Actin-binding proteins take the reins in growth cones

Emerging mechanisms in morphogen-mediated axon guidance

Early in animal development, gradients of secreted morphogenic molecules, such as Sonic hedgehog (Shh), Wnt and TGFbeta/Bmp family members, regulate cell proliferation and determine the fate and phenotype of the target cells by activating well-characterized signalling pathways, which ultimately control gene transcription. Shh, Wnt and TGFbeta/Bmp signalling also play an important and evolutionary conserved role in neural circuit assembly. They regulate neuronal polarization, axon and dendrite development and synaptogenesis, processes that require rapid and local changes in cytoskeletal organization and plasma membrane components. A key question then is whether morphogen signalling at the growth cone uses similar mechanisms and intracellular pathway components to those described for morphogen-mediated cell specification. This review discusses recent advances towards the understanding of this problem, showing how Shh, Wnt and TGFbeta/Bmp have adapted their 'classical' signalling pathways or adopted alternative and novel molecular mechanisms to influence different aspects of neuronal circuit formation.

Axon growth and guidance: receptor regulation and signal transduction

The development of precise connectivity patterns during the establishment of the nervous system depends on the regulated action of diverse, conserved families of guidance cues and their neuronal receptors. Determining how these signaling pathways function to regulate axon growth and guidance is fundamentally important to understanding wiring specificity in the nervous system and will undoubtedly shed light on many neural developmental disorders. Considerable progress has been made in defining the mechanisms that regulate the correct spatial and temporal distribution of guidance receptors and how these receptors in turn signal to the growth cone cytoskeleton to control steering decisions. This review focuses on recent advances in our understanding of the mechanisms mediating growth cone guidance with a particular emphasis on the control of guidance receptor regulation and signaling.

The Arp2/3 complex, UNC-115/abLIM, and UNC-34/Enabled regulate axon guidance and growth cone filopodia formation in Caenorhabditis elegans

BACKGROUND

While many molecules involved in axon guidance have been identified, the cellular and molecular mechanisms by which these molecules regulate growth cone morphology during axon outgrowth remain to be elucidated. The actin cytoskeleton of the growth cone underlies the formation of lamellipodia and filopodia that control growth cone outgrowth and guidance. The role of the Arp2/3 complex in growth cone filopodia formation has been controversial, and other mechanisms of growth cone filopodia formation remain to be described.

RESULTS

Here we show that mutations in genes encoding the Arp2/3 complex (arx genes) caused defects in axon guidance. Analysis of developing growth cones in vivo showed that arx mutants displayed defects in filopodia and reduced growth cone size. Time-lapse analysis of growth cones in living animals indicated that arx mutants affected the rate of growth cone filopodia formation but not filopodia stability or length. Two other actin modulatory proteins, UNC-115/abLIM and UNC-34/Enabled, that had been shown previously to affect axon guidance had overlapping roles with Arp2/3 in axon guidance and also affected the rate of filopodia initiation but not stability or length.

CONCLUSION

Our results indicate that the Arp2/3 complex is required cell-autonomously for axon guidance and growth cone filopodia initiation. Furthermore, they show that two other actin-binding proteins, UNC-115/abLIM and UNC-34/Enabled, also control growth cone filopodia formation, possibly in parallel to Arp2/3. These studies indicate that, in vivo, multiple actin modulatory pathways including the Arp2/3 complex contribute to growth cone filopodia formation during growth cone outgrowth.

Identification of functional marker proteins in the mammalian growth cone

Identification of proteins in the mammalian growth cone has the potential to advance our understanding of this critical regulator of neuronal growth and formation of neural circuit; however, to date, only one growth cone marker protein, GAP-43, has been reported. Here, we successfully used a proteomic approach to identify 945 proteins present in developing rat forebrain growth cones, including highly abundant, membrane-associated and actin-associated proteins. Almost 100 of the proteins appear to be highly enriched in the growth cone, as determined by quantitative immunostaining, and for 17 proteins, the results of RNAi suggest a role in axon growth. Most of the proteins we identified have not previously been implicated in axon growth and thus their identification presents a significant step forward, providing marker proteins and candidate neuronal growth-associated proteins.

Disruption of the cytoskeleton during Semaphorin 3A induced growth cone collapse correlates with differences in actin organization and associated binding proteins

Repulsive guidance cues induce growth cone collapse or collapse and retraction. Collapse results from disruption and loss of the actin cytoskeleton. Actin-rich regions of growth cones contain binding proteins that influence filament organization, such as Arp2/3, cortactin, and fascin, but little is known about the role that these proteins play in collapse. Here, we show that Semaphorin 3A (Sema 3A), which is repulsive to mouse dorsal root ganglion neurons, has unequal effects on actin binding proteins and their associated filaments. The immunofluorescence staining intensity of Arp-2 and cortactin decreases relative to total protein; whereas in unextracted growth cones fascin increases. Fascin and myosin IIB staining redistribute and show increased overlap. The degree of actin filament loss during collapse correlates with filament superstructures detected by rotary shadow electron microscopy. Collapse results in the loss of branched f-actin meshworks, while actin bundles are partially retained to varying degrees. Taken together with the known affects of Sema 3A on actin, this suggests a model for collapse that follows a sequence; depolymerization of actin meshworks followed by partial depolymerization of fascin associated actin bundles and their movement to the neurite to complete collapse. The relocated fascin associated actin bundles may provide the substrate for actomyosin contractions that produce retraction.

Functional versatility of transcription factors in the nervous system: the SRF paradigm

Individual transcription factors in the brain frequently display broad functional versatility, thereby controlling multiple cellular outputs. In accordance, neuron-restricted mutagenesis of the murine Srf gene, encoding the transcription factor serum response factor (SRF), revealed numerous SRF functions in the nervous system. First, SRF controls immediate early gene (IEG) activation associated with perception of synaptic activity, learning and memory. Second, processes linked to actin cytoskeletal dynamics are mediated by SRF, such as developmental neuronal migration, outgrowth and pathfinding of neurites, as well as synaptic targeting. Therefore, SRF seems to be instrumental in converting synaptic activity into plasticity-associated structural changes in neuronal connectivities. This highlights the decisive role of SRF in integrating cytoskeletal actin dynamics and nuclear gene expression. Finally, we relate SRF to the multi-functional transcription factor CREB and point out overlapping, distinct and concerted functions of these two transcriptional regulators in the brain.

Eps8 regulates axonal filopodia in hippocampal neurons in response to brain-derived neurotrophic factor (BDNF)

A novel signaling cascade controlling actin polymerization in response to extracellular signals regulates filopodia formation and likely also neuronal synapse formation.

The regulation of filopodia plays a crucial role during neuronal development and synaptogenesis. Axonal filopodia, which are known to originate presynaptic specializations, are regulated in response to neurotrophic factors. The structural components of filopodia are actin filaments, whose dynamics and organization are controlled by ensembles of actin-binding proteins. How neurotrophic factors regulate these latter proteins remains, however, poorly defined. Here, using a combination of mouse genetic, biochemical, and cell biological assays, we show that genetic removal of Eps8, an actin-binding and regulatory protein enriched in the growth cones and developing processes of neurons, significantly augments the number and density of vasodilator-stimulated phosphoprotein (VASP)-dependent axonal filopodia. The reintroduction of Eps8 wild type (WT), but not an Eps8 capping-defective mutant, into primary hippocampal neurons restored axonal filopodia to WT levels. We further show that the actin barbed-end capping activity of Eps8 is inhibited by brain-derived neurotrophic factor (BDNF) treatment through MAPK-dependent phosphorylation of Eps8 residues S624 and T628. Additionally, an Eps8 mutant, impaired in the MAPK target sites (S624A/T628A), displays increased association to actin-rich structures, is resistant to BDNF-mediated release from microfilaments, and inhibits BDNF-induced filopodia. The opposite is observed for a phosphomimetic Eps8 (S624E/T628E) mutant. Thus, collectively, our data identify Eps8 as a critical capping protein in the regulation of axonal filopodia and delineate a molecular pathway by which BDNF, through MAPK-dependent phosphorylation of Eps8, stimulates axonal filopodia formation, a process with crucial impacts on neuronal development and synapse formation.

Neurons communicate with each other via specialized cell–cell junctions called synapses. The proper formation of synapses (“synaptogenesis”) is crucial to the development of the nervous system, but the molecular pathways that regulate this process are not fully understood. External cues, such as brain-derived neurotrophic factor (BDNF), trigger synaptogenesis by promoting the formation of axonal filopodia, thin extensions projecting outward from a growing axon. Filopodia are formed by elongation of actin filaments, a process that is regulated by a complex set of actin-binding proteins. Here, we reveal a novel molecular circuit underlying BDNF-stimulated filopodia formation through the regulated inhibition of actin-capping factor activity. We show that the actin-capping protein Eps8 down-regulates axonal filopodia formation in neurons in the absence of neurotrophic factors. In contrast, in the presence of BDNF, the kinase MAPK becomes activated and phosphorylates Eps8, leading to inhibition of its actin-capping function and stimulation of filopodia formation. Our study, therefore, identifies actin-capping factor inhibition as a critical step in axonal filopodia formation and likely in new synapse formation.

Emerging roles for myosin II and cytoplasmic dynein in migrating neurons and growth cones

Motor proteins are involved in a wide range of cellular and subcellular movements. Recent studies have implicated two motor proteins in particular, myosin II and cytoplasmic dynein, in diverse aspects of cell migration. This review focuses on emerging roles for these proteins in the nervous system, with particular emphasis on migrating neurons and neuronal growth cones. The former cells exhibit unusual features of centrosome and nuclear movement, whereas growth cones offer an opportunity to evaluate motor protein function in a region of cytoplasm free of these organelles.

A nuclear actin function regulates neuronal motility by serum response factor-dependent gene transcription

Neuronal motility relies on actin treadmilling. In addition to regulating cytoskeletal dynamics in the cytoplasm, actin modulates nuclear gene expression. We present a hitherto unappreciated cross talk of actin signaling with gene expression governing neuronal motility. Toward this end, we used a novel approach using mutant actins either favoring (G15S) or inhibiting (R62D) F-actin assembly. Overexpressing these mutant actins in mouse hippocampal neurons not only modulated growth-cone function but also neurite elongation, which was ambiguous by traditional pharmacological interference. G15S actin enhanced neurite outgrowth and filopodia number. In contrast, R62D reduced neurite length and impaired growth-cone filopodia formation. Growth-cone collapse induced by ephrin-As, a family of repulsive axon guidance molecules, is impaired upon R62D expression, resulting in perseverance of ring-shaped F-actin filaments. R62D-induced phenotypes strongly resemble neurons lacking SRF (Serum Response Factor). SRF controls gene transcription of various actin isoforms (e.g., Actb, Acta1) and actin-binding proteins (e.g., Gsn) and is the archetypical transcription factor to study actin interplay with transcription. We show that neuronal motility evoked by these actin mutants requires SRF activity. Further, constitutively active SRF partially rescues R62D-induced phenotypes. Conversely, actin signaling regulates neuronal SRF-mediated gene expression. Notably, a nucleus-resident actin (R62D(NLS)) also regulates SRF's transcriptional activity. Moreover, R62D(NLS) decreases neuronal motility similar to the cytoplasmic R62D actin mutant although R62D(NLS) has no access to cytoplasmic actin dynamics. Thus, herein we provide first evidence that neuronal motility not only depends on cytoplasmic actin dynamics but also on the availability of actin to modulate nuclear functions such as gene transcription.

Regulation of GAP-43 at serine 41 acts as a switch to modulate both intrinsic and extrinsic behaviors of growing neurons, via altered membrane distribution

GAP-43 is the major neuronal substrate of protein kinase C (PKC). Its phosphorylation status dictates the severity of pathfinding errors by GAP-43 (+/-) growth cones in vivo, as well as its modulation of actin dynamics in vitro. These experiments show that stably overexpressing cDNAs mutant at its single PKC phosphorylation site at serine41 in retinoic acid treated SH-Sy5Y neuroblastoma cells regulates intrinsic and extrinsic behaviors of growing neurons. Intrinsically, only Wt and pseudophosphorylated GAP-43Ser41Asp precipitated with F-actin and potentiated F-actin - regulated filopodia formation. GAP-43Ser41Asp inhibited neurite outgrowth whereas only unphosphorylatable GAP-43Ser41Ala precipitated neurotubulin, potentiated neurotubulin accumulation in neurites and increased outgrowth. When PI3-kinase was inhibited GAP-43Ser41Asp-mediated filopodia formation was inhibited whereas GAP-43Ser41Ala-mediated neurite extension was potentiated. Extrinsically, only Wt and GAP-43Ser41Asp potentiated both homotypic adhesion and neurite outgrowth on NCAM-expressing monolayers and promoted NCAM stability. With respect to the underlying mechanism, more F-actin and NCAM colocalized with Wt and GAP-43Ser41Asp in detergent resistant membranes (DRMs) isolated from live cells and GAP-43Ser41Asp-mediated functions were insensitive to cholesterol depletion. In contrast, GAP-43Ser41Ala-mediated functions were sensitive to cholesterol depletion. Neither GAP-43Ser41Asp nor GAP-43Ser41Ala was able to protect against growth cone collapse mediated by PIP2 inhibitors. The results show that modification of GAP-43 at its PKC phosphorylation site directs its distribution to different membrane microdomains that have distinct roles in the regulation of intrinsic and extrinsic behaviors in growing neurons.

A common cofilin activity cycle in invasive tumor cells and inflammatory cells

In many cell types, the formation of membrane protrusions and directional migration depend on the spatial and temporal regulation of the actin-binding protein cofilin. Cofilin, which is important for the regulation of actin-polymerization initiation, increases the number of actin free barbed ends through three mechanisms: its intrinsic actin-nucleation activity; binding and severing of existing actin filaments; and recycling actin monomers from old filaments to new ones through its actin-depolymerization activity. The increase in free barbed ends that is caused by cofilin initiates new actin polymerization, which can be amplified by the actin-nucleating ARP2/3 complex. Interestingly, different cell systems seem to have different mechanisms of activating cofilin. The initial activation of cofilin in mammary breast tumors is dependent on PLCgamma, whereas cofilin activation in neutrophils is additionally dependent on dephosphorylation, which is promoted through Rac2 signaling. Although the literature seems to be confusing and inconsistent, we propose that all of the data can be explained by a single activity-cycle model. In this Opinion, we give an overview of cofilin activation in both tumor cells and inflammatory cells, and demonstrate how the differences in cofilin activation that are observed in various cell types can be explained by different starting points in this single common activity cycle.

The trip of the tip: understanding the growth cone machinery

The central component in the road trip of axon guidance is the growth cone, a dynamic structure that is located at the tip of the growing axon. During its journey, the growth cone comprises both 'vehicle' and 'navigator'. Whereas the 'vehicle' maintains growth cone movement and contains the cytoskeletal structural elements of its framework, a motor to move forward and a mechanism to provide traction on the 'road', the 'navigator' aspect guides this system with spatial bias to translate environmental signals into directional movement. The understanding of the functions and regulation of the vehicle and navigator provides new insights into the cell biology of growth cone guidance.

Extremely different behaviours in high and low body weight lines of chicken are associated with differential expression of genes involved in neuronal plasticity

Long-term selection (> 45 generations) for low or high body weight from the same founder population has generated two extremely divergent lines of chickens, the low (LWS) and high weight (HWS) lines, which at the age of selection (56 days) differs by more than nine-fold in body weight. The HWS line chickens are compulsive feeders, whereas, in the LWS line, some individuals are anorexic and others have very low appetites. The involvement of the central nervous system in these behavioural differences has been experimentally supported. We compared a brain region at 0 and 56 days of age containing the major metabolic regulatory regions, including the hypothalamus and brainstem, using a global cDNA array expression analysis. The results obtained show that the long-term selection has produced minor but multiple expression differences. Genes that regulate neuronal plasticity, such as actin filament polymerisation and brain-derived neurotrophic factor, were identified as being differentially expressed. Genes involved in lipid metabolism were over-represented among differentially expressed genes. The expression data confirm that neural systems regulating feeding behaviours in these lines are different. The results suggest that the lines are set in separate developmental trajectories equipped with slightly different nervous systems. We suggest that the lines adapt behaviourally different to changing situations post hatch, such as the transition from dependence on yolk to feeding, in order to obtain energy. The present study has identified and exemplifies the kind of changes that may underlie the extreme differences in such behaviours.

Formin proteins of the DAAM subfamily play a role during axon growth

The regulation of growth cone actin dynamics is a critical aspect of axonal growth control. Among the proteins that are directly involved in the regulation of actin dynamics, actin nucleation factors play a pivotal role by promoting the formation of novel actin filaments. However, the essential nucleation factors in developing neurons have so far not been clearly identified. Here, we show expression data, and use true loss-of-function analysis and targeted expression of activated constructs to demonstrate that the Drosophila formin DAAM plays a critical role in axonal morphogenesis. In agreement with this finding, we show that dDAAM is required for filopodia formation at axonal growth cones. Our genetic interaction, immunoprecipitation and protein localization studies argue that dDAAM acts in concert with Rac GTPases, Profilin and Enabled during axonal growth regulation. We also show that mouse Daam1 rescues the CNS defects observed in dDAAM mutant flies to a high degree, and vice versa, that Drosophila DAAM induces the formation of neurite-like protrusions when expressed in mouse P19 cells, strongly suggesting that the function of DAAM in developing neurons has been conserved during evolution.

Actin in axons: stable scaffolds and dynamic filaments

Actin filaments are thin polymers of the 42 kD protein actin. In mature axons a network of subaxolemmal actin filaments provide stability for membrane integrity and a substrate for short distance transport of cargos. In developing neurons dynamic regulation of actin polymerization and organization mediates axonal morphogenesis and axonal pathfinding to synaptic targets. Other changes in axonal shape, collateral branching, branch retraction, and axonal regeneration, also depend on actin filament dynamics. Actin filament organization is regulated by a diversity of actin-binding proteins (ABP). ABP are the focus of complex extrinsic and intrinsic signaling pathways, and many neurological pathologies and dysfunctions arise from defective regulation of ABP function.

Ultrastructural localization of actin and actin-binding proteins in the nucleus

Nuclear actin plays an important role in such processes as chromatin remodeling, transcriptional regulation, RNA processing, and nuclear export. Recent research has demonstrated that actin in the nucleus probably exists in dynamic equilibrium between monomeric and polymeric forms, and some of the actin-binding proteins, known to regulate actin dynamics in cytoplasm, have been also shown to be present in the nucleus. In this paper, we present ultrastructural data on distribution of actin and various actin-binding proteins (alpha-actinin, filamin, p190RhoGAP, paxillin, spectrin, and tropomyosin) in nuclei of HeLa cells and resting human lymphocytes. Probing extracts of HeLa cells for the presence of actin-binding proteins also confirmed their presence in nuclei. We report for the first time the presence of tropomyosin and p190RhoGAP in the cell nucleus, and the spatial colocalization of actin with spectrin, paxillin, and alpha-actinin in the nucleolus.

Axon guidance: asymmetric signaling orients polarized outgrowth

A network of connections is established as neural circuits form between neurons. To make these connections, neurons initiate asymmetric axon outgrowth in response to extracellular guidance cues. Within the specialized growth cones of migrating axons, F-actin and microtubules asymmetrically accumulate where an axon projects forward. Although many guidance cues, receptors and intracellular signaling components that are required for axon guidance have been identified, the means by which the asymmetry is established and maintained is unclear. Here, we discuss recent studies in invertebrate and vertebrate organisms that define a signaling module comprising UNC-6 (the Caenorhabditis elegans ortholog of netrin), UNC-40 (the C. elegans ortholog of DCC), PI3K, Rac and MIG-10 (the C. elegans ortholog of lamellipodin) and we consider how this module could establish polarized outgrowth in response to guidance cues.

The Arp2/3 activators WAVE and WASP have distinct genetic interactions with Rac GTPases in Caenorhabditis elegans axon guidance

In the developing nervous system, axons are guided to their targets by the growth cone. Lamellipodial and filopodial protrusions from the growth cone underlie motility and guidance. Many molecules that control lamellipodia and filopodia formation, actin organization, and axon guidance have been identified, but it remains unclear how these molecules act together to control these events. Experiments are described here that indicate that, in Caenorhabditis elegans, two WH2-domain-containing activators of the Arp2/3 complex, WVE-1/WAVE and WSP-1/WASP, act redundantly in axon guidance and that GEX-2/Sra-1 and GEX-3/Kette, molecules that control WAVE activity, might act in both pathways. WAVE activity is controlled by Rac GTPases, and data are presented here that suggest WVE-1/WAVE and CED-10/Rac act in parallel to a pathway containing WSP-1/WASP and MIG-2/RhoG. Furthermore, results here show that the CED-10/WVE-1 and MIG-2/WSP-1 pathways act in parallel to two other molecules known to control lamellipodia and filopodia and actin organization, UNC-115/abLIM and UNC-34/Enabled. These results indicate that at least three actin-modulating pathways act in parallel to control actin dynamics and lamellipodia and filopodia formation during axon guidance (WASP-WAVE, UNC-115/abLIM, and UNC-34/Enabled).

Cell biology of embryonic migration

Cell migration is an evolutionarily conserved mechanism that underlies the development and functioning of uni- and multicellular organisms and takes place in normal and pathogenic processes, including various events of embryogenesis, wound healing, immune response, cancer metastases, and angiogenesis. Despite the differences in the cell types that take part in different migratory events, it is believed that all of these migrations occur by similar molecular mechanisms, whose major components have been functionally conserved in evolution and whose perturbation leads to severe developmental defects. These mechanisms involve intricate cytoskeleton-based molecular machines that can sense the environment, respond to signals, and modulate the entire cell behavior. A big question that has concerned the researchers for decades relates to the coordination of cell migration in situ and its relation to the intracellular aspects of the cell migratory mechanisms. Traditionally, this question has been addressed by researchers that considered the intra- and extracellular mechanisms driving migration in separate sets of studies. As more data accumulate researchers are now able to integrate all of the available information and consider the intracellular mechanisms of cell migration in the context of the developing organisms that contain additional levels of complexity provided by extracellular regulation. This review provides a broad summary of the existing and emerging data in the cell and developmental biology fields regarding cell migration during development.

Dynamic length regulation of sensory stereocilia

Stereocilia, the mechanosensory organelles of hair cells, are a distinctive class of actin-based cellular protrusions with an unparalleled ability to regulate their lengths over time. Studies on actin turnover in stereocilia, as well as the identification of several deafness-related proteins essential for proper stereocilia structure and function, provide new insights into the mechanisms and molecules involved in stereocilia length regulation and long-term maintenance. Comparisons of ongoing investigations on stereocilia with studies on other actin protrusions offer new opportunities to further understand common principles for length regulation, the diversity of its mechanisms, and how the specific needs of each cell are met.

Relationship between the flexibility and the motility of actin filaments: effects of pH

Both the sliding velocity of fluorescently labeled actin filament and its persistence length as an index of the bending flexibility of the filament were examined in the motility assay as varying the pH values of the solution for preparing actin filaments. When the pH value was varied from 5.0 to 9.0 in the solution in which actin filaments were formed from the constituent monomers, the motile performance of Mg(2+) bound actin filaments (Mg-F-actin) was apparently suppressed compared to the case of Ca(2+) bound ones (Ca-F-actin). The persistence length for Ca-F-actin gradually increased with the increase of the pH value while the similar length for Mg-F-actin remained rather independent of the value. The largest sliding velocity of the filament, on the other hand, obtained at the persistence length of roughly 6 microm for both cases of Mg-F-actin and Ca-F-actin.

Tropomyosin and ADF/cofilin as collaborators and competitors

Dynamics of actin filaments is pivotal to many fundamental cellular processes such as Dcytokinesis, motility, morphology, vesicle and organelle transport, gene transcription and senescence. In vivo kinetics of actin filament dynamics is far from the equilibrium in vitro and these profound differences are attributed to large number of regulatory proteins. In particular, proteins of the ADF/cofilin family greatly increase actin filament dynamics by severing filaments and enhancing depolymerization of ADP-actin monomers from their pointed ends. Cofilin binds cooperatively to a minor conformer of F-actin in which the subunits are slightly under rotated along the filament helical axis. At high stoichiometry of cofilin to actin subunits, cofilin actually stabilizes actin filaments. Many isoforms oftropomyosin appear to compete with ADF/cofilin proteins for binding to actin filaments. Tropomyosin isoforms studied to date prefer binding to the "untwisted" conformer of F-actin and through their protection and stabilization of F-actin, recruit myosin II and assemble different actin superstructures from the cofilin-actin filaments. However, some tropomyosin isoforms may synergize with ADF/cofilin to enhance filament dynamics, suggesting that the different isoforms of tropomyosins, many of which show developmental or tissue specific expression profiles, play major roles in the assembly and turnover of actin superstructures. Different actin superstructures can overlap both spatially and temporally within a cell, but can be differentiated from each other based upon their kinetic and kinematic properties. Furthermore, local regulation of ADF/cofilin activity through signal transduction pathways could be one mechanism to alter the dynamic balance in F-actin-binding of certain tropomyosin isoforms in subcellular domains.