Transcriptomic analysis identifies a role of PI3K–Akt signalling in the responses of skeletal muscle to acute hypoxia in vivo

Abstract

Key points: Changes in gene expression that occur within hours of exposure to hypoxia in in vivo skeletal muscles remain unexplored. Two hours of hypoxia caused significant down-regulation of extracellular matrix genes followed by a shift at 6 h to altered expression of genes associated with the nuclear lumen while respiratory and blood gases were stabilized. Enrichment analysis of mRNAs classified by stability rates suggests an attenuation of post-transcriptional regulation within hours of hypoxic exposure, where PI3K–Akt signalling was suggested to have a nodal role by pathway analysis. Experimental measurements and bioinformatic analyses suggested that the dephosphorylation of Akt after 2 h of hypoxic exposure might deactivate RNA-binding protein BRF1, hence resulting in the selective degradation of mRNAs. Abstract: The effects of acute hypoxia have been widely studied, but there are few studies of transcriptional responses to hours of hypoxia in vivo, especially in hypoxia-tolerant tissues like skeletal muscles. We used RNA-seq to analyse gene expression in plantaris muscles while monitoring respiration, arterial blood gases, and blood glucose in mice exposed to 8% O2 for 2 or 6 h. Rapid decreases in blood gases and a slower reduction in blood glucose suggest stress, which was accompanied by widespread changes in gene expression. Early down-regulation of genes associated with the extracellular matrix was followed by a shift to genes associated with the nuclear lumen. Most of the early down-regulated genes had mRNA half-lives longer than 2 h, suggesting a role for post-transcriptional regulation. These transcriptional changes were enriched in signalling pathways in which the PI3K–Akt signalling pathway was identified as a hub. Our analyses indicated that gene targets of PI3K–Akt but not HIF were enriched in early transcriptional responses to hypoxia. Among the PI3K–Akt targets, 75% could be explained by a deactivation of adenylate–uridylate-rich element (ARE)-binding protein BRF1, a target of PI3K–Akt. Consistent decreases in the phosphorylation of Akt and BRF1 were experimentally confirmed following 2 h of hypoxia. These results suggest that the PI3K–Akt signalling pathway might play a role in responses induced by acute hypoxia in skeletal muscles, partially through the dephosphorylation of ARE-binding protein BRF1.