Transmission of growth cone traction force through apCAM-cytoskeletal linkages is regulated by Src family tyrosine kinase activity.

Tyrosine kinase activity is known to be important in neuronal growth cone guidance. However, underlying cellular mechanisms are largely unclear. Here, we report how Src family tyrosine kinase activity controls apCAM-mediated growth cone steering by regulating the transmission of traction forces through receptor-cytoskeletal linkages. Increased levels of tyrosine phosphorylation were detected at sites where beads coated with apCAM ligands were physically restrained to induce growth cone steering, but not at unrestrained bead binding sites. Interestingly, the rate and level of phosphotyrosine buildup near restrained beads were decreased by the myosin inhibitor 2,3-butanedione-2-monoxime, suggesting that tension promotes tyrosine kinase activation. While not affecting retrograde F-actin flow rates, genistein and the Src family selective tyrosine kinase inhibitors PP1 and PP2 strongly reduced the growth cone's ability to apply traction forces through apCAM-cytoskeletal linkages, assessed using the restrained bead interaction assay. Furthermore, increased levels of an activated Src family kinase were detected at restrained bead sites during growth cone steering events. Our results suggest a mechanism by which growth cones select pathways by sampling both the molecular nature of the substrate and its ability to withstand the application of traction forces.

Protein kinase C activation promotes microtubule advance in neuronal growth cones by increasing average microtubule growth lifetimes.

We describe a novel mechanism for protein kinase C regulation of axonal microtubule invasion of growth cones. Activation of PKC by phorbol esters resulted in a rapid, robust advance of distal microtubules (MTs) into the F-actin rich peripheral domain of growth cones, where they are normally excluded. In contrast, inhibition of PKC activity by bisindolylmaleimide and related compounds had no perceptible effect on growth cone motility, but completely blocked phorbol ester effects. Significantly, MT advance occurred despite continued retrograde F-actin flow-a process that normally inhibits MT advance. Polymer assembly was necessary for PKC-mediated MT advance since it was highly sensitive to a range of antagonists at concentrations that specifically interfere with microtubule dynamics. Biochemical evidence is presented that PKC activation promotes formation of a highly dynamic MT pool. Direct assessment of microtubule dynamics and translocation using the fluorescent speckle microscopy microtubule marking technique indicates PKC activation results in a nearly twofold increase in the typical lifetime of a MT growth episode, accompanied by a 1.7-fold increase and twofold decrease in rescue and catastrophe frequencies, respectively. No significant effects on instantaneous microtubule growth, shortening, or sliding rates (in either anterograde or retrograde directions) were observed. MTs also spent a greater percentage of time undergoing retrograde transport after PKC activation, despite overall MT advance. These results suggest that regulation of MT assembly by PKC may be an important factor in determining neurite outgrowth and regrowth rates and may play a role in other cellular processes dependent on directed MT advance.

Substrate-cytoskeletal coupling as a mechanism for the regulation of growth cone motility and guidance.

Growth cones are highly motile structures at the end of neuronal processes, capable of receiving multiple types of guidance cues and transducing them into directed axonal growth. Thus, to guide the axon toward the appropriate target cell, the growth cone carries out different functions: it acts as a sensor, signal transducer, and motility device. An increasing number of molecular components that mediate axon guidance have been characterized over the past years. The vast majority of these molecules include proteins that act as guidance cues and their respective receptors. In addition, more and more signaling and cytoskeleton-associated proteins have been localized to the growth cone. Furthermore, it has become evident that growth cone motility and guidance depends on a dynamic cytoskeleton that is regulated by incoming guidance information. Current and future research in the growth cone field will be focussed on how different guidance cues transmit their signals to the cytoskeleton and change its dynamic properties to affect the rate and direction of growth cone movement. In this review, we discuss recent evidence that cell adhesion molecules can regulate growth cone motility and guidance by a mechanism of substrate-cytoskeletal coupling.

Localization of unconventional myosins V and VI in neuronal growth cones.

Class V and VI myosins, two of the six known classes of actin-based motor genes expressed in vertebrate brain (Class I, II, V, VI, IX, and XV), have been suggested to be organelle motors. In this report, the neuronal expression and subcellular localization of chicken brain myosin V and myosin VI is examined. Both myosins are expressed in brain during embryogenesis. In cultured dorsal root ganglion (DRG) neurons, immunolocalization of myosin V and myosin VI revealed a similar distribution for these two myosins. Both are present within cell bodies, neurites and growth cones. Both of these myosins exhibit punctate labeling patterns that are found in the same subcellular region as microtubules in growth cone central domains. In peripheral growth cone domains, where individual puncta are more readily resolved, we observe a similar number of myosin V and myosin VI puncta. However, less than 20% of myosin V and myosin VI puncta colocalize with each other in the peripheral domains. After live cell extraction, punctate staining of myosin V and myosin VI is reduced in peripheral domains. However, we do not detect such changes in the central domains, suggesting that these myosins are associated with cytoskeletal/organelle structures. In peripheral growth cone domains myosin VI exhibits a higher extractability than myosin V. This difference between myosin V and VI was also found in a biochemical growth cone particle preparation from brain, suggesting that a significant portion of these two motors has a distinct subcellular distribution.

A diffusion barrier maintains distribution of membrane proteins in polarized neurons.

The asymmetric distribution of proteins to distinct domains in the plasma membrane is crucial to the function of many polarized cells. In epithelia, distinct apical and basolateral surfaces are maintained by tight junctions that prevent diffusion of proteins and lipids between the two domains. Polarized neurons maintain axonal and somatodendritic plasma membrane domains without an obvious physical barrier. Indeed, the artificial lipid Dil encounters no diffusion barrier at the presumptive domain boundary, the axon hillock. By measuring the lateral mobility of membrane proteins using optical tweezers, we show here that some membrane proteins exhibit markedly reduced mobility in the initial segment of the axon. Disruption of F-actin and low levels of dimethyl sulphoxide (DMSO) abolish this diffusion barrier and lead to redistribution of membrane markers that had previously been polarized. Immobilization in the initial segment may reflect, at least in part, differential tethering to cytoskeletal components. Therefore, the ability to maintain a polarized distribution of membrane proteins depends on a specialized domain at the initial segment of the axon, which restricts lateral mobility and serves as a new type of diffusion barrier that acts in the absence of cell-cell contact.

Binding of protein kinase C isoforms to actin in Aplysia.

Recently, two of the 10 vertebrate protein kinase C (PKC) isoforms, PKC betaII and PKC epsilon, have been shown to bind specifically to actin filaments, suggesting that these kinases may regulate cytoskeletal dynamics. Here, we present evidence that two PKCs from the marine mollusk Aplysia californica, PKC Apl I, a Ca2+-activated PKC, and PKC Apl II, a Ca2+-independent PKC most similar to PKC epsilon and eta, also bind F-actin. First, they both cosedimented with purified actin filaments in a phorbol ester-dependent manner. Second, they both translocated to the Triton-insoluble fraction of the nervous system after phorbol ester treatment. PKC Apl II could also partially translocate to actin filaments and associate with the Triton-insoluble fraction in the absence of phorbol esters. Translocation to purified actin filaments was increased in the presence of a PKC inhibitor, suggesting that PKC phosphorylation reduces PKC bound to actin. Although both kinases bound F-actin, actin was not sufficient to activate the kinases. In support of a physiological role for actin-PKC interactions, immunochemical localization of PKC Apl II in neuronal growth cones revealed a striking colocalization with F-actin. Our results are consistent with a role for actin-PKC interactions in regulating cytoskeletal dynamics in Aplysia.

The Ig superfamily cell adhesion molecule, apCAM, mediates growth cone steering by substrate-cytoskeletal coupling.

Dynamic cytoskeletal rearrangements are involved in neuronal growth cone motility and guidance. To investigate how cell surface receptors translate guidance cue recognition into these cytoskeletal changes, we developed a novel in vitro assay where beads, coated with antibodies to the immunoglobulin superfamily cell adhesion molecule apCAM or with purified native apCAM, replaced cellular substrates. These beads associated with retrograde F-actin flow, but in contrast to previous studies, were then physically restrained with a microneedle to simulate interactions with noncompliant cellular substrates. After a latency period of approximately 10 min, we observed an abrupt increase in bead-restraining tension accompanied by direct extension of the microtubule-rich central domain toward sites of apCAM bead binding. Most importantly, we found that retrograde F-actin flow was attenuated only after restraining tension had increased and only in the bead interaction axis where preferential microtubule extension occurred. These cytoskeletal and structural changes are very similar to those reported for growth cone interactions with physiological targets. Immunolocalization using an antibody against the cytoplasmic domain of apCAM revealed accumulation of the transmembrane isoform of apCAM around bead-binding sites. Our results provide direct evidence for a mechanical continuum from apCAM bead substrates through the peripheral domain to the central cytoplasmic domain. By modulating functional linkage to the underlying actin cytoskeleton, cell surface receptors such as apCAM appear to enable the application of tensioning forces to extracellular substrates, providing a mechanism for transducing retrograde flow into guided growth cone movement.

An emerging link between cytoskeletal dynamics and cell adhesion molecules in growth cone guidance.

It has become increasingly evident that growth cone guidance depends on the concerted actions of cytoskeletal proteins, molecular motors and cell adhesion molecules. Recent studies suggest that modulation of coupling between extracellular substrates and intracellular cytoskeletal networks via cell surface receptors is an important mechanism for regulating directed neuronal growth.

An Aplysia cell adhesion molecule associated with site-directed actin filament assembly in neuronal growth cones.

During neuronal growth cone-target interactions, a programmed sequence of cytoskeletal remodeling has been described, involving increased actin assembly at the target site and directed microtubule extension into it. The cell adhesion protein apCAM rapidly accumulates at such interaction sites, suggesting a possible role in regulating cytoskeletal remodeling. To test this hypothesis we crosslinked apCAM to varying degrees with antibodies. Secondary immunocomplexes exhibited a classical patching and capping response; in contrast, high density crosslinking of apCAM by antibody coated beads triggered localized actin assembly accompanied by formation of tail-like actin structures referred to as inductopodia. When beads were derivatized with increasing amounts of anti-apCAM they displayed three sequential dose-dependent kinetic states after binding: (1) lateral diffusion in the plane of the membrane; (2) restricted diffusion due to coupling with underlying F-actin; and (3) translocation in the plane of the membrane driven by de novo actin filament assembly local to bead binding sites, i.e. inductopodia formation. In contrast, lectin coated beads were far less efficient in triggering inductopodia formation despite demonstrated membrane protein binding. This work provides evidence that crosslinking of a diffusable membrane protein, apCAM, to threshold levels, can trigger highly localized actin filament assembly and rapid remodeling of neuronal cytoarchitecture.

Myosin drives retrograde F-actin flow in neuronal growth cones.

Actin filaments assembled at the leading edge of neuronal growth cones are centripetally transported via retrograde F-actin flow, a process fundamental to growth cone guidance and other forms of directed cell motility. Here we investigated the role of myosins in retrograde flow, using two distinct modes of myosin inhibition: microinjection of NEM inactivated myosin S1 fragments, or treatment with 2,3-butanedione-2-monoxime, and inhibitor of myosin ATPase. Both treatments resulted in dose-dependent attenuation of retrograde F-actin flow and growth of filopodia. Growth was cytochalasin sensitive and directly proportional to the degree of myosin inhibition, suggesting that retrograde flow results from superimposition of two independent processes: actin assembly and myosin-based filament retraction. These results provide the first direct evidence for myosin involvement in neuronal growth cone function.

Growth cone advance is inversely proportional to retrograde F-actin flow.

In a previous study, F-actin appeared to play a key role in guiding microtubules during growth cone-target interactions. Here, F-actin flow patterns were assessed to investigate the relationship among F-actin flow, microtubule/organelle protrusion, and rates of outgrowth. We first demonstrated conditions in which surface markers (beads) moved at the same rate as underlying F-actin. These beads were then positioned, using laser tweezers, to assess F-actin movements during target interactions. We found retrograde F-actin flow was attenuated specifically along the target interaction axis in direct proportion to the rate of growth cone advance. Retrograde actin flow adjacent to the interaction axis was unperturbed. Our results suggest that growth cones transduce retrograde F-actin flux into forward movement by modulating F-actin-substrate coupling efficiency.

In vitro motility of immunoadsorbed brain myosin-V using a Limulus acrosomal process and optical tweezer-based assay.

To facilitate functional studies of novel myosins, we have developed a strategy for characterizing the mechanochemical properties of motors isolated by immunoadsorption directly from small amounts of crude tissue extracts. In this initial study, silica beads coated with an antibody that specifically recognizes the tail of myosin-V were used to immunoadsorb this motor protein from brain extracts. The myosin-containing beads were then positioned with optical tweezers onto actin filaments nucleated from Limulus sperm acrosomal processes and observed for motility using high resolution video DIC microscopy. The addition of brush border spectrin to the motility chamber enabled the growth of stable actin filament tracks that were approximately 4-fold longer than filaments grown in the absence of this actin crosslinking protein. The velocity of myosin-V immunoadsorbed from brain extracts was similar to that observed for purified myosin-V that was antibody-linked to beads or assessed using the sliding actin filament assay. Motile beads containing myosin-V immunoadsorbed from brain extracts bound poorly to nucleated actin filaments and were incapable of linear migrations following the addition of a different antibody that specifically recognizes the motor-containing head domain of myosin-V. Myosin-V motility was most robust in the absence of Ca2+. Interestingly, skeletal muscle tropomyosin and brush border spectrin had no detectable effect on myosin-V mechanochemistry. Myosin-V containing beads were also occasionally observed migrating directly on acrosomal processes in the absence of exogenously added actin.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoskeletal reorganization underlying growth cone motility.

Recent studies have implicated cytoskeletal dynamics as an important component in directing neuronal outgrowth. By using modern imaging techniques to observe the kinetics of individual cytoskeletal elements in living cells, these results have converged upon a common theme: functional coupling between the intracellular cytoskeleton and extracellular substrates, and regulation thereof, appears to be crucial in controlling neuronal migration.

Cyclic AMP modulates fast axonal transport in Aplysia bag cell neurons by increasing the probability of single organelle movement.

Stimulation of Aplysia bag cell neurons triggers elevation of cAMP and prolonged secretion of ELH neuropeptide. Using video-enhanced microscopy to track individual organelle movements in bag cell neurons, we find that organelle translocation consists of periods of movement interrupted by stationary episodes. cAMP elevation leads to a 2- to 3-fold enhancement of the average rate of organelle transport in both anterograde and retrograde directions. This effect does not result from alteration of the instantaneous velocity of organelle transport along microtubules, but rather from an increase in the proportion of time individual organelles spend in motion. Biochemical measurements also provided evidence that cAMP elevation promotes ELH peptide translocation from the somata into axons. Enhanced transport of ELH as a result of these effects may contribute to the replenishment of neuropeptide-containing vesicles at release sites during prolonged periods of secretion.

Brain myosin-V is a two-headed unconventional myosin with motor activity.

Chicken myosin-V is a member of a recently recognized class of myosins distinct from both the myosins-I and the myosins-II. We report here the purification, electron microscopic visualization, and motor properties of a protein of this class. Myosin-V molecules consist of two heads attached to an approximately 30 nm stalk that ends in a globular region of unknown function. Myosin-V binds to and decorates F-actin, has actin-activated magnesium-ATPase activity, and is a barbed-end-directed motor capable of moving actin filaments at rates of up to 400 nm/s. Myosin-V does not form filaments. Each myosin-V heavy chain is associated with approximately four calmodulin light chains as well as two less abundant proteins of 23 and 17 kd.

In vitro motilities of the unconventional myosins, brush border myosin-I, and chick brain myosin-V exhibit assay-dependent differences in velocity.

Two types of in vitro motility assays are currently used for examining the mechanochemical properties of purified myosins. The Nitella bead movement assay (Sheetz and Spudich: Nature 303:31-35, 1983) allows determination of both velocity and directionality of movement, but is of limited utility because of the fragile nature of the dissected Nitella internodal cells. On the other hand, the sliding actin filament assay (Kron and Spudich: Proc. Natl. Acad. Sci. U.S.A. 83:6272-6276, 1986) is technically much simpler to perform than the Nitella assay, and is suitable for the study of numerous physiological parameters. As it is currently used, however, the sliding actin filament assay does not indicate the directionality of motor movement. Previous studies have demonstrated that the velocities of filament-forming conventional myosins-II from either muscle or nonmuscle cells are comparable in both motility assays (Umemoto and Sellers: J. Biol. Chem. 265:14864-14869, 1990). However, similar studies using unconventional myosins are lacking. In the present report we have compared the rates of two structurally distinct unconventional myosins: brush border (BB) myosin-I and chick brain (CB) myosin-V (p190-calmodulin), using the sliding actin filament and Nitella-based in vitro motility assays. These two unconventional myosins differ from conventional myosins in that they appear unable to associate into bipolar filaments, and have extended rod-like neck domains which bind multiple calmodulin light chains in a Ca(2+)-sensitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-calmodulin and regulation of brush border myosin-I MgATPase and mechanochemistry.

We examined the Ca(2+)-dependent regulation of brush border (BB) myosin-I by probing the possible roles of the calmodulin (CM) light chains. BB myosin-I MgATPase activity, sensitivity to chymotryptic digestion, and mechanochemical properties were assessed using 1-10 microM Ca2+ and in the presence of exogenously added CM since it has been proposed that this myosin is regulated by calcium-induced CM dissociation from the 119-kD heavy chain. Each of these BB myosin-I properties were dramatically altered by the same threshold of 2-3 microM Ca2+. Enzymatically active NH2-terminal proteolytic fragments of BB myosin-I which lack the CM binding domains (the 78-kD peptide) differ from CM-containing peptides in that the former is completely insensitive to Ca2+. Furthermore, the 78-kD peptide exhibits high levels of MgATPase activity which are comparable to that observed for BB myosin-I in the presence of Ca2+. This suggests that Ca2+ regulates BB myosin-I MgATPase by binding directly to the CM light chains, and that CM acts to repress endogenous MgATPase activity. Ca(2+)-induced CM dissociation from BB myosin-I can be prevented by the addition of exogenous CM. Under these conditions Ca2+ causes a reversible slowing of motility. In contrast, in the absence of exogenous CM, motility is stopped by Ca2+. We demonstrate this reversible slowing is not due to the presence of inactive BB myosin-I molecules exerting a "braking" effect on motile filaments. However, we did observe Ca(2+)-independent slowing of motility by acidic phospholipids, suggesting that factors other than Ca2+ and CM content can affect the mechanochemical properties of BB myosin-I.

Cytoskeletal remodeling during growth cone-target interactions.

Reorganization of the cytoskeleton of neuronal growth cones in response to environmental cues underlies the process of axonal guidance. Most previous studies addressing cytoskeletal changes during growth cone pathfinding have focused on the dynamics of a single cytoskeletal component. We report here an investigation of homophilic growth cone-target interactions between Aplysia bag cell neurons using digitally enhanced video microscopy, which addresses dynamic interactions between actin filaments and microtubules. After physical contact of a growth cone with a physiological target, mechanical coupling occurred after a delay; and then the growth cone exerted forces on and displaced the target object. Subsequent to coupling, F-actin accumulation was observed at the target contact zone, followed by preferential microtubule extension to the same site. After successful target interactions, growth cones typically moved off highly adhesive poly-L-lysine substrates into native target cell surfaces. These events were associated with modulation of both the direction and rate of neurite outgrowth: growth cone migration was typically reoriented to a trajectory along the target interaction axis and rates of advance increased by about one order of magnitude. Directed microtubule movements toward the contact site appeared to be F-actin dependent as target site-specific microtubule extension and bundling could be reversibly randomized by micromolar levels of cytochalasin B in a characteristic manner. Our results suggest that target contacts can induce focal F-actin assembly and reorganization which, in turn, guides target site-directed microtubule redistribution.