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

The Genetic Basis of Non-Contact Soft Tissue Injuries-Are There Practical Applications of Genetic Knowledge?

Physical activity increases the risk of non-contact injuries, mainly affecting muscles, tendons, and ligaments. Genetic factors are recognized as contributing to susceptibility to different types of soft tissue injuries, making this broad condition a complicated multifactorial entity. Understanding genetic predisposition seems to offer the potential for personalized injury prevention and improved recovery strategies. The candidate gene analysis approach used so far, has often yielded inconclusive results. This manuscript reviews the most commonly studied genetic variants in genes involved in the musculoskeletal system's structure and recovery processes (ACTN3, ACE, CKM, MLCK, AMPD1, IGF2, IL6, TNFα, CCL2, COL1A1, COL5A1, MMP3, and TNC). Referring to the literature, it was highlighted that single-gene analyses provide limited insight. On the other hand, novel genetic testing methods identify numerous variants of uncertain physiological relevance. Distinguishing between functionally important variants, modifying variants, and the thousands of irrelevant variants requires advanced bioinformatics methods and basic multiomics research to identify the key biological pathways contributing to injury susceptibility. Tools like the Total Genotype Score (TGS) and Polygenic Risk Score (PRS) offer a more holistic view by assessing the combined effect of multiple variants. However, these methods, while useful in research, lack clinical applicability. In conclusion, it is too early to determine the clinical implications of genetic variability as a tool for improving well-established training and injury prevention methods, as the predictive power of genetic testing for injury predisposition is currently low.

Association between Complex ACTN3 and ACE Gene Polymorphisms and Elite Endurance Sports in Koreans: A Case-Control Study

ACTN3 R577X and ACE I/D polymorphisms are associated with endurance exercise ability. This case-control study explored the association of ACTN3 and ACE gene polymorphisms with elite pure endurance in Korean athletes, hypothesizing that individuals with both ACTN3 XX and ACE II genotypes would exhibit superior endurance. We recruited 934 elite athletes (713 males, 221 females) and selected 45 pure endurance athletes (36 males, 9 females) requiring "≥90% aerobic energy metabolism during sports events", in addition to 679 healthy non-athlete Koreans (361 males, 318 females) as controls. Genomic DNA was extracted and genotyped for ACTN3 R577X and ACE I/D polymorphisms. ACE ID (p = 0.090) and ACTN3 RX+XX (p = 0.029) genotype distributions were significantly different between the two groups. Complex ACTN3-ACE genotypes also exhibited significant differences (p = 0.014), with dominant complex genotypes positively affecting endurance (p = 0.039). The presence of RX+II or XX+II was associated with a 1.763-fold higher likelihood of possessing a superior endurance capacity than that seen in healthy controls (90% CI = 1.037-3.089). Our findings propose an association of combined ACTN3 RX+XX and ACE II genotypes with enhanced endurance performance in elite Korean athletes. While causality remains to be confirmed, our study highlights the potential of ACTN3-ACE polymorphisms in predicting elite endurance.

ACTN3 XX Genotype Negatively Affects Running Performance and Increases Muscle Injury Incidence in LaLiga Football Players

The aim of this study was to investigate the association of the ACTN3 rs1815739 polymorphism with match running performance and injury incidence in top-level professional football players. A total of 315 top-level professional football players from the first division of Spanish football (i.e., LaLiga) participated in this prospective and descriptive study. The ACTN3 rs1815739 genotype was identified for each player using genomic DNA samples. During LaLiga 2021-2022, players' performance was obtained through a validated camera system in all official matches. Additionally, the incidence of non-contact injuries was obtained by each team's medical staff according to the International Olympic Committee (IOC) statement. From the study sample, 116 (36.8%) players had the RR genotype, 156 (49.5%) had the RX genotype, and 43 (13.7%) had the XX genotype. The anthropometric characteristics of the players were similar across genotypes. However, the total running distance (p = 0.046), the distance at 21.0-23.9 km/h (p = 0.042), and the number of sprints (p = 0.042) were associated with the ACTN3 genotype. In all these variables, XX players had lower match performance values than RR players. Additionally, total and match injury incidences were higher in XX players than in RR players (p = 0.026 and 0.009, respectively). The rate of muscle injuries was also higher in XX players (p = 0.016). LaLiga football players with the ACTN3 XX genotype had lower match running performance and a higher incidence of non-contact injuries over the season.

Association Between Total Genotype Score and Muscle Injuries in Top-Level Football Players: a Pilot Study

BACKGROUND

Recently, genetic predisposition to injury has become a popular area of research and the association between a few single nucleotide polymorphisms (SNPs) and the susceptibility to develop musculoskeletal injuries has been shown. This pilot study aimed to investigate the combined effect of common gene polymorphisms previously associated with muscle injuries in Italian soccer players.

RESULTS

A total of 64 Italian male top football players (age 23.1 ± 5.5 years; stature 180.2 ± 7.4 cm; weight 73.0 ± 7.9 kg) were genotyped for four gene polymorphisms [ACE I/D (rs4341), ACTN3 c.1729C > T (rs1815739), COL5A1 C > T (rs2722) and MCT1 c.1470A > T (rs1049434)]. Muscle injuries were gathered for 10 years (2009-2019). Buccal swabs were used to obtain genomic DNA, and the PCR method was used to genotype the samples. The combined influence of the four polymorphisms studied was calculated using a total genotype score (TGS: from 0 to 100 arbitrary units; a.u.). A genotype score (GS) of 2 was assigned to the "protective" genotype for injuries, a GS of 1 was assigned to the heterozygous genotype while a GS of 0 was assigned to the "worst" genotype. The distribution of genotype frequencies in the ACE I/D (rs4341), ACTN3 c.1729C > T (rs1815739) and MCT1 c.1470A > T (rs1049434) polymorphisms was different between non-injured and injured football players (p = 0.001; p = 0.016 and p = 0.005, respectively). The incidence of muscle injuries was significantly different among the ACE I/D (rs4341), ACTN3 c.1729C > T (rs1815739) and COL5A1 C > T (rs2722) genotype groups, showing a lower incidence of injuries in the "protective" genotype than "worse" genotype (ACE, p < 0.001; ACTN3, p = 0.005) or intermediate genotype (COL5A1, p = 0.029). The mean TGS in non-injured football players (63.7 ± 13.0 a.u.) was different from that of injured football players (42.5 ± 12.5 a.u., p < 0.001). There was a TGS cut-off point (56.2 a.u.) to discriminate non-injured from injured football players. Players with a TGS beyond this cut-off had an odds ratio of 3.5 (95%CI 1.8-6.8; p < 0.001) to suffer an injury when compared with players with lower TGS.

CONCLUSIONS

These preliminary data suggest that carrying a high number of "protective" gene variants could influence an individual's susceptibility to developing muscle injuries in football. Adapting the training load parameters to the athletes' genetic profile represents today the new frontier of the methodology of training.

Detyrosinated microtubule arrays drive myofibrillar malformations in mdx muscle fibers

Altered myofibrillar structure is a consequence of dystrophic pathology that impairs skeletal muscle contractile function and increases susceptibility to contraction injury. In murine Duchenne muscular dystrophy (mdx), myofibrillar alterations are abundant in advanced pathology (>4 months), an age where we formerly established densified microtubule (MT) arrays enriched in detyrosinated (deTyr) tubulin as negative disease modifiers impacting cell mechanics and mechanotransduction. Given the essential role of deTyr-enriched MT arrays in myofibrillar growth, maintenance, and repair, we examined the increased abundance of these arrays as a potential mechanism for these myofibrillar alterations. Here we find an increase in deTyr-tubulin as an early event in dystrophic pathology (4 weeks) with no evidence myofibrillar alterations. At 16 weeks, we show deTyr-enriched MT arrays significantly densified and co-localized to areas of myofibrillar malformation. Profiling the enzyme complexes responsible for deTyr-tubulin, we identify vasohibin 2 (VASH2) and small vasohibin binding protein (SVBP) significantly elevated in the mdx muscle at 4 weeks. Using the genetic increase in VASH2/SVBP expression in 4 weeks wild-type mice we find densified deTyr-enriched MT arrays that co-segregate with myofibrillar malformations similar to those in the 16 weeks mdx. Given that no changes in sarcomere organization were identified in fibers expressing sfGFP as a control, we conclude that disease-dependent densification of deTyr-enriched MT arrays underscores the altered myofibrillar structure in dystrophic skeletal muscle fibers.

Association between ACTN3 R577X genotype and risk of non-contact injury in trained athletes: A systematic review

PURPOSE

The aim of this study was to review, systematically, evidence concerning the link between the ACTN3 R577X polymorphism and the rates and severity of non-contact injuries and exercise-induced muscle damage in athletes and individuals enrolled in exercise training programs.

METHODS

A computerized literature search was performed in the electronic databases PubMed, Web of Science, and SPORTDiscus, from inception until November 2020. All included studies compared the epidemiological characteristics of non-contact injury between the different genotypes of the ACTN3 R577X polymorphism.

RESULTS

Our search identified 492 records. After the screening of titles, abstracts, and full texts, 13 studies examining the association between the ACTN3 genotypes and the rate and severity of non-contact injury were included in the analysis. These studies were performed in 6 different countries (Spain, Japan, Brazil, China, the Republic of Korea, and Italy) and involved a total participant pool of 1093 participants. Of the studies, 2 studies involved only women, 5 studies involved only men, and 6 studies involved both men and women. All the studies included were classified as high-quality studies (≥6 points in the Physiotherapy Evidence Database (PEDro) scale score). Overall, evidence suggests there is an association between the ACTN3 R577X genotype and non-contact injury in 12 investigations. Six studies observed a significant association between ACTN3 R577X polymorphism and exercise induced muscle damage: 2 with non-contact ankle injury, 3 with non-contact muscle injury, and 1 with overall non-contact injury.

CONCLUSION

The present findings support the premise that possessing the ACTN3 XX genotype may predispose athletes to a higher probability of some non-contact injuries, such as muscle injury, ankle sprains, and higher levels of exercise-induced muscle damage.

Effects of Malocclusion on Maximal Aerobic Capacity and Athletic Performance in Young Sub-Elite Athletes

Oral pathologies can cause athletic underperformance. The aim of this study was to determine the effect of malocclusion on maximal aerobic capacity in young athletes with the same anthropometric data, diet, training mode, and intensity from the same athletics training center. Sub-elite track and field athletes (middle-distance runners) with malocclusion (experimental group (EG); n = 37; 21 girls; age: 15.1 ± 1.5 years) and without malocclusion (control group (CG); n = 13; 5 girls; age: 14.7 ± 1.9 years) volunteered to participate in this study. Participants received an oral diagnosis to examine malocclusion, which was defined as an overlapping of teeth that resulted in impaired contact between the teeth of the mandible and the teeth of the upper jaw. Maximal aerobic capacity was assessed using the VAMEVAL test (calculated MAS and estimated VO_2max). The test consisted of baseline values that included the following parameters: maximum aerobic speed (MAS), maximal oxygen uptake (VO_2max), heart rate frequency, systolic (SAP) and diastolic arterial pressure (DAP), blood lactate concentration (LBP), and post-exercise blood lactate assessment (LAP) after the performance of the VAMEVAL test. There were no statistically significant differences between the two study groups related to either anthropometric data (age: EG = 15.1 ± 1.5 vs. CC = 14.7 ± 1.9 years (p = 0.46); BMI: EG = 19.25 ± 1.9 vs. CC = 19.42 ± 1.7 kg/m² (p = 0.76)) or for the following physical fitness parameters and biomarkers: MAS: EG = 15.5 (14.5-16.5) vs. CG = 15.5 (15-17) km/h (p = 0.47); VO_2max: EG = 54.2 (52.5-58.6) vs. CG = 54.2 (53.4-59.5) mL/kg/min (p = 0.62) (IQR (Q1-Q3)); heart rate before the physical test: EG = 77.1 ± 9.9 vs. CG = 74.3 ± 14.0 bpm (p = 0.43); SAP: EG = 106.6 ± 13.4 vs. CG = 106.2 ± 14.8 mmHg (p = 0.91); DAP: EG = 66.7 ± 9.1 vs. CG = 63.9 ± 10.2 mmHg (p = 0.36); LBP: EG = 1.5 ± 0.4 vs. CG = 1.3 ± 0.4 mmol/L (p = 0.12); and LAP: EG = 4.5 ± 2.36 vs. CG = 4.06 ± 3.04 mmol/L (p = 0.60). Our study suggests that dental malocclusion does not impede maximal aerobic capacity and the athletic performance of young track and field athletes.

Genotype Distribution of the ACTN3 p.R577X Polymorphism in Elite Badminton Players: A Preliminary Study

α-Actinin-3 is a protein with a structural role at the sarcomeric Z-line in skeletal muscle. As it is only present in fast-type muscle fibers, α-actinin-3 is considered a key mechanical component to produce high-intensity muscle contractions and to withstand external tension applied to the skeletal muscle. α-Actinin-3 is encoded by the gene ACTN3, which has a single-nucleotide polymorphism (p.R577X; rs1815739) that affects the expression of α-actinin-3 due to the presence of a stop codon. Individuals homozygous for the 577R allele (i.e., RR genotype) and RX heterozygotes express functional α-actinin-3, while those homozygous for the 577X (i.e., XX genotype) express a non-functional protein. There is ample evidence to support the associations between the ACTN3 genotype and athletic performance, with higher frequencies of the 577R allele in elite and professional sprint and power athletes than in control populations. This suggests a beneficial influence of possessing functional α-actinin-3 to become an elite athlete in power-based disciplines. However, no previous investigation has determined the frequency of the ACTN3 genotypes in elite badminton players, despite this sport being characterized by high-intensity actions of intermittent nature such as changes of direction, accelerations, jumps and smashes. The purpose of this study was to analyze ACTN3 R577X genotype frequencies in professional badminton players to establish whether this polymorphism is associated with elite athlete status. A total of 53 European Caucasian professional badminton players competing in the 2018 European Badminton Championships volunteered to participate in the study. Thirty-one were men (26.2 ± 4.4 years) and twenty-two were women (23.4 ± 4.5 years). Chi-squared tests were used to analyze the differences in the distribution of ACTN3 genotypes (RR, RX and XX) between categories and sexes. The ACTN3 RR genotype was the most frequent in the sample of professional badminton players (RR = 49.1%, RX = 22.6% and XX = 28.3%). None of the badminton players ranked in the world's top ten possessed the XX genotype (RX = 60%, RR = 40%). The distribution of the ACTN3 genotypes was similar between male and female professional badminton players (men: RR = 45.2%, RX = 25.8% and XX = 29.0%; women: RR = 54.5%, RX = 18.2% and XX = 27.3%; χ² = 0.58; p = 0.750). The distribution of the ACTN3 genotypes in badminton players was different from the 1000 genome database for the European population (χ² = 15.5; p < 0.001), with an overrepresentation of the RR genotype (p < 0.05) and an underrepresentation of the RX genotype (p < 0.01). In conclusion, the expression of functional α-actinin-3, associated with RR and RX genotypes in the ACTN3 gene may confer an advantage for reaching the status of elite athlete in badminton, and especially the world's top-ten ranking. Large-scale studies with different ethnic backgrounds are needed to confirm the association of the R allele of ACTN3 with badminton performance.

Influence of Alpha-Actinin-3 R577X Polymorphism on Muscle Damage and the Inflammatory Response after an Acute Strength Training Session

The objective of this study was to verify the influence of the ACTN3 R577X polymorphism on muscle damage and the inflammatory response after an acute strength training (ST) session. Twenty-seven healthy male individuals (age: 25 ± 4.3 years) participated in the study, including 18 RR/RX and 9 XX individuals. The participants were divided into two groups (RR/RX and XX groups) and subjected to an acute ST session, which consisted of a series of leg press, leg extension machine, and seated leg curl machine. The volunteers were instructed to perform the greatest volume of work until concentric muscle failure. Each volunteer's performance was analyzed as the load and total volume of training, and the blood concentrations of C-C motif chemokine ligand 2 (CCL2), interleukin-8 (IL-8), creatine kinase (CK), lactate dehydrogenase (LDH), myoglobin, testosterone, and cortisol were measured before the ST session and 30 min and 24 h postsession. The ACTN3 R577X polymorphism effect was observed, with increased concentrations of CCL2 (p < 0.01), IL-8 (p < 0.01), and LDH (p < 0.001) in XX individuals. There was an increase in the concentration of CK in the RR/RX group compared to XX at 24 h after training (p > 0.01). The testosterone/cortisol ratio increased more markedly in the XX group (p < 0.001). Regarding performance, the RR/RX group presented higher load and total volume values in the training exercises when compared to the XX group (p < 0.05). However, the XX group presented higher values of delayed onset muscle soreness (DOMS) than the RR/RX group (p < 0.05). The influence of ACTN3 R577X polymorphism on muscle damage and the inflammatory response was observed after an acute ST session, indicating that the RR/RX genotype shows more muscle damage and a catabolic profile due to a better performance in this activity, while the XX genotype shows more DOMS.

The unexpected versatility of ALP/Enigma family proteins

One of the most intriguing features of multicellular animals is their ability to move. On a cellular level, this is accomplished by the rearrangement and reorganization of the cytoskeleton, a dynamic network of filamentous proteins which provides stability and structure in a stationary context, but also facilitates directed movement by contracting. The ALP/Enigma family proteins are a diverse group of docking proteins found in numerous cellular milieus and facilitate these processes among others. In vertebrates, they are characterized by having a PDZ domain in combination with one or three LIM domains. The family is comprised of CLP-36 (PDLIM1), Mystique (PDLIM2), ALP (PDLIM3), RIL (PDLIM4), ENH (PDLIM5), ZASP (PDLIM6), and Enigma (PDLIM7). In this review, we will outline the evolution and function of their protein domains which confers their versatility. Additionally, we highlight their role in different cellular environments, focusing specifically on recent advances in muscle research using Drosophila as a model organism. Finally, we show the relevance of this protein family to human myopathies and the development of muscle-related diseases.

A Pilot Study on the Prediction of Non-Contact Muscle Injuries Based on ACTN3 R577X and ACE I/D Polymorphisms in Professional Soccer Athletes

Muscle injuries are among the main reasons for medical leavings of soccer athletes, being a major concern within professional teams and their prevention associated with sport success. Several factors are associated with a greater predisposition to injury, and genetic background is increasingly being investigated. The aim of this study was to analyze whether ACTN3 R577X and ACE I/D polymorphisms are predictors of the incidence and severity of muscle injury in professional soccer athletes from Brazil, individually and in association. Eighty-three professional athletes from the first and second divisions of the Brazilian Championship were evaluated regarding the polymorphisms through blood samples. Nighty-nine muscle injuries were identified during the seasons of 2018, 2019 and 2020 and categorized according to severity. ACTN3 XX individuals had a higher frequency of severe injuries compared to the RX and RR genotypes (p = 0.001), and in the dominant model (compared to RX+RR), with p < 0.001. The trend p-value test showed an increased number of injuries/season following the order XX > RX > RR (p = 0.045). Those with the ACE II genotype had almost 2 fold the number of injuries per season compared to those with the ID+DD genotypes (p = 0.03). Logistic regression showed that the polymorphisms are predictors of the development of severe injury (ACTN3 R577X model with p = 0.004, R2: 0.259; ACE I/D model with p = 0.045, R2: 0.163), where ACTN3 XX individuals were more likely to suffer from severe injury (OR: 5.141, 95% CI: 1.472-17.961, p = 0.010). The combination of the ACTN3 577X allele and the ACE II genotype showed an increased number of injuries per season, enhanced by 100% (1.682 injuries/season versus 0.868 injuries/season, p = 0.016). Our findings suggest that both polymorphisms ACTN3 R577X and ACE I/D (and their interaction) are associated with the susceptibility and severity of non-contact muscle injury in soccer players.

Contributions of Titin and Collagen to Passive Stress in Muscles from mdm Mice with a Small Deletion in Titin's Molecular Spring

Muscular dystrophy with myositis (mdm) is a naturally occurring mutation in the mouse Ttn gene that results in higher passive stress in muscle fibers and intact muscles compared to wild-type (WT). The goal of this study was to test whether alternative splicing of titin exons occurs in mdm muscles, which contain a small deletion in the N2A-PEVK regions of titin, and to test whether splicing changes are associated with an increase in titin-based passive tension. Although higher levels of collagen have been reported previously in mdm muscles, here we demonstrate alternative splicing of titin in mdm skeletal muscle fibers. We identified Z-band, PEVK, and C-terminus Mex5 exons as splicing hotspots in mdm titin using RNA sequencing data and further reported upregulation in ECM-associated genes. We also treated skinned mdm soleus fiber bundles with trypsin, trypsin + KCl, and trypsin + KCL + KI to degrade titin. The results showed that passive stress dropped significantly more after trypsin treatment in mdm fibers (11 ± 1.6 mN/mm2) than in WT fibers (4.8 ± 1 mN/mm2; p = 0.0004). The finding that treatment with trypsin reduces titin-based passive tension more in mdm than in WT fibers supports the hypothesis that exon splicing leads to the expression of a stiffer and shorter titin isoform in mdm fibers. After titin extraction by trypsin + KCl + KI, mdm fibers (6.7 ± 1.27 mN/mm2) had significantly higher collagen-based passive stress remaining than WT fibers (2.6 ± 1.3 mN/mm2; p = 0.0014). We conclude that both titin and collagen contribute to higher passive tension of mdm muscles.

Absence of the Z-disc protein α-actinin-3 impairs the mechanical stability of Actn3KO mouse fast-twitch muscle fibres without altering their contractile properties or twitch kinetics

BACKGROUND

A common polymorphism (R577X) in the ACTN3 gene results in the complete absence of the Z-disc protein α-actinin-3 from fast-twitch muscle fibres in ~ 16% of the world's population. This single gene polymorphism has been subject to strong positive selection pressure during recent human evolution. Previously, using an Actn3KO mouse model, we have shown in fast-twitch muscles, eccentric contractions at L₀ + 20% stretch did not cause eccentric damage. In contrast, L₀ + 30% stretch produced a significant ~ 40% deficit in maximum force; here, we use isolated single fast-twitch skeletal muscle fibres from the Actn3KO mouse to investigate the mechanism underlying this.

METHODS

Single fast-twitch fibres are separated from the intact muscle by a collagenase digest procedure. We use label-free second harmonic generation (SHG) imaging, ultra-fast video microscopy and skinned fibre measurements from our MyoRobot automated biomechatronics system to study the morphology, visco-elasticity, force production and mechanical strength of single fibres from the Actn3KO mouse. Data are presented as means ± SD and tested for significance using ANOVA.

RESULTS

We show that the absence of α-actinin-3 does not affect the visco-elastic properties or myofibrillar force production. Eccentric contractions demonstrated that chemically skinned Actn3KO fibres are mechanically weaker being prone to breakage when eccentrically stretched. Furthermore, SHG images reveal disruptions in the myofibrillar alignment of Actn3KO fast-twitch fibres with an increase in Y-shaped myofibrillar branching.

CONCLUSIONS

The absence of α-actinin-3 from the Z-disc in fast-twitch fibres disrupts the organisation of the myofibrillar proteins, leading to structural weakness. This provides a mechanistic explanation for our earlier findings that in vitro intact Actn3KO fast-twitch muscles are significantly damaged by L₀ + 30%, but not L₀ + 20%, eccentric contraction strains. Our study also provides a possible mechanistic explanation as to why α-actinin-3-deficient humans have been reported to have a faster decline in muscle function with increasing age, that is, as sarcopenia reduces muscle mass and force output, the eccentric stress on the remaining functional α-actinin-3 deficient fibres will be increased, resulting in fibre breakages.

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged mdx dystrophic mice by reducing pathological fibre branching

The common null polymorphism (R577X) in the ACTN3 gene is present in over 1.5 billion people worldwide and results in the absence of the protein α-actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin-deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this, we use a double knockout (dk)Actn3KO/mdx (dKO) mouse model, which lacks both dystrophin and sarcomere α-actinin-3. We used dKO mice and mdx dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an in vitro eccentric contraction protocol. In the aged dKO mouse, we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged mdx EDL fibres (68%), and this correlates with a graded force loss over three eccentric contractions for dKO muscles (~36% after first contraction, ~66% overall) compared to an abrupt drop in mdx upon the first eccentric contraction (~75% after first contraction, ~89% after three contractions). In dKO, protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca2+ pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.

The Impact of ACTN3 Gene Polymorphisms on Susceptibility to Exercise-Induced Muscle Damage and Changes in Running Economy Following Downhill Running

This study aimed to investigate if ACTN3 gene polymorphism impacts the susceptibility to exercise-induced muscle damage (EIMD) and changes in running economy (RE) following downhill running. Thirty-five healthy men were allocated to the two groups based on their ACTN3 gene variants: RR and X allele carriers. Neuromuscular function [knee extensor isometric peak torque (IPT), rate of torque development (RTD), and countermovement, and squat jump height], indirect markers of EIMD [muscle soreness, mid-thigh circumference, knee joint range of motion, and serum creatine kinase (CK) activity], and RE (oxygen uptake, minute ventilation, blood lactate concentration, and perceived exertion) for 5-min of running at a speed equivalent to 80% of individual maximal oxygen uptake speed were assessed before, immediately after, and 1-4 days after a 30-min downhill run (-15%). Neuromuscular function was compromised (P < 0.05) following downhill running with no differences between the groups, except for IPT, which was more affected in the RR individuals compared with the X allele carriers immediately (-24.9 ± 6.9% vs. -16.3 ± 6.5%, respectively) and 4 days (-16.6 ± 14.9% vs. -4.2 ± 9.5%, respectively) post-downhill running. EIMD manifested similarly for both the groups except for serum CK activity, which was greater for RR (398 ± 120 and 452 ± 126 U L^-1 at 2 and 4 days following downhill running, respectively) compared with the X allele carriers (273 ± 121 and 352 ± 114 U L^-1 at the same time points). RE was compromised following downhill running (16.7 ± 8.3% and 11 ± 7.5% increases in oxygen uptake immediately following downhill running for the RR and X allele carriers, respectively) with no difference between the groups. We conclude that although RR individuals appear to be more susceptible to EIMD following downhill running, this does not extend to the changes in RE.

ACTN3 Genotypes and Their Relationship with Muscle Mass and Function of Kosovan Adults

Maintaining muscle mass and function is important throughout the lifestyle. While environmental factors such as physical activity and healthy nutrition are well investigated, the contribution of genetic factors is still controversial. Therefore, we aimed to investigate the impact of a common ACTN3 polymorphism (rs1815739) on body composition, handgrip strength, knee extensor peak torque, and physical performance (gait speed, 30-s arm curl, 30-s chair stand) in Kosovan adults. In total, 308 participants (160 females and 148 males, age range from 40 to 91 years) took part in this cross-sectional study. Genomic DNA was extracted from saliva and assessed for ACTN3 genotype distribution (41.5% of RR, 53.9% of RX and 4.6% of XX). Genotype allocation did not account for differences in any of the variables. Interestingly, female XX carriers were taller (p = 0.025) and had a higher isokinetic knee extension peak torque (p = 0.024) than the RX+RR group. In males, XX carriers were also taller (p = 0.049) and had a lower BMI (p = 0.026), but did not differ in any of the strength and performance parameters. These results indicate that the ACTN3 R577X polymorphism might exert a sex-specific impact on knee extensor peak torque and BMI.

ACTN3 genotype influences skeletal muscle mass regulation and response to dexamethasone

Homozygosity for the common ACTN3 null polymorphism (ACTN3 577X) results in α-actinin-3 deficiency in ~20% of humans worldwide and is linked to reduced sprint and power performance in both elite athletes and the general population. α-Actinin-3 deficiency is also associated with reduced muscle mass, increased risk of sarcopenia, and altered muscle wasting response induced by denervation and immobilization. Here, we show that α-actinin-3 plays a key role in the regulation of protein synthesis and breakdown signaling in skeletal muscle and influences muscle mass from early postnatal development. We also show that α-actinin-3 deficiency reduces the atrophic and anti-inflammatory response to the glucocorticoid dexamethasone in muscle and protects against dexamethasone-induced muscle wasting in female but not male mice. The effects of α-actinin-3 deficiency on muscle mass regulation and response to muscle wasting provide an additional mechanistic explanation for the positive selection of the ACTN3 577X allele in recent human history.

Alpha-Actinin-3 Deficiency Might Affect Recovery from Non-Contact Muscle Injuries: Preliminary Findings in a Top-Level Soccer Team

There are recent data suggesting an association between the R577X polymorphism (rs1815739) in the gene encoding α-actinin-3 (ACTN3) and the risk of musculoskeletal injuries. The purpose of this study was to analyze the association of rs1815739 with risk of, and recovery time from non-contact soft-tissue muscle injuries in professional soccer players. Forty-six (22 male and 24 female) players from a top-level professional soccer team were assessed during five consecutive seasons: the genotype distribution was: RR, 41.3%; RX, 47.8%; and XX, 10.9%. There was a trend towards a higher risk of muscle injury associated with the XX genotype (p = 0.092, with no injury-free XX player during the 5-year study period) and a significant genotype effect for the time needed to return to play (p = 0.044, with the highest value shown for the XX genotype, i.e., 36 ± 26 days, vs. 20 ± 10 and 17 ± 12 days for RR and RX, respectively). In conclusion, the XX genotype might be associated not only with a higher risk of non-contact muscle injuries, but also of recovery time from these conditions. However, more research in larger cohorts is needed to confirm this preliminary hypothesis.

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.

Influence of the ACTN3 R577X genotype on the injury epidemiology of marathon runners

A common single nucleotide polymorphism in the ACTN3 gene might result in the complete deficiency of α-actinin-3 (i.e., XX genotype). It has been found that ACTN3 XX individuals have several traits related to lessened muscle performance. This study aimed to determine the influence, if any, of ACTN3 genotypes on injury incidence of marathoners during the year preceding to participating in a competitive marathon race. Using a cross-sectional experimental design, the type and conditions of sports injuries were documented for one year in a group of 139 marathoners. Injuries were recorded following a consensus statement on injuries in Athletics. Afterward, ACTN3 genotyping was performed, and injury epidemiology was compared among RR, RX, and XX genotypes. The distribution of the RR/RX/XX genotypes was 28.8/42.8/23.5%, respectively. A total of 67 injuries were recorded. The frequency of marathoners that reported any injury during the previous year was not different across the genotypes (55.0/38.8/40.6%, P = 0.241). Although the overall injury incidence was not different among genotypes (2.78/1.65/1.94 injuries/1000 h of running, P = 0.084), the likelihood of suffering an injury was higher in RR than in RX (OR = 1.93: 95%CI = 0.87–4.30), and higher than in XX (OR = 1.79: 0.70–4.58). There was no difference in the conditions, severity, body location, time of year, or leading cause of injury among genotypes. However, XX presented a higher frequency of sudden-onset injuries (P = 0.024), and the OR for muscle-type injuries was 2.0 (0.51–7.79) times higher compared to RR runners. Although XX marathoners did not have a higher overall incidence of injury, the OR in these runners for muscle-type injuries was superior to RR and RX runners. The likelihood of suffering a muscle injury, especially with a sudden-onset, was twice in XX than in RR endurance runners.

Genetic and Epigenetic Modulation of Cell Functions by Physical Exercise

Since ancient times, the importance of physical activity (PA) and of a wholesome diet for human health has been clearly recognized. However, only recently, it has been acknowledged that PA can reverse at least some of the unwanted effects of a sedentary lifestyle, contributing to the treatment of pathologies such as hypertension and diabetes, to the delay of aging and neurodegeneration, and even to the improvement of immunity and cognitive processes. At the same time, the cellular and molecular bases of these effects are beginning to be uncovered. The original research articles and reviews published in this Special Issue on “Genetic and Epigenetic Modulation of Cell Functions by Physical Exercise” focus on different aspects of the genetics and molecular biology of PA effects on health and, in addition, on the effects of different genotypes on the ability to perform PA. All authors have read and agreed to the published version of the manuscript.

Association Between Hematological Parameters and Iron Metabolism Response After Marathon Race and ACTN3 Genotype

α-Actinin-3 (ACTN3 R577X, rs.1815739) polymorphism is a genetic variation that shows the most consistent influence on metabolic pathway and muscle phenotype. XX genotype is associated with higher metabolic efficiency of skeletal muscle; however, the role of ACTN3 polymorphism in oxygen transport and utilization system has not yet been investigated. Therefore, the aim of this study was to determine the influence of ACTN3 polymorphisms on hematological and iron metabolism response induced by marathon race. Eighty-one Brazilian amateur male endurance runners participated in the study. Blood samples and urine were collected before; immediately after; and 1, 3, and 15 days after the marathon race. Urine, hematological parameters, iron metabolism, and ACTN3 genotyping analyses were performed. The marathon race induced a decrease in erythrocytes, Hb, and Ht, and an increase in hematuria, creatinine, myoglobin, red cell distribution width, mean corpuscular hemoglobin concentration, mean corpuscular hemoglobin, direct and indirect bilirubin and erythropoietin. Moreover, an elevation immediately or 1 day after the marathon race follows a reduction 3 or 15 days after the marathon race were observed on transferrin saturation and iron and transferrin levels. Hematological parameters and iron metabolism changes induced by marathon race were not observed in XX genotypes. Hematuria and decreased erythrocytes, Hb, Ht, and iron and transferrin levels were observed only in RR and/or RX genotypes but not in XX genotypes. The percentage of runners with hematuria, leukocyturia, iron deficiency, creatinine, myoglobin, and bilirubin imbalance was higher in RR compared to XX genotypes. ACTN3 polymorphism is associated with iron metabolism and hematological responses after endurance exercise. Despite these results being based on a small sample, they highlight a protective role of the XX genotype on hematological and renal changes induced by long-distance exercise. Therefore, these findings should be further replicated.

ACTN3 R577X Genotype and Exercise Phenotypes in Recreational Marathon Runners

Background: Homozygosity for the X-allele in the ACTN3 R577X (rs1815739) polymorphism results in the complete absence of α-actinin-3 in sarcomeres of fast-type muscle fibers. In elite athletes, the ACTN3 XX genotype has been related to inferior performance in speed and power-oriented sports; however, its influence on exercise phenotypes in recreational athletes has received less attention. We sought to determine the influence of ACTN3 genotypes on common exercise phenotypes in recreational marathon runners. Methods: A total of 136 marathoners (116 men and 20 women) were subjected to laboratory testing that included measurements of body composition, isometric muscle force, muscle flexibility, ankle dorsiflexion, and the energy cost of running. ACTN3 genotyping was performed using TaqMan probes. Results: 37 runners (27.2%) had the RR genotype, 67 (49.3%) were RX and 32 (23.5%) were XX. There was a difference in body fat percentage between RR and XX genotype groups (15.7 ± 5.8 vs. 18.8 ± 5.5%; effect size, ES, = 0.5 ± 0.4, p = 0.024), whereas the distance obtained in the sit-and-reach-test was likely lower in the RX than in the XX group (15.3 ± 7.8 vs. 18.4 ± 9.9 cm; ES = 0.4 ± 0.4, p = 0.046). Maximal dorsiflexion during the weight-bearing lunge test was different in the RR and XX groups (54.8 ± 5.8 vs. 57.7 ± 5.1 degree; ES = 0.5 ± 0.5, p = 0.044). Maximal isometric force was higher in the RR than in the XX group (16.7 ± 4.7 vs. 14.7 ± 4.0 N/kg; ES = −0.5 ± 0.3, p = 0.038). There was no difference in the energy cost of running between genotypes (~4.8 J/kg/min for all three groups, ES ~0.2 ± 0.4). Conclusions: The ACTN3 genotype might influence several exercise phenotypes in recreational marathoners. Deficiency in α-actinin-3 might be accompanied by higher body fatness, lower muscle strength and higher muscle flexibility and range of motion. Although there is not yet a scientific rationale for the use of commercial genetic tests to predict sports performance, recreational marathon runners who have performed such types of testing and have the ACTN3 XX genotype might perhaps benefit from personalized strength training to improve their performance more than their counterparts with other ACTN3 genotypes.

ACTN3 R577X Polymorphism Is Associated With the Incidence and Severity of Injuries in Professional Football Players

OBJECTIVE

The ACTN3 R577X gene variant results in the absence of the α-actinin-3 protein in ∼18% of humans worldwide and has been associated with athletic performance and increased susceptibility to eccentric muscle damage. The aim of this study was to investigate the association between ACTN3 R577X variant and indirect muscle disorders/injuries in professional football players.

DESIGN

A case-control, genotype-phenotype association study.

INTERVENTION

Two hundred fifty-seven male professional Italian football players (from Serie A, Primavera, Allievi, and Giovanissimi; age = 21.2 ± 5.3 years) and 265 nonathletic controls were recruited for the study. Genomic DNA was extracted using a buccal swab, and the ACTN3 R577X genotype was performed using a PCR method. Structural-mechanical injuries and functional muscle disorders were collected from a subgroup of 169 football players during the period of 2009 to 2014.

MAIN OUTCOME MEASURE

We hypothesized that the 577XX genotype would be associated with higher predisposition to muscle injuries (compared with the other genotypes).

RESULTS

ACTN3 XX (α-actinin-3 deficiency) players had 2.66 higher odds for an injury incidence than their ACTN3 RR counterparts (95% confidence interval [CI]: 1.09-6.63, P = 0.02), whereas RX and RR players had similar injury incidence. Furthermore, ACTN3 XX players had 2.13 higher odds for having a severe injury compared with their RR counterparts (95% CI: 1.25-3.74, P = 0.0054), whereas RX individuals had 1.63 higher odds for having a severe injury compared with the RR players (95% CI: 1.10-2.40, P = 0.015).

CONCLUSIONS

The ACTN3 R577X polymorphism is associated with the incidence and severity of muscle injuries in professional football players; players with the ACTN3 577XX genotype have higher odds of having muscle injuries than their RR counterparts.

CLINICAL RELEVANCE

Discovering the complex relationship between gene variants and muscle injuries may assist coaches, physiologists, and the medical community to development tailored injury prevention program for football players, which could provide a new edge for successful competition.

More than a 'speed gene': ACTN3 R577X genotype, trainability, muscle damage, and the risk for injuries

A common null polymorphism (rs1815739; R577X) in the gene that codes for α-actinin-3 (ACTN3) has been related to different aspects of exercise performance. Individuals who are homozygous for the X allele are unable to express the α-actinin-3 protein in the muscle as opposed to those with the RX or RR genotype. α-actinin-3 deficiency in the muscle does not result in any disease. However, the different ACTN3 genotypes can modify the functioning of skeletal muscle during exercise through structural, metabolic or signaling changes, as shown in both humans and in the mouse model. Specifically, the ACTN3 RR genotype might favor the ability to generate powerful and forceful muscle contractions. Leading to an overall advantage of the RR genotype for enhanced performance in some speed and power-oriented sports. In addition, RR genotype might also favor the ability to withstand exercise-induced muscle damage, while the beneficial influence of the XX genotype on aerobic exercise performance needs to be validated in human studies. More information is required to unveil the association of ACTN3 genotype with trainability and injury risk during acute or chronic exercise.

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.

ACTN3: More than Just a Gene for Speed

Over the last couple of decades, research has focused on attempting to understand the genetic influence on sports performance. This has led to the identification of a number of candidate genes which may help differentiate between elite and non-elite athletes. One of the most promising genes in that regard is ACTN3, which has commonly been referred to as “a gene for speed”. Recent research has examined the influence of this gene on other performance phenotypes, including exercise adaptation, exercise recovery, and sporting injury risk. In this review, we identified 19 studies exploring these phenotypes. Whilst there was large variation in the results of these studies, as well as extremely heterogeneous cohorts, there is overall a tentative consensus that ACTN3 genotype can impact the phenotypes of interest. In particular, the R allele of a common polymorphism (R577X) is associated with enhanced improvements in strength, protection from eccentric training-induced muscle damage, and sports injury. This illustrates that ACTN3 is more than just a gene for speed, with potentially wide-ranging influence on muscle function, knowledge of which may aid in the future personalization of exercise training programmes.

Human skeletal muscle type 1 fibre distribution and response of stress-sensing proteins along the titin molecule after submaximal exhaustive exercise

Early responses of stress-sensing proteins, muscle LIM protein (MLP), ankyrin repeat proteins (Ankrd1/CARP and Ankrd2/Arpp) and muscle-specific RING finger proteins (MuRF1 and MuRF2), along the titin molecule were investigated in the present experiment after submaximal exhaustive exercise. Ten healthy men performed continuous drop jumping unilaterally on a sledge apparatus with a submaximal height until complete exhaustion. Five stress-sensing proteins were analysed by mRNA measurements from biopsies obtained immediately and 3 h after the exercise from exercised vastus lateralis muscle while control biopsies were obtained from non-exercised legs before the exercise. Decreased maximal jump height and increased serum creatine kinase activities as indirect markers for muscle damage and HSP27 immunostainings on muscle biopsies as a direct marker for muscle damage indicated that the current exercised protocol caused muscle damage. mRNA levels for four (MLP, Ankrd1/CARP, MuRF1 and MuRF2) out of the five studied stress sensors significantly (p < 0.05) increased 3 h after fatiguing exercise. The magnitude of MLP and Ankrd2 responses was related to the proportion of type 1 myofibres. Our data showed that the submaximal exhaustive exercise with subject's own physical fitness level activates titin-based stretch-sensing proteins. These results suggest that both degenerative and regenerative pathways are activated in very early phase after the exercise or probably already during the exercise. Activation of these proteins represents an initial step forward adaptive remodelling of the exercised muscle and may also be involved in the initiation of myofibre repair.

Age-related Differences in Dystrophin: Impact on Force Transfer Proteins, Membrane Integrity, and Neuromuscular Junction Stability

The loss of muscle strength with age has been studied from the perspective of a decline in muscle mass and neuromuscular junction (NMJ) stability. A third potential factor is force transmission. The purpose of this study was to determine the changes in the force transfer apparatus within aging muscle and the impact on membrane integrity and NMJ stability. We measured an age-related loss of dystrophin protein that was greatest in the flexor muscles. The loss of dystrophin protein occurred despite a twofold increase in dystrophin mRNA. Importantly, this disparity could be explained by the four- to fivefold upregulation of the dystromir miR-31. To compensate for the loss of dystrophin protein, aged muscle contained increased α-sarcoglycan, syntrophin, sarcospan, laminin, β1-integrin, desmuslin, and the Z-line proteins α-actinin and desmin. In spite of the adaptive increase in other force transfer proteins, over the 48 hours following lengthening contractions, the old muscles showed more signs of impaired membrane integrity (fourfold increase in immunoglobulin G-positive fibers and 70% greater dysferlin mRNA) and NMJ instability (14- to 96-fold increases in Runx1, AchRδ, and myogenin mRNA). Overall, these data suggest that age-dependent alterations in dystrophin leave the muscle membrane and NMJ more susceptible to contraction-induced damage even before changes in muscle mass are obvious.

Persistent fibrosis, hypertrophy and sarcomere disorganisation after endoscopy-guided heart resection in adult Xenopus

Models of cardiac repair are needed to understand mechanisms underlying failure to regenerate in human cardiac tissue. Such studies are currently dominated by the use of zebrafish and mice. Remarkably, it is between these two evolutionary separated species that the adult cardiac regenerative capacity is thought to be lost, but causes of this difference remain largely unknown. Amphibians, evolutionary positioned between these two models, are of particular interest to help fill this lack of knowledge. We thus developed an endoscopy-based resection method to explore the consequences of cardiac injury in adult Xenopus laevis. This method allowed in situ live heart observation, standardised tissue amputation size and reproducibility. During the first week following amputation, gene expression of cell proliferation markers remained unchanged, whereas those relating to sarcomere organisation decreased and markers of inflammation, fibrosis and hypertrophy increased. One-month post-amputation, fibrosis and hypertrophy were evident at the injury site, persisting through 11 months. Moreover, cardiomyocyte sarcomere organisation deteriorated early following amputation, and was not completely recovered as far as 11 months later. We conclude that the adult Xenopus heart is unable to regenerate, displaying cellular and molecular marks of scarring. Our work suggests that, contrary to urodeles and teleosts, with the exception of medaka, adult anurans share a cardiac injury outcome similar to adult mammals. This observation is at odds with current hypotheses that link loss of cardiac regenerative capacity with acquisition of homeothermy.

Evidence for ACTN3 as a genetic modifier of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized by muscle degeneration and progressive weakness. There is considerable inter-patient variability in disease onset and progression, which can confound the results of clinical trials. Here we show that a common null polymorphism (R577X) in ACTN3 results in significantly reduced muscle strength and a longer 10 m walk test time in young, ambulant patients with DMD; both of which are primary outcome measures in clinical trials. We have developed a double knockout mouse model, which also shows reduced muscle strength, but is protected from stretch-induced eccentric damage with age. This suggests that α-actinin-3 deficiency reduces muscle performance at baseline, but ameliorates the progression of dystrophic pathology. Mechanistically, we show that α-actinin-3 deficiency triggers an increase in oxidative muscle metabolism through activation of calcineurin, which likely confers the protective effect. Our studies suggest that ACTN3 R577X genotype is a modifier of clinical phenotype in DMD patients.

Greater muscle damage in athletes with ACTN3 R577X (RS1815739) gene polymorphism after an ultra-endurance race: a pilot study

In this study, we aimed to investigate the influence of ACTN3 R577X gene polymorphism on muscle damage responses in athletes competing in an ultra-endurance race. Twenty moderate to well-trained ultra-runners who had entered in an official 37.1 km adventure race (22.1 km mountain biking, 10.9 km trekking, 4.1 km water trekking, 30 m rope course, and orienteering) volunteered for the study. Blood samples were collected for genotyping and analysis of muscle protein levels before and after the race. Percentage changes (pre- to post-race) of serum myoglobin [XX = 5,377% vs. RX/RR = 1,666%; P = 0.005, effect size (ES) = 1.73], creatine kinase (XX = 836.5% vs. RX/RR = 455%; P = 0.04, ES = 1.29), lactate dehydrogenase (XX = 82% vs. RX/RR = 65%; P = 0.002, ES = 1.61), and aspartate aminotransferase (XX = 148% vs. RX/RR = 75%; P = 0.02, ES = 1.77) were significantly greater for XX than RX/RR genotypes. ES analysis confirmed a large magnitude of muscle damage in XX genotype ultra-runners. Therefore, athletes with the ACTN3 577XX genotype experienced more muscle damage after an adventure race. This suggests that ultra-runners with alpha-actinin-3 deficiency may be more susceptible to rhabdomyolysis and associated health complications during ultra-endurance competitions.

Insights into the molecular etiology of exercise-induced inflammation: opportunities for optimizing performance

The study of exercise-induced muscle damage (EIMD) is of paramount importance not only because it affects athletic performance but also because it is an excellent model to study the mechanisms governing muscle cachexia under various clinical conditions. Although, a large number of studies have investigated EIMD and its associated inflammatory response, several aspects of skeletal muscles responses remain unclear. In the first section of this article, the mechanisms of EIMD are reviewed in an attempt to follow the events that result in functional and structural alterations of skeletal muscle. In the second section, the inflammatory response associated with EIMD is presented with emphasis in leukocyte accumulation through mechanisms that are largely coordinated by pro- and anti-inflammatory cytokines released either by injured muscle itself or other cells. The practical applications of EIMD and the subsequent inflammatory response are discussed with respect to athletic performance. Specifically, the mechanisms leading to performance deterioration and development of muscle soreness are discussed. Emphasis is given to the factors affecting individual responses to EIMD and the resulting interindividual variability to this phenomenon.

Exertional rhabdomyolysis: physiological response or manifestation of an underlying myopathy?

Exertional rhabdomyolysis is characterised by muscle breakdown associated with strenuous exercise or normal exercise under extreme circumstances. Key features are severe muscle pain and sudden transient elevation of serum creatine kinase (CK) levels with or without associated myoglobinuria. Mild cases may remain unnoticed or undiagnosed. Exertional rhabdomyolysis is well described among athletes and military personnel, but may occur in anybody exposed to unaccustomed exercise. In contrast, exertional rhabdomyolysis may be the first manifestation of a genetic muscle disease that lowers the exercise threshold for developing muscle breakdown. Repeated episodes of exertional rhabdomyolysis should raise the suspicion of such an underlying disorder, in particular in individuals in whom the severity of the rhabdomyolysis episodes exceeds the expected response to the exercise performed. The present review aims to provide a practical guideline for the acute management and postepisode counselling of patients with exertional rhabdomyolysis, with a particular emphasis on when to suspect an underlying genetic disorder. The pathophysiology and its clinical features are reviewed, emphasising four main stepwise approaches: (1) the clinical significance of an acute episode, (2) risks of renal impairment, (3) clinical indicators of an underlying genetic disorders and (4) when and how to recommence sport activity following an acute episode of rhabdomyolysis. Genetic backgrounds that appear to be associated with both enhanced athletic performance and increased rhabdomyolysis risk are briefly reviewed.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.

Biology of frailty: Modulation of ageing genes and its importance to prevent age-associated loss of function

Frailty is associated with loss of functional reserve as well as with the prediction of adverse events in the old population. The traditional criteria of frailty are based on five physical determinations described in the Cardiovascular Health Study. We propose that biological and genetic markers of frailty should be used to increase the predictive capacity of the established clinical indeces. In recent times, research for biological markers of frailty has gained impetus. Finding a biological markers with diagnostic and prognostic capacity would be a major milestone to identify frailty risk, and also pre-frailty status. In the first section of the manuscript, we review the available biomarkers that help to monitor and prevent the evolution and the efficacy of interventions to delay the onset of frailty and to prevent its progression to incapacity. We also discuss the contribution of genetics to frailty. There are scientific bases that support that genetics influences frailty, although environmental factors probably will have the highest contribution. We review the known SNPs of the genes associated with frailty and classify them, taking into account the pathway in which they are involved. We also highlight the importance of longevity genes and their possible relation with frailty, citing centenarians who reach a very old age as an example of successful ageing. Finally, the reversibility of frailty is discussed. It can potentially be treated with nutritional or pharmacological interventions. However, physical exercise seems to be the most effective strategy to treat and prevent frailty. The last section of the manuscript is devoted to explaining the recommendations on the appropriate design of an exercise protocol to maximize its beneficial effects in a population of frail individuals.

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.

Evidence for ACTN3 as a Speed Gene in Isolated Human Muscle Fibers

Purpose

To examine the effect of α-actinin-3 deficiency due to homozygosity for the ACTN3 577X-allele on contractile and morphological properties of fast muscle fibers in non-athletic young men.

Methods

A biopsy was taken from the vastus lateralis of 4 RR and 4 XX individuals to test for differences in morphologic and contractile properties of single muscle fibers. The cross-sectional area of the fiber and muscle fiber composition was determined using standard immunohistochemistry analyses. Skinned single muscle fibers were subjected to active tests to determine peak normalized force (P₀), maximal unloading velocity (V₀) and peak power. A passive stretch test was performed to calculate Young’s Modulus and hysteresis to assess fiber visco-elasticity.

Results

No differences were found in muscle fiber composition. The cross-sectional area of type II_a and II_x fibers was larger in RR compared to XX individuals (P<0.001). P₀ was similar in both groups over all fiber types. A higher V₀ was observed in type II_a fibers of RR genotypes (P<0.001) but not in type I fibers. The visco-elasticity as determined by Young’s Modulus and hysteresis was unaffected by fiber type or genotype.

Conclusion

The greater V₀ and the larger fast fiber CSA in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions.

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.

Rodent models for resolving extremes of exercise and health

The extremes of exercise capacity and health are considered a complex interplay between genes and the environment. In general, the study of animal models has proven critical for deep mechanistic exploration that provides guidance for focused and hypothesis-driven discovery in humans. Hypotheses underlying molecular mechanisms of disease and gene/tissue function can be tested in rodents to generate sufficient evidence to resolve and progress our understanding of human biology. Here we provide examples of three alternative uses of rodent models that have been applied successfully to advance knowledge that bridges our understanding of the connection between exercise capacity and health status. First we review the strong association between exercise capacity and all-cause morbidity and mortality in humans through artificial selection on low and high exercise performance in the rat and the consequent generation of the "energy transfer hypothesis." Second we review specific transgenic and knockout mouse models that replicate the human disease condition and performance. This includes human glycogen storage diseases (McArdle and Pompe) and α-actinin-3 deficiency. Together these rodent models provide an overview of the advancements of molecular knowledge required for clinical translation. Continued study of these models in conjunction with human association studies will be critical to resolving the complex gene-environment interplay linking exercise capacity, health, and disease.

Effects of aging, exercise, and disease on force transfer in skeletal muscle

The loss of muscle strength and increased injury rate in aging skeletal muscle has previously been attributed to loss of muscle protein (cross-sectional area) and/or decreased neural activation. However, it is becoming clear that force transfer within and between fibers plays a significant role in this process as well. Force transfer involves a secondary matrix of proteins that align and transmit the force produced by the thick and thin filaments along muscle fibers and out to the extracellular matrix. These specialized networks of cytoskeletal proteins aid in passing force through the muscle and also serve to protect individual fibers from injury. This review discusses the cytoskeleton proteins that have been identified as playing a role in muscle force transmission, both longitudinally and laterally, and where possible highlights how disease, aging, and exercise influence the expression and function of these proteins.

Association of the ACTN3 R577X polymorphism with glucose tolerance and gene expression of sarcomeric proteins in human skeletal muscle

A common polymorphism (R577X) in the α-actinin (ACTN) 3 gene, which leads to complete deficiency of a functional protein in skeletal muscle, could directly influence metabolism in the context of health and disease. Therefore, we tested the hypothesis that states of glucose tolerance are associated with the ACTN3 R577X genotype. We analyzed the prevalence of the ACTN3 R577X polymorphism in people with normal glucose tolerance (NGT) and type 2 diabetes (T2D) and measured muscle-specific α-actinin 2 and 3 mRNA and protein abundance in skeletal muscle biopsies. Furthermore, we investigated the protein abundance of the myosin heavy chain isoforms and the components of the mitochondrial electron transport chain in skeletal muscle from people with NGT or T2D. mRNA of selected sarcomeric z-disk proteins was also assessed. Although the prevalence of the ACTN3 577XX genotype was higher in T2D patients, genotype distribution was unrelated to metabolic control or obesity. ACTN2 and ACTN3 mRNA expression and protein abundance was unchanged between NGT and T2D participants. Protein abundance of mitochondrial complexes II and IV was related to genotype and glucose tolerance status. Gene expression of sarcomeric z-disk proteins was increased in skeletal muscle from NGT participants with the ACTN3 577XX genotype. While genetic variation in ACTN3 does not influence metabolic control, genotype does appear to influence gene expression of other sarcomeric proteins, which could contribute to the functional properties of skeletal muscle and the fatigue-resistant phenotype associated with the R577X polymorphism.