The evolution of life on Earth has been subject to a range of influences, from changes in the gaseous composition of the atmosphere to the movement of land masses and climatic shifts. Throughout this time, a constant and universal force was also at play, namely gravity. This has led to questions over whether the cellular and molecular functions that underpin the diverse array of organic life on the planet, require gravity in order to optimally function. Oliver Ullrich from the University of Zurich, Switzerland, and colleagues tackle this question in their study in Cell Communication & Signaling, where they investigate the impact of gravity on the oxidative burst reaction in mammalian cells. Ullrich explains how they manipulated gravity for these purposes and the effects this had on immune cells.
With a background in biochemistry and medicine, how did you become interested in space science and the cellular effects of gravity?
I was simply fascinated by the fundamental biological questions, if and how life on Earth requires and responds to gravity. Gravity has been a constant force throughout evolutionary history on Earth. It is so simple, so fundamental, but so poorly understood. In history, anatomical research elucidated in detail, how the human body is constructed to withstand and to live under the gravity conditions of Earth. Now, we try to understand how the architecture and function of human cells is related to gravitational force and therefore adapted to live on Earth.
Why did you choose to specifically study the effects of gravity on the immune system? What did your study set out to investigate?
Since the 1980s, a lot of evidence has been obtained suggesting that the function of mammalian cells and of small unicellular organisms is different under conditions of microgravity. Consequently, the question arose of how normal gravity may play a role in ‘normal’ cellular function and if gravity may provide important signals for the cell. From previous experiments it was known that cells of the immune system are severely influenced by altered gravity. The gravity-sensitive nature of these cells therefore renders them an ideal biological model in the search for general gravity-sensitive mechanisms in mammalian cells.
How are you able to test cells under altered gravity conditions?
The only opportunity to perform experiments with living mammalian cells in reduced gravity, without leaving our planet, is on board an aircraft performing parabolic flight manoeuvres that is weightless when it is flying on a Keplarian trajectory and is in free fall. For access to a parabolic flight experiment, an application either to the German Aerospace Center (DLR) or European Space Agency (ESA) is required, and once selected after peer review, a second application for funding. Then, the preparatory work can start, with design and construction of the experiment hardware, the development and testing of biological mission scenarios and much more. In the last few years, we have performed 12 parabolic flight campaigns with more than 1000 parabola. In total, I have experienced more than four hours of microgravity.
How do experiments done on parabolic flights compare to results obtained from experiments on the International Space Station (ISS)?
For a coordinated research program, both platforms are required. In the search for rapid-responsive molecular alterations, short term microgravity of 22 seconds provided by parabolic flight manoeuvres on board the Airbus A300 is an ideal instrument to elucidate these initial and primary effects. The ISS is the only research platform for the investigation of integrative, long-term and functional effects of microgravity.
Your study showed that a key step in the oxidative burst reaction of macrophages – the generation of reactive oxygen species – is highly dependent on gravity. Why do you think this is?
In our experiments we demonstrated in real microgravity (parabolic flights), in simulated microgravity (2D clinostat) and in hypergravity (centrifuges and parabolic flights) that reactive oxygen species (ROS) release during the oxidative burst reaction responds rapidly and reversibly to altered gravity within seconds. Previous studies suggest a major role for the intact cytoskeleton in NADPH oxidase activation. Because rapid modifications of the cytoskeleton are very well described in microgravity, we think that cytoskeletal-dependent processes could be the reason why the oxidative burst reacts to altered gravity.
In light of your findings, what are your thoughts on the evolutionary significance of gravity for the development of complex cellular processes on Earth? Do you think some processes may be more dependent on gravity than others?
Of course, I think that gravity-dependent mechanisms are highly specific. Phagocytes and the oxidative burst are part of the ancient innate immune system in terms of evolution, and represent the most important barrier for microbes invading the body. NADPH oxidase enzymes in the early development of life was a success story: there is no evidence of multicellular life without these enzymes. Thus, it could be possible that the gravitational conditions on Earth were one of the requirements and conditions for development of the molecular machinery of the oxidative burst reaction.
The development of cellular mechanosensitivity and mechanosensitive signal transduction was probably an evolutionary requirement to enable our cells to sense their extracellular matrix and their individual microenvironment. However, mechanosensitive mechanisms were designed to work under the condition of 1g, but never had the possibility to adapt and adjust their reaction to conditions below 1g. Therefore it is possible that the same mechanisms that enable human cells to sense and to cope with mechanical stress, are potentially dangerous in microgravity. It is a major challenge to find out if our cellular machinery is able to live and to work without gravity force or if our cellular architecture will keep us dependent on the gravity field of Earth.
Numerous studies have investigated the effects of space travel on bone, muscle and cardiovascular health. What implications do your findings at the cellular level have in this context?
Several limiting factors for human health and performance in microgravity have been clearly identified for the musculoskeletal system, the immune system and the cardiovascular system during spaceflight conditions. Considering these constraints, substantial research activities are required in order to provide the basic information for appropriate integrated risk management. In particular, bone loss during long stays in weightlessness still remains an unacceptable risk for long-term and interplanetary flights.
Recently, there is emerging evidence that the immune and skeletal system are tightly linked by cytokine and chemokine networks and direct cell-cell interactions. It has been demonstrated that the immune system influences metabolic, structural and functional changes in bones directly. Both systems share common cellular players such as the osteoclasts, which are bone-resident macrophages. Therefore, knowing the cellular and molecular mechanisms of how gravity influences macrophageal cells is an invaluable requirement for the provision of therapeutic or preventive targets to keep the bone and immune systems of astronauts fully functional during long-term space missions.
What is the potential impact on future space travel of such results?
With the completion and utilization of the International Space Station and with mission plans to the moon and Mars during the first half of our century, astronautics has entered the era of long-term space missions. Such long-term missions represent a challenge never experienced before: small or even marginal medical problems could easily evolve to substantial challenges, which could possibly endanger the entire mission. Since crew performance is the crucial factor during space missions and since evacuation or exchange of the crew is impossible during interplanetary flights, there is an urgent need to elucidate the underlying mechanisms of limiting factors for human health and performance in microgravity, such as for the immune system. Results can be used for a better risk assessment, development of in vitro tests for medical monitoring or to identify targets for preventive interventions.
What’s next for your research?
In December 2014, we will send the TRIPLE LUX A experiment to the International Space Station, performed in the BIOLAB of the European COLUMBUS module. This experiment will investigate the oxidative burst reaction of macrophages during longer periods of microgravity, determine the gravitational threshold for the burst reaction and elucidate possible adaptation mechanisms.