Spontaneous morphological changes by overexpression of the growth-associated protein B-50/GAP-43 in a PC12 cell line

Neurotrophic Effects of Mu Bie Zi (Momordica cochinchinensis) Seed Elucidated by High-Throughput Screening of Natural Products for NGF Mimetic Effects in PC-12 Cells

Post-mitotic central nervous system (CNS) neurons have limited capacity for regeneration, creating a challenge in the development of effective therapeutics for spinal cord injury or neurodegenerative diseases. Furthermore, therapeutic use of human neurotrophic agents such as nerve growth factor (NGF) are limited due to hampered transport across the blood brain barrier (BBB) and a large number of peripheral side effects (e.g. neuro-inflammatory pain/tissue degeneration etc.). Therefore, there is a continued need for discovery of small molecule NGF mimetics that can penetrate the BBB and initiate CNS neuronal outgrowth/regeneration. In the current study, we conduct an exploratory high-through-put (HTP) screening of 1144 predominantly natural/herb products (947 natural herbs/plants/spices, 29 polyphenolics and 168 synthetic drugs) for ability to induce neurite outgrowth in PC12 dopaminergic cells grown on rat tail collagen, over 7 days. The data indicate a remarkably rare event-low hit ratio with only 1/1144 tested substances (<111.25 µg/mL) being capable of inducing neurite outgrowth in a dose dependent manner, identified as; Mu Bie Zi, Momordica cochinchinensis seed extract (MCS). To quantify the neurotrophic effects of MCS, 36 images (n = 6) (average of 340 cells per image), were numerically assessed for neurite length, neurite count/cell and min/max neurite length in microns (µm) using Image J software. The data show neurite elongation from 0.07 ± 0.02 µm (controls) to 5.5 ± 0.62 µm (NGF 0.5 μg/mL) and 3.39 ± 0.45 µm (138 μg/mL) in MCS, where the average maximum length per group extended from 3.58 ± 0.42 µm (controls) to 41.93 ± 3.14 µm (NGF) and 40.20 ± 2.72 µm (MCS). Imaging analysis using immunocytochemistry (ICC) confirmed that NGF and MCS had similar influence on 3-D orientation/expression of 160/200 kD neurofilament, tubulin and F-actin. These latent changes were associated with early rise in phosphorylated extracellular signal-regulated kinase (ERK) p-Erk1 (T202/Y204)/p-Erk2 (T185/Y187) at 60 min with mild changes in pAKT peaking at 5 min, and no indication of pMEK involvement. These findings demonstrate a remarkable infrequency of natural products or polyphenolic constituents to exert neurotrophic effects at low concentrations, and elucidate a unique property of MCS extract to do so. Future research will be required to delineate in depth mechanism of action of MCS, constituents responsible and potential for therapeutic application in CNS degenerative disease or injury.

Behavioral characterization in a comprehensive mouse test battery reveals motor and sensory impairments in growth-associated protein-43 null mutant mice

The growth-associated protein (GAP)-43 is a major neuronal protein associated with axonal growth, neuronal plasticity and learning. The observation that only 5-10% of mice with a full GAP-43 gene deletion survive weaning suggests that basic neural functions are disturbed. Here we used a comprehensive test battery to characterise and quantify the motor and sensory function of surviving adult homozygous GAP-43 (-/-) mice as compared with GAP-43 (+/-) and wild-type animals. The test battery was comprised of motor, sensory, and reflex tests producing 25 measures of locomotion, as well as epicritic, auditory, olfactory and visual function. The analysis revealed significant impairments in muscle strength, limb coordination and balance in GAP-43 (-/-) mice. Furthermore, GAP-43 (-/-) animals were hyperactive and showed reduced anxiety as measured by open field and light dark tests. In sensory tests, GAP-43 (-/-) mice were tested for impaired tactile and labyrinthine function. Abnormal reflexes were found in the contact and vibrissa placing responses, and in the crossed extensor reflex. GAP-43 (+/-) animals showed only moderate abnormalities as compared with wild-type animals. We conclude that GAP-43 is necessary for the development and function of a variety of neuronal systems. The results also show that the comprehensive test battery used in the present study represents a sensitive approach to assess the functional integrity of ascending and descending pathways in genetically manipulated mice.

Expression of huntingtin-associated protein-1 in neuronal cells implicates a role in neuritic growth

Huntingtin-associated protein-1 (HAP1) binds more tightly to mutant huntingtin, but its function and distribution in neurites are not clear. Using PC12 cells and cultured hippocampal neurons, we found that two HAP1 isoforms, HAP1-A and HAP1-B, have different subcellular localizations. While most HAP1-B is diffusely distributed within the cytoplasm, HAP1-A is enriched in the growth cones and neuritic puncta of developing neurons. In mature neurons and adult brain neurons, however, HAP1-A is concentrated in axon terminals and associated with synaptic vesicles. Transfection of HAP1-A in PC12 cells significantly promotes neurite extension, with HAP1-A distributed in the distal regions of the growing neurites. Cotransfection of mutant huntingtin with HAP1-A in PC12 cells results in the accumulation of HAP1-A on huntingtin aggregates and the inhibition of neurite promotion by HAP1-A. This study suggests that HAP1-A has a function in neuritic development and synaptic function and that mutant huntingtin may alter this function.

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.

A key role for GAP-43 in the retinotectal topographic organization

To have a proper spatial visual perception, vertebrate retinal ganglion cells connect to their brain targets in a highly ordered fashion. The molecular bases for such topographic retinotectal connection in mammals still remain largely unknown. Using the gene knock-out approach in mice, we report here a key role for the GAP-43 growth cone protein in the development of the visual system. In mice bearing a targeted disruption of GAP-43 exon 1, a high proportion of retinal ganglion cell (RGC) axons was found to grow abnormally into the ipsilateral optic tract and into the hypothalamus. After leaving the optic chiasm during development, the GAP-43-deficient RGC axons generally follow the optic tracts but are unable to form proper terminal zones in the lateral geniculate nucleus. Moreover, in the superior colliculus, RGC axons lacking GAP-43 are intermingled. These results suggest an essential role for GAP-43 in development of the topographic retinotectal connection.

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.

Microinjection of activated phosphatidylinositol-3 kinase induces process outgrowth in rat PC12 cells through the Rac-JNK signal transduction pathway

We have previously shown that sustained phosphatidylinositol (PI)-3 kinase activity is necessary for neurite outgrowth of PC12 cells induced by nerve growth factor (NGF). Microinjection of a constitutively active mutant of PI-3 kinase induced process formation suggesting that PI-3 kinase is indeed involved in the neurite outgrowth. However, the processes appeared to be incomplete neurites as they had very poor organization of F-actin and GAP43 antigen. The microtubule network was enhanced in the process-bearing cells and process formation was inhibited by colchicine suggesting that microtubules play an important role in process formation downstream of PI-3 kinase. These cell responses were inhibited by dominant-negative mutants of Rac and Sek1/SAPK but not by a dominant-negative mutant Ras and PD98059, a MAP kinase kinase (MEK) inhibitor, suggesting that not the Ras-MAP kinase pathway but the Rac-Jun N-terminal kinase (JNK) pathway is involved in process formation.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

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.

Expression levels of B-50/GAP-43 in PC12 cells are decisive for the complexity of their neurites and growth cones

To study the role of the protein B-50/GAP-43 in NGF-induced neurite outgrowth, a number of stable PC12 subclones with either very low or considerably enhanced expression levels of the protein were selected. Cell bodies of subclones with suppressed B-50 expression (-B2, -B5, or -B12) possessed a relative small spherical shape and, on NGF-treatment for 7 d, developed processes that were virtually devoid of branches and that mostly bore short or blunt-ended growth cones. Cells of subclones with overexpression of B-50 (+B3, +B4, or +B11), on NGF treatment, acquired a flattened, spiky appearance with highly branched neurites possessing extended and complex growth cones. Confocal microscopy with immunofluorescence for B-50 and F-actin revealed that in neurites and growth cones of the B-50-deficient subclone -B2, no detectable B-50 and reduced amounts of filamentous F-actin were present, whereas in overexpressing +B3 cells, cell membranes, neurites, and complex growth cones were intensively stained for B-50 and exhibited numerous spikes, in which B-50 was strikingly colocalized with F-actin. These data suggest that, under normal conditions of neuritogenesis, the expression level of B-50 in PC12 cells is decisive for the complexity of neurites and growth cones.

Identified cholinergic neurones in the adult rat brain are enriched in GAP-43 mRNA: a double in situ hybridisation study

The cellular expression of growth associated protein-43 mRNA by identified choline acetyl transferase mRNA positive cells was investigated in the mature rat brain using a combined radioactive and non-radioactive in situ hybridisation technique. Cellular sites of growth associated protein-43 mRNA were detected using a 35S-oligonucleotide while choline acetyl transferase mRNA positive neurones were identified using two alkaline phosphatase-labelled probes. In the cholinergic cells of the corpus striatum, basal forebrain and laterodorsal tegmental nucleus a specific growth associated protein-43 hybridisation signal (silver grains) was detected, demonstrating that these choline acetyl transferase mRNA positive cells are enriched in growth associated protein-43 gene transcripts. By contrast, the large cholinergic cells of the motor nucleus of the trigeminal nerve did not express growth associated protein-43 mRNA. Quantification of the growth associated protein-43 hybridisation signal expressed by identified choline acetyl transferase mRNA positive cells showed regional variations in the relative cellular abundance of this transcript; cholinergic cells in the laterodorsal tegmental nucleus and corpus striatum expressed the strongest cellular hybridisation signal. Mean cross-sectional somatic area measurements of these growth associated protein-43/cholinergic positive cells confirmed the identity of these neurones as belonging to the cholinergic phenotype. A strong 35S-growth associated protein-43 hybridisation signal was detected also in numerous other non-choline acetyl transferase mRNA positive nerve cells in other regions of the brain, although the chemical phenotypes of these neurones were not determined. Our data reveal that expression of the growth-associated protein GAP-43 is maintained in identified cholinergic neurones in the postnatal rat brain, suggesting that this protein may subserve important functions in cholinergic and other neurones of the adult mammalian brain.

Inhibition of neuronal surface proteases decreases the requirement for GAP-43 in neurite outgrowth

Intracellular delivery of anti-GAP IgG inhibits the elaboration of neurites by NB2a/d1 cells. However, recent studies indicate that the extent of inhibition is minimized when cells were cultured on poly-L-lysine-coated or laminin-coated versus uncoated plates, suggesting that the role of GAP-43 in neuritogenesis may be specifically related to membrane adhesiveness. We therefore examined the influence of inhibition of thrombin, the neuronal surface protease that restricts neurite outgrowth, on GAP-43-dependent neurite outgrowth. Treatment of cells with the specific thrombin inhibitor hirudin in the presence of serum induced a similar percentage of neurite outgrowth as was observed following serum withdrawal. However, while neurite outgrowth induced by serum deprivation of cells was reduced following intracellular delivery of anti-GAP IgG, neurite outgrowth induced by hirudin treatment of cells was not. That inhibition of neuronal surface protease activity overcomes the inhibition of neurite outgrowth following intracellular delivery of anti-GAP IgG provides further evidence that the role that GAP-43 plays in neuritogenesis is related to membrane adhesiveness.

Inhibition of neurite outgrowth following intracellular delivery of anti-GAP-43 antibodies depends upon culture conditions and method of neurite induction

NB2/dl neuroblastoma cells acquire a neuronal phenotype in response to several differentiating agents, including dibutyryl cAMP (dbcAMP) and the withdrawal of serum. As shown previously, antibodies to the growth-associated protein, GAP-43, introduced intracellularly using a lipid carrier, blocked the differentiation induced by dbcAMP. Antibodies to GAP-43, at a low concentration, also blocked neurite outgrowth induced by serum withdrawal when cells were grown on a relatively unadhesive substrate. On more adhesive substrates such as poly-L-lysine and laminin, however, anti-GAP-43 antibodies had less of an effect on neurite outgrowth. Previous studies have shown that the increased adhesivity of laminin allows a small but significant population of neurites to grow from serum-deprived cells, even in the presence of the microtubule-depolymerizing drug, colchicine. The outgrowth of this population of neurites was blocked by antibodies to GAP-43. These results are in conformity with recent studies showing that the requirement for GAP-43 in neuritogenesis may be related to membrane adhesiveness, and may contribute to an understanding of some of the apparent discrepancies in the literature concerning the involvement of GAP-43 in neuronal differentiation.

Long-term potentiation and synaptic protein phosphorylation

Long-term potentiation (LTP) is a well known experimental model for studying the activity-dependent enhancement of synaptic plasticity, and because of its long duration and its associative properties, it has been proposed as a system to investigate the molecular mechanisms of memory formation. At present, there are several lines of evidence that indicate that pre- and postsynaptic kinases and their specific substrates are involved in molecular mechanisms underlying LTP. Many studies focus on the involvement of protein kinase C (PKC). One way to investigate the role of PKC in long-term potentiation is to determine the degree of phosphorylation of its substrates after in situ phosphorylation in hippocampal slices. Two possible targets are the presynaptic membrane-associated protein B-50 (a.k.a. GAP 43, neuromodulin and F1), which has been implicated in different forms of synaptical plasticity in the brain such as neurite outgrowth, hippocampal LTP and neurotransmitter release, and the postsynaptic protein neurogranin (a.k.a. RC3, BICKS and p17) which function remains to be determined. This review will focus on the protein kinase C activity in pre- and postsynaptic compartment during the early phase of LTP and the possible involvement of its substrates B-50 and neurogranin.