Triiodothyronine increases calcium loss in a bed rest antigravity model for space flight

Hypothyroidism in hibernating brown bears

Brown bears hibernate throughout half of the year as a survival strategy to reduce energy consumption during prolonged periods with scarcity of food and water. Thyroid hormones are the major endocrine regulators of basal metabolic rate in humans. Therefore, we aimed to determine regulations in serum thyroid hormone levels in hibernation compared to the active state to investigate if these are involved in the adaptions for hibernation.We used electrochemiluminescence immunoassay to quantify total triiodothyronine (T₃) and thyroxine (T₄) levels in hibernation and active state in paired serum samples from six subadult Scandinavian brown bears. Additionally, we determined regulations in the liver mRNA levels of three major thyroid hormone-binding proteins; thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin, by analysis of previously published grizzly bear RNA sequencing data.We found that bears were hypothyroid when hibernating with T₄ levels reduced to less than 44% (P = 0.008) and T₃ levels reduced to less than 36% (P = 0.016) of those measured in the active state. In hibernation, mRNA levels of TBG and albumin increased to 449% (P = 0.031) and 121% (P = 0.031), respectively, of those measured in the active state. TTR mRNA levels did not change.Hibernating bears are hypothyroid and share physiologic features with hypothyroid humans, including decreased basal metabolic rate, bradycardia, hypothermia, and fatigue. We speculate that decreased thyroid hormone signaling is a key mediator of hibernation physiology in bears. Our findings shed light on the translational potential of bear hibernation physiology to humans for whom a similar hypometabolic state could be of interest in specific conditions.

Integrative and quantitative bioenergetics: Design of a study to assess the impact of the gut microbiome on host energy balance

The literature is replete with clinical studies that characterize the structure, diversity, and function of the gut microbiome and correlate the results to different disease states, including obesity. Whether the microbiome has a direct impact on obesity has not been established. To address this gap, we asked whether the gut microbiome and its bioenergetics quantitatively change host energy balance. This paper describes the design of a randomized crossover clinical trial that combines outpatient feeding with precisely controlled metabolic phenotyping in an inpatient metabolic ward. The target population was healthy, weight-stable individuals, age 18-45 and with a body mass index ≤30 kg/m2. Our primary objective was to determine within-participant differences in energy balance after consuming a control Western Diet versus a Microbiome Enhancer Diet intervention specifically designed to optimize the gut microbiome for positive impacts on host energy balance. We assessed the complete energy-balance equation via whole-room calorimetry, quantified energy intake, fecal energy losses, and methane production. We implemented conditions of tight weight stability and balance between metabolizable energy intake and predicted energy expenditure. We explored key factors that modulate the balance between host and microbial nutrient accessibility by measuring enteroendocrine hormone profiles, appetite/satiety, gut transit and gastric emptying. By integrating these clinical measurements with future bioreactor experiments, gut microbial ecology analysis, and mathematical modeling, our goal is to describe initial cause-and-effect mechanisms of gut microbiome metabolism on host energy balance. Our innovative methods will enable subsequent studies on the interacting roles of diet, the gut microbiome, and human physiology.

CLINICALTRIALSGOV IDENTIFIER

NCT02939703. The present study reference can be found here: https://clinicaltrials.gov/ct2/show/NCT02939703.

Neurosurgery and Manned Spaceflight

There has been a renewed interest in manned spaceflight due to endeavors by private and government agencies. Publicized goals include manned trips to or colonization of Mars. These missions will likely be of long duration, exceeding existing records for human exposure to extra-terrestrial conditions. Participants will be exposed to microgravity, temperature extremes, and radiation, all of which may adversely affect their physiology. Moreover, pathological mechanisms may differ from those of a terrestrial nature. Known central nervous system (CNS) changes occurring in space include rises in intracranial pressure and spinal unloading. Intracranial pressure increases are thought to occur due to cephalad re-distribution of body fluids secondary to microgravity exposure. Spinal unloading in microgravity results in potential degenerative changes to the bony vertebrae, intervertebral discs, and supportive musculature. These phenomena are poorly understood. Trauma is of highest concern due to its potential to seriously incapacitate crewmembers and compromise missions. Traumatic pathology may also be exacerbated in the setting of altered CNS physiology. Though there are no documented instances of CNS pathologies arising in space, existing diagnostic and treatment capabilities will be limited relative to those on Earth. In instances where neurosurgical intervention is required in space, it is not known whether open or endoscopic approaches are feasible. It is obvious that prevention of trauma and CNS pathology should be emphasized. Further research into neurosurgical pathology, its diagnosis, and treatment in space are required should exploratory or colonization missions be attempted.

Bed rest and accelerated aging in relation to the musculoskeletal and cardiovascular systems and frailty biomarkers: A review

Prolonged bed rest and lifelong physical inactivity cause deleterious effects to multiple physiological systems that appear to hasten aging processes. Many such changes are similar to those seen with microgravity in space, but at a much faster rate. Head down tilt bed rest models are used to study whole-body changes that occur with spaceflight. We propose that bed rest can be used to quantify accelerated human aging in relation to frailty. In particular, frailty as a measure of the accumulation of deficits estimates the variability in aging across systems, and moves away from the traditional single-system approach. Here, we provide an overview of the impact of bed rest on the musculoskeletal and cardiovascular systems as well as frailty-related biological markers and inflammatory cytokines. We also propose future inquiries to study the accumulation of deficits with head down bed rest and bed rest in the clinical setting, specifically to understand how unrepaired and unremoved subclinical and subcellular damage give rise to clinically observable health problems.

Thyroid hormone and angiogenesis

In models of thyroid hormone-induced cardiac hypertrophy, there is appropriate, supportive angiogenesis. Twenty years ago in one such model, angiogenesis in response to the hormone was observed before hypertrophy developed and it is now understood that iodothyronines induce neovascularization in a variety of settings, including the heart, ischemic striated muscle and tumor beds. The molecular mechanism of the proangiogenic action of thyroid hormone is both nongenomic and genomic. It is initiated nongenomically at the cell surface receptor for the hormone on integrin alphavbeta3. Kinase transduction of the hormone signal and, ultimately, transcription of several anagiogenesis-relevant genes result. The genes include basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). In addition, the integrin receptor for thyroid hormone (l-thyroxine, T(4), and 3, 5, 3'-triiodo-l-thyronine, T(3)) engages in crosstalk with the VEGF and bFGF receptors. Occlusion with tetraiodothyroacetic acid (tetrac) of the hormone receptor on the integrin in the absence of T(4) and T(3) suppresses the angiogenic effects of VEGF and bFGF. Tetrac also blocks the proangiogenic actions of T(4) and T(3). Other thyroid hormone analogues that are angiogenic include diiodothyropropionic acid (DITPA) and the nuclear thyroid hormone receptor-beta-selective agonist, GC-1. Thyroid hormone sustains angiogenesis and coronary blood flow about infarcted heart tissue in experimental models and blocks deleterious heart remodeling that otherwise is predictable in such tissue. The hormone may also induce expression of the hypoxia-inducible factor 1alpha (HIF1alpha) gene, a transcription factor important to coronary artery collateralization in the setting of hypoxia. The hormone also causes transcription of the matrix Gla protein (MGP) gene that opposes vascular smooth muscle calcification.