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

Adaptive thermogenesis driving catch-up fat during weight regain: a role for skeletal muscle hypothyroidism and a risk for sarcopenic obesity

Across the spectrum of weight regain, ranging from cachexia rehabilitation and catch-up growth to obesity relapse, the recovery rate of body fat is often disproportionate relative to lean tissue recovery. Such preferential 'catch-up fat' is in part attributed to an increase in metabolic efficiency and embodied in the concept that 'metabolic adaptation' or 'adaptive thermogenesis' in response to large weight deficits can persist during weight regain to accelerate fat stores recovery. This paper reviews the evidence in humans for the existence of this thrifty metabolism driving catch-up fat within the framework of a feedback loop between fat stores depletion and suppressed thermogenesis. The search for its effector mechanisms suggests that whereas adaptive thermogenesis during weight loss results primarily from central suppression of sympathetic nervous system and hypothalamic-pituitary-thyroid axis, its persistence during weight regain for accelerating fat recovery is primarily mediated through peripheral tissue resistance to the actions of this systemic neurohormonal network. Emerging evidence linking it to an upregulation of skeletal muscle type 3 deiodinase (D3), the main thyroid hormone inactivating enzyme, along with slowed muscle metabolism and altered contractile properties, suggest that D3-induced muscle hypothyroidism is a key feature of such peripheral resistance. These findings underlying a role of skeletal muscle hypothyroidism in adaptive thermogenesis driving catch-up fat, but which can also concomitantly compromise muscle functionality, have been integrated into a mechanistic framework to explain how weight cycling and large weight fluctuations across the lifespan can predispose to sarcopenic obesity.

Mechanisms of muscle cells alterations and regeneration decline during aging

Skeletal muscles are essential for locomotion and body metabolism regulation. As muscles age, they lose strength, elasticity, and metabolic capability, leading to ineffective motion and metabolic derangement. Both cellular and extracellular alterations significantly influence muscle aging. Satellite cells (SCs), the primary muscle stem cells responsible for muscle regeneration, become exhausted, resulting in diminished population and functionality during aging. This decline in SC function impairs intercellular interactions as well as extracellular matrix production, further hindering muscle regeneration. Other muscle-resident cells, such as fibro-adipogenic progenitors (FAPs), pericytes, and immune cells, also deteriorate with age, reducing local growth factor activities and responsiveness to stress or injury. Systemic signaling, including hormonal changes, contributes to muscle cellular catabolism and disrupts muscle homeostasis. Collectively, these cellular and environmental components interact, disrupting muscle homeostasis and regeneration in advancing age. Understanding these complex interactions offers insights into potential regenerative strategies to mitigate age-related muscle degeneration.

Chronic thyrotoxic myopathy development is associated with thyroid hormone sensitivity index, predicted by lower-limb fatigue and the squat-up test

Thyroid hormones (THs) may affect chronic thyrotoxic myopathy (CTM). The relationship between TH sensitivity and CTM is inconsistent. We aimed to investigate the associations between TH sensitivity and the risk of CTM and to screen potential CTMs with strength and function tests. A total of 162 Chinese patients (36.58% men) with Graves' disease were enrolled and divided into CTM and non-CTM groups. TH and sensitivity indices were measured. Muscle power and function were assessed by grip, upper-limb fatigue (ULFT), lower-limb fatigue (LLFT), and squat-up (SUT) tests, and walking pace. Association between sensitivity to TH indices and the risk of developing CTM was assessed via multivariate logistic regression. The diagnostic effectiveness of muscle power and function for predicting CTM was evaluated via receiver operating characteristic (ROC) curves. Thyroid feedback quantile-based index FT3 (TFQIFT3) and the parametric TFQIFT3 (PTFQIFT3), TFQIFT4, and PTFQIFT4 were positively associated with CTM risk by using inverse probability of treatment weighting multivariate logistic regression. For each 1-SD increase in TFQIFT3 and PTFQIFT3, TFQIFT4 and PTFQIFT4, the odds ratios for CTM were 1.67 (95% CI = 1.17-2.48) ,1.64 (95% CI = 1.51-2.93), 1.60 (95%CI = 1.12-2.32), 1.58 (95%CI = 1.11-2.30), respectively. LLFT and SUT best predicted male/female CTM, respectively (AUC = 0.89/0.85). In Graves' disease patients, TH sensitivity is associated with CTM development, which can be predicted by SUT and LLFT results.

Thyroid dysfunction and sarcopenia: a two-sample Mendelian randomization study

Objective

Observational studies have shown positive associations between thyroid dysfunction and risk of sarcopenia. However, the causality of this association remains unknown. This study aimed to evaluate the potential causal relationship between thyroid dysfunction and sarcopenia using Mendelian randomization (MR).

Methods

This study collected pooled data from genome-wide association studies focusing on thyroid dysfunction and three sarcopenia-related features: low hand grip strength, appendicular lean mass (ALM), and walking pace, all in individuals of European ancestry. The primary analytical method used was inverse-variance weighted, with weighted median and MR-Egger serving as complementary methods to assess causal effects. Heterogeneity and pleiotropy tests were also performed, and the stability of the results was evaluated using the Leave-one-out.

Results

The MR analysis indicated that hyperthyroidism could lead to a significant decrease in ALM in the extremities (OR = 1.03; 95% CI = 1.02 to 1.05; P < 0.001). The analysis also found that hypothyroidism could cause a notable reduction in grip strength (OR = 2.03; 95% CI = 1.37 to 3.01; P < 0.001) and walking pace (OR = 0.83; 95% CI = 0.77 to 0.90; P < 0.001). There was a significant association between subclinical hyperthyroidism and a reduced walking pace (OR = 1.00; 95% CI = 0.99 to 1.00; P = 0.041).

Conclusion

This study provides evidence that hyperthyroidism, hypothyroidism, and subclinical hyperthyroidism can all increase the risk of sarcopenia.

Effects of thyroid hormones in skeletal muscle protein turnover

Thyroid hormones (THs) are critical regulators of muscle metabolism in both healthy and unhealthy conditions. Acting concurrently as powerful anabolic and catabolic factors, THs are endowed with a vital role in muscle mass maintenance. As a result, thyroid dysfunctions are the leading cause of a wide range of muscle pathologies, globally identified as myopathies. Whether muscle wasting is a common feature in patients with hyperthyroidism and is mainly caused by THs-dependent stimulation of muscle proteolysis, also muscle growth is often associated with hyperthyroid conditions, linked to THs-dependent stimulation of muscle protein synthesis. Noteworthy, also hypothyroid status negatively impacts on muscle physiology, causing muscle weakness and fatigue. Most of these symptoms are due to altered balance between muscle protein synthesis and breakdown. Thus, a comprehensive understanding of THs-dependent skeletal muscle protein turnover might facilitate the management of physical discomfort or weakness in conditions of thyroid disease. Herein, we describe the molecular mechanisms underlying the THs-dependent alteration of skeletal muscle structure and function associated with muscle atrophy and hypertrophy, thus providing new insights for targeted modulation of skeletal muscle dynamics.

Body composition changes in patients with differentiated thyroid cancer after iodine-131 treatment and short-term levothyroxine replacement and suppression therapy

PURPOSE

The purposes of this study were to assess the changes in body composition in patients who underwent thyroidectomy due to differentiated thyroid cancer (DTC) after radioactive iodine therapy (RAI) and short-term levothyroxine (LT4) supplementation and to explore the correlations between body composition distribution and corresponding blood indices.

METHODS

Fifty-seven thyroidectomized DTC patients were included. Serum was tested for several biochemical indices of thyroid function, lipids, and bone metabolism, and body composition parameters were measured via dual-energy X-ray absorptiometry before and 4-6 weeks after RAI and LT4 supplementation.

RESULTS

The body composition of DTC patients changed after RAI. Fat mass in all parts of the body decreased (range of relative change (RRC) -12.97--2.80%). Bone mineral content (BMC) increased throughout the body (relative change (RC) 12.12%), head (RC 36.23%), pelvis (RC 9.00%), and legs (RC 3.15%). Similarly, bone mineral density (BMD) increased in different regions (RRC 3.60-26.43%), except for the arms. Notably, lean mass in the arms (RC 4.30%) and legs (RC 3.67%) increased, while that in the head decreased (RC -2.75%), while total lean mass did not change at 4-6 weeks after LT4 supplementation. Furthermore, changes in fat distribution in the android region were related to the changes in total cholesterol (r = -0.390) and low-density lipoprotein cholesterol (r = -0.354), and changes in the BMC and BMD of the lumbar spine were positively associated with the changes in calcitonin (r = 0.302 and 0.325, respectively).

CONCLUSIONS

After RAI and short-term LT4 supplementation in DTC patients, body composition rapidly and positively changed and was characterized by decreased fat mass and increased BMC and BMD.

Correlation of subclinical hypothyroidism with sarcopenia and its components in the Chinese older adults

OBJECTIVE

To identify the correlation of thyroid function and subclinical hypothyroidism (SCH) with sarcopenia and its components in the older Chinese adults.

METHODS

Older adults were recruited and divided into SCH group and non-SCH group. Free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) were measured by electrochemiluminescence. Appendicular skeletal muscle mass (ASM) was measured, and skeletal muscle index (SMI) was further calculated. Grip strength was measured. Physical performance was graded by the Short Physical Performance Battery (SPPB) scores of the gait speed test, chair stand test and balance test.

RESULTS

Of the 240 older adults included, 48 (20.00%) presented with SCH. The prevalence of sarcopenia in SCH group was higher than that in non-SCH group (33.33% v.s. 18.75%). Grip strength was significantly lower in patients with SCH than those without sarcopenia. In terms of physical performance, 6-meter gait speed and SPPB score were lower in subjects with SCH than those without SCH, while 5 sit-to-stand movements was longer score in subjects with SCH than those without SCH. SCH was significantly correlated with sarcopenia, while FT3, FT4, and TSH levels were not. SCH was significantly correlated with low muscle strength and low muscle mass, but not with low physical performance. FT3 level was positively correlated with grip strength and SMI. TSH level was negatively correlated with grip strength, 6-meter gait speed, and SPPB score, but positively correlated with the time of 5 sit-to-stand movements.

CONCLUSION

SCH is a risk factor for sarcopenia in the older adults and correlated with low muscle strength and low muscle mass, but not with low physical performance. FT3, FT4 and TSH levels are associated with sarcopenia components, but not with sarcopenia.

Serum FT3/FT4, but not TSH is associated with handgrip strength in euthyroid U.S. population: evidence from NHANES

Objective

Although several studies have examined the relationship between thyroid function and muscle strength, their population primarily derived from Asian areas, and their results were controversial. Thus, this study aimed to explore the association between thyroid function and handgrip strength (HGS) in the U.S. population.

Methods

A total of 1,067 participants from NHANES were categorized into three different age groups including young (<45 years), middle (45~64 years), and old (≥65 years) age groups. Thyroid function was measured by the competitive binding immune-enzymatic assays, while HGS was examined by a trained evaluator using a dynamometer. The weighted multiple linear regression models were used to examine the association between thyroid function and handgrip strength. The restricted cubic splines were employed to explore the non-linear relationship between these two variables. All statistical analyses were performed using the SPSS version 20.0 and R software.

Results

After adjustment for potential covariates, FT3/FT4, but not TSH was positively associated with HGS in middle age group (β=0.091, t=2.428, P=0.016). The subgroup analysis by sex revealed that the positive association between FT3/FT4 and HGS was observed in the middle age group for both male and female participants (β=0.163, t=2.121, P=0.035; β=0.157, t=2.180, P=0.031). The RCS analysis showed a statistically significant non-linear association between FT3/FT4 and HGS in overall population (P for non-linear=0.026). After adjustment for covariates, men with low HGS had a significant lower FT3/FT4 than those without low HGS in old age group (P=0.013). There was a significant increase in TSH level for female participants with low HGS in old age group compared to those with normal HGS (P=0.048).

Conclusions

This study demonstrated FT3/FT4, but not TSH, was positively associated with HGS in middle age group, and the different association was observed in men in middle age group when participants were stratified by sex. Future longitudinal cohort study should be conducted to reveal the causal relationship between thyroid function and muscle strength.

Age-Dependent Developmental Changes of Selenium Content and Selenoprotein Expression and Content in Longissimus Dorsi Muscle and Liver of Duroc Pigs

The trace element selenium (Se) plays a key role in development and various physiological processes, mainly through its transformation into selenoproteins. To investigate the developmental patterns of Se content and expression of selenoproteins, the liver and longissimus dorsi (LD) muscle of Duroc pigs were collected at 1, 21, 80, and 185 days of age (7 pigs each age) for the determination of Se content, mRNA expression of selenoproteins, and concentrations of glutathione peroxidase (GPX), thioredoxin reductase (TrxR or TXNRD), and selenoprotein P (SELP). The results showed that age significantly affected the expression of GPX1, GPX2, GPX3, TXNRD1, TXNRD2, TXNRD3, iodothyronine deiodinases 2 (DIO2), DIO3, SELF, SELH, SELM, SELP, SELS, SELW, and selenophosphate synthetase2 (SPS2) in the liver, as well as GPX3, GPX4, TXNRD1, TXNRD2, DIO2, DIO3, SELF, SELN, SELP, SELR, SELS, and SELW in the LD muscle of Duroc pigs. The concentrations of GPX, TrxR, and SELP showed an increasing trend with age, and they were positively correlated with Se content at 1, 21, and 185 days of age and negatively correlated at 80 days of age, both in the liver and LD muscle. The Se content decreased at the age of 80 days, especially in the LD muscle. In summary, our study revealed developmental changes in Se content and expression of selenoproteins in the liver and LD muscle of Duroc pigs at different growth stages, which provided a theoretical basis for further study of Se nutrition and functions of selenoproteins.

Serine Administration Improves Selenium Status, Oxidative Stress, and Mitochondrial Function in Longissimus Dorsi Muscle of Piglets with Intrauterine Growth Retardation

Intrauterine growth retardation (IUGR) causes oxidative stress in the skeletal muscle. Serine and selenoproteins are involved in anti-oxidative processes; however, whether IUGR affects selenium status and whether serine has beneficial effects remain elusive. Here, we investigated the effects of serine administration on selenium nutritional status and oxidative stress in the longissimus dorsi muscle of piglets with IUGR. Six newborn Min piglets having normal birth weight were administered saline, and 12 IUGR piglets were either administered saline or 0.8% serine. The results showed a lower selenium content in skeletal muscle in IUGR piglets, which was restored after serine administration. IUGR piglets showed a disturbed expression of genes encoding selenoproteins, with decreased expression of GPX2, GPX4, TXNRD1, and TXNRD3 and increased expression of DIO1, DIO2, SELF, SELM, SELP, and SELW. Notably, serine administration restored the expression levels of these genes. In accordance with the changes in gene expression, the activity of GPX, TXNRD, and DIO and the content of GSH and SELP were also altered, whereas serine administration restored their contents and activities. Moreover, we observed severe oxidative stress in the skeletal muscle of IUGR piglets, as indicated by decreased GSH content and increased MDA and PC content, whereas serine administration alleviated these changes. In conclusion, our results indicate that IUGR piglets showed a disturbed expression of genes encoding selenoproteins, accompanied by severe oxidative stress. Serine administration can improve selenium status, oxidative stress, and mitochondrial function in the longissimus dorsi muscle of piglets with IUGR. These results suggest that serine could potentially be used in the treatment of IUGR in piglets.

Embryonic thermal manipulation leads growth inhibition and reduced hepatic insulin-like growth factor1 expression due to promoter DNA hypermethylation in broilers

When broilers cannot adapt to a high-temperature environment through self-regulation, it will cause heat stress, resulting in a large number of deaths and substantial economic losses. Studies have shown that thermal manipulation (TM) during the embryonic stage can improve broilers' ability to resist heat stress later. However, different TM strategies produce different results on broilers' growth. In this study, yellow-feathered broiler eggs were selected and randomly divided into 2 groups between E10 and E18, which the control group was incubated at 37.8°C with 56% humidity, and the TM group was subjected to 39°C with 65% humidity. After hatching, all broilers were reared normally until slaughtered at 12 d of age (D12). During D1 to D12, body weight, feed intake, and body temperature were recorded. The results showed that TM significantly decreased (P < 0.05) the final body weight, weight gain, and average daily feed intake of broilers. Meanwhile, the serum levels of Triiodothyronine (T3) and free T3 were significantly decreased in the TM group (P < 0.05). The expressions of hepatic growth regulation-associated genes, growth hormone receptor (GHR), insulin-like growth factor1, and 2 (IGF1 and IGF2) were significantly down-regulated in the TM group (P < 0.05). In addition, TM altered hepatic DNA methylation, resulting in a significant increase (P < 0.05) in the methylation of the IGF1 and GHR promoter regions. The above results indicated that TM during the embryonic stage decreased the serum thyroid hormone level and increased the methylation level of the IGF1 and GHR promoter regions to down-regulate the expression of growth-related genes, resulting in early growth inhibition of broilers.

Role of Hormones During Gestation and Early Development: Pathways Involved in Developmental Programming

Accumulating evidence suggests that an altered maternal milieu and environmental insults during the intrauterine and perinatal periods of life affect the developing organism, leading to detrimental long-term outcomes and often to adult pathologies through programming effects. Hormones, together with growth factors, play critical roles in the regulation of maternal-fetal and maternal-neonate interfaces, and alterations in any of them may lead to programming effects on the developing organism. In this chapter, we will review the role of sex steroids, thyroid hormones, and insulin-like growth factors, as crucial factors involved in physiological processes during pregnancy and lactation, and their role in developmental programming effects during fetal and early neonatal life. Also, we will consider epidemiological evidence and data from animal models of altered maternal hormonal environments and focus on the role of different tissues in the establishment of maternal and fetus/infant interaction. Finally, we will identify unresolved questions and discuss potential future research directions.

Beyond antioxidants: Selenium and skeletal muscle mitochondria

The element, Selenium (Se), has an essential nutritive and biological role as a trace mineral known primarily for its vital antioxidant functions as a constituent of the selenoenzyme, glutathione peroxidase. However, Se also has a much more global biological impact beyond antioxidant function. The objective of this review is to present an overview of prior research on the extra-antioxidant effects of Se with a key focus on skeletal muscle mitochondrial energetics. Cognizance of these additional functions of Se is requisite when formulating and recommending dietary supplementation of Se in humans or animals. Chief amongst its myriad of biological contributions, Se influences mitochondrial capacity and function and, subsequently, muscular health. Dietary Se supplementation has been shown to increase skeletal muscle mitochondrial volume density and within some cell lines, Se treatment increases mitochondrial biogenesis and respiratory capacity. In addition, the selenoproteins H, N, W, and O and deiodinases exhibit varying effects on mitochondrial and/or skeletal muscle function. Selenoprotein H enhances mitochondrial biogenesis whereas selenoproteins N and W appear to influence muscle calcium homeostasis which impacts mitochondrial function. Moreover, selenoprotein O's intramitochondrial residence facilitates Se's redox function. Deiodinases regulate thyroid hormone activation which impacts muscle cell regeneration, metabolism, and reactive oxygen species production. Although the precise relationships between dietary Se and skeletal muscle mitochondria remain unclear, previous research constitutes a firm foundation that portends promising new discoveries by future investigations.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Association between thyroid hormones and skeletal muscle and bone in euthyroid type 2 diabetes patients

Background:

The impact of thyroid hormones within their normal ranges on skeletal muscle and bone in patients with type 2 diabetes mellitus (T2DM) remains unknown. The purpose of this study was to investigate the relationships of thyroid hormones with muscle and bone in euthyroid patients with T2DM.

Methods:

This cross-sectional study included 344 euthyroid T2DM patients. Muscle mass and bone mineral density were measured by dual-energy X-ray absorptiometry. The levels of thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxin (FT4) were measured by electrochemiluminescence immunoassay.

Results:

The results revealed that FT3 was positively correlated with body mass index (BMI) in male patients after age correction. In men, FT4 was negatively correlated with body weight, BMI, total muscle mass, appendicular skeletal muscle mass (ASM), and ASM index (ASMI), while FT3/FT4 was positively correlated with body weight, BMI, total muscle mass, ASM, and ASMI after age correction. In women, FT4 was negatively correlated with ASM and ASMI, while FT3/FT4 was positively correlated with ASM and ASMI after age correction. FT3/FT4 was significantly lower in men with low muscle mass than in those with normal muscle mass. The age-adjusted odds for incident low muscle mass comparing the lowest and highest FT3/FT4 increased in men.

Conclusions:

FT3/FT4 was positively correlated with ASM and ASMI in both men and women. Therefore, FT3/FT4 may be a parameter indicative of low muscle mass in euthyroid men with T2DM.

Low Inflammatory Stimulus Increases D2 Activity and Modulates Thyroid Hormone Metabolism during Myogenesis In Vitro

Thyroid hormone (TH) signaling controls muscle progenitor cells differentiation. However, inflammation can alter muscle TH signaling by modulating the expression of TH transporters (Slc16a2), receptors (Thra1), and deiodinase enzymes (Dio2 and Dio3). Thus, a proinflammatory environment could affect myogenesis. The role of a low-grade inflammatory milieu in TH signaling during myogenesis needs further investigation. Herein, we aimed to study the impact of the bacterial lipopolysaccharide (LPS)-induced inflammatory stimulus on the TH signaling during myogenesis. C2C12 myoblasts differentiation was induced without (CTR) or with 10 ng/mL LPS presence. The myoblasts under LPS stimulus release the proinflammatory cytokines (IL-6 and IL-1β) and chemokines (CCL2 and CXCL-1). LPS decreases Myod1 expression by 28% during the initial myogenesis, thus reducing the myogenic stimulus. At the same time, LPS reduced the expression of Dio2 by 41% but doubled the D2 enzymatic activity. The late differentiation was not affected by inflammatory milieu, which only increased the Slc16a2 gene expression by 38%. LPS altered the intracellular metabolism of TH and reduced the initial myogenic stimulus. However, it did not affect late differentiation. Increased intracellular TH activation may be the compensatory pathway involved in the recovery of myogenic differentiation under a low-grade inflammatory milieu.

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.

Thyroid Hormone Receptor Isoforms Alpha and Beta Play Convergent Roles in Muscle Physiology and Metabolic Regulation

Skeletal muscle is a key energy-regulating organ, skilled in rapidly boosting the rate of energy production and substrate consumption following increased workload demand. The alteration of skeletal muscle metabolism is directly associated with numerous pathologies and disorders. Thyroid hormones (THs) and their receptors (TRs, namely, TRα and TRβ) exert pleiotropic functions in almost all cells and tissues. Skeletal muscle is a major THs-target tissue and alterations of THs levels have multiple influences on the latter. However, the biological role of THs and TRs in orchestrating metabolic pathways in skeletal muscle has only recently started to be addressed. The purpose of this paper is to investigate the muscle metabolic response to TRs abrogation, by using two different mouse models of global TRα- and TRβKO. In line with the clinical features of resistance to THs syndromes in humans, characterized by THRs gene mutations, both animal models of TRs deficiency exhibit developmental delay and mitochondrial dysfunctions. Moreover, using transcriptomic and metabolomic approaches, we found that the TRs–THs complex regulates the Fatty Acids (FAs)-binding protein GOT2, affecting FAs oxidation and transport in skeletal muscle. In conclusion, these results underline a new metabolic role of THs in governing muscle lipids distribution and metabolism.

Immunometabolic signatures predict recovery from thyrotoxic myopathy in patients with Graves' disease

BACKGROUND

Thyroid hormone excess induces protein energy wasting, which in turn promotes muscle weakness and bone loss in patients with Graves' disease. Although most studies have confirmed a relationship between thyrotoxicosis and muscle dysfunction, few have measured changes in plasma metabolites and immune cells during the development and recovery from thyrotoxic myopathy. The aim of this study was to identify specific plasma metabolites and T-cell subsets that predict thyrotoxic myopathy recovery in patients with Graves' disease.

METHODS

One hundred patients (mean age, 40.0 ± 14.2 years; 67.0% female), with newly diagnosed or relapsed Graves' disease were enrolled at the start of methimazole treatment. Handgrip strength and Five Times Sit to Stand Test performance time were measured at Weeks 0, 12, and 24. In an additional 35 patients (mean age, 38.9 ± 13.5 years; 65.7% female), plasma metabolites and immunophenotypes of peripheral blood were evaluated at Weeks 0 and 12, and the results of a short physical performance battery assessment were recorded at the same time.

RESULTS

In both patient groups, methimazole-induced euthyroidism was associated with improved handgrip strength and lower limb muscle function at 12 weeks. Elevated plasma metabolites including acylcarnitines were restored to normal levels at Week 12 regardless of gender, body mass index, or age (P trend <0.01). Senescent CD8⁺ CD28^- CD57⁺ T-cell levels in peripheral blood were positively correlated with acylcarnitine levels (P < 0.05) and decreased during thyrotoxicosis recovery (P < 0.05). High levels of senescent CD8⁺ T cells at Week 0 were significantly associated with small increases in handgrip strength after 12 weeks of methimazole treatment (P < 0.05), but not statistically associated with Five Times Sit to Stand Test performance.

CONCLUSIONS

Restoring euthyroidism in Graves' disease patients was associated with improved skeletal muscle function and performance, while thyroid hormone-associated changes in plasma acylcarnitines levels correlated with muscle dysfunction recovery. T-cell senescence-related systemic inflammation correlated with plasma acylcarnitine levels and was also associated with small increases in handgrip strength.

Mitigating the detrimental effects of heat stress in poultry through thermal conditioning and nutritional manipulation

The poultry industry faces several obstacles and challenges, including the changes in global temperature, increase in the per capita demand for meat and eggs, and the emergence and spread of various diseases. Among these, environmental challenges are one of the most severe hurdles impacting the growth and productivity of poultry. In particular, the increasing frequency and severity of heat waves over the past few years represent a major challenge, and this is expected to worsen in the coming decades. Chickens are highly susceptible to high ambient temperatures (thermal stress), which negatively affect their growth and productivity, leading to enormous economic losses. In the light of global warming, these losses are expected to increase in the near future. Specifically, the worsening of climate change and the rise in global temperatures have augmented the adverse effects of heat on poultry production worldwide. At present, the world population is approximately 7.9 billion, and it has been predicted to reach 9.3 billion by 2050 and approximately 11 billion by 2100, implying a great demand for protein supply; therefore, strategies to mitigate future poultry challenges must be urgently devised. To date, several mitigation measures have been adopted to minimize the negative effects of heat stress in poultry. Of these, thermal acclimation at the postnatal stage or throughout the embryonic stages has been explored as a promising approach; however, for large-scale application, this approach warrants further investigation to determine the suitable temperature and poultry age. Moreover, molecular mechanisms governing thermal conditioning are poorly understood. To this end, we sought to expand our knowledge of thermal conditioning in poultry, which may serve as a valuable reference to improve the thermotolerance of chickens via nutritional management and vitagene regulation. Vitagenes regulate the responses of poultry to diverse stresses. In recent years, nutritionists have paid close attention to bioactive compounds such as resveratrol, curcumin, and quercetin administered alone or in combination. These compounds activate vitagenes and other regulators of the antioxidant defense system, such as nuclear factor-erythroid 2-related factor 2. Overall, thermal conditioning may be an effective strategy to mitigate the negative effects of heat stress. In this context, the present review synthesizes information on the adverse impacts of thermal stress, elucidating the molecular mechanisms underlying thermal conditioning and its effects on the acquisition of tolerance to acute heat stress in later life. Finally, the role of some polyphenolic compounds, such as resveratrol, curcumin, and quercetin, in attenuating heat stress through the activation of the antioxidant defense system in poultry are discussed.

Association of Reduced Free T3 to Free T4 Ratio with Lower Serum Creatinine in Japanese Hemodialysis Patients

PURPOSE

Low T3 syndrome is defined by a fall in free triiodothyronine (FT3) in spite of normal serum thyroid-stimulating hormone (TSH) and often normal free thyroxin (FT4). A low FT3/FT4 ratio, a relevant marker for low T3 syndrome, is known as a risk of mortality in hemodialysis (HD) patients, as well as low muscle mass in the general population. Because of the local activation of T4 to FT3 in muscle tissue, we examined the association of FT3/FT4 ratio with serum creatinine, a marker of muscle mass and strength in HD patients to investigate the significance of muscle tissue in the development of low T3 syndrome in HD patients.

METHODS

This was a cross-sectional study derived from our prospective cohort study, named DREAM, of Japanese HD patients. After the exclusion of patients with treated and untreated thyroid dysfunction, 332 patients were analyzed in the study.

RESULTS

The serum FT4 and TSH of HD patients (n = 332) were 0.9 ± 0.1 ng/dL. and 2.0 ± 0.9 μIU/mL, which were within the respective normal range, while serum FT3 was 2.2 ± 0.3 pg/mL. As many as 101 out of 332 (30.4%) HD patients exhibited a serum FT3 less than the normal lower limit of 2.2 pg/mL. The serum FT3/FT4 ratio correlated significantly positively with serum creatinine, and inversely with serum log CRP and total cholesterol, while it exhibited a tendency towards positive correlation with serum albumin. Multiple regression analysis, which included serum creatinine, albumin, and log CRP, simultaneously, in addition to sex, age, diabetic kidney disease or not, log HD duration, body mass index, systolic blood pressure, and Kt/V, as independent variables, revealed an independent and significant positive association of serum creatinine, but not serum albumin or CRP, with the serum FT3/FT4 ratio.

CONCLUSIONS

The present study demonstrated an independent and positive correlation of serum creatinine with the serum FT3/FT4 ratio in HD patients. The lack of association of the serum FT3/FT4 ratio with serum albumin and CRP suggested the presence of a creatinine-specific mechanism to associate with serum FT3/FT4 ratio. Because of the local activation of T4 to T3 at muscle tissue, a lower muscle mass may be causatively associated with low T3 syndrome.

Effects of hormonal changes on sarcopenia in chronic kidney disease: where are we now and what can we do?

Sarcopenia or muscle wasting is a progressive and generalized skeletal muscle disorder involving the accelerated loss of muscle mass and function, often associated with muscle weakness (dynapenia) and frailty. Whereas primary sarcopenia is related to ageing, secondary sarcopenia happens independent of age in the context of chronic disease states such as chronic kidney disease (CKD). Sarcopenia has become a major focus of research and public policy debate due to its impact on patient's health-related quality of life, health-care expenditure, morbidity, and mortality. The development of sarcopenia in patients with CKD is multifactorial and it may occur independently of weight loss or cachexia including under obese sarcopenia. Hormonal imbalances can facilitate the development of sarcopenia in the general population and is a common finding in CKD. Hormones that may influence the development of sarcopenia are testosterone, growth hormone, insulin, thyroid hormones, and vitamin D. Although the relationship between free testosterone level that is low in uraemic patients and sarcopenia in CKD is not well-defined, functional improvement may be seen. Unlike testosterone, it is known that vitamin D is associated with muscle strength, muscle size, and physical performance in patients with CKD. Outcomes after vitamin D replacement therapy are still controversial. The half-life of growth hormone (GH) is prolonged in patients with CKD. Besides, IGF-1 levels are normal in patients with Stage 4 CKD-a minimal reduction is seen in the end-stage renal disease. Unresponsiveness or resistance of IGF-1 and changes in the GH/IGF-1 axis are the main causes of sarcopenia in CKD. Low serum T3 level is frequent in CKD, but the net effect on sarcopenia is not well-studied. CKD patients develop insulin resistance (IR) from the earliest period even before GFR decline begins. IR reduces glucose utilization as an energy source by hepatic gluconeogenesis, decreasing muscle glucose uptake, impairing intracellular glucose metabolism. This cascade results in muscle protein breakdown. IR and sarcopenia might also be a new pathway for targeting. Ghrelin, oestrogen, cortisol, and dehydroepiandrosterone may be other players in the setting of sarcopenia. In this review, we mainly examine the effects of hormonal changes on the occurrence of sarcopenia in patients with CKD via the available data.

Smoothelin-Like Protein 1 Regulates Development and Metabolic Transformation of Skeletal Muscle in Hyperthyroidism

Hyperthyroidism triggers a glycolytic shift in skeletal muscle (SKM) by altering the expression of metabolic proteins, which is often accompanied by peripheral insulin resistance. Our previous results show that smoothelin-like protein 1 (SMTNL1), a transcriptional co-regulator, promotes insulin sensitivity in SKM. Our aim was to elucidate the role of SMTNL1 in SKM under physiological and pathological 3,3',5-Triiodo-L-thyronine (T3) concentrations. Human hyper- and euthyroid SKM biopsies were used for microarray analysis and proteome profiler arrays. Expression of genes related to energy production, nucleic acid- and lipid metabolism was changed significantly in hyperthyroid samples. The phosphorylation levels and activity of AMPKα2 and JNK were increased by 15% and 23%, respectively, in the hyperthyroid samples compared to control. Moreover, SMTNL1 expression showed a 6-fold decrease in the hyperthyroid samples and in T3-treated C2C12 cells. Physiological and supraphysiological concentrations of T3 were applied on differentiated C2C12 cells upon SMTNL1 overexpression to assess the activity and expression level of the elements of thyroid hormone signaling, insulin signaling and glucose metabolism. Our results demonstrate that SMTNL1 selectively regulated TRα expression. Overexpression of SMTNL1 induced insulin sensitivity through the inhibition of JNK activity by 40% and hampered the non-genomic effects of T3 by decreasing the activity of ERK1/2 through PKCδ. SMTNL1 overexpression reduced IRS1 Ser307 and Ser612 phosphorylation by 52% and 53%, respectively, in hyperthyroid model to restore the normal responsiveness of glucose transport to insulin. SMTNL1 regulated glucose phosphorylation and balances glycolysis and glycogen synthesis via the downregulation of hexokinase II by 1.3-fold. Additionally, mitochondrial respiration and glycolysis were measured by SeaHorse analysis to determine cellular metabolic function/phenotype of our model system in real-time. T3 overload strongly increased the rate of acidification and a shift to glycolysis, while SMTNL1 overexpression antagonizes the T3 effects. These lines of evidence suggest that SMTNL1 potentially prevents hyperthyroidism-induced changes in SKM, and it holds great promise as a novel therapeutic target in insulin resistance.

Smoothelin-Like Protein 1 Regulates the Thyroid Hormone-Induced Homeostasis and Remodeling of C2C12 Cells via the Modulation of Myosin Phosphatase

The pathological elevation of the active thyroid hormone (T3) level results in the manifestation of hyperthyroidism, which is associated with alterations in the differentiation and contractile function of skeletal muscle (SKM). Myosin phosphatase (MP) is a major cellular regulator that hydrolyzes the phosphoserine of phosphorylated myosin II light chain. MP consists of an MYPT1/2 regulatory and a protein phosphatase 1 catalytic subunit. Smoothelin-like protein 1 (SMTNL1) is known to inhibit MP by directly binding to MP as well as by suppressing the expression of MYPT1 at the transcriptional level. Supraphysiological vs. physiological concentration of T3 were applied on C2C12 myoblasts and differentiated myotubes in combination with the overexpression of SMTNL1 to assess the role and regulation of MP under these conditions. In non-differentiated myoblasts, MP included MYPT1 in the holoenzyme complex and its expression and activity was regulated by SMTNL1, affecting the phosphorylation level of MLC20 assessed using semi-quantitative Western blot analysis. SMTNL1 negatively influenced the migration and cytoskeletal remodeling of myoblasts measured by high content screening. In contrast, in myotubes, the expression of MYPT2 but not MYPT1 increased in a T3-dependent and SMTNL1-independent manner. T3 treatment combined with SMTNL1 overexpression impeded the activity of MP. In addition, MP interacted with Na+/K+-ATPase and dephosphorylated its inhibitory phosphorylation sites, identifying this protein as a novel MP substrate. These findings may help us gain a better understanding of myopathy, muscle weakness and the disorder of muscle regeneration in hyperthyroid patients.

Transcriptome Analysis of Differentially Expressed mRNA Related to Pigeon Muscle Development

The mechanisms behind the gene expression and regulation that modulate the development and growth of pigeon skeletal muscle remain largely unknown. In this study, we performed gene expression analysis on skeletal muscle samples at different developmental and growth stages using RNA sequencing (RNA-Seq). The differentially expressed genes (DEGs) were identified using edgeR software. Weighted gene co-expression network analysis (WGCNA) was used to identify the gene modules related to the growth and development of pigeon skeletal muscle based on DEGs. A total of 11,311 DEGs were identified. WGCNA aggregated 11,311 DEGs into 12 modules. Black and brown modules were significantly correlated with the 1st and 10th day of skeletal muscle growth, while turquoise and cyan modules were significantly correlated with the 8th and 13th days of skeletal muscle embryonic development. Four mRNA-mRNA regulatory networks corresponding to the four significant modules were constructed and visualised using Cytoscape software. Twenty candidate mRNAs were identified based on their connectivity degrees in the networks, including Abca8b, TCONS-00004461, VWF, OGDH, TGIF1, DKK3, Gfpt1 and RFC5, etc. A KEGG pathway enrichment analysis showed that many pathways were related to the growth and development of pigeon skeletal muscle, including PI3K/AKT/mTOR, AMPK, FAK, and thyroid hormone pathways. Five differentially expressed genes (LAST2, MYPN, DKK3, B4GALT6 and OGDH) in the network were selected, and their expression patterns were quantified by qRT-PCR. The results were consistent with our sequencing results. These findings could enhance our understanding of the gene expression and regulation in the development and growth of pigeon muscle.

Thyroid Hormone Receptor α Regulates Autophagy, Mitochondrial Biogenesis, and Fatty Acid Use in Skeletal Muscle

Skeletal muscle (SM) weakness occurs in hypothyroidism and resistance to thyroid hormone α (RTHα) syndrome. However, the cell signaling and molecular mechanism(s) underlying muscle weakness under these conditions is not well understood. We thus examined the role of thyroid hormone receptor α (TRα), the predominant TR isoform in SM, on autophagy, mitochondrial biogenesis, and metabolism to demonstrate the molecular mechanism(s) underlying muscle weakness in these two conditions. Two genetic mouse models were used in this study: TRα1PV/+ mice, which express the mutant Thra1PV gene ubiquitously, and SM-TRα1L400R/+ mice, which express TRα1L400R in a muscle-specific manner. Gastrocnemius muscle from TRα1PV/+, SM-TRα1L400R/+, and their control mice was harvested for analyses. We demonstrated that loss of TRα1 signaling in gastrocnemius muscle from both the genetic mouse models led to decreased autophagy as evidenced by accumulation of p62 and decreased expression of lysosomal markers (lysosomal-associated membrane protein [LAMP]-1 and LAMP-2) and lysosomal proteases (cathepsin B and cathepsin D). The expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), mitochondrial transcription factor A (TFAM), and estrogen-related receptor α (ERRα), key factors contributing to mitochondrial biogenesis as well as mitochondrial proteins, were decreased, suggesting that there was reduced mitochondrial biogenesis due to the expression of mutant TRα1. Transcriptomic and metabolomic analyses of SM suggested that lipid catabolism was impaired and was associated with decreased acylcarnitines and tricarboxylic acid cycle intermediates in the SM from the mouse line expressing SM-specific mutant TRα1. Our results provide new insight into TRα1-mediated cell signaling, molecular, and metabolic changes that occur in SM when TR action is impaired.

Deiodinases: How Nonmammalian Research Helped Shape Our Present View

Iodothyronine deiodinases are enzymes capable of activating and inactivating thyroid hormones (THs) and have an important role in regulating TH action in tissues throughout the body. Three types of deiodinases (D1, D2, and D3) were originally defined based on their biochemical characteristics. Cloning of the first complementary DNAs in the 1990s (Dio1 in rat and dio2 and dio3 in frog) allowed to confirm the existence of 3 distinct enzymes. Over the years, increasing genomic information revealed that deiodinases are present in all chordates, vertebrates, and nonvertebrates and that they can even be found in some mollusks and annelids, pointing to an ancient origin. Research in nonmammalian models has substantially broadened our understanding of deiodinases. In relation to their structure, we discovered for instance that biochemical properties such as inhibition by 6-propyl-2-thiouracil, stimulation by dithiothreitol, and temperature optimum are subject to variation. Data from fish, amphibians, and birds were key in shifting our view on the relative importance of activating and inactivating deiodination pathways and in showing the impact of D2 and D3 not only in local but also whole body T3 availability. They also led to the discovery of new local functions such as the acute reciprocal changes in D2 and D3 in hypothalamic tanycytes upon photostimulation, involved in seasonal rhythmicity. With the present possibilities for rapid and precise gene silencing in any species of interest, comparative research will certainly further contribute to a better understanding of the importance of deiodinases for adequate TH action, also in humans.

Mechanism of thyroid hormone signaling in skeletal muscle of aging mice

BACKGROUND AND AIM

Skeletal muscle (SM) has been shown as a target of thyroid hormones (THs). However, the status of TH signaling in aged SM remains unclear. This study aimed to explore the mechanism of TH signaling in SM of aging mice.

METHODS

Thirty C57BL/6J male mice were divided into 6-, 15- and 22-month (6, 15 and 22M) groups according to different age. Physical parameters were evaluated by analytical balance, grip strength test and histological analysis. Thyroid function was detected by enzyme-linked immunosorbent assay. TH signaling was compared among the three groups by real-time PCR and western blotting analysis.

RESULTS

p16, p21, and p53 mRNA levels in SM increased in age-dependent manner. The muscle weight and strength decreased in 22M group compared to 6 and 15M groups. Concentrations of thyroid hormones, including free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) in 22 M mice were not shown significant difference compared to 6M or 15M mice, although FT3 showed slightly decrease and TSH appeared a mild increase accompanying with age. mRNA levels of TH transporters, including MCT8 and MCT10, as well as iodothyronine deiodinase type 2 (DIO2) and type 3 (DIO3), were higher in 22M, while TH receptor α (TRα) mRNA and protein expression was lower in 22M, compared to the other groups. Type-I myosin heavy chain (MyHC I), MyHC IIx, and MyHC IIa were upregulated and Type-IIb MyHC (MyHC IIb) was downregulated in SM with advancing age.

CONCLUSIONS

TH signaling in SM changes with aging.

Regulation of muscle stem cell function

Regeneration of skeletal muscle is a finely tuned process which is depending on muscle stem cells, a population of stem cells in skeletal muscle which is also termed satellite cells. Muscle stem cells are a prerequisite for regeneration of skeletal muscle. Of note, the muscle stem cell population is heterogeneous and subpopulations can be identified depending on gene expression or phenotypic traits. However, all muscle stem cells express the transcription factor Pax7 and their functionality is tightly controlled by intrinsic signaling pathways and extrinsic signals. The latter ones include signals form the stem cell niche as well as circulating factors such as growth factors and hormones. Among them are Wnt proteins, growth factors like IGF-1 or FGF-2 and hormones such as thyroid hormones and the anti-aging hormone Klotho. A highly orchestrated interplay between those factors and muscle stem cells is important for their full functionality and ultimately regeneration of skeletal muscle as outlined here.

The correlation between serum thyroid hormone levels and hand grip among elderly male Chinese inpatients

INTRODUCTION

Thyroid dysfunction is closely associated with skeletal muscle weakness. However, data on the optimal serum range of thyroid hormones for maintaining muscle strength in the elderly is lacking.

METHODS

We conducted a cross-sectional analysis in male elderly inpatients from the Geriatric Department of Zhongshan Hospital (affiliated to Fudan University, Shanghai, China). Serum biochemical parameters and thyroid hormones were detected for each participant. Hand grip (HG) was measured, with low hand grip defined as HG <26 kg according to the standard of the Asian Working Group for Sarcopenia. Logistic regression was used to evaluate the effects of different serum thyroid hormone levels on HG.

RESULTS

The majority of the subjects were euthyroid. The prevalence of low hand grip was 48.5%. Stratified by the free thyroxine (FT4) quartiles, the results showed HG was the highest in the third quartile. Multiple logistic regression analysis showed that compared with those in the first quartile, subjects in the third quartile of FT4 had a significantly lower risk of low hand grip (OR = 0.133, 95%CI: 0.020-0.610, p = .009), after adjusting potential confounding factors.

CONCLUSION

In elderly male inpatients, maintaining a narrower serum range of thyroid hormone might be needed to protect skeletal muscle strength.

Delayed access to feed alters gene expression associated with hormonal signaling, cellular differentiation, and protein metabolism in muscle of newly hatch chicks

Birds rely solely on utilization of the yolk sac as a means of nutritional support throughout embryogenesis and early post-hatch, before first feeding occurs. Newly hatched broiler (meat-type) chickens are frequently not given immediate access to feed, and this can result in numerous alterations to developmental processes, including those that occur in muscle. The objective of this study was to characterize the gene expression profile of newly hatched chicks' breast muscle with regards to hormonal regulation of growth and metabolism and development and differentiation of muscle tissue, and determine impacts of delayed access to feed on these profiles. Within 3 h of hatch, birds were placed in battery pens and given immediate access to feed (Fed) or delayed access to feed for 48 h (Delayed Fed). Breast muscle collected from male birds at hatch, or 4 h, 1 day (D), 2D, 4D, and 8D after hatch was used for analysis of mRNA expression by reverse transcription-quantitative PCR. Under fully fed conditions, insulin-like growth factor receptor and leptin receptor mRNA expression decreased as birds aged; however, delayed access to feed resulted in prolonged upregulation of these genes so their mRNA levels were higher in Delayed Fed birds at 2D. These expression profiles suggest that delayed feed access alters sensitivity to hormones that may regulate muscle development. Myogenin, a muscle differentiation factor, showed increasing mRNA expression in Fed birds through 2D, after which expression decreased. A similar expression pattern in Delayed Fed birds was deferred until 4D. Levels of myostatin, a negative regulator of muscle growth, increased in Fed birds starting at 2D, while levels in Delayed Fed birds began to increase at 4D. In Fed birds, levels of transcripts for two genes associated with protein catabolism, F-box protein 32 and forkhead box O3, were lower at 2D, while Delayed Fed mRNA levels did not decrease until 4D. Mechanistic target of rapamycin mRNA levels decreased from 1D through 8D in both treatments, except for a transient increase in the Delayed Fed birds between 1D and 2D. These data suggest that within breast muscle, delayed feeding alters hormonal signaling, interrupts tissue differentiation, postpones onset of growth, and may lead to increased protein catabolism. Together, these processes could ultimately contribute to a reduction in proper growth and development of birds not given feed immediately after hatch, and ultimately hinder the long-term potential of muscle accretion in meat type birds.

Timing Is Everything-The High Sensitivity of Avian Satellite Cells to Thermal Conditions During Embryonic and Posthatch Periods

Myofiber formation is essentially complete at hatch, but myofiber hypertrophy increases posthatch through the assimilation of satellite cell nuclei into myofibers. Satellite cell proliferation and differentiation occur during the early growth phase, which in meat-type poultry terminates at around 8 days posthatch. Thus, any factor that affects the accumulation of satellite cells during late-term embryogenesis or early posthatch will dictate long-term muscle growth. This review will focus on the intimate relationship between thermal conditions during chick embryogenesis and the early posthatch period, and satellite cell myogenesis and pectoralis growth and development. Satellite cells are highly sensitive to temperature changes, particularly when those changes occur during crucial periods of their myogenic activity. Therefore, timing, temperature, and duration of thermal treatments have a great impact on satellite cell activity and fate, affecting muscle development and growth in the long run. Short and mild thermal manipulations during embryogenesis or thermal conditioning in the early posthatch period promote myogenic cell proliferation and differentiation, and have long-term promotive effects on muscle growth. However, chronic heat stress during the first 2 weeks of life has adverse effects on these parameters and may lead to muscle myopathies.

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.

Transthyretin Maintains Muscle Homeostasis Through the Novel Shuttle Pathway of Thyroid Hormones During Myoblast Differentiation

Skeletal muscle, the largest part of the total body mass, influences energy and protein metabolism as well as maintaining homeostasis. Herein, we demonstrate that during murine muscle satellite cell and myoblast differentiation, transthyretin (TTR) can exocytose via exosomes and enter cells as TTR- thyroxine (T₄) complex, which consecutively induces the intracellular triiodothyronine (T₃) level, followed by T₃ secretion out of the cell through the exosomes. The decrease in T₃ with the TTR level in 26-week-old mouse muscle, compared to that in 16-week-old muscle, suggests an association of TTR with old muscle. Subsequent studies, including microarray analysis, demonstrated that T₃-regulated genes, such as FNDC5 (Fibronectin type III domain containing 5, irisin) and RXRγ (Retinoid X receptor gamma), are influenced by TTR knockdown, implying that thyroid hormones and TTR coordinate with each other with respect to muscle growth and development. These results suggest that, in addition to utilizing T₄, skeletal muscle also distributes generated T₃ to other tissues and has a vital role in sensing the intracellular T₄ level. Furthermore, the results of TTR function with T₄ in differentiation will be highly useful in the strategic development of novel therapeutics related to muscle homeostasis and regeneration.

Tissue-Specific Function of Thyroid Hormone Transporters: New Insights from Mouse Models

Thyroid hormone (TH) transporters are required for cellular transmembrane passage of TH and are thus mandatory for proper TH metabolism and action. Consequently, inactivating mutations in TH transporters such as MCT8 or OATP1C1 can cause tissue- specific changes in TH homeostasis. As the most prominent example, patients with MCT8 mutations exhibit elevated serum T3 levels, whereas their CNS appear to be in a TH deficient state. Here, we will briefly summarize recent studies of mice lacking Mct8 alone or in combination with the TH transporters Mct10 or Oatp1c1 that shed light on many aspects and pathogenic events underlying global MCT8 deficiency and also underscore the contribution of Mct10 and Oatp1c1 in tissue-specific TH transport processes. Moreover, development of conditional knock-out mice that allow a cell-specific inactivation of TH transporters in distinct tissues, disclosed cell-specific changes in TH signaling, thereby highlighting the pathophysiological significance of local control of TH action.

Thyroid Hormone Status Regulates Skeletal Muscle Response to Chronic Motor Nerve Stimulation

Although both exercise and thyroid hormone (TH) status can cause cellular and metabolic changes in skeletal muscle, the impact of TH status on exercise-associated changes is not well understood. Here, we examined the effects of TH status on muscle fiber type, cell signaling, and metabolism in a rabbit model of exercise training – chronic motor nerve stimulation (CMNS). Five rabbits were rendered hypothyroid for 7–8 weeks and three rabbits were made hyperthyroid for 2 weeks prior to CMNS of the left peroneal nerve for 10 days. We then measured markers of muscle fiber type, autophagy, and nutrient- or energy-sensing proteins, and metabolic intermediates. CMNS increased MHC-I expression in hypothyroid rabbits, whereas it was unchanged in hyperthyroid rabbits. CMNS also increased p-AMPK, p-ATGL, CPT-1α, p-Akt, GLUT4, and p-70S6K in hypothyroid rabbits. In contrast, p-AMPK and p-AKT were increased at baseline in hyperthyroid rabbits, but CMNS did not further increase them or any of the other markers. CMNS also increased TCA cycle and acylcarnitine metabolites in hypothyroid rabbits; whereas, acylcarnitines were already elevated in hyperthyroid rabbits, and were only slightly increased further by CMNS. In summary, CMNS effects on cell signaling and metabolism of skeletal muscle were more pronounced in the hypothyroid than the hyperthyroid state. Interestingly, in the hypothyroid state, CMNS caused concomitant activation of two signaling pathways that are usually reciprocally regulated – AMPK and mTOR signaling – which manifested as increased β-oxidation, MHC-I expression, and protein synthesis. Thus, our findings provide insight into the role of TH status on exercise response in muscle. Our observations suggest that TH status of patients may be an important determinant and predictor of their response to exercise training in skeletal muscle.

Deiodinases and their intricate role in thyroid hormone homeostasis

The deiodinase family of enzymes mediates the activation and inactivation of thyroid hormone. The role of these enzymes in the regulation of the systemic concentrations of thyroid hormone is well established and underpins the treatment of common thyroid diseases. Interest in this field has increased in the past 10 years as the deiodinases became implicated in tissue development and homeostasis, as well as in the pathogenesis of a wide range of human diseases. Three deiodinases have been identified, namely, types 1, 2 and 3 iodothyronine deiodinases, which differ in their catalytic properties and tissue distribution. Notably, the expression of these enzymes changes during the lifetime of an individual in relation to the different needs of each organ and to ageing. The systemic homeostatic role of deiodinases clearly emerges during changes in serum concentrations of thyroid hormone, as seen in patients with thyroid dysfunction. By contrast, the role of deiodinases at the tissue level allows thyroid hormone signalling to be finely tuned within a given cell in a precise time-space window without perturbing serum concentrations of thyroid hormone. This Review maps the overall functional role of the deiodinases and explores challenges and novel opportunities arising from the expanding knowledge of these 'master' components of the thyroid homeostatic system.

Functions of stem cells of thyroid glands in health and disease

Thyroid gland has been implicated in the regulation of many functions using endocrine, paracrine and autocrine signals. Functional thyroid follicular cells derived from stem cells attracted a great interest from researchers as a strategy for thyroid's regenerative therapy. Thyroid has a very low rate of turnover; however, studies showed that the regenerative ability is enhanced following diseases or thyroidectomy, which promotes the role of stem cell. The objective of this review is to summarize the morphological characterization and the expression of stem cell genes/markers in the thyroid. Also, to highlight the mechanisms of tumor formation in thyroid via its stem cells. The most important thyroid stem cell's markers are: stem cell antigen 1 (SCA-1), octamer-binding transcription 4 (OCT-4), p63, CD34+ CD45-, paired box gene 8 (PAX-8), thyroid transcription factor 1 (TTF-1), thyroid transcription factor 2 (TTF-2), hematopoietically expressed homeobox protein HHEX, the transcription factor GATA-4, hepatocyte nuclear factor 4-α (HNF-4-α) and homeobox transcription factor Nanog (hNanog). This review highlights the functional characterization describing the mechanisms of stem cell's differentiation into functional thyroid follicle and proposing mechanisms involving in cancer formation through one of these cell types: fetal cell, thyroblasts, prothyrocytes, certain genetic mutation in the mature thyroid cells or presence of a special type of cells (cancer stem cell) which are responsible for different types of cancer formation. Understanding the mechanisms of thyroid's stem cell in cancer formation and the expression of the biomarkers in normal and abnormal thyroid status are promising physiological tools in promoting thyroid regeneration and in provision management for thyroid cancer.

The Colorful Diversity of Thyroid Hormone Metabolites

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic "hot" metabolite 3,5-T2 and the "cool" 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the "thyronome," beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

A Global Loss of Dio2 Leads to Unexpected Changes in Function and Fiber Types of Slow Skeletal Muscle in Male Mice

The type 2 iodothyronine-deiodinase (D2) enzyme converts T4 to T3, and mice deficient in this enzyme [D2 knockout (D2KO) mice] have decreased T3 derived from T4 in skeletal muscle despite normal circulating T3 levels. Because slow skeletal muscle is particularly susceptible to changes in T3 levels, we expected D2 inactivation to result in more pronounced slow-muscle characteristics in the soleus muscle, mirroring hypothyroidism. However, ex vivo studies of D2KO soleus revealed higher rates of twitch contraction and relaxation and reduced resistance to fatigue. Immunostaining of D2KO soleus showed that these properties were associated with changes in muscle fiber type composition, including a marked increase in the number of fast, glycolytic type IIB fibers. D2KO soleus muscle fibers had a larger cross-sectional area, and this correlated with increased myonuclear accretion in myotubes formed from D2KO skeletal muscle precursor cells differentiated in vitro. Consistent with our functional findings, D2KO soleus gene expression was markedly different from that in hypothyroid wild-type (WT) mice. Comparison of gene expression between euthyroid WT and D2KO mice indicated that PGC-1α, a T3-dependent regulator of slow muscle fiber type, was decreased by ∼50% in D2KO soleus. Disruption of Dio2 in the C2C12 myoblast cell line led to a significant decrease in PGC-1α expression and a faster muscle phenotype upon differentiation. These results indicate that D2 loss leads to significant changes in soleus contractile function and fiber type composition that are inconsistent with local hypothyroidism and suggest that reduced levels of PCG-1α may contribute to the observed phenotypical changes.

Effects of perinatal di (2-ethylhexyl) phthalate exposure on thyroid function in rat offspring

Di (2-ethylhexyl) phthalate (DEHP) is a commonly used plasticizer in industry and displays the characteristics of an endocrine disruptor. Disorders of the maternal thyroid hormone (TH) during pregnancy can cause adverse effects on the fetus. We investigated the effects and possible mechanism of perinatal DEHP exposure on the thyroid function of pups. Pregnant female Wistar rats were randomly divided into four groups and received doses of DEHP of 0, 30, 300, 750 mg/kg/day by gavage at from gestational day (GD) 0 to postnatal day (PN) 21. The concentration of serum THs and the ultrastructure of thyroid follicular cells in the offspring were examined. Related protein level and gene expression of thyroid proteins in pups were analyzed by western blotting and real-time PCR. We found that DEHP significantly reduced total thyroxine (TT4) and increased thyroid stimulating hormone (TSH) in pups, while total triiodothyronine (TT3) showed no change. Thyroid follicular cells ultrastructure was damaged in DEHP exposed pups as viewed by electron microscopy. Furthermore, exposure to DEHP significantly increased protein and mRNA levels of thyroid transcription factor 1 (TTF-1), paired box 8 (PAX8), sodium iodide symporter (NIS) and thyroid peroxidase (TPO) in pups. In addition, levels of deiodinases of pups were also affected. These findings indicated that DEHP can disrupt thyroid function by damaging thyroid follicles and affecting TTF-1, PAX8, NIS, TPO and the deiodinase protein family.

Stage-dependent cardiac regeneration in Xenopus is regulated by thyroid hormone availability

Despite therapeutic advances, heart failure is the major cause of morbidity and mortality worldwide, but why cardiac regenerative capacity is lost in adult humans remains an enigma. Cardiac regenerative capacity widely varies across vertebrates. Zebrafish and newt hearts regenerate throughout life. In mice, this ability is lost in the first postnatal week, a period physiologically similar to thyroid hormone (TH)-regulated metamorphosis in anuran amphibians. We thus assessed heart regeneration in Xenopus laevis before, during, and after TH-dependent metamorphosis. We found that tadpoles display efficient cardiac regeneration, but this capacity is abrogated during the metamorphic larval-to-adult switch. Therefore, we examined the consequence of TH excess and deprivation on the efficiently regenerating tadpole heart. We found that either acute TH treatment or blocking TH production before resection significantly but differentially altered gene expression and kinetics of extracellular matrix components deposition, and negatively impacted myocardial wall closure, both resulting in an impeded regenerative process. However, neither treatment significantly influenced DNA synthesis or mitosis in cardiac tissue after amputation. Overall, our data highlight an unexplored role of TH availability in modulating the cardiac regenerative outcome, and present X. laevis as an alternative model to decipher the developmental switches underlying stage-dependent constraint on cardiac regeneration.

Impact of energy restriction during late gestation on the muscle and blood transcriptome of beef calves after preconditioning

Background

Maternal nutrition has been highlighted as one of the main factors affecting intra-uterine environment. The increase in nutritional requirements by beef cows during late gestation can cause nutritional deficiency in the fetus and impact the fetal regulation of genes associated with myogenesis and immune response.

Methods

Forty days before the expected calving date, cows were assigned to one of two diets: 100% (control) or 70% (restricted group) of the daily energy requirement. Muscle samples were collected from 12 heifers and 12 steers, and blood samples were collected from 12 steers. The objective of this work was to identify and to assess the biological relevance of differentially expressed genes (DEG) in the skeletal muscle and blood of beef calves born from cows that experienced [or not] a 30% energy restriction during the last 40 days of gestation.

Results

A total of 160, 164, and 346 DEG (q-value< 0.05) were identified in the skeletal muscle for the effects of diet, sex, and diet-by-sex interaction, respectively. For blood, 452, 1392, and 155 DEG were identified for the effects of diet, time, and diet-by-time interaction, respectively. For skeletal muscle, results based on diet identified genes involved in muscle metabolism. In muscle, from the 10 most DEG down-regulated in the energy-restricted group (REST), we identified 5 genes associated with muscle metabolism and development: SLCO3A1, ATP6V0D1, SLC2A1, GPC4, and RASD2. In blood, among the 10 most DEG, we found genes related to response to stress up-regulated in the REST after weaning, such as SOD3 and INO80D, and to immune response down-regulated in the REST after vaccination, such as OASL, KLRF1, and LOC104968634.

Conclusion

In conclusion, maternal energy restriction during late gestation may limit the expression of genes in the muscle and increase expression in the blood of calves. In addition, enrichment analysis showed that a short-term maternal energy restriction during pregnancy affects the expression of genes related to energy metabolism and muscle contraction, and immunity and stress response in the blood. Therefore, alterations in the intra-uterine environment can modify prenatal development with lasting consequences to adult life.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5089-8) contains supplementary material, which is available to authorized users.

Thyroid Hormones Play Role in Sarcopenia and Myopathies

Skeletal muscle maintains posture and enables movement by converting chemical energy into mechanical energy, further contributing to basal energy metabolism. Thyroid hormones (thyroxine, or T4, and triiodothyronine, or T3) participate in contractile function, metabolic processes, myogenesis and regeneration of skeletal muscle. T3 classically modulates gene expression after binding to thyroid hormone nuclear receptors. Thyroid hormone effects depend on nuclear receptor occupancy, which is directly related to intracellular T3 levels. Sarcolemmal thyroid hormone levels are regulated by their transport across the plasma membrane by specific transporters, as well as by the action of deiodinases types 2 and 3, which can activate or inactivate T4 and T3. Thyroid hormone level oscillations have been associated with the worsening of many myopathies such as myasthenia gravis, Duchenne muscular dystrophy (DMD) and rhabdomyolysis. During aging skeletal muscle show a decrease in mass and quality, known as sarcopenia. There is increasing evidence that thyroid hormones could have a role in the sarcopenic process. Therefore, in this review, we aim to discuss the main effects of thyroid hormones in skeletal muscular aging processes and myopathy-related pathologies.

Thyroid Hormone Transporters MCT8 and OATP1C1 Control Skeletal Muscle Regeneration

Thyroid hormone (TH) transporters are required for the transmembrane passage of TH in target cells. In humans, inactivating mutations in the TH transporter MCT8 cause the Allan-Herndon-Dudley syndrome, characterized by severe neuromuscular symptoms and an abnormal TH serum profile, which is fully replicated in Mct8 knockout mice and Mct8/Oatp1c1 double-knockout (M/O DKO) mice. Analysis of tissue TH content and expression of TH-regulated genes indicate a thyrotoxic state in Mct8-deficient skeletal muscles. Both TH transporters are upregulated in activated satellite cells (SCs). In M/O DKO mice, we observed a strongly reduced number of differentiated SCs, suggesting an impaired stem cell function. Moreover, M/O DKO mice and mice lacking both transporters exclusively in SCs showed impaired skeletal muscle regeneration. Our data provide solid evidence for a unique gate-keeper function of MCT8 and OATP1C1 in SC activation, underscoring the importance of a finely tuned TH signaling during myogenesis.

•

MCT8 and OATP1C1 expression increases upon activation of muscle stem cells

•

Loss of MCT8 and OATP1C1 expression inhibits muscle stem cell differentiation

•

Mct8- and Oatp1c1-deficient mice display impaired muscle regeneration

Restoration of type 1 iodothyronine deiodinase expression in renal cancer cells downregulates oncoproteins and affects key metabolic pathways as well as anti-oxidative system

Type 1 iodothyronine deiodinase (DIO1) contributes to deiodination of 3,5,3’,5’-tetraiodo-L-thyronine (thyroxine, T4) yielding of 3,5,3’-triiodothyronine (T3), a powerful regulator of cell differentiation, proliferation, and metabolism. Our previous work showed that loss of DIO1 enhances proliferation and migration of renal cancer cells. However, the global effects of DIO1 expression in various tissues affected by cancer remain unknown. Here, the effects of stable DIO1 re-expression were analyzed on the proteome of renal cancer cells, followed by quantitative real-time PCR validation in two renal cancer-derived cell lines. DIO1-induced changes in intracellular concentrations of thyroid hormones were quantified by L-MS/MS and correlations between expression of DIO1 and potential target genes were determined in tissue samples from renal cancer patients. Stable re-expression of DIO1, resulted in 26 downregulated proteins while 59 proteins were overexpressed in renal cancer cells. The ‘downregulated’ group consisted mainly of oncoproteins (e.g. STAT3, ANPEP, TGFBI, TGM2) that promote proliferation, migration and invasion. Furthermore, DIO1 re-expression enhanced concentrations of two subunits of thyroid hormone transporter (SLC7A5, SLC3A2), enzymes of key pathways of cellular energy metabolism (e.g. TKT, NAMPT, IDH2), sex steroid metabolism and anti-oxidative response (AKR1C2, AKR1B10). DIO1 expression resulted in elevated intracellular concentration of T4. Expression of DIO1-affected genes strongly correlated with DIO1 transcript levels in tissue samples from renal cancer patients as well as with their poor survival. This first study addressing effects of deiodinase re-expression on proteome of cancer cells demonstrates that induced DIO1 re-expression in renal cancer robustly downregulates oncoproteins, affects key metabolic pathways, and triggers proteins involved in anti-oxidative protection. This data supports the notion that suppressed DIO1 expression and changes in local availability of thyroid hormones might favor a shift from a differentiated to a more proliferation-prone state of cancer tissues and cell lines.