Filopodia: molecular architecture and cellular functions

Local ATP generation by brain-type creatine kinase (CK-B) facilitates cell motility

Background

Creatine Kinases (CK) catalyze the reversible transfer of high-energy phosphate groups between ATP and phosphocreatine, thereby playing a storage and distribution role in cellular energetics. Brain-type CK (CK-B) deficiency is coupled to loss of function in neural cell circuits, altered bone-remodeling by osteoclasts and complement-mediated phagocytotic activity of macrophages, processes sharing dependency on actomyosin dynamics.

Methodology/Principal Findings

Here, we provide evidence for direct coupling between CK-B and actomyosin activities in cortical microdomains of astrocytes and fibroblasts during spreading and migration. CK-B transiently accumulates in membrane ruffles and ablation of CK-B activity affects spreading and migration performance. Complementation experiments in CK-B-deficient fibroblasts, using new strategies to force protein relocalization from cytosol to cortical sites at membranes, confirmed the contribution of compartmentalized CK-B to cell morphogenetic dynamics.

Conclusion/Significance

Our results provide evidence that local cytoskeletal dynamics during cell motility is coupled to on-site availability of ATP generated by CK-B.

Impact of actin rearrangement and degranulation on the membrane structure of primary mast cells: a combined atomic force and laser scanning confocal microscopy investigation

Degranulation of bone marrow-derived mast cells (BMMCs) triggered by antigens (e.g., 2,4-dinitrophenylated bovine serum albumin (DNP-BSA) and secretagogues (e.g., poly-L-lysine) was investigated by combined atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). This combination enables the simultaneous visualization and correlation of membrane morphology with cytoskeletal actin arrangement and intracellular granules. Two degranulation mechanisms and detailed membrane structures that directly corresponded to the two stimuli were revealed. In DNP-BSA triggered activation, characteristic membrane ridges formed in accordance with the rearrangement of underlying F-actin networks. Individual granules were visualized after they released their contents, indicating a "kiss-and-run" pathway. In BMMCs stimulated by poly-L-lysine, lamellopodia and filopodia were observed in association with the F-actin assemblies at and near the cell periphery, whereas craters were observed on the central membrane lacking F-actin. These craters represent a new membrane feature resulting from the "kiss-and-merge" granule fusion. This work provides what we believe is important new insight into the local membrane structures in correlation with the cytoskeleton arrangement and detailed degranulation processes.

Integrins in cell migration--the actin connection

The connection between integrins and actin is driving the field of cell migration in new directions. Integrins and actin are coupled through a physical linkage, which provides traction for migration. Recent studies show the importance of this linkage in regulating adhesion organization and development. Actin polymerization orchestrates adhesion assembly near the leading edge of a migrating cell, and the dynamic cross-linking of actin filaments promotes adhesion maturation. Breaking the linkage between actin and integrins leads to adhesion disassembly. Recent quantitative studies have revealed points of slippage in the linkage between actin and integrins, showing that it is not always efficient. Regulation of the assembly and organization of adhesions and their linkage to actin relies on signaling pathways that converge on components that control actin polymerization and organization.

Involvement of headless myosin X in the motility of immortalized gonadotropin-releasing hormone neuronal cells

Myosin X (Myo X), an unconventional myosin with a tail homology 4-band 4.1/ezrin/radixin/moesin (MyTH4-FERM) tail, is expressed ubiquitously in various mammalian tissues. In addition to the full-length Myo X (Myo X FL), a headless form is synthesized in the brain. So far, little is known about the function of this motor-less Myo X. In this study, the role of the headless Myo X was investigated in immortalized gonadotropin-releasing hormone (GnRH) neuronal cells, NLT. NLT cells overexpressing the headless Myo X formed fewer focal adhesions and spread more slowly than the wild-type NLT cells and GFP-expressing NLT cells. In chemomigration assays, the NLT cells overexpressing the headless Myo X migrated shorter distances and had fewer migratory cells compared with the control NLT cells.

Filopodia: Complex models for simple rods

Filopodia are prominent cell surface projections filled with bundles of linear actin filaments that drive their protrusion. These structures are considered important sensory organelles, for instance in neuronal growth cones or during the fusion of sheets of epithelial tissues. In addition, they can serve a precursor function in adhesion site or stress fibre formation. Actin filament assembly is essential for filopodia formation and turnover, yet the precise molecular mechanisms of filament nucleation and/or elongation are controversial. Indeed, conflicting reports on the molecular requirements of filopodia initiation have prompted researchers to propose different types and/or alternative or redundant mechanisms mediating this process. However, recent data shed new light on these questions, and they indicate that the balance of a limited set of biochemical activities can determine the structural outcome of a given filopodium. Here we focus on discussing our current view of the relevance of these activities, and attempt to propose a molecular mechanism of filopodia assembly based on a single core machinery.

Nonkinase activity of MLCK in elongated filopodia formation and chemotaxis of vascular smooth muscle cells toward sphingosylphosphorylcholine

The actin-myosin interaction of vascular smooth muscle cells (VSMCs) is regulated by myosin light chain kinase (MLCK), which is a fusion protein of the central catalytic domain with the N-terminal actin-binding and C-terminal myosin-binding domains. In addition to the regulatory role of kinase activity mediated by the catalytic domain, nonkinase activity that derives from both terminals is able to exert a regulatory role as reviewed by Nakamura et al. (32). We previously showed that nonkinase activity mediated the filopodia upon the stimulation by sphingosylphosphorylcholine (SPC) (25). To explore the regulatory role of nonkinase activity in chemotaxis, we constructed VSMCs where the expression of MLCK was totally abolished by using a lentivirus-mediated RNAi system. We hypothesized that the MLCK-downregulated VSMCs were unable to form filopodia and to migrate upon SPC stimulation and confirmed the hypothesis. We further constructed a kinase-inactive mutant from bovine cDNA coding wild-type (WT) MLCK by mutating the ATP-binding sites located in the catalytic domain, followed by confirming the presence (absence) of the kinase activity of WT (kinase-inactive mutant). We transfected WT and the mutant into MLCK-downregulated VSMCs. We expected that the transfected VSMCs will recover the ability to induce filopodia and chemotaxis toward SPC and found both constructs rescued the ability. Because they share the actin- and myosin-binding domains, we concluded nonkinase activity plays a major role for SPC-induced migration.

The Toca-1-N-WASP complex links filopodial formation to endocytosis

The transducer of Cdc42-dependent actin assembly (Toca-1)-N-WASP complex was isolated as an essential cofactor for Cdc42-driven actin polymerization in vitro. Toca-1 consists of an N-terminal F-BAR domain, followed by a Cdc42 binding site (HR1 domain) and an SH3 domain, (the N-WASP interacting site). N-WASP is an activator of actin nucleation through the Arp2/3 complex. The aim of the present study was to investigate the cellular function of the Toca-1-N-WASP complex. We report that Toca-1 induces filopodia and neurites as does N-WASP in N1E115 neuroblastoma cells. Toca-1 requires the F-BAR domain, Cdc42 binding site, and SH3 domain to induce filopodia. Toca-1 and N-WASP both require each other to induce filopodia. The expression of Toca-1 and N-WASP affects the distribution, size, and number of Rab5 positive membranes. Toca-1 interacts directly with N-WASP in filopodia and Rab5 membrane as seen by Forster resonance energy transfer. Thus the Toca-1-N-WASP complex localizes to and induces the formation of filopodia and endocytic vesicles. Last, three inhibitors of endocytosis, Dynamin-K44A, Eps15Delta95/295, and clathrin heavy chain RNA interference, block Toca-1-induced filopodial formation. Taken together, these data suggest that the Toca-1-N-WASP complex can link filopodial formation to endocytosis.

Molecular mechanisms of membrane deformation by I-BAR domain proteins

BACKGROUND

Generation of membrane curvature is critical for the formation of plasma membrane protrusions and invaginations and for shaping intracellular organelles. Among the central regulators of membrane dynamics are the BAR superfamily domains, which deform membranes into tubular structures. In contrast to the relatively well characterized BAR and F-BAR domains that promote the formation of plasma membrane invaginations, I-BAR domains induce plasma membrane protrusions through a poorly understood mechanism.

RESULTS

We show that I-BAR domains induce strong PI(4,5)P(2) clustering upon membrane binding, bend the membrane through electrostatic interactions, and remain dynamically associated with the inner leaflet of membrane tubules. Thus, I-BAR domains induce the formation of dynamic membrane protrusions to the opposite direction than do BAR and F-BAR domains. Strikingly, comparison of different I-BAR domains revealed that they deform PI(4,5)P(2)-rich membranes through distinct mechanisms. IRSp53 and IRTKS I-BARs bind membranes mainly through electrostatic interactions, whereas MIM and ABBA I-BARs additionally insert an amphipathic helix into the membrane bilayer, resulting in larger tubule diameter in vitro and more efficient filopodia formation in vivo. Furthermore, FRAP analysis revealed that whereas the mammalian I-BAR domains display dynamic association with filopodia, the C. elegans I-BAR domain forms relatively stable structures inside the plasma membrane protrusions.

CONCLUSIONS

These data define I-BAR domain as a functional member of the BAR domain superfamily and unravel the mechanisms by which I-BAR domains deform membranes to induce filopodia in cells. Furthermore, our work reveals unexpected divergence in the mechanisms by which evolutionarily distinct groups of I-BAR domains interact with PI(4,5)P(2)-rich membranes.

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.

Agrin in the nervous system: synaptogenesis and beyond

The heparan sulfate proteoglycan agrin is best known for its essential role during formation, maintenance and regeneration of the neuromuscular junction. Mutations in agrin-interacting proteins are the genetic basis for a number of neuromuscular disorders. However, agrin is widely expressed in many tissues including neurons and glial cells of the brain, where its precise function is much less understood. Fewer synapses develop in brains that lack agrin, consistent with a function of agrin during CNS synaptogenesis. Recently, a specific transmembrane form of agrin (TM-agrin) was identified that is concentrated at that interneuronal synapses in the brain. Clustering or overexpression of TM-agrin leads to the formation of filopodia-like processes, which might be precursors for CNS synapses. Agrin is subject to defined and activity-dependent proteolytic cleavage by neurotrypsin at synapses and dysregulation of agrin processing might contribute to the development of mental retardation. This review summarizes what is known about the role of agrin during synapse formation at the neuromuscular junction and in the developing CNS and will discuss additional functions of agrin in the adult CNS, in particular during BBB formation, during recovery after traumatic brain injury and in the etiology of diseases, including Alzheimer’s disease and mental retardation.

Virus activated filopodia promote human papillomavirus type 31 uptake from the extracellular matrix

Human papillomaviruses (HPVs), etiological agents of epithelial tumors and cancers, initiate infection of basal human keratinocytes (HKs) facilitated by wounding. Virions bind to HKs and their secreted extracellular matrix (ECM), but molecular roles for wounding or ECM binding during infection are unclear. Herein we demonstrate that HPV31 activates signals promoting cytoskeletal rearrangements and virion transport required for internalization and infection. Activation of tyrosine and PI3 kinases precedes induction of filopodia whereon virions are transported toward the cell body. Coupled with the loss of ECM-bound virions this supports a model whereby virus activated filopodial transport contributes to an increased and protracted virion uptake into susceptible cells.

Retrograde flow and myosin II activity within the leading cell edge deliver F-actin to the lamella to seed the formation of graded polarity actomyosin II filament bundles in migrating fibroblasts

In migrating fibroblasts actomyosin II bundles are graded polarity (GP) bundles, a distinct organization to stress fibers. GP bundles are important for powering cell migration, yet have an unknown mechanism of formation. Electron microscopy and the fate of photobleached marks show actin filaments undergoing retrograde flow in filopodia, and the lamellipodium are structurally and dynamically linked with stationary GP bundles within the lamella. An individual filopodium initially protrudes, but then becomes separated from the tip of the lamellipodium and seeds the formation of a new GP bundle within the lamella. In individual live cells expressing both GFP-myosin II and RFP-actin, myosin II puncta localize to the base of an individual filopodium an average 28 s before the filopodium seeds the formation of a new GP bundle. Associated myosin II is stationary with respect to the substratum in new GP bundles. Inhibition of myosin II motor activity in live cells blocks appearance of new GP bundles in the lamella, without inhibition of cell protrusion in the same timescale. We conclude retrograde F-actin flow and myosin II activity within the leading cell edge delivers F-actin to the lamella to seed the formation of new GP bundles.

High-speed high-resolution imaging of intercellular immune synapses using optical tweezers

Imaging in any plane other than horizontal in a microscope typically requires a reconstruction from multiple optical slices that significantly decreases the spatial and temporal resolution that can be achieved. This can limit the precision with which molecular events can be detected, for example, at intercellular contacts. This has been a major issue for the imaging of immune synapses between live cells, which has generally required the reconstruction of en face intercellular synapses, yielding spatial resolution significantly above the diffraction limit and updating at only a few frames per minute. Strategies to address this issue have usually involved using artificial activating substrates such as antibody-coated slides or supported planar lipid bilayers, but synapses with these surrogate stimuli may not wholly resemble immune synapses between two cells. Here, we combine optical tweezers and confocal microscopy to realize generally applicable, high-speed, high-resolution imaging of almost any arbitrary plane of interest. Applied to imaging immune synapses in live-cell conjugates, this has enabled the characterization of complex behavior of highly dynamic clusters of T cell receptors at the T cell/antigen-presenting cell intercellular immune synapse and revealed the presence of numerous, highly dynamic long receptor-rich filopodial structures within inhibitory Natural Killer cell immune synapses.

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.