Elevated serum soluble CD200 and CD200R as surrogate markers of bone loss under bed rest conditions

Regulation of Bone by Mechanical Loading, Sex Hormones, and Nerves: Integration of Such Regulatory Complexity and Implications for Bone Loss during Space Flight and Post-Menopausal Osteoporosis

During evolution, the development of bone was critical for many species to thrive and function in the boundary conditions of Earth. Furthermore, bone also became a storehouse for calcium that could be mobilized for reproductive purposes in mammals and other species. The critical nature of bone for both function and reproductive needs during evolution in the context of the boundary conditions of Earth has led to complex regulatory mechanisms that require integration for optimization of this tissue across the lifespan. Three important regulatory variables include mechanical loading, sex hormones, and innervation/neuroregulation. The importance of mechanical loading has been the target of much research as bone appears to subscribe to the "use it or lose it" paradigm. Furthermore, because of the importance of post-menopausal osteoporosis in the risk for fractures and loss of function, this aspect of bone regulation has also focused research on sex differences in bone regulation. The advent of space flight and exposure to microgravity has also led to renewed interest in this unique environment, which could not have been anticipated by evolution, to expose new insights into bone regulation. Finally, a body of evidence has also emerged indicating that the neuroregulation of bone is also central to maintaining function. However, there is still more that is needed to understand regarding how such variables are integrated across the lifespan to maintain function, particularly in a species that walks upright. This review will attempt to discuss these regulatory elements for bone integrity and propose how further study is needed to delineate the details to better understand how to improve treatments for those at risk for loss of bone integrity, such as in the post-menopausal state or during prolonged space flight.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

Homo sapiens and their predecessors evolved in the context of the boundary conditions of Earth, including a 1 g gravity and a geomagnetic field (GMF). These variables, plus others, led to complex organisms that evolved under a defined set of conditions and define how humans will respond to space flight, a circumstance that could not have been anticipated by evolution. Over the past ~60 years, space flight and living in low Earth orbit (LEO) have revealed that astronauts are impacted to varying degrees by such new environments. In addition, it has been noted that astronauts are quite heterogeneous in their response patterns, indicating that such variation is either silent if one remained on Earth, or the heterogeneity unknowingly contributes to disease development during aging or in response to insults. With the planned mission to deep space, humans will now be exposed to further risks from radiation when traveling beyond the influence of the GMF, as well as other potential risks that are associated with the actual loss of the GMF on the astronauts, their microbiomes, and growing food sources. Experimental studies with model systems have revealed that hypogravity conditions can influence a variety biological and physiological systems, and thus the loss of the GMF may have unanticipated consequences to astronauts’ systems, such as those that are electrical in nature (i.e., the cardiovascular system and central neural systems). As astronauts have been shown to be heterogeneous in their responses to LEO, they may require personalized countermeasures, while others may not be good candidates for deep-space missions if effective countermeasures cannot be developed for long-duration missions. This review will discuss several of the physiological and neural systems that are affected and how the emerging variables may influence astronaut health and functioning.

Homo sapiens May Incorporate Daily Acute Cycles of “Conditioning–Deconditioning” to Maintain Musculoskeletal Integrity: Need to Integrate with Biological Clocks and Circadian Rhythm Mediators

Human evolution required adaptation to the boundary conditions of Earth, including 1 g gravity. The bipedal mobility of Homo sapiens in that gravitational field causes ground reaction force (GRF) loading of their lower extremities, influencing the integrity of the tissues of those extremities. However, humans usually experience such loading during the day and then a period of relative unloading at night. Many studies have indicated that loading of tissues and cells of the musculoskeletal (MSK) system can inhibit their responses to biological mediators such as cytokines and growth factors. Such findings raise the possibility that humans use such cycles of acute conditioning and deconditioning of the cells and tissues of the MSK system to elaborate critical mediators and responsiveness in parallel with these cycles, particularly involving GRF loading. However, humans also experience circadian rhythms with the levels of a number of mediators influenced by day/night cycles, as well as various levels of biological clocks. Thus, if responsiveness to MSK-generated mediators also occurs during the unloaded part of the daily cycle, that response must be integrated with circadian variations as well. Furthermore, it is also possible that responsiveness to circadian rhythm mediators may be regulated by MSK tissue loading. This review will examine evidence for the above scenario and postulate how interactions could be both regulated and studied, and how extension of the acute cycles biased towards deconditioning could lead to loss of tissue integrity.

The application of artificial gravity in medicine and space

Gravity plays a crucial role in physiology. The lack of gravity, like in long duration spaceflight missions, cause pathologies in e.g., the musculoskeletal system, cardiovascular deconditioning, immune system deprivation or brain abnormalities, to just mention a few. The application of artificial gravity through short-arm human centrifugation (SAHC) has been studied as a possible countermeasure to treat spaceflight deconditioning. However, hypergravity protocols applied by using SAHC have also been used to treat different, ground-based pathologies. Such gravitational therapies have been applied in Uruguay for more than four decades now. The aim of this overview is to summarize the most important findings about the effects of gravitational therapy in different, mainly vascular based pathologies according to the experience in the Gravitational Therapy Center and to discuss the current research in the field of hypergravity applications in medicine but also as multisystem countermeasure for near weightlessness pathologies. New insight is needed on the use of hypergravity in medicine and space research and application.

Sex Differences in Biological Systems and the Conundrum of Menopause: Potential Commonalities in Post-Menopausal Disease Mechanisms

Sex-specific differences in biology and physiology likely start at the time of conception and progress and mature during the pre-puberty time frame and then during the transitions accompanying puberty. These sex differences are impacted by both genetics and epigenetic alterations during the maturation process, likely for the purpose of preparing for successful reproduction. For females, later in life (~45–50) they undergo another transition leading to a loss of ovarian hormone production at menopause. The reasons for menopause are not clear, but for a subset of females, menopause is accompanied by an increased risk of a number of diseases or conditions that impact a variety of tissues. Most research has mainly focused on the target cells in each of the affected tissues rather than pursue the alternative option that there may be commonalities in the development of these post-menopausal conditions in addition to influences on specific target cells. This review will address some of the potential commonalities presented by an integration of the literature regarding tissue-specific aspects of these post-menopausal conditions and data presented by space flight/microgravity (a condition not anticipated by evolution) that could implicate a loss of a regulatory function of the microvasculature in the risk attached to the affected tissues. Thus, the loss of the integration of the paracrine relationships between endothelial cells of the microvasculature of the tissues affected in the post-menopausal environment could contribute to the risk for post-menopausal diseases/conditions. The validation of this concept could lead to new approaches for interventions to treat post-menopausal conditions, as well as provide new understanding regarding sex-specific biological regulation.

Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines?

The dissociation of bone formation and resorption is an important physiological process during spaceflight. It also occurs during local skeletal unloading or immobilization, such as in people with neuromuscular disorders or those who are on bed rest. Under these conditions, the physiological systems of the human body are perturbed down to the cellular level. Through the absence of mechanical stimuli, the musculoskeletal system and, predominantly, the postural skeletal muscles are largely affected. Despite in-flight exercise countermeasures, muscle wasting and bone loss occur, which are associated with spaceflight duration. Nevertheless, countermeasures can be effective, especially by preventing muscle wasting to rescue both postural and dynamic as well as muscle performance. Thus far, it is largely unknown how changes in bone microarchitecture evolve over the long term in the absence of a gravity vector and whether bone loss incurred in space or following the return to the Earth fully recovers or partly persists. In this review, we highlight the different mechanisms and factors that regulate the humoral crosstalk between the muscle and the bone. Further we focus on the interplay between currently known myokines and osteokines and their mutual regulation.

Learning From Human Responses to Deconditioning Environments: Improved Understanding of the "Use It or Lose It" Principle

Physical activity, mobility or patterned mobility (i.e., exercise) is intrinsic to the functioning of Homo sapiens, and required for maintenance of health. Thus, systems such as the musculoskeletal and cardiovascular systems appear to require constant reinforcement or conditioning to maintain integrity. Loss of conditioning or development of chronic deconditioning can have multiple consequences. The study of different types of deconditioning and their prevention or reversal can offer a number of clues to the regulation of these systems and point to how deconditioning poses risk for disease development and progression. From the study of deconditioning associated with spaceflight, a condition not predicted by evolution, prolonged bedrest, protracted sedentary behavior, as well as menopause and obesity and their consequences, provide a background to better understand human heterogeneity and how physical fitness may impact the risks for chronic conditions subsequent to the deconditioning. The effectiveness of optimized physical activity and exercise protocols likely depend on the nature of the deconditioning, the sex and genetics of the individual, whether one is addressing prevention of deconditioning-associated disease or disease-associated progression, and whether it is focused on acute or chronic deconditioning associated with different forms of deconditioning. While considerable research effort has gone into preventing deconditioning, the study of the process of deconditioning and its endpoints can provide clues to the regulation of the affected systems and their contributions to human heterogeneity that have been framed by the boundary conditions of Earth during evolution and the "use it or lose it" principle of regulation. Such information regarding heterogeneity that is elaborated by the study of deconditioning environments could enhance the effectiveness of individualized interventions to prevent deconditions or rescue those who have become deconditioned.

Optimal Human Functioning Requires Exercise Across the Lifespan: Mobility in a 1g Environment Is Intrinsic to the Integrity of Multiple Biological Systems

It is widely acknowledged that achieving and maintaining a healthier lifestyle can be enhanced through regular participation in sport and physical activity. Coevally, a growing number of health professionals regard exercise as a legitimate intervention strategy for those who have lost their health. Exercise has been shown to be effective for overweight or obese individuals, who are at risk to lose their health due to development of type II diabetes, cardiovascular disease, as well as, infiltration of muscles, bone and other organs with fat, so it can be considered medicine. However, exercise and associated mobility likely also have a strong prevention component that can effectively contribute to the maintenance of the integrity of multiple biological systems for those who do not have overt risk factors or ongoing disease. While prevention is preferred over intervention in the context of disease, it is clear that exercise and associated mobility, generally, can be an effective influence, although overtraining and excessive loading can be deleterious to health. The basis for the generally positive influence of exercise likely lies in the fact that many of our physiological systems are designed to function in the mechanically dynamic and active 1g environment of Earth (e.g., muscles, cartilage, ligaments, tendons, bones, and cardiovascular system, and neuro-cognitive function), and nearly all these systems subscribe to the "use it or lose it" paradigm. This conclusion is supported by the changes observed over the more than 50 years of space flight and exposure to microgravity conditions. Therefore, the premise advanced is: "exercise is preventative for loss of health due to age-related decline in the integrity of several physiological systems via constant reinforcement of those systems, and thus, optimal levels of exercise and physical activity are endemic to, essential for, and intrinsic to optimal health and wellbeing."

Role of the CD200-CD200R Axis During Homeostasis and Neuroinflammation

Microglia are considered to be the resident macrophages of the CNS and main effector of immune brain function. Due to their essential role in the regulation of neuroinflammatory response, microglia constitute an important target for neurological diseases, such as multiple sclerosis, Alzheimer's or Parkinson's disease. The communication between neurons and microglia contributes to a proper maintenance of homeostasis in the CNS. Research developed in the last decade has demonstrated that this interaction is mediated by "Off-signals" - molecules exerting immune inhibition - and "On signals" - molecules triggering immune activation. Among "Off signals", molecular pair CD200 and its CD200R receptor, expressed mainly in the membrane of neurons and microglia, respectively, have centered our attention due to its unexplored and powerful immunoregulatory functions. In this review, we will offer an updated global view of the CD200-CD200R role in the microglia-neuron crosstalk during homeostasis and neuroinflammation. Specifically, the effects of CD200-CD200R in the inhibition of pro-inflammatory microglial activation will be explained, and their involvement in other functions such as homeostasis preservation, tissue repair, and brain aging, among others, will be pointed out. In addition, we will depict the effects of CD200-CD200R uncoupling in the etiopathogenesis of autoimmune and neurodegenerative diseases. Finally, we will explore how to translate the scientific evidence of CD200-CD200R interaction into possible clinical therapeutic strategies to tackle neuroinflammatory CNS diseases.

Intraocular pressure and cardiovascular alterations investigated in artificial gravity as a countermeasure to spaceflight associated neuro-ocular syndrome

Artificial gravity (AG) has been proposed as a countermeasure to spaceflight-associated neuro-ocular syndrome (SANS). The etiology of SANS is unknown but mimicking gravitational loading through AG may mitigate these negative adaptations. Seventeen subjects (nine men, eight women; 18-32 yr) were analyzed in four experimental conditions: 1) standing, 2) supine, 3) AG with the center of rotation at the eye (AGEC), and 4) AG with 2 Gs at the feet (AG2G). In both AG conditions, subjects were spun to produce 1 G at their center of mass. Data included self-administered intraocular pressure (IOP, Tono-pen AVIA, Depew, NY), heart rate (HR), and mean arterial blood pressure (MAP, Omron Series 10, Omron Healthcare, Kyoto, Japan). Data were analyzed with repeated measures ANOVAs with Tukey-Kramer corrections for multiple pairwise comparisons. IOP was 15.7 ± 1.4 mmHg (mean ± 95% confidence interval) standing, 18.8 ± 1.3 mmHg supine, 18.5 ± 1.7 mmHg in AGEC, and 17.5 ± 1.5 mmHg in AG2G. Postures showed a main effect [F(3,48) = 11.0, P < 0.0005], with standing significantly lower than supine ( P = 0.0009), AGEC ( P = 0.002), and AG2G (0.036). Supine, AGEC, and AG2G were not statistically different. HR and MAP were lower in supine compared with all other postures ( P = 0.002 to P < 0.0005), but there were no differences between standing, AGEC, and AG2G. IOP in supine and standing was consistent with previous studies, but contrary to our hypothesis, remained elevated in both AG conditions. Cardiovascular parameters and hydrostatic gradients determine IOP, which remain unchanged compared with standing. These results suggest additional influence on IOP from previously unconsidered factors. NEW & NOTEWORTHY This is the first study, to the authors' knowledge, to measure intraocular pressure in short-radius centrifuge artificial gravity (AG), which has been proposed as a countermeasure to the spaceflight-associated neuro-ocular syndrome (SANS). If the etiology of SANS is related to intraocular pressure, these results have implications for whether or not short-radius AG can be used to prevent ocular changes relevant to it. Our results indicate this proposed countermeasure merits further investigation.

International roadmap for artificial gravity research

In this paper, we summarize the current and future research activities that will determine the requirements for implementing artificial gravity (AG) to mitigate the effects of long duration exposure to microgravity on board exploration class space vehicles. NASA and its international partners have developed an AG roadmap that contains a common set of goals, objectives, and milestones. This roadmap includes both ground-based and space-based projects, and involves human subjects as well as animal and cell models. It provides a framework that facilitates opportunities for collaboration using the full range of AG facilities that are available worldwide, and a forum for space physiologists, crew surgeons, astronauts, vehicle designers, and mission planners to review, evaluate, and discuss the issues of incorporating AG technologies into the vehicle design.

The impact of microgravity on bone in humans

Experiencing real weightlessness in space is a dream for many of us who are interested in space research. Although space traveling fascinates us, it can cause both short-term and long-term health problems. Microgravity is the most important influence on the human organism in space. The human body undergoes dramatic changes during a long-term spaceflight. In this review, we will mainly focus on changes in calcium, sodium and bone metabolism of space travelers. Moreover, we report on the current knowledge on the mechanisms of bone loss in space, available models to simulate the effects of microgravity on bone on Earth as well as the combined effects of microgravity and cosmic radiation on bone. The available countermeasures applied in space will also be evaluated.

Characterization of CD200 Ectodomain Shedding

We have previously reported the existence of a soluble form of CD200 (sCD200) in human plasma, and found sCD200 to be elevated in the plasma of Chronic Lymphocytic Leukemia (CLL) patients. CLL cells release CD200 at a constitutive level, which could be attenuated partially by ADAM28 silencing. In this study, we further explored mechanisms of CD200 shedding beyond that of ADAM28, and performed biochemical analysis of sCD200 using materials derived from purified CLL cells and Hek293 cells stably transfected with CD200, and antibodies generated specifically against either the extracellular or cytoplasmic regions of CD200. CD200 shedding was enhanced by PMA stimulation, and the loss of cell surface CD200 could be monitored as a reduction in CD200 cell surface expression by flow cytometry, in parallel with an increase in the detection of sCD200 in the supernatant. Western blot analyses and functional studies using CD200R1 expressing Hek293 cells showed that the shed CD200 detected in CLL and Hek293-hCD200 supernatants lacked the cytoplasmic domain of CD200 but retained the functional extracellular domain required for binding to, and phosphorylation of, CD200R. These data confirms that a functionally active CD200 extracellular moiety can be cleaved from the surface of CD200 expressing cells following ectodomain shedding.

Artificial gravity as a countermeasure for mitigating physiological deconditioning during long-duration space missions

In spite of the experience gained in human space flight since Yuri Gagarin’s historical flight in 1961, there has yet to be identified a completely effective countermeasure for mitigating the effects of weightlessness on humans. Were astronauts to embark upon a journey to Mars today, the 6-month exposure to weightlessness en route would leave them considerably debilitated, even with the implementation of the suite of piece-meal countermeasures currently employed. Continuous or intermittent exposure to simulated gravitational states on board the spacecraft while traveling to and from Mars, also known as artificial gravity, has the potential for enhancing adaptation to Mars gravity and re-adaptation to Earth gravity. Many physiological functions are adversely affected by the weightless environment of spaceflight because they are calibrated for normal, Earth’s gravity. Hence, the concept of artificial gravity is to provide a broad-spectrum replacement for the gravitational forces that naturally occur on the Earth’s surface, thereby avoiding the physiological deconditioning that takes place in weightlessness. Because researchers have long been concerned by the adverse sensorimotor effects that occur in weightlessness as well as in rotating environments, additional study of the complex interactions among sensorimotor and other physiological systems in rotating environments must be undertaken both on Earth and in space before artificial gravity can be implemented.

Effects of an artificial gravity countermeasure on orthostatic tolerance, blood volumes and aerobic power after short-term bed rest (BR-AG1)

Exposure to artificial gravity (AG) in a short-arm centrifuge has potential benefits for maintaining human performance during long-term space missions. Eleven subjects were investigated during three campaigns of 5 days head-down bed rest: 1) bed rest without countermeasures (control), 2) bed rest and 30 min of AG (AG1) daily, and 3) bed rest and six periods of 5 min AG (AG2) daily. During centrifugation, the supine subjects were exposed to AG in the head-to-feet direction with 1 G at the center of mass. Subjects participated in the three campaigns in random order. The cardiovascular effects of bed rest and countermeasures were determined from changes in tolerance to a head-up tilt test with superimposed lower body negative pressure (HUT), from changes in plasma volume (PV) and from changes in maximum aerobic power (V̇o2 peak) during upright work on a cycle ergometer. Complete data sets were obtained in eight subjects. After bed rest, HUT tolerance times were 36, 64, and 78% of pre-bed rest baseline during control, AG1 and AG2, respectively, with a significant difference between AG2 and control. PV and V̇o2 peak decreased to 85 and 95% of pre-bed rest baseline, respectively, with no differences between the treatments. It was concluded that the AG2 countermeasure should be further investigated during future long-term bed rest studies, especially as it was better tolerated than AG1. The superior effect of AG2 on orthostatic tolerance could not be related to concomitant changes in PV or aerobic power.