Isolated neuronal growth cones from developing rat forebrain possess adenylate cyclase activity which can be augmented by various receptor agonists

Distribution of VIP mRNA and two distinct VIP binding sites in the developing rat brain: relation to ontogenic events

The peptide neurotransmitter vasoactive intestinal peptide (VIP) has neurotrophic properties and influences neurobehavioral development. To assess the role of VIP during neural ontogeny, the present work traces the development of VIP mRNA with in situ hybridization and VIP receptors with in vitro autoradiography in rat central nervous system (CNS) from embryonic day 14 (E14) to the adult. VIP mRNA was not evident in the CNS until birth. Postnatally, it was expressed in several distinct brain regions, but its distribution bore little relation to that of VIP receptors. VIP receptors were present and expressed changing patterns of distribution throughout CNS development. The changing patterns were the result of 1) the transient appearance of GTP-insensitive VIP receptors in several regions undergoing mitosis or glial fasciculation and 2) the transient appearance of GTP-sensitive VIP receptors homogeneously distributed throughout the CNS during the first 2 postnatal weeks, the period of the brain growth spurt. At E14-16 VIP binding was dense throughout the brainstem and spinal cord, but limited in the rest of the brain. From E19 to postnatal day 14 (P14), while VIP binding was higher in germinal zones, it tended to be uniformly dense throughout the remainder of the brain. By P21 the adult pattern began to emerge; VIP binding was unevenly distributed and was related to specific cytoarchitectural sites. Since the expression of VIP in the CNS is limited to postnatal development but VIP receptors are abundant prenatally, we suggest that extraembryonic VIP may act upon prenatal VIP receptors to regulate ontogenic events in the brain.

Muscarinic acetylcholine receptors are expressed and enriched in growth cone membranes isolated from fetal and neonatal rat forebrain: pharmacological demonstration and characterization

Nerve growth cones, the motile tips of growing neurites, are closely related to the exact pathway finding, and their roles for synaptogenesis have been proposed to be modified by some neurotransmitters. In the present study, to clarify the expression and the ontogeny of muscarinic acetylcholine receptors in growth cones, growth cone membranes from fetal and neonatal rat forebrain were isolated, and muscarinic receptors in growth cone membrane were pharmacologically characterized, by using the [3H]quinuclidinyl benzilate as a labeled ligand. The specific binding sites for [3H]quinuclidinyl benzilate had already been detected in growth cone membrane on embryonic day (E)17 (Bmax = 557 fmol/mg protein: KD = 19.7 pM) and gradually increased in amount without significant changes in the KD values from E17 to postnatal day (P)5. [3H]Quinuclidinyl benzilate binding sites in growth cone membrane were several times higher than that in the P2-fraction-derived membranes, and in perinuclear membranes. Competitive inhibition studies showed that the proportion of high-affinity sites for pirenzepine (M1-subtype) to total [3H]quinuclidinyl benzilate binding sites in growth cone membrane was significantly lower than that in adult synaptic plasma membranes. In contrast, the proportion of high-affinity sites for AF-DX 116 (M2-subtype) was significantly higher than that in adult synaptic plasma membranes (E17 growth cone membrane: M1, 29.5%; M2, 56.9%; adult synaptic plasma membrane: M1, 63.6%, M2, 5.9%). Electron micrographic examination revealed that there were no significant morphological differences among growth cone particle fractions at the developmental stages which we examined, and that mature synaptic elements did not contaminate the growth cone particle fractions. Biochemical examination by electrophoresis and the phosphorylation study of the growth cone particle fractions showed that the protein composition and the phosphoprotein pattern did not change markedly during these stages. Our results suggest that muscarinic receptors were expressed and more concentrated in growth cone membrane than in other membrane portions from perinatal rat forebrain, and that they may play some role in the axonal guidance in growth cone via receptor subtype-specific signal transduction mechanisms.

Biochemical pharmacology of isolated neuronal growth cones: implications for synaptogenesis

The neuronal growth cone is critical to the establishment of neuronal polarity through its motile, pathfinding and target recognition properties exhibited during synaptogenesis. Subcellular fractionation procedures yielding milligram quantities of isolated growth cones has allowed for biochemical and pharmacological investigation of intrinsic growth cone components that are likely to be involved in regulation of growth cone function in neuronal development. These 'mapping' studies of growth cone components are prerequisites to elucidating the mechanisms by which extracellular factors influence the motility, adhesion and directed growth of the growth cone. For example, neurotransmitters and polypeptide growth factors which have been shown in other systems to modulate growth cone behavior are presumed to act through receptors on the growth cone, inducing second-messenger molecule formation and consequent modification and regulation of proteins effecting the response(s) of the growth cone (i.e. proteins involved in motility, adhesion and membrane turnover). In a relatively short period of time, work with the isolated growth cone preparation has identified, in independent studies, many of the elements involved in this proposed scheme of events, including transmitter receptors, second-messenger cascades, and second-messenger post-translational modifications. An obvious future goal will be to analyze in more detail the intracellular events, and the relationships between them, in the growth cone and how they transmit extracellular signals into responses such as motility and adhesivity which underly the growth cone's synaptogenic properties. It is to be expected that much of this information will come forth from experimentation with the isolated growth cone preparation.

Development of A1 adenosine receptors in the chick embryo retina

Adenosine inhibits cyclic AMP synthesis induced by dopamine in embryonic but not in post-hatched chick retinas. N6-Cyclohexyladenosine (CHA), which preferentially activates A1 receptors as well as 2-chloroadenosine, inhibits cyclic AMP accumulation induced by dopamine in retinas from 10-day-old embryos (E10) with IC50's of 0.1 and 0.5 microM, respectively, but this effect is not detectable after hatching. In order to verify if this developmental change reflects variations in the number or affinity of A1 adenosine receptors, their development during chick retina ontogeny was studied. Binding studies using 3(H)CHA revealed the presence of A1 receptors at all stages of development examined, including the post-hatched retina. The number of binding sites increased between E10 and E17, and then decreased in post-hatched animals. In the latter, 3(H)CHA binding was to a single site with a Bmax of 128.6 +/- 13.4 fmol/mg protein and a Kd of 2.1 + 0.2 nM. Various ligands showed similar hierarchies of affinity for the A1 receptor in embryonic and post-hatched retinas, namely, CHA greater than R-N6-phenylisopropyladenosine (1-PIA) greater than 5'-N-ethylcarboxamideadenosine (NECA) greater than isobuthylemethyl-xanthine (IBMX). Given that CHA inhibited forskolin-induced cyclic AMP production and Gpp(NH)p inhibited 3(H)CHA binding in both embryonic and post-hatched retinas, it appears that receptor coupling to adenylate cyclase is present since early embryonic stages. The results suggest that the A1 receptors may have different functions in the embryonic as compared to the mature chick retina.

Expression of the growth cone specific epitope CDA 1 and the synaptic vesicle protein SVP38 in the developing mammalian cerebral cortex

CDA 1 is a novel antigen that within the brain is present specifically in neuronal growth cones. Electron microscope immunohistochemistry and subcellular fractionation showed the CDA 1 epitope to be on a cytosolic molecule. In cultured neurons, it is abundant in growth cones and not detectable in neurites or cell bodies. The development of the rat cerebral cortex was investigated by using the monoclonal antibody to CDA 1 and an antibody to SVP38, the synaptic vesicle glycoprotein. CDA 1 immunoreactivity in the rat cerebral cortex peaks just before birth and disappears by postnatal day 12, a few days before the major increase in the number of mature synapses. In contrast, SVP38 is expressed in parallel with the appearance of mature synapses. CDA 1 and SVP38 thus are markers of growth cones and synapses, respectively. Their expression during development reflects some of the structural and functional changes that occur during synapse formation.

Role of the growth cone in neuronal differentiation

Nerve growth cones are motile, exploring organelles at the tip of a growing neurite. The growth cone is a highly specialized structure, equipped with a complex machinery for reversible membrane expansion and rapid cytoskeletal reorganization, a machinery required for growth cone motility and neurite elongation. It also contains perception systems that enable the growth cone to respond to external signals, thereby steering the trailing neurite to the correct target. Soluble and substrate bound guidance molecules in the environment modulate growth cone behavior either through direct interaction or classical receptor activation coupled to second messengers. A prominent phosphoprotein of the growth cone is B-50. We propose a role for this growth-associated protein kinase C substrate in signal transduction processes in the growth cone.

Cyclic AMP-dependent protein phosphorylation in isolated neuronal growth cones from developing rat forebrain

We have shown recently that neuronal growth cones isolated from developing rat forebrain possess an appreciable activity of adenylate cyclase, which produces cyclic AMP and can be stimulated by various neurotransmitter receptor agonists and by forskolin. To investigate cyclic AMP-mediated biochemical mechanisms in isolated growth cones, we have centered the present study on cyclic AMP-dependent protein phosphorylation. One-dimensional gel electrophoretic analysis showed that cyclic AMP analogs increased incorporation of 32P into several phosphoproteins in molecular mass ranges of 50-58 and 76-82 kilodaltons, including those of 82, 76, and 51 kilodaltons. Two-dimensional electrophoresis, using isoelectric focusing in the first dimension, resolved phosphorylated alpha- and beta-tubulin species, actin, a very acidic protein (isoelectric point 4.0) with a molecular mass of 93 kilodaltons, and two proteins (x and x') closely neighboring beta-tubulin. Two other phosphoproteins seen in the gels had molecular masses of 56 and 51 kilodaltons (respective isoelectric points, 4.5 and 4.4) and, along with the 93-kilodalton phosphoprotein, were highly enriched in the isolated growth cones. Only the tubulin and actin species were major proteins in the isolated growth cones. Cyclic AMP analogs enhanced incorporation of 32P into phosphoproteins x and x', and, as assessed by immunoprecipitation, into beta-tubulin. Peptide digest experiments suggested that phosphoproteins x and x' are unrelated to beta-tubulin. Nonequilibrium two-dimensional electrophoresis resolved many phosphoproteins, of which a 79- and 75-kilodalton doublet, a 74-kilodalton species, and a 58-kilodalton doublet showed enhanced incorporation of 32P in the presence of cyclic AMP.

Cyclic AMP reduces adhesion of isolated neuronal growth cones from developing rat forebrain to an astrocytic cell line from embryonic mouse striatum

We have recently shown that isolated neuronal growth cones from developing rat forebrain possess an appreciable activity of adenylate cyclase, producing cyclic adenosine monophosphate, which can be stimulated by various neurotransmitter receptor agonists and by forskolin [Lockerbie R. O., Hervé D., Blanc G., Tassin J. P. and Glowinski J. (1988) Devl Brain Res. 38, 19-25]. In the present study, we have investigated the effect of cyclic adenosine monophosphate in an in vitro adhesion assay established between [3H]GABA-labelled isolated growth cones and a Simian virus-40 transformed astrocytic cell line from embryonic mouse striatum. Adhesion of the isolated growth cones onto the astrocytic clone increased steadily up to about 45 min before it began to level off at ca 16-18% of total [3H]GABA-labelled isolated growth cones added. Adhesion of the isolated growth cones onto the astrocytic clone was much superior to that seen on polyornithine and, in particular, on non-treated tissue culture wells. Adhesion "at plateau" was independent of both temperature and extracellular Ca2+ and was markedly reduced (ca 50%) by trypsin pre-treatment of the isolated growth cones. Pre-treatment of the isolated growth cones with either forskolin or lipophilic analogues of cyclic adenosine monophosphate attenuated adhesion in a time- and concentration-dependent manner. Approximately 30% reduction in adhesion to the astrocytic clone "at plateau" was observed after a 15 min pre-treatment of the isolated growth cones with forskolin at 10(-4) M or cyclic adenosine monophosphate analogues at 10(-3) M. A cyclic guanosine monophosphate analogue was without effect on adhesion of isolated growth cones. Scanning electron microscope analysis showed that isolated growth cones pre-treated with either cyclic adenosine monophosphate analogues or forskolin had a simpler morphology when attached to the astrocytic clone than isolated growth cones under control conditions. Pre-treatment of the isolated growth cones with low concentrations of cyclic adenosine monophosphate analogues increased protein kinase activity, measured using an exogenous histone phosphate acceptor, to a level which could not be further stimulated by cyclic adenosine monophosphate. Pre-treatment with a cyclic guanosine monophosphate analogue produced the same effect but only at much higher concentrations than those required for cyclic adenosine monophosphate analogues.