Perturbations of Drosophila alpha-actinin cause muscle paralysis, weakness, and atrophy but do not confer obvious nonmuscle phenotypes

The influence of simulated microgravity on the proteome of Daphnia magna

Background:

The waterflea Daphnia is an interesting candidate for bioregenerative life support systems (BLSS). These animals are particularly promising because of their central role in the limnic food web and its mode of reproduction. However, the response of Daphnia to altered gravity conditions has to be investigated, especially on the molecular level, to evaluate the suitability of Daphnia for BLSS in space.

Methods:

In this study, we applied a proteomic approach to identify key proteins and pathways involved in the response of Daphnia to simulated microgravity generated by a two-dimensional (2D) clinostat. We analyzed five biological replicates using 2D-difference gel electrophoresis proteomic analysis.

Results:

We identified 109 protein spots differing in intensity (P<0.05). Substantial fractions of these proteins are involved in actin microfilament organization, indicating the disruption of cytoskeletal structures during clinorotation. Furthermore, proteins involved in protein folding were identified, suggesting altered gravity induced breakdown of protein structures in general. In addition, simulated microgravity increased the abundance of energy metabolism-related proteins, indicating an enhanced energy demand of Daphnia.

Conclusions:

The affected biological processes were also described in other studies using different organisms and systems either aiming to simulate microgravity conditions or providing real microgravity conditions. Moreover, most of the Daphnia protein sequences are well-conserved throughout taxa, indicating that the response to altered gravity conditions in Daphnia follows a general concept. Data are available via ProteomeXchange with identifier PXD002096.

The actinin family of actin cross-linking proteins - a genetic perspective

Actinins are one of the major actin cross-linking proteins found in virtually all cell types and are the ancestral proteins of a larger family that includes spectrin, dystrophin and utrophin. Invertebrates have a single actinin-encoding ACTN gene, while mammals have four. Mutations in all four human genes have now been linked to heritable diseases or traits. ACTN1 mutations cause macrothrombocytopenia, a platelet disorder characterized by excessive bleeding. ACTN2 mutations have been linked to a range of cardiomyopathies, and ACTN4 mutations cause a kidney condition called focal segmental glomerulosclerosis. Intriguingly, approximately 16 % of people worldwide are homozygous for a nonsense mutation in ACTN3 that abolishes actinin-3 protein expression. This ACTN3 null allele has undergone recent positive selection in specific human populations, which may be linked to improved endurance and adaptation to colder climates. In this review we discuss the human genetics of the ACTN gene family, as well as ACTN gene knockout studies in several model organisms. Observations from both of these areas provide insights into the evolution and cellular functions of actinins.

The non-muscle functions of actinins: an update

α-Actinins are a major class of actin filament cross-linking proteins expressed in virtually all cells. In muscle, actinins cross-link thin filaments from adjacent sarcomeres. In non-muscle cells, different actinin isoforms play analogous roles in cross-linking actin filaments and anchoring them to structures such as cell-cell and cell-matrix junctions. Although actinins have long been known to play roles in cytokinesis, cell adhesion and cell migration, recent studies have provided further mechanistic insights into these functions. Roles for actinins in synaptic plasticity and membrane trafficking events have emerged more recently, as has a 'non-canonical' function for actinins in transcriptional regulation in the nucleus. In the present paper we review recent advances in our understanding of these diverse cell biological functions of actinins in non-muscle cells, as well as their roles in cancer and in genetic disorders affecting platelet and kidney physiology. We also make two proposals with regard to the actinin nomenclature. First, we argue that naming actinin isoforms according to their expression patterns is problematic and we suggest a more precise nomenclature system. Secondly, we suggest that the α in α-actinin is superfluous and can be omitted.

α-Actinin2 is required for the lateral alignment of Z discs and ventricular chamber enlargement during zebrafish cardiogenesis

α-Actinin2 (Actn2) is a predominant protein in the sarcomere Z disc whose mutation can lead to cardiomyopathy. However, the function of Actn2 in Z-disc assembly and cardiomyopathy in vertebrates remains elusive. We leveraged genetic tools in zebrafish embryos to elucidate the function of Actn2. We identified a single Actn2 homologue expressed in the zebrafish heart and conducted loss-of-function studies by antisense morpholino technology. Although zebrafish Actn2 assembles early into the Z disc, depletion of actn2 did not affect the early steps of sarcomere assembly. Instead, Actn2 is required for Z bodies to register laterally, forming well-aligned Z discs. Presumably as a consequence to this structural defect in the sarcomere, the depletion of Actn2 resulted in reduced cardiac function, primarily characterized as a reduced end-diastolic diameter. The depletion of actn2 also significantly reduced the ventricle chamber size, due to both reduced cardiomyocyte (CM) size and CM number. Interestingly, reduced CM size can be rescued by the cessation of heart contractions. The genetic studies of zebrafish uncovered a function for actn2 in lateral registration of Z body. In actn2 morphant fish, the Z-disc defect sequentially affects cardiac function, which leads to morphological changes in the ventricle through a mechanical force-dependent mechanism.

The role of actin bundling proteins in the assembly of filopodia in epithelial cells

The goal of this review is to highlight how emerging new models of filopodia assembly, which include tissue specific actin-bundling proteins, could provide more comprehensive representations of filopodia assembly that would describe more adequately and effectively the complexity and plasticity of epithelial cells.  This review also describes how the true diversity of actin bundling proteins must be considered to predict the far-reaching significance and versatile functions of filopodia in epithelial cells.

α-actinin is required for the proper assembly of Z-disk/focal-adhesion-like structures and for efficient locomotion in Caenorhabditis elegans

The actin binding protein α-actinin is a major component of focal adhesions found in vertebrate cells and of focal-adhesion-like structures found in the body wall muscle of the nematode Caenorhabditis elegans. To study its in vivo function in this genetic model system, we isolated a strain carrying a deletion of the single C. elegans α-actinin gene. We assessed the cytological organization of other C. elegans focal adhesion proteins and the ultrastructure of the mutant. The mutant does not have normal dense bodies, as observed by electron microscopy; however, these dense-body-like structures still contain the focal adhesion proteins integrin, talin, and vinculin, as observed by immunofluorescence microscopy. Actin is found in normal-appearing I-bands, but with abnormal accumulations near muscle cell membranes. Although swimming in water appeared grossly normal, use of automated methods for tracking the locomotion of individual worms revealed a defect in bending. We propose that the reduced motility of α-actinin null is due to abnormal dense bodies that are less able to transmit the forces generated by actin/myosin interactions.

Conserved genes act as modifiers of invertebrate SMN loss of function defects

Spinal Muscular Atrophy (SMA) is caused by diminished function of the Survival of Motor Neuron (SMN) protein, but the molecular pathways critical for SMA pathology remain elusive. We have used genetic approaches in invertebrate models to identify conserved SMN loss of function modifier genes. Drosophila melanogaster and Caenorhabditis elegans each have a single gene encoding a protein orthologous to human SMN; diminished function of these invertebrate genes causes lethality and neuromuscular defects. To find genes that modulate SMN function defects across species, two approaches were used. First, a genome-wide RNAi screen for C. elegans SMN modifier genes was undertaken, yielding four genes. Second, we tested the conservation of modifier gene function across species; genes identified in one invertebrate model were tested for function in the other invertebrate model. Drosophila orthologs of two genes, which were identified originally in C. elegans, modified Drosophila SMN loss of function defects. C. elegans orthologs of twelve genes, which were originally identified in a previous Drosophila screen, modified C. elegans SMN loss of function defects. Bioinformatic analysis of the conserved, cross-species, modifier genes suggests that conserved cellular pathways, specifically endocytosis and mRNA regulation, act as critical genetic modifiers of SMN loss of function defects across species.

The evolution of skeletal muscle performance: gene duplication and divergence of human sarcomeric alpha-actinins

In humans, there are two skeletal muscle alpha-actinins, encoded by ACTN2 and ACTN3, and the ACTN3 genotype is associated with human athletic performance. Remarkably, approximately 1 billion people worldwide are deficient in alpha-actinin-3 due to the common ACTN3 R577X polymorphism. The alpha-actinins are an ancient family of actin-binding proteins with structural, signalling and metabolic functions. The skeletal muscle alpha-actinins diverged approximately 250-300 million years ago, and ACTN3 has since developed restricted expression in fast muscle fibres. Despite ACTN2 and ACTN3 retaining considerable sequence similarity, it is likely that following duplication there was a divergence in function explaining why alpha-actinin-2 cannot completely compensate for the absence of alpha-actinin-3. This paper focuses on the role of skeletal muscle alpha-actinins, and how possible changes in functions between these duplicates fit in the context of gene duplication paradigms.

The Rap1 guanine nucleotide exchange factor C3G is required for preservation of larval muscle integrity in Drosophila melanogaster

C3G is a guanine nucleotide exchange factor (GEF) and modulator of small G-protein activity, which primarily acts on members of the Rap GTPase subfamily. Via promotion of the active GTP bound conformation of target GTPases, C3G has been implicated in the regulation of multiple cellular and developmental events including proliferation, differentiation and apoptosis. The Drosophila C3G orthologue exhibits a domain organization similar to that of vertebrate C3G. Through deletion of the C3G locus, we have observed that loss of C3G causes semi-lethality, and that escaping adult flies are characterized by a reduction in lifespan and general fitness. In situ hybridization reveals C3G expression in the developing embryonic somatic and visceral muscles, and indeed analysis of C3G mutants suggests essential functions of C3G for normal body wall muscle development during larval stages. C3G mutants display abnormal muscle morphology and attachment, as well as failure to properly localize βPS integrins to muscle attachment sites. Moreover, we show that C3G stimulates guanine nucleotide exchange on Drosophila Rap GTPases in vitro. Taken together, we conclude that Drosophila C3G is a Rap1-specific GEF with important functions in maintaining muscle integrity during larval stages.

Accelerators, Brakes, and Gears of Actin Dynamics in Dendritic Spines

Dendritic spines are actin-rich structures that accommodate the postsynaptic sites of most excitatory synapses in the brain. Although dendritic spines form and mature as synaptic connections develop, they remain plastic even in the adult brain, where they can rapidly grow, change, or collapse in response to normal physiological changes in synaptic activity that underlie learning and memory. Pathological stimuli can adversely affect dendritic spine shape and number, and this is seen in neurodegenerative disorders and some forms of mental retardation and autism as well. Many of the molecular signals that control these changes in dendritic spines act through the regulation of filamentous actin (F-actin), some through direct interaction with actin, and others via downstream effectors. For example, cortactin, cofilin, and gelsolin are actin-binding proteins that directly regulate actin dynamics in dendritic spines. Activities of these proteins are precisely regulated by intracellular signaling events that control their phosphorylation state and localization. In this review, we discuss how actin-regulating proteins maintain the balance between F-actin assembly and disassembly that is needed to stabilize mature dendritic spines, and how changes in their activities may lead to rapid remodeling of dendritic spines.

Phylogenetic analysis of gene structure and alternative splicing in alpha-actinins

The alpha-actinins are an important family of actin-binding proteins with the ability to cross-link actin filaments when in dimer form. Members of the alpha-actinin family share a domain topology composed of highly conserved actin-binding and EF-hand domains separated by a rod domain composed of spectrin-like repeats. Functional diversity within this family has arisen through exon duplication and the formation of alternate splice isoforms as well as gene duplications during the evolution of vertebrates. In addition to the known functional domains, alpha-actinins also contain a consensus PDZ-binding site. The completed genome sequence of over 32 invertebrate species has allowed the analysis of gene structure and exon-gene duplication over a diverse range of phyla. Our analysis shows that relative to early branching metazoans, there has been considerable intron loss especially in arthropods with few cases of intron gains. The C-terminal PDZ-binding site is conserved in nearly all invertebrates but is missing in some nematodes and platyhelminths. Alternative splicing in the actin-binding domain is conserved in chordates, arthropods, and some nematodes and platyhelminths. In contrast, alternative splicing of the EF-hand domain is only observed in chordates. Finally, given the prevalence of exon duplications seen in the actin-binding domain, this may act as a significant mechanism in the modification of actin-binding properties.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Integrin-mediated adhesion maintains sarcomeric integrity

Integrin-mediated adhesion to the ECM is essential for normal development of animal tissues. During muscle development, integrins provide the structural stability required to construct such a highly tensile, force generating tissue. Mutations that disrupt integrin-mediated adhesion in skeletal muscles give rise to a myopathy in humans and mice. To determine if this is due to defects in formation or defects in maintenance of muscle tissue, we used an inducible, targeted RNAi based approach to disrupt integrin-mediated adhesion in fully formed adult fly muscles. A decrease in integrin-mediated adhesion in adult muscles led to a progressive loss of muscle function due to a failure to maintain normal sarcomeric cytoarchitecture. This defect was due to a gradual, age dependent disorganization of the sarcomeric actin, Z-line, and M-line. Electron microscopic analysis showed that reduction in integrin-mediated adhesion resulted in detachment of actin filaments from the Z-lines, separation of the Z-lines from the membrane, and eventually to disintegration of the Z-lines. Our results show that integrin-mediated adhesion is essential for maintaining sarcomeric integrity and illustrate that the seemingly stable adhesive contacts underlying sarcomeric architecture are inherently dynamic.

The ALP-Enigma protein ALP-1 functions in actin filament organization to promote muscle structural integrity in Caenorhabditis elegans

Mutations that affect the Z-disk-associated ALP-Enigma proteins have been linked to human muscular and cardiac diseases. Despite their clear physiological significance for human health, the mechanism of action of ALP-Enigma proteins is largely unknown. In Caenorhabditis elegans, the ALP-Enigma protein family is encoded by a single gene, alp-1; thus C. elegans provides an excellent model to study ALP-Enigma function. Here we present a molecular and genetic analysis of ALP-Enigma function in C. elegans. We show that ALP-1 and alpha-actinin colocalize at dense bodies where actin filaments are anchored and that the proper localization of ALP-1 at dense bodies is dependent on alpha-actinin. Our analysis of alp-1 mutants demonstrates that ALP-1 functions to maintain actin filament organization and participates in muscle stabilization during contraction. Reducing alpha-actinin activity enhances the actin filament phenotype of the alp-1 mutants, suggesting that ALP-1 and alpha-actinin function in the same cellular process. Like alpha-actinin, alp-1 also interacts genetically with a connectin/titin family member, ketn-1, to provide mechanical stability for supporting body wall muscle contraction. Taken together, our data demonstrate that ALP-1 and alpha-actinin function together to stabilize actin filaments and promote muscle structural integrity.

Muscleblind isoforms are functionally distinct and regulate alpha-actinin splicing

Drosophila Muscleblind (Mbl) proteins control terminal muscle and neural differentiation, but their molecular function has not been experimentally addressed. Such an analysis is relevant as the human Muscleblind-like homologs (MBNL1-3) are implicated in the pathogenesis of the inherited muscular developmental and degenerative disease myotonic dystrophy. The Drosophila muscleblind gene expresses four protein coding splice forms (mblA to mblD) that are differentially expressed during the Drosophila life cycle, and which vary markedly in their ability to rescue the embryonic lethal phenotype of muscleblind mutant flies. Analysis of muscleblind mutant embryos reveals misregulated alternative splicing of the transcripts encoding Z-band component alpha-Actinin, which can be replicated in human cells expressing a Drosophilaalpha-actinin minigene and epitope-tagged Muscleblind isoforms. MblC appreciably altered alpha-actinin splicing in this assay, whereas other isoforms had only a marginal or no effect, demonstrating functional specialization among Muscleblind proteins. To further analyze the molecular basis of these differences, we studied the subcellular localization of Muscleblind isoforms. Consistent with the splicing assay results, MblB and MblC were enriched in the nucleus while MblA was predominantly cytoplasmic. In myotonic dystrophy, transcripts bearing expanded non-coding CUG or CCUG repeats interfere with the function of human MBNL proteins. Co-expression of CUG repeat RNA with the alpha-actinin minigene altered splicing compared with that seen in muscleblind mutant embryos, indicating that CUG repeat expansion RNA also interferes with Drosophila muscleblind function. Moreover MblA, B, and C co-localize with CUG repeat RNA in nuclear foci in cell culture. Our observations indicate that Muscleblind isoforms perform different functions in vivo, that MblC controls muscleblind-dependent alternative splicing events, and establish the functional conservation between Muscleblind and MBNL proteins both over a physiological target (alpha-actinin) and a pathogenic one (CUG repeats).

Drosophila alpha-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

Nebulin regulates the assembly and lengths of the thin filaments in striated muscle

In many tissues, actin monomers polymerize into actin (thin) filaments of precise lengths. Although the exact mechanisms involved remain unresolved, it is proposed that "molecular rulers" dictate the lengths of the actin filaments. The giant nebulin molecule is a prime candidate for specifying thin filament lengths in striated muscle, but this idea has never been proven. To test this hypothesis, we used RNA interference technology in rat cardiac myocytes. Live cell imaging and triple staining revealed a dramatic elongation of the preexisting thin filaments from their pointed ends upon nebulin knockdown, demonstrating its role in length maintenance; the barbed ends were unaffected. When the thin filaments were depolymerized with latrunculin B, myocytes with decreased nebulin levels reassembled them to unrestricted lengths, demonstrating its importance in length specification. Finally, knockdown of nebulin in skeletal myotubes revealed its involvement in myofibrillogenesis. These data are consistent with nebulin functioning as a thin filament ruler and provide insight into mechanisms dictating macromolecular assembly.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Drosophila non-muscle alpha-actinin is localized in nurse cell actin bundles and ring canals, but is not required for fertility

The single copy Drosophila alpha-actinin gene is alternatively spliced to generate three different isoforms that are expressed in larval muscle, adult muscle and non-muscle cells, respectively. We have generated novel alpha-actinin alleles, which specifically remove the non-muscle isoform. Homozygous mutant flies are viable and fertile with no obvious defects. Using a monoclonal antibody that recognizes all three splice variants, we compared alpha-actinin distribution in wild type and mutant embryos and ovaries. We found that non-muscle alpha-actinin was present in young embryos and in the embryonic central nervous system. In ovaries, non-muscle alpha-actinin was localized in the nurse cell subcortical cytoskeleton, cytoplasmic actin cables and ring canals. In the mutant, alpha-actinin expression remained in muscle tissues, but also in a subpopulation of epithelial cells in both embryos and ovaries. This suggests that various populations of non-muscle cells regulate alpha-actinin expression in different ways. We also show that ectopically expressed adult muscle-specific alpha-actinin localizes to all F-actin containing structures in the nurse cells in the absence of endogenous non-muscle alpha-actinin.

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

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

The Drosophila bZIP transcription factor Vrille is involved in hair and cell growth

Vri is closely related to bZIP transcription factors involved in growth or cell death. vri clonal and overexpression analyses revealed defects at the cellular level. vri clones in the adult cuticle contain smaller cells with atrophic bristles. The phenotypes are strictly cell autonomous. Clones induced in the eye precursor cells lead to individuals with smaller eyes and reduced number of ommatidia with an abnormal morphology and shorter photoreceptor cell stalks. Overexpression of vri is anti-proliferative in embryonic dorsal epidermis and in imaginal discs, and induces apoptosis. On the wing surface, larger cells with multiple trichomes are observed, suggesting cytoskeletal defects. In salivary glands, vri overexpression leads to smaller cells and organs. We also show that vri is involved in locomotion and flight and interacts genetically with genes encoding actin-binding proteins. The phenotypes observed are consistent with the hypothesis that vri is required for normal cell growth and proliferation via the regulation of the actin cytoskeleton.

Drosophila paramyosin is important for myoblast fusion and essential for myofibril formation

Paramyosin is a major structural protein of thick filaments in invertebrate muscles. Coiled-coil dimers of paramyosin form a paracrystalline core of these filaments, and the motor protein myosin is arranged on the core surface. To investigate the function of paramyosin in myofibril assembly and muscle contraction, we functionally disrupted the Drosophila melanogaster paramyosin gene by mobilizing a P element located in its promoter region. Homozygous paramyosin mutants die at the late embryo stage. Mutants display defects in both myoblast fusion and in myofibril assembly in embryonic body wall muscles. Mutant embryos have an abnormal body wall muscle fiber pattern arising from defects in myoblast fusion. In addition, sarcomeric units do not assemble properly and muscle contractility is impaired. We confirmed that these defects are paramyosin-specific by rescuing the homozygous paramyosin mutant to adulthood with a paramyosin transgene. Antibody analysis of normal embryos demonstrated that paramyosin accumulates as a cytoplasmic protein in early embryo development before assembling into thick filaments. We conclude that paramyosin plays an unexpected role in myoblast fusion and is important for myofibril assembly and muscle contraction.

Fascins, and their roles in cell structure and function

The fascins are a structurally unique and evolutionarily conserved group of actin cross-linking proteins. Fascins function in the organisation of two major forms of actin-based structures: dynamic, cortical cell protrusions and cytoplasmic microfilament bundles. The cortical structures, which include filopodia, spikes, lamellipodial ribs, oocyte microvilli and the dendrites of dendritic cells, have roles in cell-matrix adhesion, cell interactions and cell migration, whereas the cytoplasmic actin bundles appear to participate in cell architecture. We discuss the current understanding of the cellular mechanisms that regulate the binding of fascin to actin and how these processes contribute to the organisation or disassembly of cell protrusions. Although the in vivo roles of fascin have been studied principally in Drosophila, several human diseases are associated with inherited or acquired alterations in the expression of fascins. Strategies to modulate fascin-containing protrusions and thereby cell adhesive and migratory behaviour could have potential for therapeutic intervention in these conditions. The supplementary material referred to in this section can be found at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/2002/v24.350.html

Eyes closed, aDrosophilap47 homolog, is essential for photoreceptor morphogenesis

Starting with a mutation impacting photoreceptor morphogenesis, we identify here a Drosophila gene, eyes closed (eyc), as a fly homolog of p47, a protein co-factor of the p97 ATPase implicated in membrane fusion. Temporal misexpression of Eyc during rhabdomere extension early in pupal life results in inappropriate retention of normally transient adhesions between developing rhabdomeres. Later Eyc misexpression results in endoplasmic reticulum proliferation and inhibits rhodopsin transport to the developing photosensitive membrane. Loss of Eyc function results in a lethal failure of nuclear envelope assembly in early zygotic divisions. Phenotypes resulting from eyc mutations provide the first in vivo evidence for a role for p47 in membrane biogenesis.

Movies available on-line

zipper Nonmuscle myosin-II functions downstream of PS2 integrin in Drosophila myogenesis and is necessary for myofibril formation

Nonmuscle myosin-II is a key motor protein that drives cell shape change and cell movement. Here, we analyze the function of nonmuscle myosin-II during Drosophila embryonic myogenesis. We find that nonmuscle myosin-II and the adhesion molecule, PS2 integrin, colocalize at the developing muscle termini. In the paradigm emerging from cultured fibroblasts, nonmuscle actomyosin-II contractility, mediated by the small GTPase Rho, is required to cluster integrins at focal adhesions. In direct opposition to this model, we find that neither nonmuscle myosin-II nor RhoA appear to function in PS2 clustering. Instead, PS2 integrin is required for the maintenance of nonmuscle myosin-II localization and we show that the cytoplasmic tail of the beta(PS) integrin subunit is capable of mediating this PS2 integrin function. We show that embryos that lack zygotic expression of nonmuscle myosin-II fail to form striated myofibrils. In keeping with this, we demonstrate that a PS2 mutant that specifically disrupts myofibril formation is unable to mediate proper localization of nonmuscle myosin-II at the muscle termini. In contrast, embryos that lack RhoA function do generate striated muscles. Finally, we find that nonmuscle myosin-II localizes to the Z-line in mature larval muscle. We suggest that nonmuscle myosin-II functions at the muscle termini and the Z-line as an actin crosslinker and acts to maintain the structural integrity of the sarcomere.

Genetics of the Drosophila flight muscle myofibril: a window into the biology of complex systems

This essay reviews the long tradition of experimental genetics of the Drosophila indirect flight muscles (IFM). It discusses how genetics can operate in tandem with multidisciplinary approaches to provide a description, in molecular terms, of the functional properties of the muscle myofibril. In particular, studies at the interface of genetics and proteomics address protein function at the cellular scale and offer an outstanding platform with which to elucidate how the myofibril works. Two generalizations can be enunciated from the studies reviewed. First, the study of mutant IFM proteomes provides insight into how proteins are functionally organized in the myofibril. Second, IFM mutants can give rise to structural and contractile defects that are unrelated, a reflection of the dual function that myofibrillar proteins play as fundamental components of the sarcomeric framework and biochemical "parts" of the contractile "engine".

Genetic analysis of the requirements for alpha-actinin function

Null alpha-actinin mutations in Drosophila are lethal and produce conspicuous defects in muscle structure and function. Here, we used transgene rescue to examine the requirements for alpha-actinin function in vivo. First, we tested the ability of a cDNA-based transgene encoding the adult muscle isoform of alpha-actinin under control of the heterologous ubiquitin promoter to rescue the lethality of null alpha-actinin mutations. Successful rescue indicated that alternative splicing, which also generates larval muscle and non-muscle isoforms, was not essential for viability and that there were no strict spatial or temporal requirements for alpha-actinin expression. Secondly, chimeric transgenes, with functional domains of alpha-actinin replaced by similar domains from spectrin, were tested for their ability to rescue alpha-actinin mutants. Replacement of either the actin binding domain or the EF hand calcium binding domain yielded inactive proteins, indicating that these conserved domains were not functionally equivalent. Thirdly, the length of alpha-actinin was modified by adding a 114 amino acid structural repeat from alpha-spectrin to the center of the rod domain of alpha-actinin. Addition of this sequence module was expected to increase the length of the native alpha-actinin molecule by at least 15%. yet was fully compatible with alpha-actinin function as measured by rescued lethality and flight. Thus, unexpectedly, the exact length of alpha-actinin was not critical to its function in the muscle Z disk.

Roles of a fimbrin and an alpha-actinin-like protein in fission yeast cell polarization and cytokinesis

Eukaryotic cells contain many actin-interacting proteins, including the alpha-actinins and the fimbrins, both of which have actin cross-linking activity in vitro. We report here the identification and characterization of both an alpha-actinin-like protein (Ain1p) and a fimbrin (Fim1p) in the fission yeast Schizosaccharomyces pombe. Ain1p localizes to the actomyosin-containing medial ring in an F-actin-dependent manner, and the Ain1p ring contracts during cytokinesis. ain1 deletion cells have no obvious defects under normal growth conditions but display severe cytokinesis defects, associated with defects in medial-ring and septum formation, under certain stress conditions. Overexpression of Ain1p also causes cytokinesis defects, and the ain1 deletion shows synthetic effects with other mutations known to affect medial-ring positioning and/or organization. Fim1p localizes both to the cortical actin patches and to the medial ring in an F-actin-dependent manner, and several lines of evidence suggest that Fim1p is involved in polarization of the actin cytoskeleton. Although a fim1 deletion strain has no detectable defect in cytokinesis, overexpression of Fim1p causes a lethal cytokinesis defect associated with a failure to form the medial ring and concentrate actin patches at the cell middle. Moreover, an ain1 fim1 double mutant has a synthetical-lethal defect in medial-ring assembly and cell division. Thus, Ain1p and Fim1p appear to have an overlapping and essential function in fission yeast cytokinesis. In addition, protein-localization and mutant-phenotype data suggest that Fim1p, but not Ain1p, plays important roles in mating and in spore formation.

Drosophila D-titin is required for myoblast fusion and skeletal muscle striation

An ethylmethane sulfonate (EMS) mutagenesis of Drosophila melanogaster aimed at discovering novel genes essential for neuromuscular development identified six embryonic lethal alleles of one genetic locus on the third chromosome at 62C. Two additional lethal P element insertion lines, l(3)S02001 and l(3)j1D7, failed to complement each other and each of the six EMS alleles. Analysis of genomic sequence bracketing the two insertion sites predicted a protein of 16,215 amino acid residues, encoded by a 70 kb genomic region. This sequence includes the recently characterized kettin, and includes all known partial D-Titin sequences. We call the genetic locus, which encodes both D-Titin and kettin, D-Titin. D-Titin has 53 repeats of the immunoglobulin C2 domain, 6 repeats of the fibronectin type III domain and two large PEVK domains. Kettin appears to be the NH2-terminal one third of D-Titin, presumably expressed via alternative splicing. Phenotype assays on the allelic series of D-Titin mutants demonstrated that D-Titin plays an essential role in muscle development. First, D-Titin has an unsuspected function in myoblast fusion during myogenesis and, second, D-Titin later serves to organize myofilaments into the highly ordered arrays underlying skeletal muscle striation. We propose that D-Titin is instrumental in the development of the two defining features of striated muscle: the formation of multi-nucleate syncitia and the organization of actin-myosin filaments into striated arrays.

Fine mapping of the alpha-actinin binding site within cysteine-rich protein

The cysteine-rich proteins (CRPs) are a family of highly conserved LIM (an acronym derived from the three gene products lin-11, isl-1 and mec-3) domain proteins that have been implicated in muscle differentiation. All CRP family members characterized so far have been shown to interact with the filamentous actin cross-linker alpha-actinin. The region of CRP required for this interaction has previously been broadly mapped to the molecule's N-terminal half. Here we report that the alpha-actinin-binding region of CRP, which we have mapped by using a combination of blot overlay and Western immunoblot techniques, is confined to an 18-residue sequence occurring within the protein's N-terminal glycine-rich repeat. A site-directed mutagenesis analysis of the binding region has revealed the critical importance of a single lysine residue (lysine 65 in human CRP1). Alterations at this site lead to a 10-fold decrease in alpha-actinin binding in comparison with wild-type CRP. The critical lysine residue localizes within a short alpha-helix, raising the possibility that mutagenesis-induced alterations in alpha-actinin-binding capacity might be attributed to the disruption of a key structural element.

Changes in the F-actin cytoskeleton during neurosensory bristle development in Drosophila: the role of singed and forked proteins

Drosophila neurosensory bristle development provides an excellent model system to study the role of the actin-based cytoskeleton in polarized cell growth. We used confocal fluorescence microscopy of isolated thoracic tissue to characterize changes in F-actin that occurred during macrochaete development in wild type flies and mutants that have aberrant bristle morphology. At the earliest stages in wild type bristle development, cortical patches of F-actin were present, but no bundles were observed. Actin bundles began to form at 31% of pupal development and became more prominent as development progressed. The F-actin patches gradually disappeared and were no longer present by 38% of pupal development. The distribution of F-actin in singed3 mutant macrochaetae was indistinguishable from wild type bristles until 35% of development when the actin bundles began to splay and appear ribbon-like. In forked36a bristles, the mutant phenotype was evident at earlier stages of development than the singed3 mutant. Wild type tissue stained with antibodies against the forked protein demonstrated that the forked protein colocalized with F-actin structures found in early and late stage developing macrochaetae. Antibodies against the singed protein showed it appeared to localize with F-actin structures only at later stages in development. These data suggested that the forked gene product was required for the initiation of fiber bundle formation and the singed gene product was required for the maintenance of fiber bundle morphology during bristle development. Similar analyses of singed3/forked36a double mutants provided additional genetic evidence that the forked gene product was required before the singed gene product. Further, the analyses suggested that at least one additional crosslinking protein was present in these bundles.

Purification and characterization of an alpha-actinin-binding PDZ-LIM protein that is up-regulated during muscle differentiation

alpha-Actinin is required for the organization and function of the contractile machinery of muscle. In order to understand more precisely the molecular mechanisms by which alpha-actinin might contribute to the formation and maintenance of the contractile apparatus within muscle cells, we performed a screen to identify novel alpha-actinin binding partners present in chicken smooth muscle cells. In this paper, we report the identification, purification, and characterization of a 36-kDa smooth muscle protein (p36) that interacts with alpha-actinin. Using a variety of in vitro binding assays, we demonstrate that the association between alpha-actinin and p36 is direct, specific, and saturable and exhibits a moderate affinity. Furthermore, native co-immunoprecipitation reveals that the two proteins are complexed in vivo. p36 is expressed in cardiac muscle and tissues enriched in smooth muscle. Interestingly, in skeletal muscle, a closely related protein of 40 kDa (p40) is detected. The expression of p36 and p40 is dramatically up-regulated during smooth and skeletal muscle differentiation, respectively, and p40 colocalizes with alpha-actinin at the Z-lines of differentiated myotubes. We have established the relationship between p36 and p40 by molecular cloning of cDNAs that encode both proteins and have determined that they are the products of a single gene. Both proteins display an identical N-terminal PDZ domain and an identical C-terminal LIM domain; an internal 63-amino acid sequence present in p36 is replaced by a unique 111-amino acid sequence in p40. Analysis of the sequences of p36 and p40 suggest that they are the avian forms of the actinin-associated LIM proteins (ALPs) recently described in rat (Xia, H., Winokur, S. T., Kuo, W.-L., Altherr, M. R., and Bredt, D. S. (1997) J. Cell Biol. 139, 507-515). The expression of the human ALP gene has been postulated to be affected by mutations that cause facioscapulohumeral muscular dystrophy; thus, the characterization of ALP function may ultimately provide insight into the mechanism of this disease.

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

Alpha-actinin in different invertebrate muscle cell types of Drosophila melanogaster, the earthworm Eisenia foetida, and the snail Helix aspersa

The presence and distribution of alpha-actinin has been studied in several invertebrate muscle cell types. These comprised transversely striated muscle (flight muscle) from the fruit fly Drosophila melanogaster, transversely striated muscle (heart muscle) from the snail Helix aspersa, obliquely striated muscle (body wall muscle) from the earthworm Eisenia foetida, smooth muscle (retractor muscle) from H. aspersa, and smooth muscle (outer muscular layer of the pseudoheart) from E. foetida. The study was carried by means of Western blot analysis, ELISA, and immunohistochemical electron microscopy, using anti alpha-actinin antibody. Immunoreaction for a protein with the same molecular weight as that of mammalian alpha-actinin was detected in all muscle types studied, although the amount and intensity of immunoreaction varied among them. In the insect muscle, immunolabelling was found along the whole Z-line. In both the transversely striated muscle from the snail and the obliquely striated muscle from the earthworm, immunolabelling did not occupy the whole Z-line but showed discontinuous, orderly arranged patches along the Z-line course. In the two smooth muscles studied (snail and earthworm), immunolabelling was limited to small patches which did not show an apparently ordered distribution. Since it is assumed that alpha-actinin is located at the anchorage sites for actin filaments, present observations suggest that, only in the Drosophila muscle, actin filaments are parallelly arranged in all their course, whereas in the other invertebrate muscles studied these filaments converge on discontinuously distributed anchorage sites.

CRP1, a LIM domain protein implicated in muscle differentiation, interacts with alpha-actinin

Members of the cysteine-rich protein (CRP) family are LIM domain proteins that have been implicated in muscle differentiation. One strategy for defining the mechanism by which CRPs potentiate myogenesis is to characterize the repertoire of CRP binding partners. In order to identify proteins that interact with CRP1, a prominent protein in fibroblasts and smooth muscle cells, we subjected an avian smooth muscle extract to affinity chromatography on a CRP1 column. A 100-kD protein bound to the CRP1 column and could be eluted with a high salt buffer; Western immunoblot analysis confirmed that the 100-kD protein is alpha-actinin. We have shown that the CRP1-alpha-actinin interaction is direct, specific, and saturable in both solution and solid-phase binding assays. The Kd for the CRP1-alpha-actinin interaction is 1.8 +/- 0.3 microM. The results of the in vitro protein binding studies are supported by double-label indirect immunofluorescence experiments that demonstrate a colocalization of CRP1 and alpha-actinin along the actin stress fibers of CEF and smooth muscle cells. Moreover, we have shown that alpha-actinin coimmunoprecipitates with CRP1 from a detergent extract of smooth muscle cells. By in vitro domain mapping studies, we have determined that CRP1 associates with the 27-kD actin-binding domain of alpha-actinin. In reciprocal mapping studies, we showed that alpha-actinin interacts with CRP1-LIM1, a deletion fragment that contains the NH2-terminal 107 amino acids (aa) of CRP1. To determine whether the alpha-actinin binding domain of CRP1 would localize to the actin cytoskeleton in living cells, expression constructs encoding epitope-tagged full-length CRP1, CRP1-LIM1(aa 1-107), or CRP1-LIM2 (aa 108-192) were microinjected into cells. By indirect immunofluorescence, we have determined that full-length CRP1 and CRP1-LIM1 localize along the actin stress fibers whereas CRP1-LIM2 fails to associate with the cytoskeleton. Collectively these data demonstrate that the NH2-terminal part of CRP1 that contains the alpha-actinin-binding site is sufficient to localize CRP1 to the actin cytoskeleton. The association of CRP1 with alpha-actinin may be critical for its role in muscle differentiation.

Deficiency of a skeletal muscle isoform of alpha-actinin (alpha-actinin-3) in merosin-positive congenital muscular dystrophy

A subset of patients with congenital muscular dystrophy (CMD) are deficient for the extracellular matrix protein, merosin. Although the aetiology of merosin-positive CMD is as yet unknown, abnormalities of other structural muscle-specific proteins are likely to be involved. The alpha-actinins are actin-binding proteins related to dystrophin. We studied expression of the skeletal muscle isoforms of alpha-actinin (alpha-actinin-2 and alpha-actinin-3) in muscle biopsies from 12 patients with pure CMD (including one with a merosin abnormality), two with unclassified CMD and central nervous system (CNS) involvement, and three with other neuromuscular disorders. Four specimens exhibited deficient alpha-actinin-3 staining by immunofluorescence and/or Western blot analysis. In one, this pattern may be a secondary consequence of marked type 1 fibre predominance, but the other three biopsies contained abundant type 2 fibres where alpha-actinin-3 is normally expressed. Three alpha-actinin-3-deficient patients had pure CMD and presented in the newborn period with muscle weakness, hypotonia and arthrogryposis. The fourth had a dystrophic muscle biopsy and CNS involvement. These results suggest that deficiency of alpha-actinin-3 may be a marker for a subset of patients with CMD. It remains to be determined whether the deficiency of alpha-actinin-3 reflects ACTN3 gene mutations or is a secondary phenomenon.

Novel actin crosslinker superfamily member identified by a two step degenerate PCR procedure

Actin-crosslinking proteins link F-actin into the bundles and networks that constitute the cytoskeleton. Dystrophin, beta-spectrin, alpha-actinin, ABP-120, ABP-280, and fimbrin share homologous actin-binding domains and comprise an actin crosslinker superfamily. We have identified a novel member of this superfamily (ACF7) using a degenerate primer-mediated PCR strategy that was optimized to resolve less-abundant superfamily sequences. The ACF7 gene is on human chromosome 1 and hybridizes to high molecular weight bands on northern blots. Sequence comparisons argue that ACF7 does not fit into one of the existing families, but represents a new class within the superfamily.

Assembly of body wall muscle and muscle cell attachment structures in Caenorhabditis elegans

C. Elegans has four muscle quadrants that are used for locomotion. Contraction is converted to locomotion because muscle cells are anchored to the cuticle (the outer covering of the worm) by a specialized basement membrane and hemidesmosome structures in the hypodermis (a cellular syncytium that covers the worm and secretes the cuticle). To study muscle assembly, we have used antibodies to determine the spatial and temporal distribution of muscle and attachment structure components in wild-type and mutant C. elegans embryos. Myofibrillar components are first observed diffusely distributed in the muscle cells, and are expressed in some dividing cells. Later, the components accumulate at the membrane adjacent to the hypodermis where the sarcomeres will form, showing that the cells have become polarized. Assembly of muscle attachment structures is spatially and temporally coordinated with muscle assembly suggesting that important developmental signals may be passed between muscle and hypodermal cells. Analysis of embryos homozygous for mutations that affect muscle assembly show that muscle components closer to the membrane than the affected protein assemble quite well, while those further from the membrane do not. Our results suggest a model where lattice assembly is initiated at the membrane and the spatial organization of the structural elements of the muscle is dictated by membrane proximal events, not by the filament components themselves.

Cell shape and interaction defects in alpha-spectrin mutants of Drosophila melanogaster

We show that the alpha-spectrin gene is essential for larval survival and development by characterizing several alpha-spectrin mutations in Drosophila. P-element minigene rescue and sequence analysis were used to identify the alpha-spectrin gene as the l(3)dre3 complementation group of the Dras-Roughened-ecdysoneless region of chromosome 3 (Sliter et al., 1988). Germ line transformants carrying an alpha-spectrin cDNA, whose expression is driven by the ubiquitin promoter, fully rescued the first to second instar lethality characteristic of the l(3)dre3 alleles. The molecular defects in two gamma-ray-induced alleles were identified. One of these mutations, which resulted in second instar lethality, contained a 73-bp deletion in alpha-spectrin segment 22 (starting at amino acid residue 2312), producing a premature stop codon between the two EF hands found in this segment. The second mutation, which resulted in first instar lethality, contained a 20 base pair deletion in the middle of segment 1 (at amino acid residue 92), resulting in a premature stop codon. Examination of the spectrin-deficient larvae revealed a loss of contact between epithelial cells of the gut and disruption of cell-substratum interactions. The most pronounced morphological change was seen in tissues of complex cellular architecture such as the middle midgut where a loss of cell contact between cup-shaped cuprophilic cells and neighboring interstitial cells was accompanied by disorganization of the cuprophilic cell brush borders. Our examination of spectrin deficient larvae suggests that an important role of non-erythroid spectrin is to stabilize cell to cell interactions that are critical for the maintenance of cell shape and subcellular organization within tissues.

Thinking about genetic redundancy

Partial functional redundancy among genes is frequently observed in a wide range of organisms and processes, but the selective value of such redundancy is not immediately apparent. Any fully redundant function should be evolutionarily unstable: unless selection acts to maintain the redundancy it will tend to be lost by mutational drift. I discuss four possible mechanisms by which selection might act to maintain genetic redundancy.

Integrins hold Drosophila together

The Drosophila position-specific (PS) integrins are members of the integrin family of cell surface receptors and are thought to be receptors for extracellular matrix components. Each PS integrin consists of an alpha subunit, alpha PS1 or alpha PS2, and a beta PS subunit. Mutations in the beta PS subunit and the alpha PS2 subunit have been characterised and reveal that the PS integrins have an essential role in the adhesion of different cell layers to each other. The PS integrins are especially required for the function of the cell-matrix-cell junctions, where the muscles attach to the epidermis and where one surface of the developing wing adheres to the other. These junctions are similar to vertebrate focal adhesions and hemidesmosomes, which also contain integrins. Integrin-mediated cell to cell adhesion via the extracellular matrix provides a way for tissues to adhere to each other without intermingling of their cells.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

The distribution of PS integrins, laminin A and F-actin during key stages in Drosophila wing development

We first summarize wing development during metamorphosis of Drosophila and identify four critical steps in the conversion of a folded single layered wing disc to a flat bilayered wing. Each step occurs twice, once during the 12 hour prepupal period and again during the 84 hour pupal period. (1) Apposition in which basal surfaces of dorsal and ventral epithelia come close together. (2) Adhesion in which basal junctions form between the apposed basal surfaces. (3) Expansion in which wing area increases as a result of cells flattening. (4) Separation in which dorsal and ventral epithelia are separated by a bulky extracellular matrix but remain connected by slender cytoplasmic processes containing the microtubules and microfilaments of the transalar cytoskeleton. Disc ultrastructure is correlated with the distribution of the beta chain of integrin, laminin A, and filamentous actin for each key stage of pupal development. Integrin and laminin exhibit a mutually exclusive distribution from the adhesion stage onwards. Integrin is present on the basal surface of intervein cells but not on vein cells whereas laminin A is absent from the basal surfaces of intervein cells but is present on vein cells. We conclude that laminin is not a ligand for integrin in this context. During apposition and adhesion stages integrin is broadly distributed over the basal and lateral surfaces of intervein cells but subsequently becomes localized to small basal foci. These foci correspond to basal contact zones between transalar processes. The distribution of filamentous actin is dynamic, changing from an apical distribution during hair morphogenesis to a basal distribution as the transalar cytoskeleton develops. Basal adherens-type junctions are first evident during the adhesion stage and become closely associated with the transalar cytoskeleton during the separation stage. Thus, basal junction formation occurs in two discrete steps; intercellular connections are established first and junction/cytoskeletal connections are formed about 20 hours later. These observations provide a basis for future investigations of integrin mediated adhesion in vivo.

A chick skeletal-muscle alpha-actinin gene gives rise to two alternatively spliced isoforms which differ in the EF-hand Ca(2+)-binding domain

A chick non-muscle alpha-actinin cDNA probe encoding the EF-hand region of molecule was used to screen a lambda gt10 chick brain cDNA library from 14-day embryos. A partial 2.1-kb alpha-actinin cDNA was isolated (8W cDNA) which encoded a protein identical to chick skeletal-muscle alpha-actinin, except in the C-terminal part of the first EF hand. In the variant, the 22 residues found in the skeletal-muscle isoform were replaced by a stretch of 26 unique residues. Analysis of the structure of the skeletal-muscle alpha-actinin gene showed that the region of divergence was encoded by two exons which are alternatively spliced. Quantitative reverse transcriptase/polymerase chain reaction (RT/PCR) was used to investigate the levels of the alpha-actinin transcripts in various tissues. The skeletal-muscle alpha-actinin variant was expressed at low levels in brain, liver and spleen, but could not be detected in skeletal muscle. Surprisingly, skeletal-muscle alpha-actinin mRNA was also expressed in brain, liver and spleen. The RT/PCR products were authenticated by using diagnostic restriction enzyme sites and by sequencing. The splice variant derived from the skeletal-muscle alpha-actinin gene was also detected in a variety of cDNA libraries from both adult and embryonic tissues by PCR. Although a transcript encoding this alpha-actinin splice variant is expressed in non-muscle tissues, neither of the two EF-hands would be predicted to be functional, making it unlikely to be a typical non-muscle isoform which are calcium-sensitive with respect to binding actin. The two vertebrate non-muscle alpha-actinins sequenced to date also have a spacer of five amino acids between the two EF hands, whereas in the variant, the spacer is just four residues in length. Further analysis will be required before this alpha-actinin isoform, which we refer to as SKv, can be classified as muscle or non-muscle alpha-actinin. We propose a new nomenclature to describe the various alpha-actinin genes and their transcripts.