XMR, a dual location protein in the XY pair and in its associated nucleolus in mouse spermatocytes

Xlr and Xmr are sex-specific genes which are expressed during the meiotic prophase I in the mouse. In spermatocytes, XMR concentrates on the asynapsed regions of the XY chromosomes, suggesting that XMR plays a role in sex chromosome condensation and silencing. The present study shows that in the mouse, XMR also concentrates in the nucleolus which is closely associated with the XY chromosome pair. In this species, the formation of a large fibrillo-granular nucleolus signals the activation of the ribosomal genes, but release of pre-ribosomal particles is inhibited. Using laser confocal microscopy we characterized the distribution of XMR in the XY body relative to the XY chromatin and the nucleolus. Immunoelectron microscopy showed that XMR concentrates in the fibrillo-granular component and the granular component (GC) of the nucleolus. In (T[X;16]16H) mouse spermatocytes, the nucleolus displays little or no activity and does not associate with the XY pair. XMR concentrated only on the XY chromosomes in (T[X;16]16H) mouse spermatocytes. These data suggest that XMR could play a role both in the XY pair and the nucleolus associated to the sex chromosomes.     

Analysis of male meiotic "sex body" proteins during XY female meiosis provides new insights into their functions

During male meiosis in mammals the X and Y chromosomes become condensed to form the sex body (XY body), which is the morphological manifestation of the process of meiotic sex chromosome inactivation (MSCI). An increasing number of sex body located proteins are being identified, but their functions in relation to MSCI are unclear. Here we demonstrate that assaying male sex body located proteins during XY female mouse meiosis, where MSCI does not take place, is one way in which to begin to discriminate between potential functions. We show that a newly identified protein, "Asynaptin" (ASY), detected in male meiosis exclusively in association with the X and Y chromatin of the sex body, is also expressed in pachytene oocytes of XY females where it coats the chromatin of the asynapsed X in the absence of MSCI. Furthermore, in pachytene oocytes of females carrying a reciprocal autosomal translocation, ASY associates with asynapsed autosomal chromatin. Thus the location of ASY to the sex body during male meiosis is likely to be a response to the asynapsis of the non-homologous regions [outside the pseudoautosomal region (PAR)] of the heteromorphic X-Y bivalent, rather than being related to MSCI. In contrast to ASY, the previously described sex body protein XY77 proved to be male sex body specific. Potential functions for MSCI and the sex body are discussed together with the possible roles of these two proteins.     

A novel Mr 77,000 protein of the XY body of mammalian spermatocytes: its localization in normal animals and in Searle’s translocation carriers

We describe a novel XY body protein of rat and mice pachytene spermatocytes called XY77. Biochemical characterization showed that protein XY77 (Mr 77,000; pH value 8.3) is present in meiotic but absent in postmeiotic stages of spermatogenesis. With the aid of an antibody against protein XY77 together with another specific for XY body-associated protein XY40 we also investigated the localization of these proteins in mice carrying Searle’s translocation, a reciprocal X-autosomal translocation. We show here that in these mice the distribution of both XY77 and XY40 is abnormal. Our results indicate that in Searle’s translocation alterations are not restricted to the translocated autosome, but also involve chromatin segments corresponding originally to the sex chromosomes X and Y. Received: 21 December 1996; in revised form: 1 February 1997 / Accepted: 15 February 1997     

Deletion of specific sequences or modification of centromeric chromatin are responsible for Y chromosome centromere inactivation

Summary Stable dicentric chromosomes behave as monocentrics because one of the centromeres is inactive. The cause of centromere inactivation is unknown; changes in centromere chromatin conformation and loss of centromeric DNA elements have been proposed as possible mechanisms. We studied the phenomenon of inactivation in two Y centromeres, having as a control genetically identical active Y centromeres. The two cases have the following karyotypes: 45,X/46,X,i(Y)(q12) and 46,XY/ 47,XY,+t(X;Y)(p22.3;p11.3). The analysis of the behaviour of the active and inactive Y chromosome centromeres after Da-Dapi staining, CREST immunofluorescence, and in situ hybridization with centromeric probes leads us to conclude that, in the case of the isochromosome, a true deletion of centromeric chromatin is responsible for its stability, whereas in the second case, stability of the dicentric (X;Y) is the result of centromere chromatin modification.     

The X-linked methylated DNA binding protein,Mecp2, is subject to X inactivation in the mouse

DNA methylation at the promoter region of X-linked genes is associated with the maintenance of X inactivation in mammals. One of the methylated DNA binding proteins, MECP2, that binds to methylated bases in DNA is encoded by a gene (Mecp2) located on the mouse X Chromosome (Chr). To determine whether this gene was expressed from the inactive X Chr, and X-autosome translocation (T(X;16)16H) system in which expression from the Mecp2 allele on the inactive X Chr could be assayed was used. Results from these experiments indicate that Mecp2 is subject to X inactivation in mouse.