Sarcomeric α-actinins and their role in human muscle disease

Muscular Strength and Quality of Life in Older Adults: The Role of ACTN3 R577X Polymorphism

As longevity is increasing, the 65-year-old and older population is projected to increase in the next decades, as are the consequences of age-related muscle deterioration on the quality of life. The purpose of this study was to examine the associations of the ACTN3R577X polymorphism with quality of life and muscular strength in an older Spanish population. In total, 281 older adults participated in this study. Anthropometric measurements, chronic diseases, prescribed medications, quality of life, hand grip strength, and physical activity and nutritional status data were collected. ACTN3 R577X genotyping was determined using Taqman probes. Multivariate regression analysis revealed in adjusted model that, in men, the ACTN3 R577X genotype was significantly associated with hand grip strength (HGS), regression coefficient (β) = 1.23, p = 0.008, dimension 1 of the five-dimension questionnaire EuroQoL (EQ-5D, mobility), (β) = -1.44, p = 0.006, and clinical group risk (CGR) category (β) = -1.38, p = 0.006. In women, a marginal association between the ACTN3 R577X genotype and the CGR category was observed, with a regression coefficient of (β) = -0.97, (p = 0.024). Our findings suggest that the ACTN3 R577X genotype may influence the decline in muscle strength and quality of life in older Spanish adult males.

Genetic Associations with Aging Muscle: A Systematic Review

The age-related decline in skeletal muscle mass, strength and function known as 'sarcopenia' is associated with multiple adverse health outcomes, including cardiovascular disease, stroke, functional disability and mortality. While skeletal muscle properties are known to be highly heritable, evidence regarding the specific genes underpinning this heritability is currently inconclusive. This review aimed to identify genetic variants known to be associated with muscle phenotypes relevant to sarcopenia. PubMed, Embase and Web of Science were systematically searched (from January 2004 to March 2019) using pre-defined search terms such as "aging", "sarcopenia", "skeletal muscle", "muscle strength" and "genetic association". Candidate gene association studies and genome wide association studies that examined the genetic association with muscle phenotypes in non-institutionalised adults aged ≥50 years were included. Fifty-four studies were included in the final analysis. Twenty-six genes and 88 DNA polymorphisms were analysed across the 54 studies. The ACTN3, ACE and VDR genes were the most frequently studied, although the IGF1/IGFBP3, TNFα, APOE, CNTF/R and UCP2/3 genes were also shown to be significantly associated with muscle phenotypes in two or more studies. Ten DNA polymorphisms (rs154410, rs2228570, rs1800169, rs3093059, rs1800629, rs1815739, rs1799752, rs7412, rs429358 and 192 bp allele) were significantly associated with muscle phenotypes in two or more studies. Through the identification of key gene variants, this review furthers the elucidation of genetic associations with muscle phenotypes associated with sarcopenia.

ACTN3 genotype and physical function and frailty in an elderly Chinese population: the Rugao Longevity and Ageing Study

Objective

To examine the associations of the actinin alpha 3 gene (ACTN3) R577X polymorphism with physical performance and frailty in an older Chinese population.

Methods

Data from 1,463 individuals (57.8% female) aged 70-87 years from the Rugao Longevity and Ageing Study were used. The associations between R577X and timed 5-m walk, grip strength, timed Up and Go test, and frailty index (FI) based on deficits of 23 laboratory tests (FI-Lab) were examined. Analysis of variance and linear regression models were used to evaluate the genetic effects of ACTN3 R577X on physical performance and FI-Lab.

Results

The XX and RX genotypes of the ACTN3 R557X polymorphism accounted for 17.1 and 46.9%, respectively. Multivariate regression analysis revealed that in men aged 70-79 years, the ACTN3 577X allele was significantly associated with physical performance (5-m walk time, regression coefficient (β) = 0.258, P = 0.006; grip strength, β = -1.062, P = 0.012; Up and Go test time β = 0.368, P = 0.019). In women aged 70-79 years, a significant association between the ACTN3 577X allele and the FI-Lab score was observed, with a regression coefficient of β = 0.019 (P = 0.003). These findings suggest an age- and gender-specific X-additive model of R577X for 5-m walk time, grip strength, Up and Go Test time, and FI-Lab score.

Conclusion

The ACTN3 577X allele is associated with an age- and sex-specific decrease in physical performance and an increase in frailty in an older population.

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.

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

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

ACTN3 genotype influences muscle performance through the regulation of calcineurin signaling

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

Molecular genetic studies of gene identification for sarcopenia

Sarcopenia, which is characterized by a progressive decrease of skeletal muscle mass and function with aging, is closely related to several common diseases (such as cardiovascular and airway diseases) and functional impairment/disability. Strong genetic determination has been reported for muscle mass and muscle strength, two most commonly recognized and studied risk phenotypes for sarcopenia, with heritability ranging from 30 to 85% for muscle strength and 45-90% for muscle mass. Sarcopenia has been the subject of increasing genetic research over the past decade. This review is designed to comprehensively summarize the most important and representative molecular genetic studies designed to identify genetic factors associated with sarcopenia. We have methodically reviewed whole-genome linkage studies in humans, quantitative trait loci mapping in animal models, candidate gene association studies, newly reported genome-wide association studies, DNA microarrays and microRNA studies of sarcopenia or related skeletal muscle phenotypes. The major results of each study are tabulated for easy comparison and reference. The findings of representative studies are discussed with respect to their influence on our present understanding of the genetics of sarcopenia. This is a comprehensive review of molecular genetic studies of gene identification for sarcopenia, and an overarching theme for this review is that the currently accumulating results are tentative and occasionally inconsistent and should be interpreted with caution pending further investigation. Consequently, this overview should enhance recognition of the need to validate/replicate the genetic variants underlying sarcopenia in large human cohorts and animal. We believe that further progress in understanding the genetic etiology of sarcopenia will provide valuable insights into important fundamental biological mechanisms underlying muscle physiology that will ultimately lead to improved ability to recognize individuals at risk for developing sarcopenia and our ability to treat this debilitating condition.

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

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