Mature pig oligodendrocytes rapidly process human recombinant pro-nerve growth factor and do not undergo cell death

Deciphering proneurotrophin actions

Like most growth factors, neurotrophins are initially synthesized as precursors that are cleaved to release C-terminal mature forms. The well-characterized mature neurotrophins bind to Trk receptors to initiate survival and differentiative responses. More recently, the precursor forms or proneurotrophins have been found to act as distinct ligands by binding to an unrelated receptor complex consisting of the p75 neurotrophin receptor (p75) and sortilin to initiate cell death. Induction of proNGF and p75 has been observed in preclinical injury models and in pathological states in the central nervous system, and strategies that block the proNGF/p75 interaction are effective in limiting neuronal apoptosis. In contrast, the mechanisms that regulate expression of other proneurotrophins, including proBDNF and proNT-3, are less well understood. Here, recent findings on the biological actions, regulation of expression, and pathophysiological effects of proneurotrophins will be reviewed.

Oligodendroglial process formation is differentially affected by modulating the intra- and extracellular cholesterol content

Cholesterol is an essential component of eukaryotic plasma membranes and plays an important role in membrane organization and signaling processes. It is the major lipid component of detergent resistant caveolin-1 containing rafts which previously had been reported as a platform for nerve growth factor (NGF) signaling in oligodendrocytes (OL). Surprisingly, a knockdown of caveolin-1 attenuated the process formation of OL (Schmitz et al. J Neurosci Res 88:572–588, 2010), for which a loss of cholesterol could be responsible. In the present report, we could show that a caveolin-1 knockdown resulted in an elevation of cellular cholesterol level; it may indicate an important role of caveolin-1 in cholesterol trafficking to the plasma membrane. Treatment with exogenous PEG cholesterol, which was incorporated to the plasma membrane, supported oligodendroglial process formation, in particular when OL were stimulated by NGF. In this context we have found that OL express NPC1L1 (Niemann–Pick disease type C1-Like 1) which could modulate cholesterol uptake. In contrast, depletion of membrane-bound cholesterol diminished NGF-induced process formation concomitant with a reduced activity of p42/44 mitogen-activated protein kinases.

Neurotrophic actions initiated by proNGF in adult sensory neurons may require peri-somatic glia to drive local cleavage to NGF

The nerve growth factor (NGF) precursor, proNGF, is implicated in various neuropathological states. ProNGF signals apoptosis by forming a complex with the receptors p75 and sortilin, however, it can also induce neurite growth, proposed to be mediated by the receptor of mature NGF, tyrosine kinase receptor A (TrkA). The way in which these dual effects occur in adult neurons is unclear. We investigated the neurotrophic effects of proNGF on peptidergic sensory neurons isolated from adult mouse dorsal root ganglia and found that proNGF stimulated neurite extension and branching, requiring p75, sortilin and TrkA. Neurite growth rarely occurred in sortilin-expressing neurons but was commonly observed in TrkA-positive, sortilin-negative neurons that associated closely with sortilin-positive glia. ProNGF was unable to induce local trophic effects at growth cones where sortilin-positive glia was absent. We propose that in adult sensory neurons the neurotrophic response to proNGF is mediated by NGF and TrkA, and that peri-somatic glia may participate in sortilin- and p-75 dependent cleavage of proNGF. The potential ability of local glial cells to provide a targeted supply of NGF may provide an important way to promote trophic (rather than apoptotic) outcomes under conditions where regeneration or sprouting is required.

Neuroglialpharmacology: myelination as a shared mechanism of action of psychotropic treatments

Current psychiatric diagnostic schema segregate symptom clusters into discrete entities, however, large proportions of patients suffer from comorbid conditions that fit neither diagnostic nor therapeutic schema. Similarly, psychotropic treatments ranging from lithium and antipsychotics to serotonin reuptake inhibitors (SSRIs) and acetylcholinesterase inhibitors have been shown to be efficacious in a wide spectrum of psychiatric disorders ranging from autism, schizophrenia (SZ), depression, and bipolar disorder (BD) to Alzheimer's disease (AD). This apparent lack of specificity suggests that psychiatric symptoms as well as treatments may share aspects of pathophysiology and mechanisms of action that defy current symptom-based diagnostic and neuron-based therapeutic schema. A myelin-centered model of human brain function can help integrate these incongruities and provide novel insights into disease etiologies and treatment mechanisms. Available data are integrated herein to suggest that widely used psychotropic treatments ranging from antipsychotics and antidepressants to lithium and electroconvulsive therapy share complex signaling pathways such as Akt and glycogen synthase kinase-3 (GSK3) that affect myelination, its plasticity, and repair. These signaling pathways respond to neurotransmitters, neurotrophins, hormones, and nutrition, underlie intricate neuroglial communications, and may substantially contribute to the mechanisms of action and wide spectra of efficacy of current therapeutics by promoting myelination. Imaging and genetic technologies make it possible to safely and non-invasively test these hypotheses directly in humans and can help guide clinical trial efforts designed to correct myelination abnormalities. Such efforts may provide insights into novel avenues for treatment and prevention of some of the most prevalent and devastating human diseases.

NGF and proNGF regulate functionally distinct mRNAs in PC12 cells: an early gene expression profiling

The biological activities of NGF and of its precursor proNGF are quite distinct, due to different receptor binding profiles, but little is known about how proNGF regulates gene expression. Whether proNGF is a purely pro-apoptotic molecule and/or simply a “less potent NGF” is still a matter of debate. We performed experiments to address this question, by verifying whether a proNGF specific transcriptional signature, distinct from that of NGF, could be identified. To this aim, we studied gene expression regulation by proNGF and NGF in PC12 cells incubated for 1 and 4 hours with recombinant NGF and proNGF, in its wild-type or in a furin-cleavage resistant form. mRNA expression profiles were analyzed by whole genome microarrays at early time points, in order to identify specific profiles of NGF and proNGF. Clear differences between the mRNA profiles modulated by the three neurotrophin forms were identified. NGF and proNGF modulate remarkably distinct mRNA expression patterns, with the gene expression profile regulated by NGF being significantly more complex than that by proNGF, both in terms of the total number of differentially expressed mRNAs and of the gene families involved. Moreover, while the total number of genes modulated by NGF increases dramatically with time, that by proNGFs is unchanged or reduced. We identified a subset of regulated genes that could be ascribed to a “pure proNGF” signalling, distinct from the “pure NGF” one. We also conclude that the composition of mixed NGF and proNGF samples, when the two proteins coexist, influences the profile of gene expression. Based on this comparison of the gene expression profiles regulated by NGF and its proNGF precursor, we conclude that the two proteins activate largely distinct transcriptional programs and that the ratio of NGF to proNGF in vivo can profoundly influence the pattern of regulated mRNAs.

Effect of cavtratin, a caveolin-1 scaffolding domain peptide, on oligodendroglial signaling cascades

Caveolin and caveolin containing rafts are involved in the signaling of growth factors in various cell types. Previous reports of our lab indicated a co-localization of caveolin and the high affinity nerve growth factor (NGF) receptor tyrosine kinase A (TrkA). Mutual effects have been observed among which a caveolin-1 knock-down resulted in an impairment of the NGF signaling cascade rather than in an increase of activity as expected from other growth factor reports. On the other hand, an over-expression of caveolin-1 impaired the NGF stimulated activity of p42/44 mitogen activated protein kinases (MAPK). In this study, we used a caveolin-1 scaffolding domain (CSD) peptide (cavtratin) of which an inhibitory effect on growth factor receptors was reported. Our data showed that cavtratin suppresses the NGF-induced phosphorylation of TrkA as well as the activation of MAPK in porcine oligodendrocytes significantly.

The pro-form of BMP-2 exhibits a delayed and reduced activity when compared to mature BMP-2

Pro-forms of growth factors receive increasing attention since it was shown that they can affect the function(s) of the mature proteins. Here, we extend our previous investigations on the biological function of the pro-form of bone morphogenetic protein-2 (BMP-2). We demonstrate that proBMP-2, upon prolonged incubation with C2C12 cells, induces alkaline phosphatase, a marker enzyme for osteoblastic differentiation. Expression studies with three different bone marker transcripts reveal that proBMP-2 induces bone-specific transcripts, however, to a smaller extent than the mature growth factor. To resolve this finding at the protein level, the fate of proBMP-2 and BMP-2 was studied in cell culture. We demonstrate that both proteins become internalized, and proBMP-2 is processed to mature BMP-2 within the cells. The data presented here suggest that proBMP-2 elicits biological functions as mature BMP-2 at a delayed and reduced level, which might depend on intracellular cleavage and subsequent secretion as mature BMP-2.

Protective effects of octacosanol on 6-hydroxydopamine-induced Parkinsonism in rats via regulation of ProNGF and NGF signaling

AIM

To investigate the protective effects of octacosanol in 6-hydroxydopamine-induced Parkinsonian rats and find whether octacosanol has effects on pro nerve growth factor (pro-NGF), NGF and the downstream effector proteins.

METHODS

Behavioral tests, enzymatic assay, tyrosine hydroxylase immunohistochemistry, TUNEL and Western blot were used to investigate the effects of octacosanol in this rat model of PD.

RESULTS

Oral administration of octacosanol (35-70 mg/kg, po for 14 d) significantly improved the behavioral impairments in rats induced by 6-OHDA and dose-dependently preserved the free radical scavenging capability of the striatum. Octacosanol treatment also effectively ameliorated morphological appearances of TH-positive neuronal cells in nigrostriatal systems and decreased the apoptotic cells induced by 6-OHDA in striatum. In addition, octacosanol strikingly blocked the 6-OHDA-induced increased expression of proNGF-p75NTR-sortilin death signaling complex and its downstream effector proteins. Meantime, octacosanol prevented the decreased levels of NGF, its receptors TrkA and p-Akt which together mediated the cell survival pathway.

CONCLUSION

The findings implicated that the anti-parkinsonism effects afforded by octacosanol might be mediated by its neuro-microenvironment improving potency through retrieving the ratios of proNGF:NGF and the respective receptors p75NTR:TrkA in vivo. Due to its excellent tolerability and non-toxicity, octacosanol may be a promising agent for PD treatment.

Understanding proneurotrophin actions: Recent advances and challenges

Neurotrophins are initially synthesized as larger precursors (proneurotrophins), which undergo proteolytic cleavage to yield mature forms. Although the functions of the mature neurotrophins have been well established during neural development and in the adult nervous system, roles for the proneurotrophins in developmental and injury-induced cell death, as well as in synaptic plasticity, have only recently been appreciated. Interestingly, both mature neurotrophins and proneurotrophins utilize dual-receptor complexes to mediate their actions. The mature neurotrophin coreceptors consist of the Trk receptor tyrosine kinases and p75(NTR), wherein Trk transduces survival and differentiative signaling, and p75(NTR) modulates the affinity and selectivity of Trk activation. On the other hand, proneurotrophins engage p75(NTR) and the structurally distinct coreceptor sortilin, to initiate p75(NTR)-dependent signal transduction cascade. Although the specificity of mature neurotrophins vs. proneurotrophins actions is due in part to the formation of distinct coreceptor complexes, a number of recent studies highlight how different p75(NTR)-mediated cellular actions are modulated. Here, we review emerging evidence for a novel transmembrane mechanism for ligand-specific p75(NTR) activation and several mechanisms by which p75(NTR)-dependent apoptotic and nonapoptotic responses can be selective activated.

Mutual effects of caveolin and nerve growth factor signaling in pig oligodendrocytes

Signaling of growth factors may depend on the recruitment of their receptors to specialized microdomains. Previous reports on PC12 cells indicated an interaction of raft-organized caveolin and TrkA signaling. Because porcine oligodendrocytes (OLs) respond to nerve growth factor (NGF), we were interested to know whether caveolin also plays a role in oligodendroglial NGF/TrkA signaling. OLs expressed caveolin at the plasma membrane but also intracellularly. This was partially organized in the classically Omega-shaped invaginations, which may represent caveolae. We could show that caveolin and TrkA colocalize by using a discontinuous sucrose gradient (Song et al. [1996] J. Biol. Chem. 271:9690-9697), MACS technology, and immunoprecipitation. However, differential extraction of caveolin and TrkA with Triton X-100 at 4 degrees C indicated that caveolin and TrkA are probably not exclusively present in detergent-resistant, caveolin-containing rafts (CCRs). NGF treatment of OLs up-regulated the expression of caveolin-1 (cav-1) and stimulated tyrosine-14 phosphorylation of cav-1. Furthermore, OLs were transfected with cav-1-specific small interfering RNA (siRNA). A knockdown of cav-1 resulted in a reduced activation of downstream components of the NGF signaling cascade, such as p21Ras and mitogen-activated protein kinase (MAPK) after NGF exposure of OLs. Subsequently, increased oligodendroglial process formation via NGF was impaired. The present study indicates that CCRs/caveolin could play a modulating role during oligodendroglial differentiation and regeneration.

Proneurotrophin-3 is a neuronal apoptotic ligand: evidence for retrograde-directed cell killing

Although mature neurotrophins are well described trophic factors that elicit retrograde survival signaling, the precursor forms of neurotrophins (i.e., proneurotrophins) can function as high-affinity apoptotic ligands for selected neural populations. An outstanding question is whether target-derived proneurotrophins might affect neuronal survival/death decisions through a retrograde transport mechanism. Since neurotrophin-3 (NT-3) is highly expressed in non-neural tissues that receive peripheral innervation, we investigated the localized actions of its precursor (proNT-3) on sympathetic neurons in the present study. Pharmacological inhibition of intracellular furin proteinase activity in 293T cells resulted in proNT-3 release instead of mature NT-3, whereas membrane depolarization in cerebellar granule neurons stimulated endogenous proNT-3 secretion, suggesting that proNT-3 is an inducible bona fide ligand in the nervous system. Our data also indicate that recombinant proNT-3 induced sympathetic neuron death that is p75(NTR)- and sortilin-dependent, with hallmark features of apoptosis including JNK (c-Jun N-terminal kinase) activation and nuclear fragmentation. Using compartmentalized culture systems that segregate neuronal cell bodies from axons, proNT-3, acting within the distal axon compartment, elicited sympathetic neuron death and overrode the survival-promoting actions of NGF. Together, these results raise the intriguing possibility that dysregulation of proneurotrophin processing/release by innervated targets can be deleterious to the neurons projecting to these sites.

Biological activity of nerve growth factor precursor is dependent upon relative levels of its receptors

Nerve growth factor (NGF) is produced as a precursor called pro-nerve growth factor (proNGF), which is secreted by many tissues and is the predominant form of NGF in the central nervous system. In Alzheimer disease brain, cholinergic neurons degenerate and can no longer transport NGF as efficiently, leading to an increase in untransported NGF in the target tissue. The protein that accumulates in the target tissue is proNGF, not the mature form. The role of this precursor is controversial, and both neurotrophic and apoptotic activities have been reported for recombinant proNGFs. Differences in the protein structures, protein expression systems, methods used for protein purification, and methods used for bioassay may affect the activity of these proteins. Here, we show that proNGF is neurotrophic regardless of mutations or tags, and no matter how it is purified or in which system it is expressed. However, although proNGF is neurotrophic under our assay conditions for primary sympathetic neurons and for pheochromocytoma (PC12) cells, it is apoptotic for unprimed PC12 cells when they are deprived of serum. The ratio of tropomyosin-related kinase A to p75 neurotrophin receptor is low in unprimed PC12 cells compared with primed PC12 cells and sympathetic neurons, altering the balance of proNGF-induced signaling to favor apoptosis. We conclude that the relative level of proNGF receptors determines whether this precursor exhibits neurotrophic or apoptotic activity.

Intrinsic structural disorder of mouse proNGF

The unprocessed precursor of the Nerve Growth Factor (NGF), proNGF, has additional functions, besides its initially described role as a chaperone for NGF folding. The precursor protein endows apoptotic and/or neurotrophic properties, in contrast to the mature part. The structural and molecular basis for such distinct activities are presently unknown. Aiming to gain insights into the specific molecular interactions that govern rm-proNGF biological activities versus those of its mature counterpart, a structural study by synchrotron small angle X-ray scattering (SAXS) in solution was carried out. The different binding properties of the two proteins were investigated by surface plasmon resonance (SPR) using, as structural probes, a panel of anti-NGF antibodies and the soluble forms of TrkA and p75(NTR) receptors. SAXS measurements revealed the rm-proNGF to be dimeric and anisometric, with the propeptide domain being intrinsically unstructured. Ab initio reconstructions assuming twofold symmetry generated two types of structural models, a globular "crab-like" and an elongated shape that resulted in equally good fits of the scattering data. A novel method accounting for possible coexistence of different conformations contributing to the experimental scattering pattern, with no symmetry constraints, suggests the "crab-like" to be a more likely proNGF conformation. To exploit the potential of chemical stabilizers affecting the existing conformational protein populations, SAXS data were also collected in the presence of ammonium sulphate. An increase of the proNGF compactness was observed. SPR data pinpoints that the propeptide of proNGF may act as an intrinsically unstructured protein domain, characterized by a molecular promiscuity in the interaction/binding to multiple partners (TrkA and p75(NTR) receptors and a panel of neutralizing anti-NGF antibodies) depending on the physiological conditions of the cell. These data provide a first insight into the structural basis for the selectivity of mouse short proNGF, versus NGF, towards its binding partners.

Commentary: Regulating proNGF action: multiple targets for therapeutic intervention

Neurotrophins are initially synthesized as precursor forms that are cleaved to release C-terminal mature forms that bind to Trk receptors to initiate survival and differentiative responses. Recent studies suggest that the precursor form of NGF (proNGF) acts as a distinct ligand by binding to a receptor complex of p75 and sortilin to initiate cell death. Induction of proNGF and p75 has been observed in multiple pathological states and injury models in the central nervous system, and blockade of proNGF/p75 interaction is efficacious in limiting neuronal apoptosis. Multiple strategies that may act to limit proNGF action are considered as potential therapeutic targets for future development.

ProNGF inhibits NGF-mediated TrkA activation in PC12 cells

Degeneration of cholinergic basal forebrain neurons (CBFN) is a hallmark in the pathology of Alzheimer's disease (AD). Critically depending upon the neurotrophic support through nerve growth factor (NGF), CBFN in the AD brain face elevated concentrations of the pro-form of NGF (proNGF) and suffer from an imbalance between TrkA and p75(NTR) expression. Research for the underlying mechanisms of CBFN death suggested a pro-apoptotic activity of proNGF. However, this finding could not be confirmed by all investigators and other studies even observed a neurotrophic function of proNGF. In the presence of these controversial findings we investigated the activity of proNGF in PC12 cells with specific emphasis on its neurotoxic versus neurotrophic action. In this study, we show that proNGF can mediate TrkA receptor signaling directly, yet in the manner of a partial agonist with a lower maximum activity than NGF. A pro-apoptotic activity of proNGF could not be confirmed in our cellular system. Interestingly and surprisingly, pre-incubation with proNGF at low, sub-active concentrations inhibited TrkA-mediated neurotrophic NGF signaling in PC12 cells. Our data support a novel hypothesis for the role of elevated proNGF levels in CBFN pathology in AD. Thus, proNGF can indirectly contribute to the slow neurodegeneration in AD by reducing NGF-mediated trophic support.

Neurotoxic and neurotrophic roles of proNGF and the receptor sortilin in the adult and ageing nervous system

The precursor form of the nerve growth factor (proNGF), forms a heterotrimeric complex with the receptors p75 and sortilin; this complex has been implicated in neuron cell death. However, it is not known whether proNGF and the receptors p75 and sortilin contribute to age- and disease-related neurodegeneration. Here we show that proNGF induces cell death in subpopulations of basal forebrain and peripheral sympathetic neurons of old, but not of young, adult rodents. In contrast, proNGF appears to induce neurite outgrowth rather than cell death of young adult sympathetic neurons. We have examined the neurotoxic role of proNGF in old age, and find that proNGF protein is elevated during ageing in the projection areas of some populations of vulnerable central and peripheral neurons; caloric restriction, which has known neuroprotective effects, partially prevents these increases. Sortilin was found to play a significant part in the observed patterns of age-related proNGF-mediated neurotoxicity. In particular, survival of aged neurons was rescued by neurotensin, an alternative sortilin ligand that blocks the sortilin-mediated effects of proNGF. Furthermore, sortilin immunoreactivity increases markedly in ageing rodent basal forebrain and sympathetic neurons; in contrast, p75 levels are either unchanged or reduced. From these data we propose that selective age-related neuronal atrophy and neurodegeneration may be mediated by increased sortilin expression in neurons, together with elevated levels of proNGF expression in some targets.

Proneurotrophins require endocytosis and intracellular proteolysis to induce TrkA activation

The uncleaved, pro-form of nerve growth factor (proNGF) functions as a pro-apoptotic ligand for the p75 neurotrophin receptor (p75NTR). However, some reports have indicated that proneurotrophins bind and activate Trk receptors. In this study, we have examined proneurotrophin receptor binding and activation properties in an attempt to reconcile these findings. We show that proNGF readily binds p75NTR expressed in HEK293T cells but does not interact with TrkA expressed under similar circumstances. Importantly, proNGF activates TrkA tyrosine phosphorylation, induces Erk and Akt activation, and causes PC12 cell differentiation. We show that inhibiting endocytosis or furin activity reduced TrkA activation induced by proNGF but not that induced by mature NGF and that proNGF123, a mutant form of NGF lacking dibasic cleavage sites in the prodomain, does not induce TrkA phosphorylation in PC12 cells. Therefore, endocytosis and cleavage appear to be prerequisites for proNGF-induced TrkA activity. We also found that proBDNF induces activation of TrkB in cerebellar granule neurons and that proBDNF cleavage by furin and metalloproteases facilitates this effect. Taken together, these data indicate that under physiological conditions, proneurotrophins do not directly bind or activate Trk receptors. However, endocytosis and cleavage of proneurotrophins produce processed forms of neurotrophins that are capable of inducing Trk activation.

Neurotrophin-induced upregulation of p75NTR via a protein kinase C-delta-dependent mechanism

Neurotrophins exert their biological effects via p75NTR and Trk receptors. Functional interplay between these two receptors has been widely explored with respect to p75NTR enhancing the activation and signalling of Trk, but few studies address the bidirectional aspects. We have previously demonstrated that the expression of p75NTR can be differentially modulated by different Trk receptor mutations. Here we investigate the mechanism of Nerve Growth Factor (NGF)-induced upregulation of p75NTR expression. We utilize pharmacological inhibition to investigate the role of various TrkA-associated signalling intermediates in this regulatory cascade. Notably, the inhibition of phospholipase C-gamma (PLC-gamma) using U73122, prevented the NGF-induced upregulation of p75NTR protein and mRNA. The inhibition of protein kinase C-delta (PKC-delta) activation by rottlerin, a selective PKC-delta inhibitor, and by small interfering RNA (siRNA) directed against PKC-delta also inhibited this NGF-induced upregulation. Finally, we also show that in cerebellar granule neurons, BDNF acting via TrkB increases p75NTR expression in a PKC-delta dependent manner. These results indicate the importance of Trk-dependent PLC-gamma and PKC-delta activation for downstream regulation of p75NTR protein expression in response to neurotrophin stimulation, a process that has implications to the survival and growth of the developing nervous system.

Neurotrophic activity of proNGF in vivo

Ectopic expression of nerve growth factor (NGF) in transgenic mice results in the directional growth of sympathetic and/or sensory fibers. For instance, mice that over-express NGF under the control of the glial fibrillary acidic protein (GFAP) promoter exhibit robust axonal sprouting into the cerebellum, with no apparent loss of neurons in peripheral ganglia. Given the disagreement in the literature over whether pro-NGF exerts neurotrophic or apoptotic effects, we assessed the relative levels of proNGF and mature NGF in the cerebella of these transgenic mice. Blinded western blot analyses revealed that proNGF was the major species in both transgenic and wild type mice, with very low levels of mature NGF expression. While transgenic mice displayed significantly higher levels of cerebellar proNGF protein as compared to wild type mice, both strains possessed comparable levels of mature NGF. These data reveal that the ectopic expression of NGF in the cerebellum results in an increase in proNGF rather than mature NGF levels. Together with the robust axonal growth and lack of neuronal death in the ganglia in these animals, our results are clearly consistent with proNGF exhibiting neurotrophic activity in vivo.

Pathophysiological mechanisms for actions of the neurotrophins

Neurotrophins provide trophic and tropic support for different neuronal subpopulations in the developing and adult nervous systems. Expression of the neurotrophins and their receptors can be altered in several different disease or injury states that impact upon the functions in the central and peripheral nervous systems. The intracellular signals used by the neurotrophins are triggered by ligand binding to the cell surface Trk and p75NTR receptors. In general, signals emanating from Trk receptors support survival, growth and synaptic strengthening, while those emanating from p75NTR induce apoptosis, attenuate growth and weaken synaptic signaling. Mature neurotrophins are the preferred ligand for Trk proteins while p75NTR binds preferentially to the proneurotrophins and serves as a signaling component of the receptor complex for growth inhibitory molecules of central nervous system myelin [ie, myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgP) and Nogo]. The functional antagonism between Trk and p75NTR signaling may significantly impact the pathogenesis of human neurodevelopmental and neurodegenerative diseases and further complicate therapeutic uses of exogenous neurotrophins. The potential for each is discussed in this review.