Abstract 11815: Prolonged Space Flight Induced Alterations in the Structure of Medaka Cardiac Muscle and Gene Expression

Abstract

Introduction: Cardiovascular deconditioning (reduction of ventricular mass and blood pressure) induced by prolonged spaceflight. Little has been reported about histology and gene expression pattern of heart after long time in the international space station (ISS). The purpose of this study is to examine how microgravity affects the structure and gene expression of medaka cardiac muscle. Methods: Fifteen medaka fishes were sent to ISS. The fish lived in a space aquarium of ISS. Control medaka lived in same aquarium on the ground. After 2 weeks and 2 months of swimming in microgravity, medaka fish (N=5, each) were fixed in 4% paraformaldehyde at ISS. Cardiac morphology and structure were assessed by hematoxylin and eosin (HE), Azan and immuno-staining. The remaining 5 medaka fish, after 2 months at ISS, were treated by RNAlater for preservation of RNA for RT-PCR and RNA-Seq analysis. Results: The fishes at ISS were swimming in loops rather than straight line and tend to standstill. The body sizes were normal. Spaceflight heart looked normal without apparent histological abnormalities in 2 weeks samples. However, 2 month sampled spaceflight heart showed thinner cardiac fibers, exhibited circular lacunas and higher cardiac fibrosis. Using RNA-seq and RT-PCR, mRNA expression of cardiac contractile genes (myosin heavy chain, myosin light chain II and troponin T) were decreased in spaceflight heart. Interestingly, autophagy related genes (atg3, atg4, atg14 and atg16) expression were higher in spaceflight heart. However, Atg genes expressions were not changed in skeletal muscle. Although, myocardium mass was lower in spaceflight heart, mRNA expressions of muscle-specific ubiquitin ligases (MuRF1 and Atrogin1) and brain natriuretic peptide (BNP) were normal. Immuno-staining showed increased ULK1 expression in 2 weeks and LC3-II level (autophagosomes formation subtype) in 2 months spaceflight samples. Prolonged weightlessness produced substantial loss of cardiac fiber mass by autophagy. Conclusion: Microgravity increases autophagy related genes and decreases sarcomeric protein gene expression, which lead cardiac atrophy after prolonged spaceflight.