Maternal obesity programs fetal skeletal muscle development: The possible role of chronic inflammation mediated by toll-like receptor 4 (TLR4)

Maternal obesity is associated with metabolic diseases such as obesity and type 2 diabetes (T2D) in offspring. Skeletal muscle is one of the principle sites for glucose and fatty acid oxidation, as well as the main tissue responding to insulin. Toll-like receptor 4 (TLR4) mediates chronic inflammatory signaling pathways and is associated with obesity and T2D. In the current study, we investigate the possible role of TLR4-mediated chronic inflammation in fetal skeletal muscle and intramuscular fat development, and the possible link of TLR4-mediated chronic inflammation to mitochondrial content and microRNA (miRNA) expression in fetal skeletal muscle.;Using anobese sheep pregnancy model, we first analyzed whether the changes in obese (OB) fetal skeletal muscle we observed previously persist in the musculature of adult offspring. Consistent with our observations in fetal muscle, we observed elevated intramuscular adiposity and fibrosis in offspring muscle of over-nourished mothers, which was associated with attenuated insulin signaling and enhanced inflammatory signaling mediated by TLR4 when compared with control (Con) muscle.We propose that maternal obesityenhancesadipogenesis and fibrogenesis in fetal and offspring muscle, which is expected to impair the physiological function of skeletal muscle.;Next, we used a maternal nutrient excess (MNE) baboon pregnancy model and detected the mitochondrial content and function in fetal skeletal muscle at late gestation. Our results showed that the content of mitochondria was decreased in fetal skeletal muscle of MNE mothers compared with the control (CTR) group. Furthermore, genes related to mitochondrial biogenesis were expressed at a lower level in the MNE group. Similar to our sheep study, we also observed an elevated chronic inflammation in MNE fetal skeletal muscle, including the expression of TLR4. Therefore, we proposed a hypothetical model, in which we believe that MNE first induces the activation of chronic inflammation, which next affects the expression of genes involved in mitochondrial biogenesis, and then results in a decreased mitochondrial content in fetal skeletal muscle. Reduced mitochondrial content in fetal skeletal muscle might account for obesity and T2D in later life.;In addition, we applied a miRNA microarray and analyzed the expression of miRNAs in fetal skeletal muscle during mid-gestation. We found more than 1,000 miRNAs expressing in sheep fetal skeletal muscle, of which several miRNAs were affected by maternal over-nutrition. Our results further showed that miRNA hsa-let-7ghas a potential target which is involved in inflammation. So we over-expressed let-7gin C3H10T1/2 cells, which led to decreased expression of inflammatory cytokines TNFalpha and IL6, and inflammatory signaling mediator, TLR4. It is highly possible that miRNAs play some role in the development of chronic inflammation in fetal muscle of obese sheep. Meanwhile, we also found that miRNA let-7g inhibited the proliferation and adipogenic differentiation of C3H10T1/2 cells, suggesting an important role of miRNA let-7g in adipocyte development.;Furthermore, we used a TLR4 knock out (KO) mouse model and assessed the role of TLR4 in pregnancy outcomes. Our data showed that TLR4 KO female mice were partially resistant to high fat diet (HFD)-induced obesity and insulin resistance. Meanwhile, TLR4 KO mice fed with HFD had a higher pregnancy rate and they were much better in taking care of their pups. In addition, the offspring of HFD TLR4 dams accumulated less subcutaneous and abdominal fat and had relatively higher insulin sensitivity compared with those of HFD C57 mice, especially female offspring.;In summary, our results showed that maternal over-nutrition up-regulated the TLR4-mediated chronic inflammatory signaling in skeletal muscle of offspring. We further demonstrated that elevated inflammatory TLR4 signaling was associated with impaired mitochondria in fetal skeletal muscle in a primate model. Chronic inflammation and TLR4 signaling are partially regulated by miRNA let-7gin the sheep, since a lower level of let-7g was associated with elevated TLR4-mediated inflammatory signaling and over-expression of let-7ghad anti-inflammatory effects. Chronic inflammation mediated by TLR4 may have a critical role in the development of fetal skeletal muscle and adipose tissue due to maternal obesity.