HIBERNATION AS A RADIOPROTECTANT: ASSESSING TELOMERE LENGTH IN SPACE-RELEVANT IRRADIATED 13-LINED GROUND SQUIRRELS

Abstract

Chronic exposure to the space radiation environment, which includes Solar Particle Events (SPEs), Galactic Cosmic Rays (GCR) consisting of high-energy protons and heavy charged (HZE) ions, and “trapped radiation” made up of protons and electrons in Van Allen Belts, represent a major hazard of long-duration deep-space flight. As missions to the moon and Mars become realities, developing effective radiation mitigation strategies is critical for protecting astronaut health and performance during a mission. Synthetic torpor mimicking hibernation has shown promise as a potential radioprotective countermeasure. Telomeres, highly conserved repetitive regions that “cap” the ends of chromosomes, shorten with age, and with a variety of lifestyle factors and stressors, including environmental exposures and ionizing radiation. Thus, telomeres can serve as sentinels and informative biomarkers of health and aging trajectories. While telomere elongation has been documented in astronauts during spaceflight onboard the International Space Station (ISS) and in ground-based hibernating mammals, the relationship between telomere length dynamics, hibernation, and space radiation exposure remains unexplored. The current study addresses this gap by investigating whether hibernation serves as a radioprotectant in a natural hibernating mammalian model; specifically, 13-lined ground squirrels (Ictidomys tridecemlineatus) chronically exposed to low dose/low-dose rate space relevant radiation using telomere length as a biological proxy. Average relative telomere length is determined utilizing an optimized (species-specific) quantitative Polymerase Chain Reaction (qPCR)-based assay to assess whether low-dose/low-dose rate space-relevant radiation exposure influences telomere length differently depending on where the animal was in its circannual cycle - active or hibernating season. Telomerase activity, the specialized reverse transcriptase associated with telomere length maintenance, is directly measured using qPCR-Telomeric Repeat Amplification Protocol (TRAP). Active and hibernating squirrels are exposed to a 252Californium mixed neutron, gamma (γ)-ray source at a dose rate of ~1-2 mSv/day over a 6-month period to simulate the space radiation environment during long-duration spaceflight. Torpor is induced during their natural hibernation season by maintaining squirrels at 4-8°C under low light conditions. Subjects are divided into four experimental groups: active control (sham; 0 Gy), active irradiated, hibernating control (sham), and hibernating irradiated. Results reveal longer average telomere length in hibernating irradiated groups compared to active control, active irradiated, and hibernating control groups. In contrast and compared to active animals, telomerase activity is lower in hibernating squirrels. Taken together, findings are suggestive of telomerase-independent pathways/mechanisms of telomere maintenance/elongation in hibernating squirrels. Implications for potential translation to humans and induced/synthetic torpor are addressed.