Regulation of chick dorsal root ganglion growth cone filopodia by protein kinase C

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.

Protein kinase C activation decreases peripheral actin network density and increases central nonmuscle myosin II contractility in neuronal growth cones

PKC activation enhances myosin II contractility in the central growth cone domain while decreasing actin density and increasing actin network flow rates in the peripheral domain. This dual mode of action has mechanistic implications for interpreting reported effects of PKC on growth cone guidance and neuronal regeneration.

Protein kinase C (PKC) can dramatically alter cell structure and motility via effects on actin filament networks. In neurons, PKC activation has been implicated in repulsive guidance responses and inhibition of axon regeneration; however, the cytoskeletal mechanisms underlying these effects are not well understood. Here we investigate the acute effects of PKC activation on actin network structure and dynamics in large Aplysia neuronal growth cones. We provide evidence of a novel two-tiered mechanism of PKC action: 1) PKC activity enhances myosin II regulatory light chain phosphorylation and C-kinase–potentiated protein phosphatase inhibitor phosphorylation. These effects are correlated with increased contractility in the central cytoplasmic domain. 2) PKC activation results in significant reduction of P-domain actin network density accompanied by Arp2/3 complex delocalization from the leading edge and increased rates of retrograde actin network flow. Our results show that PKC activation strongly affects both actin polymerization and myosin II contractility. This synergistic mode of action is relevant to understanding the pleiotropic reported effects of PKC on neuronal growth and regeneration.

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.

Interplay between cell migration and neurite outgrowth determines SH2B1β-enhanced neurite regeneration of differentiated PC12 cells

The regulation of neurite outgrowth is crucial in developing strategies to promote neurite regeneration after nerve injury and in degenerative diseases. In this study, we demonstrate that overexpression of an adaptor/scaffolding protein SH2B1β promotes neurite re-growth of differentiated PC12 cells, an established neuronal model, using wound healing (scraping) assays. Cell migration and the subsequent remodeling are crucial determinants during neurite regeneration. We provide evidence suggesting that overexpressing SH2B1β enhances protein kinase C (PKC)-dependent cell migration and phosphatidylinositol 3-kinase (PI3K)-AKT-, mitogen activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) kinase (MEK)-ERK-dependent neurite re-growth. Our results further reveal a cross-talk between pathways involving PKC and ERK1/2 in regulating neurite re-growth and cell migration. We conclude that temporal regulation of cell migration and neurite outgrowth by SH2B1β contributes to the enhanced regeneration of differentiated PC12 cells.

Activation of protein kinase C isozymes for the treatment of dementias

Memories are much more easily impaired than improved. Dementias, a lasting impairment of memory function, occur in a variety of cognitive disorders and become more clinically dominant as the population ages. Protein kinase C is one of the "cognitive kinases," and plays an essential role in both memory acquisition and maintenance. Deficits in protein kinase C (PKC) signal cascades in neurons represent one of the earliest changes in the brains of patients with Alzheimer's disease (AD) and other types of memory impairment, including those related to cerebral ischemia and ischemic stroke. Inhibition or impairment of PKC activity results in compromised learning and memory, whereas an appropriate activation of certain PKC isozymes leads to an enhancement of learning and memory and/or antidementic effects. In preclinical studies, PKC activators have been shown to increase the expression and activity of PKC isozymes, thereby restoring PKC signaling and downstream activity, including stimulation of neurotrophic activity, synaptic/structural remodeling, and synaptogenesis in the hippocampus and related cortical areas. PKC activators also reduce the accumulation of neurotoxic amyloid and tau protein hyperphosphorylation and support anti-apoptotic processes in the brain. These observations strongly suggest that PKC pharmacology may represent an attractive area for the development of effective cognition-enhancing therapeutics for the treatment of dementias.

Protein kinase C regulates neurite outgrowth in spinal cord neurons

OBJECTIVE

The functional roles of protein kinase C (PKC) in the neurite outgrowth and nerve regeneration remain controversial. The present study was aimed to investigate the role of PKC in neurite outgrowth, by studying their regulatory effects on neurite elongation in spinal cord neurons in vitro.

METHODS

The anterior-horn neurons of spinal cord from embryonic day 14 (E14) Sprague-Dawley (SD) rats were dissociated, purified and cultured in the serum-containing medium. The ratio of membrane-PKC (mPKC) activity to cytoplasm-PKC (cPKC) activity (m/c-PKC) was studied at different time points during culture.

RESULTS

Between 3-11 d of culture, the change of m/c-PKC activity ratio and PKC-betaII expression in the neurite were both significantly correlated with neurite outgrowth (r=0.95, P< 0.01; r=0.73, P< 0.01, respectively). Moreover, PMA, an activator of PKC, induced a dramatic elevation in the m/c-PKC activity ratio, accompanied with the increase in neurite length (r=0.99, P< 0.01). In contrast, GF 109203X, an inhibitor of PKC, significantly inhibited neurite elongation, which could not be reversed by PMA.

CONCLUSION

PKC activity may be important in regulating neurite outgrowth in spinal cord neurons, and betaII isoform of PKC probably plays a major role in this process.

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.

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.

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.

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.

Nervous system-derived chondroitin sulfate proteoglycans regulate growth cone morphology and inhibit neurite outgrowth: a light, epifluorescence, and electron microscopy study

Proteoglycans influence aging and plasticity in the nervous system. Particularly prominent are the chondroitin sulfate proteoglycans (CSPGs), which are generally inhibitory to neurite outgrowth. During development, CSPGs facilitate normal guidance, but following nervous system injury and in diseases of aging (e.g., Alzheimer's disease), they block successful regeneration, and are associated with axon devoid regions and degenerating nerve cells. Whereas previous studies used non-nervous system sources of CSPGs, this study analyzed the morphology and behavior of sensory (dorsal root ganglia) neurons, and a human nerve cell model (SH-SY5Y neuroblastoma cells) as they contacted nervous system-derived CSPGs, using a variety of microscopy techniques. The results of these qualitative analyses show that growth cones of both nerve cell types contact CSPGs via actin-based filopodia, sample the CSPGs repeatedly without collapse, and alter their trajectory to avoid nervous system-derived CSPGs. Turning and branching are correlated with increased filopodial sampling, and are common to both neurons and Schwann cells. We show that CSPG expression by rat CNS astrocytes in culture is correlated with sensory neuron avoidance. Further, we show for the first time the ultrastructure of sensory growth cones at a CSPG-laminin border and reveal details of growth cone and neurite organization at this choice point. This type of detailed analysis of the response of growth cones to nervous system-derived CSPGs may lead to an understanding of CSPG function following injury and in diseases of aging, where CSPGs are likely to contribute to aberrant neurite outgrowth, failed or reduced synaptic connectivity, and/or ineffective plasticity.

Regeneration of entorhinal fibers in mouse slice cultures is age dependent and can be stimulated by NT-4, GDNF, and modulators of G-proteins and protein kinase C

Axonal regeneration after lesions is normally not possible in the mature central nervous system, but occurs in the embryonic and neonatal nervous system. Slice cultures offer a convenient experimental system to study the decline of axonal regeneration with increasing maturation of central nervous system tissue. We have used mouse entorhinohippocampal slice cultures to assess regeneration of entorhinal fibers after mechanical lesions in vitro. We found that entorhinal axons regenerate well in cultures derived from postnatal days 5-7 mouse pups when the lesion is made at the second and fourth days in vitro (DIV 2 and DIV 4). Only little regenerative outgrowth is seen after lesions made at DIV 6 and DIV 10. This indicates that a maturation of the cultures occurs within a short time period in vitro resulting in a loss of the regenerative potential. We have used this system to screen for neurotrophic factors and pharmacological compounds that may promote axonal regeneration. Treatments were added to the cultures 1 day before the lesion was made. We found that most added factors did not promote regeneration. Only treatment with the neurotrophic factors NT-4 and GDNF stimulated regeneration in cultures where normally little regeneration is found. A similar improvement of regeneration was found after treatment with pertussis toxin, an inhibitor of G(i)-proteins, and with GF109203X, an inhibitor of protein kinase C. These substances may promote regeneration by interfering with intracellular signaling pathways activated by outgrowth inhibitors. Our findings indicate that the application of neurotrophic factors and the modulation of intracellular signal transduction pathways could be useful strategies to enhance axonal regeneration in a complex microenvironment.

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

Several lines of evidence suggest that phosphorylation events play an important role in transducing neurite outgrowth signals. Here we tested if such phosphorylation events altered filopodial dynamics on neuronal growth cones and thereby might affect pathfinding decisions. The general protein kinase inhibitor K252a caused an increase in the overall length of filopodia, thereby increasing the action radius of a growth cone. Application of specific kinase inhibitors demonstrated that myosin light chain kinase, Ca/calmodulin-dependent kinase II, and protein kinase A were likely not involved in this filopodial response. Inhibition of protein kinase C (PKC) with calphostin C or cerebroside, however, induced filopodial elongation similar to that seen with K252a. Activation of PKC with the phorbol ester PMA produced the opposite effect, namely filopodial shortening. Consistent with this finding, the protein phosphatase activator C(2)-ceramide resulted in a significant increase in filopodial length, whereas application of the protein phosphatase inhibitor okadaic acid caused the opposite effect, filopodial shortening. Lastly, the tyrosine kinase inhibitor genistein also caused filopodial elongation, and this effect could be negated by the tyrosine phosphatase inhibitor sodium ortho-vanadate. Using the calcium indicator fura-2, we further showed that these drugs did not cause a measurable change in the free intracellular calcium concentration ([Ca(2+)](i)) in growth cones. Taken together, these results suggest that the action radius of a growth cone and its resulting pathfinding abilities could be rapidly altered by contact with extracellular cues, leading to changes in the activity of protein kinases and phosphatases.

Protein kinase C activity modulates dendritic differentiation of rat Purkinje cells in cerebellar slice cultures

The molecular mechanisms underlying dendritic differentiation in neurons are currently poorly understood. We used slice cultures from rat cerebellum of postnatal day 8 to investigate the effect of protein kinase C (PKC) activity on dendritic development of Purkinje cells. After 12 days in culture under control conditions, Purkinje cells had developed a typical dendritic tree consisting of a few long primary dendrites with shorter side branches. Following treatment with the PKC agonist, phorbol-12-myristate-13-acetate (PMA), the dendritic tree area was strongly reduced to 32% of control and primary dendrites were short with only a few side branches. Delayed addition of PMA after 6 days resulted in a retraction of existing dendrites, whereas discontinuation of PMA treatment after 6 days resulted in a recovery of the dendritic tree to almost control values. In the presence of the PKC inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide (GF109203X), the dendritic tree area was increased to 158% of control with much more ramified branches after 12 days. The overall morphology of the cultures and the survival of Purkinje cells were unaffected by PKC modulators. Our data show that increased activity of PKC inhibits, and reduced activity of PKC promotes dendritic growth. This suggests that PKC activity is a critical regulator of dendritic growth and differentiation in cerebellar Purkinje cells.