Shallow metabolic depression and human spaceflight: a feasible first step

Mechanisms and Countermeasures for Muscle Atrophy in Microgravity

Previous studies have revealed that muscle atrophy emerges as a significant challenge faced by astronauts during prolonged missions in space. A loss in muscle mass results in a weakening of skeletal muscle strength and function, which will not only contribute to a decline in overall physical performance but also elevate the risk of various age-related diseases. Skeletal muscle atrophy in the microgravity environment is thought to be associated with changes in energy metabolism, protein metabolism, calcium ion homeostasis, myostatin levels, and apoptosis. Modulating some pathways could be a promising approach to mitigating muscle atrophy in the microgravity environment. This review serves as a comprehensive summary of research on the impact of microgravity on skeletal muscle, with the aim of providing insights into its pathogenesis and the development of effective treatments.

Identification of shared gene expression programs activated in multiple modes of torpor across vertebrate clades

Torpor encompasses diverse adaptations to extreme environmental stressors such as hibernation, aestivation, brumation, and daily torpor. Here we introduce StrokeofGenus, an analytic pipeline that identifies distinct transcriptomic states and shared gene expression patterns across studies, tissues, and species. We use StrokeofGenus to study multiple and diverse forms of torpor from publicly-available RNA-seq datasets that span eight species and two classes. We identify three transcriptionally distinct states during the cycle of heterothermia: euthermia, torpor, and interbout arousal. We also identify torpor-specific gene expression patterns that are shared both across tissues and between species with over three hundred million years of evolutionary divergence. We further demonstrate the general sharing of gene expression patterns in multiple forms of torpor, implying a common evolutionary origin for this process. Although here we apply StrokeofGenus to analysis of torpor, it can be used to interrogate any other complex physiological processes defined by transient transcriptomic states.

Alpha-2-adrenergic agonists reduce resting energy expenditure in humans during external cooling

Intravenous alpha-2-adrenergic receptor agonists reduce energy expenditure and lower the temperature when shivering begins in humans, allowing a decrease in core body temperature. Because there are few data about similar effects from oral drugs, we tested whether single oral doses of the sedative dexmedetomidine (1 µg/kg sublingual or 4 µg/kg swallowed) or the muscle relaxant tizanidine (8 mg or 16 mg), combined with surface cooling, reduce energy expenditure and core body temperature in humans. A total of 26 healthy participants completed 41 one-day laboratory studies measuring core body temperature using an ingested telemetry capsule and measuring energy expenditure using indirect calorimetry for up to 6 hours after drug ingestion. Dexmedetomidine induced a median 13% - 19% peak reduction and tizanidine induced a median 15% - 22% peak reduction in energy expenditure relative to baseline. Core body temperature decreased a median of 0.5°C - 0.6°C and 0.5°C - 0.7°C respectively. Decreases in temperature occurred after peak reductions in energy expenditure. Energy expenditure increased with a decrease in core temperature in control participants but did not occur after 4 µg/kg dexmedetomidine or 16 mg tizanidine. Plasma levels of dexmedetomidine but not tizanidine were related to mean temperature change. Decreases in heart rate, blood pressure, respiratory rate, cardiac stroke volume index, and cardiac index were associated with the change in metabolic rate after higher drug doses. We conclude that both oral dexmedetomidine and oral tizanidine reduce energy expenditure and allow decrease in core temperature in humans.

Comparing Hypothermic and Thermal Neutral Conditions to Induce Metabolic Suppression

Suppressing metabolism in astronauts could decrease CO2 production. It is unknown whether active cooling is required to suppress metabolism in sedated patients. We hypothesized that hypothermia would have an additive effect with dexmedetomidine on suppressing metabolism. This is a randomized crossover trial of healthy subjects receiving sedation with dexmedetomidine and exposure to a cold (20°C) or thermal neutral (31°C) environment for 3 hours. We measured heart rate, blood pressure, core temperature, resting oxygen consumption (VO2), resting carbon dioxide production (VCO2), and resting energy expenditure (REE) at baseline and each hour of exposure to either environment. We also evaluated components of the Defense Automated Neurobehavioral Assessment (DANA) Brief to evaluate the effect of metabolic suppression on cognition. Six subjects completed the study. Heart rate and core temperature were lower during the cold (56 bpm) condition than the thermal neutral condition (67 bpm). VO2, VCO2, and REE decreased between baseline and the 3-hour measurement in the cold condition (Δ = 0.9 mL/min, 56.94 mL/min, 487.9 Kcal/D, respectively). DANA simple response time increased between baseline and start of recovery in both conditions (20°C 136.9 cognitive efficiency [CE] and 31°C 87.83 CE). DANA procedural reaction time increased between baseline and start of recovery in the cold condition (220.6 CE) but not in the thermal neutral condition. DANA Go/No-Go time increased between baseline and start of recovery in both conditions (20°C 222.1 CE and 31°C 122.3 CE). Sedation and cold environments are required for metabolic suppression. Subjects experienced decrements in cognitive performance in both conditions. A significant recovery period may be required after metabolic suppression before completing mission critical tasks.

Metabolic Manipulation and Therapeutic Hypothermia

Hypothermia has multiple physiological effects, including decreasing metabolic rate and oxygen consumption (VO2). There are few human data about the magnitude of change in VO2 with decreases in core temperature. We aimed to quantify to magnitude of reduction in resting VO2 as we reduced core temperature in lightly sedated healthy individuals. After informed consent and physical screening, we cooled participants by rapidly infusing 20 mL/kg of cold (4°C) saline intravenously and placing surface cooling pads on the torso. We attempted to suppress shivering using a 1 mcg/kg intravenous bolus of dexmedetomidine followed by titrated infusion at 1.0 to 1.5 μg/(kg·h). We measured resting metabolic rate VO2 through indirect calorimetry at baseline (37°C) and at 36°C, 35°C, 34°C, and 33°C. Nine participants had mean age 30 (standard deviation 10) years and 7 (78%) were male. Baseline VO2 was 3.36 mL/(kg·min) (interquartile range 2.98-3.76) mL/(kg·min). VO2 was associated with core temperature and declined with each degree decrease in core temperature, unless shivering occurred. Over the entire range from 37°C to 33°C, median VO2 declined 0.7 mL/(kg·min) (20.8%) in the absence of shivering. The largest average decrease in VO2 per degree Celsius was by 0.46 mL/(kg·min) (13.7%) and occurred between 37°C and 36°C in the absence of shivering. After a participant developed shivering, core body temperature did not decrease further, and VO2 increased. In lightly sedated humans, metabolic rate decreases around 5.2% for each 1°C decrease in core temperature from 37°C to 33°C. Because the largest decrease in metabolic rate occurs between 37°C and 36°C, subclinical shivering or other homeostatic reflexes may be present at lower temperatures.

European space agency's hibernation (torpor) strategy for deep space missions: Linking biology to engineering

Long-duration space missions to Mars will impose extreme stresses of physical and psychological nature on the crew, as well as significant logistical and technical challenges for life support and transportation. Main challenges include optimising overall mass and maintaining crew physical and mental health. These key scopes have been taken up as the baseline for a study by the European Space Agency (ESA) using its Concurrent Design Facility (CDF). It focussed on the biology of hibernation in reducing metabolism and hence stress, and its links to the infrastructure and life support. We concluded that torpor of crew members can reduce the payload with respect to oxygen, food and water but will require monitoring and artificial intelligence (AI) assisted monitoring of the crew. These studies additionally offer new potential applications for patient care on Earth. Keywords: Space flight, concurrent design facility, metabolic reduction.

Contemporary biomedical engineering perspective on volitional evolution for human radiotolerance enhancement beyond low-earth orbit

A primary objective of the National Aeronautics and Space Administration (NASA) is expansion of humankind's presence outside low-Earth orbit, culminating in permanent interplanetary travel and habitation. Having no inherent means of physiological detection or protection against ionizing radiation, humans incur capricious risk when journeying beyond low-Earth orbit for long periods. NASA has made large investments to analyze pathologies from space radiation exposure, emphasizing the importance of characterizing radiation's physiological effects. Because natural evolution would require many generations to confer resistance against space radiation, immediately pragmatic approaches should be considered. Volitional evolution, defined as humans steering their own heredity, may inevitably retrofit the genome to mitigate resultant pathologies from space radiation exposure. Recently, uniquely radioprotective genes have been identified, conferring local or systemic radiotolerance when overexpressed in vitro and in vivo. Aiding in this process, the CRISPR/Cas9 technique is an inexpensive and reproducible instrument capable of making limited additions and deletions to the genome. Although cohorts can be identified and engineered to protect against radiation, alternative and supplemental strategies should be seriously considered. Advanced propulsion and mild synthetic torpor are perhaps the most likely to be integrated. Interfacing artificial intelligence with genetic engineering using predefined boundary conditions may enable the computational modeling of otherwise overly complex biological networks. The ethical context and boundaries of introducing genetically pioneered humans are considered.

Peripheral impairments of oxidative metabolism after a 10-day bed rest are upstream of mitochondrial respiration

Abstract

In order to identify peripheral biomarkers of impaired oxidative metabolism during exercise following a 10‐day bed rest, 10 males performed an incremental exercise (to determine peak pulmonary V̇O₂ (V̇O₂p)) and moderate‐intensity exercises, before (PRE) and after (POST) bed rest. Blood flow response was evaluated in the common femoral artery by Eco‐Doppler during 1 min of passive leg movements (PLM). The intramuscular matching between O₂ delivery and O₂ utilization was evaluated by near‐infrared spectroscopy (NIRS). Mitochondrial respiration was evaluated ex vivo by high‐resolution respirometry in isolated muscle fibres, and in vivo by NIRS by the evaluation of skeletal muscle V̇O₂ (V̇O₂m) recovery kinetics. Resting V̇O₂m was estimated by NIRS. Peak V̇O₂p was lower in POST vs. PRE. The area under the blood flow vs. time curve during PLM was smaller (P = 0.03) in POST (274 ± 233 mL) vs. PRE (427 ± 291). An increased (P = 0.03) overshoot of muscle deoxygenation during a metabolic transition was identified in POST. Skeletal muscle citrate synthase activity was not different (P = 0.11) in POST (131 ± 16 nmol min^–1 mg^–1) vs. PRE (138 ± 19). Maximal ADP‐stimulated mitochondrial respiration (66 ± 18 pmol s^–1 mg^–1 (POST) vs. 72 ± 14 (PRE), P = 0.41) was not affected by bed rest. Apparent K_m for ADP sensitivity of mitochondrial respiration was reduced in POST vs. PRE (P = 0.04). The V̇O₂m recovery time constant was not different (P = 0.79) in POST (22 ± 6 s) vs. PRE (22 ± 6). Resting V̇O₂m was reduced by 25% in POST vs. PRE (P = 0.006). Microvascular‐endothelial function was impaired following a 10‐day bed rest, whereas mitochondrial mass and function (both in vivo and ex vivo) were unaffected or slightly enhanced.

Key points

Ten days of horizontal bed rest impaired in vivo oxidative function during exercise.

Microvascular impairments were identified by different methods.

Mitochondrial mass and mitochondrial function (evaluated both in vivo and ex vivo) were unchanged or even improved (i.e. enhanced mitochondrial sensitivity to submaximal [ADP]).

Resting muscle oxygen uptake was significantly lower following bed rest, suggesting that muscle catabolic processes induced by bed rest/inactivity are less energy‐consuming than anabolic ones.

Be cool to be far: Exploiting hibernation for space exploration

In mammals, torpor/hibernation is a state that is characterized by an active reduction in metabolic rate followed by a progressive decrease in body temperature. Torpor was successfully mimicked in non-hibernators by inhibiting the activity of neurons within the brainstem region of the Raphe Pallidus, or by activating the adenosine A1 receptors in the brain. This state, called synthetic torpor, may be exploited for many medical applications, and for space exploration, providing many benefits for biological adaptation to the space environment, among which an enhanced protection from cosmic rays. As regards the use of synthetic torpor in space, to fully evaluate the degree of physiological advantage provided by this state, it is strongly advisable to move from Earth-based experiments to 'in the field' tests, possibly on board the International Space Station.

Breeding and hibernation of captive meadow jumping mice (Zapus hudsonius)

Hibernating mammals exhibit unique metabolic and physiological phenotypes that have potential applications in medicine or spaceflight, yet our understanding of the genetic basis and molecular mechanisms of hibernation is limited. The meadow jumping mouse, a small North American hibernator, exhibits traits-including a short generation time-that would facilitate genetic approaches to hibernation research. Here we report the collection, captive breeding, and laboratory hibernation of meadow jumping mice. Captive breeders in our colony produced a statistically significant excess of male offspring and a large number of all-male and all-female litters. We confirmed that short photoperiod induced pre-hibernation fattening, and cold ambient temperature facilitated entry into hibernation. During pre-hibernation fattening, food consumption exhibited non-linear dependence on both body mass and temperature, such that food consumption was greatest in the heaviest animals at the coldest temperatures. Meadow jumping mice exhibited a strong circadian rhythm of nightly activity that was disrupted during the hibernation interval. We conclude that it is possible to study hibernation phenotypes using captive-bred meadow jumping mice in a laboratory setting.

Aging-like metabolic and adrenal changes in microgravity: State of the art in preparation for Mars

Endocrine and metabolic changes that typically accompany aging on Earth have been consistently observed in space. Support for the role of gravity in aging has mostly come from ground simulation studies in head down bed rest. However, uncertainties remain and have to be resolved in planning for the ambitious enterprise of sending humans to Mars and back. Stress-related corticosteroid changes and metabolic adaptation to microgravity and their relationship with aging are the object of the present review mostly, albeit of course non exclusively, coming from the personal experience of the authors. The picture coming out of it is that of some, not easily proven, stress-induced cortisol increase accompanied by insulin resistance, both of which represent typical aging-like phenomena mediated by chronic low-grade inflammation. This suggests the need for humans to consider the long journey to safely land, live and work on Mars by taking advantage of integrative medicine solutions including synthetic torpor and/or continuous self-monitoring of eating, sleeping, moving to enable remotely supervised self-treatment.

Radioprotective effects of induced astronaut torpor and advanced propulsion systems during deep space travel

BACKGROUND

Human metabolic suppression is not a new concept, with 1950s scientific literature and movies demonstrating its potential use for deep space travel (Hock, 1960). An artificially induced state of metabolic suppression in the form of torpor would improve the amount of supplies required and therefore lessen weight and fuel required for missions to Mars and beyond (Choukèr et al., 2019). Transfer habitats for human stasis to Mars have been conceived (Bradford et al., 2018). Evidence suggests that animals, when hibernating, demonstrate relative radioprotection compared to their awake state. Experiments have also demonstrated relative radioprotection in conditions of hypothermia as well as during sleep (Bellesi et al., 2016 and Andersen et al., 2009). Circadian rhythm disrupted cells also appear to be more susceptible to radiation damage compared to those that are under a rhythmic control (Dakup et al., 2018). An induced torpor state for astronauts on deep space missions may provide a biological radioprotective state due to a decreased metabolism and hypothermic conditions. A regular enforced circadian rhythm might further limit DNA damage from radiation. The As Low As Reasonably Achievable (A.L.A.R.A.) radiation protection concept defines time, distance and shielding as ways to decrease radiation exposure. Whilst distance cannot be altered in space and shielding either passively or actively may be beneficial, time of exposure may be drastically decreased with improved propulsion systems. Whilst chemical propulsion systems have superior thrust to other systems, they lack high changes in velocity and fuel efficiency which can be achieved with nuclear or electric based propulsion systems. Radiation toxicity could be limited by reduced transit times, combined with the radioprotective effects of enforced circadian rhythms during a state of torpor or hibernation.

OBJECTIVES

1. Investigate how the circadian clock and body temperature may contribute to radioprotection during human torpor on deep space missions. 2. Estimate radiation dose received by astronauts during a transit to Mars with varying propulsion systems.

METHODS

We simulated three types of conditions to investigate the potential radioprotective effect of the circadian clock and decreased temperature on cells being exposed to radiation such that may be the case during astronaut torpor. These conditions were: - Circadian clock strength: strong vs weak. - Light exposure: dark-dark vs light-dark cycle - Body temperature: 37C vs hypothermia vs torpor. We estimated transit times for a mission to Mars from Earth utilizing chemical, nuclear and electrical propulsion systems. Transit times were generated using the General Mission Analysis Tool (GMAT) and Matlab. These times were then input into the National Aeronautics and Space Administration (NASA) Online Tool for the Assessment of Radiation In Space (OLTARIS) computer simulator to estimate doses received by an astronaut for the three propulsion methods.

RESULTS

Our simulation demonstrated an increase in radioprotection with decreasing temperature. The greatest degree of radioprotection was shown in cells that maintained a strong circadian clock during torpor. This was in contrast to relatively lower radioprotection in cells with a weak clock during normothermia. We were also able to demonstrate that if torpor weakened the circadian clock, a protective effect could be partially restored by an external drive such as lighting schedules to aid entrainment i.e.: Blue light exposure for periods of awake and no light for rest times For the propulsion simulation, estimated transit times from Earth to Mars were 258 days for chemical propulsion with 165.9mSv received, 209 days for nuclear propulsion with 134.4mSv received and 80 days for electrical propulsion with 51.4mSv received.

CONCLUSION

A state of torpor for astronauts on deep space missions may not only improve weight, fuel and storage requirements but also provide a potential biological radiation protection strategy. Moreover, maintaining a controlled circadian rhythm during torpor conditions may aid radioprotection. In the not too distant future, propulsion techniques will be improved to limit transit time and hence decrease radiation dose to astronauts. Limiting exposure time and enhancing physiological radioprotection during transit could provide superior radioprotection benefits compared with active and passive radiation shielding strategies alone.