[Cell Gravisensing]
Elucidation of gravisensing mechanism in single cells

RESEARCH OVERVIEW

Recent spaceflight experiments have revealed that individual animal cells can sense gravity. However, the mechanism is largely unknown and represents an important problem to be solved in space biology.

The hypothesis for the Cell Gravisensing investigation is that under microgravity conditions, tension in the cell’s stress fibers is decreased by the loss of gravity effects on the cell’s organelles to induce various cell responses. The major aim of the experiment is to prove this hypothesis by measuring the cell responses during spaceflight.

The method to conduct live imaging of cell responses to microgravity has high novelty and is expected to contribute significantly to the progress in basic biology. Expected results for understanding the molecular mechanism of cell gravisensing could promote drug development for treating muscle atrophy and osteoporosis, contributing to the improvement of human quality of life in aged society.

DESCRIPTION

Recent spaceflight experiments have revealed that individual animal cells can “feel” gravity. However, how cells can sense gravity is largely unknown. Understanding this mechanism is one of the most important problems to be solved in space biology.

This problem is studied through the analysis of the substrate-rigidity sensing of cells during spaceflight. Adherent cells sense substrate rigidity through the stress generated during the pulling of the substrate with contractile actin stress fibers via focal adhesions (integrins), and utilizing the information for regulating the differentiation axis and proliferation.

The generated stress depends on the substrate rigidity, the pulling force (tension), and the activation of the calcium (Ca²⁺) permeable mechanosensitive ion channels (MSCs) being converted to local intracellular Ca²⁺ concentration (Ca²⁺ spark). Strong evidence has been obtained that tension in the stress fiber decreases under semi-microgravity conditions. This may be due to the mechanism that relatively heavy organelle linking stress fibers, such as nucleus and/or mitochondria, induce additional tension in the stress fiber under normal Earth gravity (1g) conditions.

On the basis of these results, it is hypothesized that under microgravity conditions, tension in the stress fiber would decrease due to the loss of gravity effect on these organelles to induce various cell responses, including changes in the Ca²⁺ spark activity. Results supporting this idea were obtained in the preceding Cell Mechanosensing project.

The major aim of the Cell Gravisensing investigation is to prove the above hypothesis by measuring various cell responses during spaceflight, particularly tensile forces in the stress fiber and Ca²⁺ sparks, using confocal fluorescence microscopy and fluorescence resonance energy transfer imaging techniques. Furthermore, intracellular signaling mechanisms are also to be analyzed.

SPACE APPLICATIONS

Muscle atrophy and bone loss experienced during spaceflight pose serious potential health risks to astronauts, especially on longer duration missions. Better understanding of the molecular mechanism by which cells can sense gravity could promote development of drugs for treating these issues.

EARTH APPLICATIONS

Results from this investigation also may contribute to the development of drugs for treating muscle atrophy and osteoporosis in people on Earth, helping to improve the quality of life in an aging society.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

After SpaceX Dragon Cargo Vehicle arrival to the International Space Station, the media in the Culture Chamber Units is replaced with fresh media by using Auto Solution Exchangers. Next, the culture chambers are installed to MEU (Measurement Experiment Unit) then the MEU(s) are attached to CBEF (Cell Biology Experiment Facility) for cultivation.

During the four days of cultivation, observation by microscope is conducted six times. For the observation, an onboard crew member retrieves culture chambers from the CBEF and installs them to a stage of the microscope. Two days after cultivation start, the gravitational environment for some culture chambers is changed between microgravity and 1G conditions by ground command or crew operations.

Some culture chambers are retrieved from the CBEF for chemical fixation at 0, 4, 24 and 48 hours after the gravity condition change. The Auto Solution Exchanger and fixative cassettes are used for the chemical fixation. The fixed chambers are stowed in MELFI until return to Earth.

SOKABE Masahiro [Kanazawa Institute of Technology]