In late September 2018, we irradiated at the NASA Space Radiation Laboratory (NSRL) and the Brookhaven National Laboratory (BNL) gamma facility 660 ApoE null male mice and 480 C57Bl/6J male mice, the first two cohorts of our longitudinal lifetime studies. We completed and performed cardiac function analyses and harvested tissues and blood for 5 harvesting time points (14, 28, 365, 440, and 660 days post-irradiation). In terms of selection of radiation parameters, we utilized the following doses, energies, and ions, as recommended by Space Radiation Element Management:
(i) Gamma IR - 1, 2, and 4 Gy, using ApoE null MALE mice for all time points indicated above. (ii) 5-Ion Simplified Mixed field IR - 0.5, 1.0 and 1.5 Gy, adjusted to 500 MeV/n, using ApoE null MALE mice for all time points indicated above. (iii) Gamma IR – 1, 2 Gy, using WT-C57BL/6J MALE mice for all time points indicated in section above. (iv) 5-Ion Simplified Mixed field IR - 0.5, 1.0 Gy, adjusted to 500 MeV/n, using WT-C57BL/6J MALE mice for all time points indicated above.
Lifetime tumor burden As part of our study to assess the effects of space radiation on cardiovascular disease (CVD) risks, male mice were systematically examined for tumor development during scheduled tissue collections over 660 days after initial radiation (IR) exposure. We report:
• The incidence of tumors is higher in wild type (WT) compared to ApoE null male mice after the same doses of gamma- and simGCRsim-IR suggesting underlying genotypic variance may attenuate pathways involved in tumorigenesis;
• The highest number of tumors during the lifetime of WT male mice was detected in the no-IR, Western diet-fed (WD-fed) group, suggesting the role of high fat diet in the development of internal organ tumors, especially in the liver;
• In WT male mice, the incidence of IR-induced internal organ tumors was higher in 100 cGy gamma-IR versus 50 cGy simGCRsim-IR, suggesting higher carcinogenic potential of gamma-IR at these doses;
• In WT male mice, the liver is the most affected organ by tumor growth, followed by the spleen, and lung;
• There was a higher incidence of hepatic and splenic tumors in mice fed with WD or exposed to both types of IR in WT mice approaching end of life;
• In ApoE null male mice, the incidence of IR-induced tumors was higher in 50 cGy simCGRsim -IR mice with a higher prevalence of lung tumors.
In October 2020, we have irradiated at the NSL and the BNL gamma facility an additional 160 ApoE null female mice and 300 C57BL/6J female mice. This longitudinal life-time study is designed to have 5 harvesting time points (28, 180, 365, 440, and 660 days). We have already performed cardiac function analyses and harvested tissues for 4 of our harvesting time points (28, 180, 365, and 440 days). The final collecting time point is scheduled for July 2022. In terms of selection of radiation parameters, we utilized the following doses, energies, and ions, as recommended by Space Radiation Element Management:
(i) Gamma IR – 1 Gy, using ApoE null and WT FEMALE mice for all time points indicated above. (ii) 5-Ion Simplified Mixed field IR - 0.5 Gy, adjusted to 500 MeV/n, using ApoE null FEMALE mice. (iii) 5-Ion Simplified Mixed field IR - 0.25, 0.50 cGy adjusted to 500 MeV/n, using WT-C57BL/6J FEMALE mice for all time points indicated above.
Left ventricular (LV) cardiac response to IR.
Cardiac function was assessed non-invasively in all control and irradiated ApoE null and C57BL/6J female mice by transthoracic echocardiography at 28, 180, and 365 days post-IR. Each mouse is microchipped and followed longitudinally.
To briefly summarize our male cohort echocardiography findings:
• A single full-body IR at doses of 100-200 cGy for gamma- IR and 50-100 cGy for simGCRsim-IR decreases global LV systolic function in WT male mice at 14, 28, and 365 days post-IR.
• At 660 days post-IR, 50 cGy simGCRsim-IR WT male mice exhibited increased diastolic stiffness paired with alterations in LV size and mass, suggesting these mice may be exhibiting additional diastolic dysfunction and compensation as a result of pressure overload.
• In ApoE null male mice, global LV systolic function is impaired as early as 14 and 28 days post-IR in both simGCRsim (100, 200, 400 cGy) and gamma- IR (100, 200 cGy) mice. Interestingly, there is no intermediate time point (365, 440 days) where LV dysfunction is noted.
• By 660 days post-IR, gamma-IR ApoE null male mice exhibit significant systolic dysfunction with reduction in Left ventricular ejection fraction (LVEF), left ventricular fractional shortening (LVFS), and increases in stroke volume (SV), LV mass, and LV dimensions suggesting compensation for likely volume overload. No significant changes in LV function were noted in simGCRsim-IR ApoE null male mice at 660 days.
• No clear dose-response was observed in these studies.
In our female longitudinal cohort:
• A single full-body IR at doses of 100 cGy for gamma-IR and 25 or 50 cGy for simGCRsim-IR decreases the global systolic function of the heart in both ApoE null and WT female mice at 28 days post-IR.
• At later time points (180, 365 days), no significant alterations in global LV systolic function are noted; however, in WT female mice, there is evidence of alterations in LV structure suggesting ongoing remodeling.
• Further work is underway to collect additional data to assess long-term degenerative effects of IR (440, 660 days) in female WT and ApoE null mice. Analysis of additional echocardiography parameters to assess LV remodeling are underway.
These findings do not exclude the possibility of increased acute or degenerative CVD risks at lower doses of space-type IR and/or when combined with other space travel-associated stressors, such as microgravity.