Filopodial behavior is dependent on the phosphorylation state of neuronal growth cones

The tumor promoter-activated protein kinase Cs are a system for regulating filopodia

Different protein kinase C (PKC) isoforms have distinct roles in regulating cell functions. The conventional (α, β, γ) and novel (δ, ɛ, η, θ) classes are targets of phorbol ester tumor promoters, which are surrogates of endogenous second messenger, diacylglycerol. The promoter-stimulated disappearance of filopodia was investigated by use of blocking peptides (BPs) that inhibit PKC maturation and/or docking. Filopodia were partially rescued by a peptide representing PKC ɛ hydrophobic sequence, but also by a myristoylated PKC α/β pseudosubstrate sequence, and an inhibitor of T-cell protein tyrosine phosphatase (TC-PTP). The ability to turn over filopodia was widely distributed among PKC isoforms. PKC α and η hydrophobic sequences enhanced filopodia in cells in the absence of tumor promoter treatment. With transcriptional knockdown of PKC α, the content of PKC ɛ predominated over other isoforms. PKC ɛ could decrease filopodia significantly in promoter-treated cells, and this was attributed to ruffling. The presence of PKC α counteracted the PKC ɛ-mediated enhancement of ruffling. The results showed that there were two mechanisms of filopodia downregulation. One operated in the steady-state and relied on PKC α and η. The other was stimulated by tumor promoters and relied on PKC ɛ. Cycles of protrusion and retraction are characteristic of filopodia and are essential for the cell to orient itself during chemotaxis and haptotaxis. By suppressing filopodia, PKC ɛ can create a long-term "memory" of an environmental signal that may act in nature as a mnemonic device to mark the direction of a repulsive signal.

Possible role of cortactin phosphorylation by protein kinase Cα in actin-bundle formation at growth cone

BACKGROUND INFORMATION

Cortactin contributes to growth cone morphogenesis by forming with dynamin, ring-shaped complexes that mechanically bundle and stabilise F-actin. However, the regulatory mechanism of cortactin action is poorly understood.

RESULTS

Immunofluorescence microscopy revealed that protein kinase C (PKC) α colocalises with cortactin at growth cone filopodia in SH-SY5Y neuroblastoma cells. PKC activation by phorbol 12-myristate 13-acetate causes cortactin phosphorylation, filopodial retraction and F-actin-bundle loss. Moreover, PKCα directly phosphorylates cortactin in vitro at S135/T145/S172, mitigating both cortactin's actin-binding and actin-crosslinking activity, whereas cellular expression of a phosphorylation-mimetic cortactin mutant hinders filopodial formation with a significant decrease of actin bundles.

CONCLUSIONS

Our results indicate that PKC-mediated cortactin phosphorylation might be implicated in the maintenance of growth cone.

Filopodia as sensors

Filopodia are sensors on both excitable and non-excitable cells. The sensing function is well documented in neurons and blood vessels of adult animals and is obvious during dorsal closure in embryonic development. Nerve cells extend neurites in a bidirectional fashion with growth cones at the tips where filopodia are concentrated. Their sensing of environmental cues underpins the axon's ability to "guide," bypassing non-target cells and moving toward the target to be innervated. This review focuses on the role of filopodia structure and dynamics in the detection of environmental cues, including both the extracellular matrix (ECM) and the surfaces of neighboring cells. Other protrusions including the stereocilia of the inner ear and epididymus, the invertebrate Type I mechanosensors, and the elongated processes connecting osteocytes, share certain principles of organization with the filopodia. Actin bundles, which may be inside or outside of the excitable cell, function to transduce stress from physical perturbations into ion signals. There are different ways of detecting such perturbations. Osteocyte processes contain an actin core and are physically anchored on an extracellular structure by integrins. Some Type I mechanosensors have bridge proteins that anchor microtubules to the membrane, but bundles of actin in accessory cells exert stress on this complex. Hair cells of the inner ear rely on attachments between the actin-based protrusions to activate ion channels, which then transduce signals to afferent neurons. In adherent filopodia, the focal contacts (FCs) integrated with ECM proteins through integrins may regulate integrin-coupled ion channels to achieve signal transduction. Issues that are not understood include the role of Ca(2+) influx in filopodia dynamics and how integrins coordinate or gate signals arising from perturbation of channels by environmental cues.

Neuronal growth cone retraction relies on proneurotrophin receptor signaling through Rac

Growth of axons and dendrites is a dynamic process that involves guidance molecules, adhesion proteins, and neurotrophic factors. Although neurite extension is stimulated by the neurotrophin nerve growth factor (NGF), we found that the precursor of NGF, proNGF, induced acute collapse of growth cones of cultured hippocampal neurons. This retraction was initiated by an interaction between the p75 neurotrophin receptor (p75NTR) and the sortilin family member SorCS2 (sortilin-related VPS10 domain-containing receptor 2). Binding of proNGF to the p75NTR-SorCS2 complex induced growth cone retraction by initiating the dissociation of the guanine nucleotide exchange factor Trio from the p75NTR-SorCS2 complex, resulting in decreased Rac activity and, consequently, growth cone collapse. The actin-bundling protein fascin was also inactivated, contributing to the destabilization and collapse of actin filaments. These results identify a bifunctional signaling mechanism by which proNGF regulates actin dynamics to acutely modulate neuronal morphology.

Control of neurite outgrowth and growth cone motility by phosphatidylinositol-3-kinase

Phosphatidylinositol-3-kinase (PI-3K) has been reported to affect neurite outgrowth both in vivo and in vitro. Here we investigated the signaling pathways by which PI-3K affects neurite outgrowth and growth cone motility in identified snail neurons in vitro. Inhibition of PI-3K with wortmannin (2 microM) or LY 294002 (25 microM) resulted in a significant elongation of filopodia and in a slow-down of neurite outgrowth. Experiments using cytochalasin and blebbistatin, drugs that interfere with actin polymerization and myosin II activity, respectively, demonstrated that filopodial elongation resulting from PI-3K inhibition was dependent on actin polymerization. Inhibition of strategic kinases located downstream of PI-3K, such as Akt, ROCK, and MEK, also caused significant filopodial elongation and a slow-down in neurite outgrowth. Another growth cone parameter, filopodial number, was not affected by inhibition of PI-3K, Akt, ROCK, or MEK. A detailed study of growth cone behavior showed that the filopodial elongation induced by inhibiting PI-3K, Akt, ROCK, and MEK was achieved by increasing two motility parameters: the rate with which filopodia extend (extension rate) and the time that filopodia spend elongating. Whereas the inhibition of ROCK or Akt (both activated by the lipid kinase activity of PI-3K) and MEK (activated by the protein kinase activity of PI-3K) had additive effects, simultaneous inhibition of Akt and ROCK showed no additive effect. We further demonstrate that the effects on filopodial dynamics investigated were calcium-independent. Taken together, our results suggest that inhibition of PI-3K signaling results in filopodial elongation and a slow-down of neurite advance, reminiscent of growth cone searching behavior.

How do endothelial cells orientate?

In sprouting angiogenesis, endothelial cells must orientate in the tissue environment in order to effectively invade tissues and form vascular patterns according to the local needs. Here, we review recent data indicating that sprouting angiogenesis is a guided process resembling axonal guidance and insect trachea formation. Angiogenesis requires functional specialization of endothelial cells within the sprout. Cells situated at the tip of the sprouts sense and navigate the environment using long filopodia, whereas cells in the sprout stalks proliferate and form a vascular lumen. Migration of the tip cells depends on a graded distribution of VEGF-A and activation of VEGFR2 located on the tip-cell filopodia. Proliferation in the stalk is concomitantly regulated by the local VEGF-A levels. Thus, the shape of the VEGF-A gradient controls the balance between tip cell migration and stalk cell proliferation, which in turn determines the initial vascular pattern. An imbalance between the two processes may explain why abnormal vascular patterns develop in pathological angiogenesis.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Guiding neuronal growth cones using Ca2+ signals

Pathfinding by growing axons in the developing or regenerating nervous system is guided by gradients of molecular guidance cues. The neuronal growth cone, located at the ends of axons, uses surface receptors to sense these cues and to transduce guidance information to cellular machinery that mediates growth and turning responses. Cytoplasmic Ca2+ signals have key roles in regulating this motility. Global growth cone Ca2+ signals can regulate cytoskeletal elements and membrane dynamics to control elongation, whereas Ca2+ signals localized to one side of the growth cone can cause asymmetric activation of effector enzymes to steer the growth cone. Modulating Ca2+ levels in the growth cone might overcome inhibitory signals that normally prevent regeneration in the central nervous system.

PLA2 and secondary metabolites of arachidonic acid control filopodial behavior in neuronal growth cones

The neuronal growth cone provides the sensory and motor structure that guides neuronal processes to their target. The ability of a growth cone to navigate correctly depends on its filopodia, which sample the environment by continually extending and retracting as the growth cone advances. Several second messengers systems that are activated upon contact with extracellular cues have been reported to affect growth cone morphology by changing the length and number of filopodia. Because recent studies have suggested that guidance cues can signal via G-protein coupled receptors to regulate phospholipases, we here investigated whether phospholipase A2 (PLA2) may control filopodial dynamics and could thereby affect neuronal pathfinding. Employing identified Helisoma neurons in vitro, we demonstrate that inhibition of PLA2 with 2 microM BPB caused a 40.3% increase in average filopodial length, as well as a 37.3% reduction in the number of filopodia on a growth cone. The effect of PLA2 inhibition on filopodial length was mimicked by the inhibition of G-proteins with 500 ng/ml pertussis toxin and was partially blocked by the simultaneous activation of PLA2 with 50 nM melittin. We provide evidence that PLA2 acts via production of arachidonic acid (AA), because (1) the effect of inhibition of PLA2 could be counteracted by supplying AA exogenously, and (2) the inhibition of cyclooxygenase, which metabolizes AA into prostaglandins, also increased filopodial length. We conclude that filopodial contact with extracellular signals that alter the activity of PLA2 can control growth cone morphology and may affect neuronal pathfinding by regulating the sensory radius of navigating growth cones.

Imaging filopodia dynamics in the mouse blastocyst

During mammalian development, the first cell lineage diversification event occurs in the blastocyst, when the trophectoderm (TE) and the inner cell mass (ICM) become established. Part of the TE (polar) remains in contact with the ICM and differs from the mural TE (mTE) which is separated from the ICM by a cavity known as the blastocoele. The presence of filopodia connecting ICM cells with the distant mural TE cells through the blastocoelic fluid was investigated in this work. We describe two types of actin-based cell projections found in freshly dissected and in vitro cultured expanding blastocysts: abundant short filopodia projecting into the blastocoelic cavity that present a continuous undulating behavior; and long, thin traversing filopodia connecting the mural TE with the ICM. Videomicroscopy analyses revealed the presence of vesicle-like structures moving along traversing filopodia and dynamic cytoskeletal rearrangements. These observations, together with immunolocalization of the FGFR2 and the ErbB3 receptors to these cell extensions, suggest that they display signal transduction activity. We propose that traversing filopodia are employed by mitotic mTE cells to receive the required signals for cell division after they become distant to the ICM.

The phosphorylation state of neuronal processes determines growth cone formation after neuronal injury

Growth cones are essential for neuronal pathfinding during embryonic development and again after injury, when they aid in neuronal regeneration. This study was aimed at investigating the role of kinases in the earliest events in neuronal regeneration, namely, the formation of new growth cones from injured neuronal processes. Neurites of identified snail neurons grown in vitro were severed, and the formation of growth cones was observed from the ends of such transected processes. Under control conditions, all neurites formed a new growth cone within 45 min of transection. In contrast, growth cone formation in the presence of a general kinase inhibitor, K252a, was significantly inhibited. Moreover, decreasing the phosphorylation state of neurites by activating protein phosphatases with C2-ceramide also reduced growth cone formation. Pharmacological analysis with specific kinase inhibitors suggested that targets of protein kinase C (PKC) and tyrosine kinase (PTK) phosphorylation control growth cone formation. Inhibition of PKC with calphostin C and cerebroside completely blocked growth cone formation, whereas the inhibition of PTK with erbstatin analog significantly reduced growth cone formation. In contrast, inhibitors of protein kinase A, protein kinase G, CaM-kinase II, myosin light-chain kinase, Rho kinase, and PI-3 kinase had little or no effect 45 min after transection. These results suggest that the transformation underlying the formation of a growth cone from an injured (transected) neurite stump is highly sensitive to the phosphorylation state of key target proteins. Therefore, injury-induced signaling events will determine the outcome of neuronal regeneration through their action on kinase and phosphatase activities.

CD151 regulates epithelial cell-cell adhesion through PKC- and Cdc42-dependent actin cytoskeletal reorganization

CD151, a member of the tetraspanin family proteins, tightly associates with integrin α3β1 and localizes at basolateral surfaces of epithelial cells. We found that overexpression of CD151 in A431 cells accelerated intercellular adhesion, whereas treatment of cells with anti-CD151 mAb perturbed the integrity of cortical actin filaments and cell polarity. E-Cadherin puncta formation, indicative of filopodia-based adhesion zipper formation, as well as E-cadherin anchorage to detergent-insoluble cytoskeletal matrix, was enhanced in CD151-overexpressing cells. Levels of GTP-bound Cdc42 and Rac were also elevated in CD151-overexpressing cells, suggesting the role of CD151 in E-cadherin–mediated cell–cell adhesion as a modulator of actin cytoskeletal reorganization. Consistent with this possibility, engagement of CD151 by the substrate-adsorbed anti-CD151 mAb induced prominent Cdc42-dependent filopodial extension, which along with E-cadherin puncta formation, was strongly inhibited by calphostin C, a protein kinase C (PKC) inhibitor. Together, these results indicate that CD151 is involved in epithelial cell–cell adhesion as a modulator of PKC- and Cdc42-dependent actin cytoskeletal reorganization.

Eicosanoid activation of protein kinase C epsilon: involvement in growth cone repellent signaling

Exposure of growing neurons to thrombin or semaphorin 3A stimulates a receptor-mediated signaling cascade that results in collapse of their growth cones. This collapse response necessitates eicosanoid production, as we have shown earlier. The present report investigates whether and which protein kinase C (PKC) isoforms may be activated by such eicosanoids. To examine these questions, we isolated growth cones from fetal rat brain and tested whether thrombin or the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), could activate endogenous growth cone PKC. We show that both thrombin and 12(S)-HETE stimulate the phosphorylation of the myristoylated alanine-rich protein kinase C substrate, an 87-kDa adhesion site protein. Furthermore, we show both with immunoprecipitated and with recombinant PKC that 12(S)-HETE activation is selective for the epsilon isoform and does not require accessory proteins. Last, we demonstrate that PKC activation is necessary for thrombin-induced growth cone collapse. These data indicate that eicosanoid-mediated repellent effects result from the direct and selective activation of PKCepsilon and suggest the involvement of myristoylated alanine-rich protein kinase C substrate phosphorylation in growth cone collapse.

Spatial localization of mono-and diphosphorylated myosin II regulatory light chain at the leading edge of motile HeLa cells

Nonmuscle myosin II activity is regulated by phosphorylation of the myosin II regulatory light chain (MRLC) at Ser19 or at both Thr18 and Ser19, and the phosphorylation of MRLC promotes the contractility and stability of actomyosin. To analyze the states of MRLC phosphorylation at the leading edge in the motile HeLa cells, we have examined the subcellular distribution of monophosphorylated or diphosphorylated form of MRLC using a confocal microscope. The cross-sectional imaging revealed that monophosphorylated MRLC distributed throughout the cortical region and the leading edge, but its fluorescent signal was much stronger at the leading edge. This distribution pattern of monophosphorylated MRLC was almost identical to those of myosin II and F-actin. On the other hand, diphosphorylated MRLC is localized at the base of leading edge, spatially very close to the substrate, and colocalized with F-actin in part at the base of filopodia. Diphosphorylated MRLC was hardly detectable at the tip of filopodia and the cell cortical region, where monophosphorylated MRLC was clearly detected. These localization patterns suggest that myosin II is activated at the leading edge, especially at the base but not the tip of filopodia in motile cells. Next, we analyzed the cells expressing GFP-tagged recombinant MRLCs. Expression of GFP-tagged diphosphorylatable and monophosphorylatable MRLCs led to a significant increase in the filopodial number, compared with the cells expressing nonphosphorylatable MRLC. This result indicated that expression of phosphorylatable MRLC enhances the formation of filopodia at the wound edge.

Local calcium changes regulate the length of growth cone filopodia

Previous studies have demonstrated that the free intracellular calcium concentration ([Ca(2+)](i)) in growth cones can act as an important regulator of growth cone behavior. Here we investigated whether there is a spatial and temporal correlation between [Ca(2+)](i) and one particular aspect of growth cone behavior, namely the regulation of growth cone filopodia. Calcium was released from the caged compound NP-EGTA (o-nitrophenyl EGTA tetrapotassium salt) to simulate a signaling event in the form of a transient increase in [Ca(2+)](i). In three different experimental paradigms, we released calcium either globally (within an entire growth cone), regionally (within a small area of the lamellipodium), or locally (within a single filopodium). We demonstrate that global photolysis of NP-EGTA in growth cones caused a transient increase in [Ca(2+)](i) throughout the growth cone and elicited subsequent filopodial elongation that was restricted to the stimulated growth cone. Pharmacological blockage of either calmodulin or the Ca(2+)-dependent phosphatase, calcineurin, inhibited the effect of uncaging calcium, suggesting that these enzymes are acting downstream of calcium. Regional uncaging of calcium in the lamellipodium caused a regional increase in [Ca(2+)](i), but induced filopodial elongation on the entire growth cone. Elevation of [Ca(2+)](i) locally within an individual filopodium resulted in the elongation of only the stimulated filopodium. These findings suggest that the effect of an elevation of [Ca(2+)](i) on filopodial behavior depends on the spatial distribution of the calcium signal. In particular, calcium signals within filopodia can cause filopodial length changes that are likely a first step towards directed filopodial steering events seen during pathfinding in vivo.

Knowing how to navigate: mechanisms of semaphorin signaling in the nervous system

Neuronal connections are made during embryonic development with astonishing precision to ultimately form the physical basis for the central nervous system's main capacity: information processing. Over the past few decades, much has been learned about the general principles of axon guidance. A key finding to emerge is that extracellular cues play decisive roles in establishing the connections. One family of such cues, the semaphorin proteins, was first identified as repellents for navigating axons during brain wiring. Recent studies have implicated these molecules in many other processes of neuronal development, including axonal fasciculation, target selection, neuronal migration, and dendritic guidance, as well as in the remodeling and repair of the adult nervous system. It appears that responding neuronal processes sense these semaphorin signals by a family of transmembrane molecules, namely the plexins, even though neuropilins were also found to be required for mediating the interaction between plexins and class 3 semaphorins. Our understanding of the intracellular signaling machinery linking the receptors to the cytoskeleton machinery is still incomplete, but several molecules have been implicated in mediating or modulating semaphorin-induced responses. Adding to the complexity of semaphorin biology, new findings implicate semaphorins in functioning not only as signaling ligands, but also as signal-transducing receptors. Thus, semaphorins may serve as important probes for exploring the mechanisms of intercellular communication during the development and function of the nervous system.

Alpha4 integrin is expressed during peripheral nerve regeneration and enhances neurite outgrowth

We have shown previously that repair in the peripheral nervous system is associated with a reversion to an embryonic pattern of alternative splicing of the extracellular matrix molecule fibronectin. One of the consequent changes is a relative increase in the number of fibronectins expressing the binding site for alpha4 integrins. Here we show that alpha4 integrins are expressed on dorsal root ganglion neuron cell bodies and growth cones in the sciatic nerve during regeneration and that the interaction of alpha4 integrin with alternatively spliced isoforms of recombinant fibronectins containing the alpha4 binding site enhances neurite outgrowth in dorsal root ganglion neurons. The pheochromocytoma (PC12) neuronal cell line, which normally extends neurites poorly on fibronectin, does so efficiently when alpha4 is expressed in the cells. Experiments using chimeric integrins expressed in PC12 cells show that the alpha4 cytoplasmic domain is necessary and sufficient for this enhanced neurite outgrowth. In both dorsal root ganglion neurons and PC12 cells the alpha4 cytoplasmic domain is tightly linked to the intracellular adapter protein paxillin. These experiments suggest an important role for alpha4 integrin and paxillin in peripheral nerve regeneration and show how alternative splicing of fibronectin may provide a mechanism to enhance repair after injury.

A mammalian homolog of yeast MOB1 is both a member and a putative substrate of striatin family-protein phosphatase 2A complexes

Striatin and S/G(2) nuclear autoantigen (SG2NA) are related proteins that contain membrane binding domains and associate with protein phosphatase 2A (PP2A) and many additional proteins that may be PP2A regulatory targets. Here we identify a major member of these complexes as class II mMOB1, a mammalian homolog of the yeast protein MOB1, and show that its phosphorylation appears to be regulated by PP2A. Yeast MOB1 is critical for cytoskeletal reorganization during cytokinesis and exit from mitosis. We show that mMOB1 associated with PP2A is not detectably phosphorylated in asynchronous murine fibroblasts. However, treatment with the PP2A inhibitor okadaic acid induces phosphorylation of PP2A-associated mMOB1 on serine. Moreover, specific inhibition of PP2A also results in hyperphosphorylation of striatin, SG2NA, and three unidentified proteins, suggesting that these proteins may also be regulated by PP2A. Indirect immunofluorescence produced highly similar staining patterns for striatin, SG2NA, and mMOB1, with the highest concentrations for each protein adjacent to the nuclear membrane. We also present evidence that these complexes may interact with each other. These data are consistent with a model in which PP2A may regulate mMOB1, striatin, and SG2NA to modulate changes in the cytoskeleton or interactions between the cytoskeleton and membrane structures.