Evaluating the effects of low-dose simulated galactic cosmic rays on murine hippocampal-dependent cognitive performance

Abstract

Space exploration has advanced substantially over recent decades and plans to increase the duration of deep space missions are in preparation. One of the primary health concerns is potential damage to the central nervous system (CNS), resulting in loss of cognitive abilities and function. The majority of ground-based research on space radiation-induced health risks has been conducted using single particle simulations, which do not effectively model real-world scenarios. Thus, to improve the safety of space missions, we must expand our understanding of the effects of simulated galactic cosmic rays (GCRs) on the CNS. To assess the effects of low-dose GCR, we subjected 6-month-old male BALB/c mice to 50 cGy 5-beam simplified GCR spectrum (¹H, ²⁸Si, ⁴He, ¹⁶O, and ⁵⁶Fe) whole-body irradiation at the NASA Space Radiation Laboratory. Animals were tested for cognitive performance with Y-maze and Morris water maze tests 3 months after irradiation. Irradiated animals had impaired short-term memory and lacked spatial memory retention on day 5 of the probe trial. Glial cell analysis by flow cytometry showed no significant changes in oligodendrocytes, astrocytes, microglia or neural precursor cells (NPC's) between the sham group and GCR group. Bone marrow cytogenetic data showed a significant increase in the frequency of chromosomal aberrations after GCR exposure. Finally, tandem mass tag proteomics identified 3,639 proteins, 113 of which were differentially expressed when comparing sham versus GCR exposure (fold change > 1.5; p < 0.05). Our data suggest exposure to low-dose GCR induces cognitive deficits by impairing short-term memory and spatial memory retention.

Keywords: Mars; central nervous system; galactic cosmic radiation; risk; space radiation.

FIGURE 1

Y maze. (A) Sham-irradiated mice (n = 6) had significantly more entries into the novel arm during the testing phase of the Y maze, indicating normal spatial recognition. (B) Irradiated mice (n = 6) (50 cGy GCR) were unable to distinguish between the 3 Y-maze arms, indicating that GCR exposure impaired short-term memory. *P < 0.05.

FIGURE 2

Spatial memory retention during probe trials on day 5 of Morris water maze testing. During the day of probe trials, the sham-irradiated group (n = 6) showed significant preference for the target quadrant; whereas, the irradiated group (n = 6) (50 cGy GCR) was unable to differentiate the target quadrant. *P < 0.0001.

FIGURE 3

Flow cytometry was used to measure changes in astrocytes (ACSA-2–positive), neural precursor cells (PSA-NCAM–positive), microglia (CD11b-positive), and oligodendrocytes (O4-positive) of hippocampal tissue in sham-irradiated (n = 6) and irradiated (n = 6) (50 cGy GCR) mice. There were no significant changes in the proportions of each cell type between either treatment group. NPC, neural precursor cells.

FIGURE 4

Representative photomicrographs of G-banded chromosomes from (A) sham-irradiated and (B) irradiated mice (arrows indicate change in banding pattern). (C) Aberration frequency in sham-irradiated (n = 4) and irradiated (n = 4) mice as estimated by observing change in banding pattern of chromosomes following G-banding. Statistical uncertainty between the groups was determined by t-test.

FIGURE 5

Representative photomicrographs of chromosomes following spectral karyotyping (SKY) hybridization in (A) sham-irradiated and (B) irradiated mice (arrows indicate breaks and translocations). (C) Aberration frequency in sham-irradiated (n = 4) and irradiated (n = 4) mice as estimated by observing change in length or color junction. Statistical uncertainty between the groups was determined by t-test.

FIGURE 6

Representative photomicrographs of chromosomes following centromere hybridization (Kreatech Biotechnology B.V, Amsterdam, Netherland). Each centromere shows 2 signals except Y-chromosome (circled) in both sham-irradiated and irradiated groups. The image was captured under 63x magnification.

FIGURE 7

Graphic representation of mouse hippocampus protein network 1, identified by Ingenuity Pathway Analysis (IPA) as being affected by sham irradiation compared to galactic cosmic rays (GCR) irradiation. Functions associated with the network include free-radical scavenging, cellular assembly and organization, and cellular function and maintenance. The network is overlayed with the disease and function tool to display the 2 key molecules involved with behavior. The node color indicates expression value, and color intensity indicates degree of up-or down-regulation: red indicates upregulation, and green indicates downregulation. Gray nodes are dataset molecules that were not significantly expressed and therefore did not pass the IPA analysis cutoff. Uncolored nodes were not part of our dataset but were incorporated into the pathway based on evidence stored in the Ingenuity Knowledge Base. Known direct and indirect interactions between network proteins, as well as the direction of the interaction, are indicated by arrows or blocked lines. Reproduced with a license obtained from QIAGEN.

FIGURE 8

Graphic representation of mouse hippocampus protein network 1, identified by Ingenuity Pathway Analysis (IPA) as being affected by sham irradiation compared to galactic cosmic rays (GCR) irradiation. Functions associated with the network include free-radical scavenging, cellular assembly and organization, and cellular function and maintenance. The network is overlayed with the disease and function tool to display the 14 key molecules involved with cell morphology. Reproduced with a license obtained from QIAGEN.

FIGURE 9

Graphic representation of mouse hippocampus protein network 1, identified by Ingenuity Pathway Analysis (IPA) as being affected by sham irradiation compared to galactic cosmic rays (GCR) irradiation. Functions associated with the network include free radical-scavenging, cellular assembly and organization, and cellular function and maintenance. The network is overlayed with the disease and function tool to display the 15 key molecules involved with neurological disease.

FIGURE 10

Ingenuity pathway analysis (IPA) legend of protein networks and predicted interactions.

FIGURE 11

Heatmap of proteins identified in network 1 with their corresponding samples. S, Sham; R, Radiation.

FIGURE 12

A chart identifying the GO biological processes for the genes identified in network 1.