A gene for speed? The evolution and function of alpha-actinin-3

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.

Protective role of alpha-actinin-3 in the response to an acute eccentric exercise bout

The ACTN3 gene encodes for the alpha-actinin-3 protein, which has an important structural function in the Z line of the sarcomere in fast muscle fibers. A premature stop codon (R577X) polymorphism in the ACTN3 gene causes a complete loss of the protein in XX homozygotes. This study investigates a possible role for the alpha-actinin-3 protein in protecting the fast fiber from eccentric damage and studies repair mechanisms after a single eccentric exercise bout. Nineteen healthy young men (10 XX, 9 RR) performed 4 series of 20 maximal eccentric knee extensions with both legs. Blood (creatine kinase; CK) and muscle biopsy samples were taken to study differential expression of several anabolic (MyoD1, myogenin, MRF4, Myf5, IGF-1), catabolic (myostatin, MAFbx, and MURF-1), and contraction-induced muscle damage marker genes [cysteine- and glycine-rich protein 3 (CSRP3), CARP, HSP70, and IL-6] as well as a calcineurin signaling pathway marker (RCAN1). Baseline mRNA content of CSRP3 and MyoD1 was 49 + or - 12 and 67 + or - 25% higher in the XX compared with the RR group (P = 0.01-0.045). However, satellite cell number was not different between XX and RR individuals. After eccentric exercise, XX individuals tended to have higher serum CK activity (P = 0.10) and had higher pain scores than RR individuals. However, CSRP3 (P = 0.058) and MyoD1 (P = 0.08) mRNA expression tended to be higher after training in RR individuals compared with XX alpha-actinin-3-deficient subjects. This study suggests a protective role of alpha-actinin-3 protein in muscle damage after eccentric training and an improved stress-sensor signaling, although effects are small.

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

In humans, there are two skeletal muscle alpha-actinins, encoded by ACTN2 and ACTN3, and the ACTN3 genotype is associated with human athletic performance. Remarkably, approximately 1 billion people worldwide are deficient in alpha-actinin-3 due to the common ACTN3 R577X polymorphism. The alpha-actinins are an ancient family of actin-binding proteins with structural, signalling and metabolic functions. The skeletal muscle alpha-actinins diverged approximately 250-300 million years ago, and ACTN3 has since developed restricted expression in fast muscle fibres. Despite ACTN2 and ACTN3 retaining considerable sequence similarity, it is likely that following duplication there was a divergence in function explaining why alpha-actinin-2 cannot completely compensate for the absence of alpha-actinin-3. This paper focuses on the role of skeletal muscle alpha-actinins, and how possible changes in functions between these duplicates fit in the context of gene duplication paradigms.

Elite Athletes: Are the Genes the Champions?

Recent research has analyzed the genetic factors that influence world-class athletic status. Much of what we know comes from association studies, with the ACE I/D and ACTN3 R577X polymorphisms having been extensively studied. The association between the ACTN3 R577X variation and elite athlete status in power sports is strongly documented, yet whether the current body of knowledge on other variants can be extrapolated to athletic champion status remains to be determined. Athletic champion status is a complex polygenic trait in which numerous candidate genes, complex gene–gene interactions, and environment–gene interactions are involved. Besides the need for more studies and new approaches taking into account the complexity of the problem, we believe that factors beyond genetic endowment are likely to have a stronger influence in the attainment of athletic champion status.

The human genome and sport, including epigenetics, gene doping, and athleticogenomics

Hugh Montgomery's discovery of the first of more than 239 fitness genes together with rapid advances in human gene therapy have created a prospect of using genes, genetic elements, and cells that have the capacity to enhance athletic performance (to paraphrase the World Anti-Doping Agency's definition of gene doping). This brief overview covers the main areas of interface between genetics and sport, attempts to provide a context against which gene doping may be viewed, and predicts a futuristic legitimate use of genomic (and possibly epigenetic) information in sport.

Phylogenetic analysis of gene structure and alternative splicing in alpha-actinins

The alpha-actinins are an important family of actin-binding proteins with the ability to cross-link actin filaments when in dimer form. Members of the alpha-actinin family share a domain topology composed of highly conserved actin-binding and EF-hand domains separated by a rod domain composed of spectrin-like repeats. Functional diversity within this family has arisen through exon duplication and the formation of alternate splice isoforms as well as gene duplications during the evolution of vertebrates. In addition to the known functional domains, alpha-actinins also contain a consensus PDZ-binding site. The completed genome sequence of over 32 invertebrate species has allowed the analysis of gene structure and exon-gene duplication over a diverse range of phyla. Our analysis shows that relative to early branching metazoans, there has been considerable intron loss especially in arthropods with few cases of intron gains. The C-terminal PDZ-binding site is conserved in nearly all invertebrates but is missing in some nematodes and platyhelminths. Alternative splicing in the actin-binding domain is conserved in chordates, arthropods, and some nematodes and platyhelminths. In contrast, alternative splicing of the EF-hand domain is only observed in chordates. Finally, given the prevalence of exon duplications seen in the actin-binding domain, this may act as a significant mechanism in the modification of actin-binding properties.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Genetics of athletic performance

Performance enhancing polymorphisms (PEPs) are examples of natural genetic variation that affect the outcome of athletic challenges. Elite athletes, and what separates them from the average competitor, have been the subjects of discussion and debate for decades. While training, diet, and mental fitness are all clearly important contributors to achieving athletic success, the fact that individuals reaching the pinnacle of their chosen sports often share both physical and physiological attributes suggests a role for genetics. That multiple members of a family often participate in highly competitive events, such as the Olympics, further supports this argument. In this review, we discuss what is known regarding the genes and gene families, including the mitochondrial genome, that are believed to play a role in human athletic performance. Where possible, we describe the physiological impact of the critical gene variants and consider predictions about other potentially important genes. Finally, we discuss the implications of these findings on the future for competitive athletics.

Different adaptations of alpha-actinin isoforms to exercise training in rat skeletal muscles

AIM

Alpha (alpha)-actinins are located in the skeletal muscle Z-line and form actin-actin cross-links. Mammalian skeletal muscle has two isoforms: alpha-actinin-2 and alpha-actinin-3. However, the response of alpha-actinin to exercise training is little understood. Therefore, the current study examined the effects of exercise training on the expression level of two alpha-actinin isoforms in skeletal muscles.

METHODS

Twelve male Wistar rats were assigned randomly to a control (C; n = 6) or exercise training (T; n = 6) group. After T animals were trained on an animal treadmill for 9 weeks, alpha-actinin-2 and alpha-actinin-3 levels in the plantaris, white and red gastrocnemius muscles were analysed. In addition, changes in the myosin heavy chain (MyHC) composition were assessed, and muscle bioenergetic enzyme activities were measured.

RESULTS

Results show that exercise training increased alpha-actinin-2 expression levels in all muscles (P < 0.05). However, no significant difference was found in alpha-actinin-3 expression levels between C and T animals. Subsequent MyHC analyses of all muscle showed an MyHC shift with direction from IIb to IIa. Furthermore, enzymatic analysis revealed that exercise training improved enzyme activities related to aerobic metabolism.

CONCLUSION

The results of this study demonstrate that exercise training alters the expression level of alpha-actinin at the isoform level. Moreover, the increase in expression levels of alpha-actinin-2 is apparently related to alteration of skeletal muscle: its aerobic capacity is improved.

A genome-wide survey of the prevalence and evolutionary forces acting on human nonsense SNPs

Nonsense SNPs introduce premature termination codons into genes and can result in the absence of a gene product or in a truncated and potentially harmful protein, so they are often considered disadvantageous and are associated with disease susceptibility. As such, we might expect the disrupted allele to be rare and, in healthy people, observed only in a heterozygous state. However, some, like those in the CASP12 and ACTN3 genes, are known to be present at high frequencies and to occur often in a homozygous state and seem to have been advantageous in recent human evolution. To evaluate the selective forces acting on nonsense SNPs as a class, we have carried out a large-scale experimental survey of nonsense SNPs in the human genome by genotyping 805 of them (plus control synonymous SNPs) in 1,151 individuals from 56 worldwide populations. We identified 169 genes containing nonsense SNPs that were variable in our samples, of which 99 were found with both copies inactivated in at least one individual. We found that the sampled humans differ on average by 24 genes (out of about 20,000) because of these nonsense SNPs alone. As might be expected, nonsense SNPs as a class were found to be slightly disadvantageous over evolutionary timescales, but a few nevertheless showed signs of being possibly advantageous, as indicated by unusually high levels of population differentiation, long haplotypes, and/or high frequencies of derived alleles. This study underlines the extent of variation in gene content within humans and emphasizes the importance of understanding this type of variation.

Strength, power, fiber types, and mRNA expression in trained men and women with different ACTN3 R577X genotypes

Alpha-actinins are structural proteins of the Z-line. Human skeletal muscle expresses two alpha-actinin isoforms, alpha-actinin-2 and alpha-actinin-3, encoded by their respective genes ACTN2 and ACTN3. ACTN2 is expressed in all muscle fiber types, while only type II fibers, and particularly the type IIb fibers, express ACTN3. ACTN3 (R577X) polymorphism results in loss of alpha-actinin-3 and has been suggested to influence skeletal muscle function. The X allele is less common in elite sprint and power athletes than in the general population and has been suggested to be detrimental for performance requiring high power. The present study investigated the association of ACTN3 genotype with muscle power during 30-s Wingate cycling in 120 moderately to well-trained men and women and with knee extensor strength and fatigability in a subset of 21 men performing isokinetic exercise. Muscle biopsies were obtained from the vastus lateralis muscle to determine fiber-type composition and ACTN2 and ACTN3 mRNA levels. Peak and mean power and the torque-velocity relationship and fatigability output showed no difference across ACTN3 genotypes. Thus this study suggests that R577X polymorphism in ACTN3 is not associated with differences in power output, fatigability, or force-velocity characteristics in moderately trained individuals. However, repeated exercise bouts prompted an increase in peak torque in RR but not in XX genotypes, suggesting that ACTN3 genotype may modulate responsiveness to training. Our data further suggest that alpha-actinins do not play a significant role in determining muscle fiber-type composition. Finally, we show that ACTN2 expression is affected by the content of alpha-actinin-3, which implies that alpha-actinin-2 may compensate for the lack of alpha-actinin-3 and hence counteract the phenotypic consequences of the deficiency.

Why is alpha-actinin-3 deficiency so common in the general population? The evolution of athletic performance

'We can now explain how this common genetic variation influences athletic performance as well as why it has become so common in the general population. There is a fascinating link between factors that influence survival in ancient humans and the factors that contribute to athletic abilities in modern man.' The human ACTN3 gene encodes the protein alpha-actinin-3, a component of the contractile apparatus in fast skeletal muscle fibers. In 1999, we identified a common polymorphism in ACTN3 (R577X) that results in absence of alpha-actinin-3 in more than one billion people worldwide, despite the ACTN3 gene being highly conserved during human evolution. In 2003, we demonstrated that ACTN3 genotype influences elite athletic performance, and the association between ACTN3 genotype and skeletal muscle performance has since been replicated in athletes and non-athlete cohorts. We have also studied the evolution of the R577X allele during human evolution and demonstrated that the null (X) allele has undergone strong, recent positive selection in Europeans and Asian populations. We have developed an Actn3 knockout mouse model that replicates alpha-actinin-3 deficiency in humans and has already provided insight into the role of alpha-actinin-3 in the regulation of skeletal muscle metabolism, fibre size, muscle mass and contractile properties. In particular, mouse muscle lacking alpha-actinin-3 uses energy more efficiently, with the fast fibers displaying metabolic and contractile properties of slow oxidative fibers. While this favors endurance activities, the trade off is that the muscle cannot generate the rapid contractions needed to excel in sprinting. We propose that the shift towards more efficient aerobic muscle metabolism associated with alpha-actinin-3 deficiency also underlies the adaptive benefit of the 577X allele. Our future studies will focus on the effect of ACTN3 genotype on response to exercise and ageing, and the onset and severity of muscle disease phenotype.

alpha-Actinin interacts with rapsyn in agrin-stimulated AChR clustering

AChR is concentrated at the postjunctional membrane at the neuromuscular junction. However, the underlying mechanism is unclear. We show that α-actinin, a protein known to cross-link F-actin, interacts with rapsyn, a scaffold protein essential for neuromuscular junction formation. α-Actinin, rapsyn, and surface AChR form a ternary complex. Moreover, the rapsyn-α-actinin interaction is increased by agrin, a factor known to stimulate AChR clustering. Downregulation of α-actinin expression inhibits agrin-mediated AChR clustering. Furthermore, the rapsyn-α-actinin interaction can be disrupted by inhibiting Abl and by cholinergic stimulation. Together these results indicate a role for α-actinin in AChR clustering.

ACTN3 genotype is associated with muscle phenotypes in women across the adult age span

The R577X polymorphism in the alpha-actinin-3 encoding gene (ACTN3) has been associated with elite athletic performance, and recently with differences in isometric and dynamic muscle strength and power in the general population. In this study we sought to determine the association of ACTN3 R577X genotype with muscle strength and mass phenotypes in men and women across the adult age span. Eight hundred forty-eight (n = 848) adult volunteers (454 men and 394 women) aged 22-90 yr were genotyped for ACTN3 R577X. Knee extensor (KE) shortening and lengthening peak torque values were determined using isokinetic dynamometry and fat-free mass (FFM) by dual-energy X-ray absorptiometry. Women deficient in alpha-actinin-3 (X/X; n = 53) displayed lower KE shortening peak torque (30 degrees /s: 89.5 +/- 3.5 vs. 99.3 +/- 1.4 N.m, P = 0.011; 180 degrees /s: 60.3 +/- 2.6 vs. 67.0 +/- 1.0 N.m, P = 0.019) and KE lengthening peak torque (30 degrees /s: 122.8 +/- 5.7 vs. 137.0 +/- 2.2 N.m, P = 0.022; 180 degrees /s: 121.8 +/- 5.8 vs. 138.5 +/- 2.2 N.m, P = 0.008) compared with R/X + R/R women (n = 341). Women X/X homozygotes also displayed lower levels of both total body FFM (38.9 +/- 0.5 vs. 40.1 +/- 0.2 kg, P = 0.040) and lower limb FFM (11.9 +/- 0.2 vs. 12.5 +/- 0.1 kg, P = 0.044) compared with R/X + R/R women. No genotype-related differences were observed in men. In conclusion, our results indicate that the absence of alpha-actinin-3 protein (i.e., ACTN3 X/X genotype) influences KE peak torque and FFM in women but not men.

A gene for speed: contractile properties of isolated whole EDL muscle from an alpha-actinin-3 knockout mouse

The actin-binding protein alpha-actinin-3 is one of the two isoforms of alpha-actinin that are found in the Z-discs of skeletal muscle. alpha-Actinin-3 is exclusively expressed in fast glycolytic muscle fibers. Homozygosity for a common polymorphism in the ACTN3 gene results in complete deficiency of alpha-actinin-3 in about 1 billion individuals worldwide. Recent genetic studies suggest that the absence of alpha-actinin-3 is detrimental to sprint and power performance in elite athletes and in the general population. In contrast, alpha-actinin-3 deficiency appears to be beneficial for endurance athletes. To determine the effect of alpha-actinin-3 deficiency on the contractile properties of skeletal muscle, we studied isolated extensor digitorum longus (fast-twitch) muscles from a specially developed alpha-actinin-3 knockout (KO) mouse. alpha-Actinin-3-deficient muscles showed similar levels of damage to wild-type (WT) muscles following lengthening contractions of 20% strain, suggesting that the presence or absence of alpha-actinin-3 does not significantly influence the mechanical stability of the sarcomere in the mouse. alpha-Actinin-3 deficiency does not result in any change in myosin heavy chain expression. However, compared with alpha-actinin-3-positive muscles, alpha-actinin-3-deficient muscles displayed longer twitch half-relaxation times, better recovery from fatigue, smaller cross-sectional areas, and lower twitch-to-tetanus ratios. We conclude that alpha-actinin-3 deficiency results in fast-twitch, glycolytic fibers developing slower-twitch, more oxidative properties. These changes in the contractile properties of fast-twitch skeletal muscle from alpha-actinin-3-deficient individuals would be detrimental to optimal sprint and power performance, but beneficial for endurance performance.

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) binds to alpha-actinin 1: novel pathways in skeletal muscle?

Background

Hereditary inclusion body myopathy (HIBM) is a rare neuromuscular disorder caused by mutations in GNE, the key enzyme in the biosynthetic pathway of sialic acid. While the mechanism leading from GNE mutations to the HIBM phenotype is not yet understood, we searched for proteins potentially interacting with GNE, which could give some insights about novel putative biological functions of GNE in muscle.

Methodology/Principal Findings

We used a Surface Plasmon Resonance (SPR)-Biosensor based assay to search for potential GNE interactors in anion exchanged fractions of human skeletal muscle primary culture cell lysate. Analysis of the positive fractions by in vitro binding assay revealed α-actinin 1 as a potential interactor of GNE. The direct interaction of the two proteins was assessed in vitro by SPR-Biosensor based kinetics analysis and in a cellular environment by a co-immunoprecipitation assay in GNE overexpressing 293T cells. Furthermore, immunohistochemistry on stretched mouse muscle suggest that both GNE and α-actinin 1 localize to an overlapping but not identical region of the myofibrillar apparatus centered on the Z line.

Conclusions/Significance

The interaction of GNE with α-actinin 1 might point to its involvement in α-actinin mediated processes. In addition these studies illustrate for the first time the expression of the non-muscle form of α-actinin, α-actinin 1, in mature skeletal muscle tissue, opening novel avenues for its specific function in the sarcomere. Although no significant difference could be detected in the binding kinetics of α-actinin 1 with either wild type or mutant GNE in our SPR biosensor based analysis, further investigation is needed to determine whether and how the interaction of GNE with α-actinin 1 in skeletal muscle is relevant to the putative muscle-specific function of α-actinin 1, and to the muscle-restricted pathology of HIBM.

Alpha-actinin-3 levels increase concomitantly with fast fibers in rat soleus muscle

Alpha (alpha)-actinin-3 is located in the skeletal muscle Z-line and forms actin-actin crosslinks. An interesting property of alpha-actinin-3 is its expression pattern, which is restricted to fast type II skeletal muscle fibers. However, little is known about the response of alpha-actinin-3 levels to changes in skeletal muscle such as fiber type transformation. This study examined alpha-actinin-3 levels in the soleus muscles of rats subjected to hindlimb unloading, which causes a slow-to-fast fiber transformation in the soleus muscle. After unloading, type II myosin heavy chain (MyHC) and fast myosin levels increased significantly (P<0.0001 for type II MyHC, P<0.005 for fast myosin). Along with these increases in fast fibers, alpha-actinin-3 expression levels increased significantly (P<0.0007) and dramatically. These results indicate that alpha-actinin-3 levels increase concomitantly with increases in skeletal muscle fast fibers.

Modulation of the dystrophin-associated protein complex in response to resistance training in young and older men

The dystrophin-associated protein complex (DAPC) is a scaffold of proteins linking the intracellular cytoskeleton with the extracellular matrix that is integral to structural stability and integrity, signaling and mechanotransduction, and force transmission. We hypothesized that the expression of DAPC component proteins would be altered by resistance loading during progressive resistance training (PRT)-mediated myofiber hypertrophy, and we investigated whether aging influenced these changes. Seventeen young (27 yr) and 13 older (65 yr) men completed 16 wk of PRT with muscle biopsies at baseline (T1), 24 h after bout 1 (T2), and 24 h after the final bout at week 16 (T3). Myofiber hypertrophy in the young (type I 31%, P < 0.005; type II 40%, P < 0.001) far exceeded hypertrophy in the old (type II only, 19.5%, P < 0.05). PRT altered protein expression for caveolin-3 (decreased 24% by T3, P < 0.01), alpha(1)-syntrophin (increased 16% by T3, P < 0.05), alpha-dystrobrevin (fell 23% from T2 to T3, P < 0.01), and dystrophin [rose acutely (30% by T2, P < 0.05) and returned to baseline by T3]. The phosphorylation state of membrane neuronal nitric oxide synthase (Ser(1417)) decreased 70% (P < 0.005) by T3, particularly in the old (81%), whereas p38 MAPK phosphorylation increased twofold by T3 in the old (P < 0.01). We conclude that component proteins of the DAPC are modulated by PRT, which may serve to improve both structural and signaling functions during load-mediated myofiber hypertrophy. The blunted hypertrophic adaptation seen in old vs. young men may have resulted from overstress, as suggested by marked p38 MAPK activation in old men only.

Gene doping

Gene doping is the misuse of gene therapy to enhance athletic performance. It has recently been recognised as a potential threat and subsequently been prohibited by the World Anti-Doping Agency. Despite concerns with safety and efficacy of gene therapy, the technology is progressing steadily. Many of the genes/proteins which are involved in determining key components of athletic performance have been identified. Naturally occurring mutations in humans as well as gene-transfer experiments in adult animals have shown that altered expression of these genes does indeed affect physical performance. For athletes, however, the gains in performance must be weighed against the health risks associated with the gene-transfer process, whereas the detection of such practices will provide new challenges for the anti-doping authorities.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

ACTN3 (R577X) genotype is associated with fiber type distribution

α-Actinin-3 is a Z-disc structural protein found only in type II muscle fibers. The X allele of the R577X polymorphism in the ACTN3 gene results in a premature stop codon and α-actinin-3 deficiency in XX homozygotes. Associations between the R577X polymorphism and the muscle-power performance of elite athletes have been described earlier. About 45% of the fiber type proportions are determined by genetic factors. The ACTN3 variant could be one of the contributing genes in the heritability of fiber type distribution through its interaction with calcineurin. The aim of this study was to quantify the association between the polymorphism and muscle fiber type distribution and fast-velocity knee extension strength. Ninety healthy young men (18–29 y) were genotyped for ACTN3 R577X. Knee extensor strength was measured isometrically (45°) and at different dynamic velocities (100–300°/s) on a programmable dynamometer. Twenty-two XX and twenty-two RR subjects underwent a biopsy of the right vastus lateralis muscle. Fiber type composition was determined by immunohistochemistry. Homozygotes for the R allele show significantly higher relative dynamic quadriceps torques at 300°/s, compared with XX carriers ( P < 0.05). Fiber type characteristics differed significantly between the two genotype groups. The percentage surface and number of type IIx fibers were greater in the RR than the XX genotype group ( P < 0.05), and α-actinin-3 protein content is systematically higher in type IIx compared with type IIa fibers (staining intensity ratio IIx to IIa = 1.17). This study shows that the mechanism, by which the ACTN3 polymorphism has its effect on muscle power, might rely on a control function of fiber type proportions.

The 577X allele of the ACTN3 gene is associated with improved exercise capacity in women with McArdle's disease

We assessed the possible association existing between alpha-actinin-3 (ACTN3) R577X genotypes and the capacity for performing aerobic exercise in McArdle's patients. Forty adult McArdle's disease patients and forty healthy, age and gender-matched sedentary controls (21 men, 19 women in both groups) performed a graded test until exhaustion and a constant-load test on a cycle-ergometer to determine clinically relevant indices of exercise capacity as peak oxygen uptake (VO(2peak)) and the ventilatory threshold (VT). In the group of diseased women, carriers of the X allele had a higher (P<0.01) VO(2peak) (15.0+/-1.2 ml/kg/min) and a higher (P<0.05) oxygen uptake (VO(2)) at the VT (11.2+/-1 ml/kg/min) than R/R homozygotes (VO(2peak): 9.6+/-0.5 ml/kg/min; VO(2) at the VT: 8.2+/-0.7 ml/kg/min). No differences were found in male patients. In women with McArdle's disease, ACTN3 genotypes might partly explain the large individual variability that exists in the phenotypic manifestation of this disorder.

Polimorfismos genéticos determinantes da performance física em atletas de elite

Este artigo direciona-se à revisão de publicações sobre os "genes candidatos" e sua relação com os fenótipos de performance física humana em atletas de elite. Nosso objetivo é trazer ao conhecimento do leitor informações atualizadas sobre marcadores e variantes genéticas que podem levar certos indivíduos a sobressair-se em modalidades esportivas específicas. Além disso, serão descritos os mecanismos pelos quais um gene pode contribuir para a performance física, detalhando em cada momento as propriedades celulares, fisiológicas e moleculares do sistema em questão. Por esse motivo, limitamos nossa discussão a um número pequeno de variantes genéticas: polimorfismos R577X do gene da alfa-actinina 3 (ACTN3), C34T do gene da AMP deaminase (AMPD1), I/D da enzima conversora de angiotensina (ECA), -9/+9 do receptor beta2 de bradicinina (BDKRB2) e 985+185/1170 do gene da enzima creatina quinase M (CK-M). Esperamos com este artigo informar e sensibilizar o leitor para o fato de que a identificação de talentos e a otimização do potencial individual do atleta, com conseqüente sucesso no esporte, estão diretamente associados a variantes genéticas.

Alpha-actinin-3 (ACTN3) R577X polymorphism influences knee extensor peak power response to strength training in older men and women

BACKGROUND

The alpha-actinin-3 (ACTN3) R577X polymorphism has been associated with muscle power performance in cross-sectional studies.

METHODS

We examined baseline knee extensor concentric peak power (PP) and PP change with approximately 10 weeks of unilateral knee extensor strength training (ST) using air-powered resistance machines in 71 older men (65 [standard deviation = 8] years) and 86 older women (64 [standard deviation = 9] years).

RESULTS

At baseline in women, the XX genotype group had an absolute (same resistance) PP that was higher than the RR (p =.005) and RX genotype groups (p =.02). The women XX group also had a relative (70% of one-repetition maximum [1-RM]) PP that was higher than that in the RR (p =.002) and RX groups (p =.008). No differences in baseline absolute or relative PP were observed between ACTN3 genotype groups in men. In men, absolute PP change with ST in the RR (n = 16) group approached a significantly higher value than in the XX group (n = 9; p =.07). In women, relative PP change with ST in the RR group (n = 16) was higher than in the XX group (n = 17; p =.02).

CONCLUSIONS

The results indicate that the ACTN3 R577X polymorphism influences the response of quadriceps muscle power to ST in older adults.

ACTN3 Genotyping by Real-Time PCR in the Italian Population and Athletes

PURPOSE

Development of two novel sets of primers and probes to detect R577X and Q523R polymorphisms of the alpha-actinin-3 (ACTN3) gene by real-time PCR. We report the allelic frequencies observed in Italian individuals from the general population and athletes. Athletic performance is influenced by training, environmental factors, and genetic predisposition. Actn3 belongs to a family of actin-binding proteins and is supposed to influence sport performance.

METHODS

Primer-probe set design and protocol optimization for real-time PCR genotyping of R577X and Q523R polymorphisms. The assay was verified using a traditional PCR-RFLP approach and applied on an Italian population sample (102 male subjects and 42 athletes).

RESULTS

Haplotype distribution confirmed the presence of linkage disequilibrium between the polymorphisms, both in the Italian general population and athletes (respectively: chi = 54.4, P < or = 0.001 and chi = 24.5, P < or = 0.001). Within the general population, a large percentage of homozygous subjects (21.6%) was deficient for Actn3. No significant differences were observed in athletes. The concordance between PCR-RFLP and real-time PCR results was 100and 93% for polymorphisms Q523R and R577X, respectively.

CONCLUSION

Real-time PCR represents an effective approach for typing ACTN3 alleles. Allelic frequencies in the Italian population are consistent with those seen in other studies on Caucasians.

Citius and longius (faster and longer) with no alpha-actinin-3 in skeletal muscles?

The muscle protein alpha-actinin-3 (ACTN3) is normally thought to be expressed in type II muscle fibres and to be necessary for high-power, high-velocity muscle contractions, such as those typically seen in speed/power athletes. The authors report the case of a Spanish elite long jumper (two times Olympian, personal best of 8.26 m) whose genotype for the ACTN3 gene is 577XX (ACTN3 deficient). These data suggest that there might be notable exceptions to the concept that ACTN3 is the "gene for speed".

ACTN3

A common variant of the ACTN3 gene, R577X, results in complete deficiency of the alpha-actinin-3 protein in the fast skeletal muscle fibers of more than a billion humans worldwide. We review the evidence that this genetic variant is strongly associated with elite athlete status and with normal variation in human muscle strength and sprinting speed.

Association analysis of the ACTN3 R577X polymorphism and complex quantitative body composition and performance phenotypes in adolescent Greeks

The functional allele (577R) of ACTN3, which encodes human alpha-actinin-3, has been reported to be associated with elite athletic status and with response to resistance training, while the nonfunctional allele (577X) has been proposed as a candidate metabolically thrifty allele. In a study of 992 adolescent Greeks, we show that there is a significant association (P=0.003) between the ACTN3 R577X polymorphism and 40 m sprint time in males that accounts for 2.3% of phenotypic variance, with the 577R allele contributing to faster times in an additive manner. The R577X polymorphism is not associated with other power phenotypes related to 40 m sprint, nor with an endurance phenotype. Furthermore, the polymorphism is not associated with obesity-related phenotypes in our population, suggesting that the 577X allele is not a thrifty allele, and thus the persistence of this null allele must be explained in other terms.

Physiological characteristics of the best Eritrean runners—exceptional running economy

Despite their young age, limited training history, and lack of running tradition compared with other East African endurance athletes (e.g., Kenyans and Ethiopians), male endurance runners from Eritrea have recently attained important running successes. The purposes of our study were (i) to document the main physical and physiological characteristics of elite black Eritrean distance runners (n = 7; age: 22 ± 3 years) and (ii) to compare them with those of their elite white Spanish counterparts. For this second purpose we selected a control group of elite Spanish runners (n = 9; 24 ± 2 years), owing to the traditionally high success of Spanish athletes in long-distance running compared with other white runners, especially in cross-country competitions. The subjects’ main anthropometric characteristics were determined, together with their maximum oxygen uptake (VO2 max) and VO2 (mL·kg–1·min–1), blood lactate, and ammonia concentrations while running at 17, 19, or 21 km·h–1. The body mass index (18.9 ± 1.5 kg·m–2) and maximal calf circumference (30.9 ± 1.5 cm) was lower in Eritreans than in Spaniards (20.5 ± 1.7 kg·m–2 and 33.9 ± 2.0 cm, respectively) (p < 0.05 and p < 0.01, respectively) and their lower leg (shank) length was longer (44.1 ± 3.0 cm vs. 40.6 ± 2.7 cm, respectively) (p < 0.05). VO2 max did not differ significantly between Eritreans and Spaniards (73.8 ± 5.6 mL·kg–1·min–1 vs. 77.8 ± 5.7 mL·kg–1·min–1, respectively), whereas the VO2 cost of running was lower (p < 0.01) in the former (e.g., 65.9 ± 6.8 mL·kg–1·min–1 vs. 74.8  ± 5.0 mL·kg–1·min–1 when running at 21 km·h–1). Our data suggest that the excellent running economy of Eritreans is associated, at least partly, with anthropometric variables. Comparison of their submaximal running cost with other published data suggests that superior running economy, rather than enhanced aerobic capacity, may be the common denominator in the success of black endurance runners of East African origin.

Brain-specific regulator of G-protein signaling 9-2 selectively interacts with alpha-actinin-2 to regulate calcium-dependent inactivation of NMDA receptors

Regulator of G-protein signaling 9-1 (RGS9-1) and RGS9-2 are highly related RGS proteins with distinctive C termini arising from alternative splicing of RGS9 gene transcripts. RGS9-1 is expressed in photoreceptors where it functions as a regulator of transducin. In contrast, RGS9-2 is abundantly expressed in the brain, especially in basal ganglia, where its specific function remains poorly understood. To gain insight into the function of RGS9-2, we screened a human cDNA library for potential interacting proteins. This screen identified a strong interaction between RGS9-2 and alpha-actinin-2, suggesting a possible functional relationship between these proteins. Consistent with this idea, RGS9-2 and alpha-actinin-2 coimmunoprecipitated after coexpression in human embryonic kidney 293 (HEK-293) cells. Furthermore, endogenous RGS9-2 and alpha-actinin-2 could also be coimmunoprecipitated from extracts of rat striatum, an area highly enriched in both these proteins. These results supported the idea that RGS9-2 and alpha-actinin-2 could act in concert in central neurons. Like alpha-actinin-2, RGS9-2 coimmunoprecipitated NMDA receptors from striatal extracts, suggesting an interaction between RGS9-2, alpha-actinin-2, and NMDA receptors. Previous studies have shown that alpha-actinin mediates calcium-dependent inactivation of NMDA receptors. In HEK-293 cells expressing NMDA receptors, expression of RGS9-2 significantly modulated this form of NMDA receptor inactivation. Furthermore, this modulation showed remarkable preference for NMDA receptor inactivation mediated by alpha-actinin-2. Using a series of deletion constructs, we localized this effect to the RGS domain of the protein. These results identify an unexpected functional interaction between RGS9-2 and alpha-actinin-2 and suggest a potential novel role for RGS9-2 in the regulation of NMDA receptor function.

Does complete deficiency of muscle alpha actinin 3 alter functional capacity in elderly women? A preliminary report

The sarcomeric protein alpha actinin 3 is localised to the Z line of fast fibres, which are responsible for generating forceful muscle contractions at high velocity. However, a substantial proportion of healthy humans are totally deficient in this protein as they are homozygous for a premature stop codon polymorphism (R577X) in the ACTN3 gene. The purpose of this preliminary study was to assess if the presence or absence of alpha actinin 3 influences the deleterious effects of ageing on muscle output and functional capacity.

Isoform diversity of giant proteins in relation to passive and active contractile properties of rabbit skeletal muscles

The active and passive contractile performance of skeletal muscle fibers largely depends on the myosin heavy chain (MHC) isoform and the stiffness of the titin spring, respectively. Open questions concern the relationship between titin-based stiffness and active contractile parameters, and titin's importance for total passive muscle stiffness. Here, a large set of adult rabbit muscles (n = 37) was studied for titin size diversity, passive mechanical properties, and possible correlations with the fiber/MHC composition. Titin isoform analyses showed sizes between ∼3300 and 3700 kD; 31 muscles contained a single isoform, six muscles coexpressed two isoforms, including the psoas, where individual fibers expressed similar isoform ratios of 30:70 (3.4:3.3 MD). Gel electrophoresis and Western blotting of two other giant muscle proteins, nebulin and obscurin, demonstrated muscle type–dependent size differences of ≤70 kD. Single fiber and single myofibril mechanics performed on a subset of muscles showed inverse relationships between titin size and titin-borne tension. Force measurements on muscle strips suggested that titin-based stiffness is not correlated with total passive stiffness, which is largely determined also by extramyofibrillar structures, particularly collagen. Some muscles have low titin-based stiffness but high total passive stiffness, whereas the opposite is true for other muscles. Plots of titin size versus percentage of fiber type or MHC isoform (I-IIB-IIA-IID) determined by myofibrillar ATPase staining and gel electrophoresis revealed modest correlations with the type I fiber and MHC-I proportions. No relationships were found with the proportions of the different type II fiber/MHC-II subtypes. Titin-based stiffness decreased with the slow fiber/MHC percentage, whereas neither extramyofibrillar nor total passive stiffness depended on the fiber/MHC composition. In conclusion, a low correlation exists between the active and passive mechanical properties of skeletal muscle fibers. Slow muscles usually express long titin(s), predominantly fast muscles can express either short or long titin(s), giving rise to low titin-based stiffness in slow muscles and highly variable stiffness in fast muscles. Titin contributes substantially to total passive stiffness, but this contribution varies greatly among muscles.

Phosphorylation-dependent translocation of glycogen synthase to a novel structure during glycogen resynthesis

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood. It is well accepted that the rate-limiting enzymes in glycogenesis and glycogenolysis are glycogen synthase (GS) and glycogen phosphorylase (GPh), respectively. Both enzymes are regulated by reversible phosphorylation and by allosteric effectors. However, evidence in the literature indicates that changes in muscle GS and GPh intracellular distribution may constitute a new regulatory mechanism of glycogen metabolism. Already in the 1960s, it was proposed that glycogen was present in dynamic cellular organelles that were termed glycosomas but no such cellular entities have ever been demonstrated. The aim of this study was to characterize muscle GS and GPh intracellular distribution and to identify possible translocation processes of both enzymes. Using in situ stimulation of rabbit tibialis anterior muscle, we show GS and GPh intracellular redistribution at the beginning of glycogen resynthesis after contraction-induced glycogen depletion. We identify a new "player," a new intracellular compartment involved in skeletal muscle glycogen metabolism. They are spherical structures that were not present in basal muscle, and we present evidence that indicate that they are products of actin cytoskeleton remodeling. Furthermore, for the first time, we show a phosphorylation-dependent intracellular distribution of GS. Here, we present evidence of a new regulatory mechanism of skeletal muscle glycogen metabolism based on glycogen enzyme intracellular compartmentalization.

Genes and human elite athletic performance

Physical fitness is a complex phenotype influenced by a myriad of environmental and genetic factors, and variation in human physical performance and athletic ability has long been recognised as having a strong heritable component. Recently, the development of technology for rapid DNA sequencing and genotyping has allowed the identification of some of the individual genetic variations that contribute to athletic performance. This review will examine the evidence that has accumulated over the last three decades for a strong genetic influence on human physical performance, with an emphasis on two sets of physical traits, viz. cardiorespiratory and skeletal muscle function, which are particularly important for performance in a variety of sports. We will then review recent studies that have identified individual genetic variants associated with variation in these traits and the polymorphisms that have been directly associated with elite athlete status. Finally, we explore the scientific implications of our rapidly growing understanding of the genetic basis of variation in performance.

ACTN3 genotype is associated with increases in muscle strength in response to resistance training in women

The alpha-actinin 3 (ACTN3) gene encodes a protein of the Z disk of myofibers, and a polymorphism of ACTN3 results in complete loss of the protein. The ACTN3 genotype (R577X) has been found to be associated with performance in Australian elite athletes (Yang N, MacArthur DG, Gulbin JP, Hahn AG, Beggs AH, Easteal S, and North K. Am J Hum Genet 73: 627-631, 2003). We studied associations between ACTN3 genotype and muscle size [cross-sectional area of the biceps brachii via magnetic resonance imaging (MRI)] and elbow flexor isometric (MVC) and dynamic [1-repetition maximum (1-RM)] strength in a large group of men (N = 247) and women (N = 355) enrolled in a 12-wk standardized elbow flexor/extensor resistance training program of the nondominant arm at one of eight study centers. We found no association between ACTN3 R577X genotype and muscle phenotype in men. However, women homozygous for the ACTN3 577X allele (XX) had lower baseline MVC compared with heterozygotes (P < 0.05) when adjusted for body mass and age. Women homozygous for the mutant allele (577X) demonstrated greater absolute and relative 1-RM gains compared with the homozygous wild type (RR) after resistance training when adjusted for body mass and age (P < 0.05). There was a trend for a dose-response with genotype such that gains were greatest for XX and least for RR. Significant associations were validated in at least one ethnic subpopulation (Caucasians, Asians) and were independent of training volume. About 2% of baseline MVC and of 1-RM strength gain after training were attributable to ACTN3 genotype (likelihood-ratio test P value, P = 0.01), suggesting that ACTN3 is one of many genes contributing to genetic variation in muscle performance and adaptation to exercise.

Layer-specific differences of gene expression in extraocular muscles identified by laser-capture microscopy

In mammals, separate muscles are typically specialized as a whole to provide distinct functional roles leading to well-recognized adaptations. This is exemplified in the lower limb by the slow, fatigue-resistant soleus, which provides a postural role vs. the fast, fatiguable tibialis anterior (TA), which provides rapid movements. A unique characteristic of extraocular muscles (EOMs) is their compartmentalization into two distinct layers, the orbital layer (OL) and global layer (GL), presumably to subserve diverse functions within the same muscle. However, molecular evidence of this diversity has been limited. We used laser-capture microscopy coupled with microarray-based expression profiling to identify molecular differences between the OL and GL of rat EOMs. We found that 210 genes were differentially regulated between these layers at a twofold expression cutoff. Differences in genes related to metabolic pathways and related to structural elements of muscle and nerve formed the largest functional clusters. Layer-specific differential expression was validated at both mRNA and protein level for MYH3, MYH6, and ACTN3. The expected layer-specific differences among genes encoding vascular elements were not evident by profiling; morphometric analysis demonstrated that the differences exist, but at a magnitude below the cutoff level established by our statistical methods. Comparison of these results with previous results comparing whole EOMs and TA suggest evolutionary mechanisms may play a role in achieving functional distinctions between OL and GL.