Actin filament turnover regulated by cross-linking accounts for the size, shape, location, and number of actin bundles in Drosophila bristles

The actin cytoskeleton plays multiple roles in structural colour formation in butterfly wing scales

Vivid structural colours in butterflies are caused by photonic nanostructures scattering light. Structural colours evolved for numerous biological signalling functions and have important technological applications. Optically, such structures are well understood, however insight into their development in vivo remains scarce. We show that actin is intimately involved in structural colour formation in butterfly wing scales. Using comparisons between iridescent (structurally coloured) and non-iridescent scales in adult and developing H. sara, we show that iridescent scales have more densely packed actin bundles leading to an increased density of reflective ridges. Super-resolution microscopy across three distantly related butterfly species reveals that actin is repeatedly re-arranged during scale development and crucially when the optical nanostructures are forming. Furthermore, actin perturbation experiments at these later developmental stages resulted in near total loss of structural colour in H. sara. Overall, this shows that actin plays a vital and direct templating role during structural colour formation in butterfly scales, providing ridge patterning mechanisms that are likely universal across lepidoptera.

Tyrosyl-tRNA synthetase has a noncanonical function in actin bundling

Dominant mutations in tyrosyl-tRNA synthetase (YARS1) and six other tRNA ligases cause Charcot-Marie-Tooth peripheral neuropathy (CMT). Loss of aminoacylation is not required for their pathogenicity, suggesting a gain-of-function disease mechanism. By an unbiased genetic screen in Drosophila, we link YARS1 dysfunction to actin cytoskeleton organization. Biochemical studies uncover yet unknown actin-bundling property of YARS1 to be enhanced by a CMT mutation, leading to actin disorganization in the Drosophila nervous system, human SH-SY5Y neuroblastoma cells, and patient-derived fibroblasts. Genetic modulation of F-actin organization improves hallmark electrophysiological and morphological features in neurons of flies expressing CMT-causing YARS1 mutations. Similar beneficial effects are observed in flies expressing a neuropathy-causing glycyl-tRNA synthetase. Hence, in this work, we show that YARS1 is an evolutionary-conserved F-actin organizer which links the actin cytoskeleton to tRNA-synthetase-induced neurodegeneration.

Plastin and spectrin cooperate to stabilize the actomyosin cortex during cytokinesis

Cytokinesis, the process that partitions the mother cell into two daughter cells, requires the assembly and constriction of an equatorial actomyosin network. Different types of non-motor F-actin crosslinkers localize to the network, but their functional contribution remains poorly understood. Here, we describe a synergy between the small rigid crosslinker plastin and the large flexible crosslinker spectrin in the C. elegans one-cell embryo. In contrast to single inhibitions, co-inhibition of plastin and the βH-spectrin (SMA-1) results in cytokinesis failure due to progressive disorganization and eventual collapse of the equatorial actomyosin network. Cortical localization dynamics of non-muscle myosin II in co-inhibited embryos mimic those observed after drug-induced F-actin depolymerization, suggesting that the combined action of plastin and spectrin stabilizes F-actin in the contractile ring. An in silico model predicts that spectrin is more efficient than plastin at stabilizing the ring and that ring formation is relatively insensitive to βH-spectrin length, which is confirmed in vivo with a sma-1 mutant that lacks 11 of its 29 spectrin repeats. Our findings provide the first evidence that spectrin contributes to cytokinesis and highlight the importance of crosslinker interplay for actomyosin network integrity.

IKKε inhibits PKC to promote Fascin-dependent actin bundling

Signaling molecules have pleiotropic functions and are activated by various extracellular stimuli. Protein kinase C (PKC) is activated by diverse receptors, and its dysregulation is associated with diseases including cancer. However, how the undesired activation of PKC is prevented during development remains poorly understood. We have previously shown that a protein kinase, IKKε, is active at the growing bristle tip and regulates actin bundle organization during Drosophila bristle morphogenesis. Here, we demonstrate that IKKε regulates the actin bundle localization of a dynamic actin cross-linker, Fascin. IKKε inhibits PKC, thereby protecting Fascin from inhibitory phosphorylation. Excess PKC activation is responsible for the actin bundle defects in IKKε-deficient bristles, whereas PKC is dispensable for bristle morphogenesis in wild-type bristles, indicating that PKC is repressed by IKKε in wild-type bristle cells. These results suggest that IKKε prevents excess activation of PKC during bristle morphogenesis.

Summary: The protein kinase IKKϵ is active at the growing tip of Drosophila bristles and prevents excess PKC activation during bristle actin bundle organization and morphogenesis.

Cofilin-mediated actin dynamics promotes actin bundle formation during Drosophila bristle development

The actin bundle is an array of linear actin filaments cross-linked by actin-bundling proteins, but its assembly and dynamics are not as well understood as those of the branched actin network. Here we used the Drosophila bristle as a model system to study actin bundle formation. We found that cofilin, a major actin disassembly factor of the branched actin network, promotes the formation and positioning of actin bundles in the developing bristles. Loss of function of cofilin or AIP1, a cofactor of cofilin, each resulted in increased F-actin levels and severe defects in actin bundle organization, with the defects from cofilin deficiency being more severe. Further analyses revealed that cofilin likely regulates actin bundle formation and positioning by the following means. First, cofilin promotes a large G-actin pool both locally and globally, likely ensuring rapid actin polymerization for bundle initiation and growth. Second, cofilin limits the size of a nonbundled actin-myosin network to regulate the positioning of actin bundles. Third, cofilin prevents incorrect assembly of branched and myosin-associated actin filament into bundles. Together these results demonstrate that the interaction between the dynamic dendritic actin network and the assembling actin bundles is critical for actin bundle formation and needs to be closely regulated.

Drosophila melanogaster clip-domain serine proteases: Structure, function and regulation

Mammalian chymotrypsin-like serine proteases (SPs) are one of the best-studied family of enzymes with roles in a wide range of physiological processes, including digestion, blood coagulation, fibrinolysis and humoral immunity. Extracellular SPs can form cascades, in which one protease activates the zymogen of the next protease in the chain, to amplify physiological or pathological signals. These extracellular SPs are generally multi-domain proteins, with pro-domains that are involved in protein-protein interactions critical for the sequential organization of the cascades, the control of their intensity and their proper localization. Far less is known about invertebrate SPs than their mammalian counterparts. In insect genomes, SPs and their proteolytically inactive homologs (SPHs) constitute large protein families. In addition to the chymotrypsin fold, many of these proteins contain additional structural domains, often with conserved mammalian orthologues. However, the largest group of arthropod SP regulatory modules is the clip domains family, which has only been identified in arthropods. The clip-domain SPs are extracellular and have roles in the immune response and embryonic development. The powerful reverse-genetics tools in Drosophila melanogaster have been essential to identify the functions of clip-SPs and their organization in sequential cascades. This review focuses on the current knowledge of Drosophila clip-SPs and presents, when necessary, data obtained in other insect models. We will first cover the biochemical and structural features of clip domain SPs and SPHs. Clip-SPs are implicated in three main biological processes: the control of the dorso-ventral patterning during embryonic development; the activation of the Toll-mediated response to microbial infections and the prophenoloxydase cascade, which triggers melanization. Finally, we review the regulation of SPs and SPHs, from specificity of activation to inhibition by endogenous or pathogen-encoded inhibitors.

Septin 9 Exhibits Polymorphic Binding to F-Actin and Inhibits Myosin and Cofilin Activity

Septins are a highly conserved family of proteins in eukaryotes that is recognized as a novel component of the cytoskeleton. Septin 9 (SEPT9) interacts directly with actin filaments and functions as an actin stress fiber cross-linking protein that promotes the maturation of nascent focal adhesions and cell migration. However, the molecular details of how SEPT9 interacts with F-actin remain unknown. Here, we use electron microscopy and image analysis to show that SEPT9 binds to F-actin in a highly polymorphic fashion. We demonstrate that the basic domain (B-domain) of the N-terminal tail of SEPT9 is responsible for actin cross-linking, while the GTP-binding domain (G-domain) does not bundle F-actin. We show that the B-domain of SEPT9 binds to three sites on F-actin, and the two of these sites overlap with the binding regions of myosin and cofilin. SEPT9 inhibits actin-dependent ATPase activity of myosin and competes with the weakly bound state of myosin for binding to F-actin. At the same time, SEPT9 significantly reduces the extent of F-actin depolymerization by cofilin. Taken together, these data suggest that SEPT9 protects actin filaments from depolymerization by cofilin and myosin and indicate a mechanism by which SEPT9 could maintain the integrity of growing and contracting actin filaments.

Fascin, may the Forked be with you

The FGFR pathway triggers a wide range of key biological responses. Among others, the Breathless (Btl, Drosophila FGFR1) receptor cascade promotes cell migration during embryonic tracheal system development. However, how the actin cytoskeleton responds to Btl pathway activation to induce cell migration has remained largely unclear. Our recent results shed light into this issue by unveiling a link between the actin-bundling protein Singed (Sn) and the Btl pathway. We showed that the Btl pathway regulates sn, which leads to the stabilization of the actin bundles required for filopodia formation and actin cytoskeleton rearrangement. This regulation contributes to tracheal migration, tracheal branch fusion and tracheal cell elongation. Parallel actin bundles (PABs) are usually cross-linked by more than one actin-bundling protein. Accordingly, we have also shown that sn synergistically interacts with forked (f), another actin crosslinker. In this Extra View we extend f analysis and hypothesize how both actin-bundling proteins may act together to regulate the PABs during tracheal embryonic development. Although both proteins are required for similar tracheal events, we suggest that Sn is essential for actin bundle initiation and stiffening, while F is required for the lengthening and further stabilization of the PABs.

Fascin links Btl/FGFR signalling to the actin cytoskeleton during Drosophila tracheal morphogenesis

A key challenge in normal development and in disease is to elucidate the mechanisms of cell migration. Here we approach this question using the tracheal system of Drosophila as a model. Tracheal cell migration requires the Breathless/FGFR pathway; however, how the pathway induces migration remains poorly understood. We find that the Breathless pathway upregulates singed at the tip of tracheal branches, and that this regulation is functionally relevant. singed encodes Drosophila Fascin, which belongs to a conserved family of actin-bundling proteins involved in cancer progression and metastasis upon misregulation. We show that singed is required for filopodia stiffness and proper morphology of tracheal tip cells, defects that correlate with an abnormal actin organisation. We propose that singed-regulated filopodia and cell fronts are required for timely and guided branch migration and for terminal branching and branch fusion. We find that singed requirements rely on its actin-bundling activity controlled by phosphorylation, and that active Singed can promote tip cell features. Furthermore, we find that singed acts in concert with forked, another actin cross-linker. The absence of both cross-linkers further stresses the relevance of tip cell morphology and filopodia for tracheal development. In summary, our results on the one hand reveal a previously undescribed role for forked in the organisation of transient actin structures such as filopodia, and on the other hand identify singed as a new target of Breathless signal, establishing a link between guidance cues, the actin cytoskeleton and tracheal morphogenesis.

SelR reverses Mical-mediated oxidation of actin to regulate F-actin dynamics

Actin's polymerization properties are markedly altered by oxidation of its conserved Met 44 residue. Mediating this effect is a specific oxidation-reduction (redox) enzyme, Mical, that works with Semaphorin repulsive guidance cues and selectively oxidizes Met 44. We now find that this actin-regulatory process is reversible. Employing a genetic approach, we identified a specific methionine sulfoxide reductase (MsrB) enzyme SelR that opposes Mical redox activity and Semaphorin-Plexin repulsion to direct multiple actin-dependent cellular behaviours in vivo. SelR specifically catalyses the reduction of the R isomer of methionine sulfoxide (methionine-R-sulfoxide) to methionine, and we found that SelR directly reduced Mical-oxidized actin, restoring its normal polymerization properties. These results indicate that Mical oxidizes actin stereospecifically to generate actin Met-44-R-sulfoxide (actin(Met(R)O-44)), and also implicate the interconversion of specific Met/Met(R)O residues as a precise means to modulate protein function. Our results therefore uncover a specific reversible redox actin regulatory system that controls cell and developmental biology.

β-Actin and fascin-2 cooperate to maintain stereocilia length

Stereocilia are actin-based protrusions on auditory sensory hair cells that are deflected by sound waves to initiate the conversion of mechanical energy to neuronal signals. Stereocilia maintenance is essential because auditory hair cells are not renewed in mammals. This process requires both β-actin and γ-actin as knock-out mice lacking either isoform develop distinct stereocilia pathology during aging. In addition, stereocilia integrity may hinge on immobilizing actin, which outside of a small region at stereocilia tips turns over with a very slow, months-long half-life. Here, we establish that β-actin and the actin crosslinking protein fascin-2 cooperate to maintain stereocilia length and auditory function. We observed that mice expressing mutant fascin-2 (p.R109H) or mice lacking β-actin share a common phenotype including progressive, high-frequency hearing loss together with shortening of a defined subset of stereocilia in the hair cell bundle. Fascin-2 binds β-actin and γ-actin filaments with similar affinity in vitro and fascin-2 does not depend on β-actin for localization in vivo. Nevertheless, double-mutant mice lacking β-actin and expressing fascin-2 p.R109H have a more severe phenotype suggesting that each protein has a different function in a common stereocilia maintenance pathway. Because the fascin-2 p.R109H mutant binds but fails to efficiently crosslink actin filaments, we propose that fascin-2 crosslinks function to slow actin depolymerization at stereocilia tips to maintain stereocilia length.

A new role for the architecture of microvillar actin bundles in apical retention of membrane proteins

The bundled architecture of actin filaments is not needed for intestinal microvillar morphogenesis, as shown in knockout mice devoid of microvillar actin-bundling proteins. This architecture is essential for the apical anchorage of digestive proteins, probably via the recruitment of key players in apical retention, such as myosin-1a, and, as a result, for intestinal physiology.

Actin-bundling proteins are identified as key players in the morphogenesis of thin membrane protrusions. Until now, functional redundancy among the actin-bundling proteins villin, espin, and plastin-1 has prevented definitive conclusions regarding their role in intestinal microvilli. We report that triple knockout mice lacking these microvillar actin-bundling proteins suffer from growth delay but surprisingly still develop microvilli. However, the microvillar actin filaments are sparse and lack the characteristic organization of bundles. This correlates with a highly inefficient apical retention of enzymes and transporters that accumulate in subapical endocytic compartments. Myosin-1a, a motor involved in the anchorage of membrane proteins in microvilli, is also mislocalized. These findings illustrate, in vivo, a precise role for local actin filament architecture in the stabilization of apical cargoes into microvilli. Hence, the function of actin-bundling proteins is not to enable microvillar protrusion, as has been assumed, but to confer the appropriate actin organization for the apical retention of proteins essential for normal intestinal physiology.

Actin cross-linkers and the shape of stereocilia

Stereocilia are actin-based cellular protrusions essential for hearing. We propose that they are shaped by the detachment dynamics of actin cross-linkers, in particular espin. We account for experimentally observed stereocilium shapes, treadmilling velocity to length relationship, espin 1 localization profile, and microvillus length to espin level relationship. If the cross-linkers are allowed to reattach, our model yields a dynamical phase transition toward unbounded growth. Considering the simplified case of a noninteracting, one-filament system, we calculate the length probability distribution in the growing phase and its stationary form in a continuum approximation of the finite-length phase. Numerical simulations of interacting filaments suggest an anomalous power-law divergence of the protrusion length at the growth transition, which could be a universal feature of cross-linked depolymerizing systems.

Cytoskeletal regulation of epithelial barrier function during inflammation

Increased epithelial permeability is a common and important consequence of mucosal inflammation that results in perturbed body homeostasis and enhanced exposure to external pathogens. The integrity and barrier properties of epithelial layers are regulated by specialized adhesive plasma membrane structures known as intercellular junctions. It is generally believed that inflammatory stimuli increase transepithelial permeability by inducing junctional disassembly. This review highlights molecular events that lead to disruption of epithelial junctions during inflammation. We specifically focus on key mechanisms of junctional regulation that are dependent on reorganization of the perijunctional F-actin cytoskeleton. We discuss critical roles of myosin-II-dependent contractility and actin filament turnover in remodeling of the F-actin cytoskeleton that drive disruption of epithelial barriers under different inflammatory conditions. Finally, we highlight signaling pathways induced by inflammatory mediators that regulate reorganization of actin filaments and junctional disassembly in mucosal epithelia.

The Drosophila Fry protein interacts with Trc and is highly mobile in vivo

Background

Cell polarity is a common feature of eukaryotic cells. The NDR kinases have been found to regulate polarized growth in both animal cells and fungi. Drosophila Tricornered is an NDR kinase that is essential for the normal polarized growth of extensions of epidermal cells and for the tiling and branching of dendrites of da sensory neurons. Tricornered function requires interacting with the large Furry protein (3479 amino acid).

Results

We constructed a furry (fry) transgene and established that it rescued the lethality of fry null mutations. The encoded protein was tagged at both its amino and carboxy termini and this allowed us to demonstrate that the protein existed as an uncleaved protein in vivo. We used the C terminal GFP tag to follow the protein in vivo and found it to be highly mobile. Interestingly Fry accumulated at the distal tip of growing bristles. We established that Fry and Trc could be co-immunoprecipitated from wing discs.

Conclusions

The mobility of Fry in both bristles and dendrites suggests that it could function in directing/mediating the intracellular transport needed for polarized growth. Our observations that full length Fry and Trc show only partial co-localization in growing bristles while an amino terminal fragment of Fry shows close to complete co-localization with Trc suggests that the interaction between these proteins is transient and regulated.

Asymmetric microtubule function is an essential requirement for polarized organization of the Drosophila bristle

While previous studies have shown that microtubules (MTs) are essential for maintaining the highly biased axial growth of the Drosophila bristle, the mechanism for this process has remained vague. We report that the MT minus-end marker, Nod-KHC, accumulates at the bristle tip, suggesting that the MT network in the bristle is organized minus end out. Potential markers for studying the importance of properly polarized MTs to bristle axial growth are Ik2 and Spindle-F (Spn-F), since mutations in spn-F and ik2 affect bristle development. We demonstrate that Spn-F and Ik2 are localized to the bristle tip and that mutations in ik2 and spn-F affect bristle MT and actin organization. Specifically, mutation in ik2 affects polarized bristle MT function. It was previously found that the hook mutant exhibited defects in bristle polarity and that hook is involved in endocytic trafficking. We found that Hook is localized at the bristle tip and that this localization is affected in ik2 mutants, suggesting that the contribution of MTs within the bristle shaft is important for correct endocytic trafficking. Thus, our results show that MTs are organized in a polarized manner within the highly elongated bristle and that this organization is essential for biased bristle axial growth.

Rab35 controls actin bundling by recruiting fascin as an effector protein

Actin filaments are key components of the eukaryotic cytoskeleton that provide mechanical structure and generate forces during cell shape changes, growth, and migration. Actin filaments are dynamically assembled into higher-order structures at specified locations to regulate diverse functions. The Rab family of small guanosine triphosphatases is evolutionarily conserved and mediates intracellular vesicle trafficking. We found that Rab35 regulates the assembly of actin filaments during bristle development in Drosophila and filopodia formation in cultured cells. These effects were mediated by the actin-bundling protein fascin, which directly associated with active Rab35. Targeting Rab35 to the outer mitochondrial membrane triggered actin recruitment, demonstrating a role for an intracellular trafficking protein in localized actin assembly.

Gamma-actin is required for cytoskeletal maintenance but not development

Beta(cyto)-actin and gamma(cyto)-actin are ubiquitous proteins thought to be essential building blocks of the cytoskeleton in all non-muscle cells. Despite this widely held supposition, we show that gamma(cyto)-actin null mice (Actg1(-/-)) are viable. However, they suffer increased mortality and show progressive hearing loss during adulthood despite compensatory up-regulation of beta(cyto)-actin. The surprising viability and normal hearing of young Actg1(-/-) mice means that beta(cyto)-actin can likely build all essential non-muscle actin-based cytoskeletal structures including mechanosensory stereocilia of hair cells that are necessary for hearing. Although gamma(cyto)-actin-deficient stereocilia form normally, we found that they cannot maintain the integrity of the stereocilia actin core. In the wild-type, gamma(cyto)-actin localizes along the length of stereocilia but re-distributes to sites of F-actin core disruptions resulting from animal exposure to damaging noise. In Actg1(-/-) stereocilia similar disruptions are observed even without noise exposure. We conclude that gamma(cyto)-actin is required for reinforcement and long-term stability of F-actin-based structures but is not an essential building block of the developing cytoskeleton.

Plastin 1 binds to keratin and is required for terminal web assembly in the intestinal epithelium

Plastin 1 (I-plastin, fimbrin) along with villin and espin is a prominent actin-bundling protein of the intestinal brush border microvilli. We demonstrate here that plastin 1 accumulates in the terminal web and interacts with keratin 19, possibly contributing to anchoring the rootlets to the keratin network. This prompted us to investigate the importance of plastin 1 in brush border assembly. Although in vivo neither villin nor espin is required for brush border structure, plastin 1-deficient mice have conspicuous ultrastructural alterations: microvilli are shorter and constricted at their base, and, strikingly, their core actin bundles lack true rootlets. The composition of the microvilli themselves is apparently normal, whereas that of the terminal web is profoundly altered. Although the plastin 1 knockout mice do not show any overt gross phenotype and present a normal intestinal microanatomy, the alterations result in increased fragility of the epithelium. This is seen as an increased sensitivity of the brush border to biochemical manipulations, decreased transepithelial resistance, and increased sensitivity to dextran sodium sulfate-induced colitis. Plastin 1 thus emerges as an important regulator of brush border morphology and stability through a novel role in the organization of the terminal web, possibly by connecting actin filaments to the underlying intermediate filament network.

Dynamic length regulation of sensory stereocilia

Stereocilia, the mechanosensory organelles of hair cells, are a distinctive class of actin-based cellular protrusions with an unparalleled ability to regulate their lengths over time. Studies on actin turnover in stereocilia, as well as the identification of several deafness-related proteins essential for proper stereocilia structure and function, provide new insights into the mechanisms and molecules involved in stereocilia length regulation and long-term maintenance. Comparisons of ongoing investigations on stereocilia with studies on other actin protrusions offer new opportunities to further understand common principles for length regulation, the diversity of its mechanisms, and how the specific needs of each cell are met.

Asymmetric elastic properties of Dictyostelium discoideum in relation to chemotaxis

In this study we used an AFM to investigate the cytoskeletal properties of live Dictyostelium discoideum cells by measuring the local stiffness across individual living cells. We have examined differences in elastic properties of polarized and unpolarized AX3 wild type and the mutant DAip1- cells, as well as the differences in the front and rear of the cells in relation to organization of the actin cytoskeleton. We found that the average Young's modulus increases upon polarization for the thin regions of the cell and that in polarized cells, the cell front was stiffer than the cell back. We also found that AX3 cells were stiffer than DAip1- cells. This finding suggests that actin polymerization is one of the major determinants of cell motility in Dictyostelium. In addition, a thin agarose film was studied as a model system to examine the influence of the substrate of thin materials probed with the AFM.

LTP consolidation: Substrates, explanatory power, and functional significance

Long-term potentiation (LTP) resembles memory in that it is initially unstable and then, over about 30 min, becomes increasingly resistant to disruption. Here we present an hypothesis to account for this initial consolidation effect and consider implications that follow from it. Anatomical studies indicate that LTP is accompanied by changes in spine morphology and therefore likely involves cytoskeletal changes. Accordingly, theta bursts initiate calpain-mediated proteolysis of the actin cross-linking protein spectrin and trigger actin polymerization in spine heads, two effects indicative of cytoskeletal reorganization. Polymerization occurs within 2 min, has the same threshold as LTP, is dependent on integrins, and becomes resistant to disruption over 30 min. We propose that the stabilization of the new cytoskeletal organization, and thus of a new spine morphology, underlies the initial phase of LTP consolidation. This hypothesis helps explain the diverse array of proteins and signaling cascades implicated in LTP, as well as the often-contradictory results about contributions of particular molecules. It also provides a novel explanation for why LTP is potently modulated by factors likely to be released during theta trains (e.g., BDNF). Finally, building on evidence that normal patterns of activity reverse LTP, we suggest that consolidation provides a delay that allows brain networks to sculpt newly formed memories.

Deformations in actin comets from rocketing beads

The mechanical and dynamical properties of the actin network are essential for many cellular processes like motility or division, and there is a growing body of evidence that they are also important for adhesion and trafficking. The leading edge of migrating cells is pushed out by the polymerization of actin networks, a process orchestrated by cross-linkers and other actin-binding proteins. In vitro physical characterizations show that these same proteins control the elastic properties of actin gels. Here we use a biomimetic system of Listeria monocytogenes, beads coated with an activator of actin polymerization, to assess the role of various actin-binding proteins in propulsion. We find that the properties of actin-based movement are clearly affected by the presence of cross-linkers. By monitoring the evolution of marked parts of the comet, we provide direct experimental evidence that the actin gel continuously undergoes deformations during the growth of the comet. Depending on the protein composition in the motility medium, deformations arise from either gel elasticity or monomer diffusion through the actin comet. Our findings demonstrate that actin-based movement is governed by the mechanical properties of the actin network, which are fine-tuned by proteins involved in actin dynamics and assembly.

Capping protein and the Arp2/3 complex regulate nonbundle actin filament assembly to indirectly control actin bundle positioning during Drosophila melanogaster bristle development

Drosophila melanogaster bristle development is dependent on actin assembly, and prominent actin bundles form against the elongating cell membrane, giving the adult bristle its characteristic grooved pattern. Previous work has demonstrated that several actin-regulating proteins are required to generate normal actin bundles. Here we have addressed how two actin regulators, capping protein, a barbed end binding protein, and the Arp2/3 complex, a potent actin assembly nucleator, function to generate properly organized bundles. As predicted from studies in motile cells, we find that capping protein and the Arp2/3 complex act antagonistically to one another during bristle development. However, these proteins do not primarily act directly on bundles, but rather on a dynamic population of actin filaments that are not part of the bundles. These nonbundle filaments, termed snarls, play an important role in determining the number and spacing of the actin bundles. Reduction of capping protein leads to an increase in snarls, which prevents actin bundles from properly attaching to the membrane. Conversely, loss of an Arp2/3 complex component leads to a loss of snarls and accumulation of excess membrane-attached bundles. These results indicate that in nonmotile cells dynamic actin filaments can function to regulate the positioning of stable actin structures. In addition, our results suggest that the Arpc1 subunit may have an additional function, independent of the rest of the Arp2/3 complex.

Balanced levels of Espin are critical for stereociliary growth and length maintenance

Hearing and balance depend on microvilli-like actin-based projections of sensory hair cells called stereocilia. Their sensitivity to mechanical displacements on the nanometer scale requires a highly organized hair bundle in which the physical dimension of each stereocilium is tightly controlled. The length and diameter of each stereocilium are established during hair bundle maturation and maintained by life-long continuing dynamic regulation. Here, we studied the role of the actin-bundling protein Espin in stereociliary growth by examining the hair cell stereocilia of Espin-deficient jerker mice (Espn(je)), and the effects of transiently overexpressing Espin in the neuroepithelial cells of the organ of Corti cultures. Using fluorescence scanning confocal and electron microscopy, we found that a lack of Espin results in inhibition of stereociliary growth followed by progressive degeneration of the hair bundle. In contrast, overexpression of Espin induced lengthening of stereocilia and microvilli that mirrored the elongation of the actin filament bundle at their core. Interestingly, Espin deficiency also appeared to influence the localization of Myosin XVa, an unconventional myosin that is normally present at the stereocilia tip at levels proportional to stereocilia length. These results indicate that Espin is important for the growth and maintenance of the actin-based protrusions of inner ear neuroepithelial cells.

How to make a curved Drosophila bristle using straight actin bundles

This, our Inaugural Article as Academy Members, is ironically our swan song from the field of the actin cytoskeleton. By reviewing what we have learned and what we think is going on during development, we hope to lure you, the reader, into applying your skills to the bristle cell. The processes of the assembly and disassembly of actin bundles is laid out in time and space in an organism that lends itself to genetic manipulation. The cell provides every process you could want: filament nucleation, growth of microvilli, joining of microvillar bundles into modules, assembly of modules into bundles, time-dependent use of at least two crossbridging proteins, filament turnover, treadmilling, disassembly, and filament translocation.

Mutational analysis of Stubble-stubbloid gene structure and function in Drosophila leg and bristle morphogenesis

The Stubble-stubbloid (Sb-sbd) gene is required for ecdysone-regulated epithelial morphogenesis of imaginal tissues during Drosophila metamorphosis. Mutations in Sb-sbd are associated with defects in apical cell shape changes critical for the evagination of the leg imaginal disc and with defects in assembly and extension of parallel actin bundles in growing mechanosensory bristles. The Sb-sbd gene encodes a type II transmembrane serine protease (TTSP). Here we use a Sb-sbd transgenic construct to rescue both bristle and leg morphogenesis defects in Sb-sbd mutations. Molecular characterization of Sb-sbd mutations and rescue experiments with wild-type and modified Sb-sbd transgenic constructs show that the protease domain is required for both leg and bristle functions. Truncated proteins that express the noncatalytic domains without the protease have dominant effects in bristles but not in legs. Leg morphogenesis, but not bristle growth, is sensitive to Sb-sbd overexpression. Antibody localization of the Sb-sbd protein shows apical expression in elongating legs. Sb-sbd protein is found in the base and shaft in budding bristles and then concentrates at the growing tip when bristles are elongating rapidly. We propose a model whereby Sb-sbd helps coordinate proteolytic modification of extracellular matrix attachments with cytoskeletal changes in both legs and bristles.

Actin filament bundles in Drosophila wing hairs: hairs and bristles use different strategies for assembly

Actin filament bundles can shape cellular extensions into dramatically different forms. We examined cytoskeleton formation during wing hair morphogenesis using both confocal and electron microscopy. Hairs elongate with linear kinetics (approximately 1 microm/h) over the course of approximately 18 h. The resulting structure is vividly asymmetric and shaped like a rose thorn--elongated in the distal direction, curved in two dimensions with an oval base and a round tip. High-resolution analysis shows that the cytoskeleton forms from microvilli-like pimples that project actin filaments into the cytoplasm. These filaments become cross-linked into bundles by the sequential use of three cross-bridges: villin, forked and fascin. Genetic loss of each cross-bridge affects cell shape. Filament bundles associate together, with no lateral membrane attachments, into a cone of overlapping bundles that matures into an oval base by the asymmetric addition of bundles on the distal side. In contrast, the long bristle cell extension is supported by equally long (up to 400 microm) filament bundles assembled together by end-to-end grafting of shorter modules. Thus, bristle and hair cells use microvilli and cross-bridges to generate the common raw material of actin filament bundles but employ different strategies to assemble these into vastly different shapes.

Stress release drives symmetry breaking for actin-based movement

By using a simple assay composed of purified proteins, we studied the spontaneous polarization of actin networks polymerizing on spherical beads, which subsequently undergo movement. We show evidence that this symmetry breaking is based on the release of elastic energy, analogous to the fracture of polymer gels. The dynamics of this process and the thickness at which it occurs depend on the growth rate and mechanical properties of the actin gel. We explain our experimental results with a model based on elasticity theory and fracture mechanics.

Drosophila crinkled, mutations of which disrupt morphogenesis and cause lethality, encodes fly myosin VIIA

Myosin VIIs provide motor function for a wide range of eukaryotic processes. We demonstrate that mutations in crinkled (ck) disrupt the Drosophila myosin VIIA heavy chain. The ck/myoVIIA protein is present at a low level throughout fly development and at the same level in heads, thoraxes, and abdomens. Severe ck alleles, likely to be molecular nulls, die as embryos or larvae, but all allelic combinations tested thus far yield a small fraction of adult "escapers" that are weak and infertile. Scanning electron microscopy shows that escapers have defects in bristles and hairs, indicating that this motor protein plays a role in the structure of the actin cytoskeleton. We generate a homology model for the structure of the ck/myosin VIIA head that indicates myosin VIIAs, like myosin IIs, have a spectrin-like, SH3 subdomain fronting their N terminus. In addition, we establish that the two myosin VIIA FERM repeats share high sequence similarity with only the first two subdomains of the three-lobed structure that is typical of canonical FERM domains. Nevertheless, the approximately 100 and approximately 75 amino acids that follow the first two lobes of the first and second FERM domains are highly conserved among myosin VIIs, suggesting that they compose a conserved myosin tail homology 7 (MyTH7) domain that may be an integral part of the FERM domain or may function independently of it. Together, our data suggest a key role for ck/myoVIIA in the formation of cellular projections and other actin-based functions required for viability.

Arabidopsis VILLIN1 generates actin filament cables that are resistant to depolymerization

Dynamic cytoplasmic streaming, organelle positioning, and nuclear migration use molecular tracks generated from actin filaments arrayed into higher-order structures like actin cables and bundles. How these arrays are formed and stabilized against cellular depolymerizing forces remains an open question. Villin and fimbrin are the best characterized actin-filament bundling or cross-linking proteins in plants and each is encoded by a multigene family of five members in Arabidopsis thaliana. The related villins and gelsolins are conserved proteins that are constructed from a core of six homologous gelsolin domains. Gelsolin is a calcium-regulated actin filament severing, nucleating and barbed end capping factor. Villin has a seventh domain at its C terminus, the villin headpiece, which can bind to an actin filament, conferring the ability to crosslink or bundle actin filaments. Many, but not all, villins retain the ability to sever, nucleate, and cap filaments. Here we have identified a putative calcium-insensitive villin isoform through comparison of sequence alignments between human gelsolin and plant villins with x-ray crystallography data for vertebrate gelsolin. VILLIN1 (VLN1) has the least well-conserved type 1 and type 2 calcium binding sites among the Arabidopsis VILLIN isoforms. Recombinant VLN1 binds to actin filaments with high affinity (K(d) approximately 1 microM) and generates bundled filament networks; both properties are independent of the free Ca(2+) concentration. Unlike human plasma gelsolin, VLN1 does not nucleate the assembly of filaments from monomer, does not block the polymerization of profilin-actin onto barbed ends, and does not stimulate depolymerization or sever preexisting filaments. In kinetic assays with ADF/cofilin, villin appears to bind first to growing filaments and protects filaments against ADF-mediated depolymerization. We propose that VLN1 is a major regulator of the formation and stability of actin filament bundles in plant cells and that it functions to maintain the cable network even in the presence of stimuli that result in depolymerization of other actin arrays.

The co-workers of actin filaments: from cell structures to signals

Cells have various surface architectures, which allow them to carry out different specialized functions. Actin microfilaments that are associated with the plasma membrane are important for generating these cell-surface specializations, and also provide the driving force for remodelling cell morphology and triggering new cell behaviour when the environment is modified. This phenomenon is achieved through a tight coupling between cell structure and signal transduction, a process that is modulated by the regulation of actin-binding proteins.

An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal

We have previously shown that the seemingly static paracrystalline actin core of hair cell stereocilia undergoes continuous turnover. Here, we used the same approach of transfecting hair cells with actin–green fluorescent protein (GFP) and espin-GFP to characterize the turnover process. Actin and espin are incorporated at the paracrystal tip and flow rearwards at the same rate. The flux rates (∼0.002–0.04 actin subunits s⁻¹) were proportional to the stereocilia length so that the entire staircase stereocilia bundle was turned over synchronously. Cytochalasin D caused stereocilia to shorten at rates matching paracrystal turnover. Myosins VI and VIIa were localized alongside the actin paracrystal, whereas myosin XVa was observed at the tips at levels proportional to stereocilia lengths. Electron microscopy analysis of the abnormally short stereocilia in the shaker 2 mice did not show the characteristic tip density. We argue that actin renewal in the paracrystal follows a treadmill mechanism, which, together with the myosins, dynamically shapes the functional architecture of the stereocilia bundle.

The role actin filaments play in providing the characteristic curved form of Drosophila bristles

Drosophila bristles display a precise orientation and curvature. An asymmetric extension of the socket cell overlies the newly emerging bristle rudiment to provide direction for bristle elongation, a process thought to be orchestrated by the nerve dendrite lying between these cells. Scanning electron microscopic analysis of individual bristles showed that curvature is planar and far greater near the bristle base. Correlated with this, as development proceeds the pupa gradually recedes from the inner pupal case (an extracellular layer that encloses the pupa) leading to less bristle curvature along the shaft. We propose that the inner pupal case induces elongating bristles to bend when they contact this barrier. During elongation the actin cytoskeleton locks in this curvature by grafting together the overlapping modules that comprise the long filament bundles. Because the bristle is curved, the actin bundles on the superior side must be longer than those on the inferior side. This is accomplished during grafting by greater elongation of superior side modules. Poor actin cross-bridging in mutant bristles results in altered curvature. Thus, the pattern of bristle curvature is a product of both extrinsic factors-the socket cell and the inner pupal case--and intrinsic factors--actin cytoskeleton assembly.

Microvilli appear to represent the first step in actin bundle formation in Drosophila bristles

During bristle development the emerging bristle shaft, socket cell, and the apical surface of thoracic epithelial cells form tiny protuberances or pimples that contain electron-dense material located on the cytoplasmic surface of the pimple tip. In a few cases short actin filaments extend from this material into the cortical cytoplasm. When cultured in the presence of jasplakinolide, an agent that prevents filament disassembly, pimples elongate to form microvilli containing a core of crosslinked filaments. Emerging-bristle mutants delay cortical bundle formation and are aggregated by forked protein crossbridges. Using these mutants and enhancing core bundle formation with jasplakinolide we found that microvillar formation represents the first stage in the morphogenesis of much larger actin bundles in Drosophila bristle shaft cells. Evidence is presented showing that socket cells do not contain forked protein crossbridges, a fact that may explain why cortical bundles only appear in bristle shaft cells. Furthermore, as pimples and microvilli form in the absence of both forked and fascin crossbridges, we also conclude that neither of these crossbridges account for core bundle formation in microvilli, but there must exist a third, as yet unidentified crossbridge in this system. Immunocytochemisty suggested that this new crossbridge is not Drosophila villin. Finally, ultrastructural comparisons suggest that microspikes and microvilli form very differently.

Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments

The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation. Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing. These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.