Kristian Svensson

Expression of a novel member of estrogen response element-binding nuclear receptors is restricted to the early stages of chorion formation during mouse embryogenesis

Members of the nuclear hormone receptor gene family of transcription factors have been shown to be expressed in characteristic patterns during mouse organogenesis and postnatal development. Using an RT-PCR based screening assay, we have identified nuclear receptors expressed in embryonal carcinoma stem cells. One of the cDNAs characterized, mERR-2, was found to be expressed exclusively during a narrow developmental window in trophoblast progenitor cells between days 6.5 and 7.5 post coitum (p.c.). From 8.5 days p.c. and onwards, the mERR-2 gene activity evaded detection as analysed by in situ hybridization. We also show that the mERR-2 gene product and the estrogen receptor share a common target DNA-sequence recognition specificity unique among members of the gene family. Furthermore, efficient homodimerization and DNA-binding of the orphan receptor mERR-2 was found to be dependent on interaction with the heat shock protein 90, a molecular chaperone hitherto recognized to interact only with the steroid hormone receptor subgroup of nuclear receptors. Based on our results we suggest that the mouse orphan receptor mERR-2 has the potential to regulate overlapping gene networks with the estrogen receptor and may participate in signal transduction pathways during a short developmental period coinciding with the formation of the chorion.

A novel pleckstrin homology-related gene family defined by Ipl/Tssc3, TDAG51, and Tih1: tissue-specific expression, chromosomal location, and parental imprinting

Abstract. We previously described a gene, Ipl (Tssc3), that is expressed selectively from the maternal allele in placenta, yolk sac, and fetal liver and that maps within the imprinted domain of mouse distal Chromosome (Chr) 7/human Chr 11p15.5 (Hum Mol Genet 6, 2021, 1997). Ipl is similar to TDAG51, a gene that is involved in FAS/CD95 expression. Here we describe another gene, Tih1 (TDAG/Ipl homologue 1), with equivalent sequence similarity to Ipl. Structural prediction indicates that the products of these three genes share a central motif resembling a pleckstrin-homology (PH) domain, and TIH1 protein has weak sequence similarity to the PH-domain protein SEC7/CYTOHESIN. Like Ipl, Tih1 is a small gene with a single small intron. Tih1 maps to distal mouse Chr 1 and human Chr 1q31, chromosomal regions that have not shown evidence for imprinting and, in contrast to Ipl, Tih1 is expressed equally from both parental alleles. Ipl, Tih1, and TDAG51 have overlapping but distinct patterns of expression. Tih1 and TDAG51 are expressed in multiple fetal and adult tissues. In contrast, during early mouse development Ipl mRNA and protein are highly specific for two tissues involved in maternal/fetal exchange: visceral endoderm of the yolk sac and labyrinthine trophoblast of the placenta. These findings highlight the dominance of chromosomal context over gene structure in some examples of parental imprinting and extend previous evidence for placenta-specific expression of imprinted genes. The data also define a new subfamily of PH domain genes.

Overexpression of Semicarbazide-Sensitive Amine Oxidase in Smooth Muscle Cells Leads to an Abnormal Structure of the Aortic Elastic Laminas

Elevated semicarbazide-sensitive amine oxidase (SSAO) activity has been observed in several human conditions, eg, diabetes, and it has been speculated that SSAO contributes to the development of vasculopathies associated with this disease. To investigate in vivo consequences of elevated expression of SSAO in vascular tissues, we have developed a transgenic model for overexpression of human SSAO in mice. A smooth muscle-specific promoter, smooth muscle alpha-actin promoter 8 (SMP8) was used. Transgenic expression of human SSAO in tissues with a high content of smooth muscle cells was confirmed by Northern blot analysis. Enzymatic analysis of homogenates from transgenic tissues showed elevated levels of SSAO activity compared to non-transgenic littermates. Furthermore, when plasma SSAO activity was analyzed, much higher activity was detected compared to plasma from control mice, indicating that plasma SSAO may originate from smooth muscle cells. Histopathological evaluation of aorta and renal artery from transgenic mice revealed an abnormal structure of the elastin tissue. Instead of the regularly folded elastic laminae normally found in tunica media of sacrificed mice, the elastic laminae were straight and unfolded with irregularly arranged elastic fibers, forming tangled webs, between the intercalating elastic laminae. These alterations of the elastin structures suggest that overexpression of SSAO has led to a reduced elasticity of the arteries. Moreover, the mean femoral arterial pressure of the SMP8 SSAO transgenic mice was significantly lower in comparison to non-transgenic littermates. This suggests that the transgenic mice have a defect in their ability to regulate blood pressure.

H19 is imprinted in the choroid plexus and leptomeninges of the mouse foetus

It has been proposed that either the Igf-2 gene or the H19 gene--but not both--can be expressed from a given chromosome. Igf-2 is known to be biallelically expressed in the choroid plexus and leptomeninges, however, raising the question of whether H19 is down-regulated or absent there. We found by in situ hybridization that H19 is indeed expressed in the choroid plexus and leptomeninges of the developing mouse foetus. Comparison with the expression pattern of Igf-2 showed that the genes are coexpressed in all areas, with the exception of the choroid plexus epithelium. To evaluate whether H19 is also biallelically expressed in these tissues, we microdissected embryos from interspecific crosses and performed RNAse protection analysis on the isolated RNA. This revealed that H19 maintains its imprint in the choroid plexus/leptomeninges, being transcribed from the maternal allele at a level comparable to that in normal liver. We discuss the significance of these results for current models of Igf-2 and H19 imprinting.

Oligodendrocyte precursor hypercellularity and abnormal retina development in mice overexpressing PDGF-B in myelinating tracts

Platelet‐derived growth factor (PDGF) influences the generation of neurons and glia during embryogenesis and in early postnatal life. In an attempt to determine the consequences of an overexpression of PDGF‐B during the first weeks of life, we targeted transgenic expression of a human PDGF‐B cDNA to myelinating tracts using the promoter region of the myelin basic protein (MBP) gene. Transgenic mRNA and protein were expressed in the brain and the expression profile of the human PDGF‐B during early postnatal development closely paralleled that of the endogenous mouse MBP gene. The gross morphological appearance of transgenic brains was normal but at the cellular level several phenotypic alterations could be identified. In white matter tracts such as the corpus callosum and cerebellar medulla, there was a marked hypercellularity. The number of oligodendrocyte precursors was increased and astrocytes were more abundant. In adult mice carrying the MBP–PDGF‐B transgene, however, myelination appeared normal and the amount of oligodendrocytes was similar to that of control littermates. In addition to the phenotypic alterations in the brain, investigation of eye structure revealed a striking disorganization of retinal architecture. The retina was folded with cells collected in papillar or follicular‐like structures. Retinal whole mount preparations after India ink perfusion revealed capillary disorganization with large‐caliber vessels supporting only a few fine branches. Our observations strengthen the notion that PDGF is an important effector molecule in postnatal CNS development. GLIA 41:276–289, 2003.

The genotype and epigenotype synergize to diversify the spatial pattern of expression of the imprinted H19 gene.

Little is known of how the genetic background effects the phenomenon of genomic imprinting. The H19 gene belongs to a cluster of imprinted genes on human chromosome 11. Here we show that the alternative splicing of a human H19 transcript is genotype-specific. Moreover, this variant transcript, which lacks exon 4, is either not found at all, is widely expressed or is confined to extra-villous cytotrophoblasts in first trimester placenta, depending on a combination of the genotype and the sex of the transmitting parent.

Allele-specific in situ hybridization (ASISH).

An unexpected outcome of the diploid genome is that evolutionary strategies have evolved to express only one of the alleles (1). The rapidly expanding list of genes that are expressed monoallelically fall into three main categories: random inactivation, allelic exclusion, and genomic imprinting. These categories are distinguished by whether the expressed and inactivated alleles are maintained from one cell division to the next (random inactivation occurs with each cell division, compared to stable propagation through subsequent cell divisions as seen for allelic exclusion and genomic imprinting) and whether allele inactivation or expression is determined by parent of origin of inheritance (this differentiates allelic exclusion from genomic imprinting). Genomically imprinted genes are currently more numerous than members of the other categories. This fact is likely to reflect that the persistent (in)activation of one allele in a parent of origin-specific manner has facilitated the detection of monoallelic expression patterns in RNA extracted from homogenized tissue. This crude approach does not take into account, however, different imprinted states within a tissue, or, indeed, random allelic (in)activation. It is not surprising, therefore, that well-known genes, such as Il2 (2), were only recently found to be monoallelically expressed in a random manner (1).