l organization Development Development Development Development Development Development Structure Immune and stress response Anti tumor Humoral immune response Immune response Oxidative stress response TIMP metallopeptidase inhibitor 3 Diablo homolog Endoglin LIM and senescent cell antigen-like domains 1 Stromal interaction molecule 1 Transforming growth factor, beta 1 Retinoblastoma-like 2 RAS homolog gene family, member g Hcls1 associated protein x-1 Endothelial pas domain protein 1 Kelch domain containing 3 Calpain, small subunit 1 TIMP3 DIABLO ENG LIMS1 STIM1 TGFB1 RBL2 RHOG HAX1 EPAS1 KLHDC3 CAPNS1 1.36 21.25 22.85 1.31 21.60 21.54 1.61 21.04 21.56 21.69 21.57 21.36 1.90 22.18 23.61 3.13 21.95 22.12 3.45 21.65 21.80 22.10 21.77 21.41 Phosphodiesterase 4 d 11741928 interacting protein Myosin-10 Septin 11 22761436 Actin binding lim protein 1 MYBPC1 myosin binding protein C, slow type Nebulin Titin Myocyte enhancer factor 2C Integrin beta 1 binding protein 2 Actin, gamma 2, smooth muscle, enteric DMN desmuslin PDE4DIP MYH10 SEPT11 ABLIM1 MYBPC1 NEB TTN MEF2C ITGB1BP2 ACTG2 DMN 1.30 3.19 2.52 2.23 1.82 1.86 1.96 1.38 22.02 22.72 2.13 1.90 6.22 3.79 3.17 4.14 3.16 8.95 4.90 22.28 26.73 8.41 Dermatan sulfate epimerase Adenosine deaminase Gamma-interferon inducible lysosomal thiol reductase precursor Peroxiredoxin 5 DSE ADA IFI30/GILT PRDX5 4.81 22.21 24.70 21.90 4.49 22.58 25.30 21.90 Denotes statistically significant differences. doi:10.1371/journal.pone.0004481.t001 Decreased expression of ENG and TGFB1 suggests decreased cell turnover in geriatric dogs. ENG or endoglin is a co-receptor for TGFB1 that controls endothelial cell proliferation and angiogenesis. ENG has been identified in muscle satellite cells or myogenic progenitors, but in contrast to our findings, Beggs et al. noted increased expression of ENG in aged MPs compared to adult MPs. Interestingly, STIM1, a gene related to cell growth suppression, was up-regulated in young adult dogs. STIM1 controls intracellular calcium homeostasis in conjunction with STIM2. Calcium is an important second messenger, which may affect cell functions, including growth, and, specifically for muscle, contraction. Moreover, calcium is a co-factor for CAPNS1, which controls the stability of calpains . Therefore, by dependency, calcium may control CAPNS1 and have strong control over cell proliferation and apoptosis. Additionally, the calpain system may play a role in stress or damage response. In general, the up-regulation of genes related to cell cycle and proliferation in young adult dogs is not surprising as they have a higher rate of tissue turnover. For example, HAX1 was increased in human psoriasis patients, indicating its effect on cell proliferation and suppression of apoptosis. Expression of EPAS1, a hypoxia-induced gene, was also decreased in geriatric dogs. A lack of EPAS1 can have severe metabolic consequences, including multiple organ pathogenesis, dysregulated TCA cycle and fatty acid oxidation, and impaired homeostasis of reactive oxygen species. In the healthy 
individuals, EPAS1 stimulates angiogenesis. KLHDC3 bears similarity to RAG2 normally found in the testis where it is thought to play a role in meiotic recombination, but its function in skeletal muscle is unknown. Collectively, these data suggest that cell cycle, Canine Muscle Gene Expression Functional classification Gene name Gene 718630-59-2 symbol Fold change APB PPB Metabolism Amino acid metabolism Carbohydrate metabolism Electron transport Electron tr