Changes in the phosphorylation and distribution of vinculin during nerve growth factor induced neurite outgrowth

Phosphoproteomic profiling of selenate-treated Alzheimer's disease model cells

The reversible phosphorylation of proteins regulates most biological processes, while abnormal phosphorylation is a cause or consequence of many diseases including Alzheimer's disease (AD). One of the hallmarks of AD is the formation of neurofibrillary tangles (NFTs), which is composed of hyperphosphorylated tau proteins. Sodium selenate has been recently found to reduce tau hyperphosphorylation and NFTs formation, and to improve spatial learning and motor performance in AD mice. In the current study, the phosphoproteomics of N2aSW cells treated with selenate were investigated. To avoid missing low-abundance phosphoproteins, both the total proteins of cells and the phosphor-enriched proteins were extracted and subjected to the two-dimensional gel electrophoresis with Pro-Q diamond staining and then LC-MS/MS analysis. A total of 65 proteins were altered in phosphorylation level, of which 39 were up-regulated and 26 were down-regulated. All identified phosphoproteins were bioinformatically annotated according to their physiochemical features, subcellular location, and biological function. Most of these significantly changed phosphoproteins are involved in crucial neural processes such as protesome activity, oxidative stress, cysteine and methionine metabolism, and energy metabolism. Furthermore, decreases were found in homocysteine, phosphor-tau and amyloid β upon selenate treatment. Our results suggest that selenate may intervene in the pathological process of AD by altering the phosphorylation of some key proteins involved in oxidative stress, energy metabolism and protein degradation, thus play important roles in maintaining redox homeostasis, generating ATP, and clearing misfolded proteins and aggregates. The present paper provides some new clues to the mechanism of selenate in AD prevention.

Centrosome-declustering drugs mediate a two-pronged attack on interphase and mitosis in supercentrosomal cancer cells

Classical anti-mitotic drugs have failed to translate their preclinical efficacy into clinical response in human trials. Their clinical failure has challenged the notion that tumor cells divide frequently at rates comparable to those of cancer cells in vitro and in xenograft models. Given the preponderance of interphase cells in clinical tumors, we asked whether targeting amplified centrosomes, which cancer cells carefully preserve in a tightly clustered conformation throughout interphase, presents a superior chemotherapeutic strategy that sabotages interphase-specific cellular activities, such as migration. Herein we have utilized supercentrosomal N1E-115 murine neuroblastoma cells as a test-bed to study interphase centrosome declustering induced by putative declustering agents, such as Reduced-9-bromonoscapine (RedBr-Nos), Griseofulvin and PJ-34. We found tight ‘supercentrosomal' clusters in the interphase and mitosis of ~80% of patients' tumor cells with excess centrosomes. RedBr-Nos was the strongest declustering agent with a declustering index of 0.36 and completely dispersed interphase centrosome clusters in N1E-115 cells. Interphase centrosome declustering caused inhibition of neurite formation, impairment of cell polarization and Golgi organization, disrupted cellular protrusions and focal adhesion contacts—factors that are crucial prerequisites for directional migration. Thus our data illustrate an interphase-specific potential anti-migratory role of centrosome-declustering agents in addition to their previously acknowledged ability to induce spindle multipolarity and mitotic catastrophe. Centrosome-declustering agents counter centrosome clustering to inhibit directional cell migration in interphase cells and set up multipolar mitotic catastrophe, suggesting that disbanding the nuclear–centrosome–Golgi axis is a potential anti-metastasis strategy.

New insights into vinculin function and regulation

Vinculin is a cytoplasmic actin-binding protein enriched in focal adhesions and adherens junctions that is essential for embryonic development. Much is now known regarding the role of vinculin in governing cell-matrix adhesion. In the past decade that the crystal structure of vinculin and the molecular details for how vinculin regulates adhesion events have emerged. The recent data suggests a critical function for vinculin in regulating integrin clustering, force generation, and strength of adhesion. In addition to an important role in cell-matrix adhesion, vinculin is also emerging as a regulator of apoptosis, Shigella entry into host cells, and cadherin-based cell-cell adhesion. A close inspection of this work reveals that there are similarities between vinculin's role in focal adhesions and these processes and also some intriguing differences.

Phorbol myristate acetate induces changes on F-actin and vinculin content in immature rat Sertoli cells

Actin and vinculin are two of the most abundant cytoskeletal proteins, widely expressed in nearly all types of eukaryotic cells. It has been well established that long-term exposure to the tumor promoter phorbol myristate acetate (PMA) affects Sertoli cell morphology, as well as F-actin and vinculin organization in vitro. To analyze in a quantitative manner the F-actin and vinculin content of rat immature Sertoli cells in vitro in response to PMA exposure, cytoskeletal fractions were prepared following extraction with Triton X-100. Analysis of the isolated cytoskeletal fractions by immunoblotting showed that exposure of immature rat Sertoli cells to PMA for 8h has an appreciable effect on the cellular level of both the actin and vinculin. Interestingly, as revealed by using calphostin C, a specific protein kinase C inhibitor, the observed F-actin and vinculin changes are most probably mediated by a mechanism that depends on protein kinase activity. A discussion is made concerning PKC modulation by PMA and the subsequent actin and vinculin quantitative changes and reorganization, phenomena that have been closely related to cell transformation.

Essential role of type I(alpha) phosphatidylinositol 4-phosphate 5-kinase in neurite remodeling

Rapid neurite remodeling is fundamental to nervous system development and plasticity and is regulated by Rho family GTPases that signal f-actin reorganization in response to various receptor ligands. Neuronal N1E-115 cells show dramatic neurite retraction and cell rounding in response to serum factors such as lysophosphatidic acid (LPA), sphingosine-1 phosphate (S1P), and thrombin, due to activation of the RhoA-Rho kinase pathway. Type I phosphatidylinositol 4-phosphate 5-kinases (PIPkinase), which regulate cellular levels of PtdIns(4,5)P(2), have been suggested as targets of the RhoA-Rho kinase pathway able to modulate cytoskeletal dynamics. Here, we show that the introduction of Type Ialpha PIPkinase into N1E-115 cells leads to cell rounding and complete inhibition of neurite outgrowth, perhaps through the dissociation of vinculin and the destabilization of focal adhesions. This occurs independently of RhoA, Rho kinase, and the activation of actomyosin contraction. Strikingly, expression of kinase-dead PIPkinase promotes the outgrowth of neurites, which fail to retract in response to LPA, S1P, thrombin, or active RhoA. Moreover, neurite retraction in response to an endogenous neuronal guidance cue, Semaphorin3A, was also dependent on Type Ialpha PIPkinase. Our results suggest an essential role for a Type I PIPkinase during neurite retraction in response to a number of diverse stimuli.

Vinculin but not alpha-actinin is a target of PKC phosphorylation during junctional assembly induced by calcium

The establishment of the junctional complex in epithelial cells requires the presence of extracellular calcium, and is controlled by a network of reactions involving G-proteins, phospholipase C and protein kinase C. Since potential candidates for phosphorylation are the tight junction associated proteins ZO1, ZO2 and ZO3, in a previous work we specifically explored these molecules but found no alteration in their phosphorylation pattern. To continue the search for the target of protein kinase C, in the present work we have studied the subcellular distribution and phosphorylation of vinculin and alpha-actinin, two actin binding proteins of the adherent junctions. We found that during the junctional sealing induced by Ca2+, both proteins move towards the cell periphery and, while there is a significant increase in the phosphorylation of vinculin, alpha-actinin remains unchanged. The increased phosphorylation of vinculin is due to changes in phosphoserine and phosphothreonine content and seems to be regulated by protein kinase C, since: (1) DiC8 (a kinase C stimulator) added to monolayers cultured without calcium significantly increases the vinculin phosphorylation level; (2) H7 and calphostin C (both protein kinase C inhibitors) completely abolish this increase during a calcium switch; (3) inhibition of phosphorylation during a calcium switch blocks the subcellular redistribution of vinculin and alpha-actinin. These results therefore suggest that vinculin phosphorylation by protein kinase C is a crucial step in the correct assembly of the epithelial junctional complex.

Acute morphogenic and chemotropic effects of neurotrophins on cultured embryonic Xenopus spinal neurons

Neurotrophins constitute a family of trophic factors with profound effects on the survival and differentiation of the nervous system. Addition of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), but not nerve growth factor (NGF), increased the survival of embryonic Xenopus spinal neurons in culture, although all three neurotrophins enhanced neurite outgrowth. Here we report that neurotrophins also exert acute actions on the morphology and motility of 1-day-old cultured Xenopus spinal neurons. Bath application of BDNF induced extensive formation of lamellipodia simultaneously at multiple sites along the neurite shaft as well as at the growth cone. The BDNF-induced lamellipodia appeared within minutes, rapidly protruded to their greatest extent in about 10 min, and gradually disappeared thereafter, leaving behind newly formed thin lateral processes. When applied as microscopic concentration gradients, both BDNF and NT-3, but not NGF, induced the growth cone to grow toward the neurotrophin source. Our results suggest that neurotrophic factors, when delivered to responsive neurons, may serve as morphogenic and chemotropic agents during neuronal development.

Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons

Membrane excitability in different tissues is due, in large part, to the selective expression of distinct genes encoding the voltage-dependent sodium channel. Although the predominant sodium channels in brain, skeletal muscle, and cardiac muscle have been identified, the major sodium channel types responsible for excitability within the peripheral nervous system have remained elusive. We now describe the deduced primary structure of a sodium channel, peripheral nerve type 1 (PN1), which is expressed at high levels throughout the peripheral nervous system and is targeted to nerve terminals of cultured dorsal root ganglion neurons. Studies using cultured PC12 cells indicate that both expression and targeting of PN1 is induced by treatment of the cells with nerve growth factor. The preferential localization suggests that the PN1 sodium channel plays a specific role in nerve excitability.

Intramolecular interactions regulate serine/threonine phosphorylation of vinculin

Using protein kinase C, we have studied the influence of intramolecular interactions on phosphorylation in vinculin. We show that vinculin and its 90 kDa head and 29/27 kDa tail fragments, generated by V8 proteolytic cleavage, are differentially phosphorylated. While intact vinculin and the isolated head domain are only weakly labelled, the isolated tail fragment is much more strongly phosphorylated. In the presence of the tail, the head is fully protected from the kinase. These data are consistent with our observation that native vinculin is primarily phosphorylated within the tail domain and suggest a function of vinculin phosphorylation in the regulation of the vinculin conformation.

Role of laminin and integrin interactions in growth cone guidance

Laminin is well known to promote neuronal adhesion and axonal growth, but recent experiments suggest laminin has a wider role in guiding axons, both in development and regeneration. In vitro experiments demonstrate that laminin can alter the rate and direction of axonal growth, even when growth cone contact with laminin is transient. Investigations focused on a single neuronal type, such as retinal ganglion cells (RGCs), strongly implicate laminin as an important guidance molecule in development and suggest the involvement of integrins. Integrins are receptors for laminin, and neurons express multiple types of integrins that bind laminin. Morphologically, integrins cluster in point contacts, specialized regions of the growth cone that may coordinately regulate adhesion and motility. Recent evidence suggests that the structure and regulation of point contacts may differ from that of their nonneuronal counterpart, focal contacts. In part, this may be because the interaction of the cytoplasmic domain of integrin with the cytoskeleton is different in point contacts and focal contracts. Mutational studies where the cytoplasmic domain is truncated or altered are leading to a better understanding of the role of the alpha and beta subunit in regulating integrin clustering and binding to the cytoskeleton. In addition, whereas integrins may regulate motility through direct physical linkages to the growth cone cytoskeleton, an equally important role is their ability to elicit signaling, both through protein tyrosine phosphorylation and modulating calcium levels. Through such mechanisms integrins likely regulate the dynamic attachment and detachment of the growth cone as it moves on laminin substrates.

Vinculin-deficient PC12 cell lines extend unstable lamellipodia and filopodia and have a reduced rate of neurite outgrowth

We have studied the role of vinculin in regulating integrin-dependent neurite outgrowth in PC12 cells, a neuronal cell line. Vinculin is a cytoskeletal protein believed to mediate interactions between integrins and the actin cytoskeleton. In differentiated PC12 cells, the cell body, the neurite, and the growth cone contain vinculin. Within the growth cone, both the proximal region of "consolidation" and the more distal region consisting of lamellipodia and filopodia contain vinculin. To study the role of vinculin in neurite outgrowth, we generated vinculin-deficient isolates of PC12 cell lines by transfection with vectors expressing antisense vinculin RNA. In some of these cell lines, vinculin levels were reduced to 18-23% of normal levels. In the vinculin-deficient cell lines, neurite outgrowth on laminin was significantly reduced. In time-lapse analysis, growth cones advanced much more slowly than normal. Further analysis indicated that this deficit could be explained in large part by changes in the behaviors of filopodia and lamellipodia. Filopodia were formed in normal numbers, extended at normal rates, and extended to approximately normal lengths, but were much less stable in the vinculin deficient compared to control PC12 cells. Similarly, lamellipodia formed and grew nearly normally, but were dramatically less stable in the vinculin-deficient cells. This can account for the reduction in rate of growth cone advance. These results indicate that interactions between integrins and the actin-based cytoskeleton are necessary for stability of both filopodia and lamellipodia.

Impairment of integrin-mediated cell-matrix adhesion in oxidant-stressed PC12 cells

Oxidants are believed to play an important and complex role in neuronal injury and death in the aging process and various neurode generative diseases. We studied the effect of oxidative stress on integrin-mediated cell-extracellular matrix (ECM) interactions using the PC12 neuronal cell line. In assays in which attachment was measured between 30 and 90 min, addition of hydrogen peroxide (H2O2) to the attachment medium resulted in a dose-dependent inhibition of initial cell attachment to collagen. Addition of H2O2 also caused previously attached cells to detach from collagen. The inhibition by H2O2 was specific for integrin-mediated adhesion, since attachment to substrata coated with non-ECM molecules was much less affected. Exposure of cells to H2O2 resulted in a rapid and profound reduction of intracellular ATP, accompanied by only a slight increase in intracellular free Ca2+ concentration ([Ca2+]i). Treatment of cells with the microfilament-disrupting agent, cytochalasin B, like that with H2O2, inhibited cell adhesion to collagen. We propose that integrin-mediated cell adhesion, which requires interactions between cytoplasmic portions of integrin subunits and cytoskeletal microfilaments, is impaired by oxidative stress as a result of the depletion of intracellular ATP and that such depletion is an early event in the process of oxidant-induced neuronal injury.

Nerve growth factor regulates the abundance and distribution of K+ channels in PC12 cells

We examined the effect of nerve growth factor (NGF) treatment on expression of a neuronal delayed rectifler K+ channel subtype, Kv2.1 (drk1), in PC12 cells. Anti-Kv2.1 antibodies recognized a single polypeptide population of M(r) = 132 kD in PC12 cell membranes, distinct from the more heterogeneous population found in adult rat brain. In response to NGF treatment, levels of Kv2.1 polypeptide in PC12 membranes increased fourfold. This increase in polypeptide levels could be seen within 12 h, and elevated levels were maintained for at least 6 d of continuous NGF treatment. RNase protection assays indicate that this increase in Kv2.1 protein occurs without an increase in steady state levels of Kv2.1 mRNA following NGF treatment. Immunofluorescent localization of the Kv2.1 polypeptide revealed plasma membrane-associated staining of cell bodies in both untreated and NGF-treated PC12 cells. In undifferentiated cells, intense staining is seen at sites of cell-cell and cell-substratum contact. In differentiated cells the most intense Kv2.1 staining is observed in neuritic growth cones. These studies show that in PC12 cells both the abundance and distribution of the Kv2.1 k+ channel are regulated by NGF, and suggest that PC12 cells provide a model for the selective expression of Kv2.1 in neuritic endings.

Adherens junctions: demonstration in human epidermis

Adherens junctions are intercellular and cell-matrix junctions that, like desmosomes and hemidesmosomes, mediate adhesion of cells to each other or to matrix structures. These junctions have been detected recently in cultured human keratinocytes, indicating that they may be of importance in epidermis. To investigate the localization of adherens junctions in normal epidermis, we examined human epidermis, human oral mucosa, and monkey esophagus for the presence of vinculin, a major protein of the intracellular plaques of adherens junctions that is thought to be present in all adherens junctions. Western blot analysis demonstrated vinculin in extracts of epidermis. Immunohistochemistry of vinculin in these tissues displayed two distinct locations for adherens junctions: i) at the dermal-epidermal junction, and ii) in the region of cell-cell contacts in all layers of the epidermis. The location of vinculin in the region of the epidermal-dermal junction is reminiscent of the distribution of vinculin-containing focal contacts in cultured keratinocytes, and the intercellular staining of vinculin in epidermis is consistent with the presence of vinculin in adherens junctions in cultured keratinocytes at sites of cell-cell contact. These results demonstrate that adherens junctions are present in human epidermis, oral mucosa, and monkey esophagus. Vinculin-containing junctions in epidermis may be important in the pathogenesis of skin diseases involving alterations in intercellular integrity.

Signal transduction pathways in neuronal differentiation

New insights into the signal transduction pathways for neuronal growth factors and cell adhesion molecules are affording us a better understanding of the intracellular mechanisms for neuronal differentiation, and of the ways in which the various signals are integrated during this process.

Cytokeratin reorganization induced by adenosine diphosphate in kidney epithelial cells

Exogenous adenosine diphosphate (ADP), the most potent mitogen for nontransformed African green monkey kidney epithelial cells of the BSC-1 line, rapidly alters the appearance of the cell monolayer. Examination of the cells with indirect immunofluorescence using monoclonal antibodies reveals a considerable reorganization of cytokeratin filaments without a major change in the pattern of microtubules or microfilaments. In untreated confluent cells, cytokeratin filaments are predominantly confined to a star-like spot in the perinuclear area, but these can be seen to begin to spread within 2 min after addition of ADP. The effect is particularly notable using anti-cytokeratin 8 antibodies. At 6 h this process is complete and produces a well-developed filamentous network throughout the cell. By 12 h, the network appears to collapse, so that the filaments again form a spot in the perinuclear area, a process that is complete by 24 h. Immunoblotting of total cellular proteins reveals no apparent alterations in the amounts of several species of cytokeratins, including cytokeratin 8 and 18, at 3 or 24 h after exposure to ADP. Other purine and pyrimidine nucleotides which do not stimulate DNA synthesis in these cells fail to alter cytokeratin organization, and there is no apparent alteration in the distribution of vimentin, another intermediate filament protein. The rapid ADP-induced cytokeratin reorganization appears to coincide with the induction of early growth-response gene transcription in these cells and may be correlated with the capacity of ADP to subsequently initiate DNA synthesis. This dramatic and reversible cytokeratin reorganization immediately after exposure to ADP may be an important step in the mitogenic signal transduction pathway.

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.

Tyrosine phosphorylation and immunodetection of vinculin in growth cone particle (GCP) fraction and in GCP-cytoskeletal subfractions

The growth cone, the motile tip of developing neuronal processes, is considered responsible for the exact guidance of axons and synaptogenesis. High activity of tyrosine kinases in growth cones may contribute to the functions of growth cones. Our previous work revealed that vinculin is one of the endogenous substrates for intrinsic tyrosine kinases in the growth cone particle (GCP) fraction isolated from fetal rat brain. In the present study, we examined tyrosine phosphorylation and immunoblot analysis of vinculin in various fractions from fetal rat brains and adult synaptosomal fraction. Tyrosine phosphorylation of vinculin in the GCP fraction was more prominent than in any other fraction from fetal brain or synaptosomes from adult. Compared to other fractions, however, the enrichment of vinculin in the GCP fraction was not observed. Tyrosine phosphorylation of vinculin in the fraction was inhibited by genistein, a specific tyrosine kinase inhibitor. Although vinculin was also phosphorylated by protein kinase C in the GCP fraction, it incorporated a much smaller amount of 32P than MARCKS protein or GAP-43. The cytoskeletal subfraction from the GCP fraction contained a considerable amount of vinculin and it was one of the major substrates for tyrosine kinases in the GCP cytoskeleton. The membrane skeleton from the GCP fraction contained a low amount of vinculin but showed high kinase activity that phosphorylated vinculin. Taken together, our results suggest that tyrosine phosphorylation of vinculin contributes to the cytoskeletal organization of growth cones.

Quantitative effects of laminin concentration on neurite outgrowth in vitro

Recent studies indicate that mediation of neurite outgrowth by the glycoprotein laminin may be a significant factor in the outgrowth of neurites to their targets during embryogenesis. To further characterize the possible role of this extracellular matrix molecule during development, we have systematically measured several features of outgrowth by neonatal rat sympathetic neurons on different concentrations of laminin. Individual neurons, obtained by mechanical dissociation of superior cervical ganglia (SCG), were cultured at low density on laminin substrates ranging from 0.01 to 1.0 microgram/cm2. Outgrowth characteristics were subsequently analyzed for noninteracting cells in both fixed and live cultures. Data obtained from neurons fixed after 11 hr of culture showed approximately twofold increases in neurite initiation and outgrowth, and a twofold decrease in branching for a corresponding 100-fold increase in adsorbed laminin concentration. In time-lapse videomicroscopy observations, the root-mean square speed of growth cone movement increased from 60 to 90 microns/hr over the same range in concentration, while the persistence time remained constant at 0.10 hr. In general, neurite outgrowth parameters were relatively insensitive to changes in laminin concentration, supporting the idea that laminin is a permissive rather than an "instructive" substrate during development. Data obtained from fixed cultures were examined in terms of probability models to suggest possible mechanisms contributing to the dose-dependent effects observed.

Microtubule-associated-protein (MAP) kinase activated by nerve growth factor and epidermal growth factor in PC12 cells. Identity with the mitogen-activated MAP kinase of fibroblastic cells

Treatment of PC12 cells with either nerve growth factor (NGF), a differentiating factor, or epidermal growth factor (EGF), a mitogen, resulted in 7-15-fold activation of a protein kinase activity in cell extracts that phosphorylated microtubule-associated protein (MAP) 2 on serine and threonine residues in vitro. Both the NGF-activated kinase and the EGF-activated kinase could be partially purified by sequential chromatography on DEAE-cellulose, phenyl-Sepharose and hydroxylapatite, and were identical with each other in their chromatographic behavior, apparent molecular mass (approximately 40 kDa) on gel filtration, substrate specificity, and phosphopeptide-mapping pattern of MAP2 phosphorylated by each kinase. Moreover, both kinases were found to be indistinguishable from a mitogen-activated MAP kinase previously described in growth-factor-stimulated or phorbol-ester-stimulated fibroblastic cells, based on the same criteria. Kinase assays in gels after SDS/polyacrylamide gel electrophoresis revealed further that the NGF- or EGF-activated MAP kinase in PC12 cells, as well as the EGF-activated MAP kinase in fibroblastic 3Y1 cells resided in two closely spaced polypeptides with an apparent molecular mass of approximately 40 kDa. In addition, these MAP kinases were inactivated by either acid phosphatase treatment or protein phosphatase 2A treatment. These results indicate that MAP kinase may be activated through phosphorylation by a differentiating factor as well as by a mitogen. MAP kinase activation by EGF was protein kinase C independent; it reached an almost maximal level 1 min after EGF treatment and subsided rapidly within 30-60 min. On the other hand, NGF-induced activation of MAP kinase was partly protein kinase C dependent and continued for at least 2-3 h.

Vinculin is one of the major endogenous substrates for intrinsic tyrosine kinases in neuronal growth cones isolated from fetal rat brain

Neuronal growth cones, the motile tips of growing neuronal processes, are responsible for the exact guidance of extending neurites. To elucidate the mechanisms of their biochemical signal transduction in growth cones, the growth-cone-enriched fraction was isolated biochemically from fetal rat brain and the endogenous protein phosphorylation in the fraction was analyzed under the conditions where tyrosine residues were preferentially phosphorylated. One of the major phosphoproteins was a 130-kDa slightly acidic protein which reacted with antiphosphotyrosine antibody. Its phosphoryl residues were alkali-stable. Thus, the 130-kDa protein was concluded to be susceptible to tyrosine phosphorylation. This protein was a component of cytoskeletal proteins thought to be associated indirectly with membranes. All the behavior of the 130-kDa protein was compatible with the properties of vinculin, a component of focal contacts which are responsible for the stable or motile adhesion between cells or between a cell and the substratum. Immunochemical analyses showed that the 130-kDa protein was specifically recognized by anti-vinculin antibody. Therefore, the 130-kDa protein was concluded to be vinculin. Tyrosine phosphorylation of the protein appeared to be relatively more pronounced in the growth-cone-enriched fraction than in adult synaptosomes. The results suggest that tyrosine phosphorylation of vinculin may be regulated developmentally and it may be involved in the functions of growth cones.

Nerve growth factor-induced transient increase in the phosphorylation of ribosomal protein S6 mediated through a mechanism independent of cyclic AMP-dependent protein kinase and protein kinase C

Treatment of PC12h cells with nerve growth factor (NGF) induced a transient increase in the phosphorylation of a 35,000-dalton protein. This transient increase was observed also when extracts of NGF-treated cells were incubated with [gamma-32P]ATP. In the intact-cell phosphorylation system, treatment with N,2'-dibutyryladenosine 3',5'-cyclic monophosphate (dBcAMP) or 12-O-tetradecanoylphorbol 13-acetate (TPA) also induced a transient increase in the phosphorylation of the 35,000-dalton protein, but the effect was less than that of NGF. An effect comparable to that of NGF was obtained by the combination of dBcAMP and TPA. Pretreatment of PC12h cells with dBcAMP plus TPA for 3 days, which deprived the cells of their ability to respond to a rechallenge with dBcAMP, TPA, or dBcAMP plus TPA by increasing the rate of 35,000-dalton protein phosphorylation, caused only a slight attenuation of the NGF effect, directly indicating a minimal role of cyclic AMP (cAMP)-dependent protein kinase and protein kinase C in the mechanism of the NGF action. Pretreatment of the cells with K-252a, a protein kinase inhibitor, at a concentration of 300 nM almost completely blocked the action of NGF, but scarcely affected the action of dBcAMP, TPA, or dBcAMP plus TPA in intact-cell phosphorylation experiments. This NGF-sensitive 35,000-dalton protein was a ribosomal protein and identified as ribosomal protein S6. The results lead us to conclude that NGF activates some NGF-sensitive component(s), probably some specific protein kinase(s) other than cAMP-dependent protein kinase or protein kinase C, which is suppressed by K-252a and directly or indirectly activates a 35,000-dalton protein kinase(s) [S6 kinase(s)] to increase the rate of phosphorylation of the 35,000-dalton ribosomal protein (S6).

Nerve growth factor-induced rapid reorganization of microfilaments in PC12 cells: possible roles of different second messenger systems

Nerve growth factor (NGF) induces in 2 to 10 min the redistribution of F-actin in rat pheochromocytoma PC12 cells. The NGF specificity of this phenomenon was shown by blocking it with anti-NGF antibodies. We used the rapid F-actin redistribution as an assay to study NGF second messenger systems and their inhibition or activation by specific agents. The results show that the NGF-induced effect on the microfilament system of PC12 cells can be specifically inhibited by lithium chloride and neomycin, inhibitors of the phosphoinositol system, but cannot be mimicked by TPA and acetylcholine, the activators of the phosphoinositol system. An increase in the intracellular concentration of cyclic AMP by addition of dBcAMP (but not dBcGMP) caused rapid F-actin redistribution that nonetheless differed from the NGF-induced effect. Changes in the intracellular calcium level did not have any influence on the microfilament system of PC12 cells. The specificity of the inhibition of NGF-induced effects by methylase inhibitors was questionable, since MTA- or SAH-treated PC12 cells acquired an altered morphology even in the absence of NGF or dBcAMP. Using the microfilament- and microtubule-disrupting drugs cytochalasin B and colchicine, we showed that the microtubule system in PC12 cells is required for the initiation of neurite outgrowth and that microfilament-associated filopodial activity does not appear to be necessary.

Blockage of nerve growth factor action in PC12h cells by staurosporine, a potent protein kinase inhibitor

Staurosporine, which has a structure similar to that of K-252a, a potent protein kinase inhibitor that blocks nerve growth factor (NGF) action in PC12 and PC12h cells, is also known as a potent inhibitor of several protein kinases. This study shows that in PC12h cells staurosporine has a dual action: at lower concentrations than that required by K-252a, it is an inhibitor of NGF induction of neurite formation and of changes in the phosphorylation of specific proteins, whereas at concentrations higher than that required to inhibit NGF-induced neurite outgrowth, it rapidly enhances outgrowth by itself.

Role of protein tyrosine phosphorylation in the NGF response

The role of protein tyrosine phosphorylation in the response of PC12 cells to NGF was investigated by using a variety of agents which affect NGF-induced neurite outgrowth. K-252a, a kinase inhibitor, was previously found to selectively inhibit many of the actions of NGF on PC12 cells. In the present study, it was shown to inhibit NGF-induced protein tyrosine phosphorylation. However, sphingosine, an inhibitor of protein kinase C and NGF-induced differentiation of PC 12 cells, did not alter the phosphorylation of proteins on tyrosine stimulated by NGF. Disruption of either actin microfilaments or microtubules also had no effect on NGF-induced protein tyrosine phosphorylation in PC12 cells. The effect of vanadate, an inhibitor of phosphotyrosyl phosphatases, on the differentiation of PC12 cells was also examined. Vanadate did not promote neurite outgrowth but did stimulate protein tyrosine phosphorylation. Taken together, these results suggest that protein tyrosine phosphorylation is one of the first events in the NGF pathway in PC12 cells but alone is not sufficient to induce morphological differentiation. Finally, the distribution of phosphotyrosine-containing proteins in untreated and NGF-treated cells was examined by immunofluorescence microscopy. The distribution of these proteins was altered by treatment of the cells with NGF and appeared to correlate with the distribution of actin filaments, particularly in growth cones.

Enhancement of vinculin synthesis by migrating stratified squamous epithelium

A 110-115-kD protein is present at levels 27-fold higher in migratory epithelium in the rat cornea than in stationary epithelium. This protein represents 2.7% of the total protein in migratory epithelium 6-h postabrasion wound and 0.1% of the total protein in stationary epithelium. Our findings demonstrate that this 110-115-kD protein is vinculin. In Western blots comparing proteins from migratory and control epithelium, antibody against vinculin cross-reacted with the 110-115-kD protein. Using immunoslot blots, vinculin was determined to be present at maximal levels 6 h postabrasion wound, at levels 22- and 8-fold higher than control at 18 and 48 h, respectively, returning to control levels 72 h postwounding. Vinculin was also localized by indirect immunohistochemistry in migrating corneal epithelium. 3-mm scrape wounds were allowed to heal in vivo for 20 h. In flat mounts of these whole wounded corneas, vinculin was localized as punctate spots in the leading edge of migrating epithelium. In cryostat sections, vinculin was localized as punctate spots along the basal cell membranes of the migrating sheet adjacent to the basement membrane and in patches between cells as well as diffusely throughout the cell. Only very diffuse localization with occasional punctate spots between adjacent superficial cells was present in stationary epithelium. The increased synthesis of vinculin during migration and the localization of vinculin at the leading edge of migratory epithelium suggest that vinculin may be involved in cell-cell and cell-substrate adhesion as the sheet of epithelium migrates to cover a wound.

Distribution and possible interactions of actin-associated proteins and cell adhesion molecules of nerve growth cones

Actin filaments and their interactions with cell surface molecules have key roles in tissue cell behaviour. Axonal pathfinding during embryogenesis, an especially complex cell behaviour, is based on the migration of nerve growth cones. We have used fluorescence immunocytochemistry to examine the distribution in growth cones, their filopodia and lamellipodia of several actin-associated proteins and nerve cell adhesion molecules. The leading margins of chick dorsal root ganglion nerve growth cones and their protrusions stain strongly for f-actin, filamin, alpha-actinin, myosin, tropomyosin, talin and vinculin. MAP2 is absent from DRG growth cones, and staining for spectrin fodrin extends into growth cones, but not along filopodia. Thus, organization of the leading margins of growth cones may strongly resemble the leading lamella of migrating fibroblasts. The adhesion-mediating molecules integrin, L1, N-CAM and A-CAM are all found on DRG neurites and growth cones. However, filopodia stain relatively more strongly for integrin and L1 than for A-CAM or N-CAM. In fact, the 180 X 10(3) Mr form of N-CAM may be absent from most of the length of filopodia. DRG neurones cultured in cytochalasin B display differences in immunofluorescence staining which further emphasize that these adhesion molecules interact differentially with the actin filament system of migrating growth cones. Several models for neuronal morphogenesis emphasize the importance of regulation of the expression of adhesion molecules. Our results support hypotheses that cellular distribution and transmembrane interactions are key elements in the functions of these adhesion molecules during axonal pathfinding.

Modulation of growth factor induced fiber outgrowth in rat pheochromocytoma (PC12) cells by a fibronectin receptor antibody

Rat pheochromocytoma PC12 cells respond to the binding of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) by extending neurites in a manner resembling sympathetic neurons. This response requires cell attachment to an appropriate substratum (Fujii et al., J. Neurosci., 2:1157, 1982); attachment factors which function in this capacity include the adhesive proteins fibronectin and laminin. Incubating PC12 cells with a polyclonal antiserum directed against a putative 140-kDa fibroblast cell surface fibronectin receptor (anti-gp140) perturbed spreading but not attachment of the cells to fibronectin and laminin substrates. However, in the presence of anti-gp 140 or its Fab fragments, NGF-stimulated neurite outgrowth was dramatically reduced. The antibody also caused a retraction of previously extended neurites. SDS-PAGE analysis of immunoprecipitates of PC12 cells surface labeled with 125I identified a prominent 120-140-kDa band, suggesting that the site of anti-gp140 action in PC12 cells is also through a fibronectin receptor.

Axon guidance and the patterning of neuronal projections in vertebrates

Over the past decade, new insights have been obtained into the cellular strategies and molecular mechanisms that guide axons to their targets in the developing vertebrate nervous system. Axons select pathways by recognizing specific cues in their environment. These cues include cell surface and extracellular matrix molecules that mediate cell and substrate adhesion and axon fasciculation, molecules with contact-dependent inhibitory properties, and diffusible tropic factors. Several guidance cues may operate in a coordinated way to generate the distinct axonal trajectories of individual neurons.

cDNA-derived sequence of chicken embryo vinculin

We report the complete primary structure of chicken embryo vinculin. The amino acid sequence was derived from the nucleotide sequence of five overlapping cDNA clones isolated from a lambda gt11 phage library. Chicken embryo vinculin contains 1066 amino acids, has a calculated Mr of 116,990, a calculated pI of 5.9, and a hydropathy index of -4.22. A search of the National Biomedical Research Foundation protein sequence data base found no proteins with significant homology to vinculin. A striking feature of the linear sequence is a proline-rich region extending between residues 837 and 879. This region contains 45% proline and 19% aspartic plus glutamic acids; it is also the longest hydrophilic stretch in the molecule. The proline-rich region separates an amino-terminal domain with a calculated pI of 5.4 from a carboxyl-terminal domain with a calculated pI of 9.7. This feature suggests a structural basis for the specific interaction of vinculin with acidic phospholipids and a mechanism for the shuttling of vinculin between cytoplasm and membrane-associated junctional plaque.

The mode of action of nerve growth factor in PC12 cells

This review deals with the mechanism of nerve growth factor action. In view of the many and diversified effects of this growth factor, and since it could utilize different mechanism(s) in distinct types of cells, we have confined our analysis to the best characterized and more extensively studied target, the clonal cell line PC12. When exposed to NGF in vitro, these neoplastic cells recapitulate the last major steps of neuronal differentiation, i.e., the commitment to become a neuron and the acquisition of the neuronal phenotype. This is characterized by electrically excitable neurites, a display of a highly organized cytoskeleton, and the specific chemical and molecular neuronal properties. These effects are elicited upon the interaction of NGF with a receptor whose gene has been cloned and whose kinetic properties are now relatively well characterized. It is not yet clear, on the contrary, if and which of the several potential second messengers (cAMP, Ca, or phosphoinositides) that undergo marked fluctuations following NGF binding, transduce and amplify the NGF message. Among both the early and late effects of NGF is the modulation of expression of several genes. Some of the products of these genes are mainly restricted to nerve cells and others appear to play a crucial role in regulating the proper assembly of cytoskeletal elements. It is hypothesized that this complex array of chemical, molecular, and ultrastructural changes is triggered by NGF, not through activation of a single pathway, but more likely via combinatorial processes whereby several intracellular signals interplay before the irreversible commitment of becoming a neuron is undertaken.

The ultrastructure of the neuronal growth cone: new insights from subcellular fractionation and rapid freezing studies

In this review I have discussed the ultrastructure of the growth cone in relation to two aspects of growth cone behaviour; motility and membrane recycling. There are obvious and severe limitations in studying such a dynamic entity as the growth cone with the static images produced by the electron microscope, but these notwithstanding, electron microscopy, as I have tried to show here, has made important contributions in this area. Notable amongst these contributions is the fairly complete catalogue we now have of the organelles within the growth cone and their spatial relations, in particular the cytoskeletal and membrane bounded elements. Among the important questions that remain unanswered are those relating to the source and destiny of plasma membrane components, especially those concerned with recognising extrinsic cues, and the control of the cytoskeleton in relation to neurite extension and growth cone guidance. These questions can be approached using electron microscopy especially the rapid freezing and deep-etching methods used in conjunction with specific probes such as antibodies and we can look forward to progress in these areas in the near future.