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

p75NTR-dependent, myelin-mediated axonal degeneration regulates neural connectivity in the adult brain

Axonal degeneration is important during development but has not been thought to function in the intact mature nervous system. Here, we provide evidence that degeneration of adult axons occurs in the intact rodent brain through a p75 neurotrophin receptor (p75NTR)- and myelin-dependent mechanism. Specifically, we show that p75NTR-mediated axonal degeneration prevents septal cholinergic axons from aberrantly growing onto myelinated tracts in vivo or on a myelin substrate in culture. Myelin also triggers local degeneration of p75NTR-expressing sympathetic axons that is rescued by increasing TrkA signaling or elevating intracellular cyclic AMP. Myelin-mediated degeneration occurs when neurotrophins bind to p75NTR, and involves p75NTR-dependent sequestration of Rho guanine nucleotide dissociation inhibitor (Rho-GDI). Moreover, degeneration, but not growth inhibition, requires downstream activation of Rho and caspase-6. These data indicate that p75NTR maintains the specificity of neural connectivity by preventing inappropriate sprouting onto myelinated tracts and provide a physiological explanation for myelin inhibition after neural injury.

Carboxypeptidase E knockout mice exhibit abnormal dendritic arborization and spine morphology in central nervous system neurons

Carboxypeptidase E (CPE) is involved in maturation of neuropeptides and sorting of brain-derived neurotrophic factor (BDNF) to the regulated pathway for activity-dependent secretion from CNS neurons. CPE knockout (CPE-KO) mice have many neurological deficits, including deficits in learning and memory. Here, we analyzed the dendritic arborization and spine morphology of CPE-KO mice to determine a possible correlation of defects in such structures with the neurological deficits observed in these animals. Analysis of pyramidal neurons in layer V of cerebral cortex and in hippocampal CA1 region in 14-week-old CPE-KO mice showed more dendritic complexity compared with wild type (WT) mice. There were more dendritic intersections and more branch points in CPE-KO vs. WT neurons. Comparison of pyramidal cortical neurons in 6- vs. 14-week-old WT mice showed a decrease in dendritic arborization, reflecting the occurrence of normal dendritic pruning. However, this did not occur in CPE-KO neurons. Furthermore, analysis of spine morphology demonstrated a significant increase in the number of D-type spines regarded as nonfunctional in the cortical neurons of CPE-KO animals. Our findings suggest that CPE is an important, novel player in mediating appropriate dendritic patterning and spine formation in CNS neurons.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Extracellular signals regulating sympathetic neuron survival and target innervation during development

The comparative ease with which paravertebral sympathetic neurons are studied in vitro and in vivo at stages throughout their development has facilitated major advances in our understanding of several key aspects of neuronal development. Detailed anatomical descriptions of the in vivo development of these neurons, studies of the effects of various extracellular signalling molecules on these neurons in vitro and analysis of the sympathetic phenotype of relevant transgenic mice have provided an in-depth understanding of how different extracellular signals orchestrate sequential steps in the establishment and refinement of sympathetic innervation. In this review, I will document the roles of neurotrophic factors, cytokines and other extracellular signals in regulating sympathetic neuron survival and target innervation at sequential stages of development.

Autonomic innervation of the developing heart: origins and function

Maintenance of homeostatic circulation in mammals and birds is reliant upon autonomic innervation of the heart. Neural branches of mixed cellular origin and function innervate the heart at the arterial and venous poles as it matures, eventually coupling autonomic output to the cardiac components, including the conduction system. The development of neural identity is controlled by specific networks of genes and growth factors, whereas functional properties are governed by the use of different neurotransmitters. In this review, we summarize briefly the anatomic arrangement of the vertebrate autonomic nervous system and describe, in detail, the innervation of the heart. We discuss the timing of cardiac innervation in the chick and mouse, emphasizing the relationship of the cardiac neural networks to the anatomical structures within the heart. We also discuss the variable contribution of the neural crest to vagal cardiac nerves, and summarize the main neurotransmitters secreted by the developing sympathetic and parasympathetic autonomic divisions. We provide an overview of the main growth factor and gene families involved in neural development, discussing how these factors may impact upon the development of cardiac abnormalities in congenital syndromes associated with autonomic dysfunction.

Proteomic assessment of sympathetic ganglia from adult mice that possess null mutations of ExonIII or ExonIV in the p75 neurotrophin receptor gene

Neurotrophins, such as nerve growth factor (NGF), are capable of binding to the transmembrane p75 neurotrophin receptor (p75NTR), which regulates a variety of cellular responses including apoptosis and axonal elongation. While the development of mutant mouse strains that lack functional p75NTR expression has provided further insight into the importance of this neurotrophin receptor, there remains a paucity of information concerning how the loss of p75NTR expression may alter neural phenotypes. To address this issue, we assessed the proteome of the cervical sympathetic ganglia from two mutant lines of mice, which were compared to the ganglionic proteome of age-matched wild type mice. The ganglionic proteome of mice possessing two mutant alleles of either exonIII or exonIV for the p75NTR gene displayed detectable alterations in levels of Lamin A, tyrosine hydroxylase, and Annexin V, as compared to ganglionic proteome of wild type mice. Decreased expression of the basic isoform of tyrosine hydroxylase may be linked to perturbed NGF signaling in the absence of p75NTR in mutant mice. Stereological measurement showed significant increases in the number of sympathetic neurons in both lines of p75NTR-deficient mice, relative to wild type mice. This enhanced survival of sympathetic neurons coincides with shifts toward the more basic isoforms of Annexin V in mutant mice. This study, in addition to providing the first comparative proteomic assessment of sympathetic ganglia, sheds new light onto the phenotypic changes that occur as a consequence of a loss of p75NTR expression in adult mice.

Neurotrimin is an estrogen-regulated determinant of peripheral sympathetic innervation

Mechanisms underlying axon degeneration in peripheral neuropathies and during normal remodeling are poorly understood. Because estrogen induces widespread sympathetic axon degeneration in female reproductive tract smooth muscle, we surveyed estrogen-regulated genes in rat myometrium. Microarray analysis revealed that the neural cell adhesion protein neurotrimin (Ntm) was markedly up-regulated 6 hr and down-regulated 24 hr after injection of 17beta-estradiol, and real time RT-PCR confirmed this pattern of expression. Protein analysis by Western blotting showed that uterine Ntm protein is also up-regulated in vivo 6-24 hr following estrogen injection and that Ntm protein is increased selectively in the myometrium during the high-estrogen phase of the estrous cycle. Cultured myometrial smooth muscle cells display perinuclear accumulations of Ntm protein, and 17beta-estradiol also increases intracellular levels of Ntm and its secretion into the culture medium. To determine if neurotrimin is required for estrogen-induced sympathetic pruning, sympathetic neurons were cocultured with uterine smooth muscle cells transfected with siRNA directed against Ntm. Although estrogen inhibited neurite outgrowth in nontransfected cocultures, estrogen's ability to reduce sympathetic outgrowth was impaired substantially following Ntm down-regulation. This supports a role for neurotrimin in mediating estrogen-induced sympathetic pruning in some peripheral targets. Together with earlier studies, these findings support the idea that physiological sympathetic axon degeneration is a multifactorial process requiring dynamic regulation of multiple repellant proteins.

Modulation of rat parasympathetic cardiac ganglion phenotype and NGF synthesis by adrenergic nerves

Cardiac function is regulated by interactions among intrinsic and extrinsic autonomic neurons, and the mechanisms responsible for organizing these circuits are poorly understood. Parasympathetic neurons elsewhere synthesize the neurotrophin NGF, which may promote postganglionic axonal associations where parasympathetic axons inhibit sympathetic transmitter release. Previous studies have shown that parasympathetic NGF content and neurochemical phenotype are regulated by sympathetic innervation. In this study we assessed contributions of sympathetic input on cardiac ganglion neuronal phenotype and NGF expression. Because cardiac ganglia are reported to contain putative noradrenergic neurons, we eliminated sympathetic input both surgically (extrinsic) and chemically (extrinsic plus intrinsic). In controls, most cardiac ganglion neurons expressed vesicular acetylcholine transporter, frequently colocalized with vesicular monoamine transporter, but lacked catecholamine histofluorescence. Most cardiac ganglion neurons expressed NGF transcripts, and 40% contained mature and 47% proNGF immunoreactivity. Guanethidine treatment for 7 days decreased numbers of neurons expressing vesicular acetylcholine transporter, NGF transcripts and NGF immunoreactivity, but did not affect proNGF or vesicular monoamine transporter immunoreactivity. Stellate ganglionectomy had comparable effects on neurochemical phenotype and mature NGF immunoreactivity, but proNGF expression was additionally reduced. These findings show that individual cardiac ganglion neurons display markers of both cholinergic and noradrenergic transmission. Sympathetic noradrenergic innervation maintains levels of cholinergic but not noradrenergic marker protein. Sympathetic innervation also promotes cardiac ganglion neuronal NGF synthesis. Because chemical blockade of all noradrenergic transmission is no more effective than extrinsic sympathectomy, local intrinsic noradrenergic transmission is not a factor in regulating ganglion neuron phenotype.

Neurite outgrowth is enhanced by laminin-mediated down-regulation of the low affinity neurotrophin receptor, p75NTR

Laminin (LN), an extracellular matrix component, is a key factor in promoting axonal regeneration, coordinately regulating growth in conjunction with trophic signals provided by the neurotrophins, including nerve growth factor (NGF). This study investigated potential interactions between the LN and NGF-mediated signaling pathways in PC12 cells and primary neurons. Neurite outgrowth stimulated by NGF was enhanced on a LN substrate. Western blot analysis of pertinent signal transduction components revealed both enhanced phosphorylation of early signaling intermediates upon co-stimulation, and a LN-induced down-regulation of p75NTR which could be prevented by the addition of integrin inhibitory arginine-glycine-aspartate (RGD) peptides. This p75NTR down-regulation was associated with a LN-mediated up-regulation of PTEN and resulted in a decrease in Rho activity. Studies using over-expression or siRNA-mediated knock-down of PTEN demonstrate a consistent inverse relationship with p75NTR, and the over-expression of p75NTR impaired neurite outgrowth on a LN substrate, as well as resulting in sustained activation of Rho which is inhibitory to neurite outgrowth. p75NTR is documented for its role in the transduction of inhibitory myelin-derived signals, and our results point to extracellular matrix regulation of p75NTR as a potential mechanism to ameliorate inhibitory signaling leading to optimized neurite outgrowth.

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.

Developmental axon pruning mediated by BDNF-p75NTR-dependent axon degeneration

The mechanisms that regulate the pruning of mammalian axons are just now being elucidated. Here, we describe a mechanism by which, during developmental sympathetic axon competition, winning axons secrete brain-derived neurotrophic factor (BDNF) in an activity-dependent fashion, which binds to the p75 neurotrophin receptor (p75NTR) on losing axons to cause their degeneration and, ultimately, axon pruning. Specifically, we found that pruning of rat and mouse sympathetic axons that project to the eye requires both activity-dependent BDNF and p75NTR. p75NTR and BDNF are also essential for activity-dependent axon pruning in culture, where they mediate pruning by directly causing axon degeneration. p75NTR, which is enriched in losing axons, causes axonal degeneration by suppressing TrkA-mediated signaling that is essential for axonal maintenance. These data provide a mechanism that explains how active axons can eliminate less-active, competing axons during developmental pruning by directly promoting p75NTR-mediated axonal degeneration.

SIV infection decreases sympathetic innervation of primate lymph nodes: the role of neurotrophins

The sympathetic nervous system regulates immune responses in part through direct innervation of lymphoid organs. Recent data indicate that viral infections can alter the structure of lymph node innervation. To determine the molecular mechanisms underlying sympathetic denervation during Simian Immunodeficiency Virus (SIV) infection, we assessed the expression of neurotrophic factors and neuromodulatory cytokines within lymph nodes from experimentally infected rhesus macaques. Transcription of nerve growth factor (NGF), brain-derived neurotropic factor (BDNF) and neurotrophin-4 (NT4) decreased significantly in vivo during chronic SIV infection, whereas expression of the neuro-inhibitory cytokine interferon-gamma (IFN gamma) was up-regulated. Acute SIV infection of macaque leukocytes in vitro induced similar changes in the expression of neurotrophic and neuro-inhibitory factors, indicative of an innate immune response. Statistical mediation analyses of data from in vivo lymph node gene expression suggested that coordinated changes in expression of multiple neuromodulatory factors may contribute to SIV-induced depletion of catecholaminergic varicosities within lymphoid tissue. Given previous evidence that lymph node catecholaminergic varicosities can enhance SIV replication in vivo, these results are consistent with the hypothesis that reduced expression of neurotrophic factors during infection could constitute a neurobiological component of the innate immune response to viral infection.

Factors regulating vagal sensory development: potential role in obesities of developmental origin

Contributors to increased obesity in children may include perinatal under- or overnutrition. Humans and rodents raised under these conditions develop obesity, which like obesities of other etiologies has been associated with increased meal size. Since vagal sensory innervation of the gastrointestinal (GI) tract transmits satiation signals that regulate meal size, one mechanism through which abnormal perinatal nutrition could increase meal size is by altering vagal development, possibly by causing changes in the expression of factors that control it. Therefore, we have begun to characterize development of vagal innervation of the GI tract and the expression patterns and functions of the genes involved in this process. Important events in development of mouse vagal GI innervation occurred between midgestation and the second postnatal week, suggesting they could be vulnerable to effects of abnormal nutrition pre- or postnatally. One gene investigated was brain- derived neurotrophic factor (BDNF), which regulates survival of a subpopulation of vagal sensory neurons. BDNF was expressed in some developing stomach wall tissues innervated by vagal afferents. At birth, mice deficient in BDNF exhibited a 50% reduction of putative intraganglionic laminar ending mechanoreceptor precursors, and a 50% increase in axons that had exited fiber bundles. Additionally, BDNF was required for patterning of individual axons and fiber bundles in the antrum and differentiation of intramuscular array mechanoreceptors in the forestomach. It will be important to determine whether abnormal perinatal environments alter development of vagal sensory innervation of the GI tract, involving effects on expression of BDNF, or other factors regulating vagal development.

Transient receptor potential channels as novel effectors of brain-derived neurotrophic factor signaling: potential implications for Rett syndrome

In addition to their prominent role as survival signals for neurons in the developing nervous system, neurotrophins have established their significance in the adult brain as well, where their modulation of synaptic transmission and plasticity may participate in associative learning and memory. These crucial activities are primarily the result of neurotrophin regulation of intracellular Ca(2+) homeostasis and, ultimately, changes in gene expression. Outlined in the following review is a synopsis of neurotrophin signaling with a particular focus upon brain-derived neurotrophic factor (BDNF) and its role in hippocampal synaptic plasticity and neuronal Ca(2+) homeostasis. Neurotrophin signaling through tropomyosin-related kinase (Trk) and pan-neurotrophin receptor 75 kD (p75(NTR)) receptors are also discussed, reviewing recent results that indicate signaling through these two receptor modalities leads to opposing cellular outcomes. We also provide an intriguing look into the transient receptor potential channel (TRPC) family of ion channels as distinctive targets of BDNF signaling; these channels are critical for capacitative Ca(2+) entry, which, in due course, mediates changes in neuronal structure including dendritic spine density. Finally, we expand these topics into an exploration of mental retardation (MR), in particular Rett Syndrome (RTT), where dendritic spine abnormalities may underlie cognitive impairments. We propose that understanding the role of neurotrophins in synapse formation, plasticity, and maintenance will make fundamental contributions to the development of therapeutic strategies to improve cognitive function in developmental disorders associated with MR.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Site-specific interactions of neurotrophin-3 and fibroblast growth factor (FGF2) in the embryonic development of the mouse cochlear nucleus

Neurotrophins and FGF2 contribute to formation of the cochlea, but their roles in cochlear nucleus development are unknown. The effects of these factors may differ in the cochlea and cochlear nucleus, which may influence each other's development. It is important to analyze the effects of these factors on cellular structures at well-defined steps in the normal morphogenetic sequence. The present study used immunohistochemistry to localize factors in situ and to test hypotheses about their roles in an in vitro model. Specific antibody staining revealed that TrkC, the NT3 receptor, is present in neural precursors prior to embryonic day E11 until after birth. NT3 appeared in precursor cells during migration (E13-E15) and disappeared at birth. TrkC and NT3 occurred in the same structures, including growing axons, terminals, and their synaptic targets. Thus, NT3 tracks the migration routes and the morphogenetic sequences within a window defined by TrkC. In vitro, the cochlear nucleus anlage was explanted from E11 embryos. Cultures were divided into groups fed with defined medium, with or without FGF2, BDNF, and NT3 supplements, alone or in combinations, for 7 days. When neuroblasts migrated and differentiated, immunostaining was used for locating NT3 and TrkC in the morphogenetic sequence, bromodeoxyuridine for proliferation, and synaptic vesicle protein for synaptogenesis. By time-lapse imaging and quantitative measures, the results support the hypothesis that FGF2 promotes proliferation and migration. NT3 interacts with FGF2 and BDNF to promote neurite outgrowth, fasciculation, and synapse formation. Factors and receptors localize to the structural sites undergoing critical changes.

Does the p75 neurotrophin receptor mediate Abeta-induced toxicity in Alzheimer's disease?

Alzheimer's disease is characterized by the over-production and accumulation of amyloidogenic Abeta peptide, which can induce cell death in vitro. It has been suggested that the death signal could be transduced by the pan neurotrophin receptor (p75NTR). p75NTR is well known for its ability to mediate neuronal death in neurodegenerative conditions and is inextricably linked with changes that occur in Alzheimer's disease. Moreover, Abeta binds to p75NTR, activating signalling cascades. However, the complexity of p75NTR-mediated signalling, which does not always promote cell death, leaves open the possibly of Abeta promoting death via an alternative signalling pathway or the regulation of other p75NTR-mediated actions. This review focuses on the interactions between Abeta and p75NTR in the context of the broader p75NTR signalling field, and offers alternative explanations for how p75NTR might contribute to the aetiology of Alzheimer's disease.

The localization, trafficking and retrograde transport of BDNF bound to p75NTR in sympathetic neurons

BDNF, through p75NTR, promotes apoptosis and inhibits axonal growth of sympathetic neurons, antagonizing the pro-survival and axon growth-promoting actions of NGF through TrkA. While the trafficking of the TrkA:NGF complex is well characterized, little is known about p75NTR:BDNF trafficking in these neurons. Here we show that BDNF binds to and appears inside sympathetic neurons relatively slowly, although the temperature-sensitive internalization step itself is rapid. P75NTR internalization is partially sensitive to disruption of clathrin- or raft-mediated internalization, while that of TrkA is entirely clathrin-mediated. P75NTR, but not Trk, associates with neurotrophins in lipid rafts and coimmunoprecipitates with the truncated beta-caveolin-1 isoform. Finally, we directly visualize the retrograde transport of p75NTR ligands to cell bodies, which is insensitive to inhibitors of Trk retrograde transport, suggesting mechanistic differences. We postulate that beta-caveolin-1-containing lipid rafts and possibly intracellular endosomes might be compartments to which p75NTR:BDNF complexes are trafficked separately from Trk.

Chronic stress decreases the expression of sympathetic markers in the pineal gland and increases plasma melatonin concentration in rats

Chronic stress affects brain areas involved in learning and emotional responses. Although most studies have concentrated on the effect of stress on limbic-related brain structures, in this study we investigated whether chronic stress might induce impairments in diencephalic structures associated with limbic components of the stress response. Specifically, we analyzed the effect of chronic immobilization stress on the expression of sympathetic markers in the rat epithalamic pineal gland by immunohistochemistry and western blot, whereas the plasma melatonin concentration was determined by radioimmunoassay. We found that chronic stress decreased the expression of three sympathetic markers in the pineal gland, tyrosine hydroxylase, the p75 neurotrophin receptor and alpha-tubulin, while the same treatment did not affect the expression of the non-specific sympathetic markers Erk1 and Erk2, and glyceraldehyde-3-phosphate dehydrogenase. Furthermore, these results were correlated with a significant increase in plasma melatonin concentration in stressed rats when compared with control animals. Our findings indicate that stress may impair pineal sympathetic inputs, leading to an abnormal melatonin release that may contribute to environmental maladaptation. In addition, we propose that the pineal gland is a target of glucocorticoid damage during stress.

Postsynaptic inositol 1,4,5-trisphosphate signaling maintains presynaptic function of parallel fiber-Purkinje cell synapses via BDNF

The maintenance of synaptic functions is essential for neuronal information processing, but cellular mechanisms that maintain synapses in the adult brain are not well understood. Here, we report an activity-dependent maintenance mechanism of parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum. When postsynaptic metabotropic glutamate receptor (mGluR) or inositol 1,4,5-trisphosphate (IP(3)) signaling was chronically inhibited in vivo, PF-PC synaptic strength decreased because of a decreased transmitter release probability. The same effects were observed when PF activity was inhibited in vivo by the suppression of NMDA receptor-mediated inputs to granule cells. PF-PC synaptic strength similarly decreased after the in vivo application of an antibody against brain-derived neurotrophic factor (BDNF). Furthermore, the weakening of synaptic connection caused by the blockade of mGluR-IP(3) signaling was reversed by the in vivo application of BDNF. These results indicate that a signaling cascade comprising PF activity, postsynaptic mGluR-IP(3) signaling and subsequent BDNF signaling maintains presynaptic functions in the mature cerebellum.

Expression of Nerve Growth Factor and Its Receptors NTRK1 and TNFRSF1B Is Regulated by Estrogen and Progesterone in the Uteri of Golden Hamsters1

Experiments were conducted using female golden hamsters to identify the presence of nerve growth factor (NGF) and its receptors NTRK1 and TNFRSF1B in the uteri of female animals and regulation on their expression by estrogen and progesterone. NGF and its receptor NTRK1 were immunolocalized to luminal epithelial cells, glandular cells, and stromal cells. TNFRSF1B was immunolocalized in luminal epithelial and glandular cells, with no staining found in stromal cells of the uterine horns of normal cyclic golden hamsters. Strong immunostaining of NGF and its receptors NTRK1 and TNFRSF1B was observed in uteri on the day of proestrus as compared to the other stages of the estrous cycle. Results of immunoblot analysis of NGF revealed that there was a positive correlation between uterine NGF expression and plasma concentrations of estradiol-17beta. To clarify the effects of estrogen and progesterone on NGF, NTRK1, and TNFRSF1B expression, adult female golden hamsters were ovariectomized and treated with estradiol-17beta and/or progesterone. Immunoblot analysis and immunohistochemistry indicated that estradiol-17beta stimulated expression of NGF and its two receptors in the uterus. Treatment with progesterone also increased NGF and NTRK1 expression in the uterus. However, no additive effect of these steroids on expression of NGF and its receptors was observed. Changes in uterine weights induced by estradiol-17beta and/or progesterone showed the same profile with that of NGF, suggesting that a proliferative act of NGF may be involved in uterine growth. These results suggest that NGF may play important roles in action of steroids on uterine function.

Chronic stress induces upregulation of brain-derived neurotrophic factor (BDNF) mRNA and integrin alpha5 expression in the rat pineal gland

Chronic stress affects brain areas involved in learning and emotional responses. These alterations have been related with the development of cognitive deficits in major depression. Moreover, stress induces deleterious actions on the epithalamic pineal organ, a gland involved in a wide range of physiological functions. The aim of this study was to investigate whether the stress effects on the pineal gland are related with changes in the expression of neurotrophic factors and cell adhesion molecules. Using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, we analyzed the effect of chronic immobilization stress on the BDNF mRNA and integrin alpha5 expression in the rat pineal gland. We found that BDNF is produced in situ in the pineal gland. Chronic immobilization stress induced upregulation of BDNF mRNA and integrin alpha5 expression in the rat pineal gland but did not produce changes in beta-actin mRNA or in GAPDH expression. Stressed animals also evidenced an increase in anxiety-like behavior and acute gastric lesions. These results suggest that BDNF and integrin alpha5 may have a counteracting effect to the deleterious actions of immobilization stress on functionally stimulated pinealocytes. Furthermore, this study proposes that the pineal gland may be a target of glucocorticoid damage during stress.

Null mutations for exon III and exon IV of the p75 neurotrophin receptor gene enhance sympathetic sprouting in response to elevated levels of nerve growth factor in transgenic mice

Under normal conditions, expression of the p75 neurotrophin receptor (p75NTR) by sympathetic neurons can increase the affinity of the signaling receptor, trkA, to target-derived nerve growth factor (NGF) at distal axons. We have previously reported that sprouting of sympathetic axons into NGF-rich target tissues is enhanced when p75NTR expression is perturbed, leading to the postulate that p75NTR may restrain sympathetic sprouting in response to elevated NGF levels. These observations were made using mice having a null mutation of the third p75NTR exon, a line that may express a hypomorphic form of this receptor. Since mice carrying a null mutation of the fourth p75NTR exon may not express a similar splice variant, we sought to determine whether these animals possess the same phenotype of enhanced sympathetic sprouting in response to elevated levels of NGF. Both lines of transgenic mice lacking p75NTR displayed similar degrees of sympathetic axonal sprouting into the cerebellum and trigeminal ganglia, two target tissues having elevated levels of NGF protein. Furthermore, the densities of sympathetic axons in both targets were significantly greater than those observed in age-matched NGF transgenic siblings expressing full-length p75NTR. Our new findings provide a comparative analysis of the phenotype in two independent mutations of the same neurotrophin receptor, revealing that p75NTR plays an important role in restricting sympathetic sprouting in response to higher NGF levels.

Plasticity in rat uterine sympathetic nerves: the role of TrkA and p75 nerve growth factor receptors

Uterine sympathetic innervation undergoes profound remodelling in response to physiological and experimental changes in the circulating levels of sex hormones. It is not known, however, whether this plasticity results from changes in the innervating neurons, the neuritogenic properties of the target tissue or both. Using densitometric immunohistochemistry, we analysed the effects of prepubertal chronic oestrogen treatment (three subcutaneous injections of 20 microg of beta-oestradiol 17-cypionate on days 25, 27 and 29 after birth), natural peripubertal transition and late pregnancy (19-20 days post coitum) on the levels of TrkA and p75 nerve growth factor receptors in uterine-projecting sympathetic neurons of the thoraco-lumbar paravertebral sympathetic chain (T7-L2) identified using the retrograde tracer Fluorogold. For comparative purposes, levels of TrkA and p75 were assessed in the superior cervical ganglion (SCG) following prepubertal chronic oestrogen treatment. These studies showed that the vast majority of uterine-projecting neurons expressed both TrkA and p75. Both prepubertal chronic oestrogen treatment and the peripubertal transition increased the ratio p75 to TrkA in uterine-projecting neurons, whereas pregnancy elicited the opposite effect. Prepubertal chronic oestrogen treatment had no effects on levels of TrkA or p75 in sympathetic neurons of the SCG. Taken together, our data suggest that neurotrophin receptor-mediated events may contribute to regulate sex hormone-induced plasticity in uterine sympathetic nerves, and are in line with the idea that, in vivo, plasticity in uterine nerves involves changes in both the target and the innervating neurons.

Effects of castration on the expression of brain-derived neurotrophic factor (BDNF) in the vas deferens and male accessory genital glands of the rat

Brain-derived neurotrophic factor (BDNF) is a growth factor belonging to the family of neurotrophins. Although neurotrophins in the male genital organs have been well documented, their role in the biology of these organs is far from clear. In particular, little is known about the influence of sex hormones on neurotrophin expression. In the present study, using immunohistochemistry and enzyme-linked immunosorbent assay (ELISA), we investigated the distribution and tissue concentration of BDNF in the vas deferens and accessory male genital glands in normal and castrated rats. The expression of BDNF mRNA was also investigated. In normal rats, BDNF immunoreactivity was localized in the musculature of the vas deferens and vesicular gland and in the fibromuscular stromal cells of the prostate. In the ventral prostatic lobes, BDNF immunoreactivity was localized in basal, secretory and neuroendocrine epithelial cells. Innervating ganglia and nerves were immunoreactive in all the examined tracts. After castration, BDNF immunoreactivity increased in the musculature of the vesicular gland and in the fibromuscular stromal cells of both dorsal and ventral prostatic lobes. BDNF immunoreactivity also increased in the nerves. ELISA and reverse transcription/real-time polymerase chain reaction confirmed the findings of the immunohistochemical study. In the accessory glands, castration induced an increase of both BDNF tissue concentration and mRNA expression. These results suggest that BDNF is expressed in the internal male genital organs of the rat and that its expression is downregulated by androgen hormones. We hypothesize that the observed BDNF increases are related to the castration-induced regression of the sympathetic nerves.

Growth and survival signals controlling sympathetic nervous system development

The precise coordination of the many events in nervous system development is absolutely critical for the correct establishment of functional circuits. The postganglionic sympathetic neuron has been an amenable model for studying peripheral nervous system formation. Factors that control several developmental events, including multiple stages of axon extension, neuron survival and death, dendritogenesis, synaptogenesis, and establishment of functional diversity, have been identified in this neuron type. This knowledge allows us to integrate the various intricate processes involved in the formation of a functional sympathetic nervous system and thereby create a paradigm for understanding neuronal development in general.

The influences of p75 neurotrophin receptor and brain-derived neurotrophic factor in the sympathetic innervation of target tissues during murine postnatal development

Post-ganglionic sympathetic neurons express the p75 neurotrophin receptor (p75NTR) and brain-derived neurotrophic factor (BDNF), which together have been implicated in controlling the degree of efferent innervation of peripheral organs [Kohn, J., Aloyz, R.S., Toma, J.G., Haak-Frendscho, M., Miller, F.D. 1999. Functionally Antagonistic Interactions between the TrkA and p75 Neurotrophin Receptors Regulate Sympathetic Neuron Growth and Target Innervation. J. Neurosci. 19, 5393-5408]. To examine this concept further, we developed null mutant mice lacking both p75NTR and BDNF, and assessed whether the loss of this receptor-ligand interaction negatively impacts the degree of sympathetic innervation to various target tissues. Between postnatal days 10 and 14, hearts, urinary bladders, kidneys, and submandibular salivary glands were isolated from p75(-/-)/BDNF-/-, p75-/-, BDNF-/-, and wild type siblings. Sympathetic axons were visualized using tyrosine hydroxylase (TH) immunohistochemistry, and TH protein levels were quantified by immunoblotting. Concerning the sympathetic innervation of the heart, urinary bladder and kidneys, no differences were seen in single and double null mutant mice, as compared with their wild type siblings. Sympathetic innervation of the submandibular salivary gland was, however, increased in both p75-/- and p75(-/-)/BDNF-/- mice over control mice. These results reveal that an absence of p75NTR and/or BDNF expression does not perturb the degree of sympathetic innervation of many peripheral tissues during postnatal development, and that a lack of p75NTR expression may actually enhance the density of these efferent fibers in other target tissues, such as the salivary glands.

Activity regulates positive and negative neurotrophin-derived signals to determine axon competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

Maturation but not survival of dopaminergic nigrostriatal neurons is affected in developing and aging BDNF-deficient mice

Brain-derived neurotrophic factor (BDNF) promotes survival of injured dopaminergic nigrostriatal neurons of the adult rodent substantia nigra pars compacta, as well their development in vitro. BDNF deficiency may play a role in Parkinson's disease, as the surviving dopaminergic nigrostriatal neurons have reduced levels of BDNF, and a BDNF gene polymorphism is present in a subpopulation of patients. Here, we investigated whether a lack of BDNF in early postnatal BDNF-/- mice or a chronic 50% reduction in BDNF levels in aging BDNF+/- mice would affect the survival of the dopaminergic nigrostriatal neurons. In general terms, BDNF-/- and BDNF+/- mice had morphologically and quantitatively normal nigrostriatal neurons at any time between postnatal day 14 (P14) and 18 months, when compared to their wild-type littermates. BDNF-/- mice (P14 and P21 only) had fewer dopaminergic dendrites in the substantia nigra, suggesting that BDNF plays a role in phenotypic maturation, but not in neuronal birth or survival. BDNF-/- mice also had aberrant tyrosine hydroxylase (TH) positive cell bodies in the pars reticulata. During adulthood and aging, BDNF+/- mice performed equally well as their wild-type littermates in tests of motor coordination, and both showed aging-related decreases in the size of the dopaminergic neurons as well as in motor coordination. These results suggest that chronic deficits in BDNF alone do not affect survival or function of dopaminergic nigrostriatal neurons during aging or potentially even in Parkinson's disease.

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

Here we review mechanisms and molecules that necessitate protection and oppose axonal growth in the injured spinal cord, representing not only a cast of villains but also a company of therapeutic targets, many of which have yet to be fully exploited. We next discuss recent progress in the fields of bridging, overcoming conduction block and rehabilitation after spinal cord injury (SCI), where several treatments in each category have entered the spotlight, and some are being tested clinically. Finally, studies that combine treatments targeting different aspects of SCI are reviewed. Although experiments applying some treatments in combination have been completed, auditions for each part in the much-sought combination therapy are ongoing, and performers must demonstrate robust anatomical regeneration and/or significant return of function in animal models before being considered for a lead role.

Functional interaction between p75NTR and TrkA: the endocytic trafficking of p75NTR is driven by TrkA and regulates TrkA-mediated signalling

The topology and trafficking of receptors play a key role in their signalling capability. Indeed, receptor function is related to the microenvironment inside the cell, where specific signalling molecules are compartmentalized. The response to NGF (nerve growth factor) is strongly dependent on the trafficking of its receptor, TrkA. However, information is still scarce about the role of the cellular localization of the TrkA co-receptor, p75NTR (where NTR is neurotrophin receptor), following stimulation by NGF. It has been shown that these two receptors play a key role in epithelial tissue and in epithelial-derived tumours, where the microenvironment at the plasma membrane is defined by the presence of tight junctions. Indeed, in thyroid carcinomas, rearrangements of TrkA are frequently found, which produce TrkA mutants that are localized exclusively in the cytoplasm. We used a thyroid cellular model in which it was possible to dissect the trafficking of the two NGF receptors upon neurotrophin stimulation. In FRT (Fischer rat thyroid) cells, endogenous TrkA is localized exclusively on the basolateral surface, while transfected p75NTR is selectively distributed on the apical membrane. This cellular system enabled us to selectively stimulate either p75NTR or TrkA and to analyse the role of receptor trafficking in their signalling capability. We found that, after binding to NGF, p75NTR was co-immunoprecipitated with TrkA and was transcytosed at the basolateral membrane. We showed that the TrkA-p75NTR interaction is necessary for this relocation of p75NTR to the basolateral side. Interestingly, TrkA-specific stimulation by basolateral NGF loading also induced the TrkA-p75NTR interaction and subsequent p75NTR transcytosis at the basolateral surface. Moreover, specific stimulation of p75NTR by NGF activated TrkA and the MAPK (mitogen-activated protein kinase) pathway. Our data indicate that TrkA regulates the subcellular localization of p75NTR upon stimulation with neurotrophins, thus affecting the topology of the signal transduction molecules, driving the activation of a specific signal transduction pathway.

Estrogen alters trkA and p75 neurotrophin receptor expression within sympathetic neurons

Survival and growth of sympathetic neurons is regulated by nerve growth factor acting through trkA and p75NTR receptors. Sympathetic neurons are also affected by gonadal steroid hormones, particularly estrogen. To determine if estrogen may influence sympathetic neurons via altered neurotrophin receptor expression, we investigated effects of acute or chronic estrogen administration on levels of trkA and p75NTR proteins, numbers of immunoreactive neurons, and numbers of neurons expressing trkA, p75NTR, and estrogen receptor-alpha transcripts. Superior cervical ganglia from ovariectomized or estradiol-treated rats were processed for in situ hybridization or immunohistochemistry, and percentages of stained neurons quantitated or processed for Western blot analysis. In ovariectomized rats, approximately 50% of sympathetic neurons expressed trkA mRNA and protein. Acute estrogen administration did not affect trkA transcript expression, but reduced trkA protein significantly. Chronic treatment did not alter neuronal trkA expression. Approximately 70% of sympathetic neurons in ovariectomized rats expressed p75NTR transcripts and about 50% showed p75NTR immunoreactivity. Acute estrogen did not affect p75NTR expression. However, chronic estrogen reduced p75NTR mRNA and protein expression significantly. Fifty to sixty percent of sympathetic neurons in ovariectomized rats displayed estrogen receptor-alpha mRNA. After acute estrogen administration, estrogen receptor-alpha transcript expression increased by 35%, although this was not maintained chronically. These findings indicate that estrogen can influence sympathetic neuronal neurotrophin receptor expression as well as estrogen receptor-alpha. Reduced trkA expression after acute estrogen may transiently predispose neurons to degenerative events, while diminished p75NTR expression by chronic estrogen administration may exert long-term effects on survival or axonal outgrowth in sympathetic neurons.

Gene deletion of dopamine β-hydroxylase and α1-adrenoceptors demonstrates involvement of catecholamines in vascular remodeling

In vitro studies have shown that stimulation of α1-adrenoceptors (ARs) directly induces proliferation, hypertrophy, and migration of arterial smooth muscle cells and adventitial fibroblasts. In vivo studies confirmed these findings and showed that catecholamine trophic activity becomes excessive after experimental balloon injury and contributes to neointimal growth, adventitial thickening, and lumen loss. However, past studies have been limited by selectivity of pharmacological agents. The aim of this study, in which mice devoid of norepinephrine and epinephrine synthesis [dopamine β-hydroxylase (DBH−/−)] or deficient in α1-AR subtypes expressed in murine carotid (α1B-AR−/− and α1D-AR−/−) were used, was to test the hypothesis that catecholamines contribute to wall hypertrophy after injury. At 3 wk after injury of wild-type mice, lumen area and carotid circumference increased significantly, and hypertrophy of media and adventitia was in excess of that needed to restore circumferential wall stress to normal. In DBH−/− and α1B-AR−/− mice, increases in lumen area, circumference, and hypertrophy of the media and adventitia were reduced by 50–91%, resulting in restoration of wall tension to nearly normal (DBH−/−) or normal (α1B-AR−/−). In contrast, in α1D-AR−/− mice, increases in lumen area, circumference, and wall hypertrophy were unaffected and wall thickening remained in excess of that required to return tension to normal. When examined 5 days after injury, proliferation and leukocyte infiltration were inhibited in DBH−/− mice. These studies suggest that the trophic effects of catecholamines are mediated primarily by α1B-ARs in mouse carotid and contribute to hypertrophic growth after vascular injury.

Comparison of target innervation by sympathetic axons in adult wild type and heterozygous mice for nerve growth factor or its receptor trkA

Nerve growth factor (NGF), a neurotrophin required for the survival and maintenance of postganglionic sympathetic neurons, mediates its trophic effects by activation of its high-affinity receptor trkA. Null mutant mice lacking either NGF or trkA have profound sympathetic deficits, thus revealing the vital importance of NGF synthesis in target tissues and trkA expression by sympathetic neurons. In this study, we sought to assess whether sympathetic neurons of the superior cervical ganglion (SCG) display alterations in their neurochemical phenotype in adult mice carrying one mutated allele for either NGF or trkA, and whether such differences result in altered patterns of innervation to the submandibular salivary gland and pineal gland. In comparison with adult siblings, levels of trkA protein in the SCG were reduced in age-matched NGF(+/-) and trkA(+/-) mice. While NGF(+/-) mice also had significantly fewer sympathetic axons innervating both the submandibular salivary gland and pineal gland, densities of sympathetic axons in both tissues reached normal levels in trkA(+/-) mice. These findings reveal that while levels of trkA are reduced in SCG neurons of adult NGF(+/-) and trkA(+/-) mice (compared with their wild type counterparts), sympathetic axons are capable of achieving normal patterns of target innervation in trkA(+/-) mice but not in NGF(+/-) mice. As NGF protein levels are not depleted in the submandibular salivary gland and pineal gland of NGF(+/-) mice, a loss of sympathetic neurons [Nat Neurosci 1999; 2:699-705], in combination with reduced levels of trkA protein, may account for perturbed patterns of sympathetic innervation to peripheral tissues.

The Nogo signaling pathway for regeneration block

A hostile environment and decreased regenerative capacity may contribute to the failure of axon regeneration in the adult central nervous system. Recent studies leading to the identification of several myelin-associated inhibitors and their signaling molecules provide opportunitities to assess the contribution of these inhibitory molecules in restricting axon regeneration. These findings may ultimately allow for the development of strategies to alleviate the inhibitory effects of such molecules in an effort to encourage axon regeneration after spinal cord and brain injury.

Immobilized concentration gradients of nerve growth factor guide neurite outgrowth

Axons are guided to their targets by a combination of haptotactic and chemotactic cues. We previously demonstrated that soluble neurotrophic factor concentration gradients guide axons in a model system. In an attempt to translate this model system to a device for implantation, our goal was to immobilize a stable neurotrophic concentration gradient for axonal (or neurite) guidance. Nerve growth factor (NGF) was immobilized within poly(2-hydroxyethylmethacrylate) [p(HEMA)] microporous gels using a gradient maker. The NGF was stably immobilized, with only approximately 0.05% of the amount originally incorporated into the gel released over an 8-day period. Immobilized NGF was bioactive: the percent of PC12 cells extending neurites on NGF-immobilized p(HEMA) gels was 16 +/- 2%, which was statistically the same as those exposed to soluble NGF (22 +/- 6%). We were able to predict and reproducibly create stable NGF concentration gradients in the gel. At an NGF concentration gradient of 357 ng/mL/mm, PC12 cell neurites were guided up the gradient. The facile, flexible, and reproducible nature of this method allowed us to translate soluble growth factor gradient models to stable growth factor gradient devices that may ultimately enhance axonal guidance and regeneration in vivo.

TrkA and mitogen-activated protein kinase phosphorylation are enhanced in sympathetic neurons lacking functional p75 neurotrophin receptor expression

This study examined the effects of hypomorphic p75 neurotrophin receptor (p75NTR) expression and high levels of nerve growth factor (NGF) on trkA phosphorylation and downstream activation of p44/42 mitogen-activated protein kinase (MAPK). Post-ganglionic sympathetic neurons from postnatal day 1 p75NTR exon III null mutant (p75(-/-)) and 129/SvJ mice were cultured in the presence of 50 ng/mL NGF and analysed by Western blotting. Levels of phosphorylated trkA are increased in p75(-/-) neurons compared with 129/SvJ neurons, and these higher levels are maintained with continuous exposure to NGF. MAPK is also phosphorylated to a greater extent in p75(-/-) neurons than in 129/SvJ neurons, both within 10 min of exposure to NGF, and with continuous NGF treatment for 5 days. These data provide new insight into the mechanism underlying enhanced neurite outgrowth in p75(-/-) neurons, demonstrating that trkA and MAPK signalling in sympathetic neurons are increased when p75NTR function is disrupted.

The effects of deafferentation and exogenous NGF on neurotrophins and neurotrophin receptor mRNA expression in the adult superior cervical ganglion

Levels of nerve growth factor (NGF) and neurotrophin-3 (NT-3) protein and neurotrophin receptor mRNA in adult sympathetic neurons were investigated following surgical removal of preganglionic input and/or in vivo administration of NGF. Expression of trkC and p75, but not trkA, was significantly decreased following a 3-week deafferentation of the superior cervical ganglion (SCG). Protein levels of NGF and NT-3 in the SCG were unchanged by deafferentation. A 2-week intracerebroventricular infusion of NGF without deafferentation resulted in enhanced mRNA levels of trkA, trkC, and p75 as well as significantly increased NGF and NT-3 protein in the SCG. When NGF infusion followed deafferentation, both trkA and p75 showed significant increases while trkC levels were similar to control values. NGF protein was not increased in the SCG when deafferentation preceded exogenous NGF, yet NT-3 was elevated and levels were similar to cases receiving NGF infusion only. These results support a role for preganglionic input in trkC and p75 expression in adult sympathetic neurons. The increased levels of NT-3 protein and trkC gene expression observed following NGF infusion suggest that NGF influences NT-3 regulation in adult sympathetic neurons. In addition, the present findings provide evidence that, when preganglionic input is removed prior to the NGF infusion, NT-3 effectively competes with NGF for trkA binding. Taken together, we propose that NT-3 may play a role in the robust sprouting of sympathetic cerebrovascular axons previously observed following NGF administration, particularly when deafferentation precedes the NGF infusion period.

Treatment of cerebellar granule cell neurons with the neurotrophic factor pigment epithelium-derived factor in vitro enhances expression of other neurotrophic factors as well as cytokines and chemokines

Microarray analyses demonstrated that a variety of genes was affected by treatment of cerebellar granule cell neurons with the neurotrophic factor pigment epithelium-derived factor (PEDF). The genes for neurotrophins, glial cell-derived neurotrophic factor (GDNF), and their receptors were regulated differentially in immature versus mature neurons; however, nerve growth factor (NGF), neurotrophin (NT)-3, and GDNF did not contribute to the protective effect of PEDF. Brain-derived neurotrophic factor (BDNF) seemed capable of inducing apoptosis, because a blocking antibody enhanced the protective effect of PEDF. In addition, PEDF exposure also stimulated expression of several cytokine and chemokine genes. Removal of the less than 1% of microglia in the cultures by treatment with L-leucine methyl ester, combined with enzyme-linked immunosorbent assays (ELISAs), demonstrated that the cerebellar granule cells constitutively produce three chemokines, macrophage inflammatory protein (MIP)-1alpha, MIP-2, and MIP-3alpha, whose production is enhanced further by treatment with PEDF. Blocking antibodies to each of the chemokines was protective under control conditions, suggesting that they may contribute to the "natural" apoptosis occurring in the cultures, and enhanced the effects of PEDF. Although PEDF enhanced production of all three chemokines, the blocking antibodies did not increase its protective effect against induced apoptosis. These results suggest that although PEDF enhances expression of other neurotrophic factors or chemokines, it does not exert its neuroprotective effect on cerebellar granule cells through their production.

The p75 neurotrophin receptor: multiple interactors and numerous functions

The neurotrophin receptor p75 (p75NTR), is involved in a diverse array of cellular responses, including apoptosis, neurite outgrowth and myelination. Stimulation of p75NTR with neurotrophin can activate multiple downstream signals, including the small GTP binding protein Rac, the transcription factor NF-kappa B and the stress activated kinase, JNK. How these signals are generated and regulated to produce a specific cellular effect has yet to be fully elucidated. A number of proteins have recently been shown to interact with the intracellular domain of p75NTR. Here, we review these p75NTR interacting factors and the current evidence as to how they contribute to the functional effects of p75NTR activation.

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.