Introduction

Several undesirable consequences of space flight have been described since the very beginning of space exploration, including: bone loss, muscle atrophy, immune response impairment, nervous system functional derangement, cardiovascular deconditioning, metabolic dysregulation, spaceflight associated neuro-ocular syndrome (SANS) etc.1,2,3,4,5,6,7,8.

All of these alterations represent models of chronic diseases commonly observed on Earth, mostly affecting the elderly, and potentially sharing the same pathophysiological mechanism(s), involving oxidative stress and vascular dysfunction through chronic inflammation9,10,11,12,13,14. In addition, stressful tasks and diet changes, confinement-related artificial light, isolation and reduced motor activity might worsen the effects of weightlessness by further contributing to musculoskeletal and cardiovascular deconditioning, dysregulation of energy metabolism and endocrine balance14,15,16,17,18,19,20,21.

Despite the continuously increasing knowledge gathered on mechanisms underlying the multiple effects of space flight on astronauts, studies conducted so far have not been able to generate a synthetic integrated view of changes occurring at different system and functional levels. Therefore, it remains difficult to elaborate credible models predicting long-term consequences of human adaptation to the space environment, as well as, to plan the medical support and the necessary countermeasures for long-term missions.

Recent research progress points to new approaches to understand the inner mechanisms leading from health to the onset of pathological conditions. The ability to maintain/restore homeostasis is of critical importance to allow tissues to restore their functions and e.g. to heal from injuries and diseases allows the organism to survive. In contrast, persistence of altered homeostatic conditions and here the lack of e.g., efficient healing ability predispose to the onset of chronic diseases, accelerated aging, disability and death22,23,24,25.

Most recent research suggests the importance to study the connections among oxidative stress, increased endotoxin levels, metabolic dysregulation (MetS), low grade inflammation and immune system changes, which in turn may be related to altered microbiome/virome expressions6,20,26,27,28,29,30,31,32,33,34,35,36,37. A case study carried out on one subject engaged in a one-year mission has recently reported that alterations of the inflammatory, metabolic and immune profiles are among the more persistent alterations after return to Earth, with an early peak and a slow decreasing trend thereafter without coming back to pre-flight levels for months28. In other studies, a stress related shift toward inflammageing has been observed in cosmonauts after long-duration spaceflight38. Therefore, the microbiome-immune profile-inflammation-metabolism (including mitochondrial dysfunction and oxidative stress) seems to play a central role in adaptation mechanisms to spaceflight.

Key knowledge gaps

To plan long-lasting deep space missions, the effects of altered gravity on animal and human systems (in particular the immune, musculoskeletal, cardiovascular, neurosensory, neuroendocrine, excretory, respiratory, metabolic and integumentary systems) should be further explored, focusing on molecular/cellular and cell-communication processes. These effects should be framed in the more general context of systemic adaptation to the space environment and contemporary alterations in different organs and functions, which can be induced also by factors different from altered gravity (e.g. radiation, isolation, confinement, celestial dust)18,39,40.

Efforts should be made to untangle the confounding effects of multiple stressors in the space environment (the space exposome), and differentiate the effects of space-related stressors (altered gravity and cosmic rays from the effects of confinement, isolation and psychophysical stress). In order to provide appropriate countermeasures, it is extremely important to clarify the causes and contributing factors of the various possible alterations.

It is mandatory to study the evolution of fundamental physiological processes of adaptation, crucial for organism’s survival, (e.g. hemorrhage and blood coagulation, acute and chronic inflammation, stromal activation, tissue repair mechanisms, metabolic pathways) in altered gravity, in order to predict long-term alterations and implement countermeasures. In fact, long-duration space exploration missions might reduce the organism’s ability to respond to acute injuries/infections and predispose astronauts to the onset of chronic diseases (e.g. cardiovascular and metabolic diseases) and premature aging by strongly influencing redox and metabolic processes, microbiota, immune function, as well as, endotoxin and pro-inflammatory signal production6,21,28,31,32,33,34,35,37,41,42,43,44. Indeed, it is known that low grade chronic inflammation (LGI) involves many of the above mechanisms and is a distinctive feature of major chronic diseases in the aging population45,46,47.

Moreover, we need to know the long-term consequences of the above effects. In other words, we need to know if and how changes in the crucial biological processes mentioned above can increase the risk of acute and chronic disease onset and/or influence disease progression.

In summary, we need to gain knowledge about the effects of space stressors on the onset and evolution of serious diseases.

It is also important to unravel the role of gravity in wound and trauma healing, tissue regeneration, remodelling and development. In all these processes mechanical factors play important, but still not completely known, roles. Due to unloading conditions, the space environment is particularly suitable for these studies48.

A better understanding of the scientific problems described above, could not only help manage a number of diseases both in space and on Earth, but it should also enable crew members to be better fitted to return to a gravity environment like on Earth or when arriving on Mars.

Last but not least, cancer is caused by an accumulation of genetic mutations49. Its onset and progression also depend on alterations of the microenvironment50,51. Cancer onset could be caused by chronic exposure to environmental stressors as encountered during long-term space missions52. Identifying mechanisms of cancer development and progression during spaceflights is crucial. Since the impairment of immune system activity plays an important role in cancer progression, and a healthy immune system can counteract tumor growth53, it would be interesting to elucidate how altered gravity may affect the existing balance between immunity and immune escape mechanisms in cancer. Moreover, the role played by the cancer microenvironment, namely the stromal components and the extracellular matrix, should be further investigated.

The development of countermeasures for the different space stressors is a fundamental aspect in the preparation of future space exploration missions. Before application, identified countermeasures must be tested and validated to know their effectiveness and possible side effects when applied during spaceflight or in suitable space analogs.

When physical countermeasures (exercise, physical therapies) are considered, investigations on the mechanisms underlying their therapeutic effects carried out in unloading conditions will be very important to elucidate any difference in comparison with their application in loading conditions54.

Drug treatments are also included among the countermeasures to prevent or counteract the effects induced by spaceflight. However, due to the pathophysiological adaptation occurring during spaceflight, the absorption, distribution, metabolism and elimination of drugs is expected to be different in space. The recent episode on the ISS in which the administration of anticoagulants became necessary to treat a case of asymptomatic venous thrombosis (VT) of the internal jugular vein of an astronaut, further highlights the importance of studying the pharmacodynamics/kinetics of drugs in space55. Therefore, assessing gravity-induced changes in the pharmacodynamics/kinetics of drugs is certainly a topic that requires many research efforts56.

Proposed research activities

Summarizing what has been reported in the previous paragraph, five main scientific questions that are partially/totally unanswered have been identified (Fig. 1):

  1. 1.

    What are the effects of altered gravity on key biological systems such as the immune, musculoskeletal, cardiovascular, neurosensory, neuroendocrine, excretory, respiratory, and integumentary systems as well as on their metabolism and homeostasis?

  2. 2.

    What are the effects of altered gravity on tissue regeneration and development as well as on tissue engineering technologies?

  3. 3.

    What are the effects of altered gravity on the development and progression of diseases (e.g., cardiovascular and metabolic diseases, autoimmune diseases and allergies, cancer, genetic predisposition).

  4. 4.

    What is the effectiveness of countermeasures (e.g., physical countermeasures, nutrition, pharmaceuticals, nanoparticles and nanomaterials, other therapeutics) and what are the possible side effects when the countermeasures are applied during spaceflight?

  5. 5.

    What are the changes in pharmacodynamics/kinetics of drugs when these are used by crew members during spaceflight?

Fig. 1: How to fill knowledge gaps.
figure 1

Main issues regarding how gravity alterations affect animal and human systems at a cellular/tissue level.

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To answer these questions, a corresponding number of research lines should be implemented, with activities including both on ground and in-flight experiments, using all the available platforms and spanning from short to long timeline.

In more detail, the first research line should concern in-depth studies on effects of long-term exposure to spaceflight in order to understand basic processes in life sciences, and being able to support plans for long-duration and exploration missions (moon, Mars).

Studies should be designed with a dual purpose: i) on one side, evaluating the sum of multiple effects due to contemporary exposure to altered gravity, radiation, isolation, confinement, and psychophysical stress, which result in contemporary alterations in different organs and functions. ii) on the other side, trying to untangle the confounding effects of multiple stressors of the space exposome and to define the causes and contributing causes triggering different biological responses to help uncover the reasons and sequences of (mal)adaptations to space and place them in the context of human evolution. Moreover, a better understanding of the relationships between causes and effects is of crucial importance to implement effective countermeasures.

Indeed, basic research and human subjects´ research experiments should be bridged in the effort to connect biology, physiology and evolutionary anthropology, obtaining a more holistic approach to the process of human adaptation to space. Furthermore, the role of sex/gender in the biological response to spaceflight should be investigated more thoroughly.

Studies involving animal models can help explore the role of gravity with respect to spaceflight-related phenomena, like accelerated ageing, metabolic disorders or more social and behavioral-related phenomena. Animal models could also be applied to develop and test long duration Space Exploration related countermeasures such as in-flight artificial gravity, hibernation related protocols, nanotechnology, physical therapies, drugs, and probiotics. Small animal models (vertebrates like mice, fish, amphibians, other rodents or invertebrates like fruit flies, planarians, C. elegans, leeches and rotifers) could be of particular interest to study processes, functions and morphologies that have been conserved throughout evolution.

To heighten studies with animal models, in particular with rodents, access of European scientists to rodent research opportunities should be entertained either via collaboration with ISS partners for use of their facilities and/or via a close collaboration and use of the Italian Mouse Drawer System (MDS). Furthermore, when organizing animal/rodent studies both in-flight (altered gravity) or on the ground (unloading conditions, 1 g and hyper-g) a comprehensive tissue sharing program should be implemented.

Whichever model is used, it is recommended to consider the following key points when designing the experiments:

  1. a.

    The spaceflight exposure time, namely the duration of the experiments, should be increased, even to the length of the life cycle of the organism chosen as a model. An extended time exposure helps understand the risks associated to long duration missions;

  2. b.

    Higher statistical significance and reproducibility are needed to obtain “more quantitative” results. Therefore, the number of samples/subjects involved in the experiments should increase.

  3. c.

    Real time analysis and monitoring is important to avoid bias due to the return to Earth of the samples. Therefore, the development of suitable devices is needed;

  4. d.

    A highly integrated approach should be anticipated to make optimal use of the animal models (e.g. with elaborate Tissue Sharing Program).

A second research line should be addressed to explore and monitor the role of gravity (and the consequences of its alteration) in fundamental biological processes, such as growth and development, differentiation and organ morphogenesis, wound healing, tissue regeneration and restoration of homeostasis. Since the fine regulation of these processes is crucial for the development of organisms, their reaction to injuries and survival, a better knowledge of these topics in space environment is essential in view of future interplanetary missions, which will require long stay of crew members in space. In this perspective, research activities aimed at investigating the impact of spaceflight on reproduction, embryonic development, and multi-generational exposure should be increasingly carried out. Furthermore, the role of nutrition in growth, development, and response to injuries during spaceflight should be examined.

Beyond the investigation on embryogenesis in the space environment, research should be extended to other 3/4D biological processes, such as the growth and maturation of tissue constructs. Biotechnologies focused on tissue regeneration and engineering, not only have been indicated as enabling technologies for interplanetary missions, but they could also play a crucial role in coping with serious diseases on Earth. In this perspective, the identification of key omics-signatures for potential therapeutic applications both on Earth and long-term spaceflights would be extremely important.

One of the most important problems in programming crewed exploration missions beyond LEO is the increase in health risks due to prolonged exposure to radiation and microgravity. The condition of isolation and confinement and the prolonged psychophysical stress worsen the overall picture. Therefore, it is mandatory to implement a third line of research focused on the on-set, development and progression of serious acute and chronic diseases possibly induced by spaceflight. The identification of key omics-signatures of cancer and other diseases, such as metabolic and cardiovascular pathologies could be very important for their prevention and treatment.

It is well known that the cell microenvironment has a crucial role in cancer evolution, while experiments carried out in real and modeled microgravity demonstrated that extracellular matrix (ECM) properties can change in unloading conditions. Therefore, the cancer-associated microenvironment should be carefully studied in space. As regards the mechanisms underlying metabolic and cardiovascular diseases, studies should focus on low grade inflammation and mitochondrial dysfunction, which often characterize these kind of diseases.

A fourth line of research should focus efforts on the development of countermeasures enabling, from a biomedical point of view, future interplanetary missions. Protective (preventative) and therapeutic strategies based on both pharmacological and physical therapies should be implemented. The mechanisms of action of the countermeasures must be studied in the space environment, or by using facilities that model the space exposome, because differences are expected in comparison to what has been observed on Earth. Studies to elucidate the mechanisms of action of the countermeasures, evaluate their effectiveness, and to validate them, might be conducted using 2D and 3D in vitro models, ex vivo tissue models, and animal models, and then combined with studies on human subjects in space to confirm the results obtained with the models. Also, research on drug vectors (including nanomaterials) for medical applications in space is needed.

An important aspect to consider when specifically evaluating pharmacological treatments for use during spaceflight, is related to possible changes in drug pharmacodynamics/pharmacokinetics due to changes in human physiology induced by space adaptation mechanisms. Indeed, these changes could induce differences in absorption, distribution, metabolism and elimination of drugs. Studies carried out in rodents have reported that microgravity induces alterations in liver metabolism, causing a pathophysiological state similar to fatty liver disease, a condition that can affect the processes of absorption, distribution, metabolism, and excretion of drugs57,58. Therefore, a fifth research line should be specifically aimed at: i) evaluating the effectiveness of drugs in spaceflight conditions; ii) investigating the possible changes induced by the space environment in the action mechanisms of drugs and possible adverse reactions; iii) defining treatment protocols and dosages for use during spaceflight. Studies in spaceflight conditions should be systematically compared to suitable ground-controls in order to estimate the impact of spaceflight conditions on drug effects, drug metabolism, side effects, drug toxicity and adverse reactions.

The most relevant knowledge gaps and activities are reported in Table 1.

Table 1 Recommendations in short (3 years), middle (6 years) and long term (> 10 years).
Full size table

On-ground research activities require the use of microgravity analogs and microgravity simulators. Ground-based microgravity analogs include long-duration bed-rest and water immersion. The ground-based microgravity simulators include the following: clinostat, random positioning machine (RPM), rotating wall vessel (RWV) or Rotary Cell Culture System (RCCS) and Magnetic Levitation (ML)59. While microgravity analogs replicate some microgravity-induced physiological responses, microgravity simulators are platforms that can be used to model the exposure of cells, biological tissue samples and small animal models to microgravity conditions. Since conducting experiments in space is not always a viable option, ground-based microgravity analogs and simulators are widely used in space biology/physiology studies. Moreover, also on ground analogues such as submarines and Concordia Station in Antarctica are used to study the effects of isolation and confinement on human physiology. Thus, it is necessary to use these facilities coherently and to be careful when comparing results obtained by using different platforms60.

Priorities for the ESA space programme (altered gravity and/or Exploration relevance)

The results of the above research activities will contribute to understand the mechanisms underlying temporary functional alterations in the human organism during spaceflight, their causes, recovery timelines and effects possibly leading to health risks and a progression towards chronic diseases.

The results will contribute to implementing preventative and therapeutic countermeasures as well as diagnostic tools to monitor astronauts’ health, manage health risks and supply effective therapies and health care facilities in future long-lasting space exploration missions. With these results, novel and original therapeutic solutions for spaceflight could also be found.

It is very important to highlight that these studies also offer the possibility of investigating topics of basic and applied research. The extended microgravity exposure will allow to explore the effect of sub g accelerations and make a comparison with the response in microgravity analogue or hypergravity and gravity transitions explored by ground-based studies. Moreover, it will be possible to increase the understanding of the impact of spaceflight on drug shelf-life and kinetics and to evaluate the effect of spaceflight-induced physiological alterations on pharmacodynamics.

The space environment offers a unique opportunity to study the role of gravity and mechanical factors on biological processes. By carrying out these studies, a better understanding of how decreased mechanical load can impact fundamental processes as tissue regeneration or disease onset and progression, could be reached. The perspectives of human exploration of the moon and Mars will need to further expand the knowledge on G-level transitions and especially the effects of life under at 1/6 or 1/3 of Earth gravity.

Benefit for earth and industrial relevance

As above reported, many pathophysiological changes observed during and after spaceflight can represent models of a series of chronic diseases commonly observed on Earth, mostly affecting the elderly and frail subjects. The possibility to conduct studies on pathophysiological alterations in healthy and well-characterized subjects in space can shed new light on the cellular and molecular mechanisms underlying chronic diseases, help to find therapeutic targets and implement effective countermeasures to prevent the onset of these diseases or slow their progression, and, potentially increase our understanding of the molecular basis of frailty.

Conclusions

In conclusion, for planning future manned interplanetary missions, it is necessary to expand our knowledge about the effects induced in human and animal organisms by the different stressors of the space exposome, namely microgravity, radiation, confinement, isolation and psychophysical stress. It is also required to have a greater awareness of the long-term consequences that could occur at a systemic level due to the multiple and concomitant alterations due to adaptation to spaceflight. Efforts should be made to progress in the implementation of effective countermeasures to monitor astronaut’s health as well as to prevent and treat diseases occurring during spaceflight. The application of advanced technologies (nanotechnologies, 3D printing, tissue engineering and regeneration, etc.) has a crucial role in the implementation of the research programs recommended in the roadmap.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.