The SNARE Sec22b has a non-fusogenic function in plasma membrane expansion

Development of the nervous system requires extensive axonal and dendritic growth during which neurons massively increase their surface area. Here we report that the endoplasmic reticulum (ER)-resident SNARE Sec22b has a conserved non-fusogenic function in plasma membrane expansion. Sec22b is closely apposed to the plasma membrane SNARE syntaxin1. Sec22b forms a trans-SNARE complex with syntaxin1 that does not include SNAP23/25/29, and does not mediate fusion. Insertion of a long rigid linker between the SNARE and transmembrane domains of Sec22b extends the distance between the ER and plasma membrane, and impairs neurite growth but not the secretion of VSV-G. In yeast, Sec22 interacts with lipid transfer proteins, and inhibition of Sec22 leads to defects in lipid metabolism at contact sites between the ER and plasma membrane. These results suggest that close apposition of the ER and plasma membrane mediated by Sec22 and plasma membrane syntaxins generates a non-fusogenic SNARE bridge contributing to plasma membrane expansion, probably through non-vesicular lipid transfer.

Dosage of amyloid precursor protein affects axonal contact guidance in Down syndrome

Amyloid precursor protein (APP), encoded on Hsa21, functions as a cell adhesion molecule (CAM) in axonal growth cones (GCs) of the developing brain. We show here that axonal GCs of human fetal Down syndrome (DS) neurons (and of a DS mouse model) overexpress APP protein relative to euploid controls. We investigated whether DS neurons generate an abnormal, APP-dependent GC phenotype in vitro. On laminin, which binds APP and β1 integrins (Itgb1), DS neurons formed enlarged and faster-advancing GCs compared to controls. On peptide matrices that bind APP only, but not on those binding exclusively Itgb1 or L1CAM, DS GCs were significantly enlarged (2.0-fold), formed increased close adhesions (1.8-fold), and advanced faster (1.4-fold). In assays involving alternating stripes of monospecific matrices, human control GCs exhibited no preference for any of the substrates, whereas DS GCs preferred the APP-binding matrix (cross-over decreased significantly from 48.2 to 27.2%). Reducing APP expression in DS GCs with siRNA normalized most measures of the phenotype, including substrate choice. These experiments show that human DS neurons exhibit an APP-dependent, abnormal GC phenotype characterized by increased adhesion and altered contact guidance. The results suggest that APP overexpression may perturb axonal pathfinding and circuit formation in developing DS brain.

Amyloid precursor protein is an autonomous growth cone adhesion molecule engaged in contact guidance

Amyloid precursor protein (APP), a transmembrane glycoprotein, is well known for its involvement in the pathogenesis of Alzheimer disease of the aging brain, but its normal function is unclear. APP is a prominent component of the adult as well as the developing brain. It is enriched in axonal growth cones (GCs) and has been implicated in cell adhesion and motility. We tested the hypothesis that APP is an extracellular matrix adhesion molecule in experiments that isolated the function of APP from that of well-established adhesion molecules. To this end we plated wild-type, APP-, or β1-integrin (Itgb1)- misexpressing mouse hippocampal neurons on matrices of either laminin, recombinant L1, or synthetic peptides binding specifically to Itgb1 s or APP. We measured GC adhesion, initial axonal outgrowth, and substrate preference on alternating matrix stripes and made the following observations: Substrates of APP-binding peptide alone sustain neurite outgrowth; APP dosage controls GC adhesion to laminin and APP-binding peptide as well as axonal outgrowth in Itgb1- independent manner; and APP directs GCs in contact guidance assays. It follows that APP is an independently operating cell adhesion molecule that affects the GC's phenotype on APP-binding matrices including laminin, and that it is likely to affect axon pathfinding in vivo.

Functional complexity of the axonal growth cone: a proteomic analysis

The growth cone, the tip of the emerging neurite, plays a crucial role in establishing the wiring of the developing nervous system. We performed an extensive proteomic analysis of axonal growth cones isolated from the brains of fetal Sprague-Dawley rats. Approximately 2000 proteins were identified at ≥ 99% confidence level. Using informatics, including functional annotation cluster and KEGG pathway analysis, we found great diversity of proteins involved in axonal pathfinding, cytoskeletal remodeling, vesicular traffic and carbohydrate metabolism, as expected. We also found a large and complex array of proteins involved in translation, protein folding, posttranslational processing, and proteasome/ubiquitination-dependent degradation. Immunofluorescence studies performed on hippocampal neurons in culture confirmed the presence in the axonal growth cone of proteins representative of these processes. These analyses also provide evidence for rough endoplasmic reticulum and reveal a reticular structure equipped with Golgi-like functions in the axonal growth cone. Furthermore, Western blot revealed the growth cone enrichment, relative to fetal brain homogenate, of some of the proteins involved in protein synthesis, folding and catabolism. Our study provides a resource for further research and amplifies the relatively recently developed concept that the axonal growth cone is equipped with proteins capable of performing a highly diverse range of functions.

Group IVA phospholipase A₂ is necessary for growth cone repulsion and collapse

The repellent semaphorin 3A (Sema3A) causes growth cone turning or collapse by triggering cytoskeletal rearrangements and detachment of adhesion sites. Growth cone detachment is dependent on eicosanoid activation of protein kinase C epsilon (PKCε), but the characterization of the phospholipase A(2) (PLA(2) ) that releases arachidonic acid (AA) for eicosanoid synthesis has remained elusive. Here, we show, in rat dorsal root ganglion (DRG) neurons, that Sema3A stimulates PLA(2) activity, that Sema3A-induced growth cone turning and collapse are dependent on the release of AA, and that the primary PLA(2) involved is the group IV α isoform (GIVA). Silencing GIVA expression renders growth cones resistant to Sema3A-induced collapse, and GIVA inhibition reverses Sema3A-induced repulsion into attraction. These studies identify a novel, early step in Sema3A-signaling and a PLA(2) necessary for growth cone repulsion and collapse.

Dynamic adhesions and MARCKS in melanoma cells

Cell motility necessitates the rapid formation and disassembly of cell adhesions. We have studied adhesions in a highly motile melanoma cell line using various biochemical approaches and microscopic techniques to image close adhesions. We report that WM-1617 melanoma cells contain at least two types of close adhesion: classic focal adhesions and more extensive, irregularly shaped adhesions that tend to occur along lamellipodial edges. In contrast to focal adhesions, these latter adhesions are highly dynamic and can be disassembled rapidly via protein kinase C (PKC) activation (e.g. by eicosanoid) and MARCKS phosphorylation. MARCKS overexpression, however, greatly increases the area of close adhesions and renders them largely refractory to PKC stimulation. This indicates that nonphosphorylated MARCKS is an adhesion stabilizer. Unlike focal adhesions, the dynamic adhesions contain alpha3 integrin and MARCKS, but they do not contain the focal adhesion marker vinculin. Overall, these results begin to define the molecular and functional properties of dynamic close adhesions involved in cell motility.

NPY and its involvement in axon guidance, neurogenesis, and feeding

OBJECTIVES

The role of neuropeptides in nervous system function is still in many cases undefined. In the present study we examined a possible role of the 36-amino acid neuropeptide Y (NPY) with regard to three functions: axon guidance and attraction/repulsion, adult neurogenesis, and control of food intake.

METHODS

Growth cones from embryonic dorsal root ganglion neurons were studied in culture during asymmetrical gradient application of NPY. Growth cones were monitored over a 60-min period, and final turning angle and growth rate were recorded. In the second part the NPY Y(1) and Y(2) receptors were studied in the subventricular zone, the rostral migratory stream, and the olfactory bulb in normal mice and mice with genetically deleted NPY Y(1) or Y(2) receptors. In the third part an anorectic mouse was analyzed with immunohistochemistry.

RESULTS

1) NPY elicited an attractive turning response and an increase in growth rate, effects exerted via the NPY Y(1) receptor. 2) The NPY Y(1) receptor was expressed in neuroblasts in the anterior rostral migratory stream. Mice deficient in the Y(1) or Y(2) receptor had fewer proliferating precursor cells and neuroblasts in the subventricular zone and rostral migratory stream and fewer neurons in the olfactory bulb expressing calbindin, calretinin or tyrosine hydroxylase. 3) In the anorectic mouse markers for microglia were strongly upregulated in the arcuate nucleus and in projection areas of the NPY/agouti gene-related protein arcuate system.

CONCLUSION

NPY participates in several mechanisms involved in the development of the nervous system and is of importance in the control of food intake.

Myristoylated, alanine-rich C-kinase substrate phosphorylation regulates growth cone adhesion and pathfinding

Repellents evoke growth cone turning by eliciting asymmetric, localized loss of actin cytoskeleton together with changes in substratum attachment. We have demonstrated that semaphorin-3A (Sema3A)-induced growth cone detachment and collapse require eicosanoid-mediated activation of protein kinase C epsilon (PKC epsilon) and that the major PKC epsilon target is the myristoylated, alanine-rich C-kinase substrate (MARCKS). Here, we show that PKC activation is necessary for growth cone turning and that MARCKS, while at the membrane, colocalizes with alpha3-integrin in a peripheral adhesive zone of the growth cone. Phosphorylation of MARCKS causes its translocation from the membrane to the cytosol. Silencing MARCKS expression dramatically reduces growth cone spread, whereas overexpression of wild-type MARCKS inhibits growth cone collapse triggered by PKC activation. Expression of phosphorylation-deficient, mutant MARCKS greatly expands growth cone adhesion, and this is characterized by extensive colocalization of MARCKS and alpha3-integrin, resistance to eicosanoid-triggered detachment and collapse, and reversal of Sema3A-induced repulsion into attraction. We conclude that MARCKS is involved in regulating growth cone adhesion as follows: its nonphosphorylated form stabilizes integrin-mediated adhesions, and its phosphorylation-triggered release from adhesions causes localized growth cone detachment critical for turning and collapse.

IGF-1 receptor is essential for the establishment of hippocampal neuronal polarity

How a neuron becomes polarized remains largely unknown. Results obtained with a function-blocking antibody and an siRNA targeting the insulin-like growth factor-1 (IGF-1) receptor suggest that an essential step in the establishment of hippocampal neuronal polarity and the initiation of axonal outgrowth is the activation of the phosphatidylinositol 3-kinase (PI3k)-Cdc42 pathway by the IGF-1 receptor, but not by the TrkA or TrkB receptors.

CSMD1 is a novel multiple domain complement-regulatory protein highly expressed in the central nervous system and epithelial tissues

In this study, we describe the identification and in vitro functional activity of a novel multiple domain complement regulatory protein discovered based on its homology to short consensus repeat (SCR)-containing proteins of the regulators of complement activation (RCA) gene family. The rat cDNA encodes a predicted 388-kDa protein consisting of 14 N-terminal CUB domains that are separated from each other by a SCR followed by 15 tandem SCR domains, a transmembrane domain, and a short cytoplasmic tail. This protein is the homolog of the human protein of unknown function called the CUB and sushi multiple domains 1 (CSMD1) protein. A cloning strategy that incorporates the two C-terminal CUB-SCR domains and 12 of the tandem SCR repeats was used to produce a soluble rat CSMD1 protein. This protein blocked classical complement pathway activation in a comparable fashion with rat Crry but did not block alternative pathway activation. Analysis of CSMD1 mRNA expression by in situ hybridization and immunolabeling of neurons indicates that the primary sites of synthesis are the developing CNS and epithelial tissues. Of particular significance is the enrichment of CSMD1 in the nerve growth cone, the amoeboid-leading edge of the growing neuron. These results suggest that CSMD1 may be an important regulator of complement activation and inflammation in the developing CNS, and that it may also play a role in the context of growth cone function.

Functional analysis of SIRPα in the growth cone

The `signal regulatory protein' SIRPα is an Ig superfamily, transmembrane glycoprotein with a pair of cytoplasmic domains that can bind the phosphatase SHP-2 when phosphorylated on tyrosine. SIRPα is prominent in growth cones of rat cortical neurons and located, together with the tetraspanin CD81, in the growth cone periphery. SIRPα is dynamically associated with Triton-X-100-sensitive, but Brij-98-resistant, lipid microdomains, which also contain CD81. Challenge of growth cones with the integrin-binding extracellular-matrix (ECM) protein, laminin, or with the growth factors, IGF-1 or BDNF, increases SIRPα phosphorylation and SHP-2 binding rapidly and transiently, via Src family kinase activation; phosphorylated SIRPα dissociates from the lipid microdomains. A cytoplasmic tail fragment of SIRPα (cSIRPα), when expressed in primary cortical neurons, also is phosphorylated and binds SHP-2. Expression of wild-type cSIRPα, but not of a phosphorylation-deficient mutant, substantially decreases IGF-1-stimulated axonal growth on laminin. On poly-D-lysine and in control conditions, axonal growth is slower than on laminin, but there is no further reduction in growth rate induced by the expression of cSIRPα. Thus, the effect of cSIRPα on axon growth is dependent upon integrin activation by laminin. These results suggest that SIRPα functions in the modulation of axonal growth by ECM molecules, such as laminin.

PI3K activation by IGF-1 is essential for the regulation of membrane expansion at the nerve growth cone

Exocytotic incorporation of plasmalemmal precursor vesicles (PPVs) into the cell surface is necessary for axonal outgrowth and is known to occur mainly at the nerve growth cone. We have demonstrated recently that plasmalemmal expansion is regulated at the growth cone by IGF-1, but not by BDNF, in a manner that is quasi independent of the neuron's perikaryon. To begin elucidating the signaling pathway by which exocytosis of the plasmalemmal precursor is regulated, we studied activation of the IRS/PI3K/Akt pathway in isolated growth cones and hippocampal neurons in culture stimulated with IGF-1 or BDNF. Our results show that IGF-1, but not BDNF, significantly and rapidly stimulates IRS/PI3K/Akt and membrane expansion. Inhibition of PI3K with Wortmannin or LY294002 blocked IGF-1-stimulated plasmalemmal expansion at the growth cones of cultured neurons. Finally, our results show that, upon stimulation with IGF-1, most active PI3K becomes associated with distal microtubules in the proximal or central domain of the growth cone. Taken together, our results suggest a critical role for IGF-1 and the IRS/PI3K/Akt pathway in the process of membrane assembly at the axonal growth cone.

The heterogeneous growth cone glycoprotein gp93 is identical to the signal regulatory protein SIRPalpha/SHPS-1/BIT

Growth cone gp93 is a highly heterogeneous membrane glycoprotein with an Mr of about 93 kDa. It was purified from adult rat brain and microsequenced. The sequences of four different peptide fragments of gp93 matched those of the 'signal regulatory protein' SIRPalpha (also known as SHPS-1, BIT or P84), an Ig superfamily member. SIRPalpha contains a cytoplasmic tail that is a tyrosine kinase substrate and binds the protein tyrosine phosphatase SHP-2. SIRPalpha and gp93 also were immunochemically cross-reactive. A PCR strategy was used to determine whether gp93/SIRPalpha heterogeneity in the brain depended upon the presence of different transcripts and, thus, sequence heterogeneity. However, we observed only a single full-length transcript. A short splice variant also was detected. These data identify gp93 as the Ig superfamily member SIRPalpha. Together with our previous results, the data also demonstrate that, in rat brain, gp93/SIRPalpha heterogeneity is the result of differential glycosylation (plus phosphorylation), rather than sequence heterogeneity.

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.

Regulation of membrane expansion at the nerve growth cone

Exocytotic incorporation of plasmalemmal precursor vesicles (PPVs) into the cell surface is necessary for neurite extension and is known to occur mainly at the growth cone. This report examines whether this is a regulated event controlled by growth factors. The Golgi complex and nascent PPVs of hippocampal neurons in culture were pulse-labeled with fluorescent ceramide. We studied the dynamics of labeled PPVs upon arrival at the axonal growth cone. In controls and cultures stimulated with brain-derived neurotrophic factor (BDNF), PPV clusters persisted in growth cones with a half-life (t(1/2)) of >14 minutes. Upon challenge with IGF-1, however, fluorescent elements cleared from the growth cones with a t(1/2) of only 6 minutes. Plasmalemmal expansion was measured directly as externalization of membrane glycoconjugates in resealed growth cone particles (GCPs) isolated from fetal forebrain. These assays demonstrated that membrane expansion could be stimulated by IGF-1 in a dose-dependent manner but not by BDNF, even though intact, functional BDNF receptor was present on GCPs. Because both BDNF and IGF-1 are known to enhance neurite growth, but BDNF did not stimulate membrane expansion at the growth cone, we studied the effect of BDNF on the IGF-1 receptor. BDNF was found to cause the translocation of the growth-cone-specific IGF-1 receptor subunit beta(gc) to the distal axon, in a KIF2-dependent manner. We conclude that IGF-1 stimulates axonal assembly at the growth cone, and that this occurs via regulated exocytosis of PPVs. This mechanism is affected by BDNF only indirectly, by regulation of the beta(gc) level at the growth cone.

Growth cone collapse induced by semaphorin 3A requires 12/15-lipoxygenase

Detection of a repellent factor, such as a semaphorin (Sema), causes localized collapse of the growth cone and directs the neurite away from the repellent. Growth cone collapse results from concomitant cytoskeletal rearrangements and detachment of adhesion sites from the extracellular matrix, via mostly unknown signaling mechanisms. In cultures of dorsal root ganglion neurons, we found that Sema3A treatment stimulates the synthesis of the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (HETE), whereas Sema3A-induced growth cone collapse is prevented when 12(S)-HETE synthesis is blocked with an inhibitor of 12/15-lipoxygenase (LO). Exogenously applied product of 12/15-LO, 12(S)-HETE, mimics Sema3A-induced collapse. As observed by interference reflection and confocal microscopy, 12(S)-HETE causes the loss of growth cone adhesion sites. The adhesion site effect seems partially independent of the actin cytoskeleton because growth cones treated with Sema3A and 12/15-LO inhibitor remain spread despite actin cytoskeleton loss. These studies demonstrate that 12/15-LO activity is a necessary step in Sema3A collapse signaling in growth cones and suggest a mechanism for its action.

Thrombin causes pseudopod detachment via a pathway involving cytosolic phospholipase A2 and 12/15-lipoxygenase products

Thrombin causes rapid pseudopod detachment and shortening in Dunning rat prostatic carcinoma (MAT-Lu) cells. As seen by interference reflection microscopy and by immunofluorescence analysis with antibodies to paxillin and talin, the primary event is disassembly of adhesion sites. Biochemically, thrombin is a potent activator of cytosolic phospholipase A2 and increases eicosanoid production in these cells. The pseudopod effects are blocked by lipoxygenase (but not cyclooxygenase) inhibitors. Arachidonic acid and 12(S)-hydroxyeicosatetraenoic acid or 15(S)-hydroxyeicosatetraenoic acid mimic the thrombin effect. We conclude that in certain cancer cells, thrombin is a pseudopod repellent that exerts its effect via a cascade involving cytosolic phospholipase A2, 12/15-lipoxygenase, and 12(S)- and/or 15(S)-hydroxyeicosatetraenoic acid.

Thrombin-induced growth cone collapse: involvement of phospholipase A(2) and eicosanoid generation

The studies presented here explore intracellular signals resulting from the action of repellents on growth cones. Growth cone challenge with thrombin or thrombin receptor-activating peptide (TRAP) triggers collapse via a receptor-mediated process. The results indicate that this involves activation of cytosolic phospholipase A(2) (PLA(2)) and eicosanoid synthesis. The collapse response to repellents targets at least two functional units of the growth cone, the actin cytoskeleton and substratum adhesion sites. We show in a cell-free assay that thrombin and TRAP cause the detachment of isolated growth cones from laminin. Biochemical analyses of isolated growth cones reveal that thrombin and TRAP stimulate cytosolic PLA(2) but not phospholipase C. In addition, thrombin stimulates synthesis of 12- and 15-hydroxyeicosatetraenoic acid (HETE) from the released arachidonic acid via a lipoxygenase (LO) pathway. A selective LO inhibitor blocks 12/15-HETE synthesis in growth cones and inhibits thrombin-induced growth cone collapse. Exogenously applied 12(S)-HETE mimics the thrombin effect and induces growth cone collapse in culture. These observations indicate that thrombin-induced growth cone collapse occurs by a mechanism that involves the activation of cytosolic PLA(2) and the generation of 12/15-HETE.

Anatomical distribution of glycoprotein 93 (gp93) on nerve fibers during rat brain development

Recent studies have implicated glycoconjugates on the membrane of growth cones as the necessary markers and intermediaries for axonal recognition, axonal motility, and pathway development. One such glycoconjugate, glycoprotein 93 (gp93), has been characterized, but the relative distribution of gp93 has yet to be described for the embryonic brain. In this study, the anatomical distribution of gp93 has been analyzed at embryonic day 15 (E15) and E18, and on postnatal day 3 in the rat by using a polyclonal gp93 antibody. Furthermore, fetal brain tissue transplanted into the adult rat eye has been tested for gp93 immunoreactivity, since central noradrenergic neurons in brainstem transplants are known to provide a continuous source of growing axons, even in adult tissue. In general, a greater abundance of gp93 immunoreactivity is apparent in the earlier embryonic stages (E15 and E18), whereas less is seen in the postnatal brain. The regions showing unique dispersal patterns of gp93 are the neuroepithelium, cerebral cortex, septo-hippocampal pathways, brainstem, and midbrain. This study has therefore focused on these areas and found implications for gp93 distribution appearing in the early development of specific neuronal pathways. Moreover, axons stain densely for gp93 within brain tissue transplants. The presence of gp93 in areas of extensive axonal outgrowth in the normal brain and in transplants suggests that this antibody is used as an early marker for axonal growth. Furthermore, gp93 might be used to map normal development in order to improve our understanding of diseases arising from developmental abnormalities.

SRC binding to the cytoskeleton, triggered by growth cone attachment to laminin, is protein tyrosine phosphatase-dependent

The interaction of the non-receptor tyrosine kinase, Src, with the cytoskeleton of adhesion sites was studied in nerve growth cones isolated from fetal rat brain. Of particular interest was the role of protein tyrosine phosphatases in the regulation of Src-cytoskeleton binding. Growth cones were found to contain a high level of protein tryrosine phosphatase activity, most of it membrane-associated and forming large, multimeric and wheat germ agglutinin-binding complexes. The receptor tyrosine phosphatase PTPalpha seems to be the most prevalent species among the membrane-associated enzymes. As seen by immunofluorescence, PTPalpha is present throughout the plasmalemma of the growth cone including filopodia, and it forms a punctate pattern consistent with that of integrin beta1. For adhesion site analysis, isolated growth cones were either plated onto the neurite growth substratum, laminin, or kept in suspension. Plating growth cones on laminin triggered an 8-fold increase in Src binding to the adherent cytoskeleton. This effect was blocked completely with the protein tyrosine phosphatase inhibitor, vanadate. Growth cone plating also increased the association with adhesion sites of tyrosine phosphatase activity (14-fold) and of PTPalpha immunoreactivity (6-fold). Vanadate blocked the enzyme activity but not the recruitment of PTPalpha to the adhesion sites. In conjunction with our previous results on growth cones, these data suggest that integrin binding to laminin triggers the recruitment of PTPalpha (and perhaps other protein tyrosine phosphatases) to adhesion sites, resulting in de-phosphorylation of Src's tyr 527. As a result Src unfolds, becomes kinase-active, and its SH2 domain can bind to an adhesion site protein. This implies a critical role for protein tyrosine phosphatase activity in the earliest phases of adhesion site assembly.

Expression and distribution of IGF-1 receptors containing a beta-subunit variant (betagc) in developing neurons

Betagc is a beta-subunit variant of the insulin-like growth factor-1 (IGF-1) receptor highly enriched in growth cone membranes prepared by subcellular fractionation of fetal rat brain (). The present study is focused on the expression and on the cellular and subcellular distribution of betagc in developing neurons and differentiating PC12 cells. In the developing cerebral cortex and, at least at early stages, in cultured primary neurons, betagc expression was found to be correlated with neurite outgrowth. In PC12 cells betagc expression was nerve growth factor (NGF)-dependent and also paralleled neurite outgrowth. In contrast, beta-subunits of the insulin receptor and/or of other IGF-1 receptors ("betaP5"; detected with antibody AbP5) were downregulated as betagc expression increased. Immunofluorescence studies confirmed the enrichment of betagc at growth cones and demonstrated morphologically its spatial separation from betaP5, which is confined to the perikaryon. At the growth cone, betagc colocalizes and associates in a proximal region with microtubules, but it seems independent of the more peripheral microfilaments. Some betagc immunoreactivity is detected in the perinuclear region of PC12 cells, most likely the Golgi complex and its vicinity. betagc seems to emerge from the periphery of this structure in an apparently vesicular compartment distinct from that carrying synaptophysin to the growth cones. The facts that (1) betagc expression is correlated closely with neurite outgrowth, that (2) it is regulated in PC12 cells by a neurotrophin, NGF, and that (3) betagc is concentrated in the proximal growth cone region raise new questions regarding a possible role of IGF-1 receptors containing betagc in the regulation of neurite growth.

Characterization of phospholipase A2 activity enriched in the nerve growth cone

Nerve growth cones isolated from fetal rat brain exhibit in their cytosol a robust level of phospholipase A2 activity hydrolyzing phosphatidylinositol (PI) and phosphatidylethanolamine (PE) but not phosphatidylcholine (PC). Western blot analysis with an antibody to the well-characterized cytosolic phospholipase A2 (mol wt 85,000) reveals only trace amounts of this PC- and PE-selective enzyme in growth cones. By gel filtration on Superose 12, growth cone phospholipase A2 activity elutes essentially as two peaks of high molecular mass, at approximately 65 kDa and at well over 100 kDa. Anion exchange chromatography completely separates a PI-selective from a PE-selective activity, indicating the presence of two different, apparently monoselective phospholipase A2 species. The PI-selective enzyme, the predominant phospholipase A2 activity in whole growth cones, is enriched greatly in these structures relative to their parent fractions from fetal brain. This phospholipase A2 is resistant to reducing agents and is found in the cytosol as well as membrane-associated in the presence of Ca2+. However, its catalytic activity is Ca(2+)-independent regardless of whether the enzyme is associated with pure substrate or mixed-lipid growth cone vesicles. The PE-selective phospholipase A2 in growth cones was studied in less detail but shares with the PI-selective enzyme several properties, including intracellular localization, the existence of cytosolic and membrane-associated forms, and Ca2+ independence. Our data indicate growth cones contain two high-molecular-weight forms of phospholipase A2 that share many properties with known, Ca(2+)-independent cytosolic phospholipase A2 species but that appear to be monoselective for PI and PE, respectively. In particular, the PI-selective enzyme may represent a new member of the growing family of cytoplasmic phospholipases A2. The enrichment of the PI-selective phospholipase A2 in growth cones suggests it plays a major role in the regulation of growth cone function.

Axonal origin and purity of growth cones isolated from fetal rat brain

The investigation of the molecular properties of nerve growth cones depends to a significant degree on their isolation from fetal brain in the form of 'growth cone particles' (GCPs). The availability of markers for developing axons and dendrites, as well as glial cells, has made it possible to characterize the GCP fraction in much greater detail than before and to optimize its yield. Marker analyses show that a member of the N-CAM family (5B4-CAM), synaptophysin, and especially GAP-43 and non-phosphorylated tau, are enriched in the GCP fraction. In contrast, MAP2 and, particularly, glial fibrillary acidic protein and vimentin are fractionated away from GCPs. Furthermore, GCP yield can be doubled relative to the original procedure, without compromising purity, by raising the sucrose concentration of the fractionation gradient's uppermost layer. The results indicate that GCPs are highly purified growth cone fragments with very little glial contamination, and that they are primarily of axonal origin.

Na+ channel changes in the growth cone and developing nerve terminal

Previous saxitoxin binding studies indicated two forms of the sodium channel in the fetal rat brain; a low-affinity precursor located in an internal membrane compartment, present exclusively in growth cones and a high-affinity mature form present in the plasmalemma of growth cones and characteristic of synapses. This raises the questions (1) of the presence or absence of the beta2 subunit in these channel forms and (2) of the developmental regulation of the the beta2 subunit. Antibodies against the alpha and beta2 channel subunits were used to probe Western blots of subcellular fractions from rat brains at embryonic day 18 (E18), pups at postnatal (P) days 7-25, and adults, as well as purified sodium channels from adult brain. In both synaptosomes and the purified sodium channel the beta2 antibody recognized the expected band at 38 kDa under reducing conditions. However, in contrast to the alpha subunit, this band was absent at E18 and became apparent only from P7 onwards. At the earlier time intervals a very prominent immunoreactive band of unknown identity was evident at 260--300kDa, which declined in intensity concomitant with the appearance of the 38 kDa beta2 band. These data indicate that beta2 subunits are regulated independently from alpha subunits, are absent in differentiating neurons, and hence are not necessary for insertion of the sodium channel into the plasmalemma, at least during early development of the neuron.

Insulin-like growth factor I receptors of fetal brain are enriched in nerve growth cones and contain a beta-subunit variant

Nerve growth cones isolated from fetal rat brain are highly enriched in a 97-kDa glycoprotein, termed beta gc, that comigrates with the beta subunit of the IGF-I receptor upon two-dimensional PAGE and is disulfide-linked to this receptor's alpha subunit. Antibodies prepared to a conserved domain shared by the insulin and IGF-I receptor beta subunits (AbP2) or to beta gc were used to study receptor distribution further. Subcellular fractionation of the fetal brain segregated most AbP2 immunoreactivity away from growth cones, whereas most beta gc immunoreactivity copurified with growth cones. Experiments involving ligand-activated receptor autophosphorylation confirmed the concentration of IGF-I but not of insulin receptors in growth cone fractions. These results indicate the enrichment of IGF-I receptors in (presumably axonal) growth cones of the differentiating neuron. Furthermore, the segregation of beta gc from AbP2 immunoreactivity suggests that such neurons express an immunochemically distinct variant of the IGF-I receptor beta subunit at the growth cone.

Growth cone enrichment and cytoskeletal association of non-receptor tyrosine kinases

Fetal rat brain (E18) expresses at least three c-src-like, membrane-associated non-receptor tyrosine kinases: c-src, fyn, and lyn. c-src and fyn are the most abundant and are highly enriched in a subcellular fraction of nerve growth cones (GCPs). To study the cytoskeletal association of these tyrosine kinases, Triton X-100-resistant fractions were prepared from GCPs. All three non-receptor tyrosine kinases are associated with the cytoskeleton to a significant degree with the relative affinities: fyn > c-src > lyn. The binding is sensitive to ionic strength and to phosphotyrosine, but not to phosphoserine or phosphothreonine. To investigate the regulation of this association we used phosphatase inhibitors to increase phosphotyrosine levels in GCPs. This resulted in the release of c-src from the cytoskeleton. Under these conditions tyrosine phosphorylation was increased selectively in released c-src and primarily on tyrosine 527. Cytoskeletally bound c-src had a higher specific kinase activity than Triton X-100-soluble c-src. These findings indicate that src family members interact in a regulated manner with the cytoskeleton in non-transformed cells. This regulation is explained by a model in which c-src binds to the cytoskeleton via its SH2 domain and is released when phosphorylated tyrosine-527 binds to this domain intramolecularly, inhibiting kinase activity.

Characterization of gp93, a novel, highly heterogeneous glycoprotein present in growth cone membranes

gp93 was first described in growth cones from fetal rat brain as a 90-97-kDa glycoprotein family that binds wheat-germ agglutinin and consists of at least 12 different isoelectric variants (pl range approximately 4.9-6.4). Of particular interest is that different sets of gp93 variants are expressed in growth cones isolated from different brain regions. The preparation of a polyclonal antibody to gp93 allowed further characterization of this glycoprotein. The carbohydrate groups of gp93 were partially characterized by digestion with different glycosidases. The results indicate that most or all oligosaccharide units are N-linked (asparagine-linked) and contain sialic acid. Two-dimensional polyacrylamide gel electrophoresis and western blot with anti-gp93 show that deglycosylated gp93 is an only slightly heterogeneous polypeptide of 66 kDa, indicating that gp93 heterogeneity is due, primarily or exclusively, to differential glycosylation. Analysis of the tissue distribution in fetal rat showed gp93 to be highly enriched in the brain. Immunoblots and immunostaining of cross sections of developing cerebellum revealed that gp93 is developmentally regulated in this tissue, associated primarily with growing parallel fibers and Purkinje dendrites. Immunostaining of neurons in culture shows significant amounts of gp93 in elongating neurites and growth cones. Our results indicate that gp93 is a developmentally regulated glycoprotein of the brain that is most prominent in growth cones and growing neurites and that appears to be glycosylated differentially by different neurons.

A chloride channel reconstituted from fetal rat brain growth cones

Chloride channels were reconstituted into planar lipid bilayers isolated from a preparation of growth cone particles (GCPs) isolated from fetal rat brain. One type of channel was predominantly seen and some of its biophysical and pharmacological properties were studied. The single channel i-V relationship was curvilinear with a chord conductance of 75 pS at +30 mV in symmetric 200 mM NaCl solutions buffered with phosphate. The channel was inactivated by depolarization, and this inactivation was reversed rapidly upon returning to -25 mV. The Cl- channel was significantly permeant to Na+ ions (PNa/PCl = 0.26), and the relative halide permeabilities were determined to be: I(1.92) > Br(1.73) > Cl(1.0) > F(0.34). The channel was inhibited by the common stilbene compounds (DIDS, SITS, DNDS), as well as by Zn2+ ions and an indanyloxyacetic acid derivative. A developmental role for the GCP Cl- channel is suggested by the observation that adult rat brain synaptosomal membranes were nearly devoid of this type of Cl- channel.

Arachidonic acid turnover and phospholipase A2 activity in neuronal growth cones

We analyzed de novo synthesis and local turnover of phospholipids in the growing neuron and the isolated nerve growth cone. The metabolism of phosphatidylinositol (PI) was studied with regard to the incorporation of saturated and unsaturated fatty acids and inositol. A comparison of de novo phospholipid synthesis in the intact neuron (whole brain, cell cultures) versus local turnover in isolated growth cone particles (GCPs) from fetal rat brain revealed different incorporation patterns and, in particular, high arachidonic acid (AA) turnover in PI of GCPs. These observations, together with elevated levels of free AA (2.5% of total AA content) in GCPs, demonstrate the predominance of acylation/deacylation in the sn-2 position of PI. GCP phospholipase A2 (PLA2) activity was demonstrated using [3H]-or [14C]AA-phosphatidylcholine (PC) or -PI as the substrate in vitro and GCPs or a cytosolic GCP extract as the source of enzyme. In contrast to PC, which is hydrolyzed very slowly, PI is a very good GCP PLA2 substrate. PLA2 activity is much higher in GCPs than that of phospholipase C, as demonstrated by the comparison of AA and inositol turnover, by the low levels of 1,2-diacylglycerol generated by GCPs, and by the resistance of AA release to treatment of GCPs with RHC-80267, a specific inhibitor of diacylglycerol lipase. The predominance of PLA2 activity in GCPs raises questions regarding its regulation and the functional roles of PI metabolites, especially lysocompounds, in growth cones.

Sites of plasmalemmal expansion in growth cones

Studies on plasmalemmal expansion in isolated nerve growth cones identified large, clear vesicles characteristically found in growth cones as the plasmalemmal precursor. The present article examines these plasmalemmal precursor vesicles (PPVs) in greater detail in the intact cell. (a) Pulse-chase experiments with the phospholipid precursor, [3H]glycerol, followed by radioautographic analysis show that PPVs in distal neurites and growth cones are labeled prior to equilibration of the label with the plasmalemma. (b) Pulse-chase experiments with lectin-ferritin conjugates demonstrate that PPVs are not endocytotic, that they contain lectin receptors, and that, during growth, patches of lectin receptors appear on the plasmalemma covering PPV clusters. (c) Freeze-fracture studies show that this plasmalemma shares with PPVs a paucity of intramembrane particles. (d) Lectin labeling experiments and freeze-fracture analysis demonstrate, furthermore, that the plasmalemma forms a network of invaginations at the base of PPV clusters. (e) Correlative studies indicate that the refractive 'vacuoles' seen in growth cones by phase-contrast light microscopy correspond to the PPV clusters seen at the ultrastructural level. These results confirm the identity of the plasmalemmal precursor in the intact cell and demonstrate that PPV clusters form distinct, dynamic organelles specialized for plasmalemmal expansion in the growth cone.

A complex consisting of pp60c-src/pp60c-srcN and a 38 kDa protein is highly enriched in growth cones from differentiated SH-SY5Y neuroblastoma cells

Nerve growth cones of primary neurons are highly enriched in the proto-oncogene product pp60c-src. In order to investigate this molecule further in growing neuronal cells, growth cone and cell body fractions were prepared from human SH-SY5Y neuroblastoma cells differentiated neuronally in vitro under the influence of phorbol ester. The fractions were characterized ultrastructurally and by biochemical criteria. The neuronal (pp60c-srcN) and the fibroblastic (pp60c-src) forms of pp60src are slightly enriched and activated in the growth cones relative to the perikarya. Immunoprecipitates of pp60src from differentiated SH-SY5Y growth cones contain at least four phosphoproteins in addition to pp60src. One of these, pp38, migrates as a 100–140 kDa complex with pp60src under non-reducing conditions of gel electrophoresis. The pp38/pp60src complex is not easily detected in non-differentiated SH-SY5Y cells or perikarya of differentiated SH-SY5Y cells, but it is highly enriched in the growth cone preparation. These data suggest that growth-cone pp60src exists in a disulfide-linked oligomeric complex. The complex appears to be assembled only in the cell periphery and may be dependent upon neuronal differentiation.

Plasmalemmal insertion and modification of sodium channels at the nerve growth cone

We have characterized voltage-dependent sodium channels in growth cones (GCPs) isolated from fetal rat brain using saxitoxin and TTX binding as well as recordings from channels reconstituted into lipid bilayer membranes. Both high- and low-affinity binding sites are present in GCP membranes. However, the two binding sites are segregated largely or completely, with the high-affinity binding sites in the plasmalemma, and the low-affinity sites in an internal membrane compartment. Plasmalemmal insertion of these internal sites can be triggered by high-potassium depolarization and depends on a metalloendoprotease-requiring mechanism. These observations indicate that a precursor-product relationship exists between the internal and external sodium channels of the growth cone, and therefore suggest that channel externalization causes conversion of low-affinity to high-affinity saxitoxin receptors. This conversion may represent a step of channel capacitation.

A developmentally regulated plasmalemmal antigen present in synaptosomes but not in growth cones

Monoclonal antibody 2L4 was generated against rat synaptosomes but does not cross-react with nerve growth cones. Expression of the 2L4 antigen is developmentally regulated in a manner that is, in part, the opposite of the expression of the 5B4-CAM antigen, a marker of neuronal outgrowth belonging to the N-CAM family. While 5B4-CAM appears and increases during sprouting and then decreases to reach low levels in the adult, the 2L4 antigen appears only late in development, when neuronal outgrowth ceases, at or around the time of synaptogenesis. Once expressed, the antigen is found on the entire plasmalemmal surface of the neuron, but seems to be enriched at synaptic endings, at least of some neuron types. Biochemical analyses involving blotting of non-denaturing gels and immunoaffinity chromatography identify the antigen as a pair of polypeptides with similar, basic isoelectric points. These polypeptides form a somewhat diffuse, probably glycosylated band at 67 kDa and may be part of a hetero-oligomeric complex. The localization, biochemical, and developmental results suggest that the 2L4 antigen is a plasmalemmal marker of maturing and/or mature neurons whose expression may be triggered by synaptogenesis.

Variable membrane glycoproteins in different growth cone populations

The question of whether growth cones generated by different neurons contain distinctive membrane glycoproteins was examined. Growth cone particles (GCPs) were isolated from specific regions of fetal or early postnatal brain, and their membrane proteins were analyzed by 2D gel electrophoresis and Western blotting, using WGA as a probe. These blots were compared to those generated by synaptosomes from adult brain. The patterns reveal a number of WGA-binding glycoproteins that are uniformly present in these subcellular fractions and others that are found in GCPs from selected brain regions only. The results indicate, therefore, substantial pattern diversity for the different, restricted growth cone populations. Some of the WGA-binding glycoproteins seen in GCPs disappear with increasing age and are absent from synaptosomes, while others seem to become more prominent. One of the glycoprotein complexes present in all GCP and synaptosome fractions analyzed is gp93. It has an apparent molecular weight of 90-97 kDa and exhibits unusually high heterogeneity in GCPs from whole fetal brain. The gp93 complex covers a pI range from about 4.9 to about 6.4 and consists of at least 12 different species, probably isoelectric variants. In GCPs from different brain regions, the sets of gp93 species observed are different and characteristic. Neuraminidase digestion shifts the gp93 pattern to a more neutral pI but simplifies it only partially, indicating that variable sialic acid content explains the molecular diversity to some extent. Thus, gp93 is a glycoprotein complex whose members are expressed and/or posttranslationally processed differentially in different growth cone populations. Such a glycoprotein family may be involved in selective cell-cell recognition.

5B4-CAM expression parallels neurite outgrowth and synaptogenesis in the developing rat brain

In order to study whether 5B4-CAM expression parallels neurite outgrowth and synaptogenesis, a monoclonal antibody, 5B4, was used, which recognizes both fetal (185-250 kD) and adult (140 kD, 180 kD) forms of the neural cell adhesion molecule (N-CAM), to identify and localize the antigen in rat tissue during developmental ages P1 through P31 and in adults between P60 and 2 years of age. A ubiquitous pattern of intense immunolabelling was detected during the earliest stages of development. 5B4-CAM expression paralleled process outgrowth and the early stages of synaptogenesis in the cerebral cortex, hippocampal formation, and cerebellum. In the adult, immunoreactivity was generally less intense, but the cerebral cortex and hippocampal and cerebellar molecular layers, all areas implicated in learning-associated plasticity, retained substantial immunoreactivity. The inner one-third of the dentate gyrus molecular layer, an area implicated in axonal sprouting and reactive synaptogenesis, was particularly intensely labelled. Evidence from this work suggests that 5B4-CAM expression may be useful in monitoring neurite outgrowth and the early stages of synapse formation during development and possibly axonal sprouting and reactive synaptogenesis in the adult.

N-type and L-type calcium channels are present in nerve growth cones. Numbers increase on synaptogenesis

We have demonstrated the presence of both N- and L-type calcium channels in growth cone and other subcellular fractions of fetal rat brain, using the ligands omega-conotoxin GVIA for N-type channels and nitrendipine for L-type channels. The N-type channels seem to be distributed evenly throughout the perikaryon, neurite shaft and growing tip of the neurons. In contrast, the L-type channels appear to have a lower density in the growth cone than on the rest of the neuron. These observations apply at least within the limitations of cell fractionation technology. We have also studied both calcium channel subtypes in rat brain synaptosomal membranes. In both adult and fetal fractions there are approximately 6 times more N-type than L-type channels. Synaptosomal membranes contain more N- and L-type channels than any of the fetal subfractions, indicating that there is a substantial increase in calcium channel numbers upon synaptogenesis.

Growth-regulated proteins and neuronal plasticity. A commentary

Growth-regulated proteins (GRPs) of the neuron are synthesized during outgrowth and regeneration at an increased rate and enriched in nerve growth cones. Therefore, they can be used to some degree as markers of neurite growth. However, these proteins are not unique to the growing neuron, and their properties are not known sufficiently to assign them a functional and/or causal role in the mechanisms of outgrowth. During synaptogenesis, GRPs decrease in abundance, and growth cone functions of motility and organelle assembly are being replaced by junctional contact and transmitter release. However, there is a stage during which growth cone and synaptic properties overlap to some degree. We propose that it is this overlap and its continuation that allow for synaptic plasticity in developing and adult nervous systems. We also propose a hypothesis involving (a) trophic factor(s) that might explain the regulation of synaptic sizes and collateral sprouting. Some GRPs, especially GAP43/B50/pp46/F1, are more prominent in adult brain regions of high plasticity, and they undergo change, such as phosphorylation, during long-term potentiation (LTP). Without precise functional knowledge of GRPs, it is impossible to use changes in such proteins to explain the plasticity mechanism. However, changes in these "growth markers" are likely to be an indication of sprouting activity, which would explain well the various phenomena associated with plasticity and learning in the adult. Thus, plasticity and memory may be viewed as a continuation of the developmental process into adulthood.

Regulated plasmalemmal expansion in nerve growth cones

To study the mechanisms underlying plasmalemmal expansion in the nerve growth cone, a cell-free assay was developed to quantify membrane addition, using ligand binding and sealed growth cone particles isolated by subcellular fractionation from fetal rat brain. Exposed versus total binding sites of 125I-wheat germ agglutinin were measured in the absence or presence of saponin, respectively, after incubation with various agents. Ca2(+)-ionophore A23187 in the presence of Ca2+ increases the number of binding sites (Bmax) but does not change their affinity (KD), indicating that new receptors appear on the plasma membrane. Similarly, membrane depolarization by high K+ or veratridine significantly induces, in a Ca2(+)-dependent manner, the externalization of lectin binding sites from an internal pool. Morphometric analysis of isolated growth cones indicates that A23187 and high K+ treatment cause a significant reduction in a specific cytoplasmic membrane compartment, thus confirming the lectin labeling results and identifying the plasmalemmal precursor. The isolated growth cones take up gamma-amino-butyric acid and serotonin, but show no evidence for Ca2(+)-dependent transmitter release so that transmitter exocytosis is dissociated from plasmalemmal expansion. The data demonstrate that plasmalemmal expansion in the growth cone is a regulated process and identify an internal pool of precursor membrane.

Membrane proteins of the nerve growth cone and their developmental regulation

The membrane polypeptides of growth cone fragments ("growth cone particles," GCPs) isolated from fetal rat brain by subcellular fractionation have been analyzed in further detail. The major polypeptides of salt-washed GCP membranes detected by 1-dimensional gel electrophoresis (Ellis et al., 1985b) resolve in 2-dimensional gels as a spot of 52 kDa that comigrates with beta-tubulin and reacts with anti-beta-tubulin; a 46 kDa, pl 4.3, polypeptide (pp46) that has no equivalent in the soluble fraction and is identical to one of the GCP's major phosphoproteins (Katz et al., 1985) and to GAP43 (Willard et al., 1985); a spot of 42 kDa that comigrates with actin; and a species of 34 kDa (p34) without soluble equivalent. The prominent 38 kDa doublet identified in 1-dimensional gels is difficult to resolve in 2-dimensional gels. The major phosphoproteins pp80ac, pp46, and pp40 (Katz et al., 1985), as well as p34 partition into the oil phase of Triton X-114 extracts, suggesting that they are integral membrane proteins, at least in our experimental conditions. The properties of pp46 reported here are in conflict with the highly hydrophilic amino acid sequence predicted for GAP43/B50/F1 (Basi et al., 1987; Karns et al., 1987). Growth-cone and presynaptic membrane proteins are compared as follows. After eye injection of 35S-methionine, GCPs and synaptosomes are isolated from the target areas of optic nerve of fetal and adult rats, respectively. Polypeptides are separated by 1- and 2-dimensional gel electrophoresis and the radiolabeled species identified fluorographically. The comparison of labeled GCP and synaptosome polypeptides shows that all 5 major Coomassie blue-stained polypeptides of GCP membranes (52, 46, 42, 38, 34 kDa) are intensely labeled after eye injection. However, in synaptosomes, these polypeptides are weakly labeled if at all; instead, an intensely labeled polypeptide of 28 kDa, and several additional species not seen in GCPs, have appeared. Therefore, the major growth cone membrane proteins are developmentally regulated, and the rates of synthesis and transport into the axonal ending of neuronal polypeptides change dramatically at the time of synaptogenesis.

The two major phosphoproteins in growth cones are probably identical to two protein kinase C substrates correlated with persistence of long-term potentiation

Regulation of neural protein kinase C (PKC) activity appears to directly affect the persistence of long-term potentiation (LTP; Akers and Routtenberg, 1985; Lovinger et al., 1985, 1986, 1987; Routtenberg et al., 1985, 1986; Akers et al., 1986; Linden et al., 1987), a model of neural plasticity (Bliss and Lomo, 1973). In addition, the in vitro phosphorylation of a brain-specific PKC substrate, protein F1 (Mr 47 kDa, pl 4.5), has been directly correlated with persistence of LTP (Lovinger et al., 1986). Because PKC has been implicated in neurite outgrowth and is present at high levels in growth cone-rich areas of fetal brain, we investigated and characterized PKC substrates in a preparation of isolated nerve growth cone fragments from fetal rat brain and compared them with PKC substrates found in adult rat hippocampus. Four major proteins in the growth cone preparation showed endogenous phosphorylation levels at least 10-fold greater than any other phosphoproteins. Three of these 4 phosphoproteins, termed pp40, pp46, and pp80 (Katz et al., 1985), were phosphorylated by exogenous PKC in a dose-dependent manner, indicating that PKC activity might be of particular importance relative to other kinases in growth cone function. The 2 most highly labeled PKC substrates, pp46 and pp80, comigrated on 2-dimensional gels with the adult hippocampal proteins F1 and "80k" (Mr 78-80 kDa, pl 4.0), respectively. In addition, similarities in charge heterogeneity, 2-dimensional phosphopeptide maps, and increased phosphorylation in the presence of exogenous PKC or PKC stimulators suggest that protein F1 and 80k are highly homologous to, if not identical to, pp46 and pp80, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Gangliosides and other lipids of the growth cone membrane

Growth cone membranes, derived from growth cone particles isolated from 16- to 18-day-old fetal rat brain, were found to be rich in overall lipid content with a lipid-to-protein ratio of 3.5. The phospholipid-to-cholesterol ratio indicated considerably less cholesterol than plasma membranes from mature neurons. All major classes of phospholipid were present in the usual proportions except sphingomyelin, which could not be detected. Gangliosides expressed in relation to protein were present at somewhat higher levels compared to previously reported values for synaptic plasma membranes (73 versus 44 micrograms/mg protein), but when related to phospholipid their level was well below that of the latter (26 versus 62 micrograms/mg phospholipid). The ganglioside pattern was generally similar to that of mature synaptic membranes except for the presence of relatively more GD3 and less GD1a, a phenomenon also observed in whole fetal brain of the same age. Several neutral glycosphingolipids were detected, glucosylceramide being the major one of this group. Their total level in growth cone membranes was roughly comparable to that of gangliosides, but unlike the latter their concentration in whole brain decreased with development. For comparison we analyzed the ganglioside composition of mixed membrane fractions from the same fetal brains and found no significant differences between these and growth cone membranes, suggesting that these glycoconjugates are not localized specifically in the growth cones. Neutral glycosphingolipids, on the other hand, appeared somewhat more concentrated in growth cones than in the mixed membranes.

c-src gene product in developing rat brain is enriched in nerve growth cone membranes

Differentiating rat neurons express high levels of the protooncogene product pp60c-src, a 60-kDa tyrosine kinase of unknown function encoded by c-src. pp60c-src was found to be concentrated at least 9-fold in membranes from a subcellular fraction of nerve growth cones, the motile tips of outgrowing neuronal processes. Indirect immunofluorescence staining of cultured chick retinal explants showed pp60c-src in neuronal growth cones and processes, with the antigen particularly concentrated in growth cones of long neurites. pp60c-src in growth cone membranes was an active tyrosine-specific protein kinase with elevated tyrosine-specific protein kinase activity and reduced electrophoretic mobility characteristic of the form of pp60c-src in central nervous system neurons. pp60c-src was present at lower levels in subcellular fractions from mature rat brain but synaptosomal membranes were not enriched. Preferential localization of an active form of pp60c-src in nerve growth cone membranes and persistence of pp60c-src in mature neurons suggest that this tyrosine kinase is important in growth cone-mediated neurite extension and synaptic plasticity.

Estrogen and insulin synergism in neurite growth enhancement in vitro: mediation of steroid effects by interactions with growth factors?

Addition of estradiol to organotypic cultures of the fetal murine hypothalamus, preoptic area and cerebral cortex has been shown to elicit a striking enhancement of neurite growth which appears restricted to estrogen receptor-containing explant regions. The mechanisms underlying this response are unknown. An important question is whether the neurite enhancement which follows exposure to estradiol is due directly to the interaction of estrogen with the cell that was stimulated (the receptor-containing cell) or whether intermediate steps involving the possible interaction of estrogen and the endogenous polypeptide neurite-promoting growth factors or their receptors may play an important role. Recent findings in the cultures suggest that the effect of estrogen on neurite growth may involve synergistic interactions between estradiol and insulin-related peptides and may be important in regulating estrogen-responsive neurite growth in the central nervous system. Concurrent addition of estradiol and high levels of insulin (10 micrograms/ml or 50 micrograms/ml) to cultures of the olfactory bulb, hypothalamus, preoptic area and cerebral cortex of the fetal rat and mouse results in a dramatic acceleration and increase of neurite outgrowth which appears localized to estrogen receptor-containing explant regions. The supraphysiological concentrations of insulin required to elicit this response suggest that the factor(s) involved is unlikely to be insulin per se. Insulin may activate the receptor of different but closely related molecules such as the insulin-like growth factors (IGF)-I or -II to which it exhibits a relatively low affinity. Interactions between hormones and endogenous growth factors have been implicated in the modulation or mediation of an increasing number of endocrine-dependent, differentiative processes in vivo and in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular regulators of neuronal sprouting: II. Phosphorylation reactions in isolated growth cones

Mechanisms potentially involved in the regulation of neurite growth were investigated. Since both the phosphatidylinositol (PI) pathway and protein kinase C have been implicated in transmembrane signal transduction, protein and lipid phosphorylation reactions were examined in intact growth cone particles (GCPs) isolated from fetal rat brain. Three major substrates of Ca2+-dependent phosphorylation were observed: proteins of 40 and 46 kDa and an acidic 80 kDa species separated in 2D PAGE (pp40, pp46, and pp80ac). The pp40 and pp80ac substrates had similar rates of 32P incorporation, whereas that of pp46 was more rapid. The importance of protein kinase C in growth cone function is indicated by the enhancement of phosphorylation of the 3 major substrates by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). An examination of the Ca2+-dependent 32P incorporation into pp40 and pp46 revealed serine to be the only amino acid phosphorylated under these conditions. A rapidly metabolized pool of phosphoinositides was observed in GCPs. This suggests the presence of the Pl pathway's enzymes in this fraction. Inositol trisphosphate (IP3) was found to stimulate the phosphorylation of pp40 and pp80ac, indicating a possible link between the activation of the PI pathway and protein phosphorylation. Our findings demonstrate the prominence of the PI pathway and of Ca2+-dependent protein phosphorylation in the growth cone and may suggest the involvement of these mechanisms in growth-factor signal transduction.

Membrane glycoproteins of the nerve growth cone: diversity and growth regulation of oligosaccharides

A subcellular fraction prepared from fetal rat brain and enriched in growth cone membranes is analyzed for its lectin-binding proteins. Growth-associated glycoproteins are identified by comparing the growth cone glycoproteins with those of synaptosomes. Protein was resolved in one- or two-dimensional gels, electroblotted, and blots probed with radioiodinated concanavalin A, wheat germ agglutinin, and Ricinus communis agglutinins I and II. In one-dimensional gels, each lectin recognizes approximately 20 polypeptides (with substantial overlap) most of which migrate diffusely and have relatively high molecular masses (range 30-200 kD). The seven major Coomassie-staining proteins of the membrane fraction (34-52 kD) are not the major lectin-binding proteins. In two-dimensional gels, the lectin-binding proteins are either streaked across the pH gradient or exist as multiple spots, indicating broad charge heterogeneity. Seven wheat germ agglutinin- and Ricinus communis agglutinin II-binding glycoproteins are present in greater abundance in growth cone fractions compared with synaptosomes. Most notably, an acidic, sialic acid-rich protein (27-30 kD, pI 4.0; termed gp27-30) is most abundant at postnatal day 4, but absent from adult brain. The protein's very acidic isoelectric point is due, at least in part, to its high sialic acid content. Growth regulation of specific protein-linked oligosaccharides suggests that they play a special role in growth cone function. In addition, the great diversity of growth cone glycoproteins from whole brain suggests glycoprotein heterogeneity among growth cones from different neuron types.