Regulation of specific neuronal and nonneuronal proteins during development and following injury in the rat central nervous system

Claulansine F promotes neuritogenesis in PC12 cells via the ERK signaling pathway

AIM

To study the effects of Claulansine F (Clau F), a carbazole alkaloid isolated from the stem of Clausena lansium (Lour) Skeels, on neuritogenesis of PC12 cells, and to elucidate the mechanism of action.

METHODS

Neuritogenesis of PC12 cells was quantified under an inverted microscope. Expression of the neurite outgrowth marker GAP-43 was detected using immunofluorescence. GAP-43 transcription was measured using RT-PCR. Cell viability was evaluated with MTT assay. The levels of phosphor-ERK1/2, phosphor-CREB, phosphor-AKT and acetylate-p53 in the cells were examined using Western blotting analyses.

RESULTS

Clau F (10-100 μmol/L) significantly increased the percentage of PC12 cells bearing neurites. Clau F markedly increased the expression of GAP-43 in the cells. The efficiency of Clau F (10 μmol/L) in increasing neuritogenesis and GAP-43 expression was comparable to that of nerve growth factor (50 ng/mL). In addition, Clau F completely blocked the proliferation of PC12 cells within 7 d of incubation, whereas it did not cause cell death in cultured rat cortical neurons. Treatment of PC12 cells with Clau F activated both ERK and AKT signaling pathways. Co-treatment of PC12 cells with the specific ERK inhibitor PD98059, but not the specific PI3K inhibitor LY294002, blocked Clau F-induced neuritogenesis and GAP-43 upregulation.

CONCLUSION

Clau F promotes neuritogenesis in PC12 cells specifically via activation of the ERK signaling pathway.

Vital role of protein kinase C-related kinase in the formation and stability of neurites during hypoxia

Exposure of pheochromocytoma cells to hypoxia (1% O(2)) favors differentiation at the expense of cell viability. Additional incubation with nerve growth factor (NGF) and guanosine, a purine nucleoside with neurotrophin characteristics, rescued cell viability and further enhanced the extension of neurites. In parallel, an increase in the activity of protein kinase C-related kinase (PRK1), which is known to be involved in regulation of the actin cytoskeleton, was observed in hypoxic cells. NGF and guanosine further enhanced PRK1 in normoxic and hypoxic cells. To study the role of PRK1 during cellular stress response and neurotrophin-mediated signaling, pheochromocytoma cells were transfected with small interfering RNA directed against PRK1. Loss of functional PRK1 initiated a significant loss of viability and inhibited neurite formation. SiRNA-mediated knockdown of PRK1 also completely stalled guanosine-mediated neuroprotective effects. Additionally, the F-actin-associated cytoskeleton and the expression of the plasticity protein growth associated protein-43 were disturbed upon PRK1 knockdown. A comparable dependency of neurite formation and growth associated protein-43 immunoreactivity on functional PRK1 expression was observed in cerebellar granule neurons. Based on these data, a putative role of PRK1 as a key-signaling element for the successive NGF- and purine nucleoside-mediated protection of hypoxic neuronal cells is hypothesized.

An 8.5-kb segment of the PMP22 promoter responds to loss of axon signals during Wallerian degeneration, but does not respond to specific axonal signals during nerve regeneration

Altered expression of the PMP22 gene causes Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP). We have examined the promoter activity of 8.5 kb upstream of the first coding exon of the rat peripheral myelin protein-22 (rPmp22) gene in transgenic mice. We found that the -8.5 kb rPmp22/chloramphenicol acetyl transferase (CAT)/beta-galactosidase (lacZ) construct directs reporter gene expression in a weakly developmental and tissue-specific pattern, consistent with the expression pattern of the endogenous Pmp22 gene. The -8.5 kb rPmp22/CAT/lacZ transgene responds to loss of axonal signals during Wallerian degeneration but unlike the endogenous Pmp22 gene, the transgene fails to respond to axonal signals during nerve regeneration after a sciatic nerve crush injury. In conclusion, the function of the -8.5 kb rPmp22/CAT/lacZ transgene suggests that there are separable regulatory elements in the rPmp22 gene that respond differently to axonal signals received by Schwann cells during nerve development, and during remyelination.

Up-regulation of fast-axonally transported proteins in retinal ganglion cells of adult rats with optic-peroneal nerve grafts

Metabolic labeling and quantitative 2D gel fluorography were used to assess changes in the synthesis and transport of five fast-axonally transported and developmentally regulated proteins (GAP-43, SNAP-25, and proteins of 18, 22, and 23/24 kDa) after grafting of a peroneal nerve segment onto a transected optic nerve in adult rats. After optic nerve transection alone, only GAP-43 was up-regulated significantly compared to normal adult controls. The other proteins showed little change or were down-regulated following axotomy. By 4 weeks following optic nerve transection and peroneal nerve grafting, however, GAP-43, proteins 22 and 23/24 kDa showed a sustained up-regulation in synthesis and transport compared to normal controls; SNAP-25 and protein 18 kDa showed levels of expression similar to or slightly greater than normal controls. Increased expression of GAP-43 in retinal ganglion cells was also examined with immunocytochemistry. While a transient up-regulation of GAP-43 in retinal ganglion cells was observed following optic nerve transection, a sustained increase in GAP-43 immunoreactivity was present only in animals with nerve grafts. Backfilling of retinal ganglion cells from the grafts with horseradish peroxidase combined with GAP-43 immunocytochemistry revealed that all retinal ganglion cells with axons growing into the grafts were positive for GAP-43, but not all retinal ganglion cells showing GAP-43 immunoreactivity were extending axons into the grafts. We conclude that the presence of a nerve graft sustains the up-regulation of a number of proteins including GAP-43, and that this up-regulation is correlated with an increased potential for nerve growth, but other as yet unknown factors or conditions appear to play a role in determining if this growth potential will be realized.

Secretion and processing of apolipoprotein A-I in the avian sciatic nerve during development

Apolipoprotein A-I (apo A-I), a major apolipoprotein synthesized by liver and intestine to facilitate transport of plasma lipids as lipoproteins, has been detected also in the avian sciatic nerve. The mRNA and protein levels of apo A-I have been shown to increase during the period of rapid myelination (LeBlanc et al.: J Cell Biol 109:1245-1256, 1989). In order to assess the synthesis of apo A-I protein and the processing of apo A-I isoforms during development, endoneurial slices of avian sciatic nerves from chicks during active myelination at 15 and 17 days embryonic and 1 day posthatch age were incubated with [35]S-methionine. The incubations were fractionated into secreted and intracellular fractions, and incorporation of the label was assessed for apo A-I protein. The pattern of labeling of Po protein, as a marker of myelination, was also determined in the intracellular and compact myelin fractions. Methionine incorporation into Po protein was highest in the intracellular compartment at the 15-day embryonic stage and decreased thereafter, with a corresponding increase in the myelin fraction. During these developmental periods, the levels of nascent apo A-I increased in both the secreted and intracellular fractions. The synthesis of apo A-I specifically increases in the secreted fraction compared with total protein synthesis. The processing of the pro-apo A-I is also developmentally regulated. In the intracellular compartment, there are approximately equal proportions of the acidic and basic isoforms. However, with increasing age, a higher proportion of the apo A-I is secreted as acidic isoforms. It is concluded that the secretion and processing of apo A-I is developmentally regulated in the chick sciatic nerve, in parallel with the process of active myelination.

Immunosuppressant FK506 prevents mossy fiber sprouting induced by kindling stimulation

Kindling stimulation induces expansive growth of the axons of the dentate granule cells, the mossy fiber, into several areas of the hippocampus. An intraperitoneal injection of the immunosuppressant drug FK506, which is a specific inhibitor of Ca(2+)-calmodulin dependent phosphatase, calcineurin, prevented the full development of kindling as well as mossy fiber sprouting. The results show a correlation between mossy fiber sprouting and the development of kindling. The results suggest also that calcineurin may have a promoting role in mossy fiber sprouting and subsequent synaptogenesis.

Post-translational modifications of apolipoprotein A-I and Po proteins in the avian peripheral nerve

Apolipoprotein A-I (apo A-I), a soluble lipid transporter, and Po, the major glycoprotein of myelin, are actively synthesized during myelination. To explore the status of post-translational modifications of these proteins in the avian PNS during rapid myelination, endoneurial slices from one day old chick sciatic nerves were incubated with various radioactive precursors that could serve as indicators of such processes. The proteins were isolated from the incubation medium (secreted fraction), the 1% Triton-X-100-soluble intracellular-endoneurial (intracellular) fraction, and myelin-related and purified compact myelin fractions by immunoprecipitation with monospecific anti-apo A-I and or anti-Po antisera. Our results demonstrated that secreted apo A-I is fatty acylated, but not phosphorylated or sulfated. Avian Po protein was phosphorylated by a phorbol ester sensitive protein kinase. Sulfation, as well as fatty acylation, of avian Po protein was observed in organ culture using highly sensitive methods of detection. These results indicate that fatty acylation of secreted apo A-I and phosphorylation, sulfation and fatty acylation of Po have been conserved during evolution, and that these post-translational modifications may play a common function in various species.

Immunosuppressant FK506 promotes neurite outgrowth in cultures of PC12 cells and sensory ganglia

The immunosuppressant drug FK506 acts by binding to receptor proteins, FK506-binding proteins (FKBPs), which in turn can bind to and regulate a Ca(2+)-dependent phosphatase, calcineurin, and a Ca2+ release channel, the ryanodine receptor. Based on our findings in regeneration models that levels of FKBPs during neural regeneration parallel those of growth-associated protein GAP43, a calcineurin substrate that regulates neurite extension, we examined effects of FK506 in PC12 rat pheochromocytoma cells and in rat sensory ganglia. FK506 enhances neurite outgrowth in both systems by increasing sensitivity to nerve growth factor. Blockade of FK506 actions in sensory ganglia by rapamycin, an FK506 antagonist, establishes that these effects involve FKBPs. Rapamycin itself stimulates neurite outgrowth in PC12 cells. These drug effects are detected at subnanomolar concentrations, suggesting therapeutic application in diseases involving neural degeneration.

GAP 43-like immunoreactivity in normal adult rat sciatic nerve, spinal cord, and motoneurons: axonal transport and effect of spinal cord transection

Using immunofluorescence and cytofluorimetric scanning techniques in the rat, the fast anterograde and retrograde axonal transport of growth-associated protein-43-like immunoreactivity in normal sciatic nerves, and after spinal cord transection in the lower thoracic region, were investigated. Spinal roots and motor endplates in the peroneal muscles were also studied. For comparison, anti-synaptophysin (p38) was used. In intact adult animals, the amounts of immunoreactive growth-associated protein-43 increased linearly, both proximally and distally to the crush site, between 1 and 24 h after crushing the sciatic nerve. The accumulations were present in thick as well as in thin axons. Distal accumulations in the sciatic nerve were about 40-60% of the proximal amounts, indicating a recycling of organelles with growth-associated protein-43-like immunoreactivity. During the week after spinal cord transection, no clear changes were observed; the anterograde transport of growth-associated protein-43-like immunoreactivity showed a tendency to decrease at day 1 and then a tendency to increase, reaching 120% of control at seven days (not significant). Transported p38-like immunoreactivity showed similar but smaller changes. In the lumbar spinal cord gray matter many nerve terminals with growth-associated protein-43-like immunoreactivity were seen in intact animals. After spinal transection, these terminals gradually decreased, suggesting that they belonged to descending pathways. However, p38-positive terminals were not obviously decreased. After crushing ventral and dorsal roots, accumulations of pf growth-associated protein-43-like immunoreactivity were present in thick axons in the ventral roots and in thin to medium-sized axons in the dorsal roots. In peroneal muscles, growth-associated protein-43-like immunoreactivity was present in some (but not all) motor endplates in all groups. These results indicate that: (i) growth-associated protein-43 is normally present in nerve terminals of many descending projections of the spinal cord; (ii) growth-associated protein-43-like immunoreactivity is expressed and bidirectionally transported in neurons (motor as well as sensory) of normal sciatic nerves; (iii) growth-associated protein-43-like immunoreactivity is present in some adult motor endplates; and (iv) inhibited supraspinal input causes minor, if any, alterations--paralleled by p38--in axonal transport of growth-associated protein-43-like immunoreactivity.

Distribution of GAP-43 in relation to CGRP and synaptic vesicle markers in rat skeletal muscles during development

GAP 43 in nerve terminal structures of rat skeletal muscles, was investigated during postnatal development using immunofluorescence and confocal laser scanning microscopy. Comparison with synaptophysin, synapsin, SV2, CGRP, SP and NF was done in double immunoincubation studies. GAP 43-like immunoreactivity (LI) was demonstrated in preterminal axons and motor endplates in all age groups (from E18 to adult), although the intensity of immunofluorescence was considerably higher in the younger rats. The outgrowing nerve sprouts in E18 muscles were strongly GAP 43-positive. The intensity decreased with increasing age, but even in adult animals GAP 43-LI was present in some p38- or SV2-positive endplates. GAP 43-LI was also present in muscle spindles and preterminal nerve branches, and likewise decreased with age. Perivascular nerve terminals (around arteries mainly) were, however, strong in GAP 43-LI during both development and adulthood. GAP 43-LI was strong, and present in both small and large granules. SP-LI was observed in a few thin, presumably sensory, axons around vessels, which also contained a few GAP 43-positive large granules. Most of the strongly GAP 43-positive terminals around vessels were probably autonomic postganglionic terminals. The results suggest that GAP 43, in addition to development and regeneration, may play a significant role also in normal adult rats, especially in perivascular nerve terminals, possibly connected with a high potential for plasticity in this kind of nerve terminals.

Beta APP gene expression is increased in the rat brain after motor neuron axotomy

The response of the beta APP gene to neuronal injury was studied in the facial and hypoglossal nerve nuclei of the rat after corresponding nerve axotomy. Increased levels of beta APP 695, 714, 751 and 770 mRNAs were observed after either facial or hypoglossal nerve axotomy in the parent ipsilateral motor neurons. The increase was gradual, with maximal values 7 days after axotomy. beta APP mRNA expression returned to normal values 60 days after the lesion. Increased beta APP immunostaining was also detected in ipsilateral chromatolytic motor neurons. No change in beta APP immunoreactivity was observed in oligodendrocytes, another cell type expressing beta APP under normal conditions. A rapid increase in the expression of the GFAP gene was observed in reactive astrocytes surrounding chromatolytic neurons in the ipsilateral facial or hypoglossal nuclei. Thus, in contrast with other models of neuronal injury, where only the Kunitz protease inhibitor-containing beta APP mRNA isoforms are increased, all beta APP mRNAs are increased in the axotomy model. Furthermore, although beta APP expression has been shown to be increased in reactive astrocytes following neuronal injury, in the present study the increase was essentially found in the motor neurons reacting to axotomy.

Differential expression of growth-associated protein (GAP-43) mRNA in rat primary sensory neurons after peripheral nerve lesion: a non-radioactive in situ hybridisation study

An alkaline phosphatase-labelled anti-sense oligodeoxynucleotide probe specific for growth-associated protein messenger RNA (GAP-43 mRNA) was used for non-radioactive in situ hybridisation histochemistry to follow relative changes in GAP-43 mRNA content in lumbar primary sensory neurons (L4-6) after unilateral ligation of the sciatic nerve. In normal dorsal root ganglia (DRG) 16% of neurons expressed GAP-43 mRNA, and these cells belonged to a sub-group of intermediate-sized (32-50 microns diameter) and large (> 50 microns) neurons. The hybridisation signal detected in these cells was weak to moderate. One day after nerve ligature a significant increase in the number of GAP-43 mRNA expressing neurons in the ipsilateral DRG was detected involving particularly the very small (12-20 microns) cells, and small cell population (20-32 microns), though the hybridisation signal was less pronounced in this latter cell group. A significant increase in the cellular content of GAP-43 mRNA was detected in both cell groups when compared to the normal DRG by 2 days after the lesion. At later times (4, 7, and 10 days postinjury) the intermediate-sized and large cell subpopulations also showed an increase in the number of GAP-43 mRNA positive neurons, followed by a significant rise in their content of GAP-43 mRNA. However, they did not reach the same intensity of hybridisation signal as seen in the small and very small neurons. All DRG neurons showed a maximum of GAP-43 mRNA expression by 10 days postsurgery. At longer times there was a slight decrease in the content of GAP-43 mRNA towards 14 days postinjury, but mRNA levels remained elevated up to 28 days after nerve ligature, the longest time point examined in this study. The different onset and levels of GAP-43 gene expression in the rat primary sensory neurons after lesion of their peripheral branch axons further characterize the different subclasses of these cells and may reflect their different involvement in the plastic changes following peripheral nerve injury.

Synthesis and transport of GAP-43 in entorhinal cortex neurons and perforant pathway during lesion-induced sprouting and reactive synaptogenesis

Metabolic labeling and quantitative 2D gel autoradiography were used to assess changes in the synthesis and transport of GAP-43 in entorhinal cortex (EC) neurons and perforant pathway during lesion-induced sprouting and reactive synaptogenesis. In normal adult rats, there is a high constitutive level of GAP-43 synthesis and transport in EC neurons projecting to the hippocampus. Following unilateral EC lesions, there is a 2-fold (100%) increase in the transport of newly synthesized GAP-43 to the contralateral or 'sprouting' hippocampus. The timing of this upregulation (between 6 and 15 days) suggests that changes in GAP-43 expression occur in response to the growth of presynaptic terminals during sprouting.

GAP-43 immunoreactivity is widespread in the autonomic neurons and sensory neurons of the rat

GAP-43 is a membrane-bound phosphoprotein generally associated with axon growth during development and regeneration. Using immunohistochemical and immunoblotting techniques this study shows that GAP-43 is expressed extensively in the unperturbed adult autonomic nervous system. Strong immunoreactivity was seen in the developing and mature enteric subdivision of the autonomic nervous system and in nerves of the iris and various blood vessels. The presence of GAP-43 immunoreactivity in varicose nerve fibres, and a comparison of the labelling pattern of GAP-43 with the nerve associated marker PGP 9.5 suggests that GAP-43 is present in most or all autonomic nerve fibres in these organs. Immunoblotting of gut samples on 10% polyacrylamide gels revealed a single band of approximately 45,000 mol. wt that co-migrated with pure central nervous system GAP-43. Surgical sympathectomy experiments resulting in almost complete elimination of sympathetic fibres did not markedly affect the pattern of GAP-43 immunoreactivity in the iris, indicating that GAP-43 is expressed not only in sympathetic nerves but also in parasympathetic and sensory fibres. These findings show that GAP-43 is expressed extensively in autonomic nerves of the adult rat, at levels comparable to those seen during development. High levels of GAP-43 are not therefore restricted to development and regeneration in this part of the nervous system.

Dibutyryl-cAMP induces SNAP-25 translocation into the neurites in PC12

SNAP-25 immunoreactivity was translocated into the endings of the processes induced in PC12 cells by dibutyryl-cAMP-treatment. Conversely, the protein was not present in the endings of the processes seen after NGF-treatment unless dibutyryl-cAMP was used simultaneously. This redistribution of SNAP-25 immunoreactivity appeared to be dependent upon new protein synthesis. Finally, dibutyryl-cAMP was capable of inducing SNAP-25 expression.

GAP-43 expression in developing cutaneous and muscle nerves in the rat hindlimb

The expression of the growth associated protein, GAP-43, in developing rat hindlimb peripheral nerves has been studied using immunocytochemistry. GAP-43, is first detected in lumbar spinal nerves at embryonic day (E)12 as the axons grow to the base of the hindlimb. It is expressed along the whole length of the nerves as well as in the growth cones. GAP-43 staining becomes very intense over the next 36 h while the axons remain in the plexus region at the base of the limb bud before forming peripheral nerves at E14. It remains intense along the length of the growing peripheral nerves, the first of which are cutaneous, branching away from the plexus and growing specifically to the skin, their axon tips penetrating the epidermis of the proximal skin at E15 and the toes at E19. GAP-43-containing terminals form a dense plexus throughout the epidermis which subsequently withdraws subepidermally in the postnatal period. GAP-43 staining is also evident along the growing muscle nerves during muscle innervation, which follows behind that of skin. Axons branch over the surface of proximal muscles at E15 but do not form terminals until E17. As target innervation proceeds, GAP-43 staining declines in the proximal part of the nerve but remains intense in the distal portions. Overall GAP-43 expression in the hindlimb decreases in the second postnatal week as axon growth and peripheral terminal formation decline.

The growth-associated protein GAP-43 appears in dorsal root ganglion cells and in the dorsal horn of the rat spinal cord following peripheral nerve injury

When adult dorsal root ganglion cells are dissociated and maintained in vitro, both the small dark and the large light neurons show increases in the growth-associated protein GAP-43, a membrane phosphoprotein associated with neuronal development and plasticity. Immunoreactivity for GAP-43 appears in the cytoplasm of the cell bodies as early as 3.5 h post axotomy and is present in neurites and growth cones as soon as they develop. At early stages of culture (4 h to eight days) satellite/Schwann cells are also immunoreactive for GAP-43. Neurons in isolated whole dorsal root ganglion maintained in vitro become GAP-43-immunoreactive between 2 and 3 h after axotomy. It takes three days however, after cutting or crushing the sciatic nerve in adult rats in vivo, for GAP-43 immunoreactivity to appear in the axotomized dorsal root ganglion cells. GAP-43 immunoreactivity can be detected in the central terminals of primary afferent neurons in the superficial laminae of the dorsal horn of the lumbar enlargement four days after sciatic cut or crush. The intensity of the GAP-43 staining reaches a peak at 21 days and becomes undetectable nine weeks following crush injury and 36 weeks following sciatic nerve cut. The pattern of GAP-43 staining is identical to the distribution of sciatic small-calibre afferent terminals. Little or no staining is present in the deep dorsal horn, but GAP-43 does appear in the ipsilateral gracile nucleus 22 days after sciatic injury. In investigating the mechanism of GAP-43 regulation, blockade of axon transport in the sciatic nerve with vinblastine (10(-5) M-10(-4) M) or capsaicin (1.5%) was found to produce a pattern of GAP-43 immunoreactivity in the dorsal horn identical to that found with crush, while electrical stimulation of the sciatic nerve had no effect. Axotomy of primary sensory neurons or the interruption of axon transport in the periphery therefore acts to trigger GAP-43 production in the cell body. The GAP-43 is transported to both the peripheral and the central terminals of the afferents. In the CNS the elevated GAP-43 levels may contribute to an inappropriate synaptic reorganization of afferent terminals that could play a role in the sensory disorders that follow nerve injury.

The apolipoprotein A-I gene is actively expressed in the rapidly myelinating avian peripheral nerve

The expression of the apolipoprotein A-I (apo A-I) gene was investigated in the myelinating sciatic nerve. Hybridization analysis with an apo A-I cDNA probe obtained from a cDNA library of mRNA isolated from rapidly myelinating chick sciatic nerve indicated that apo A-I coding transcripts increase during development in the chick sciatic nerve in parallel with the increase of myelin lamellae. Substantial apo A-I-like immunoreactivity in chick sciatic nerve homogenates was detected by Western blotting. The amount of antigen increased from the 15-d embryonic stage to 1 d posthatch and then decreased. Two subcellular fractions corresponding to the cytoplasmic compartments were particularly enriched in apo A-I. apo A-I immunoreactivity was also found in highly purified myelin preparations. Immunohistochemical staining provided further evidence for the presence of apo A-I in the endoneurial compartment of the sciatic nerve. Electron microscopic examination of these fractions after negative staining showed the presence of spherical and disc-shaped particles resembling high density lipoproteins. The presence of apo A-I, cholesterol esters, phospholipids, and triacylglycerols in ultracentrifugal fractions corresponding to serum lipoproteins and the behavior of apo A-I on nondenaturing gradient gels implied that apo A-I was associated with lipid. Studies with short-term organ cultures of sciatic nerves from 1-d chicks strengthened the evidence for local synthesis and secretion of apo A-I and apo A-I-containing lipoproteins by this tissue. These results establish that the apo A-I gene is actively expressed in developing sciatic nerve during the period of rapid myelination. These findings support the hypothesis that apo A-I synthesized within the nerve participates in the local transport of lipids used in myelin biosynthesis.

Nerve growth factor-induced changes in the intracellular localization of the protein kinase C substrate B-50 in pheochromocytoma PC12 cells

High levels of the neuron-specific protein kinase C substrate, B-50 (= GAP43), are present in neurites and growth cones during neuronal development and regeneration. This suggests a hitherto nonelucidated role of this protein in neurite outgrowth. Comparable high levels of B-50 arise in the pheochromocytoma PC12 cell line during neurite formation. To get insight in the putative growth-associated function of B-50, we compared its ultrastructural localization in naive PC12 cells with its distribution in nerve growth factor (NGF)- or dibutyryl cyclic AMP (dbcAMP)-treated PC12 cells. B-50 immunogold labeling of cryosections of untreated PC12 cells is mainly associated with lysosomal structures, including multivesicular bodies, secondary lysosomes, and Golgi apparatus. The plasma membrane is virtually devoid of label. However, after 48-h NGF treatment of the cells, B-50 immunoreactivity is most pronounced on the plasma membrane. Highest B-50 immunoreactivity is observed on plasma membranes surrounding sprouting microvilli, lamellipodia, and filopodia. Outgrowing neurites are scattered with B-50 labeling, which is partially associated with chromaffin granules. In NGF-differentiated PC12 cells, B-50 immunoreactivity is, as in untreated cells, also associated with organelles of the lysosomal family and Golgi stacks. B-50 distribution in dbcAMP-differentiated cells closely resembles that in NGF-treated cells. The altered distribution of B-50 immunoreactivity induced by differentiating agents indicates a shift of the B-50 protein towards the plasma membrane. This translocation accompanies the acquisition of neuronal features of PC12 cells and points to a neurite growth-associated role for B-50, performed at the plasma membrane at the site of protrusion.

Role of the growth cone in neuronal differentiation

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

Light-microscopic immunolocalization of the growth- and plasticity-associated protein GAP-43 in the developing rat brain

Growth-associated protein-43 (GAP-43) is a developmentally regulated, fast-axonally transported phosphoprotein whose synthesis and transport are enhanced during periods of growth and synaptic terminal formation. GAP-43 is a substrate of protein kinase C and is identical to protein F1, a phosphoprotein which is regulated during long-term potentiation in the hippocampus. In order to characterize the cellular localization of GAP-43, we have raised a specific antiserum against it, and used this as a probe to show that GAP-43 is neuron-specific, and is localized to growing neuronal processes in developing rat brain, and to presynaptic terminals in both the peripheral and central nervous system. In the mature CNS, GAP-43 immunoreactivity is present in most neuropil areas, but is especially dense in the molecular layers of the cerebellum, neocortex, and the hippocampus, structures known to exhibit synaptic plasticity. Its localization, together with biochemical data concerning the dynamics of its synthesis and its identity as protein F1, suggest that GAP-43 may be involved in axon growth in the developing nervous system, and in some aspect of synaptic plasticity in the mature CNS. These data also suggest that axon growth and synaptic plasticity in the brain may be regulated by a common mechanism, both involving the protein kinase C-mediated phosphorylation of GAP-43.