Myogenic cells express multiple myosin isoforms

Formation of Aberrant Myotubes by Myoblasts Lacking Myosin VI Is Associated with Alterations in the Cytoskeleton Organization, Myoblast Adhesion and Fusion

We have previously postulated that unconventional myosin VI (MVI) could be involved in myoblast differentiation. Here, we addressed the mechanism(s) of its involvement using primary myoblast culture derived from the hindlimb muscles of Snell’s waltzer mice, the natural MVI knockouts (MVI-KO). We observed that MVI-KO myotubes were formed faster than control heterozygous myoblasts (MVI-WT), with a three-fold increase in the number of myosac-like myotubes with centrally positioned nuclei. There were also changes in the levels of the myogenic transcription factors Pax7, MyoD and myogenin. This was accompanied by changes in the actin cytoskeleton and adhesive structure organization. We observed significant decreases in the levels of proteins involved in focal contact formation, such as talin and focal adhesion kinase (FAK). Interestingly, the levels of proteins involved in intercellular communication, M-cadherin and drebrin, were also affected. Furthermore, time-dependent alterations in the levels of the key proteins for myoblast membrane fusion, myomaker and myomerger, without effect on their cellular localization, were observed. Our data indicate that in the absence of MVI, the mechanisms controlling cytoskeleton organization, as well as myoblast adhesion and fusion, are dysregulated, leading to the formation of aberrant myotubes.

Actomyosin contractility scales with myoblast elongation and enhances differentiation through YAP nuclear export

Skeletal muscle fibers are formed by the fusion of mononucleated myoblasts into long linear myotubes, which differentiate and reorganize into multinucleated myofibers that assemble in bundles to form skeletal muscles. This fundamental process requires the elongation of myoblasts into a bipolar shape, although a complete understanding of the mechanisms governing skeletal muscle fusion is lacking. To address this question, we consider cell aspect ratio, actomyosin contractility and the Hippo pathway member YAP as potential regulators of the fusion of myoblasts into myotubes. Using fibronectin micropatterns of different geometries and traction force microscopy, we investigated how myoblast elongation affects actomyosin contractility. Our findings indicate that cell elongation enhances actomyosin contractility in myoblasts, which regulate their actin network to their spreading area. Interestingly, we found that the contractility of cell pairs increased after their fusion and raise on elongated morphologies. Furthermore, our findings indicate that myoblast elongation modulates nuclear orientation and triggers cytoplasmic localization of YAP, increasing evidence that YAP is a key regulator of mechanotransduction in myoblasts. Taken together, our findings support a mechanical model where actomyosin contractility scales with myoblast elongation and enhances the differentiation of myoblasts into myotubes through YAP nuclear export.

How myosin organization of the actin cytoskeleton contributes to the cancer phenotype

The human genome contains 39 genes that encode myosin heavy chains, classified on the basis of their sequence similarity into 12 classes. Most cells express at least 12 different genes, from at least 8 different classes, which are typically composed of several class 1 genes, at least one class 2 gene and classes 5, 6, 9, 10, 18 and 19. Although the different myosin isoforms all have specific and non-overlapping roles in the cell, in combination they all contribute to the organization of the actin cytoskeleton, and the shape and phenotype of the cell. Over (or under) expression of these different myosin isoforms can have strong effects on actin organization, cell shape and contribute to the cancer phenotype as discussed in this review.

Specific Myosins Control Actin Organization, Cell Morphology, and Migration in Prostate Cancer Cells

We investigated the myosin expression profile in prostate cancer cell lines and found that Myo1b, Myo9b, Myo10, and Myo18a were expressed at higher levels in cells with high metastatic potential. Moreover, Myo1b and Myo10 were expressed at higher levels in metastatic tumors. Using an siRNA-based approach, we found that knockdown of each myosin resulted in distinct phenotypes. Myo10 knockdown ablated filopodia and decreased 2D migration speed. Myo18a knockdown increased circumferential non-muscle myosin 2A-associated actin filament arrays in the lamella and reduced directional persistence of 2D migration. Myo9b knockdown increased stress fiber formation, decreased 2D migration speed, and increased directional persistence. Conversely, Myo1b knockdown increased numbers of stress fibers but did not affect 2D migration. In all cases, the cell spread area was increased and 3D migration potential was decreased. Therefore, myosins not only act as molecular motors but also directly influence actin organization and cell morphology, which can contribute to the metastatic phenotype.

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Myo1b, Myo9b, Myo10, and Myo18a are highly expressed in metastatic prostate cancer

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Knockdown of individual myosins distinctly affects the cytoskeleton and cell migration

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Myosins act in concert to directly influence actin organization and cell migration

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Misregulation of myosin expression may drive the metastatic phenotype

The Extent of mRNA Editing Is Limited in Chicken Liver and Adipose, but Impacted by Tissular Context, Genotype, Age, and Feeding as Exemplified with a Conserved Edited Site in COG3

RNA editing is a posttranscriptional process leading to differences between genomic DNA and transcript sequences, potentially enhancing transcriptome diversity. With recent advances in high-throughput sequencing, many efforts have been made to describe mRNA editing at the transcriptome scale, especially in mammals, yielding contradictory conclusions regarding the extent of this phenomenon. We show, by detailed description of the 25 studies focusing so far on mRNA editing at the whole-transcriptome scale, that systematic sequencing artifacts are considered in most studies whereas biological replication is often neglected and multi-alignment not properly evaluated, which ultimately impairs the legitimacy of results. We recently developed a rigorous strategy to identify mRNA editing using mRNA and genomic DNA sequencing, taking into account sequencing and mapping artifacts, and biological replicates. We applied this method to screen for mRNA editing in liver and white adipose tissue from eight chickens and confirm the small extent of mRNA recoding in this species. Among the 25 unique edited sites identified, three events were previously described in mammals, attesting that this phenomenon is conserved throughout evolution. Deeper investigations on five sites revealed the impact of tissular context, genotype, age, feeding conditions, and sex on mRNA editing levels. More specifically, this analysis highlighted that the editing level at the site located on COG3 was strongly regulated by four of these factors. By comprehensively characterizing the mRNA editing landscape in chickens, our results highlight how this phenomenon is limited and suggest regulation of editing levels by various genetic and environmental factors.

Myosin VI localization and expression in striated muscle pathology

Myosin VI (MVI) is a unique unconventional myosin translocating, unlike other myosins, towards the minus end of actin filaments. It is involved in numerous cellular processes such as endocytosis, intracellular trafficking, cell migration, and transcription. In mammalian skeletal muscles it localizes mainly to sarcoplasmic reticulum and is also present within the muscle nuclei and at the neuromuscular junction (Karolczak et al. Histochem Cell Biol 2013; 23:219-228). We have also shown that in denervated rat hindlimb muscle the MVI expression level is significantly increased and its localization is changed, indicating an important role of MVI in striated muscle pathology. Here, we addressed this problem by examining the distribution and expression levels of myosin VI in biopsies of skeletal muscles from patients with different myopathies. We found that, particularly in myopathies associated with fiber atrophy, the amount of MVI was enhanced and its localization in affected fibers was changed. Also, since a mutation within the human MVI gene was shown to be associated with cardiomyopathy, we assessed MVI localization and expression level in cardiac muscle using wild type and MLP(-/-) mice, a dilated cardiomyopathy model. No significant difference in MVI expression level was observed for both types of animals. MVI was found at intercalated discs and also at the sarcoplasmic reticulum. In the knockout mice, it was also present in ring-like structures surrounding the nuclei. The data indicate that in striated muscle MVI could be engaged in sarcoplasmic reticulum maintenance and/or functioning, vesicular transport, signal transmission and possibly in gene transcription.

Localization of sarcomeric proteins during myofibril assembly in cultured mouse primary skeletal myotubes

It is important to understand how muscle forms normally in order to understand muscle diseases that result in abnormal muscle formation. Although the structure of myofibrils is well understood, the process through which the myofibril components form organized contractile units is not clear. Based on the staining of muscle proteins in avian embryonic cardiomyocytes, we previously proposed that myofibrils formation occurred in steps that began with premyofibrils followed by nascent myofibrils and ending with mature myofibrils. The purpose of this study was to determine whether the premyofibril model of myofibrillogenesis developed from studies developed from studies in avian cardiomyocytes was supported by our current studies of myofibril assembly in mouse skeletal muscle. Emphasis was on establishing how the key sarcomeric proteins, F-actin, nonmuscle myosin II, muscle myosin II, and α-actinin were organized in the three stages of myofibril assembly. The results also test previous reports that nonmuscle myosins II A and B are components of the Z-bands of mature myofibrils, data that are inconsistent with the premyofibril model. We have also determined that in mouse muscle cells, telethonin is a late assembling protein that is present only in the Z-bands of mature myofibrils. This result of using specific telethonin antibodies supports the approach of using YFP-tagged proteins to determine where and when these YFP-sarcomeric fusion proteins are localized. The data presented in this study on cultures of primary mouse skeletal myocytes are consistent with the premyofibril model of myofibrillogenesis previously proposed for both avian cardiac and skeletal muscle cells.

Involvement of unconventional myosin VI in myoblast function and myotube formation

The important role of unconventional myosin VI (MVI) in skeletal and cardiac muscle has been recently postulated (Karolczak et al. in Histochem Cell Biol 139:873-885, 2013). Here, we addressed for the first time a role for this unique myosin motor in myogenic cells as well as during their differentiation into myotubes. During myoblast differentiation, the isoform expression pattern of MVI and its subcellular localization underwent changes. In undifferentiated myoblasts, MVI-stained puncti were seen throughout the cytoplasm and were in close proximity to actin filaments, Golgi apparatus, vinculin-, and talin-rich focal adhesion as well as endoplasmic reticulum. Colocalization of MVI with endoplasmic reticulum was enhanced during myotube formation, and differentiation-dependent association was also seen in sarcoplasmic reticulum of neonatal rat cardiomyocytes (NRCs). Moreover, we observed enrichment of MVI in myotube regions containing acetylcholine receptor-rich clusters, suggesting its involvement in the organization of the muscle postsynaptic machinery. Overexpression of the H246R MVI mutant (associated with hypertrophic cardiomyopathy) in myoblasts and NRCs caused the formation of abnormally large intracellular vesicles. MVI knockdown caused changes in myoblast morphology and inhibition of their migration. On the subcellular level, MVI-depleted myoblasts exhibited aberrations in the organization of actin cytoskeleton and adhesive structures as well as in integrity of Golgi apparatus and endoplasmic reticulum. Also, MVI depletion or overexpression of H246R mutant caused the formation of significantly wider or aberrant myotubes, respectively, indicative of involvement of MVI in myoblast differentiation. The presented results suggest an important role for MVI in myogenic cells and possibly in myoblast differentiation.

Myosin VI in skeletal muscle: its localization in the sarcoplasmic reticulum, neuromuscular junction and muscle nuclei

Myosin VI (MVI) is a unique unconventional motor moving backwards on actin filaments. In non-muscle cells, it is involved in cell migration, endocytosis and intracellular trafficking, actin cytoskeleton dynamics, and possibly in gene transcription. An important role for MVI in striated muscle functioning was suggested in a report showing that a point mutation (H236R) within the MVI gene was associated with cardiomyopathy (Mohiddin et al., J Med Genet 41:309–314, 2004). Here, we have addressed MVI function in striated muscle by examining its expression and distribution in rat hindlimb skeletal muscle. We found that MVI was present predominantly at the muscle fiber periphery, and it was also localized within muscle nuclei. Analysis of both the hindlimb and cardiac muscle longitudinal sections revealed ~3 μm striation pattern, corresponding to the sarcoplasmic reticulum. Moreover, MVI was detected in the sarcoplasmic reticulum fractions isolated from skeletal and cardiac muscle. The protein also localized to the postsynaptic region of the neuromuscular junction. In denervated muscle, the defined MVI distribution pattern was abolished and accompanied by significant increase in its amount in the muscle fibers. In addition, we have identified several novel potential MVI-binding partners, which seem to aid our observations that in striated muscle MVI could be involved in postsynaptic trafficking as well as in maintenance of and/or transport within the sarcoplasmic reticulum and non-sarcomeric cytoskeleton.

At the Start of the Sarcomere: A Previously Unrecognized Role for Myosin Chaperones and Associated Proteins during Early Myofibrillogenesis

The development of striated muscle in vertebrates requires the assembly of contractile myofibrils, consisting of highly ordered bundles of protein filaments. Myofibril formation occurs by the stepwise addition of complex proteins, a process that is mediated by a variety of molecular chaperones and quality control factors. Most notably, myosin of the thick filament requires specialized chaperone activity during late myofibrillogenesis, including that of Hsp90 and its cofactor, Unc45b. Unc45b has been proposed to act exclusively as an adaptor molecule, stabilizing interactions between Hsp90 and myosin; however, recent discoveries in zebrafish and C. elegans suggest the possibility of an earlier role for Unc45b during myofibrillogenesis. This role may involve functional control of nonmuscle myosins during the earliest stages of myogenesis, when premyofibril scaffolds are first formed from dynamic cytoskeletal actin. This paper will outline several lines of evidence that converge to build a model for Unc45b activity during early myofibrillogenesis.

Nap1-mediated actin remodeling is essential for mammalian myoblast fusion

Myoblast fusion is crucial for the formation, growth, maintenance and regeneration of healthy skeletal muscle. Unfortunately, the molecular machinery, cell behaviors, and membrane and cytoskeletal remodeling events that govern fusion and myofiber formation remain poorly understood. Using time-lapse imaging approaches on mouse C2C12 myoblasts, we identify discrete and specific molecular events at myoblast membranes during fusion and myotube formation. These events include rearrangement of cell shape from fibroblast to spindle-like morphologies, changes in lamellipodial and filopodial extensions during different periods of differentiation, and changes in membrane alignment and organization during fusion. We find that actin-cytoskeleton remodeling is crucial for these events: pharmacological inhibition of F-actin polymerization leads to decreased lamellipodial and filopodial extensions and to reduced myoblast fusion. Additionally, shRNA-mediated inhibition of Nap1, a member of the WAVE actin-remodeling complex, results in accumulations of F-actin structures at the plasma membrane that are concomitant with a decrease in myoblast fusion. Our data highlight distinct and essential roles for actin cytoskeleton remodeling during mammalian myoblast fusion, provide a platform for cellular and molecular dissection of the fusion process, and suggest a functional conservation of Nap1-regulated actin-cytoskeleton remodeling during myoblast fusion between mammals and Drosophila.

Transcriptional response of the neuromuscular system to exercise training and potential implications for ALS

The transcriptional adaptive response of motoneurons and muscles to voluntary exercise has been investigated by using laser capture microdissection and microarray analysis. Our results show that motoneurons respond to physical activity by activating a complex transcriptional plan, with changes involved in neurotrophic factor signalling, electrophysiological changes and synaptic reorganization. Gastrocnemius muscle shows increases in transcripts responsible for neovascularization and new myogenesis. Both tissues show transcriptional changes involved in the growth and reinforcement of the neuromuscular junction. This study indicates that the neuromuscular system undergoes significant structural and functional alterations, aiming to optimize the transmission of both chemical and electrical stimuli, thus prompting axonal outgrowth and mechanisms similar to long-term potentiation in hippocampal neurons. Understanding the response of these cells during exercise has potentially important implications for human neuromuscular disease, including amyotrophic lateral sclerosis, by highlighting candidate genes pivotal for the balance between the physiology and the pathology of the neuromuscular system in terms of the stress response to physical exercise.

Engineering a multi-nucleated myotube, the role of the actin cytoskeleton

Myoblasts in vitro form characteristic arrays of bipolar-shaped cells prior to fusion. We have shown that the actin cytoskeleton re-organizes in these fusing cells and that the interaction of non-muscle myosin 2A with actin at the plasma membrane helps to generate the bipolar shape of myoblasts, which is key for fusion. Here we discuss how fusion occurs, and in particular how the actin cytoskeleton is involved. Myoblast fusion is essential to form the multi-nucleated muscle fibres that make up the skeletal muscle. Skeletal muscle fibres contain many nuclei, roughly one nucleus to every 15 sarcomeres (35 microm) in adult muscle, although this varies with muscle type (Bruusgaard et al., 2006). Thus a muscle fibre 30 cm long contains about 8000 nuclei and is formed by the fusion of about 8000 cells during development. The formation of multi-nucleated myotubes has been intensively studied for many years using a number of different systems. Many different proteins have been identified using Drosophila as a model system (e.g. see reviews by Taylor, 2000, 2002) that have given an insight into what happens in mammals. However, the process of fusion of mammalian cells is less well understood, and this paper will cover some of the aspects of mammalian myoblast fusion, with a particular focus on the role of the actin cytoskeleton.

Unconventional myosins in muscle

Myosins, actin-based molecular motors originally isolated from muscle tissues, are ubiquitously expressed in all eukaryotic cells. They are involved in a panoply of cellular functions, including cell migration, intracellular trafficking, adhesion, and cytokinesis. Several unconventional myosins belonging to classes I, V, VI, VII, IX, and XVIII have been detected in myogenic cells and/or adult muscle where they seem to play important roles in muscle functioning and/or differentiation. For example, a point mutation within the myosin VI gene leads to a cardiac dysfunction, and myosin XVIIIB (expressed predominantly in striated muscle) may be involved in muscle gene transcription. This review summarizes data addressing the functioning of these unconventional myosins in muscle and/or myogenic cells.

Non-muscle myosins 2A and 2B drive changes in cell morphology that occur as myoblasts align and fuse

The interaction of non-muscle myosins 2A and 2B with actin may drive changes in cell movement, shape and adhesion. To investigate this, we used cultured myoblasts as a model system. These cells characteristically change shape from triangular to bipolar when they form groups of aligned cells. Antisense oligonucleotide knockdown of non-muscle myosin 2A, but not non-muscle myosin 2B, inhibited this shape change, interfered with cell-cell adhesion, had a minor effect on tail retraction and prevented myoblast fusion. By contrast, non-muscle myosin 2B knockdown markedly inhibited tail retraction, increasing cell length by over 200% by 72 hours compared with controls. In addition it interfered with nuclei redistribution in myotubes. Non-muscle myosin 2C is not involved as western analysis showed that it is not expressed in myoblasts, but only in myotubes. To understand why non-muscle myosins 2A and 2B have such different roles, we analysed their distributions by immuno-electron microscopy, and found that non-muscle myosin 2A was more tightly associated with the plasma membrane than non-muscle myosin 2B. This suggests that non-muscle myosin 2A is more important for bipolar shape formation and adhesion owing to its preferential interaction with membrane-associated actin, whereas the role of non-muscle myosin 2B in retraction prevents over-elongation of myoblasts.

Actin filament organization in aligned prefusion myoblasts

The organization of the actin cytoskeleton in prefusion aligning myoblasts is likely to be important for their shape and interaction. We investigated actin filament organization and polarity by transmission electron microscopy (TEM) in these cells. About 84% of the filaments counted were either found in a subplasmalemma sheet up to 0.5 microm thick that was aligned with the long axis of the cell, or in protrusions. The remaining filaments were found in the cytoplasm, where they were randomly orientated and not organized into bundles. The polarity of the subplasmalemma filaments changed progressively from one end of the cell to the other. At the ends of the cells and in protrusions, the majority of filaments were organized such that their barbed ends faced the tip of the protrusion. We did not find any actin filament bundles or stress fibres in these cells. Time-lapse phase microscopy demonstrated that aligned cells were still actively migrating at the time of our TEM observations, and their direction of movement was restricted to the long axis of the cell group. The ability of these cells to locomote actively in the absence of actin filament bundles suggests that in these cells the subplasmalemma actin sheet contributes not only to cell shape but also to cell locomotion.

Ventricular myosin heavy chain isoform expression is altered in vitro in low score normal chickens

The low score normal (LSN) chicken exhibits a genetic muscle weakness and altered in vitro myogenesis compared to the normal White Leghorn chicken. The ventricular myosin heavy chain isoform has been reported to be the initial muscle-specific contractile protein expressed during myogenesis. The goals of this study were to determine whether altered myogenesis of the LSN satellite cells in culture was accompanied by delayed ventricular myosin heavy chain expression and to further characterize the altered myogenic events exhibited by the LSN chicken. Immunocytochemical and ELISA analyses were employed to document the temporal expression of the ventricular myosin heavy chain during LSN chicken myogenesis. Satellite cells derived from the LSN chicken pectoralis major exhibited lower (P </= 0.05) expression of ventricular myosin heavy chain during proliferation and differentiation in culture than did satellite cells derived from White Leghorn chickens. Low score normal cells failing to express the ventricular myosin heavy chain generally remained mononucleated and unfused, whereas cells that were multinucleated appeared to express ventricular myosin heavy chain regardless of the avian source. These results suggest that the altered myogenesis observed in LSN chickens is associated with delayed ventricular myosin heavy chain expression.

Intermediate filament-co-localized molecules with myosin heavy chain epitopes define distinct cellular domains in hair follicles and epidermis

BACKGROUND

Proteins linking intermediate filaments to other cytoskeletal components have important functions in maintaining tissue integrity and cell shape.

RESULTS

We found a set of monoclonal antibodies raised against specific human sarcomeric myosin heavy chain (MyHC) isoforms labels cells in distinct regions of the mammalian epidermis. The antigens co-localize with intermediate filament-containing structures. A slow MyHC-related antigen is punctate on the cell surface and co-localizes with desmoplakin at desmosomal junctions of all suprabasal epidermal layers from rat foetal day 16 onwards, in the root sheath of the hair follicle and in intercalated disks of cardiomyocytes. A fast MyHC-related antigen occurs in cytoplasmic filaments in a subset of basal cells of skin epidermis and bulb, but not neck, of hair follicles. A fast IIA MyHC-related antigen labels filaments of a single layer of cells in hair bulb. This 230,000 Mr antigen co-purifies with keratin. No obvious candidate for any of the antigens appears in the literature.

CONCLUSIONS

We describe a set of molecules that co-localize with intermediate filament in specific cell subsets in epithelial tissues. These antigens presumably influence intermediate filament structure or function.

Alignment of myoblasts on ultrafine gratings inhibits fusion in vitro

During development, skeletal muscle precursor cells fuse to form multi-nucleated myotubes. However, it is unclear how this fusion is regulated such that linear myotubes are produced. In a previous study, we found that linear arrays of myoblasts cultured on micropatterns of laminin fused to form linear myotubes of a constant diameter, independent of the width of the laminin track. This suggested that a mechanism exists to prevent myoblasts from fusing laterally [Exp. Cell Res. 230 (1997) 275]. In this study, we have investigated this further by culturing myoblasts on ultrafine grooved surfaces previously shown to align fibroblasts and epithelial cells. We found that all the individual myoblasts were highly aligned along the groove axis, and time-lapse recordings showed that motility was mostly restricted to a direction parallel to the grooves. In contrast to the previous study, however, there was a strong tendency for early differentiating cells to form aggregates either at an angle of approximately 45 degrees or perpendicular to the groove axis. Nevertheless, we rarely saw myotubes formed at those angles, supporting our earlier idea that the ability of cells to fuse laterally is prohibited. Our data strongly suggest that myoblasts are most likely to fuse in an end-to-end configuration, and it is this that enables them to form linear, rather than irregular myotubes.

Specific changes to the mechanism of cell locomotion induced by overexpression of (β)-actin

Overexpression of (β)-actin is known to alter cell morphology, though its effect on cell motility has not been documented previously. Here we show that overexpressing (β)-actin in myoblasts has striking effects on motility, increasing cell speed to almost double that of control cells. This occurs by increasing the areas of protrusion and retraction and is accompanied by raised levels of (β)-actin in the newly protruded regions. These regions of the cell margin, however, show decreased levels of polymerised actin, indicating that protrusion can outpace the rate of actin polymerisation in these cells. Moreover, the expression of (β)*-actin (a G244D mutant, which shows defective polymerisation in vitro) is equally effective at increasing speed and protrusion. Concomitant changes in actin binding proteins show no evidence of a consistent mechanism for increasing the rate of actin polymerisation in these actin overexpressing cells. The increase in motility is confined to poorly spread cells in both cases and the excess motility can be abolished by blocking myosin function with butanedione monoxime (BDM). Our observations on normal myoblasts are consistent with the view that they protrude by the assembly and cross linking of actin filaments. In contrast, the additional motility shown by cells overexpressing (β)-actin appears not to result from an increase in the rate of actin polymerisation but to depend on myosin function. This suggests that the additional protrusion arises from a different mechanism. We discuss the possibility that it is related to retraction-induced protrusion in fibroblasts. In this phenomenon, a wave of increased protrusion follows a sudden collapse in cell spreading. This view could explain why it is only the additional motility that depends on spreading, and has implications for understanding the differences in locomotion that distinguish tissue cells from highly invasive cell types such as leucocytes and malignant cells.

Myosin diversity in Apicomplexa

A polymerase chain reaction (PCR) screen was used to examine the diversity of myosins in 7 Apicomplexan parasites: Toxoplasma gondii, Plasmodium falciparum, Neospora caninum, Eimeria tenella, Sarcocystis muris, Babesia bovis, and Cryptosporidium parvum. Using degenerate PCR primers compatible with the majority of known myosin classes, putative myosin sequences were obtained from all of these species. All of the sequences obtained showed greatest similarity to previously identified apicomplexan myosins, suggesting that the diversity of myosins in these parasites is limited. Myosin classes that are known to be widespread across the phylogenetic spectrum, e.g., the myosins I, II, and V, were not seen in the Apicomplexa. Thus, like the plants, the Apicomplexa may have evolved their own unique cohort of myosins that are responsible for the myosin-driven cellular functions observed in these parasites.

Cloning of the murine unconventional myosin gene Myo9b and identification of alternative splicing

We report the cloning of a cDNA for the mouse unconventional myosin Myo9b, the orthologue of the rat myr5 and human MYOIXb genes. A full-length spleen cDNA of 7087bp encoding a protein of 1961 amino acids was isolated. By RT-PCR, we show that Myo9b is expressed in a wide range of tissues, including heart, brain, muscle and inner ear. In addition, we have identified two alternatively spliced exons. Equivalent exons have not been previously reported for either the human or rat homologues. These exons are located in the Myo9b specific actin-binding site insert of the head domain and in the tail region. A third splice form utilizing an alternative reading frame within the 3'UTR is also described. Several polymorphisms within the coding region were identified; of interest is an in-frame 33bp imperfect duplication within the tail region that was observed only in the C57Bl/6 strain. Myo9b has been previously mapped to mouse chromosome 8 and is a candidate for the mouse mutations myodystrophy and quinky.