Gene-Metabolite Network Linked to Inhibited Bioenergetics in Association With Spaceflight-Induced Loss of Male Mouse Quadriceps Muscle

Abstract

Prolonged residence of mice in spaceflight is a scientifically robust and ethically ratified model of muscle atrophy caused by continued unloading. Under the Rodent Research Program of the National Aeronautics and Space Administration (NASA), we assayed the large-scale mRNA and metabolomic perturbations in the quadriceps of C57BL/6j male mice that lived in spaceflight (FLT) or on the ground (control or CTR) for approximately 4 weeks. The wet weights of the quadriceps were significantly reduced in FLT mice. Next-generation sequencing and untargeted mass spectroscopic assays interrogated the gene-metabolite landscape of the quadriceps. A majority of top-ranked differentially suppressed genes in FLT encoded proteins from the myosin or troponin families, suggesting sarcomere alterations in space. Significantly enriched gene-metabolite networks were found linked to sarcomeric integrity, immune fitness, and oxidative stress response; all inhibited in space as per in silico prediction. A significant loss of mitochondrial DNA copy numbers in FLT mice underlined the energy deprivation associated with spaceflight-induced stress. This hypothesis was reinforced by the transcriptomic sequencing-metabolomics integrative analysis that showed inhibited networks related to protein, lipid, and carbohydrate metabolism, and adenosine triphosphate (ATP) synthesis and hydrolysis. Finally, we discovered important upstream regulators, which could be targeted for next-generation therapeutic intervention for chronic disuse of the musculoskeletal system.

Alternative splicing regulates the physiological adaptation of the mouse hind limb postural and phasic muscles to microgravity

“…Therefore, it is unlikely that the transcriptome alterations we characterized are simply a product of the age of the mice employed. In fact, investigations of microgravity-induced differential gene expression in younger mice (Gambara et al, 2017;Chakraborty et al, 2020) showed downregulation of metabolic and mitochondrial pathways similar to what we identified in our older mice. Therefore, while AS has not been adequately investigated in the context of spaceflight, it can be speculated that extensive microgravity-induced AS would be identified regardless of age.…”

“…Therefore, the presence of a microgravity-induced slow-to-fast fiber transition suggests that, despite the opposing force of aging on fiber type composition, the slow-to-fast fiber plasticity of skeletal muscle fibers is maintained even at an increased age. In contrast to fiber transitions, aging mirrors the effect of microgravity on muscle atrophy (Ohlendiek, 2011); it is possible that the advanced age of our mice compared to early age mice in other studies (Harrison et al, 2003;Gambara et al, 2017;Camerino et al, 2013;Chakraborty et al, 2020) amplified the microgravity-induced atrophy that was observed here. Most important to our work, however, is the relationship between modes of transcriptomic regulation and age.…”

“…While our experiments employed adult mice (32 weeks), it is unlikely that age alone can account for our findings. Previous characterizations of the musculoskeletal adaptation to spaceflight have occurred in the context of mice ranging from 8-20 weeks at the time of spaceflight (Harrison et al, 2003; Gambara et al, 2017; Camerino et al, 2013; Chakraborty et al, 2020), requiring inquiry as to the role age may have played in generating the phenotypes we observed and the transcriptomic alterations we characterized.…”

“…There is evidence in humans (Trappe et al, 2009) as well as mice (Sandonà et al, 2012), rats (Martin et al, 1985), and monkeys (Shenkman et al, 1994) to support microgravity-induced downregulation of the myosin heavy chain gene MYHC7 , which encodes the slow twitch muscle fiber protein MYHC I and upregulation of the myosin heavy chain genes MYH2, MYH1, and MYH4 which encode fast twitch muscle fiber proteins MYHC IIA, MYHC IID/X, and MYHC IIB, respectively. In addition, microgravity-induced DGE has been shown to regulate gene networks related to protein, lipid, and carbohydrate metabolism as well as mitochondrial dysfunction in skeletal muscle (Chakraborty et al, 2020). However, a focus restricted to the level of gene expression fails to fully characterize the amplifying actions of alternative splicing (AS) on the host’s transcriptome; for example, AS can affect a multi-fold increase in the diversity translatable mRNA isoforms over what can be accounted for by the 18,000 genes in the human genome.…”

Global transcriptomic analysis of a murine osteocytic cell line subjected to spaceflight

“…66 Our findings support emerging spaceflight evidence that altered bioenergetic forms a universal cellular response to microgravity. 67 To further identify novel mechanosensitive gene(s), we compared gene datasets available in the NASA GeneLab database. We identified two upregulated genes and eight downregulated genes in-flight vs ground (Table 1).…”