposed monitoring function, act to promote normal meiotic recombination.