Dominant negative effect of cytoplasmic actin isoproteins on cardiomyocyte cytoarchitecture and function

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

A solution to the long-standing problem of actin expression and purification

Actin is the most abundant protein in the cytoplasm of eukaryotic cells and interacts with hundreds of proteins to perform essential functions, including cell motility and cytokinesis. Numerous diseases are caused by mutations in actin, but studying the biochemistry of actin mutants is difficult without a reliable method to obtain recombinant actin. Moreover, biochemical studies have typically used tissue-purified α-actin, whereas humans express six isoforms that are nearly identical but perform specialized functions and are difficult to obtain in isolation from natural sources. Here, we describe a solution to the problem of actin expression and purification. We obtain high yields of actin isoforms in human Expi293F cells. Experiments along the multistep purification protocol demonstrate the removal of endogenous actin and the functional integrity of recombinant actin isoforms. Proteomics analysis of endogenous vs. recombinant actin isoforms confirms the presence of native posttranslational modifications, including N-terminal acetylation achieved after affinity-tag removal using the actin-specific enzyme Naa80. The method described facilitates studies of actin under fully native conditions to determine differences among isoforms and the effects of disease-causing mutations that occur in all six isoforms.

The remodelling of actin composition as a hallmark of cancer

Actin is a key structural protein that makes up the cytoskeleton of cells, and plays a role in functions such as division, migration, and vesicle trafficking. It comprises six different cell-type specific isoforms: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Abnormal actin isoform expression has been reported in many cancers, which led us to hypothesize that it may serve as an early biomarker of cancer. We show an overview of the different actin isoforms and highlight mechanisms by which they may contribute to tumorigenicity. Furthermore, we suggest how the aberrant expression of actin subunits can confer cells with greater proliferation ability, increased migratory capability, and chemoresistance through incorporation into the normal cellular F-actin network and altered actin binding protein interaction. Studying this fundamental change that takes place within cancer cells can further our understanding of neoplastic transformation in multiple tissue types, which can ultimately aid in the early-detection, diagnosis and treatment of cancer.

Extracellular Matrix in Regulation of Contractile System in Cardiomyocytes

The contractile apparatus of cardiomyocytes is considered to be a stable system. However, it undergoes strong rearrangements during heart development as cells progress from their non-muscle precursors. Long-term culturing of mature cardiomyocytes is also accompanied by the reorganization of their contractile apparatus with the conversion of typical myofibrils into structures of non-muscle type. Processes of heart development as well as cell adaptation to culture conditions in cardiomyocytes both involve extracellular matrix changes, which appear to be crucial for the maturation of contractile apparatus. The aim of this review is to analyze the role of extracellular matrix in the regulation of contractile system dynamics in cardiomyocytes. Here, the remodeling of actin contractile structures and the expression of actin isoforms in cardiomyocytes during differentiation and adaptation to the culture system are described along with the extracellular matrix alterations. The data supporting the regulation of actin dynamics by extracellular matrix are highlighted and the possible mechanisms of such regulation are discussed.

Supporting the heart: Functions of the cardiomyocyte's non-sarcomeric cytoskeleton

The non-contractile cytoskeleton in cardiomyocytes is comprised of cytoplasmic actin, microtubules, and intermediate filaments. In addition to providing mechanical support to these cells, these structures are important effectors of tension-sensing and signal transduction and also provide networks for the transport of proteins and organelles. The majority of our knowledge on the function and structure of these cytoskeletal networks comes from research on proliferative cell types. However, in recent years, researchers have begun to show that there are important cardiomyocyte-specific functions of the cytoskeleton. Here we will discuss the current state of cytoskeletal biology in cardiomyocytes, as well as research from other cell types, that together suggest there is a wealth of knowledge on cardiac health and disease waiting to be uncovered through exploration of the complex signaling networks of cardiomyocyte non-sarcomeric cytoskeletal proteins.

The makings of the 'actin code': regulation of actin's biological function at the amino acid and nucleotide level

The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles in vivo We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.

Critical roles for multiple formins during cardiac myofibril development and repair

Cardiac and skeletal muscle function depends on the proper formation of myofibrils, which are tandem arrays of highly organized actomyosin contractile units called sarcomeres. How the architecture of these colossal molecular assemblages is established during development and maintained over the lifetime of an animal is poorly understood. We investigate the potential roles in myofibril formation and repair of formin proteins, which are encoded by 15 different genes in mammals. Using quantitative real-time PCR analysis, we find that 13 formins are differentially expressed in mouse hearts during postnatal development. Seven formins immunolocalize to sarcomeres in diverse patterns, suggesting that they have a variety of functional roles. Using RNA interference silencing, we find that the formins mDia2, DAAM1, FMNL1, and FMNL2 are required nonredundantly for myofibrillogenesis. Knockdown phenotypes include global loss of myofibril organization and defective sarcomeric ultrastructure. Finally, our analysis reveals an unanticipated requirement specifically for FMNL1 and FMNL2 in the repair of damaged myofibrils. Together our data reveal an unexpectedly large number of formins, with diverse localization patterns and nonredundant roles, functioning in myofibril development and maintenance, and provide the first evidence of actin assembly factors being required to repair myofibrils.

Distinct functional interactions between actin isoforms and nonsarcomeric myosins

Despite their near sequence identity, actin isoforms cannot completely replace each other in vivo and show marked differences in their tissue-specific and subcellular localization. Little is known about isoform-specific differences in their interactions with myosin motors and other actin-binding proteins. Mammalian cytoplasmic β- and γ-actin interact with nonsarcomeric conventional myosins such as the members of the nonmuscle myosin-2 family and myosin-7A. These interactions support a wide range of cellular processes including cytokinesis, maintenance of cell polarity, cell adhesion, migration, and mechano-electrical transduction. To elucidate differences in the ability of isoactins to bind and stimulate the enzymatic activity of individual myosin isoforms, we characterized the interactions of human skeletal muscle α-actin, cytoplasmic β-actin, and cytoplasmic γ-actin with human myosin-7A and nonmuscle myosins-2A, -2B and -2C1. In the case of nonmuscle myosins-2A and -2B, the interaction with either cytoplasmic actin isoform results in 4-fold greater stimulation of myosin ATPase activity than was observed in the presence of α-skeletal muscle actin. Nonmuscle myosin-2C1 is most potently activated by β-actin and myosin-7A by γ-actin. Our results indicate that β- and γ-actin isoforms contribute to the modulation of nonmuscle myosin-2 and myosin-7A activity and thereby to the spatial and temporal regulation of cytoskeletal dynamics. FRET-based analyses show efficient copolymerization abilities for the actin isoforms in vitro. Experiments with hybrid actin filaments show that the extent of actomyosin coupling efficiency can be regulated by the isoform composition of actin filaments.

Novel proteins associated with human dilated cardiomyopathy: selective reduction in α(1A)-adrenergic receptors and increased desensitization proteins

Abstract Therapeutics to treat human heart failure (HF) and the identification of proteins associated with HF are still limited. We analyzed α(1)-adrenergic receptor (AR) subtypes in human HF and performed proteomic analysis on more uniform samples to identify novel proteins associated with human HF. Six failing hearts with end-stage dilated cardiomyopathy (DCM) and four non-failing heart controls were subjected to proteomic analysis. Out of 48 identified proteins, 26 proteins were redundant between samples. Ten of these 26 proteins were previously reported to be associated with HF. Of the newly identified proteins, we found several muscle proteins and mitochondrial/electron transport proteins, while novel were functionally similar to previous reports. However, we also found novel proteins involved in functional classes such as β-oxidation and G-protein coupled receptor signaling and desensitization not previously associated with HF. We also performed radioligand-binding studies on the heart samples and not only confirmed a large loss of β(1)-ARs in end-stage DCM, but also found a selective decrease in the α(1A)-AR subtype not previously reported. We have identified new proteins and functional categories associated with end-stage DCM. We also report that similar to the previously characterized loss of β(1)-AR in HF, there is also a concomitant loss of α(1A)-ARs, which are considered cardioprotective proteins.

How and why does β-actin mRNA target?

beta-Actin mRNA is localized near the leading edge in several cell types where actin polymerization is actively promoting forward protrusion. The localization of the beta-actin mRNA near the leading edge is facilitated by a short sequence in the 3'UTR (untranslated region), the 'zipcode'. Localization of the mRNA at this region is important physiologically. Treatment of chicken embryo fibroblasts with antisense oligonucleotides complementary to the localization sequence (zipcode) in the 3'UTR leads to delocalization of beta-actin mRNA, alteration of cell phenotype and a decrease in cell motility. The dynamic image analysis system (DIAS) used to quantify movement of cells in the presence of sense and antisense oligonucleotides to the zipcode showed that net pathlength and average speed of antisense-treated cells were significantly lower than in sense-treated cells. This suggests that a decrease in persistence of direction of movement and not in velocity results from treatment of cells with zipcode-directed antisense oligonucleotides. We postulate that delocalization of beta-actin mRNA results in delocalization of nucleation sites and beta-actin protein from the leading edge followed by loss of cell polarity and directional movement. Hence the physiological consequences of beta-actin mRNA delocalization affect the stability of the cell phenotype.

Smooth muscle α actin is specifically required for the maintenance of lactation

Smooth muscle α-actin (Acta2) is one of six highly conserved mammalian actin isoforms that appear to exhibit functional redundancy. Nonetheless, we have postulated a specific functional role for the smooth muscle specific isoform. Here, we show that Acta2 deficient mice have a remarkable mammary phenotype such that dams lacking Acta2 are unable to nurse their offspring effectively. The phenotype was rescued in cross fostering experiments with wild type mice, excluding a developmental defect in Acta2 null pups. The mechanism for the underlying phenotype is due to myoepithelial dysfunction postpartum resulting in precocious involution. Further, we demonstrate a specific defect in myoepithelial cell contractility in Acta2 null mammary glands, despite normal expression of cytoplasmic actins. We conclude that Acta2 specifically mediates myoepithelial cell contraction during lactation and that this actin isoform therefore exhibits functional specificity.

New approaches to targeting the actin cytoskeleton for chemotherapy

The actin cytoskeleton is indispensable for normal cellular function. In particular, several actin-based structures coordinate cellular motility, a process hijacked by tumor cells in order to facilitate their propagation to distant sites. The actin cytoskeleton, therefore, represents a point for chemotherapeutic intervention. The challenge in disrupting the actin cytoskeleton is in preserving actin-driven contraction of cardiac and skeletal muscle. By targeting actin-binding proteins with altered expression in malignancy, it may be possible to achieve tumor-specific toxicity. A number of actin-binding proteins act cooperatively and synergistically to regulate actin structures required for motility. The actin cytoskeleton is characterized by a significant degree of plasticity. Targeting specific actin-binding proteins for chemotherapy will only be successful if no other compensatory mechanisms exist.

Evidence for somatic gene conversion and deletion in bipolar disorder, Crohn's disease, coronary artery disease, hypertension, rheumatoid arthritis, type-1 diabetes, and type-2 diabetes

BACKGROUND

During gene conversion, genetic information is transferred unidirectionally between highly homologous but non-allelic regions of DNA. While germ-line gene conversion has been implicated in the pathogenesis of some diseases, somatic gene conversion has remained technically difficult to investigate on a large scale.

METHODS

A novel analysis technique is proposed for detecting the signature of somatic gene conversion from SNP microarray data. The Wellcome Trust Case Control Consortium has gathered SNP microarray data for two control populations and cohorts for bipolar disorder (BD), cardiovascular disease (CAD), Crohn's disease (CD), hypertension (HT), rheumatoid arthritis (RA), type-1 diabetes (T1D) and type-2 diabetes (T2D). Using the new analysis technique, the seven disease cohorts are analyzed to identify cohort-specific SNPs at which conversion is predicted. The quality of the predictions is assessed by identifying known disease associations for genes in the homologous duplicons, and comparing the frequency of such associations with background rates.

RESULTS

Of 28 disease/locus pairs meeting stringent conditions, 22 show various degrees of disease association, compared with only 8 of 70 in a mock study designed to measure the background association rate (P < 10-9). Additional candidate genes are identified using less stringent filtering conditions. In some cases, somatic deletions appear likely. RA has a distinctive pattern of events relative to other diseases. Similarities in patterns are apparent between BD and HT.

CONCLUSIONS

The associations derived represent the first evidence that somatic gene conversion could be a significant causative factor in each of the seven diseases. The specific genes provide potential insights about disease mechanisms, and are strong candidates for further study.

Hypertrophic stimulation increases beta-actin dynamics in adult feline cardiomyocytes

The myocardium responds to hemodynamic stress through cellular growth and organ hypertrophy. The impact of cytoskeletal elements on this process, however, is not fully understood. While α-actin in cardiomyocytes governs muscle contraction in combination with the myosin motor, the exact role of β-actin has not been established. We hypothesized that in adult cardiomyocytes, as in non-myocytes, β-actin can facilitate cytoskeletal rearrangement within cytoskeletal structures such as Z-discs. Using a feline right ventricular pressure overload (RVPO) model, we measured the level and distribution of β-actin in normal and pressure overloaded myocardium. Resulting data demonstrated enriched levels of β-actin and enhanced translocation to the Triton-insoluble cytoskeletal and membrane skeletal complexes. In addition, RVPO in vivo and in vitro hypertrophic stimulation with endothelin (ET) or insulin in isolated adult cardiomyocytes enhanced the content of polymerized fraction (F-actin) of β-actin. To determine the localization and dynamics of β-actin, we adenovirally expressed GFP-tagged β-actin in isolated adult cardiomyocytes. The ectopically expressed β-actin-GFP localized to the Z-discs, costameres, and cell termini. Fluorescence recovery after photobleaching (FRAP) measurements of β-actin dynamics revealed that β-actin at the Z-discs is constantly being exchanged with β-actin from cytoplasmic pools and that this exchange is faster upon hypertrophic stimulation with ET or insulin. In addition, in electrically stimulated isolated adult cardiomyocytes, while β-actin overexpression improved cardiomyocyte contractility, immunoneutralization of β-actin resulted in a reduced contractility suggesting that β-actin could be important for the contractile function of adult cardiomyocytes. These studies demonstrate the presence and dynamics of β-actin in the adult cardiomyocyte and reinforce its usefulness in measuring cardiac cytoskeletal rearrangement during hypertrophic stimulation.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Context-dependent functional substitution of alpha-skeletal actin by gamma-cytoplasmic actin

We generated transgenic mice that overexpressed gamma-(cyto) actin 2000-fold above wild-type levels in skeletal muscle. gamma-(cyto) actin comprised 40% of total actin in transgenic skeletal muscle, with a concomitant 40% decrease in alpha-actin. Surprisingly, transgenic muscle was histologically and ultrastructurally identical to wild-type muscle despite near-stoichiometric incorporation of gamma-(cyto) actin into sarcomeric thin filaments. Furthermore, several parameters of muscle physiological performance in the transgenic animals were not different from wild type. Given these surprising results, we tested whether overexpression of gamma-(cyto) actin could rescue the early postnatal lethality in alpha-(sk) actin-null mice (Acta1(-/-)). By quantitative Western blot analysis, we found total actin levels were decreased by 35% in Acta1(-/-) muscle. Although transgenic overexpression of gamma-(cyto) actin on the Acta1(-/-) background restored total actin levels to wild type, resulting in thin filaments composed of 60% gamma-(cyto) actin and a 40% mixture of cardiac and vascular actin, the life span of transgenic Acta1(-/-) mice was not extended. These results indicate that sarcomeric thin filaments can accommodate substantial incorporation of gamma-(cyto) actin without functional consequences, yet gamma-(cyto) actin cannot fully substitute for alpha-(sk) actin.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Expression and function of alpha-smooth muscle actin during embryonic-stem-cell-derived cardiomyocyte differentiation

Three alpha-muscle actin isoforms are sequentially expressed during in vivo cardiac development. alpha-Smooth muscle actin is first and transiently expressed, followed by alpha-skeletal and finally alpha-cardiac actin. The significance of these transitions in actin gene expression during myogenesis remains to be determined. To understand whether actin isoforms have specific functions during cardiac development and cardiomyocyte contractility, we have hampered alpha-smooth muscle and alpha-skeletal actin expression and organization during embryonic stem cell differentiation towards cardiomyocyte. We show that the sequence of actin isoform expression displays similar pattern in the in vitro model and in mouse heart embryogenesis. Treatment with an interfering fusion peptide containing the N-terminal sequence of alpha-smooth muscle actin during a time window preceding spontaneous beating, prevents proper cardiac sarcomyogenesis, whereas alpha-skeletal actin-fusion peptide has no effect. Knockdown of alpha-smooth muscle actin in embryonic stem cells using RNA interference also affects cardiac differentiation. The application of both fusion peptides on beating embryoid bodies impairs frequency. These results suggest specific functional activities for actin isoforms in cardiogenesis and cardiomyocyte contractility.

Cytoplasmic gamma-actin is not required for skeletal muscle development but its absence leads to a progressive myopathy

Nonmuscle gamma(cyto)-actin is expressed at very low levels in skeletal muscle but uniquely localizes to costameres, the cytoskeletal networks that couple peripheral myofibrils to the sarcolemma. We generated and analyzed skeletal muscle-specific gamma(cyto)-actin knockout (Actg1-msKO) mice. Although muscle development proceeded normally, Actg1-msKO mice presented with overt muscle weakness accompanied by a progressive pattern of muscle fiber necrosis/regeneration. Functional deficits in whole-body tension and isometric twitch force were observed, consistent with defects in the connectivity between muscle fibers and/or myofibrils or at the myotendinous junctions. Surprisingly, gamma(cyto)-actin-deficient muscle did not demonstrate the fibrosis, inflammation, and membrane damage typical of several muscular dystrophies but rather presented with a novel progressive myopathy. Together, our data demonstrate an important role for minimally abundant but strategically localized gamma(cyto)-actin in adult skeletal muscle and describe a new mouse model to study the in vivo relevance of subcellular actin isoform sorting.

Cytoplasmic gamma-actin contributes to a compensatory remodeling response in dystrophin-deficient muscle

Dystrophin mechanically links the costameric cytoskeleton and sarcolemma, yet dystrophin-deficient muscle exhibits abnormalities in cell signaling, gene expression, and contractile function that are not clearly understood. We generated new antibodies specific for cytoplasmic gamma-actin and confirmed that gamma-actin most predominantly localized to the sarcolemma and in a faint reticular lattice within normal muscle cells. However, we observed that gamma-actin levels were increased 10-fold at the sarcolemma and within the cytoplasm of striated muscle cells from dystrophin-deficient mdx mice. Transgenic overexpression of the dystrophin homologue utrophin, or functional dystrophin constructs in mdx muscle, restored gamma-actin to normal levels, whereas gamma-actin remained elevated in mdx muscle expressing nonfunctional dystrophin constructs. We conclude that increased cytoplasmic gamma-actin in dystrophin-deficient muscle may be a compensatory response to fortify the weakened costameric lattice through recruitment of parallel mechanical linkages. However, the presence of excessive myoplasmic gamma-actin may also contribute to altered cell signaling or gene expression in dystrophin-deficient muscle.

Cardiomyocyte cytoskeleton and myofibrillogenesis in healthy and diseased heart

The unique cytoarchitecture of cardiomyocytes arises by complex interactions of different filamentous structures of the cytoskeleton. Intermediate filaments of the non-sarcomeric cytoskeleton are not essential for development but important for maintenance of myofibrils. Myofibrils consist of contractile proteins involved in force generation and the muscle cytoskeleton framework. The latter is essential for proper assembly and maintenance as well as for interaction with other cardiomyocytes or the extracellular matrix, thus being involved in force transmission. The information for sarcomere assembly is encoded in the proteins and some domains essential for faithful incorporation have been identified by epitope tagging experiments. Many KO mutations result in embryonic lethal phenotypes and new techniques e.g. using cardiomyocytes derived from ES cell-lines will have to be developed that allow to study such mutations in cardiomyocytes rather than whole organisms. Alterations in the expression levels of several proteins of the muscle cytoskeleton or impairment of their function by point mutations can result in increased mechanical stress in the cardiomyocytes which finally leads to cellular responses such as the development of dilated cardiomyopathy (DCM). MLP (muscle-LIM-protein) deficient mice develop DCM and changes in the mechanical coupling of cardiomyocytes result in alterations at the intercalated disks and enhanced accumulation of adherens junction proteins. Therefore, controlled interactions between proteins of the muscle cytoskeleton and contractile proteins are essential to ensure proper cardiac function and a more detailed insight in these processes might provide new tools to improve the contractile efficiency of the cardiomyocytes and thus working output in cardiomyopathies.

Contribution of sequence variation inDrosophilaactins to their incorporation into actin-based structures in vivo

Actin is a highly conserved protein important for many cellular functions including motility, contraction in muscles and intracellular transport. Many eukaryotic genomes encode multiple actin protein isoforms that differ from each other by only a few residues. We addressed whether the sequence differences between actin paralogues in one species affect their ability to integrate into the large variety of structures generated by filamentous actin. We thus ectopically expressed all six Drosophila actins as fusion proteins with green fluorescent protein (GFP) in a variety of embryonic, larval and adult fly tissues. We found that each actin was able to integrate into most actin structures analysed. For example, in contrast to studies in mammalian cells, the two Drosophila cytoplasmic actins were incorporated into muscle sarcomeres. However, there were differences in the efficiency with which each actin was incorporated into specific actin structures. The most striking difference was observed within the Z-lines of the sarcomeres: one actin was specifically excluded and we mapped this feature to one or both of two residues within the C-terminal half of the protein. Thus, in Drosophila, the primary sequence of different actins does affect their ability to incorporate into actin structures, and so specific GFPactins may be used to label certain actin structures particularly well.

Specific features of neuronal size and shape are regulated by tropomyosin isoforms

Spatially distinct populations of microfilaments, characterized by different tropomyosin (Tm) isoforms, are present within a neuron. To investigate the impact of altered tropomyosin isoform expression on neuronal morphogenesis, embryonic cortical neurons from transgenic mice expressing the isoforms Tm3 and Tm5NM1, under the control of the beta-actin promoter, were cultured in vitro. Exogenously expressed Tm isoforms sorted to different subcellular compartments with Tm5NM1 enriched in filopodia and growth cones, whereas the Tm3 was more broadly localized. The Tm5NM1 neurons displayed significantly enlarged growth cones accompanied by an increase in the number of dendrites and axonal branching. In contrast, Tm3 neurons displayed inhibition of neurite outgrowth. Recruitment of Tm5a and myosin IIB was observed in the peripheral region of a significant number of Tm5NM1 growth cones. We propose that enrichment of myosin IIB increases filament stability, leading to the enlarged growth cones. Our observations support a role for different tropomyosin isoforms in regulating interactions with myosin and thereby regulating morphology in specific intracellular compartments.

The N-terminal Ac-EEED sequence plays a role in alpha-smooth-muscle actin incorporation into stress fibers

We have previously shown that the N-terminal sequence AcEEED of alpha-smooth-muscle actin causes the loss of alpha-smooth-muscle actin from stress fibers and a decrease in cell contractility when introduced in myofibroblasts as a cell-penetrating fusion peptide. Here, we have investigated the function of this sequence on stress fiber organization in living cells, using enhanced green fluorescent protein (EGFP)-tagged alpha-smooth-muscle actin. The fusion peptide provokes the gradual disappearance of EGFP fluorescence of alpha-smooth-muscle actin from stress fibers and the formation of hitherto unknown rod-like structures. In addition to alpha-smooth-muscle actin, these structures contain cytoplasmic actins, gelsolin and cofilin but not other major actin-binding proteins. These rod-like structures are also visible in wild-type fibroblasts during normal cell spreading, suggesting that they represent a physiological step in the organization of alpha-smooth-muscle actin in stress fibers. Fluorescence-recovery-after-photobleaching experiments suggest that the fusion peptide reduces the dynamics of alpha-smooth-muscle actin and its incorporation in stress fibers. Here, we propose a new mechanism of how alpha-smooth-muscle actin is incorporated in stress fibers involving the sequence Ac-EEED.

Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

A novel role for non-muscle gamma-actin in skeletal muscle sarcomere assembly

Existing models describing sarcomere assembly have arisen primarily from studies using cardiac muscle. In contrast to cardiac muscle, skeletal muscle differentiation is characterised by dramatic changes in protein expression, from non-muscle to muscle-specific isoforms before organisation of the sarcomeres. Consequently, little is understood of the potential influence of non-muscle cytoskeletal proteins on skeletal sarcomere assembly. To address this issue, transfectant (gamma33-B1) and control mouse C2 myoblasts were differentiated to form myotubes, and various stages of skeletal sarcomere assembly were studied. Organisation of non-muscle gamma-actin and co-localisation with sarcomeric alpha-actinin, an early marker of sarcomere assembly and a major component of Z lines, was noted. gamma-Actin was also identified in young myotubes with developing sarcomeric myofibrils in regenerating adult mouse muscle. Localisation of gamma-actin in a different area of the myotube to the muscle-specific sarcomeric alpha-actin also indicated a distinct role for gamma-actin. The effects of aberrant gamma-actin expression in other myoblast lines, further suggested a sequestering role for gamma-actin. These observations make the novel suggestion that non-muscle gamma-actin plays a role in skeletal sarcomere assembly both in vitro and in vivo. Consequently, a modified model is proposed which describes the role of gamma-actin in skeletal sarcomere assembly.

Myofibrillogenesis in the developing chicken heart: Role of actin isoforms and of the pointed end actin capping protein tropomodulin during thin filament assembly

Recently, important differences between myofibrillogenesis in cultured cardiomyocytes vs. the three-dimensional setting in situ could be determined. We investigated thin filament assembly in situ by confocal microscopy of whole-mount preparations of immunostained embryonic chicken hearts. Of interest, a distinct localisation of different actin isoforms was observed in immature thin filaments. Cardiac alpha-actin is restricted to filaments with a length comparable to mature thin filaments as soon as the first contractions occur, while vascular alpha-actin makes up filaments that extend toward the M-band. The pointed-end actin filament capping protein tropomodulin can be found initially in close association with the plasma membrane, but attains its mature localisation pattern at the ends of the thin filaments only comparatively late during myofibrillogenesis. Thus tropomodulin acts as a length stabilising element of actin filaments also in developing cardiomyocytes in situ, but plays an additional role together with membrane-associated actin filaments in the earliest steps of myofibril assembly.

The interaction of tropomodulin with tropomyosin stabilizes thin filaments in cardiac myocytes

Actin (thin) filament length regulation and stability are essential for striated muscle function. To determine the role of the actin filament pointed end capping protein, tropomodulin1 (Tmod1), with tropomyosin, we generated monoclonal antibodies (mAb17 and mAb8) against Tmod1 that specifically disrupted its interaction with tropomyosin in vitro. Microinjection of mAb17 or mAb8 into chick cardiac myocytes caused a dramatic loss of the thin filaments, as revealed by immunofluorescence deconvolution microscopy. Real-time imaging of live myocytes expressing green fluorescent protein-alpha-tropomyosin and microinjected with mAb17 revealed that the thin filaments depolymerized from their pointed ends. In a thin filament reconstitution assay, stabilization of the filaments before the addition of mAb17 prevented the loss of thin filaments. These studies indicate that the interaction of Tmod1 with tropomyosin is critical for thin filament stability. These data, together with previous studies, indicate that Tmod1 is a multifunctional protein: its actin filament capping activity prevents thin filament elongation, whereas its interaction with tropomyosin prevents thin filament depolymerization.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Ca(2+) activation and tension cost in myofilaments from mouse hearts ectopically expressing enteric gamma-actin

To determine the significance of actin isoforms in chemomechanical coupling, we compared tension and ATPase rate in heart myofilaments from nontransgenic (NTG) and transgenic (TG) mice in which enteric gamma-actin replaced >95% of the cardiac alpha-actin. Enteric gamma-actin was expressed against three backgrounds: mice expressing cardiac alpha-actin, heterozygous null cardiac alpha-actin mice, and homozygous null cardiac alpha-actin mice. There were no differences in maximum Ca(2+) activated tension or maximum rate of tension redevelopment after a quick release and rapid restretch protocol between TG and NTG skinned fiber bundles. However, compared with NTG controls, Ca(2+) sensitivity of tension was significantly decreased and economy of tension development was significantly increased in myofilaments from all TG hearts. Shifts in myosin isoform population could not fully account for this increase in the economy of force production of TG myofilaments. Our results indicate that an exchange of cardiac alpha-actin with an actin isoform differing in only five amino acids has a significant impact on both Ca(2+) regulation of cardiac myofilaments and the cross-bridge cycling rate.

Cloning and characterization of betaCAP73, a novel regulator of beta-actin assembly

In non-muscle cells, the isoactins are differentially localized, with beta-actin specifically enriched at the cell cortex within motile structures, such as lamellae, while gamma-actin shows no specific localization. To understand the sorting and regulation of beta-actin within moving cells, we previously isolated betaCAP73, a novel beta-actin-specific binding protein (Cell Motil. Cytoskel. 35 (1996) 175). Here, we have cloned and characterized the 4718 nucleotide betaCAP73 cDNA from an endothelial cell library. betaCAP73 cDNA contains six predicted ankyrin-like repeats at the amino terminus and is partially homologous to three previously reported sequences of unknown function. Northern analysis reveals betaCAP73 expression in all tissues tested, with highest levels in skeletal muscle. Consistent with previously demonstrated interactions between native betaCAP73 and beta-actin filament barbed-ends, recombinant betaCAP73 inhibits pyrene-actin assembly in an isoactin-specific manner. Compared to stationary cells betaCAP73 mRNA is down regulated in crawling cells. Similarly, motility-defective cells have increased betaCAP73 protein. Overexpression of full-length betaCAP73 induces the formation of novel membrane protrusions that are enriched in betaCAP73, while overexpression of betaCAP73 domains alters cell morphology. Combined, these results indicate that betaCAP73 modulates isoactin dynamics to regulate the morphological alterations required for cell growth and motility.

Transient production of alpha-smooth muscle actin by skeletal myoblasts during differentiation in culture and following intramuscular implantation

alpha-smooth muscle actin (SMA) is typically not present in post-embryonic skeletal muscle myoblasts or skeletal muscle fibers. However, both primary myoblasts isolated from neonatal mouse muscle tissue, and C2C12, an established myoblast cell line, produced SMA in culture within hours of exposure to differentiation medium. The SMA appeared during the cells' initial elongation, persisted through differentiation and fusion into myotubes, remained abundant in early myotubes, and was occasionally observed in a striated pattern. SMA continued to be present during the initial appearance of sarcomeric actin, but disappeared shortly thereafter leaving only sarcomeric actin in contractile myotubes derived from primary myoblasts. Within one day after implantation of primary myoblasts into mouse skeletal muscle, SMA was observed in the myoblasts; but by 9 days post-implantation, no SMA was detectable in myoblasts or muscle fibers. Thus, both neonatal primary myoblasts and an established myoblast cell line appear to similarly reprise an embryonic developmental program during differentiation in culture as well as differentiation within adult mouse muscles.

Actin cytoskeleton and small heat shock proteins: how do they interact?

Actin and small heat shock proteins (sHsps) are ubiquitous and multifaceted proteins that exist in 2 reversible forms, monomers and multimers, ie, the microfilament of the cytoskeleton and oligomers of the sHsps, generally, supposed to be in a spherical and hollow form. Two situations are described in the literature, where the properties of actin are modulated by sHsps; the actin polymerization is inhibited in vitro by some sHsps acting as capping proteins, and the actin cytoskeleton is protected by some sHsps against the disruption induced by various stressful conditions. We propose that a direct actin-sHsp interaction occurs to inhibit actin polymerization and to participate in the in vivo regulation of actin filament dynamics. Protection of the actin cytoskeleton would result from an F-actin-sHsp interaction in which microfilaments would be coated by small oligomers of phosphorylated sHsps. Both proteins share common structural motives suggesting direct binding sites, but they remain to be demonstrated. Some sHsps would behave with the actin cytoskeleton as actin-binding proteins capable of either capping a microfilament when present as a nonphosphorylated monomer or stabilizing and protecting the microfilament when organized in small, phosphorylated oligomers.

Targeted disruption of nebulette protein expression alters cardiac myofibril assembly and function

To evaluate nebulette's role in cardiac myofibrils, cardiomyocytes expressing green fluorescent protein (GFP)-nebulette constructs were monitored for their ability to contract and myofilament protein distribution was analyzed. Cells expressing full-length GFP-nebulette appear unaffected and exhibit normal beating frequencies. Expression of the GFP linker and SH3 results in loss of the endogenous nebulette and tropomyosin; however, Z-line and thick filaments are undisturbed. Cells expressing either of these domains have dramatically reduced beating frequencies, consistent with the loss of thin filament proteins. This loss was inhibited by the addition of protease inhibitors during culturing. The GFP repeat domain disrupts both myofibrillogenesis and contraction in spreading cardiomyocytes, whereas introduction of this protein into well-spread cardiomyocytes results in localization at the Z-line and a 50% reduction in beating frequency. Ultimately, these cells form bundles containing the GFP repeat and many myofilament proteins. Interestingly, butanedione monoxime inhibition of contraction inhibited the formation of these bundles. These results show that the GFP-nebulette domains have a dominant-negative effect on the distribution and function of the sarcomeric proteins. Taken together with the observation that nebulette colocalizes with alpha-actinin in the pre-, nascent, and mature myofibrils, our data demonstrate the importance of this cardiac-specific nebulin isoform in myofibril organization and function.

Expression and function of the Drosophila ACT88F actin isoform is not restricted to the indirect flight muscles

Most higher eukaryotic genomes contain multiple actin genes, yet the sequence differences between isoforms are few. In Drosophila melanogaster it was previously established that one of the six actin genes, Act88F, is expressed only in the indirect flight muscles (IFMs). These muscles are highly specialised for oscillatory contractions to power flight. The implication was that this isoform had tissue-specific properties. In this paper we show using two reporter constructs expressing either beta-galactosidase, Act88F-lacZ, or the green fluorescent protein, Act88F-GFP, that the Act88F promoter is active in a small number of other muscles, including leg (femoral) and uterine muscles. However, the levels of Act88F driven non-IFM expression are much less than in the IFMs. We have confirmed endogenous Act88F gene expression in these other muscles by in situ hybridisation studies. Using null and antimorphic mutants to show decreased walking ability and delayed/reduced oviposition we demonstrated that Act88F expression is functionally important in multiple muscle groups. Since the mutant effects are mild, this supports the expectation that other actin genes are also expressed in these muscles. The Act88F-GFP promoter-reporter also detects Act88F-driven expression in the bristle-forming cells in the pupal wings. The implications of these results for the functions and developmental expression of the Drosophila ACT88F isoform are discussed.

Alpha-smooth muscle actin expression upregulates fibroblast contractile activity

To evaluate whether alpha-smooth muscle actin (alpha-SMA) plays a role in fibroblast contractility, we first compared the contractile activity of rat subcutaneous fibroblasts (SCFs), expressing low levels of alpha-SMA, with that of lung fibroblasts (LFs), expressing high levels of alpha-SMA, with the use of silicone substrates of different stiffness degrees. On medium stiffness substrates the percentage of cells producing wrinkles was similar to that of alpha-SMA-positive cells in each fibroblast population. On high stiffness substrates, wrinkle production was limited to a subpopulation of LFs very positive for alpha-SMA. In a second approach, we measured the isotonic contraction of SCF- and LF-populated attached collagen lattices. SCFs exhibited 41% diameter reduction compared with 63% by LFs. TGFbeta1 increased alpha-SMA expression and lattice contraction by SCFs to the levels of LFs; TGFbeta-antagonizing agents reduced alpha-SMA expression and lattice contraction by LFs to the level of SCFs. Finally, 3T3 fibroblasts transiently or permanently transfected with alpha-SMA cDNA exhibited a significantly higher lattice contraction compared with wild-type 3T3 fibroblasts or to fibroblasts transfected with alpha-cardiac and beta- or gamma-cytoplasmic actin. This took place in the absence of any change in smooth muscle or nonmuscle myosin heavy-chain expression. Our results indicate that an increased alpha-SMA expression is sufficient to enhance fibroblast contractile activity.

The physiological significance of beta -actin mRNA localization in determining cell polarity and directional motility

beta-actin mRNA is localized near the leading edge in several cell types, where actin polymerization is actively promoting forward protrusion. The localization of the beta-actin mRNA near the leading edge is facilitated by a short sequence in the 3' untranslated region, the "zip code." Localization of the mRNA at this region is important physiologically. Treatment of chicken embryo fibroblasts with antisense oligonucleotides complementary to the localization sequence (zip code) in the 3' untranslated region leads to delocalization of beta-actin mRNA, alteration of cell phenotype, and a decrease in cell motility. To determine the components of this process responsible for the change in cell behavior after beta-actin mRNA delocalization, the Dynamic Image Analysis System was used to quantify movement of cells in the presence of sense and antisense oligonucleotides to the zip code. It was found that net path length and average speed of antisense-treated cells were significantly lower than in sense-treated cells. Total path length and the velocity of protrusion of antisense-treated cells were not affected compared with those of control cells. These results suggest that a decrease in persistence of direction of movement and not in velocity results from treatment of cells with zip code-directed antisense oligonucleotides. To test this, direct analysis of directionality was performed on antisense-treated cells and showed a decrease in directionality (net path/total path) and persistence of movement. Less directional movement of antisense-treated cells correlated with a unpolarized and discontinuous distribution of free barbed ends of actin filaments and of beta-actin protein. These results indicate that delocalization of beta-actin mRNA results in delocalization of nucleation sites and beta-actin protein from the leading edge followed by loss of cell polarity and directional movement.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

The effect of dibutyryl cyclic AMP on the expression of actin isoforms in astroglia

Mammalian cells contain at least 8 actin isoforms. The functional significance and the mechanisms that regulate the expression of each actin isoform are not yet known. Using immunofluorescence staining, it was found that all astroglia in tissue culture express beta-actin isoform and about 86% of astroglia express alpha-smooth muscle actin isoform. When astroglia were treated with dibutyryl cyclic AMP for 4, 7, 14 and 21 days, it was found that the number of the cells expressing alpha-smooth muscle actin isoform progressively decreased, whereas, the number of the cells expressing beta-actin isoform remained constant. The western blot experiment showed that the amount of total alpha-smooth muscle actin isoform (soluble and insoluble) and of the insoluble isoform expressed by astroglia treated with dibutyryl cAMP decreased whereas, the amount of total and insoluble beta-actin isoform expressed by the same cells did not show any significant changes. The cells treated with the cAMP failed to migrate and to close the area created by the scratch wound in monolayer culture. However, the non-treated cells migrated and closed the area created by the scratch after 3 days. This study shows that the astroglia have different mechanisms in regulating the expression of different actin isoforms and that the alpha-sm actin isoform is important in migration of astroglia.

Vertebrates have conserved capping protein alpha isoforms with specific expression patterns

Capping protein (CP), a ubiquitous actin binding protein composed of an alpha and a beta subunit, is important for actin assembly and cell motility. Lower organisms have one gene and one isoform of each subunit. Chickens have two very similar alpha-subunit isoforms. To determine if vertebrates in general contain multiple alpha isoforms and if those alpha isoforms have conserved sequences, we isolated and analyzed alpha subunit cDNA's in mice and humans. Both mice and humans also have two alpha isoforms. Phylogenetic analysis of the alpha isoform sequences reveals that vertebrates have two highly conserved subfamilies, alpha1 and alpha2. The alpha1 and alpha2 subfamilies are very similar to each other but can be defined and distinguished from each other by a small number of key amino acid residues. In addition, 3' untranslated cDNA sequences are conserved within the isoform subfamilies. To investigate the function of the alpha isoforms, we examined their expression in mouse cells and tissues. Endothelial cells contain only the alpha2 isoform, and erythrocytes contain almost exclusively the alpha1 isoform. Most tissues have both alpha1 and alpha2 isoforms but the ratio of alpha1:alpha2 varies widely. Together, these findings support the hypothesis that the CP alpha isoforms have conserved, unique and essential roles in vertebrates.