Effects of spaceflight and thyroid deficiency on hindlimb development. I. Muscle mass and IGF-I expression

Factors mediating spaceflight-induced skeletal muscle atrophy

Skeletal muscle atrophy is a well-known consequence of spaceflight. Because of the potential significant impact of muscle atrophy and muscle dysfunction on astronauts and to their mission, a thorough understanding of the mechanisms of this atrophy and the development of effective countermeasures is critical. Spaceflight-induced muscle atrophy is similar to atrophy seen in many terrestrial conditions, and therefore our understanding of this form of atrophy may also contribute to the treatment of atrophy in humans on Earth. The unique environmental features humans encounter in space include the weightlessness of microgravity, space radiation, and the distinctive aspects of living in a spacecraft. The disuse and unloading of muscles in microgravity are likely the most significant factors that mediate spaceflight-induced muscle atrophy, and have been extensively studied and reviewed. However, there are numerous other direct and indirect effects on skeletal muscle that may be contributing factors to the muscle atrophy and dysfunction seen as a result of spaceflight. This review offers a novel perspective on the issue of muscle atrophy in space by providing a comprehensive overview of the unique aspects of the spaceflight environment and the various ways in which they can lead to muscle atrophy. We systematically review the potential contributions of these different mechanisms of spaceflight-induced atrophy and include findings from both actual spaceflight and ground-based models of spaceflight in humans, animals, and in vitro studies.

The Role of Incubation Conditions in the Onset of Avian Myopathies

White striping, wooden breast, and spaghetti muscle have become common myopathies in broilers worldwide. Several research reports have indicated that the origin of these lesions is metabolic disorders. These failures in normal metabolism can start very early in life, and suboptimal incubation conditions may trigger some of the key alterations on muscle metabolism. Incubation conditions affect the development of muscle and can be associated with the onset of myopathies. A series of experiments conducted with broilers, turkeys, and ducks are discussed to overview primary information showing the main changes in breast muscle histomorphology, metabolism, and physiology caused by suboptimal incubation conditions. These modifications may be associated with current myopathies. Those effects of incubation on myopathy occurrence and severity have also been confirmed at slaughter age. The impact of egg storage, temperature profiles, oxygen concentrations, and time of hatch have been evaluated. The effects have been observed in diverse species, genetic lines, and both genders. Histological and muscle evaluations have detected that myopathies could be induced by extended hypoxia and high temperatures, and those effects depend on the genetic line. Thus, these modifications in muscle metabolic responses may make hatchlings more susceptible to develop myopathies during grow out due to thermal stress, high-density diets, and fast growth rates.

Regulation of Protein Synthesis in Inactivated Skeletal Muscle: Signal Inputs, Protein Kinase Cascades, and Ribosome Biogenesis

Disuse atrophy of skeletal muscles is characterized by a significant decrease in the mass and size of muscle fibers. Disuse atrophy develops as a result of prolonged reduction in the muscle functional activity caused by bed rest, limb immobilization, and real or simulated microgravity. Disuse atrophy is associated with the downregulation of protein biosynthesis and simultaneous activation of protein degradation. This review is focused on the key molecular mechanisms regulating the rate of protein synthesis in mammalian skeletal muscles during functional unloading.

Spaceflight affects postnatal development of the aortic wall in rats

We investigated effect of microgravity environment during spaceflight on postnatal development of the rheological properties of the aorta in rats. The neonate rats were randomly divided at 7 days of age into the spaceflight, asynchronous ground control, and vivarium control groups (8 pups for one dam). The spaceflight group rats at 9 days of age were exposed to microgravity environment for 16 days. A longitudinal wall strip of the proximal descending thoracic aorta was subjected to stress-strain and stress-relaxation tests. Wall tensile force was significantly smaller in the spaceflight group than in the two control groups, whereas there were no significant differences in wall stress or incremental elastic modulus at each strain among the three groups. Wall thickness and number of smooth muscle fibers were significantly smaller in the spaceflight group than in the two control groups, but there were no significant differences in amounts of either the elastin or collagen fibers among the three groups. The decreased thickness was mainly caused by the decreased number of smooth muscle cells. Plastic deformation was observed only in the spaceflight group in the stress-strain test. A microgravity environment during spaceflight could affect postnatal development of the morphological and rheological properties of the aorta.

An assessment of the long-term effects of simulated microgravity on cranial neural crest cells in zebrafish embryos with a focus on the adult skeleton

It is becoming increasingly important to address the long-term effects of exposure to simulated microgravity as the potential for space tourism and life in space become prominent topics amongst the World's governments. There are several studies examining the effects of exposure to simulated microgravity on various developmental systems and in various organisms; however, few examine the effects beyond the juvenile stages. In this study, we expose zebrafish embryos to simulated microgravity starting at key stages associated with cranial neural crest cell migration. We then analyzed the skeletons of adult fish. Gross observations and morphometric analyses show that exposure to simulated microgravity results in stunted growth, reduced ossification and severe distortion of some skeletal elements. Additionally, we investigated the effects on the juvenile skull and body pigmentation. This study determines for the first time the long-term effects of embryonic exposure to simulated microgravity on the developing skull and highlights the importance of studies investigating the effects of altered gravitational forces.

Growth inhibition and compensation in response to neonatal hypoxia in rats

BACKGROUND

Hypoxia (Hx) is an important disease mechanism in prematurity, childhood asthma, and obesity. In children, Hx results in chronic inflammation.

METHODS

We investigated the effects of Hx (12% O2) during postnatal days 2-20 in rats. Control groups were normoxic control (Nc), and normoxic growth restricted (Gr) (14-pup litters).

RESULTS

The Hx-exposed and Gr rats had similar decreases in growth. Hx increased plasma tumor necrosis factor-α (TNF-α) and interleukin 6 (IL-6) levels and decreased insulin-like growth factor 1 (IGF-I) and vascular endothelial growth factor (VEGF) levels. Hx resulted in hypertrophy of the right ventricle (RV) but disproportionate decrements in limb skeletal muscle (SM) growth. miR-206 was depressed in the hypertrophied RV of Hx rats but was increased in growth-retarded SM. Hx resulted in decreased RV messenger RNA (mRNA) level for myostatin but had no effect on SM myostatin. The mRNA for Hx-sensitive factors such as hypoxia inducible factor-1α (HIF-1α) was depressed in the RV of Hx rats, suggesting negative feedback.

CONCLUSION

The results indicate that Hx induces a proinflammatory state that depresses growth-regulating mechanisms and that tissues critical for survival, such as the heart, can escape from this general regulatory program to sustain life. This study identifies accessible biomarkers for evaluating the impact of interventions designed to mitigate the long-term deleterious consequences of Hx that all too often occur in babies born prematurely.

Thermal manipulations in late-term chick embryos have immediate and longer term effects on myoblast proliferation and skeletal muscle hypertrophy

We investigated the cellular and molecular bases for the promotion of muscle development and growth by temperature manipulations (TMs) during late-term chick embryogenesis. We show that incubation at 39.5 degrees C (increase of 1.7 degrees C from normal conditions) from embryonic days 16 to 18 (E16 to E18) for 3 or 6 h daily increased diameter of myofibers as of day 13 of age and enhanced absolute muscle growth relative to controls, until day 35 of age. TMs had immediate (E17) and later (up to 2 wk posthatch) effects in elevating muscle cell proliferation relative to controls. This was indicated by higher DNA incorporation of thymidine and a higher number of cells expressing PCNA in intact muscle, accompanied by higher Pax7 levels, all reflecting a higher number of myogenic cells, and suggesting that the increased hypertrophy can be attributed to a higher reservoir of myogenic progeny cells produced in response to the TM. IGF-I levels were higher in the TM groups than in controls, implying a mechanism by which heat manipulations in chicks affect muscle development, with locally secreted IGF-I playing a major role. Whereas hypertrophy was similar in both TM groups, cell proliferation and Pax7 levels were more robust in the 6-h muscle, mainly posthatch, suggesting a differential effect of various TM periods on cell reservoir vs. hypertrophy and a high sensitivity of myoblasts to relatively small changes in heat duration with respect to these processes, which is manifested in the short and long term.

Effects of artificial rearing on contractile properties of genioglossus muscle in Sprague–Dawley rat

Mammals suckle from a nipple during the early neonatal period to obtain nourishment. The genioglossus muscle helps position and move the tongue for efficient suckling. The purpose of this study was to examine the contractile properties and myosin heavy chain (MHC) phenotype of the genioglossus following an early period of artificial rearing, which reduced nutritive suckling. Beginning at 3 days of age, rats were fed via gastric cannula until postnatal day 14 (P14). At P14, artificially reared rat pups were either allowed to grow to postnatal day 42 (P42) or anaesthetised and prepared for experimentation. Comparisons were made between artificially reared and dam reared groups at P14 and P42. At P14 maximum tetanic tension and fatigue index were lower in the artificially reared group than the dam reared group. By P42, artificially reared rats had a higher fatigue index and lower percentage of MHCIIa than dam reared rats. The artificial rearing technique employed in this study was adequate to produce chronic changes in fatigue resistance and MHC distribution in genioglossus muscle of rat; the changes observed here may be similar to changes that occur in premature human infants requiring early artificial feedings.

Determinants of Disuse-Induced Skeletal Muscle Atrophy: Exercise and Nutrition Countermeasures to Prevent Protein Loss

Muscle atrophy results from a variety of conditions such as disease states, neuromuscular injuries, disuse, and aging. Absence of gravitational loading during spaceflight or long-term bed rest predisposes humans to undergo substantial loss of muscle mass and, consequently, become unfit and/or unhealthy. Disuse- or inactivity-induced skeletal muscle protein loss takes place by differential modulation of proteolytic and synthetic systems. Transcriptional, translational, and posttranslational events are involved in the regulation of protein synthesis and degradation in myofibers, and these regulatory events are known to be responsive to contractile activity. However, regardless of the numerous studies which have been performed, the intracellular signals that mediate skeletal muscle wasting due to muscular disuse are not completely comprehended. Understanding the triggers of atrophy and the mechanisms that regulate protein loss in unloaded muscles may lead to the development of effective countermeasures such as exercise and dietary intervention. The objective of the present review is to provide a window into the molecular processes that underlie skeletal muscle remodeling and to examine what we know about exercise and nutrition countermeasures designed to minimize muscle atrophy.

The effects of microgravity on the development of surface righting in rats

The active interaction of neonatal animals with their environment has been shown to be a decisive factor in the postnatal development of sensory systems, which demonstrates a critical period in their maturation. The direct demonstration of such a dependence on the rearing environment has not been demonstrated for motor system function. Nor has the role of gravity in mammalian motor system development been investigated. Here we report the results of two space flight missions examining the effect of removing gravity on the development of surface righting. Since the essential stimulus that drives this synergy, gravitation, was missing, righting did not occur while the animals were in the microgravity environment. We hypothesize that this absence of contextual motor experience arrested the maturation of the motor tactics for surface righting. Such effects were permanent in rats spending 16 days (from postnatal day (P), P14 to P30), but were transient in animals spending nine days (from P15 to P24) in microgravity. Thus, active, contextual interaction with the environment during a critical period of development is necessary for the postnatal maturation of motor tactics as exemplified by surface righting, and such events must occur within a particular time period. Further, Earth's gravitational field is not assumed by the developing motor system. Rather, postnatal motor system development is appropriate to the gravitational field in which the animal is reared.

Long-term effects of microgravity on the swimming behaviour of young rats

The postnatal development of sensory systems has been shown in studies over the last four decades to be influenced by experience during critical periods of development. We report here that similar experience-dependent development can be observed in the swimming behaviour of young rats reared from postnatal day 14 (P14) to P30 in the reduced gravitational field of low earth orbit. Animals flown in space when placed in the water on the day of landing maintained their head and forelimbs in a balanced posture. However, until the animals began to swim, their hindquarters showed little lateral postural control resulting in rotation about the longitudinal axis (60 degrees+/-4 deg). Such results suggest an 'unlinking' of postural control of the forequarters from the hindquarters in the early hours after landing. Similar instability seen in animals age-matched to the day of launch (97+/-7 deg) and in ground control animals (9+/-3 deg) was corrected within one or two rotations, even in the absence of swimming. Animals flown in space began to swim sooner after being placed in the water, and the duration of swimming strokes was shorter than in control animals. Motion analysis revealed a difference in the swimming style on landing day. In flight animals, the knee joint was more flexed throughout the stroke, there was a narrower range of movement, and the linear velocity of the tip of the foot was faster throughout most of the stroke than in age-matched control animals. Thus, posture in the water as well as swimming speed and style were altered in the animals flown in space. Some of these characteristics persisted for as long as the animals were followed (30 days). These included the short pre-swimming interval and short stroke duration in flight animals. These findings clearly show that an altered gravitational field influences the postnatal development of motor function. The nature of the differences between animals reared in space for 16 days and those remaining on the ground reflects an adaptation of the flight animals to the microgravity environment. The data suggest that the most fundamental of these adaptations is a resetting of the basic motor rhythm to a higher frequency.

Insulin facilitation of muscle protein synthesis following resistance exercise in hindlimb-suspended rats is independent of a rapamycin-sensitive pathway

Hindlimb suspension (HS) results in rapid losses of muscle mass, which may in part be explained by attenuated rates of protein synthesis. Mammalian target of rapamycin (mTOR) regulates protein synthesis and has been implicated as a potential mediator of the muscle mass decrement with HS. This study examined the effect of resistance exercise, a muscle hypertrophy stimulant, on rates of protein synthesis after 4 days of HS in mature male Sprague-Dawley rats. Flywheel resistance exercise (2 sets x 25 repetitions) was conducted on days 2 and 4 of HS (HSRE). Sixteen hours after the last exercise bout, soleus muscles were assessed for in vitro rates of protein synthesis, with and without insulin (signaling agonist) and/or rapamycin (mTOR inhibitor). Results demonstrated that soleus mass was reduced (P < 0.05) with HS, but this loss of mass was not observed (P > 0.05) with HSRE. Muscle protein synthesis was diminished (P < 0.05) with HS, with or without insulin. HSRE also had reduced rates of synthesis without insulin; however, insulin administration yielded higher (P < 0.05) rates in HSRE compared with HS or control. Rapamycin diminished protein synthesis in all groups (P < 0.05), but insulin rescued synthesis rates in HS and HSRE to levels similar to insulin alone for each group, suggesting that alternate signaling pathways develop to increase protein synthesis with HS. These results demonstrate that the capacity for an augmented anabolic response to resistance exercise is maintained after 4 days of HS and is independent of a rapamycin-sensitive pathway.

IL-6-induced skeletal muscle atrophy

Chronic, low-level elevation of circulating interleukin (IL)-6 is observed in disease states as well as in many outwardly healthy elderly individuals. Increased plasma IL-6 is also observed after intense, prolonged exercise. In the context of skeletal muscle, IL-6 has variously been reported to regulate carbohydrate and lipid metabolism, increase satellite cell proliferation, or cause muscle wasting. In the present study, we used a rodent local infusion model to deliver modest levels of IL-6, comparable to that present after exercise or with chronic low-level inflammation in the elderly, directly into a single target muscle in vivo. The aim of this study was to examine the direct effects of IL-6 on skeletal muscle in the absence of systemic changes in this cytokine. Data included cellular and molecular markers of cytokine and growth factor signaling (phosphorylation and mRNA content) as well as measurements to detect muscle atrophy. IL-6 infusion resulted in muscle atrophy characterized by a preferential loss of myofibrillar protein (-17%). IL-6 induced a decrease in the phosphorylation of ribosomal S6 kinase (-60%) and STAT5 (-33%), whereas that of STAT3 was increased approximately twofold. The changes seen in the IL-6-infused muscles suggest alterations in the balance of growth factor-related signaling in favor of a more catabolic profile. This suggests that downregulation of growth factor-mediated intracellular signaling may be a mechanism contributing to the development of muscle atrophy induced by elevated IL-6.

Mammalian development in space

Life on Earth, and thus the reproductive and ontogenetic processes of all extant species and their ancestors, evolved under the constant influence of the Earth's l g gravitational field. These considerations raise important questions about the ability of mammals to reproduce and develop in space. In this chapter, I review the current state of our knowledge of spaceflight effects on developing mammals. Recent studies are revealing the first insights into how the space environment affects critical phases of mammalian reproduction and development, viz., those events surrounding fertilization, embryogenesis, pregnancy, birth, postnatal maturation and parental care. This review emphasizes fetal and early postnatal life, the developmental epochs for which the greatest amounts of mammalian spaceflight data have been amassed. The maternal-offspring system, the coordinated aggregate of mother and young comprising mammalian development, is of primary importance during these early, formative developmental phases. The existing research supports the view that biologically meaningful interactions between mothers and offspring are changed in the weightlessness of space. These changes may, in turn, cloud interpretations of spaceflight effects on developing offspring. Whereas studies of mid-pregnant rats in space have been extraordinarily successful, studies of young rat litters launched at 9 days of postnatal age or earlier, have been encumbered with problems related to the design of in-flight caging and compromised maternal-offspring interactions. Possibilities for mammalian birth in space, an event that has not yet transpired, are considered. In the aggregate, the results indicate a strong need for new studies of mammalian reproduction and development in space. Habitat development and systematic ground-based testing are important prerequisites to future research with young postnatal rodents in space. Together, the findings support the view that the environment within which young mammals develop, comprised of its mother and siblings, is of paramount importance in interpreting spaceflight effects.

A rat resistance exercise regimen attenuates losses of musculoskeletal mass during hindlimb suspension

Exposure to microgravity and/or spaceflight causes dramatic losses in both muscle and bone mass. In normal gravity, resistance exercise has been effectively used to increase muscle and bone mass. We tested a novel form of resistance exercise training using flywheel technology as a countermeasure to offset the loss of musculoskeletal mass during 4 weeks of adult rat hindlimb suspension (HS), an unloading model of microgravity. Male, Sprague-Dawley rats (6-month old) were operantly conditioned to perform resistance exercise, and then randomly assigned to groups of sedentary control (CON), HS, and HS with resistance exercise training (HSRT; 2 sets of approximately 21 repetitions, 3 days week(-1) for 4 weeks during suspension). In soleus, HS resulted in lower (P < 0.05) muscle mass to body mass ratio (approximately 50% of controls) and rates of protein synthesis. HSRT significantly attenuated the loss of muscle mass in soleus muscle, and rates of protein synthesis for soleus were similar for HSRT and controls. There were no differences among groups for mass or rates of protein synthesis in extensor digitorum longus. In cancellous regions of the distal femur, HS resulted in significant reductions of bone mineral density (BMD), but this was restored to control levels with HSRT. Cortical regions of the femur were not different among HS, HSRT or control groups. Together, these data suggest that resistance training using flywheel technology may be a promising tool to attenuate losses of the musculoskeletal system during periods of hindlimb unloading.

Chronic heart failure and skeletal muscle catabolism: effects of exercise training

Although the clinical picture of cardiac cachexia is well-known in patients with advanced chronic heart failure (CHF) the factors that determine who is at risk for this progressive catabolic syndrome and who is not remain unclear. Different endocrine systems have been accused of being involved in this process: an imbalance between catabolic and anabolic steroids with an elevated cortisol/dihydroepiandrosterone ratio, an increased resting metabolic rate due to high levels of circulating catecholamines, various cytokines are activated in CHF (i.e. TNF-alpha, IL-6, IL-1beta and others), and elevated levels of growth hormone (GH) with inappropriately normal or low serum levels of insulin-like growth factor-I (IGF-I) have been described in cardiac cachexia. These catabolic factors contribute to peripheral muscle atrophy, augment the expression of the inducible nitric oxide synthase (iNOS), which in turn inhibits the aerobic cellular metabolism. The present review examines whether the catabolic factors can be influenced by a classical anabolic intervention: regular physical exercise training. Long-term training programs increase skeletal muscle cytochrome c oxidase activity and are associated with reduced local expression of pro-inflammatory cytokines as well as iNOS, and augment local IGF-I production. In concert, these beneficial effects of exercise training may help to retard the catabolic process in CHF finally leading to cardiac cachexia and death.

Effects of microgravity on myogenic factor expressions during postnatal development of rat skeletal muscle

To clarify the role of gravity in the postnatal development of skeletal muscle, we exposed neonatal rats at 7 days of age to microgravity. After 16 days of spaceflight, tibialis anterior, plantaris, medial gastrocnemius, and soleus muscles were removed from the hindlimb musculature and examined for the expression of MyoD-family transcription factors such as MyoD, myogenin, and MRF4. For this purpose, we established a unique semiquantitative method, based on RT-PCR, using specific primers tagged with infrared fluorescence. The relative expression of MyoD in the tibialis anterior and plantaris muscles and that of myogenin in the plantaris and soleus muscles were significantly reduced (P < 0.001) in the flight animals. In contrast, MRF4 expression was not changed in any muscle. These results suggest that MyoD and myogenin, but not MRF4, are sensitive to gravity-related stimuli in some skeletal muscles during postnatal development.

Effects of spaceflight and thyroid deficiency on rat hindlimb development. II. Expression of MHC isoforms

Both slow-twitch and fast-twitch muscles are undifferentiated after birth as to their contractile protein phenotype. Thus we examined the separate and combined effects of spaceflight (SF) and thyroid deficiency (TD) on myosin heavy chain (MHC) gene expression (protein and mRNA) in muscles of neonatal rats (7 and 14 days of age at launch) exposed to SF for 16 days. Spaceflight markedly reduced expression of the slow, type I MHC gene by approximately 55%, whereas it augmented expression of the fast IIx and IIb MHCs in antigravity skeletal muscles. In fast muscles, SF caused subtle increases in the fast IIb MHC relative to the other adult MHCs. In contrast, TD prevented the normal expression of the fast MHC phenotype, particularly the IIb MHC, whereas TD maintained expression of the embryonic/neonatal MHC isoforms; this response occurred independently of gravity. Collectively, these results suggest that normal expression of the type I MHC gene requires signals associated with weight-bearing activity, whereas normal expression of the IIb MHC requires an intact thyroid state acting independently of the weight-bearing activities typically encountered during neonatal development of laboratory rodents. Finally, MHC expression in developing muscles is chiefly regulated by pretranslational processes based on the tight relationship between the MHC protein and mRNA data.