The growth cone cytoskeleton. Glycoprotein association, calmodulin binding, and tyrosine/serine phosphorylation of tubulin

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

G protein alpha subunits activate tubulin GTPase and modulate microtubule polymerization dynamics

G proteins serve many functions involving the transfer of signals from cell surface receptors to intracellular effector molecules. Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton. In this report, G protein alpha subunits Gi1alpha, Gsalpha, and Goalpha are shown to activate the GTPase activity of tubulin, inhibit microtubule assembly, and accelerate microtubule dynamics. Gialpha inhibited polymerization of tubulin-GTP into microtubules by 80-90% in the absence of exogenous GTP. Addition of exogenous GTP, but not guanylylimidodiphosphate, which is resistant to hydrolysis, overcame the inhibition. Analysis of the dynamics of individual microtubules by video microscopy demonstrated that Gi1alpha increases the catastrophe frequency, the frequency of transition from growth to shortening. Thus, Galpha may play a role in modulating microtubule dynamic instability, providing a mechanism for the modification of the cytoskeleton by extracellular signals.

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.

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.

In vitro substrate specificity of protein tyrosine kinases

Synthetic peptides such as P60src autophosphorylation site peptides and angiotensin are indiscriminately phosphorylated by protein tyrosine kinases. The observation has led to the general belief that protein tyrosine kinases are highly promiscuous, displaying little in vitro site specificity. In recent years, evidence has been accumulating to indicate that such a belief requires close examination. Synthetic peptides showing high substrate activity for specific groups of protein tyrosine kinases have been obtained. Systematic modification of certain substrate peptides suggests that kinase substrate determinants reside with specific amino acid residues proximal to the target tyrosine. A number of protein kinases have been shown to be regulated by tyrosine phosphorylation at specific sites by highly specific protein tyrosine kinases. These and other selected biochemical studies that contribute to the evolving view of in vitro substrate specificity of protein tyrosine kinases are reviewed.

Organization of microtubules in axonal growth cones: a role for microtubule-associated protein MAP 1B

Neuronal growth cones guide growing axons and dendrites (neurites) through developing embryos by detecting extrinsic guidance cues and transducing the signal into changes in motile behaviour. In this brief review, the role of the growth cone cytoskeleton in these events, in particular the microtubules, is discussed. Microtubules in the neurite are mainly bundled into fascicles whereas on entering the growth cone they diverge from each other and traverse the central (C)-domain of the growth cone. Occasionally, individual microtubules extend as far as the peripheral (P)-domain and may even enter filopodia. Microtubules in the growth cone are probably dynamically unstable, exchanging dimer with a large pool of soluble tubulin. It is proposed that the 'capture' of dynamically unstable microtubules by filopodial actin filament bundles is a crucial step underlying directed growth. Localised assembly of microtubules at the growth cone, rather than at the cell body followed by transport of polymer to the growth cone, may facilitate the delivery of material to specific regions of the growth cone and hence allow vectorial growth. Bundling of microtubules and capture of microtubules by filopodia both imply roles for microtubule-associated proteins (MAPs). Several microtubule-associated proteins are present within growth cones, including MAP 1B, MAP2 and tau. Recent experiments point toward a phosphorylated form of MAP 1B as an important component in neurite elongation and in particular in the bundling of microtubules in the growth cone.

Malnutrition increases insoluble-to-soluble tubulin ratio and in vitro incorporation of 32ATP in rat cerebral cortex

Wistar rats were fed a normal protein (25% casein) or an isoenergetic low protein (8% casein) diet from the day of birth to weaning on day 21. Litters were killed at weaning and cerebral cortex was removed. Tubulin was prepared by centrifugation at 100,000 g, 4 degrees C, as described by Shelansky et al. [Proc. Natn. Acad. Sci. U.S.A. 70, 765-768 (1973)]. Cold-insoluble tubulin was recovered in the pellet (P1) fraction and cold-soluble tubulin in the supernatant (S1) fraction. Alpha and beta tubulin were quantified by electrophoretic and immunological methods in both fractions. Our results indicated that malnutrition enhanced the ratio of cold-insoluble-tubulin-to-cold-soluble-tubulin. Furthermore malnutrition induced an increased in vitro incorporation of 32P into both soluble and insoluble tubulins. Although tubulin phosphorylation has been related to tubulin stability properties, we cannot unequivocally ascribe the increased insoluble/soluble tubulin ratio with malnutrition to increased in vitro incorporation of 32P.

Neuronal protein tyrosine kinases associated with synaptosomal glycoproteins

Protein tyrosine kinase (PTK) activity associated with synaptosomal membrane glycoprotein (SMGP) fractions of rat brain was examined. The synthetic substrate poly(Glu4-Tyr) was phosphorylated by SMGP in the presence of Mg2+ and Mn2+, whose stimulatory effects were additive. In contrast, endogenous tyrosine phosphorylation in SMGPs was strictly dependent on Mn2+. Anti-phosphotyrosine antibodies (PY20) immunoprecipitated two polypeptides in SMGPs of Mr 170K and 60K. Upon preincubation with IGF-I, 97/90K polypeptides were phosphorylated, corresponding to the IGF-I receptor beta-subunits, and were immunoprecipitated with both PY20 and anti-IGF-I-receptor antibodies. Immunoblot analysis using anti-src antibody revealed that there was src protein associated with the glycoprotein fractions of solubilized synaptosomal membranes. Additional experiments revealed that the 60K tyrosine-phosphorylated polypeptide present in the PY20 precipitates was indeed pp60c-src. This was confirmed by subjecting the PY20 immunoprecipitates to immunoblotting using anti-src antibodies. In addition, src protein was directly immunoprecipitated by anti-src antibodies from the SMGP preparations. Hence, IGF-I receptors and glycoprotein-associated PTKs including pp60c-src may play an important role in synaptic transmembrane signalling, plasticity, and neuronal survival.

Noncytoplasmic and filamentous appearance of calbindin-D28k and tubulin in double, indirect immunofluorescent staining of embryonic chick tissue

Double indirect immunofluorescent labeling of embryonic chick tissue was undertaken for the vitamin D-induced calcium binding protein, calbindin-D28k, and microtubules. Immunoreactivities for both calbindin-D28k and tubulin were found to exhibit a filamentous staining pattern in mesonephros, metanephros, intestine, and brain. In the intestine, staining was absent at 19 days, while immunolabeling of tubulin became evident at 20 days, and both antigens were present in 21-day tissue. In intestinal epithelium, as well as in 10-day metanephros, it was strikingly evident that cells either stained for both antigens or were negative for both calbindin-D28k and tubulin. In 11-12-day metanephros, an increased number of cells with both immunoreactivities were found. In 15-17-day brain, tubulin was evident within all cells but stained most intensely in Purkinje cells which were also positive for calbindin-D28k. Mesonephros from 4-5-day embryos revealed immunolabeling of both tubulin and the calcium binding protein. A statistical analysis of the various cell types revealed that the vast majority contained either both antigens or neither of the immunoreactivities. Of the more than 600 cells scored, none were found to be positive for calbindin-D28k, while at the same time negative for tubulin. It is concluded that calbindin-D28k exhibits a noncytoplasmic distribution in all tissues tested and that the filamentous appearance may reflect localization of the antigen in tubulo-vesicular organelles associated with cytoskeleton.

Differential hormonal modulation of brain antigens recognized by the AB-2 monoclonal antibody

The expression of monoclonal antibody AB-2 immunoreactivity is age- and sex-dependent in radial glia of developing rat hypothalamus and is regulated by prenatal exposure to gonadal steroids. In the present study, several proteins were recognized by AB-2 and were distributed selectively in subcellular fractions from neonatal hypothalamus (HYP), remaining forebrain (FB), and brainstem regions. Immunoblots revealed polypeptide bands in 3 major molecular weight classes: one at approximately 195 kDa in the cytosolic compartment; and two doublets at 220 kDa and 340 kDa in both microsomal and crude mitochondrial membrane fractions. The 220 kDa and 340 kDa doublets were also Triton-insoluble, suggesting a cytoskeletal association. The 195 kDa-AB-2-immunoreactive band was present in both Triton-soluble and insoluble fractions. AB-2 also recognized several acidic glycolipids extracted from postnatal rat brain regions on immunoblots following high performance thin layer chromatography. One of the bands from postnatal rat brain extracts migrated similarly to purified bovine brain sulfatide, which was also immunoreactive with AB-2. AB-2 immunoreactivity with proteins, polar lipids, and sulfatide suggests that the epitope is a carbohydrate present in multiple cellular compartments. AB-2 recognized the same molecular bands in males and females. Testosterone treatment selectively decreased the level of the 195 kDa AB-2-immunoreactive polypeptide. The 195 kDa AB-2-immunoreactive polypeptide possibly acts in radial glia in the determination of sexually dimorphic neurons in the preoptic area/hypothalamus.

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.

Growth cones: the mechanism of neurite advance

Growth cones are the highly motile structures found at the tips of growing axons and dendrites (neurites), which extend from neurones, during the development of the nervous system. They function both as detectors and transducers of extrinsic guidance cues and as regions where the neurite assembly, advance cannot occur. Assembly of the neurite cytoskeleton in growing neurites chiefly involves microtubule assembly at the growth cone. Some of the factors that may influence microtubule assembly in growth cones are becoming apparent and include post-translational modification of tubulin itself and microtubule associated proteins, particularly tau and MAP1B.

Synthesis of phosphotyrosyl-containing phosphopeptides by solid-phase peptide synthesis

The synthesis of phosphotyrosine-containing phosphopeptides using solid-phase peptide synthesis (SPPS) techniques is described. We present the synthesis of a Boc-phosphotyrosine derivative, which when used with modifications of the conventional SPPS protocol permits the incorporation of phosphotyrosine into synthetic peptides. The resulting phosphopeptides were authenticated by fast atom bombardment mass spectrometry, amino acid analysis, and phosphate assay. Alkaline phosphatase was found to dephosphorylate synthetic phosphopeptides at different rates, supporting the potential use of these synthetic substrates for studies of phosphoprotein phosphatases. Synthesis of a phosphopeptide using the described protocol has several advantages over the preparation of phosphopeptides via enzymatic phosphorylation.

The p38 and p34 polypeptides of growth cone particle membranes are the alpha- and beta-subunits of G proteins

Growth cone particle (GCP) membranes prepared from fetal day 17 rat brain are comprised of 5 major polypeptides as analyzed by SDS-PAGE: tubulin (p52), actin (p42), pp46/GAP-43 and two unidentified species, p38 and p34. Antibodies specific for the alpha- and beta-subunits of G proteins recognize p38 and p34, respectively, on immunoblots following one- and two-dimensional electrophoretic separation. That G protein subunits comprise major species of GCP membrane-associated polypeptides suggests a role for G proteins in transmembrane signaling in nerve growth cones.

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.