Essential role of Fkbp6 in male fertility and homologous chromosome pairing in meiosis

Ultrastructural analysis of chromatin in meiosis I + II of rye (Secale cereale L.)

Scanning electron microscopy (SEM) proves to be an appropriate technique for imaging chromatin organization in meiosis I and II of rye (Secale cereale) down to a resolution of a few nanometers. It could be shown for the first time that organization of basic structural elements (coiled and parallel fibers, chromomeres) changes dramatically during the progression to metaphase I and II. Controlled loosening with proteinase K (after fixation with glutaraldehyde) provides an enhanced insight into chromosome architecture even of highly condensed stages of meiosis. By selective staining with platinum blue, DNA content and distribution can be visualized within compact chromosomes as well as in a complex arrangement of fibers. Chromatin interconnecting threads, which are typically observed in prophase I between homologous and non-homologous chromosomes, stain clearly for DNA. In zygotene transversion of chromatid strands to their homologous counterparts becomes evident. In pachytene segments of synapsed and non-synapsed homologs alternate. At synapsed regions pairing is so intimate that homologous chromosomes form one filament of structural entity. Chiasmata are characterized by chromatid strands which traverse from one homolog to its counterpart. Bivalents are characteristically fused at their telomeric regions. In metaphase I and II there is no structural evidence for primary and secondary constrictions.

Specific arrests of spermatogenesis in genetically modified and mutant mice

In naturally occurring mutant mice but also in mice genetically modified for the study of other organs, relatively often a spermatogenic arrest is seen. In a number of cases the arrests appear to be very specific causing apoptosis of germ cells at a particular step in their development, while before this step cells progress normally. These steps include: proliferation/migration of primordial germ cells, the production of differentiating spermatogonia by gonocytes, the regulation of stem cell renewal/differentiation, the differentiation of A(al) into A1 spermatogonia, proliferation of A1-A4 spermatogonia, germ cell density regulation, start of meiosis, epithelial stage IV checkpoint of pachytene spermatocytes, the first meiotic division, the formation of the acrosomic vesicle in spermatids and several other steps in spermatid development. In addition, there are many mice that have not been studied in enough detail for a proper categorization. In this review an overview is given of the various mutations and genetically modified mice showing a direct effect on specific spermatogenic cell types. In addition, the relevance of these models to our understanding of the spermatogenic process is discussed.

Irregular telomeres impair meiotic synapsis and recombination in mice

Telomere shortening can lead to chromosome instability, replicative senescence, and apoptosis in both somatic and male germ cells. To study roles for mammalian telomeres in homologous pairing and recombination, we characterized effects of telomere shortening on spermatogenesis and oogenesis in late-generation telomerase-deficient mice. We show that shortened telomeres of late-generation telomerase-deficient mice impair meiotic synapsis and decrease recombination, in particular, in females. In response to telomere shortening, male germ cells mostly undergo apoptosis, whereas female germ cells preferentially arrest in early meiosis, suggesting sexually dimorphic surveillance mechanisms for telomere dysfunction during meiosis in mice. Further, meiocytes of late-generation telomerase-deficient females with shortened telomeres, bred with early-generation males harboring relatively long telomeres, exhibit severely impaired chromosome pairing and synapsis and reduced meiotic recombination. These findings imply that functional telomeres are important in mammalian meiotic synapsis and recombination.

A note on clustering the functionally-related paralogues and orthologues of proteins: a case of the FK506-binding proteins (FKBPs)

The expression patterns of 18 FK506-binding proteins (FKBPs) encoded in the human genome have been established whereas the functional significance of the numerous ORFs coding for FKBP-like sequences remains unknown. Nominal masses of the human FKBPs vary from 12 to 135 kDa. Some large FKBPs consist up to four repeats of the 12 kDa FK506-like binding domain (FKBD) whereas other large FKBPs contain one FKBD linked to different functional domains such as TPRs, leucine-zipper, calmodulin-binding domain etc. The genomes of other eukaryotic organisms, namely D. melanogaster, C. elegans, A. thaliana, S. pombe and S. cerevisiae encode different numbers of the FKBPs' paralogues some of which are orthologues to the human FKBPs. A library of novel algorithms was developed and used for computation of the level of conservation of the hydrophobicity and bulkiness profiles, and the amino acid compositions (AACs) of 247 aligned sequences of FKBPs. The pairwisely-compared hydrophobicity and bulkiness profiles for some combinations of the aligned sequences of the FKBDs yielded high values of the correlation coefficients (CCF). The AACs of some combinations of the aligned sequences of the FKBDs also differed to a low degree. The functionally-related orthologues and paralogues of the FKBPs were clustered by using the following criteria: 1 degrees apparent conservation of the crucial amino acid (AA) residues for peptidylprolyl cis/trans isomerase (PPIase) acitity and binding of some immunosuppressive drugs; 2 degrees convergence of the three mentioned above properties of the polypeptide chain; 3 degrees similarity in the sequence attributes pI and total hydrophobicity index (HI). The clustering method was used for setting up several hypotheses on the emergence of certain classes of the FKBPs in the eukaryotic kingdom.

Insights into regulation of the mammalian cell cycle from studies on spermatogenesis using genetic approaches in animal models

The genetic hierarchy controlling mitosis and especially meiosis during gamete formation is not well understood, even in less complicated systems such as the yeasts. Meiotic divisions are obviously restricted to germ line cells and as such likely require mechanisms of cell cycle control that do not function and may not exist in somatic cells. While male and female germ cells have stages of cell cycle regulation in common, the timing of these events and the stage of development at which these events occur differ in the two sexes. Understanding the genetic program controlling the mitotic and meiotic divisions of the germ line represents a unique opportunity for providing insight into cell cycle control in vivo. Elucidating the key control points and proteins may also enhance our understanding of the etiology of infertility and provide new directions for contraception.

TopBP1 and ATR colocalization at meiotic chromosomes: role of TopBP1/Cut5 in the meiotic recombination checkpoint

Mammalian TopBP1 is a BRCT domain-containing protein whose function in mitotic cells is linked to replication and DNA damage checkpoint. Here, we study its possible role during meiosis in mice. TopBP1 foci are abundant during early prophase I and localize mainly to histone gamma-H2AX-positive domains, where DNA double-strand breaks (required to initiate recombination) occur. Strikingly, TopBP1 showed a pattern almost identical to that of ATR, a PI3K-like kinase involved in mitotic DNA damage checkpoint. In the synapsis-defective Fkbp6(-/-) mouse, TopBP1 heavily stains unsynapsed regions of chromosomes. We also tested whether Schizosaccharomyces pombe Cut5 (the TopBP1 homologue) plays a role in the meiotic recombination checkpoint, like spRad3, the ATR homologue. Indeed, we found that a cut5 mutation suppresses the checkpoint-dependent meiotic delay of a meiotic recombination defective mutant, indicating a direct role of the Cut5 protein in the meiotic checkpoint. Our findings suggest that ATR and TopBP1 monitor meiotic recombination and are required for activation of the meiotic recombination checkpoint.

Fine Mapping of a Region of Rat Chromosome 12 Close to the Aspermia (as) Locus and Comparison with the Human Orthologous Regions

The aspermia mutation of the rat exhibits male sterility caused by arrest of spermatogenesis, which is controlled by an autosomal single recessive gene (as). The as locus has been mapped on rat chromosome 12. We recently identified a causative mutation for the aspermia phenotype of the as homozygous rats in the gene encoding Fkbp6, a member of the immunophilins FK506 binding proteins. In this paper, we report the fine mapping of the as locus by linkage analysis combined with comparative mapping using rat, mouse, and human genomic sequences and expression analysis of genes located in the as region. We constructed a fine linkage map of the region of rat chromosome 12 close to the as locus by using 13 microsatellite markers and localized the as locus to a 1.0-cM interval. Comparison of the linkage map with physical maps of rat, mouse, and human refined the as critical region in a 2.2-Mb segment of the rat physical map between the D12Nas3 and D12Nas8 genes, which includes the Fkbp6 gene. A centromeric part of this segment corresponds to the region commonly deleted in Williams syndrome, a human complex developmental disorder, on human chromosome 7q11.23. The expression analysis of 23 genes located on the 2.2-Mb segments in various mouse tissues identified genes exclusively or strongly expressed in the testis.

Williams-Beuren syndrome: a challenge for genotype-phenotype correlations

Many human chromosomal abnormality syndromes include specific cognitive and behavioural components. Children with Prader-Willi syndrome lack a paternally derived copy of the proximal long arm of chromosome 15, and eat uncontrollably; in Angelman syndrome lack of a maternal contribution of 15q11-q13 results in absence of speech, frequent smiling and episodes of paroxysmal laughter; deletions on 22q11 can be associated with obsessive behaviour and schizophrenia. The neurodevelopmental disorder Williams-Beuren syndrome (WBS), is caused by a microdeletion at 7q11.23 and provides us with one of the most convincing models of a relationship that links genes with human cognition and behaviour. The hypothesis is that deletion of one or a series of genes causes neurodevelopmental abnormalities that manifest as the fractionation of mental abilities typical of WBS. Detailed molecular characterization of the deletion alongside well-defined cognitive profiling in WBS provides a unique opportunity to investigate the neuromolecular basis of complex cognitive behaviour, and develop integrated approaches to study gene function and genotype-phenotype correlations.