Amorphin is phosphorylase; phosphorylase is an alpha-actinin-binding protein

Case report: A novel ACTA1 variant in a patient with nemaline rods and increased glycogen deposition

Background

Congenital myopathies are a group of heterogeneous inherited disorders, mainly characterized by early-onset hypotonia and muscle weakness. The spectrum of clinical phenotype can be highly variable, going from very mild to severe presentations. The course also varies broadly resulting in a fatal outcome in the most severe cases but can either be benign or lead to an amelioration even in severe presentations. Muscle biopsy analysis is crucial for the identification of pathognomonic morphological features, such as core areas, nemaline bodies or rods, nuclear centralizations and congenital type 1 fibers disproportion. However, multiple abnormalities in the same muscle can be observed, making more complex the myopathological scenario.

Case presentation

Here, we describe an Italian newborn presenting with severe hypotonia, respiratory insufficiency, inability to suck and swallow, requiring mechanical ventilation and gastrostomy feeding. Muscle biopsy analyzed by light microscopy showed the presence of vacuoles filled with glycogen, suggesting a metabolic myopathy, but also fuchsinophilic inclusions. Ultrastructural studies confirmed the presence of normally structured glycogen, and the presence of minirods, directing the diagnostic hypothesis toward a nemaline myopathy. An expanded Next Generation Sequencing analysis targeting congenital myopathies genes revealed the presence of a novel heterozygous c.965 T > A p. (Leu322Gln) variant in the ACTA1 gene, which encodes the skeletal muscle alpha-actin.

Conclusion

Our case expands the repertoire of molecular and pathological features observed in actinopathies. We highlight the value of ultrastructural examination to investigate the abnormalities detected at the histological level. We also emphasized the use of expanded gene panels in the molecular analysis of neuromuscular patients, especially for those ones presenting multiple bioptic alterations.

The ACTN3 R577X polymorphism is associated with metabolic alterations in a sex-dependent manner in subjects from western Mexico

BACKGROUND

The ACTN3 gene is primarily expressed in fast skeletal muscle fibres. A common nonsense polymorphism in this gene is ACTN3 R577X (rs1815739), which causes an absolute deficiency of α-actinin-3 protein and alterations in muscle metabolism. Considering metabolic alterations are influenced by nutrition and genetic factors, as well as lifestyle factors, we hypothesise a possible association of the ACTN3 R577X polymorphism with metabolic alterations.

METHODS

In this cross-sectional study, 397 adults met the inclusion criteria. Body composition was measured by electrical bioimpedance. Dietary data were analysed using Nutritionist Pro™ software. Biochemical variables were determined by dry chemistry. Genomic DNA was extracted from peripheral leukocytes and genotyping of the ACTN3 R577X polymorphism was determined by allelic discrimination using TaqMan probes. The statistical analyses were performed using SPSS statistical software. p < 0.05 was considered statistically significant.

RESULTS

The ACTN3 577XX genotype was associated with high glucose, triglyceride and very low density lipoprotein-cholesterol levels and a higher frequency of hypertriglyceridaemia and insulin resistance in women. In males, the genetic variant showed a trend towards significance for insulin resistance.

CONCLUSIONS

The ACTN3 R577X polymorphism was associated with metabolic alterations in women and a tendency was observed in men variant carriers. Thus, this common genetic variant could be implicated in the development of chronic metabolic diseases.

Mechanosignaling pathways alter muscle structure and function by post-translational modification of existing sarcomeric proteins to optimize energy usage

A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca²⁺ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.

Intracellular calcium leak lowers glucose storage in human muscle, promoting hyperglycemia and diabetes

Most glucose is processed in muscle, for energy or glycogen stores. Malignant Hyperthermia Susceptibility (MHS) exemplifies muscle conditions that increase [Ca2+]cytosol. 42% of MHS patients have hyperglycemia. We show that phosphorylated glycogen phosphorylase (GPa), glycogen synthase (GSa) - respectively activated and inactivated by phosphorylation - and their Ca2+-dependent kinase (PhK), are elevated in microsomal extracts from MHS patients' muscle. Glycogen and glucose transporter GLUT4 are decreased. [Ca2+]cytosol, increased to MHS levels, promoted GP phosphorylation. Imaging at ~100 nm resolution located GPa at sarcoplasmic reticulum (SR) junctional cisternae, and apo-GP at Z disk. MHS muscle therefore has a wide-ranging alteration in glucose metabolism: high [Ca2+]cytosol activates PhK, which inhibits GS, activates GP and moves it toward the SR, favoring glycogenolysis. The alterations probably cause these patients' hyperglycemia. For basic studies, MHS emerges as a variable stressor, which forces glucose pathways from the normal to the diseased range, thereby exposing novel metabolic links.

Sequence analysis and expression profiling of the equine ACTN3 gene during exercise in Arabian horses

The ACTN3 gene codes for α-actinin-3, a protein localized in the Z-line in the skeletal muscle. Actinin-3 is critical in anchoring the myofibrillar actin filaments and plays a key role in muscle contraction. ACTN3 (α-actinin-3) cross-links glycogen phosphorylase (GP), which is the key enzyme catalysing glycogen metabolism. The aim of present study was to establish the expression level of the ACTN3 gene (for both isoforms separately and together in the gene expression analysis) in the gluteus medius muscle in order to verify if the α-actinin-3 gene can be related to training intensity in Arabian horses. A structural analysis of the ACTN3 gene was performed simultaneously to identify polymorphisms that can be related to racing performance traits. Our results showed the significant decrease (p < 0.05) of ACTN3 expression in the skeletal muscle of Arabian horses during the training periods preparing for flat-racing, and this decrease differed by the intensity of the exercises. The highest mRNA abundance measured for all ACTN3 genes was detected in the muscle of untrained horses, while the lowest expression was identified at the end of the racing season when horses had fully adapted to the physical effort. This gene expression profile was confirmed for both ACTN3 isoforms. The analysis of the ACTN3 sequence allowed us to identify 14 polymorphisms, which were localized in the promoter region, the 5'UTR (7 SNPs), exons (2 SNPs) and introns (5 SNPs). Two of them, a novel c.2334C>T - splice variant and the g.1104G>A polymorphism in the promoter region, were proposed as the causative mutations that might affect gene expression. The presented gene expression analyses indicated the significant role of the ACTN3 gene in adaptation to physiological effort in horses. Due to previous reports and our findings, further studies should be conducted to verify the usage of the ACTN3 gene as a potential genetic marker for determining exercise performance in Arabian horses and other horse breeds.

Overview of the Muscle Cytoskeleton

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

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.

The 3T3-L1 adipocyte glycogen proteome

Background

Glycogen is a branched polysaccharide of glucose residues, consisting of α-1-4 glycosidic linkages with α-1-6 branches that together form multi-layered particles ranging in size from 30 nm to 300 nm. Glycogen spatial conformation and intracellular organization are highly regulated processes. Glycogen particles interact with their metabolizing enzymes and are associated with a variety of proteins that intervene in its biology, controlling its structure, particle size and sub-cellular distribution. The function of glycogen in adipose tissue is not well understood but appears to have a pivotal role as a regulatory mechanism informing the cells on substrate availability for triacylglycerol synthesis. To provide new molecular insights into the role of adipocyte glycogen we analyzed the glycogen-associated proteome from differentiated 3T3-L1-adipocytes.

Results

Glycogen particles from 3T3-L1-adipocytes were purified using a series of centrifugation steps followed by specific elution of glycogen bound proteins using α-1,4 glucose oligosaccharides, or maltodextrins, and tandem mass spectrometry. We identified regulatory proteins, 14-3-3 proteins, RACK1 and protein phosphatase 1 glycogen targeting subunit 3D. Evidence was also obtained for a regulated subcellular distribution of the glycogen particle: metabolic and mitochondrial proteins were abundant. Unlike the recently analyzed hepatic glycogen proteome, no endoplasmic proteins were detected, along with the recently described starch-binding domain protein 1. Other regulatory proteins which have previously been described as glycogen-associated proteins were not detected, including laforin, the AMPK beta-subunit and protein targeting to glycogen (PTG).

Conclusions

These data provide new molecular insights into the regulation of glycogen-bound proteins that are associated with the maintenance, organization and localization of the adipocyte glycogen particle.

Effects of preslaughter stress levels on the post-mortem sarcoplasmic proteomic profile of gilthead seabream muscle

Fish welfare is an important concern in aquaculture, not only due to the ethical implications but also for productivity and quality-related reasons. The purpose of this study was to track soluble proteome expression in post-mortem gilthead seabream muscle and to observe how preslaughter stress affects these post-mortem processes. For the experiment, two groups of gilthead seabream (n = 5) were subjected to distinct levels of preslaughter stress, with three muscle samples being taken from each fish. Proteins were extracted from the muscle samples, fractionated, and separated by 2DE. Protein identification was performed by MALDI-TOF-TOF MS. Analysis of the results indicates changes on several cellular pathways, with some of these changes being attributable to oxidative and proteolytic activity on sarcoplasmic proteins, together with leaking of myofibrillar proteins. These processes appear to have been hastened by preslaughter stress, confirming that it induces clear post-mortem changes in the muscle proteome of gilthead seabream.

Clock is not a component of Z-bands

The process of Z-band assembly begins with the formation of small Z-bodies composed of a complex of proteins rich in alpha-actinin. As additional proteins are added to nascent myofibrils, Z-bodies are transformed into continuous bands that form coherent discs of interacting proteins at the boundaries of sarcomeres. The steps controlling the transition of Z-bodies to Z-bands are not known. The report that a circadian protein, Clock, was localized in the Z-bands of neonatal rat cardiomyocytes raised the question whether this transcription factor could be involved in Z-band assembly. We found that the anti-Clock antibody used in the reported study also stained the Z-bands and Z-bodies of mouse and avian cardiac and skeletal muscle cells. YFP constructs of Clock that were assembled, however, did not localize to the Z-bands of muscle cells. Controls of Clock's activity showed that cotransfection of muscle cells with pYFP-Clock and pCeFP-BMAL1 led to the expected nuclear localization of YFP-Clock with its binding partner CeFP-BMAL1. Neither CeFP-BMAL1 nor antibodies directed against BMAL1 localized to Z-bands. A bimolecular fluorescence complementation assay (VC-BMAL1 and VN-Clock) confirmed the absence of Clock and BMAL1 from Z-bands, and their nuclear colocalization. A second anti-Clock antibody stained nuclei, but not Z-bands, of cells cotransfected with Clock and BMAL1 plasmids. Western blots of reactions of muscle extracts and purified alpha-actinins with the two anti-Clock antibodies showed that the original antibody cross-reacted with alpha-actinin and the second did not. These results cannot confirm Clock as an active component of Z-bands. © 2012 Wiley Periodicals, Inc.

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

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

Arg/Abl-binding protein, a Z-body and Z-band protein, binds sarcomeric, costameric, and signaling molecules

ArgBP2 (Arg/Abl-Binding Protein) is expressed at high levels in the heart and is localized in the Z-bands of mature myofibrils. ArgBP2 is a member of a small family of proteins that also includes vinexin and CAP (c-Cbl-associated protein), all characterized by having one sorbin homology (SOHO) domain and three C-terminal SH3 domains. Antibodies directed against ArgBP2 also react with the Z-bodies of myofibril precursors: premyofibrils and nascent myofibrils. Expression in cardiomyocytes of plasmids encoding Yellow Fluorescent Protein (YFP) fused to either full length ArgBP2, the SOHO, mid-ArgBP or the SH3 domains of ArgBP2 led to Z-band targeting of the fusion proteins, whereas an N-terminal fragment lacking these domains did not target to Z-bands. Although ArgBP2 is not found in skeletal muscle cells, YFP-ArgBP2 did target to Z-bodies and Z-bands in cultured myotubes. GST-ArgBP2-SH3 bound actin, α-actinin and vinculin proteins in blot overlays, cosedimentation assays, and EM negative staining techniques. Over-expression of ArgBP2 and ArgBP2-SH3 domains, but not YFP alone, led to loss of myofibrils in cardiomyocytes. Fluorescence recovery after photobleaching was used to measure the rapid dynamics of both the full length and some truncated versions of ArgBP2. Our results indicate that ArgBP2 may play an important role in the assembly and maintenance of myofibrils in cardiomyocytes.

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.

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.

Sequence and expression of the zebrafish alpha-actinin gene family reveals conservation and diversification among vertebrates

Alpha-actinins are actin microfilament crosslinking proteins. Vertebrate actinins fall into two classes: the broadly-expressed actinins 1 and 4 (actn1 and actn4) and muscle-specific actinins, actn2 and actn3. Members of this family have numerous roles, including regulation of cell adhesion, cell differentiation, directed cell motility, intracellular signaling, and stabilization of f-actin at the sarcomeric Z-line in muscle. Here we identify five zebrafish actinin genes including two paralogs of ACTN3. We describe the temporal and spatial expression patterns of these genes through embryonic development. All zebrafish actinin genes have unique expression profiles, indicating specialization of each gene. In particular, the muscle actinins display preferential expression in different domains of axial, pharyngeal, and cranial musculature. There is no identified avian actn3 and approximately 16% of humans are null for ACTN3. Duplication of actn3 in the zebrafish indicates that variation in actn3 expression may promote physiological diversity in muscle function among vertebrates.

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.

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.

How to build a myofibril

Building a myofibril from its component proteins requires the interactions of many different proteins in a process whose details are not understood. Several models have been proposed to provide a framework for understanding the increasing data on new myofibrillar proteins and their localizations during muscle development. In this article we discuss four current models that seek to explain how the assembly occurs in vertebrate cross-striated muscles. The models hypothesize: (a) stress fiber-like structures as templates for the assembly of myofibrils, (b) assembly in which the actin filaments and Z-bands form subunits independently from A-band subunits, with the two subsequently joined together to form a myofibril, (c) premyofibrils as precursors of myofibrils, or (d) assembly occurring without any intermediary structures. The premyofibril model, proposed by the authors, is discussed in more detail as it could explain myofibrillogenesis under a variety of different conditions: in ovo, in explants, and in tissue culture studies on cardiac and skeletal muscles.

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.

Dynamics of Z-band based proteins in developing skeletal muscle cells

During myofibril formation, Z-bodies, small complexes of alpha-actinin and associated proteins, grow in size, fuse and align to produce Z-bands. To determine if there were changes in protein dynamics during the assembly process, Fluorescence Recovery after Photobleaching was used to measure the exchange of Z-body and Z-band proteins with cytoplasmic pools in cultures of quail myotubes. Myotubes were transfected with plasmids encoding Yellow, Green, or Cyan Fluorescent Protein linked to the Z-band proteins: actin, alpha-actinin, cypher, FATZ, myotilin, and telethonin. Each Z-band protein showed a characteristic recovery rate and mobility. All except telethonin were localized in both Z-bodies and Z-bands. Proteins that were present both early in development in Z-bodies and later in Z-bands had faster exchange rates in Z-bodies. These results suggest that during myofibrillogenesis, molecular interactions develop between the Z-band proteins that decrease their mobility and increase the stability of the Z-bands. A truncated construct of alpha-actinin, which localized in Z-bands in myotubes and exhibited a very low rate of exchange, led to disruption of myofibrils, suggesting the importance of dynamic, intact alpha-actinin molecules for the formation and maintenance of Z-bands. Our experiments reveal the Z-band to be a much more dynamic structure than its appearance in electron micrographs of cross-striated muscle cells might suggest.

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.

The regulation of the interaction between F-actin and muscle fructose 1,6-bisphosphatase

Interaction between rabbit muscle fructose 1,6-bisphosphatase (FBPase) and rabbit muscle F-actin results in heterologous complex formation [A. Gizak, D. Rakus, A. Dzugaj, Histol. Histopathol. 18 (2003) 135]. Calculated on the basis of co-sedimentation-binding experiments and ELISA assay-binding constant (Ka) revealed that FBPase binds to F-actin with Ka equal to 7.4 x 10(4) M(-1). The binding is down-regulated by ligands interacting with the FBPase active site (fructose 6-phosphate, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate) and with the FBPase allosteric inhibitory site (AMP). The binding and the kinetic data suggests that FBPase may bind F-actin using a bipartite motif which includes the amino acids residues involved in the binding of the substrate as well as of the allosteric inhibitor of the enzyme. The in situ co-localization experiment, in which FBPase was diffused into skinned muscle fibres pre-incubated with phalloidin (polymeric actin-interacting toxin), has shown that FBPase binds predominantly to the region of the Z-line.

Calcium inhibits muscle FBPase and affects its intracellular localization in cardiomyocytes

As our recent investigation revealed, in mammalian heart muscle, fructose 1,6-bisphosphatase (FBPase)--a key enzyme of glyconeogenesis--is located around the Z-line, inside cells' nuclei and, as we demonstrate here for the first time, it associates with intercalated discs. Since the degree of association of numerous enzymes with subcellular structures depends on the metabolic state of the cell, we studied the effect of elevated Ca2+ concentration on localization of FBPase in cardiomyocytes. In such conditions, FBPase dissociated from the Z-line, but no visible effect on FBPase associated with intercalated discs or on the nuclear localization of the enzyme was observed. Additionally, Ca2+ appeared to be a strong inhibitor of muscle FBPase.

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

The alpha-actinins are an ancient family of actin-binding proteins that play structural and regulatory roles in cytoskeletal organisation and muscle contraction. alpha-actinin-3 is the most-highly specialised of the four mammalian alpha-actinins, with its expression restricted largely to fast glycolytic fibres in skeletal muscle. Intriguingly, a significant proportion ( approximately 18%) of the human population is totally deficient in alpha-actinin-3 due to homozygosity for a premature stop codon polymorphism (R577X) in the ACTN3 gene. Recent work in our laboratory has revealed a strong association between R577X genotype and performance in a variety of athletic endeavours. We are currently exploring the function and evolutionary history of the ACTN3 gene and other alpha-actinin family members. The alpha-actinin family provides a fascinating case study in molecular evolution, illustrating phenomena such as functional redundancy in duplicate genes, the evolution of protein function, and the action of natural selection during recent human evolution.

Myofibrillogenesis in skeletal muscle cells in the presence of taxol

We address the controversy of whether mature myofibrils can form in the presence of taxol, a microtubule-stabilizing compound. Previous electron microscopic studies reported the absence of actin filaments and Z-bands in taxol-treated myocytes [Antin et al., 1981: J Cell Biol 90:300-308; Toyoma et al., 1982: Proc Natl Acad Sci USA 79:6556-6560]. Quail skeletal myoblasts were isolated from 10-day-old embryos and grown in the presence or absence of taxol. Taxol inhibited the formation of multinucleated elongated myotubes. Myocytes cultured in the continual presence of taxol progressed from rounded to stellate shapes. Groups of myocytes that were clustered together after the isolation procedure fused in the presence of taxol but did not form elongated myotubes. Actin filaments and actin-binding proteins were detected with several different fluorescent probes in all myofibrils that formed in the presence of taxol. The Z-bands contained both alpha-actinin and titin, and the typical arrays of A-Bands were always associated with actin filaments in the myofibrils. Myofibril formation was followed by fixing cells each day in culture and staining with probes for actin, muscle-specific alpha-actinin, myosin II, nebulin, troponin, tropomyosin, and non-muscle myosin II. Small linear aggregates of alpha-actinin or Z-bodies, premyofibrils, were detected at the edges of the myocytes and in the arms of the taxol-treated cells and were always associated with actin filaments. Non-muscle myosin II was detected at the edges of the taxol-treated cells. Removal of the taxol drug led to the cells assuming a normal compact elongated shape. During the recovery process, additional myofibrils formed at the spreading edges of these elongated and thicker myotubes. Staining of these taxol-recovering cells with specific fluorescent reagents reveals three different classes of actin fibers. These results are consistent with a model of myofibrillogenesis that involves the transition of premyofibrils to mature myofibrils.

Colocalization of muscle FBPase and muscle aldolase on both sides of the Z-line

Previously we have reported that in vitro muscle aldolase binds to muscle FBPase [Biochem. Biophys. Res. Commun. 275 (2000) 611-616] which results in the changes of regulatory properties of the latter enzyme. In the present paper, the evidence that aldolase binds to FBPase in living cell is presented. The colocalization experiment, in which aldolase was diffused into skinned fibres that had been pre-incubated with FBPase, has shown that aldolase in the presence of FBPase binds predominantly to the Z-line. The existence of a triple aldolase-FBPase-alpha-actinin complex was confirmed through a real-time interaction analysis using the BIAcore biosensor. The colocalization of FBPase and aldolase on alpha-actinin of the Z-line indicates the existence of glyconeogenic metabolon in vertebrates' myocytes.

ACTN3 genotype is associated with human elite athletic performance

There is increasing evidence for strong genetic influences on athletic performance and for an evolutionary "trade-off" between performance traits for speed and endurance activities. We have recently demonstrated that the skeletal-muscle actin-binding protein alpha-actinin-3 is absent in 18% of healthy white individuals because of homozygosity for a common stop-codon polymorphism in the ACTN3 gene, R577X. alpha-Actinin-3 is specifically expressed in fast-twitch myofibers responsible for generating force at high velocity. The absence of a disease phenotype secondary to alpha-actinin-3 deficiency is likely due to compensation by the homologous protein, alpha-actinin-2. However, the high degree of evolutionary conservation of ACTN3 suggests function(s) independent of ACTN2. Here, we demonstrate highly significant associations between ACTN3 genotype and athletic performance. Both male and female elite sprint athletes have significantly higher frequencies of the 577R allele than do controls. This suggests that the presence of alpha-actinin-3 has a beneficial effect on the function of skeletal muscle in generating forceful contractions at high velocity, and provides an evolutionary advantage because of increased sprint performance. There is also a genotype effect in female sprint and endurance athletes, with higher than expected numbers of 577RX heterozygotes among sprint athletes and lower than expected numbers among endurance athletes. The lack of a similar effect in males suggests that the ACTN3 genotype affects athletic performance differently in males and females. The differential effects in sprint and endurance athletes suggests that the R577X polymorphism may have been maintained in the human population by balancing natural selection.

Skeletal muscles express the xenobiotic-metabolizing enzyme arylamine N-acetyltransferase

The human arylamine N-acetyltransferases (NATs) NAT1 and NAT2 are enzymes responsible for the acetylation of many arylamines and hydrazines, thereby playing an important role in both detoxification and activation of many drugs and carcinogens. Both enzymes show polymorphisms but exhibit key differences in substrate selectivity and tissue expression. In the present study, reverse transcriptase-PCR, Western blotting, and immunohistochemistry were used to investigate the expression of the NATs in human skeletal muscle. Despite the presence of its mRNA, NAT2 enzyme level was below the limit of detection. In contrast, both NAT1 mRNA and enzyme were readily detected in fetal, newborn, and adult muscles. In addition, punctate cytoplasmic and perinuclear NAT1 immunostaining was observed in all tissue sections, the staining being more intense in the fetal tissue. High expression of NAT1 enzyme in fetal muscle was also suggested by Western blotting. Because skeletal muscle accounts for a large proportion of body mass, muscle NAT1 expression may contribute significantly to the total activity in the body. These results further support the involvement of skeletal muscle in the metabolism of xenobiotics.

Myofibrillogenesis in the first cardiomyocytes formed from isolated quail precardiac mesoderm

De novo assembly of myofibrils was investigated in explants of precardiac mesoderm from quail embryos to address a controversy about different models of myofibrillogenesis. The sequential expression of sarcomeric components was visualized in double- and triple-stained explants before, during, and just after the first cardiomyocytes began to beat. In explants from stage 6 embryos, cultured for 10 h, ectoderm, endoderm, and the precardiac mesoderm displayed arrays of stress fibers with alternating bands of the nonmuscle isoforms of alpha-actinin and myosin IIB. With increasing time in culture, mesoderm cells contained fibrils composed of actin, nonmuscle myosin IIB, and sarcomeric alpha-actinin. Several hours later, before beating occurred, both nonmuscle and muscle myosin II localized in some of the fibrils in the cells. Concentrations of muscle myosin began as thin bundles, dispersed in the cytoplasm, often overlapping one another, and progressed to small, aligned A-band-sized aggregates. The amount of nonmuscle myosin decreased dramatically when Z-bands formed, the muscle myosin became organized into A-bands, and the cells began beating. The sequential changes in protein composition of the fibrils in the developing muscle cells supports the model of myofibrillogenesis in which assembly begins with premyofibrils and progresses through nascent myofibrils to mature myofibrils.

Myofibrillogenesis in skeletal muscle cells

How are myofibrils assembled in skeletal muscles? The current authors present evidence that myofibrils assemble through a three-step model: premyofibrils to nascent myofibrils to mature myofibrils. This three-step sequence was based initially on studies of living and fixed cultured cells from cardiac muscle. Data from avian primary muscle cells and from a transgenic skeletal mouse cell line indicate that a premyofibril model for myofibrillogenesis also holds for skeletal muscle cells. Premyofibrils are characterized by minisarcomeres bounded by Z-bodies composed of the muscle isoform of alpha-actinin. Actin filaments are connected to these Z-bodies and to the mini-A-bands composed of nonmuscle myosin II filaments. Nascent myofibrils are formed when premyofibrils align and are modified by the addition of titin and muscle myosin II filaments. Mature myofibrils result when nonmuscle myosin II is eliminated from the myofibrils and the alpha-actinin rich Z-bodies fuse as the distance between them increases from 0.5 microm in premyofibrils to 2 to 2.5 microm in the mature myofibrils.