Responsible Center: NASA JSC
Grant Monitor: Brocato, Becky
Center Contact: becky.brocato@nasa.gov
Unique ID: 12262
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Solicitation / Funding Source: 2017-2018 HERO 80JSC017N0001-BPBA Topics in Biological, Physiological, and Behavioral Adaptations to Spaceflight. Appendix C
Grant/Contract No.: 80NSSC19K0392
Project Type: Flight
Flight Program:
TechPort: No |
No. of Post Docs: 6
No. of PhD Candidates: 2
No. of Master's Candidates: 0
No. of Bachelor's Candidates: 2
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No. of PhD Degrees: 30
No. of Master's Degrees: 1
No. of Bachelor's Degrees: 11
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Task Description: |
Spaceflight-associated neuro-ocular syndrome (SANS) develops in astronauts completing long duration spaceflights. It is reported to affect ~70% of astronauts completing long-duration spaceflights (NASA Evidence Report 4/2022) and has been characterized as the development of one or more findings: optic disc edema, hyperopic shifts, globe flattening, cotton-wool spots, or choroidal folds. These changes in ocular function could lead to vision alteration that is uncorrectable and mission threatening. The leading hypothesis for the development of ocular changes is that prolonged exposure to the headward fluid shift that occurs in weightlessness is the primary instigating factor, and additional factors such as genetic disposition, anatomic variations, ambient CO2 on the International Space Station, or on-orbit exercise countermeasures may augment or diminish the development of ocular symptoms. However, the pathophysiology of SANS remains unclear. The changes in fluid pressures associated with the loss of the gravitational vector and the resulting headward fluid shifts can influence many ocular-associated fluid compartments, CSF in the brain and optic nerve, blood and lymph vessels in the brain, retina and eye. Previous work in the brain microcirculation found alteration in the regulation of their function. However studies of the vessels in the eye have not previously been done. Therefore, studies of ocular vascular hydrodynamics are required to clarify if chronic mild elevations of ocular pressure variables compromise ocular structure and function. Since all blood and lymph vessels are compliant, fluid-filled structures whose pressures are strongly influenced by gravity, we focused the RR23 studies on the potential changes directly to the ocular vasculature caused by microgravity. Perfusion of the optic nerve and inner retina for sufficient delivery of oxygen and nutrients is dependent on retinal blood flow. The pressure gradient for driving blood flow through the inner retina begins with the arterial pressure in the feed artery, which is the central retinal artery in humans. Changes in retinal blood flow or pressure may contribute to the formation of cotton wool spots and optic disc edema. Optic disc edema, choroidal folds, and optic nerve thickening may also result from ocular venous congestion and/or elevated venous compliance, disruption of the blood-retinal barrier, and/or reduction in ocular lymph flow. There had been no systematic analysis of the ocular vascular changes in microgravity. We assembled a team of experts in SANS and all 3 main vascular types (arteries, veins, and lymphatics) to address this information gap. Thus, the primary objective of this application is to determine whether microgravity alters the structure and function of the ocular vasculature at the level of feed arteries, venous exchange and capacitance vessels, and lymph vessels. This provides a novel comprehensive evaluation of the ocular vascular elements after spaceflight. The central hypothesis of this proposal is that microgravity/spaceflight-induced changes in the structure/function of the ocular vasculature lead to alterations in ocular hydrodynamics and promoted symptoms of SANS. We accomplished this objective using in vivo measures of ocular characteristics, ocular vascular function (retinal artery blood flow, retinal arteriole and venular diameter measurements, and retinal venular permeability measures) and in vitro studies of freshly isolated vascular structure and function (vessel/tissue histology, arterial vasomotor regulation, venous compliance measures, and lymphatic transport characteristics). These studies were conducted in mice flown in space for ~37 days immediately upon arrival back on earth (10.5 hours after splashdown) and the corresponding ground controls to address the following specific aims:
1: Evaluate the effects of microgravity on ocular artery structure/function. • In vivo Measurements (anesthetized mice): • VisualSonics Vevo 2100 ultrasound system was used for blood velocity measurement. The central retinal artery was located by color Doppler. • A TonoLab tonometer was used to measure intraocular pressure (IOP). • The Heidelberg Spectralis HRA + OCT system was used to acquire fundus and cross-sectional images of the mouse retina. • In vitro Functional Studies of the Ophthalmic Artery: • Isolated ophthalmic arteries were cannulated with glass micropipettes and pressurized to 75 cmH2O. Vessel diameter changes/contractile status to acetylcholine, sodium nitroprusside, and endothelin-1 were monitored and recorded.
2: Evaluate the effects of microgravity on ocular vein structure/function. • In vitro Functional Studies of the Angular Vein: • The mouse angular vein, which drains the supratrochlear and supraorbital vein, was isolated and mounted in a wire myograph. • Assessment of Retinal Microvascular Barrier Function: • Intravital microscopy (multiphoton laser scanning microscope) was used to acquire real-time images of FITC-dextran extravasation in anesthetized mice to measure microvascular permeability in the mouse retina. • Mice were injected with FITC-dextran (10 kDa) via the jugular vein, followed by intravital imaging of the retinal microcirculation every 5 min for 40 min. Retinal microvascular permeability/barrier integrity was measured based on extravasation fluorescence intensity of FITC-dextran.
3: Evaluate the effects of microgravity on ocular lymphatic structure/function. • Assessment of Ocular Lymphatic Vessel Function: • Isolated ocular collecting lymphatics were cannulated with glass micropipettes and pressurized from 0.5 to 4 cmH2O. Diameter changes were tracked and recorded to measure phasic and tonic contractile/pump function. • Assessment of Ocular Lymphatic Tissue Structure: • Perilymphatic tissue was collected for immunohistochemical analysis of local immune cells (mast cells, macrophages, dendritic cells, and lymphocytes) and for multiplex analysis of cytokines.
In addition to conducting experiments to address these primary missions we also0used a team of scientists to conduct the dissection and specimen storage for NASA's Biospecimen Sharing Program, since this occurred during the COVID pandemic and NASA did not allow their normal BSP team top travel. As a result of our BSP efforts we were granted permission to acquire some additional tissues for study.
A brief summary of the results from these novel studies (as seen below) provided the first comprehensive analysis of the effects of microgravity on ocular vascular function where the predominant changes associated with SANS in astronauts occur. These results will help define the roles the ocular vascular function may play in the etiology of SANS and could lead to the development of countermeasures for SANS.
• Aim 1: Effects of microgravity on ocular artery structure/function • Central retinal artery blood flow velocity was lower in the flight mice. • Trends towards higher IOP in the flight mice. • Trends towards increased total retinal thickness in the flight mice. • Ophthalmic arteries from flight mice exhibited normal vasodilator function but diminished basal vessel tone and endothelin-1-induced vasoconstriction.
• Aim 2: Effects of microgravity on ocular vein structure/function • Increase in retinal microvascular permeability. • The trends observed in isolated angular veins were attenuated vasoconstriction and vasodilation to adrenergic stimulus and exogenous nitric oxide donors, respectively.
• Aim 3: Effects of microgravity on ocular lymphatic structure/function • There was a large decrease (~40%) in ocular lymphatics from flight that had pumping activity. • Trends towards decreased lymph pump strength and increased pump frequency in those that exhibited intrinsic pumping activity. • Complete loss of shear-dependent dilation and trends towards impaired shear-dependent reductions in pumping |