Balance of two secretion pathways of nerve growth factor in PC12 cells changes during the progression of their differentiation, with a decrease in constitutive secretion in more differentiated cells

The expression system affects the binding affinity between p75NTR and proNGF

ProNGF (nerve growth factor) is a precursor of NGF and a signaling peptide exerting opposite effects on neuronal cells, i.e., apoptotic or neuritogenic. The conflicting biological activity of proNGF depends on the relative levels of two membrane receptors, TrkA and p75NTR. The effect of proNGF depends on the expression levels of these receptor proteins and their affinity to proNGF. Since the affinity of proteins has been studied with various recombinant proteins, it is worth comparing the affinity of these proteins within one experiment with the same method. This study examined the affinity between a recombinant proNGF and p75NTR expressed in common systems: bacterial, insect, and mammalian cells. The extracellular domain of p75NTR expressed in the insect or mammalian systems bound to native mature NGF, with a higher affinity for the insect receptor. The uncleavable proNGF was expressed in the three systems and they showed neuritogenic activity in PC12 cells. These recombinant proteins were used to compare their binding affinity to p75NTR. The insect p75NTR showed a higher binding affinity to proNGF than the mammalian p75NTR. The insect p75NTR bound proNGF from the insect system with the highest affinity, then from the mammalian system, and the lowest from the bacterial system. Conversely, the mammalian p75NTR showed no such preference for proNGF. Because the recombinant proNGF and p75NTR from different expression systems are supposed to have the same amino acid sequences, these differences in the affinity depend likely on their post-translational modifications, most probably on their glycans. Each recombinant proNGF and p75NTR in various expression systems exhibited different mobilities on SDS-PAGE and reactivities with glycosidases and lectins.

Intracellular interaction of newly synthesized nerve growth factor and its receptors

In autocrine cells, both a ligand and its receptors are synthesized in the same cell, but their intracellular interaction is not well known. We examined it using PC84 cells, a mutant PC12 cell line expressing nerve growth factor (NGF). We have already reported that the intracellular precursor of TrkA was phosphorylated and that MAP kinase was phosphorylated in PC84 cells. In this paper we found that the NGF receptors, TrkA and p75NTR, existed mainly as precursors, and most p75NTR localized inside PC84 cells. The phosphorylation of MAP kinase was also observed even when PC84 cells were incubated with anti-NGF antibody to block the extracellular interaction. These results suggest the possibility that newly synthesized NGF could interact intracellularly with the receptors in PC84 cells.

Molecular basis for peroxisomal localization of tetrameric carbonyl reductase

Pig heart peroxisomal carbonyl reductase (PerCR) belongs to the short-chain dehydrogenase/reductase family, and its sequence comprises a C-terminal SRL tripeptide, which is a variant of the type 1 peroxisomal targeting signal (PTS1) Ser-Lys-Leu. PerCR is imported into peroxisomes of HeLa cells when the cells are transfected with vectors expressing the enzyme. However, PerCR does not show specific targeting when introduced into the cells with a protein transfection reagent. To understand the structural basis for peroxisomal localization of PerCR, we determined the crystal structure of PerCR. Our data revealed that the C-terminal PTS1 of each subunit of PerCR was involved in intersubunit interactions and was buried in the interior of the tetrameric molecule. These findings indicate that the PTS1 receptor Pex5p in the cytosol recognizes the monomeric form of PerCR whose C-terminal PTS1 is exposed, and that this PerCR is targeted into the peroxisome, thereby forming a tetramer.

Pro-region of neurotrophins determines the processing efficiency

Neurotrophins are synthesized as precursors called pro-neurotrophins and then mature neurotrophins are formed proteolytically from them. Recent findings revealed that pro- and mature neurotrophins elicit opposite functional effects on cell survival, highlighting the importance of this processing step. Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) belong to the neurotrophin family and are mutually homologous, but BDNF is less efficiently processed. In order to find the reason for this, we examined some possibilities by using PC12 cells, and found that the pro-region, especially the last half of it, affected very much the processing efficiency of these neurotrophins.

Processing of nerve growth factor: the role of basic amino acid clusters in the pro-region

Neurotrophins are synthesized first as precursors called pro-neurotrophins, and their propeptides are then proteolytically removed to form mature neurotrophins. However, a significant proportion of total neurotrophins has been shown to be secreted as pro-neurotrophins. Furthermore, pro- and mature neurotrophins have been shown to elicit opposite effects on cell survival. Thus, the processing step of neurotrophins is very important. In order to understand the mechanism of neurotrophin processing, we focused on the two basic amino acid clusters in the pro-region of nerve growth factor (NGF). Various NGFs mutated at basic amino acids in the pro-region were introduced in COS7 and PC12 cells. The results indicated that these basic amino acid clusters were actually cleaved in the cells by furin, but that their cleavage contributed little to the production of mature NGF. However, one of the two sites was considered to contribute to mature NGF production depending on conditions used.

Identification of AMP N1-oxide in royal jelly as a component neurotrophic toward cultured rat pheochromocytoma PC12 cells

An extract of royal jelly (RJ) induced processes from cultured rat pheochromocytoma PC12 cells. Active components were isolated, and identified as adenosine monophosphate (AMP) and AMP N1-oxide. AMP N1-oxide was more than 20 times as active as AMP, judging from the minimal concentration to elicit activity. AMP N1-oxide was thought to be responsible for about half of the process-forming activity of whole RJ. Chemically-synthesized AMP N1-oxide was active similarly to the molecule purified from RJ, confirming AMP N1-oxide as the active entity. AMP N1-oxide also suppressed proliferation of PC12 cells and stimulated expression of neurofilament M, a specific protein of mature neurons, demonstrating the stimulatory activity of AMP N1-oxide to induce neuronal differentiation of PC12 cells. Pharmacological experiments suggested that AMP N1-oxide actions are mediated by adenyl cyclase-coupled adenosine receptors, including A2A. Thus AMP N1-oxide is a key molecule that characterizes RJ, and is not found in natural products other than RJ.

Differential vesicular targeting and time course of synaptic secretion of the mammalian neurotrophins

Neurotrophins are a family of secreted neuronal survival and plasticity factors comprising NGF, BDNF, neurotrophin-3 (NT-3), and NT-4. Whereas synaptic secretion of BDNF has been described, the routes of intracellular targeting and secretion of NGF, NT-3, and NT-4 in neurons are poorly understood. To allow for a direct comparison of intracellular targeting and release properties, all four mammalian neurotrophins were expressed as green fluorescent protein fusion proteins in cultured rat hippocampal neurons. We show that BDNF and NT-3 are targeted more efficiently to dendritic secretory granules of the regulated pathway of secretion (BDNF, in 98% of cells; NT-3, 85%) than NGF (46%) and NT-4 (23%). For all NTs, the remaining cells showed targeting to the constitutive secretory pathway. Fusing the BDNF pre-pro sequence to NT-4 directed NT-4 more efficiently to the regulated pathway of secretion. All neurotrophins, once directed to the regulated secretion pathway, were detected near synapsin I-positive presynaptic terminals and colocalized with PSD-95-DsRed (postsynaptic density-95-Discosoma red), suggesting postsynaptic targeting of the neurotrophins to glutamatergic synapses. Depolarization-induced release of all neurotrophins from synaptic secretory granules was slow (delay in onset, 10-30 s; tau = 120-307 s) compared with transmitter release kinetics monitored with FM4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-diethylamino)phenyl)hexatrienyl)pyridinium dibromide] destaining (onset, <5 s; tau = 13 +/- 2 s). Among the neurotrophins, NT-4 secretion was most rapid but still proceeded 10 times more slowly than transmitter secretion. Preincubation of neurons with monensin (neutralizing intragranular pH, thus solubilizing the peptide core) increased the speed of secretion of BDNF, NGF, and NT-3 to the value of NT-4. These data suggest that peptide core dissolution in secretory granules is the critical determinant of the speed of synaptic secretion of all mammalian NTs and that the speed of release is not compatible with fast transmitter-like actions of neurotrophins.

Nerve growth factor expression by PLG-mediated lipofection

Biomaterials capable of efficient gene delivery provide a fundamental tool for basic and applied research models, such as promoting neural regeneration. We developed a system for the encapsulation and sustained release of plasmid DNA complexed with a cationic lipid and investigated their efficacy using in vitro models of neurite outgrowth. Sustained lipoplex release was obtained for up to 50 days, with rates controlled by the fabrication conditions. Released lipoplexes retained their activity, transfecting 48.2+/-8.3% of NIH3T3 cells with luciferase activity of 3.97x10(7)RLU/mg. Expression of nerve growth factor (NGF) was employed in two models of neurite outgrowth: PC12 and primary dorsal root ganglia (DRG) co-culture. Polymer-mediated lipofection of PC12 produced bioactive NGF, eliciting robust neurite outgrowth. An EGFP/NGF dual-expression vector identified transfected cells (GFP-positive) while neurite outgrowth verified NGF secretion. A co-culture model examined the ability of NGF secretion by an accessory cell population to stimulate DRG neurite outgrowth. Polymer-mediated transfection of HEK293T with an NGF-encoding plasmid induced outgrowth by DRG neurons. This system could be fabricated as implants or nerve guidance conduits to support cellular and tissue regeneration. Combining this physical support with the ability to locally express neurotrophic factors will potentiate regeneration in nerve injury and disease models.

Establishment and characterization of pheochromocytoma tumor models expressing different levels of trkA receptors

To date experimental in vivo pheochromocytoma (PC) models have not been available. A major in vitro PC model consists of PC12 cells that respond to nerve growth factor (NGF) by differentiation, mediated by the trkA receptor. We report the establishment of PC12 tumor models expressing low and high levels of trkA receptor in CD1 nude mice. The tumors are characterized by their responsiveness to NGF, karyotype, presence of enolase, and chromaffin granules, as well as dopamine release. These novel PC models facilitate research on the role of the trkA receptor in cancer and the development of trkA-selective anti-cancer agents.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

Role of low-affinity p75 receptor in nerve growth factor-inducible growth arrest of PC12 cells

Mutant PC12 cell clones (PC84 cells) were obtained by transfection with nerve growth factor (NGF) cDNA. These cells secreted active NGF, extended short processes, and proliferated faster than the parental PC12 cells. These features are of great interest because the parental PC12 cells cease proliferation and extend long processes when transfected with NGF cDNA. PC84 cells expressed a high level of acetylcholinesterase activity and neurofilament M, which indicates that PC84 cells were differentiated. The inhibition of TrkA by K252a diminished the short processes of PC84 cells but had no effect on their fast proliferation. The expression level of TrkA in PC84 cells was comparable to that in PC12 cells; whereas that of another NGF receptor, p75, was significantly lower. These data suggest that the decrease of p75 contributed to the continuous growth of PC84 cells, which was confirmed by suppressing p75 activity of PC12 cells with the antisense oligonucleotide of p75 or with anti-p75 neutralizing antibody. The treated cells did not cease proliferation in the presence of NGF and extended short processes. Our results suggest that NGF signaling via TrkA affects the differentiation characteristics of PC12 cells but that an additional signaling via p75 is necessary for the growth arrest of the cells.

Regional differences in neurotrophin availability regulate selective expression of VGF in the developing limbic cortex

Gene and protein expression patterns in the cerebral cortex are complex and often change spatially and temporally through development. The signals that regulate these patterns are primarily unknown. In the present study, we focus on the regulation of VGF expression, which is limited to limbic cortical areas early in development but later expands into sensory and motor areas. We isolated neurons from embryonic day 17 rat cortex and demonstrate that the profile of VGF expression in perirhinal (expressing) and occipital (nonexpressing) populations in vitro is similar to that in the perinatal cortex in vivo. The addition of neutralizing neurotrophin antibodies indicates that endogenous brain-derived neurotrophic factor (BDNF) is necessary for the normal complement of VGF-expressing neurons in the perirhinal cortex, although endogenous neurotrophin-3 (NT-3) regulates the expression of VGF in a subpopulation of cells. ELISA analysis demonstrates that there is significantly more BDNF present in the perirhinal cortex compared with the occipital cortex in the perinatal period. However, the total amount of NT-3 is similar between the two regions and, moreover, there is considerably more NT-3 than BDNF in both areas, a finding seemingly in conflict with regional VGF expression. Quantification of the extracellular levels of neurotrophins in perirhinal and occipital cultures using ELISA in situ analysis indicates that perirhinal neurons release significantly more BDNF than the occipital population. Furthermore, the amount of NT-3 released by the perirhinal neurons is significantly less than the amount of BDNF. Local injection of BDNF in vivo into a normally negative VGF region results in robust ectopic expression of VGF. These data suggest that the local availability of specific neurotrophins for receptor occupation, rather than the total amount of neurotrophin, is a critical parameter in determining the selective expression of VGF in the developing limbic cortex.

Anterograde axonal transport, transcytosis, and recycling of neurotrophic factors: the concept of trophic currencies in neural networks

Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded upon arrival in cell bodies. Increasing evidence indicates that several trophic factors such as brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF-2), glial cell-line derived neurotrophic factor (GDNF), insulin-like growth factor (IGF-I), and neurotrophin-3 (NT-3), can move anterogradely along axons. They can escape the degradative pathway upon internalization and are recycled for future uses. Internalized ligands can move through intermediary cells by transcytosis, presumably by endocytosis via endosomes to the Golgi system, by trafficking of the factor to dendrites or by sorting into anterograde axonal transport with subsequent release from axon terminals and uptake by second- or third-order target neurons. Such data suggest the existence of multiple "trophic currencies," which may be used over several steps in neural networks to enable nurturing relationships between connected neurons or glial cells, not unlike currency exchanges between trading partners in the world economy. Functions of multistep transfer of trophic material through neural networks may include regulation of neuronal survival, differentiation of phenotypes and dendritic morphology, synapse plasticity, as well as excitatory neurotransmission. The molecular mechanisms of sorting, trafficking, and release of trophic factors from distinct neuronal compartments are important for an understanding of neurotrophism, but they present challenging tasks owing to the low levels of the endogenous factors.