Mechano-sensitive Gene Expression with RNA-Seq: Revisiting the Osteocytic Cell Response to Fluid Flow

Bone adaptation to mechanical loading is regulated via signal transduction by mechano-sensing osteocytes. Mineral-embedded osteocytes experience strain-induced interstitial fluid flow and fluid shear stress, and broad shifts in gene expression are key components in the signaling pathways that regulate bone turnover. RNA sequencing analysis, or RNA-Seq, enables more complete characterization of mechano-sensitive transcriptome regulation than previously possible. We hypothesized that RNA-Seq of osteocytic MLO-Y4 cells reveals both expected and novel gene transcript regulation in cells previously fluid flowed and analyzed using gene microarrays (Govey et al., J Biomech, 2014). MLO-Y4 cells were flowed for 2 h with 1 Pa oscillating fluid shear stress and post-incubated 2 h. RNA-Seq of original samples detected 58 fluid flow-regulated gene transcripts (p-corrected<0.05) versus 65 transcripts detected by microarray. However, RNA-Seq demonstrated greater dynamic range, with all 58 transcripts >1.5 fold-change whereas 10 of 65 met this cut-off by microarray. Analyses were complimentary in patterns of regulation, though only 6 transcripts were significant in both analyses: Cxcl5, Cxcl1, Zc3h12a, Ereg, Slc2a1, and Egln1. As part of a broad inflammatory response inferred by gene ontology analyses, we again observed greatest up-regulation of inflammatory C-X-C motif chemokines, and newly implicated HIF-1? and AMPK signaling pathways. Importantly, we detected both expected mechano-sensitive transcripts (e.g. Nos2, Ptgs2, Ccl7) and transcripts not previously identified as mechano-sensitive, e.g. Ccl2. We found RNA-Seq advantageous over microarrays because of its ability to analyze unbiased estimation of gene expression, informing our understanding of osteocyte signaling. Overall design: Osteocyte-like MLO-Y4 cells were subjected to 2 hours of 10 dyn/cm^2 oscillating fluid flow in parallel-plate fluid flow chambers and harvested for analysis after an additional 2 hours post-flow incubation in fresh medium. Parallel control samples from sham treated cells were also collected. Triplicate samples of both flow and non-flow control conditions were collected to analyze flow vs. non-flow gene transcript regulation.

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Meng F, Murray GF, Kurgan L, Donahue HJ