Nerve growth factor-mediated induction of tyrosine hydroxylase in rat superior cervical ganglia in vitro

The biology of neurotrophins: cardiovascular function

This chapter addresses the role of neurotrophins in the development of the heart, blood vessels, and neural circuits that control cardiovascular function, as well as the role of neurotrophins in the mature cardiovascular system. The cardiovascular system includes the heart and vasculature whose functions are tightly controlled by the nervous system. Neurons, cardiomyocytes, endothelial cells, vascular smooth muscle cells, and pericytes are all targets for neurotrophin action during development. Neurotrophin expression continues throughout life, and several common pathologies that impact cardiovascular function involve changes in the expression or activity of neurotrophins. These include atherosclerosis, hypertension, diabetes, acute myocardial infarction, and heart failure. In many of these conditions, altered expression of neurotrophins and/or neurotrophin receptors has direct effects on vascular endothelial and smooth muscle cells in addition to effects on nerves that modulate vascular resistance and cardiac function. This chapter summarizes the effects of neurotrophins in cardiovascular physiology and pathophysiology.

Altered norepinephrine content and ventricular function in p75NTR-/- mice after myocardial infarction

Cardiac sympathetic neurons stimulate heart rate and the force of contraction through release of norepinephrine. Nerve growth factor modulates sympathetic transmission through activation of TrkA and p75NTR. Nerve growth factor plays an important role in post-infarct sympathetic remodeling. We used mice lacking p75NTR to examine the effect of altered nerve growth factor signaling on sympathetic neuropeptide expression, cardiac norepinephrine, and ventricular function after myocardial infarction. Infarct size was similar in wildtype and p75NTR-/- mice after ischemia-reperfusion surgery. Likewise, mRNAs encoding vasoactive intestinal peptide, galanin, and pituitary adenylate cyclase activating peptides were identical in wildtype and p75NTR-/- cardiac sympathetic neurons, as was expression of the TrkA neurotrophin receptor. Norepinephrine content was elevated in the base of the p75NTR-/- ventricle compared to wildtype, but levels were identical below the site of occlusion. Left ventricular pressure, dP/dt(MAX), and dP/dt(MIN) were measured under isoflurane anesthesia 3 and 7 days after surgery. Ventricular pressure decreased significantly 3 days after infarction, and deficits in dP/dt(MAX) were revealed by stimulating beta receptors with dobutamine and release of endogenous norepinephrine with tyramine. dP/dt(MIN) was not altered by genotype or surgical group. Few differences were observed between genotypes 3 days after surgery, in contrast to low pressure and dP/dt(MAX) previously reported in control p75NTR-/- animals. Seven days after surgery ventricular pressure and dP/dt(MAX) were significantly lower in p75NTR-/- hearts compared to WT hearts. Thus, the lack of p75NTR did not enhance cardiac function after myocardial infarction.

Paracrine control of vascular innervation in health and disease

Proper vascular regulation is of paramount importance for the control of blood flow to tissues. In particular, the regulation of peripheral resistance arteries is essential for several physiological processes, including control of blood pressure, thermoregulation and increase of blood flow to central nervous system and heart under stress conditions such as hypoxia. Arterial tone is regulated by the periarterial autonomic nervous plexus, as well as by endothelium-dependent, myogenic and humoral mechanisms. Underscoring the importance of proper vascular regulation, defects in these processes can lead to diseases such as hypertension, orthostatic hypotension, Raynaud's phenomenon, defective thermoregulation, hand-foot syndrome, migraine and congestive heart failure. Here, we review the molecular mechanisms controlling the development of the periarterial nerve plexus, retrograde and localized signalling at neuro-effector junctions, the molecular and cellular mechanisms of vascular regulation and adult plasticity and maintenance of periarterial innervation. We particularly highlight a newly discovered role for vascular endothelial growth factor in the structural and functional maintenance of arterial neuro-effector junctions. Finally, we discuss how defects in neuronal vascular regulation can lead to disease.

Heterogeneous ventricular sympathetic innervation, altered beta-adrenergic receptor expression, and rhythm instability in mice lacking the p75 neurotrophin receptor

Sympathetic nerves stimulate cardiac function through the release of norepinephrine and the activation of cardiac beta(1)-adrenergic receptors. The sympathetic innervation of the heart is sculpted during development by chemoattractive factors including nerve growth factor (NGF) and the chemorepulsive factor semaphorin 3a. NGF acts through the TrkA receptor and the p75 neurotrophin receptor (p75(NTR)) in sympathetic neurons. NGF stimulates sympathetic axon extension into the heart through TrkA, but p75(NTR) modulates multiple coreceptors that can either stimulate or inhibit axon outgrowth. In mice lacking p75(NTR), the sympathetic innervation density in target tissues ranges from denervation to hyperinnervation. Recent studies have revealed significant changes in the sympathetic innervation density of p75NTR-deficient (p75(NTR-/-)) atria between early postnatal development and adulthood. We examined the innervation of adult p75(NTR-/-) ventricles and discovered that the subendocardium of the p75(NTR-/-) left ventricle was essentially devoid of sympathetic nerve fibers, whereas the innervation density of the subepicardium was normal. This phenotype is similar to that seen in mice overexpressing semaphorin 3a, and we found that sympathetic axons lacking p75(NTR) are more sensitive to semaphorin 3a in vitro than control neurons. The lack of subendocardial innervation was associated with decreased dP/dt, altered cardiac beta(1)-adrenergic receptor expression and sensitivity, and a significant increase in spontaneous ventricular arrhythmias. The lack of p75(NTR) also resulted in increased tyrosine hydroxylase content in cardiac sympathetic neurons and elevated norepinephrine in the right ventricle, where innervation density was normal.

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.

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.

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.

Opposing functions of GDNF and NGF in the development of cholinergic and noradrenergic sympathetic neurons

We identified a population of mature sympathetic neurons in which Ret, the receptor for glial cell line-derived neurotrophic factor (GDNF), is coexpressed with the neurotrophin-3 (NT3) receptor TrkC and choline acetyltransferase. In a complementary population the nerve growth factor receptor TrkA is coexpressed with the norepinephrine transporter. In accordance with these in vivo results, GDNF and neurturin promote the expression of cholinergic marker genes in sympathetic chain explants, similar to NT3 and ciliary neuronotrophic factor (CNTF). To define intracellular signaling mechanisms commonly activated by NT3, GDNF, or CNTF to promote cholinergic differentiation, we have analyzed the activation of intracellular signaling cascades. Signal transducer and activator of transcription-3 (STAT3) was strongly activated by CNTF but not by GDNF or NT3 and hence is not essential for cholinergic differentiation. We conclude that cholinergic properties can be regulated by neurotrophic factors from three different protein families, whereas noradrenergic properties are promoted by NGF.

Tyrosine hydroxylase and neuropeptide Y are increased in ciliary ganglia of sympathectomized rats

We have examined the expression of tyrosine hydroxylase and neuropeptide Y in ciliary ganglia of normal adult rats and of adult rats in which the environment of these neurons was altered by sympathectomy at birth. Following neonatal 6-hydroxydopamine treatment, the proportion of tyrosine hydroxylase-immunoreactive and neuropeptide Y-immunoreactive neurons in ciliary ganglia was significantly increased. In ciliary neurons of both control and sympathectomized rats, neuropeptide Y immunoreactivity was preferentially co-localized with tyrosine hydroxylase. Immunoblot analysis confirmed the presence of tyrosine hydroxylase and its increase following sympathectomy. In situ hybridization studies revealed that many ciliary neurons contain mRNA for tyrosine hydroxylase and for neuropeptide Y. Like tyrosine hydroxylase immunoreactivity, the number of ciliary neurons containing tyrosine hydroxylase mRNA and the amount of mRNA per cell were increased in 6-hydroxydopamine-treated rats. In contrast, neuropeptide Y mRNA levels were the same in control and 6-hydroxydopamine-treated rats. Nerve growth factor is a candidate for mediating the effects of sympathectomy and most ciliary neurons in control and sympathectomized rats expressed immunoreactivity for the low-affinity nerve growth factor receptor. In addition, ciliary neurons from 6-hydroxydopamine-treated animals possessed increased nerve growth factor receptor immunoreactivity. These studies indicate that both tyrosine hydroxylase and neuropeptide Y in the ciliary ganglion are regulated by alterations in their environment. Their expression was enhanced by chemical sympathectomy which does not affect ciliary neurons directly but, rather, removes sympathetic innervation of shared targets, including the iris. In situ hybridization analysis suggests that the increased tyrosine hydroxylase and neuropeptide Y levels result from different mechanisms and provides evidence that neuropeptide levels can be regulated without changes in mRNA levels.

Enucleation of the rat pheochromocytoma clonal cell line, PC12: effect on neurite outgrowth

The effect of removal of PC12 cell nuclei on neurite outgrowth was studied. Enucleation (80-90%) was accomplished in the presence of cytochalasin B using a centrifugation technique that exploited the very tight adhesivity of PC12 cells for a substratum composed of an extracellular matrix secreted by bovine corneal endothelial cells in response to epidermal growth factor treatment. Neither nucleated nor enucleated PC12 cells showed significant neurite outgrowth on this particular matrix in the absence of nerve growth factor. In the presence of nerve growth factor both PC12 cell types initiated neurite outgrowth, but whereas neurites from nucleated cells grew continuously for two days, those from enucleated cells reached a maximum length after one day. The results suggest that neurite initiation but not continued neurite growth or stabilization can occur in the absence of transcription.

Directed expression of NGF to pancreatic beta cells in transgenic mice leads to selective hyperinnervation of the islets

Nerve growth factor (NGF) is implicated in the differentiation of neurons in both the central and peripheral nervous systems. As a new approach to its role in neuronal development, we have used transgenic mice to selectively overexpress NGF in an innervated peripheral tissue, the islets of the pancreas. In two lines of mice, directed expression of NGF in the beta cells elicits a dramatic increase in the innervation of the islets, but not the surrounding exocrine tissue, by one class of sympathetic neurons. In contrast, the innervation by sensory and parasympathetic neurons appears unchanged. The results indicate that expression of NGF by a target tissue during neuronal development selectively influences the characteristics of its innervation.

Differential effects of nerve growth factor and ciliary neuronotrophic factor on catecholamine storage and catecholamine synthesizing enzymes of cultured rat chromaffin cells

The effects of nerve growth factor (NGF) and ciliary neuronotrophic factor (CNTF) on catecholamine content and in vitro activities of tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) were studied in adrenal chromaffin cells cultured from 8-day-old rats. Both NGF and CNTF enhanced chromaffin cell survival and partially prevented losses of adrenaline during the 4-day culture period in a dose-dependent manner. CNTF was more potent, although cellular levels of adrenaline and noradrenaline were not maintained. NGF did not add to the effect of CNTF. The effect of CNTF on catecholamine storage was not accompanied by changes in the activities of TH and PNMT. In contrast, NGF induced TH but not PNMT activity. These data indicate differences between the mechanisms by which NGF and CNTF affect adrenal chromaffin cells.

Tyrosine hydroxylase in "leaky" adrenal medullary cells: evidence for in situ phosphorylation by separate Ca2+ and cyclic AMP-dependent systems

The systems responsible for phosphorylating tyrosine hydroxylase, the rate-limiting enzyme of catecholamine biosynthesis, were investigated in situ in adrenal medullary cells made permeable to solutes of up to 1,000 dalton by exposure to brief intense electric fields. Two different phosphorylation systems were found. One is dependent on Ca2+, the other on cyclic AMP. The Ca2+-dependent system is half-maximally activated by 1-2 microM Ca2+ and 0.5 mM ATP, and follows a time course similar to that of secretion of catecholamines. Trifluoperazine (0.1 mM) does not inhibit significantly Ca2+-dependent phosphorylation of tyrosine hydroxylase in situ. The cyclic AMP-dependent system is half-maximally activated by addition of 0.5 microM cyclic AMP and about 0.3 mM ATP. Ca2+-dependent and cyclic AMP-dependent phosphorylations of tyrosine hydroxylase have roughly the same time course and are additive under conditions where one system is already saturated. Peptide maps of immunoprecipitated tyrosine hydroxylase, after in situ phosphorylation of the enzyme either in the presence of 10(-8)M Ca2+ plus 2 X 10(-5)M cyclic AMP or of 10(-5)M Ca2+, show a marked difference indicating that the enzyme contains several phosphorylation sites. At least one of these sites is phosphorylated only by the Ca2+-dependent system, whereas the other site(s) are phosphorylated by both the Ca2+- and cyclic AMP-dependent systems. The effect of in situ phosphorylation of tyrosine hydroxylase on its enzymatic activity was also investigated.

Monoclonal antibodies to tyrosine hydroxylase from rat pheochromocytoma PC12h cells with special reference to nerve growth factor-mediated increase of the immunoprecipitable enzymes

Twelve hybridomas, which secrete the monoclonal antibodies to rat pheochromocytoma tyrosine hydroxylase (TH), were obtained. All of these antibodies immunoprecipitated the TH molecules from rat pheochromocytoma PC12h cells, adrenal medulla and brain. Two antibodies, namely PCTH-3 and -7 (both IgG1), directly inhibited the catalytic activity of TH. Another antibody, PCTH-4 (IgG1), bound to the enzyme without inhibition of the catalytic activity. The antibodies PCTH-3 and -4 immunostained the 60 K bands on a nitrocellulose sheet, which were electrotransfered from the gel after the SDS-polyacrylamide gel electrophoresis of PC12h cell and rat adrenal medulla homogenates. The antibody PCTH-4 immunocytochemically labeled specific neurons in the locus ceruleus, hypothalamus and substantia nigra of rat brain. Immunotitration using PCTH-4 antibody revealed that the nerve growth factor-mediated increase of TH activity in PC12h cells is due to the increase of the enzyme molecules.

Target organ regulation of substance P in sympathetic neurons in culture

Target organ regulation of the putative, peptide neurotransmitter, substance P (SP) was examined in explants and dissociated cell cultures of the neonatal rat sympathetic superior cervical ganglion (SCG). SP levels increased dramatically in explants, rising more than 30-fold after 72 hr in culture. By contrast, peptide levels did not increase in dissociated ganglion cultures. However, SP increased almost 10-fold in cell cultures grown on a monolayer of cells derived from the pineal or salivary gland, targets of the SCG. By contrast, SP content did not increase in cultures grown on a substrate of cells derived from heart or intestine. Peptide identity in the SCG-target cocultures was authenticated by means of combined high-pressure liquid chromatography (HPLC)-radioimmunoassay. Moreover, immunohistochemical examination localized the peptide virtually exclusively to sympathetic neurons and nerve processes. Mechanisms mediating the sympathetic-target interaction were examined in SCG-pineal cocultures. The increase in peptide required interactions with living tissue, since substrates of killed target cells did not elevate SP levels. The target influences were not mediated by nerve growth factor or indoleamines, potential secretory products of pineal in culture. Veratridine treatment prevented the increase in SP in the cocultures, and tetrodotoxin blocked the veratridine effect, suggesting that sodium influx and membrane depolarization prevent SP elevation. Our observations suggest that sympathetic neuron interactions with target organs influence peptidergic expression, and that this interaction may be restricted to certain appropriate target structures.

3',5'-cyclic adenosine monophosphate- and Ca2+-calmodulin-dependent endogenous protein phosphorylation activity in membranes of the bovine chromaffin secretory vesicles: identification of two phosphorylated components as tyrosine hydroxylase and protein kinase regulatory subunit type II

Membranes of the secretory vesicles from bovine adrenal medulla were investigated for the presence of the endogenous protein phosphorylation activity. Seven phosphoprotein bands in the molecular weight range of 250,000 to 30,000 were observed by means of the sodium dodecyl sulphate electrophoresis and autoradiography. On the basis of the criteria of molecular weight, selective stimulation of the phosphorylation by cyclic AMP (as compared with cyclic GMP) and immunoprecipitation by specific antibodies, band 5 (molecular weight 60,300) was found to represent the phosphorylated form of the secretory vesicle-bound tyrosine hydroxylase. The electrophoretic mobility, the stimulatory and inhibitory effects of cyclic AMP in presence of Mg2+ and Zn,2+ respectively, and immunoreactivity toward antibodies showed band 6 to contain two forms of the regulatory subunits of the type II cyclic AMP-dependent protein kinase, distinguishable by their molecular weights (56,000 and 52,000, respectively). Phosphorylation of band 7 (molecular weight 29,800) was stimulated about 2 to 3 times by Ca2+ and calmodulin in the concentration range of both agents believed to occur in the secretory tissues under physiological conditions.