Functionally antagonistic interactions between the TrkA and p75 neurotrophin receptors regulate sympathetic neuron growth and target innervation

Oestrogen regulates sympathetic neurite outgrowth by modulating brain derived neurotrophic factor synthesis and release by the rodent uterus

Sympathetic innervation of the adult rodent uterus undergoes cyclic remodelling. Terminal sympathetic axons degenerate when oestrogen levels rise and regenerate when oestrogen levels decline. This study examined the role of neurotrophins in oestrogen-mediated uterine sympathetic nerve remodelling. Oestrogen injection of ovariectomized female rats did not affect uterine NT-3 levels 24 h postinjection, and increased endometrial NGF protein, indicating that reduced NGF or NT-3 is not responsible for the oestrogen-induced denervation. Oestrogen also raised BDNF protein and mRNA in myometrium and endometrium. To assess whether increased BDNF affects uterine receptivity to sympathetic outgrowth, sympathetic ganglion explants were co-cultured with myometrium. Myometrium from ovariectomized rats induced neuritogenesis in oestrogen-free conditions, and this was abolished when BDNF was added to the medium. Neuritogenesis induced by ovariectomized myometrium was suppressed by oestrogen, and restored by a BDNF function-blocking antibody. To determine if target BDNF synthesis is required for oestrogen to suppress sympathetic neurite outgrowth, uteri from wild-type mice and mice homozygous or heterozygous for recombinant mutations of the BDNF gene were cultured with rat sympathetic ganglia. Neuritogenesis induced by wild-type uteri was diminished by oestrogen. Neurite formation in the presence of homozygous BDNF mutant uteri was not affected by oestrogen, but was lower than that of wild-type mice. Uteri from mice heterozygous for the BDNF mutation, who have reduced BDNF synthesis, showed normal neuritogenic properties, but were not affected by oestrogen. These findings suggest that oestrogen alters neuritogenic properties of the rodent uterus by regulating BDNF synthesis, which inhibits sympathetic neurite outgrowth.

Sympathetic neurons synthesize and secrete pro-nerve growth factor protein

Postmitotic sympathetic neuronal survival is dependent upon nerve growth factor (NGF) provided by peripheral targets, and this dependency serves as a central tenet of the neurotrophic hypothesis. In some other systems, NGF has been shown to play an autocrine role, although the pervasiveness and significance of this phenomenon within the nervous system remain unclear. We show here that rat sympathetic neurons synthesize and secrete NGF. NGF mRNA is expressed in nearly half of superior cervical ganglion sympathetic neurons at embryonic day 17, rising to over 90% in the early postnatal period, and declining in the adult. Neuronal immunoreactivity is reduced when retrograde transport is interrupted by axotomy, but persists in a subpopulation of neurons despite diminished mRNA expression, suggesting that intrinsic protein synthesis occurs. Cultured neonatal neurons express NGF mRNA, which is maintained even when they are undergoing apoptosis. To determine which NGF isoforms are secreted, we performed metabolic labeling and immunoprecipitation of NGF-immunoreactive proteins synthesized by cultured NGF-dependent and -independent neurons. Conditioned medium contained high molecular weight NGF precursor proteins, which varied depending upon the state of NGF dependence. Mature NGF was undetectable by these methods. High molecular weight NGF isoforms were also detected in ganglion homogenates, and persisted at diminished levels following axotomy. We conclude that sympathetic neurons express NGF mRNA, and synthesize and secrete pro-NGF protein. These findings suggest that a potential NGF-sympathetic neuron autocrine loop may exist in this prototypic target-dependent system, but that the secreted forms of this neurotrophin apparently do not support neuronal survival.

Endogenously produced neurotrophins regulate survival and differentiation of cortical progenitors via distinct signaling pathways

Cultured embryonic cortical progenitor cells will mimic the temporal differentiation pattern observed in vivo, producing neurons first and then glia. Here, we investigated the role of two endogenously produced growth factors, the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 (NT-3), in the early progenitor-to-neuron transition. Cultured cortical progenitors express BDNF and NT-3, as well as their receptors TrkB (tyrosine kinase receptor B) and TrkC. Inhibition of these endogenously expressed neurotrophins using function-blocking antibodies resulted in a marked decrease in the survival of cortical progenitors, accompanied by decreased proliferation and inhibition of neurogenesis. Inhibition of neurotrophin function also suppressed the downstream Trk receptor signaling pathways, PI3-kinase (phosphatidyl inositol-3-kinase) and MEK-ERK (MAP kinase kinase-extracellular signal-regulated kinase), indicating the presence of autocrine-paracrine neurotrophin:Trk receptor signaling in these cells. Moreover, specific inhibition of these two Trk signaling pathways led to distinct biological effects; inhibition of PI3-kinase decreased progenitor cell survival, whereas inhibition of MEK selectively blocked the generation of neurons, with no effects on survival or proliferation. Thus, neurotrophins made by cortical progenitor cells themselves signal through the TrkB and TrkC receptors to mediate cortical progenitor cell survival and neurogenesis via two distinct downstream signaling pathways.

Reduced age-related plasticity of neurotrophin receptor expression in selected sympathetic neurons of the rat

Selective vulnerability of particular groups of neurons is a characteristic of the aging nervous system. We have studied the role of neurotrophin (NT) signalling in this phenomenon using rat sympathetic (SCG) neurons projecting to cerebral blood vessels (CV) and iris which are, respectively, vulnerable to and protected from atrophic changes during old age. RT-PCR was used to examine NT expression in iris and CV in 3- and 24-month-old rats. NGF and NT3 expression in iris was substantially higher compared to CV; neither target showed any alterations with age. RT-PCR for the principal NT receptors, trkA and p75, in SCG showed increased message during early postnatal life. However, during mature adulthood and old age, trkA expression remained stable while p75 declined significantly over the same period. In situ hybridization was used to examine receptor expression in subpopulations of SCG neurons identified using retrograde tracing. Eighteen to 20 h following local treatment of iris and CV with NGF, NT3 or vehicle, expression of NT receptor protein and mRNA was higher in iris- compared with CV-projecting neurons from both young and old rats. NGF and NT3 treatment had no effect on NT receptor expression in CV-projecting neurons at either age. However, similar treatment up-regulated p75 and trkA expression in iris-projecting neurons from 3-month-old, but not 24-month-old, rats. We conclude that lifelong exposure to low levels of NTs combined with impaired plasticity of NT receptor expression are predictors of neuronal vulnerability to age-related atrophy.

Regional expression of p75NTR contributes to neurotrophin regulation of cerebellar patterning

Neurotrophins were initially identified as critical regulators of neuronal survival. However, these factors have many additional functions. In the developing cerebellum the roles of the neurotrophins BDNF and NT3 include a surprising effect on patterning, as revealed by changes in foliation in neurotrophin-deficient mice. Here we examine the potential role of p75NTR in cerebellar development and patterning. We show that p75NTR is expressed at highest levels in the region of the cerebellum where foliation is altered in BDNF and NT3 mutants. Although the cerebellar phenotype of p75NTR mutant animals is indistinguishable from wild type, mutation of p75NTR in BDNF heterozygotes results in defects in foliation and in Purkinje cell morphologic development. Taken together, these data suggest that p75NTR activity is critical for cerebellar development under pathologic circumstances where neurotrophin levels are reduced.

Neurotrophic control of ovarian development

Substantial evidence now exists indicating that the neurotrophins, a family of growth factors required for the survival, development, and differentiation of various neuronal populations of the nervous system, are also important for the development of nonneuronal tissues. Such a function was first suggested by studies showing the presence of high-affinity neurotrophin receptors in a variety of nonneuronal tissues including those of the cardiovascular, endocrine, immune, and reproductive systems. Within the latter, the gonads appear to be a preferential site of neurotrophin action as suggested by the presence in the mammalian ovary of at least four of the five known neurotrophins and all of the neurotrophin receptors thus far identified. While the various functions that the neurotrophins may have in the ovary are still being elucidated, it is now clear that in addition to recruiting the ovarian innervation, they play a direct role in the regulation of two different maturational periods that are critical for the acquisition of female reproductive function: early follicular development and ovulation. Neurotrophins facilitate the development of newly formed follicles by promoting the initial differentiation and the subsequent growth of primordial follicles. These actions appear to be related to the ability of neurotrophins to sustain the proliferation of both mesenchymal and granulosa cells, and to induce the synthesis of follicle stimulating hormone (FSH) receptors. At the time of the first ovulation, neurotrophins contribute to the ovulatory cascade by increasing prostaglandin E(2) release, reducing gap junction communication, and inducing cell proliferation within the thecal compartment of preovulatory follicles.

SHP-2 mediates target-regulated axonal termination and NGF-dependent neurite growth in sympathetic neurons

The tyrosine phosphatase SHP-2 has been implicated in a variety of signaling pathways, including those mediated by neurotrophins in neurons. To examine the role of SHP-2 in the development of sympathetic neurons, we inhibited the function of SHP-2 in transgenic mice by overexpressing a catalytically inactive SHP-2 mutant under the control of the human dopamine beta-hydroxylase promoter. Expression of mutant SHP-2 did not influence the survival, axon initiation, or pathfinding abilities of the sympathetic neurons. However, mutant SHP-2 expression resulted in an overproduction of sympathetic fibers in sympathetic target organs. This was due to interference with SHP-2 function, as overexpression of wild type SHP-2 had no such effect. In vitro, NGF-dependent neurite growth was inhibited in neurons expressing mutant SHP-2 but not in those expressing wild type SHP-2. Mutant (but not wt) SHP-2 expression also inhibited NGF-stimulated ERK activation. The NGF-dependent survival pathway was less affected than the neurite growth pathway. Our results suggest that NGF-regulated axon growth signals, and to a lesser degree survival signals, are mediated through a SHP-2-dependent pathway in sympathetic neurons. The increased sympathetic innervation in target tissues of neurons expressing mutant SHP-2 may result from interference with normal "stop" signals dependent on signaling by gradients of NGF.

The p75 receptor transduces the signal from myelin-associated glycoprotein to Rho

Myelin-associated glycoprotein (MAG) is a potent inhibitor of neurite outgrowth from a variety of neurons. The receptor for MAG or signals that elicit morphological changes in neurons remained to be established. Here we show that the neurotrophin receptor p75 (p75(NTR)) is the signal transducing element for MAG. Adult dorsal root ganglion neurons or postnatal cerebellar neurons from mice carrying a mutation in the p75(NTR) gene are insensitive to MAG with regard to neurite outgrowth. MAG activates small GTPase RhoA, leading to retarded outgrowth when p75(NTR)) is present. Colocalization of p75(NTR) and MAG binding is seen in neurons. Ganglioside GT1b, which is one of the binding partners of MAG, specifically associates with p75(NTR). Thus, p75(NTR) and GT1b may form a receptor complex for MAG to transmit the inhibitory signals in neurons.

Control of postganglionic neurone phenotype by the rat pineal gland

As neurones develop they are faced with choices as to which genes to express, to match their final phenotype to their role in the nervous system. A number of processes can guide these decisions. Within the autonomic and sensory nervous systems, there are a handful of examples that suggest that one mechanism that may match phenotype to function is the presence of target-derived differentiation factors. We tested whether the rat pineal gland controls the expression of a neuropeptide (neuropeptide Y) and a calcium-binding protein (calbindin) in sympathetic postganglionic neurones that innervate it. We first showed that the chemical phenotype of sympathetic neurones innervating the rat pineal includes the expression of both neuropeptide Y and the calcium-binding protein, calbindin. After transplanting the pineal gland of neonatal rats into the submandibular salivary gland of neonatal hosts, it was innervated by sympathetic axons from the surrounding salivary gland tissue, which do not normally express neuropeptide Y and calbindin. The presence of the pineal gland led to the appearance of neuropeptide Y and calbindin in many of the postganglionic neurones that innervated the graft. From these findings we suggest that, like the rodent sweat gland, the pineal gland generates a signal that can direct the neurochemical phenotype of innervating sympathetic neurones.

Testosterone and nerve growth factor have distinct but interacting effects on structure and neurotransmitter expression of adult pelvic ganglion cells in vitro

Circulating testosterone has potent effects on the structure and function of many pelvic ganglion cells in adult rats in vivo. However not all androgen-sensitive pelvic neurones possess androgen receptors and testosterone effects may therefore be indirect, by an action on the target organs. Here we have examined if testosterone influences neuronal structure in vitro in pelvic ganglion cells cultured from adult male rats. We have also used multiple label immunofluorescence to monitor the expression of transmitter-synthesising enzymes and peptides under various culture conditions. Testosterone was a more potent stimulant of noradrenergic soma growth in culture than nerve growth factor. Whereas nerve growth factor increased the number, branching and length of neurites, testosterone stimulated growth of a small number of very short processes, each of which bore numerous short protrusions. Testosterone also impeded the longer neurite growth induced by nerve growth factor. Many pelvic ganglion cells altered their expression of transmitters/neuropeptides under different culture conditions. In particular, under control conditions or during nerve growth factor treatment, vasoactive intestinal peptide was up-regulated in noradrenergic and cholinergic neurones; testosterone impeded this up-regulation in noradrenergic neurones. Choline acetyltransferase immunoreactivity could only be visualised when nerve growth factor was present in the cultures, and cholinergic neurones showed less neurite outgrowth than noradrenergic neurones under all culture conditions. Nerve growth factor did not stimulate levels of this enzyme as strongly if testosterone was present. This study has shown that testosterone has potent effects on the structure of many pelvic ganglion cells in vitro. It is possible that these effects are mediated indirectly, e.g. by stimulating glial-derived substances, however our results suggest that the effects are not mediated by nerve growth factor. The results also show that testosterone influences some of the actions of nerve growth factor, suggesting that there may be complex interactions between steroid signalling and neurotrophic factors in maintaining neuronal structure and function in vivo.

A dose-dependent facilitation and inhibition of peripheral nerve regeneration by brain-derived neurotrophic factor

The time-dependent decline in the ability of motoneurons to regenerate their axons after axotomy is one of the principle contributing factors to poor functional recovery after peripheral nerve injury. A decline in neurotrophic support may be partially responsible for this effect. The up-regulation of BDNF after injury, both in denervated Schwann cells and in axotomized motoneurons, suggests its importance in motor axonal regeneration. In adult female Sprague-Dawley rats, we counted the number of freshly injured or chronically axotomized tibial motoneurons that had regenerated their axons 1 month after surgical suture to a freshly denervated common peroneal distal nerve stump. Motor axonal regeneration was evaluated by applying fluorescent retrograde neurotracers to the common peroneal nerve 20 mm distal to the injury site and counting the number of fluorescently labelled motoneurons in the T11-L1 region of the spinal cord. We report that low doses of BDNF (0.5-2 microg/day for 28 days) had no detectable effect on axonal regeneration after immediate nerve repair, but promoted axonal regeneration of motoneurons whose regenerative capacity was reduced by chronic axotomy 2 months prior to nerve resuture, completely reversing the negative effects of delayed nerve repair. In contrast, high doses of BDNF (12-20 microg/day for 28 days) significantly inhibited motor axonal regeneration, after both immediate nerve repair and nerve repair after chronic axotomy. The inhibitory actions of high dose BDNF could be reversed by functional blockade of p75 receptors, thus implicating these receptors as mediators of the inhibitory effects of high dose exogenous BDNF.

BDNF/trkB signaling in the developmental sculpting of visual connections

Neurotrophins are a family of secreted molecules that have multiple, profound actions on the structure and function of both developing and mature neurons. Neurotrophins exert their influences by signaling through the trk family of receptor tyrosine kinases and the p75 low affinity neurotrophin receptor. Here we review the contributions of neurotrophins to the development of neural circuitry in the mammalian visual system. We emphasize: (1) the role of neurotrophins as components of the cellular mechanisms by which neuroelectric activity sculpts pattern of brain connectivity; and (2) the results of recent experiments suggesting that the trafficking of neurotrophin proteins may be activity dependent.

TrkA mediates developmental sympathetic neuron survival in vivo by silencing an ongoing p75NTR-mediated death signal

Developmental sympathetic neuron death is determined by functional interactions between the TrkA/NGF receptor and the p75 neurotrophin receptor (p75NTR). A key question is whether p75NTR promotes apoptosis by directly inhibiting or modulating TrkA activity, or by stimulating cell death independently of TrkA. Here we provide evidence for the latter model. Specifically, experiments presented here demonstrate that the presence or absence of p75NTR does not alter Trk activity or NGF- and NT-3-mediated downstream survival signaling in primary neurons. Crosses of p75NTR-/- and TrkA-/- mice indicate that the coincident absence of p75NTR substantially rescues TrkA-/- sympathetic neurons from developmental death in vivo. Thus, p75NTR induces death regardless of the presence or absence of TrkA expression. These data therefore support a model where developing sympathetic neurons are "destined to die" by an ongoing p75NTR-mediated apoptotic signal, and one of the major ways that TrkA promotes neuronal survival is by silencing this ongoing death signal.

The neurotrophin receptors, trkB and p75, differentially regulate motor axonal regeneration

Neurotrophic factors that support neuronal survival are implicated in axonal regeneration after injury. Specifically, a strong role for BDNF in motor axonal regeneration has been suggested based on its pattern of expression after injury, as well as the expression of its receptors, trkB and p75. Despite considerable in vitro evidence, which demonstrate specific and distinct physiological responses elicited following trkB and p75 activation, relatively little is known about the function of these receptors in vivo. To investigate the roles of the trkB and p75 receptors in motor axonal regeneration, we have used a tibial (TIB)- common peroneal (CP) cross suture paradigm in p75 homozygous (-/-) knockout mice, trkB heterozygous (+/-) knockout mice, as well as in their wild-type controls. Contralateral intact TIB motoneurons, and axotomized TIB motoneurons that regenerated their axons 10 mm into the CP distal nerve stump were identified by fluorescent retrograde tracers and counted in the T11-L1 spinal segments. Regeneration was evaluated 2, 3, 4, 6, and 8 weeks after nerve repair. Compared to wild-type animals, there are significantly fewer intact TIB motoneurons in p75 (-/-), but not trkB (+/-) mice. The number of motoneurons that regenerated their axons was significantly increased in the p75 (-/-) knockout mice, but significantly attenuated in the trkB (+/-) mice compared to wild-type controls. These results suggest that p75 is important for motoneuronal survival during development, but p75 expression after injury serves to inhibit motor axonal regeneration. In addition, full expression of trkB is critical for complete axonal regeneration to proceed.

Developmental changes in BDNF protein levels in the hamster retina and superior colliculus

Quantitative studies of ontogenetic changes in the levels of brain-derived neurotrophic factor (BDNF) mRNA and its effector, BDNF protein, are not available for the retinal projection system. We used an electrochemiluminescence immunoassay to measure developmental changes in the tissue concentration of BDNF within the hamster retina and superior colliculus (SC). In the SC, we first detected BDNF (about 9 pg/mg tissue) on embryonic day 14 (E14). BDNF protein concentration in the SC rises about fourfold between (E14) and postnatal day 4 (P4), remains at a plateau through P15, then declines by about one-third to attain its adult level by P18. By contrast, BDNF protein concentration in the retina remains low (about 1 pg/mg tissue) through P12, then increases 4.5-fold to attain its adult level on P18. The developmental changes in retinal and collicular BDNF protein concentrations are temporally correlated with multiple events in the structural and functional maturation of the hamster retinal projection system. Our data suggest roles for BDNF in the cellular mechanisms underlying some of these events and are crucial to the design of experiments to examine those roles.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

p75 Co-receptors regulate ligand-dependent and ligand-independent Trk receptor activation, in part by altering Trk docking subdomains

Neurotrophins signal via Trk tyrosine kinase receptors and a common receptor called p75. Nerve growth factor is the cognate ligand for TrkA, brain-derived neurotrophic factor for TrkB, and neurotrophin-3 (NT-3) for TrkC. NT-3 also binds TrkA and TrkB as a heterologous ligand. All neurotrophins bind p75, which regulates ligand affinity and Trk signals. Trk extracellular domain has five subdomains: a leucine-rich motif, two cysteine-rich clusters, and immunoglobulin-like subdomains IgG-C1 and IgG-C2. The IgG-C1 subdomain is surface exposed in the tertiary structure and regulates ligand-independent activation. The IgG-C2 subdomain is less exposed but regulates cognate ligand binding and Trk activation. NT-3 as a heterologous ligand of TrkA and TrkB optimally requires the IgG-C2 but also binds other subdomains of these receptors. When p75 is co-expressed, major changes are observed; NGF-TrkA activation can occur also via the cysteine 1 subdomain, and brain-derived neurotrophic factor-TrkB activation requires the TrkB leucine-rich motif and cysteine 2 subdomains. We propose a two-site model of Trk binding and activation, regulated conformationally by the IgG-C1 subdomain. Moreover, p75 affects Trk subdomain utilization in ligand-dependent activation, possibly by conformational or allosteric control.

The extracellular domain of p75NTR is necessary to inhibit neurotrophin-3 signaling through TrkA

The TrkA receptor is activated primarily by nerve growth factor (NGF), but it can also be activated by high concentrations of neurotrophin 3 (NT-3). The pan-neurotrophin receptor p75(NTR) strongly inhibits activation of TrkA by NT-3 but not by NGF. To examine the role of p75(NTR) in regulating the specificity of TrkA signaling, we expressed both receptors in Xenopus oocytes. Application of NGF or NT-3 to oocytes expressing TrkA alone resulted in efflux of (45)Ca(2+) by a phospholipase C-gamma-dependent pathway. Coexpression of p75(NTR) with TrkA inhibited (45)Ca(2+) efflux in response to NT-3 but not NGF. The inhibitory effect on NT-3 activation of TrkA increased with increasing expression of p75(NTR). Coexpression of a truncated p75(NTR) receptor lacking all but the first 9 amino acids of the cytoplasmic domain inhibited NT-3 stimulation of (45)Ca(2+) efflux, whereas coexpression of an epidermal growth factor receptor/p75(NTR) chimera (extracellular domain of epidermal growth factor receptor with transmembrane and cytoplasmic domains of p75(NTR)) did not inhibit NT-3 signaling through TrkA. These studies demonstrated that the extracellular domain of p75(NTR) was necessary to inhibit NT-3 signaling through TrkA. Remarkably, p75(NTR) binding to NT-3 was not required to prevent signaling through TrkA, since occupying p75(NTR) with brain-derived neurotrophic factor or anti-p75 antibody (REX) did not rescue the ability of NT-3 to activate (45)Ca(2+) efflux. These data suggested a physical association between TrkA and p75(NTR). Documenting this physical interaction, we showed that p75(NTR) and TrkA could be coimmunoprecipitated from Xenopus oocytes. Our results suggest that the interaction of these two receptors on the cell surface mediated the inhibition of NT-3-activated signaling through TrkA.

Contribution of BDNF-mediated inhibition in patterning avian skin innervation

Multiple factors are involved in the development and regulation of sensory innervation in skin. The findings we report here suggest that brain-derived neurotrophic factor (BDNF)-mediated inhibition may play an important role in determining the pattern of sensory innervation in avian skin. In birds, cutaneous innervation is restricted to dermis, where axons form a ring of innervation around the base of each feather. Here we show that both BDNF message and protein are more abundant in avian epidermis than dermis when innervation is being established; the BDNF in dermis is localized to feather buds. In vitro, BDNF caused growth cones of NGF-dependent dorsal root ganglion neurons to collapse. Similarly, outgrowth of neurites toward BDNF-secreting fibroblasts was inhibited. The inhibitory effects of BDNF appear to be mediated by the low-affinity p75 neurotrophin receptor, rather than a trk receptor. Thus, the distribution of BDNF in embryonic avian skin and the inhibitory effects of BDNF on cutaneous neurites in vitro suggest that BDNF may be important in restricting axons from entering the epidermis and the core of feather buds during development in vivo.

Phosphoinositide 3-kinase regulates crosstalk between Trk A tyrosine kinase and p75(NTR)-dependent sphingolipid signaling pathways

The mechanism of crosstalk between signaling pathways coupled to the Trk A and p75(NTR) neurotrophin receptors in PC12 cells was examined. In response to nerve growth factor (NGF), Trk A activation inhibited p75(NTR)-dependent sphingomyelin (SM) hydrolysis. The phosphoinositide 3-kinase (PI 3-kinase) inhibitor, LY294002, reversed this inhibition suggesting that Trk A activation of PI 3-kinase is necessary to inhibit sphingolipid signaling by p75(NTR). In contrast, SM hydrolysis induced by neurotrophin-3 (NT-3), which did not activate PI-3 kinase, was uneffected by LY294002. However, transient expression of a constituitively active PI 3-kinase inhibited p75(NTR)-dependent SM hydrolysis by both NGF and NT-3. Intriguingly, NGF induced an association of activated PI 3-kinase with acid sphingomyelinase (SMase). This interaction localized to caveolae-related domains and correlated with a 50% decrease in immunoprecipitated acid SMase activity. NGF-stimulated PI 3-kinase activity was necessary for inhibition of acid SMase but was not required for ligand-induced association of the p85 subunit of PI 3-kinase with the phospholipase. Finally, this interaction was specific for NGF since EGF did not induce an association of PI 3-kinase with acid SMase. In summary, our data suggest that PI 3-kinase regulates the inhibitory crosstalk between Trk A tyrosine kinase and p75(NTR)-dependent sphingolipid signaling pathways and that this interaction localizes to caveolae-related domains.

Differential regulation of survival and growth in adult sympathetic neurons: Aninvitro study of neurotrophin responsiveness

The survival and growth of embryonic and postnatal sympathetic neurons is dependent on both NGF and NT3. While it has been established that adult sensory neurons survive independently of neurotrophins, the case is less clear for adult sympathetic neurons, where the studies of survival responses to neurotrophins have relied upon using long-term cultures of embryonic neurons. We have previously established a method to culture purified young (7 day) and adult (12 week) sympathetic neurons isolated from adult rat superior cervical ganglia (SCG) in order to examine their survival and growth responses to neurotrophins. We now show that by 12 weeks after birth virtually all neurons (90%) survive for 24 h in the absence of neurotrophins. Neuron survival is unaffected by treatment with anti-NGF antibodies (anti-NGF) or with the tyrosine kinase inhibitor, K252a, confirming the lack of dependence on extrinsic neurotrophins. Duration of neuron survival in culture increases significantly between E19 and day 7 and week 12 posnatally, and is similarly unaffected by the presence of anti-NGF or K252a. Saturating concentrations of NGF and NT3 are equipotent in promoting neurite extension and branching. However, we find that NGF is more potent than NT3 in promoting neurite growth, irrespective of postnatal age. The growth-promoting effects of NGF and NT3 are almost entirely blocked by K252a, demonstrating that these effects are mediated via activation of Trk receptors, which therefore appear to remain crucial to plasticity of adult neurons. Our results indicate that maturing neurons acquire protection against cell death, induced in the absence of neurotrophin, while retaining their growth responsiveness to these factors.

Neurotrophins: roles in neuronal development and function

Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems. Neurotrophins activate two different classes of receptors, the Trk family of receptor tyrosine kinases and p75NTR, a member of the TNF receptor superfamily. Through these, neurotrophins activate many signaling pathways, including those mediated by ras and members of the cdc-42/ras/rho G protein families, and the MAP kinase, PI-3 kinase, and Jun kinase cascades. During development, limiting amounts of neurotrophins function as survival factors to ensure a match between the number of surviving neurons and the requirement for appropriate target innervation. They also regulate cell fate decisions, axon growth, dendrite pruning, the patterning of innervation and the expression of proteins crucial for normal neuronal function, such as neurotransmitters and ion channels. These proteins also regulate many aspects of neural function. In the mature nervous system, they control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.

Primary olfactory axons form ectopic glomeruli in mice lacking p75NTR

The restricted expression of the low affinity nerve growth factor receptor p75NTR by olfactory ensheathing cells suggests that this molecule is involved in the development of the olfactory nerve pathway. To begin to understand the role of p75NTR, we examined the development of the primary olfactory system in p75NTR(-/-) and wild-type mice. Our results demonstrate that, although p75NTR is not essential for the initial assembly of the olfactory nerve, it plays an important role in the postnatal maturation of the olfactory bulb. In the absence of p75NTR, there is exuberant growth of some primary olfactory axons into the olfactory bulb. These axons either aberrantly bypass the glomerular layer and project into deeper lamina or grow into an abnormal bleb of tissue protruding from the medial surface of the dorsocaudal olfactory bulb. These blebs become apparent in neonatal mice and contain axons expressing olfactory marker protein that form ectopic glomerular-like tufts. Histochemical staining with the plant lectin Dolichos biflorus agglutinin revealed that axons sorted out and selectively converged on glomeruli within these blebs. Our results suggest that p75NTR indirectly influences axon growth but not glomerular targeting and plays a role in the postnatal maturation of laminar cytoarchitecture in the olfactory bulb.

Spontaneous synaptogenesis in ex vivo sympathetic ganglion and the blockade by serum treatment

Central denervation for more than 1 month has been shown to cause an increase in the number of adrenergic synapses in sympathetic ganglia in vivo. Here, we report several lines of evidence that adrenergic synapses may be generated de novo in ex vivo superior cervical ganglion (SCG) of adult rats only several hours after the isolation. Structures immunoreactive for synaptophysin, a marker of presynaptic elements, were drastically decreased 6 days after the preganglionic denervation. A significant increase in number of synaptophysin positive boutons was observed over 3-8 hours in the denervated SCGs maintained ex vivo at 36 degrees C in oxygenated physiologic saline, and this increase was blocked by adding normal serum in the saline. Electron microscopic analysis confirmed that the number of adrenergic synapses specifically labeled with 5-hydroxydopamine was increased by several-fold under the same condition. Intracellular labeling of SCG neurons revealed an increase in the incidence (from 8 to 50%) of neurons having dendritic plexus after the in vitro incubation. No evidence of axonal sprouting within the ganglion was observed. Intracellular recordings from single neurons of denervated SCGs revealed that maximum amplitudes of inhibitory postsynaptic potentials, which were completely blocked by yohimbine, an alpha2-adrenoceptor antagonist, in response to focal stimulation were increased over the several hours. These results suggest that dendrites of SCG neurons rapidly develop and exhibit local efferent characteristics that underlie the inhibitory synaptic transmission once they are subjected to serum deprivation.

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.

p75 reduces TrkB tyrosine autophosphorylation in response to brain-derived neurotrophic factor and neurotrophin 4/5

Neurotrophins mediate their signals through two different receptors: the family of receptor tyrosine kinases, Trks, and the low affinity pan-neurotrophin receptor p75. Trk receptors show more restricted ligand specificity, whereas all neurotrophins are able to bind to p75. One important function of p75 is the enhancement of nerve growth factor signaling via TrkA by increasing TrkA tyrosine autophosphorylation. Here, we have examined the importance of p75 on TrkB- and TrkC-mediated neurotrophin signaling in an MG87 fibroblast cell line stably transfected with either p75 and TrkB or p75 and TrkC, as well as in PC12 cells stably transfected with TrkB. In contrast to TrkA signaling, p75 had a negative effect on TrkB tyrosine autophosphorylation in response to its cognate neurotrophins, brain-derived neurotrophic factor and neurotrophin 4/5. On the other hand, p75 had no effect on TrkB or TrkC activation in neurotrophin 3 treatment. p75 did not effect extracellular signal-regulated kinase 2 tyrosine phosphorylation in response to brain-derived neurotrophic factor, neurotrophin 3, or neurotrophin 4/5. These results suggest that the observed reduction in TrkB tyrosine autophosphorylation caused by p75 does not influence Ras/mitogen-activated protein kinase signaling pathway in neurotrophin treatments.

Regulation of NGF-family ligands and receptors in adulthood and senescence: correlation to degenerative and regenerative changes in cutaneous innervation

During development, a highly differential neurotrophin dependency is reported for various types of nerve endings in the whisker follicle. To what extent these dependencies extend and play a role in adulthood is largely unresolved. We show here, using in situ hybridization and immunohistochemistry that the expression of neurotrophins and trk/p75 receptors persists in adulthood. As suggested by their expression profiles, many classes of cutaneous nerve endings disclose similar ligand-receptor dependencies in adult animals as during development, while other populations appear to switch their dependency. Furthermore, our data suggest that sensory endings that have a high turnover due to mechanical wear and tear, e. g. Merkel cell-neurite complexes at the level of ring sinus show a more complex ligand-receptor expression phenotype than do endings with a less vulnerable location, e.g. the Merkel cell-neurite complexes at the rete ridge collar. Thus, neurotrophin-3 (NT3)/trkA signalling is suggested to be important for a continuous terminal plasticity of Merkel cell-neurite complexes at the level of ring sinus in adulthood. Evidence supporting a role for neurotrophin signalling in maintaining the adult cutaneous innervation also comes from the close correlation between altered ligand-receptor expression(s) and axonal/terminal aberrations in senescence. Thus, an ageing-related decrease in target neurotrophin expression, notably NT3 and NT4, results in a site-specific loss of sensory terminals concomitant with an aberrant growth of regenerating/sprouting axons into new target fields. Ageing of the cutaneous innervation, manifested in degenerative and regenerative events, seems strongly associated with changes in neurotrophic interactions between sensory neurons and target tissues.

Neurotrophin signal transduction in the nervous system

Neurotrophins use two types of receptors, the Trk tyrosine kinase receptors and the p75 neurotrophin receptor (p75NTR), to regulate the growth, development, survival and repair of the nervous system. These receptors can either collaborate with or inhibit each other's actions to mediate neurotrophin effects. The development and survival of neurons is thus based upon the functional interplay of the signals generated by Trk and p75NTR. In the past two years, the signaling pathways used by these receptors, including Akt and MAPK-induced signaling via Trk, and JNK, p53, and NF-kappaB signaling via p75NTR, have been identified. In addition, a number of novel p75NTR-interacting proteins have been identified that transmit growth, survival, and apoptotic signals.

Neurotrophic and cell-cell dependent control of early follicular development

Neurotrophins (NTs) and their receptors play an essential role in the differentiation and survival of defined neuronal populations of the central and peripheral nervous systems. Their actions, however, do not appear to be limited to the nervous system, as both NTs and their receptors have been found in non neuronal cells, including cells of the endocrine system. At least four of the five known neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), and their receptors (p75 NTR, trkA, trkB and trkC) are present in the developing ovary. Using mice carrying null mutations of the genes encoding neurotrophins (NGF, NT-4, BDNF) or the receptor that mediates the actions of NT-4 and BDNF (trkB), we have obtained initial results consistent with the notion that neurotrophins are required for the growth of primordial follicles. NGF-deficient mice show a decreased formation of both primary and secondary preantral follicles. Null mutation of the NT-4 gene failed to affect either folliculogenesis or follicular development. However, formation of primary and secondary follicles was compromised in mice carrying a null mutation of both the NT-4 and BDNF genes, suggesting compensation of function by BDNF in NT-4 knockouts. Support for this concept is provided by the similar deficiency in follicular growth observed in animals carrying a null mutation of the gene encoding trkB, the receptors mediating NT-4 and BDNF actions. Initial experiments, using differential display, to isolate genes that may be involved in the process of folliculogenesis and/or early follicular development, resulted in the isolation of a recently identified cell adhesion molecule and a novel transcription factor originally shown to induce cell transformation. It thus appears that formation and development of mammalian follicles requires the concerted action of genes originally thought to be only involved in cell differentiation/survival of neuronal cells, and genes that may control the growth, differentiation, and cell-cell interactions of somatic and germ cells in the ovary.

Neurotrophins: neurotrophic modulation of neurite growth

As well as regulating neuronal survival, neurotrophins control the growth of axons and dendrites. New light has been shed on the mechanism of this latter control process by the discovery that the common neurotrophin receptor p75(NTR) interacts in a ligand-dependent manner with RhoA, a known regulator of actin assembly.

Brain-derived neurotrophic factor induces excitotoxic sensitivity in cultured embryonic rat spinal motor neurons through activation of the phosphatidylinositol 3-kinase pathway

Neurotrophic factors (NTFs) can protect against or sensitize neurons to excitotoxicity. We studied the role played by various NTFs in the excitotoxic death of purified embryonic rat motor neurons. Motor neurons cultured in brain-derived neurotrophic factor, but not neurotrophin 3, glial-derived neurotrophic factor, or cardiotrophin 1, were sensitive to excitotoxic insult. BDNF also induces excitotoxic sensitivity (ES) in motor neurons when BDNF is combined with these other NTFs. The effect of BDNF depends on de novo protein and mRNA synthesis. Reagents that either activate or inhibit the 75-kDa NTF receptor p75NTR do not affect BDNF-induced ES. The low EC50 for BDNF-induced survival and ES suggests that TrkB mediates both of these biological activities. BDNF does not alter glutamate-evoked rises of intracellular Ca2+, suggesting BDNF acts downstream. Both wortmannin and LY294002, which specifically block the phosphatidylinositol 3-kinase (PI3K) intracellular signaling pathway in motor neurons, inhibit BDNF-induced ES. We confirm this finding using a herpes simplex virus (HSV) that expresses the dominant negative p85 subunit of PI3K. Infecting motor neurons with this HSV, but not a control HSV, blocks activation of the PI3K pathway and BDNF-induced ES. Through the activation of TrkB and the PI3K signaling pathway, BDNF renders developing motor neurons susceptible to glutamate receptor-mediated cell death.