4. Skeletal Muscle in Inbred Mice Exhibits Divergent Hypoxia and Angiogenesis Transcriptome Expression as Compared to Humans in Aging

Abstract

Hypoxia signaling has been previously demonstrated to be a critical regulator in age-dependent skeletal muscle regeneration. While animal models provide a crucial scientific substrate for medical innovation and translation, discordant findings have been commonly described in existing literature between animals and humans. The nature and extent of this divergence in the context of aging and its effect on skeletal muscle regeneration remains incompletely understood. Quadriceps muscles from young (Y, 12 weeks, n=3) and old (O, 25 months, n=3) C57BL/6 mice were harvested and analyzed via RNA bulk-sequencing. These results were compared to healthy human vastus lateralis muscles (Y, 22-30 years, n=8; O, 80-83 years, n=5) from the GESTALT study (NIA/NIH) analyses as found in the Gene Expression Omnibus (NIH) database. Gross examination of the most differentially expressed genes demonstrated low variance between samples within each mouse cohort (Disp = 0.04467, BCV = 0.2114) relative to young/old human (Disp = 0.61261, BCV = 0.7827) specimens. Enrichment analyses were agnostically performed on filtered differentially expressed human genes via edgeR (Bioconductor) resulting in significant overlap with the HIF-1 signaling pathway KEGG term (Eno1, Tfrc, Ldha, Eno3, and Pdha1) with significant upregulation in young human muscles. However, these same genes demonstrated no significant differences within mice. Interestingly, when examining angiogenesis gene lists determined a priori, mice demonstrated several significantly upregulated genes in the young cohort (Anxa2, Lox, Mmp9, Angptl4, Adora2b, and Pgf) that thematically correspond to modulators of the extracellular matrix in the process of neoangiogenesis. Conversely, none of these genes were found to be significantly different between old and young humans. To further compare the muscle transcriptomes between humans and mice in the context of aging, genes with the largest magnitude fold changes and most significant p-values were identified. The most significant differentially expressed genes in mice demonstrated robust upregulation of Mss51, Igkv3-2, Ighg2c, Rasd2, Igkc, Eda2r, and Gdf11 in old mice. These same genes in human specimens demonstrated no comparable segregation. Similarly, the most upregulated genes in old human specimens (Gda, Mtcybp32, Fgf7, Fam83b, Lyve1, Plag1, Eda2r, Dkk2, and Col21a1) showed no similar segregations in mice. The most differentially expressed genes for hypoxia and angiogenesis in both mice and humans were not found to be concordantly upregulated when cross-examined across the other species. Thus, skeletal muscle in inbred mice exhibits differential transcriptome expression and decreased variance as compared to humans in aging. Given these findings, great care should be taken in making inferences between findings between mice and humans. As NextGen sequencing becomes routine, incorporation of genetically diverse mice to better approximate human genetic variation may provide further insights in translation from bench to bedside.