The Drosophila FHOD1-like formin Knittrig acts through Rok to promote stress fiber formation and directed macrophage migration during the cellular immune response

Peripheral thickening of the sarcomeres and pointed end elongation of the thin filaments are both promoted by SALS and its formin interaction partners

During striated muscle development the first periodically repeated units appear in the premyofibrils, consisting of immature sarcomeres that must undergo a substantial growth both in length and width, to reach their final size. Here we report that, beyond its well established role in sarcomere elongation, the Sarcomere length short (SALS) protein is involved in Z-disc formation and peripheral growth of the sarcomeres. Our protein localization data and loss-of-function studies in the Drosophila indirect flight muscle strongly suggest that radial growth of the sarcomeres is initiated at the Z-disc. As to thin filament elongation, we used a powerful nanoscopy approach to reveal that SALS is subject to a major conformational change during sarcomere development, which might be critical to stop pointed end elongation in the adult muscles. In addition, we demonstrate that the roles of SALS in sarcomere elongation and radial growth are both dependent on formin type of actin assembly factors. Unexpectedly, when SALS is present in excess amounts, it promotes the formation of actin aggregates highly resembling the ones described in nemaline myopathy patients. Collectively, these findings helped to shed light on the complex mechanisms of SALS during the coordinated elongation and thickening of the sarcomeres, and resulted in the discovery of a potential nemaline myopathy model, suitable for the identification of genetic and small molecule inhibitors.

Formin tails act as a switch, inhibiting or enhancing processive actin elongation

Formins are large, multidomain proteins that nucleate new actin filaments and accelerate elongation through a processive interaction with the barbed ends of filaments. Their actin assembly activity is generally attributed to their eponymous formin homology (FH) 1 and 2 domains; however, evidence is mounting that regions outside of the FH1FH2 stretch also tune actin assembly. Here, we explore the underlying contributions of the tail domain, which spans the sequence between the FH2 domain and the C terminus of formins. Tails vary in length from ∼0 to >200 residues and contain a number of recognizable motifs. The most common and well-studied motif is the ∼15-residue-long diaphanous autoregulatory domain. This domain mediates all or nothing regulation of actin assembly through an intramolecular interaction with the diaphanous inhibitory domain in the N-terminal half of the protein. Multiple reports demonstrate that the tail can enhance both nucleation and processivity. In this study, we provide a high-resolution view of the alternative splicing encompassing the tail in the formin homology domain (Fhod) family of formins during development. While four distinct tails are predicted, we found significant levels of only two of these. We characterized the biochemical effects of the different tails. Surprisingly, the two highly expressed Fhod-tails inhibit processive elongation and diminish nucleation, while a third supports activity. These findings demonstrate a new mechanism of modulating actin assembly by formins and support a model in which splice variants are specialized to build distinct actin structures during development.

FHODs: Nuclear tethered formins for nuclear mechanotransduction

In this review, we discuss FHOD formins with a focus on recent studies that reveal a new role for them as critical links for nuclear mechanotransduction. The FHOD family in vertebrates comprises two structurally related proteins, FHOD1 and FHOD3. Their similar biochemical properties suggest overlapping and redundant functions. FHOD1 is widely expressed, FHOD3 less so, with highest expression in skeletal (FHOD1) and cardiac (FHOD3) muscle where specific splice isoforms are expressed. Unlike other formins, FHODs have strong F-actin bundling activity and relatively weak actin polymerization activity. These activities are regulated by phosphorylation by ROCK and Src kinases; bundling is additionally regulated by ERK1/2 kinases. FHODs are unique among formins in their association with the nuclear envelope through direct, high affinity binding to the outer nuclear membrane proteins nesprin-1G and nesprin-2G. Recent crystallographic structures reveal an interaction between a conserved motif in one of the spectrin repeats (SRs) of nesprin-1G/2G and a site adjacent to the regulatory domain in the amino terminus of FHODs. Nesprins are components of the LINC (linker of nucleoskeleton and cytoskeleton) complex that spans both nuclear membranes and mediates bidirectional transmission of mechanical forces between the nucleus and the cytoskeleton. FHODs interact near the actin-binding calponin homology (CH) domains of nesprin-1G/2G enabling a branched connection to actin filaments that presumably strengthens the interaction. At the cellular level, the tethering of FHODs to the outer nuclear membrane mechanically couples perinuclear actin arrays to the nucleus to move and position it in fibroblasts, cardiomyocytes, and potentially other cells. FHODs also function in adhesion maturation during cell migration and in the generation of sarcomeres, activities distant from the nucleus but that are still influenced by it. Human genetic studies have identified multiple FHOD3 variants linked to dilated and hypertrophic cardiomyopathies, with many mutations mapping to "hot spots" in FHOD3 domains. We discuss how FHOD1/3's role in reinforcing the LINC complex and connecting to perinuclear actin contributes to functions of mechanically active tissues such as striated muscle.

Polarized branched Actin modulates cortical mechanics to produce unequal-size daughters during asymmetric division

The control of cell shape during cytokinesis requires a precise regulation of mechanical properties of the cell cortex. Only few studies have addressed the mechanisms underlying the robust production of unequal-sized daughters during asymmetric cell division. Here we report that unequal daughter-cell sizes resulting from asymmetric sensory organ precursor divisions in Drosophila are controlled by the relative amount of cortical branched Actin between the two cell poles. We demonstrate this by mistargeting the machinery for branched Actin dynamics using nanobodies and optogenetics. We can thereby engineer the cell shape with temporal precision and thus the daughter-cell size at different stages of cytokinesis. Most strikingly, inverting cortical Actin asymmetry causes an inversion of daughter-cell sizes. Our findings uncover the physical mechanism by which the sensory organ precursor mother cell controls relative daughter-cell size: polarized cortical Actin modulates the cortical bending rigidity to set the cell surface curvature, stabilize the division and ultimately lead to unequal daughter-cell size.

Contractile and expansive actin networks in Drosophila: Developmental cell biology controlled by network polarization and higher-order interactions

Actin networks are central to shaping and moving cells during animal development. Various spatial cues activate conserved signal transduction pathways to polarize actin network assembly at sub-cellular locations and to elicit specific physical changes. Actomyosin networks contract and Arp2/3 networks expand, and to affect whole cells and tissues they do so within higher-order systems. At the scale of tissues, actomyosin networks of epithelial cells can be coupled via adherens junctions to form supracellular networks. Arp2/3 networks typically integrate with distinct actin assemblies, forming expansive composites which act in conjunction with contractile actomyosin networks for whole-cell effects. This review explores these concepts using examples from Drosophila development. First, we discuss the polarized assembly of supracellular actomyosin cables which constrict and reshape epithelial tissues during embryonic wound healing, germ band extension, and mesoderm invagination, but which also form physical borders between tissue compartments at parasegment boundaries and during dorsal closure. Second, we review how locally induced Arp2/3 networks act in opposition to actomyosin structures during myoblast cell-cell fusion and cortical compartmentalization of the syncytial embryo, and how Arp2/3 and actomyosin networks also cooperate for the single cell migration of hemocytes and the collective migration of border cells. Overall, these examples show how the polarized deployment and higher-order interactions of actin networks organize developmental cell biology.

Evolutionary changes in gene expression profiles associated with the coevolution of male and female genital parts among closely related ground beetle species

Background

The coevolutionary dynamics of corresponding male and female sexual traits, including genitalia, may be driven by complex genetic mechanisms. Carabus (Ohomopterus) ground beetles show correlated evolution in the size of their functionally corresponding male and female genital parts. To reveal the genetic mechanisms involved in the evolution of size, we investigated interspecific differences in gene expression profiles in four closely related species (two species each with long and short genital parts) using transcriptome data from genital tissues in the early and late pupal stages.

Results

We detected 1536 and 1306 differentially expressed genes (DEGs) among the species in males and 546 and 1959 DEGs in females in the two pupal stages, respectively. The DEGs were clustered by species-specific expression profiles for each stage and sex to identify candidate gene clusters for genital size based on the expression patterns among the species and gene ontology. We identified one and two gene clusters in females and males, respectively, all from the late pupal stage; one cluster of each sex showed similar expression profiles in species with similar genital size, which implies a common gene expression change associated with similar genital size in each sex. However, the remaining male cluster showed different expression profiles between species with long genital parts, which implies species-specific gene expression changes. These clusters did not show sex-concordant expression profiles for genital size differences.

Conclusion

Our study demonstrates that sex-independent and partly species-specific gene expression underlies the correlated evolution of male and female genital size. These results may reflect the complex evolutionary history of male and female genitalia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08865-2.

Calcium bursts allow rapid reorganization of EFhD2/Swip-1 cross-linked actin networks in epithelial wound closure

Changes in cell morphology require the dynamic remodeling of the actin cytoskeleton. Calcium fluxes have been suggested as an important signal to rapidly relay information to the actin cytoskeleton, but the underlying mechanisms remain poorly understood. Here, we identify the EF-hand domain containing protein EFhD2/Swip-1 as a conserved lamellipodial protein strongly upregulated in Drosophila macrophages at the onset of metamorphosis when macrophage behavior shifts from quiescent to migratory state. Loss- and gain-of-function analysis confirm a critical function of EFhD2/Swip-1 in lamellipodial cell migration in fly and mouse melanoma cells. Contrary to previous assumptions, TIRF-analyses unambiguously demonstrate that EFhD2/Swip-1 proteins efficiently cross-link actin filaments in a calcium-dependent manner. Using a single-cell wounding model, we show that EFhD2/Swip-1 promotes wound closure in a calcium-dependent manner. Mechanistically, our data suggest that transient calcium bursts reduce EFhD2/Swip-1 cross-linking activity and thereby promote rapid reorganization of existing actin networks to drive epithelial wound closure.

LSP1-myosin1e bimolecular complex regulates focal adhesion dynamics and cell migration

Several cytoskeleton-associated proteins and signaling pathways work in concert to regulate actin cytoskeleton remodeling, cell adhesion, and migration. Although the leukocyte-specific protein 1 (LSP1) has been shown to interact with the actin cytoskeleton, its function in the regulation of actin cytoskeleton dynamics is, as yet, not fully understood. We have recently demonstrated that the bimolecular complex between LSP1 and myosin1e controls actin cytoskeleton remodeling during phagocytosis. In this study, we show that LSP1 downregulation severely impairs cell migration, lamellipodia formation, and focal adhesion dynamics in macrophages. Inhibition of the interaction between LSP1 and myosin1e also impairs these processes resulting in poorly motile cells, which are characterized by few and small lamellipodia. Furthermore, cells in which LSP1-myosin1e interaction is inhibited are typically associated with inefficient focal adhesion turnover. Collectively, our findings show that the LSP1-myosin1e bimolecular complex plays a pivotal role in the regulation of actin cytoskeleton remodeling and focal adhesion dynamics required for cell migration.

Active FHOD1 promotes the formation of functional actin stress fibers

The formin FHOD1 acts as a nucleating, capping and bundling protein of actin filaments. In cells, release from the C-terminal diaphanous autoregulatory domain (DAD) of FHOD1 stimulates the protein into the active form. However, the cellular physiological relevance of active form FHOD1 and the phenotypic regulation by FHOD1 depletion are not completely understood. Here, we show that in contrast with the cytosolic diffused expression of auto-inhibited FHOD1, active FHOD1 by C-terminal truncation was recruited into all three types of actin stress fibers in human osteosarcoma cells. Notably, the recruited active FHOD1 was more incorporated with myosin II than α-actinin, and associated with both naïve and mature focal adhesions. Active FHOD1 displayed faster turnover than actin molecules on ventral stress fibers. Moreover, we witnessed the emergence of active FHOD1 from the cell periphery, which subsequently moved centripetally together with transverse arcs. Furthermore, FHOD1 knockdown resulted in defective maturation of actomyosin bundles and subsequently longer non-contractile dorsal stress fibers, whereas the turnover of both actin and myosin II were maintained normally. Importantly, the loss of FHOD1 led to slower actin centripetal flow, resulting in abnormal cell spreading and migration defects. Taken together, these results reveal a critical role of FHOD1 in temporal- and spatial- control of the morphology and dynamics of functional actin stress fibers during variable cell behavior.

Gene expression atlas of a developing tissue by single cell expression correlation analysis

The Drosophila wing disc has been a fundamental model system for the discovery of key signaling pathways and for our understanding of developmental processes. However, a complete map of gene expression in this tissue is lacking. To obtain a complete gene expression atlas in the wing disc, we employed single-cell sequencing (scRNA-seq) and developed a new method for analyzing scRNA-seq data based on gene expression correlations rather than cell mapping. This enables us to compute expression maps for all detected genes in the wing disc and to discover 824 genes with spatially restricted expression patterns. This approach identifies both known and new clusters of genes with similar expression patterns and functional relevance. As proof of concept, we characterize the previously unstudied gene CG5151 and show that it regulates Wnt signaling. This novel method will enable the leveraging of scRNA-seq data for generating expression atlases of undifferentiated tissues during development.

Actin assembly mechanisms at a glance

The actin cytoskeleton and associated motor proteins provide the driving forces for establishing the astonishing morphological diversity and dynamics of mammalian cells. Aside from functions in protruding and contracting cell membranes for motility, differentiation or cell division, the actin cytoskeleton provides forces to shape and move intracellular membranes of organelles and vesicles. To establish the many different actin assembly functions required in time and space, actin nucleators are targeted to specific subcellular compartments, thereby restricting the generation of specific actin filament structures to those sites. Recent research has revealed that targeting and activation of actin filament nucleators, elongators and myosin motors are tightly coordinated by conserved protein complexes to orchestrate force generation. In this Cell Science at a Glance article and the accompanying poster, we summarize and discuss the current knowledge on the corresponding protein complexes and their modes of action in actin nucleation, elongation and force generation.

Analysis of Cell Shape and Cell Migration of Drosophila Macrophages In Vivo

The most abundant immune cells in Drosophila are macrophage-like plasmatocytes that fulfill central roles in morphogenesis, immune and tissue damage response. The various genetic tools available in Drosophila together with high-resolution and live-imaging microscopy techniques make Drosophila macrophages an excellent model system that combines many advantages of cultured cells with in vivo genetics. Here, we describe the isolation and staining of macrophages from larvae for ex vivo structured illumination microscopy (SIM), the preparation of white prepupae for in vivo 2D random cell migration analysis, and the preparation of pupae (18 h after puparium formation, APF) for in vivo 3D directed cell migration analysis upon wounding using spinning disk microscopy.

The neuron-specific formin Delphilin nucleates nonmuscle actin but does not enhance elongation

The formin Delphilin binds the glutamate receptor, GluRδ2, in dendritic spines of Purkinje cells. Both proteins play a role in learning. To understand how Delphilin functions in neurons, we studied the actin assembly properties of this formin. Formins have a conserved formin homology 2 domain, which nucleates and associates with the fast-growing end of actin filaments, influencing filament growth together with the formin homology 1 (FH1) domain. The strength of nucleation and elongation varies widely across formins. Additionally, most formins have conserved domains that regulate actin assembly through an intramolecular interaction. Delphilin is distinct from other formins in several ways: its expression is limited to Purkinje cells, it lacks classical autoinhibitory domains, and its FH1 domain has minimal proline-rich sequence. We found that Delphilin is an actin nucleator that does not accelerate elongation, although it binds to the barbed end of filaments. In addition, Delphilin exhibits a preference for actin isoforms, nucleating nonmuscle actin but not muscle actin, which has not been described or systematically studied in other formins. Finally, Delphilin is the first formin studied that is not regulated by intramolecular interactions. We speculate how the activity we observe is consistent with its localization in the small dendritic spines.

α-synuclein Induces Mitochondrial Dysfunction through Spectrin and the Actin Cytoskeleton

Genetics and neuropathology strongly link α-synuclein aggregation and neurotoxicity to the pathogenesis of Parkinson's disease and related α-synucleinopathies. Here we describe a new Drosophila model of α-synucleinopathy based on widespread expression of wild-type human α-synuclein, which shows robust neurodegeneration, early-onset locomotor deficits, and abundant α-synuclein aggregation. We use results of forward genetic screening and genetic analysis in our new model to demonstrate that α-synuclein expression promotes reorganization of the actin filament network and consequent mitochondrial dysfunction through altered Drp1 localization. Similar changes are present in a mouse α-synucleinopathy model and in postmortem brain tissue from patients with α-synucleinopathy. Importantly, we provide evidence that the interaction of α-synuclein with spectrin initiates pathological alteration of the actin cytoskeleton and downstream neurotoxicity. These findings suggest new therapeutic approaches for α-synuclein induced neurodegeneration.

Drosophila and human FHOD family formin proteins nucleate actin filaments

Formins are a conserved group of proteins that nucleate and processively elongate actin filaments. Among them, the formin homology domain-containing protein (FHOD) family of formins contributes to contractility of striated muscle and cell motility in several contexts. However, the mechanisms by which they carry out these functions remain poorly understood. Mammalian FHOD proteins were reported not to accelerate actin assembly in vitro; instead, they were proposed to act as barbed end cappers or filament bundlers. Here, we show that purified Drosophila Fhod and human FHOD1 both accelerate actin assembly by nucleation. The nucleation activity of FHOD1 is restricted to cytoplasmic actin, whereas Drosophila Fhod potently nucleates both cytoplasmic and sarcomeric actin isoforms. Drosophila Fhod binds tightly to barbed ends, where it slows elongation in the absence of profilin and allows, but does not accelerate, elongation in the presence of profilin. Fhod antagonizes capping protein but dissociates from barbed ends relatively quickly. Finally, we determined that Fhod binds the sides of and bundles actin filaments. This work establishes that Fhod shares the capacity of other formins to nucleate and bundle actin filaments but is notably less effective at processively elongating barbed ends than most well studied formins.

Evolutionary conserved role of eukaryotic translation factor eIF5A in the regulation of actin-nucleating formins

Elongation factor eIF5A is required for the translation of consecutive prolines, and was shown in yeast to translate polyproline-containing Bni1, an actin-nucleating formin required for polarized growth during mating. Here we show that Drosophila eIF5A can functionally replace yeast eIF5A and is required for actin-rich cable assembly during embryonic dorsal closure (DC). Furthermore, Diaphanous, the formin involved in actin dynamics during DC, is regulated by and mediates eIF5A effects. Finally, eIF5A controls cell migration and regulates Diaphanous levels also in mammalian cells. Our results uncover an evolutionary conserved role of eIF5A regulating cytoskeleton-dependent processes through translation of formins in eukaryotes.

Actin stress fiber organization promotes cell stiffening and proliferation of pre-invasive breast cancer cells

Studies of the role of actin in tumour progression have highlighted its key contribution in cell softening associated with cell invasion. Here, using a human breast cell line with conditional Src induction, we demonstrate that cells undergo a stiffening state prior to acquiring malignant features. This state is characterized by the transient accumulation of stress fibres and upregulation of Ena/VASP-like (EVL). EVL, in turn, organizes stress fibres leading to transient cell stiffening, ERK-dependent cell proliferation, as well as enhancement of Src activation and progression towards a fully transformed state. Accordingly, EVL accumulates predominantly in premalignant breast lesions and is required for Src-induced epithelial overgrowth in Drosophila. While cell softening allows for cancer cell invasion, our work reveals that stress fibre-mediated cell stiffening could drive tumour growth during premalignant stages. A careful consideration of the mechanical properties of tumour cells could therefore offer new avenues of exploration when designing cancer-targeting therapies.

When cells acquire a malignant phenotype they become less stiff and this helps migration and invasion favouring metastasis. Here the authors show that Src-driven cell transformation and transition to a less stiff state follows an event of membrane stiffening due to stress fibres accumulation.

Transmission of cytokinesis forces via E-cadherin dilution and actomyosin flows

During epithelial cytokinesis, the remodelling of adhesive cell-cell contacts between the dividing cell and its neighbours has profound implications for the integrity, arrangement and morphogenesis of proliferative tissues. In both vertebrates and invertebrates, this remodelling requires the activity of non-muscle myosin II (MyoII) in the interphasic cells neighbouring the dividing cell. However, the mechanisms that coordinate cytokinesis and MyoII activity in the neighbours are unknown. Here we show that in the Drosophila notum epithelium, each cell division is associated with a mechanosensing and transmission event that controls MyoII dynamics in neighbouring cells. We find that the ring pulling forces promote local junction elongation, which results in local E-cadherin dilution at the ingressing adherens junction. In turn, the reduction in E-cadherin concentration and the contractility of the neighbouring cells promote self-organized actomyosin flows, ultimately leading to accumulation of MyoII at the base of the ingressing junction. Although force transduction has been extensively studied in the context of adherens junction reinforcement to stabilize adhesive cell-cell contacts, we propose an alternative mechanosensing mechanism that coordinates actomyosin dynamics between epithelial cells and sustains the remodelling of the adherens junction in response to mechanical forces.

The genetic architecture of defence as resistance to and tolerance of bacterial infection in Drosophila melanogaster

Defence against pathogenic infection can take two forms: resistance and tolerance. Resistance is the ability of the host to limit a pathogen burden, whereas tolerance is the ability to limit the negative consequences of infection at a given level of infection intensity. Evolutionarily, a tolerance strategy that is independent of resistance could allow the host to avoid mounting a costly immune response and, theoretically, to avoid a co-evolutionary arms race between pathogen virulence and host resistance. Biomedically, understanding the mechanisms of tolerance and how they relate to resistance could potentially yield treatment strategies that focus on health improvement instead of pathogen elimination. To understand the impact of tolerance on host defence and identify genetic variants that determine host tolerance, we defined genetic variation in tolerance as the residual deviation from a binomial regression of fitness under infection against infection intensity. We then performed a genomewide association study to map the genetic basis of variation in resistance to and tolerance of infection by the bacterium Providencia rettgeri. We found a positive genetic correlation between resistance and tolerance, and we demonstrated that the level of resistance is highly predictive of tolerance. We identified 30 loci that predict tolerance, many of which are in genes involved in the regulation of immunity and metabolism. We used RNAi to confirm that a subset of mapped genes have a role in defence, including putative wound repair genes grainy head and debris buster. Our results indicate that tolerance is not an independent strategy from resistance, but that defence arises from a collection of physiological processes intertwined with canonical immunity and resistance.

Drosophila WASH is required for integrin-mediated cell adhesion, cell motility and lysosomal neutralization

The Wiskott-Aldrich syndrome protein and SCAR homolog (WASH; also known as Washout in flies) is a conserved actin-nucleation-promoting factor controlling Arp2/3 complex activity in endosomal sorting and recycling. Previous studies have identified WASH as an essential regulator in Drosophila development. Here, we show that homozygous wash mutant flies are viable and fertile. We demonstrate that Drosophila WASH has conserved functions in integrin receptor recycling and lysosome neutralization. WASH generates actin patches on endosomes and lysosomes, thereby mediating both aforementioned functions. Consistently, loss of WASH function results in cell spreading and cell migration defects of macrophages, and an increased lysosomal acidification that affects efficient phagocytic and autophagic clearance. WASH physically interacts with the vacuolar (V)-ATPase subunit Vha55 that is crucial to establish and maintain lysosome acidification. As a consequence, starved flies that lack WASH function show a dramatic increase in acidic autolysosomes, causing a reduced lifespan. Thus, our data highlight a conserved role for WASH in the endocytic sorting and recycling of membrane proteins, such as integrins and the V-ATPase, that increase the likelihood of survival under nutrient deprivation.

The Drosophila formin Fhos is a primary mediator of sarcomeric thin-filament array assembly

Actin-based thin filament arrays constitute a fundamental core component of muscle sarcomeres. We have used formation of the Drosophila indirect flight musculature for studying the assembly and maturation of thin-filament arrays in a skeletal muscle model system. Employing GFP-tagged actin monomer incorporation, we identify several distinct phases in the dynamic construction of thin-filament arrays. This sequence includes assembly of nascent arrays after an initial period of intensive microfilament synthesis, followed by array elongation, primarily from filament pointed-ends, radial growth of the arrays via recruitment of peripheral filaments and continuous barbed-end turnover. Using genetic approaches we have identified Fhos, the single Drosophila homolog of the FHOD sub-family of formins, as a primary and versatile mediator of IFM thin-filament organization. Localization of Fhos to the barbed-ends of the arrays, achieved via a novel N-terminal domain, appears to be a critical aspect of its sarcomeric roles.

DOI:http://dx.doi.org/10.7554/eLife.16540.001

Muscles owe their ability to contract to structural units called sarcomeres, and a single muscle fiber can contain many thousands of these structures, aligned one next to the other. Each mature sarcomere is made up of precisely arranged and intertwined thin filaments of actin and thicker bundles of motor proteins, surrounded by other proteins. Sliding the motors along the filaments provides the force needed to contract the muscle. However, it was far from clear how sarcomeres, especially the arrays of thin-filaments, are assembled from scratch in developing muscles.

When the fruit fly Drosophila transforms from a larva into an adult, it needs to build muscles to move its newly forming wings. While smaller in size, these flight muscles closely resemble the skeletal muscles of animals with backbones, and therefore serve as a good model for muscle formation in general. New muscles require new sarcomeres too, and now Shwartz et al. have observed and monitored sarcomeres assembling in developing flight muscles of fruit flies, a process that takes about three days.

The analysis made use of genetically engineered flies in which the gene for a fluorescently labeled version of actin, the building block of the thin filaments, could be switched on at specific points in time. Looking at how these green-glowing proteins become incorporated into the growing sarcomere revealed that the assembly process involves four different phases. First, a large store of unorganized and newly-made thin filaments is generated for future use. These filaments are then assembled into rudimentary structures in which the filaments are roughly aligned. Once these core structures are formed, the existing filaments are elongated, while additional filaments are brought in to expand the structure further. Finally, actin proteins are continuously added and removed at the part of the sarcomere where the thin filaments are anchored.

Shwartz et al. went on to identify a protein termed Fhos as the chief player in the process. Fhos is a member of a family of proteins that are known to elongate and organize actin filaments in many different settings. Without Fhos, the thin-filament arrays cannot properly begin to assemble, and the subsequent steps of growth and expansion are blocked as well.

The next challenges will be to understand what guides the initial stages in the assembly of the thin-filament array, and how the coordination between assembly of actin filament arrays and motor proteins is executed. It will also be important to determine how sarcomeres are maintained throughout the life of the organism when defective actin filaments are replaced, and which proteins are responsible for carrying out this process.

DOI:http://dx.doi.org/10.7554/eLife.16540.002

Cooperative functions of the two F-BAR proteins Cip4 and Nostrin in the regulation of E-cadherin in epithelial morphogenesis

F-BAR proteins are prime candidates to regulate membrane curvature and dynamics during different developmental processes. Here, we analyzed nostrin, a so-far-unknown Drosophila melanogaster F-BAR protein related to Cip4. Genetic analyses revealed a strong synergism between nostrin and cip4 functions.Whereas single mutant flies are viable and fertile, combined loss of nostrin and cip4 results in reduced viability and fertility. Double mutant escaper flies show enhanced wing polarization defects and females exhibit strong egg chamber encapsulation defects. Live imaging analysis suggests that the observed phenotypes are caused by an impaired turnover of E-cadherin at the membrane. Simultaneous knockdown of Cip4 and Nostrin strongly increases the formation of tubular E-cadherin vesicles at adherens junctions. Cip4 and Nostrin localize at distinct membrane subdomains. Both proteins prefer similar membrane curvatures but seem to form distinct membrane coats and do not heterooligomerize. Our data suggest an important synergistic function of both F-BAR proteins in membrane dynamics. We propose a cooperative recruitment model, in which Cip4 initially promotes membrane invagination and early-actin-based endosomal motility, and Nostrin makes contacts with microtubules through the kinesin Khc-73 for trafficking of recycling endosomes.

Molecular Control of Actin Dynamics In Vivo: Insights from Drosophila

The actin cytoskeleton provides mechanical support for cells and generates forces to drive cell shape changes and cell migration in morphogenesis. Molecular understanding of actin dynamics requires a genetically traceable model system that allows interdisciplinary experimental approaches to elucidate the regulatory network of cytoskeletal proteins in vivo. Here, we will discuss some examples of how advances in Drosophila genetics and high-resolution imaging techniques contribute to the discovery of new actin functions, signaling pathways, and mechanisms of actin regulation in vivo.

WHAMY is a novel actin polymerase promoting myoblast fusion, macrophage cell motility and sensory organ development in Drosophila

Wiskott-Aldrich syndrome proteins (WASPs) are nucleation-promoting factors (NPF) that differentially control the Arp2/3 complex. In Drosophila, three different family members, SCAR (also known as WAVE), WASP and WASH (also known as CG13176), have been analyzed so far. Here, we characterized WHAMY, the fourth Drosophila WASP family member. whamy originated from a wasp gene duplication and underwent a sub-neofunctionalization. Unlike WASP, we found that WHAMY specifically interacted with activated Rac1 through its two CRIB domains, which were sufficient for targeting WHAMY to lamellipodial and filopodial tips. Biochemical analyses showed that WHAMY promoted exceptionally fast actin filament elongation, although it did not activate the Arp2/3 complex. Loss- and gain-of-function studies revealed an important function of WHAMY in membrane protrusions and cell migration in macrophages. Genetic data further implied synergistic functions between WHAMY and WASP during morphogenesis. Double mutants were late-embryonic lethal and showed severe defects in myoblast fusion. Trans-heterozygous mutant animals showed strongly increased defects in sensory cell fate specification. Thus, WHAMY is a novel actin polymerase with an initial partitioning of ancestral WASP functions in development and subsequent acquisition of a new function in cell motility during evolution.

FHOD proteins in actin dynamics--a formin' class of its own

Eukaryotic cells have evolved a variety of actin-binding proteins to regulate the architecture and the dynamics of the actin cytoskeleton in time and space. The Diaphanous-related formins (DRF) represent a diverse group of Rho-GTPase-regulated actin regulators that control a range of actin structures composed of tightly-bundled, unbranched actin filaments as found in stress fibers and in filopodia. Under resting conditions, DRFs are auto-inhibited by an intra-molecular interaction between the C-terminal and the N-terminal domains. The auto-inhibition is thought to be released by binding of an activated RhoGTPase to the N-terminal GTPase-binding domain (GBD). However, there is growing evidence for more sophisticated variations from this simplified linear activation model. In this review we focus on the formin homology domain-containing proteins (FHOD), an unconventional group of DRFs. Recent findings on the molecular control and cellular functions of FHOD proteins in vivo are discussed in the light of the phylogeny of FHOD proteins.

Functional and Genetic Analysis of Spectraplakins in Drosophila

The cytoskeleton is a dynamic network of filamentous protein polymers required for virtually all cellular processes. It consists of three major classes, filamentous actin (F-actin), intermediate filaments, and microtubules, all displaying characteristic structural properties, functions, cellular distributions, and sets of interacting regulatory proteins. One unique class of proteins, the spectraplakins, bind, regulate, and integrate the functions of all three classes of cytoskeleton proteins. Spectraplakins are giant, evolutionary conserved multidomain proteins (spanning up to 9000 aa) that are true members of the plakin, spectrin, and Gas2-like protein families. They have OMIM-listed disease links to epidermolysis bullosa and hereditary sensory and autonomic neuropathy. Their role in disease is likely underrepresented since studies in model animal systems have revealed critical roles in polarity, morphogenesis, differentiation and maintenance, migration, signaling, and intracellular trafficking in a variety of tissues. This enormous diversity of spectraplakin function is consistent with the numerous isoforms produced from single genomic loci that combine different sets of functional domains in distinct cellular contexts. To study the broad range of functions and complexity of these proteins, Drosophila is a powerful model. Thus, the fly spectraplakin Short stop (Shot) acts as an actin-microtubule linker and plays important roles in many developmental processes, which provide experimentally amenable and relevant contexts in which to study spectraplakin functions. For these studies, a versatile range of relevant experimental resources that facilitate genetics and transgenic approaches, highly refined genomics tools, and an impressive set of spectraplakin-specific genetic and molecular tools are readily available. Here, we use the example of Shot to illustrate how the various tools and strategies available for Drosophila can be employed to decipher and dissect cellular roles and molecular mechanisms of spectraplakins.

Cell rigidity and shape override CD47's "self"-signaling in phagocytosis by hyperactivating myosin-II

A macrophage engulfs another cell or foreign particle in an adhesive process that often activates myosin-II, unless the macrophage also engages "marker of self" CD47 that inhibits myosin. For many cell types, adhesion-induced activation of myosin-II is maximized by adhesion to a rigid rather than a flexible substrate. Here we demonstrate that rigidity of a phagocytosed cell also hyperactivates myosin-II, which locally overwhelms self-signaling at a phagocytic synapse. Cell stiffness is one among many factors including shape that changes in erythropoiesis, in senescence and in diseases ranging from inherited anemias and malaria to cancer. Controlled stiffening of normal human red blood cells (RBCs) in different shapes does not compromise CD47's interaction with the macrophage self-recognition receptor signal regulatory protein alpha (SIRPA). Uptake of antibody-opsonized RBCs is always fastest with rigid RBC discocytes, which also show that maximal active myosin-II at the synapse can dominate self-signaling by CD47. Rigid but rounded RBC stomatocytes signal self better than rigid RBC discocytes, highlighting the effects of shape on CD47 inhibition. Physical properties of phagocytic targets thus regulate self signaling, as is relevant to erythropoiesis, to clearance of rigid RBCs after blood storage, clearance of rigid pathological cells such as thalassemic or sickle cells, and even to interactions of soft/stiff cancer cells with macrophages.

Drosophila blood cell chemotaxis

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Integrin-mediated adhesion used by Drosophila blood cells to migrate in vivo.

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SCAR/WAVE is required for lamellipodia but also for clearance of apoptotic cells.

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The formins Fhos and Diaphanous regulate Drosophila macrophage migration and morphology.

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Calcium waves drive hydrogen peroxide production to regulate inflammatory migrations.

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The steroid hormone Ecdysone controls the onset of immune competence.

Drosophila melanogaster contains a population of blood cells called hemocytes that represent the functional equivalent of vertebrate macrophages. These cells undergo directed migrations to disperse during development and reach sites of tissue damage or altered self. These chemotactic behaviors are controlled by the expression of PDGF/Vegf-related ligands in developing embryos and local production of hydrogen peroxide at wounds. Recent work reveals that many molecules important in vertebrate cell motility, including integrins, formins, Ena/VASP proteins and the SCAR/WAVE complex, have a conserved function in these innate immune cells. The use of this model organism has elucidated how damage signals are activated by calcium signaling during inflammation and that the steroid hormone ecdysone activates immune competence at key developmental stages.