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BACKGROUND: Maternal exercise (ME) improves fetal and offspring muscle development, but mechanisms remain to be established. Since the thyroid hormone (TH) is critical for cell differentiation during embryonic development, we hypothesized that ME elevates TH receptor (THR) signaling in embryos, which promotes embryonic myogenesis. METHODS: Female mice were exercised daily on a treadmill or received a daily TH, triiodothyronine (T3) injection. Embryos (embryonic day 12.5 [E12.5]) and P19 cells were used for studying effects of TH on embryonic myogenesis. TH levels in serum and embryos after ME or T3I were analyzed. Expression of TH signaling related genes and myogenic genes was assessed. THRα binding to the promoters of myogenic genes was investigated by chromatin immunoprecipitation-qantitative polymerase chain reaction (ChIP-qPCR). A CRISPR/CAS9 plasmid was utilized to knock out THRα in P19 cells. RESULTS: ME elevated TH levels in both maternal circulation and embryos, which were correlated with enhanced TH signaling and myogenesis. At E12.5, both myogenic determinants (Pax3, Pax7) and myogenic regulatory factors (Myf5, Myod) were upregulated in ME embryos. ME increased THRα content and elevated messenger RNA (mRNA) expression of TH transporter Slc16a2 and deiodinase Dio2. In addition, the THRα binding to the promoters of Pax3/7 was increased. In P19 embryoid bodies, T3 promoted myogenic differentiation, which was abolished by ablating THRα. Furthermore, maternal daily injection of T3 at a level matching exercised mothers promoted embryonic myogenesis. CONCLUSIONS: ME promotes TH delivery to the embryos and enhances embryonic myogenesis, which is partially mediated by enhanced TH signaling in ME embryos.

X chromosome inactivation (XCI) in females needs to be maintained in newly synthesized chromosome during each cell proliferation cycle, and improper XCI may reactivate X chromosome oncogenes which links to carcinogenesis. However, mechanisms maintaining XCI during cell proliferation remain unclear. During XCI, PRC1 canonical or PRC1-like proteins are assumed to ubiquitinate H2A, which forms a "reversibly" silenced chromatin, while during the maintenance stage of XCI, macroH2As instead of H2As are enriched in the inactivated X chromosome to form a stable silencing status. Recently studies suggested that Breast cancer type 1 (BRCA1), a tumor suppressor commonly mutated in breast cancer cells, can catalyze macroH2A ubiquitination, which prompted us to hypothesize that BRCA1 instead of PRC1 mediates macroH2A ubiquitination and maintains XCI during cell proliferation. Based on RNA immunoprecipitation and confocal microscopic imaging, we found that BRCA1 directly interacts with XIST, a key LncRNA mediating XCI, which colocalizes with mH2A in inactivated X chromosome. Loss of either XIST or BRCA1 decreased the ubiquitination level of macroH2A, suppressing DNA methylation of X chromosome genes, which leads to failure of heterochromatin formation in proliferating breast cancer and mammary gland epithelial cells. Furthermore, this ubiquitination by BRCA1 was not replaceable by PRC1. Using domain-specific deletion, the interaction between XIST and BRCA1 was narrowed to exon 2-7. Our data showed that BRCA1 interacts with XIST to ubiquitinate the macroH2A on X chromosome, which is required for XCI maintenance in mammary gland epithelial cells during cell proliferation. These data provide a novel mechanism explaining the frequent BRCA1 mutations occurred during breast carcinogenesis.