BackgroundIn animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes.ResultsUsing microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation.ConclusionBirds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.

The first postmitotic neurons in the developing neocortex establish the preplate layer. These early-born neurons have a significant influence on the circuitry of the developing cortex. However, the exact timing and trajectory of their projections, between cortical hemispheres and intra- and extra-cortical regions, remain unresolved. Here, we describe the creation of a transgenic mouse using a 1.3 kb golli promoter element of the myelin basic protein gene to target expression of a tau-green fluorescent protein (GFP) fusion protein in the cell bodies and processes of pioneer cortical neurons. During embryonic and early neonatal development, the timing and patterning of process extension from these neurons was examined. Analysis of tau-GFP fluorescent fibers revealed that progression of early labeled projections was interrupted unexpectedly by transient pauses at the corticostriatal and telencephalic-diencephalic boundaries before invading the thalamus just prior to birth. After birth the pioneering projections differentially invaded the thalamus, excluding some nuclei, e.g. medial and lateral geniculate, until postnatal days 10-14. Early labeled projections were also found to cross to the contralateral hemisphere as well as to the superior colliculus. These results indicate that early corticothalamic projections appear to pause before invading specific subcortical regions during development, that there is developmental regulation of innervation of individual thalamic nuclei, and that these early-generated neurons also establish early projections to commissural and subcortical targets.

Two human Golli (for gene expressed in the oligodendrocyte lineage)-MBP (for myelin basic protein) cDNAs have been isolated from a human oligodendroglioma cell line. Analysis of these cDNAs has enabled us to determine the entire structure of the human Golli-MBP gene. The Gofli-MBP gene, which encompasses the MBP transcription unit, is =179 kb in length and consists of 10 exons, seven of which constitute the MBP gene. The human Goli-MBP gene contains two transcription start sites, each of which gives rise to a family of alternatively spliced transcripts. At least two Golli-MBP transcripts, containing the first three exons ofthe gene and one or more MBP exons, are produced from the first transcription start site. The second family of transcripts contains only MBP exons and produces the well-known MBPs. In humans, RNA blot analysis revealed that Golli-MBP transcripts were expressed in fetal thymus, spleen, and human B-cell and macrophage cell lines, as well as in fetal spinal cord. These rmdings clearly link the expression of exons encoding the autoimmunogen/encephalitogen MBP in the central nervous system to cells and tissues of the immune system through normal expression of the Golli-MBP gene. They also establish that this genetic locus, which includes the MBP gene, is conserved among species, providing further evidence that the MBP transcription unit is an integral part of the Golli transcription unit and suggest that this structural arrangement is important for the genetic function and/or regulation of these genes.

The myelin basic protein (MBP) gene is part of the golli-mbp gene complex. In mouse, the golli-mbp gene produces two families of mRNAs from different transcription start sites that generate either MBPs or golli proteins (which contain MBP sequences in addition to unique peptide sequences). In situ hybridization and immunocytochemical analyses indicate that golli products are expressed in selected neuronal populations in postnatal mouse brain, in addition to oligodendrocytes, as shown earlier. The principal subcellular location of golli proteins in neurons was in axonal and dendritic processes. In a small subset of neurons, golli proteins were located in nuclei. With development and neuronal maturation, golli-mbp expression decreased and/or there was a striking shift in subcellular localization from nuclei and cell soma to the cell processes in specific neuronal populations. Golli protein was localize in neurites of migrating cerebellar granule cells, but it shifted to a nuclear localization when the cells took up residence in the internal granule cell layer. In some regions, (e.g., olfactory bulb and cerebellum) golli proteins were expressed over the entire postnatal period examined (birth to 75 d). The unique patterns of developmental expression within individual populations of neurons, and the unusual shift in subcellular localization of golli proteins with neuronal migration and maturation, suggest a complex regulation of this gene at both the transcriptional and posttranslational levels. The data also suggest that the cellular function(s) of the golli proteins is very different from the structurally related MBPs.

The myelin proteolipid protein (PLP) gene (i.e., the PLP/DM20 gene) has been of some interest because of its role in certain human demyelinating diseases, such as Pelizaeus-Merzbacher disease. A substantial amount of evidence, including neuronal pathology in knock-out and transgenic animals, suggests the gene also has functions unrelated to myelin structure, but the products of the gene responsible for these putative functions have not yet been identified. Here we report the identification of a new exon of the PLP/DM20 gene and at least two new products of the gene that contain this exon. The new exon, located between exons 1 and 2, is spliced into PLP and DM20 mRNAs creating a new translation initiation site that generates PLP and DM20 proteins with a 12 amino acid leader sequence. This leader sequence appears to target these proteins to a different cellular compartment within the cell bodies of oligodendrocytes and away from the myelin membranes. Furthermore, these new products are also expressed in a number of neuronal populations within the postnatal mouse brain, including the cerebellum, hippocampus, and olfactory system. We term these products somal-restricted PLP and DM20 proteins to distinguish them from the classic PLP and DM20 proteolipids. They represent putative candidates for some of the nonmyelin-related functions of the PLP/DM20 gene.

The myelin basic protein (MBP) gene encodes two families of proteins, the classic MBP constituents of myelin and the golli-MBPs, the function of which is less well understood. In this study, targeted ablation of the golli-MBPs, but not the classic MBPs, resulted in a distinct phenotype unlike that of knock-outs (KOs) of the classic MBPs or other myelin proteins. Although the golli KO animals did not display an overt dysmyelinating phenotype, they did exhibit delayed and/or hypomyelination in selected areas of the brain, such as the visual cortex and the optic nerve, as determined by Northern and Western blots and immunohistochemical analysis with myelin protein markers. Hypomyelination in some areas, such as the visual cortex, persisted into adulthood. Ultrastructural analysis of the KOs confirmed both the delay and hypomyelination and revealed abnormalities in myelin structure and in some oligodendrocytes. Abnormal visual-evoked potentials indicated that the hypomyelination in the visual cortex had functional consequences in the golli KO brain. Evidence that the abnormal myelination in these animals was a consequence of intrinsic problems with the oligodendrocyte was indicated by an impaired ability of oligodendrocytes to form myelin sheets in culture and by the presence of abnormal Ca 2ϩ transients in purified cortical oligodendrocytes studied in vitro. The Ca 2ϩ results reported in this study complement previous results implicating golli proteins in modulating intracellular signaling in T-cells. Together, all these findings suggest a role for golli proteins in oligodendrocyte differentiation, migration, and/or myelin elaboration in the brain.

The myelin basic protein (MBP) gene produces two families of structurally related proteins from three different promoters-the golli products, generated from the most upstream promoter, and the MBPs, produced from the two downstream promoters. In this report we describe the expression of golli proteins within some of the earliest neuronal populations of the brain, including Cajal-Retzius cells and preplate neurons of the forebrain, representing a new marker for these cells. To identify elements responsible for neuronal expression of the golli products, we generated transgenic animals from constructs containing different portions of the upstream promoter. A construct containing 1.1 kb immediately upstream of the golli transcription start site targeted expression of ␤-galactosidase to preplate neurons and a subset of Cajal-Retzius cells in transgenic mice-the first reported genetic element to target expression to these pioneer cortical populations. Although expression in Cajal-Retzius cells declined with embryonic development, preplate cells continued to express the transgene after arriving at their final destination in the subplate. Interestingly, expression persisted in subplate neurons found within a distinct layer between the white matter and cortical layer VI well into postnatal life. Birth dating studies with bromodeoxyuridine indicated that these neurons were born between E10.5 and E12.5. Thus, the transgene marked subplate neurons from their birth, providing a fate marker for these cells. This work suggests a role for the MBP gene in the early developing brain long before myelination and especially in the pioneer cortical neurons important in the formation of the cortical layers.

BackgroundThe “four core genotypes” (FCG) mouse model has emerged as a major model testing if sex differences in phenotypes are caused by sex chromosome complement (XX vs. XY) or gonadal hormones or both. The model involves deletion of the testis-determining gene Sry from the Y chromosome and insertion of an Sry transgene onto an autosome. It produces XX and XY mice with testes, and XX and XY mice with ovaries, so that XX and XY mice with the same type of gonad can be compared to assess phenotypic effects of sex chromosome complement in cells and tissues.FindingsWe used PCR to amplify the Sry transgene and adjacent genomic sequences, to resolve the location of the Sry transgene to chromosome 3 and confirmed this location by fluorescence in situ hybridization (FISH) of the Sry construct to metaphase chromosomes. Using quantitative PCR, we estimate that 12–14 copies of the transgene were inserted. The anogenital distance (AGD) of FCG pups at 27–29 days after birth was not different in XX vs. XY males, or XX vs. XY females, suggesting that differences between XX and XY mice with the same type of gonad are not caused by difference in prenatal androgen levels.ConclusionThe Sry transgene in FCG mice is present in multiple copies at one locus on chromosome 3, which does not interrupt known genes. XX and XY mice with the same type of gonad do not show evidence of different androgen levels prenatally.