Exposure to a Single Dose of Space-Relevant Proton Radiation Alters the Intestinal One-Carbon Metabolism Pathway and Microbiome in Mice

Space radiation, primarily originating from galactic cosmic rays, is mainly composed of protons. Given NASA's plans for manned lunar and Mars missions, it is critical to assess the risk of proton radiation in disrupting tissue homeostasis, including in the intestine, which is a highly radiosensitive organ that harbors trillions of bacteria on the luminal surface. One-carbon metabolism encompasses the folate and methionine cycle and plays a crucial role in maintaining tissue homeostasis by regulating methylation, reductive metabolism, and nucleotide synthesis. However, the effects of proton radiation on intestinal one-carbon metabolism and the luminal microbiome profile are unknown. To address this, 6-month-old male C57BL/6J mice were exposed to a single dose of 0.5 Gy or 1.0 Gy of protons (150 MeV/n; dose rate = 35-55 cGy/min). Nine months after irradiation, significant shifts in the one-carbon metabolism pathway were detected in the mouse proximal jejunum and colon. These changes were exhibited as a loss of intra-intestinal methionine, s-adenosylmethionine, and glutathione tissue concentrations, with more pronounced effects being observed in the proximal jejunum compared to the colon. This resulted in the loss of DNA methylation within long-interspersed nucleotide element-1 (LINE-1), indicative of a global hypomethylative phenotype. Molecular changes were characterized by substantial dysregulation of gene expression in the proximal jejunum, where the most pronounced changes were associated with the dramatic loss of Nos2 expression and reactivation of Casp14, suggesting potential shifts in amino acid utilization and restoration of epithelial barriers in the gut. Furthermore, claudins Cldn5, Cldn6, and Cldn10 were substantially modulated in the proximal jejunum of exposed mice. Gross shifts in the microbiota profiles were exhibited as increases in both overall richness and diversity, however, at the expense of commensal bacterial species, like Akkermansia. The extent of the observed alterations was not congruent with the relatively low doses used in the study, the late time-point, and the overall lack of histomorphological alterations. Altogether, our findings demonstrate that exposure to space-relevant proton radiation causes substantial and persistent changes in the mouse gut. The degree and nature of the observed effects suggest the potential for negative health consequences after exposure to proton radiation during deep space exploration.