Recombination Of Homologous Chromosomes Research Articles

Centromeres Convert but Don't Cross

A long-standing problem in chromosome biology concerns the dynamic nature of centromeres. These chromosomal sites assemble the protein machines called kinetochores that connect chromosomes to the spindle microtubules for segregation to daughter cells during mitosis and meiosis. In multicelluar eukaryotes, centromeres are typically composed of highly homogeneous tandem repeats that evolve rapidly despite their highly conserved function [1]. For tandem repeats to evolve, a mutation must spread by some recombinational process, but a persistent dogma is that centromeres do not undergo homologous chromosome recombination (the shuffling of genetic segments between chromosomal pairs). New evidence [2] challenges this dogma and addresses the problem of rapidly evolving centromeres.

From meiosis to postmeiotic events: Alignment and recognition of homologous chromosomes in meiosis

Recombination of homologous chromosomes is essential for correct reductional segregation of homologous chromosomes, which characterizes meiosis. To accomplish homologous recombination, chromosomes must find their homologous partners and pair with them within the spatial constraints of the nucleus. Although various mechanisms have developed in different organisms, two major steps are involved in the process of pairing: first, alignment of homologous chromosomes to bring them close to each other for recognition; and second, recognition of the homologous partner of each chromosome so that they can form an intimate pair. Here, we discuss the various mechanisms used for alignment and recognition of homologous chromosomes in meiosis.

PHS1 regulates meiotic recombination and homologous chromosome pairing by controlling the transport of RAD50 to the nucleus

Recombination and pairing of homologous chromosomes are critical for bivalent formation in meiotic prophase. In many organisms, including yeast, mammals, and plants, pairing and recombination are intimately interconnected. The POOR HOMOLOGOUS SYNAPSIS1 (PHS1) gene acts in coordination of chromosome pairing and early recombination steps in plants, ensuring pairing fidelity and proper repair of meiotic DNA double-strand-breaks. In phs1 mutants, chromosomes exhibit early recombination defects and frequently associate with non-homologous partners, instead of pairing with their proper homologs. Here, we show that the product of the PHS1 gene is a cytoplasmic protein that functions by controlling transport of RAD50 from cytoplasm to the nucleus. RAD50 is a component of the MRN protein complex that processes meiotic double-strand-breaks to produce single-stranded DNA ends, which act in the homology search and recombination. We demonstrate that PHS1 plays the same role in homologous pairing in both Arabidopsis and maize, whose genomes differ dramatically in size and repetitive element content. This suggests that PHS1 affects pairing of the gene-rich fraction of the genome rather than preventing pairing between repetitive DNA elements. We propose that PHS1 is part of a system that regulates the progression of meiotic prophase by controlling entry of meiotic proteins into the nucleus. We also document that in phs1 mutants in Arabidopsis, centromeres interact before pairing commences along chromosome arms. Centromere coupling was previously observed in yeast and polyploid wheat while our data suggest that it may be a more common feature of meiosis.

A high incidence of meiotic silencing of unsynapsed chromatin is not associated with substantial pachytene loss in heterozygous male mice carrying multiple simple robertsonian translocations.

Meiosis is a complex type of cell division that involves homologous chromosome pairing, synapsis, recombination, and segregation. When any of these processes is altered, cellular checkpoints arrest meiosis progression and induce cell elimination. Meiotic impairment is particularly frequent in organisms bearing chromosomal translocations. When chromosomal translocations appear in heterozygosis, the chromosomes involved may not correctly complete synapsis, recombination, and/or segregation, thus promoting the activation of checkpoints that lead to the death of the meiocytes. In mammals and other organisms, the unsynapsed chromosomal regions are subject to a process called meiotic silencing of unsynapsed chromatin (MSUC). Different degrees of asynapsis could contribute to disturb the normal loading of MSUC proteins, interfering with autosome and sex chromosome gene expression and triggering a massive pachytene cell death. We report that in mice that are heterozygous for eight multiple simple Robertsonian translocations, most pachytene spermatocytes bear trivalents with unsynapsed regions that incorporate, in a stage-dependent manner, proteins involved in MSUC (e.g., γH2AX, ATR, ubiquitinated-H2A, SUMO-1, and XMR). These spermatocytes have a correct MSUC response and are not eliminated during pachytene and most of them proceed into diplotene. However, we found a high incidence of apoptotic spermatocytes at the metaphase stage. These results suggest that in Robertsonian heterozygous mice synapsis defects on most pachytene cells do not trigger a prophase-I checkpoint. Instead, meiotic impairment seems to mainly rely on the action of a checkpoint acting at the metaphase stage. We propose that a low stringency of the pachytene checkpoint could help to increase the chances that spermatocytes with synaptic defects will complete meiotic divisions and differentiate into viable gametes. This scenario, despite a reduction of fertility, allows the spreading of Robertsonian translocations, explaining the multitude of natural Robertsonian populations described in the mouse.

Protein SYCP2 provides a link between transverse filaments and lateral elements of mammalian synaptonemal complexes

Synaptonemal complexes (SCs) are evolutionarily conserved meiosis-specific nuclear structures critically involved in synapsis, recombination, and segregation of homologous chromosomes. SCs are proteinaceous structures composed of (a) two lateral elements (LEs), to which the chromatin of the homologs is attached, (b) numerous transverse filaments (TFs) that link the LEs, and (c) a central element (CE). Major protein components of mammalian SCs are the TF protein SYCP1 and the LE proteins SYCP2 and SCYP3. How SCs become assembled is presently poorly understood, in particular, it is not known how TFs assemble at the plane of LEs to interconnect the homologous chromosomes. Therefore, we have investigated possible interactions between SYCP1 and other SC proteins. In immunoprecipitation experiments we could find that SYCP1 and SYCP2 interact in extracts of meiotic cells. Using the yeast two-hybrid system, we were able to demonstrate that the C-terminus of SYCP1 directly interacts with SYCP2. These results were confirmed by different interaction traps. Furthermore, we could narrow down the interacting domain of the SYCP2 molecule to its C-terminal region. We propose that SYCP2 acts as a linker between SYCP1 and SYCP3 and therefore would be the missing connecting link between LEs and TFs essential for proper chromosome synapsis.

Meiotic recombination in normal and clone bulls and their offspring

Homologous chromosome pairing and recombination are essential components of meiosis and sexual reproduction. The reshuffling of genetic material through breakage and reunion of chromatids ensure proper segregation of homologous chromosomes in reduction division and genetic diversity in the progeny. The advent of somatic cell nuclear transfer (SCNT) as a reproductive biotechnology for use in livestock industry has made it easy to bypass these vital steps. However, few studies have been carried out on the impact of SCNT on the reproductive characteristics of cloned animals and, none to date, on the meiotic processes in animals, which were created by circumventing meiosis. In an attempt to assess the impact of cloning by SCNT on the meiotic processes, we undertook an immunocytological comparison of recombination in normal and clone bulls using antibodies raised against the synaptonemal complex protein 3 (SCP3) to label the lateral elements and the mismatch repair protein 1 (MLH1) foci on bivalents as indicators of recombination events. Our studies involving five normal bulls of proven fertility, two SCNT-derived bulls, and four mature offspring of SCNT bulls showed that the mean number of crossing over per spermatocyte for normal bulls (42 ± 4 SD; ranging from 33 to 56), was not significantly different from that of SCNT-derived bulls (43 ± 5 SD; ranging from 35 to 56), and the offspring of SCNT-derived bulls (43 ± 5 SD; ranging from 37 to 58). It would appear that circumventing meiosis to produce these animals does not influence the meiotic processes revealed by MLH1 foci detected in spermatocytes.

The Arabidopsis MutS homolog AtMSH5 is required for normal meiosis

MSH5, a member of the MutS homolog DNA mismatch repair protein family, has been shown to be required for proper homologous chromosome recombination in diverse organisms such as mouse, budding yeast and Caenorhabditis elegans. In this paper, we show that a mutant Arabidopsis plant carrying the putative disrupted AtMSH5 gene exhibits defects during meiotic division, producing a proportion of nonviable pollen grains and abnormal embryo sacs, and thereby leading to a decrease in fertility. AtMSH5 expression is confined to meiotic floral buds, which is consistent with a possible role during meiosis. Cytological analysis of male meiosis revealed the presence of numerous univalents from diplotene to metaphase I, which were associated with a great reduction in chiasma frequencies. The average number of residual chiasmata in the mutant is reduced to 2.54 per meiocyte, which accounts for approximately 25% of the amount in the wild type. Here, quantitative cytogenetical analysis reveals that the residual chiasmata in Atmsh5 mutants are randomly distributed among meiocytes, suggesting that AtMSH5 has an essential role during interference-sensitive chiasma formation. Taken together, the evidence indicates that AtMSH5 promotes homologous recombination through facilitating chiasma formation during prophase I in Arabidopsis.

250. Importin α2-recognised testis cargoes; relevance to spermatogenesis

Spermatogenesis, the morphogenetic process of generating haploid sperm capable of fertilising the female gamete, requires transport into the nucleus of transcription and chromatin remodelling factors to implement genome compaction. This transport is mediated by members of the importin (IMP) superfamily, which exhibit distinct expression patterns during male germ cell development, consistent with the idea that each IMP carries specific cargo(es) at discrete stages of spermatogenesis. To identify IMP cargoes in the testis to help identify potential developmental switches critical to the spermatogenic process, we performed a yeast two-hybrid screen using full length IMPα2 as bait and an adult mouse testis library. Binding partners identified included coilin, Hop2, Chrp (cysteine and histidine rich protein), TAF9 (TATA Binding Protein Associated Factor 9) and Cdyl (Chromodomain Y chromosome-like). These proteins may be important in cell cycle regulation, homologous chromosome pairing and recombination, transcriptional regulation, splicing, mRNA storage or histone-protamine exchange during spermatogenesis. IMPα2 interaction with these cargoes was verified by binding assays, coimmunoprecipation and cotransfection approaches, while immunohistochemical staining of rodent testis sections indicated their co-expression with IMPα2 in specific testicular cell types. A key cargo in this context is the chromatin remodelling component, Cdyl where we could show that interaction with IMPα2 (and not IMPα4 or IMPα6 which are known to be expressed in the same testicular cell types as IMPα2) is necessary for its efficient nuclear import and function in facilitating histone H4 acetylation, the prerequisite step for incorporation of protamines during the final stages of spermatogenesis. A similar approach is currently in progress to demonstrate the importance of IMPα2 for the efficient nuclear transport of the other cargoes identified for their function(s) during spermatogenesis. IMPα2-mediated nuclear localisation of specific cargoes would thus seem to represent a critical developmental switch during spermatogenesis.

Transmembrane protein Sun2 is involved in tethering mammalian meiotic telomeres to the nuclear envelope

Dynamic repositioning of telomeres is a unique feature of meiotic prophase I that is highly conserved among eukaryotes. At least in fission yeast it was shown to be required for proper alignment and recombination of homologous chromosomes. On entry into meiosis telomeres attach to the nuclear envelope and transiently cluster at a limited area to form a chromosomal bouquet. Telomere clustering is thought to promote chromosome recognition and stable pairing of the homologs. However, the molecular basis of telomere attachment and movement is largely unknown. Here we report that mammalian SUN-domain protein Sun2 specifically localizes to the nuclear envelope attachment sites of meiotic telomeres. Sun2-telomere association is maintained throughout the dynamic movement of telomeres. This association does not require the assembly of chromosomal axial elements or the presence of A-type lamins. Detailed EM analysis revealed that Sun2 is part of a membrane-spanning fibrillar complex that interconnects attached telomeres with cytoplasmic structures. Together with recent findings in fission yeast, our study indicates that the molecular mechanisms required for tethering meiotic telomeres and their dynamic movements during bouquet formation are conserved among eukaryotes.

Mlh1 Deficiency in Zebrafish Results in Male Sterility and Aneuploid as Well as Triploid Progeny in Females

In most eukaryotes, recombination of homologous chromosomes during meiosis is necessary for proper chromosome pairing and subsequent segregation. The molecular mechanisms of meiosis are still relatively unknown, but numerous genes are known to be involved, among which are many mismatch repair genes. One of them, mlh1, colocalizes with presumptive sites of crossing over, but its exact action remains unclear. We studied meiotic processes in a knockout line for mlh1 in zebrafish. Male mlh1 mutants are sterile and display an arrest in spermatogenesis at metaphase I, resulting in increased testis weight due to accumulation of prophase I spermatocytes. In contrast, females are fully fertile, but their progeny shows high rates of dysmorphology and mortality within the first days of development. SNP-based chromosome analysis shows that this is caused by aneuploidy, resulting from meiosis I chromosomal missegregation. Surprisingly, the small percentage of progeny that develops normally has a complete triploid genome, consisting of both sets of maternal and one set of paternal chromosomes. As adults, these triploid fish are infertile males with wild-type appearance. The frequency of triploid progeny of mlh1 mutant females is much higher than could be expected for random chromosome segregation. Together, these results show that multiple solutions exist for meiotic crossover/segregation problems.

Synaptonemal complex protein SYCP3: Conserved polymerization properties among vertebrates

Synaptonemal complexes (SCs) are meiosis-specific, nuclear structures that are critically involved in synapsis, recombination and segregation of homologous chromosomes. Although the SC structure is conserved in evolution this is not the case for its protein components. To provide information on SC proteins which would be important for our understanding of the conserved SC structure and function, here we compared ortholog SYCP3 proteins of two evolutionary distant vertebrate species, namely rat and medaka fish. To this end we have investigated the polymerization properties of both proteins by immunocytochemistry, electron microscopy and cell fractionation. We found that despite of the sequence differences that have accumulated over the last 450 million years mammalian and fish SYCP3 have similar properties that allow them to co-assemble higher order structures under experimental conditions. We also provide a likely explanation as to how heterozygous mutations in the SYCP3 gene can lead to a defective meiosis.

34 MEIOTIC RECOMBINATION IN SOMATIC CELL NUCLEAR TRANSFER BULLS AND THEIR OFFSPRING

In mammals, homologous chromosome pairing and recombination are essential events for meiosis. The generation of reciprocal exchanges of genetic material ensure both genetic diversity and the proper segregation of homologous chromosomes. With the advent of reproductive biotechnologies such as somatic cell nuclear transfer (SCNT) in the livestock industry, these vital steps have begun to be bypassed. Despite this, there have been few studies carried out on the reproductive characteristics of cloned animals and none to date regarding the consequences on the meiotic process. As these procedures grow in popularity and use, the importance of evaluating the long-term viability, health, and productivity of cloned animals and any subsequent offspring in future generations to validate potential applications of the technology is increasingly evident. Previously, studies of recombination and synapsis have focused on the physical observation of chiasmata formation in meiotic chromosomes; however, in recent years, the characterization of proteins that localize to the sites of crossing-over and of proteins present in the synaptonemal complex have permitted the study of meiotic recombination using a precise direct immunocytogenetic approach. Cytological analysis of meiotic cells obtained from the testicular tissue of five normal bulls of proven fertility, two SCNT transgenic bulls (Powell et al. 2004 Biol. Reprod. 71, 210–216, 2004), and four reproductively mature offspring of SCNT bulls was performed in order to detect the effects of SCNT on the meiotic process. Over 50 pachytene cells per animal were analyzed by immunofluorescence using antibodies against the synaptonemal complex protein 3 (SCP3) and the mismatch repair protein 1 (MLH1) located on the crossover sites. Data were analyzed using a mixed model analysis of variance with repeated measurements to determine group effects (SAS 9.1; SAS Institute, Inc., Cary, NC, USA). The average number of crossovers per spermatocyte for the non-SCNT bulls (42 � 4 SD, min: 33, max: 56), SCNT bulls (43 � 5 SD, min: 35, max: 56), and SCNT offspring (46 � 4 SD, min: 37, max: 58) were quite similar among the cells of the same individual; however, inter-individual variation was observed. These results are the first documentation of the normal range of variability of recombination distribution within the cattle genome and suggest that the SCNT process does not affect meiotic recombination. This work was funded by NSERC and the CRC program.

P63 protects the female germ line during meiotic arrest

Meiosis in the female germ line of mammals is distinguished by a prolonged arrest in prophase of meiosis I between homologous chromosome recombination and ovulation. How DNA damage is detected in these arrested oocytes is poorly understood, but it is variably thought to involve p53, a central tumour suppressor in mammals. While the function of p53 in monitoring the genome of somatic cells is clear, a consensus for the importance of p53 for germ line integrity has yet to emerge. Here we show that the p53 homologue p63 (refs 5, 6), and specifically the TAp63 isoform, is constitutively expressed in female germ cells during meiotic arrest and is essential in a process of DNA damage-induced oocyte death not involving p53. We also show that DNA damage induces both the phosphorylation of p63 and its binding to p53 cognate DNA sites and that these events are linked to oocyte death. Our data support a model whereby p63 is the primordial member of the p53 family and acts in a conserved process of monitoring the integrity of the female germ line, whereas the functions of p53 are restricted to vertebrate somatic cells for tumour suppression. These findings have implications for understanding female germ line fidelity, the regulation of fertility and the evolution of tumour suppressor mechanisms.

GENETICS OF MEIOTIC PROPHASE I IN PLANTS

During meiotic prophase I, traits are reassorted as a result of a highly organized process involving sister chromatid cohesion, homologous chromosome alignment, pairing, synapsis, and recombination. In the past two years, a number of components involved in this pathway, including Structure Maintenance of Chromosomes (SMC), MRE11, the RAD51 homologs, BRCA2, MSH4, MER3, and ZIP1, have been characterized in plants; in addition, several genes that encode components unique to plants, such as POOR HOMOLOGOUS SYNAPSIS 1 and AMEIOTIC 1, have been cloned. Based on these recent data, essentially from maize and Arabidopsis, we discuss the conserved and plant-specific aspects of meiosis commitment and meiotic prophase I features.

Mcp5, a meiotic cell cortex protein, is required for nuclear movement mediated by dynein and microtubules in fission yeast

During meiotic prophase I of the fission yeast Schizosaccharomyces pombe, oscillatory nuclear movement occurs. This promotes homologous chromosome pairing and recombination and involves cortical dynein, which plays a pivotal role by generating a pulling force with the help of an unknown dynein anchor. We show that Mcp5, the homologue of the budding yeast dynein anchor Num1, may be this putative dynein anchor. mcp5 + is predominantly expressed during meiotic prophase, and GFP-Mcp5 localizes at the cell cortex. Moreover, the mcp5Δ strain lacks the oscillatory nuclear movement. Accordingly, homologous pairing and recombination rates of the mcp5Δ strain are significantly reduced. Furthermore, the cortical localization of dynein heavy chain 1 appears to be reduced in mcp5Δ cells. Finally, the full function of Mcp5 requires its coiled-coil and pleckstrin homology (PH) domains. Our results suggest that Mcp5 localizes at the cell cortex through its PH domain and functions as a dynein anchor, thereby facilitating nuclear oscillation.

The synaptonemal complex and meiotic recombination in humans: new approaches to old questions

Meiotic prophase serves as an arena for the interplay of two important cellular activities, meiotic recombination and synapsis of homologous chromosomes. Synapsis is mediated by the synaptonemal complex (SC), originally characterized as a structure linked to pairing of meiotic chromosomes (Moses (1958) J Biophys Biochem Cytol 4:633-638). In 1975, the first electron micrographs of human pachytene stage SCs were presented (Moses et al. (1975) Science 187:363-365) and over the next 15 years the importance of the SC to normal meiotic progression in human males and females was established (Jhanwar and Chaganti (1980) Hum Genet 54:405-408; Pathak and Elder (1980) Hum Genet 54:171-175; Solari (1980) Chromosoma 81:315-337; Speed (1984) Hum Genet 66:176-180; Wallace and Hulten (1985) Ann Hum Genet 49(Pt 3):215-226). Further, these studies made it clear that abnormalities in the assembly or maintenance of the SC were an important contributor to human infertility (Chaganti et al. (1980) Am J Hum Genet 32:833-848; Vidal et al. (1982) Hum Genet 60:301-304; Bojko (1983) Carlsberg Res Commun 48:285-305; Bojko (1985) Carlsberg Res Commun 50:43-72; Templado et al. (1984) Hum Genet 67:162-165; Navarro et al. (1986) Hum Reprod 1:523-527; Garcia et al. (1989) Hum Genet 2:147-53). However, the utility of these early studies was limited by lack of information on the structural composition of the SC and the identity of other SC-associated proteins. Fortunately, studies of the past 15 years have gone a long way toward remedying this problem. In this minireview, we highlight the most important of these advances as they pertain to human meiosis, focusing on temporal aspects of SC assembly, the relationship between the SC and meiotic recombination, and the contribution of SC abnormalities to human infertility.

The AtRAD51C Gene Is Required for Normal Meiotic Chromosome Synapsis and Double-Stranded Break Repair in Arabidopsis

Meiotic prophase I is a complex process involving homologous chromosome (homolog) pairing, synapsis, and recombination. The budding yeast (Saccharomyces cerevisiae) RAD51 gene is known to be important for recombination and DNA repair in the mitotic cell cycle. In addition, RAD51 is required for meiosis and its Arabidopsis (Arabidopsis thaliana) ortholog is important for normal meiotic homolog pairing, synapsis, and repair of double-stranded breaks. In vertebrate cell cultures, the RAD51 paralog RAD51C is also important for mitotic homologous recombination and maintenance of genome integrity. However, the function of RAD51C in meiosis is not well understood. Here we describe the identification and analysis of a mutation in the Arabidopsis RAD51C ortholog, AtRAD51C. Although the atrad51c-1 mutant has normal vegetative and flower development and has no detectable abnormality in mitosis, it is completely male and female sterile. During early meiosis, homologous chromosomes in atrad51c-1 fail to undergo synapsis and become severely fragmented. In addition, analysis of the atrad51c-1 atspo11-1 double mutant showed that fragmentation was nearly completely suppressed by the atspo11-1 mutation, indicating that the fragmentation largely represents a defect in processing double-stranded breaks generated by AtSPO11-1. Fluorescence in situ hybridization experiments suggest that homolog juxtaposition might also be abnormal in atrad51c-1 meiocytes. These results demonstrate that AtRAD51C is essential for normal meiosis and is probably required for homologous synapsis.

Disruption of p53 by the Viral oncoprotein HPV16-E6 does not Deregulate Chromosomal Homologous Recombination in a Transcriptional Interference-Free Assay System

In addition to its well established role in the maintenance of genome integrity by regulating transcription of genes involved in cell cycle arrest and programmed cell death, the tumour suppressor p53 has also been shown to inhibit spontaneous chromosomal homologous recombination (HR) between adjacent transcription units, raising the possibility that p53 may prevent chromosomal rearrangements by suppressing HR between repetitive DNA elements (ectopic HR). Consistent with its role in the maintenance of genome integrity is that p53 does not suppress HR between homologous chromosomes (allelic HR) or identical sister chromatids, raising the question of how p53 discriminates between ectopic and allelic HR events. Here, we report that disruption of human p53 by the viral oncoprotein HPV16-E6 does not result in increased rates of chromosomal HR between adjacent DNA repeats in a transcriptional interference-free assay system in which a HR reporter gene can escape transcription repression. These results argue against a direct role for p53 in the regulation of HR mechanisms, imply that HR assay systems may be important determinants of the outcome, and suggest that p53 may suppress ectopic HR through its known ability to repress transcription and alter chromatin structure.

217. Interaction of cdyl (chromodomain y-chromosome like) with the nuclear transport protein importin α2

Spermatogenesis is a unique, ordered process governed by the precise expression of a specific set of genes at each stage. Progression through successive stages requires the shuttling of proteins and transcription factors into and out of the nucleus to implement changes in gene transcription. Major factors that mediate nucleocytoplasmic transport are members of the importin superfamily, of which there are five and 20 different importin α and β genes, respectively, in mouse. We have previously demonstrated that several importins display distinct mRNA and protein expression patterns in adult mouse testis1 indicating that specific importins carry a specific cargo at discrete stages of spermatogenesis. Identification of importin cargoes in the testis should help describe the potential developmental switches critical to the spermatogenic process. We performed a yeast two-hybrid screen using full length importin α2 as bait and an adult mouse testis library, identifying nine target proteins. Some of these proteins include nuclear components that may be important in eliciting changes in the nuclear structure during spermatogenesis, as well as those involved in cell cycle regulation, homologous chromosome pairing and recombination, transcriptional regulation and guanine nucleotide biosynthesis. One key candidate is CDYL, which has been implicated in male infertility. It is a chromodomain-containing protein that is predominantly expressed during spermiogenesis and has been previously described to participate in hyperacetylation of histone H4, which is believed to facilitate protamine replacement of histones during spermiogenesis. Verification of CDYL-importin α2 interaction was demonstrated using co-immunoprecipation and co-transfection, while immunohistochemical staining of testis sections indicated colocalisation in the same cell types (mainly elongating spermatids). Importantly, preliminary experiments indicated that increasing CDYL nuclear accumulation by over-expressing importin α2 can increase histone H4 acetylation. Our hypothesis is that importin α2 is central in nuclear targeting of CDYL to facilitate its hyperacetylation role during protamine-histone exchange. (1)Hogarth et al. (2005). Dev. Dynamics (submitted).

Oocyte-specific gene signaling and its regulation of mammalian reproductive potential

Oocyte-specific genes play important roles in regulating ovarian development, principally through the proper and timely progression of oogenesis and folliculogenesis. Development of transgenic mouse models has been critical in revealing how oocyte-specific transcripts influence oocyte development and growth, integrity of the oocyte-granulosa cell complex, oocyte maturation, fertilization, and early embryonic development. Oocyte-derived genes that mediate recombination of homologous chromosomes and DNA mismatch repair include Spo11, Atm, Dmc1, Msh5, Mlh1, and Msh4. Transcripts such as Dazla and Fig-alpha regulate initial proliferation of the primordial germ cell and follicle. Transition from the primordial to primary follicle relies on the expression of growth factors bFGF, Gdf9 and Bmp15, as well as on the expression of various transcripts that mediate oocyte-granulosa cell interactions. Oocyte growth is predominantly under exogenous control, however resumption of meiotic progression is dictated by genes that influence proper chromatin and spindle regulation, such as Cdk, Histone H1oo, Fmn-2, Mad2, and Bub3. Maintenance of meiotic metaphase II arrest prior to fertilization is mediated primarily by c-mos, and successful fertilization requires the expression of zona pellucida glycoproteins (Zp1, Zp2, and Zp3) and Cd9. Following fertilization, maternal-effect and maternally expressed 'imprinting' genes are necessary for the completion of meiosis and for patterning early embryonic development. Recent utilization of suppressive subtractive hybridization (SSH), PCR amplification, and cDNA microarray analysis techniques alongside established transgenesis models are expanding the classification of novel oocyte-specific genes required for reproductive fitness in various species, including human.