Regulation of expression of mRNAs encoding the nerve growth factor receptors p75 and trkA in developing sensory neurons

Ligand-induced dynamics of neurotrophin receptors investigated by single-molecule imaging approaches

Neurotrophins are secreted proteins that regulate neuronal development and survival, as well as maintenance and plasticity of the adult nervous system. The biological activity of neurotrophins stems from their binding to two membrane receptor types, the tropomyosin receptor kinase and the p75 neurotrophin receptors (NRs). The intracellular signalling cascades thereby activated have been extensively investigated. Nevertheless, a comprehensive description of the ligand-induced nanoscale details of NRs dynamics and interactions spanning from the initial lateral movements triggered at the plasma membrane to the internalization and transport processes is still missing. Recent advances in high spatio-temporal resolution imaging techniques have yielded new insight on the dynamics of NRs upon ligand binding. Here we discuss requirements, potential and practical implementation of these novel approaches for the study of neurotrophin trafficking and signalling, in the framework of current knowledge available also for other ligand-receptor systems. We shall especially highlight the correlation between the receptor dynamics activated by different neurotrophins and the respective signalling outcome, as recently revealed by single-molecule tracking of NRs in living neuronal cells.

Low-density lipoprotein receptor related protein-1 (LRP1)-dependent cell signaling promotes neurotrophic activity in embryonic sensory neurons

Developing sensory neurons require neurotrophic support for survival, neurite outgrowth and myelination. The low-density lipoprotein receptor-related protein-1 (LRP1) transactivates Trk receptors and thereby functions as a putative neurotrophin. Herein, we show that LRP1 is abundantly expressed in developing dorsal root ganglia (DRG) and that LRP1-dependent cell signaling supports survival, neurite extension and receptivity to Schwann cells even in the absence of neurotrophins. Cultured embryonic DRG neurons (E15) were treated with previously characterized LRP1 ligands, LRP1-receptor binding domain of α2-macroglobulin (RBD), hemopexin domain of MMP-9 (PEX) or controls (GST) for two weeks. These structurally diverse LRP1 ligands significantly activated and sustained extracellular signal-regulated kinases (ERK1/2) 5-fold (p<0.05), increased expression of growth-associated protein-43(GAP43) 15-fold (P<0.01), and increased neurite outgrowth 20-fold (P<0.01). Primary sensory neurons treated with LRP1 ligands survived > 2 weeks in vitro, to an extent equaling NGF, a finding associated with canonical signaling mechanisms and blockade of caspase-3 cleavage. LRP1 ligand-induced survival and sprouting were blocked by co-incubation with the LRP1 antagonist, receptor associated protein (RAP), whereas RAP had no effect on NGF-induced activity. Site directed mutagenesis of the LRP1 ligand, RBD, in which Lys¹³⁷⁰ and Lys¹³⁷⁴ are converted to alanine to preclude LRP1 binding, were ineffective in promoting cell signaling, survival or inducing neurite extension in primary sensory neurons, confirming LRP1 specificity. Furthermore, LRP1-induced neurite sprouting was mediated by Src-family kinase (SFK) activation, suggesting transactivation of Trk receptors. Co-cultures of primary embryonic neurons and Schwann cells showed that LRP1 agonists promoted axonal receptivity to myelination to Schwann cells. Collectively, these findings identify LRP1 as a novel and perhaps essential trophic molecule for sensory neuronal survival and development.

Selective regulation of nerve growth factor expression in developing cutaneous tissue by early sensory innervation

Background

In the developing vertebrate peripheral nervous system, the survival of sympathetic neurons and the majority of sensory neurons depends on a supply of nerve growth factor (NGF) from tissues they innervate. Although neurotrophic theory presupposes, and the available evidence suggests, that the level of NGF expression is completely independent of innervation, the possibility that innervation may regulate the timing or level of NGF expression has not been rigorously investigated in a sufficiently well-characterized developing system.

Results

To address this important question, we studied the influence of innervation on the regulation of NGF mRNA expression in the embryonic mouse maxillary process in vitro and in vivo. The maxillary process receives its innervation from predominantly NGF-dependent sensory neurons of the trigeminal ganglion and is the most densely innervated cutaneous territory with the highest levels of NGF in the embryo. When early, uninnervated maxillary processes were cultured alone, the level of NGF mRNA rose more slowly than in maxillary processes cultured with attached trigeminal ganglia. In contrast to the positive influence of early innervation on NGF mRNA expression, the levels of brain-derived neurotrophic factor (BDNF) mRNA and neurotrophin-3 (NT3) mRNA rose to the same extent in early maxillary processes grown with and without trigeminal ganglia. The level of NGF mRNA, but not BDNF mRNA or NT3 mRNA, was also significantly lower in the maxillary processes of erbB3^-/- mice, which have substantially fewer trigeminal neurons than wild-type mice.

Conclusions

This selective effect of initial innervation on target field NGF mRNA expression provokes a re-evaluation of a key assertion of neurotrophic theory that the level of NGF expression is independent of innervation.

Neurotrophin receptors TrkA and TrkC cause neuronal death whereas TrkB does not

Neurons of the peripheral nervous system have long been known to require survival factors to prevent their death during development. But why they selectively become dependent on secretory molecules has remained a mystery, as is the observation that in the central nervous system, most neurons do not show this dependency. Using engineered embryonic stem cells, we show here that the neurotrophin receptors TrkA and TrkC (tropomyosin receptor kinase A and C, also known as Ntrk1 and Ntrk3, respectively) instruct developing neurons to die, both in vitro and in vivo. By contrast, TrkB (also known as Ntrk2), a closely related receptor primarily expressed in the central nervous system, does not. These results indicate that TrkA and TrkC behave as dependence receptors, explaining why developing sympathetic and sensory neurons become trophic-factor-dependent for survival. We suggest that the expansion of the Trk gene family that accompanied the segregation of the peripheral from the central nervous system generated a novel mechanism of cell number control.

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.

Revisiting the Koebner phenomenon: role of NGF and its receptor system in the pathogenesis of psoriasis

Nerve growth factor (NGF) influences the key pathological events of psoriasis: keratinocyte proliferation, angiogenesis, and T-cell activation. We have systematically examined the kinetics of NGF expression, keratinocyte proliferation, and migration of T lymphocytes in the epidermis in Koebner-induced developing psoriatic plaques. In skin traumatized by the tape-stripping method (n = 12), a marked up-regulation of NGF in Koebner-positive lesions (n = 7) was observed 24 hours after trauma. Synthesis of NGF reached its maximum level in the 2nd week. Furthermore, cultured keratinocytes from nonlesional skin of psoriasis patients produced 10 times higher levels of NGF compared with keratinocytes from healthy individuals. To substantiate the in vivo effect of NGF secreted by keratinocytes in psoriatic plaques, we studied psoriatic plaques and normal human skin in a SCID-human skin xenograft model. The transplanted psoriatic plaques demonstrated marked proliferation of NGF-R (p75)-positive nerve fibers compared with only a few nerves in the transplanted normal human skin. Our results demonstrate that 1) in a developing psoriatic lesion, up-regulation of NGF together with keratinocyte proliferation are early events and precede epidermotropism of T lymphocytes; 2) keratinocytes in patients with psoriasis are primed to produce elevated levels of NGF; and 3) NGF synthesized by these keratinocytes is functionally active.

Tumor necrosis factor alpha regulates responses to nerve growth factor, promoting neural cell survival but suppressing differentiation of neuroblastoma cells

Although nerve growth factor (NGF) promotes survival of neurons, tumor necrosis factor alpha (TNF-alpha) contributes to cell death triggered by NGF depletion, through TNF-alpha receptor (TNFR) 1. In contrast to this effect, TNF-alpha can promote neural cell survival via TNF-alpha receptor TNFR2. Although these findings demonstrate pivotal roles of TNF-alpha and NGF in cell fate decisions, cross-talk between these signaling pathways has not been clarified. We find that NGF can induce TNF-alpha synthesis through the nuclear factor-kappaB transcription factor. This provides a new basis for examining the cross-talk between NGF and TNF-alpha. Inhibition of TNFR2 shows opposite effects on two downstream kinases of NGF, extracellular signal-regulated kinase (Erk) and Akt. It increases Erk activation by NGF, and this increased activation induces differentiation of neuroblastoma cell lines. Reciprocally, inhibition of TNFR2 decreases Akt activation by NGF. Consistent with an essential role of Akt in survival signaling, inhibition of TNF-alpha signaling decreases NGF-dependent survival of neurons from rat dorsal root ganglia. Thus, NGF and NGF-induced TNF-alpha cooperate to activate Akt, promoting survival of normal neural cells. However, the NGF-induced TNF-alpha suppresses Erk activation by NGF, blocking NGF-induced differentiation of neuroblastoma cells. TNFR2 signaling could be a novel target to modulate cell responses to NGF.

Novel Brn3a cis-acting sequences mediate transcription of human trkA in neurons

TrkA, the receptor tropomyosin-related kinase for nerve growth factor, is critical not only for the correct spatial and temporal development of sensory neurons during embryogenesis but also for the survival of sensory neurons, the differentiation and apoptosis of neuronal tumors and suppression of latent herpes simplex virus genomes. While the regulation of the expression of trkA is a complex process, the transcription factor Brn3a is known to play an important role as an enhancer of trkA transcription during development in the mouse. Despite considerable information on the regulation of trkA during embryogenesis, the mechanisms by which the expression of trkA is regulated in differentiated neurons, or the factors that influence its expression in tumor cells, have not been identified. We initiated studies to determine whether Brn3a/trkA promoter interactions may be important in a model of differentiated neurons and in medulloblastoma cells. We constructed a plasmid that contains 1043 base pairs of genomic sequences that extend to 30 nucleotides upstream of trkA coding region. In contrast to previous data, a short 190 bp region that lies proximal to the trkA initiation codon was sufficient for Brn3a responsiveness in Vero cells. This region was also sufficient for Brn3a trans-activation in nerve growth factor-differentiated PC12 cells. At least two portions of the 190 bp fragment bind to Brn3a with an affinity high enough to be detected in electromobility shift assays. In addition, Brn3a increased levels of endogenous trkA transcripts in PC12 cells and initiated trkA expression in medulloblastoma cells, which normally do not express trkA.

Regulation of sympathetic neuron differentiation by endogenous nerve growth factor and neurotrophin-3

Nerve growth factor (NGF) and neurotrophin-3 (NT3) play distinctive roles in sympathetic axon growth and target field innervation and are required for sympathetic neuron survival in vivo. To ascertain if these neurotrophins selectively regulate the expression of genes that determine the functional characteristics of differentiated sympathetic neurons, we measured the mRNA levels for several such genes in the superior cervical ganglion of NGF(-/-), NT3(-/-) and wild type mouse embryos at a stage before excessive neuronal loss occurs in the absence of these neurotrophins. Despite the extensively documented ability of NGF to regulate the noradrenergic phenotype of sympathetic neurons, we found that tyrosine hydroxylase (TH) and dopamine beta hydroxylase (DbetaH) mRNA levels were normal in NGF(-/-) embryos, but significantly reduced in NT3(-/-) embryos. In contrast, the beta2 nicotinic acetylcholine receptor and PACAP receptor 1 mRNA levels were normal in NT3(-/-) embryos, but significantly reduced in NGF(-/-) embryos. Studies of mice lacking neurotrophin receptors suggested that the effects of NGF on gene expression require TrkA whereas those of NT3 require TrkA and p75(NTR). These findings demonstrate that endogenous NGF and NT3 have distinctive and separate effects on gene expression in early sympathetic neurons and that these selective effects on gene expression require a different combination of neurotrophin receptors.

Neurotrophic factors as pharmacological agents for the treatment of injured auditory neurons

Mature auditory neurons degenerate in response to a loss of target-derived trophic factors. Neurotrophic factors influence the health and viability of auditory neurons. This suggests that neurotrophic factors can be used as therapeutic agents to prevent neuronal cell death and to initiate repair of damaged neuronal processes in the injured auditory system. Both in vitro and in vivo experiments have been performed to determine which of the vast array of neurotrophic factors affect mature auditory neurons and how they can be delivered to the sites of injury within the auditory system. Neurotrophin 3 was found to be a major survival factor for auditory neurons. Brain-derived neurotrophic factor is a minor survival factor for auditory neurons and nerve growth factor, although not promoting survival, does elicit the repair of neuronal processes. Fibroblast growth factor 2, transforming growth factor beta 1 and ciliary neurotrophic factor function as injury-response factors in the auditory system. Combination of different classes of growth factors has an additive effect on neuronal survival. Growth factors may be able to be delivered to sites of injury within the cochlea by either direct perfusion or gene therapy (e.g. a defective virus expressing a growth factor gene).

A cell-biological model of p75NTR signaling

Neurotrophin stimulation of tropomyosin-related kinase (Trk) and p75 receptors influences cellular processes such as proliferation, growth, differentiation, and other cell-specific functions, as well as regeneration. In contrast to Trk receptors, which have a well-defined trophic role, p75 has activities ranging from trophism to apoptosis. Continued neurotrophin stimulation of differentiating neurons transforms the initially trophic character of p75 signaling into negative growth control and overstimulation leads to apoptosis. This function shift reflects the signaling effects of ceramide that is generated upon stimulation of p75. The use of ceramide signaling by p75 may provide a key to understanding the cell-biological role of p75. The review presents arguments that the control of cell shape formation and cell selection can serve as an organizing principle of p75 signaling. Concurrent stimulation by neurotrophins of p75 and Trk receptors constitutes a dual growth control with antagonistic and synergistic elements aimed at optimal morphological and functional integration of cells and cell populations into their context.

Zinc-mediated neuronal death is dependent on Trk activation

Zinc release is a primary mediator of neuronal death. Here we show that zinc-mediated death of neurons in vitro is dependent on nerve growth factor (NGF) stimulation and does not occur in response to exposure to leukemia inhibitory factor. NGF priming is regulated, not by the traditional neurotrophin death receptor, p75NTR, but by TrkA, in a protein- and mRNA synthesis-dependent manner. Furthermore, Trk signaling promotes raised free intracellular zinc, mediating neuronal death after extracellular application of zinc. Thus, regulators of Trk signaling provide attractive targets for future treatment of zinc-associated neurological diseases, including stroke, epilepsy and brain trauma.

Invariant mantling of growth cones by Schwann cell precursors characterize growing peripheral nerve fronts

Little is known about the cytoarchitecture of growth fronts in developing mammalian nerves. We report here the first quantitative, ultrastructural analysis of growth cones (GCs) and their immediate cellular and tissue environment at tips of growing nerves that are nearing their targets in fore limbs of E14 rat embryos. Schwann cell precursor (SCP) marker, p75 neurotrophin receptor, and growth cone marker, SCG10, were used to identify nerve fronts, respectively. Using confocal 3D reconstructions and immunoelectron microscopy, we found that growth cone and Schwann cell precursor migrate together at the nerve front, where growth cone contact adjacent growth cone and Schwann cell precursor with similar frequency. Schwann cell precursor are extensively connected by adherens junctions and form elaborate scaffolds that enmantle growth cone at nerve fronts, so that 80% of the nerve front surface is covered by Schwann cell precursor. Although they interdigitate in complex ways among growth cone, the total contact area between growth cone and glial membranes is remarkably constant among the 100 growth fronts analyzed. In contrast to this consistency, other growth cone contacts varied markedly from front to front such that the frequencies of GC-GC contacts are increasing proportional to their decreasing contacts with mesenchymal tissue. Thus, at the nerve front, it is the Schwann cell precursor that are most exposed to extracellular environment while forming a surprisingly invariant substrate for advancing growth cone. This study shows for the first time that Schwann cell precursor are close and consistent cellular companions of growth cone in their approach to their final targets in the developing limb and suggests a previously unappreciated role for Schwann cell precursor in growth cone advance through the limb mesenchyme.

TrkA NGF receptor plays a role in the modulation of p75NTR expression

The cellular response to nerve growth factor (NGF) is mediated by two structurally unrelated receptors, TrkA and p75 neurotrophin receptor (p75NTR), which have been shown to interact resulting in reciprocal modulation of function. In this study, we have examined the modulation of p75NTR protein expression by specific TrkA autophosphorylation sites in the presence or absence of NGF. We have used cell lines derived from PC12 cells that express either no endogenous TrkA (PC12nnr5) or TrkA receptors mutated via site-directed mutagenesis to abrogate individual tyrosine autophosphorylation sites on the cytoplasmic tail (Y490F, Y785F and Y490/785F). Results indicate that in the absence of TrkA in PC12nnr5 cells there is reduced constitutive p75NTR expression, which can be restored to different degrees by transfection of the Y490F TrkA or the Y490/785F TrkA, but not by transfection of the Y785F TrkA. In addition, the expression of p75NTR was upregulated in the presence of NGF in the parental and Y490F cell lines only. Together these results indicate a role for the individual tyrosine autophosphorylation sites of TrkA in regulating p75NTR expression.

Dose and age-dependent axonal responses of embryonic trigeminal neurons to localized NGF via p75NTR receptor

Nerve growth factor (NGF) and related neurotrophins are target-derived survival factors for sensory neurons. In addition, these peptides modulate neuronal differentiation, axon guidance, and synaptic plasticity. We tested axonal behavior of embryonic trigeminal neurons towards localized sources of NGF in collagen gel assays. Trigeminal axons preferentially grow towards lower doses of localized NGF and grow away from higher concentrations at earlier stages of development, but do not show this response later. Dorsal root ganglion axons also show similar responses to NGF, but NGF-dependent superior cervical ganglion axons do not. Such axonal responses to localized NGF sources were also observed in Bax-/- mice, suggesting that the axonal effects are largely independent of cell survival. Immunocytochemical studies indicated that axons, which grow towards or away from localized NGF are TrkA-positive, and TrkA-/- TG axons do not respond to any dose of NGF. We further show that axonal responses to NGF are absent in TG derived from mice that lack the p75 neurotrophin receptor (p75NTR). Collectively, our results suggest that localized sources of NGF can direct axon outgrowth from trigeminal ganglion in a dose- and age-dependent fashion, mediated by p75NTR signaling through TrkA expressing axons.

HGF promotes survival and growth of maturing sympathetic neurons by PI-3 kinase- and MAP kinase-dependent mechanisms

Hepatocyte growth factor (HGF) is a pleiotrophic factor whose many functions include promoting neuronal survival and growth. Hitherto, these effects have been observed in the presence of other neurotrophic factors like NGF and CNTF, and this requirement for an accessory factor has made it difficult to elucidate the signaling pathways that mediate its survival and growth-enhancing effects. Here, we show that HGF promotes the survival of mature sympathetic neurons of the superior cervical ganglion (SCG) grown at low density in defined medium lacking other neurotrophic factors. This effect was first clearly observed in cultures established from postnatal day 20 (P20) mice and became maximal by P40. HGF also enhanced the growth of neurite arbors from neurons throughout postnatal development and in the adult. HGF treatment resulted in phosphorylation of Akt and ERK1/ERK2. Preventing Akt activation with the phosphatidylinositol-3 (PI-3) kinase inhibitor LY294002 blocked the HGF survival response, and inhibition of ERK activation with the MEK inhibitors PD98059 or U0126 reduced the HGF survival response and the neurite growth-promoting effects of HGF. These results indicate that HGF promotes the survival and growth of maturing sympathetic neurons by both PI-3 kinase- and MAP kinase-dependent mechanisms.

K252a, a high-affinity nerve growth factor receptor blocker, improves psoriasis: an in vivo study using the severe combined immunodeficient mouse-human skin model

The peripheral nervous system, in addition to its sensory and motor functions, can induce a local inflammatory response known as neurogenic inflammation. This phenomenon plays a critical role in several inflammatory diseases, e.g., asthma, atopy, rheumatoid arthritis, psoriasis, and ulcerative colitis. Neurogenic inflammation and the role of nerve growth factor (NGF) have been extensively studied in psoriasis. There are increased levels of NGF in the keratinocytes and upregulation of NGF receptor (NGF-R) in the cutaneous nerves of psoriatic plaques. NGF can influence all the salient pathologic events noticed in psoriasis such as proliferation of keratinocytes, angiogenesis, T cell activation, expression of adhesion molecules, proliferation of cutaneous nerves, and upregulation of neuropeptides. In this double-blinded, placebo-controlled study, we addressed the role of NGF/NGF-R in psoriasis in an in vivo system using the severe combined immunodeficient (SCID) mouse-human skin model of psoriasis. The transplanted psoriatic plaques on the SCID mice (n=12) were treated with K252a, a high-affinity NGF receptor blocker. Psoriasis significantly improved following 2 wk of therapy. The length of the rete pegs changed from 308.57+/-98.72 to 164.64+/-46.78 microm (p<0.01, Student's t test). A similar improvement of psoriasis was observed by directly inhibiting NGF with NGF-neutralizing antibody. NGF-neutralizing antibody in normal saline at 10 ng (n=4) and 20 ng (n=4) per kilogram of body weight doses were used. Both doses of NGF-neutralizing antibody reduced rete peg lengths significantly, e.g., from 298.5+/-42.69 to 150.52+/-32.93 microm (p<0.05, Student's t test). This study provides evidence for the role of NGF and its high-affinity receptor in the pathogenesis of psoriasis and insights to develop novel therapeutic modalities.

Distribution of central sensory axons in transgenic mice overexpressing nerve growth factor and lacking functional p75 neurotrophin receptor expression

This study examined the roles of nerve growth factor (NGF) and the p75 neurotrophin receptor (p75NTR) in the growth of dorsal root ganglion (DRG) central processes in the dorsal horn. Two genetically modified mouse strains were used: transgenic mice that overexpress NGF in the CNS under the control of the glial fibrillary acidic protein promoter, and p75NTR exon III null mutant mice that express a hypomorphic form of this receptor. In both NGF transgenic and nontransgenic mice with hypomorphic expression of p75NTR, there is a significant loss of DRG neurons compared to mice with normal p75NTR expression. This reduction in neuron number has been shown to underlie a corresponding decrease in peripheral nociceptive sensory innervation. Within the CNS, however, nociceptive innervation of the dorsal horn appears to be unaffected by hypomorphic expression of p75NTR. Comparisons of calcitonin gene-related peptide immunoreactivity in the dorsal horn revealed that the area occupied by DRG central processes was not significantly different between p75NTR hypomorphic mice and wild-type siblings, or between NGF transgenic mice with either hypomorphic or normal expression of p75NTR. We propose that DRG central processes arborize extensively in both NGF-transgenic and nontransgenic p75NTR hypomorphic mice in order to compensate for the loss of DRG neurons and restore dorsal horn innervation to normal levels. We also present evidence suggesting that NGF plays only a minor role in the growth of DRG central processes.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

p75 and TrkA receptors are both required for uptake of NGF in adult sympathetic neurons: use of a novel fluorescent NGF conjugate

We have developed and tested the biological activity and specificity of a novel fluorescent dextran-Texas Red-nerve growth factor (DTR-NGF) conjugate. DTR-NGF was found to promote survival and neurite outgrowth in cultured dissociated sympathetic neurons similarly to native NGF. The conjugate was taken up and transported retrogradely by terminal sympathetic nerves innervating the iris to neurons in the ipsilateral superior cervical ganglion (SCG) of young adult rats. Uptake and transport was assessed by counting numbers of labelled neurons and by measuring intensity of neuronal labelling using confocal microscopy and image analysis. DTR-NGF labelling in SCG neurons was shown to be dose-dependent with an EC(50) of 75 ng. Similar concentrations of unconjugated DTR resulted in no neuronal labelling. DTR-NGF uptake was competed off using a 50-fold excess of native NGF, resulting in a 73% reduction in numbers of labelled neurons. Pretreatment of nerve terminals with function-blocking antibodies against the low (p75) and high (TrkA) affinity NGF receptors resulted in a large (85-93%) reduction in numbers of DTR-NGF labelled neurons. Anti-p75 and anti-TrkA antibodies had comparable effects which were concentration-dependent. These findings indicate that both receptors are required for uptake of NGF in adult rat sympathetic neurons. In particular, the results provide strong evidence that the p75 receptor plays a more active role in transducing the NGF signal than has been proposed.

Reciprocal developmental changes in the roles of Bcl-w and Bcl-x(L) in regulating sensory neuron survival

We have compared the roles of two anti-apoptotic members of the Bcl2 family, Bcl-w and Bcl-x(L), in regulating the survival of sensory neurons during development. We used microinjection to introduce expression plasmids containing Bcl-w and Bcl-x(L) cDNAs in the sense and antisense orientations into the nuclei of BDNF-dependent nodose neurons and NGF-dependent trigeminal neurons at stages during and after the period of naturally occurring neuronal death. Whilst overexpression of either protein promoted neuronal survival in the absence of neurotrophins and microinjection of antisense constructs reduced neuronal survival in the presence of neurotrophins, the magnitude of these effects changed with age. Whereas Bcl-w overexpression became more effective in promoting neuronal survival with age, Bcl-x(L) overexpression became less effective, and whereas antisense Bcl-w became much more effective in killing neurotrophin-supplemented neurons with age, antisense Bcl-x(L) became much less effective in killing these neurons. There was a marked increased in Bcl-w mRNA and Bcl-w immunoreactive neurons and a decrease in Bcl-x(L) mRNA and Bcl-x(L) immunoreactive neurons in the trigeminal and nodose ganglia over this period of development. Our results demonstrate that both Bcl-w and Bcl-x(L)play an important anti-apoptotic role in regulating the survival of NGF- and BDNF-dependent neurons, and that reciprocal changes occur in the relative importance of these proteins with age. Whereas Bcl-x(L) plays a more important role during the period of naturally occurring neuronal death, Bcl-w plays a more important role at later stages.

Attenuation of a caspase-3 dependent cell death in NT4- and p75-deficient embryonic sensory neurons

Neuronal survival during the developmental period of naturally occurring cell death is mediated through a successful competition for limiting concentrations of neurotrophic factors, and the deprived neurons will die. New results show that induced death through the p75 neurotrophin receptor (p75(NTR)), a member of the p55TNF/Fas family of cell death receptors, may also influence survival during development. We find that eliminating p75(NTR) or neurotrophin 4 (NT4) in mice leads to a marked attenuation of apoptosis during the programmed cell death period of the trigeminal ganglion neurons, suggesting that NT4 can induce the death of these neurons through the p75(NTR). These in vivo findings were reproduced in primary cell cultures, where NT4 was found to induce death in a p75(NTR)-dependent pathway. Analysis of p75 deficient and wild-type cells revealed two separate cell death pathways, a p75(NTR)- and caspase-3-independent pathway activated by trophic factor deprivation, and a p75(NTR)- and caspase-3-dependent pathway initiated by NT4. Crossing in the NT4 null alleles in brain-derived neurotrophic factor (BDNF) null mutant mice led to a rescue of a large proportion of BDNF-dependent neurons from excessive cell death, indicating that trophic factor deprivation is not sufficient for the death of many neurons and that additional death inducing signals might be required. Our results suggest that NT4 competitively signals survival and death of sensory neurons through trkB and p75(NTR), respectively.

Dexamethasone induces TrkA and p75NTR immunoreactivity in the cerebral cortex and hippocampus

Nerve growth factor (NGF) plays a crucial role in synaptic plasticity during brain development and adulthood by activating a dual receptor system composed of TrkA and p75 (p75NTR) receptors. Exogenous NGF modulates the expression of both receptors. Little is known about the ability of endogenous NGF to regulate the expression of these receptors in basal forebrain cholinergic terminals. The ability of glucocorticoids to increase NGF expression in the hippocampus prompted us to investigate whether the synthetic glucocorticoid dexamethasone (DEX) increases TrkA and p75NTR expression in NGF-target cholinergic neurons in developing rats. We first examined the effect of DEX on NGF mRNA by in situ hybridization. DEX given systemically (0.5 mg/kg, sc) for 1 week to 7-day-old rats elicited an increase in NGF mRNA levels in the dentate gyrus of the hippocampus and superficial layers II and III of the cerebral cortex. Immunohistochemical analysis of p75NTR and TrkA levels revealed a dramatic increase in p75NTR immunoreactivity (IR) in both basal forebrain and hippocampus and TrkA IR in the hippocampus. Interestingly, in DEX-treated rats more axonal terminals were immunopositive for p75NTR in the hippocampus and cortex, suggesting an increase in p75NTR IR in cell bodies as well as in terminals. Our data indicate that the endogenously produced NGF elicits biological changes similar to those of the exogenously delivered NGF. We suggest that glucocorticoids might regulate and coordinate cholinergic neuronal maturation by increasing the biosynthesis of NGF.

Depolarisation causes reciprocal changes in GFR(alpha)-1 and GFR(alpha)-2 receptor expression and shifts responsiveness to GDNF and neurturin in developing neurons

GDNF and neurturin are structurally related neurotrophic factors that promote the survival of many different kinds of neurons and influence axonal and dendritic growth and synaptic function. These diverse effects are mediated via multicomponent receptors consisting of the Ret receptor tyrosine kinase plus one of two structurally related GPI-linked receptors, GFR(alpha)-1 and GFR(alpha)-2. To ascertain how the expression of these receptors is regulated during development, we cultured embryonic neurons under different experimental conditions and used competitive RT/PCR to measure the levels of the mRNAs encoding these receptors. We found that depolarising levels of KCl caused a marked increase in GFR(alpha)-1 mRNA and a marked decrease in GFR(α)-2 mRNA in sympathetic, parasympathetic and sensory neurons. These changes were accompanied by increased responsiveness to GDNF and decreased responsiveness to neurturin, and were inhibited by L-type Ca(2+) channel antagonists, suggesting that they were due to elevated intracellular free-Ca(2+). There was no consistent effect of depolarising levels of KCl on ret mRNA expression, and neither GDNF nor neurturin significantly affected receptor expression. These results show that depolarisation has marked and opposing actions on the expression of GFR(α)-1 and GFR(α)-2, which are translated into corresponding changes in neuronal responsiveness to GDNF and neurturin. This provides evidence for a mechanism of regulating the neurotrophic factor responses of neurons by neural activity that has important implications for structural and functional plasticity in the developing nervous system.

Neurotrophin receptor structure and interactions

Although ligand-induced dimerization or oligomerization of receptors is a well established mechanism of growth factor signaling, increasing evidence indicates that biological responses are often mediated by receptor trans-signaling mechanisms involving two or more receptor systems. These include G protein-coupled receptors, cytokine, growth factor and trophic factor receptors. Greater flexibility is provided when different signaling pathways are merged through multiple receptor signaling systems. Trophic factors exemplified by NGF and its family members, ciliary neurotrophic factor (CNTF) and glial derived neurotrophic factor (GDNF) all utilize increased tyrosine phosphorylation of cellular substrates to mediate neuronal cell survival. Actions of the NGF family of neurotrophins are not only dictated by ras activation through the Trk family of receptor tyrosine kinases, but also a survival pathway defined by phosphatidylinositol-3-kinase activity (Yao and Cooper, 1995), which gives rise to phosphoinositide intermediates that activate the serine/threonine kinase Akt/PKB (Dudek et al., 1997). Induction of the serine-threonine kinase activity is critical for cell survival, as well as cell proliferation. Hence, for many trophic factors, multiple proteins constitute a functional multisubunit receptor complex that activates ras-dependent and ras-independent intracellular signaling. The NGF receptors provide an example of bidirectional crosstalk. In the presence of TrkA receptors, p75 can participate in the formation of high affinity binding sites and enhanced neurotrophin responsiveness leading to a survival or differentiation signal. In the absence of TrkA receptors, p75 can generate, in only specific cell populations, a death signal. These activities include the induction of NF kappa B (Carter et al., 1996); the hydrolysis of sphingomyelin to ceramide (Dobrowsky et al., 1995); and the pro-apoptotic functions attributed to p75. Receptors are generally drawn and viewed as isolated integral membrane proteins which span the lipid bilayer, with signal transduction proceeding in a linear step-wise fashion. There are now numerous examples which indicate that each receptor acts not only in a linear, independent manner, but can also influence the activity of other cell surface receptors, either directly or through signaling intermediates. Which step and which intermediates are utilized for crosstalk between the receptors is a critical question. For neurotrophins, their primary function in sustaining the viability of neurons is counterbalanced by a receptor mechanism to eliminate cells by an apoptotic mechanism. It is conceivable that this bidirectional system may be utilized selectively during development and in neurodegenerative diseases.

Subpopulations of rat B2(+) neuroblasts exhibit differential neurotrophin responsiveness during sympathetic development

Sympathetic neurons comprise a population of postmitotic, tyrosine hydroxylase expressing cells whose survival is dependent upon nerve growth factor (NGF) both in vivo and in vitro. However, during development precursors to rat sympathetic neurons in the thoracolumbar region are not responsive to NGF because they lack the signal transducing NGF receptor, trkA. We have previously shown that acquisition of trkA expression is sufficient to confer a functional response to NGF. Here we describe four subpopulations of thoracolumbar sympathetic neuroblasts which are mitotically active and unresponsive to NGF at E13.5 of rat gestation, but differ based upon their neurotrophic responsiveness in vitro. The survival in culture of the largest sympathetic subpopulation is mediated by neurotrophin-3 (NT-3) or glial-derived neurotrophic factor (GDNF), whereas the cell survival of two smaller subpopulations of neuroblasts are mediated by either solely GDNF or solely NT-3. Finally, we identify a subpopulation of sympathetic neuroblasts in the thoracolumbar region whose survival, exit from the cell cycle, induction of trkA expression, and consequent acquisition of NGF responsiveness in culture appear to be neurotrophin independent and cell autonomous. These subpopulations reflect the diversity of neurotrophic actions that occur in the proper development of sympathetic neurons.

In vivo survival requirement of a subset of nodose ganglion neurons for nerve growth factor

The sensory neurons of the nodose ganglion are the classic example of a population of peripheral nervous system neurons that do not require nerve growth factor (NGF) for survival during development but are dependent on other neurotrophins. We have re-examined this assertion by studying the development of the nodose ganglion of mice that have a null mutation in the NGF gene. Compared with wild-type embryos, the number of neurons undergoing apoptosis was elevated in NGF -/- mice, resulting in a significant reduction in the total number of neurons in the ganglion by the end of embryonic development. TrkA, the NGF receptor tyrosine kinase, was expressed in the nodose ganglion throughout development and there was a marked decrease in TrkA mRNA expression in the nodose ganglion of NGF -/- embryos. Although the in vitro survival of the majority of nodose neurons was promoted by brain-derived neurotrophic factor (BDNF), a minor proportion was supported by NGF in cultures established over a range of embryonic stages. These results clearly demonstrate that a subset of nodose ganglion neurons depends on NGF for survival during development. The finding that the expression of tyrosine hydroxylase (TH) mRNA was unaffected in the nodose ganglia of NGF-deficient embryos indicates that this NGF-dependent subset is distinct from the subset of catacholaminergic neurons in the nodose ganglion.

TrkB expression and early sensory neuron survival are independent of endogenous BDNF

Sensory neurons initially survive independently of neurotrophins in culture during the stage of development when their axons are growing to their targets. Because mRNAs encoding brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase TrkB are detectable in subsets of sensory neurons from the earliest stages of their development, we investigated whether a BDNF autocrine loop is responsible for sustaining the survival of these neurons during this early stage in their development. Low-density dissociated cultures of nodose and dorsal root ganglion neurons were established from wild type and BDNF(-/-) mouse embryos at this stage and were grown in defined medium without added neurotrophins. Wild type and BDNF-deficient neurons survived equally well under these conditions, indicating that a BDNF autocrine loop does not play a role in sustaining the survival of sensory neurons during the earliest stages of their development. As sensory axons approach their targets, TrkB expression increases in a subset of neurons that becomes dependent on BDNF produced by other cells. Because numerous studies have shown that neurotrophins, including BDNF, increase expression of their receptors, we investigated whether endogenous BDNF is required for the increase in TrkB expression observed during stage of development. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that the developmental increase in TrkB mRNA expression occurred normally in the sensory ganglia of BDNF(-/-) embryos. Taken together, our studies of sensory neuron development in BDNF-deficient embryos have demonstrated that endogenous BDNF is neither required for the early survival of these neurons nor for the induction of TrkB expression.

The role of POU domain proteins in the regulation of mammalian pituitary and nervous system development

POU domain proteins represent a subfamily of homeodomain-containing transcription factors that are expressed in many animal orders in a number of distinct regions in the developing and adult organism. In mammals, the expression profiles of these factors have suggested roles for class I, class III, and class IV POU domain proteins in the development, maintenance, and function of the endocrine and nervous systems. The genetic characterizations of the functions of these proteins during the generation, differentiation, and maturation of cells comprising these tissues have revealed a requirement for the individual actions of these transcription factors in the development of various elements of the anterior pituitary, the brain, and the somatosensory, vestibular/cochlear, and visual systems.

Developmental changes in the response of trigeminal neurons to neurotrophins: influence of birthdate and the ganglion environment

Previous studies have shown that most neurons in cultures established during the early stages of neurogenesis in the embryonic mouse trigeminal ganglion are supported by BDNF whereas most neurons cultured from older ganglia survive with NGF. To ascertain to what extent these developmental changes in neurotrophin responsiveness result from separate phases of generation of BDNF- and NGF-responsive neurons or from a developmental switch in the response of neurons from BDNF to NGF, we administered BrdU to pregnant mice at different stages of gestation to identify neurons born at different times and studied the survival of labelled neurons in dissociated cultures established shortly after BrdU administration. Most early-generated neurons responded to BDNF, neurons generated at intermediate times responded to both factors and late-generated neurons responded to NGF, indicating that there are overlapping phases in the generation of BDNF- and NGF-responsive neurons and that late-generated neurons do not switch responsiveness from BDNF to NGF. To ascertain if early-generated neurons do switch their response to neurotrophins during development, we used repeated BrdU injection to label all neurons generated after an early stage in neurogenesis and studied the neurotrophin responsiveness of the unlabelled neurons in cultures established after neurogenesis had ceased. The response of these early-generated neurons had decreased to BDNF and increased to NGF, indicating that at least a proportion of early-generated neurons switch responsiveness to neurotrophins in vivo. Because early-generated neurons do not switch responsiveness from BDNF to NGF in long-term dissociated cultures, we cultured early trigeminal ganglion explants with and without their targets for 24 hours before establishing dissociated cultures. This period of explant culture was sufficient to enable many early-generated neurons to switch their response from BDNF to NGF and this switch occurred irrespective of presence of target tissue. Our findings conclusively demonstrate for the first time that individual neurons switch their neurotrophin requirements during development and that this switch depends on cell interactions within the ganglion. In addition, we show that there are overlapping phases in the generation of BDNF- and NGF-responsive neurons in the trigeminal ganglion.

Identification of maxillary factor, a maxillary process-derived chemoattractant for developing trigeminal sensory axons

Trigeminal sensory axons project to several epithelial targets, including those of the maxillary and mandibular processes. Previous studies identified a chemoattractant activity, termed Maxillary Factor, secreted by these processes, which can attract developing trigeminal axons in vitro. We report that Maxillary Factor activity is composed of two neurotrophins, neurotrophin-3 (NT-3) and Brain-Derived Neurotrophic Factor (BDNF), which are produced by both target epithelium and pathway mesenchyme and which are therefore more likely to have a trophic effect on the neurons or their axons than to provide directional information, at least at initial stages of trigeminal axon growth. Consistent with this, the initial trajectories of trigeminal sensory axons are largely or completely normal in mice deficient in both BDNF and NT-3, indicating that other cues must be sufficient for the initial stages of trigeminal axon guidance.

Effects of aging and axotomy on the expression of neurotrophin receptors in primary sensory neurons

Aging is accompanied by declined sensory perception, paralleled by widespread dystrophic and degenerative changes in both central and peripheral sensory pathways. Several lines of evidence indicate that neurotrophic interactions are of importance for a maintained plasticity in the adult and aging nervous system, and that changes in the expression of neurotrophins and/or their receptors may underpin senile neurodegeneration. We have here examined the expression of neurotrophin receptor (p75NTR, trkA, trkB, and trkC) mRNA and protein in intact and axotomized primary sensory neurons of young adult (3 months) and aged (30 months) rats. To examine possible differences among primary sensory neuron populations, we have studied trigeminal ganglia (TG) as well as cervical and lumbar dorsal root ganglia (DRG). In intact aged rats, a decrease in trk (A/B/C) mRNA labeling densities and protein-like immunoreactivities was observed. The decrease was most pronounced in lumbar DRG. In contrast, a small, not statistically significant, increase of p75NTR expression was observed in aged DRG neuron profiles. After axotomy, a down-regulation of mRNA and protein levels was observed for all neurotrophin receptors (p75NTR, trkA, trkB and trkC) in both young adult and aged rats. Consistent with the higher expression levels of neurotrophin receptors in unlesioned young adult primary sensory neurons, the relative effect of axotomy was more pronounced in the young adult than aged rats. Although a decrease in mean cell profile cross-sectional areas was found during aging and after axotomy, the characteristic distribution of neurotrophin receptor expression in different populations of NRG neurons was conserved. The present findings suggest an attenuation of neurotrophic signaling in primary sensory neurons with advancing age and that the expression of p75NTR and trks is regulated differently during aging. A similar dissociation of p75NTR and trk regulation has previously been reported in other neuronal systems during aging, suggesting that there may be a common underlying mechanism. Decreased access to ligands, disturbed axon function and systemic changes in androgen/estrogen levels are discussed as inducing and/or contributing factors.

p75-mediated NF-kappaB activation enhances the survival response of developing sensory neurons to nerve growth factor

We have investigated whether the transcription factor NF-kappaB plays a role in regulating neuronal survival by manipulating NF-kappaB activation in the nerve growth factor (NGF)-dependent sensory neurons of the embryonic mouse trigeminal ganglion. Overexpression of either the p65 or the p50 NF-kappaB subunits resulted in NF-kappaB activation and promoted in vitro survival as effectively as NGF. Expression of a superrepressor IkappaB-alpha protein prevented NF-kappaB activation in p65/p50-overexpressing neurons and caused the neurons to die as rapidly as NGF-deprived neurons. NGF treatment also activated NF-kappaB, and preventing this activation with superrepressor IkappaB-alpha reduced the NGF survival response. Antibodies that block binding of NGF to the p75 receptor prevented NGF-induced NF-kappaB activation and reduced the NGF survival response to the same extent as superrepressor IkappaB-alpha. Trigeminal neurons cultured from p65(-/-) embryos showed a reduced survival response to NGF compared with neurons from wild-type embryos and there was increased apoptosis of neurons in the trigeminal ganglia of p65(-/-) embryos in vivo. However, as with p75-deficient sensory neurons, p65-deficient sensory neurons showed a normal survival response to BDNF. These results reveal a role for NF-kappaB in regulating neuronal survival during embryonic development and suggest that in addition to the well-established Trk receptor tyrosine kinase signaling cascade, NGF enhances neuronal survival by signaling via a p75-mediated pathway.

Differences and developmental changes in the responsiveness of PNS neurons to GDNF and neurturin

We have studied the ability of GDNF and neurturin to promote the in vitro survival of populations of embryonic chicken parasympathetic, sympathetic, and sensory neurons. We show that these neurons are more responsive to one or other of these factors at particular stages of development. Whereas the parasympathetic neurons are more sensitive to neurturin at late embryonic stages, sympathetic neurons are more sensitive to neurturin at early stages. In contrast, sensory neurons of the nodose ganglion are more sensitive to GDNF throughout embryonic development. Using competitive RT/PCR, we measured the levels of mRNAs encoding GDNF and neurturin receptors in purified neurons. All neurons expressed Ret mRNA, which encodes the common receptor tyrosine kinase for GDNF and neurturin. Neurons that were more sensitive to GDNF expressed higher levels of GFRalpha-1 mRNA than GFRalpha-2 mRNA and neurons that were more sensitive to neurturin expressed higher levels of GFRalpha-2 mRNA than GFRalpha-1 mRNA. These results show that populations of PNS neurons differ markedly in their responsiveness to GDNF and neurturin at certain stages of the development and suggest that these differences are governed in part by the relative levels of expression of members of the GFRalpha family of GPI-linked receptors.

Expression of Trk receptors in the developing mouse trigeminal ganglion: in vivo evidence for NT-3 activation of TrkA and TrkB in addition to TrkC

Animals lacking neurotrophin-3 (NT-3) are born with deficits in almost all sensory ganglia. Among these, the trigeminal ganglion is missing 70% of the normal number of neurons, a deficit which develops during the major period of neurogenesis between embryonic stages (E) 10.5 and E13.5. In order to identify the mechanisms for this deficit, we used antisera specific for TrkA, TrkB, and TrkC to characterize and compare the expression patterns of each Trk receptor in trigeminal ganglia of wild type and NT-3 mutants between E10.5 and E15.5. Strikingly, TrkA, TrkB, and TrkC proteins appear to be exclusively associated with neurons, not precursors. While some neurons show limited co-expression of Trk receptors at E11.5, by E13. 5 each neuron expresses only one Trk receptor. Neuronal birth dating and cell counts show that in wild-type animals all TrkB- and TrkC-expressing neurons are generated before E11.5, while the majority of TrkA-expressing neurons are generated between E11.5 and E13.5. In mice lacking NT-3, the initial formation of the ganglion, as assessed at E10.5, is similar to that in wild-type animals. At E11.5, however, the number of TrkC-expressing neurons is dramatically reduced and the number of TrkC-immunopositive apoptotic profiles is markedly elevated. By E13.5, TrkC-expressing neurons are virtually eliminated. At E11.5, compared to wild type, the number of TrkB-expressing neurons is also reduced and the number of TrkB immunoreactive apoptotic profiles is increased. TrkA neurons are also reduced in the NT-3 mutants, but the major deficit develops between E12.5 and E13.5 when elevated numbers of TrkA-immunoreactive apoptotic profiles are detected. Normal numbers of TrkA- and TrkB-expressing neurons are seen in a TrkC-deficient mutant. Therefore, our data provide evidence that NT-3 supports the survival of TrkA-, TrkB- and TrkC-expressing neurons in the trigeminal ganglion by activating directly each of these receptors in vivo.

NGF binding to p75 enhances the sensitivity of sensory and sympathetic neurons to NGF at different stages of development

To clarify the role of the common neurotrophin receptor p75 in modulating the survival response of sensory and sympathetic neurons to NGF at different stages of development, we compared the actions of wild-type NGF with a mutated NGF protein that binds normally to TrkA, the NGF receptor tyrosine kinase, but has greatly reduced binding to p75. At saturating concentrations, the NGF mutant promoted the survival of similar numbers of trigeminal sensory and sympathetic neurons as NGF. At subsaturating concentrations, the NGF mutant was less effective than wild-type NGF in promoting the survival of embryonic sensory neurons and postnatal sympathetic neurons but was equally effective as wild-type NGF in promoting the survival of embryonic sympathetic neurons. Whereas the levels of trkA and p75 were similar in embryonic sensory neurons and postnatal sympathetic neurons, the level of p75 was significantly lower than that of trkA in embryonic sympathetic neurons. These results indicate that binding of NGF to p75 enhances the sensitivity of NGF-dependent neurons to NGF at stages in their development when the levels of p75 and TrkA are similar.

NT-3 regulates expression of Brn3a but not Brn3b in developing mouse trigeminal sensory neurons

We have used a quantitative RT-PCR approach to determine the levels of Brn3a and Brn3b POU domain transcription factor mRNAs in the developing mouse trigeminal ganglion from E10 to E18. Using low density neuronal cultures, we have shown that NT-3 can regulate the expression of Brn3a mRNA in trigeminal neurons during the periods that they are differentiating and innervating their peripheral and central targets. In contrast to Brn3a, Brn3b mRNA is expressed at extremely low levels in the early trigeminal ganglion. Trigeminal neurons from early ganglia express low levels of Brn3b mRNA in culture and do not up-regulate Brn3b mRNA in response to a number of growth factors and experimental conditions. However, at later ages, when in vivo levels of Brn3b mRNA are high, FGF2, TGFbeta1 and retinoic acid all up-regulate Brn3b mRNA expression in cultured trigeminal neurons. Since NT-3 regulates the developmental expression of Brn3a, Brn3a may mediate some of the effects that NT-3 exerts on sensory neurons and their progenitors. Similarly, Brn3b may mediate some of the effects that FGF2, TGFbeta1 and retinoic acid have on neurons.

Cytokines promote the survival of mouse cranial sensory neurones at different developmental stages

To investigate when the neurotrophic cytokines ciliary neurotrophic factor (CNTF), leukaemia inhibitory factor (LIF), oncostatin-M (OSM), interleukin-6 (IL-6) and cardiotrophin-1 (CT-1) act on developing sensory neurones and whether they co-operate with neurotrophins in regulating neuronal survival, we studied the in vitro trophic effects of these factors on two well-characterized populations of cranial sensory neurones at closely staged intervals throughout embryonic development. The cutaneous sensory neurones of the trigeminal ganglion, which show an early, transient survival response to BDNF and NT3 before becoming NGF-dependent, were supported by CNTF, LIF, OSM and CT-1 during the late fetal period, several days after the neurones become NGF-dependent. At this stage of development, these cytokines promoted the survival of a subset of NGF-responsive neurones. The enteroceptive neurones of the nodose ganglion, which retain dependence on BDNF throughout fetal development, were supported throughout their development by CNTF, LIF, OSM and CT-1, and displayed an additional survival response to IL-6 in the late fetal period. These findings indicate that populations of sensory neurones display different developmental patterns of cytokine responsiveness and show that embryonic trigeminal neurones pass through several phases of differing neurotrophic factor survival requirements.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Quantitative analysis of long-term survival and neuritogenesis in vitro: cochleovestibular ganglion of the chick embryo in BDNF, NT-3, NT-4/5, and insulin

The dynamics of survival and growth were examined for cochleovestibular ganglion (CVG) cells maintained in long-term cultures. CVG cells were explanted from chick embryos after 90 h of incubation into a defined-medium containing BDNF, NT-3, or NT-4/5 and an insulin, transferrin, selenium, and progesterone supplement. Explant survival and neuritogenesis was measured for 23 to 24 days in vitro. All three neurotrophins prolonged CVG survival in a dose-dependent manner although insulin acted as a cofactor. In 0.872 microM insulin-containing medium the ED50 for BDNF and NT-3 was 100 pg/ml, whereas the ED50 for NT-4/5 was 600-1200 pg/ml. However, at later ages in vitro, survival decreased with concentrations of BDNF greater than 2 ng/ml. In insulin-free medium, concentrations of 5-200 ng/ml of BDNF or 30-200 ng/ml of NT-4/5 maintained the survival of explants at a rate that was equivalent to or less than the survival rate of cultures treated with insulin but not with neurotrophin. In contrast, NT-3-treated explants in insulin-free medium did not survive the duration of the experiment. Dose-dependent effects of BDNF and NT-3 on explant neuritogenesis were reflected as an initial delay in outgrowth, whereas NT-4/5 had no effect. Insulin regulation of neuritogenesis was suggested when outgrowth decreased in the presence of an antibody to the insulin receptor. These data suggest that while all three of these neurotrophins protect the CVG from death the long-term consequences of cofactors and certain dose levels should be considered when treating CVG cells in vivo.

Embryonic precursor cells that express Trk receptors: induction of different cell fates by NGF, BDNF, NT-3, and CNTF

Epidermal growth factor (EGF)-treated neurosphere cultures from embryonal striatum contain multipotential cells capable of neuronal, astrocytic, and oligodendroglial differentiation. In this study, we tested whether these neural precursor cells differentiate in the presence of neurotrophic factors. We first assayed neurosphere cells for expression of neurotrophin receptors. TrkA, TrkB, TrkC, and gp75 were detected by immunofluorescence microscopy in 60-80% of cells. In addition, the ciliary neurotrophic factor receptor alpha was expressed in 50-60% of cells. In the presence of the mitogen, EGF, treatment of stem cells with neurotrophic factors had no apparent effect. Removal of EGF from cells resulted in cessation of cell proliferation and pronounced astrocytic (glial fibrillary acidic protein+) differentiation. Neuronal (neurofilament+) and oligodendroglial (galactocerebroside+) cells appeared in cultures treated with neurotrophic factors. Nerve growth factor (NGF) resulted in bipolar neuronal cells, and brain-derived neurotrophic factor led to multipolar neuronal cells. Treatment with neurotrophin-3 or ciliary neurotrophic factor resulted in bipolar neuronal cells and oligodendrocytes. Neuronal differentiation in the presence of NGF was enhanced by extracellular matrix, and the resulting neuronal cells expressed choline acetyltransferase and, to a lesser degree, tyrosine hydroxylase. These studies demonstrate that neurotrophic factors influence the fates of these multipotential precursor cells. Indeed, the true utility of multipotential precursor cells is the production of different types of cells in different situations. Local cues, such as neurotrophic factors and extracellular matrix, may regulate production of different types of neural cells during development or in response to other stimuli, such as injury.

Neurotrophin switching: where does it stand?

In vitro and in vivo studies suggest that certain populations of neurons switch their survival requirements from one neurotrophin to another during an early stage in their development. Although there is good evidence for neurotrophin switching in sensory neurons, the evidence for switching in sympathetic neurons has become more controversial, as has the identity of the factors that regulate their responsiveness to particular neurotrophins.

Timing and regulation of trkB and BDNF mRNA expression in placode-derived sensory neurons and their targets

The sensory neurons of the vestibular and nodose ganglia of the chicken embryo have nearby and distant targets, respectively. In vitro studies have shown that these neurons survive independently of neurotrophins when their axons are growing to their targets and become dependent on brain-derived neurotrophic factor (BDNF) for survival when their axons reach the vicinity of their targets. Although the timing of BDNF dependence is principally controlled by an intrinsic timing mechanism in the neurons, the onset of dependence can be accelerated by BDNF exposure toward the end of the phase of neurotrophin independence. We have used quantitative reverse transcription/polymerase chain reaction to study the expression of transcripts coding for BDNF and the catalytic isoform of its receptor tyrosine kinase, TrkB, in these neurons and their targets at different stages of development. We show that the peripheral and central target tissues of these neurons express BDNF mRNA prior to the arrival of sensory axons. Vestibular neurons express trkB mRNA before nodose neurons, which accords with the earlier response of vestibular neurons to BDNF. In culture, early nodose neurons start expressing trkB mRNA after 36 h incubation, which is 36 h before these neurons become dependent on BDNF for survival. Although BDNF does not affect the timing and level of trkB mRNA expression during the first 48 h in vitro, it increases the level of trkB mRNA after this time. The timing of BDNF-induced elevation of trkB mRNA correlates with the period during which BDNF exposure accelerates the onset of BDNF dependence in nodose neurons. These results suggest that the timing of BDNF dependence in developing sensory neurons is due in part to expression of catalytic TrkB and demonstrate that a BDNF autocrine loop is not required for the survival of sensory neurons during the earliest stages of their development.