Mutagenesis of ser41 to ala inhibits the association of GAP-43 with the membrane skeleton of GAP-43-deficient PC12B cells: effects on cell adhesion and the composition of neurite cytoskeleton and membrane

Protein kinase C-dependent growth-associated protein 43 phosphorylation regulates gephyrin aggregation at developing GABAergic synapses

Growth-associated protein 43 (GAP43) is known to regulate axon growth, but whether it also plays a role in synaptogenesis remains unclear. Here, we found that GAP43 regulates the aggregation of gephyrin, a pivotal protein for clustering postsynaptic GABA(A) receptors (GABA(A)Rs), in developing cortical neurons. Pharmacological blockade of either protein kinase C (PKC) or neuronal activity increased both GAP43-gephyrin association and gephyrin misfolding-induced aggregation, suggesting the importance of PKC-dependent regulation of GABAergic synapses. Furthermore, we found that PKC phosphorylation-resistant GAP43(S41A), but not PKC phosphorylation-mimicking GAP43(S41D), interacted with cytosolic gephyrin to trigger gephyrin misfolding and its sequestration into aggresomes. In contrast, GAP43(S41D), but not GAP43(S41A), inhibited the physiological aggregation/clustering of gephyrin, reduced surface GABA(A)Rs under physiological conditions, and attenuated gephyrin misfolding under transient oxygen-glucose deprivation (tOGD) that mimics pathological neonatal hypoxia. Calcineurin-mediated GAP43 dephosphorylation that accompanied tOGD also led to GAP43-gephyrin association and gephyrin misfolding. Thus, PKC-dependent phosphorylation of GAP43 plays a critical role in regulating postsynaptic gephyrin aggregation in developing GABAergic synapses.

Interplay between phosphorylation and palmitoylation mediates plasma membrane targeting and sorting of GAP43

A combination of biochemical, genetic, and imaging approaches is used to show that phosphorylation and lipidation exhibit a complex interplay in sorting of GAP43. Palmitoylation tags GAP43 for global sorting by inducing piggybacking on exocytic vesicles, whereas phosphorylation locally regulates plasma membrane targeting of palmitoylated GAP43.

Phosphorylation and lipidation provide posttranslational mechanisms that contribute to the distribution of cytosolic proteins in growing nerve cells. The growth-associated protein GAP43 is susceptible to both phosphorylation and S-palmitoylation and is enriched in the tips of extending neurites. However, how phosphorylation and lipidation interplay to mediate sorting of GAP43 is unclear. Using a combination of biochemical, genetic, and imaging approaches, we show that palmitoylation is required for membrane association and that phosphorylation at Ser-41 directs palmitoylated GAP43 to the plasma membrane. Plasma membrane association decreased the diffusion constant fourfold in neuritic shafts. Sorting to the neuritic tip required palmitoylation and active transport and was increased by phosphorylation-mediated plasma membrane interaction. Vesicle tracking revealed transient association of a fraction of GAP43 with exocytic vesicles and motion at a fast axonal transport rate. Simulations confirmed that a combination of diffusion, dynamic plasma membrane interaction and active transport of a small fraction of GAP43 suffices for efficient sorting to growth cones. Our data demonstrate a complex interplay between phosphorylation and lipidation in mediating the localization of GAP43 in neuronal cells. Palmitoylation tags GAP43 for global sorting by piggybacking on exocytic vesicles, whereas phosphorylation locally regulates protein mobility and plasma membrane targeting of palmitoylated GAP43.

Structural basis for the interaction of unstructured neuron specific substrates neuromodulin and neurogranin with Calmodulin

Neuromodulin (Nm) and neurogranin (Ng) are neuron-specific substrates of protein kinase C (PKC). Their interactions with Calmodulin (CaM) are crucial for learning and memory formation in neurons. Here, we report the structure of IQ peptides (24aa) of Nm/Ng complexed with CaM and their functional studies with full-length proteins. Nm/Ng and their respective IQ peptides are intrinsically unstructured; however, upon binding with CaM, IQ motifs adopt a helical conformation. Ser41 (Ser36) of Nm (Ng) is located in a negatively charged pocket in the apo CaM and, when phosphorylated, it will repel Nm/Ng from CaM. These observations explain the mechanism by which PKC-induced Ser phosphorylation blocks the association of Nm/Ng with CaM and interrupts several learning- and memory-associated functions. Moreover, the present study identified Arg as a key CaM interacting residue from Nm/Ng. This residue is crucial for CaM-mediated function, as evidenced by the inability of the Ng mutant (Arg-to-Ala) to potentiate synaptic transmission in CA1 hippocampal neurons.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

Regulation of GAP-43 at serine 41 acts as a switch to modulate both intrinsic and extrinsic behaviors of growing neurons, via altered membrane distribution

GAP-43 is the major neuronal substrate of protein kinase C (PKC). Its phosphorylation status dictates the severity of pathfinding errors by GAP-43 (+/-) growth cones in vivo, as well as its modulation of actin dynamics in vitro. These experiments show that stably overexpressing cDNAs mutant at its single PKC phosphorylation site at serine41 in retinoic acid treated SH-Sy5Y neuroblastoma cells regulates intrinsic and extrinsic behaviors of growing neurons. Intrinsically, only Wt and pseudophosphorylated GAP-43Ser41Asp precipitated with F-actin and potentiated F-actin - regulated filopodia formation. GAP-43Ser41Asp inhibited neurite outgrowth whereas only unphosphorylatable GAP-43Ser41Ala precipitated neurotubulin, potentiated neurotubulin accumulation in neurites and increased outgrowth. When PI3-kinase was inhibited GAP-43Ser41Asp-mediated filopodia formation was inhibited whereas GAP-43Ser41Ala-mediated neurite extension was potentiated. Extrinsically, only Wt and GAP-43Ser41Asp potentiated both homotypic adhesion and neurite outgrowth on NCAM-expressing monolayers and promoted NCAM stability. With respect to the underlying mechanism, more F-actin and NCAM colocalized with Wt and GAP-43Ser41Asp in detergent resistant membranes (DRMs) isolated from live cells and GAP-43Ser41Asp-mediated functions were insensitive to cholesterol depletion. In contrast, GAP-43Ser41Ala-mediated functions were sensitive to cholesterol depletion. Neither GAP-43Ser41Asp nor GAP-43Ser41Ala was able to protect against growth cone collapse mediated by PIP2 inhibitors. The results show that modification of GAP-43 at its PKC phosphorylation site directs its distribution to different membrane microdomains that have distinct roles in the regulation of intrinsic and extrinsic behaviors in growing neurons.

Ral GTPases regulate neurite branching through GAP-43 and the exocyst complex

Neurite branching is essential for the establishment of appropriate neuronal connections during development and regeneration. We identify the small GTPase Ral as a mediator of neurite branching. Active Ral promotes neurite branching in cortical and sympathetic neurons, whereas Ral inhibition decreases laminin-induced branching. In addition, depletion of endogenous Ral by RNA interference decreases branching in cortical neurons. The two Ral isoforms, RalA and -B, promote branching through distinct pathways, involving the exocyst complex and phospholipase D, respectively. Finally, Ral-dependent branching is mediated by protein kinase C-dependent phosphorylation of 43-kD growth-associated protein, a crucial molecule involved in pathfinding, plasticity, and regeneration. These findings highlight an important role for Ral in the regulation of neuronal morphology.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Isolation and characterization of detergent-resistant microdomains responsive to NCAM-mediated signaling from growth cones

It is still largely unclear how cell adhesion molecule (CAM)-mediated signaling evokes responses from the growth cone cytoskeleton. Here we used TX-114 extraction of growth cones followed by equilibrium gradient centrifugation to isolate subfractions of detergent-resistant microdomains (DRMs) that could be structurally and functionally distinguished on the basis of localization and activation of components of CAM-mediated signaling pathways. DRMs enriched in cholesterol, caveolin, NCAM140, GPI-linked NCAM120, fyn, and GAP-43, all conventional markers of microdomains or rafts, were located in areas 2 and 3 of the gradient. Coimmunoprecipitation of specific components of CAM signaling pathways by GAP-43 then identified distinct subpopulations of DRMs. GAP-43 from area 2 DRMs coprecipitated GPI-linked NCAM120 and was inactive, i.e., PKC phosphorylation had not been stimulated. In contrast the GAP-43 from area 3 DRMs coprecipitated both transmembrane NCAM140 and caveolin and was active, i.e., highly phosphorylated by PKC. A different subset of DRMs from both area 2 and area 3 contained fyn that could not be coprecipitated with GAP-43 antibodies. In this case area 2 DRMs contained activated fyn that was phosphorylated on Y415. In contrast area 3 DRMs contained inactive fyn. Hence fyn and GAP-43, both targets of NCAM signaling, are located in distinct populations of DRMs, and their activated forms are reciprocally distributed on the gradient. A detergent-resistant membrane fraction recovered from area 4 was enriched in NCAM140, phosphorylated GAP-43, and actin, but not cholesterol, caveolin, or fyn. Immunoelectron microscopy revealed that phosphorylated GAP-43 was localized where the membranes and F-actin interacted. Our results provide evidence for NCAM-mediated signaling in DRMs and suggest that the DRMs responsible for fyn and PKC/GAP-43-mediated NCAM signaling are structurally distinct and differentially distributed in growth cones.

bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.

The RNA-binding protein HuD is required for GAP-43 mRNA stability, GAP-43 gene expression, and PKC-dependent neurite outgrowth in PC12 cells

The RNA-binding protein HuD binds to a regulatory element in the 3' untranslated region (3' UTR) of the GAP-43 mRNA. To investigate the functional significance of this interaction, we generated PC12 cell lines in which HuD levels were controlled by transfection with either antisense (pDuH) or sense (pcHuD) constructs. pDuH-transfected cells contained reduced amounts of GAP-43 protein and mRNA, and these levels remained low even after nerve growth factor (NGF) stimulation, a treatment that is normally associated with protein kinase C (PKC)-dependent stabilization of the GAP-43 mRNA and neuronal differentiation. Analysis of GAP-43 mRNA stability demonstrated that the mRNA had a shorter half-life in these cells. In agreement with their deficient GAP-43 expression, pDuH cells failed to grow neurites in the presence of NGF or phorbol esters. These cells, however, exhibited normal neurite outgrowth when exposed to dibutyryl-cAMP, an agent that induces outgrowth independently from GAP-43. We observed opposite effects in pcHuD-transfected cells. The GAP-43 mRNA was stabilized in these cells, leading to an increase in the levels of the GAP-43 mRNA and protein. pcHuD cells were also found to grow short spontaneous neurites, a process that required the presence of GAP-43. In conclusion, our results suggest that HuD plays a critical role in PKC-mediated neurite outgrowth in PC12 cells and that this protein does so primarily by promoting the stabilization of the GAP-43 mRNA.

Regulation of calcineurin by growth cone calcium waves controls neurite extension

Growth cones generate spontaneous transient elevations of intracellular Ca(2+) that regulate the rate of neurite outgrowth. Here we report that these Ca(2+) waves inhibit neurite extension via the Ca(2+)-dependent phosphatase calcineurin (CN) in Xenopus spinal neurons. Pharmacological blockers of CN (cyclosporin A and deltamethrin) and peptide inhibitors of CN [the Xenopus CN (xCN) autoinhibitory domain and African swine fever virus protein A238L] block the Ca(2+)-dependent reduction of neurite outgrowth in cultured neurons. Time-lapse microscopy of growing neurites demonstrates directly that the reduction in the rate of outgrowth by Ca(2+) transients is blocked by cyclosporin A. In contrast, expression of a constitutively active form of xCN in the absence of waves results in shorter neurite lengths similar to those seen in the presence of waves. The developmental expression pattern of xCN transcripts in vivo coincides temporally with axonal pathfinding by spinal neurons, supporting a role of CN in regulating Ca(2+)-dependent neurite extension in the spinal cord. Ca(2+) wave frequency and Ca(2+)-dependent expression of GABA are not affected by inhibition or activation of CN. However, phosphorylation of the cytoskeletal element GAP-43, which promotes actin polymerization, is reduced by Ca(2+) waves and enhanced by suppression of CN activity. CN ultimately acts on the growth cone actin cytoskeleton, because disrupting actin microfilaments with cytochalasin D or stabilizing them with jasplakinolide negates the effects of suppressing or activating CN. Destabilization or stabilization of microtubules with colcemide or taxol results in Ca(2+)-independent inhibition of neurite outgrowth. The results identify components of the cascade by which Ca(2+) waves act to regulate neurite extension.

Targeted disruption of GAP-43 in P19 embryonal carcinoma cells inhibits neuronal differentiation. As well as acquisition of the morphological phenotype

GAP-43 is expressed in proliferating neuroblasts in vivo and in vitro, but its role during early neurogenesis has not been investigated. Here we show that neuroectodermal differentiation stimulated by retinoic acid (RA) in the embryonal carcinoma (EC) line P19 is accompanied by upregulation of GAP-43 expression in neuroepithelial precursor cells. In contrast, when upregulation of GAP-43 expression was prevented in 3 independent P19 lines because of a targeted insertion into the gene, generation of neuroepithelial precursors was inhibited. Consequently, neuronal number was significantly decreased, neuronal morphology was abnormal and fewer than 20% of all neurons were able to initiate neuritogenesis. Extracellular matrix (ECM) was unable to rescue initiation of neuritogenesis in the mutant cells, however those neurites that were extended responded normally to ECM-stimulated neurite outgrowth-promoting signals. These data suggest that GAP-43 function is required for commitment to a neuronal phenotype as well as initiation of neurite extension. However, stimulation of neurite outgrowth by ECM in P19s occurs independently of GAP-43.

1,2-dioctanoyl-s,n-glycerol-induced activation of protein kinase C results in striking, but reversible growth cone shape changes and an accumulation of f-actin and serine 41-phosphorylated GAP-43 in the axonal process

In spinal cord explant cultures from embryonic chicken (E7) we found that both a long-time downregulation of PKC by phorbol-12,13-dibutyrate (PDBu) and an inhibition of PKC by RO-31-8220 strongly reduce neurite outgrowth. Unlike this, in the presence of a high dose of 1,2-dioctanoyl-s,n-glycerol (diC8, 60 microM), PKCalpha,beta isoforms are not downregulated, but neurite outgrowth appeared reduced up to 37 %. A low dose of diC8 (5 microM), however, was found to stimulate neurite outgrowth up to 25 %. Using this tissue culture system as well as neuronal cell culture we then studied the effects of diC8 on the shapes and actin-based motility of distal axonal processes and growth cones as well as on the spatial distribution of f-actin and serine 41-phosphorylated GAP-43 (neuromodulin, B50). High-resolution microscopy showed that addition of 30-60 microM diC8 leads within a few minutes to a retraction of filopodia and to an increased protrusion of lamellipodia followed by the formation of club-shaped dense growing tips, axonal varicosities, and a cessation of any actin dynamics. These striking shape changes are completely reversed after replacement of the medium by drug-free medium. Presence of cytochalasins and a panel of different PKC inhibitors prevent or respectively attenuate the diC8 effects. Immuno- and phalloidin-staining confirmed that in control neurons f-actin and serine 41-phosphorylated GAP-43 are confined to and enriched in the growth cones. In parallel with diC8-induced shape changes there is an accretion of f-actin and serine 41-phosphorylated GAP-43 in the entire axonal processes and the rounded growing tips. With respect to the fundamental role of the actin dynamics in growth cone steering and neuronal pathfinding, the data supports the view that in neurons local PKC-regulated phosphorylation of GAP-43 may represent an important mechanism to transduce guiding signals into actincytoskeletal responses mediating directed axonal growth.

B-50/GAP-43 potentiates cytoskeletal reorganization in raft domains

B-50 (GAP-43) is a neural, membrane-associated protein that has been implicated in neurite outgrowth and guidance. Following stable transfection of Rat1 fibroblasts with B-50 cDNA we observed a dispersed distribution of B-50 immunoreactivity in flattened resting cells. In contrast, motile cells exhibited high concentrations of B-50 at the leading edge of ruffling membranes, coinciding with actin polymerization. Time-lapse studies on Rat1 fibroblasts transiently transfected with B-50/EGFP revealed that large vesicles originated from the ruffling membranes. These large vesicles (pinocytes) were found positive for Thy-1, a GPI-anchored protein, but negative for rab-5, an early endosome marker. In primary hippocampal neurons B-50 also colocalized completely with the raft marker Thy-1. Antibody-mediated cross-linking of Thy-1 in hippocampal neurons resulted in a redistribution of the intracellular protein B-50 to Thy-1-immunopositive membrane patches, whereas syntaxin was mainly excluded from the patches, showing that B-50 is associated with rafts. Academic Press.

Local accumulations of B-50/GAP-43 evoke excessive bleb formation in PC12 cells

B-50 (GAP-43) is an axonal, plasma membrane-associated protein involved in growth cone morphology and function. We have conducted immunocytochemical, electron microscopic, and time-lapse experiments to visualize morphological consequences of local accumulations of B-50 at the plasma membrane of B-50-transfected PC-B2 cells, a clonal PC12 cell line with very low expression of endogenous B-50. The distribution of the transfected B-50 within these cells was inhomogeneous. At sites where the B-50 concentration was locally increased up to twofold, numerous filopodia were present in growth cone-like, substrate-attached regions. When local B-50 concentrations were even higher (up to 6.2-fold), blebs were formed, often containing vesicular structures, heavily decorated with B-50 immunoreactivity. Double labeling with f-actin binding phalloidin revealed that local B-50 accumulations were accompanied by increased actin filament concentrations. Colocalization of B-50 with actin filaments was prominent in filopodia, but was virtually absent in blebs, suggesting a disconnection of the bleb plasma membrane from the actin cytoskeleton. We conclude that B-50 evokes distinct effects on cell-surface activity in PC12 cells depending on its local concentration.

Hypoxia/ischemia induces dephosphorylation of rat brain neuromodulin/GAP-43 in vivo

The in vivo state of phosphorylation and the modification of two Cys residues of neuromodulin/ GAP-43 (Nm) were analyzed by electrospray ionization-mass spectrometry (ES-MS). The protein was purified from rat brain with homogenization buffer containing 1% Nonidet P-40, protease inhibitors, protein phosphatase inhibitors, and sulfhydryl reagent, 4-vinylpyridine. Nm was purified by HPLC and ion-exchange chromatography, and the various fractions were identified by ES-MS as unphosphorylated and mono-, di-, tri-, and tetraphosphorylated species. All of these Nm species contained 2 mol of added 4-vinylpyridine per mol of Nm, suggesting that the two Cys residues are in the reduced form in the brain. In vivo, the majority of Nm is in the phosphorylated form (approximately 80%), of which the levels of the mono- and diphospho forms are higher than those of the tri- and tetraphospho species. Four in vivo phosphorylation sites, Ser41, Thr95, Ser142, and Thr172, were identified by amino acid sequencing and tandem ES-MS of the peptides derived from Lys-C endoproteinase digestion. Among these sites, only Ser41 is a known target of PKC, whereas the kinases responsible for the phosphorylation of the other three novel sites are unknown. Hypoxia/ischemia caused a preferential dephosphorylation of Ser41 and Thr172, whereas Thr95 is the least susceptible to dephosphorylation.

cis-acting regulatory elements in the GAP-43 mRNA 3'-untranslated region can function in trans to suppress endogenous GAP-43 gene expression

The expression of the GAP-43 gene is controlled partly by changes in the stability of its mRNA, a process that is mediated by the interaction of specific sequences in the 3'-untranslated region (3'UTR) with neuronal-specific RNA-binding proteins. Limiting amounts of these trans-acting factors are available in the cell, thus we proposed that overexpression of the GAP-43 3'UTR could affect the levels of the endogenous mRNA via competitive binding to specific RNA-binding proteins. In this study, we show that chronic expression of GAP-43 3'UTR sequences in PC12 cells causes the depletion of the endogenous mRNA and consequent reduction of GAP-43 protein levels. The levels of the mRNAs for c-fos, the amyloid precursor protein (APP) and the microtubule associated protein tau, all three containing similar 3'UTR sequences, were not affected by the treatment. These results thus suggest that the effect of excess GAP-43 3'UTR is specific for its corresponding mRNA. We also used an HSV (herpes simplex virus)-1 vector and a mammalian expression vector with an inducible promoter to acutely express a 10 to 50 fold excess of 3'UTR sequences. Under these conditions, we found that transient expression of the GAP-43 3'UTR was effective in inhibiting both GAP-43 gene expression and neurite outgrowth in nerve growth factor (NGF)-treated PC12 cells and in primary neuronal cultures. These results underscore the role of 3'UTR sequences in the control of GAP-43 gene expression and suggest that overexpression of specific 3'UTR sequences could be used as a potential tool for probing the function of other post-transcriptionally-regulated proteins during neuronal differentiation.

The neuronal growth-associated protein GAP-43 interacts with rabaptin-5 and participates in endocytosis

Structural plasticity of nerve cells is a requirement for activity-dependent changes in the brain. The growth-associated protein GAP-43 is thought to be one determinant of such plasticity, although the molecular mechanism by which it mediates dynamic structural alterations at the synapse is not known. GAP-43 is bound by calmodulin when Ca2+ levels are low, and releases the calmodulin when Ca2+ levels rise, suggesting that calmodulin may act as a negative regulator of GAP-43 during periods of low activity in the neurons. To identify the function of GAP-43 during activity-dependent increases in Ca2+ levels, when it is not bound to calmodulin, we sought proteins with which GAP-43 interacts in the presence of Ca2+. We show here that rabaptin-5, an effector of the small GTPase Rab5 that mediates membrane fusion in endocytosis, is one such protein. We demonstrate that GAP-43 regulates endocytosis and synaptic vesicle recycling. Modulation of endocytosis by GAP-43, in association with rabaptin-5, may constitute a common molecular mechanism by which GAP-43 regulates membrane dynamics during its known roles in activity-dependent neurotransmitter release and neurite outgrowth.

B-50/GAP-43-induced formation of filopodia depends on Rho-GTPase

In the present study we show that expression of the neural PKC-substrate B-50 (growth-associated protein [GAP-43]) in Rat-1 fibroblasts induced the formation of filopodial extensions during spreading. This morphological change was accompanied by an enhanced formation of peripheral actin filaments and by accumulation of vinculin immunoreactivity in filopodial focal adhesions, colocalizing with B-50. In time lapse experiments, the B-50-induced filopodial extensions were shown to stay in close contact with the substratum and appeared remarkably stable, resulting in a delayed lamellar spreading of the fibroblasts. The morphogenetic effects of the B-50 protein were entirely dependent on the integrity of the two N-terminal cysteines involved in membrane association (C3C4), but were not significantly affected by mutations of the PKC-phosphorylation site (S41) or deletion of the C terminus (177-226). Cotransfection of B-50 with dominant negative Cdc42 or Rac did not prevent B-50-induced formation of filopodial cells, whereas this process could be completely blocked by cotransfection with dominant negative Rho or Clostridium botulinum C3-transferase. Conversely, constitutively active Rho induced a similar filopodial phenotype as B-50. We therefore propose that the induction of surface extensions by B-50 in spreading Rat-1 fibroblasts depends on Rho-guanosine triphosphatase function.

Distribution of phosphorylated GAP-43 (neuromodulin) in growth cones directly reflects growth cone behavior

Phosphorylation of GAP-43 (neuromodulin) by protein kinase C (PKC) occurs at a single site, serine41. In vivo, phosphorylation is induced after initiation of axonogenesis and is confined to distal axons and growth cones. Within individual growth cones, phosphorylation is nonuniformly distributed. Here, we have used high-resolution video-enhanced microscopy of cultured dorsal root ganglia neurons together with immunocytochemistry with a monoclonal antibody that recognizes PKC-phosphorylated GAP-43 to correlate the distribution of phosphorylated GAP-43 with growth cone behavior. In "quiescent," nontranslocating growth cones, phosphorylated GAP-43 was confined to the proximal neurite and the central organelle-rich region, and was low in organelle-poor lamellae. However, levels in lamellae were elevated when they became motile. Conversely, levels of phosphorylated GAP-43 were low in either lamellae that were actively retracting or in the central organelle-rich region and proximal neurite of growth cones that had totally collapsed. The results suggest a mechanism whereby phosphorylation of GAP-43 by PKC, potentially in response to extracellular signals, could direct the functional behavior of the growth cone.

S100A1 regulates neurite organization, tubulin levels, and proliferation in PC12 cells

As a first step in determining what cellular processes are regulated by the calcium-modulated protein S100A1 isoform in neurons, the effects of ablated S100A1 expression on neurite organization and microtubule/tubulin levels in PC12 cells were examined. A mammalian expression vector containing the rat S100A1 cDNA in the antisense orientation with respect to a cytomegalovirus promoter was constructed and transfected into PC12 cells. Indirect immunofluorescence microscopy confirmed decreased S100A1 protein levels in all three stable transfectants (pAntisense clones) that expressed exogenous S100A1 antisense mRNA. In response to nerve growth factor, pAntisense clones extended significantly more neurites than control cells (4.01 +/- 0.16 versus 2.93 +/- 0.16 neurites/cell). This increase in neurite number was accompanied by an increase in total alpha-tubulin levels in untreated (4.0 +/- 0.6 versus 1.76 +/- 0.4 ng of alpha-tubulin/mg of total protein) and nerve growth factor-treated pAntisense clones (4.15 +/- 0.4 versus 2. 04 +/- 0.5 ng of alpha-tubulin/mg of total protein) when compared with control cells. At high cell densities, pAntisense clones exhibited a significant decrease in anchorage-dependent growth. In soft agar, pAntisense clones formed significantly more colonies (153 +/- 8%) than control cells (116 +/- 5%). However, the pAntisense soft agar colonies were significantly smaller than those observed in control cells (40.6 +/- 3.0 versus 59.5 +/- 1.2 micron). These data suggest that cell density inhibits both anchorage-independent and -dependent growth of pAntisense clones. In summary, ablation of S100A1 expression in PC12 cells results in increased tubulin levels, altered neurite organization, and decreased cell growth. Thus, S100A1 may directly link the cytoskeleton and calcium signal transduction pathways to cell proliferation.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

The motility-associated proteins GAP-43, MARCKS, and CAP-23 share unique targeting and surface activity-inducing properties

Local regulation of the cortical cytoskeleton controls cell surface dynamics. GAP-43 and MARCKS are two abundant cytosolic protein kinase C substrates that are anchored to the cell membrane via acyl groups and interact with the cortical cytoskeleton. Each of them has been implicated in several forms of motility involving the cell surface. Although their primary sequences do not reveal significant homologies, GAP-43, MARCKS, and the cortical cytoskeleton-associated protein CAP-23 (in the following, the three proteins will be abbreviated as GMC) share a number of characteristic biochemical and biophysical properties and an unusual amino acid composition. In this study we determined whether GMC may be related functionally. In double-labeling immunocytochemistry experiments GMC accumulated at unique surface-associated structures, where they codistributed. In transfected cells GMC induced the same range of characteristic changes in cell morphology and cell surface activities, including prominent blebs and filopodia. These activities correlated with local accumulation of transgene and had characteristic features of locally elevated actin dynamics, including loss of stress fiber structures, accumulation of beta-(cytosolic) actin at cell surface protrusions, and dynamic blebbing activity. Analysis of appropriate deletion and fusion constructs revealed that the surface accumulation pattern and cell surface activities were correlated and that minimal structural requirements included acylation-mediated targeting to the cell membrane and the presence of a predominantly GMC-type sequence composition. Based on these experiments and on the results of previous studies on GAP-43, MARCKS, and CAP-23, we propose that GMC may define a class of functionally related proteins whose local accumulation promotes actin dynamics and the formation of dynamic structures at the cell periphery. Superimposed on these general properties, differences in the regulation of membrane association and binding properties of effector domains would confer individual properties to each of these proteins.

Ultrastructural co-localization of calmodulin and B-50/growth-associated protein-43 at the plasma membrane of proximal unmyelinated axon shafts studied in the model of the regenerating rat sciatic nerve

Calmodulin and de-phosphorylated B-50/growth-associated protein-43 (GAP-43) have been shown to bind in vitro in a molecular complex, but evidence for an in situ association in the nervous system does not exist. Previously, we have reported that, in the model of the regenerating rat sciatic nerve, the B-50/GAP-43 immunoreactivity is increased and concentrated at the axolemma of unmyelinated axons located proximal to the site of injury and axon outgrowth. To explore a putative function of B-50/GAP-43, namely, the capacity of binding calmodulin to the plasma membrane, we examined the ultrastructural distribution of calmodulin in the proximal unmyelinated axon shafts of this model, using double immunolabelling and detection by fluorescent or gold probes conjugated to second antibodies. Immunofluorescence showed that seven days post-sciatic nerve crush the calmodulin immunoreactivity, similar to B-50/GAP-43 immunoreactivity, was intense in unmyelinated axon shafts located proximal to the site of injury of the regenerating nerve. Ultrastructurally, calmodulin was located at the axolemma of these regenerating unmyelinated axon shafts and inside the axoplasm, where it was associated with vesicles and microtubules. The plasma membrane labelling (approximately 69%) was significantly higher than the axoplasmic labelling. Over 60% of the plasma membrane-associated calmodulin co-localized with B-50/GAP-43 in a non-random distribution. Since normally calmodulin is largely present in the cytoplasm, these data suggest that calmodulin has been concentrated at the plasma membrane of unmyelinated axons, most probably by B-50/GAP-43. If the concentrating effect is due to B-50/GAP-43, then there is a possibility that these proteins may be present as a molecular complex in situ. The physiological significance could be that this association regulates the local availability of both B-50/GAP-43 and calmodulin for other interactions.

Modulation of actin filament behavior by GAP-43 (neuromodulin) is dependent on the phosphorylation status of serine 41, the protein kinase C site

Synthesis of GAP-43 (also known as neuromodulin) in neurons is induced during axon growth, and high concentrations (estimated between 50 and 100 microM) accumulate in the growth cone. GAP-43 is tightly associated with the growth cone membrane skeleton, the structure that transduces extracellular guidance cues into alterations in morphology by spatially regulating polymerization of actin filaments, thereby causing directional changes in axon growth. GAP-43 cosediments with actin filaments, and its phosphorylation on serine 41 by PKC, too, is spatially regulated so that phosphorylated GAP-43 is found in areas where growth cones make productive, stable contacts with other cells. In contrast, unphosphorylated GAP-43, which binds calmodulin, is always found in parts of the growth cone that are retracting. Here we have used a cell-free assay to investigate how the phosphorylation status of GAP-43 affects its interactions with actin and show that both phosphorylated and unphosphorylated GAP-43 have different, independent effects on actin filament structure. Phosphorylated GAP-43 stabilizes long actin filaments (Kd = 161 nM), and antibodies to phosphorylated GAP-43 inhibit binding of actin to phalloidin, implying a lateral interaction with filaments. In contrast, unphosphorylated GAP-43 reduces filament length distribution (Kd = 1.2 microM) and increases the critical concentration for polymerization. Prebinding calmodulin potentiates this effect. The results show that spatially regulated post-translational modifications of GAP-43 within the growth cone, which can be regulated in response to extracellular signals, have the ability to directly influence the structure of the actin cytoskeleton.

Analysis of the role of calmodulin binding and sequestration in neuromodulin (GAP-43) function

We demonstrated previously that forced expression of the neuronal phosphoprotein neuromodulin (also known as GAP-43, F1, B-50, and p57) in mouse anterior pituitary AtT-20 cells enhances depolarization-mediated secretion and alters cellular morphology. Here we analyze the role of calmodulin binding by neuromodulin in these responses. In cells expressing wild-type neuromodulin, a complex with calmodulin that is sensitive to intracellular calcium and phosphorylation is localized to the plasma membrane. Transfection of several mutant forms of neuromodulin shows that the effects of this protein on secretion are dependent on both calmodulin binding and association with the plasma membrane. In contrast, the morphological changes depend only on membrane association. Thus, the multitude of effects of neuromodulin noted in previous studies may result from divergent properties of this protein.