Regulation of vertebrate muscle differentiation by thyroid hormone: the role of the myoD gene family

Associations between short-term exposure to air pollution and thyroid function in a representative sample of the Korean population

BACKGROUND

Disruption of thyroid function can profoundly affect various organ systems. However, studies on the association between air pollution and thyroid function are relatively scarce and most studies have focused on the long-term effects of air pollution among pregnant women.

OBJECTIVES

This study aimed to explore the associations between short-term exposure to air pollution and thyroid function in the general population.

METHODS

Data from the Korea National Health and Nutrition Examination Survey (2013-2015) were analyzed (n = 5,626). Air pollution concentrations in residential addresses were estimated using Community Multiscale Air Quality models. The moving averages of air pollution over 7 days were set as exposure variables through exploratory analyses. Linear regression and quantile g-computation models were constructed to assess the effects of individual air pollutants and air pollution mixture, respectively.

RESULTS

A 10-ppb increase in NO2 (18.8-μg/m3 increase) and CO (11.5-μg/m3 increase) was associated with 2.43% [95% confidence interval (CI): 0.42, 4.48] and 0.19% (95% CI: 0.01, 0.36) higher thyroid-stimulating hormone (TSH) levels, respectively. A 10-μg/m3 increase in PM2.5 and a 10-ppb increase in O3 (19.6-μg/m3 increment) were associated with 0.87% (95% CI: 1.47, -0.27) and 0.59% (95% CI: 1.18, -0.001) lower free thyroxine (fT4) levels, respectively. A simultaneous quartile increase in PM2.5, NO2, O3, and CO levels was associated with lower fT4 but not TSH levels.

CONCLUSIONS

As the subtle changes in thyroid function can affect various organ systems, the present results may have substantial public health implications despite the relatively modest effect sizes. Because this was a cross-sectional study, it is necessary to conduct further experimental or repeated-measures studies to consolidate the current results.

Maternal Exercise Before and During Pregnancy Facilitates Embryonic Myogenesis by Enhancing Thyroid Hormone Signaling

Background: Maternal exercise (ME) improves fetal and offspring muscle development, but mechanisms remain to be established. Since the thyroid hormone (TH) is critical for cell differentiation during embryonic development, we hypothesized that ME elevates TH receptor (THR) signaling in embryos, which promotes embryonic myogenesis. Methods: Female mice were exercised daily on a treadmill or received a daily TH, triiodothyronine (T3) injection. Embryos (embryonic day 12.5 [E12.5]) and P19 cells were used for studying effects of TH on embryonic myogenesis. TH levels in serum and embryos after ME or T3I were analyzed. Expression of TH signaling related genes and myogenic genes was assessed. THRα binding to the promoters of myogenic genes was investigated by chromatin immunoprecipitation-qantitative polymerase chain reaction (ChIP-qPCR). A CRISPR/CAS9 plasmid was utilized to knock out THRα in P19 cells. Results: ME elevated TH levels in both maternal circulation and embryos, which were correlated with enhanced TH signaling and myogenesis. At E12.5, both myogenic determinants (Pax3, Pax7) and myogenic regulatory factors (Myf5, Myod) were upregulated in ME embryos. ME increased THRα content and elevated messenger RNA (mRNA) expression of TH transporter Slc16a2 and deiodinase Dio2. In addition, the THRα binding to the promoters of Pax3/7 was increased. In P19 embryoid bodies, T3 promoted myogenic differentiation, which was abolished by ablating THRα. Furthermore, maternal daily injection of T3 at a level matching exercised mothers promoted embryonic myogenesis. Conclusions: ME promotes TH delivery to the embryos and enhances embryonic myogenesis, which is partially mediated by enhanced TH signaling in ME embryos.

A Significant Association of Upper Limb Muscle Strength with Thyroid Function in Overweight and Obese Population: A Study of the Sixth Korea National Health and Nutrition Examination Survey (KNHANES 2014-2015)

Background. As skeletal muscle is one of main targets of thyroid hormone signalling, an association of thyroid function and muscle strength could be expected. The aim of study is to evaluate the association of free thyroxine (FT4) and thyrotropin (TSH) with upper limb muscle strength, measured by hand grip strength, in subjects with normal FT4 from national representative data. The study utilized the sixth edition of the Korea National Health and Nutrition Examination Survey. After exclusion of subjects with FT4 level out of normal range, a history of thyroid disease or cerebral disease, restricted activity, and incomplete data, a total of 3503 were recruited (age range 19-80 years, 51% male). FT4 positively correlated with upper limb muscle strength (β coefficient = -12.84, p < 0.001), while TSH did negatively (β coefficient = -0.37, p=0.002). After adjusting for confounding factors, statistical significance disappeared. However, among subjects with BMI above 23 kg/m², a negative correlation of TSH with upper limb muscle strength was found in a younger age group (19-39 years old) (β coefficient = -0.56, p=0.021), while FT4 positively correlated with upper limb muscle strength (β coefficient = 3.24, p=0.019) in an older group (above 40 years old). In overweight and obese subjects, a significant association of thyroid function with upper limb muscle strength was observed in nation-wide representative data. High TSH in a younger group and low FT4 in an older group could be risk factors for decreased upper limb muscle strength in obese population.

Thyroid Hormone Protects from Fasting-Induced Skeletal Muscle Atrophy by Promoting Metabolic Adaptation

Thyroid hormones regulate a wide range of cellular responses, via non-genomic and genomic actions, depending on cell-specific thyroid hormone transporters, co-repressors, or co-activators. Skeletal muscle has been identified as a direct target of thyroid hormone T3, where it regulates stem cell proliferation and differentiation, as well as myofiber metabolism. However, the effects of T3 in muscle-wasting conditions have not been yet addressed. Being T3 primarily responsible for the regulation of metabolism, we challenged mice with fasting and found that T3 counteracted starvation-induced muscle atrophy. Interestingly, T3 did not prevent the activation of the main catabolic pathways, i.e., the ubiquitin-proteasome or the autophagy-lysosomal systems, nor did it stimulate de novo muscle synthesis in starved muscles. Transcriptome analyses revealed that T3 mainly affected the metabolic processes in starved muscle. Further analyses of myofiber metabolism revealed that T3 prevented the starvation-mediated metabolic shift, thus preserving skeletal muscle mass. Our study elucidated new T3 functions in regulating skeletal muscle homeostasis and metabolism in pathological conditions, opening to new potential therapeutic approaches for the treatment of skeletal muscle atrophy.

Expression profiles of microRNAs in skeletal muscle of sheep by deep sequencing

Objective

MicroRNAs are a class of endogenous small regulatory RNAs that regulate cell proliferation, differentiation and apoptosis. Recent studies on miRNAs are mainly focused on mice, human and pig. However, the studies on miRNAs in skeletal muscle of sheep are not comprehensive.

Methods

RNA-seq technology was used to perform genomic analysis of miRNAs in prenatal and postnatal skeletal muscle of sheep. Targeted genes were predicted using miRanda software and miRNA-mRNA interactions were verified by quantitative real-time polymerase chain reaction. To further investigate the function of miRNAs, candidate targeted genes were enriched for analysis using gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) enrichment.

Results

The results showed total of 1,086 known miRNAs and 40 new candidate miRNAs were detected in prenatal and postnatal skeletal muscle of sheep. In addition, 345 miRNAs (151 up-regulated, 94 down-regulated) were differentially expressed. Moreover, miRanda software was performed to predict targeted genes of miRNAs, resulting in a total of 2,833 predicted targets, especially miR-381 which targeted multiple muscle-related mRNAs. Furthermore, GO and KEGG pathway analysis confirmed that targeted genes of miRNAs were involved in development of skeletal muscles.

Conclusion

This study supplements the miRNA database of sheep, which provides valuable information for further study of the biological function of miRNAs in sheep skeletal muscle.

Thyroid hormone signaling and deiodinase actions in muscle stem/progenitor cells

Thyroid hormone (TH) regulates such crucial biological functions as normal growth, development and metabolism of nearly all vertebrate tissues. In skeletal muscle, TH plays a critical role in regulating the function of satellite cells, the bona fide skeletal muscle stem cells. Deiodinases (D2 and D3) have been found to modulate the expression of various TH target genes in satellite cells. Regulation of the expression and activity of the deiodinases constitutes a cell-autonomous, pre-receptor mechanism that controls crucial steps during the various phases of myogenesis. Here, we review the roles of deiodinases in skeletal muscle stem cells, particularly in muscle homeostasis and upon regeneration. We focus on the role of T3 in stem cell functions and in commitment towards lineage progression. We also discuss how deiodinases might be therapeutically exploited to improve satellite-cell-mediated muscle repair in skeletal muscle disorders or injury.

Permanent Deiodinase Type 2 Deficiency Strongly Perturbs Zebrafish Development, Growth, and Fertility

Iodothyronine deiodinases are selenocysteine-containing enzymes that activate or inactivate thyroid hormones (THs). Deiodinase type 2 (Dio2) catalyzes the conversion of the prohormone T4 into the transcriptionally active T3 and is the predominant activating deiodinase in zebrafish. Using zinc finger nucleases, we generated two different dio2(-/-) mutant zebrafish lines to investigate the physiological function of this TH activator. The first line contains a deletion of 9 bp, resulting in an in-frame elimination of three conserved amino acids. The other line is characterized by an insertion of 4 bp, leading to the introduction of a premature stop-codon. Both lines completely lack Dio2 activity, resulting in a strong reduction of T3 abundancy in all tissues tested. Early development is clearly perturbed in these animals, as shown by a diverse set of morphometric parameters, defects in swim bladder inflation, and disturbed locomotor activity tested between 1 and 7 days after fertilization. Permanent Dio2 deficiency also provokes long-term effects because growth and especially fertility are severely hampered. Possible compensatory mechanisms were investigated in adult dio2(-/-) mutants, revealing a down-regulation of the inactivating deiodinase Dio3 and TH receptor transcript levels. As the first nonmammalian model with permanent Dio2 deficiency, these mutant zebrafish lines provide evidence that Dio2 is essential to assure normal development and to obtain a normal adult phenotype.

Thyroid Hormone Receptor α Plays an Essential Role in Male Skeletal Muscle Myoblast Proliferation, Differentiation, and Response to Injury

Thyroid hormone plays an essential role in myogenesis, the process required for skeletal muscle development and repair, although the mechanisms have not been established. Skeletal muscle develops from the fusion of precursor myoblasts into myofibers. We have used the C2C12 skeletal muscle myoblast cell line, primary myoblasts, and mouse models of resistance to thyroid hormone (RTH) α and β, to determine the role of thyroid hormone in the regulation of myoblast differentiation. T3, which activates thyroid hormone receptor (TR) α and β, increased myoblast differentiation whereas GC1, a selective TRβ agonist, was minimally effective. Genetic approaches confirmed that TRα plays an important role in normal myoblast proliferation and differentiation and acts through the Wnt/β-catenin signaling pathway. Myoblasts with TRα knockdown, or derived from RTH-TRα PV (a frame-shift mutation) mice, displayed reduced proliferation and myogenic differentiation. Moreover, skeletal muscle from the TRα1PV mutant mouse had impaired in vivo regeneration after injury. RTH-TRβ PV mutant mouse model skeletal muscle and derived primary myoblasts did not have altered proliferation, myogenic differentiation, or response to injury when compared with control. In conclusion, TRα plays an essential role in myoblast homeostasis and provides a potential therapeutic target to enhance skeletal muscle regeneration.

Effect of hypothyroidism on growth performance, carcass composition and meat quality of fat-tailed Lori-Bakhtiari lambs

The aim of the present study was to investigate the effects of induction hypothyroidism by propylthiouracil (PTU) on the growth performance and meat quality of fat-tailed Lori-Bakhtiari lambs. Eighteen Lori-Bakhtiari male lambs were randomly assigned to one of three groups (n = 6) and received daily treatments (gavage) consisting of 0 (Control: C), 10 (Low: L) or 20 (High: H) mg PTU/kg bodyweight/day for 60 days. PTU decreased plasma triiodothyronine and thyroxine concentration in both L and H (P < 0.0001). Lambs treated with PTU (L and H) had lower feed intake (P < 0.004), feed conversion efficiency (P < 0.003), and greater intramuscular fat than C lambs (P < 0.035). Meat from the L and H lambs had lower cooking loss and shear force, and also higher L* (lightness) than C lambs (P < 0.004, P < 0.015 and P < 0.025, respectively). The meat of H and L lambs was more tender than C lambs (P < 0.032). However, the meat of H lambs required fewer chews before swallowing than C lambs (P < 0.041). Generally, induction of mild hypothyroidism appeared to improve feed conversion efficiency and meat quality of lambs.

Intracellular inactivation of thyroid hormone is a survival mechanism for muscle stem cell proliferation and lineage progression

Precise control of the thyroid hormone (T3)-dependent transcriptional program is required by multiple cell systems, including muscle stem cells. Deciphering how this is achieved and how the T3 signal is controlled in stem cell niches is essentially unknown. We report that in response to proliferative stimuli such as acute skeletal muscle injury, type 3 deiodinase (D3), the thyroid hormone-inactivating enzyme, is induced in satellite cells where it reduces intracellular thyroid signaling. Satellite cell-specific genetic ablation of dio3 severely impairs skeletal muscle regeneration. This impairment is due to massive satellite cell apoptosis caused by exposure of activated satellite cells to the circulating TH. The execution of this proapoptotic program requires an intact FoxO3/MyoD axis, both genes positively regulated by intracellular TH. Thus, D3 is dynamically exploited in vivo to chronically attenuate TH signaling under basal conditions while also being available to acutely increase gene programs required for satellite cell lineage progression.

Thyroid hormones and skeletal muscle--new insights and potential implications

Thyroid hormone signalling regulates crucial biological functions, including energy expenditure, thermogenesis, development and growth. The skeletal muscle is a major target of thyroid hormone signalling. The type 2 and 3 iodothyronine deiodinases (DIO2 and DIO3, respectively) have been identified in skeletal muscle. DIO2 expression is tightly regulated and catalyses outer-ring monodeiodination of the secreted prohormone tetraiodothyronine (T4) to generate the active hormone tri-iodothyronine (T3). T3 can remain in the myocyte to signal through nuclear receptors or exit the cell to mix with the extracellular pool. By contrast, DIO3 inactivates T3 through removal of an inner-ring iodine. Regulation of the expression and activity of deiodinases constitutes a cell-autonomous, pre-receptor mechanism for controlling the intracellular concentration of T3. This local control of T3 activity is crucial during the various phases of myogenesis. Here, we review the roles of T3 in skeletal muscle development and homeostasis, with a focus on the emerging local deiodinase-mediated control of T3 signalling. Moreover, we discuss these novel findings in the context of both muscle homeostasis and pathology, and examine how skeletal muscle deiodinase activity might be therapeutically harnessed to improve satellite-cell-mediated muscle repair in patients with skeletal muscle disorders, muscle atrophy or injury.

Knockdown of type 3 iodothyronine deiodinase severely perturbs both embryonic and early larval development in zebrafish

Exposure to appropriate levels of thyroid hormones (THs) at the right time is of key importance for normal development in all vertebrates. Type 3 iodothyronine deiodinase (D3) is the prime TH-inactivating enzyme, and its expression is highest in the early stages of vertebrate development, implying that it may be necessary to shield developing tissues from overexposure to THs. We used antisense morpholino knockdown to examine the role of D3 during early development in zebrafish. Zebrafish possess 2 D3 genes, dio3a and dio3b. Here, we show that both genes are expressed during development and both contribute to in vivo D3 activity. However, dio3b mRNA levels in embryos are higher, and the effects of dio3b knockdown on D3 activity and on the resulting phenotype are more severe. D3 knockdown induced an overall delay in development, as determined by measurements of otic vesicle length, eye and ear size, and body length. The time of hatching was also severely delayed in D3-knockdown embryos. Importantly, we also observed a severe disturbance of several aspects of development. Swim bladder development and inflation was aberrant as was the development of liver and intestine. Furthermore, D3-knockdown larvae spent significantly less time moving, and both embryos and larvae exhibited perturbed escape responses, suggesting that D3 knockdown affects muscle development and/or functioning. These data indicate that D3 is essential for normal zebrafish embryonic and early larval development and show the value of morpholino knockdown in this model to further elucidate the specific role of D3 in some aspects of vertebrate development.

How does Trichinella spiralis make itself at home?

The nurse cell-parasite complex of Trichinella spiralis is unlike anything else in Nature. It is derived from a normal portion of striated skeletal muscle cell and develops in a matter of 15 to 20 days after the larva invades that cell type. What are the molecular mechanisms at work that result in this unique relationship? Here, Dickson Despommier presents a hypothesis to account for its formation, in which secreted tyvelosylated proteins of the larva play a central role. These proteins are always present in the intracellular niche of the larva from Day 7 after infection and may be responsible for redirecting host genomic expression, leading to nurse cell formation.

Thyroid hormone accelerates the differentiation of adult hippocampal progenitors

Disrupted thyroid hormone function evokes severe physiological consequences in the immature brain. In adulthood, although clinical reports document an effect of thyroid hormone status on mood and cognition, the molecular and cellular changes underlying these behavioural effects are poorly understood. More recently, the subtle effects of thyroid hormone on structural plasticity in the mature brain, in particular on adult hippocampal neurogenesis, have come to be appreciated. However, the specific stages of adult hippocampal progenitor development that are sensitive to thyroid hormone are not defined. Using nestin-green fluorescent protein reporter mice, we demonstrate that thyroid hormone mediates its effects on hippocampal neurogenesis by influencing Type 2b and Type 3 progenitors, although it does not alter proliferation of either the Type 1 quiescent progenitor or the Type 2a amplifying neural progenitor. Thyroid hormone increases the number of doublecortin (DCX)-positive Type 3 progenitors, and accelerates neuronal differentiation into both DCX-positive immature neurones and neuronal nuclei-positive granule cell neurones. Furthermore, we show that this increase in neuronal differentiation is accompanied by a significant induction of specific transcription factors involved in hippocampal progenitor differentiation. In vitro studies using the neurosphere assay support a direct effect of thyroid hormone on progenitor development because neurospheres treated with thyroid hormone are shifted to a more differentiated state. Taken together, our results indicate that thyroid hormone mediates its neurogenic effects via targeting Type 2b and Type 3 hippocampal progenitors, and suggests a role for proneural transcription factors in contributing to the effects of thyroid hormone on neuronal differentiation of adult hippocampal progenitors.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

The FSHD atrophic myotube phenotype is caused by DUX4 expression

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is linked to deletions in 4q35 within the D4Z4 repeat array in which we identified the double homeobox 4 (DUX4) gene. We found stable DUX4 mRNAs only derived from the most distal D4Z4 unit and unexpectedly extended to the flanking pLAM region that provided an intron and a polyadenylation signal. DUX4 encodes a transcription factor expressed in FSHD but not control primary myoblasts or muscle biopsies. The DUX4 protein initiates a large transcription deregulation cascade leading to muscle atrophy and oxidative stress, which are FSHD key features.

METHODOLOGY/PRINCIPAL FINDINGS

We now show that transfection of myoblasts with a DUX4 expression vector leads to atrophic myotube formation associated with the induction of E3 ubiquitin ligases (MuRF1 and Atrogin1/MAFbx) typical of muscle atrophy. DUX4 induces expression of downstream targets deregulated in FSHD such as mu-crystallin and TP53. We developed specific siRNAs and antisense oligonucleotides (AOs) targeting the DUX4 mRNA. Addition of these antisense agents to primary FSHD myoblast cultures suppressed DUX4 protein expression and affected expression of the above-mentioned markers.

CONCLUSIONS/SIGNIFICANCE

These results constitute a proof of concept for the development of therapeutic approaches for FSHD targeting DUX4 expression.

The FoxO3/type 2 deiodinase pathway is required for normal mouse myogenesis and muscle regeneration

The active thyroid hormone 3,5,3' triiodothyronine (T3) is a major regulator of skeletal muscle function. The deiodinase family of enzymes controls the tissue-specific activation and inactivation of the prohormone thyroxine (T4). Here we show that type 2 deiodinase (D2) is essential for normal mouse myogenesis and muscle regeneration. Indeed, D2-mediated increases in T3 were essential for the enhanced transcription of myogenic differentiation 1 (MyoD) and for execution of the myogenic program. Conversely, the expression of T3-dependent genes was reduced and after injury regeneration markedly delayed in muscles of mice null for the gene encoding D2 (Dio2), despite normal circulating T3 concentrations. Forkhead box O3 (FoxO3) was identified as a key molecule inducing D2 expression and thereby increasing intracellular T3 production. Accordingly, FoxO3-depleted primary myoblasts also had a differentiation deficit that could be rescued by high levels of T3. In conclusion, the FoxO3/D2 pathway selectively enhances intracellular active thyroid hormone concentrations in muscle, providing a striking example of how a circulating hormone can be tissue-specifically activated to influence development locally.

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.

Effects of maternal hyperthermia on myogenesis-related factors in developing upper limb

BACKGROUND

Maternal hyperthermia is one causative factor in various congenital anomalies in experimental animals and humans. In the present study, we assessed the effects of high temperature on limb myogenesis in mice.

METHODS

Pregnant mice, C57BL/6 strain, were exposed to hyperthermia (43 degrees C, 5 minutes) on embryonic day (ED) 8. Fetuses on ED 11, 13, 15, and 17 and neonates on postnatal day (PD) 1 were collected. To characterize the effects of hyperthermia on myogenesis-related factors Pax3, MyoD, myogenin, and myosin heavy chain (MyHC) during skeletal muscle development, we performed RT-PCR, western blotting, immunohistochemistry, and transmission electron microscopy.

RESULTS

Pax3 gene expression was still detected on ED 13 in hyperthermia-exposed fetuses. The expression of MyoD protein was down-regulated in fetuses exposed to hyperthermia. In contrast, myogenin and MyHC protein expression were up-regulated on PD 1 and ED 17, respectively, in the group exposed to hyperthermia. Immunohistochemical analysis confirmed the findings from western blot analysis. Compared with control neonates, a TEM study revealed immature muscle fibers in PD 1 hyperthermia neonates. Thus, our studies showed that maternal hyperthermia induced delayed expression of Pax3 and inhibited expression of MyoD proteins, which are known to play important roles in migration of myogenic progenitor cells, and in myoblast proliferation. In addition, maternal hyperthermia also delayed the expression of myogenin protein for the formation of myotubes, and MyHC protein, which is one of the final muscle differentiation factors.

CONCLUSION

Our data suggest that maternal hyperthermia delays limb myogenesis in part by disregulating the expression of key myogenesis-related factors.

Influence of hyperthyroid conditions on gene expression in extraocular muscles of rats

Extraocular muscles (EOMs) are a highly specialized type of tissue with a wide range of unique properties, including characteristic innervation, development, and structural proteins. Even though EOMs are frequently and prominently affected by thyroid-associated diseases, little is known about the direct effects of thyroid hormone on these muscles. To create a comprehensive profile of changes in gene expression levels in EOMs induced by thyroid hormone, hyperthyroid conditions were simulated by treating adult Sprague-Dawley rats with intraperitoneal injections of the thyroid hormone 3,3',5-triiodo-L-thyronine (T(3)); subsequently, microarray analysis was used to determine changes in mRNA levels in EOMs from T(3)-treated animals relative to untreated control animals. The expression of 468 transcripts was found to be significantly altered, with 466 of these transcripts downregulated in EOMs from T(3)-treated animals. The biological processes into which the affected genes could be grouped included cellular metabolism, transport, biosynthesis, protein localization, and cell homeostasis. Moreover, 15 distinct biochemical canonical pathways were represented among the genes with altered transcription levels. Strikingly, myostatin (Gdf8), a potent negative regulator of muscle growth, was found to be strongly downregulated in EOMs from T(3)-treated animals. Together, these findings suggest that pathological concentrations of thyroid hormone have a unique effect on gene expression in EOMs, which is likely to play a hitherto neglected role in thyroid-associated ophthalmopathies.

Identification of a novel modulator of thyroid hormone receptor-mediated action

Background

Diabetes is characterized by reduced thyroid function and altered myogenesis after muscle injury. Here we identify a novel component of thyroid hormone action that is repressed in diabetic rat muscle.

Methodology/Principal Findings

We have identified a gene, named DOR, abundantly expressed in insulin-sensitive tissues such as skeletal muscle and heart, whose expression is highly repressed in muscle from obese diabetic rats. DOR expression is up-regulated during muscle differentiation and its loss-of-function has a negative impact on gene expression programmes linked to myogenesis or driven by thyroid hormones. In agreement with this, DOR enhances the transcriptional activity of the thyroid hormone receptor TR_α1. This function is driven by the N-terminal part of the protein. Moreover, DOR physically interacts with TR _α1 and to T₃-responsive promoters, as shown by ChIP assays. T₃ stimulation also promotes the mobilization of DOR from its localization in nuclear PML bodies, thereby indicating that its nuclear localization and cellular function may be related.

Conclusions/Significance

Our data indicate that DOR modulates thyroid hormone function and controls myogenesis. DOR expression is down-regulated in skeletal muscle in diabetes. This finding may be of relevance for the alterations in muscle function associated with this disease.

Abnormal expression of mu-crystallin in facioscapulohumeral muscular dystrophy

To identify proteins expressed abnormally in facioscapulohumeral muscular dystrophy (FSHD), we extracted soluble proteins from deltoid muscle biopsies from unaffected control and FSHD patients and analyzed them using two-dimensional electrophoresis, mass spectrometry and immunoblotting. Muscles from patients with FSHD showed large increases over controls in a single soluble, 34 kDa protein (pI=5.08) identified by mass spectrometry and immunoblotting as mu-crystallin (CRYM). Soluble fractions of biopsies of several other myopathies and muscular dystrophies showed no appreciable increases in mu-crystallin. Mu-crystallin has thyroid hormone and NADPH binding activity and so may influence differentiation and oxidative stress responses, reported to be altered in FSHD. It is also linked to retinal and inner ear defects, common in FSHD, suggesting that its up-regulation may play a specific and important role in pathogenesis of FSHD.

Hepatic and muscle expression of thyroid hormone receptors in association with body and muscle growth in large yellow croaker, Pseudosciaena crocea (Richardson)

The role of thyroid hormone (TH) and its receptors (TRs) in the regulation of body growth and muscle accretion is well established in mammals and birds, whereas the involvement of THs and TRs in fish growth, especially during the muscle accretion period of juvenile-adult transition, is unknown. This study describes the cloning of the partial cDNA sequences of TRalpha and TRbeta in large yellow croaker, Pseudosciaena crocea (Richardson) and the patterns of TRalpha and TRbeta mRNA expression in liver and muscle of 1- and 2-year-old large yellow croaker, associated with changes in body mass and muscle characteristics. Two TRalpha isoforms (TRalpha1, TRalpha2) and TRbeta were identified in large yellow croaker. The deduced amino acid sequences showed high homology to the TRs of human and other teleosts. Hepatic TRbeta mRNA expression was markedly lower in 2-year-old large yellow croaker compared with the 1-year-old, while no significant age difference was observed for hepatic TRalpha mRNA expression. Muscle expression of TRalpha mRNA was significantly higher in 2-year-old large yellow croaker, whereas TRbeta exhibited no significant age difference. Meanwhile, serum concentration of T(4) was significantly decreased in 2-year-old large yellow croaker, but no change was observed for T(3). The body mass, fork length and body height of 2-year-old large yellow croaker were 4.7, 1.6 and 1.7 times greater, respectively compared with that of 1-year-old. Average diameters of skeletal muscle in 2-year-old large yellow croaker were remarkably larger than that in 1-year-old with no significant difference in muscle crude fat content. The down-regulation of hepatic TRbeta expression was associated with the decrease in general growth rate and the increase in muscle expression of TRalpha was accompanied with muscle accretion and myofiber hypertrophy, implicating the different roles of TRs in the regulation of growth in large yellow croaker.

Differential regulation of imprinting in the murine embryo and placenta by the Dlk1-Dio3 imprinting control region

Genomic imprinting is an epigenetic mechanism controlling parental-origin-specific gene expression. Perturbing the parental origin of the distal portion of mouse chromosome 12 causes alterations in the dosage of imprinted genes resulting in embryonic lethality and developmental abnormalities of both embryo and placenta. A 1 Mb imprinted domain identified on distal chromosome 12 contains three paternally expressed protein-coding genes and multiple non-coding RNA genes, including snoRNAs and microRNAs, expressed from the maternally inherited chromosome. An intergenic, parental-origin-specific differentially methylated region, the IG-DMR, which is unmethylated on the maternally inherited chromosome, is necessary for the repression of the paternally expressed protein-coding genes and for activation of the maternally expressed non-coding RNAs: its absence causes the maternal chromosome to behave like the paternally inherited one. Here, we characterise the developmental consequences of this epigenotype switch and compare these with phenotypes associated with paternal uniparental disomy of mouse chromosome 12. The results show that the embryonic defects described for uniparental disomy embryos can be attributed to this one cluster of imprinted genes on distal chromosome 12 and that these defects alone, and not the mutant placenta, can cause prenatal lethality. In the placenta, the absence of the IG-DMR has no phenotypic consequence. Loss of repression of the protein-coding genes occurs but the non-coding RNAs are not repressed on the maternally inherited chromosome. This indicates that the mechanism of action of the IG-DMR is different in the embryo and the placenta and suggests that the epigenetic control of imprinting differs in these two lineages.

The mitochondrion as a primary site of action of steroid and thyroid hormones: presence and action of steroid and thyroid hormone receptors in mitochondria of animal cells

Mitochondria are key cellular organelles that regulate events related to energy production and apoptosis. These processes are modulated, in turn, by steroid and thyroid hormones in the course of their actions on metabolism, growth and development. In this context, a direct effect of these hormones on the mitochondrial-linked processes, possibly by way of cognate mitochondrial receptors, has been proposed. In this paper we review data from the literature and present new findings supporting this concept. Receptors for steroid hormones, glucocorticoids and estrogens, and for T(3), have been detected in mitochondria by immunofluorescence labeling and confocal laser microscopy, by Western blotting of mitochondrial proteins and by immunogold electron microscopy. Furthermore, the mitochondrial genome contains nucleotide sequences with high similarity to known hormone-responsive elements, which interact with the appropriate receptors to confer hormone-dependent activation of reporter genes in transfection experiments. Thus, thyroid hormone stimulates mitochondrial transcription mediated by the cognate receptor when added to an in organello mitochondrial system, capable of faithful transcription.

Skeletal muscle and nuclear hormone receptors: implications for cardiovascular and metabolic disease

Skeletal muscle is a major mass peripheral tissue that accounts for approximately 40% of the total body mass and a major player in energy balance. It accounts for >30% of energy expenditure, is the primary tissue of insulin stimulated glucose uptake, disposal, and storage. Furthermore, it influences metabolism via modulation of circulating and stored lipid (and cholesterol) flux. Lipid catabolism supplies up to 70% of the energy requirements for resting muscle. However, initial aerobic exercise utilizes stored muscle glycogen but as exercise continues, glucose and stored muscle triglycerides become important energy substrates. Endurance exercise increasingly depends on fatty acid oxidation (and lipid mobilization from other tissues). This underscores the importance of lipid and glucose utilization as an energy source in muscle. Consequently skeletal muscle has a significant role in insulin sensitivity, the blood lipid profile, and obesity. Moreover, caloric excess, obesity and physical inactivity lead to skeletal muscle insulin resistance, a risk factor for the development of type II diabetes. In this context skeletal muscle is an important therapeutic target in the battle against cardiovascular disease, the worlds most serious public health threat. Major risk factors for cardiovascular disease include dyslipidemia, hypertension, obesity, sedentary lifestyle, and diabetes. These risk factors are directly influenced by diet, metabolism and physical activity. Metabolism is largely regulated by nuclear hormone receptors which function as hormone regulated transcription factors that bind DNA and mediate the patho-physiological regulation of gene expression. Metabolism and activity, which directly influence cardiovascular disease risk factors, are primarily driven by skeletal muscle. Recently, many nuclear receptors expressed in skeletal muscle have been shown to improve glucose tolerance, insulin resistance, and dyslipidemia. Skeletal muscle and nuclear receptors are rapidly emerging as critical targets in the battle against cardiovascular disease risk factors. Understanding the function of nuclear receptors in skeletal muscle has enormous pharmacological utility for the treatment of cardiovascular disease. This review focuses on the molecular regulation of metabolism by nuclear receptors in skeletal muscle in the context of dyslipidemia and cardiovascular disease.

Creatine synthesis and transport during rat embryogenesis: spatiotemporal expression of AGAT, GAMT and CT1

BACKGROUND

Creatine (Cr) is synthesized by a two-step mechanism involving arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT), and is taken up by cells through a specific Cr transporter, CT1. Recently, genetic defects of this pathway have been described, that lead to Cr deficiency, neurological symptoms in early infancy and severe neurodevelopmental delay. To investigate the involvement of Cr synthesis and uptake pathways during embryonic development, we determined the spatiotemporal expression of AGAT, GAMT and CT1 during the rat embryogenesis, at the mRNA and protein level.

RESULTS

We show that AGAT and GAMT are expressed in hepatic primordium as soon as 12.5 days, then progressively acquire their adult pattern of expression, with high levels of AGAT in kidney and pancreas, and high levels of GAMT in liver and pancreas. AGAT and CT1 are prominent in CNS, skeletal muscles and intestine, where they appear earlier than GAMT. High levels of CT1 are found in epithelia.

CONCLUSION

Our results suggest that de novo synthesis of Cr by AGAT and GAMT, as well as cellular Cr uptake by CT1, are essential during embryonic development. This work provides new clues on how creatine can be provided to developing tissues, and suggests that Cr deficiencies might induce irreversible damages already in utero, particularly on the nervous system.

Aberrant expression of myosin isoforms in skeletal muscles from mice lacking the rev-erbAalpha orphan receptor gene

The rev-erbAalpha orphan protein belongs to the steroid nuclear receptor superfamily. No ligand has been identified for this protein, and little is known of its function in development or physiology. In this study, we focus on 1) the distribution of the rev-erbAalpha protein in adult fast- and slow-twitch skeletal muscles and muscle fibers and 2) how the rev-erbAalpha protein influences myosin heavy chain (MyHC) isoform expression in mice heterozygous (+/-) and homozygous (-/-) for a rev-erbAalpha protein null allele. In the fast-twitch extensor digitorum longus muscle, rev-erbAalpha protein expression was linked to muscle fiber type; however, MyHC isoform expression did not differ between wild-type, +/-, or -/- mice. In the slow-twitch soleus muscle, the link between rev-erbAalpha protein and MyHC isoform expression was more complex than in the extensor digitorum longus. Here, a significantly higher relative amount of the beta/slow (type I) MyHC isoform was observed in both rev-erbAalpha -/- and +/- mice vs. that shown in wild-type controls. A role for the ratio of thyroid hormone receptor proteins alpha1 to alpha2 in modulating MyHC isoform expression can be ruled out because no differences were seen in MyHC isoform expression between thyroid hormone receptor alpha2-deficient mice (heterozygous and homozygous) and wild-type mice. Therefore, our data are compatible with the rev-erbAalpha protein playing an important role in the regulation of skeletal muscle MyHC isoform expression.

Glucocorticoid and thyroid hormone receptors in mitochondria of animal cells

This article concerns the localization of glucocorticoid and thyroid hormone receptors in mitochondria of animal cells. The receptors are discussed in terms of their potential role in the regulation of mitochondrial transcription and energy production by the oxidative phosphorylation pathway, realized both by nuclear-encoded and mitochondrially encoded enzymes. A brief survey of the role of glucocorticoid and thyroid hormones on energy metabolism is presented, followed by a description of the molecular mode of action of these hormones and of the central role of the receptors in regulation of transcription. Subsequently, the structure and characteristics of glucocorticoid and thyroid hormone receptors are described, followed by a section on the effects of glucocorticoid and thyroid hormones on the transcription of mitochondrial and nuclear genes encoding subunits of OXPHOS and by an introduction to the mitochondrial genome and its transcription. A comprehensive description of the data demonstrates the localization of glucocorticoid and thyroid hormone receptors in mitochondria as well as the detection of potential hormone response elements that bind to these receptors. This leads to the conclusion that the receptors potentially play a role in the regulation of transcription of mitochondrial genes. The in organello mitochondrial system, which is capable of sustaining transcription in the absence of nuclear participation, is presented, responding to T3 with increased transcription rates, and the central role of a thyroid receptor isoform in the transcription effect is emphasized. Lastly, possible ways of coordinating nuclear and mitochondrial gene transcription in response to glucocorticoid and thyroid hormones are discussed, the hormones acting directly on the genes of the two compartments by way of common hormone response elements and indirectly on mitochondrial genes by stimulation of nuclear-encoded transcription factors.

Selecting cells with different Alzheimer's disease gamma-secretase activity using FACS. Differential effect on presenilin exon 9 gamma- and epsilon-cleavage

The ultimate step in Alzheimer's disease Abeta generation involves gamma-secretase, which releases Abeta from its membrane-bound precursor. A similar presenilin-dependent proteolytic activity is implicated in the release of the Notch intracellular domain. We have developed a novel assay for gamma-secretase activity based on green fluorescent protein detection. This involves cotransfection of a substrate-activator based on the amyloid precursor protein or the Notch sequence and a fluorescent reporter gene. Stable fluorescent cell populations were selected by fluorescent activated cell sorting and characterized. This assay enabled the identification and sorting of populations, which differ in their levels of gamma-secretase activity, with high fluorescent cells producing more Abeta than low fluorescent cells. Specific gamma-secretase inhibitors, L-685,458 and MW167, reduced cell fluorescence in a dose-dependent manner that paralleled inhibition of Abeta secretion. Overexpression of presenilin 1 increased the cell fluorescence. Cells expressing presenilin with different aspartate mutations (D257A, D385A and D257A/D385A) or exon 9 deletion mutation showed reduced fluorescence. The single aspartate mutations showed a concomitant reduction in Abeta secretion, whereas the D257A/D385A and DeltaE9 mutations had no effect on Abeta secretion.

Embryological origins and development of the rat diaphragm

Textbooks of embryology provide a standard set of drawings and text reflecting the traditional interpretation of phrenic nerve and diaphragm development based on anatomical dissections of embryonic tissue. Here, we revisit this issue, taking advantage of immunohistochemical markers for muscle precursors in conjunction with mouse mutants to perform a systematic examination of phrenic-diaphragm embryogenesis. This includes examining the spatiotemporal relationship of phrenic axon outgrowth and muscle precursors during different stages of myogenesis. Additionally, mutant mice lacking c-met receptors were used to visualize the mesenchymal substratum of the developing diaphragm in the absence of myogenic cells. We found no evidence for contributions to the diaphragm musculature from the lateral body wall, septum transversum, or esophageal mesenchyme, as standard dogma would state. Nor did the data support the hypothesis that the crural diaphragm is of distinct embryological origins. Rather, we found that myogenic cells and axons destined to form the neuromuscular component of the diaphragm coalesce within the pleuroperitoneal fold (PPF). It is the expansion of these components of the PPF that leads to the formation of the diaphragm. Furthermore, we extended these studies to examine the developing diaphragm in an animal model of congenital diaphragmatic hernia (CDH). We find that malformation of the PPF mesenchymal substratum leads to the defect characteristic of CDH. In summary, the data demonstrates that a significant revision of narratives describing normal and pathological development of the diaphragm is warranted.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

A novel functional co-operation between MyoD, MEF2 and TRalpha1 is sufficient for the induction of GLUT4 gene transcription

We report tripartite co-operation between MyoD, myocyte enhancer factor-2 (MEF2) and the thyroid hormone receptor (TRalpha1) that takes place in the context of an 82-bp muscle-specific enhancer in the rat insulin-responsive glucose transporter (GLUT4) gene that is active in both cardiac and skeletal muscle. In the L6E9 skeletal muscle cell line and in 10T1/2 fibroblasts, a powerful synergistic activation of the GLUT4 enhancer relied on the over-expression of MyoD, MEF2 and TRalpha1 and the integrity of their respective binding sites, and occurred when linked to either a heterologous promoter or in the context of the native GLUT4 promoter. In cardiac myocytes, enhancer activity was dependent on the binding sites for MEF2 and TRalpha1. Furthermore, we show that in 10T1/2 fibroblasts, the forced expression of MyoD, MEF2 and TRalpha1 induced the expression of the endogenous, otherwise silent, GLUT4 gene. In all, our results indicate a novel functional co-operation between these three factors which is required for full activation of GLUT4 transcription.

Molecular regulation of tongue and craniofacial muscle differentiation

The molecular regulation of muscle development is tightly controlled at three distinct stages of the process: determination, differentiation, and maturation. Developmentally, specific populations of myoblasts exhibit distinct molecular phenotypes that begin to limit the ultimate characteristics of the muscle fibers. The expression of the myogenic regulatory factor family of the transcription process plays a key role in muscle development and, ultimately, in the subset of contractile genes expressed in a specific muscle. Craniofacial muscles have distinct functional requirements and associated molecular phenotypes that distinguish them from other skeletal muscles. The general principles of muscle molecular differentiation with specific reference to craniofacial muscles, such as the tongue, are discussed in this review.

Molecular cloning and sequence of Sparus aurata skeletal myosin light chains expressed in white muscle: developmental expression and thyroid regulation

Two full-length cDNA clones encoding the skeletal myosin light chain 2 (MLC2; 1452bp) and myosin light chain 3 (MLC3; 972bp) were isolated from a cDNA library prepared from gilthead sea bream Sparus aurata larvae. The MLC2 cDNA encoded a predicted protein of 170 residues that was 79% identical to rabbit MLC2 over the entire length and 87% identical within the Ca2+-binding region. The deduced amino acid sequence of MLC3 was 153 residues in length and was 91% and 69% identical to the zebrafish and rabbit MLC3, respectively. Northern blot analysis revealed that in adults both transcripts were expressed in fast white muscle only. MLC2 appeared earlier in development: MLC2 transcripts were detectable from the beginning of segmentation, whereas MLC3 transcripts did not appear until 27h post-fertilisation. At this developmental stage, a second MLC2 transcript of 0.89 kilobase-pairs was present. MLCs exhibited a different age-related pattern of response to varied thyroidal states, which were experimentally induced by the administration of 1μgg−1bodymass of thyroxine (T4) or triiodothyronine (T3), or 5ngg−1bodymass of the hypothyroidal compound thiourea; MLC3 expression was not significantly affected, whereas levels of MLC2 transcripts were significantly elevated in the white muscle only of juvenile sea bream after administration of T4. Although the mechanism of thyroidal regulation of MLC expression remains unknown, the present results suggest that different regulatory mechanisms exist for different MLCs.

Developmental expression analysis of thyroid hormone receptor isoforms reveals new insights into their essential functions in cardiac and skeletal muscles

Nuclear thyroid hormone (TH) receptors (TR) play a critical role in mediating the diverse actions of TH in development, differentiation, and metabolism of most tissues, but the role of TR isoforms in muscle development and function is unclear. Therefore, we have undertaken a comprehensive expression analysis of TRalpha 1, TRbeta 1, TRbeta 2 (TH binding), and TRalpha 2 (non-TH binding) in functionally distinct porcine muscles during prenatal and postnatal development. Use of a novel and highly sensitive RNase protection assay revealed striking muscle-specific developmental profiles of all four TR isoform mRNAs in cardiac, longissimus, soleus, rhomboideus, and diaphragm. Distribution of TR isoforms varied markedly between muscles; TRalpha expression was considerably greater than TRbeta and there were significant differences in the ratios TRalpha 1:TRalpha 2, and TRbeta 1:TRbeta 2. Together with immunohistochemistry of myosin heavy chain isoforms and data on myogenesis and maturation of the TH axis, these findings provide new evidence that highlights central roles for 1) TRalpha isoforms in fetal myogenesis, 2) the ratio TRalpha 1:TRalpha 2 in determining cardiac and skeletal muscle phenotype and function; 3) TRbeta in maintaining a basal level of cellular response to TH throughout development and a specific maturational function around birth. These findings suggest that events disrupting normal developmental profiles of TR isoforms may impair optimal function of cardiac and skeletal muscles.

Juvenile coho salmon locomotion and mosaic muscle are modified by 3′,3′,5′-tri-iodo-l-thyronine (T(3))

Studies of maximum aerobic swimming performance in smolting juvenile salmonids indicate that these animals may be aerobically compromised during downstream migration. To test our hypothesis that hyperthyroid status contributes to decreased swimming performance through modification of muscle contractility in juvenile (112 mm mean total length) coho salmon (Oncorhynchus kisutch), we measured swimming performance and isolated muscle bundle contractility of fish implanted with 3′,3′,5′-tri-iodo-l-thyronine (T(3)) pellets, of fish implanted with sham pellets and of fish with no pellet implantation (control group). After 3 weeks (N=12-13), critical swimming speeds (maximum aerobic swimming speed or U(crit)) were measured. Muscle bundles (N=15-16) were dissected from the hypaxial musculature and stimulated to measure the force and velocity of an isometric twitch and tetani. T(3)-treated fish demonstrated visible morphological changes associated with smoltification. Mean values of U(crit) were significantly decreased and the prolonged contraction (tetani) and twitch rates of contraction, relaxation and maximum force were significantly increased by T(3) treatment compared with both the sham and control fish. Hematocrit, body mass and body length were not significantly affected by T(3) treatment. In conclusion, we suggest that the reported decrease in U(crit) during salmonid smoltification may be mediated by endogenous T(3)-induced contractile modification of mosaic muscle fibers.

Nutrition-hormone receptor-gene interactions: implications for development and disease

Nutrition profoundly alters the phenotypic expression of a given genotype, particularly during fetal and postnatal development. Many hormones act as nutritional signals and their receptors play a key role in mediating the effects of nutrition on numerous genes involved in differentiation, growth and metabolism. Polypeptide hormones act on membrane-bound receptors to trigger gene transcription via complex intracellular signalling pathways. By contrast, nuclear receptors for lipid-soluble molecules such as glucocorticoids (GC) and thyroid hormones (TH) directly regulate transcription via DNA binding and chromatin remodelling. Nuclear hormone receptors are members of a large superfamily of transcriptional regulators with the ability to activate or repress many genes involved in development and disease. Nutrition influences not only hormone synthesis and metabolism but also hormone receptors, and regulation is mediated either by specific nutrients or by energy status. Recent studies on the role of early environment on development have implicated GC and their receptors in the programming of adult disease. Intrauterine growth restriction and postnatal undernutrition also induce striking differences in TH-receptor isoforms in functionally-distinct muscles, with critical implications for gene transcription of myosin isoforms. glucose transporters, uncoupling proteins and cation pumps. Such findings highlight a mechanism by which nutritional status can influence normal development, and modify nutrient utilization. thermogenesis. peripheral sensitivity to insulin and optimal cardiac function. Diet and stage of development will also influence the transcriptional activity of drugs acting as ligands for nuclear receptors. Potential interactions between nuclear receptors, including those for retinoic acid and vitamin D, should not be overlooked in intervention programmes using I or vitamin A supplementation of young and adult human populations

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.

Neural and hormonal control of expression of myogenic regulatory factor genes during regeneration of Xenopus fast muscles: myogenin and MRF4 mRNA accumulation are neurally regulated oppositely

With the aim to investigate the influence of both innervation and thyroid hormone, on the expression of the MRFs during muscle regeneration, we performed cardiotoxin injury-induced regeneration experiments on fast muscles of adult Xenopus laevis subjected to different experimental conditions, including denervation and T3 treatment, and analyzed the accumulation of the four myogenic regulatory factors (MRFs) using RT-PCR and in situ hybridization. We show here that manipulation of hormone levels or innervation resulted in differential alterations of MRF expression. Denervation and T3 treatment transiently down-regulated Myf-5 mRNA levels at the beginning of the regeneration process. Myf-5 was the only myogenic factor subject to thyroid hormone influence. Muscle denervation persistently reduces the levels of MRF4 transcripts as early as the first stages of regeneration, whereas the levels of myogenin mRNA were increased in the late stages of regeneration. This suggests that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation and that myogenin expression may compensate for the down-regulation of MRF4 gene. This switch in MRF gene expression following denervation could have important consequences for the ability of Xenopus regenerating muscles to recover function after denervation.

Recent advances in understanding the pathogenesis of nitrofen-induced congenital diaphragmatic hernia

In this review, we discuss recent advances in the study of the pathogenesis of congenital diaphragmatic hernia (CDH). Much of the research has involved the use of an animal model of CDH in which diaphragmatic defects are produced in fetal rats by administering the herbicide nitrofen to dams during mid-gestation. The animal model is described and the relevance to the human condition is discussed. The data derived from the animal studies are critically assessed in the context of commonly cited hypotheses proposed for the pathogenesis of CDH. Finally, experimental strategies are proposed for systematically examining the normal and pathological formation of the pleuroperitoneal fold. We conclude that a malformation of the primordial diaphragm, the pleuroperitoneal fold, underlies the muscle defects associated with CDH.

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.

The p23 molecular chaperones act at a late step in intracellular receptor action to differentially affect ligand efficacies

Multiple molecular chaperones, including Hsp90 and p23, interact with members of the intracellular receptor (IR) family. To investigate p23 function, we compared the effects of three p23 proteins on IR activities, yeast p23 (sba1p) and the two human p23 homologs, p23 and tsp23. We found that Sba1p was indistinguishable from human p23 in assays of seven IR activities in both animal cells and in yeast; in contrast, certain effects of tsp23 were specific to that homolog. Transcriptional activation by two IRs was increased by expression of any of the p23 species, whereas activation by five other IRs was decreased by Sba1p or p23, and unaffected by tsp23. p23 was expressed in all tissues examined except striated and cardiac muscle, whereas tsp23 accumulated in a complementary pattern; hence, p23 proteins might contribute to tissue-specific differences in IR activities. Unlike Hsp90, which acts on IR aporeceptors to stimulate ligand potency (i.e., hormone-binding affinity), p23 proteins acted on IR holoreceptors to alter ligand efficiencies (i.e., transcriptional activation activity). Moreover, the p23 effects developed slowly, requiring prolonged exposure to hormone. In vitro, p23 interacted preferentially with hormone–receptor–response element ternary complexes, and stimulated receptor–DNA dissociation. The dissociation was reversed by addition of a fragment of the GRIP1 coactivator, suggesting that the two reactions may be in competition in vivo. Our findings suggest that p23 functions at one or more late steps in IR-mediated signal transduction, perhaps including receptor recycling and/or reversal of the response.

The influence of thyroid hormone on myosin isoform composition and shortening velocity of single skeletal muscle fibres with special reference to ageing and gender

This review summarizes the effects of altered thyroid hormone levels on the expression of myosin isoforms and contractility in single muscle fibres from fast- and slow-twitch muscles from young and old male and female rats. The differences between male and female hyperthyroid soleus muscles are suggested to be related to an interaction of thyroid hormones and sex hormones in the regulation of myosin gene expression. Additionally, the mismatch between the protein and mRNA levels of MyHCs between male and female hyperthyroid extensor digitorum longus (EDL) muscles raises the possibility of a gender-related difference in post-transcriptional, translational or post-translational regulation of MyHC isoforms by T3.

The nuclear receptor corepressor N-CoR regulates differentiation: N-CoR directly interacts with MyoD

Classical ligand-activated nuclear receptors (e.g. thyroid hormone receptor, retinoic acid receptor), orphan nuclear receptors (e.g. Rev-erbAalpha/beta), Mad/Max bHLH (basic helix loop helix)-LZ proteins, and oncoproteins, PLZF and LAZ3/BCL6, bind DNA and silence transcription by recruiting a repressor complex that contains N-CoR (nuclear receptor corepressor)/SMRT (silencing mediator of retinoic acid and thyroid hormone receptor), Sin3A/B, and HDAc-1/-2 proteins. The function of the corepressor, N-CoR, in the process of cellular differentiation and coupled phenotypic acquisition, has not been investigated. We examined the functional role of N-CoR in myogenesis (muscle differentiation), an ideal paradigm for the analysis of the determinative events that govern the cell's decision to divide or differentiate. We observed that the mRNA encoding N-CoR was suppressed as proliferating myoblasts exited the cell cycle, and formed morphologically and biochemically differentiated myotubes. Exogenous expression of N-CoR (but not RIP13) in myogenic cells ablated 1) myogenic differentiation, 2) the expression of the myoD gene family that encode the myogenic specific bHLH proteins, and 3) the crucial cell cycle regulator, p21Waf-1/Cip-1 mRNA. Furthermore, N-CoR expression efficiently inhibits the myoD-mediated myogenic conversion of pluripotential C3H10T1/2 cells. We demonstrate that MyoD-mediated transactivation and activity are repressed by N-CoR. The mechanism involves direct interactions between MyoD and N-CoR; moreover, the interaction was dependent on the amino-terminal repression domain (RD1) of N-CoR and the bHLH region of MyoD. Trichostatin A treatment significantly stimulated the activity of MyoD by approximately 10-fold and inhibited the ability of N-CoR to repress MyoD-mediated transactivation, consistent with the involvement of the corepressor and the recruitment of a histone deacteylase activity in the process. This work demonstrates that the corepressor N-CoR is a key regulator of MyoD activity and mammalian differentiation, and that N-CoR has a multifaceted role in myogenesis.

An E-box within the MHC IIB gene is bound by MyoD and is required for gene expression in fast muscle

The myosin heavy chain (MHC) IIB gene is selectively expressed in skeletal muscles, imparting fast contractile kinetics. Why the MHC IIB gene product is expressed in muscles like the tibialis anterior (TA) and not expressed in muscles like the soleus is currently unclear. It is shown here that the mutation of an E-box within the MHC IIB promoter decreased reporter gene activity in the fast-twitch TA muscle 90-fold as compared with the wild-type promoter. Reporter gene expression within the TA required this E-box for activation of a heterologous construct containing upstream regulatory regions of the MHC IIB promoter linked to the basal 70-kDa heat shock protein TATA promoter. Electrophoretic mobility shift assays demonstrated that mutation of the E-box prevented the binding of both MyoD and myogenin to this element. In cotransfected C2C12 myotubes and Hep G2 cells, MyoD preferentially activated the MHC IIB promoter in an E-box-dependent manner, whereas myogenin activated the MHC IIB promoter to a lesser extent, and in an E-box-independent manner. A time course analysis of hindlimb suspension demonstrated that the unweighted soleus muscle activated expression of MyoD mRNA before the de novo expression of MHC IIB mRNA. These data suggest a possible causative role for MyoD in the observed upregulation of MHC IIB in the unweighted soleus muscle.

Time course of myosin heavy chain transitions in neonatal rats: importance of innervation and thyroid state

During the postnatal period, rat limb muscles adapt to weight bearing via the replacement of embryonic (Emb) and neonatal (Neo) myosin heavy chains (MHCs) by the adult isoforms. Our aim was to characterize this transition in terms of the six MHC isoforms expressed in skeletal muscle and to determine the importance of innervation and thyroid hormone status on the attainment of the adult MHC phenotype. Neonatal rats were made hypothyroid via propylthiouracil (PTU) injection. In normal and PTU subgroups, leg muscles were unilaterally denervated at 15 days of age. The MHC profiles of plantaris (PLN) and soleus (Sol) muscles were determined at 7, 14, 23, and 30 days postpartum. At day 7, the Sol MHC profile was 55% type I, 30% Emb, and 10% Neo; in the PLN, the pattern was 60% Neo and 25% Emb. By day 30 the Sol and PLN had essentially attained an adult MHC profile in the controls. PTU augmented slow MHC expression in the Sol, whereas in the PLN it markedly repressed IIb MHC by retaining neonatal MHC expression. Denervation blunted the upregulation of IIb in the PLN and of Type I in the Sol and shifted the pattern to greater expression of IIa and IIx MHCs in both muscles. In contrast to previous observations, these findings collectively suggest that both an intact thyroid and innervation state are obligatory for the attainment of the adult MHC phenotype, particularly in fast-twitch muscles.

Coding sequence, genomic organization, chromosomal localization, and expression pattern of the signalosome component Cops2: the mouse homologue of Drosophila alien

The Drosophila alien gene is highly homologous to the human thyroid receptor interacting protein, TRIP15/COPS2, which is a component of the recently identified signalosome protein complex. We identified the mouse homologue of Drosophila alien through homology searches of the EST database. We found that the mouse cDNA encodes a predicted 443-amino-acid protein, which migrates at approximately 50 kDa. The gene for the mouse alien homologue, named Cops2, includes 12 coding exons spanning approximately 30 kb of genomic DNA on the central portion of mouse chromosome 2. Mouse Cops2 is widely expressed in embryonic, fetal, and adult tissues beginning as early as E7.5. Mouse Cops2 cDNA hybridizes to two mRNA bands in all tissues at approximately 2.3 and approximately 4 kb, with an additional approximately 1.9-kb band in liver. Immunostaining of native and epitope tagged proteins localized the mouse Cops2 protein in both the cytoplasm and the nucleus, with larger amounts in the nucleus in some cells.

Ski is a component of the histone deacetylase complex required for transcriptional repression by Mad and thyroid hormone receptor

The N-CoR/SMRT complex containing mSin3 and histone deacetylase (HDAC) mediates transcriptional repression by nuclear hormone receptors and Mad. The proteins encoded by the ski proto-oncogene family directly bind to N-CoR/SMRT and mSin3A, and forms a complex with HDAC. c-Ski and its related gene product Sno are required for transcriptional repression by Mad and thyroid hormone receptor (TRbeta). The oncogenic form, v-Ski, which lacks the mSin3A-binding domain, acts in a dominant-negative fashion, and abrogates transcriptional repression by Mad and TRbeta. In ski-deficient mouse embryos, the ornithine decarboxylase gene, whose expression is normally repressed by Mad-Max, is expressed ectopically. These results show that Ski is a component of the HDAC complex and that Ski is required for the transcriptional repression mediated by this complex. The involvement of c-Ski in the HDAC complex indicates that the function of the HDAC complex is important for oncogenesis.