pH control of actin polymerization by cofilin

Cofilin-mediated actin filament network flexibility facilitates 2D to 3D actomyosin shape change

The organization of actin filaments (F-actin) into crosslinked networks determines the transmission of mechanical stresses within the cytoskeleton and subsequent changes in cell and tissue shape. Principally mediated by proteins such as α-actinin, F-actin crosslinking increases both network connectivity and rigidity, thereby facilitating stress transmission at low crosslinking yet attenuating transmission at high crosslinker concentration. Here, we engineer a two-dimensional model of the actomyosin cytoskeleton, in which myosin-induced mechanical stresses are controlled by light. We alter the extent of F-actin crosslinking by the introduction of oligomerized cofilin. At pH 6.5, F-actin severing by cofilin is weak, but cofilin bundles and crosslinks filaments. Given its effect of lowering the F-actin bending stiffness, cofilin- crosslinked networks are significantly more flexible and softer in bending than networks crosslinked by α-actinin. Thus, upon local activation of myosin-induced contractile stress, the network bends out-of-plane in contrast to the in-plane compression as observed with networks crosslinked by α-actinin. Here, we demonstrate that local effects on filament mechanics by cofilin introduces novel large-scale network material properties that enable the sculpting of complex shapes in the cell cytoskeleton.

Troponin and a Myopathy-Linked Mutation in TPM3 Inhibit Cofilin-2-Induced Thin Filament Depolymerization

Uniform actin filament length is required for synchronized contraction of skeletal muscle. In myopathies linked to mutations in tropomyosin (Tpm) genes, irregular thin filaments are a common feature, which may result from defects in length maintenance mechanisms. The current work investigated the effects of the myopathy-causing p.R91C variant in Tpm3.12, a tropomyosin isoform expressed in slow-twitch muscle fibers, on the regulation of actin severing and depolymerization by cofilin-2. The affinity of cofilin-2 for F-actin was not significantly changed by either Tpm3.12 or Tpm3.12-R91C, though it increased two-fold in the presence of troponin (without Ca2+). Saturation of the filament with cofilin-2 removed both Tpm variants from the filament, although Tpm3.12-R91C was more resistant. In the presence of troponin (±Ca2+), Tpm remained on the filament, even at high cofilin-2 concentrations. Both Tpm3.12 variants inhibited filament severing and depolymerization by cofilin-2. However, the inhibition was more efficient in the presence of Tpm3.12-R91C, indicating that the pathogenic variant impaired cofilin-2-dependent actin filament turnover. Troponin (±Ca2+) further inhibited but did not completely stop cofilin-2-dependent actin severing and depolymerization.

Differential sensitivity of ADF isovariants to a pH gradient promotes pollen tube growth

The actin cytoskeleton is one of the targets of the pH gradient in tip-growing cells, but how cytosolic pH regulates the actin cytoskeleton remains largely unknown. We here demonstrate that Arabidopsis ADF7 and ADF10 function optimally at different pH levels when disassembling actin filaments. This differential pH sensitivity allows ADF7 and ADF10 to respond to the cytosolic pH gradient to regulate actin dynamics in pollen tubes. ADF7 is an unusual actin-depolymerizing factor with a low optimum pH in in vitro actin depolymerization assays. ADF7 plays a dominant role in promoting actin turnover at the pollen tube apex. ADF10 has a typically high optimum pH in in vitro assays and plays a dominant role in regulating the turnover and organization of subapical actin filaments. Thus, functional specification and cooperation of ADF isovariants with different pH sensitivities enable the coordination of the actin cytoskeleton with the cytosolic pH gradient to support pollen tube growth.

The influence of polarized membrane ion carriers and extracellular electrical/pH gradients on cell ionic homeostasis and locomotion

Anisotropic environmental signals or polarized membrane ion/solute carriers can generate spatially-varying intracellular gradients, leading to polarized cell dynamics. For example, directional migration of neutrophils, galvanotaxis of glioblastoma, and water flux in kidney cells, all result from the polarized distribution of membrane ion carriers and other intracellular components. The underlying physical mechanisms behind how polarized ion carriers interact with environmental signals are not well studied. Here, we use a physiologically-relevant, physics-based mathematical model to reveal how ion carriers generate intracellular ionic and voltage gradients. The model is able to discern the contribution of individual ion carriers to the intracellular pH gradient, electric potential, and water flux. We discover that an extracellular pH gradient leads to an intracellular pH gradient via chloride-bicarbonate exchangers, whereas an extracellular electric field leads to an intracellular electric potential gradient via passive potassium channels. In addition, the mechanical-biochemical coupling can modulate actin distribution and flow, and create biphasic dependence of the cell speed on water flux. Moreover, we find that F-actin interaction with NHE alone can generate cell movement, even when other ion carriers are not polarized. Taken together, the model shows the importance of cell ion dynamics in modulating cell migration and cytoskeletal dynamics.

Activation of actin-depolymerizing factor by CDPK16-mediated phosphorylation promotes actin turnover in Arabidopsis pollen tubes

As the stimulus-responsive mediator of actin dynamics, actin-depolymerizing factor (ADF)/cofilin is subject to tight regulation. It is well known that kinase-mediated phosphorylation inactivates ADF/cofilin. Here, however, we found that the activity of Arabidopsis ADF7 is enhanced by CDPK16-mediated phosphorylation. We found that CDPK16 interacts with ADF7 both in vitro and in vivo, and it enhances ADF7-mediated actin depolymerization and severing in vitro in a calcium-dependent manner. Accordingly, the rate of actin turnover is reduced in cdpk16 pollen and the amount of actin filaments increases significantly at the tip of cdpk16 pollen tubes. CDPK16 phosphorylates ADF7 at Serine128 both in vitro and in vivo, and the phospho-mimetic mutant ADF7S128D has enhanced actin-depolymerizing activity compared to ADF7. Strikingly, we found that failure in the phosphorylation of ADF7 at Ser128 impairs its function in promoting actin turnover in vivo, which suggests that this phospho-regulation mechanism is biologically significant. Thus, we reveal that CDPK16-mediated phosphorylation up-regulates ADF7 to promote actin turnover in pollen.

Loss of mTORC2-induced metabolic reprogramming in monocytes uncouples migration and maturation from production of proinflammatory mediators

Monocyte migration to the sites of inflammation and maturation into macrophages are key steps for their immune effector function. Here, we show that mechanistic target of rapamycin complex 2 (mTORC2)-dependent Akt activation is instrumental for metabolic reprogramming at the early stages of macrophage-mediated immunity. Despite an increased production of proinflammatory mediators, monocytes lacking expression of the mTORC2 component Rictor fail to efficiently migrate to inflammatory sites and fully mature into macrophages, resulting in reduced inflammatory responses in vivo. The mTORC2-dependent phosphorylation of Akt is instrumental for the enhancement of glycolysis and mitochondrial respiration, required to sustain monocyte maturation and motility. These observations are discussed in the context of therapeutic strategies aimed at selective inhibition of mTORC2 activity.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

The multiple functions of actin in apicomplexan parasites

The cytoskeletal protein actin is highly abundant and conserved in eukaryotic cells. It occurs in two different states- the globular (G-actin) form, which can polymerise into the filamentous (F-actin) form, fulfilling various critical functions including cytokinesis, cargo trafficking and cellular motility. In higher eukaryotes, there are several actin isoforms with nearly identical amino acid sequences. Despite the high level of amino acid identity, they display regulated expression patterns and unique non-redundant roles. The number of actin isoforms together with conserved sequences may reflect the selective pressure exerted by scores of actin binding proteins (ABPs) in higher eukaryotes. In contrast, in many protozoans such as apicomplexan parasites which possess only a few ABPs, the regulatory control of actin and its multiple functions are still obscure. Here, we provide a summary of the regulation and biological functions of actin in higher eukaryotes and compare it with the current knowledge in apicomplexans. We discuss future experiments that will help us understand the multiple, critical roles of this fascinating system in apicomplexans.

The quest for the central players governing pollen tube growth and guidance

Recent insights into the mechanism of pollen tube growth and guidance point to the importance of H⁺ dynamics, which are regulated by the plasma membrane H⁺-ATPase.

Role of Cofilin in Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative neurological disease and has an inconspicuous onset and progressive development. Clinically, it is characterized by severe dementia manifestations, including memory impairment, aphasia, apraxia, loss of recognition, impairment of visual-spatial skills, executive dysfunction, and changes in personality and behavior. Its etiology is unknown to date. However, several cellular biological signatures of AD have been identified such as synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies which are related to the actin cytoskeleton. Cofilin is one of the most affluent and common actin-binding proteins and plays a role in cell motility, migration, shape, and metabolism. They also play an important role in severing actin filament, nucleating, depolymerizing, and bundling activities. In this review, we summarize the structure of cofilins and their functional and regulating roles, focusing on the synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies of AD.

Plasma membrane H+-ATPases sustain pollen tube growth and fertilization

Pollen tubes are highly polarized tip-growing cells that depend on cytosolic pH gradients for signaling and growth. Autoinhibited plasma membrane proton (H+) ATPases (AHAs) have been proposed to energize pollen tube growth and underlie cell polarity, however, mechanistic evidence for this is lacking. Here we report that the combined loss of AHA6, AHA8, and AHA9 in Arabidopsis thaliana delays pollen germination and causes pollen tube growth defects, leading to drastically reduced fertility. Pollen tubes of aha mutants had reduced extracellular proton (H+) and anion fluxes, reduced cytosolic pH, reduced tip-to-shank proton gradients, and defects in actin organization. Furthermore, mutant pollen tubes had less negative membrane potentials, substantiating a mechanistic role for AHAs in pollen tube growth through plasma membrane hyperpolarization. Our findings define AHAs as energy transducers that sustain the ionic circuit defining the spatial and temporal profiles of cytosolic pH, thereby controlling downstream pH-dependent mechanisms essential for pollen tube elongation, and thus plant fertility.

Long-Range and Directional Allostery of Actin Filaments Plays Important Roles in Various Cellular Activities

A wide variety of uniquely localized actin-binding proteins (ABPs) are involved in various cellular activities, such as cytokinesis, migration, adhesion, morphogenesis, and intracellular transport. In a micrometer-scale space such as the inside of cells, protein molecules diffuse throughout the cell interior within seconds. In this condition, how can ABPs selectively bind to particular actin filaments when there is an abundance of actin filaments in the cytoplasm? In recent years, several ABPs have been reported to induce cooperative conformational changes to actin filaments allowing structural changes to propagate along the filament cables uni- or bidirectionally, thereby regulating the subsequent binding of ABPs. Such propagation of ABP-induced cooperative conformational changes in actin filaments may be advantageous for the elaborate regulation of cellular activities driven by actin-based machineries in the intracellular space, which is dominated by diffusion. In this review, we focus on long-range allosteric regulation driven by cooperative conformational changes of actin filaments that are evoked by binding of ABPs, and discuss roles of allostery of actin filaments in narrow intracellular spaces.

Golgi Acidification by NHE7 Regulates Cytosolic pH Homeostasis in Pancreatic Cancer Cells

Cancer cells reprogram their metabolism to meet elevated energy demands and favor glycolysis for energy production. This boost in glycolytic flux supports proliferation, but also generates acid in the form of hydrogen ions that must be eliminated from the cytoplasm to maintain the alkaline intracellular pH (pHi) associated with transformation. To cope with acid production, tumor cells employ ion transport systems, including the family of sodium-hydrogen exchangers (NHE). Here, we identify NHE7 as a novel regulator of pHi in pancreatic ductal adenocarcinoma (PDAC). We determine that NHE7 suppression causes alkalinization of the Golgi, leading to a buildup of cytosolic acid that diminishes tumor cell fitness mainly through the dysregulation of actin. Importantly, NHE7 knockdown in vivo leads to the abrogation of tumor growth. These results identify Golgi acidification as a mechanism to control pHi and point to the regulation of pHi as a possible therapeutic vulnerability in PDAC. SIGNIFICANCE: NHE7 regulates cytosolic pH through Golgi acidification, which points to the Golgi as a "proton sink" for metabolic acid. Disruption of cytosolic pH homeostasis via NHE7 suppression compromises PDAC cell viability and tumor growth.See related commentary by Ward and DeNicola, p. 768.This article is highlighted in the In This Issue feature, p. 747.

K336I mutant actin alters the structure of neighbouring protomers in filaments and reduces affinity for actin-binding proteins

Mutation of the Lys-336 residue of actin to Ile (K336I) or Asp (K336E) causes congenital myopathy. To understand the effect of this mutation on the function of actin filaments and gain insight into the mechanism of disease onset, we prepared and biochemically characterised K336I mutant actin from Dictyostelium discoideum. Subtilisin cleavage assays revealed that the structure of the DNase-I binding loop (D-loop) of monomeric K336I actin, which would face the adjacent actin-protomer in filaments, differed from that of wild type (WT) actin. Although K336I actin underwent normal salt-dependent reversible polymerisation and formed apparently normal filaments, interactions of K336I filaments with alpha-actinin, myosin II, and cofilin were disrupted. Furthermore, co-filaments of K336I and WT actins also exhibited abnormal interactions with cofilin, implying that K336I actin altered the structure of the neighbouring WT actin protomers such that interaction between cofilin and the WT actin protomers was prevented. We speculate that disruption of the interactions between co-filaments and actin-binding proteins is the primary reason why the K336I mutation induces muscle disease in a dominant fashion.

Identification of Novel Protein Targets of Dimethyl Fumarate Modification in Neurons and Astrocytes Reveals Actions Independent of Nrf2 Stabilization

The fumarate ester dimethyl fumarate (DMF) has been introduced recently as a treatment for relapsing remitting multiple sclerosis (RRMS), a chronic inflammatory condition that results in neuronal demyelination and axonal loss. DMF is known to act by depleting intracellular glutathione and modifying thiols on Keap1 protein, resulting in the stabilization of the transcription factor Nrf2, which in turn induces the expression of antioxidant response element genes. We have previously shown that DMF reacts with a wide range of protein thiols, suggesting that the complete mechanisms of action of DMF are unknown. Here, we investigated other intracellular thiol residues that may also be irreversibly modified by DMF in neurons and astrocytes. Using mass spectrometry, we identified 24 novel proteins that were modified by DMF in neurons and astrocytes, including cofilin-1, tubulin and collapsin response mediator protein 2 (CRMP2). Using an in vitro functional assay, we demonstrated that DMF-modified cofilin-1 loses its activity and generates less monomeric actin, potentially inhibiting its cytoskeletal remodeling activity, which could be beneficial in the modulation of myelination during RRMS. DMF modification of tubulin did not significantly impact axonal lysosomal trafficking. We found that the oxygen consumption rate of N1E-115 neurons and the levels of proteins related to mitochondrial energy production were only slightly affected by the highest doses of DMF, confirming that DMF treatment does not impair cellular respiratory function. In summary, our work provides new insights into the mechanisms supporting the neuroprotective and remyelination benefits associated with DMF treatment in addition to the antioxidant response by Nrf2.

Quantitative Variations with pH of Actin Depolymerizing Factor/Cofilin's Multiple Actions on Actin Filaments

Actin depolymerizing factor (ADF)/cofilin is the main protein family promoting the disassembly of actin filaments, which is essential for numerous cellular functions. ADF/cofilin proteins disassemble actin filaments through different reactions, as they bind to their sides, sever them, and promote the depolymerization of the resulting ADF/cofilin-saturated filaments. Moreover, the efficiency of ADF/cofilin is known to be very sensitive to pH. ADF/cofilin thus illustrates two challenges in actin biochemistry: separating the different regulatory actions of a single protein and characterizing them as a function of specific biochemical conditions. Here, we investigate the different reactions of ADF/cofilin on actin filaments, at four different pH values ranging from 6.6 to 7.8, using single-filament microfluidics techniques. We show that decreasing the pH decreases the effective filament severing rate by increasing the rate at which filaments become saturated by ADF/cofilin, thereby reducing the number of ADF/cofilin domain boundaries, where severing can occur. The severing rate per domain boundary, however, remains unchanged at different pH values. The ADF/cofilin-decorated filaments (“cofilactin” filaments) depolymerize from both ends. We show here that, at physiological pH (7.0–7.4), the pointed end depolymerization of cofilactin filaments is barely faster than that of bare filaments. In contrast, cofilactin barbed ends undergo an “unstoppable” depolymerization (depolymerizing for minutes despite the presence of free actin monomers and capping protein in solution), throughout our pH range. We thus show that, at physiological pH, the main contribution of ADF/cofilin to filament depolymerization is at the barbed end.

Frequency-induced morphology alterations in microconfined biological cells

Low-intensity therapeutic ultrasound has demonstrated an impetus in bone signaling and tissue healing for decades now. Though this technology is clinically well proven, still there are breaches in studies to understand the fundamental principle of how osteoblast tissue regenerates physiologically at the cellular level with ultrasound interaction as a form of acoustic wave stimuli. Through this article, we illustrate an analysis for cytomechanical changes of cell membrane periphery as a basic first physical principle for facilitating late downstream biochemical pathways. With the help of in situ single-cell direct analysis in a microfluidic confinement, we demonstrate that alteration of low-intensity pulse ultrasound (LIPUS) frequency would physically perturb cell membrane and establish inherent cell oscillation. We experimentally demonstrate here that, at LIPUS resonance near 1.7 MHz (during 1-3 MHz alteration), cell membrane area would expand to 6.85 ± 0.7% during ultrasound exposure while it contracts 44.68 ± 0.8% in post actuation. Conversely, cell cross-sectional area change (%) from its previous morphology during and after switching off LIPUS was reversibly different before and after resonance. For instance, at 1.5 MHz, LIPUS exposure produced 1.44 ± 0.5% expansion while in contrast 2 MHz instigates 1.6 ± 0.3% contraction. We conclude that alteration of LIPUS frequency from 1-3 MHz keeping other ultrasound parameters like exposure time, pulse repetition frequency (PRF), etc., constant, if applied to a microconfined biological single living cell, would perturb physical structure reversibly based on the system resonance during and post exposure ultrasound pulsing. We envision, in the near future, our results would constitute the foundation of mechanistic effects of low-intensity therapeutic ultrasound and its allied potential in medical applications. Graphical Abstract Frequency Dependent Characterization of Area Strain in Cell Membrane by Microfluidic Based Single Cell Analysis.

Structural basis for cofilin binding and actin filament disassembly

Actin depolymerizing factor (ADF) and cofilin accelerate actin dynamics by severing and disassembling actin filaments. Here, we present the 3.8 Å resolution cryo-EM structure of cofilactin (cofilin-decorated actin filament). The actin subunit structure of cofilactin (C-form) is distinct from those of F-actin (F-form) and monomeric actin (G-form). During the transition between these three conformations, the inner domain of actin (subdomains 3 and 4) and the majority of subdomain 1 move as two separate rigid bodies. The cofilin–actin interface consists of three distinct parts. Based on the rigid body movements of actin and the three cofilin–actin interfaces, we propose models for the cooperative binding of cofilin to actin, preferential binding of cofilin to ADP-bound actin filaments and cofilin-mediated severing of actin filaments.

Cofilin is a small actin-binding protein that accelerates actin turnover by disassembling actin filaments. Here the authors present the 3.8 Å cryo-EM structure of a cofilin-decorated actin filament and discuss mechanistic implications.

F-actin reorganization by V-ATPase inhibition in prostate cancer

The vacuolar ATPase (V-ATPase) proton pump sustains cellular pH homeostasis, and its inhibition triggers numerous stress responses. However, the cellular mechanisms involved remain largely elusive in cancer cells. We studied V-ATPase in the prostate cancer (PCa) cell line PC-3, which has characteristics of highly metastatic PCa. V-ATPase inhibitors impaired endo-lysosomal pH, vesicle trafficking, migration, and invasion. V-ATPase accrual in the Golgi and recycling endosomes suggests that traffic of internalized membrane vesicles back to the plasma membrane was particularly impaired. Directed movement provoked co-localization of V-ATPase containing vesicles with F-actin near the leading edge of migrating cells. V-ATPase inhibition prompted prominent F-actin cytoskeleton reorganization. Filopodial projections were reduced, which related to reduced migration velocity. F-actin formed novel cytoplasmic rings. F-actin rings increased with extended exposure to sublethal concentrations of V-ATPase inhibitors, from 24 to 48 h, as the amount of alkalinized endo-lysosomal vesicles increased. Studies with chloroquine indicated that F-actin rings formation was pH-dependent. We hypothesize that these novel F-actin rings assemble to overcome widespread traffic defects caused by V-ATPase inhibition, similar to F-actin rings on the surface of exocytic organelles.

Summary: V-ATPase activates multiple stress responses. In prostate cancer, sub-lethal concentrations of V-ATPase inhibitors trigger widespread traffic defects. F-actin assembles into rings that mimic those seen during regulated exocytosis.

Functions of actin-interacting protein 1 (AIP1)/WD repeat protein 1 (WDR1) in actin filament dynamics and cytoskeletal regulation

Actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), are conserved among eukaryotes and play critical roles in dynamic reorganization of the actin cytoskeleton. AIP1 preferentially promotes disassembly of ADF/cofilin-decorated actin filaments but exhibits minimal effects on bare actin filaments. Therefore, AIP1 has been often considered to be an ancillary co-factor of ADF/cofilin that merely boosts ADF/cofilin activity level. However, genetic and cell biological studies show that AIP1 deficiency often causes lethality or severe abnormalities in multiple tissues and organs including muscle, epithelia, and blood, suggesting that AIP1 is a major regulator of many biological processes that depend on actin dynamics. This review summarizes recent progress in studies on the biochemical mechanism of actin filament severing by AIP1 and in vivo functions of AIP1 in model organisms and human diseases.

Complement C5a-Induced Changes in Neutrophil Morphology During Inflammation

The complement and neutrophil defence systems, as major components of innate immunity, are activated during inflammation and infection. For neutrophil migration to the inflamed region, we hypothesized that the complement activation product C5a induces significant changes in cellular morphology before chemotaxis. Exposure of human neutrophils to C5a dose- and time-dependently resulted in a rapid C5a receptor-1 (C5aR1)-dependent shape change, indicated by enhanced flow cytometric forward-scatter area values. Similar changes were observed after incubation with zymosan-activated serum and in blood neutrophils during murine sepsis, but not in mice lacking the C5aR1. In human neutrophils, Amnis high-resolution digital imaging revealed a C5a-induced decrease in circularity and increase in the cellular length/width ratio. Biomechanically, microfluidic optical stretching experiments indicated significantly increased neutrophil deformability early after C5a stimulation. The C5a-induced shape changes were inhibited by pharmacological blockade of either the Cl-/HCO3--exchanger or the Cl^- -channel. Furthermore, actin polymerization assays revealed that C5a exposure resulted in a significant polarization of the neutrophils. The functional polarization process triggered by ATP-P2X/Y-purinoceptor interaction was also involved in the C5a-induced shape changes, because pretreatment with suramin blocked not only the shape changes but also the subsequent C5a-dependent chemotactic activity. In conclusion, the data suggest that the anaphylatoxin C5a regulates basic neutrophil cell processes by increasing the membrane elasticity and cell size as a consequence of actin-cytoskeleton polymerization and reorganization, transforming the neutrophil into a migratory cell able to invade the inflammatory site and subsequently clear pathogens and molecular debris.

Higher-Ordered Actin Structures Remodeled by Arabidopsis ACTIN-DEPOLYMERIZING FACTOR5 Are Important for Pollen Germination and Pollen Tube Growth

Dynamics of the actin cytoskeleton are essential for pollen germination and pollen tube growth. ACTIN-DEPOLYMERIZING FACTORs (ADFs) typically contribute to actin turnover by severing/depolymerizing actin filaments. Recently, we demonstrated that Arabidopsis subclass III ADFs (ADF5 and ADF9) evolved F-actin-bundling function from conserved F-actin-depolymerizing function. However, little is known about the physiological function, the evolutional significance, and the actin-bundling mechanism of these neofunctionalized ADFs. Here, we report that loss of ADF5 function caused delayed pollen germination, retarded pollen tube growth, and increased sensitive to latrunculin B (LatB) treatment by affecting the generation and maintenance of actin bundles. Examination of actin filament dynamics in living cells revealed that the bundling frequency was significantly decreased in adf5 pollen tubes, consistent with its biochemical functions. Further biochemical and genetic complementation analyses demonstrated that both the N- and C-terminal actin-binding domains of ADF5 are required for its physiological and biochemical functions. Interestingly, while both are atypical actin-bundling ADFs, ADF5, but not ADF9, plays an important role in mature pollen physiological activities. Taken together, our results suggest that ADF5 has evolved the function of bundling actin filaments and plays an important role in the formation, organization, and maintenance of actin bundles during pollen germination and pollen tube growth.

Intercalated Cell Depletion and Vacuolar H+-ATPase Mistargeting in an Ae1 R607H Knockin Model

Distal nephron acid secretion is mediated by highly specialized type A intercalated cells (A-ICs), which contain vacuolar H+-ATPase (V-type ATPase)-rich vesicles that fuse with the apical plasma membrane on demand. Intracellular bicarbonate generated by luminal H+ secretion is removed by the basolateral anion-exchanger AE1. Chronically reduced renal acid excretion in distal renal tubular acidosis (dRTA) may lead to nephrocalcinosis and renal failure. Studies in MDCK monolayers led to the proposal of a dominant-negative trafficking mechanism to explain AE1-associated dominant dRTA. To test this hypothesis in vivo, we generated an Ae1 R607H knockin mouse, which corresponds to the most common dominant dRTA mutation in human AE1, R589H. Compared with wild-type mice, heterozygous and homozygous R607H knockin mice displayed incomplete dRTA characterized by compensatory upregulation of the Na+/HCO3- cotransporter NBCn1. Red blood cell Ae1-mediated anion-exchange activity and surface polypeptide expression did not change. Mutant mice expressed far less Ae1 in A-ICs, but basolateral targeting of the mutant protein was preserved. Notably, mutant mice also exhibited reduced expression of V-type ATPase and compromised targeting of this proton pump to the plasma membrane upon acid challenge. Accumulation of p62- and ubiquitin-positive material in A-ICs of knockin mice suggested a defect in the degradative pathway, which may explain the observed loss of A-ICs. R607H knockin did not affect type B intercalated cells. We propose that reduced basolateral anion-exchange activity in A-ICs inhibits trafficking and regulation of V-type ATPase, compromising luminal H+ secretion and possibly lysosomal acidification.

NHE1 is upregulated in gastric cancer and regulates gastric cancer cell proliferation, migration and invasion

Na+/H+ exchanger isoform 1 (NHE1) is known to play a key role in regulating intracellular pH and osmotic homeostasis and is involved in the development and progression of several types of cancer. However, the function and specific mechanism of NHE1 in gastric cancer (GC) are not clearly understood. In the present study, we report that NHE1 is overexpressed in tissues and cell lines from GC patients, and knockdown or inhibition of NHE1 suppressed GC cell proliferation via regulation of G1/S and G2/M cell cycle phase transitions, concomitant with a marked decrease in positive cell cycle regulators, including cyclin D1 and cyclin B1. Likewise, NHE1 was required for GC cell migration and invasion through the regulation of epithelial-mesenchymal transition (EMT) proteins, and NHE1 inhibition resulted in an acidic intracellular environment, providing possible mechanisms underlying NHE1-mediated GC progression both in vitro and in vivo. These data highlight the important role of NHE1 in GC progression and suggest that NHE1 may be a useful target for GC therapy.

Cofilin-1 and Other ADF/Cofilin Superfamily Members in Human Malignant Cells

Identification of actin-depolymerizing factor homology (ADF-H) domains in the structures of several related proteins led first to the formation of the ADF/cofilin family, which then expanded to the ADF/cofilin superfamily. This superfamily includes the well-studied cofilin-1 (Cfl-1) and about a dozen different human proteins that interact directly or indirectly with the actin cytoskeleton, provide its remodeling, and alter cell motility. According to some data, Cfl-1 is contained in various human malignant cells (HMCs) and is involved in the formation of malignant properties, including invasiveness, metastatic potential, and resistance to chemotherapeutic drugs. The presence of other ADF/cofilin superfamily proteins in HMCs and their involvement in the regulation of cell motility were discovered with the use of various OMICS technologies. In our review, we discuss the results of the study of Cfl-1 and other ADF/cofilin superfamily proteins, which may be of interest for solving different problems of molecular oncology, as well as for the prospects of further investigations of these proteins in HMCs.

Spectroscopic characterization of the effect of mouse twinfilin-1 on actin filaments at different pH values

The effect of mammalian twinfilin-1 on the structure and dynamics of actin filaments were studied with steady state fluorescence spectroscopy, total internal reflection fluorescence microscopy and differential scanning calorimetry techniques. It was proved before that the eukaryotic budding yeast twinfilin-1 can efficiently bind and severe actin filaments in vitro at low pH values. In the present work steady-state anisotropy measurements revealed that twinfilin can bind efficiently to F-actin. Dilution-induced depolymerization assay proved that mammalian twinfilin-1 has an actin filament severing activity. This severing activity was more pronounced at low pH values. Total internal reflection fluorescence microscopy measurements could support the severing activity of mouse twinfilin-1. The average rate of depolymerization was more apparent at low pH values. The differential scanning calorimetry measurements demonstrated that mammalian twinfilin-1 could reduce the stiffness within the actin filaments before the detachment of the actin protomers. The structural and dynamic reorganization of actin can support the twinfilin-1 induced separation of actin protomers. The measured data indicated that mammalian twinfilin-1 was able to accelerate the monomers dissociation and/or sever the filaments effectively at low pH values. It was concluded that twinfilin-1 can affect the F-actin in biological processes or under stress situations when the pH is markedly under the physiological level.

Allosteric regulation by cooperative conformational changes of actin filaments drives mutually exclusive binding with cofilin and myosin

Heavy meromyosin (HMM) of myosin II and cofilin each binds to actin filaments cooperatively and forms clusters along the filaments, but it is unknown whether the two cooperative bindings are correlated and what physiological roles they have. Fluorescence microscopy demonstrated that HMM-GFP and cofilin-mCherry each bound cooperatively to different parts of actin filaments when they were added simultaneously in 0.2 μM ATP, indicating that the two cooperative bindings are mutually exclusive. In 0.1 mM ATP, the motor domain of myosin (S1) strongly inhibited the formation of cofilin clusters along actin filaments. Under this condition, most actin protomers were unoccupied by S1 at any given moment, suggesting that transiently bound S1 alters the structure of actin filaments cooperatively and/or persistently to inhibit cofilin binding. Consistently, cosedimentation experiments using copolymers of actin and actin-S1 fusion protein demonstrated that the fusion protein affects the neighboring actin protomers, reducing their affinity for cofilin. In reciprocal experiments, cofilin-actin fusion protein reduced the affinity of neighboring actin protomers for S1. Thus, allosteric regulation by cooperative conformational changes of actin filaments contributes to mutually exclusive cooperative binding of myosin II and cofilin to actin filaments, and presumably to the differential localization of both proteins in cells.

Cofilin-induced cooperative conformational changes of actin subunits revealed using cofilin-actin fusion protein

To investigate cooperative conformational changes of actin filaments induced by cofilin binding, we engineered a fusion protein made of Dictyostelium cofilin and actin. The filaments of the fusion protein were functionally similar to actin filaments bound with cofilin in that they did not bind rhodamine-phalloidin, had quenched fluorescence of pyrene attached to Cys374 and showed enhanced susceptibility of the DNase loop to cleavage by subtilisin. Quantitative analyses of copolymers made of different ratios of the fusion protein and control actin further demonstrated that the fusion protein affects the structure of multiple neighboring actin subunits in copolymers. Based on these and other recent related studies, we propose a mechanism by which conformational changes induced by cofilin binding is propagated unidirectionally to the pointed ends of the filaments, and cofilin clusters grow unidirectionally to the pointed ends following this path. Interestingly, the fusion protein was unable to copolymerize with control actin at pH 6.5 and low ionic strength, suggesting that the structural difference between the actin moiety in the fusion protein and control actin is pH-sensitive.

Actin-interacting Protein 1 Promotes Disassembly of Actin-depolymerizing Factor/Cofilin-bound Actin Filaments in a pH-dependent Manner

Actin-interacting protein 1 (AIP1) is a conserved WD repeat protein that promotes disassembly of actin filaments when actin-depolymerizing factor (ADF)/cofilin is present. Although AIP1 is known to be essential for a number of cellular events involving dynamic rearrangement of the actin cytoskeleton, the regulatory mechanism of the function of AIP1 is unknown. In this study, we report that two AIP1 isoforms from the nematode Caenorhabditis elegans, known as UNC-78 and AIPL-1, are pH-sensitive in enhancement of actin filament disassembly. Both AIP1 isoforms only weakly enhance disassembly of ADF/cofilin-bound actin filaments at an acidic pH but show stronger disassembly activity at neutral and basic pH values. However, a severing-defective mutant of UNC-78 shows pH-insensitive binding to ADF/cofilin-decorated actin filaments, suggesting that the process of filament severing or disassembly, but not filament binding, is pH-dependent. His-60 of AIP1 is located near the predicted binding surface for the ADF/cofilin-actin complex, and an H60K mutation of AIP1 partially impairs its pH sensitivity, suggesting that His-60 is involved in the pH sensor for AIP1. These biochemical results suggest that pH-dependent changes in AIP1 activity might be a novel regulatory mechanism of actin filament dynamics.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

Haplotype combination of the bovine CFL2 gene sequence variants and association with growth traits in Qinchuan cattle

The aim of this study was to examine the association of cofilin2 (CFL2) gene polymorphisms with growth traits in Chinese Qinchuan cattle. Three single nucleotide polymorphisms (SNPs) were identified in the bovine CFL2 gene using DNA sequencing and (forced) PCR-RFLP methods. These polymorphisms included a missense mutation (NC_007319.5: g. C 2213 G) in exon 4, one synonymous mutation (NC_007319.5: g. T 1694 A) in exon 4, and a mutation (NC_007319.5: g. G 1500 A) in intron 2, respectively. In addition, we evaluated the haplotype frequency and linkage disequilibrium coefficient of three sequence variants in 488 individuals in QC cattle. All the three SNPs in QC cattle belonged to an intermediate level of genetic diversity (0.25<PIC<0.5). Haplotype analysis of three SNPs showed that 8 different haplotypes were identified in all, but only 5 haplotypes were listed except for those with a frequency of <0.03. Hap4 (-GTC-) had the highest haplotype frequencies (34.70%). However in the three SNPs there were no significant associations between the 13 combined genotypes of the CFL2 gene and growth traits. LD analysis showed that the SNP T 1694 A and C 2213 G loci had a strong linkage (r(2)>0.33). Association analysis indicated that SNP G 1500 A, T 1694 A and C 2213 G were significantly associated with growth traits in the QC population. The results of our study suggest that the CFL2 gene may be a strong candidate gene that affects growth traits in the QC cattle breeding program.

Self-incompatibility-induced programmed cell death in field poppy pollen involves dramatic acidification of the incompatible pollen tube cytosol

Self-incompatibility (SI) is an important genetically controlled mechanism to prevent inbreeding in higher plants. SI involves highly specific interactions during pollination, resulting in the rejection of incompatible (self) pollen. Programmed cell death (PCD) is an important mechanism for destroying cells in a precisely regulated manner. SI in field poppy (Papaver rhoeas) triggers PCD in incompatible pollen. During SI-induced PCD, we previously observed a major acidification of the pollen cytosol. Here, we present measurements of temporal alterations in cytosolic pH ([pH]cyt); they were surprisingly rapid, reaching pH 6.4 within 10 min of SI induction and stabilizing by 60 min at pH 5.5. By manipulating the [pH]cyt of the pollen tubes in vivo, we show that [pH]cyt acidification is an integral and essential event for SI-induced PCD. Here, we provide evidence showing the physiological relevance of the cytosolic acidification and identify key targets of this major physiological alteration. A small drop in [pH]cyt inhibits the activity of a soluble inorganic pyrophosphatase required for pollen tube growth. We also show that [pH]cyt acidification is necessary and sufficient for triggering several key hallmark features of the SI PCD signaling pathway, notably activation of a DEVDase/caspase-3-like activity and formation of SI-induced punctate actin foci. Importantly, the actin binding proteins Cyclase-Associated Protein and Actin-Depolymerizing Factor are identified as key downstream targets. Thus, we have shown the biological relevance of an extreme but physiologically relevant alteration in [pH]cyt and its effect on several components in the context of SI-induced events and PCD.

Cofilin-induced unidirectional cooperative conformational changes in actin filaments revealed by high-speed atomic force microscopy

High-speed atomic force microscopy was employed to observe structural changes in actin filaments induced by cofilin binding. Consistent with previous electron and fluorescence microscopic studies, cofilin formed clusters along actin filaments, where the filaments were 2-nm thicker and the helical pitch was ∼25% shorter, compared to control filaments. Interestingly, the shortened helical pitch was propagated to the neighboring bare zone on the pointed-end side of the cluster, while the pitch on the barbed-end side was similar to the control. Thus, cofilin clusters induce distinctively asymmetric conformational changes in filaments. Consistent with the idea that cofilin favors actin structures with a shorter helical pitch, cofilin clusters grew unidirectionally toward the pointed-end of the filament. Severing was often observed near the boundaries between bare zones and clusters, but not necessarily at the boundaries.

DOI:http://dx.doi.org/10.7554/eLife.04806.001

Actin is one of the most abundant proteins found inside all eukaryotic cells including plant, animal, and fungal cells. This protein is involved in a wide range of biological processes that are essential for an organism's survival. Actin proteins form long filaments that help cells to maintain their shape and also provide the force required for cells to divide and/or move.

Actin filaments are helical in shape and are made up of many actin subunits joined together. Actin filaments are changeable structures that continuously grow and shrink as new actin subunits are added to or removed from the ends of the filaments. One end of an actin filament grows faster than the other; the fast-growing end is known as the barbed-end, while the slow-growing end is referred to as the pointed-end.

Over 100 other proteins are known to bind to and work with actin to regulate its roles in cells and how it forms into filaments. Cofilin is one such protein that binds to and forms clusters on actin filaments and it can also sever actin filaments. Severing an actin filament can encourage the filament to disassemble, or it can help produce new barbed ends that can then grow into new filaments. Previous work had suggested that cofilin severs actin filaments at the junction between regions on the filament that are coated with cofilin and those that are not. It was also known that cofilin binding to a filament causes the filament to change shape, and that the shape change is propagated to neighboring sections of the filaments not coated with cofilin. However, the details of where cofilin binds and how changes in shape are propagated along an actin filament were not known. Furthermore, the findings of these previous studies were largely based on examining still images of actin filaments, which are unlike the constantly changing filaments of living cells.

Ngo, Kodera et al. have now analyzed what happens when cofilin binds to and forms clusters along actin filaments using a recently developed imaging technique called high-speed atomic force microscopy. This technique can be used to directly visualize individual proteins in action. Consistent with previous findings, Ngo, Kodera et al. observed that filaments coated with cofilin are thicker than those filaments without cofilin; and that cofilin binding also substantially reduces the helical twist of the filament. Ngo, Kodera et al. also found that these changes in shape are propagated along the filament but in only one direction—towards the pointed-end. Moreover, cofilin clusters also only grew towards the pointed-end of the actin filament—and the filaments were often severed near, but not exactly at, the junctions between cofilin-coated and uncoated regions. Such one-directional changes in shape of the actin filaments presumably help to regulate how other actin binding proteins can interact with the filament and consequently regulate the roles of the filaments themselves.

DOI:http://dx.doi.org/10.7554/eLife.04806.002

Ion channels and transporters in metastasis

An elaborate interplay between ion channels and transporters, components of the cytoskeleton, adhesion molecules, and signaling cascades provides the basis for each major step of the metastatic cascade. Ion channels and transporters contribute to cell motility by letting through or transporting ions essential for local Ca2+, pH and--in cooperation with water permeable aquaporins--volume homeostasis. Moreover, in addition to the actual ion transport they, or their auxiliary subunits, can display non-conducting activities. They can exert kinase activity in order to phosphorylate cytoskeletal constituents or their associates. They can become part of signaling processes by permeating Ca2+, by generating local pH-nanodomains or by being final downstream effectors. A number of channels and transporters are found at focal adhesions, interacting directly or indirectly with proteins of the extracellular matrix, with integrins or with components of the cytoskeleton. We also include the role of aquaporins in cell motility. They drive the outgrowth of lamellipodia/invadopodia or control the number of β1 integrins in the plasma membrane. The multitude of interacting ion channels and transporters (called transportome) including the associated signaling events holds great potential as therapeutic target(s) for anticancer agents that are aimed at preventing metastasis. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

The Dictyostelium discoideum GPHR ortholog is an endoplasmic reticulum and Golgi protein with roles during development

Dictyostelium discoideum GPHR (Golgi pH regulator)/Gpr89 is a developmentally regulated transmembrane protein present on the endoplasmic reticulum (ER) and the Golgi apparatus. Transcript levels are low during growth and vary during development, reaching high levels during the aggregation and late developmental stages. The Arabidopsis ortholog was described as a G protein-coupled receptor (GPCR) for abscisic acid present at the plasma membrane, whereas the mammalian ortholog is a Golgi apparatus-associated anion channel functioning as a Golgi apparatus pH regulator. To probe its role in D. discoideum, we generated a strain lacking GPHR. The mutant had different growth characteristics than the AX2 parent strain, exhibited changes during late development, and formed abnormally shaped small slugs and fruiting bodies. An analysis of development-specific markers revealed that their expression was disturbed. The distributions of the endoplasmic reticulum and the Golgi apparatus were unaltered at the immunofluorescence level. Likewise, their functions did not appear to be impaired, since membrane proteins were properly processed and glycosylated. Also, changes in the external pH were sensed by the ER, as indicated by a pH-sensitive ER probe, as in the wild type.

Taurine chloramine-induced inactivation of cofilin protein through methionine oxidation

Cofilin regulates reorganization of actin filaments (F-actin) in eukaryotes. A recent finding has demonstrated that oxidation of cofilin by taurine chloramine (TnCl), a physiological oxidant derived from neutrophils, causes cofilin to translocate to the mitochondria inducing apoptosis (F. Klamt et al. Nat. Cell Biol.11:1241-1246; 2009). Here we investigated the effect of TnCl on biological activities of cofilin in vitro. Our data show that TnCl-induced oxidation of recombinant human cofilin-1 inhibits its F-actin-binding and depolymerization activities. Native cofilin contains four free Cys and three Met residues. Incubation of oxidized cofilin with DTT does not lead to its reactivation. A double Cys to Ala mutation on the two C-terminal Cys shows similar biological activities as the wild type, but does not prevent the TnCl-induced inactivation. In contrast, incubation of oxidized cofilin with methionine sulfoxide reductases results in its reactivation. Phosphorylation is known to inhibit cofilin activities. We found that Met oxidation also prevents phosphorylation of cofilin, which is reversed by incubating oxidized cofilin with methionine sulfoxide reductases. Interestingly, intact protein mass spectrometry of the oxidized mutant indicated one major oxidation product with an additional mass of 16 Da, consistent with oxidation of one specific Met residue. This residue was identified as Met-115 by peptide mapping and tandem mass spectrometry. It is adjacent to Lys-114, a known residue on globular-actin-binding site, implying that oxidation of Met-115 disrupts the globular-actin-binding site of cofilin, which causes TnCl-induced inactivation. The findings identify Met-115 as a redox switch on cofilin that regulates its biological activity.

Single-molecule imaging and kinetic analysis of cooperative cofilin-actin filament interactions

The actin filament-severing protein actin depolymerizing factor (ADF)/cofilin is ubiquitously distributed among eukaryotes and modulates actin dynamics. The cooperative binding of cofilin to actin filaments is crucial for the concentration-dependent unconventional modulation of actin dynamics by cofilin. In this study, the kinetic parameters associated with the cooperative binding of cofilin to actin filaments were directly evaluated using a single-molecule imaging technique. The on-rate of cofilin binding to the actin filament was estimated to be 0.06 µM(-1)⋅s(-1) when the cofilin concentration was in the range of 30 nM to 1 µM. A dwell time histogram of cofilin bindings decays exponentially to give an off-rate of 0.6 s(-1). During long-term cofilin binding events (>0.4 s), additional cofilin bindings were observed in the vicinity of the initial binding site. The on-rate for these events was 2.3-fold higher than that for noncontiguous bindings. Super-high-resolution image analysis of the cofilin binding location showed that the on-rate enhancement occurred within 65 nm of the original binding event. By contrast, the cofilin off-rate was not affected by the presence of prebound cofilin. Neither decreasing the temperature nor increasing the viscosity of the test solution altered the on-rates, off-rates, or the cooperative parameter (ω) of the binding. These results indicate that cofilin binding enhances additional cofilin binding in the vicinity of the initial binding site (ca. 24 subunits), but it does not affect the off-rate, which could be the molecular mechanism of the cooperative binding of cofilin to actin filaments.

The role of cyclase-associated protein in regulating actin filament dynamics - more than a monomer-sequestration factor

Dynamic reorganization of the actin cytoskeleton is fundamental to a number of cell biological events. A variety of actin-regulatory proteins modulate polymerization and depolymerization of actin and contribute to actin cytoskeletal reorganization. Cyclase-associated protein (CAP) is a conserved actin-monomer-binding protein that has been studied for over 20 years. Early studies have shown that CAP sequesters actin monomers; recent studies, however, have revealed more active roles of CAP in actin filament dynamics. CAP enhances the recharging of actin monomers with ATP antagonistically to ADF/cofilin, and also promotes the severing of actin filaments in cooperation with ADF/cofilin. Self-oligomerization and binding to other proteins regulate activities and localization of CAP. CAP has crucial roles in cell signaling, development, vesicle trafficking, cell migration and muscle sarcomere assembly. This Commentary discusses the recent advances in our understanding of the functions of CAP and its implications as an important regulator of actin cytoskeletal dynamics, which are involved in various cellular activities.

Ion channels and transporters in tumour cell migration and invasion

Cell migration is a central component of the metastatic cascade requiring a concerted action of ion channels and transporters (migration-associated transportome), cytoskeletal elements and signalling cascades. Ion transport proteins and aquaporins contribute to tumour cell migration and invasion among other things by inducing local volume changes and/or by modulating Ca(2+) and H(+) signalling. Targeting cell migration therapeutically bears great clinical potential, because it is a prerequisite for metastasis. Ion transport proteins appear to be attractive candidate target proteins for this purpose because they are easily accessible as membrane proteins and often overexpressed or activated in cancer. Importantly, a number of clinically widely used drugs are available whose anticipated efficacy as anti-tumour drugs, however, has now only begun to be evaluated.

Cofilin: a redox sensitive mediator of actin dynamics during T-cell activation and migration

Cofilin is an actin-binding protein that depolymerizes and/or severs actin filaments. This dual function of cofilin makes it one of the major regulators of actin dynamics important for T-cell activation and migration. The activity of cofilin is spatio-temporally regulated. Its main control mechanisms comprise a molecular toolbox of phospho-, phospholipid, and redox regulation. Phosphorylated cofilin is inactive and represents the dominant cofilin fraction in the cytoplasm of resting human T cells. A fraction of dephosphorylated cofilin is kept inactive at the plasma membrane by binding to phosphatidylinositol 4,5-bisphosphate. Costimulation via the T-cell receptor/CD3 complex (signal 1) together with accessory receptors (signal 2) or triggering through the chemokine SDF1α (stromal cell-derived factor 1α) induce Ras-dependent dephosphorylation of cofilin, which is important for immune synapse formation, T-cell activation, and T-cell migration. Recently, it became evident that cofilin is also highly sensitive for microenvironmental changes, particularly for alterations in the redox milieu. Cofilin is inactivated by oxidation, provoking T-cell hyporesponsiveness or necrotic-like programmed cell death. In contrast, in a reducing environment, even phosphatidylinositol 4,5-bisphosphate-bound cofilin becomes active, leading to actin dynamics in the vicinity of the plasma membrane. In addition to the well-established three signals for T-cell activation, this microenvironmental control of cofilin delivers a modulating signal for T-cell-dependent immune reactions. This fourth modulating signal highly impacts both initial T-cell activation and the effector phase of T-cell-mediated immune responses.

Non-overlapping activities of ADF and cofilin-1 during the migration of metastatic breast tumor cells

BACKGROUND

ADF/cofilin proteins are key modulators of actin dynamics in metastasis and invasion of cancer cells. Here we focused on the roles of ADF and cofilin-1 individually in the development of polarized migration of rat mammary adenocarcinoma (MTLn3) cells, which express nearly equal amounts of each protein. Small interference RNA (siRNA) technology was used to knockdown (KD) the expression of ADF and cofilin-1 independently.

RESULTS

Either ADF KD or cofilin KD caused cell elongation, a reduction in cell area, a decreased ability to form invadopodia, and a decreased percentage of polarized cells after 180 s of epidermal growth factor stimulation. Moreover, ADF KD or cofilin KD increased the rate of cell migration and the time of lamellipodia protrusion but through different mechanisms: lamellipodia protrude more frequently in ADF KD cells and are more persistent in cofilin KD cells. ADF KD cells showed a significant increase in F-actin aggregates, whereas cofilin KD cells showed a significant increase in prominent F-actin bundles and increased cell adhesion. Focal adhesion area and cell adhesion in cofilin KD cells were returned to control levels by expressing exogenous cofilin but not ADF. Return to control rates of cell migration in ADF KD cells was achieved by expression of exogenous ADF but not cofilin, whereas in cofilin KD cells, expression of cofilin efficiently rescued control migration rates.

CONCLUSION

Although ADF and cofilin have many redundant functions, each of these isoforms has functional differences that affect F-actin structures, cell adhesion and lamellipodial dynamics, all of which are important determinants of cell migration.

Stimulation of Na(+)/H(+) exchanger isoform 1 promotes microglial migration

Regulation of microglial migration is not well understood. In this study, we proposed that Na(+)/H(+) exchanger isoform 1 (NHE-1) is important in microglial migration. NHE-1 protein was co-localized with cytoskeletal protein ezrin in lamellipodia of microglia and maintained its more alkaline intracellular pH (pHi). Chemoattractant bradykinin (BK) stimulated microglial migration by increasing lamellipodial area and protrusion rate, but reducing lamellipodial persistence time. Interestingly, blocking NHE-1 activity with its potent inhibitor HOE 642 not only acidified microglia, abolished the BK-triggered dynamic changes of lamellipodia, but also reduced microglial motility and microchemotaxis in response to BK. In addition, NHE-1 activation resulted in intracellular Na(+) loading as well as intracellular Ca(2+) elevation mediated by stimulating reverse mode operation of Na(+)/Ca(2+) exchange (NCXrev). Taken together, our study shows that NHE-1 protein is abundantly expressed in microglial lamellipodia and maintains alkaline pHi in response to BK stimulation. In addition, NHE-1 and NCXrev play a concerted role in BK-induced microglial migration via Na(+) and Ca(2+) signaling.

Rapid nucleotide exchange renders Asp-11 mutant actins resistant to depolymerizing activity of cofilin, leading to dominant toxicity in vivo

Conserved Asp-11 of actin is a part of the nucleotide binding pocket, and its mutation to Gln is dominant lethal in yeast, whereas the mutation to Asn in human α-actin dominantly causes congenital myopathy. To elucidate the molecular mechanism of those dominant negative effects, we prepared Dictyostelium versions of D11N and D11Q mutant actins and characterized them in vitro. D11N and D11Q actins underwent salt-dependent reversible polymerization, although the resultant polymerization products contained small anomalous structures in addition to filaments of normal appearance. Both monomeric and polymeric D11Q actin released bound nucleotides more rapidly than the wild type, and intriguingly, both monomeric and polymeric D11Q actins hardly bound cofilin. The deficiency in cofilin binding can be explained by rapid exchange of bound nucleotide with ATP in solution, because cofilin does not bind ATP-bound actin. Copolymers of D11Q and wild type actins bound cofilin, but cofilin-induced depolymerization of the copolymers was slower than that of wild type filaments, which may presumably be the primary reason why this mutant actin is dominantly toxic in vivo. Purified D11N actin was unstable, which made its quantitative biochemical characterization difficult. However, monomeric D11N actin released nucleotides even faster than D11Q, and we speculate that D11N actin also exerts its toxic effects in vivo through a defective interaction with cofilin. We have recently found that two other dominant negative actin mutants are also defective in cofilin binding, and we propose that the defective cofilin binder is a major class of dominant negative actin mutants.