The evolution of skeletal muscle performance: gene duplication and divergence of human sarcomeric alpha-actinins

Investigation of the Relationship Between ACTN3 rs1815739 Polymorphism and Openbite Cases: A Prospective Study

OBJECTIVE

The aim of this study was to examine whether ACTN3 rs1815739 polymorphism, which causes the deficiency of the alpha-actinin-3 muscle protein, is related to the formation of open bite malocclusion.

MATERIALS AND METHODS

Fifty-eight participants (18.5 ± 3.6 years old) with anterior open bite (n = 29) and normal overbite (n = 29) who presented to Marmara University, Department of Orthodontics for treatment were included in the study. Initial cephalometric radiographs were used for the diagnosis of malocclusion. The case group was divided into three subgroups according to degree of open bite. For DNA isolation, oral epithelial cells were collected with buccal swabs (Van Allen Way, Carlsbad, USA), and the real-time PCR method was used for the genotyping of all polymorphisms. The results were statistically analysed, and the threshold for statistical significance was set at p < 0.05.

RESULTS

The frequencies of RR, RX and XX genotypes of ACTN3 rs1815739 polymorphism were found as 6 (20.7%), 14 (48.3%) and 9 (31.0%) in the control group and 8 (8%), 9 (31.0%) and 12 (41.4%) in the case group, respectively. There was no statistically significant difference between the groups in terms of the presence of the examined polymorphism (p > 0.05). However, the intra-group evaluation of case group revealed a significant difference in the prevalence of XX genotype (83.3%) for the subgroup with an open bite of -5 mm or above.

CONCLUSION

Although no significant difference was observed between the case and control groups, a possible association was identified between ACTN3 rs1815739 polymorphism and an increased severity of open bite malocclusion.

Screening of potential regulatory genes in carotid atherosclerosis vascular immune microenvironment

BACKGROUND

Immune microenvironment is one of the essential characteristics of carotid atherosclerosis (CAS), which cannot be reversed by drug therapy alone. Thus, there is a pressing need to develop novel immunoregulatory strategies to delay this pathological process that drives cardiovascular-related diseases. This study aimed to detect changes in the immune microenvironment of vascular tissues at various stages of carotid atherosclerosis, as well as cluster and stratify vascular tissue samples based on the infiltration levels of immune cell subtypes to distinguish immune phenotypes and identify potential hub genes regulating the immune microenvironment of carotid atherosclerosis.

MATERIALS AND METHODS

RNA sequencing datasets for CAS vascular tissue and healthy vascular tissue (GSE43292 and GSE28829) were downloaded from the Gene Expression Omnibus (GEO) database. To begin, the immune cell subtype infiltration level of all samples in both GSE43292 and GSE28829 cohorts was assessed using the ssGSEA algorithm. Following this, consensus clustering was performed to stratify CAS samples into different clusters. Finally, hub genes were identified using the maximum neighborhood component algorithm based on the construction of interaction networks, and their diagnostic efficiency was evaluated.

RESULTS

Compared to the controls, a higher number of immune cell subtypes were enriched in CAS samples with higher immune scores in the GSE43292 cohort. Advanced CAS was characterized by high immune cell infiltration, whereas early CAS was characterized by low immune cell infiltration in the GSE28829 cohort. Moreover, CAS progression may be related to the immune response pathway. Biological processes associated with muscle cell development may impede the progression of CAS. Finally, the hub genes PTPRC, ACTN2, ACTC1, LDB3, MYOZ2, and TPM2 had satisfactory efficacy in the diagnosis and prediction of high and low immune cell infiltration in CAS and distinguishing between early and advanced CAS samples.

CONCLUSION

The enrichment of immune cells in vascular tissues is a primary factor driving pathological changes in CAS. Additionally, CAS progression may be related to the immune response pathway. Biological processes linked to muscle cell development may delay the progression of CAS. PTPRC, ACTN2, ACTC1, LDB3, MYOZ2, and TPM2 may regulate the immune microenvironment of CAS and participate in the occurrence and progression of the disease.

In-depth exploration of the high and normal pH beef proteome: First insights emphasizing the dynamic protein changes in Longissimus thoracis muscle from pasture-finished Nellore bulls over different postmortem times

This study aimed to evaluate for the first time the temporal dynamic changes in early postmortem proteome of normal and high ultimate pH (pHu) beef samples from the same cattle using a shotgun proteomics approach. Ten selected carcasses classified as normal (pHu < 5.8; n = 5) or high (pHu ≥ 6.2; n = 5) pHu beef from pasture-finished Nellore (Bos taurus indicus) bulls were sampled from Longissimus thoracis muscle at 30 min, 9 h and 44 h postmortem for proteome comparison. The temporal proteomics profiling quantified 863 proteins, from which 251 were differentially abundant (DAPs) between high and normal pHu at 30 min (n = 33), 9 h (n = 181) and 44 h (n = 37). Among the myriad interconnected pathways regulating pH decline during postmortem metabolism, this study revealed the pivotal role of energy metabolism, cellular response to stress, oxidoreductase activity and muscle system process pathways throughout the early postmortem. Twenty-three proteins overlap among postmortem times and may be suggested as candidate biomarkers to the dark-cutting condition development. The study further evidenced for the first time the central role of ribosomal proteins and histones in the first minutes after animal bleeding. Moreover, this study revealed the disparity in the mechanisms underpinning the development of dark-cutting beef condition among postmortem times, emphasizing multiple dynamic changes in the muscle proteome. Therefore, this study revealed important insights regarding the temporal dynamic changes that occur in early postmortem of high and normal muscle pHu beef, proposing specific pathways to determine the biological mechanisms behind dark-cutting determination.

Genome-wide mapping of the binding sites of myocyte enhancer factor 2A in chicken primary myoblasts

Myocyte enhancer factor 2A (MEF2A) is a transcription factor that plays a critical role in cell proliferation, differentiation and apoptosis. In contrast to the wide characterization of its regulation mechanism in mammalian skeletal muscle, its role in chickens is limited. Especially, its wide target genes remain to be identified. Therefore, we utilized Cleavage Under Targets and Tagmentation (CUT&Tag) technology to reveal the genome-wide binding profile of MEF2A in chicken primary myoblasts thus gaining insights into its potential role in muscle development. Our results revealed that MEF2A binding sites were primarily distributed in intergenic and intronic regions. Within the promoter region, although only 8.87% of MEF2A binding sites were found, these binding sites were concentrated around the transcription start site (TSS). Following peak annotation, a total of 1903 genes were identified as potential targets of MEF2A. Gene Ontology (GO) enrichment analysis further revealed that MEF2A target genes may be involved in the regulation of embryonic development in multiple organ systems, including muscle development, gland development, and visual system development. Moreover, a comparison of the MEF2A target genes identified in chicken primary myoblasts with those in mouse C2C12 cells revealed 388 target genes are conserved across species, 1515 target genes are chicken specific. Among these conserved genes, ankyrin repeat and SOCS box containing 5 (ASB5), transmembrane protein 182 (TMEM182), myomesin 2 (MYOM2), leucyl and cystinyl aminopeptidase (LNPEP), actinin alpha 2 (ACTN2), sorbin and SH3 domain containing 1 (SORBS1), ankyrin 3 (ANK3), sarcoglycan delta (SGCD), and ORAI calcium release-activated calcium modulator 1 (ORAI1) exhibited consistent expression patterns with MEF2A during embryonic muscle development. Finally, TMEM182, as an important negative regulator of muscle development, has been validated to be regulated by MEF2A by dual-luciferase and quantitative real-time PCR (qPCR) assays. In summary, our study for the first time provides a wide landscape of MEF2A target genes in chicken primary myoblasts, which supports the active role of MEF2A in chicken muscle development.

Genome-wide identification of quantitative trait loci and candidate genes for seven carcass traits in a four-way intercross porcine population

BACKGROUND

Carcass traits are essential economic traits in the commercial pig industry. However, the genetic mechanism of carcass traits is still unclear. In this study, we performed a genome-wide association study (GWAS) based on the specific-locus amplified fragment sequencing (SLAF-seq) to study seven carcass traits on 223 four-way intercross pigs, including dressing percentage (DP), number of ribs (RIB), skin thinkness (ST), carcass straight length (CSL), carcass diagonal length (CDL), loin eye width (LEW), and loin eye thickness (LET).

RESULTS

A total of 227,921 high-quality single nucleotide polymorphisms (SNPs) were detected to perform GWAS. A total of 30 SNPs were identified for seven carcass traits using the mixed linear model (MLM) (p < 1.0 × 10- 5), of which 9 SNPs were located in previously reported quantitative trait loci (QTL) regions. The phenotypic variation explained (PVE) by the significant SNPs was from 2.43 to 16.32%. Furthermore, 11 candidate genes (LYPLAL1, EPC1, MATN2, ZFAT, ZBTB10, ZNF704, INHBA, SMYD3, PAK1, SPTBN2, and ACTN3) were found for carcass traits in pigs.

CONCLUSIONS

The GWAS results will improve our understanding of the genetic basis of carcass traits. We hypothesized that the candidate genes associated with these discovered SNPs would offer a biological basis for enhancing the carcass quality of pigs in swine breeding.

Identification of crucial circRNAs in skeletal muscle during chicken embryonic development

Background

Chicken provides humans with a large amount of animal protein every year, in which skeletal muscle plays a leading role. The embryonic skeletal muscle development determines the number of muscle fibers and will affect the muscle production of chickens. CircRNAs are involved in a variety of important biological processes, including muscle development. However, studies on circRNAs in the chicken embryo muscle development are still lacking.

Results

In the study, we collected chicken leg muscles at 14 and 20-day embryo ages both in the fast- and slow-growing groups for RNA-seq. We identified 245 and 440 differentially expressed (DE) circRNAs in the comparison group F14vsF20 and S14vsS20 respectively. GO enrichment analysis for the host genes of DE circRNAs showed that biological process (BP) terms in the top 20 related to growth in F14vsF20 were found such as positive regulation of transcription involved in G1/S phase of mitotic cell cycle, multicellular organismal macromolecule metabolic process, and multicellular organismal metabolic process. In group S14vsS20, we also found some BP terms associated with growth in the top 20 including actomyosin structure organization, actin cytoskeleton organization and myofibril assembly. A total of 7 significantly enriched pathways were obtained, containing Adherens junction and Tight junction. Further analysis of those pathways found three crucial host genes MYH9, YBX3, IGF1R in both fast- and slow-growing groups, three important host genes CTNNA3, AFDN and CREBBP only in the fast-growing group, and six host genes FGFR2, ACTN2, COL1A2, CDC42, DOCK1 and MYL3 only in the slow-growing group. In addition, circRNA-miRNA network also revealed some key regulation pairs such as novel_circ_0007646-miR-1625-5p, novel_circ_0007646-miR-1680-5p, novel_circ_0008913-miR-148b-5p, novel_circ_0008906-miR-148b-5p and novel_circ_0001640-miR-1759-3p.

Conclusions

Comprehensive analysis of circRNAs and their targets would contribute to a better understanding of the molecular mechanisms in poultry skeletal muscle and it also plays an important guiding role in the next research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08588-4.

Skeletal Muscle Expression of Actinin-3 (ACTN3) in Relation to Feed Efficiency Phenotype of F2Bos indicus - Bos taurus Steers

In this study, actinin-3 (ACTN3) gene expression was investigated in relation to the feed efficiency phenotype in Bos indicus - Bos taurus crossbred steers. A measure of relative feed efficiency based on residual feed intake relative to predictions from the NRC beef cattle model was analyzed by the use of a mixed linear model that included sire and family nested within sire as fixed effects and age, animal type, sex, condition, and breed as random effects for 173 F₂ Nellore-Angus steers. Based on these residual intake observations, individuals were ranked from most efficient to least efficient. Skeletal muscle samples were analyzed from 54 steers in three groups of 18 (high efficiency, low efficiency, and a statistically average group). ACTN3, which encodes a muscle-specific structural protein, was previously identified as a candidate gene from a microarray analysis of RNA extracted from muscle samples obtained from a subset of steers from each of these three efficiency groups. The expression of ACTN3 was evaluated by quantitative reverse transcriptase PCR analysis. The expression of ACTN3 in skeletal muscle was 1.6-fold greater in the inefficient steer group than in the efficient group (p = 0.007). In addition to expression measurements, blocks of SNP haplotypes were assessed for breed or parent of origin effects. A maternal effect was observed for ACTN3 inheritance, indicating that a maternal B. indicus block conferred improved residual feed efficiency relative to the B. taurus copy (p = 0.03). A SNP haplotype analysis was also conducted for m-calpain (CAPN2) and fibronectin 1 (FN1), and a significant breed effect was observed for both genes, with B. indicus and B. taurus alleles each conferring favorable efficiency when inherited maternally (p = 0.03 and p = 0.04). Because the ACTN3 structural protein is specific to fast-twitch (type II) muscle fibers and not present in slow-twitch muscle fibers (type I), muscle samples used for expression analysis were also assayed for fiber type ratio (type II/type I). Inefficient animals had a fast fiber type ratio 1.8-fold greater than the efficient animals (p = 0.027). Because these fiber-types exhibit different metabolic profiles, we hypothesize that animals with a greater proportion of fast-twitch muscle fibers are also less feed efficient.

The Extent and Rate of the Appearance of the Major 110 and 30 kDa Proteolytic Fragments during Post-Mortem Aging of Beef Depend on the Glycolysing Rate of the Muscle and Aging Time: An LC-MS/MS Approach to Decipher Their Proteome and Associated Pathways

Post-mortem (p-m) muscle undergoes a myriad of complex physical and biochemical changes prior to its conversion to meat, which are influential on proteolysis and hence tenderization. A more in-depth understanding of the mechanisms underpinning these dynamics is a key to consistently providing tender beef. Using an LC-MS/MS approach, with state-of-art mass spectrometry Q Exactive HF-X, the proteome and associated pathways contributing to the appearance of the proteolytic breakdown products appearing over 14 days p-m, at two important molecular weights (110 and 30 kDa) on 1D SDS-PAGE gels, have been investigated in beef longissimus thoracis et lumborum muscles exhibiting four rates of pH decline differentiated on the basis of time at pH 6 (fast glycolysing, <3 h; medium, 3-5 h; slow, 5-8 h; and very slow, 8+ h). Both 110 and 30 kDa bands appeared during aging and increased in intensity as a function of p-m time in a pH decline-dependent manner. The 110 kDa band appeared as early as 3 h p-m and displayed an incremental increase in all groups through to 14 days p-m. From 2 days p-m, this increase in abundance during aging was significantly (P < 0.001) influenced by the glycolytic rate: fast > or = medium > slow > very slow. The day 2 p-m appearance of the 30 kDa band was most evident for the fast glycolysing muscle with little or no evidence of appearance in slow and very slow. For days 7 and 14 p-m, the strength of appearance was dependent on glycolysing groups fast > medium > or = slow > very slow. LC-MS/MS analysis yielded a total of 22 unique proteins for the 110 kDa fragment and 13 for the 30 kDa, with 4 common proteins related to both the actin and fibrinogen complex. The Gene Ontology analysis revealed that a myriad of biological pathways are influential with many related to proteins involved primarily in muscle contraction and structure. Other pathways of interest include energy metabolism, apoptotic mitochondrial changes, calcium and ion transport, and so on. Interestingly, most of the proteins composing the fragments were so far identified as biomarkers of beef tenderness and other quality traits.

Genetics of Muscle Stiffness, Muscle Elasticity and Explosive Strength

Muscle stiffness, muscle elasticity and explosive strength are the main components of athletes' performance and they show a sex-based as well as ethnicity variation. Muscle stiffness is thought to be one of the risk factors associated with sports injuries and is less common in females than in males. These observations may be explained by circulating levels of sex hormones and their specific receptors. It has been shown that higher levels of estrogen are associated with lower muscle stiffness responsible for suppression of collagen synthesis. It is thought that these properties, at least in part, depend on genetic factors. Particularly, the gene encoding estrogen receptor 1 (ESR1) is one of the candidates that may be associated with muscle stiffness. Muscle elasticity increases with aging and there is evidence suggesting that titin (encoded by the TTN gene), a protein that is expressed in cardiac and skeletal muscles, is one of the factors responsible for elastic properties of the muscles. Mutations in the TTN gene result in some types of muscular dystrophy or cardiomyopathy. In this context, TTN may be regarded as a promising candidate for studying the elastic properties of muscles in athletes. The physiological background of explosive strength depends not only on the muscle architecture and muscle fiber composition, but also on the central nervous system and functionality of neuromuscular units. These properties are, at least partly, genetically determined. In this context, the ACTN3 gene code for α-actinin 3 has been widely researched.

Effect of ACTN3 Polymorphism on Self-reported Running Times

Kreutzer, A, Martinez, CA, Kreutzer, M, Stone, JD, Mitchell, JB, and Oliver, JM. Effect of ACTN3 polymorphism on self-reported running times. J Strength Cond Res 33(1): 80-88, 2019-This investigation examined the effect of ACTN3 genotype on self-reported distance running personal records (PRs). Of 94 (n = 94) recreationally active men and women, 82 (f = 42, m = 40; age: 22.6 ± 4.5 years; body mass index [BMI]: 23.5 ± 3.4 kg·m) reported 1-mile running PRs, whereas 57 (f = 33, m = 24; age: 23.4 ± 5.3 years; BMI: 22.9 ± 9.3 kg·m) reported 5K running PRs. Subjects were grouped by the presence (ACTN3) or absence (ACTN3) of α-actinin-3, as well as by individual genotype (RR, RX, and XX). Among female participants, ACTN3 reported 64.5 seconds faster (p = 0.048) 1-mile PRs compared with their ACTN3 counterparts. No differences were observed when comparing 5K PRs between genotypes. Two one-sided test equivalence testing revealed that none of the effects observed when comparing ACTN3 and ACTN3 were equivalent to zero. Our study confirms a reportedly greater prevalence of XX benefits for endurance performance in females when compared with males but fails to strongly link ACTN3 genotype to endurance performance. Practitioners should continue to be cautious when using genetic information for talent identification and sport selection.

The Effect of ACTN3 Gene Doping on Skeletal Muscle Performance

Loss of expression of ACTN3, due to homozygosity of the common null polymorphism (p.Arg577X), is underrepresented in elite sprint/power athletes and has been associated with reduced muscle mass and strength in humans and mice. To investigate ACTN3 gene dosage in performance and whether expression could enhance muscle force, we performed meta-analysis and expression studies. Our general meta-analysis using a Bayesian random effects model in elite sprint/power athlete cohorts demonstrated a consistent homozygous-group effect across studies (per allele OR = 1.4, 95% CI 1.3-1.6) but substantial heterogeneity in heterozygotes. In mouse muscle, rAAV-mediated gene transfer overexpressed and rescued α-actinin-3 expression. Contrary to expectation, in vivo "doping" of ACTN3 at low to moderate doses demonstrated an absence of any change in function. At high doses, ACTN3 is toxic and detrimental to force generation, to demonstrate gene doping with supposedly performance-enhancing isoforms of sarcomeric proteins can be detrimental for muscle function. Restoration of α-actinin-3 did not enhance muscle mass but highlighted the primary role of α-actinin-3 in modulating muscle metabolism with altered fatiguability. This is the first study to express a Z-disk protein in healthy skeletal muscle and measure the in vivo effect. The sensitive balance of the sarcomeric proteins and muscle function has relevant implications in areas of gene doping in performance and therapy for neuromuscular disease.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Analysis of the ACTN3 heterozygous genotype suggests that α-actinin-3 controls sarcomeric composition and muscle function in a dose-dependent fashion

A common null polymorphism (R577X) in ACTN3 causes α-actinin-3 deficiency in ∼ 18% of the global population. There is no associated disease phenotype, but α-actinin-3 deficiency is detrimental to sprint and power performance in both elite athletes and the general population. However, despite considerable investigation to date, the functional consequences of heterozygosity for ACTN3 are unclear. A subset of studies have shown an intermediate phenotype in 577RX individuals, suggesting dose-dependency of α-actinin-3, while others have shown no difference between 577RR and RX genotypes. Here, we investigate the effects of α-actinin-3 expression level by comparing the muscle phenotypes of Actn3(+/-) (HET) mice to Actn3(+/+) [wild-type (WT)] and Actn3(-/-) [knockout (KO)] littermates. We show reduction in α-actinin-3 mRNA and protein in HET muscle compared with WT, which is associated with dose-dependent up-regulation of α-actinin-2, z-band alternatively spliced PDZ-motif and myotilin at the Z-line, and an incremental shift towards oxidative metabolism. While there is no difference in force generation, HET mice have an intermediate endurance capacity compared with WT and KO. The R577X polymorphism is associated with changes in ACTN3 expression consistent with an additive model in the human genotype-tissue expression cohort, but does not influence any other muscle transcripts, including ACTN2. Overall, ACTN3 influences sarcomeric composition in a dose-dependent fashion in mouse skeletal muscle, which translates directly to function. Variance in fibre type between biopsies likely masks this phenomenon in human skeletal muscle, but we suggest that an additive model is the most appropriate for use in testing ACTN3 genotype associations.

How does α-actinin-3 deficiency alter muscle function? Mechanistic insights into ACTN3, the 'gene for speed'

An estimated 1.5 billion people worldwide are deficient in the skeletal muscle protein α-actinin-3 due to homozygosity for the common ACTN3 R577X polymorphism. α-Actinin-3 deficiency influences muscle performance in elite athletes and the general population. The sarcomeric α-actinins were originally characterised as scaffold proteins at the muscle Z-line. Through studying the Actn3 knockout mouse and α-actinin-3 deficient humans, significant progress has been made in understanding how ACTN3 genotype alters muscle function, leading to an appreciation of the diverse roles that α-actinins play in muscle. The α-actinins interact with a number of partner proteins, which broadly fall into three biological pathways-structural, metabolic and signalling. Differences in functioning of these pathways have been identified in α-actinin-3 deficient muscle that together contributes to altered muscle performance in mice and humans. Here we discuss new insights that have been made in understanding the molecular mechanisms that underlie the consequences of α-actinin-3 deficiency.

A genome-wide association study on amyotrophic lateral sclerosis in the Taiwanese Han population

Identification of mutations in patients with amyotrophic lateral sclerosis (ALS) in a genome-wide association study can reveal possible biomarkers of such a rapidly progressive and fatal neurodegenerative disease. It was observed that significant single nucleotide polymorphisms vary when the tested population changes from one ethnic group to another. To identify new loci associated with ALS susceptibility in the Taiwanese Han population, we performed a genome-wide association study on 94 patients with sporadic ALS and 376 matched controls. We uncovered two new susceptibility loci at 13q14.3 (rs2785946) and 11q25 (rs11224052). In addition, we analyzed the functions of all the associated genes among 54 significant single nucleotide polymorphisms using Gene Ontology annotations, and the results showed several statistically significant neural- and muscle-related Gene Ontology terms and the associated diseases.

The actinin family of actin cross-linking proteins - a genetic perspective

Actinins are one of the major actin cross-linking proteins found in virtually all cell types and are the ancestral proteins of a larger family that includes spectrin, dystrophin and utrophin. Invertebrates have a single actinin-encoding ACTN gene, while mammals have four. Mutations in all four human genes have now been linked to heritable diseases or traits. ACTN1 mutations cause macrothrombocytopenia, a platelet disorder characterized by excessive bleeding. ACTN2 mutations have been linked to a range of cardiomyopathies, and ACTN4 mutations cause a kidney condition called focal segmental glomerulosclerosis. Intriguingly, approximately 16 % of people worldwide are homozygous for a nonsense mutation in ACTN3 that abolishes actinin-3 protein expression. This ACTN3 null allele has undergone recent positive selection in specific human populations, which may be linked to improved endurance and adaptation to colder climates. In this review we discuss the human genetics of the ACTN gene family, as well as ACTN gene knockout studies in several model organisms. Observations from both of these areas provide insights into the evolution and cellular functions of actinins.

The heritable path of human physical performance: from single polymorphisms to the "next generation"

Human physical performance is a complex multifactorial trait. Historically, environmental factors (e.g., diet, training) alone have been unable to explain the basis of all prominent phenotypes for physical performance. Therefore, there has been an interest in the study of the contribution of genetic factors to the development of these phenotypes. Support for a genetic component is found with studies that shown that monozygotic twins were more similar than were dizygotic twins for many physiological traits. The evolution of molecular techniques and the ability to scan the entire human genome enabled association of several genetic polymorphisms with performance. However, some biases related to the selection of cohorts and inadequate definition of the study variables have complicated the already difficult task of studying such a large and polymorphic genome, often resulting in inconsistent results about the influence of candidate genes. This review aims to provide a critical overview of heritable genetic aspects. Novel molecular technologies, such as next-generation sequencing, are discussed and how they can contribute to improving understanding of the molecular basis for athletic performance. It is important to ensure that the large amount of data that can be generated using these tools will be used effectively by ensuring well-designed studies.

α-Actinin involvement in Z-disk assembly during skeletal muscle C2C12 cells in vitro differentiation

α-Actinin is involved in the assembly and maintenance of muscle fibers. α-Actinin is required to cross-link actin filaments and to connect the actin cytoskeleton to the cell membrane and it is necessary for the attachment of actin filaments to Z-disks in skeletal muscle fibers and to dense bodies in smooth muscle ones. In addition to its mechanical role, sarcomeric α-actinin interacts with proteins involved in a variety of signaling and metabolic pathways. The aim of this work is to monitor Z-disk formation, in order to clear up the role of sarcomeric α-actinin in undifferentiated stage, after 4 days of differentiation (intermediate differentiation stage) and after 7 days of differentiation (fully differentiated stage). For this purpose, C2C12 murine skeletal muscle cells, grown in vitro, were analyzed at three time points of differentiation. Confocal laser scanner microscopy and transmission electron microscopy have been utilized for α-actinin immunolocalization. Both techniques reveal that in undifferentiated cells labeling appears uniformly distributed in the cytoplasm with punctate α-actinin Z-bodies. Moreover, we found that when differentiation is induced, α-actinin links at first membrane-associated proteins, then it aligns longitudinally across the cytoplasm and finally binds actin, giving rise to Z-disks. These findings evidence α-actinin involvement in sarcomeric development, suggesting for this protein an important role in stabilizing the muscle contractile apparatus.

Differential skeletal muscle proteome of high- and low-active mice

Physical inactivity contributes to cardiovascular disease, type II diabetes, obesity, and some types of cancer. While the literature is clear that there is genetic regulation of physical activity with existing gene knockout data suggesting that skeletal muscle mechanisms contribute to the regulation of activity, actual differences in end-protein expression between high- and low-active mice have not been investigated. This study used two-dimensional differential gel electrophoresis coupled with mass spectrometry to evaluate the proteomic differences between high-active (C57L/J) and low-active (C3H/HeJ) mice in the soleus and extensor digitorum longus (EDL). Furthermore, vivo-morpholinos were used to transiently knockdown candidate proteins to confirm their involvement in physical activity regulation. Proteins with higher expression patterns generally fell into the calcium-regulating and Krebs (TCA) cycle pathways in the high-active mice (e.g., annexin A6, P = 0.0031; calsequestrin 1; P = 0.000025), while the overexpressed proteins in the low-active mice generally fell into cytoskeletal structure- and electron transport chain-related pathways (e.g., ATPase, P = 0.031; NADH dehydrogenase, P = 0.027). Transient knockdown of annexin A6 and calsequestrin 1 protein of high-active mice with vivo-morpholinos resulted in decreased physical activity levels (P = 0.001). These data suggest that high- and low-active mice have unique protein expression patterns and that each pattern contributes to the peripheral capability to be either high- or low-active, suggesting that different specific mechanisms regulate activity leading to the high- or low-activity status of the animal.

ACE I/D and ACTN3 R/X polymorphisms as potential factors in modulating exercise-related phenotypes in older women in response to a muscle power training stimuli

Genetic variation of the human ACE I/D and ACTN3 R577X polymorphisms subsequent to 12 weeks of high-speed power training on maximal strength (1RM) of the arm and leg muscles, muscle power performance (counter-movement jump), and functional capacity (sit-to-stand test) was examined in older Caucasian women [n = 139; mean age 65.5 (8.2) years; 67.0 (10.0) kg and 1.57 (0.06) m]. Chelex 100 was used for DNA extraction, and genotype was determined by PCR-RFLP methods. Muscular strength, power, and functional testing were conducted at baseline (T1) and after 12 weeks (T2) of high-speed power training. At baseline, the ACE I/D and ACTN3 R/X polymorphisms were not associated with muscle function or muscularity phenotypes in older Caucasian women. After the 12-week high-speed training program, subjects significantly increased their muscular and functional capacity performance (p < 0.05). For both polymorphisms, significant genotype-training interaction (p < 0.05) was found in all muscular performance indices, except for 1RM leg extension in the ACE I/D (p = 0.187). Analyses of the combined effects between genotypes showed significant differences in all parameters (p < 0.05) in response to high-speed power training between the power (ACTN3 RR + RX & ACE DD) versus "non-power" muscularity-oriented genotypes (ACTN3 XX & ACE II + ID)]. Our data suggest that the ACE and ACTN3 genotypes (single or combined) exert a significant influence in the muscle phenotypes of older Caucasian women in response to high-speed power training. Thus, the ACE I/D and ACTN3 R/X polymorphisms are likely factors in modulating exercise-related phenotypes in older women, particularly in response to a resistance training stimuli.

ACTN3 genotype influences muscle performance through the regulation of calcineurin signaling

α-Actinin-3 deficiency occurs in approximately 16% of the global population due to homozygosity for a common nonsense polymorphism in the ACTN3 gene. Loss of α-actinin-3 is associated with reduced power and enhanced endurance capacity in elite athletes and nonathletes due to "slowing" of the metabolic and physiological properties of fast fibers. Here, we have shown that α-actinin-3 deficiency results in increased calcineurin activity in mouse and human skeletal muscle and enhanced adaptive response to endurance training. α-Actinin-2, which is differentially expressed in α-actinin-3-deficient muscle, has higher binding affinity for calsarcin-2, a key inhibitor of calcineurin activation. We have further demonstrated that α-actinin-2 competes with calcineurin for binding to calsarcin-2, resulting in enhanced calcineurin signaling and reprogramming of the metabolic phenotype of fast muscle fibers. Our data provide a mechanistic explanation for the effects of the ACTN3 genotype on skeletal muscle performance in elite athletes and on adaptation to changing physical demands in the general population. In addition, we have demonstrated that the sarcomeric α-actinins play a role in the regulation of calcineurin signaling.

Genetic polymorphisms associated with exertional rhabdomyolysis

Exertional rhabdomyolysis (ER) occurs in young, otherwise healthy, individuals principally during strenuous exercise, athletic, and military training. Although many risk factors have been offered, it is unclear why some individuals develop ER when participating in comparable levels of physical exertion under identical environmental conditions and others do not. This study investigated possible genetic polymorphisms that might help explain ER. DNA samples derived from a laboratory-based study of persons who had never experienced an episode of ER (controls) and clinical ER cases referred for testing over the past several years were analyzed for single nucleotide polymorphisms (SNPs) in candidate genes. These included angiotensin I converting enzyme (ACE), α-actinin-3 (ACTN3), creatine kinase muscle isoform (CKMM), heat shock protein A1B (HSPA1B), interleukin 6 (IL6), myosin light chain kinase (MYLK), adenosine monophosphate deaminase 1 (AMPD1), and sickle cell trait (HbS). Population included 134 controls and 47 ER cases. The majority of ER cases were men (n = 42/47, 89.4 %); the five women with ER were Caucasian. Eighteen African Americans (56.3 %) were ER cases. Three SNPs were associated with ER: CKMM Ncol, ACTN3 R577X, and MYLK C37885A. ER cases were 3.1 times more likely to have the GG genotype of CKMM (odds ratio/OR = 3.1, confidence interval/CI 1.33-7.10), 3.0 times for the XX genotype of ACTN3 SNP (OR = 2.97, CI 1.30-3.37), and 5.7 times for an A allele of MYLK (OR = 21.35, CI 2.60-12.30). All persons with HbS were also ER cases. Three distinct polymorphisms were associated with ER. Further work will be required to replicate these findings and determine the mechanism(s) whereby these variants might confer susceptibility.

ACTN3 allele frequency in humans covaries with global latitudinal gradient

A premature stop codon in ACTN3 resulting in α-actinin-3 deficiency (the ACTN3 577XX genotype) is common in humans and reduces strength, muscle mass, and fast-twitch fiber diameter, but increases the metabolic efficiency of skeletal muscle. Linkage disequilibrium data suggest that the ACTN3 R577X allele has undergone positive selection during human evolution. The allele has been hypothesized to be adaptive in environments with scarce resources where efficient muscle metabolism would be selected. Here we test this hypothesis by using recently developed comparative methods that account for evolutionary relatedness and gene flow among populations. We find evidence that the ACTN3 577XX genotype evolved in association with the global latitudinal gradient. Our results suggest that environmental variables related to latitudinal variation, such as species richness and mean annual temperature, may have influenced the adaptive evolution of ACTN3 577XX during recent human history.

Human alpha-actinin-3 genotype association with exercise-induced muscle damage and the repeated-bout effect

Alpha-actinin-3 (ACTN3) is an integral part of the Z line of the sarcomere. The ACTN3 R577X (rs1815739) polymorphism determines the presence or absence of functional ACTN3, which may influence the extent of exercise-induced muscle damage. This study aimed to compare the impact of, and recovery from, muscle-damaging eccentric exercise on subjects with or without functional ACTN3. Seventeen young men (20-33 years old), homozygous for the R (n = 9) or X (n = 8) alleles, performed two bouts of stretch-shortening exercise (50 drop jumps) two weeks apart. Muscle soreness, plasma creatine kinase (CK) activity, jump height, maximal voluntary isometric torque (MVC), peak concentric isokinetic torque (IT), and electrically stimulated knee extension torques at 20 and 100 Hz were measured at baseline and at a number of time points up to 14 days after each bout. There were no significant baseline differences between the groups. However, significant time point × genotype interactions were observed for MVC (p = 0.021) and IT (p = 0.011) for the immediate effect of eccentric exercise in bout 1. The RR group showed greater voluntary force decrements (RR vs. XX: MVC, -33.3% vs. -24.5%; IT, -35.9% vs. -23.2%) and slower recovery. A repeated-bout effect was clearly observed, but there were no differences by genotype group. The ACTN3 genotype modulates the response of muscle function to plyometric jumping exercise, although the differences are modest. The ACTN3 genotype does not influence the clearly observed repeated-bout effect; however, XX homozygotes recover baseline voluntary torque values faster and thus may be able to undertake more frequent training sessions.

The ACTN3 R577X polymorphism is associated with inflammatory myopathies in a Mexican population

BACKGROUND

The ACTN3 gene encodes the fast muscle protein α-actinin-3. The ACTN3 R577X polymorphism is a premature stop codon and results in absence of α-actinin-3 in 577XX homozygotes. The aim of this study was to determine the ACTN3 genotype in idiopathic inflammatory myopathies (IIMs).

METHODS

We performed ACTN3 genotyping on 27 patients with dermatomyositis (DM), 10 with polymyositis (PM), and 85 healthy subjects. Muscle enzyme levels of creatine phosphokinase (CPK), lactic dehydrogenase (LDH), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) were recorded at the time of diagnosis and recruitment. Genotyping was performed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and the allele frequency was analysed.

RESULTS

A total of 36% of healthy subjects had the ACTN3 577XX polymorphism (α-actinin-3 deficiency), 18% had the 577RR (homozygous wild type) genotype, and 46% 577RX (heterozygous). In DM/PM, 70% had the ACTN3 577XX polymorphism, 6% RR, and 24% RX [odds ratio (OR) 4.12, 95% confidence interval (CI) 1.67-10.33, p < 0.001]. In healthy subjects, the R allele was present in 41% and the X allele in 59% compared to 18% and 82%, respectively, in the IIM group (OR 3.21, 95% CI 1.57-6.66, p < 0.001). Thus, the ACTN3 577X allele seemed to increase the risk of developing IIM, and DM in particular, although this was not related to severity of expression of the phenotype.

CONCLUSIONS

The ACTN3 577X allele appeared to increase the risk of developing IIM; 70% of IIM patients were deficient in α-actinin-3. By contrast, ACTN3 577XX patients seemed to have less severe disease as reflected in lower muscle enzyme levels.

Fiber-type specific expression of α-actinin isoforms in rat skeletal muscle

α-Actinins are actin-binding proteins, and two isoforms (α-actinin-2 and -3) are major structural components of the sarcomeric Z line in mammalian skeletal muscle. Based on human and knockout mice studies, α-actinin-3 is thought to be associated with muscle force output and high contraction velocities. However, fiber-type specific expression of α-actinin isoforms is not well understood and may vary among species. In this study, we investigated the expression of α-actinin isoforms and the difference between fiber types in rat skeletal muscle and compared it with those of humans and mice from previous reports. Soleus and plantaris muscles were analyzed immunohistochemically to identify muscle fiber types and α-actinin protein expression. α-Actinin-2 was stained in all muscle fibers in both the soleus and plantaris muscles; i.e., all α-actinin-3 co-expressed with α-actinin-2 in rat skeletal muscles. The proportions of α-actinin-3 expression, regardless of fiber type, were significantly higher in the plantaris (75.8 ± 0.6%) than the soleus (8.0 ± 1.7%). No α-actinin-3 expression was observed in type I fibers, whereas all type IIx+b fibers expressed α-actinin-3. α-Actinin-3 was also expressed in type IIa fibers; however, approximately 75% of type IIa fibers were not stained by α-actinin-3, and the proportion varied between muscles. The proportion of α-actinin-3 expression in type IIa fibers was significantly higher in the soleus muscle than the plantaris muscle. Our results showed that fiber-type specific expression of α-actinin isoforms in rats is more similar to that in humans compared to that of the mouse, whereas the proportion of α-actinin-3 protein varied between muscles.

Deficiency of α-actinin-3 is associated with increased susceptibility to contraction-induced damage and skeletal muscle remodeling

Sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-disk in skeletal muscle, where they crosslink actin and other structural proteins to maintain an ordered myofibrillar array. Homozygosity for the common null polymorphism (R577X) in ACTN3 results in the absence of fast fiber-specific α-actinin-3 in ∼20% of the general population. α-Actinin-3 deficiency is associated with decreased force generation and is detrimental to sprint and power performance in elite athletes, suggesting that α-actinin-3 is necessary for optimal forceful repetitive muscle contractions. Since Z-disks are the structures most vulnerable to eccentric damage, we sought to examine the effects of α-actinin-3 deficiency on sarcomeric integrity. Actn3 knockout mouse muscle showed significantly increased force deficits following eccentric contraction at 30% stretch, suggesting that α-actinin-3 deficiency results in an increased susceptibility to muscle damage at the extremes of muscle performance. Microarray analyses demonstrated an increase in muscle remodeling genes, which we confirmed at the protein level. The loss of α-actinin-3 and up-regulation of α-actinin-2 resulted in no significant changes to the total pool of sarcomeric α-actinins, suggesting that alterations in fast fiber Z-disk properties may be related to differences in functional protein interactions between α-actinin-2 and α-actinin-3. In support of this, we demonstrated that the Z-disk proteins, ZASP, titin and vinculin preferentially bind to α-actinin-2. Thus, the loss of α-actinin-3 changes the overall protein composition of fast fiber Z-disks and alters their elastic properties, providing a mechanistic explanation for the loss of force generation and increased susceptibility to eccentric damage in α-actinin-3-deficient individuals.

A gene for speed: the emerging role of alpha-actinin-3 in muscle metabolism

A common polymorphism (R577X) in the ACTN3 gene results in complete deficiency of alpha-actinin-3 protein in approximately 16% of humans worldwide. The presence of alpha-actinin-3 protein is associated with improved sprint/power performance in athletes and the general population. Despite this, there is evidence that the null genotype XX has been acted on by recent positive selection, likely due to its emerging role in the regulation of muscle metabolism. alpha-Actinin-3 deficiency reduces the activity of glycogen phosphorylase and results in a fundamental shift toward more oxidative pathways of energy utilization.

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.