Mapping of drebrin binding site on F-actin

Drebrin Protects Assembled Actin from INF2-FFC-mediated Severing and Stabilizes Cell Protrusions

Highly specialized cells, such as neurons and podocytes, have arborized morphologies that serve their specific functions. Actin cytoskeleton and its associated proteins are responsible for the distinctive shapes of cells. The mechanism of their cytoskeleton regulation - contributing to cell shape maintenance - is yet to be fully clarified. Inverted formin 2 (INF2), one of the modulators of the cytoskeleton, is an atypical formin that can both polymerize and depolymerize actin filaments depending on its molar ratio to actin. Prior work has established that INF2 binds to the sides of actin filaments and severs them. Drebrin is another actin-binding protein that also binds filaments laterally and stabilizes them, but the interplay between drebrin and INF2 on actin filament stabilization is not well understood. Here, we have used biochemical assays, electron microscopy, and total internal reflection fluorescence microscopy imaging to show that drebrin protects actin filaments from severing by INF2 without inhibiting its polymerization activity. Notably, truncated drebrin - DrbA1-300 - is sufficient for this protection, though not as effective as the full-length protein. INF2 and drebrin are abundantly expressed in highly specialized cells and are crucial for the temporal regulation of their actin cytoskeleton, consistent with their involvement in peripheral neuropathy.

Mapping Molecular Interaction Interface Between Diaphanous Formin-2 and Neuron-Specific Drebrin A

Actin cytoskeleton is critical for neuronal shape and function. Drebrin and formins are key regulators of neuronal actin networks. Neuron-specific drebrin A is highly enriched in dendritic spines (postsynaptic terminals) of mature excitatory neurons. Decreased levels of drebrin in dendritic spines is a hallmark of Alzheimer's disease, epilepsy, and other complex disorders, which calls for better understanding of its regulatory functions. Drebrin A was previously shown to inhibit actin nucleation and bundling by the diaphanous formin-2 (mDia2) - an actin nucleator that is involved in the initiation of dendritic spines. Characterization of the molecular binding interface between mDia2 and drebrin is necessary to better understand the functional consequences of this interaction and its biological relevance. Prior work suggested a multi-pronged interface between mDia2 and drebrin, which involves both N-terminal and C-terminal regions of the drebrin molecule. Here we used mass spectrometry analysis, deletion mutagenesis, and an array of synthetic peptides of neuronal drebrin A to map its formin-binding interface. The mDia2-interacting interface on drebrin was narrowed down to three highly conserved 9-16 residue sequences that were used to identify some of the key residues involved in this interaction. Deletion of the C-terminal region of drebrin greatly reduces its binding to mDia2 and the extent of its inhibition of formin-driven actin assembly. Moreover, our experiments with formins from different subfamilies showed that drebrin is a specific rather than general inhibitor of these proteins. This work contributes to a molecular level understanding of the formin-drebrin interaction and will help to unravel its biological significance.

Exposure to bisphenol A induces neurotoxicity associated with synaptic and cytoskeletal dysfunction in neuro-2a cells

Bisphenol A (BPA) has been reported to injure the developing and adult brain. However, the underlying mechanism still remains elusive. This study used neuro-2a cells as a cellular model to investigate the neurotoxic effects of BPA. Microtubule-associated protein 2 (MAP2) and tau protein maintain microtubule normal function and promote the normal development of the nervous system. Synaptophysin (SYP) and drebrin (Dbn) proteins are involved in regulating synaptic plasticity. Cells were exposed to the minimum essential medium (MEM), 0.01% (v/v) DMSO, and 150 μM BPA for 12, 24, or 36 h. Morphological analysis revealed that the cells in the BPA-treated groups shrank and collapsed compared with those in the control groups. CCK-8 and lactate dehydrogenase assay (LDH) assays showed that the mortality of neuro-2a cells increased as the BPA treatment time was prolonged. Ultrastructural analysis further revealed that cells demonstrated nucleolar swelling, dissolution of nuclear and mitochondrial membranes, and partial mitochondrial condensation following exposure to BPA. BPA also decreased the relative protein expression levels of MAP2, tau, and Dbn. Interestingly, the relative protein expression levels of SYP increased. These results indicated that BPA inhibited the proliferation and disrupted cytoskeleton and synaptic integrity of neuro-2a cells.

Microscopic Temperature Control Reveals Cooperative Regulation of Actin-Myosin Interaction by Drebrin E

Drebrin E is a regulatory protein of intracellular force produced by actomyosin complexes, that is, myosin molecular motors interacting with actin filaments. The expression level of drebrin E in nerve cells decreases as the animal grows, suggesting its pivotal but unclarified role in neuronal development. Here, by applying the microscopic heat pulse method to actomyosin motility assay, the regulatory mechanism is examined from the room temperature up to 37 °C without a thermal denaturing of proteins. We show that the inhibition of actomyosin motility by drebrin E is eliminated immediately and reversibly during heating and depends on drebrin E concentration. The direct observation of quantum dot-labeled drebrin E implies its stable binding to actin filaments during the heat-induced sliding. Our results suggest that drebrin E allosterically modifies the actin filament structure to regulate cooperatively the actomyosin activity at the maintained in vivo body temperature.

Effects of neuronal drebrin on actin dynamics

Drebrin is a key regulator of actin cytoskeleton in neuronal cells which is critical for synaptic plasticity, neuritogenesis, and neuronal migration. It is also known to orchestrate a cross-talk between actin and microtubules. Decreased level of drebrin is a hallmark of multiple neurodegenerative disorders such as Alzheimer's disease. Despite its established importance in health and disease, we still have a lot to learn about drebrin's interactome and its effects on cytoskeletal dynamics. This review aims to summarize the recently reported novel effects of drebrin on actin and its regulators. Here I will also reflect on the most recent progress made in understanding of the role of drebrin isoforms and posttranslational modifications on its functionality.

Drebrin controls scar formation and astrocyte reactivity upon traumatic brain injury by regulating membrane trafficking

The brain of mammals lacks a significant ability to regenerate neurons and is thus particularly vulnerable. To protect the brain from injury and disease, damage control by astrocytes through astrogliosis and scar formation is vital. Here, we show that brain injury in mice triggers an immediate upregulation of the actin-binding protein Drebrin (DBN) in astrocytes, which is essential for scar formation and maintenance of astrocyte reactivity. In turn, DBN loss leads to defective astrocyte scar formation and excessive neurodegeneration following brain injuries. At the cellular level, we show that DBN switches actin homeostasis from ARP2/3-dependent arrays to microtubule-compatible scaffolds, facilitating the formation of RAB8-positive membrane tubules. This injury-specific RAB8 membrane compartment serves as hub for the trafficking of surface proteins involved in astrogliosis and adhesion mediators, such as β1-integrin. Our work shows that DBN-mediated membrane trafficking in astrocytes is an important neuroprotective mechanism following traumatic brain injury in mice.

Direct Visualization of Actin Filaments and Actin-Binding Proteins in Neuronal Cells

Actin networks and actin-binding proteins (ABPs) are most abundant in the cytoskeleton of neurons. The function of ABPs in neurons is nucleation of actin polymerization, polymerization or depolymerization regulation, bundling of actin through crosslinking or stabilization, cargo movement along actin filaments, and anchoring of actin to other cellular components. In axons, ABP–actin interaction forms a dynamic, deep actin network, which regulates axon extension, guidance, axon branches, and synaptic structures. In dendrites, actin and ABPs are related to filopodia attenuation, spine formation, and synapse plasticity. ABP phosphorylation or mutation changes ABP–actin binding, which regulates axon or dendritic plasticity. In addition, hyperactive ABPs might also be expressed as aggregates of abnormal proteins in neurodegeneration. Those changes cause many neurological disorders. Here, we will review direct visualization of ABP and actin using various electron microscopy (EM) techniques, super resolution microscopy (SRM), and correlative light and electron microscopy (CLEM) with discussion of important ABPs in neuron.

Biased localization of actin binding proteins by actin filament conformation

The assembly of actin filaments into distinct cytoskeletal structures plays a critical role in cell physiology, but how proteins localize differentially to these structures within a shared cytoplasm remains unclear. Here, we show that the actin-binding domains of accessory proteins can be sensitive to filament conformational changes. Using a combination of live cell imaging and in vitro single molecule binding measurements, we show that tandem calponin homology domains (CH1-CH2) can be mutated to preferentially bind actin networks at the front or rear of motile cells. We demonstrate that the binding kinetics of CH1-CH2 domain mutants varies as actin filament conformation is altered by perturbations that include stabilizing drugs and other binding proteins. These findings suggest that conformational changes of actin filaments in cells could help to direct accessory binding proteins to different actin cytoskeletal structures through a biophysical feedback loop.

Neuronal drebrin A directly interacts with mDia2 formin to inhibit actin assembly

Dendritic spines (DS) are actin-rich postsynaptic terminals of neurons that are critical for higher-order brain functions. Maturation of DS is accompanied by a change in actin architecture from linear to branched filamentous structures. Presumably, the underlying cause of this is a switch in a mode of actin assembly from formin-driven to Arp2/3-mediated via an undefined mechanism. Here we present data suggesting that neuron-specific actin-binding drebrin A may be a part of such a switch. It is well documented that DS are highly enriched in drebrin A, which is critical for their plasticity and function. At the same time, mDia2 is known to mediate the formation of filopodia-type (immature) spines. We found that neuronal drebrin A directly interacts with mDia2 formin. Drebrin inhibits formin-mediated nucleation of actin and abolishes mDia2-induced actin bundling. Using truncated protein constructs we identified the domain requirements for drebrin–mDia2 interaction. We hypothesize that accumulation of drebrin A in DS (that coincides with spine maturation) leads to inhibition of mDia2-driven actin polymerization and, therefore, may contribute to a change in actin architecture from linear to branched filaments.

Drebrin Isoforms Critically Regulate NMDAR- and mGluR-Dependent LTD Induction

Drebrin is an actin-binding protein that is preferentially expressed in the brain. It is highly localized in dendritic spines and regulates spine shapes. The embryonic-type (drebrin E) is expressed in the embryonic and early postnatal brain and is replaced by the adult-type (drebrin A) during development. In parallel, NMDA receptor (NMDAR)-dependent long-term depression (LTD) of synaptic transmission, induced by low-frequency stimulation (LFS), is dominant in the immature brain and decreases during development. Here, we report that drebrin regulates NMDAR-dependent and group 1 metabotropic glutamate receptor (mGluR)-dependent LTD induction in the hippocampus. While LFS induced NMDAR-dependent LTD in the developing hippocampus in wild-type (WT) mice, it did not induce LTD in developing drebrin E and A double knockout (DXKO) mice, indicating that drebrin is required for NMDAR-dependent LTD. On the other hand, LFS induced robust LTD dependent on mGluR5, one of group 1 mGluRs, in both developing and adult brains of drebrin A knockout (DAKO) mice, in which drebrin E is expressed throughout development and adulthood. Agonist-induced mGluR-dependent LTD was normal in WT and DXKO mice; however, it was enhanced in DAKO mice. Also, mGluR1, another group 1 mGluR, was involved in agonist-induced mGluR-dependent LTD in DAKO mice. These data suggest that abnormal drebrin E expression in adults promotes group 1 mGluR-dependent LTD induction. Therefore, while drebrin expression is critical for NMDAR-dependent LTD induction, developmental conversion from drebrin E to drebrin A prevents robust group 1 mGluR-dependent LTD.

The role of drebrin in dendritic spines

Dendritic spines form typical excitatory synapses in the brain and their shapes vary depending on synaptic inputs. It has been suggested that the morphological changes of dendritic spines play an important role in synaptic plasticity. Dendritic spines contain a high concentration of actin, which has a central role in supporting cell motility, and polymerization of actin filaments (F-actin) is most likely involved in spine shape changes. Drebrin is an actin-binding protein that forms stable F-actin and is highly accumulated within dendritic spines. Drebrin has two isoforms, embryonic-type drebrin E and adult-type drebrin A, that change during development from E to A. Inhibition of drebrin A expression results in a delay of synapse formation and inhibition of postsynaptic protein accumulation, suggesting that drebrin A has an important role in spine maturation. In mature synapses, glutamate stimulation induces rapid spine-head enlargement during long-term potentiation (LTP) formation. LTP stimulation induces Ca²⁺ entry through N-methyl-d-aspartate (NMDA) receptors, which causes drebrin exodus from dendritic spines. Once drebrin exits from dendritic spine heads, the dynamic actin pool increases in spine heads to facilitate F-actin polymerization. To maintain enlarged spine heads, drebrin-decorated F-actin is thought to reform within the spine heads. Thus, drebrin plays a pivotal role in spine plasticity through regulation of F-actin.

The role of the drebrin/EB3/Cdk5 pathway in dendritic spine plasticity, implications for Alzheimer's disease

The drebrin/EB3/Cdk5 intracellular signalling pathway couples actin filaments to dynamic microtubules in cellular settings where cells are changing shape. The pathway has been most intensively studied in neuronal development, particularly neuritogenesis and neuronal migration, and in synaptic plasticity at dendritic spines in mature neurons. Drebrin is an actin filament side-binding and bundling protein that stabilises actin filaments. The end-binding (EB) proteins are microtubule plus-end tracking proteins (+TIPs) that localise to the growing plus-ends of dynamic microtubules and regulate their behavior and the binding of other +TIP proteins. EB3 binds specifically to drebrin when drebrin is bound to actin filaments, for example at the base of a growth cone filopodium, and EB3 is located at the plus-end of a growing microtubule inserting into the filopodium. This interaction therefore forms the basis for coupling dynamic microtubules to actin filaments in growth cones of developing neurons. Appropriate responses to growth cone guidance cues depend on actin filament/microtubule co-ordination in the growth cone, although the role of the drebrin/EB3/Cdk5 pathway in this context has not been directly tested. A similar cytoskeleton coupling pathway operates in dendritic spines in mature neurons where the activity-dependent insertion of dynamic microtubules into dendritic spines is facilitated by drebrin binding to EB3. Microtubule insertion into dendritic spines drives spine maturation during long-term potentiation and therefore has a role in synaptic plasticity and memory formation. In Alzheimer's disease and related chronic neurodegenerative diseases, there is an early and dramatic loss of drebrin from dendritic spines that precedes synapse loss and neurodegeneration and might contribute to a failure of synaptic plasticity and hence to cognitive decline.

Connexin43 Forms Supramolecular Complexes through Non-Overlapping Binding Sites for Drebrin, Tubulin, and ZO-1

Gap junctions are membrane specialization domains identified in most tissue types where cells abut each other. The connexin channels found in these membrane domains are conduits for direct cell-to-cell transfer of ions and molecules. Connexin43 (Cx43) is the most ubiquitous connexin, with critical roles in heart, skin, and brain. Several studies described the interaction between Cx43 and the cytoskeleton involving the actin binding proteins Zonula occludens (ZO-1) and drebrin, as well as with tubulin. However, a direct interaction has not been identified between drebrin and Cx43. In this study, co-IP and NMR experiments were used to demonstrate that the Cx43-CT directly interacts with the highly conserved N-terminus region of drebrin. Three Cx43-CT areas were found to be involved in drebrin binding, with residues 264–275 being critical for the interaction. Mimicking Src phosphorylation within this region (Y265) significantly disrupted the interaction between the Cx43-CT and drebrin. Immunofluorescence showed colocalization of Cx43, drebrin, and F-actin in astrocytes and Vero cells membrane, indicating that Cx43 forms a submembrane protein complex with cytoskeletal and scaffolding proteins. The co-IP data suggest that Cx43 indirectly interacts with F-actin through drebrin. Along with the known interaction of the Cx43-CT with ZO-1 and tubulin, the data presented here for drebrin indicate non-overlapping and separated binding sites for all three proteins for which simultaneous binding could be important in regulating cytoskeleton rearrangements, especially for neuronal migration during brain development.

ATP and ADP actin states

This minireview is dedicated to the memory of Henryk Eisenberg and honors his major contributions to many areas of biophysics and to the analysis of macromolecular states and interactions in particular. This work reviews the ATP and ADP states of a ubiquitous protein, actins, and considers the present evidence for and against unique, nucleotide-dependent conformations of this protein. The effects of ATP and ADP on specific structural elements of actins, its loops and clefts, as revealed by mutational, crosslinking, spectroscopic, and EPR methods are discussed. It is concluded that the existing evidence points to dynamic equilibria of these structural elements among various conformational states in both ATP- and ADP-actins, with the nucleotides impacting the equilibria distributions.

Drebrin A regulates hippocampal LTP and hippocampus-dependent fear learning in adult mice

Structural plasticity of dendritic spines, which underlies higher brain functions including learning and memory, is dynamically regulated by the actin cytoskeleton and its associated proteins. Drebrin A is an F-actin-binding protein preferentially expressed in the brain and localized in the dendritic spines of mature neurons. Isoform conversion from drebrin E to drebrin A and accumulation of the latter in dendritic spines occurs during synapse maturation. We have previously demonstrated that drebrin A plays a pivotal role in spine morphogenesis and plasticity. However, it is unclear whether drebrin A plays a specific role in processes required for structural plasticity, and whether drebrin E can substitute in this role. To answer these questions, we analyzed mutant mice (named DAKO mice), in which isoform conversion from drebrin E to drebrin A is disrupted. In DAKO mouse brain, drebrin E continues to be expressed throughout life instead of drebrin A. Electrophysiological studies using hippocampal slices revealed that long-term potentiation of CA1 synapses was impaired in adult DAKO mice, but not in adolescents. In parallel with this age-dependent impairment, DAKO mice exhibited impaired hippocampus-dependent fear learning in an age-dependent manner; the impairment was evident in adult mice, but not in adolescents. In addition, histological investigation revealed that the spine length of the apical dendrite of CA1 pyramidal cells was significantly longer in adult DAKO mice than in wild-type mice. Our data indicate that the roles of drebrin E and drebrin A in brain function are different from each other, that the isoform conversion of drebrin is critical, and that drebrin A is indispensable for normal synaptic plasticity and hippocampus-dependent fear memory in the adult brain.

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

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

Cryo-EM reveals different coronin binding modes for ADP- and ADP-BeFx actin filaments

Essential cellular processes involving the actin cytoskeleton are regulated by auxiliary proteins which can sense the nucleotide state of actin. Here we report cryo electron microscopy (cryoEM) structures at 8.6 Å resolution for ADP- and ADP-BeFx- (mimicking ADP-Pi) bound actin filaments in complex with the β-propeller domain (residues 1–600) of yeast coronin 1 (crn1). Our structures identify the main differences in the interaction of coronin with the two nucleotide states of F-actin. We derived pseudo-atomic models by fitting the atomic structures of actin and coronin into these structures. The identified binding interfaces on actin were confirmed by chemical crosslinking, fluorescence spectroscopy and actin mutagenesis. Importantly, the structures of actin and coronin mapped in this study offer a structural explanation for the nucleotide-dependent effects of coronin on cofilin-assisted remodeling of F-actin.

Actin-binding protein drebrin E is involved in junction dynamics during spermatogenesis

The actin-based cytoskeleton plays a critical role in the seminiferous epithelium during spermatogenesis by conferring cell shape, adhesion, structural support and cell polarity to both Sertoli and developing germ cells, which are essential for spermatogonial stem cell renewal, maintenance of the stem cell niche, cell cycle progression, mitosis, meiosis, spermiogenesis and spermiation. However, few functional studies are found in the literature, which explore the functional significance of actin dynamics in these events. This by and large is due to a lack of information on the proteins that regulate actin dynamics. Herein, we report drebrin E is an integrated component of the apical ectoplasmic specialization (apical ES) and the basal ES at the blood-testis barrier (BTB) in the seminiferous epithelium of the adult rat testis. Using immunohistochemistry and dual-labeled immunofluorescence analysis, drebrin E was found to display a stage-specific localization at the apical ES, as well as at the basal ES at the BTB during the seminiferous epithelial cycle of spermatogenesis. Drebrin E was first detected in stage V tubules at the basal ES with the highest expression at the BTB at stages V and VI, but it diminished considerably by stages VII and VIII and was almost non-detectable until stage IV. At the apical ES, drebrin E was also first detected at stage V, surrounding the entire head of the elongating spermatid, but by stage VI its localization had "shifted" to localize most intensely and almost exclusively to the concave side of the spermatid head. In stage VII tubules, drebrin E co-localized with actin, as well as with two other actin regulatory proteins Eps8 (epidermal growth factor receptor pathway substrate 8, an actin capping and bundling protein) and Arp3 (actin-related protein 3, a component of the Arp2/3 complex known to regulate actin nucleation and branching). The localization of drebrin E at the apical ES was compromised following treatment of rats with adjudin, which is known to exert its destructive effects primarily at the apical ES by inducing premature loss of elongating/elongated spermatids from the epithelium, mimicking "spermiation." Instead of being restricted to the concave side of spermatid heads, drebrin E was found to be mis-localized in the seminiferous epithelium of adjudin-treated rats; it was also present on the convex side of elongating spermatids, but these cells were mis-oriented so that their heads no longer pointed toward the basement membrane. The expression of drebrin E by Sertoli cells was also found to be modulated by TGFβ3 and TNFα. Since Arp3, but not Eps8, was found to bind drebrin E; and cytokines were also shown to affect the cellular distribution of drebrin E and enhance the interaction between drebrin E and Arp3, these findings illustrate that cytokines may regulate BTB dynamics during the epithelial cycle by recruiting drebrin E and Arp3 to the BTB microenvironment to induce changes in the configuration of actin filament bundles at the basal ES. In summary, these findings illustrate drebrin E is working in concert with Arp3 to regulate actin filament bundles at both the apical and the basal ES in the testis, conferring adhesion and cell polarity at both sites during spermatogenesis.

Actin binding proteins and spermiogenesis: Some unexpected findings

Drebrin E, an actin-binding protein lacking intrinsic activity in the regulation of actin dynamics (e.g., polymerization, capping, nucleation, branching, cross-linking, bundling and severing), is known to recruit actin regulatory proteins to a specific cellular site. Herein, we critically evaluate recent findings in the field which illustrate that drebrin E works together with two other actin-binding proteins, namely Arp3 (actin-related protein 3, a component of the Arp2/3 complex that simultaneously controls actin nucleation for polymerization and branching of actin filaments) and Eps8 (epidermal growth factor receptor pathway substrate 8 that controls capping of the barbed ends of actin filaments, as well as actin filament bundling) to regulate the homeostasis of F-actin filament bundles at the ectoplasmic specialization (ES), a testis-specific atypical adherens junction (AJ) in the seminiferous epithelium. This is mediated by the strict temporal and spatial expression of these three actin-binding proteins at the apical and basal ES at the Sertoli cell-spermatid (step 8-19) and Sertoli-Sertoli cell interface, respectively, during the seminiferous epithelial cycle of spermatogenesis. In this Commentary, we put forth a possible model by which drebrin E may be acting as a platform upon which proteins (e.g., Arp3) that are needed to alter the conformation of actin filament bundles at the ES can be recruited to the site, thus facilitating changes in cell shape and cell position in the epithelium during spermiogenesis and spermiation. In short, drebrin E may be acting as a "logistic" distribution center to manage different regulatory proteins at the apical ES, thereby regulating the dynamics of actin filament bundles and modulating the plasticity of the apical ES. This would allow adhesion to be altered continuously throughout the epithelial cycle to accommodate spermatid movement in the seminiferous epithelium during spermiogenesis and spermiation. We also describe a hypothetical model, upon which functional studies can be designed in the future.

Drebrin inhibits cofilin-induced severing of F-actin

Molecular cross-talk between neuronal drebrin A and cofilin is believed to be a part of the activity-dependent cytoskeleton-modulating pathway in dendritic spines. Impairments in this pathway are implicated also in synaptic dysfunction in Alzheimer's disease, Down syndrome, epilepsy, and normal aging. However, up to now the molecular interplay between cofilin and drebrin has not been elucidated. TIRF microscopy and solution experiments revealed that full length drebrin A or its actin binding core (Drb1-300) inhibits, but do not abolish cofilin-induced severing of actin filaments. Cosedimentation experiments showed that F-actin can be fully occupied with combination of these two proteins. The dependence of cofilin binding on fractional saturation of actin filaments with drebrin suggests direct competition between these two proteins for F-actin binding. This implies that cofilin and drebrin can either overcome or reverse the allosteric changes in F-actin induced by the competitor's binding. The ability of cofilin to displace drebrin from actin filaments is pH dependent and is facilitated at acidic pH (6.8). Pre-steady state kinetic experiments reveal that both binding and dissociation of drebrin to/from actin filaments is faster than that reported for cooperative binding of cofilin. We found, that drebrin displacement by cofilin is greatly inhibited when actin severing is abolished, which might be linked to the cooperativity of drebrin binding to actin filaments. Our results contribute to molecular understanding of the competitive interactions of drebrin and cofilin with actin filaments.

Epithelial protein lost in neoplasm modulates platelet-derived growth factor-mediated adhesion and motility of mesangial cells

Mesangial cell migration, regulated by several growth factors, is crucial after glomerulopathy and during glomerular development. Directional migration requires the establishment of a polarized cytoskeletal arrangement, a process regulated by coordinated actin dynamics and focal adhesion turnover at the peripheral ruffles in migrating cells. Here we found high expression of the actin cross-linking protein EPLIN (epithelial protein lost in neoplasm) in mesangial cells. EPLIN was localized in mesangial angles, which consist of actin-containing microfilaments extending underneath the capillary endothelium, where they attach to the glomerular basement membrane. In cultured mesangial cells, EPLIN was localized in peripheral actin bundles at focal adhesions and formed a protein complex with paxillin. The MEK-ERK (extracellular signal-regulated kinase) cascade regulated EPLIN-paxillin interaction and induced translocalization of EPLIN from focal adhesion sites to peripheral ruffles. Knockdown of EPLIN in mesangial cells enhanced platelet-derived growth factor-induced focal adhesion disassembly and cell migration. Furthermore, EPLIN expression was decreased in mesangial proliferative nephritis in rodents and humans in vivo. These results shed light on the coordinated actin remodeling in mesangial cells during restorative remodeling. Thus, changes in expression and localization of cytoskeletal regulators underlie phenotypic changes in mesangial cells in glomerulonephritis.

Myosin II ATPase activity mediates the long-term potentiation-induced exodus of stable F-actin bound by drebrin A from dendritic spines

The neuronal actin-binding protein drebrin A forms a stable structure with F-actin in dendritic spines. NMDA receptor activation causes an exodus of F-actin bound by drebrin A (DA-actin) from dendritic spines, suggesting a pivotal role for DA-actin exodus in synaptic plasticity. We quantitatively assessed the extent of DA-actin localization to spines using the spine-dendrite ratio of drebrin A in cultured hippocampal neurons, and found that (1) chemical long-term potentiation (LTP) stimulation induces rapid DA-actin exodus and subsequent DA-actin re-entry in dendritic spines, (2) Ca²⁺ influx through NMDA receptors regulates the exodus and the basal accumulation of DA-actin, and (3) the DA-actin exodus is blocked by myosin II ATPase inhibitor, but is not blocked by myosin light chain kinase (MLCK) or Rho-associated kinase (ROCK) inhibitors. These results indicate that myosin II mediates the interaction between NMDA receptor activation and DA-actin exodus in LTP induction. Furthermore, myosin II seems to be activated by a rapid actin-linked mechanism rather than slow MLC phosphorylation. Thus the myosin-II mediated DA-actin exodus might be an initial event in LTP induction, triggering actin polymerization and spine enlargement.

The other side of the coin: functional and structural versatility of ADF/cofilins

Several cellular processes rely on the fine tuning of actin cytoskeleton. A central component in the regulation of this cellular machinery is the ADF-H domain proteins. Despite sharing the same domain, ADF-H domain proteins produce a diverse functional landscape in the regulation of the actin cytoskeleton. Recent findings emphasize that the functional and structural features of these proteins can differ not only between ADF-H families but even within the same family. The structural and evolutional background of this functional diversity is poorly understood. This review focuses on the specific functional characteristics of ADF-H domain proteins and how these features can be linked to structural differences in the ADF-H domain and also to different conformational transitions in actin. In the light of recent discoveries we pay special attention to the ADF/cofilin proteins to find tendencies along which the functional and structural diversification is governed through the evolution.

Spikar, a novel drebrin-binding protein, regulates the formation and stabilization of dendritic spines

Dendritic spines are small, actin-rich protrusions on dendrites, the development of which is fundamental for the formation of neural circuits. The actin cytoskeleton is central to dendritic spine morphogenesis. Drebrin is an actin-binding protein that is thought to initiate spine formation through a unique drebrin-actin complex at postsynaptic sites. However drebrin overexpression in neurons does not increase the final density of dendritic spines. In this study, we have identified and characterized a novel drebrin-binding protein, spikar. Spikar is localized in cell nuclei and dendritic spines, and accumulation of spikar in dendritic spines directly correlates with spine density. A reporter gene assay demonstrated that spikar acts as a transcriptional co-activator for nuclear receptors. We found that dendritic spine, but not nuclear, localization of spikar requires drebrin. RNA-interference knockdown and overexpression experiments demonstrated that extranuclear spikar regulates dendritic spine density by modulating de novo spine formation and retraction of existing spines. Unlike drebrin, spikar does not affect either the morphology or function of dendritic spines. These findings indicate that drebrin-mediated postsynaptic accumulation of spikar regulates spine density, but is not involved in regulation of spine morphology.

Drebrin contains a cryptic F-actin-bundling activity regulated by Cdk5 phosphorylation

Drebrin activity in F-actin bundling and filopodia induction relies on two adjacent F-actin binding sites and a Cdk5 phosphorylation-regulated intramolecular inhibitory interaction.

Drebrin is an actin filament (F-actin)–binding protein with crucial roles in neuritogenesis and synaptic plasticity. Drebrin couples dynamic microtubules to F-actin in growth cone filopodia via binding to the microtubule-binding +TIP protein EB3 and organizes F-actin in dendritic spines. Precisely how drebrin interacts with F-actin and how this is regulated is unknown. We used cellular and in vitro assays with a library of drebrin deletion constructs to map F-actin binding sites. We discovered two domains in the N-terminal half of drebrin—a coiled-coil domain and a helical domain—that independently bound to F-actin and cooperatively bundled F-actin. However, this activity was repressed by an intramolecular interaction relieved by Cdk5 phosphorylation of serine 142 located in the coiled-coil domain. Phospho-mimetic and phospho-dead mutants of serine 142 interfered with neuritogenesis and coupling of microtubules to F-actin in growth cone filopodia. These findings show that drebrin contains a cryptic F-actin–bundling activity regulated by phosphorylation and provide a mechanistic model for microtubule–F-actin coupling.

Actin filaments and microtubules in dendritic spines

Dendritic spines are small protrusions emerging from their parent dendrites, and their morphological changes are involved in synaptic plasticity. These tiny structures are composed of thousands of different proteins belonging to several subfamilies such as membrane receptors, scaffold proteins, signal transduction proteins, and cytoskeletal proteins. Actin filaments in dendritic spines consist of double helix of actin protomers decorated with drebrin and ADF/cofilin, and the balance of the two is closely related to the actin dynamics, which may govern morphological and functional synaptic plasticity. During development, the accumulation of drebrin-binding type actin filaments is one of the initial events occurring at the nascent excitatory postsynaptic site, and plays a pivotal role in spine formation as well as small GTPases. It has been recently reported that microtubules transiently appear in dendritic spines in correlation with synaptic activity. Interestingly, it is suggested that microtubule dynamics might couple with actin dynamics. In this review, we will summarize the contribution of both actin filaments and microtubules to the formation and regulation of dendritic spines, and further discuss the role of cytoskeletal deregulation in neurological disorders.

PakB binds to the SH3 domain of Dictyostelium Abp1 and regulates its effects on cell polarity and early development

Dictyostelium p21-activated kinase B (PakB) phosphorylates and activates class I myosins. PakB colocalizes with myosin I to actin-rich regions of the cell, including macropinocytic and phagocytic cups and the leading edge of migrating cells. Here we show that residues 1-180 mediate the cellular localization of PakB. Yeast two-hybrid and pull-down experiments identify two proline-rich motifs in PakB-1-180 that directly interact with the SH3 domain of Dictyostelium actin-binding protein 1 (dAbp1). dAbp1 colocalizes with PakB to actin-rich regions in the cell. The loss of dAbp1 does not affect the cellular distribution of PakB, whereas the loss of PakB causes dAbp1 to adopt a diffuse cytosolic distribution. Cosedimentation studies show that the N-terminal region of PakB (residues 1-70) binds directly to actin filaments, whereas dAbp1 exhibits only a low affinity for filamentous actin. PakB-1-180 significantly enhances the binding of dAbp1 to actin filaments. When overexpressed in PakB-null cells, dAbp1 completely blocks early development at the aggregation stage, prevents cell polarization, and significantly reduces chemotaxis rates. The inhibitory effects are abrogated by the introduction of a function-blocking mutation into the dAbp1 SH3 domain. We conclude that PakB plays a critical role in regulating the cellular functions of dAbp1, which are mediated largely by its SH3 domain.

Drebrin-induced stabilization of actin filaments

Drebrin is a mammalian neuronal protein that binds to and organizes filamentous actin (F-actin) in dendritic spines, the receptive regions of most excitatory synapses that play a crucial role in higher brain functions. Here, the structural effects of drebrin on F-actin were examined in solution. Depolymerization and differential scanning calorimetry assays show that F-actin is stabilized by the binding of drebrin. Drebrin inhibits depolymerization mainly at the barbed end of F-actin. Full-length drebrin and its C-terminal truncated constructs were used to clarify the domain requirements for these effects. The actin binding domain of drebrin decreases the intrastrand disulfide cross-linking of Cys-41 (in the DNase I binding loop) to Cys-374 (C-terminal) but increases the interstrand disulfide cross-linking of Cys-265 (hydrophobic loop) to Cys-374 in the yeast mutants Q41C and S265C, respectively. We also demonstrate, using solution biochemistry methods and EM, the rescue of filament formation by drebrin in different cases of longitudinal interprotomer contact perturbation: the T203C/C374S yeast actin mutant and grimelysin-cleaved skeletal actin (between Gly-42 and Val-43). Additionally, we show that drebrin rescues the polymerization of V266G/L267G, a hydrophobic loop yeast actin mutant with an impaired lateral interface formation between the two filament strands. Overall, our data suggest that drebrin stabilizes actin filaments through its effect on their interstrand and intrastrand contacts.

Regulation of actin dynamics and protein trafficking during spermatogenesis--insights into a complex process

In the mammalian testis, extensive restructuring takes place across the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface during the epithelial cycle of spermatogenesis, which is important to facilitate changes in the cell shape and morphology of developing germ cells. However, precise communications also take place at the cell junctions to coordinate the discrete events pertinent to spermatogenesis, namely spermatogonial renewal via mitosis, cell cycle progression and meiosis, spermiogenesis and spermiation. It is obvious that these cellular events are intimately related to the underlying actin-based cytoskeleton which is being used by different cell junctions for their attachment. However, little is known on the biology and regulation of this cytoskeleton, in particular its possible involvement in endocytic vesicle-mediated trafficking during spermatogenesis, which in turn affects cell adhesive function and communication at the cell-cell interface. Studies in other epithelia in recent years have shed insightful information on the intimate involvement of actin dynamics and protein trafficking in regulating cell adhesion and communications. The goal of this critical review is to provide an updated assessment of the latest findings in the field on how these complex processes are being regulated during spermatogenesis. We also provide a working model based on the latest findings in the field including our laboratory to provide our thoughts on an apparent complicated subject, which also serves as the framework for investigators in the field. It is obvious that this model will be rapidly updated when more data are available in future years.

Drebrin preserves endothelial integrity by stabilizing nectin at adherens junctions

Regulation of cell-cell contacts is essential for integrity of the vascular endothelium. Here, a critical role of the F-actin binding protein drebrin in maintaining endothelial integrity is revealed under conditions mimicking vascular flow. Drebrin knockdown leads to weakening of cell-cell contacts, characterized by loss of nectin from adherens junctions and its subsequent lysosomal degradation. Immunoprecipitation, FRAP and mitochondrial retargeting experiments show that nectin stabilization occurs through a chain of interactions: drebrin binding to F-actin, interaction of drebrin and afadin through their polyproline and PR1-2 regions, and recruitment of nectin through afadin's PDZ region. Key elements are drebrin's modules that confer binding to afadin and F-actin. Evidence is provided by constructs containing afadin's PDZ region coupled to drebrin's F-actin binding region or to lifeact, which restore junctional nectin under knockdown of drebrin or of both drebrin and afadin. Drebrin, containing binding sites for both afadin and F-actin, is thus uniquely equipped to stabilize nectin at endothelial junctions and to preserve endothelial integrity under vascular flow.

Molecular cooperativity of drebrin1-300 binding and structural remodeling of F-actin

Drebrin A, an actin-binding protein, is a key regulatory element in synaptic plasticity of neuronal dendrites. Understanding how drebrin binds and remodels F-actin is important for a functional analysis of their interactions. Conventionally, molecular models for protein-protein interactions use binding parameters derived from bulk solution measurements with limited spatial resolution, and the inherent assumption of homogeneous binding sites. In the case of actin filaments, their structural and dynamic states-as well as local changes in those states-may influence their binding parameters and interaction cooperativity. Here, we probed the structural remodeling of single actin filaments and the binding cooperativity of DrebrinA(1-300) -F-actin using AFM imaging. We show direct evidence of DrebrinA(1-300)-induced cooperative changes in the helical structure of F-actin and observe the binding cooperativity of drebrin to F-actin with nanometer resolution. The data confirm at the in vitro molecular level that variations in the F-actin helical structure can be modulated by cooperative binding of actin-binding proteins.

Drebrin controls neuronal migration through the formation and alignment of the leading process

Formation of a functional nervous system requires neurons to migrate to the correct place within the developing brain. Tangentially migrating neurons are guided by a leading process which extends towards the target and is followed by the cell body. How environmental cues are coupled to specific cytoskeletal changes to produce and guide leading process growth is unknown. One such cytoskeletal modulator is drebrin, an actin-binding protein known to induce protrusions in many cell types and be important for regulating neuronal morphology.

Using the migration of oculomotor neurons as a model, we have shown that drebrin is necessary for the generation and guidance of the leading process. In the absence of drebrin, leading processes are not formed and cells fail to migrate although axon growth and pathfinding appear grossly unaffected. Conversely, when levels of drebrin are elevated the leading processes turn away from their target and as a result the motor neuron cell bodies move along abnormal paths within the brain. The aberrant trajectories were highly reproducible suggesting that drebrin is required to interpret specific guidance cues. The axons and growth cones of these neurons display morphological changes, particularly increased branching and filopodial number but despite this they extend along normal developmental pathways.

Collectively these results show that drebrin is initially necessary for the formation of a leading process and subsequently for this to respond to navigational signals and grow in the correct direction. Furthermore, we have shown that the actions of drebrin can be segregated within individual motor neurons to direct their migration independently of axon guidance.

Putting structure into context: fitting of atomic models into electron microscopic and electron tomographic reconstructions

A complete understanding of complex dynamic cellular processes such as cell migration or cell adhesion requires the integration of atomic level structural information into the larger cellular context. While direct atomic-level information at the cellular level remains inaccessible, electron microscopy, electron tomography and their associated computational image processing approaches have now matured to a point where sub-cellular structures can be imaged in three dimensions at the nanometer scale. Atomic-resolution information obtained by other means can be combined with this data to obtain three-dimensional models of large macromolecular assemblies in their cellular context. This article summarizes some recent advances in this field.

Actin-depolymerizing factor homology domain: a conserved fold performing diverse roles in cytoskeletal dynamics

Actin filaments form contractile and protrusive structures that play central roles in many processes such as cell migration, morphogenesis, endocytosis, and cytokinesis. During these processes, the dynamics of the actin filaments are precisely regulated by a large array of actin-binding proteins. The actin-depolymerizing factor homology (ADF-H) domain is a structurally conserved protein motif, which promotes cytoskeletal dynamics by interacting with monomeric and/or filamentous actin, and with the Arp2/3 complex. Despite their structural homology, the five classes of ADF-H domain proteins display distinct biochemical activities and cellular roles, only parts of which are currently understood. ADF/cofilin promotes disassembly of aged actin filaments, whereas twinfilin inhibits actin filament assembly via sequestering actin monomers and interacting with filament barbed ends. GMF does not interact with actin, but instead binds Arp2/3 complex and promotes dissociation of Arp2/3-mediated filament branches. Abp1 and drebrin are multidomain proteins that interact with actin filaments and regulate the activities of other proteins during various actin-dependent processes. The exact function of coactosin is currently incompletely understood. In this review article, we discuss the biochemical functions, cellular roles, and regulation of the five groups of ADF-H domain proteins.