The Characteristic Response of the Human Leukocyte Transcriptome to 60 days of Bed Rest and to Reambulation.

INTRODUCTION: We sought to isolate the microgravity effect of spaceflight from other space stressors by characterizing the leukocytes' transcriptome of participants to a 60-day bed rest study; an Earth model of microgravity.

METHODS: Twenty healthy men received a nutritional supplement or not and ten blood samples were collected throughout 3 study phases: baseline (BDC-12, BDC-11), head-down tilt bed rest (HDT1, HDT2, HDT30, HDT60), and reambulation (R1, R2, R12, R30). We measured gene expression through RNA sequencing of leukocytes, applied generalized linear models to assess differential expression followed by enrichment analysis to identify temporal changes (Model 1) and to measure the impact of a nutritional supplement (Model 2).

RESULTS: Baseline transcriptomes included 14,624 protein-coding transcripts and showed both high intra-individual correlations (mean Kendall coefficient 0.91 ± 0.04) and inter-individual homogeneity (0.89 ± 0.03). We identified 2,415 differentially expressed protein-coding transcripts grouping into 6 clusters (C1-C6). At phase transitions, clusters showed either a decrease-then-increase (C3 and C5) or an increase-then-decrease (C1, C2, C6) pattern. All 6 clusters converged towards average expression at HDT30 and HDT60. Gene Ontology (GO) terms at baseline related to immune functions while bed rest and reambulation related to sequestration of ions, immune response, cellular stress, and mineralization. The nutritional intervention had no effect.

CONCLUSIONS: The temporal profiles of leukocytes' transcriptomes emphasized the dynamic nature of gene expression occurring during and after bed rest. Enriched biological processes among the differentially expressed genes included immune related and unrelated responses. The convergence towards no differential expression at days 30 and 60 of bed rest suggests a hypometabolic state. Current findings can guide future work on the complex responses and adaptation mechanisms to microgravity.