Actin nucleation and elongation factors: mechanisms and interplay

The WAVE complex forms linear arrays at negative membrane curvature to instruct lamellipodia formation

Cells generate a wide range of actin-based membrane protrusions for various cell behaviors. These protrusions are organized by different actin nucleation promoting factors. For example, N-WASP controls finger-like filopodia, whereas the WAVE complex controls sheet-like lamellipodia. These different membrane morphologies likely reflect different patterns of nucleator self-organization. N-WASP phase separation has been successfully studied through biochemical reconstitutions, but how the WAVE complex self-organizes to instruct lamellipodia is unknown. Because WAVE complex self-organization has proven refractory to cell-free studies, we leverage in vivo biochemical approaches to investigate WAVE complex organization within its native cellular context. With single molecule tracking and molecular counting, we show that the WAVE complex forms highly regular multi-layered linear arrays at the plasma membrane that are rem-iniscent of a microtubule-like organization. Similar to the organization of microtubule protofilaments in a curved array, membrane curvature is both necessary and sufficient for formation of these WAVE complex linear arrays, though actin polymerization is not. This dependency on negative membrane curvature could explain both the templating of lamellipodia and their emergent behaviors, including barrier avoidance. Our data uncover the key biophysical properties of mesoscale WAVE complex patterning and highlight an integral relationship between NPF self-organization and cell morphogenesis.

Force-activated zyxin assemblies coordinate actin nucleation and crosslinking to orchestrate stress fiber repair

As the cytoskeleton sustains cell and tissue forces, it incurs physical damage that must be repaired to maintain mechanical homeostasis. The LIM-domain protein zyxin detects force-induced ruptures in actin-myosin stress fibers, coordinating downstream repair factors to restore stress fiber integrity through unclear mechanisms. Here, we reconstitute stress fiber repair with purified proteins, uncovering detailed links between zyxin's force-regulated binding interactions and cytoskeletal dynamics. In addition to binding individual tensed actin filaments (F-actin), zyxin's LIM domains form force-dependent assemblies that bridge broken filament fragments. Zyxin assemblies engage repair factors through multi-valent interactions, coordinating nucleation of new F-actin by VASP and its crosslinking into aligned bundles by ɑ-actinin. Through these combined activities, stress fiber repair initiates within the cores of micron-scale damage sites in cells, explaining how these F-actin depleted regions are rapidly restored. Thus, zyxin's force-dependent organization of actin repair machinery inherently operates at the network scale to maintain cytoskeletal integrity.

Tumor invasiveness is regulated by the concerted function of APC, formins, and Arp2/3 complex

Tumor cell invasion is the initial step in metastasis, the leading cause of death from cancer. Invasion requires protrusive cellular structures that steer the migration of leader cells emanating from the tumor mass toward neighboring tissues. Actin is central to these processes and is therefore the prime target of drugs known as migrastatics. However, the broad effects of general actin inhibitors limit their therapeutic use. Here, we delineate the roles of specific actin nucleators in tuning actin-rich invasive protrusions and pinpoint potential pharmacological targets. We subject colorectal cancer spheroids embedded in collagen matrix-a preclinical model mirroring solid tumor invasiveness-to pharmacologic and/or genetic treatment of specific actin arrays to assess their roles in invasiveness. Our data reveal coordinated yet distinct involvement of actin networks nucleated by adenomatous polyposis coli, formins, and actin-related protein 2/3 complex in the biogenesis and maintenance of invasive protrusions. These findings may open avenues for better targeted therapies.

CLIP170 inhibits the metastasis and EMT of papillary thyroid cancer through the TGF-β pathway

Metastasis poses a significant challenge in combating tumors. Even in papillary thyroid cancer (PTC), which typically exhibits a favorable prognosis, high recurrence rates are attributed to metastasis. Cytoplasmic linker protein 170 (CLIP170) functions as a classical microtubule plus-end tracking protein (+TIP) and has shown close association with cell migration. Nevertheless, the specific impact of CLIP170 on PTC cells remains to be elucidated. Our analysis of the GEO and TCGA databases unveiled an association between CLIP170 and the progression of PTC. To explore the impact of CLIP170 on PTC cells, we conducted various assays. We evaluated its effects through CCK-8, wound healing assay, and transwell assay after knocking down CLIP170. Additionally, the influence of CLIP170 on the cellular actin structure was examined via immunofluorescence; we further investigated the molecular expressions of epithelial-mesenchymal transition (EMT) and the transforming growth factor-β (TGF-β) signaling pathways through Western blotting and RT-qPCR. These findings were substantiated through an in vivo nude mouse model of lung metastasis. We observed a decreased expression of CLIP170 in PTC in contrast to normal thyroid tissue. Functionally, the knockdown of CLIP170 (CLIP170KD) notably enhanced the metastatic potential and EMT of PTC cells, both in vitro and in vivo. Mechanistically, CLIP170KD triggered the activation of the TGF-β pathway, subsequently promoting tumor cell migration, invasion, and EMT. Remarkably, the TGF-β inhibitor LY2157299 effectively countered TGF-β activity and significantly reversed tumor metastasis and EMT induced by CLIP170 knockdown. In summary, these findings collectively propose CLIP170 as a promising therapeutic target to mitigate metastatic tendencies in PTC.

Collective dynamics of actin and microtubule and its crosstalk mediated by FHDC1

The coordination between actin and microtubule network is crucial, yet this remains a challenging problem to dissect and our understanding of the underlying mechanisms remains limited. In this study, we used travelling waves in the cell cortex to characterize the collective dynamics of cytoskeletal networks. Our findings show that Cdc42 and F-BAR-dependent actin waves in mast cells are mainly driven by formin-mediated actin polymerization, with the microtubule-binding formin FH2 domain-containing protein 1 (FHDC1) as an early regulator. Knocking down FHDC1 inhibits actin wave formation, and this inhibition require FHDC1's interaction with both microtubule and actin. The phase of microtubule depolymerization coincides with the nucleation of actin waves and microtubule stabilization inhibit actin waves, leading us to propose that microtubule shrinking and the concurrent release of FHDC1 locally regulate actin nucleation. Lastly, we show that FHDC1 is crucial for multiple cellular processes such as cell division and migration. Our data provided molecular insights into the nucleation mechanisms of actin waves and uncover an antagonistic interplay between microtubule and actin polymerization in their collective dynamics.

K-Means Clustering Coarse-Graining (KMC-CG): A Next Generation Methodology for Determining Optimal Coarse-Grained Mappings of Large Biomolecules

Coarse-grained (CG) molecular dynamics (MD) has become a method of choice for simulating various large scale biomolecular processes; therefore, the systematic definition of the CG mappings for biomolecules remains an important topic. Appropriate CG mappings can significantly enhance the representability of a CG model and improve its ability to capture critical features of large biomolecules. In this work, we present a systematic and more generalized method called K-means clustering coarse-graining (KMC-CG), which builds on the earlier approach of essential dynamics coarse-graining (ED-CG). KMC-CG removes the sequence-dependent constraints of ED-CG, allowing it to explore a more extensive space and thus enabling the discovery of more physically optimal CG mappings. Furthermore, the implementation of the K-means clustering algorithm can variationally optimize the CG mapping with efficiency and stability. This new method is tested in three cases: ATP-bound G-actin, the HIV-1 CA pentamer, and the Arp2/3 complex. In these examples, the CG models generated by KMC-CG are seen to better capture the structural, dynamic, and functional domains. KMC-CG therefore provides a robust and consistent approach to generating CG models of large biomolecules that can then be more accurately parametrized by either bottom-up or top-down CG force fields.

DANGER Signals Activate G -Protein Receptor Kinases Suppressing Neutrophil Function and Predisposing to Infection After Tissue Trauma

OBJECTIVE

Injured tissue predisposes the subject to local and systemic infection. We studied injury-induced immune dysfunction seeking novel means to reverse such predisposition.

BACKGROUND

Injury mobilizes primitive "DANGER signals" [danger-associated molecular patterns (DAMPs)] activating innate immunocyte (neutrophils, PMN) signaling and function. Mitochondrial formyl peptides activate G -protein coupled receptors (GPCR) like formyl peptide receptor-1. Mitochondrial DNA and heme activate toll-like receptors (TLR9 and TLR2/4). GPCR kinases (GRKs) can regulate GPCR activation.

METHODS

We studied human and mouse PMN signaling elicited by mitochondrial DAMPs (GPCR surface expression; protein phosphorylation, or acetylation; Ca 2+ flux) and antimicrobial functions [cytoskeletal reorganization, chemotaxis (CTX), phagocytosis, bacterial killing] in cellular systems and clinical injury samples. Predicted rescue therapies were assessed in cell systems and mouse injury-dependent pneumonia models.

RESULTS

Mitochondrial formyl peptides activate GRK2, internalizing GPCRs and suppressing CTX. Mitochondrial DNA suppresses CTX, phagocytosis, and killing through TLR9 through a novel noncanonical mechanism that lacks GPCR endocytosis. Heme also activates GRK2. GRK2 inhibitors like paroxetine restore functions. GRK2 activation through TLR9 prevented actin reorganization, implicating histone deacetylases (HDACs). Actin polymerization, CTX, bacterial phagocytosis, and killing were also rescued, therefore, by the HDAC inhibitor valproate. Trauma repository PMN showed GRK2 activation and cortactin deacetylation, which varied with severity and was most marked in patients developing infections. Either GRK2 or HDAC inhibition prevented loss of mouse lung bacterial clearance, but only the combination rescued clearance when given postinjury.

CONCLUSIONS

Tissue injury-derived DAMPs suppress antimicrobial immunity through canonical GRK2 activation and a novel TLR-activated GRK2-pathway impairing cytoskeletal organization. Simultaneous GRK2/HDAC inhibition rescues susceptibility to infection after tissue injury.

Cyclase-associated protein interacts with actin filament barbed ends to promote depolymerization and formin displacement

Cyclase-associated protein (CAP) has emerged as a central player in cellular actin turnover, but its molecular mechanisms of action are not yet fully understood. Recent studies revealed that the N terminus of CAP interacts with the pointed ends of actin filaments to accelerate depolymerization in conjunction with cofilin. Here, we use in vitro microfluidics-assisted TIRF microscopy to show that the C terminus of CAP promotes depolymerization at the opposite (barbed) ends of actin filaments. In the absence of actin monomers, full-length mouse CAP1 and C-terminal halves of CAP1 (C-CAP1) and CAP2 (C-CAP2) accelerate barbed end depolymerization. Using mutagenesis and structural modeling, we show that these activities are mediated by the WH2 and CARP domains of CAP. In addition, we observe that CAP collaborates with profilin to accelerate barbed end depolymerization and that these effects depend on their direct interaction, providing the first known example of CAP-profilin collaborative effects in regulating actin. In the presence of actin monomers, CAP1 attenuates barbed end growth and promotes formin dissociation. Overall, these findings demonstrate that CAP uses distinct domains and mechanisms to interact with opposite ends of actin filaments and drive turnover. Further, they contribute to the emerging view of actin barbed ends as sites of dynamic molecular regulation, where numerous proteins compete and cooperate with each other to tune polymer dynamics, similar to the rich complexity seen at microtubule ends.

Cholesterol dependent cytolysins and the brain: Revealing a potential therapeutic avenue for bacterial meningitis

Bacterial meningitis is a catastrophic nervous system disorder with high mortality and wide range of morbidities. Some of the meningitis-causing bacteria occupy cholesterol dependent cytolysins (CDCs) to increase their pathogenicity and arrange immune-evasion strategy. Studies have observed that the relationship between CDCs and pathogenicity in these meningitides is complex and involves interactions between CDC, blood-brain barrier (BBB), glial cells and neurons. In BBB, these CDCs acts on capillary endothelium, tight junction (TJ) proteins and neurovascular unit (NVU). CDCs also observed to elicit intriguing effects on brain inflammation which involves microglia and astrocyte activations, along with neuronal damage as the end-point of pathological pathways in bacterial meningitis. As some studies mentioned potential advantage of CDC-targeted therapeutic mechanisms to combat CNS infections, it might be a fruitful avenue to deepen our understanding of CDC as a candidate for adjuvant therapy to combat bacterial meningitis.

Polarized SCAR and the Arp2/3 complex regulate apical cortical remodeling in asymmetrically dividing neuroblasts

Although the formin-nucleated actomyosin cortex has been shown to drive the changes in cell shape that accompany animal cell division in both symmetric and asymmetric cell divisions, the mitotic role of cortical Arp2/3-nucleated actin networks remain unclear. Here using asymmetrically dividing Drosophila neural stem cells as a model system, we identify a pool of membrane protrusions that form at the apical cortex of neuroblasts as they enter mitosis. Strikingly, these apically localized protrusions are enriched in SCAR, and depend on SCAR and Arp2/3 complexes for their formation. Because compromising SCAR or the Arp2/3 complex delays the apical clearance of Myosin II at the onset of anaphase and induces cortical instability at cytokinesis, these data point to a role for an apical branched actin filament network in fine-tuning the actomyosin cortex to enable the precise control of cell shape changes during an asymmetric cell division.

EVL and MIM/MTSS1 regulate actin cytoskeletal remodeling to promote dendritic filopodia in neurons

Dendritic spines are the postsynaptic compartment of a neuronal synapse and are critical for synaptic connectivity and plasticity. A developmental precursor to dendritic spines, dendritic filopodia (DF), facilitate synapse formation by sampling the environment for suitable axon partners during neurodevelopment and learning. Despite the significance of the actin cytoskeleton in driving these dynamic protrusions, the actin elongation factors involved are not well characterized. We identified the Ena/VASP protein EVL as uniquely required for the morphogenesis and dynamics of DF. Using a combination of genetic and optogenetic manipulations, we demonstrated that EVL promotes protrusive motility through membrane-direct actin polymerization at DF tips. EVL forms a complex at nascent protrusions and DF tips with MIM/MTSS1, an I-BAR protein important for the initiation of DF. We proposed a model in which EVL cooperates with MIM to coalesce and elongate branched actin filaments, establishing the dynamic lamellipodia-like architecture of DF.

Activation of an actin signaling pathway in pre-malignant mammary epithelial cells by P-cadherin is essential for transformation

Alterations in the expression or function of cell adhesion molecules have been implicated in all steps of tumor progression. Among those, P-cadherin is highly enriched in basal-like breast carcinomas, playing a central role in cancer cell self-renewal, collective cell migration and invasion. To establish a clinically relevant platform for functional exploration of P-cadherin effectors in vivo, we generated a humanized P-cadherin Drosophila model. We report that actin nucleators, Mrtf and Srf, are main P-cadherin effectors in fly. We validated these findings in a human mammary epithelial cell line with conditional activation of the SRC oncogene. We show that, prior to promoting malignant phenotypes, SRC induces a transient increase in P-cadherin expression, which correlates with MRTF-A accumulation, its nuclear translocation and the upregulation of SRF target genes. Moreover, knocking down P-cadherin, or preventing F-actin polymerization, impairs SRF transcriptional activity. Furthermore, blocking MRTF-A nuclear translocation hampers proliferation, self-renewal and invasion. Thus, in addition to sustaining malignant phenotypes, P-cadherin can also play a major role in the early stages of breast carcinogenesis by promoting a transient boost of MRTF-A-SRF signaling through actin regulation.

Polymerization power: effectors of actin polymerization as regulators of T lymphocyte migration through complex environments

During their life span, T cells are tasked with patrolling the body for potential pathogens. To do so, T cells migrate through numerous distinct anatomical sites and tissue environments with different biophysical characteristics. To migrate through these different environments, T cells use various motility strategies that rely on actin network remodeling to generate shape changes and mechanical forces. In this review, we initially discuss the migratory journey of T cells and then cover the actin polymerization effectors at play in T cells, and finally, we focus on the function of these effectors of actin cytoskeleton remodeling in mediating T-cell migration through diverse tissue environments. Specifically, we will discuss the current state of the field pertaining to our understanding of the roles in T-cell migration played by members of the three main families of actin polymerization machinery: the Arp2/3 complex; formin proteins; and Ena/VASP proteins.

Links of Cytoskeletal Integrity with Disease and Aging

Aging is a complex feature and involves loss of multiple functions and nonreversible phenotypes. However, several studies suggest it is possible to protect against aging and promote rejuvenation. Aging is associated with many factors, such as telomere shortening, DNA damage, mitochondrial dysfunction, and loss of homeostasis. The integrity of the cytoskeleton is associated with several cellular functions, such as migration, proliferation, degeneration, and mitochondrial bioenergy production, and chronic disorders, including neuronal degeneration and premature aging. Cytoskeletal integrity is closely related with several functional activities of cells, such as aging, proliferation, degeneration, and mitochondrial bioenergy production. Therefore, regulation of cytoskeletal integrity may be useful to elicit antiaging effects and to treat degenerative diseases, such as dementia. The actin cytoskeleton is dynamic because its assembly and disassembly change depending on the cellular status. Aged cells exhibit loss of cytoskeletal stability and decline in functional activities linked to longevity. Several studies reported that improvement of cytoskeletal stability can recover functional activities. In particular, microtubule stabilizers can be used to treat dementia. Furthermore, studies of the quality of aged oocytes and embryos revealed a relationship between cytoskeletal integrity and mitochondrial activity. This review summarizes the links of cytoskeletal properties with aging and degenerative diseases and how cytoskeletal integrity can be modulated to elicit antiaging and therapeutic effects.

Reforming the Barrier: The Role of Formins in Wound Repair

The restoration of an intact epidermal barrier after wound injury is the culmination of a highly complex and exquisitely regulated physiological process involving multiple cells and tissues, overlapping dynamic events and protein synthesis and regulation. Central to this process is the cytoskeleton, a system of intracellular proteins that are instrumental in regulating important processes involved in wound repair including chemotaxis, cytokinesis, proliferation, migration, and phagocytosis. One highly conserved family of cytoskeletal proteins that are emerging as major regulators of actin and microtubule nucleation, polymerization, and stabilization are the formins. The formin family includes 15 different proteins categorized into seven subfamilies based on three formin homology domains (FH1, FH2, and FH3). The formins themselves are regulated in different ways including autoinhibition, activation, and localization by a range of proteins, including Rho GTPases. Herein, we describe the roles and effects of the formin family of cytoskeletal proteins on the fundamental process of wound healing and highlight recent advances relating to their important functions, mechanisms, and regulation at the molecular and cellular levels.

VASP localization to lipid bilayers induces polymerization driven actin bundle formation

Actin bundles constitute important cytoskeleton structures and enable a scaffold for force transmission inside cells. Actin bundles are formed by proteins, with multiple F-actin binding domains cross-linking actin filaments to each other. Vasodilator-stimulated phosphoprotein (VASP) has mostly been reported as an actin elongator, but it has been shown to be a bundling protein as well and is found in bundled actin structures at filopodia and adhesion sites. Based on in vitro experiments, it remains unclear when and how VASP can act as an actin bundler or elongator. Here we demonstrate that VASP bound to membranes facilitates the formation of large actin bundles during polymerization. The alignment by polymerization requires the fluidity of the lipid bilayers. The mobility within the bilayer enables VASP to bind to filaments and capture and track growing barbed ends. VASP itself phase separates into a protein-enriched phase on the bilayer. This VASP-rich phase nucleates and accumulates at bundles during polymerization, which in turn leads to a reorganization of the underlying lipid bilayer. Our findings demonstrate that the nature of VASP localization is decisive for its function. The up-concentration based on VASP’s affinity to actin during polymerization enables it to simultaneously fulfill the function of an elongator and a bundler.

Ena/VASP Protein-Mediated Actin Polymerization Contributes to Naïve CD8+ T Cell Activation and Expansion by Promoting T Cell-APC Interactions In Vivo

Naïve T cell activation in secondary lymphoid organs such as lymph nodes (LNs) occurs upon recognition of cognate antigen presented by antigen presenting cells (APCs). T cell activation requires cytoskeleton rearrangement and sustained interactions with APCs. Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are a family of cytoskeletal effector proteins responsible for actin polymerization and are frequently found at the leading edge of motile cells. Ena/VASP proteins have been implicated in motility and adhesion in various cell types, but their role in primary T cell interstitial motility and activation has not been explored. Our goal was to determine the contribution of Ena/VASP proteins to T cell–APC interactions, T cell activation, and T cell expansion in vivo. Our results showed that naïve T cells from Ena/VASP-deficient mice have a significant reduction in antigen-specific T cell accumulation following Listeria monocytogenes infection. The kinetics of T cell expansion impairment were further confirmed in Ena/VASP-deficient T cells stimulated via dendritic cell immunization. To investigate the cause of this T cell expansion defect, we analyzed T cell–APC interactions in vivo by two-photon microscopy and observed fewer Ena/VASP-deficient naïve T cells interacting with APCs in LNs during priming. We also determined that Ena/VASP-deficient T cells formed conjugates with significantly less actin polymerization at the T cell–APC synapse, and that these conjugates were less stable than their WT counterparts. Finally, we found that Ena/VASP-deficient T cells have less LFA-1 polarized to the T cell–APC synapse. Thus, we conclude that Ena/VASP proteins contribute to T cell actin remodeling during T cell–APC interactions, which promotes the initiation of stable T cell conjugates during APC scanning. Therefore, Ena/VASP proteins are required for efficient activation and expansion of T cells in vivo.

Structure of Arp2/3 complex at a branched actin filament junction resolved by single-particle cryo-electron microscopy

SignificanceActin filament nucleation by Arp2/3 complex must be triggered by activators like WASP family proteins. Understanding how WASP proteins activate Arp2/3 complex has been a major challenge due to a lack of high-resolution structures of the complex in an activated state. We determined a high-resolution (∼3.9 Å) structure of the WASP-activated Arp2/3 complex at a branch junction and used biochemical, cell biological, and molecular dynamic simulations to understand the mechanism of WASP-mediated activation. This work shows in detail the contacts between the fully activated Arp2/3 complex, the nucleated daughter actin filament, and the mother actin filament and provides important insights into how conformational rearrangements in the Arp2/3 complex are stimulated during activation.

Cell cortex regulation by the planar cell polarity protein Prickle1

The planar cell polarity pathway regulates cell polarity, adhesion, and rearrangement. Its cytoplasmic core components Prickle (Pk) and Dishevelled (Dvl) often localize as dense puncta at cell membranes to form antagonizing complexes and establish cell asymmetry. In vertebrates, Pk and Dvl have been implicated in actomyosin cortex regulation, but the mechanism of how these proteins control cell mechanics is unclear. Here we demonstrate that in Xenopus prechordal mesoderm cells, diffusely distributed, cytoplasmic Pk1 up-regulates the F-actin content of the cortex. This counteracts cortex down-regulation by Dvl2. Both factors act upstream of casein kinase II to increase or decrease cortical tension. Thus, cortex modulation by Pk1 and Dvl2 is translated into mechanical force and affects cell migration and rearrangement during radial intercalation in the prechordal mesoderm. Pk1 also forms puncta and plaques, which are associated with localized depletion of cortical F-actin, suggesting opposite roles for diffuse and punctate Pk1.

N-WASP Attenuates Cell Proliferation and Migration through ERK2-Dependent Enhanced Expression of TXNIP

Simple Summary

Neural Wiskott–Aldrich Syndrome Protein (N-WASP) regulates actin cytoskeleton remodeling and can, it has been suggested, suppress several cancers. In this study, HSC-5 cells, a mammalian cell line with reduced N-WASP expression, were used to generate control cells and HSC-5 cells with increased N-WASP expression that is comparable to that of normal keratinocytes. The two cell lines were used to elucidate the regulation of cell proliferation and migration by N-WASP. Our findings suggest that N-WASP increases ERK2-dependent phosphorylation of FOXO1 and increases TXNIP expression, which reduces cell proliferation and migration. This study is the first to propose an antiproliferative role of N-WASP, which is mediated via ERK2, and it suggests new avenues for cancer therapeutic research and treatment.

Abstract

Neural Wiskott–Aldrich Syndrome Protein (N-WASP) regulates actin cytoskeleton remodeling. It has been known that reduced N-WASP expression in breast and colorectal cancers is associated with poor prognosis. Here, we found reduced N-WASP expression in squamous cell carcinoma (SCC) patient samples. The SCC cell line HSC-5 with reduced N-WASP expression was used to generate HSC-5^CN (control) and HSC-5^NW (N-WASP overexpression) cells. HSC-5^NW cells had reduced cell proliferation and migration compared to HSC-5^CN cells. HSC-5^NW cells had increased phospho-ERK2 (extracellular signal-regulated kinase 2), phosphorylated Forkhead box protein class O1 (FOXO1) and reduced nuclear FOXO1 staining compared to HSC-5^CN cells. Proteasome inhibition stabilized total FOXO1, however, not nuclear staining, suggesting that FOXO1 could be degraded in the cytoplasm. Inhibition of ERK2 enhanced nuclear FOXO1 levels and restored cell proliferation and migration of HSC-5^NW to those of HSC-5^CN cells, suggesting that ERK2 regulates FOXO1 activity. The expression of thioredoxin-interacting protein (TXNIP), a FOXO1 target that inhibits thioredoxin and glucose uptake, was higher in HSC-5^NW cells than in HSC-5^CN cells. Knockdown of TXNIP in HSC-5^NW cells restored cell proliferation and migration to those of HSC-5^CN cells. Thus, we propose that N-WASP regulates cell proliferation and migration via an N-WASP-ERK2-FOXO1-TXNIP pathway.

Interactions of Liberibacter Species with Their Psyllid Vectors: Molecular, Biological and Behavioural Mechanisms

Liberibacter is a group of plant pathogenic bacteria, transmitted by insect vectors, psyllids (Hemiptera: Psylloidea), and has emerged as one of the most devastating pathogens which have penetrated into many parts of the world over the last 20 years. The pathogens are known to cause plant diseases, such as Huanglongbing (citrus greening disease), Zebra chip disease, and carrot yellowing, etc., threatening some very important agricultural sectors, including citrus, potato and others. Candidatus Liberibacter asiaticus (CLas), the causative agent of citrus greening disease, is one of the most important pathogens of this group. This pathogen has infected most of the citrus trees in the US, Brazil and China, causing tremendous decline in citrus productivity, and, consequently, a severely negative impact on economic and personnel associated with citrus and related industries in these countries. Like other members in this group, CLas is transmitted by the Asian citrus psyllid (ACP, Diaphorina citri) in a persistent circulative manner. An additional important member of this group is Ca. L. solanacearum (CLso), which possesses nine haplotypes and infects a variety of crops, depending on the specific haplotype and the insect vector species. Ongoing pathogen control strategies, that are mainly based on use of chemical pesticides, lack the necessary credentials of being technically feasible, and environmentally safe. For this reason, strategies based on interference with Liberibacter vector transmission have been adopted as alternative strategies for the prevention of infection by these pathogens. A significant amount of research has been conducted during the last 10-15 years to understand the aspects of transmission of these bacterial species by their psyllid vectors. These research efforts span biological, ecological, behavioural and molecular aspects of Liberibacter–psyllid interactions, and will be reviewed in this manuscript. These attempts directed towards devising new means of disease control, endeavoured to explore alternative strategies, instead of relying on using chemicals for reducing the vector populations, which is the sole strategy currently employed and which has profound negative effects on human health, beneficial organisms and the environment.

Analysis tools for single-monomer measurements of self-assembly processes

Protein assembly plays an important role throughout all phyla of life, both physiologically and pathologically. In particular, aggregation and polymerization of proteins are key-strategies that regulate cellular function. In recent years, methods to experimentally study the assembly process on a single-molecule level have been developed. This progress concomitantly has triggered the question of how to analyze this type of single-filament data adequately and what experimental conditions are necessary to allow a meaningful interpretation of the analysis. Here, we developed two analysis methods for single-filament data: the visitation analysis and the average-rate analysis. We benchmarked and compared both approaches with the classic dwell-time-analysis frequently used to study microscopic association and dissociation rates. In particular, we tested the limitations of each analysis method along the lines of the signal-to-noise ratio, the sampling rate, and the labeling efficiency and bleaching rate of the fluorescent dyes used in single-molecule fluorescence experiments. Finally, we applied our newly developed methods to study the monomer assembly of actin at the single-molecule-level in the presence of the class II nucleator Cappuccino and the WH2 repeats of Spire. For Cappuccino, our data indicated fast elongation circumventing a nucleation phase whereas, for Spire, we found that the four WH2 motifs are not sufficient to promote de novo nucleation of actin.

ARPC1B binds WASP to control actin polymerization and curtail tonic signaling in B cells

Immune cells exhibit low-level, constitutive signaling at rest (tonic signaling). Such tonic signals are required for fundamental processes, including the survival of B lymphocytes, but when they are elevated by genetic or environmental causes, they can lead to autoimmunity. Events that control ongoing signal transduction are, therefore, tightly regulated by submembrane cytoskeletal polymers like F-actin. The actin-binding proteins that underpin the process, however, are poorly described. By investigating patients with ARPC1B deficiency, we report that ARPC1B-containing ARP2/3 complexes are stimulated by Wiskott Aldrich Syndrome protein (WASP) to nucleate the branched actin networks that control tonic signaling from the B cell receptor (BCR). Despite an upregulation of ARPC1A, ARPC1B-deficient cells were not capable of WASP-mediated nucleation by ARP2/3, and this caused the loss of WASP-dependent structures, including podosomes in macrophages and lamellipodia in B cells. In the B cell compartment, ARPC1B deficiency also led to weakening of the cortical F-actin cytoskeleton that normally curtails the diffusion of BCRs and ultimately resulted in increased tonic lipid signaling, oscillatory calcium release from the endoplasmic reticulum (ER), and phosphorylated Akt. These events contributed to skewing the threshold for B cell activation in response to microbial-associated molecular patterns (MAMPs). Thus, ARPC1B is critical for ARP2/3 complexes to control steady-state signaling of immune cells.

A teamwork promotion of formin-mediated actin nucleation by Bud6 and Aip5 in Saccharomyces cerevisiae

Actin nucleation is achieved by collaborative teamwork of actin nucleator factors (NFs) and nucleation-promoting factors (NPFs) into functional protein complexes. Selective inter- and intramolecular interactions between the nucleation complex constituents enable diverse modes of complex assembly in initiating actin polymerization on demand. Budding yeast has two formins, Bni1 and Bnr1, which are teamed up with different NPFs. However, the selective pairing between formin NFs and NPFs into the nucleation core for actin polymerization is not completely understood. By examining the functions and interactions of NPFs and NFs via biochemistry, genetics, and mathematical modeling approaches, we found that two NPFs, Aip5 and Bud6, showed joint teamwork effort with Bni1 and Bnr1, respectively, by interacting with the C-terminal intrinsically disordered region (IDR) of formin, in which two NPFs work together to promote formin-mediated actin nucleation. Although the C-terminal IDRs of Bni1 and Bnr1 are distinct in length, each formin IDR orchestrates the recruitment of Bud6 and Aip5 cooperatively by different positioning strategies to form a functional complex. Our study demonstrated the dynamic assembly of the actin nucleation complex by recruiting multiple partners in budding yeast, which may be a general feature for effective actin nucleation by formins.

Nucleation limits the lengths of actin filaments assembled by formin

Formins stimulate actin polymerization by promoting both filament nucleation and elongation. Because nucleation and elongation draw upon a common pool of actin monomers, the rate at which each reaction proceeds influences the other. This interdependent mechanism determines the number of filaments assembled over the course of a polymerization reaction, as well as their equilibrium lengths. In this study, we used kinetic modeling and in vitro polymerization reactions to dissect the contributions of filament nucleation and elongation to the process of formin-mediated actin assembly. We found that the rates of nucleation and elongation evolve over the course of a polymerization reaction. The period over which each process occurs is a key determinant of the total number of filaments that are assembled, as well as their average lengths at equilibrium. Inclusion of formin in polymerization reactions speeds filament nucleation, thus increasing the number and shortening the lengths of filaments that are assembled over the course of the reaction. Modulation of the elongation rate produces modest changes in the equilibrium lengths of formin-bound filaments. However, the dependence of filament length on the elongation rate is limited by the number of filament ends generated via formin's nucleation activity. Sustained elongation of small numbers of formin-bound filaments, therefore, requires inhibition of nucleation via monomer sequestration and a low concentration of activated formin. Our results underscore the mechanistic advantage for keeping formin's nucleation efficiency relatively low in cells, where unregulated actin assembly would produce deleterious effects on cytoskeletal dynamics. Under these conditions, differences in the elongation rates mediated by formin isoforms are most likely to impact the kinetics of actin assembly.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

The WAVE complex associates with sites of saddle membrane curvature

How local interactions of actin regulators yield large-scale organization of cell shape and movement is not well understood. Here we investigate how the WAVE complex organizes sheet-like lamellipodia. Using super-resolution microscopy, we find that the WAVE complex forms actin-independent 230-nm-wide rings that localize to regions of saddle membrane curvature. This pattern of enrichment could explain several emergent cell behaviors, such as expanding and self-straightening lamellipodia and the ability of endothelial cells to recognize and seal transcellular holes. The WAVE complex recruits IRSp53 to sites of saddle curvature but does not depend on IRSp53 for its own localization. Although the WAVE complex stimulates actin nucleation via the Arp2/3 complex, sheet-like protrusions are still observed in ARP2-null, but not WAVE complex-null, cells. Therefore, the WAVE complex has additional roles in cell morphogenesis beyond Arp2/3 complex activation. Our work defines organizing principles of the WAVE complex lamellipodial template and suggests how feedback between cell shape and actin regulators instructs cell morphogenesis.

Functional interdependence of the actin nucleator Cobl and Cobl-like in dendritic arbor development

Local actin filament formation is indispensable for development of the dendritic arbor of neurons. We show that, surprisingly, the action of single actin filament-promoting factors was insufficient for powering dendritogenesis. Instead, this required the actin nucleator Cobl and its only evolutionary distant ancestor Cobl-like acting interdependently. This coordination between Cobl-like and Cobl was achieved by physical linkage by syndapins. Syndapin I formed nanodomains at convex plasma membrane areas at the base of protrusive structures and interacted with three motifs in Cobl-like, one of which was Ca²⁺/calmodulin-regulated. Consistently, syndapin I, Cobl-like’s newly identified N terminal calmodulin-binding site and the single Ca²⁺/calmodulin-responsive syndapin-binding motif all were critical for Cobl-like’s functions. In dendritic arbor development, local Ca²⁺/CaM-controlled actin dynamics thus relies on regulated and physically coordinated interactions of different F-actin formation-promoting factors and only together they have the power to bring about the sophisticated neuronal morphologies required for neuronal network formation in mammals.

Phosphorylation of the WH2 domain in yeast Las17/WASP regulates G-actin binding and protein function during endocytosis

Actin nucleation is the key rate limiting step in the process of actin polymerization, and tight regulation of this process is critical to ensure actin filaments form only at specific times and at defined regions of the cell. WH2 domains are short sequence motifs found in many different actin binding proteins including WASP family proteins which regulate the actin nucleating complex Arp2/3. In this study we reveal a phosphorylation site, Serine 554, within the WH2 domain of the yeast WASP homologue Las17. Both phosphorylation and a phospho-mimetic mutation reduce actin monomer binding affinity while an alanine mutation, generated to mimic the non-phosphorylated state, increases actin binding affinity. The effect of these mutations on the Las17-dependent process of endocytosis in vivo was analysed and leads us to propose that switching of Las17 phosphorylation states may allow progression through distinct phases of endocytosis from site assembly through to the final scission stage. While the study is focused on Las17, the sole WASP family protein in yeast, our results have broad implications for our understanding of how a key residue in this conserved motif can underpin the many different actin regulatory roles with which WH2 domains have been associated.

The Role of Protein Arginine Methylation as Post-Translational Modification on Actin Cytoskeletal Components in Neuronal Structure and Function

The brain encompasses a complex network of neurons with exceptionally elaborated morphologies of their axonal (signal-sending) and dendritic (signal-receiving) parts. De novo actin filament formation is one of the major driving and steering forces for the development and plasticity of the neuronal arbor. Actin filament assembly and dynamics thus require tight temporal and spatial control. Such control is particularly effective at the level of regulating actin nucleation-promoting factors, as these are key components for filament formation. Arginine methylation represents an important post-translational regulatory mechanism that had previously been mainly associated with controlling nuclear processes. We will review and discuss emerging evidence from inhibitor studies and loss-of-function models for protein arginine methyltransferases (PRMTs), both in cells and whole organisms, that unveil that protein arginine methylation mediated by PRMTs represents an important regulatory mechanism in neuritic arbor formation, as well as in dendritic spine induction, maturation and plasticity. Recent results furthermore demonstrated that arginine methylation regulates actin cytosolic cytoskeletal components not only as indirect targets through additional signaling cascades, but can also directly control an actin nucleation-promoting factor shaping neuronal cells—a key process for the formation of neuronal networks in vertebrate brains.

Involvement of actin cytoskeletal modifications in the inhibition of triple-negative breast cancer growth and metastasis by nimbolide

Triple-negative breast cancers (TNBCs) are aggressive cancers, which currently do not have effective treatment options. Migration and establishment of metastatic colonies require dynamic cytoskeletal modifications characterized by polymerization and depolymerization of actin. Studies have demonstrated a direct molecular link between the integrin-focal adhesion kinase (FAK) pathway and cytoskeletal modifications. Nimbolide, a major bioactive compound present in neem leaves, shows promising anti-cancer effect on various cancers. In this study, we have demonstrated the growth and metastasis inhibitory potential of nimbolide on TNBC cells. Nimbolide inhibited cell proliferation, migratory, and invasive abilities of TNBC cells and also changed the shape of MDA-MB-231 cells, which is correlated with cytoskeletal changes including actin depolymerization. Furthermore, analysis revealed that integrins αV and β3, ILK, FAK, and PAK levels were downregulated by nimbolide. Even in cells where Rac1/Cdc42 was constitutively activated, nimbolide inhibited the formation of filopodial structures. Immunofluorescence analysis of phosphorylated p21 activated kinase (pPAK) showed reduced expression in nimbolide-treated cells. Nimbolide significantly reduced the metastatic colony formation in lung, liver, and brain of athymic nude mice. In conclusion, our data demonstrate that nimbolide inhibits TNBC by altering the integrin and FAK signaling pathway.

Developmental axon regrowth and primary neuron sprouting utilize distinct actin elongation factors

Intrinsic neurite growth potential is a key determinant of neuronal regeneration efficiency following injury. The stereotypical remodeling of Drosophila γ-neurons includes developmental regrowth of pruned axons to form adult specific connections, thereby offering a unique system to uncover growth potential regulators. Motivated by the dynamic expression in remodeling γ-neurons, we focus here on the role of actin elongation factors as potential regulators of developmental axon regrowth. We found that regrowth in vivo requires the actin elongation factors Ena and profilin, but not the formins that are expressed in γ-neurons. In contrast, primary γ-neuron sprouting in vitro requires profilin and the formin DAAM, but not Ena. Furthermore, we demonstrate that DAAM can compensate for the loss of Ena in vivo. Similarly, DAAM mutants express invariably high levels of Ena in vitro. Thus, we show that different linear actin elongation factors function in distinct contexts even within the same cell type and that they can partially compensate for each other.

Arp2/3-dependent mechanical control of morphogenetic robustness in an inherently challenging environment

Epithelial sheets undergo highly reproducible remodeling to shape organs. This stereotyped morphogenesis depends on a well-defined sequence of events leading to the regionalized expression of developmental patterning genes that finally triggers downstream mechanical forces to drive tissue remodeling at a pre-defined position. However, how tissue mechanics controls morphogenetic robustness when challenged by intrinsic perturbations in close proximity has never been addressed.

Using Drosophila developing leg, we show that a bias in force propagation ensures stereotyped morphogenesis despite the presence of mechanical noise in the environment. We found that knockdown of the Arp2/3 complex member Arpc5 specifically affects fold directionality while altering neither the developmental nor the force generation patterns. By combining in silico modeling, biophysical tools, and ad hoc genetic tools, our data reveal that junctional myosin II planar polarity favors long-range force channeling and ensures folding robustness, avoiding force scattering and thus isolating the fold domain from surrounding mechanical perturbations.

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Drosophila developing leg folding is extremely robust

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Fold orientation becomes variable in Arp2/3 knockdown condition

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Arp2/3 controls junctional myosin II planar polarity

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Myosin II planar polarity ensures fold robustness through force channeling

Induced Arp2/3 Complex Depletion Increases FMNL2/3 Formin Expression and Filopodia Formation

The Arp2/3 complex generates branched actin filament networks operating in cell edge protrusion and vesicle trafficking. Here we employ a conditional knockout mouse model permitting tissue- or cell-type specific deletion of the murine Actr3 gene (encoding Arp3). A functional Actr3 gene appeared essential for fibroblast viability and growth. Thus, we developed cell lines for exploring the consequences of acute, tamoxifen-induced Actr3 deletion causing near-complete loss of functional Arp2/3 complex expression as well as abolished lamellipodia formation and membrane ruffling, as expected. Interestingly, Arp3-depleted cells displayed enhanced rather than reduced cell spreading, employing numerous filopodia, and showed little defects in the rates of random cell migration. However, both exploration of new space by individual cells and collective migration were clearly compromised by the incapability to efficiently maintain directionality of migration, while the principal ability to chemotax was only moderately affected. Examination of actin remodeling at the cell periphery revealed reduced actin turnover rates in Arp2/3-deficient cells, clearly deviating from previous sequestration approaches. Most surprisingly, induced removal of Arp2/3 complexes reproducibly increased FMNL formin expression, which correlated with the explosive induction of filopodia formation. Our results thus highlight both direct and indirect effects of acute Arp2/3 complex removal on actin cytoskeleton regulation.

Intra-bundle contractions enable extensile properties of active actin networks

The cellular cortex is a dynamic and contractile actomyosin network modulated by actin-binding proteins. We reconstituted a minimal cortex adhered to a model cell membrane mimicking two processes mediated by the motor protein myosin: contractility and high turnover of actin monomers. Myosin reorganized these networks by extensile intra‑bundle contractions leading to an altered growth mechanism. Hereby, stress within tethered bundles induced nicking of filaments followed by repair via incorporation of free monomers. This mechanism was able to break the symmetry of the previously disordered network resulting in the generation of extensile clusters, reminiscent of structures found within cells.

The Actin Cytoskeleton Mediates Transmission of "Candidatus Liberibacter solanacearum" by the Carrot Psyllid

Several vector-borne plant pathogens have evolved mechanisms to exploit and to hijack vector host cellular, molecular, and defense mechanisms for their transmission. In the past few years, Liberibacter species, which are transmitted by several psyllid vectors, have become an economically important group of pathogens that have devastated the citrus industry and caused tremendous losses to many other important crops worldwide. The molecular mechanisms underlying the interactions of Liberibacter species with their psyllid vectors are poorly studied. "Candidatus Liberibacter solanacearum," which is associated with important vegetable diseases, is transmitted by the carrot psyllid Bactericera trigonica in a persistent manner. Here, we elucidated the role of the B. trigonica Arp2/3 protein complex, which plays a major role in regulation of the actin cytoskeleton, in the transmission of "Ca Liberibacter solanacearum." "Ca Liberibacter solanacearum" colocalized with ArpC2, a key protein in this complex, and this colocalization was strongly associated with actin filaments. Silencing of the psyllid ArpC2 disrupted the colocalization and the dynamics of F-actin. Silencing of RhoGAP21 and Cdc42, which act in the signaling cascade leading to upregulation of Arp2/3 and F-actin bundling, showed similar results. On the other hand, silencing of ArpC5, another component of the complex, did not induce any significant effects on F-actin formation. Finally, ArpC2 silencing caused a 73.4% reduction in "Ca Liberibacter solanacearum" transmission by psyllids, strongly suggesting that transmission of "Ca Liberibacter solanacearum" by B. trigonica is cytoskeleton dependent and "Ca Liberibacter solanacearum" interacts with ArpC2 to exploit the intracellular actin nucleation process for transmission. Targeting this unique interaction could lead to the development of a novel strategy for the management of Liberibacter-associated diseases.IMPORTANCE Plant diseases caused by vector-borne pathogens are responsible for tremendous losses and threaten some of the most important agricultural crops. A good example is the citrus greening disease, which is caused by bacteria of the genus Liberibacter and is transmitted by psyllids; it has devastated the citrus industry in the United States, China, and Brazil. Here, we show that psyllid-transmitted "Candidatus Liberibacter solanacearum" employs the actin cytoskeleton of psyllid gut cells, specifically the ArpC2 protein in the Arp2/3 complex of this system, for movement and transmission in the vector. Silencing of ArpC2 dramatically influenced the interaction of "Ca Liberibacter solanacearum" with the cytoskeleton and decreased the bacterial transmission to plants. This system could be targeted to develop a novel approach for the control of Liberibacter-associated diseases.

Unravelling the distinct contribution of cell shape changes and cell intercalation to tissue morphogenesis: the case of the Drosophila trachea

Intercalation allows cells to exchange positions in a spatially oriented manner in an array of diverse processes, spanning convergent extension in embryonic gastrulation to the formation of tubular organs. However, given the co-occurrence of cell intercalation and changes in cell shape, it is sometimes difficult to ascertain their respective contribution to morphogenesis. A well-established model to analyse intercalation, particularly in tubular organs, is the Drosophila tracheal system. There, fibroblast growth factor (FGF) signalling at the tip of the dorsal branches generates a ‘pulling’ force believed to promote cell elongation and cell intercalation, which account for the final branch extension. Here, we used a variety of experimental conditions to study the contribution of cell elongation and cell intercalation to morphogenesis and analysed their mutual requirements. We provide evidence that cell intercalation does not require cell elongation and vice versa. We also show that the two cell behaviours are controlled by independent but simultaneous mechanisms, and that cell elongation is sufficient to account for full extension of the dorsal branch, while cell intercalation has a specific role in setting the diameter of this structure. Thus, rather than viewing changes in cell shape and cell intercalation as just redundant events that add robustness to a given morphogenetic process, we find that they can also act by contributing to different features of tissue architecture.

Lysine acetylation of cytoskeletal proteins: Emergence of an actin code

Reversible lysine acetylation of nuclear proteins such as histones is a long-established important regulatory mechanism for chromatin remodeling and transcription. In the cytoplasm, acetylation of a number of cytoskeletal proteins, including tubulin, cortactin, and the formin mDia2, regulates both cytoskeletal assembly and stability. More recently, acetylation of actin itself was revealed to regulate cytoplasmic actin polymerization through the formin INF2, with downstream effects on ER-to-mitochondrial calcium transfer, mitochondrial fission, and vesicle transport. This finding raises the possibility that actin acetylation, along with other post-translational modifications to actin, might constitute an "actin code," similar to the "histone code" or "tubulin code," controlling functional shifts to these central cellular proteins. Given the multiple roles of actin in nuclear functions, its modifications might also have important roles in gene expression.

Myosin 1b flattens and prunes branched actin filaments

Motile and morphological cellular processes require a spatially and temporally coordinated branched actin network that is controlled by the activity of various regulatory proteins, including the Arp2/3 complex, profilin, cofilin and tropomyosin. We have previously reported that myosin 1b regulates the density of the actin network in the growth cone. Here, by performing in vitro F-actin gliding assays and total internal reflection fluorescence (TIRF) microscopy, we show that this molecular motor flattens (reduces the branch angle) in the Arp2/3-dependent actin branches, resulting in them breaking, and reduces the probability of new branches forming. This experiment reveals that myosin 1b can produce force sufficient enough to break up the Arp2/3-mediated actin junction. Together with the former in vivo studies, this work emphasizes the essential role played by myosins in the architecture and dynamics of actin networks in different cellular regions.This article has an associated First Person interview with the first author of the paper.

Formin-like 1 mediates effector T cell trafficking to inflammatory sites to enable T cell-mediated autoimmunity

Lymphocyte migration is essential for the function of the adaptive immune system, and regulation of T cell entry into tissues is an effective therapy in autoimmune diseases. Little is known about the specific role of cytoskeletal effectors that mediate mechanical forces and morphological changes essential for migration in complex environments. We developed a new Formin-like-1 (FMNL1) knock-out mouse model and determined that the cytoskeletal effector FMNL1 is selectively required for effector T cell trafficking to inflamed tissues, without affecting naïve T cell entry into secondary lymphoid organs. Here, we identify a FMNL1-dependent mechanism of actin polymerization at the back of the cell that enables migration of the rigid lymphocyte nucleus through restrictive barriers. Furthermore, FMNL1-deficiency impairs the ability of self-reactive effector T cells to induce autoimmune disease. Overall, our data suggest that FMNL1 may be a potential therapeutic target to specifically modulate T cell trafficking to inflammatory sites.

EhC2B, a C2 domain-containing protein, promotes erythrophagocytosis in Entamoeba histolytica via actin nucleation

Remodelling of the actin cytoskeleton in response to external stimuli is obligatory for many cellular processes in the amoebic cell. A rapid and local rearrangement of the actin cytoskeleton is required for the development of the cellular protrusions during phagocytosis, trogocytosis, migration, and invasion. Here, we demonstrated that EhC2B, a C2 domain-containing protein, is an actin modulator. EhC2B was first identified as an effector of EhRab21 from E. histolytica. In vitro interaction studies including GST pull-down, fluorescence-based assay and ITC also corroborated with our observation. In the amoebic trophozoites, EhC2B accumulates at the pseudopods and the tips of phagocytic cups. FRAP based studies confirmed the recruitment and dynamics of EhC2B at the phagocytic cup. Moreover, we have shown the role of EhC2B in erythrophagocytosis. It is well known that calcium-dependent signal transduction is essential for the cytoskeletal dynamics during phagocytosis in the amoebic parasite. Using liposome pelleting assay, we demonstrated that EhC2B preferentially binds to the phosphatidylserine in the presence of calcium. The EhC2B mutants defective in calcium or lipid-binding failed to localise beneath the plasma membrane. The cells overexpressing these mutants have also shown a significant reduction in erythrophagocytosis. The role of EhC2B in erythrophagocytosis and pseudopod formation was also validated by siRNA-based gene knockdown approach. Finally, with the help of in vitro nucleation assay using fluorescence spectroscopy and total internal reflection fluorescence microscopy, we have established that EhC2B is an actin nucleator. Collectively, based on the results from the study, we propose that EhC2B acts like a molecular bridge which promotes membrane deformation via its actin nucleation activity during the progression of the phagocytic cup in a calcium-dependent manner.

Formins specify membrane patterns generated by propagating actin waves

Circular actin waves separate two distinct areas on the substrate-attached cell surface from each other: an external area from an inner territory that is circumscribed by the wave. These areas differ in composition of actin-associated proteins and of phosphoinositides in the membrane. At the propagating wave, one area is converted into the other. By photo-conversion of Eos-actin and analysis of actin network structures we show that both in the inner territory and the external area the actin network is subject to continuous turnover. To address the question of whether areas in the wave pattern are specified by particular actin polymerizing machines, we locate five members of the formin family to specific regions of the wave landscape using TIRF microscopy and constitutively active formin constructs tagged with fluorescent protein. Formin ForB favors the actin wave and ForG the inner territory, whereas ForA, ForE, and ForH are more strongly recruited to the external area. Fluctuations of membrane binding peculiar to ForB indicate transient states in the specification of membrane domains before differentiation into ForB decorated and depleted ones. Annihilation of the patterns by 1 µM of the formin inhibitor SMIFH2 supports the implication of formins in their generation.

Cytoskeleton and Nucleotide Signaling in Glioma C6 Cells

This chapter describes signaling pathways, stimulated by the P2Y₂ nucleotide receptor (P2Y₂R), that regulate cellular processes dependent on actin cytoskeleton dynamics in glioma C6 cells. P2Y₂R coupled with G-proteins, in response to ATP or UTP, regulates the level of iphosphatidylinositol-4,5-bisphosphate (PIP₂) which modulates a variety of actin binding proteins and is involved in calcium response and activates Rac1 and RhoA proteins. The RhoA/ROCK signaling pathway plays an important role in contractile force generation needed for the assembly of stress fibers, focal adhesions and for tail retraction during cell migration. Blocking of this pathway by a specific Rho-kinase inhibitor induces changes in F-actin organization and cell shape and decreases the level of phosphorylated myosin II and cofilin. In glioma C6 cells these changes are reversed after UTP stimulation of P2Y₂R. Signaling pathways responsible for this compensation are calcium signaling which regulates MLC kinase activation via calmodulin, and the Rac1/PAK/LIMK cascade. Stimulation of the Rac1 mediated pathway via G_o proteins needs additional interaction between α_vβ₅ integrins and P2Y₂Rs. Calcium free medium, or growing of the cells in suspension, prevents Gα_o activation by P2Y₂ receptors. Rac1 activation is necessary for cofilin phosphorylation as well as integrin activation needed for focal complexes formation and stabilization of lamellipodium. Inhibition of positive Rac1 regulation prevents glioma C6 cells from recovery of control cell like morphology.

Distinct molecular mechanisms control levels of synaptic F-actin

Polymerization of filamentous (F)-actin at the neuronal synapse plays an important role in neuronal function. However, the regulatory mechanisms controlling the levels of synaptic actin remain incompletely understood. Here, I used established pharmacological blockers to acutely disrupt the function of actin polymerization machinery, then quantified their effect on synaptic F-actin levels. Synaptic F-actin was modestly decreased by inhibition of Arp2/3-dependent actin branching. Blockade of formin-dependent actin elongation resulted in an Arp2/3-dependent increase in synaptic actin that could be mimicked by limited actin depolymerization. Limited actin depolymerization was also sufficient to reverse a decrease in synaptic F-actin caused by prolonged blockade of synaptic NMDA-type glutamate receptors. These results suggest that interplay between different actin polymerization pathways may regulate synaptic actin dynamics.

Profilin-Mediated Actin Allocation Regulates the Growth of Epithelial Microvilli

Transporting epithelial cells, like those that line the intestinal tract, are specialized for solute processing and uptake. One defining feature is the brush border, an array of microvilli that serves to amplify apical membrane surface area and increase functional capacity. During differentiation, upon exit from stem-cell-containing crypts, enterocytes build thousands of microvilli, each supported by a parallel bundle of actin filaments several microns in length. Given the high concentration of actin residing in mature brush borders, we sought to determine whether enterocytes were resource (i.e., actin monomer) limited in assembling this domain. To examine this possibility, we inhibited Arp2/3, the ubiquitous branched actin nucleator, to increase G-actin availability during brush border assembly. In native intestinal tissues, Arp2/3 inhibition led to increased microvilli length on the surface of crypt, but not villus, enterocytes. In a cell culture model of brush border assembly, Arp2/3 inhibition accelerated the growth and increased the length of microvilli; it also led to a redistribution of F-actin from cortical lateral networks into the brush border. Effects on brush border growth were rescued by treatment with the G-actin sequestering drug, latrunculin A. G-actin binding protein, profilin-1, colocalized in the terminal web with G-actin, and knockdown of this factor compromised brush border growth in a concentration-dependent manner. Finally, the acceleration in brush border assembly induced by Arp2/3 inhibition was abrogated by profilin-1 knockdown. Thus, brush border assembly is limited by G-actin availability, and profilin-1 directs unallocated actin monomers into microvillar core bundles during enterocyte differentiation.

Membrane curvature underlies actin reorganization in response to nanoscale surface topography

Surface topography profoundly influences cell adhesion, differentiation, and stem cell fate control. Numerous studies using a variety of materials demonstrate that nanoscale topographies change the intracellular organization of actin cytoskeleton and therefore a broad range of cellular dynamics in live cells. However, the underlying molecular mechanism is not well understood, leaving why actin cytoskeleton responds to topographical features unexplained and therefore preventing researchers from predicting optimal topographic features for desired cell behavior. Here we demonstrate that topography-induced membrane curvature plays a crucial role in modulating intracellular actin organization. By inducing precisely controlled membrane curvatures using engineered vertical nanostructures as topographies, we find that actin fibers form at the sites of nanostructures in a curvature-dependent manner with an upper limit for the diameter of curvature at ∼400 nm. Nanotopography-induced actin fibers are branched actin nucleated by the Arp2/3 complex and are mediated by a curvature-sensing protein FBP17. Our study reveals that the formation of nanotopography-induced actin fibers drastically reduces the amount of stress fibers and mature focal adhesions to result in the reorganization of actin cytoskeleton in the entire cell. These findings establish the membrane curvature as a key linkage between surface topography and topography-induced cell signaling and behavior.

The tomato Arp2/3 complex is required for resistance to the powdery mildew fungus Oidium neolycopersici

The actin-related protein 2/3 complex (Arp2/3 complex), a key regulator of actin cytoskeletal dynamics, has been linked to multiple cellular processes, including those associated with response to stress. Herein, the Solanum habrochaites ARPC3 gene, encoding a subunit protein of the Arp2/3 complex, was identified and characterized. ShARPC3 encodes a 174-amino acid protein possessing a conserved P21-Arc domain. Silencing of ShARPC3 resulted in enhanced susceptibility to the powdery mildew pathogen Oidium neolycopersici (On-Lz), demonstrating a role for ShARPC3 in defence signalling. Interestingly, a loss of ShARPC3 coincided with enhanced susceptibility to On-Lz, a process that we hypothesize is the result of a block in the activity of SA-mediated defence signalling. Conversely, overexpression of ShARPC3 in Arabidopsis thaliana, followed by inoculation with On-Lz, showed enhanced resistance, including the rapid induction of hypersensitive cell death and the generation of reactive oxygen. Heterologous expression of ShARPC3 in the arc18 mutant of Saccharomyces cerevisiae (i.e., ∆arc18) resulted in complementation of stress-induced phenotypes, including high-temperature tolerance. Taken together, these data support a role for ShARPC3 in tomato through positive regulation of plant immunity in response to O. neolycopersici pathogenesis.

Microridges are apical epithelial projections formed of F-actin networks that organize the glycan layer

Apical projections are integral functional units of epithelial cells. Microvilli and stereocilia are cylindrical apical projections that are formed of bundled actin. Microridges on the other hand, extend laterally, forming labyrinthine patterns on surfaces of various kinds of squamous epithelial cells. So far, the structural organization and functions of microridges have remained elusive. We have analyzed microridges on zebrafish epidermal cells using confocal and electron microscopy methods including electron tomography, to show that microridges are formed of F-actin networks and require the function of the Arp2/3 complex for their maintenance. During development, microridges begin as F-actin punctae showing signatures of branching and requiring an active Arp2/3 complex. Using inhibitors of actin polymerization and the Arp2/3 complex, we show that microridges organize the surface glycan layer. Our analyses have unraveled the F-actin organization supporting the most abundant and evolutionarily conserved apical projection, which functions in glycan organization.