The filamentous actin cross-linking/bundling activity of mammalian formins

Identification of an FMNL2 Interactome by Quantitative Mass Spectrometry

Formin Homology Proteins (Formins) are a highly conserved family of cytoskeletal regulatory proteins that participate in a diverse range of cellular processes. FMNL2 is a member of the Diaphanous-Related Formin sub-group, and previous reports suggest FMNL2's role in filopodia assembly, force generation at lamellipodia, subcellular trafficking, cell-cell junction assembly, and focal adhesion formation. How FMNL2 is recruited to these sites of action is not well understood. To shed light on how FMNL2 activity is partitioned between subcellular locations, we used biotin proximity labeling and proteomic analysis to identify an FMNL2 interactome. The interactome identified known and new FMNL2 interacting proteins with functions related to previously described FMNL2 activities. In addition, our interactome predicts a novel connection between FMNL2 and extracellular vesicle assembly. We show directly that FMNL2 protein is present in exosomes.

The formin DAAM1 regulates the deubiquitinase activity of USP10 and integrin homeostasis

The differentiation of fibroblasts into pathological myofibroblasts during wound healing is characterized by increased cell surface expression of αv-integrins. Our previous studies found that the deubiquitinase (DUB) USP10 removes ubiquitin from αv-integrins, leading to cell surface integrin accumulation, subsequent TGFβ1 activation, and pathological myofibroblast differentiation. In this study, a yeast two-hybrid screen revealed a novel binding partner for USP10, the formin, DAAM1. We found that DAAM1 binds to and inhibits USP10's DUB activity through the FH2 domain of DAAM1 independent of its actin functions. The USP10/DAAM1 interaction was also supported by proximity ligation assay (PLA) in primary human corneal fibroblasts. Treatment with TGFβ1 significantly increased USP10 and DAAM1 protein expression, PLA signal, and co-localization to actin stress fibers. DAAM1 siRNA knockdown significantly reduced co-precipitation of USP10 and DAAM1 on purified actin stress fibers, and β1- and β5-integrin ubiquitination. This resulted in increased αv-, β1-, and β5-integrin total protein levels, αv-integrin recycling, and extracellular fibronectin (FN) deposition. Together, our data demonstrate that DAAM1 inhibits USP10's DUB activity on integrins subsequently regulating cell surface αv-integrin localization and FN accumulation.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

F-actin architecture determines constraints on myosin thick filament motion

Active stresses are generated and transmitted throughout diverse F-actin architectures within the cell cytoskeleton, and drive essential behaviors of the cell, from cell division to migration. However, while the impact of F-actin architecture on the transmission of stress is well studied, the role of architecture on the ab initio generation of stresses remains less understood. Here, we assemble F-actin networks in vitro, whose architectures are varied from branched to bundled through F-actin nucleation via Arp2/3 and the formin mDia1. Within these architectures, we track the motions of embedded myosin thick filaments and connect them to the extent of F-actin network deformation. While mDia1-nucleated networks facilitate the accumulation of stress and drive contractility through enhanced actomyosin sliding, branched networks prevent stress accumulation through the inhibited processivity of thick filaments. The reduction in processivity is due to a decrease in translational and rotational motions constrained by the local density and geometry of F-actin.

β-heavy-spectrin stabilizes the constricting contractile ring during cytokinesis

Cytokinesis requires the constriction of an actomyosin-based contractile ring and involves multiple F-actin crosslinkers. We show that partial depletion of the C. elegans cytokinetic formin generates contractile rings with low F-actin levels that constrict but are structurally fragile, and we use this background to investigate the roles of the crosslinkers plastin/PLST-1 and β-heavy-spectrin/SMA-1 during ring constriction. We show that the removal of PLST-1 or SMA-1 has opposite effects on the structural integrity of fragile rings. PLST-1 loss reduces cortical tension that resists ring constriction and makes fragile rings less prone to ruptures and regressions, whereas SMA-1 loss exacerbates structural defects, leading to frequent ruptures and cytokinesis failure. Fragile rings without SMA-1 or containing a shorter SMA-1, repeatedly rupture at the same site, and SMA-1::GFP accumulates at repair sites in fragile rings and in rings cut by laser microsurgery. These results establish that β-heavy-spectrin stabilizes the constricting ring and reveals the importance of β-heavy-spectrin size for network connectivity at low F-actin density.

Cooperative bundling by fascin generates actin structures with architectures that depend on filament length

The assembly of actin-based structures with precisely defined architectures supports essential cellular functions, including motility, intracellular transport, and division. The geometric arrangements of the filaments within actin structures are stabilized via the association of crosslinking proteins, which bind two filaments simultaneously. Because actin polymerization and crosslinking occur concurrently within the dynamic environment of the cell, these processes likely play interdependent roles in shaping the architectures of actin-based structures. To dissect the contribution of polymerization to the construction of higher-order actin structures, we investigated how filament elongation affects the formation of simple, polarized actin bundles by the crosslinking protein fascin. Using populations of actin filaments to represent distinct stages of elongation, we found that the rate of bundle assembly increases with filament length. Fascin assembles short filaments into discrete bundles, whereas bundles of long filaments merge with one another to form interconnected networks. Although filament elongation promotes bundle coalescence, many connections formed between elongating bundles are short-lived and are followed by filament breakage. Our data suggest that initiation of crosslinking early in elongation aligns growing filaments, creating a template for continued bundle assembly as elongation proceeds. This initial alignment promotes the assembly of bundles that are resistant to large changes in curvature that are required for coalescence into interconnected networks. As a result, bundles of short filaments remain straighter and more topologically discrete as elongation proceeds than bundles assembled from long filaments. Thus, uncoordinated filament elongation and crosslinking can alter the architecture of bundled actin networks, highlighting the importance of maintaining precise control over filament length during the assembly of specialized actin structures.

Crosstalk between myosin II and formin functions in the regulation of force generation and actomyosin dynamics in stress fibers

REF52 fibroblasts have a well-developed contractile machinery, the most prominent elements of which are actomyosin stress fibers with highly ordered organization of actin and myosin IIA filaments. The relationship between contractile activity and turnover dynamics of stress fibers is not sufficiently understood. Here, we simultaneously measured the forces exerted by stress fibers (using traction force microscopy or micropillar array sensors) and the dynamics of actin and myosin (using photoconversion-based monitoring of actin incorporation and high-resolution fluorescence microscopy of myosin II light chain). Our data revealed new features of the crosstalk between myosin II-driven contractility and stress fiber dynamics. During normal stress fiber turnover, actin incorporated all along the stress fibers and not only at focal adhesions. Incorporation of actin into stress fibers/focal adhesions, as well as actin and myosin II filaments flow along stress fibers, strongly depends on myosin II activity. Myosin II-dependent generation of traction forces does not depend on incorporation of actin into stress fibers per se, but still requires formin activity. This previously overlooked function of formins in maintenance of the actin cytoskeleton connectivity could be the main mechanism of formin involvement in traction force generation.

Actin regulators in cancer progression and metastases: From structure and function to cytoskeletal dynamics

The cytoskeleton is a central factor contributing to various hallmarks of cancer. In recent years, there has been increasing evidence demonstrating the involvement of actin regulatory proteins in malignancy, and their dysregulation was shown to predict poor clinical prognosis. Although enhanced cytoskeletal activity is often associated with cancer progression, the expression of several inducers of actin polymerization is remarkably reduced in certain malignancies, and it is not completely clear how these changes promote tumorigenesis and metastases. The complexities involved in cytoskeletal induction of cancer progression therefore pose considerable difficulties for therapeutic intervention; it is not always clear which cytoskeletal regulator should be targeted in order to impede cancer progression, and whether this targeting may inadvertently enhance alternative invasive pathways which can aggravate tumor growth. The entire constellation of cytoskeletal machineries in eukaryotic cells are numerous and complex; the system is comprised of and regulated by hundreds of proteins, which could not be covered in a single review. Therefore, we will focus here on the actin cytoskeleton, which encompasses the biological machinery behind most of the key cellular functions altered in cancer, with specific emphasis on actin nucleating factors and nucleation-promoting factors. Finally, we discuss current therapeutic strategies for cancer which aim to target the cytoskeleton.

Actin Disassembly: How to Contract without Motors?

Contractile actin networks take on various functions in cells. How disordered actin networks contract is still poorly understood. A recent study proposes a contractile mechanism that is driven by actin disassembly and required to prevent chromosome losses in starfish oocytes.

Tropomyosin concentration but not formin nucleators mDia1 and mDia3 determines the level of tropomyosin incorporation into actin filaments

The majority of actin filaments in human cells exist as a co-polymer with tropomyosin, which determines the functionality of actin filaments in an isoform dependent manner. Tropomyosin isoforms are sorted to different actin filament populations and in yeast this process is determined by formins, however it remains unclear what process determines tropomyosin isoform sorting in mammalian cells. We have tested the roles of two major formin nucleators, mDia1 and mDia3, in the recruitment of specific tropomyosin isoforms in mammals. Despite observing poorer cell-cell attachments in mDia1 and mDia3 KD cells and an actin bundle organisation defect with mDia1 knock down; depletion of mDia1 and mDia3 individually and concurrently did not result in any significant impact on tropomyosin recruitment to actin filaments, as observed via immunofluorescence and measured via biochemical assays. Conversely, in the presence of excess Tpm3.1, the absolute amount of Tpm3.1-containing actin filaments is not fixed by actin filament nucleators but rather depends on the cell concentration of Tpm3.1. We conclude that mDia1 and mDia3 are not essential for tropomyosin recruitment and that tropomyosin incorporation into actin filaments is concentration dependent.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

The carboxy-terminus of the formin FMNL1ɣ bundles actin to potentiate adenocarcinoma migration

The formin family of proteins contributes to spatiotemporal control of actin cytoskeletal rearrangements during motile cell activities. The FMNL subfamily exhibits multiple mechanisms of linear actin filament formation and organization. Here we report novel actin-modifying functions of FMNL1 in breast adenocarcinoma migration models. FMNL1 is required for efficient cell migration and its three isoforms exhibit distinct localization. Suppression of FMNL1 protein expression results in a significant impairment of cell adhesion, migration, and invasion. Overexpression of FMNL1ɣ, but not FMNL1β or FMNL1α, enhances cell adhesion independent of the FH2 domain and FMNL1ɣ rescues migration in cells depleted of all three endogenous isoforms. While FMNL1ɣ inhibits actin assembly in vitro, it facilitates bundling of filamentous actin independent of the FH2 domain. The unique interactions of FMNL1ɣ with filamentous actin provide a new understanding of formin domain functions and its effect on motility of diverse cell types suggest a broader role than previously realized.

Mutation of microphthalmia-associated transcription factor (mitf) in zebrafish sensitizes for glomerulopathy

Different glomerular diseases that affect podocyte homeostasis can clinically present as nephrotic syndrome with massive proteinuria, hypoalbuminemia, hyperlipidemia and edema. Up to now, no drugs that specifically target the actin cytoskeleton of podocytes are on the market and model systems for library screenings to develop anti-proteinuric drugs are of high interest. We developed a standardized proteinuria model in zebrafish using puromycin aminonucleoside (PAN) via treatment in the fish water to allow for further drug testing to develop anti-proteinuric drugs for the treatment of glomerular diseases. We noticed that fish that carry the nacre-mutation show a significantly higher susceptibility for the disruption of the glomerular filtration barrier following PAN treatment, which results in a more pronounced proteinuria phenotype. Nacre zebrafish inherit a mutation yielding a truncated version of microphthalmia-associated transcription factor/melanogenesis associated transcription factor (mitf). We hypothesized that the nacre mutation may lead to reduced formin expression and defects in cytoskeletal rearrangement. Based on the observations in zebrafish, we carried out a PAN treatment on cultured human podocytes after knockdown with MITF siRNA causing a rearrangement of the actin cytoskeleton.

Formins, Golgi, and the Centriole

Formin homology proteins (formins) are a highly conserved family of cytoskeletal remodeling proteins that are involved in a diverse array of cellular functions. Formins are best known for their ability to regulate actin dynamics, but the same functional domains also govern stability and organization of microtubules. It is thought that this dual activity allows them to coordinate the activity of these two major cytoskeletal networks and thereby influence cellular architecture. Golgi ribbon assembly is dependent upon cooperative interactions between actin filaments and cytoplasmic microtubules originating both at the Golgi itself and from the centrosome. Similarly, centrosome assembly, centriole duplication, and centrosome positioning are also reliant on a dialogue between both cytoskeletal networks. As presented in this chapter, a growing body of evidence suggests that multiple formin proteins play essential roles in these central cellular processes.

A disassembly-driven mechanism explains F-actin-mediated chromosome transport in starfish oocytes

While contraction of sarcomeric actomyosin assemblies is well understood, this is not the case for disordered networks of actin filaments (F-actin) driving diverse essential processes in animal cells. For example, at the onset of meiosis in starfish oocytes a contractile F-actin network forms in the nuclear region transporting embedded chromosomes to the assembling microtubule spindle. Here, we addressed the mechanism driving contraction of this 3D disordered F-actin network by comparing quantitative observations to computational models. We analyzed 3D chromosome trajectories and imaged filament dynamics to monitor network behavior under various physical and chemical perturbations. We found no evidence of myosin activity driving network contractility. Instead, our observations are well explained by models based on a disassembly-driven contractile mechanism. We reconstitute this disassembly-based contractile system in silico revealing a simple architecture that robustly drives chromosome transport to prevent aneuploidy in the large oocyte, a prerequisite for normal embryonic development.

Non-canonical activity of the podosomal formin FMNL1γ supports immune cell migration

Having previously located the formin FMNL1 in macrophage podosomes, we developed an in vivo model to assess the role of FMNL1 in the migration activities of primary macrophages. Deletion of FMNL1 in mice was genetically lethal; however, targeted deletion in macrophages was achieved by employing macrophage-specific Cre. Unchallenged FMNL1-deficient mice exhibited an unexpected reduction in tissue-resident macrophages despite normal blood monocyte numbers. Upon immune stimulus, the absence of FMNL1 resulted in reduced macrophage recruitment in vivo, decreased migration in two-dimensional in vitro culture and a decrease in the number of macrophages exhibiting podosomes. Of the three described isoforms of FMNL1 - α, β and γ - only FMNL1γ rescued macrophage migration when expressed exogenously in depleted macrophages. Surprisingly, mutation of residues in the FH2 domain of FMNL1γ that disrupt barbed-end actin binding did not limit rescue of macrophage migration and podosome numbers. These observations suggest that FMNL1 contributes to macrophage migration activity by stabilizing the lifespan of podosomes without interaction of fast-growing actin termini.

Cell sensing of physical properties at the nanoscale: Mechanisms and control of cell adhesion and phenotype

The chemistry, geometry, topography and mechanical properties of biomaterials modulate biochemical signals (in particular ligand-receptor binding events) that control cells-matrix interactions. In turn, the regulation of cell adhesion by the biochemical and physical properties of the matrix controls cell phenotypes such as proliferation, motility and differentiation. In particular, nanoscale geometrical, topographical and mechanical properties of biomaterials are essential to achieve control of the cell-biomaterials interface. The design of such nanoscale architectures and platforms requires understanding the molecular mechanisms underlying adhesion formation and the assembly of the actin cytoskeleton. This review presents some of the important molecular mechanisms underlying cell adhesion to biomaterials mediated by integrins and discusses the nanoscale engineered platforms used to control these processes. Such nanoscale understanding of the cell-biomaterials interface offers exciting opportunities for the design of biomaterials and their application to the field of tissue engineering.

STATEMENT OF SIGNIFICANCE

Biomaterials design is important in the fields of regenerative medicine and tissue engineering, in particular to allow the long term expansion of stem cells and the engineering of scaffolds for tissue regeneration. Cell adhesion to biomaterials often plays a central role in regulating cell phenotype. It is emerging that physical properties of biomaterials, and more generally the microenvironment, regulate such behaviour. In particular, cells respond to nanoscale physical properties of their matrix. Understanding how such nanoscale physical properties control cell adhesion is therefore essential for biomaterials design. To this aim, a deeper understanding of molecular processes controlling cell adhesion, but also a greater control of matrix engineering is required. Such multidisciplinary approaches shed light on some of the fundamental mechanisms via which cell adhesions sense their nanoscale physical environment.

Formin 2 regulates the stabilization of filopodial tip adhesions in growth cones and affects neuronal outgrowth and pathfinding in vivo

Growth cone filopodia are actin-based mechanosensory structures that are essential for chemoreception and the generation of contractile forces necessary for directional motility. However, little is known about the influence of filopodial actin structures on substrate adhesion and filopodial contractility. Formin 2 (Fmn2) localizes along filopodial actin bundles and its depletion does not affect filopodia initiation or elongation. However, Fmn2 activity is required for filopodial tip adhesion maturation and the ability of filopodia to generate traction forces. Dysregulation of filopodia in Fmn2-depleted neurons leads to compromised growth cone motility. Additionally, in mouse fibroblasts, Fmn2 regulates ventral stress fiber assembly and affects the stability of focal adhesions. In the developing chick spinal cord, Fmn2 activity is required cell-autonomously for the outgrowth and pathfinding of spinal commissural neurons. Our results reveal an unanticipated function for Fmn2 in neural development. Fmn2 regulates structurally diverse bundled actin structures, parallel filopodial bundles in growth cones and anti-parallel stress fibers in fibroblasts, in turn modulating the stability of substrate adhesions. We propose Fmn2 as a mediator of actin bundle integrity, enabling efficient force transmission to the adhesion sites.

Reliable and inexpensive expression of large, tagged, exogenous proteins in murine bone marrow-derived macrophages using a second generation lentiviral system

Over the past two decades, researchers have struggled to efficiently express foreign DNA in primary macrophages, impeding research progress. The applications of lipofection, electroporation, microinjection, and viral-mediated transfer typically result in disruptions in macrophage differentiation and function, low expression levels of exogenous proteins, limited efficiency and high cell mortality. In this report, after extensive optimization, we present a method of expressing large tagged proteins at high efficiency, consistency, and low cost using lentiviral infection. This method utilizes laboratory-propagated second generation plasmids to produce efficient virus that can be stored for later use. The expression of proteins up to 150 kDa in size is achieved in 30-70% of cells while maintaining normal macrophage differentiation and morphology as determined by fluorescence microscopy and Western blot analysis. This manuscript delineates the reagents and methods used to produce lentivirus to express exogenous DNA in murine bone marrow-derived macrophages sufficient for single cell microscopy as well as functional assays requiring large numbers of murine bone marrow-derived macrophages.

RhoA-mediated FMNL1 regulates GM130 for actin assembly and phosphorylates MAPK for spindle formation in mouse oocyte meiosis

Formin-like 1 (FMNL1) is a member of Formin family proteins which are the actin nucleators. Although FMNL1 activities have been shown to be essential for cell adhesion, cytokinesis, cell polarization and migration in mitosis, the functional roles of mammalian FMNL1 during oocyte meiosis remain uncertain. In this study, we investigated the functions of FMNL1 in mouse oocytes using specific morpholino (MO) microinjection and live cell imaging. Immunofluorescent staining showed that in addition to its cytoplasmic distribution, FMNL1 was primarily localized at the spindle poles after germinal vesicle breakdown (GVBD). FMNL1 knockdown caused the low rate of polar body extrusion and resulted in large polar bodies. Time-lapse microscopic and immunofluorescence intensity analysis indicated that this might be due to the aberrant actin expression levels. Cortical polarity was disrupted as shown by a loss of actin cap and cortical granule free domain (CGFD) formation, which was confirmed by a failure of meiotic spindle positioning. And this might be the reason for the large polar body formation. Spindle formation was also disrupted, which might be due to the abnormal localization of p-MAPK. These results indicated that FMNL1 affected both actin dynamics and spindle formation for the oocyte polar body extrusion. Moreover, FMNL1 depletion resulted in aberrant localization and expression patterns of a cis-Golgi marker protein, GM130. Finally, we found that the small GTPase RhoA might be the upstream regulator of FMNL1. Taken together, our data indicate that FMNL1 is required for spindle organization and actin assembly through a RhoA-FMNL1-GM130 pathway during mouse oocyte meiosis.

The structure of FMNL2-Cdc42 yields insights into the mechanism of lamellipodia and filopodia formation

Formins are actin polymerization factors that elongate unbranched actin filaments at the barbed end. Rho family GTPases activate Diaphanous-related formins through the relief of an autoregulatory interaction. The crystal structures of the N-terminal domains of human FMNL1 and FMNL2 in complex with active Cdc42 show that Cdc42 mediates contacts with all five armadillo repeats of the formin with specific interactions formed by the Rho-GTPase insert helix. Mutation of three residues within Rac1 results in a gain-of-function mutation for FMNL2 binding and reconstitution of the Cdc42 phenotype in vivo. Dimerization of FMNL1 through a parallel coiled coil segment leads to formation of an umbrella-shaped structure that—together with Cdc42—spans more than 15 nm in diameter. The two interacting FMNL–Cdc42 heterodimers expose six membrane interaction motifs on a convex protein surface, the assembly of which may facilitate actin filament elongation at the leading edge of lamellipodia and filopodia.

FMNL formins polymerize actin filaments to generate cellular protrusions such as lamellipodia and filopodia at the leading edge of a cell. Here the authors provide detailed mechanistic insights into the formation of actin-based protrusions through GTPase dependent activation and membrane localization of FMNL1 and FMNL2.

Human Macrophages Utilize the Podosome Formin FMNL1 for Adhesion and Migration

Macrophages play a crucial role in detecting, regulating, and resolving immune crises, requiring migration through complex extracellular matrices. Unwarranted macrophage inflammatory activity potentiates kidney disease, rheumatoid arthritis, and transplant rejection. Proper remodeling of the actin cytoskeleton, especially at adhesion structures, is essential to the translocation of macrophages. Macrophages form actin-rich adhesions termed "podosomes", giving them the capacity to make contacts with the substratum for traction through interstitial tissues. Macrophages express multiple formins, including FMNL1, Dia1, and Fhod1, with potential to impact actin remodeling involved in migration. Formins are a family of proteins that are best known for modifying the actin cytoskeleton via nucleation, elongation, bundling, and/or severing actin filaments. In this study we demonstrate that the formin FMNL1 is a key regulator of podosomes and is required for normal macrophage migration. Additionally, this is the first study to demonstrate defects in primary human cell migration resulting from specific formin silencing. Pharmacologic inhibition of all formin activity results in a significant decrease in podosome formation and normal macrophage migration. Furthermore, targeted suppression of FMNL1 results in decreases in macrophage migration similar to inhibition of all expressed macrophage formins. These novel findings suggest FMNL1 as a possible chemotherapeutic target to hinder macrophage migration, which could offer an innovative method for limiting unnecessary macrophage-mediated inflammation. We hypothesize that formins are required in podosome actin dynamics to support macrophage migration.

Formin mDia1, a downstream molecule of FMNL1, regulates Profilin1 for actin assembly and spindle organization during mouse oocyte meiosis

Mammalian diaphanous1 (mDia1) is a homologue of Drosophila diaphanous and belongs to the Formin-homology family of proteins that catalyze actin nucleation and polymerization. Although Formin family proteins, such as Drosophila diaphanous, have been shown to be essential for cytokinesis, whether and how mDia1 functions during meiosis remain uncertain. In this study, we explored possible roles and the signaling pathway involved for mDia1 using a mouse oocyte model. mDia1 depletion reduced polar body extrusion, which may have been due to reduced cortical actin assembly. mDia1 and Profilin1 had similar localization patterns in mouse oocytes and mDia1 knockdown resulted in reduced Profilin1 expression. Depleting FMNL1, another Formin family member, resulted in reduced mDia1 expression, while RhoA inhibition did not alter mDia1 expression, which indicated that there was a FMNL1-mDia1-Profilin1 signaling pathway in mouse oocytes. Additionally, mDia1 knockdown resulted in disrupting oocyte spindle morphology, which was confirmed by aberrant p-MAPK localization. Thus, these results demonstrated indispensable roles for mDia1 in regulating mouse oocyte meiotic maturation through its effects on actin assembly and spindle organization.

Myosin IIA is essential for Shigella flexneri cell-to-cell spread

A key feature of Shigella pathogenesis is the ability to spread from cell-to-cell post-invasion. This is dependent on the bacteria's ability to initiate de novo F-actin tail polymerisation, followed by protrusion formation, uptake of bacteria-containing protrusion and finally, lysis of the double membrane vacuole in the adjacent cell. In epithelial cells, cytoskeletal tension is maintained by the actin-myosin II networks. In this study, the role of myosin II and its specific kinase, myosin light chain kinase (MLCK), during Shigella intercellular spreading was investigated in HeLa cells. Inhibition of MLCK and myosin II, as well as myosin IIA knockdown, significantly reduced Shigella plaque and infectious focus formation. Protrusion formation and intracellular bacterial growth was not affected. Low levels of myosin II were localised to the Shigella F-actin tail. HeLa cells were also infected with Shigella strains defective in cell-to-cell spreading. Unexpectedly loss of myosin IIA labelling was observed in HeLa cells infected with these mutant strains. This phenomenon was not observed with WT Shigella or with the less abundant myosin IIB isoform, suggesting a critical role for myosin IIA.

Formins as effector proteins of Rho GTPases

Formin proteins were recognized as effectors of Rho GTPases some 15 years ago. They contribute to different cellular actin cytoskeleton structures by their ability to polymerize straight actin filaments at the barbed end. While not all formins necessarily interact with Rho GTPases, a subgroup of mammalian formins, termed Diaphanous-related formins or DRFs, were shown to be activated by small GTPases of the Rho superfamily. DRFs are autoinhibited in the resting state by an N- to C-terminal interaction that renders the central actin polymerization domain inactive. Upon the interaction with a GTP-bound Rho, Rac, or Cdc42 GTPase, the C-terminal autoregulation domain is displaced from its N-terminal recognition site and the formin becomes active to polymerize actin filaments. In this review we discuss the current knowledge on the structure, activation, and function of formin-GTPase interactions for the mammalian formin families Dia, Daam, FMNL, and FHOD. We describe both direct and indirect interactions of formins with GTPases, which lead to formin activation and cytoskeletal rearrangements. The multifaceted function of formins as effector proteins of Rho GTPases thus reflects the diversity of the actin cytoskeleton in cells.

Daam1 is a regulator of filopodia formation and phagocytic uptake of Borrelia burgdorferi by primary human macrophages

Borrelia burgdorferi is the causative agent of Lyme disease, an infectious disease that primarily affects the skin, nervous system, and joints. Uptake of borreliae by immune cells is decisive for the course of the infection, and remodelling of the host actin cytoskeleton is crucial in this process. In this study, we showed that the actin-regulatory formin Daam1 is important in Borrelia phagocytosis by primary human macrophages. Uptake of borreliae proceeds preferentially through capture by filopodia and formation of coiling pseudopods that enwrap the spirochetes. Using immunofluorescence, we localized endogenous and overexpressed Daam1 to filopodia and to F-actin-rich uptake structures. Live-cell imaging further showed that Daam1 is enriched at coiling pseudopods that arise from the macrophage surface. This filopodia-independent step was corroborated by control experiments of phagocytic cup formation with latex beads. Moreover, siRNA-mediated knockdown of Daam1 led to a 65% reduction of borreliae-induced filopodia, and, as shown by the outside-inside staining technique, to a 50% decrease in phagocytic uptake of borreliae, as well as a 37% reduction in coiling pseudopod formation. Collectively, we showed that Daam1 plays a dual role in the phagocytic uptake of borreliae: first, as a regulator of filopodia, which are used for capturing spirochetes, and second, in the formation of the coiling pseudopod that enwraps the bacterial cell. These data identify Daam1 as a novel regulator of B. burgdorferi phagocytosis. At the same time, this is the first demonstration of a role for Daam1 in phagocytic processes in general.-Hoffmann, A.-K., Naj, X., Linder, S. Daam1 is a regulator of filopodia formation and phagocytic uptake of Borrelia burgdorferi by primary human macrophages.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.

Impact of dynasore an inhibitor of dynamin II on Shigella flexneri infection

Shigella flexneri remains a significant human pathogen due to high morbidity among children < 5 years in developing countries. One of the key features of Shigella infection is the ability of the bacterium to initiate actin tail polymerisation to disseminate into neighbouring cells. Dynamin II is associated with the old pole of the bacteria that is associated with F-actin tail formation. Dynamin II inhibition with dynasore as well as siRNA knockdown significantly reduced Shigella cell to cell spreading in vitro. The ocular mouse Sereny model was used to determine if dynasore could delay the progression of Shigella infection in vivo. While dynasore did not reduce ocular inflammation, it did provide significant protection against weight loss. Therefore dynasore's effects in vivo are unlikely to be related to the inhibition of cell spreading observed in vitro. We found that dynasore decreased S. flexneri-induced HeLa cell death in vitro which may explain the protective effect observed in vivo. These results suggest the administration of dynasore or a similar compound during Shigella infection could be a potential intervention strategy to alleviate disease symptoms.

SIN-dependent phosphoinhibition of formin multimerization controls fission yeast cytokinesis

In Schizosaccharomyces pombe, the septation initiation network (SIN) controls cytokinetic ring (CR) formation, maintenance, and constriction. Bohnert et al. identify Cdc12 as a key CR substrate of SIN kinase Sid2. Eliminating Sid2-mediated Cdc12 phosphorylation allows multimerization of a domain that confers F-actin bundling activity, which leads to persistent Cdc12 clustering, causing CRs to collapse when cytokinesis is delayed. These findings identify a SIN-triggered oligomeric switch that modulates cytokinetic formin function, revealing a novel mechanism of actin cytoskeleton regulation during cell division.

Many eukaryotes accomplish cell division by building and constricting a medial actomyosin-based cytokinetic ring (CR). In Schizosaccharomyces pombe, a Hippo-related signaling pathway termed the septation initiation network (SIN) controls CR formation, maintenance, and constriction. However, how the SIN regulates integral CR components was unknown. Here, we identify the essential cytokinetic formin Cdc12 as a key CR substrate of SIN kinase Sid2. Eliminating Sid2-mediated Cdc12 phosphorylation leads to persistent Cdc12 clustering, which prevents CR assembly in the absence of anillin-like Mid1 and causes CRs to collapse when cytokinesis is delayed. Molecularly, Sid2 phosphorylation of Cdc12 abrogates multimerization of a previously unrecognized Cdc12 domain that confers F-actin bundling activity. Taken together, our findings identify a SIN-triggered oligomeric switch that modulates cytokinetic formin function, revealing a novel mechanism of actin cytoskeleton regulation during cell division.

FMNL1 promotes proliferation and migration of leukemia cells

The human FMNL1 is expressed predominantly in hematopoietic cells and has been described previously as overexpressed in hematopoietic malignancies. However, it is not known whether FMNL1 contributes to leukemogenesis. Here, we investigate the FMNL1 function using two different human leukemia models: Namalwa and K562 cell lines. FMNL1 depletion reduced cell proliferation and colony formation in both leukemic cell types, as well as a decrease in the tumor growth of FMNL1-depleted Namalwa cell xenografts. In addition, there was a decrease in migration and in TEM in FMNL1-depleted Namalwa cells. FMNL1 endogenously associates with Rac1, and FMNL1 silencing resulted in an increased Rac1 activity. The reduced migration observed in FMNL1-depleted cells was restored by inhibiting Rac activity. Our results indicate that FMNL1 stimulates leukemia cell proliferation as well as migration. This suggests that FMNL1 contributes to leukemogenesis and could act in part through Rac1 regulation.

The formin Daam1 and fascin directly collaborate to promote filopodia formation

Filopodia are slender cellular protrusions that dynamically extend and retract to facilitate directional cell migration, pathogen sensing, and cell-cell adhesion. Each filopodium contains a rigid and organized bundle of parallel actin filaments, which are elongated at filopodial tips by formins and Ena/VASP proteins. However, relatively little is known about how the actin filaments in the filopodial shaft are spatially organized to form a bundle with appropriate dimensions and mechanical properties. Here, we report that the mammalian formin Daam1 (Disheveled-associated activator of morphogenesis 1) is a potent actin-bundling protein and localizes all along the filopodial shaft, which differs from other formins that localize specifically to the tips. Silencing of Daam1 led to severe defects in filopodial number, integrity, and architecture, similar to silencing of the bundling protein fascin. This led us to investigate the potential relationship between Daam1 and fascin. Fascin and Daam1 coimmunoprecipitated from cell extracts, and silencing of fascin led to a striking loss of Daam1 localization to filopodial shafts, but not tips. Furthermore, purified fascin bound directly to Daam1, and multicolor single-molecule TIRF imaging revealed that fascin recruited Daam1 to and stabilized Daam1 on actin bundles in vitro. Our results reveal an unanticipated and direct collaboration between Daam1 and fascin in bundling actin, which is required for proper filopodial formation.

Thermodynamic and structural characterization of an antibody gel

Although extensively studied, protein-protein interactions remain highly elusive and are of increasing interest in drug development. We show the assembly of a monoclonal antibody, using multivalent carboxylate ions, into highly-ordered structures. While the presence and function of similar structures in vivo are not known, the results may present a possible unexplored area of antibody structure-function relationships. Using a variety of tools (e.g., mechanical rheology, electron microscopy, isothermal calorimetry, Fourier transform infrared spectroscopy), we characterized the physical, biochemical, and thermodynamic properties of these structures and found that citrate may interact directly with the amino acid residue histidine, after which the individual protein units assemble into a filamentous network gel exhibiting high elasticity and interfilament interactions. Citrate interacts exothermically with the monoclonal antibody with an association constant that is highly dependent on solution pH and temperature. Secondary structure analysis also reveals involvement of hydrophobic and aromatic residues.

Two distinct phosphorylation events govern the function of muscle FHOD3

Posttranslational modifications such as phosphorylation are universally acknowledged regulators of protein function. Recently we characterised a striated muscle-specific isoform of the formin FHOD3 that displays distinct subcellular targeting and protein half-life compared to its non-muscle counterpart and which is dependent on phosphorylation by CK2 (formerly casein kinase 2). We now show that the two isoforms of FHOD3 are already expressed in the vertebrate embryonic heart. Analysis of CK2 alpha knockout mice showed that phosphorylation by CK2 is also required for proper targeting of muscle FHOD3 to the myofibrils in embryonic cardiomyocytes in situ. The localisation of muscle FHOD3 in the sarcomere varies depending on the maturation state, being either broader or restricted to the Z-disc proper in the adult heart. Following myofibril disassembly, such as that in dedifferentiating adult rat cardiomyocytes in culture, the expression of non-muscle FHOD3 is up-regulated, which is reversed once the myofibrils are reassembled. The shift in expression levels of different isoforms is accompanied by an increased co-localisation with p62, which is involved in autophagy, and affects the half-life of FHOD3. Phosphorylation of three amino acids in the C-terminus of FHOD3 by ROCK1 is sufficient for activation, which results in increased actin filament synthesis in cardiomyocytes and also a broader localisation pattern of FHOD3 in the myofibrils. ROCK1 can directly phosphorylate FHOD3, and FHOD3 seems to be the downstream mediator of the exaggerated actin filament formation phenotype that is induced in cardiomyocytes upon the overexpression of constitutively active ROCK1. We conclude that the expression of the muscle FHOD3 isoform is characteristic of the healthy mature heart and that two distinct phosphorylation events are crucial to regulate the activity of this isoform in thin filament assembly and maintenance.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Bi-modal regulation of a formin by srGAP2

The maintenance of rapid and efficient actin dynamics in vivo requires coordination of filament assembly and disassembly. This regulation requires temporal and spatial integration of signaling pathways by protein complexes. However, it remains unclear how these complexes form and then regulate the actin cytoskeleton. Here, we identify a srGAP2 and formin-like 1 (FMNL1, also known as FRL1 or FRLα) complex whose assembly is regulated by Rac signaling. Our data suggest srGAP2 regulates FMNL1 in two ways; 1) Rac-mediated activation of FMNL1 leads to the recruitment of srGAP2, which contains a Rac-specific GAP domain; 2) the SH3 domain of srGAP2 binds the formin homology 1 domain of FMNL1 to inhibit FMNL1-mediated actin severing. Thus, srGAP2 can efficiently terminate the upstream activating Rac signal while also opposing an important functional output of FMNL1, namely actin severing. We also show that FMNL1 and srGAP2 localize to the actin-rich phagocytic cup of macrophage-derived cells, suggesting the complex may regulate this Rac- and actin-driven process in vivo. We propose that after Rac-dependent activation of FMNL1, srGAP2 mediates a potent mechanism to limit the duration of Rac action and inhibit formin activity during rapid actin dynamics.

Actin cross-link assembly and disassembly mechanics for alpha-Actinin and fascin

Self-assembly of complex structures is commonplace in biology but often poorly understood. In the case of the actin cytoskeleton, a great deal is known about the components that include higher order structures, such as lamellar meshes, filopodial bundles, and stress fibers. Each of these cytoskeletal structures contains actin filaments and cross-linking proteins, but the role of cross-linking proteins in the initial steps of structure formation has not been clearly elucidated. We employ an optical trapping assay to investigate the behaviors of two actin cross-linking proteins, fascin and alpha-actinin, during the first steps of structure assembly. Here, we show that these proteins have distinct binding characteristics that cause them to recognize and cross-link filaments that are arranged with specific geometries. alpha-Actinin is a promiscuous cross-linker, linking filaments over all angles. It retains this flexibility after cross-links are formed, maintaining a connection even when the link is rotated. Conversely, fascin is extremely selective, only cross-linking filaments in a parallel orientation. Surprisingly, bundles formed by either protein are extremely stable, persisting for over 0.5 h in a continuous wash. However, using fluorescence recovery after photobleaching and fluorescence decay experiments, we find that the stable fascin population can be rapidly competed away by free fascin. We present a simple avidity model for this cross-link dissociation behavior. Together, these results place constraints on how cytoskeletal structures assemble, organize, and disassemble in vivo.

Activation of the mitochondrial protein quality control system and actin cytoskeletal alterations in cells harbouring the MELAS mitochondrial DNA mutation

Point mutations in the mitochondrial genome are associated with a variety of metabolic disorders. The myopathy, encephalopathy, lactic acidosis, stroke-like episodes syndrome (MELAS), is most frequently associated with an A to G transition at position 3243 of the mitochondrial tRNA(Leu(UUR)) gene, and is characterized by biochemical and structural alterations of mitochondria. In the present study, we analyzed proteomic changes in an immortalized B-cell line harbouring the MELAS A3243G mutation by two-dimensional difference gel electrophoresis and immunoblot analysis. Although the cell line contained only 10% mutated mitochondrial genomes, we detected significant alterations in numerous proteins associated with the actin cytoskeleton and in nuclear-encoded subunits of mitochondrial respiratory chain complexes. Notably, mitochondrial Lon protease and Hsp60 were deregulated in MELAS cells, indicating an effect on the mitochondrial protein quality control system. By immunofluorescence microscopy, we detected mitochondrial Lon protease accumulation and changes in actin-binding proteins preferentially in MELAS cells containing numerous mitochondria with mutated genomes. Enzymatic assays revealed that Lon protease activity is increased in MELAS cell lysates. Although Lon protease has been shown to degrade misfolded proteins and to stabilize respiratory chain complexes within mitochondria, our MELAS cell line exhibited a higher sensitivity to mitochondrial stress. These findings provide novel insights into the cellular response to dysfunctional mitochondria containing mutated genomes.

Characterization of the biochemical properties and biological function of the formin homology domains of Drosophila DAAM

We characterized the properties of Drosophila melanogaster DAAM-FH2 and DAAM-FH1-FH2 fragments and their interactions with actin and profilin by using various biophysical methods and in vivo experiments. The results show that although the DAAM-FH2 fragment does not have any conspicuous effect on actin assembly in vivo, in cells expressing the DAAM-FH1-FH2 fragment, a profilin-dependent increase in the formation of actin structures is observed. The trachea-specific expression of DAAM-FH1-FH2 also induces phenotypic effects, leading to the collapse of the tracheal tube and lethality in the larval stages. In vitro, both DAAM fragments catalyze actin nucleation but severely decrease both the elongation and depolymerization rate of the filaments. Profilin acts as a molecular switch in DAAM function. DAAM-FH1-FH2, remaining bound to barbed ends, drives processive assembly of profilin-actin, whereas DAAM-FH2 forms an abortive complex with barbed ends that does not support profilin-actin assembly. Both DAAM fragments also bind to the sides of the actin filaments and induce actin bundling. These observations show that the D. melanogaster DAAM formin represents an extreme class of barbed end regulators gated by profilin.

A nucleator arms race: cellular control of actin assembly

For over a decade, the actin-related protein 2/3 (ARP2/3) complex, a handful of nucleation-promoting factors and formins were the only molecules known to directly nucleate actin filament formation de novo. However, the past several years have seen a surge in the discovery of mammalian proteins with roles in actin nucleation and dynamics. Newly recognized nucleation-promoting factors, such as WASP and SCAR homologue (WASH), WASP homologue associated with actin, membranes and microtubules (WHAMM), and junction-mediating regulatory protein (JMY), stimulate ARP2/3 activity at distinct cellular locations. Formin nucleators with additional biochemical and cellular activities have also been uncovered. Finally, the Spire, cordon-bleu and leiomodin nucleators have revealed new ways of overcoming the kinetic barriers to actin polymerization.

Fifteen formins for an actin filament: a molecular view on the regulation of human formins

The regulation of the actin cytoskeleton is a key process for the stability and motility of eukaryotic cells. Besides the Arp2/3 complex and its nucleation promoting factors, WH2 domain-containing proteins and a diverse family of formin proteins have recently been recognized as actin nucleators and potent polymerization factors of actin filaments. Formins are defined by the presence of a catalytic formin homology 2 (FH2) domain, yet, the modular domain architecture appears significantly different for the eight formin families identified in humans. A diverse picture of protein localization, interaction partners and cell specific regulation emerged, suggesting various functions of formins in the building and maintenance of actin filaments. This review focuses on the domain architecture of human formins, the regulation mechanisms of their activation and the diversity in formin cellular functions.

The effects of formins on the conformation of subdomain 1 in actin filaments

In this study we investigated the effects of formins on the conformation of actin filaments by using the method of fluorescence quenching. Actin was labelled with IAEDANS at Cys(374) and the quencher was acrylamide. The results showed that formin binding induced structural changes in the subdomain 1 of actin protomers which were reflected by greater quenching constants (K(SV)). Simultaneously the fraction of the fluorophore population accessible for the quencher (alpha) decreased. These observations suggest that the conformational distribution characteristic for the actin protomers became broader after the binding of formins, for which the structural framework was provided by a more flexible protein matrix in the microenvironment of the label. The effects of formins depended on the formin:actin molar ratio, and also on the ionic strength of the medium. These observations are in agreement with previous results and underline the importance of the intramolecular conformational changes induced by formins in the structure of actin filaments.

Vinculin nucleates actin polymerization and modifies actin filament structure

Vinculin links integrins to the actin cytoskeleton by binding F-actin. Little is known with respect to how this interaction occurs or affects actin dynamics. Here we assess the consequence of the vinculin tail (VT) on actin dynamics by examining its binding to monomeric and filamentous yeast actins. VT causes pyrene-labeled G-actin to polymerize in low ionic strength buffer (G-buffer), conditions that normally do not promote actin polymerization. Analysis by electron microscopy shows that, under these conditions, the filaments form small bundles at low VT concentrations, which gradually increase in size until saturation occurs at a ratio of 2 VT:1 actin. Addition of VT to pyrene-labeled mutant yeast G-actin (S265C) produced a fluorescence excimer band, which requires a relatively normal filament geometry. In higher ionic strength polymerization-promoting F-buffer, substoichiometric amounts of VT accelerate the polymerization of pyrene-labeled WT actin. However, the amplitude of the pyrene fluorescence caused by actin polymerization is quenched as the VT concentration increases without an effect on net actin polymerization as determined by centrifugation assays. Finally, addition of VT to preformed pyrene-labeled S265C F-actin causes a concentration-dependent decrease in the maximum amplitude of the pyrene fluorescence band demonstrating the ability of VT to remodel the conformation of the actin filament. These observations support the idea that vinculin can link adhesion plaques to the cytoskeleton by initiating the formation of bundled actin filaments or by remodeling existing filaments.