Characterization of mouse ocular response to a 35-day spaceflight mission: Evidence of blood-retinal barrier disruption and ocular adaptations

Abstract

The health risks associated with spaceflight-induced ocular structural and functional damage has become a recent concern for NASA. The goal of the present study was to characterize the effects of spaceflight and reentry to 1 g on the structure and integrity of the retina and blood-retinal barrier (BRB) in the eye. To investigate possible mechanisms, changes in protein expression profiles were examined in mouse ocular tissue after spaceflight. Ten week old male C57BL/6 mice were launched to the International Space Station (ISS) on Space-X 12 at the Kennedy Space Center (KSC) on August, 2017. After a 35-day mission, mice were returned to Earth alive. Within 38 +/− 4 hours of splashdown, mice were euthanized and ocular tissues were collected for analysis. Ground control (GC) and vivarium control mice were maintained on Earth in flight hardware or normal vivarium cages respectively. Repeated intraocular pressure (IOP) measurements were performed before the flight launch and re-measured before the mice were euthanized after splashdown. IOP was significantly lower in post-flight measurements compared to that of pre-flight (14.4–19.3 mmHg vs 16.3–20.3 mmHg) (p < 0.05) for the left eye. Flight group had significant apoptosis in the retina and retinal vascular endothelial cells compared to control groups (p < 0.05). Immunohistochemical analysis of the retina revealed that an increased expression of aquaporin-4 (AQP-4) in the flight mice compared to controls gave strong indication of disturbance of BRB integrity. There were also a significant increase in the expression of platelet endothelial cell adhesion molecule-1 (PECAM-1) and a decrease in the expression of the BRB-related tight junction protein, Zonula occludens-1 (ZO-1). Proteomic analysis showed that many key proteins and pathways responsible for cell death, cell cycle, immune response, mitochondrial function and metabolic stress were significantly altered in the flight mice compared to ground control animals. These data indicate a complex cellular response that may alter retina structure and BRB integrity following long-term spaceflight.

Integrated RNA-seq Analysis Indicates Asynchrony in Clock Genes between Tissues under Spaceflight

“…The genes in MODE1 are known to play a role in a core loop that regulates circadian rhythm cycles via a feedback mechanism, altogether synchronizing circadian rhythms between tissues from signals of the central nervous system via light input to the eye ( Figure 3 B) [ 30 ]. Therefore, we predicted that clock genes under spaceflight would show gene expression patterns that were consistent between peripheral tissues.…”

“…The central clock localized in the suprachiasmatic nucleus (SCN) unifies circadian rhythms between tissues through neurotransmission and hormonal signals [ 30 ]. After the SCN receives light input from the eye, the resulting signal from the SCN synchronizes the peripheral clocks of peripheral tissues.…”

Validation of a New Rodent Experimental System to Investigate Consequences of Long Duration Space Habitation

“…The Rodent Habitat differs from other long duration spaceflight housing systems 12,17 in that it provides for both group-housing and internal wire grating that enable grabbing and purposeful locomotion throughout the cage, much like human crew do throughout the cabin. To accommodate unique aspects of the Dragon transport vehicle and the ISS, flight hardware was modified from the Animal Enclosure Module (AEM), that was used successfully in 27 missions with rats or mice aboard the Space Shuttle 3 followed by multiple RR missions 14,[18][19][20][21][22][23][24][25] .…”

“…Female mice were selected for this mission for two reasons: first, because only female mice were flown on earlier missions that used the heritage AEM group-housed hardware system, and second, to avoid potential complications due to aggression which is more common in males than females. After this first Validation mission, pre-flight experiments were performed to optimize conditions for acclimating males in group-housed conditions 22 , and subsequently flight experiments were conducted successfully with males 19 . Important aspects of the study design included use of adult animals, a cage system that enabled purposeful locomotion of the animals throughout the habitat, and measurements of basic indices of animal health that were compromised in past spaceflight experiments.…”

Spaceflight decelerates the epigenetic clock orchestrated with a global alteration in DNA methylome and transcriptome in the mouse retina

“…Previous research designed to better understand space-induced visual impairment in astronauts has primarily utilized proteomics and microarray technologies to better understand the biological effects of spaceflight on the mouse retina; these studies collectively reveal that spaceflight alters the expression of certain genes and proteins involved in cell death, mitochondrial function, endoplasmic reticulum (ER) stress, neuronal and glial cell loss, and axonal degeneration in mice 8,11 . Although there are studies analyzing the transcriptome in human retinas, none have been done under spaceflight conditions 12,13,14,15.…”