Fast Myosin Heavy Chain Isoforms Research Articles

Fiber type composition of the architecturally distinct regions of human supraspinatus muscle: a cadaveric study.

The human supraspinatus muscle is clinically important as it is frequently injured in older adults and the elderly. We have previously shown that the supraspinatus has a complex architecture with two distinct regions each consisting of three parts. Further we have found dynamic changes in architectural parameters such as fiber bundle length markedly vary between these regions. Fiber types of the supraspinatus have not been thoroughly investigated throughout its volume and are of interest to clinicians treating supraspinatus pathologies. In this study we investigated the distribution of fiber types within the distinct regions and parts of supraspinatus. Samples of supraspinatus were excised from six distinct parts of each muscle from five formalin embalmed specimens (one male, four female; mean age 77±11.1 years) free of tendon pathology. Samples were frozen in liquid nitrogen and then cryosectioned. Serial sections were labeled using immunohistochemical techniques and antibodies against fast or slow myosin heavy chain isoforms. The mean percentage of Type I (slow) fibers ranged from 56.73% to 63.97%. Results demonstrated significant variations in fiber type distribution. The middle part of the anterior region has a significantly greater percentage of Type I fibers compared to that of the posterior. The superficial part of the anterior region has a greater percentage of Type II (fast) fibers compared to the middle and deep parts. Findings aid in highlighting the distinct functions of the anterior and posterior regions, and prompt the need to re-evaluate assessment and treatment techniques established on a limited understanding of the fiber type distribution.

Myosin Isoforms and Contractile Properties of Single Fibers of Human Latissimus Dorsi Muscle

The aim of our study was to investigate fiber type distribution and contractile characteristics of Latissimus Dorsi muscle (LDM). Samples were collected from 18 young healthy subjects (9 males and 9 females) through percutaneous fine needle muscle biopsy. The results showed a predominance of fast myosin heavy chain isoforms (MyHC) with 42% of MyHC 2A and 25% of MyHC 2X, while MyHC 1 represented only 33%. The unbalance toward fast isoforms was even greater in males (71%) than in females (64%). Fiber type distribution partially reflected MyHC isoform distribution with 28% type 1/slow fibers and 5% hybrid 1/2A fibers, while fast fibers were divided into 30% type 2A, 31% type A/X, 4% type X, and 2% type 1/2X. Type 1/slow fibers were not only less abundant but also smaller in cross-sectional area than fast fibers. During maximal isometric contraction, type 1/slow fibers developed force and tension significantly lower than the two major groups of fast fibers. In conclusion, the predominance of fast fibers and their greater size and strength compared to slow fibers reveal that LDM is a muscle specialized mainly in phasic and powerful activity. Importantly, such specialization is more pronounced in males than in females.

Feed restriction delays developmental fast skeletal muscle myosin heavy chain isoforms in turkey poults selected for differential growth

Genetic selection has been very successful at significantly increasing BW and breast muscle proportion in commercial broiler and turkey strains. The mechanisms of breast muscle growth in poultry and the interactive effects of nutritional status and selection are not fully understood. The hypothesis underlying the current study is that feed restriction, simply as a vehicle for controlling early growth, would delay the temporal expression pattern of neonatal (nMyHC) and adult (aMyHC) fast skeletal muscle myosin heavy chain (MyHC) isoforms in the pectoralis major muscle of turkey poults. The poultry growth model used to evaluate this hypothesis consisted of a randombred control turkey line (RBC2) that represents commercial turkeys of the 1960s and a line developed from the RBC2 by selection for BW at 16 wk of age (F line). The F line has significantly heavier breast muscles than the RBC2 concomitant with increased BW, but the proportion of breast muscle relative to BW is similar. A quantitative indirect ELISA using fast skeletal MyHC isoform specific monoclonal antibodies revealed no significant line differences in the temporal expression of posthatch fast skeletal muscle MyHC in ad libitum fed poults. Feed restriction, however, altered the temporal expression patterns of nMyHC and aMyHC in both F line and RBC2 poults compared with the poults fed ad libitum.

Changes in titin and myosin heavy chain isoform composition in skeletal muscles of Mongolian Gerbil (Meriones unguiculatus) after 12-day spaceflight

Changes of titin and myosin heavy chain isoform composition in skeletal muscles (m. soleus, m. gastrocnemius, m. tibialis anterior, m. psoas major) in Mongolian Gerbil (Meriones unguiculatus ) were investigated after 12-day spaceflight on board of Russian space vehicle "Foton-M3". In m. psoas and m. soleus in the gerbils from "Flight" group the expected increase in the content of fast myosin heavy chain isoforms (IIxd and IIa, respectively) were observed. No significant differences were found in the content of IIxd and IIa isoforms of myosin heavy chain in m. tibialis anterior in the gerbils from control group as compared to that in "Flight" group. An unexpected increase in the content of slow myosin heavy chain I isoform and a decrease in the content of fast IIx/d isoform in m. gastrocnemius of the gerbils from "Flight" group were observed. In skeletal muscles of the gerbils from "Flight" group the relative content of titin N2A-isoform was reduced (by 1,2-1,7 times), although the content of its NT-isoform, which was revealed in striated muscles of mammals in our experiments earlier, remained the same. When the content of titin N2A-isoform was decreased, no predictable abnormalities in sarcomeric structure and contractile ability of skeletal muscles in the gerbils from "Flight" group were found. An assumption on the leading role of titin NT-isoform in maintenance of structural and functional properties of striated muscles of mammals was made.

Effects of R92 mutations in mouse cardiac troponin T are influenced by changes in myosin heavy chain isoform

One limitation in understanding how different familial hypertrophic cardiomyopathy (FHC)-related mutations lead to divergent cardiac phenotypes is that such mutations are often studied in transgenic (TG) mouse hearts which contain a fast cycling myosin heavy chain isoform (α-MHC). However, the human heart contains a slow cycling MHC isoform (β-MHC). Given the physiological significance of MHC-troponin interplay effects on cardiac contractile function, we hypothesized that cardiac troponin T (cTnT) mutation-mediated effects on contractile function depend on the type of MHC isoform present in the sarcomere. We tested our hypothesis using two variants of cTnT containing mutations at FHC hotspot R92 (R92L or R92Q), expressed against either an α-MHC or β-MHC background in TG mouse hearts. One finding from our study was that R92L attenuated the length-dependent increase in tension and abolished the length-dependent increase in myofilament Ca2+ sensitivity only when β-MHC was present. In addition, α- and β-MHC isoforms differentially affected how R92 mutations altered crossbridge (XB) recruitment dynamics. For example, the rate of XB recruitment was faster in R92L or R92Q fibers when β-MHC was present, but was unaffected when α-MHC was present. The R92Q mutation sped XB detachment in the presence of β-MHC, but not in the presence of α-MHC. R92Q affected the XB strain-dependent influence on XB recruitment dynamics, an effect not observed for R92L. Our findings have major implications for understanding not only the divergent effects of R92 mutations on cardiac phenotype, but also the distinct effects of MHC isoforms in determining the outcome of mutations in cTnT.

MiR-23a inhibits myogenic differentiation through down regulation of fast myosin heavy chain isoforms

MicroRNAs (miRNAs) are a class of small non-coding RNAs that repress the expression of their target genes post-transcriptionally. MiRNAs participate in the regulation of a variety of biological processes, including development and diseases. However, the functional role and molecular mechanism by which miRNAs regulate skeletal muscle development and differentiation are not fully understood. In this report, we identified miR-23a as a key regulator of skeletal muscle differentiation. Using bioinformatics analyses, miR-23a is predicted to target multiple adult fast myosin heavy chain (Myh) genes, including Myh 1, 2 and 4. Luciferase reporter assays show that miR-23a directly targets the 3′ untranslated regions (UTRs) of these mRNAs. Interestingly, the expression level of mature miR-23a is inversely correlated with myogenic progression in mouse skeletal muscle. Both gain- and loss-of-function studies using C2C12 myoblasts demonstrate that miR-23a inhibits myogenic differentiation. These findings therefore reveal a novel role of miR-23a in regulating myogenic differentiation via inhibiting the expression of fast myosin heavy chain isoforms.

Myosin heavy chain and parvalbumin expression in swimming and feeding muscles of centrarchid fishes: The molecular basis of the scaling of contractile properties

In centrarchid fishes, such as bluegill (Lepomis macrochirus, Rafinesque) and largemouth bass (Micropterus salmoides, Lacepède), the contractile properties of feeding and swimming muscles show different scaling patterns. While the maximum shortening velocity (Vmax) and rate of relaxation from tetanus of swimming or myotomal muscle slow with growth, the feeding muscle shows distinctive scaling patterns. Cranial epaxial muscle, which is used to elevate the head during feeding strikes, retains fast contractile properties across a range of fish sizes in both species. In bass, the sternohyoideous muscle, which depresses the floor of the mouth during feeding strikes, shows faster contractile properties with growth. The objective of this study was to determine the molecular basis of these different scaling patterns. We examined the expression of two muscle proteins, myosin heavy chain (MyHC) and parvalbumin (PV), that affect contractile properties. We hypothesized that the relative contribution of slow and fast MyHC isoforms will modulate Vmax in these fishes, while the presence of PV in muscle will enhance rates of muscle relaxation. Myotomal muscle displays an increase in sMyHC expression with growth, in agreement with its physiological properties. Feeding muscles such as epaxial and sternohyoideus show no change or a decrease in sMyHC expression with growth, again as predicted from contractile properties. PV expression in myotomal muscle decreases with growth in both species, as has been seen in other fishes. The feeding muscles again show no change or an increase in PV expression with growth, contributing to faster contractile properties in these fishes. Both MyHC and PV appear to play important roles in modulating muscle contractile properties of swimming and feeding muscles in centrarchid fishes.

1α,25‐dihydroxyvitamin D3 enhances fast‐myosin heavy chain expression in differentiated C2C12 myoblasts

We investigated the effect of VD3 (1α,25-dihydroxyvitamin D3) on the proliferating, differentiating and differentiated phases of C2C12 myoblasts, a mouse skeletal muscle cell line. VD3 treatment in 10% FBS (fetal bovine serum) inhibited the proliferation and viability of the cells in a dose-dependent manner. It also dose-dependently increased the percentage of cells in the G0/G1 phase as shown by flow cytometry. In the differentiating phase, VD3 treatment inhibited the formation of myotubes and the expression of total myosin heavy chain at both the mRNA and protein levels. In the differentiated phase, treatment had no significant effect on the amount of total myosin heavy chain, as Western blot analysis with MF20 antibody [DSHB (Developmental Studies Hybridoma Bank)] showed. However, significantly greater expression of fast myosin heavy chain in 1 nM VD3 was found by Western blot analysis with MY-32 (Sigma). Thus VD3 inhibited the proliferation of myoblasts during proliferating and differentiating phases, whereas it increased the expression of the fast myosin heavy chain isoform in the differentiated phase. The data indicate that an adequate concentration of VD3 might have an anabolic effect on differentiated skeletal muscle.

Role of NO-synthase in regulation of protein metabolism of stretched rat m. soleus muscle during functional unloading

Gravitational unloading causes atrophy of muscle fibers and can lead to destruction of cytoskeletal and contractile proteins. Along with the atrophic changes, unloaded muscle frequently demonstrates significant shifts in the ratio of muscle fibers expressing fast and slow myosin heavy chain isoforms. Stretching of the m. soleus during hindlimb suspension prevents its atrophy. We supposed that neuronal NO-synthase (NOS) (which is attached to membrane dystrophin-sarcoglycan complex) can contribute to maintenance of protein metabolism in the muscle and prevent its atrophy when m. soleus is stretched. To test this hypothesis, we used Wistar rats (56 animals) in experiments with hindlimb suspension during 14 days. The group of hindlimb suspended rats with stretched m. soleus was injected with L-NAME to block NOS activity. We found that m. soleus mass and its protein content in hindlimb-suspended rats with stretched m. soleus were preserved due to prevention of protein degradation. NOS is involved in maintenance of expression of some muscle proteins. Proliferation of satellite cells in stretched m. soleus may be due to nNOS activity, but maintenance of muscle mass upon stretching is regulated not by NOS alone.

Effects of myosin heavy chain (MHC) plasticity induced by HMGCoA‐reductase inhibition on skeletal muscle functions

The rate-limiting step of cholesterol biosynthetic pathway is catalyzed by 3-hydroxy-3-methylglutaryl coenzyme reductase (HGMR), whose inhibitors, the statins, widely used in clinical practice to treat hypercholesterolemia, often cause myopathy, and rarely rhabdomyolysis. All studies to date are limited to the definition of statin-induced myotoxicity omitting to investigate whether and how HMGR inhibition influences muscle functions. To this end, 3-mo-old male rats (Rattus norvegicus) were treated for 3 wk with a daily intraperitoneal injection of simvastatin (1.5 mg/kg/d), and biochemical, morphological, mechanical, and functional analysis were performed on extensor digitorum longus (EDL) muscle. Our results show that EDL muscles from simvastatin-treated rats exhibited reduced HMGR activity; a 15% shift from the fastest myosin heavy-chain (MHC) isoform IIb to the slower IIa/x; and reduced power output and unloaded shortening velocity, by 41 and 23%, respectively, without any change in isometric force and endurance. Moreover, simvastatin-treated rats showed a decrease of maximum speed reached and the latency to fall off the rotaroad (∼-30%). These results indicate that the molecular mechanism of the impaired muscle function following statin treatment could be related to the plasticity of fast MHC isoform expression.

There is no slowing of motility speed with increased body size in rat, human, horse and rhinoceros independent on temperature and skeletal muscle myosin isoform

The predictions of scaling of skeletal muscle shortening velocity made by A.V. Hill 60-years ago have proven to be remarkably accurate at the cellular level. The current investigation looks to extend the study of scaling of contractile speed to the level of the molecular motor protein myosin at both physiological and unphysiological low temperatures. A single muscle cell in vitro motility assay to test myosin function, i.e. myosin extracted from short single muscle fibre segments, was used in four species representing a 5 500-fold difference in body mass (rat, man, horse and rhinoceros) at temperatures ranging from 15 to 35 °C. The in vitro motility speed increased as the temperature of the assay increased, but a more profound effect was observed on the slower isoforms, narrowing the relative differences between fast and slow myosin heavy chain (MyHC) isoforms at physiological temperature in all species. The in vitro motility speed varied according to MyHC isoform within each species: I < IIa < IIx < IIb, but the expected scaling relationship within orthologous myosin isoforms was not observed at any temperature. The scaling effect of body size and limb length on shortening velocity at the muscle fibre level, i.e. the decreasing shortening velocity associated with increasing body weight and limb length, was not confirmed at the motor protein level when including mammals of very large size. Thus, other factors than myosin structure and function appear to cause this scaling effect and thin filament isoform expression or myofilament lattice spacing are forwarded as alternative underlying factors.

Myosin Heavy Chain Plasticity in Aging Skeletal Muscle with Aerobic Exercise Training

PURPOSE: To comprehensively assess myosin heavy chain (MHC) plasticity in aging skeletal muscle in response to aerobic exercise training. METHODS: Muscle biopsy samples were obtained from the vastus lateralis of eight older women (70±2y) before and after 12-wks of cycle ergometer training. MHC composition was measured at the mRNA level via qPCR and at the protein level in muscle homogenates and individual myofibers using SDS-PAGE. RESULTS: At the mRNA level, aerobic training reduced (p<0.05) MHC IIx and tended to elevate (p=0.10) MHC I mRNA expression. The homogenate proportion of MHC I protein increased (54±4 to 61±2%, p<0.05) and MHC IIx protein decreased (8±1 to 4±1%, p<0.05) after training while the MHC IIa proportion was unaltered. Single myofiber analysis demonstrated a robust elevation (42±4 to 52±3%, p<0.05) in the percentage of MHC I fibers with a reciprocal decrease (48±3 to 38±4% p<0.05) in the percentage of MHC IIa fibers after training. CONCLUSION: These results document a reduction in the relative amount of fast MHC isoforms (MHC IIa or IIx) with a concomitant increase in slow MHC I isoforms independent of the method employed. The shift to an oxidative myosin heavy chain phenotype indicates that aerobic exercise training augments skeletal muscle health of older women and may serve as a countermeasure to age-related skeletal muscle atrophy. Supported by NIH AG032127

Prdm1 (Blimp-1) and the Expression of Fast and Slow Myosin Heavy Chain Isoforms during Avian Myogenesis In Vitro

BackgroundMultiple types of fast and slow skeletal muscle fibers form during early embryogenesis in vertebrates. In zebrafish, formation of the earliest slow myofibers in fin muscles requires expression of the zinc-finger transcriptional repressor Prdm1 (also known as Blimp1). To further understand how the role of Prdm1 in early myogenesis may vary through evolution and during development, we have now analyzed Prdm1 expression in the diverse types of myotubes that form in culture from somitic, embryonic, and fetal chicken myoblasts.Principal FindingsIn cultures of somitic, embryonic limb, and fetal limb chicken cells, we found that Prdm1 was expressed in all of the differentiated muscle cells that formed, including those that expressed only fast myosin heavy chain isoforms, as well as those that co-expressed both fast and slow myosin heavy chain isoforms. Prdm1 was also expressed in Pax7-positive myoblasts, as well as in non-myogenic cells in the cultures. Furthermore, though all differentiated cells in control somite cultures co-expressed fast and slow myosin heavy chains, antisense knockdown of Prdm1 expression inhibited the formation of these co-expressing cells in somite cultures.ConclusionsIn chicken myogenic cell cultures, Prdm1 was expressed in most Pax7-positive myoblasts and in all differentiated muscle cells, irrespective of the developmental stage of cell donor or the pattern of fast and slow myosin heavy chains expressed in the differentiated cells that were formed. Thus, Prdm1 was expressed in myogenic cells prior to terminal differentiation; and, after differentiation, Prdm1 expression was not limited to cells that expressed slow myosin heavy chain isoforms. In addition, Prdm1 appeared to be required for differentiation of the somitic myocytes, which are the earliest myocytes to form in the avian embryo.

Force deficits of human type I, IIa, and IIa/IIx muscle fibers following a single in vitro eccentric contraction

Fast muscles from small laboratory animals show greater force deficits after eccentric contractions than slow muscles. How this applies to humans is unclear as humans do not express the same fast myosin heavy chain (MHC) isoforms as these animal models. To investigate relationships between eccentric damage and fiber type in human muscle, we measured peak Ca2+ activated force (15°C) of chemically skinned fiber segments (vastus lateralis, 6 subjects, age = 24 ± 2) before and after a single eccentric contraction (25% strain, 50% of pre‐eccentric unloaded shortening velocity). Fiber type was determined by gel electrophoresis. Fibers showing visible post‐eccentric myofibrillar tearing (N = 8) were excluded from analysis. At baseline, type IIa (125 ± 6 kN/m2; n = 28) and IIa/IIx (133 ± 7 kN/m2; n = 12) fibers produced greater Ca2+ activated force than type I fibers (111 ± 3 kN/m2; n = 38). A significant fiber type × time interaction revealed similar force deficits for type I (11 ± 1 kN/m2 or 10 ± 1% of pre force) and IIa (14 ± 2 kN/m2, 11 ± 1%) fibers but a 3‐fold greater deficit for IIa/IIx fibers (40 ± 5 kN/m2, 30 ± 3%). Control fibers subjected to a single isometric contraction showed force deficits of only 2 ± 1 kN/m2 (1 ± 1 %) with no difference between fiber types. These results indicate that human fast hybrid fibers (IIa/IIx) are more sensitive to a standardized in vitro eccentric contraction then either type I or type IIa fibers.

Changing sexes in a complex environment ‐ shifts in skeletal myosin isoforms in coastal populations of California sheephead fish

Myosin heavy chain (MyHC) isoform expression was investigated in California sheephead, Semicossyphus pulcher, a protogynous hermaphroditic fish found in coastal Southern California and Northern Mexico. This species was collected from nine capture sites near the Channel Islands where both overall size, and size of individuals at the transitional sexual stage, varies considerably. We therefore cloned MyHC isoforms and analyzed proteins by SDS‐PAGE to determine if gender, size, water temperature, or geographic location influenced contractile protein expression. The relative percentage of the fast MyHC isoform decreased as sheephead fish transitioned from female to male. The highest expression of slow MyHC isoform was observed in male sheephead. The lowest expression of slow MyHC was seen in Northern and off‐shore female populations, when compared to Southern and near‐shore female populations. Relative percentage of slow MyHC isoform positively correlated with standard length. Thus, in this interesting system where muscle mass undergoes dramatic growth accompanying gender transitions, MyHC isoforms are likely influenced by both sexual development and environment.

Systemic Myostatin Inhibition via Liver-Targeted Gene Transfer in Normal and Dystrophic Mice

BackgroundMyostatin inhibition is a promising therapeutic strategy to maintain muscle mass in a variety of disorders, including the muscular dystrophies, cachexia, and sarcopenia. Previously described approaches to blocking myostatin signaling include injection delivery of inhibitory propeptide domain or neutralizing antibodies.Methodology/Principal FindingsHere we describe a unique method of myostatin inhibition utilizing recombinant adeno-associated virus to overexpress a secretable dominant negative myostatin exclusively in the liver of mice. Systemic myostatin inhibition led to increased skeletal muscle mass and strength in control C57 Bl/6 mice and in the dystrophin-deficient mdx model of Duchenne muscular dystrophy. The mdx soleus, a mouse muscle more representative of human fiber type composition, demonstrated the most profound improvement in force production and a shift toward faster myosin-heavy chain isoforms. Unexpectedly, the 11-month-old mdx diaphragm was not rescued by long-term myostatin inhibition. Further, mdx mice treated for 11 months exhibited cardiac hypertrophy and impaired function in an inhibitor dose–dependent manner.Conclusions/SignificanceLiver-targeted gene transfer of a myostatin inhibitor is a valuable tool for preclinical investigation of myostatin blockade and provides novel insights into the long-term effects and shortcomings of myostatin inhibition on striated muscle.

Inferior oblique muscles from patients with inferior oblique overaction: Examination of myosin heavy chain isoforms

Purpose: Few studies have examined the potential changes in the inferior oblique muscles of patients who have been diagnosed with inferior oblique overaction. This study examined the percentage of fibers positive for fast, slow or embryonic myosin heavy chain isoforms (MHC) in patients with and without prior surgery compared to normal control inferior oblique muscles.

Myosin heavy chain composition of tiger (Panthera tigris) and cheetah (Acinonyx jubatus) hindlimb muscles

Felids have a wide range of locomotor activity patterns and maximal running speeds, including the very fast cheetah (Acinonyx jubatas), the roaming tiger (Panthera tigris), and the relatively sedentary domestic cat (Felis catus). As previous studies have suggested a relationship between the amount and type of activity and the myosin heavy chain (MHC) isoform composition of a muscle, we assessed the MHC isoform composition of selected hindlimb muscles from these three felid species with differing activity regimens. Using gel electrophoresis, western blotting, histochemistry, and immunohistochemistry with MHC isoform-specific antibodies, we compared the MHC composition in the tibialis anterior, medial gastrocnemius (MG), plantaris (Plt), and soleus muscles of the tiger, cheetah, and domestic cat. The soleus muscle was absent in the cheetah. At least one slow (type I) and three fast (types IIa, IIx, and IIb) MHC isoforms were present in the muscles of each felid. The tiger had a high combined percentage of the characteristically slower isoforms (MHCs I and IIa) in the MG (62%) and the Plt (86%), whereas these percentages were relatively low in the MG (44%) and Plt (55%) of the cheetah. In general, the MHC isoform characteristics of the hindlimb muscles matched the daily activity patterns of these felids: the tiger has daily demands for covering long distances, whereas the cheetah has requirements for speed and power.

Muscle after spinal cord injury

The morphological and contractile changes of muscles below the level of the lesion after spinal cord injury (SCI) are dramatic. In humans with SCI, a fiber-type transformation away from type I begins 4-7 months post-SCI and reaches a new steady state with predominantly fast glycolytic IIX fibers years after the injury. There is a progressive drop in the proportion of slow myosin heavy chain (MHC) isoform fibers and a rise in the proportion of fibers that coexpress both the fast and slow MHC isoforms. The oxidative enzymatic activity starts to decline after the first few months post-SCI. Muscles from individuals with chronic SCI show less resistance to fatigue, and the speed-related contractile properties change, becoming faster. These findings are also present in animals. Future studies should longitudinally examine changes in muscles from early SCI until steady state is reached in order to determine optimal training protocols for maintaining skeletal muscle after paralysis.

Rapid muscle atrophy response to unloading: pretranslational processes involving MHC and actin

Skeletal muscles, especially weight-bearing muscles, are very sensitive to changes in loading state. The aim of this paper was to characterize the dynamic changes in the unloaded soleus muscle in vivo following a short bout of hindlimb suspension (HS), testing the hypothesis that transcriptional events respond early to the atrophic stimulus. In fact, we observed that after only 1 day of HS, primary transcript levels of skeletal alpha-actin and type I myosin heavy chain (MHC) genes were significantly reduced by more than 50% compared with ground control levels. The degree of the decline for the mRNA expression of actin and type I MHC lagged behind that of the pre-mRNA levels after 1 day of HS, but by 2 and 7 days of HS, large decreases were observed. Although the faster MHC isoforms, IIx and IIb, began to be expressed in soleus after 1 day of HS, a relatively significant shift in mRNA expression from the slow MHC isoform type I toward these fast MHC isoforms did not emerge until 7 days of HS. One day of HS was sufficient to show significant decreases in mRNA levels of putative signaling factors serum response factor (SRF), suppressor of cytokine signaling-3 (SOCS3), and striated muscle activator of Rho signaling (STARS), although transcription factors yin-yang-1 (YY1) and transcriptional enhancing factor-1 (TEF-1) were not significantly affected by HS. The protein levels of actin and type I MHC were significantly decreased after 2 days of HS, and SRF protein was significantly decreased after 7 days HS. Our results show that after only 1 day of unloading, pre-mRNA and mRNA expression of muscle proteins and muscle-specific signaling factors are significantly reduced, suggesting that the downregulation of the synthesis side of the protein balance equation that occurs in atrophying muscle is initiated rapidly.