Immunolocalization of muscle and nonmuscle isoforms of actin in myogenic cells and adult skeletal muscle

Machine learning inference of continuous single-cell state transitions during myoblast differentiation and fusion

Cells modify their internal organization during continuous state transitions, supporting functions from cell division to differentiation. However, tools to measure dynamic physiological states of individual transitioning cells are lacking. We combined live-cell imaging and machine learning to monitor ERK1/2-inhibited primary murine skeletal muscle precursor cells, that transition rapidly and robustly from proliferating myoblasts to post-mitotic myocytes and then fuse, forming multinucleated myotubes. Our models, trained using motility or actin intensity features from single-cell tracking data, effectively tracked real-time continuous differentiation, revealing that differentiation occurs 7.5-14.5 h post induction, followed by fusion ~3 h later. Co-inhibition of ERK1/2 and p38 led to differentiation without fusion. Our model inferred co-inhibition leads to terminal differentiation, indicating that p38 is specifically required for transitioning from terminal differentiation to fusion. Our model also predicted that co-inhibition leads to changes in actin dynamics. Mass spectrometry supported these in silico predictions and suggested novel fusion and maturation regulators downstream of differentiation. Collectively, this approach can be adapted to various biological processes to uncover novel links between dynamic single-cell states and their functional outcomes.

Nucleotide- and Protein-Dependent Functions of Actg1

Cytoplasmic β- and γ-actin proteins are 99% identical but support unique organismal functions. The cytoplasmic actin nucleotide sequences Actb and Actg1, respectively, are more divergent but still 89% similar. Actb-/- mice are embryonic lethal and Actb-/- cells fail to proliferate, but editing the Actb gene to express γ-actin (Actbc-g) resulted in none of the overt phenotypes of the knockout revealing protein-independent functions for Actb. To determine if Actg1 has a protein-independent function, we crossed Actbc-g and Actg1-/- mice to generate the bG/0 line, where the only cytoplasmic actin expressed is γ-actin from Actbc-g. The bG/0 mice were viable but showed a survival defect despite expressing γ-actin protein at levels no different from bG/gG with normal survival. A unique myopathy phenotype was also observed in bG/0 mice. We conclude that impaired survival and myopathy in bG/0 mice are due to loss of Actg1 nucleotide-dependent function(s). On the other hand, the bG/0 genotype rescued functions impaired by Actg1-/-, including cell proliferation and auditory function, suggesting a role for γ-actin protein in both fibroblasts and hearing. Together, these results identify nucleotide-dependent functions for Actg1 while implicating γ-actin protein in more cell-/tissue-specific functions.

Nuclear Mechanotransduction in Skeletal Muscle

Skeletal muscle is composed of multinucleated, mature muscle cells (myofibers) responsible for contraction, and a resident pool of mononucleated muscle cell precursors (MCPs), that are maintained in a quiescent state in homeostatic conditions. Skeletal muscle is remarkable in its ability to adapt to mechanical constraints, a property referred as muscle plasticity and mediated by both MCPs and myofibers. An emerging body of literature supports the notion that muscle plasticity is critically dependent upon nuclear mechanotransduction, which is transduction of exterior physical forces into the nucleus to generate a biological response. Mechanical loading induces nuclear deformation, changes in the nuclear lamina organization, chromatin condensation state, and cell signaling, which ultimately impacts myogenic cell fate decisions. This review summarizes contemporary insights into the mechanisms underlying nuclear force transmission in MCPs and myofibers. We discuss how the cytoskeleton and nuclear reorganizations during myogenic differentiation may affect force transmission and nuclear mechanotransduction. We also discuss how to apply these findings in the context of muscular disorders. Finally, we highlight current gaps in knowledge and opportunities for further research in the field.

Regulation of actin isoforms in cellular and developmental processes

Actin is one of the most abundant and essential intracellular proteins that mediates nearly every form of cellular movement and underlies such key processes as embryogenesis, tissue integrity, cell division and contractility of all types of muscle and non-muscle cells. In mammals, actin is represented by six isoforms, which are encoded by different genes but produce proteins that are 95-99 % identical to each other. The six actin genes have vastly different functions in vivo, and the small amino acid differences between the proteins they encode are rigorously maintained through evolution, but the underlying differences behind this distinction, as well as the importance of specific amino acid sequences for each actin isoform, are not well understood. This review summarizes different levels of actin isoform-specific regulation in cellular and developmental processes, starting with the nuclear actin's role in transcription, and covering the gene-level, mRNA-level, and protein-level regulation, with a special focus on mammalian actins in non-muscle cells.

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.

Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms

While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic β_cyto - or γ_cyto -actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific β_cyto - and γ_cyto -actin KO mice. We found β_cyto - and γ_cyto -actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking β_cyto - and/or γ_cyto -actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific β_cyto - and γ_cyto -actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old β_cyto - and γ_cyto -actin muscle-specific KO mice. Our data suggest that impaired Ca²⁺ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.

Actin in hair cells and hearing loss

Hereditary deafness is genetically heterogeneous such that mutations of many different genes can cause hearing loss. This review focuses on the evidence and implications that several of these deafness genes encode actin-interacting proteins or actin itself. There is a growing appreciation of the contribution of the actin interactome in stereocilia development, maintenance, mechanotransduction and malfunction of the auditory system.

γ-Actin regulates cell migration and modulates the ROCK signaling pathway

Cell migration plays a crucial role in numerous cellular functions, and alterations in the regulation of cell migration are required for invasive transformation of a tumor cell. While the mechanistic process of actin-based migration has been well documented, little is known as to the specific function of the nonmuscle actin isoforms in mammalian cells. Here, we present a comprehensive examination of γ-actin's role in cell migration using an RNAi approach. The partial suppression of γ-actin expression in SH-EP neuroblastoma cells resulted in a significant decrease in wound healing and transwell migration. Similarly, the knockdown of γ-actin significantly reduced speed of motility and severely affected the cell's ability to explore, which was, in part, due to a loss of cell polarity. Moreover, there was a significant increase in the size and number of paxillin-containing focal adhesions, coupled with a significant decrease in phosphorylated paxillin in γ-actin-knockdown cells. In addition, there was a significant increase in the phosphorylation of cofilin and myosin regulatory light chain, suggesting an overactivated Rho-associated kinase (ROCK) signaling pathway in γ-actin-knockdown cells. The alterations in the phosphorylation of paxillin and myosin regulatory light chain were unique to γ-actin and not β-actin knockdown. Inhibition of the ROCK pathway with the inhibitor Y-27632 restored the ability of γ-actin-knockdown cells to migrate. This study demonstrates γ-actin as a potential upstream regulator of ROCK mediated cell migration.

Cytoplasmic gamma-actin and tropomodulin isoforms link to the sarcoplasmic reticulum in skeletal muscle fibers

Tropomodulins, cytoplasmic γ-actin, and small ankyrin 1.5 mechanically stabilize the sarcoplasmic reticulum and maintain myofibril alignment in skeletal muscle fibers.

The sarcoplasmic reticulum (SR) serves as the Ca²⁺ reservoir for muscle contraction. Tropomodulins (Tmods) cap filamentous actin (F-actin) pointed ends, bind tropomyosins (Tms), and regulate F-actin organization. In this paper, we use a genetic targeting approach to examine the effect of Tmod1 deletion on the organization of cytoplasmic γ-actin (γ_cyto-actin) in the SR of skeletal muscle. In wild-type muscle fibers, γ_cyto-actin and Tmod3 defined an SR microdomain that was distinct from another Z line–flanking SR microdomain containing Tmod1 and Tmod4. The γ_cyto-actin/Tmod3 microdomain contained an M line complex composed of small ankyrin 1.5 (sAnk1.5), γ_cyto-actin, Tmod3, Tm4, and Tm5NM1. Tmod1 deletion caused Tmod3 to leave its SR compartment, leading to mislocalization and destabilization of the Tmod3–γ_cyto-actin–sAnk1.5 complex. This was accompanied by SR morphological defects, impaired Ca²⁺ release, and an age-dependent increase in sarcomere misalignment. Thus, Tmod3 regulates SR-associated γ_cyto-actin architecture, mechanically stabilizes the SR via a novel cytoskeletal linkage to sAnk1.5, and maintains the alignment of adjacent myofibrils.

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.

Arginyltransferase regulates alpha cardiac actin function, myofibril formation and contractility during heart development

Post-translational arginylation mediated by arginyltransferase (Ate1) is essential for cardiovascular development and angiogenesis in mammals and directly affects myocardium structure in the developing heart. We recently showed that arginylation exerts a number of intracellular effects by modifying proteins involved in the functioning of the actin cytoskeleton and in cell motility. Here, we investigated the role of arginylation in the development and function of cardiac myocytes and their actin-containing structures during embryogenesis. Biochemical and mass spectrometry analyses showed that alpha cardiac actin undergoes arginylation at four sites during development. Ultrastructural analysis of the myofibrils in wild-type and Ate1 knockout mouse hearts showed that the absence of arginylation results in defects in myofibril structure that delay their development and affect the continuity of myofibrils throughout the heart, predicting defects in cardiac contractility. Comparison of cardiac myocytes derived from wild-type and Ate1 knockout mouse embryos revealed that the absence of arginylation results in abnormal beating patterns. Our results demonstrate cell-autonomous cardiac myocyte defects in arginylation knockout mice that lead to severe congenital abnormalities similar to those observed in human disease, and outline a new function of arginylation in the regulation of the actin cytoskeleton in cardiac myocytes.

The decrease of the cytoskeleton tubulin follows the decrease of the associating molecular chaperone alphaB-crystallin in unloaded soleus muscle atrophy without stretch

The cytoskeletal component tubulin/microtubule commonly allows the cell to respond mechanically to the environment. The concentration of free tubulin dimer is autoregulated in the balance of free dimer and polymeric forms of microtubule (MT) protein, having an intrinsic property of "dynamic instability", and through cotranslational beta-tubulin mRNA degradation. Recently, we have demonstrated that alphaB-crystallin is a key molecule of muscle atrophy, since alphaB-crystallin has a chaperone-like-activity that suppresses tubulin aggregation and protects the MT disassembly against both Ca2+ and depolymelizing alkaloid in vitro. Most of the small heat-shock proteins (sHsps), including alphaB-crystallin, are expressed in skeletal muscle. However, no report to date has studied the changes of tubulin/MT during muscle adaptation. Here, we examined changes in tubulin content in rat soleus muscles after hindlimb suspension (HS) with/without passive stretch and the recovery. HS induced rapid decreases of soleus muscle mass, most Hsps (alphaB-crystallin, Hsp90, Hsp70, Hsp27, and p20) and tubulin contents in soleus muscle, while heat-shock cognate 70-kDa protein (Hsc70) did not decrease. Soleus muscle mass, most Hsps, and tubulin were maintained with passive stretch. After 5 days' recovery, the levels of tubulin and Hsps, but not Hsc70, were restored to control levels. The interactions of alphaB-crystallin and tubulin/MT were observed with immunoprecipitation with an anti-alpha-tubulin antibody and taxol-dependent MT assembly. Other sHsps were also associated with alphaB-crystallin and MT, whereas Hsp90 and Hsp70 did not co-precipitate with them. These data imply an interaction and close relationship between alphaB-crystallin and tubulin/MTs in muscle tissues. The amount of mRNA of alphaB-crystallin decreased with the muscle atrophy level, whereas the gene expression level of betaI-tubulin was maintained during HS. This means a significant role of post-transcriptional regulation in tubulin/MT system in muscle adaptation, whereas alphaB-crystallin and most sHsps are regulated at the transcriptional level. Additional functional contribution of alphaB-crystallin to tubulin/MTs during myotube formation was examined using C2C12 myoblast cultured cells, the alphaB-crystallin expression of which was decreased or increased. It indicated the necessity of alphaB-crystallin during microtubule reorganization. In conclusion, tubulin/MTs were revealed to be one of the substrates of alphaB-crystallin, and also serial decreases of alphaB-crystallin and tubulin/MT in early soleus muscle atrophy suggest that the chaperone effect of alphaB-crystallin on the cytoskeleton, which may be also dynamically regulated in the muscle cell, is a key mechanism for muscle adaptation and protection of the atrophy and also muscle differentiation.

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.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Cytoplasmic gamma actin as a Z-disc protein

To investigate the precise localization of cytoplasmic gamma actin in skeletal muscle and the relationship to dystrophin molecules, we designed an antibody against the N-terminal peptide of cytoplasmic gamma actin. Western blot analysis using SDS-PAGE and isoelectric focusing (IEF) gel revealed that the antibody reacted only with the actin isoforms having gamma motility, confirming that the antibody is specific to the cytoplasmic (nonmuscle) gamma actin. Immunohistochemical analysis of the skeletal muscle of the adult mouse revealed a dot-like staining pattern of the antibody in transverse sections and a striated staining pattern in longitudinal sections. The double immunostaining technique revealed the colocalization of cytoplasmic gamma actin with alpha-actinin, implying the localization of the actin on the Z-disc. Contrary to previous findings (1), we did not detect the colocalization of cytochrome oxidase, a mitochondria marker, with this actin.

Alpha-skeletal actin induces a subset of muscle genes independently of muscle differentiation and withdrawal from the cell cycle

Muscle differentiation is characterized by the induction of genes encoding contractile structural proteins and the repression of nonmuscle isoforms from these gene families. We have examined the importance of this regulated order of gene expression by expressing the two sarcomeric muscle actins characteristic of the differentiated state, i.e. alpha-skeletal and alpha-cardiac actin, in C2 mouse myoblasts. Precocious accumulation of transcripts and proteins for a group of differentiation-specific genes was elicited by alpha-skeletal actin only: four muscle tropomyosins, two muscle actins, desmin and MyoD. The nonmuscle isoforms of tropomyosin and actin characteristic of the undifferentiated state continued to be expressed, and no myosin heavy or light chain or troponin transcripts characteristic of muscle differentiation were induced. Stable transfectants displayed a substantial reduction in cell surface area and in the levels of nonmuscle tropomyosins and beta-actin, consistent with a relationship between the composition of the actin cytoskeleton and cell surface area. The transfectants displayed normal cell cycle progression. We propose that alpha-skeletal actin can activate a regulatory pathway linking a subset of muscle genes that operates independently of normal differentiation and withdrawal from the cell cycle.

The dystrophin complex forms a mechanically strong link between the sarcolemma and costameric actin

The absence of dystrophin complex leads to disorganization of the force-transmitting costameric cytoskeleton and disruption of sarcolemmal membrane integrity in skeletal muscle. However, it has not been determined whether the dystrophin complex can form a mechanically strong bond with any costameric protein. We performed confocal immunofluorescence analysis of isolated sarcolemma that were mechanically peeled from skeletal fibers of mouse hindlimb muscle. A population of gamma-actin filaments was stably associated with sarcolemma isolated from normal muscle and displayed a costameric pattern that precisely overlapped with dystrophin. However, costameric actin was absent from all sarcolemma isolated from dystrophin-deficient mdx mouse muscle even though it was localized to costameres in situ. Vinculin, alpha-actinin, beta-dystroglycan and utrophin were all retained on mdx sarcolemma, indicating that the loss of costameric actin was not due to generalized membrane instability. Our data demonstrate that the dystrophin complex forms a mechanically strong link between the sarcolemma and the costameric cytoskeleton through interaction with gamma-actin filaments. Destabilization of costameric actin filaments may also be an important precursor to the costamere disarray observed in dystrophin-deficient muscle. Finally, these methods will be broadly useful in assessing the mechanical integrity of the membrane cytoskeleton in dystrophic animal models lacking other costameric proteins.

The cytoskeleton of the electric tissue of Electrophorus electricus, L

The electric eel Electrophorus electricus is a fresh water teleost showing an electrogenic tissue that produces electric discharges. This electrogenic tissue is distributed in three well-defined electric organs which may be found symmetrically along both sides of the eel. These electric organs develop from muscle and exhibit several biochemical properties and morphological features of the muscle sarcolema. This review examines the contribution of the cytoskeletal meshwork to the maintenance of the polarized organization of the electrocyte, the cell that contains all electric properties of each electric organ. The cytoskeletal filaments display an important role in the establishment and maintenance of the highly specialized membrane model system of the electrocyte. As a muscular tissue, these electric organs expresses actin and desmin. The studies that characterized these cytoskeletal proteins and their implications on the electrophysiology of the electric tissues are revisited.

Distinct families of Z-line targeting modules in the COOH-terminal region of nebulin

To learn how nebulin functions in the assembly and maintenance of I-Z-I bands, MYC- and GFP- tagged nebulin fragments were expressed in primary cultured skeletal myotubes. Their sites of incorporation were visualized by double staining with anti-MYC, antibodies to myofibrillar proteins, and FITC- or rhodamine phalloidin. Contrary to expectations based on in vitro binding studies, none of the nebulin fragments expressed in maturing myotubes were incorporated selectively into I-band approximately 1.0-micrometer F-alpha-actin-containing thin filaments. Four of the MYC/COOH-terminal nebulin fragments were incorporated exclusively into periodic approximately 0.1-micrometer Z-bands. Whereas both anti-MYC and Rho-phalloidin stained intra-Z-band F-alpha-actin oligomers, only the latter stained the pointed ends of the polarized approximately 1.0-micrometer thin filaments. Z-band incorporation was independent of the nebulin COOH-terminal Ser or SH3 domains. In vitro cosedimentation studies also demonstrated that nebulin SH3 fragments did not bind to F-alpha-actin or alpha-actinin. The remaining six fragments were not incorporated into Z-bands, but were incorporated (a) diffusely throughout the sarcoplasm and into (b) fibrils/patches of varying lengths and widths nested among normal striated myofibrils. Over time, presumably in response to the mediation of muscle-specific homeostatic controls, many of the ectopic MYC-positive structures were resorbed. None of the tagged nebulin fragments behaved as dominant negatives; they neither blocked the assembly nor induced the disassembly of mature striated myofibrils. Moreover, they were not cytotoxic in myotubes, as they were in the fibroblasts and presumptive myoblasts in the same cultures.

Dystrophin binding to nonmuscle actin

We purified actin from bovine brain by DNase I affinity chromatography in order to compare the binding of dystrophin to muscle actin with its binding to nonmuscle actin. While both beta- and gamma-nonmuscle actins are expressed in brain, Western blot analysis with isoform-specific antibodies indicated that our purified brain actin was exclusively the gamma-isoform. The recombinant amino-terminal, actin-binding domain of dystrophin bound to muscle and brain actin in a saturable manner (approximately 1 mol/mol actin) with similar Kd values of 13.7+/-3.5 and 10.6+/-3.7 microM, respectively. We further demonstrate that intact dystrophin in the dystrophin-glycoprotein complex bound with equal avidity to muscle and brain F-actin. These data argue that a preferential binding of dystrophin to nonmuscle actin is not the basis for its targeting to the muscle cell plasmalemma but do support the hypothesis that dystrophin is capable of interacting with filamentous actin in nonmuscle tissues.

Isoform sorting and the creation of intracellular compartments

The generation of isoforms via gene duplication and alternative splicing has been a valuable evolutionary tool for the creation of biological diversity. In addition to the formation of molecules with related but different functional characteristics, it is now apparent that isoforms can be segregated into different intracellular sites within the same cell. Sorting has been observed in a wide range of genes, including those encoding structural molecules, receptors, channels, enzymes, and signaling molecules. This results in the creation of intracellular compartments that (a) can be independently controlled and (b) have different functional properties. The sorting mechanisms are likely to operate at the level of both proteins and mRNAs. Isoform sorting may be an important consequence of the evolution of isoforms and is likely to have contributed to the diversity of functional properties within groups of isoforms.

Subcutaneous tissue fibroblasts transfected with muscle and nonmuscle actins: A good in vitro model to study fibroblastic cell plasticity

Cultured fibroblasts develop several biochemical and morphological properties of smooth muscle cells, particularly the expression of alpha-smooth muscle actin, the actin isoform typical of vascular smooth muscle cells. They resemble modified fibroblasts or myofibroblasts observed in granulation tissue during wound repair and in fibrotic situations. We have analysed by immunolabeling the fate of exogenous epitope-tagged actin isoforms by transfection of the corresponding cDNAs into fibroblasts cultured from rat subcutaneous tissue. Tagged muscle actins were efficiently integrated into stress fibers and did not produce obvious changes in cell shape of transfected cells. Transfected nonmuscle actins in contrast changed the morphology and were not or poorly incorporated into stress fibers. These cultured subcutaneous fibroblasts behave similarly to smooth muscle cells when transfected with the same actin encoding cDNAs, indicating another common characteristic of these two cell types in sorting and targeting actin isoforms. Subcutaneous fibroblasts transfected with muscle and nonmuscle actin isoforms provide a good in vitro model to analyze the intracellular sorting of isoactins and to improve our knowledge of myofibroblast characterization and differentiation during tissue repair as well as to understand the relationships between modifications of actin cytoskeleton, adhesion and extracellular matrix proteins.

beta-Actin messenger RNA localization and protein synthesis augment cell motility

In chicken embryo fibroblasts (CEFs), beta-actin mRNA localizes near an actin-rich region of cytoplasm specialized for motility, the lamellipodia. This localization is mediated by isoform-specific 3'-untranslated sequences (zipcodes) and can be inhibited by antizipcode oligodeoxynucleotides (ODNs) (Kislauskis, E.H., X.-C. Zhu, and R.H. Singer. 1994. J. Cell Biol. 127: 441-451). This inhibition of beta-actin mRNA localization resulted in the disruption of fibroblast polarity and, presumably, cell motility. To investigate the role of beta-actin mRNA in motility, we correlated time-lapse images of moving CEFs with the distribution of beta-actin mRNA in these cells. CEFs with localized beta-actin mRNA moved significantly further over the same time period than did CEFs with nonlocalized mRNA. Antizipcode ODN treatment reduced this cell translocation while control ODN treatments showed no effect. The temporal relationship of beta-actin mRNA localization to cell translocation was investigated using serum addition to serum-deprived cultures. beta-actin mRNA was not localized in serum-deprived cells but became localized within minutes after serum addition (Latham, V.M., E.H. Kislauskis, R.H. Singer, and A.F. Ross. 1994. J. Cell Biol. 126:1211-1219). Cell translocation increased over the next 90 min, and actin synthesis likewise increased. Puromycin reduced this cell translocation and blocked this induction in cytosolic actin content. The serum induction of cell movement was also inhibited by antizipcode ODNs. These observations support the hypothesis that beta-actin mRNA localization and consequent protein synthesis augment cell motility.

Sorting of actin isoforms in chicken auditory hair cells

Most nonmuscle cells of higher vertebrates contain two different actin isoforms, beta- and gamma-cytoplasmic actin. The beta-isoform is with few exceptions the predominant isoform in nonmuscle cells and tissues. Perturbation of the beta:gamma ratio has been shown to affect the organization of bundled actin filaments indicating that the beta- and gamma-genes encode functionally distinct cytoarchitectural information. In the present study we localized by immunostaining beta- and gamma-actin in chicken auditory hair cells. These highly specialized cells serve as model system for studying certain developmental and structural aspects of a complex actin filament system with high architectural precision. We show that gamma-actin is the predominant actin isoform in auditory hair cells with an apparent beta:gamma ratio of approximately 1:2. gamma-Actin is not sorted and occurs in all three actin assemblies of the hair border, i.e. the cores of sensory hairs (stereocilia), the subjacent gel-like actin filament meshwork (cuticular plate) and the zonula adherens ring. In contrast to gamma-actin, the beta-isoform is specifically sorted to the actin filament core bundle of stereocilia that is extensively crosslinked by fimbrin. In view of recent studies showing that L-plastin, the leukocyte homolog of fimbrin, has a higher binding affinity for beta-actin than for gamma-actin, a mechanism is proposed for how hair cells might restrict formation of actin filament bundles to a single cellular site (i.e. the stereocilia). The limited level of expression of beta-actin in hair cells may help to prevent ectopic bundle formation in other cellular compartments.

Tropomyosin localization reveals distinct populations of microfilaments in neurites and growth cones

The functional and structural differences between neurites and growth cones suggests the possibility that distinct microfilament populations may exist in each domain. Tropomyosins are integral components of the actin-based microfilament system. Using antibodies which detect three different sets of tropomyosin isoforms, we found that the vast majority of tropomyosin was found in a microfilament-enriched fraction of cultured cortical neurons, therefore enabling us to use the antisera to evaluate compositional differences in neuritic and growth cone microfilaments. An antibody which reacts with all known nonmuscle isoforms of the alpha Tms gene (Tm5NM1-4) stains both neurites and growth cones, whereas a second antibody against the isoform subset, Tm5NM1-2, reacts only with the neurite. A third antibody which reacts with the Tm5a/5b isoforms encoded by a separate gene from alpha Tms was strongly reactive with both neurites and growth cones in 16-h cultures but only with the neurite shaft in 40-h cultures. Treatment of neurons with cytochalasin B allowed neuritic Tm5NM1-2 to spread into growth cones. Removal of the drug resulted in the disappearance of Tm5NM1-2 from the growth cone, indicating that isoform segregation is an active process dependent on intact microfilaments. Treatment of 40-h cultures with nocodazole resulted in the removal of Tm5NM1-2 from the neurite whereas Tm5a/5b now spread back into the growth cone. We conclude that the organization of Tm5NM1-2 and Tm5a/5b in the neurite is at least partially dependent on microtubule integrity. These results indicate that tropomyosin isoforms Tm5NM1-2, Tm5NM3-4, and Tm5a/5b mark three distinct populations of actin filaments in neurites and growth cones. Further, the composition of microfilaments differs between neurites and growth cones and is subject to temporal regulation.

Dynamics of capping protein and actin assembly in vitro: uncapping barbed ends by polyphosphoinositides

Bursts of actin polymerization in vivo involve the transient appearance of free barbed ends. To determine how rapidly barbed ends might appear and how long they might remain free in vivo, we studied the kinetics of capping protein, the major barbed end capper, binding to barbed ends in vitro. First, the off-rate constant for capping protein leaving a barbed end is slow, predicting a half-life for a capped barbed end of approximately 30 min. This half-life implies that cells cannot wait for capping protein to spontaneously dissociate from capped barbed ends in order to create free barbed ends. However, we find that phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-mono-phosphate (PIP) cause rapid and efficient dissociation of capping protein from capped filaments. PIP2 is a strong candidate for a second messenger regulating actin polymerization; therefore, the ability of PIP2 to remove capping protein from barbed ends is a potential mechanism for stimulating actin polymerization in vivo. Second, the on-rate constant for capping protein binding to free barbed ends predicts that actin filaments could grow to the length of filaments observed in vivo during one lifetime. Third, capping protein beta-subunit isoforms did not differ in their actin binding properties, even in tests with different actin isoforms. A major hypothesis for why capping protein beta-subunit isoforms exist is thereby excluded. Fourth, the proposed capping protein regulators, Hsc70 and S100, had no effect on capping protein binding to actin in vitro.

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

The intracompartmental sorting and functional consequences of ectopic expression of the six vertebrate actin isoforms was investigated in different types of cultured cells. In transfected fibroblasts all isoactin species associated with the endogenous microfilament cytoskeleton, even though cytoplasmic actins also showed partial localization to peripheral submembranous sites. Functional and structural studies were performed in neonatal and adult rat cardiomyocytes. All the muscle isoactin constructs sorted preferentially to sarcomeric sites and, to a lesser extent, also to stress-fiber-like structures. The expression of muscle actins did not interfere with cell contractility, and did not disturb the localization of endogenous sarcomeric proteins. In sharp contrast, ectopic expression of the two cytoplasmic actin isoforms resulted in rapid cessation of cellular contractions and induced severe morphological alterations characterized by an exceptional outgrowth of filopodia and cell flattening. Quantitative analysis in neonatal cardiomyocytes indicated that the levels of accumulation of the different isoactins are very similar and cannot be responsible for the observed isoproteins-specific effects. Structural analysis revealed a remodeling of the cytoarchitecture including a specific alteration of sarcomeric organization; proteins constituting the sarcomeric thin filaments relocated to nonmyofibrillar sites while thick filaments and titin remained unaffected. Experiments with chimeric proteins strongly suggest that isoform specific residues in the carboxy-terminal portion of the cytoplasmic actins are responsible for the dominant negative effects on function and morphology.

Indirect association of ezrin with F-actin: isoform specificity and calcium sensitivity

Whereas it has been demonstrated that muscle and nonmuscle isoactins are segregated into distinct cytoplasmic domains, the mechanism regulating subcellular sorting is unknown (Herman, 1993a). To reveal whether isoform-specific actin-binding proteins function to coordinate these events, cell extracts derived from motile (Em) versus stationary (Es) cytoplasm were selectively and sequentially fractionated over filamentous isoactin affinity columns prior to elution with a KCl step gradient. A polypeptide of interest, which binds specifically to beta-actin filament columns, but not to muscle actin columns has been conclusively identified as the ERM family member, ezrin. We studied ezrin-beta interactions in vitro by passing extracts (Em) over isoactin affinity matrices in the presence of Ca(2+)-containing versus Ca(2+)-free buffers, with or without cytochalasin D. Ezrin binds and can be released from beta-actin Sepharose-4B in the presence of Mg2+/EGTA and 100 mM NaCl (at 4 degrees C and room temperature), but not when affinity fractionation of Em is carried out in the presence of 0.2 mM CaCl2 or 2 microM cytochalasin D. N-acetyl-(leucyl)2-norleucinal and E64, two specific inhibitors of the calcium-activated protease, calpain I, protect ezrin binding to beta actin in the presence of calcium. Moreover, biochemical analysis of endothelial lysates reveals that a calpain I cleavage product of ezrin emerges when cell locomotion is stimulated in response to monolayer injury. Immunofluorescence analysis of leading lamellae reveals that anti-ezrin and anti-beta-actin IgGs can be simultaneously co-localized, extending the results of isoactin affinity fractionation of Em-derived extracts and suggesting that ezrin and beta-actin interact in vivo. To test the hypothesis that ezrin binds directly to beta-actin, we performed three sets of studies under a wide range of physiological conditions (pH 7.0-8.5) using purified pericyte ezrin and either alpha- or beta-actin. These included co-sedimentation, isoactin affinity fractionation, and co-immunoprecipitation. Results of these experiments reveal that purified ezrin does not directly bind to beta-actin filaments, either in solution or while isoactins are covalently cross-linked to Sepharose-4B. This is in contrast to our finding that ezrin and beta-actin could be co-immunoprecipitated or co-sedimented from Em-derived cell lysates. To explore whether calcium transients occur in cellular domains enriched in ezrin and beta-actin, we mapped cellular free calcium in endothelial monolayers crawling in response to injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential localization and sequence analysis of capping protein beta-subunit isoforms of vertebrates

Capping protein nucleates the assembly of actin filaments and stabilizes actin filaments by binding to their barbed ends. We describe here a novel isoform of the beta subunit of chicken capping protein, the beta 2 isoform, which arises by alternative splicing. The chicken beta 1 isoform and the beta 2 isoform are identical in their amino acid sequence except for a short region at the COOH terminus; this region of the beta subunit has been implicated in binding actin. Human and mouse cDNAs of the beta 1 and beta 2 isoforms also were isolated and among these vertebrates, the COOH-terminal region of each isoform is highly conserved. In contrast, comparison of the sequences of the vertebrate beta subunit COOH-termini to those of lower eukaryotes shows no similarities. The beta 2 isoform is the predominant isoform of nonmuscle tissues and the beta 1 isoform, which was first characterized in studies of capping protein from chicken muscle, is the predominant isoform of muscle tissues, as shown by immunoblots probed with isoform-specific antibodies and by RNAse protection analysis of mRNAs. The beta 2 isoform also is a component of dynactin complex from brain, which contains the actin-related protein Arp1. Both beta-subunit isoforms are expressed in cardiac muscle but they have non-overlapping subcellular distributions. The beta 1 isoform is at Z-discs of myofibrils, and the beta 2 isoform is enriched at intercalated discs; in cardiac myocytes grown in culture, the beta 2 isoform also is a component of cell-cell junctions and at sites where myofibrils contact the sarcolemma. The biochemical basis for the differential distribution of capping protein isoforms is likely due to interaction with specific proteins at Z-discs and cell-cell junctions, or to preferential association with different actin isoforms. Thus, vertebrates have developed isoforms of capping protein that associate with distinct actin-filament arrays.

Ultrastructural and immunohistochemical studies in callitrichid renal glomeruli

The "normal" mesangium of callitrichids exhibits certain features pointing to enhanced activity. Protrusions of the mesangial cellular cytoplasm (blebs) into the capillary lumen were observed very frequently as were electron-dense granules in mesangial matrix channels. Histochemical, alpha-actin expression was invariably observed in the mesangial cells of callitrichids.

Actin isoform compartments in chicken gizzard smooth muscle cells

Differentiated smooth muscle cells typically contain a mixture of muscle (alpha and gamma) and cytoplasmic (beta and gamma) actin isoforms. Of the cytoplasmic actins the beta-isoform is the more dominant, making up from 10% to 30% of the total actin complement. Employing an antibody raised against the N-terminal peptide specific to beta-actin, which labels only the beta-isoform on two-dimensional gel immunoblots, we have shown that this isoform has a restricted localisation in smooth muscle. Using double-label immunofluorescence and immunoelectron microscopy of ultrathin sections of chicken gizzard, beta-actin was localised in the dense bodies and in longitudinal channels linking consecutive dense bodies that were also occupied by desmin. It was additionally found in the membrane-associated dense plaques, but was excluded from the actomyosin-containing regions of the contractile apparatus. Taken together with earlier results these findings identify a cytoskeletal compartment containing intermediate filaments, cytoplasmic actin and the actin cross-linking protein filamin. Using an antibody specific only for muscle actin, labelling was found generally around the myosin filaments of the contractile apparatus, but was absent from the core of the dense bodies that contained beta-actin. Thus, if dense bodies act as dual-purpose anchorage sites, for the cytoskeletal actin and the contractile actin, the thin filaments of the contractile apparatus must be anchored at the periphery of the dense bodies. A model of the structural organisation of the cell is presented and the possible roles of the cytoskeleton are discussed.

Expression of actin isoforms in the guinea pig organ of Corti: muscle isoforms are not detected

The specificity of antibodies to actin was assayed by use of immunoblots and histological sections of control tissues enriched for each of six different isoforms. On immunoblots, all antibodies stained at most one band of protein in most of the control materials, with a molecular weight of approximately 43 kDa. Their pattern of staining of muscle and nonmuscle tissues indicated their isoform specificity. On tissue sections, immunocytochemical staining demonstrated cellular and subcellular localization of the different isoforms. Once characterized with regard to specificity, these antibodies were used to probe actin in the guinea pig organ of Corti. None of the four muscle isoforms of actin were found in either immunoblots or tissue sections of the organ of Corti. Both beta- and gamma-cytoplasmic isoforms of actin were present in hair cells and supporting cells. This leaves open to investigation the role which cytoplasmic actins play in these cells of the organ of Corti.

Beta-actin specific monoclonal antibody

Using a synthetic peptide mimicking the NH2-terminus of beta-actin we have raised a monoclonal antibody specific for this cytoplasmic actin isoform. Specificity of the antibody was demonstrated by its labelling of the actin polypeptide only in tissues containing the beta isoform, by its exclusive recognition of the synthetic beta-actin peptide amongst those mimicking all six vertebrate isoactins, and by its selective recognition of the beta-actin spot in two-dimensional electrophoresis gels of smooth muscle extracts. The antibody bound to actin filaments in both living and fixed fibroblasts where it labelled the stress fiber bundles and, more predominantly, the peripheral actin rich lamellipodia. The characteristics of the antibody indicate that it should serve as a useful tool for studying isoactin distribution and function.

Isoform-specific 3'-untranslated sequences sort alpha-cardiac and beta-cytoplasmic actin messenger RNAs to different cytoplasmic compartments

We demonstrate that in differentiating myoblasts, the mRNAs encoding two actin isoforms, beta-cytoplasmic, and alpha-cardiac, can occupy different cytoplasmic compartments within the same cytoplasm. beta-actin mRNA is localized to the leading lamellae and alpha-actin mRNA is associated with a perinuclear compartment. This was revealed by co-hybridizing, in situ, fluorochrome-conjugated oligonucleotide probes specific for each isoform. To address the mechanism of isoform-specific mRNA localization, molecular chimeras were constructed by insertion of actin sequences between the Lac Z coding region and SV-40 3'UTR in a reporter plasmid. These constructs were transiently expressed in a mixed culture of embryonic fibroblasts, myoblasts and myotubes, beta-galactosidase activity within transfectants was revealed by a brief incubation with its substrate (X-gal). Since the blue-insoluble reaction product co-localized with the specific mRNAs expressed from each construct, it was used as a bioassay for mRNA localization. Transfectants were scored as either perinuclear, peripheral or nonlocalized with respect to the distribution of the blue product. The percentage of transfectants within those categories was quantitated as a function of the various constructs. This analysis revealed that for each actin mRNA its 3'UTR is necessary and sufficient to direct reporter transcripts to its appropriate compartment; beta-actin peripheral and alpha-actin perinuclear. In contrast, sequences from the 5'UTR through the coding region of either actin gene did not localize the blue product. Therefore, 3'UTR sequences play a key role in modulating the distribution of actin mRNAs in muscle cells. We propose that the mechanism of mRNA localization facilitates actin isoform sorting in the cytoplasm.

Beta and gamma actin mRNAs are differentially located within myoblasts

Actin is of fundamental importance to all eukaryotic cells. Of the six mammalian actins, beta (beta) and gamma (gamma) cytoplasmic are the isoforms found in all nonmuscle cells and differ by only four amino acids at the amino-terminal region. Both genes are regulated temporally and spatially, though no differences in protein function have been described. Using fluorescent double in situ hybridization we describe the simultaneous intracellular localization of both beta and gamma actin mRNA. This study shows that myoblasts differentially segregate the beta and gamma actin mRNAs. The distribution of gamma actin mRNA, only to perinuclear and nearby cytoplasm, suggests a distribution based on diffusion or restriction to nearby cytoplasm. The distribution of beta actin mRNA, perinuclear and at the cell periphery, implicates a peripheral localizing signal which is unique to the beta isoform. The peripheral beta actin mRNA corresponded to cellular morphologies, extending processes, and ruffling edges that reflect cell movement. Total actin and gamma actin protein steady-state distributions were identified by specific antibodies. gamma actin protein was found in both stress fibers and at the cell plasma membrane and does not correspond to its mRNA distribution. We suggest that localized protein synthesis rather than steady-state distribution functionally differentiates between the actin isoforms.

Molecular genetics of actin function

Images

Noncoding regions of the gamma-actin gene influence the impact of the gene on myoblast morphology

We have addressed the question of whether two highly conserved noncoding regions of the gamma-actin gene are of functional importance. Human gamma-actin gene constructs deleted for either the entire 3' untranslated region (UTR) and 3' flank or intron III sequences were transfected into mouse myoblasts and the resulting clones were analyzed for cell morphology and actin protein expression. Transfectants carrying the gamma-actin gene deleted for the 3' end (gamma 22) exhibited numerous long pseudopods and increased surface area. In contrast, transfectants expressing the gamma-actin gene deleted for intron III (gamma 156) were rounded with blebs over the cell surface and showed decreased surface area. The relative expression of beta- to gamma-actin protein decreased for both transfectant types. The total actin protein levels remained constant for the gamma 22 cells but decreased for the gamma 156 cells. The results indicate that alterations to transfectant cell morphology can be influenced by the presence or absence of different noncoding regions in the transfected gamma-actin gene. The mechanisms by which noncoding regions of the gamma-actin gene influence the impact of the gene are unknown. Nevertheless, these noncoding regions are isoform specific and highly conserved in evolution. We propose that the functional significance of the different actin isoforms may involve the properties of these noncoding regions in addition to the differences in protein sequence.

Actin isoforms

The actin supergene family encodes a number of structurally related, but perhaps functionally distinct, protein isoforms that regulate contractile potential in muscle tissues and help to control the shape as well as the motility of non-muscle cells. In spite of the documented conservation amongst isoactin genes and their encoded proteins, recent results of biochemical, antibody localization, molecular mutagenesis and isoactin gene replacement studies lend credence to the notion that functional differences amongst muscle and non-muscle actin isoforms exist. Furthermore, the discovery of a new class of actin isoforms, the actin-related proteins, reveals that the actin gene and protein isoform family is more complex than was previously believed.

High level expression of transfected beta- and gamma-actin genes differentially impacts on myoblast cytoarchitecture

The impact of the human beta- and gamma-actin genes on myoblast cytoarchitecture was examined by their stable transfection into mouse C2 myoblasts. Transfectant C2 clones expressing high levels of human beta-actin displayed increases in cell surface area. In contrast, C2 clones with high levels of human gamma-actin expression showed decreases in cell surface area. The changes in cell morphology were accompanied by changes in actin stress-fiber organization. The beta-actin transfectants displayed well-defined filamentous organization of actin; whereas the gamma-actin transfectants displayed a more diffuse organization of the actin cables. The role of the beta-actin protein in generating the enlarged cell phenotype was examined by transfecting a mutant form of the human beta-actin gene. Transfectant cells were shown to incorporate the aberrant actin protein into stress-fiber-like structures. High level expression of the mutant beta-actin produced decreases in cell surface area and disruption of the actin microfilament network similar to that seen with transfection of the gamma-actin gene. In contrast, transfection of another mutant form of the beta-actin gene which encodes an unstable protein had no impact on cell morphology or cytoarchitecture. These results strongly suggest that it is the nature of the encoded protein that determines the morphological response of the cell. We conclude that the relative gene expression of beta- and gamma-actin is of relevance to the control of myoblast cytoarchitecture. In particular, we conclude that the beta- and gamma-actin genes encode functionally distinct cytoarchitectural information.

Onset of expression and regional deposition of alpha-smooth and sarcomeric actin during avian heart development

The sequential appearance of mRNAs for smooth, cardiac, and skeletal alpha-actin has been described during development of the chicken heart (Ruzicka, D.L., and R.J. Schwartz 1988 J. Cell Biol., 107:2575-2586). To assess whether this reflects the deposition of corresponding isoproteins, we have immunocytochemically localized smooth and sarcomeric (cardiac and skeletal) alpha-actin in Hamburger-Hamilton (H-H) stage 7-18 embryos using monoclonal antibodies. Within the developing embryo at stage 9-, smooth muscle alpha-actin was exclusively detected in the developing heart, upon fusion of the endocardial tubes; sarcomeric alpha-actin was observed later (stage 9). By the onset of contraction at stage 10+, intense immunostaining of both smooth and sarcomeric isoproteins was observed in the ventricle; at this time smooth muscle alpha-actin was also detected in splanchnic mesoderm of the pre-vitelline area, in a cellular layer adjacent to the only embryonic cells that exhibited factor VIII (von Willebrand factor) antigens. Double immunostaining of the myocardium at stage 11, at which time striations were first detected, revealed the co-existence of smooth and sarcomeric actin in developing sarcomeres. Intense expression of sarcomeric actin continued in the heart after stage 11, whereas smooth muscle alpha-actin was down-regulated in the ventricle and became regionalized to the inflow and outflow tracts. As expected, smooth muscle alpha-actin was detected around intra- and extra-embryonic vascular structures at later developmental stages, while sarcomeric actin was observed in somites.

Chicken cardiac myofibrillogenesis studied with antibodies specific for titin and the muscle and nonmuscle isoforms of actin and tropomyosin

Myofibrillogenesis was studied in cultured chick cardiomyocytes using indirect immunofluorescence microscopy and antibodies against alpha- and gamma-actin, muscle and nonmuscle tropomyosin, muscle myosin, and titin. Initially, cardiomyocytes, devoid of myofibrils, developed variable numbers of stress fiber-like structures with uniform staining for anti-muscle and nonmuscle actin and tropomyosin, and diffuse, weak staining with anti-titin. Anti-myosin labeled bundles of filaments that exhibited variable degrees of association with the stress fiber-like structures. Myofibrillogenesis occurred with a progressive, and generally simultaneous, longitudinal reorganization of stress fiber-like structures to form primitive sarcomeric units. Titin appeared to attain its mature pattern before the other major contractile proteins. Changes in the staining patterns of actin, tropomyosin, and myosin as myofibrils matured were interpreted as due to longitudinal filament alignment occurring before ordering in the axial direction. Non-muscle actin and tropomyosin were found with sarcomeric periodicity in the initial stages of sarcomere myofibrillogenesis, although their staining patterns were not identical. The localization of the "sarcomeric" proteins alpha-actin and muscle tropomyosin in stress fiber-like structures and the incorporation of non-muscle proteins in the initial stages of sarcomere organization bring into question the meaning of "sarcomeric" proteins in regard to myofibrillogenesis.

Distribution of myosin mRNA during development and regeneration of skeletal muscle fibers

Myosin mRNA distribution among subcellular compartments of anterior tibialis muscles in rabbit is monitored by in situ hybridization. A high density of mRNA was widely distributed throughout myotubes from 29-day fetal muscle and from regenerating adult muscle. All cytoplasmic spaces contained mRNA except where scattered myofibrils and centrally located nuclei were found. In fibers from 22-week-old rabbits, myosin mRNA was concentrated under the sarcolemma and excluded from the consolidated myofibrils and peripheral nuclei. The dispersal of mRNA through the cytoplasm in myotubes suggests that translation of myosin is widespread and that rapid myofibril assembly can occur throughout the fiber.

Post-translational incorporation of actin into myofibrils in vitro: evidence for isoform specificity

The incorporation of actin into myofibrils has been examined in a cell-free system [Bouché et al.: Journal of Cell Biology 107:587-596, 1988; Goldfine et al.: Cellular and Molecular Biology of Muscle Development, 1989]. Actin was translated in a reticulocyte lysate in the presence of 35S-methionine (35S-actin) or purified from muscle and labeled with fluorescein-5-isothiocyanate (FITC-actin). Myofibrils were incubated with either 35S-actin or FITC-actin and then analyzed by gel electrophoresis or fluorescence microscopy. When myofibrils were incubated with FITC-actin monomer in the reticulocyte lysate buffer, strong fluorescent labeling was observed in Z-band regions and less so in I-bands. No fluorescence was detected in non-overlap regions of A-bands. Confocal microscopic analysis of these myofibrils indicated that FITC-actin was distributed evenly across the diameter of the myofibrils. These observations suggest that actin incorporation in the reticulocyte lysate buffer occurred at sites in the sarcomere which contain actin. In contrast, FITC-actin showed a variety of non-physiological incorporation patterns when incubated with myofibrils in the presence of an isotonic buffer (I-buffer). However, when ATP was added to I-buffer, FITC-actin showed a pattern of incorporation into myofibrils similar to that seen in the reticulocyte lysate buffer. Immunoblots indicated that actin of native size was released from myofibrils during incubation in the reticulocyte lysate buffer. No actin release was detected when the myofibrils were incubated in I-buffer lacking ATP. We used this system to compare the incorporation of actin isoforms into myofibrils. Both alpha- and beta-actins exhibited incorporation into the myofibrils but there was a three-fold greater incorporation of the alpha isoform. We propose that the differential affinities of actin isoforms for myofibrils and other cytoskeletal structures could provide a mechanism for actin isoform targeting within the cytoplasm.

Extra actin filaments at the periphery of skeletal muscle myofibrils

Myofibrils isolated from a variety of vertebrate muscle fibers have a set of peripheral filaments associated with the periphery of the Z line free to move away from the surface of the myofibril. Decoration with myosin subfragment 1 shows that these are actin filaments.