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Proteins of the ADF/cofilin family: essential regulators of actin dynamics.

Ubiquitous among eukaryotes, the ADF/cofilins are essential proteins responsible for the high turnover rates of actin filaments in vivo. In vertebrates, ADF and cofilin are products of different genes. Both bind to F-actin cooperatively and induce a twist in the actin filament that results in the loss of the phalloidin-binding site. This conformational change may be responsible for the enhancement of the off rate of subunits at the minus end of ADF/cofilin-decorated filaments and for the weak filament-severing activity. Binding of ADF/cofilin is competitive with tropomyosin. Other regulatory mechanisms in animal cells include binding of phosphoinositides, phosphorylation by LIM kinases on a single serine, and changes in pH. Although vertebrate ADF/cofilins contain a nuclear localization sequence, they are usually concentrated in regions containing dynamic actin pools, such as the leading edge of migrating cells and neuronal growth cones. ADF/cofilins are essential for cytokinesis, phagocytosis, fluid phase endocytosis, and other cellular processes dependent upon actin dynamics.

Actin Depolymerizing Factors↗

Formation of actin-ADF/cofilin rods transiently retards decline of mitochondrial potential and ATP in stressed neurons.

When neurons in culture are transiently stressed by inhibition of ATP synthesis, they rapidly form within their neurites rodlike actin inclusions that disappear when the insult is removed. Oxidative stress, excitotoxic insults, and amyloid beta-peptide oligomers also induce rods. Immunostaining of neurites indicates that these rods also contain the majority of the actin filament dynamizing proteins, actin-depolymerizing factor (ADF) and cofilin (AC). If the rods reappear within 24 h after the stress is removed, the neurite degenerates distal to the rod but with no increase in neuronal death. Here, rods were generated in cultured rat E18 hippocampal cells by overexpression of a green fluorescent protein chimera of AC. Surprisingly, we have found that, for a short period (approximately 60 min) immediately after initial rod formation, the loss of mitochondrial membrane potential (Delta Psi(m)) and ATP in neurites with rods is slower than in neurites without them. The Delta Psi(m) was monitored with the fluorescent dye tetramethylrhodamine methyl ester, and ATP was monitored with the fluorescent ion indicator mag-fura 2. Actin in rods is less dynamic than is filamentous actin in other cytoskeletal structures. Because Delta Psi(m) depends on cellular ATP and because ATP hydrolysis associated with actin filament turnover is responsible for a large fraction of neuronal energy consumption (approximately 50%), the formation of rods transiently protects neurites by slowing filament turnover and its associated ATP hydrolysis.

Actins↗

Renal ischemia induces tropomyosin dissociation-destabilizing microvilli microfilaments.

Ischemic-induced cell injury results in rapid duration-dependent actin-depolymerizing factor (ADF)/cofilin-mediated disruption of the apical microvilli microfilament cores. Because intestinal microvillar microfilaments are bound and stabilized in the terminal web by the actin-binding protein tropomyosin, we questioned whether a protective effect of tropomyosin localization to the terminal web of the proximal tubule microfilament cores is disrupted during ischemic injury. With tropomyosin-specific antibodies, we examined rat cortical sections under physiological conditions and following ischemic injury by confocal microscopy. In addition, Western blot analysis of cortical extracts and urine was undertaken. Our studies demonstrated the presence of tropomyosin isoforms in the proximal tubule microvillar terminal web under physiological conditions and their dissociation in response to 25 min of ischemic injury. This correlated with the excretion of tropomyosin-containing plasma membrane vesicles in urine from ischemic rats. In addition, we noted increased tropomyosin Triton X-100 solubility following ischemia in cortical extracts. These studies suggest tropomyosin binds to and stabilizes the microvillar microfilament core in the terminal web under physiological conditions. With the onset of ischemic injury, we propose that tropomyosin dissociates from the microfilament core providing access to microfilaments in the terminal web for F-actin binding, severing and depolymerizing actions of ADF/cofilin proteins.

Actin Cytoskeleton↗

ADF/cofilin mediates actin cytoskeletal alterations in LLC-PK cells during ATP depletion.

Ischemic injury induces actin cytoskeleton disruption and aggregation, but mechanisms affecting these changes remain unclear. To determine the role of actin-depolymerizing factor (ADF)/ cofilin participation in ischemic-induced actin cytoskeletal breakdown, we utilized porcine kidney cultured cells, LLC-PK(A4.8), and adenovirus containing wild-type (wt), constitutively active, and inactive Xenopus ADF/cofilin linked to green fluorescence protein [XAC(wt)-GFP] in an ATP depletion model. High adenoviral infectivity (70%) in LLC-PK(A4.8) cells resulted in linearly increasing XAC(wt)-GFP and phosphorylated (p)XAC(wt)-GFP (inactive) expression. ATP depletion rapidly induced dephosphorylation, and, therefore, activation, of endogenous pcofilin as well as pXAC(wt)-GFP in conjunction with the formation of fluorescent XAC(wt)-GFP/actin aggregates and rods. No significant actin cytoskeletal alterations occurred with short-term ATP depletion of LLC-PK(A4.8) cells expressing GFP or the constitutively inactive mutant XAC(S3E)-GFP, but cells expressing the constitutively active mutant demonstrated nearly instantaneous actin disruption with aggregate and rod formation. Confocal image three-dimensional volume reconstructions of normal and ATP-depleted LLC-PK(A4.8) cells demonstrated that 25 min of ATP depletion induced a rapid increase in XAC(wt)-GFP apical and basal signal in addition to XAC-GFP/actin aggregate formation. These data demonstrate XAC(wt)-GFP participates in ischemia-induced actin cytoskeletal alterations and determines the rate and extent of these ATP depletion-induced cellular alterations.

Actin Depolymerizing Factors↗

Ischemia activates actin depolymerizing factor: role in proximal tubule microvillar actin alterations.

Apical membrane of renal proximal tubule cells is extremely sensitive to ischemia, with structural alterations occurring within 5 min. These changes are felt secondary to actin cytoskeletal disruption, yet the mechanism responsible is unknown. Actin depolymerizing factor (ADF), a 19-kDa actin-binding protein, has recently been shown to play an important role in regulation of actin filament dynamics. Because ADF is known to mediate pH-dependent F-actin binding, depolymerization, and severing, and because ADF activation occurs by dephosphorylation, we questioned whether ADF played a role in microvilli microfilament disruption during ischemia. To test our hypothesis, we induced renal ischemia in the rat with the clamp model. Initial immunofluorescence and Western blot studies on cortical tissue documented the presence of ADF in proximal tubule cells. Under physiological conditions, ADF was distributed homogeneously throughout the cytoplasm, primarily in the Triton X-100-soluble fraction, and both phosphorylated (pADF) and nonphosphorylated forms were identified. During ischemia, marked alterations occurred. Intraluminal vesicle/bleb structures contained extremely high concentrations of ADF along with G-actin, but not F-actin. Western blot showed a rapidly occurring duration-dependent dephosphorylation of ADF. At 0-30 min of ischemia, total ADF levels were unchanged, whereas pADF decreased significantly to 72% and 19% of control levels, at 5 and 15 min, respectively. Urine collected under physiological conditions did not contain ADF or actin, whereas urine collected after 30 min of ischemia contained both ADF and actin. Reperfusion was associated with normalization of cellular pADF levels, pADF intracellular distribution, and repair of apical microvilli. These data suggest that activation of ADF during ischemia via dephosphorylation is, in part, responsible for apical actin disruption resulting in microvillar destruction and formation of intraluminal vesicles.

Actin Depolymerizing Factors↗

Ischemic injury induces ADF relocalization to the apical domain of rat proximal tubule cells.

Breakdown of proximal tubule cell apical membrane microvilli is an early-occurring hallmark of ischemic acute renal failure. Intracellular mechanisms responsible for these apical membrane changes remain unknown, but it is known that actin cytoskeleton alterations play a critical role in this cellular process. Our laboratory previously demonstrated that ischemia-induced cell injury resulted in dephosphorylation and activation of the actin-binding protein, actin depolymerizing factor [(ADF); Schwartz, N, Hosford M, Sandoval RM, Wagner MC, Atkinson SJ, Bamburg J, and Molitoris BA. Am J Physiol Renal Fluid Electrolyte Physiol 276: F544-F551, 1999]. Therefore, we postulated that ischemia-induced ADF relocalization from the cytoplasm to the apical microvillar microfilament core was an early event occurring before F-actin alterations. To directly investigate this hypothesis, we examined the intracellular localization of ADF in ischemic rat cortical tissues by immunofluorescence and quantified the concentration of ADF in brush-border membrane vesicles prepared from ischemic rat kidneys by using Western blot techniques. Within 5 min of the induction of ischemia, ADF relocalized to the apical membrane region. The length of ischemia correlated with the time-related increase in ADF in isolated brush-border membrane vesicles. Finally, depolymerization of microvillar F-actin to G-actin was documented by using colocalization studies for G- and F-actin. Collectively, these data indicate that ischemia induces ADF activation and relocalization to the apical domain before microvillar destruction. These data further suggest that ADF plays a critical role in microvillar microfilament destruction and apical membrane damage during ischemia.

Actin Depolymerizing Factors↗

Regulation of growth cone actin dynamics by ADF/cofilin.

Nervous system development is reliant on neuronal pathfinding, the process in which axons are guided to their target cells by specific extracellular cues. The ability of neurons to extend over long distances in response to environmental guidance signals is made possible by the growth cone, a highly motile structure found at the end of neuronal processes. Growth cones detect directional cues and respond with either attractive or repulsive movements. The motility of growth cones is dependent on rapid reorganization of the actin cytoskeleton, presumably mediated by actin-associated proteins under the control of incoming guidance signals. This article reviews how one such family of proteins, the ADF/cofilins, are emerging as key regulators of growth cone actin dynamics. These proteins are essential for rapid actin turnover in a variety of different cell types. ADF/cofilins are heavily co-localized with actin in growth cones and are necessary for neurite outgrowth. ADF/cofilin activities are regulated through reversible phosphorylation by LIM kinases and slingshot phosphatases. LIM kinases are downstream effectors of the Rho GTPases Rho, Rac, and Cdc42. Growing evidence suggests that extracellular guidance cues may locally alter actin dynamics by regulating the activity of LIM kinase and ADF/cofilin phosphatases via the Rho GTPases. In this way, ADF/cofilins and their upstream effectors may be pivotal to our understanding of how guidance information is translated into physical alterations of the growth cone actin cytoskeleton.

Actin Depolymerizing Factors↗

The ADF/cofilin family: actin-remodeling proteins.

The ADF/cofilins are a family of actin-binding proteins expressed in all eukaryotic cells so far examined. Members of this family remodel the actin cytoskeleton, for example during cytokinesis, when the actin-rich contractile ring shrinks as it contracts through the interaction of ADF/cofilins with both monomeric and filamentous actin. The depolymerizing activity is twofold: ADF/cofilins sever actin filaments and also increase the rate at which monomers leave the filament's pointed end. The three-dimensional structure of ADF/cofilins is similar to a fold in members of the gelsolin family of actin-binding proteins in which this fold is typically repeated three or six times; although both families bind polyphosphoinositide lipids and actin in a pH-dependent manner, they share no obvious sequence similarity. Plants and animals have multiple ADF/cofilin genes, belonging in vertebrates to two types, ADF and cofilins. Other eukaryotes (such as yeast, Acanthamoeba and slime moulds) have a single ADF/cofilin gene. Phylogenetic analysis of the ADF/cofilins reveals that, with few exceptions, their relationships reflect conventional views of the relationships between the major groups of organisms.

Actin Depolymerizing Factors↗

Specific requirement for two ADF/cofilin isoforms in distinct actin-dependent processes in Caenorhabditis elegans.

Actin depolymerizing factor (ADF)/cofilin is an essential enhancer of actin turnover. Multicellular organisms express multiple ADF/cofilin isoforms in different patterns of tissue distribution. However, the functional significance of different ADF/cofilin isoforms is not understood. The Caenorhabditis elegans unc-60 gene generates two ADF/cofilins, UNC-60A and UNC-60B, by alternative splicing. These two ADF/cofilin proteins have different effects on actin dynamics in vitro, but their functional difference in vivo remains unclear. Here, we demonstrate that the two isoforms are expressed in different tissues and are required for distinct morphogenetic processes. UNC-60A was ubiquitously expressed in most embryonic cells and enriched in adult gonads, intestine and oocytes. In contrast, UNC-60B was specifically expressed in the body wall muscle, vulva and spermatheca. RNA interference of UNC-60A caused embryonic lethality with variable defects in cytokinesis and developmental patterning. In severely affected embryos, a cleavage furrow was formed and progressed but reversed before completion of the cleavage. Also, in some affected embryos, positioning of the blastomeres became abnormal, which resulted in embryonic arrest. In contrast, an unc-60B-null mutant was homozygous viable, underwent normal early embryogenesis and caused disorganization of actin filaments specifically in body wall muscle. These results suggest that the ADF/cofilin isoforms play distinct roles in specific aspects of actin reorganization in vivo.

Actin Depolymerizing Factors↗

Organization and translation of mRNA in sympathetic axons.

Many axons carry out the synthesis of macromolecules independent of their cell bodies but the nature, organization and magnitude of axonal protein synthesis remain unclear. We have examined these features in axons of chick sympathetic neurons in cell culture. In situ hybridization showed that poly(A) mRNA is abundant and non-uniformly distributed in nearly all axons. The specific transcripts for beta-actin and actin-depolymerizing factor (ADF) were also present and non-uniformly distributed in axons, with an approximately hundredfold higher concentration in growth cones, branch points and axonal varicosities than in the axon shaft. Immunoprecipitation using specific antibodies indicates that beta-actin, ADF and neurofilament protein (NF) are translated in axons independently of cell bodies. Quantification of the distribution of beta-actin and ADF mRNAs showed that their ability to enter the axon was likely to be a property of the neuron as a whole rather than of individual axons. To compare the distribution of axonally translated protein to that of mRNA, we performed 35S metabolic labeling with axons separated from their cell bodies. Axonally synthesized proteins were distributed throughout the axons and their synthesis was inhibited by cycloheximide but not by chloramphenicol. Proteins translated mainly or exclusively in axons or cell bodies were both detected by metabolic labeling. Axons separated from their cell bodies synthesized up to 5% as much protein in a 3-hour period as did intact neurons. Because axons in our culture conditions contain approximately 50% of the non-nuclear volume of the neurons, we estimate that axoplasm of sympathetic neurons has a protein synthetic capacity per unit volume equal to 10% that of cell body cytoplasm.

Actin Depolymerizing Factors↗

Low molecular-weight G-actin binding proteins involved in the regulation of actin assembly during myofibrillogenesis.

We previously demonstrated that small G-actin binding proteins, cofilin, ADF and profilin, are involved in the actin dynamics during myofibrillogenesis (OBINATA, T. (1993). Int. Rev. Cytol., 143: 153-189.). To better understand how they are responsible for the regulation of actin assembly, the amounts of the actin-binding proteins were quantified by means of quantitative immunoblotting and compared with that of G-actin pool. The sum of the amounts of cofilin, ADF and profilin was insufficient at early developmental stages but sufficient at later stages to account for the pool of G-actin in muscle cells. We detected expression of thymosin beta 4 at a considerable level in young embryonic but not in adult skeletal muscles. We, therefore, conclude that the G-actin pool in young embryonic skeletal muscle is mainly due to cofilin, ADF, profilin and thymosin beta 4. Switching from a non-muscle-type (NM-) cofilin to a muscle-type (M-) cofilin was observed during muscle development of mammals. In order to clarify cofilin-dependent regulation of actin assembly in muscle cells, cofilin tagged with fluorescence dyes was introduced into C2 myoblasts by a micro injection method. The exogeneous cofilin, but not ADF, caused quick disassembly of actin filaments and accumulated in furrow region of dividing cells. The analogs of the unphosphorylated form (A3-cofilin) and the phosphorylated form (D3-cofilin) were prepared by converting Ser3, a regulatory phosphorylation site, to Ala or Asp. When A3-cofilin and D3-cofilin were injected into living cells, the former was concentrated at the membrane ruffles and cleavage furrow, while the latter showed only diffuse distribution in the cytoplasm. These results suggest that the subcellular distribution of cofilin as well as its interaction with actin in vivo is regulated by its phosphorylation and dephosphorylation.

Actin Depolymerizing Factors↗

Signal-regulated ADF/cofilin activity and growth cone motility.

It is becoming increasingly evident that proteins of the actin depolymerizing factor (ADF)/cofilin family are essential regulators of actin turnover required for many actin-based cellular processes, including motility. ADF can increase actin turnover by either increasing the rate of actin filament treadmilling or by severing actin filaments. In neurons ADF is highly expressed in neuronal growth cones and its activity is regulated by many signals that affect growth cone motility. In addition, increased activity of ADF causes an increase in neurite extension. ADF activity is inhibited upon phosphorylation by LIM kinases (LIMK), kinases activated by members of the Rho family of small GTPases. ADF become dephosphorylated downstream of signal pathways that activate PI-3 kinase or increase levels of intracellular calcium. The growth-regulating effects of ADF together with its ability to be regulated by a wide variety of guidance cues, suggest that ADF may regulate growth cone advance and navigation.

Actin Depolymerizing Factors↗

Regulation of spine morphology and synaptic function by LIMK and the actin cytoskeleton.

Filamentous actin (F-actin) is highly enriched in the dendritic spine, a specialized postsynaptic structure on which the great majority of the excitatory synapses are formed in the mammalian central nervous system (CNS). The protein kinases of the Lim-kinase (LIMK) family are potent regulators of actin dynamics in many cell types and they are abundantly expressed in the CNS, including the hippocampus. Using a combination of genetic manipulations and electrophysiological recordings in mice, we have demonstrated that LIMK-1 signaling is important in vivo in the regulation of the actin cytoskeleton, spine morphology, and synaptic function, including hippocampal long-term potentiation (LTP), a prominent form of long lasting synaptic plasticity thought to be critical to memory formation. Our results provide strong genetic evidence that LIMK and its substrate ADF/cofilin are involved in spine morphology and synaptic properties and are consistent with the notion that the Rho family small GTPases and the actin cytoskeleton are critical to spine structure and synaptic regulation.

Actin Depolymerizing Factors↗

Brain-derived neurotrophic factor regulation of retinal growth cone filopodial dynamics is mediated through actin depolymerizing factor/cofilin.

The molecular mechanisms by which neurotrophins regulate growth cone motility are not well understood. This study investigated the signaling involved in transducing BDNF-induced increases of filopodial dynamics. Our results indicate that BDNF regulates filopodial length and number through a Rho kinase-dependent mechanism. Additionally, actin depolymerizing factor (ADF)/cofilin activity is necessary and sufficient to transduce the effects of BDNF. Our data indicate that activation of ADF/cofilin mimics the effects of BDNF on filopodial dynamics, whereas ADF/cofilin inactivity blocks the effects of BDNF. Furthermore, BDNF promotes the activation of ADF/cofilin by reducing the phosphorylation of ADF/cofilin. Although inhibition of myosin II also enhances filopodial length, our results indicate that BDNF signaling is independent of myosin II activity and that the two pathways result in additive effects on filopodial length. Thus, filopodial extension is regulated by at least two independent mechanisms. The BDNF-dependent pathway works via regulation of ADF/cofilin, independently of myosin II activity.

14-3-3 Proteins↗

Deformations in actin comets from rocketing beads.

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

Actin-Related Protein 2-3 Complex↗

Increase in neurite outgrowth mediated by overexpression of actin depolymerizing factor.

Growth cone motility is regulated by changes in actin dynamics. Actin depolymerizing factor (ADF) is an important regulator of actin dynamics, and extracellular signal-induced changes in ADF activity may influence growth cone motility and neurite extension. To determine this directly, we overexpressed ADF in primary neurons and analyzed neurite lengths. Recombinant adenoviruses were constructed that express wild-type Xenopus ADF/cofilin [XAC(wt)], as well as two mutant forms of XAC, the active but nonphosphorylatable XAC(A3) and the less active, pseudophosphorylated XAC(E3). XAC expression was detectable on Western blots 24 hr after infection and peaked at 3 d in cultured rat cortical neurons. Peak expression was approximately 75% that of endogenous ADF. XAC(wt) expression caused a slight increase in growth cone area and filopodia but decreased filopodia numbers on neurite shafts. At maximal XAC levels, neurite lengths increased >50% compared with controls infected with a green fluorescent protein-expressing adenovirus. Increased neurite extension was directly related to the expression of active XAC. Expression of the XAC(E3) mutant did not increase neurite extension, whereas expression of the XAC(A3) mutant increased neurite extension but to a lesser extent than XAC(wt), which was partially phosphorylated. XAC expression had minimal, if any, impact on F-actin levels and did not result in compensatory changes in the expression of endogenous ADF or actin. However, F-actin turnover appeared to increase based on F-actin loss after treatment with drugs that block actin polymerization. These results provide direct evidence that increased ADF activity promotes process extension and neurite outgrowth.

Actin Depolymerizing Factors↗

Regulating actin dynamics in neuronal growth cones by ADF/cofilin and rho family GTPases.

Growth cone motility and navigation in response to extracellular signals are regulated by actin dynamics. To better understand actin involvement in these processes we determined how and in what form actin reaches growth cones, and once there, how actin assembly is regulated. A continuous supply of actin is maintained at the axon tip by slow transport, the mobile component consisting of an unassembled form of actin. Actin is co-transported with actin-binding proteins, including ADF and cofilin, structurally related proteins essential for rapid turnover of actin filaments in vivo. ADF and cofilin activity is regulated through phosphorylation by LIM kinases, downstream effectors of the Rho family of GTPases, Cdc42, Rac and Rho. Attractive and repulsive extracellular guidance cues might locally alter actin dynamics by binding specific GTPase-linked receptors, activating LIM kinases, and subsequently modulating the activity of ADF/cofilin. ADF is enriched in growth cones and is required for neurite outgrowth. In addition, signals that influence growth cone behavior alter ADF/cofilin phosphorylation, and overexpression of ADF enhances neurite outgrowth. Growth promoting effects of laminin are mimicked by expression of constitutively active Cdc42 and blocked by expression of the dominant negative Cdc42. Repulsive effects of myelin and sema3D on growth cones are blocked by expression of constitutively active Rac1 and dominant negative Rac1, respectively. Thus a series of complex pathways must exist for regulating effectors of actin dynamics. The bifurcating nature of the ADF/cofilin phosphorylation pathway may provide the integration necessary for this complex regulation.

Actin Depolymerizing Factors↗

Searching for the missing link: a role for the actin capping protein in heart failure.

Cardiac Z-discs have historically been classified as passive myocardial elements. Z-discs are positioned at the junction between the cytoskeleton and the myofilaments, providing a physical connection between the sarcomere, nucleus, membrane and sarcoplasmic reticulum. Moreover, numerous molecular messengers congregate at the Z-disc. The combination of physical and chemical signals moving through the Z-disc makes this myocardial element a vital switching station of the heart, and suggests significant regulatory potential. Using the actin capping protein (CapZ) as a representative of the Z-disc, it was found that decreasing CapZ enhances force development and inhibits protein kinase C, a messenger of heart failure. These results indicate the potential for CapZ as a therapeutic target in the management of heart failure. Future research is required to determine the mechanisms by which changes in CapZ impact myocardial function and intracellular signalling, and to develop feasible strategies that can manipulate CapZ in the intact heart.

Actin Capping Proteins↗