Sympathetic neuron survival and TrkA expression in NT3-deficient mouse embryos

Studying the pathophysiologic connection between cardiovascular and nervous systems using stem cells

The cardiovascular and nervous systems are deeply connected during development, health, and disease. Both systems affect and regulate the development of each other during embryogenesis and the early postnatal period. Specialized neural crest cells contribute to cardiac structures, and a number of growth factors released from the cardiac tissue (e.g., glial cell line-derived neurotrophic factor, neurturin, nerve growth factor, Neurotrophin-3) ensure proper maturation of the incoming parasympathetic and sympathetic neurons. Physiologically, the cardiovascular and nervous systems operate in harmony to adapt to various physical and emotional conditions to maintain homeostasis through sympathetic and parasympathetic nervous systems. Moreover, neurocardiac regulation involves a neuroaxis consisting of cortex, amygdala, and other subcortical structures, which have the ability to modify lower-level neurons in the hierarchy. Given the interconnectivity of cardiac and neural systems, when one undergoes pathological changes, the other is affected to a certain extent. In addition, there are specific neurocardiac diseases that affect both systems simultaneously, such as Huntington disease, Lewy body diseases, Friedreich ataxia, congenital heart diseases, Danon disease, and Timothy syndrome. Over the last decade, in vitro modeling of neurocardiac diseases using induced pluripotent stem cells (iPSCs) has provided an invaluable opportunity to elevate our knowledge about the brain-heart connection, since previously primary cardiomyocytes and neurons had been extremely difficult to maintain long-term in vitro. Ultimately, the ability of iPSC technology to model abnormal functional phenotypes of human neurocardiac disorders, combined with the ease of therapeutic screening using this approach, will transform patient care through personalized medicine in the future. © 2016 Wiley Periodicals, Inc.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

Region-specific role of growth differentiation factor-5 in the establishment of sympathetic innervation

BACKGROUND

Nerve growth factor (NGF) is the prototypical target-derived neurotrophic factor required for sympathetic neuron survival and for the growth and ramification of sympathetic axons within most but not all sympathetic targets. This implies the operation of additional target-derived factors for regulating terminal sympathetic axon growth and branching.

RESULTS

Here report that growth differentiation factor 5 (GDF5), a widely expressed member of the transforming growth factor beta (TGFβ) superfamily required for limb development, promoted axon growth from mouse superior cervical ganglion (SCG) neurons independently of NGF and enhanced axon growth in combination with NGF. GDF5 had no effect on neuronal survival and influenced axon growth during a narrow window of postnatal development when sympathetic axons are ramifying extensively in their targets in vivo. SCG neurons expressed all receptors capable of participating in GDF5 signaling at this stage of development. Using compartment cultures, we demonstrated that GDF5 exerted its growth promoting effect by acting directly on axons and by initiating retrograde canonical Smad signalling to the nucleus. GDF5 is synthesized in sympathetic targets, and examination of several anatomically circumscribed tissues in Gdf5 null mice revealed regional deficits in sympathetic innervation. There was a marked, highly significant reduction in the sympathetic innervation density of the iris, a smaller though significant reduction in the trachea, but no reduction in the submandibular salivary gland. There was no reduction in the number of neurons in the SCG.

CONCLUSIONS

These findings show that GDF5 is a novel target-derived factor that promotes sympathetic axon growth and branching and makes a distinctive regional contribution to the establishment of sympathetic innervation, but unlike NGF, plays no role in regulating sympathetic neuron survival.

Signaling molecules and transcription factors involved in the development of the sympathetic nervous system, with special emphasis on the superior cervical ganglion

The cells that constitute the sympathetic nervous system originate from the neural crest. This review addresses the current understanding of sympathetic ganglion development viewed from molecular and morphological perspectives. Development of the sympathetic nervous system is categorized into three main steps, as follows: (1) differentiation and migration of cells in the neural crest lineage for formation of the primary sympathetic chain, (2) differentiation of sympathetic progenitors, and (3) growth and survival of sympathetic ganglia. The signaling molecules and transcription factors involved in each of these developmental stages are elaborated mostly on the basis of the results of targeted mutation of respective genes. Analyses in mutant mice revealed differences between the superior cervical ganglion (SCG) and the other posterior sympathetic ganglia. This review provides a summary of the similarities and differences in the development of the SCG and other posterior sympathetic ganglia. Relevant to the development of sympathetic ganglia is the demonstration that neuroendocrine cells, such as adrenal chromaffin cells and carotid body glomus cells, share a common origin with the sympathetic ganglia. Neural crest cells at the trunk level give rise to common sympathoadrenal progenitors of sympathetic neurons and chromaffin cells, while progenitors segregated from the SCG give rise to glomus cells. After separation from the sympathetic primordium, the progenitors of both chromaffin cells and glomus cells colonize the anlage of the adrenal gland and carotid body, respectively. This review highlights the biological properties of chromaffin cells and glomus cells, because, although both cell types are derivatives of sympathetic primordium, they are distinct in many respects.

Cardiac fibroblasts regulate sympathetic nerve sprouting and neurocardiac synapse stability

Sympathetic nervous system (SNS) plays a key role in cardiac homeostasis and its deregulations always associate with bad clinical outcomes. To date, little is known about molecular mechanisms regulating cardiac sympathetic innervation. The aim of the study was to determine the role of fibroblasts in heart sympathetic innervation. RT-qPCR and western-blots analysis performed in cardiomyocytes and fibroblasts isolated from healthy adult rat hearts revealed that Pro-Nerve growth factor (NGF) and pro-differentiating mature NGF were the most abundant neurotrophins expressed in cardiac fibroblasts while barely detectable in cardiomyocytes. When cultured with cardiac fibroblasts or fibroblast-conditioned medium, PC12 cells differentiated into/sympathetic-like neurons expressing axonal marker Tau-1 at neurites in contact with cardiomyocytes. This was prevented by anti-NGF blocking antibodies suggesting a paracrine action of NGF secreted by fibroblasts. When co-cultured with cardiomyocytes to mimic neurocardiac synapse, differentiated PC12 cells exhibited enhanced norepinephrine secretion as quantified by HPLC compared to PC12 cultured alone while co-culture with fibroblasts had no effect. However, when supplemented to PC12-cardiomyocytes co-culture, fibroblasts allowed long-term survival of the neurocardiac synapse. Activated fibroblasts (myofibroblasts) isolated from myocardial infarction rat hearts exhibited significantly higher mature NGF expression than normal fibroblasts and also promoted PC12 cells differentiation. Within the ischemic area lacking cardiomyocytes and neurocardiac synapses, tyrosine hydroxylase immunoreactivity was increased and associated with local anarchical and immature sympathetic hyperinnervation but tissue norepinephrine content was similar to that of normal cardiac tissue, suggesting depressed sympathetic function. Collectively, these findings demonstrate for the first time that fibroblasts are essential for the setting of cardiac sympathetic innervation and neurocardiac synapse stability. They also suggest that neurocardiac synapse functionality relies on a triptych with tight interaction between sympathetic nerve endings, cardiomyocytes and fibroblasts. Deregulations of this triptych may be involved in pathophysiology of cardiac diseases.

The transcription factor Hmx1 and growth factor receptor activities control sympathetic neurons diversification

The sympathetic nervous system relies on distinct populations of neurons that use noradrenaline or acetylcholine as neurotransmitter. We show that fating of the sympathetic lineage at early stages results in hybrid precursors from which, genetic cell-lineage tracing reveals, all types progressively emerge by principal mechanisms of maintenance, repression and induction of phenotypes. The homeobox transcription factor HMX1 represses Tlx3 and Ret, induces TrkA and maintains tyrosine hydroxylase (Th) expression in precursors, thus driving segregation of the noradrenergic sympathetic fate. Cholinergic sympathetic neurons develop through cross-regulatory interactions between TRKC and RET in precursors, which lead to Hmx1 repression and sustained Tlx3 expression, thereby resulting in failure of TrkA induction and loss of maintenance of Th expression. Our results provide direct evidence for a model in which diversification of noradrenergic and cholinergic sympathetic neurons is based on a principle of cross-repressive functions in which the specific cell fates are directed by an active suppression of the expression of transcription factors and receptors that direct the alternative fate.

Glia cell line-derived neurotrophic factor mediates survival of murine sympathetic precursors

During embryonic development, neurons are first produced in excess, and final numbers are adjusted by apoptosis at later stages. Crucial to this end is the amount of target-derived growth factor available for the neurons. By this means, the target size correctly matches the innervating neuron number. This target-derived survival has been well studied for sympathetic neurons, and nerve growth factor (NGF) was identified to be the crucial factor for maintaining sympathetic neurons at late embryonic and early postnatal stages, with a virtual complete loss of sympathetic neurons in NGF knockout (KO) mice. This indicates that all sympathetic neurons are dependent on NGF. However, also different glia cell line-derived neurotrophic factor (GDNF) KO mice consistently presented a loss of sympathetic neurons. This was the rationale for investigating the role of GDNF for sympathetic precursor/neuron survival. Here we show that GDNF is capable of promoting survival of 30% sympathetic precursors dissociated at E13. This is in line with data from GDNF KOs in which a comparable sympathetic neuron loss was observed at late embryonic stages, although the onset of the phenotype was unclear. We further present data showing that GDNF ligand and canonical receptors are expressed in sympathetic neurons especially at embryonic stages, raising the possibility of an autocrine/paracrine GDNF action. Finally, we show that GDNF also maintained neonatal sympathetic neurons (40%) cultured for 2 days. However, the GDNF responsiveness was lost at 5 days in vitro.

Sequential requirement of Sox4 and Sox11 during development of the sympathetic nervous system

The highly related transcription factors Sox4 and Sox11 are expressed in the developing sympathetic nervous system. In the mouse, Sox11 appears first, whereas Sox4 is prevalent later. Using mouse mutagenesis and overexpression strategies in chicken, we studied the role of both SoxC proteins in this tissue. Neither Sox4 nor Sox11 predominantly functioned by promoting pan-neuronal or noradrenergic differentiation of sympathetic neurons as might have been expected from studies in neuronal precursors of the central nervous system. The transcriptional network that regulates the differentiation of sympathetic neurons remained intact and expression of noradrenergic markers showed only minor alterations. Instead, Sox11 was required in early sympathetic ganglia for proliferation of tyrosine hydroxylase-expressing cells, whereas Sox4 ensured the survival of these cells at later stages. In the absence of both Sox4 and Sox11, sympathetic ganglia remained hypoplastic throughout embryogenesis because of consecutive proliferation and survival defects. As a consequence, sympathetic ganglia were rudimentary in the adult and sympathetic innervation of target tissues was impaired leading to severe dysautonomia.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

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.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Neurotrophin-4/5-depletion induces a delay in maturation of the periodontal Ruffini endings in mice

Neurotrophin-4/5 (NT-4/5) - a member of the neurotrophic factors - is a ligand for TrkB, which has been reported to be expressed in the mechanoreceptive Ruffini endings of the periodontal ligament. The present study examined developmental changes in the terminal morphology and neural density in homozygous mice with a targeted disruption of the nt-4/5 gene and wild-type mice by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by quantitative analysis using an image analyzer. Postnatal development of terminal Schwann cells was also investigated by enzymatic histochemistry for non-specific cholinesterase activity (ChE). Furthermore, the immuno-expression of TrkB and low affinity nerve growth factor receptor (p75-NGFR) was surveyed in the periodontal Ruffini endings as well as trigeminal ganglion. At postnatal 1 week, the lingual periodontal ligament of both types of mice contained PGP 9.5-positive nerve fibers showing a tree-like ramification with axonal swellings in their course. In both types of mice at 2 weeks of age, comparatively thick nerve fibers with a smooth outline increased in number, and frequently ramified to form nerve terminals with dendritic profiles. However, no typical Ruffini endings with irregular outlines observed in the adult wild-type mice were found in the periodontal ligament at this stage. At postnatal 3 weeks, typical Ruffini endings with irregular outlines were discernable in the periodontal ligament of the wild-type mice while the dendritic endings showing smooth outlines were restricted to the homozygous mice. After postnatal 8 weeks, both types of mice showed an increase in the number of Ruffini endings, but the morphology differed between the wild-type and NT-4/5 homozygous mice. In the wild-type mice, a major population of the Ruffini endings expanded their axonal branches and developed many microprojections, resulting in a reduction of endings with smooth outlines. In contrast, we failed to find such typical Ruffini endings in the periodontal ligament of the homozygous mice: A majority of the periodontal Ruffini endings continued to show smooth outlines at postnatal 12 weeks. Quantitative analysis on neural density demonstrated a reduction in the homozygous mice with a significant difference by postnatal 8 weeks. Enzymatic histochemistry for non-specific ChE did not exhibit a distinct difference in the distribution and density of terminal Schwann cells between wild-type and homozygous mice. Furthermore, TrkB and p75-NGFR mRNA and proteins did not differ in the trigeminal ganglion between the two types. The periodontal Ruffini endings also displayed immunoreactivities for TrkB and p75- NGFR in both phenotypes. These findings suggest that the nt-4/5 gene depletion caused a delay in the formation and maturation of the periodontal Ruffini endings in the mice by inhibiting the growth of the periodontal nerves at an early stage, and indicate that multiple neurotrophins such as NT- 4/5 and BDNF might play roles in the development and/or maturation of the periodontal Ruffini endings.

Impaired activation of celiac ganglion neurons in vivo after damage to their sympathetic nerve terminals

Because damage to sympathetic nerve terminals occurs in a variety of diseases, we tested the hypothesis that nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo. First, we measured the effect of nerve terminal damage produced by the sympathetic nerve terminal toxin 6-hydroxydopamine (6-OHDA) on ganglionic levels of several neurotrophins thought to promote neurotransmission. 6-OHDA-induced nerve terminal damage did not decrease the expression of neurotrophin-4 or brain-derived neurotrophic factor mRNA in the celiac ganglia but did decrease the ganglionic content of both nerve growth factor protein (nadir = -63%) and the mRNA of the alpha-3 subunit of the nicotinic cholinergic receptor (nadir = -49%), a subunit required for neurotransmission. Next, we tested whether this degree of receptor deficiency was sufficient to impair activation of celiac ganglia neurons. Impaired fos mRNA responses to nicotine administration in the celiac ganglia of 6-OHDA-pretreated rats correlated temporally with suppressed expression of functional nicotinic receptors. We verified by Fos protein immunohistochemistry that this ganglionic impairment was specific to principal ganglionic neurons. Last, we tested whether centrally initiated ganglionic neurotransmission is also impaired following nerve terminal damage. The principal neurons in rat celiac ganglia were reflexively activated by 2-deoxy-glucose-induced glucopenia, and the Fos response in the celiac ganglia was markedly inhibited by pretreatment with 6-OHDA. We conclude that sympathetic nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo and that decreased nicotinic receptor production is a likely mediator.

Abnormal sympathetic nervous system development and physiological dysautonomia in Egr3-deficient mice

Sympathetic nervous system development depends upon many factors that mediate neuron migration, differentiation and survival. Target tissue-derived nerve growth factor (NGF) signaling-induced gene expression is required for survival, differentiation and target tissue innervation of post-migratory sympathetic neurons. However, the transcriptional regulatory mechanisms mediated by NGF signaling are very poorly defined. Here, we identify Egr3, a member of the early growth response (Egr) family of transcriptional regulators, as having an important role in sympathetic nervous system development. Egr3 is regulated by NGF signaling and it is expressed in sympathetic neurons during development when they depend upon NGF for survival and target tissue innervation. Egr3-deficient mice have severe sympathetic target tissue innervation abnormalities and profound physiological dysautonomia. Unlike NGF, which is essential for sympathetic neuron survival and for axon branching within target tissues, Egr3 is required for normal terminal axon extension and branching, but not for neuron survival. The results indicate that Egr3 is a novel NGF signaling effector that regulates sympathetic neuron gene expression required for normal target tissue innervation and function. Egr3-deficient mice have a phenotype that is remarkably similar to humans with sympathetic nervous system disease, raising the possibility that it may have a role in some forms of human dysautonomia, most of which have no known cause.

NGF-promoted axon growth and target innervation requires GITRL-GITR signaling

Nerve growth factor (NGF) has an important role in regulating sympathetic neuron survival and target field innervation during development. Here we show that glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR), a member of the TNF superfamily, and its ligand (GITRL) are co-expressed in mouse sympathetic neurons when their axons are innervating their targets under the influence of target-derived NGF. In culture, GITRL enhanced NGF-promoted neurite growth from neonatal sympathetic neurons, and preventing GITR-GITRL interaction in these neurons or knocking down GITR inhibited NGF-promoted neurite growth without affecting neuronal survival. Tnfrsf18(-/-) (Gitr) neonates have reduced sympathetic innervation density in vivo compared with Gitr(+/+) littermates. GITR activation is required for the phosphorylation of extracellular signal-regulated kinases 1 and 2 by NGF that is necessary for neurite growth. Our results reveal a previously unknown signaling loop in developing sympathetic neurons that is crucial for NGF-dependent axon growth and target innervation.

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

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

The novel neurotrophin-regulated neuronal development-associated protein, NDAP, mediates apoptosis

We identified a novel gene and named it, "neuronal development-associated protein (NDAP)". We detected NDAP mRNA presence in most tissues including the brain where it was present in the area from the external granular layer to the multiform layer in the cerebral cortex, and in CA1, CA2, CA3 and the dentate gyrus in the hippocampus. Its expression increased transiently in primary cultures of 2-4 day neurons and 1-2 week astrocytes and was significantly reduced in older cultures. Treatment by the neurotrophin, NT-3, significantly attenuated the decline of NDAP in neurons from days 2 to 10, whereas growth factors such as GDNF and insulin, and high potassium levels did not. To elucidate the effects of neurotrophins, we treated day 5 neurons with NT-3, BDNF or NGF for 48 h. NT-3 and BDNF both inhibited downregulation of NDAP mRNA levels but NGF slightly enhanced the already present downregulation; this effect of NGF was significant when examined in day 3 neurons. To investigate the potential function of NDAP, we over-expressed an NDAP-EGFP fusion protein in 4-week-old astrocytes. The newly expressed NDAP gradually aggregated into membrane-bound structures and eventually led to cell death through apoptosis by 24 h. Significant levels of cell death were also observed in NDAP-EGFP transfected HEK293 cells. Thus maintenance of high NDAP levels may cause apoptosis. The different regulations of NDAP expression by neurotrophins indicate that the expression of NDAP might be a checkpoint for apoptosis during neuronal development.

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.

Regulation of neural development by glial cell line-derived neurotrophic factor family ligands

Glial cell line-derived neurotrophic factor (GDNF) and its three relatives constitute a novel family of neurotrophic factors, the GDNF family ligands. These factors signal through a multicomponent receptor complex comprising a glycosylphosphatidylinositol-anchored cell surface molecule (GDNF family receptor (GFR) alpha) and RET tyrosine kinase, triggering the activation of multiple signaling pathways in responsive cells. Recent gene-targeting studies have demonstrated that GDNF family ligands are essential for the development of a diverse set of neuronal populations and we have now started to understand how these ligands uniquely regulate the formation and sculpting of the nervous system. Recent studies have also revealed interactions by multiple extracellular signals during neural development. The deciphering of GDNF family ligand signaling in neural cells promises to provide vital new insights into the development and pathology of the nervous system.

Mesenchymal stem cells that produce neurotrophic factors reduce ischemic damage in the rat middle cerebral artery occlusion model

Mesenchymal stem cells (MSC) were reported to ameliorate functional deficits after stroke in rats, with some of this improvement possibly resulting from the action of cytokines secreted by these cells. To enhance such cytokine effects, we previously transfected the telomerized human MSC with the BDNF gene using a fiber-mutant adenovirus vector and reported that such treatment contributed to improved ischemic recovery in a rat transient middle cerebral artery occlusion (MCAO) model. In the present study, we investigated whether other cytokines in addition to BDNF, i.e., GDNF, CNTF, or NT3, might have a similar or greater effect in this model. Rats that received MSC-BDNF (P < 0.05) or MSC-GDNF (P < 0.05) showed significantly more functional recovery as demonstrated by improved behavioral test results and reduced ischemic damage on MRI than did control rats 7 and 14 days following MCAO. On the other hand, rats that received MSC-CNTF or MSC-NT3 showed neither functional recovery nor ischemic damage reduction compared to control rats. Thus, MSC transfected with the BDNF or GDNF gene resulted in improved function and reduced ischemic damage in a rat model of MCAO. These data suggest that gene-modified cell therapy may be a useful approach for the treatment of stroke.

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.

A Neurotrophin Signaling Cascade Coordinates Sympathetic Neuron Development through Differential Control of TrkA Trafficking and Retrograde Signaling

A fundamental question in developmental biology is how a limited number of growth factors and their cognate receptors coordinate the formation of tissues and organs endowed with enormous morphological complexity. We report that the related neurotrophins NGF and NT-3, acting through a common receptor, TrkA, are required for sequential stages of sympathetic axon growth and, thus, innervation of target fields. Yet, while NGF supports TrkA internalization and retrograde signaling from distal axons to cell bodies to promote neuronal survival, NT-3 cannot. Interestingly, final target-derived NGF promotes expression of the p75 neurotrophin receptor, in turn causing a reduction in the sensitivity of axons to intermediate target-derived NT-3. We propose that a hierarchical neurotrophin signaling cascade coordinates sequential stages of sympathetic axon growth, innervation of targets, and survival in a manner dependent on the differential control of TrkA internalization, trafficking, and retrograde axonal signaling.

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.

Neurotrophins

Nerve growth factor was the first identified protein with anti-apoptotic activity on neurons. This prototypic neurotrophic factor, together with the three structurally and functionally related growth factors brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4/5 (NT4/5), forms the neurotrophin protein family. Target T cells for neurotrophins include many neurons affected by neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and peripheral polyneuropathies. In addition, the neurotrophins act on neurons affected by other neurological and psychiatric pathologies including ischemia, epilepsy, depression and eating disorders. Work with cell cultures and animal models provided solid support for the hypothesis that neurotrophins prevent neuronal death. While no evidence exists that a lack of neurotrophins underlies the etiology of any neurodegenerative disease, these studies have spurred on hopes that neurotrophins might be useful symptomatic-therapeutic agents. However first clinical trials led to variable results and severe side effects were observed. For future therapeutic use of the neurotrophins it is therefore crucial to expand our knowledge about their physiological functions as well as their pharmacokinetic properties. A major challenge is to develop methods for their application in effective doses and in a precisely timed and localized fashion.

Regulation of neuronal survival and death by extracellular signals during development

Cell death is a prominent feature of the developing vertebrate nervous system, affecting neurons, glial cells and their progenitors. The most extensively studied and best understood phase of cell death occurs in populations of neurons shortly after they begin establishing connections with other neurons and/or non-neural tissues. This phase of cell death makes appropriate adjustments to the relative sizes of interconnected groups of neurons and matches the size of neuronal populations that innervate non-neural tissues to the optimal requirements of these tissues. The fate of neurons during this period of development is regulated by a variety of secreted proteins that either promote survival or bring about cell death after binding to receptors expressed on the neurons. These proteins may be derived from the targets the neurons innervate, the afferents they receive or from associated glial cells, or they may be secreted by the neurons themselves. In this review, I will outline the established and emerging principles that modulate neuronal number in the developing nervous system.

Macrophage stimulating protein is a target-derived neurotrophic factor for developing sensory and sympathetic neurons

Macrophage stimulating protein (MSP) is a pleiotropic growth factor that signals via the Ron receptor tyrosine kinase. We report that Ron mRNA is expressed by NGF-dependent sensory and sympathetic neurons and that these neurons survive and grow with MSP at different stages of development. Whereas NGF-dependent sensory neurons become increasingly responsive to MSP with age, sympathetic neurons exhibit an early response to MSP that is lost by birth. MSP mRNA expression increases with age in sensory neuron targets and decreases in sympathetic targets. After the phase of naturally occurring neuronal death, significant numbers of NGF-dependent sensory neurons, but not sensory neurons, dependent on other neurotrophins, are lost in mice lacking a functional Ron receptor. These results show that MSP is a target-derived neurotrophic factor for subsets of sensory and sympathetic neurons at different times during their development.

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.

Geldanamycin decreases Raf-1 and Akt levels and induces apoptosis in neuroblastomas

Neuroblastomas are the most common extracranial solid tumors of childhood. These tumors are associated with an overall poor prognosis, particularly for advanced stage disease. The benzoquinone ansamycin antibiotic, geldanamycin (GA), exhibits potent antitumor activity in certain cancer cell lines by destabilizing important signal transduction proteins (e.g., Raf-1 and Akt). The purpose of our study was to determine whether GA can alter the expression of Raf-1 and Akt, which have been shown to be critical for neuronal cell survival, and induce apoptosis of neuroblastoma cells. Human neuroblastoma cells (SH-SY5Y, SK-N-SH and LAN-1) were treated with GA for a variable period of time. Cell viability was assessed with MTT assays. Apoptosis was assessed with DNA fragmentation ELISA, TUNEL-flow cytometric assay, Western blot and caspase activities. We found that GA decreases cell viability and induces apoptosis in the SH-SY5Y human neuroblastoma cell line. These effects were mediated through activation of caspase-9 and -3, mitochondrial release of cytochrome c and subsequent PARP cleavage. GA-induced apoptosis was associated with a reduction in the level and activity of Raf-1 and Akt. The importance of these proteins was further demonstrated by induction of apoptosis in SH-SY5Y cells by a combination of U0126 (MEK1/2 inhibitor) and LY294002 (an inhibitor of PI3K). Similar to SH-SY5Y cells, other human neuroblastoma cells (SK-N-SH and LAN-1) were sensitive to the effects of GA-induced apoptosis. Taken together, our findings suggest that GA may be a novel therapeutic agent, which may be effective in the treatment of neuroblastomas.

Neurotrophin-3 promotes cell death induced in cerebral ischemia, oxygen-glucose deprivation, and oxidative stress: possible involvement of oxygen free radicals

To explore the role of neurotrophin-3 (NT-3) during cerebral ischemia, NT-3-deficient brains were subjected to transient focal ischemia. Conditional mutant brains produced undetectable amounts of NT-3 mRNA, whereas the expression of the neurotrophin, BDNF, the NT-3 receptor, TrkC, and the nonselective, low-affinity neurotrophin receptor p75NTR, were comparable to wild-type. Baseline absolute blood flow, vascular and neuroanatomical features, as well as physiological measurements were also indistinguishable from wild-type. Interestingly, the absence of NT-3 led to a significantly decreased infarct volume 23 h after middle cerebral artery occlusion. Consistent with this, the addition of NT-3 to primary cortical cell cultures exacerbated neuronal death caused by oxygen-glucose deprivation. Coincubation with the oxygen free radical chelator, trolox, diminished potentiation of neuronal death. NT-3 also enhanced neuronal cell death and the production of reactive oxygen species caused by oxidative damage inducing agents. We conclude that endogenous NT-3 enhanced neuronal injury during acute stroke, possible by increasing oxygen-radical mediated cell death.

Populations of NGF-dependent neurones differ in their requirement for BAX to undergo apoptosis in the absence of NGF/TrkA signalling in vivo

Reports that apoptosis within populations of neurotrophin-dependent neurones is virtually eliminated in BAX-deficient mice and that BAX-deficient neurones survive indefinitely in culture without neurotrophins have led to the view that BAX is required for the death of neurotrophin-deprived neurones. To further examine this assertion in vivo, we have studied two populations of NGF-dependent neurones during the period of naturally occurring neuronal death in mice that lack BAX, NGF or the NGF receptor TrkA, alone and in combination. In the superior cervical ganglion (SCG), naturally occurring neuronal death and the massive loss of neurones that took place in the absence of NGF or TrkA were completely prevented by elimination of BAX. However, in the trigeminal ganglion, naturally occurring neuronal death was only partly abrogated by the elimination of BAX, and although the massive neuronal death that took place in this ganglion in the absence of NGF or TrkA was initially delayed in embryos lacking BAX, this subsequently occurred unabated. Accordingly, BAX-deficient neurones survived in defined without NGF whereas BAX-deficient trigeminal neurones died in the absence of NGF. These results indicate that whereas BAX is required for the death of SCG neurones during normal development and when these neurones are deprived of NGF/TrkA signalling in vivo, the death of trigeminal ganglion neurones occurs independently of BAX when they are deprived of NGF/TrkA signalling. We conclude that BAX is not universally required for neuronal death induced by neurotrophin deprivation, but that there are major differences for the requirement for BAX among different populations of NGF-dependent neurones.

RET signaling is essential for migration, axonal growth and axon guidance of developing sympathetic neurons

Sympathetic axons use blood vessels as an intermediate path to reach their final target tissues. The initial contact between differentiating sympathetic neurons and blood vessels occurs following the primary sympathetic chain formation, where precursors of sympathetic neurons migrate and project axons along or toward blood vessels. We demonstrate that, in Ret-deficient mice, neuronal precursors throughout the entire sympathetic nervous system fail to migrate and project axons properly. These primary deficits lead to mis-routing of sympathetic nerve trunks and accelerated cell death of sympathetic neurons later in development. Artemin is expressed in blood vessels during periods of early sympathetic differentiation, and can promote and attract axonal growth of the sympathetic ganglion in vitro. This analysis identifies RET and artemin as central regulators of early sympathetic innervation.

Multiple effects of artemin on sympathetic neurone generation, survival and growth

To define the role of artemin in sympathetic neurone development, we have studied the effect of artemin on the generation, survival and growth of sympathetic neurones in low-density dissociated cultures of mouse cervical and thoracic paravertebral sympathetic ganglia at stages throughout embryonic and postnatal development. Artemin promoted the proliferation of sympathetic neuroblasts and increased the generation of new neurones in cultures established from E12 to E14 ganglia. Artemin also exerted a transient survival-promoting action on newly generated neurones during these early stages of development. Between E16 and P8, artemin exerted no effect on survival, but by P12, as sympathetic neurones begin to acquire neurotrophic factor independent survival, artemin once again enhanced survival, and by P20 it promoted survival as effectively as nerve growth factor (NGF). During this late period of development, artemin also enhanced the growth of neurites from cultured neurones more effectively than NGF. Confirming the physiological relevance of the mitogenic action of artemin on cultured neuroblasts, there was a marked reduction in the rate of neuroblast proliferation in the sympathetic ganglia of mice lacking the GFRα3 subunit of the artemin receptor. These results indicate that artemin exerts several distinct effects on the generation, survival and growth of sympathetic neurones at different stages of development.

Role of PI 3-kinase, Akt and Bcl-2-related proteins in sustaining the survival of neurotrophic factor-independent adult sympathetic neurons

By adulthood, sympathetic neurons have lost dependence on NGF and NT-3 and are able to survive in culture without added neurotrophic factors. To understand the molecular mechanisms that sustain adult neurons, we established low density, glial cell-free cultures of 12-wk rat superior cervical ganglion neurons and manipulated the function and/or expression of key proteins implicated in regulating cell survival. Pharmacological inhibition of PI 3-kinase with LY294002 or Wortmannin killed these neurons, as did dominant-negative Class IA PI 3-kinase, overexpression of Rukl (a natural inhibitor of Class IA PI 3-kinase), and dominant-negative Akt/PKB (a downstream effector of PI 3-kinase). Phospho-Akt was detectable in adult sympathetic neurons grown without neurotrophic factors and this was lost upon PI 3-kinase inhibition. The neurons died by a caspase-dependent mechanism after inhibition of PI 3-kinase, and were also killed by antisense Bcl-xL and antisense Bcl-2 or by overexpression of Bcl-xS, Bad, and Bax. These results demonstrate that PI 3-kinase/Akt signaling and the expression of antiapoptotic members of the Bcl-2 family are required to sustain the survival of adult sympathetic neurons.

Retinoic acid combined with neurotrophin-3 enhances the survival and neurite outgrowth of embryonic sympathetic neurons

Both nerve growth factor (NGF) and neurotrophin-3 (NT-3) are necessary for the survival of embryonic sympathetic neurons in vivo. All-trans retinoic acid (atRA) has been shown to promote neurite outgrowth and long-term survival of chick embryonic sympathetic neurons cultured in the presence of NGF. The present study shows that atRA can also potentiate the survival and neurite outgrowth-promoting activities of NT-3. This was accomplished by enhancing the survival of existing neurons, as cell proliferation was unaffected by exposure to atRA. atRA also enhanced neurite outgrowth of the NT-3-treated cells; however, the neurites appeared thicker and less branched than cells treated with atRA in combination with NGF. Using a quantitative PCR assay, trkA and p75(NTR) mRNAs, but not trkC mRNA, were increased ( approximately 1.5- to 2-fold) after 72 and 48 hr of exposure of the cultures to atRA, respectively. The atRA-induced increase in trkA mRNA may play a role in the enhanced survival of neurons cultured in the presence of either NGF or NT-3, as both neurotrophins have been shown to signal through this receptor. The time course of these mRNA changes would indicate that atRA does not regulate the neurotrophin receptor mRNA directly, rather, intervening gene transcription is required. Thus, during development, atRA may play a role in fine-tuning embryonic responsiveness to both NT-3 and NGF.

Msx-2 and p21 mediate the pro-apoptotic but not the anti-proliferative effects of BMP4 on cultured sympathetic neuroblasts

The differentiation, survival, and proliferation of developing sympathetic neuroblasts are all coordinately promoted by neurotrophins. In this study, we demonstrate that bone morphogenetic protein 4 (BMP4), a factor known to be necessary for the differentiation of sympathetic neurons (Schneider et al., 1999), conversely reduces both survival and proliferation of cultured E14 sympathetic neuroblasts. The anti-proliferative effects of BMP4 occur more rapidly than the pro-apoptotic actions and appear to involve different intracellular mechanisms. BMP4 treatment induces expression of the transcription factor Msx-2 and the cyclin-dependent kinase inhibitor p21(CIP1/WAF1) (p21). Treatment of cells with oligonucleotides antisense to either of these genes prevents cell death after BMP4 treatment but does not significantly alter the anti-proliferative effects. Thus Msx-2 and p21 are necessary for BMP4-mediated cell death but not for promotion of exit from cell cycle. Although treatment of cultured E14 sympathetic neuroblasts with neurotrophins alone did not alter cell numbers, BMP4-induced cell death was prevented by co-treatment with either neurotrophin-3 (NT-3) or nerve growth factor (NGF). This suggests that BMP4 may also induce dependence of the cells on neurotrophins for survival. Thus, sympathetic neuron numbers may be determined in part by factors that inhibit the proliferation and survival of neuroblasts and make them dependent upon exogenous factors for survival.

Serum-free culture of dissociated, purified adult and aged sympathetic neurons and quantitative assays of growth and survival

In vitro studies of dissociated neurons have provided crucial data regarding the regulation of plasticity in embryonic and perinatal neurons from both central and peripheral nervous systems. There have been few attempts to apply these methods to adult or aged neurons and the methods that have been reported have not been able to dissect the possible confounding contributions of non-neuronal cells and serum. Furthermore, quantitative assays of cultured neurons, particularly of their growth, have rarely been described. We report here the development of a novel method for the dissociation, purification and culture of sympathetic neurons from the adult and aged rat SCG under serum free conditions and in defined media. The technique results in a relatively high yield of viable, growing neurons. We describe methods for assaying the total yield of neurons, the proportion of surviving neurons and the proportion of neurons initiating neurite outgrowth after plating. A novel semiautomated assay of neurite outgrowth is outlined using image analysis of composite images of immunofluorescence-stained single neurons.

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.

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

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

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.

The mammalian ShcB and ShcC phosphotyrosine docking proteins function in the maturation of sensory and sympathetic neurons

Shc proteins possess SH2 and PTB domains and serve a scaffolding function in signaling by a variety of receptor tyrosine kinases. There are three known mammalian Shc genes, of which ShcB and ShcC are primarily expressed in the nervous system. We have generated null mutations in ShcB and ShcC and have obtained mice lacking either ShcB or ShcC or both gene products. ShcB-deficient animals exhibit a loss of peptidergic and nonpeptidergic nociceptive sensory neurons, which is not enhanced by additional loss of ShcC. Mice lacking both ShcB and ShcC exhibit a significant loss of neurons within the superior cervical ganglia, which is not observed in either mutant alone. The results indicate that these Shc family members possess both unique and overlapping functions in regulating neural development and suggest physiological roles for ShcB/ShcC in TrkA signaling.

Neurotrophin-3 promotes the cholinergic differentiation of sympathetic neurons

Neurotrophins influence the epigenetic shaping of the vertebrate nervous system by regulating neuronal numbers during development and synaptic plasticity. Here we attempt to determine whether these growth factors can also regulate neurotransmitter plasticity. As a model system we used the selection between noradrenergic and cholinergic neurotransmission by paravertebral sympathetic neurons. Developing sympathetic neurons express the neurotrophin receptors TrkA and TrkC, two highly related receptor tyrosine kinases. Whereas the TrkA ligand nerve growth factor (NGF) has long been known to regulate both the survival and the expression of noradrenergic traits in sympathetic neurons, the role of TrkC and of its ligand neurotrophin-3 (NT3) has remained unclear. We found that TrkC expression in the avian sympathetic chain overlaps substantially with that of choline acetyltransferase. In sympathetic chain explants, transcripts of the cholinergic marker genes choline acetyltransferase and vasoactive intestinal polypeptide were strongly enriched in the presence of NT3 compared with NGF, whereas the noradrenergic markers tyrosine hydroxylase and norepinephrine transporter were reduced. The transcription factor chicken achaete scute homolog 1 was coexpressed with cholinergic markers. The effects of NT3 are reversed and antagonized by NGF. They are independent of neuronal survival and developmentally regulated. These results suggest a role for NT3 as a differentiation factor for cholinergic neurons and establish a link between neurotrophins and neurotransmitter plasticity.

Levels of nerve growth factor and neurotrophin-3 are affected differentially by the presence of p75 in sympathetic neurons in vivo

The development and survival of sympathetic neurons is critically dependent on the related neurotrophic factors nerve growth factor (NGF) and neurotrophin-3 (NT3), the actions of which must be executed appropriately despite spatial and temporal overlaps in their activities. The tyrosine receptor kinases, trkA and trkC, are the cognate receptors for NGF and NT3, respectively. The p75 neurotrophin receptor has been implicated in neurotrophin binding and signaling for both NGF and NT3. In this study, the authors used mice that overexpressed NGF (NGF-OE) or NT3 (NT3-OE) in skin and mice that lacked p75 (p75(-/-)) to understand the dynamics of sympathetic neuron response to each neurotrophin and to address the role of p75. NGF and NT3 were measured in sympathetic ganglia and skin (a major target of sympathetic neurons) by using the enzyme-linked immunosorbent assay (ELISA) technique. A three- to four-fold increase in skin NT3 was seen in both NT3-OE and p75(-/-) mice. Moreover, both mouse lines exhibited a three-fold increase in ganglionic NT3. However, the increase in ganglionic NT3 was accompanied by a decrease in ganglionic NGF in p75(-/-) mice but not in NT3-OE mice. This indicated that p75 plays an important role in determining the level of NGF within sympathetic neurons. In NGF-OE mice, the overexpression of NGF was correlated with increased ganglionic NGF and increased ganglionic expression of p75 mRNA. In addition, in NGF-OE mice, ganglionic trkC expression was decreased, as was the amount of NT3 present within sympathetic ganglia. These results indicate that the level of p75 is integral in determining the level of sympathetic NGF and that NGF competes with NT3 by increasing the expression of p75 and decreasing the expression of trkC.

Expression of neurexin Ialpha splice variants in sympathetic neurons: selective changes during differentiation and in response to neurotrophins

Neurexins are a surprisingly diverse group of alternatively spliced proteins possibly involved in neural cell recognition processes. We find neurexin Ialpha and its splice variants highly conserved between mammals and birds. In vivo, neurexin Ialpha is expressed in sympathetic neurons during target innervation and relative expression levels of splice variants change with development. In vitro, no such changes are observed in the absence of growth factors, indicating that interactions with the environment are required to modify the splicing pattern. Specific alterations in splice variant expression are induced in vitro by neurotrophins. Expression patterns of splice variants in vivo and neurotrophin-induced regulation without changes in cell composition in vitro demonstrate that neurexin splice variant expression varies during differentiation of individual neurons. Our data suggest that changes in neurexin splice variants contribute to alterations of neuronal cell surface properties during target innervation.

Inactivation of one copy of the mouse neurotrophin-3 gene induces cardiac sympathetic deficits

Whether two copies of the neurotrophin-3 (NT3) gene are necessary for proper development of cardiac sympathetic innervation was investigated in mice carrying a targeted inactivation of the NT3 gene. Heterozygous (+/-) and null (-/-) mutant mice had fewer stellate ganglion neurons than did wild-type (+/+) mice at postnatal day 0 (P0 or birth), and this deficit was maintained between adult (P60) +/- and +/+ mice. The sympathetic innervation of the heart matured postnatally in +/+ and +/- mice. Tyrosine hydroxylase (TH)-positive axons were restricted largely to the epicardium at P0, were concentrated around large blood vessels in the myocardium at P21, and were present among cardiac myocytes at P60. Cardiac norepinephrine (NE) concentrations paralleled the growth of the sympathetic axons into the heart. NE concentrations were equivalent among +/+, +/-, and -/- mice at birth, but differences between +/- and +/+ mice increased with age. Adult +/- mice also exhibited lower resting heart rates and sympathetic tonus than +/+ mice. Thus deletion of one copy of the NT3 gene translates into anatomical, biochemical, and functional deficits in cardiac sympathetic innervation of postnatal mice, thereby indicating a gene-dosage effect for the NT3 gene.

Autocrine expression and ontogenetic functions of the PACAP ligand/receptor system during sympathetic development

The superior cervical ganglion (SCG) is a well-characterized model of neural development, in which several regulatory signals have been identified. Vasoactive intestinal peptide (VIP) has been found to regulate diverse ontogenetic processes in sympathetics, though functional requirements for high peptide concentrations suggest that other ligands are involved. We now describe expression and functions of pituitary adenylate cyclase-activating polypeptide (PACAP) during SCG ontogeny, suggesting that the peptide plays critical roles in neurogenesis. PACAP and PACAP receptor (PAC(1)) mRNA's were detected at embryonic days 14.5 (E14.5) through E17.5 in vivo and virtually all precursors exhibited ligand and receptor, indicating that the system is expressed as neuroblasts proliferate. Exposure of cultured precursors to PACAP peptides, containing 27 or 38 residues, increased mitogenic activity 4-fold. Significantly, PACAP was 1000-fold more potent than VIP and a highly potent and selective antagonist entirely blocked effects of micromolar VIP, consistent with both peptides acting via PAC(1) receptors. Moreover, PACAP potently enhanced precursor survival more than 2-fold, suggesting that previously defined VIP effects were mediated via PAC(1) receptors and that PACAP is the more significant developmental signal. In addition to neurogenesis, PACAP promoted neuronal differentiation, increasing neurite outgrowth 4-fold and enhancing expression of neurotrophin receptors trkC and trkA. Since PACAP potently activated cAMP and PI pathways and increased intracellular Ca(2+), the peptide may interact with other developmental signals. PACAP stimulation of precursor mitosis, survival, and trk receptor expression suggests that the signaling system plays a critical autocrine role during sympathetic neurogenesis.