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Retinoid X receptor-gamma (RXRgamma) is a transcription factor that mediates retinoid signalling and is expressed in rat heart during adult life. However, its expression in embryonic and neonatal heart has not been investigated and it is not known whether ventricular cardiomyocytes express RXRgamma or whether all- trans -retinoic acid (tRA) and thyroid hormone (T3) could influence RXRgamma transcript levels in these cells. First, in situ hybridization experiments were used to test for any spatio-temporal correlation between RXRgamma gene expression and the previously shown requirement for retinoid signalling in embryonic ventricular cardiomyocytes. It was shown that RXRgamma transcripts are not detectable in embryonic heart at all developmental stages examined under conditions where they are detectable in other embryonic tissues. Second, Northern blotting was used to examine whether there is a difference in RXRgamma transcript levels between neonatal and adult heart. We show that levels of two RXRgamma transcripts are developmentally regulated during the postnatal period because they differ between neonatal and adult hearts. Third, it was demonstrated that primary cultures of neonatal rat ventricular cardiomyocytes express RXRgamma transcripts, making them a novel in vitro system for the study of RXRgamma gene regulation in heart-derived cells. Finally, this system was used to examine whether tRA and T3can influence levels of RXRgamma transcripts because they have been shown to have antagonistic effects in this system and to influence RXRgamma RNA levels in other systems. It was shown by Northern blot experiments, that in this system, RXRgamma transcript levels are differentially influenced by these two hormones. The significance of these findings in relation to previously published work is discussed.

The retinoid X receptor-gamma (RXRgamma) is a transcription factor that belongs to the thyroid hormone/retinoid family of nuclear receptors. Previous studies have shown that RXRgamma is expressed in the developing peripheral nervous system (PNS) of chick embryos, but such expression has not been reported previously in rodent embryos. Indeed, the pattern of RXRgamma expression appears to be different between avian and rodent species. Using in situ hybridisation and northern blot experiments we show that RXRgamma is expressed in elements of the PNS in rat and mouse embryos and in postnatal rats. However, unlike the chick, where RXRgamma is expressed from the onset of neural crest migration, rat RXRgamma expression in the dorsal root ganglia and trigeminal ganglia was detectable at 14.5 days post coitum (dpc), but not at 13, 12, or 11 dpc. These data suggest that RXRgamma may have a role in the dorsal root ganglia and trigeminal ganglia of late embryos, and that this could be evolutionarily conserved between chick and rodent. On the other hand, rat RXRgamma transcripts in the facio-acoustic (VII-VIII) ganglion were detectable at 11 dpc in neural crest cells condensing to form this ganglion. We discuss the possible significance of the timing of RXRgamma expression in the developing PNS and suggest that closer examination of the structure and function of the PNS of RXRgamma null mutant mice would be of interest.

Measurements of aerosol composition, volatile organic compounds, and CO are used to determine biogenic secondary organic aerosol (SOA) concentrations at a rural site 70 km north of Toronto. These biogenic SOA levels are many times higher than past observations and occur during a period of increasing temperatures and outflow from Northern Ontario and Quebec forests in early summer. A regional chemical transport model approximately predicts the event timing and accurately predicts the aerosol loading, identifying the precursors as monoterpene emissions from the coniferous forest. The agreement between the measured and modeled biogenic aerosol concentrations contrasts with model underpredictions for polluted regions. Correlations of the oxygenated organic aerosol mass with tracers such as CO support a secondary aerosol source and distinguish biogenic, pollution, and biomass burning periods during the field campaign. Using the Master Chemical Mechanism, it is shown that the levels of CO observed during the biogenic event are consistent with a photochemical source arising from monoterpene oxidation. The biogenic aerosol mass correlates with satellite measurements of regional aerosol optical depth, indicating that the event extends across the eastern Canadian forest. This regional event correlates with increased temperatures, indicating that temperature-dependent forest emissions can significantly affect climate through enhanced direct optical scattering and higher cloud condensation nuclei numbers.

Even though previous studies have shown that transcripts encoding the murine retinoid X receptor γ (RXRγ) are present in skeletal muscle of mouse embryos and that cultured myoblasts are induced to differentiate upon retinoid treatment, a function for RXRγ and retinoids in mammalian myogenesis has not yet been identified. To begin to understand the possible role of RXRγ during mammalian myogenesis we isolated novel rat RXRγ cDNA sequences and examined in detail the spatio-temporal expression pattern of RXRγ transcripts in relation to skeletal muscle differentiation in rat embryos and cultured myoblasts. We show that the onset of RXRγ expression coincides with the differentiation of limb myoblasts in vivo. In vitro, RXRγ is expressed in differentiating myoblasts, but not in proliferating myoblasts. In the myotome, however, RXRγ is first expressed after myoblast differentiation, with RXRγ transcripts being confined initially to its ventral region. Subsequently, RXRγ becomes expressed throughout limb and myotome-derived muscle masses, and by the end of the primary myogenic wave, RXRγ transcripts are mainly confined to their periphery. This dynamic expression pattern of RXRγ during myogenesis suggests its possible involvement in the differentiation of limb myoblasts but excludes a role in the differentiation of early myotomal myoblasts. 210 227 235 Cited By :10

The major histocompatibility complex (MHC) class II-like molecules, H2-M, have an essential role in processing and presentation of antigens by the MHC class II molecules, because functional inactivation of these genes lead to surface expression of MHC class II molecules devoid of associated foreign peptides. We have used in situ hybridization to examine the expression of MHC class II and H2-Mb genes in embryonic and neonatal mice and show here that expression of H2-Ab and H2-Mb mRNA is absent in 13.5-day-old mice. However, mRNA for both genes could be detected in the thymus of 14.5- and 15.5-day-old embryos, and the patterns of hybridization suggested that the two genes were expressed in the same cells. In spleen and thymus of neonatal mice the H2-Ab and H2-Mb genes were also coexpressed, with expression being localized to the white pulp of the spleen and to the thymic medulla, which are rich in antigen-presenting cells. The steady-state levels of H2-Ab mRNA appeared to be approximately 10 to 14 times greater that the level of H2-Mb mRNA molecules, irrespectively of the tissue and age, reflecting the different functions of the two molecules. 88 447 451 Cited By :1