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

The differential regulation of Gap43 gene in the neuronal differentiation of P19 cells

Growth associated protein 43 (Gap43) is a neuron-specific phosphoprotein, which plays critical role in axon growth and synapses functions during neurogenesis. Here we identified two transcription start sites (TSSs) of the mouse Gap43 gene designated as a proximal site at +1, and a distal TSS at -414. RT-qPCR data reveal that the transcripts from +1 increase 10-fold on day-1 post-all-trans retinoic acid (RA) treatment, reached a peak value at day-4 and gradually reduced. By contrast, the distal TSS directs a late, remarkably sharp increase of the transcripts from the day-5 on. An intense signal of Gap43 at the neurites and neural network is determined by the efficient transcription of the distal promoter as shown in Northern blot and RT-qPCR assay. In addition, the targeting of p300 in combination with a differential enrichment of Brm to Brg1 change at the distal promoter region of the gene is induced under RA treatment. The over hundreds of GA rich stretches and the GAGAG elements located between the two TSSs may take parts in the differential transcription of the two TSSs of the Gap43. Our findings provide the first evidence on the identification and differential transcription of the two TSSs of the mouse Gap43 gene, and the preferential distribution of their protein products in the specific stages of RA induced P19 differentiation. These data suggest the efficient transcription of the distal promoter of Gap43 is an important mark for the transition of P19 cells from the progenitor stage into neuronal differentiation.

Changes in Ca(2+) channel expression upon differentiation of SN56 cholinergic cells

The SN56 cell line, a fusion of septal neurons and neuroblastoma cells, has been used as a model for central cholinergic neurons. These cells show increased expression of cholinergic neurochemical features upon differentiation, but little is known about how differentiation affects their electrophysiological properties. We examined the changes in Ca(2+) channel expression that occur as these cells undergo morphological differentiation in response to serum withdrawal and exposure to dibutyryl-cAMP. Undifferentiated cells expressed a T-type current with biophysical and pharmacological properties similar, although not identical, to those reported for the current generated by the alpha(1H) (CaV3.2) Ca(2+) channel subunit. Differentiated cells expressed, in addition to this T-type current, high voltage activated currents which were inhibited 38% by the L-type channel antagonist nifedipine (5 microM), 37% by the N-type channel antagonist omega-conotoxin-GVIA (1 microM), and 15% by the P/Q-type channel antagonist omega-agatoxin-IVA (200 nM). Current resistant to these inhibitors accounted for 15% of the high voltage activated current in differentiated SN56 cells. Our data demonstrate that differentiation increases the expression of neuronal type voltage gated Ca(2+) channels in this cell line, and that the channels expressed are comparable to those reported for native basal forebrain cholinergic neurons. This cell line should thus provide a useful model system to study the relationship between calcium currents and cholinergic function and dysfunction.

Cyclic AMP prevents an increase in GAP-43 but promotes neurite growth in cultured adult rat dorsal root ganglion neurons

High expression of the growth-associated protein GAP-43 in neurons is correlated with developmental and regenerative axon growth. It has been postulated that during development and after injury, GAP-43 expression is elevated due to the unavailability of a target-derived repressive signal, but that GAP-43 expression then declines upon target contact. Here we examine the cyclic AMP second messenger signaling pathway to determine if it might mediate retrograde transmission of a signal which represses GAP-43 expression and inhibits growth. Cultures of adult rat dorsal root ganglia were chronically exposed to membrane-permeable analogs of cyclic AMP and activators of adenyl cyclase. These treatments caused GAP-43 protein levels to decrease in a dose-dependent manner, although neuronal survival was not affected. GAP-43 mRNA was also decreases by cyclic AMP. GAP-43 protein levels were not repressed by neurotrophins, cytokines, or other agents. Surprisingly, cyclic AMP caused an increase in the rate of neurite outgrowth, even though the neurons were partially depleted of GAP-43. Growth stimulation was quickly inducible and reversible, could occur in the presence of transcription inhibitors, and did not entail alterations in branching pattern. These findings suggest that axon growth involving high levels of GAP-43 is distinct from the growth stimulation which is rapidly induced by cyclic AMP.

Synaptic vesicle proteins and neuronal plasticity in adrenergic neurons

The neurons in the superior cervical ganglion are active in plasticity and re-modelling in order to adapt to requirements. However, so far, only a few studies dealing with synaptic vesicle related proteins during adaptive processes have been published. In the present paper, changes in content and expression of the synaptic vesicle related proteins in the neurons after decentralization (cutting the cervical sympathetic trunk) or axotomy (cutting the internal and external carotid nerves) were studied. Immunofluorescence studies were carried out using antibodies and antisera against integral membrane proteins, vesicle associated proteins, NPY, and the enzymes TH and PNMT. For colocalization studies, the sections were simultaneously double labelled. Confocal laser scanning microscopy was used for colocalization studies as well as for semi-quantification analysis, using the computer software. Westen blot analysis, in situ 3'-end DNA labelling, and in situ hybridization were also employed. After decentralization of the ganglia several of the synaptic vesicle proteins (synaptotagmin I, synaptophysin, SNAP-25, CLC and GAP-43) were increased in the iris nerve terminal network, but with different time patterns, while TH-immunoreactivity had clearly decreased. In the ganglia, these proteins had decreased at 1 day after decentralization, probably due to degeneration of the pre-ganglionic nerve fibres and terminals. At later intervals, these proteins, except SNAP-25, had increased in the nerve fibre bundles and re-appeared in nerve fibres outlining the principal neurons.

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.

Neurite outgrowth induced by cyclic AMP can be modulated by the alpha subunit of Go

Although abundant Go has been found in nervous tissues and it has been implicated in neuronal differentiation, the mechanism of how Go modulates neuronal differentiation has not been defined. Here, we report that the alpha subunit of Go (alphao) modulates neurite outgrowth by interfering with the signaling pathway initiated by cyclic AMP (cAMP). In F11 cells, cAMP induced neurite outgrowth and activated cAMP-responsive element binding protein (CREB). Specific inhibition of cAMP-dependent protein kinase reduced both CREB activity and neurite outgrowth (NOG). Interestingly, cAMP reduced phosphorylation of extracellular signal-regulated kinase (Erk). Neither a dominant negative form nor an active form of Ras altered neurite outgrowth. Expression of alphao (alphao(wt)) decreased the average length of neurites but increased the number of neurites per cell. An active mutant, alphaoQ205L, which lost GTPase activity and thus could not bind to Gbetagamma, gave similar results, suggesting that the effect of alphao is not mediated through Gbetagamma. Expression of ao(wt) or alphaoQ205L also prohibited CREB activation. Thus, activation of Erk may not be essential for neuronal differentiation in F11 cells and alphao may cause changes in NOG by inhibiting CREB activation.

Growth-associated protein-43 is increased in cerebrum of immature rats following induction of hydrocephalus

Hydrocephalus is associated with gradual progressive impairment and destruction of cerebral axons and neurons. Growth associated protein-43 appears to be permissive for neuro-axonal regeneration and synaptic remodeling. Hydrocephalus was induced in three-week-old rats by injection of kaolin into the cisterna magna. Compared to controls, cerebral growth-associated protein-43 messenger RNA was significantly up-regulated one week after kaolin injection and the overall cerebral growth-associated protein-43 protein level was significantly higher at four weeks when the ventricles were severely enlarged. One and three weeks after kaolin injection, growth-associated protein-43-like immunoreactivity was increased in periventricular axons, and also in the cerebral cortex at three weeks. Hydrocephalic rats that had been treated by shunting after one week, exhibited growth-associated protein-43 messenger RNA and protein levels intermediate between hydrocephalic rats and control rats. The increase in periventricular axon growth-associated protein-43, early in the course of experimental hydrocephalus, suggests that through early intervention there may be a chance for preventing or reversing the axonal injury. Cortical expression of growth associated protein-43 suggests that an alteration in synaptogenesis may also occur.

Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules

We analyzed changes in the expression of mRNAs for the axonal growth-promoting cell recognition molecules L1.1, L1.2, and neural cell adhesion molecule (NCAM) after a rostral (proximal) or caudal (distal) spinal cord transection in adult zebrafish. One class of cerebrospinal projection nuclei (represented by the nucleus of the medial longitudinal fascicle, the intermediate reticular formation, and the magnocellular octaval nucleus) showed a robust regenerative response after both types of lesions as determined by retrograde tracing and/or in situ hybridization for GAP-43. A second class (represented by the nucleus ruber, the nucleus of the lateral lemniscus, and the tangential nucleus) showed a regenerative response only after proximal lesion. After distal lesion, upregulation of L1.1 and L1.2 mRNAs, but not NCAM mRNA expression, was observed in the first class of nuclei. The second class of nuclei did not show any changes in their mRNA expression after distal lesion. After proximal lesion, both classes of brain nuclei upregulated L1.1 mRNA expression (L1.2 and NCAM were not tested after proximal lesion). In the glial environment distal to the spinal lesion, labeling for L1.2 mRNA but not L1.1 or NCAM mRNAs was increased. These results, combined with findings in the lesioned retinotectal system of zebrafish (Bernharnhardt et al., 1996), indicate that the neuron-intrinsic regulation of cell recognition molecules after axotomy depends on the cell type as well as on the proximity of the lesion to the neuronal soma. Glial reactions differ for different regions of the CNS.

Retinal ganglion cell axon progression from the optic chiasm to initiate optic tract development requires cell autonomous function of GAP-43

Pathfinding mechanisms underlying retinal ganglion cell (RGC) axon growth from the optic chiasm into the optic tract are unknown. Previous work has shown that mouse embryos deficient in GAP-43 have an enlarged optic chiasm within which RGC axons were reportedly stalled. Here we have found that the enlarged chiasm of GAP-43 null mouse embryos appears subsequent to a failure of the earliest RGC axons to progress laterally through the chiasm-tract transition zone to form the optic tract. Previous work has shown that ventral diencephalon CD44/stage-specific embryonic antigen (SSEA) neurons provide guidance information for RGC axons during chiasm formation. Here we found that in the chiasm-tract transition zone, axons of CD44/SSEA neurons precede RGC axons into the lateral diencephalic wall and like RGC axons also express GAP-43. However unlike RGC axons, CD44/SSEA axon trajectories are unaffected in GAP-43 null embryos, indicating that GAP-43-dependent guidance at this site is RGC axon specific or occurs only at specific developmental times. To determine whether the phenotype results from loss of GAP-43 in RGCs or in diencephalon components such as CD44/SSEA axons, wild-type, heterozygous, or homozygous GAP-43 null donor retinal tissues were grafted onto host diencephalons of all three genotypes, and graft axon growth into the optic tract region was assessed. Results show that optic tract development requires cell autonomous GAP-43 function in RGC axons and not in cellular elements of the ventral diencephalon or transition zone.

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.

Regulated binding of the protein kinase C substrate GAP-43 to the V0/C2 region of protein kinase C-delta

The interaction between protein kinase C-delta and its neuronal substrate, GAP-43, was studied. Two forms of protein kinase C-delta were isolated from COS cells and characterized by differences in gel mobility, GAP-43 binding, and specific GAP-43 and histone kinase activities. A slow migrating, low specific activity form of protein kinase C-delta bound directly to immobilized GAP-43. Binding was abolished in the presence of EGTA, suggesting Ca2+ dependence of the interaction. The free catalytic domain of protein kinase C-delta did not bind GAP-43, suggesting the existence of a binding site in the regulatory domain. Glutathione S-transferase-protein kinase C-delta regulatory domain fusion proteins were generated and tested for binding to GAP-43. The V0/C2-like amino-terminal domain was defined as the GAP-43-binding site. GAP-43 binding to this region is inhibited by EGTA and regulated at Ca2+ levels between 10(-7) and 10(-6) M. The interaction between protein kinase C-delta and GAP-43 was studied in intact cells by coexpression of the two proteins in human embryonic kidney cells followed by immunoprecipitation. Complex formation occurred only after treatment of the cells with the Ca2+ ionophore ionomycin, indicating that elevation of intracellular Ca2+ is required for interaction in vivo. It is concluded that protein kinase C-delta interacts with GAP-43 through the V0/C2-like domain, outside the catalytic site, and that this interaction is modulated by intracellular Ca2+.

Role of vimentin in early stages of neuritogenesis in cultured hippocampal neurons

Vimentin is expressed initially by nearly all neuronal precursors in vivo, and is replaced by neurofilaments shortly after the immature neurons become post-mitotic. Moreover, both vimentin and neurofilaments can be detected transiently within the same neurite, leaving open the possibility that vimentin may play a role in the early stages of neuritogenesis. In the present study, cultured hippocampal neurons, which transiently express vimentin in culture, were treated with sense- and antisense-oriented deoxyoligonucleotides encoding regions of the vimentin sequence that overlap the translation initiation codon. Antisense oligonucleotide treatment reduced vimentin-immunoreactivity to background levels. Moreover, while 90-100% of cultured hippocampal neurons elaborated neurites within the first 24 hr following plating, only 24-30% did so in the presence of vimentin antisense oligonucleotides. Inhibition of neurite outgrowth was reversible following removal of antisense oligonucleotide. These findings substantiate earlier studies in neuroblastoma cells, indicating a possible role for vimentin in the initiation of neurite outgrowth.