'Mature' nerve growth factor is a minor species in most peripheral tissues

Neurocardiology: translational advancements and potential

In our original white paper published in the The Journal of Physiology in 2016, we set out our knowledge of the structural and functional organization of cardiac autonomic control, how it remodels during disease, and approaches to exploit such knowledge for autonomic regulation therapy. The aim of this update is to build on this original blueprint, highlighting the significant progress which has been made in the field since and major challenges and opportunities that exist with regard to translation. Imbalances in autonomic responses, while beneficial in the short term, ultimately contribute to the evolution of cardiac pathology. As our understanding emerges of where and how to target in terms of actuators (including the heart and intracardiac nervous system (ICNS), stellate ganglia, dorsal root ganglia (DRG), vagus nerve, brainstem, and even higher centres), there is also a need to develop sensor technology to respond to appropriate biomarkers (electrophysiological, mechanical, and molecular) such that closed-loop autonomic regulation therapies can evolve. The goal is to work with endogenous control systems, rather than in opposition to them, to improve outcomes.

Pro Nerve Growth Factor and Its Receptor p75NTR Activate Inflammatory Responses in Synovial Fibroblasts: A Novel Targetable Mechanism in Arthritis

We have recently provided new evidence for a role of p75NTR receptor and its preferential ligand proNGF in amplifying inflammatory responses in synovial mononuclear cells of chronic arthritis patients. In the present study, to better investigate how activation of the p75NTR/proNGF axis impacts synovial inflammation, we have studied the effects of proNGF on fibroblast-like synoviocytes (FLS), which play a central role in modulating local immune responses and in activating pro-inflammatory pathways. Using single cell RNA sequencing in synovial tissues from active and treatment-naïve rheumatoid arthritis (RA) patients, we demonstrated that p75NTR and sortilin, which form a high affinity receptor complex for proNGF, are highly expressed in PRG4pos lining and THY1posCOL1A1pos sublining fibroblast clusters in RA synovia but decreased in RA patients in sustained clinical remission. In ex vivo experiments we found that FLS from rheumatoid arthritis patients (RA-FLS) retained in vitro a markedly higher expression of p75NTR and sortilin than FLS from osteoarthritis patients (OA-FLS). Inflammatory stimuli further up-regulated p75NTR expression and induced endogenous production of proNGF in RA-FLS, leading to an autocrine activation of the proNGF/p75NTR pathway that results in an increased release of pro-inflammatory cytokines. Our data on the inhibition of p75NTR receptor, which reduced the release of IL-1β, IL-6 and TNF-α, further confirmed the key role of p75NTR activation in regulating inflammatory cytokine production. In a set of ex vivo experiments, we used RA-FLS and cultured them in the presence of synovial fluids obtained from arthritis patients that, as we demonstrated, are characterized by a high concentration of proNGF. Our data show that the high levels of proNGF present in inflamed synovial fluids induced pro-inflammatory cytokine production by RA-FLS. The blocking of NGF binding to p75NTR using specific inhibitors led instead to the disruption of this pro-inflammatory loop, reducing activation of the p38 and JNK intracellular pathways and decreasing inflammatory cytokine production. Overall, our data demonstrate that an active proNGF/p75NTR axis promotes pro-inflammatory responses in synovial fibroblasts, thereby contributing to chronic synovial inflammation, and point to the possible use of p75NTR inhibitors as a novel therapeutic approach in chronic arthritis.

Understanding pain perception through genetic painlessness diseases: The role of NGF and proNGF

Nerve growth factor (NGF), by binding to TrkA and p75^NTR receptors, regulates the survival and differentiation of sensory neurons during development and mediates pain transmission and perception during adulthood, by acting at different levels of the nervous system. Key to understanding the role of NGF as a pain mediator is the finding that mutations (namely, R121W, V232fs and R221W) in the NGF gene cause painlessness disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). Here we shall review the consequences of these NGF mutations, each of which results in specific clinical signs: R221W determines congenital pain insensitivity with no overt cognitive disabilities, whereas V232fs and R121W also result in intellectual disability, thus showing similarities to HSAN IV, which is caused by mutations in TrkA, rather than to HSAN V. Comparing the cellular, biochemical and clinical findings of these mutations could help in better understanding not only the possible mechanisms underlying HSAN V, but also mechanisms of NGF signalling and roles. These mutations alter the balance between NGF and proNGF in favour of an accumulation of the latter, suggesting a possible role of proNGF as a molecule with an analgesic role. Furthermore, the neurotrophic and pronociceptive functions of NGF are split by the R221W mutation, making NGF variants based on this mutation interesting for designing therapeutic applications for many diseases. This review emphasizes the possibility of using the mutations involved in "painlessness" clinical disorders as an innovative approach to identify new proteins and pathways involved in pain transmission and perception. OUTSTANDING QUESTIONS: Why do homozygous HSAN V die postnatally? What is the cause of this early postnatal lethality? Is the development of a mouse or a human feeling less pain affecting higher cognitive and perceptual functions? What is the consequence of the HSAN V mutation on the development of joints and bones? Are the multiple fractures observed in HSAN V patients due exclusively to the carelessness consequent to not feeling pain, or also to an intrinsic frailty of their bones? Are heterodimers of NGF^WT and NGF^R221W in the heterozygote state formed? And if so, what are the properties of these heterodimeric proteins? How is the processing of proNGF^R221W to NGF^R221W affected by the mutation?

Effects of intranasally-delivered pro-nerve growth factors on the septo-hippocampal system in healthy and diabetic rats

Pro-nerve growth factor (proNGF) is the predominant form of NGF in the brain and its levels increase in neurodegenerative diseases. The balance between NGF receptors may explain the contradictory biological activities of proNGF. However, the specific role of the two main proNGF variants is mostly unexplored. proNGF-A is prevalently expressed in healthy brain, while proNGF-B content increases in the neuro-degenerating brain. Recently we have investigated in vitro the biological action of native mouse proNGF variants. To gain further insights into the specific functions of the two proNGFs, here we intranasally delivered mouse-derived proNGF-A and proNGF-B to the brain parenchyma of healthy and diabetic rats, the latter characterized by dysfunction in spatial learning and memory, in the septo-hippocampal circuitry and by relative increase in proNGF-B hippocampal levels. Exogenous proNGF-B induces depression of hippocampal DG-LTP and impairment of hippocampal neurogenesis in healthy animals, with concomitant decrease in basal forebrain cholinergic neurons and cholinergic fibers projecting to the hippocampus. proNGF-A, while ineffective in healthy animals, rescues the diabetes-induced impairment in DG-LTP and hippocampal neurogenesis, promoting the concomitant recovery of the basal forebrain cholinergic phenotype. Our experimental evidences suggest that the balance between different proNGFs may influence the development and progression of neurodegenerative diseases.

Construction and preliminary characterization of human recombinant proNGF-A variant

The precursor of Nerve Growth Factor (proNGF) is the predominant form of NGF in the brain, where its tissue levels are increased in neurodegenerative diseases. proNGF exists in two main splicing variants, the long proNGF-A and the short proNGF-B. We demonstrated that proNGF-B is selectively increased in the hippocampus of rats affected by early diabetic encephalopathy and that native, purified proNGFs elicit different responses when used to stimulate PC12 cells. Therefore, the evaluation of the proNGF-B/proNGF-A ratio may be of important diagnostic and prognostic value in pathologies characterized by dysfunctions of NGF system. To date there is not clear pharmacological characterization of the different proNGFs variants, due to the lack of a proper recombinant proNGF-A. Using a bioinformatics approach, we predicted aminoacid sites involved in proNGF-A intracellular cleavage/conversion into proNGF-B, we cloned and expressed non-cleavable proNGF-A in HeLa cells and pursued a first characterization of their secretion modalities. Finally, we studied the biological effects of different proNGF-A mutants, stimulating PC12 cells with conditioned media from transfected HeLa cells. Based on our results, we propose the A73Y mutation as essential to obtaining an intact proNGF-A, limiting its conversion to proNGF-B. proNGF-A A73Y is probably released in an activity dependent manner and, when supplied to PC12 cells, shows a moderate differentiative capacity opposed to high neuroprotective potential. This preliminary study lays the foundation for future research aimed at uncovering the selective biological activities of proNGF-A and proNGF-B, and at developing pharmacological treatments that target the unbalance of proNGF system, induced by neurodegeneration.

Muscarinic receptors modulate Nerve Growth Factor production in rat Schwann-like adipose-derived stem cells and in Schwann cells

Regenerative capability of the peripheral nervous system after injury is enhanced by Schwann cells (SCs) producing several growth factors. The clinical use of SCs in nerve regeneration strategies is hindered by the necessity of removing a healthy nerve to obtain the therapeutic cells. Adipose-derived stem cells (ASCs) can be chemically differentiated towards a SC-like phenotype (dASCs), and represent a promising alternative to SCs. Their physiology can be further modulated pharmacologically by targeting receptors for neurotransmitters such as acetylcholine (ACh). In this study, we compare the ability of rat dASCs and native SCs to produce NGF in vitro. We also evaluate the ability of muscarinic receptors, in particular the M2 subtype, to modulate NGF production and maturation from the precursor (proNGF) to the mature (mNGF) form. For the first time, we demonstrate that dASCs produce higher basal levels of proNGF and mature NGF compared to SCs. Moreover, muscarinic receptor activation, and in particular M2 subtype stimulation, modulates NGF production and maturation in both SCs and dASCs. Indeed, both cell types express both proNGF A and B isoforms, as well as mNGF. After M2 receptor stimulation, proNGF-B (25 kDa), which is involved in apoptotic processes, is strongly reduced at transcript and protein level. Thus, we demonstrate that dASCs possess a stronger neurotrophic potential compared to SCs. ACh, via M2 muscarinic receptors, contributes to the modulation and maturation of NGF, improving the regenerative properties of dASCs.

Different responses of PC12 cells to different pro-nerve growth factor protein variants

The present work aimed to explore the innovative hypothesis that different transcript/protein variants of a pro-neurotrophin may generate different biological outcomes in a cellular system. Nerve growth factor (NGF) is important in the development and progression of neurodegenerative and cancer conditions. Mature NGF (mNGF) originates from a precursor, proNGF, produced in mouse in two major variants, proNGF-A and proNGF-B. Different receptors bind mNGF and proNGF, generating neurotrophic or neurotoxic outcomes. It is known that dysregulation in the proNGF/mNGF ratio and in NGF-receptors expression affects brain homeostasis. To date, however, the specific roles of the two major proNGF variants remain unexplored. Here we attempted a first characterization of the possible differential effects of proNGF-A and proNGF-B on viability, differentiation and endogenous ngf gene expression in the PC12 cell line. We also investigated the differential involvement of NGF receptors in the actions of proNGF. We found that native mouse mNGF, proNGF-A and proNGF-B elicited different effects on PC12 cell survival and differentiation. Only mNGF and proNGF-A promoted neurotrophic responses when all NGF receptors are exposed at the cell surface. Tropomyosine receptor kinase A (TrkA) blockade inhibited cell differentiation, regardless of which NGF was added to culture media. Only proNGF-A exerted a pro-survival effect when TrkA was inhibited. Conversely, proNGF-B exerted differentiative effects when the p75 neurotrophin receptor (p75^NTR) was antagonized. Stimulation with NGF variants differentially regulated the autocrine production of distinct proNgf mRNA. Overall, our findings suggest that mNGF and proNGF-A may elicit similar neurotrophic effects, not necessarily linked to activation of the same NGF-receptor, while the action of proNGF-B may be determined by the NGF-receptors balance. Thus, the proposed involvement of proNGF/NGF on the development and progression of neurodegenerative and tumor conditions may depend on the NGF-receptors balance, on specific NGF trancript expression and on the proNGF protein variant ratio.

ProNGF and Neurodegeneration in Alzheimer's Disease

Profound and early basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Alzheimer's disease (AD). Loss of synapses between basal forebrain and hippocampal and cortical target tissue correlates highly with the degree of dementia and is thought to be a major contributor to memory loss. BFCNs depend for their survival, connectivity and function on the neurotrophin nerve growth factor (NGF) which is retrogradely transported from its sites of synthesis in the cortex and hippocampus. The form of NGF found in human brain is proNGF. ProNGF binds to the NGF receptors TrkA and p75^NTR, but it binds more strongly to p75^NTR and more weakly to TrkA than does mature NGF. This renders proNGF more sensitive to receptor balance than mature NGF. In the healthy brain, where BFCNs express both TrkA and p75^NTR, proNGF is neurotrophic, activating TrkA-dependent signaling pathways such as MAPK and Akt-mTOR and eliciting cell survival and neurite outgrowth. However, if TrkA is lost or if p75^NTR is increased, proNGF activates p75^NTR-dependent apoptotic pathways such as JNK. This receptor sensitivity serves as a neurotrophic/apoptotic switch that eliminates BFCNs that cannot maintain TrkA/p75^NTR balance and therefore synaptic connections with their targets. TrkA is increasingly lost in mild cognitive impairment (MCI) and AD. In addition, proNGF accumulates at BFCN terminals in cortex and hippocampus, reducing the amount of trophic factor that reaches BFCN cell bodies. The loss of TrkA and accumulation of proNGF occur early in MCI and correlate with cognitive impairment. Increased levels of proNGF and reduced levels of TrkA lead to BFCN neurodegeneration and eventual p75NTR-dependent apoptosis. In addition, in AD BFCNs suffer from reduced TrkA-dependent retrograde transport which reduces neurotrophic support. Thus, BFCNs are particularly vulnerable to AD due to their dependence upon retrograde trophic support from proNGF signaling and transport.

ProNGF-p75NTR axis plays a proinflammatory role in inflamed joints: a novel pathogenic mechanism in chronic arthritis

Objective

To identify the role of mature nerve growth factor (mNGF), its immature form proNGF and their receptors in arthritis inflammation.

Methods

Real-time PCR, western blot and ELISA were performed to evaluate NGF, proNGF, their receptor and cytokine expression in synovial tissue and cells of patients with juvenile idiopathic arthritis (JIA) and rheumatoid arthritis (RA), and controls.

Results

proNGF and not mNGF is the prevalent form measured in synovial fluids of patients with JIA and RA with synovial fibroblasts as a major source of proNGF in the inflamed synoviae. p75NTR, the specific receptor for proNGF, is the NGF receptor most expressed in mononuclear cells of patients with JIA, while TrkA is the prevalent receptor in healthy donors. In ex vivo experiments the effects of proNGF differ from those of mNGF, suggesting that the balance of p75NTR and TrkA expression represents a critical factor in regulating mNGF/proNGF functions, determining which intracellular pathways and biological activities are triggered. Contrary to NGF, proNGF administration increased inflammatory cytokines but not interleukin (IL)-10 expression, inducing a stronger activation of p38 and JNK pathways. proNGF effects depend on its binding to p75NTR, as inhibition of p75NTR with neutralising antibodies or LM11A-31 abolished proNGF-induced production of IL-6 in patients’ mononuclear cells, while inhibition of TrkA did not. There is a correlation in patients with arthritis between high p75NTR levels and severity of clinical symptoms.

Conclusions

Our data suggest that an active proNGF-p75NTR axis promotes proinflammatory mechanisms contributing to chronic tissue inflammation, and that the use of p75NTR inhibitors may represent a new therapeutic approach in chronic arthritis.

Recovery of hippocampal functions and modulation of muscarinic response by electroacupuncture in young diabetic rats

The muscarinic receptor response to acetylcholine regulates the hippocampal-related learning, memory, neural plasticity and the production and processing of the pro-nerve growth factor (proNGF) by hippocampal cells. The development and progression of diabetes generate a mild cognitive impairment reducing the functions of the septo-hippocampal cholinergic circuitry, depressing neural plasticity and inducing proNGF accumulation in the brain. Here we demonstrate, in a rat model of early type-1 diabetes, that a physical therapy, the electroacupuncture, counteracts the diabetes-induced deleterious effects on hippocampal physiology by ameliorating hippocampal-related memory functions; recovering the impaired long-term potentiation at the dentate gyrus (DG-LTP) and the lowered expression of the vesicular glutamate transporter 1; normalizing the activity-dependent release of proNGF in diabetic rat hippocampus. Electroacupuncture exerted its therapeutic effects by regulating the expression and activity of M1- and M2-acetylcholine muscarinic receptors subtypes in the dentate gyrus of hippocampus. Our results suggest that a physical therapy based on repetitive sensory stimulation could promote hippocampal neural activity, neuronal metabolism and functions, and conceivably improve the diabetes-induced cognitive impairment. Our data can support the setup of therapeutic protocols based on a better integration between physical therapies and pharmacology for the cure of diabetes-associated neurodegeneration and possibly for Alzheimer’s disease.

Maternal testosterone and placental function: Effect of electroacupuncture on placental expression of angiogenic markers and fetal growth

Women with polycystic ovary syndrome (PCOS) have elevated circulating androgens during pregnancy and are at an increased risk of adverse pregnancy outcomes. Here we tested the hypotheses that maternal androgen excess decrease placental and fetal growth, and placental expression of markers of steroidogenesis, angiogenesis and sympathetic activity, and that acupuncture with low-frequency electrical stimulation prevents these changes. Pregnant rats were exposed to vehicle or testosterone on gestational day (GD)15-19. Low-frequency electroacupuncture (EA) or handling, as a control for the EA procedure, was given to control or testosterone exposed dams on GD16-20. On GD21, blood pressure was measured and maternal blood, fetuses and placentas collected. Placental steroid receptor expression and proteins involved in angiogenic, neurotrophic and adrenergic signaling were analyzed. EA did not affect any variables in control rats except maternal serum corticosterone, which was reduced. EA in testosterone exposed dams compared with controls increased systolic pressure by 30%, decreased circulating norepinephrine and corticosterone, fetal and placental weight and placental VEGFR1 and proNGF protein expression, and increased the VEGFA/VEGFR1 ratio, mature NGF (mNGF) and the mNGF/proNGF ratio. In conclusion, low-frequency EA in control animals did not have any negative influence on any of the studied variables. In contrast, EA in pregnant dams exposed to testosterone increased blood pressure and impaired placental growth and function, leading to decreased fetal growth.

NGF and proNGF Reciprocal Interference in Immunoassays: Open Questions, Criticalities, and Ways Forward

The homeostasis between mature neurotrophin NGF and its precursor proNGF is thought to be crucial in physiology and in pathological states. Therefore, the measurement of the relative amounts of NGF and proNGF could serve as a footprint for the identification of disease states, for diagnostic purposes. Since NGF is part of proNGF, their selective identification with anti-NGF antibodies is not straightforward. Currently, many immunoassays for NGF measurement are available, while the proNGF assays are few and not validated by published information. The question arises, as to whether the commercially available assays are able to distinguish between the two forms. Also, since in biological samples the two forms coexist, are the measurements of one species affected by the presence of the other? We describe experiments addressing these questions. For the first time, NGF and proNGF were measured together and tested in different immunoassays. Unexpectedly, NGF and proNGF were found to reciprocally interfere with the experimental outcome. The interference also calls into question the widely used NGF ELISA methods, applied to biological samples where NGF and proNGF coexist. Therefore, an immunoassay, able to distinguish between the two forms is needed. We propose possible ways forward, toward the development of a selective assay. In particular, the use of the well validated anti-NGF αD11 antibody in an alphaLISA assay with optimized incubation times would be a solution to avoid the interference in the measurement of a mixed sample containing NGF and proNGF. Furthermore, we explored the possibility of measuring proNGF in a biological sample. But the available commercial kit for the detection of proNGF does not allow the measurement of proNGF in mouse brain tissues. Therefore, we validated an SPR approach for the measurement of proNGF in a biological sample. Our experiments help in understanding the technical limits in the measurement of the NGF/proNGF ratio in biological samples, and propose concrete solutions toward the solution of this problem.

HBpF-proBDNF: A New Tool for the Analysis of Pro-Brain Derived Neurotrophic Factor Receptor Signaling and Cell Biology

Neurotrophins activate intracellular signaling pathways necessary for neuronal survival, growth and apoptosis. The most abundant neurotrophin in the adult brain, brain-derived neurotrophic factor (BDNF), is first synthesized as a proBDNF precursor and recent studies have demonstrated that proBDNF can be secreted and that it functions as a ligand for a receptor complex containing p75NTR and sortilin. Activation of proBDNF receptors mediates growth cone collapse, reduces synaptic activity, and facilitates developmental apoptosis of motoneurons but the precise signaling cascades have been difficult to discern. To address this, we have engineered, expressed and purified HBpF-proBDNF, an expression construct containing a 6X-HIS tag, a biotin acceptor peptide (BAP) sequence, a PreScission^™ Protease cleavage site and a FLAG-tag attached to the N-terminal part of murine proBDNF. Intact HBpF-proBDNF has activities indistinguishable from its wild-type counterpart and can be used to purify proBDNF signaling complexes or to monitor proBDNF endocytosis and retrograde transport. HBpF-proBDNF will be useful for characterizing proBDNF signaling complexes and for deciphering the role of proBDNF in neuronal development, synapse function and neurodegenerative disease.

Putting together the clues of the everlasting neuro-cardiac liaison

Starting from the late embryonic development, the sympathetic nervous system extensively innervates the heart and modulates its activity during the entire lifespan. The distribution of myocardial sympathetic processes is finely regulated by the secretion of limiting amounts of pro-survival neurotrophic factors by cardiac cells. Norepinephrine release by the neurons rapidly modulates myocardial electrophysiology, and increases the rate and force of cardiomyocyte contractions. Sympathetic processes establish direct interaction with cardiomyocytes, characterized by the presence of neurotransmitter vesicles and reduced cell-cell distance. Whether such contacts have a functional role in both neurotrophin- and catecholamine-dependent communication between the two cell types, is poorly understood. In this review we will address the effects of the sympathetic neuron activity on the myocardium and the hypothesis that the direct neuro-cardiac contact might have a key role both in norepinephrine and neurotrophin mediated signaling. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

The mature/pro nerve growth factor ratio is decreased in the brain of diabetic rats: Analysis by ELISA methods

Nerve growth factor (NGF) is essential for the survival and functional maintenance of forebrain cholinergic neurons projecting mainly to the cortex and hippocampus. NGF is produced in these brain areas but while mature NGF (mNGF) has a survival/differentiative effect its precursor proNGF elicits apoptosis in cholinergic neurons. Impaired neurotransmission, loss of cholinergic phenotype and abnormal NGF content characterize the cholinergic circuitries in animal models of diabetic encephalopathy (DE). It is not known whether defective production or maturation of NGF could play a key role in cholinergic neurodegeneration in DE. Quantification of the mNGF/proNGF ratio is therefore needed to characterize the development and progression of NGF-related neuronal diseases. In our work, we aimed at developing ELISA methods to measure either mNGF or proNGF tissue concentration; and to define the mNGF/proNGF ratio in the rat cortex and hippocampus during the early stage of streptozotocin-induced type 1 diabetes. Using commercially available NGF ELISA kits and antibodies, we set up ELISAs for human and rat mNGF and proNGF. We then analyzed the mNGF/proNGF ratio in the cortex and hippocampus of DE rats and found that it decreased in both tissues starting from the fourth week after diabetes induction. In diabetic brain the increase in proNGF involves accumulation of the isoforms with molecular weights of 50 and 34 kDa. Our study for the first time specifically quantifies the absolute content of mature and proNGF and the mNGF/proNGF ratio in brain tissues, suggesting that early progression of experimental DE is characterized by defective maturation of NGF.

Synovial TRPV1 is upregulated by 17-β-estradiol and involved in allodynia of inflamed temporomandibular joints in female rats

Women with reproductive capability are more likely to suffer from temporomandibular disorders (TMD), with orofacial pain as the most common complaint. In the past, we focused on the role of estradiol in TMD pain through the nervous system. In this study, we explored estradiol's influence on synoviocyte gene expressions involved in the allodynia of the inflamed TMJ. The influence of 17-β-estradiol on NGF and TRPV1 expression in TMJ synovium was determined in vivo and in vitro and analyzed by Western blot and real-time PCR. Complete Freund's adjuvant (CFA) injection into the TMJ was used to induce TMJ arthritis. Capsazepine served as a TRPV1 antagonist. Head withdrawal threshold was examined using a von Frey Anesthesiometer. We observed that estradiol upregulated the expressions of TRPV1 and NGF in a dose-dependent manner. In the primary cultured synoviocytes, TRPV1 was upregulated by lipopolysaccharide (LPS), estradiol, and NGF, while NGF antibodies fully blocked LPS and estradiol-induced upregulation of TRPV1. Activation of TRPV1 in the primary synoviocytes with capsaicin, a TRPV1 agonist, dose-dependently enhanced COX-2 transcription. Moreover, intra-TMJ injection of TRPV1 antagonist, capsazepine, significantly attenuated allodynia of the inflamed TMJ induced by intra-TMJ injection of CFA in female rats. This article presents a possible local mechanism for estradiol that may be involved in TMJ inflammation or pain in the synovial membrane through the pain-related gene TRPV1. This finding could potentially help clinicians understand the sexual dimorphism of TMD pain.

NGF in Early Embryogenesis, Differentiation, and Pathology in the Nervous and Immune Systems

The physiology of NGF is extremely complex, and although the study of this neurotrophin began more than 60 years ago, it is far from being concluded. NGF, its precursor molecule pro-NGF, and their different receptor systems (i.e., TrkA, p75NTR, and sortilin) have key roles in the development and adult physiology of both the nervous and immune systems. Although the NGF receptor system and the pathways activated are similar for all types of cells sensitive to NGF, the effects exerted during embryonic differentiation and in committed mature cells are strikingly different and sometimes opposite. Bearing in mind the pleiotropic effects of NGF, alterations in its expression and synthesis, as well as variations in the types of receptor available and in their respective levels of expression, may have profound effects and play multiple roles in the development and progression of several diseases. In recent years, the use of NGF or of inhibitors of its receptors has been prospected as a therapeutic tool in a variety of neurological diseases and injuries. In this review, we outline the different roles played by the NGF system in various moments of nervous and immune system differentiation and physiology, from embryonic development to aging. The data collected over the past decades indicate that NGF activities are highly integrated among systems and are necessary for the maintenance of homeostasis. Further, more integrated and multidisciplinary studies should take into consideration these multiple and interactive aspects of NGF physiology in order to design new therapeutic strategies based on the manipulation of NGF and its intracellular pathways.

Pro-neurotrophins, sortilin, and nociception

Nerve growth factor (NGF) signaling is important in the development and functional maintenance of nociceptors, but it also plays a central role in initiating and sustaining heat and mechanical hyperalgesia following inflammation. NGF signaling in pain has traditionally been thought of as primarily engaging the classic high-affinity receptor tyrosine kinase receptor TrkA to initiate sensitization events. However, the discovery that secreted proforms of nerve NGF have biological functions distinct from the processed mature factors raised the possibility that these proneurotrophins (proNTs) may have distinct function in painful conditions. ProNTs engage a novel receptor system that is distinct from that of mature neurotrophins, consisting of sortilin, a type I membrane protein belonging to the VPS10p family, and its co-receptor, the classic low-affinity neurotrophin receptor p75NTR. Here, we review how this new receptor system may itself function with or independently of the classic TrkA system in regulating inflammatory or neuropathic pain.

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.

proNGF/NGF mixtures induce gene expression changes in PC12 cells that neither singly produces

BACKGROUND

Growing evidence shows that, in vivo, the precursor of Nerve Growth Factor (NGF), proNGF, displays biological activities different from those of its mature NGF counterpart, mediated by distinct, and somewhat complementary, receptor binding properties. NGF and proNGF induce distinct transcriptional signatures in target cells, highlighting their different bioactivities. In vivo, proNGF and mature NGF coexist. It was proposed that the relative proNGF/NGF ratio is important for their biological outcomes, especially in pathological conditions, since proNGF, the principal form of NGF in Central Nervous System (CNS), is increased in Alzheimer's disease brains. These observations raise a relevant question: does proNGF, in the presence of NGF, influence the NGF transcriptional response and viceversa? In order to understand the specific proNGF effect on NGF activity, depending on the relative proNGF/NGF concentration, we investigated whether proNGF affects the pattern of well-known NGF-regulated mRNAs.

RESULTS

To test any influence of proNGF on pure NGF expression fingerprinting, the expression level of a set of candidate genes was analysed by qReal-Time PCR in rat adrenal pheochromocytoma cell line PC12, treated with a mixture of NGF and proNGF recombinant proteins, in different stoichiometric ratios. These candidates were selected amongst a set of genes well-known as being rapidly induced by NGF treatment. We found that, when PC12 cells are treated with proNGF/NGF mixtures, a unique pattern of gene expression, which does not overlap with that deriving from treatment with either proNGF or NGF alone, is induced. The specific effect is also dependent on the stoichiometric composition of the mixture. The proNGF/NGF equimolar mixture seems to partially neutralize the specific effects of the proNGF or NGF individual treatments, showing a weaker overall response, compared to the individual contributions of NGF and proNGF alone.

CONCLUSIONS

Using gene expression as a functional read-out, our data demonstrate that the relative availability of NGF and proNGF in vivo might modulate the biological outcome of these ligands.

Decreased adrenoceptor stimulation in heart failure rats reduces NGF expression by cardiac parasympathetic neurons

Postganglionic cardiac parasympathetic and sympathetic nerves are physically proximate in atrial cardiac tissue allowing reciprocal inhibition of neurotransmitter release, depending on demands from central cardiovascular centers or reflex pathways. Parasympathetic cardiac ganglion (CG) neurons synthesize and release the sympathetic neurotrophin nerve growth factor (NGF), which may serve to maintain these close connections. In this study we investigated whether NGF synthesis by CG neurons is altered in heart failure, and whether norepinephrine from sympathetic neurons promotes NGF synthesis. NGF and proNGF immunoreactivity in CG neurons in heart failure rats following chronic coronary artery ligation was investigated. NGF immunoreactivity was decreased significantly in heart failure rats compared to sham-operated animals, whereas proNGF expression was unchanged. Changes in neurochemistry of CG neurons included attenuated expression of the cholinergic marker vesicular acetylcholine transporter, and increased expression of the neuropeptide vasoactive intestinal polypeptide. To further investigate norepinephrine's role in promoting NGF synthesis, we cultured CG neurons treated with adrenergic receptor (AR) agonists. An 82% increase in NGF mRNA levels was detected after 1h of isoproterenol (β-AR agonist) treatment, which increased an additional 22% at 24h. Antagonist treatment blocked isoproterenol-induced increases in NGF transcripts. In contrast, the α-AR agonist phenylephrine did not alter NGF mRNA expression. These results are consistent with β-AR mediated maintenance of NGF synthesis in CG neurons. In heart failure, a decrease in NGF synthesis by CG neurons may potentially contribute to reduced connections with adjacent sympathetic nerves.

The proform of glia cell line-derived neurotrophic factor: a potentially biologically active protein

Growing evidences have revealed that the proforms of several neurotrophins including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT3), by binding to p75 neurotrophin receptor and sortilin, could induce neuronal apoptosis and are implicated in the pathogenesis of various neurodegenerative diseases. The glial cell line-derived neurotrophic factor (GDNF), one of the most potent useful neurotrophic factors for the treatment of Parkinson's disease (PD), is firstly synthesized as the proform (proGDNF) like other neurotrophin NGF, BDNF, and NT3. However, little is known about proGDNF expression and secretion under physiological as well as pathological states in vivo or in vitro. In this study, we investigated the expression profile and dynamic changes of proGDNF in brains of aging and PD animal models, with the interesting finding that proGDNF was a predominant form of GDNF with molecular weight of about 36 kDa by reducing and nonreducing immunoblots in adult brains and was unregulated in the aging, lipopolysaccharide (LPS), and 1-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine (MPTP) insult. We further provided direct evidence that accompanied activation of primary astrocytes as well as C6 cell line induced by LPS stimulation, proGDNF was increasingly synthesized and released as the uncleaved form in cell culture. Taken together, our results strongly suggest that proGDNF may be a biologically active protein and has specific effects on the cells close to its secreting site, and a potentially important role of proGDNF signaling in the brains, in the glia-neuronal interaction or in the pathogenesis of PD, should merit further investigation.

Neurotrophins and nerve regeneration-associated genes are expressed in the cornea after lamellar flap surgery

PURPOSE

To determine the in vivo expression of neurotrophins (NTs) and nerve regeneration-associated genes (RAGs) after surgically creating a hinged lamellar corneal flap in thy1-YFP mice.

METHODS

Lamellar corneal flaps with multiple hinges were created in thy1-YFP mice. Mice were killed at weeks 2, 4, and 8. Quantitative polymerase chain reaction was performed to determine the expression of NTs and RAGs in the corneas after lamellar transection. Nerve growth factor (Ngf), brain-derived neurotrophic factor (Bdnf), glial cell-derived neurotrophic factor (Gdnf), neurotrophin 3, neurotrophin 5, small proline-rich repeat protein 1A (Sprr1a), growth-associated protein 43 (Gap43), and beta III tubulin (Tubb3) gene expressions were analyzed. Whole-mount confocal immunofluorescence and Western analyses were performed for localization and abundance of robustly expressed genes.

RESULTS

Sprouts of fine YFP-positive fronds emanating from transected (injured) nerve bundles were seen in the flap area at 2 weeks onward. Bdnf and Sprr1a were robustly and significantly expressed at 2 weeks postoperatively (>2-fold increase in expression; P<0.05). Bdnf localized to thy1-YFP+ cells in operated corneas. Sprr1a localized to corneal epithelial cell membranes. At 8 weeks, none of the NTs and RAGs had increased expression. Bdnf (ρ=0.73, P=0.001) and Sprr1a (ρ=0.76, P=0.001) showed a significant positive correlation with beta III tubulin.

CONCLUSIONS

The neurotrophin Bdnf and RAG Sprr1a are robustly and significantly expressed during corneal nerve regeneration in vivo.

[Precursors and propeptides of neurotrophic factors as the modulators of biological activity of its mature forms]

Here, we review the problems of neurotrophic factors' folding, the role of its precursors (proneurotrophins) and the contribution of elements deleted during its maturation (propeptides) in biological functioning of these growth factors.

Semaphorin 7a links nerve regeneration and inflammation in the cornea

PURPOSE

We determined Semaphorin 7a (Sema7a) localization and abundance in naive corneas and in corneas after nerve-transecting lamellar flap surgery, and determined the effect of Sema7a supplementation on corneal nerve regeneration and inflammation.

METHODS

Immunolocalization and Western blot analyses were performed to evaluate the abundance of Sema7a in naive corneas and corneas undergoing nerve regeneration after lamellar corneal surgery in thy1-YFP+ neurofluorescent mice. We used compartmental cultures of dissociated trigeminal ganglion cells to determine the effect of Sema7a exposure on neurite outgrowth in vitro. Finally, a Sema7a pellet was implanted under the corneal flap after lamellar transection surgery to determine the neuronal and inflammatory effects of Sema7a supplementation in vivo.

RESULTS

Sema7a was expressed in the corneal epithelium and stromal keratocytes, but was more abundant in the epithelium (74.3%) compared to the stroma (25.7%, P = 0.02). Sema7a expression was increased significantly in the cornea after lamellar corneal surgery and was localized to stromal cells near the regenerating nerve fronds. Exposure of trigeminal neurites to Sema7a (20 nM) in the side compartment increased neurite length significantly. The implanted Sema7a pellet increased significantly YFP+ inflammatory cell influx into the cornea as well as increased corneal nerve length.

CONCLUSIONS

Sema7a is expressed constitutively in the cornea, and potently stimulates nerve regeneration and inflammatory cell influx. Therefore, this immune semaphorin links nerve regeneration and inflammatory processes in the cornea.

Neurotrophic actions initiated by proNGF in adult sensory neurons may require peri-somatic glia to drive local cleavage to NGF

The nerve growth factor (NGF) precursor, proNGF, is implicated in various neuropathological states. ProNGF signals apoptosis by forming a complex with the receptors p75 and sortilin, however, it can also induce neurite growth, proposed to be mediated by the receptor of mature NGF, tyrosine kinase receptor A (TrkA). The way in which these dual effects occur in adult neurons is unclear. We investigated the neurotrophic effects of proNGF on peptidergic sensory neurons isolated from adult mouse dorsal root ganglia and found that proNGF stimulated neurite extension and branching, requiring p75, sortilin and TrkA. Neurite growth rarely occurred in sortilin-expressing neurons but was commonly observed in TrkA-positive, sortilin-negative neurons that associated closely with sortilin-positive glia. ProNGF was unable to induce local trophic effects at growth cones where sortilin-positive glia was absent. We propose that in adult sensory neurons the neurotrophic response to proNGF is mediated by NGF and TrkA, and that peri-somatic glia may participate in sortilin- and p-75 dependent cleavage of proNGF. The potential ability of local glial cells to provide a targeted supply of NGF may provide an important way to promote trophic (rather than apoptotic) outcomes under conditions where regeneration or sprouting is required.

Hippocampal nerve growth factor potentiated by 17β-estradiol and involved in allodynia of inflamed TMJ in rat

The hippocampus is believed to play an important role in sex-based differences of pain perception. Whether estrogen potentiates allodynia in the inflamed temporomandibular joint (TMJ) through affecting the expressions of pain-related genes in the hippocampus remains largely unknown. Because the nerve growth factor (NGF) is an important gene related to inflammatory pain, we tested whether hippocampal NGF may be involved in TMJ inflammatory pain. Here we showed that the rat hippocampal NGF was upregulated by TMJ inflammation induced by complete Freund adjuvant. NGF upregulation was further potentiated by estradiol in a dose-dependent manner. In contrast, NGF transcription in the amygdala, prefrontal cortex, and thalamus was not affected by TMJ inflammation and estradiol. An intrahippocampal injection of NGF antibody or NGF receptor inhibitor K252a (inhibitor for tropomyosin receptor kinase A, TrkA) reduced the allodynia of inflamed TMJ in proestrous rats. Our data suggest that the hippocampal NGF is involved in estradiol-sensitized allodynia of inflammatory TMJ pain.

PERSPECTIVE

We report that complete Freund adjuvant-induced temporomandibular joint (TMJ) inflammation upregulated hippocampal nerve growth factor (NGF) expression, and estradiol replacement potentiated this upregulation. These results propose that estradiol could modulate TMJ pain through the NGF signaling pathway in the hippocampus to exacerbate TMJ pain and offer a possible mechanism of sexual dimorphism of temporomandibular disorder pain.

Dysregulation of astrocyte-motoneuron cross-talk in mutant superoxide dismutase 1-related amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a neurodegenerative disease in which death of motoneurons leads to progressive failure of the neuromuscular system resulting in death frequently within 2-3 years of symptom onset. Focal onset and propagation of the disease symptoms to contiguous motoneuron groups is a striking feature of the human disease progression. Recent work, using mutant superoxide dismutase 1 murine models and in vitro culture systems has indicated that astrocytes are likely to contribute to the propagation of motoneuron injury and disease progression. However, the basis of this astrocyte toxicity and/or failure of motoneuron support has remained uncertain. Using a combination of in vivo and in vitro model systems of superoxide dismutase 1-related amyotrophic lateral sclerosis, linked back to human biosamples, we set out to elucidate how astrocyte properties change in the presence of mutant superoxide dismutase 1 to contribute to motoneuron injury. Gene expression profiling of spinal cord astrocytes from presymptomatic transgenic mice expressing mutant superoxide dismutase 1 revealed two striking changes. First, there was evidence of metabolic dysregulation and, in particular, impairment of the astrocyte lactate efflux transporter, with resultant decrease of spinal cord lactate levels. Second, there was evidence of increased nerve growth factor production and dysregulation of the ratio of pro-nerve growth factor to mature nerve growth factor, favouring p75 receptor expression and activation by neighbouring motoneurons. Functional in vitro studies showed that astrocytes expressing mutant superoxide dismutase 1 are toxic to normal motoneurons. We provide evidence that reduced metabolic support from lactate release and activation of pro-nerve growth factor-p75 receptor signalling are key components of this toxicity. Preservation of motoneuron viability could be achieved by increasing lactate provision to motoneurons, depletion of increased pro-nerve growth factor levels or p75 receptor blockade. These findings are likely to be relevant to human amyotrophic lateral sclerosis, where we have demonstrated increased levels of pro-nerve growth factor in cerebrospinal fluid and increased expression of the p75 receptor by spinal motoneurons. Taken together, these data confirm that altered properties of astrocytes are likely to play a crucial role in the propagation of motoneuron injury in superoxide dismutase 1-related amyotrophic lateral sclerosis and indicate that manipulation of the energy supply to motoneurons as well as inhibition of p75 receptor signalling may represent valuable neuroprotective strategies.

Transsynaptic activity-dependent regulation of axon branching and neurotrophin expression in vivo

The two major classes of activity-dependent neuroplasticity predict different consequences of activity alteration on circuit response. Hebbian plasticity (positive feedback) posits that alteration of neuronal activity causes a parallel response within a circuit. In contrast, homeostatic plasticity (negative feedback) predicts that altering neuronal activity results in compensatory responses within a circuit. The relative roles of these modes of plasticity in vivo are unclear, since neuronal circuits are difficult to manipulate in the intact organism. In this study, we tested the in vivo effects of activity deprivation in the superior cervical ganglion-pineal circuit of adult rats, which can be noninvasively silenced by exposing animals to constant light. We demonstrated that total deprivation of sympathetic activity markedly decreased the presence of axonal proteins in the pineal and reduced the density and thickness of sympathetic axonal arbors. In addition, we demonstrated that sympathetic inactivity eliminated pineal function and markedly decreased pineal expression of neurotrophins. Administration of β-adrenergic agonist restored the expression of presynaptic and postsynaptic proteins. Furthermore, compensatory axonal growth through collateral sprouting, normally seen following unilateral denervation of the pineal, was profoundly impaired in the absence of neural activity. Thus, these data suggest that sympathetic axonal terminals are maintained by neural activity that induces neurotrophins, which may act through a retrograde mechanism to preserve the integrity of axonal arbors via a positive feedback loop. Conversely, by using Hebbian-like neuroplasticity, silent yet intact circuits enter a hibernation mode marked by reduction of presynaptic axonal structures and dramatically reduced postsynaptic expression of neurotrophins.

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.

Biological activity of nerve growth factor precursor is dependent upon relative levels of its receptors

Nerve growth factor (NGF) is produced as a precursor called pro-nerve growth factor (proNGF), which is secreted by many tissues and is the predominant form of NGF in the central nervous system. In Alzheimer disease brain, cholinergic neurons degenerate and can no longer transport NGF as efficiently, leading to an increase in untransported NGF in the target tissue. The protein that accumulates in the target tissue is proNGF, not the mature form. The role of this precursor is controversial, and both neurotrophic and apoptotic activities have been reported for recombinant proNGFs. Differences in the protein structures, protein expression systems, methods used for protein purification, and methods used for bioassay may affect the activity of these proteins. Here, we show that proNGF is neurotrophic regardless of mutations or tags, and no matter how it is purified or in which system it is expressed. However, although proNGF is neurotrophic under our assay conditions for primary sympathetic neurons and for pheochromocytoma (PC12) cells, it is apoptotic for unprimed PC12 cells when they are deprived of serum. The ratio of tropomyosin-related kinase A to p75 neurotrophin receptor is low in unprimed PC12 cells compared with primed PC12 cells and sympathetic neurons, altering the balance of proNGF-induced signaling to favor apoptosis. We conclude that the relative level of proNGF receptors determines whether this precursor exhibits neurotrophic or apoptotic activity.

p75 and TrkA signaling regulates sympathetic neuronal firing patterns via differential modulation of voltage-gated currents

Neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) act through the tropomyosin-related receptor tyrosine kinases (Trk) and the pan-neurotrophin receptor (p75) to regulate complex developmental and functional properties of neurons. While NGF activates both receptor types in sympathetic neurons, differential signaling through TrkA and p75 can result in widely divergent functional outputs for neuronal survival, growth, and synaptic function. Here we show that TrkA and p75 signaling pathways have opposing effects on the firing properties of sympathetic neurons, and define a mechanism whereby the relative level of signaling through these two receptors sets firing patterns via coordinate regulation of a set of ionic currents. We show that signaling through the p75 pathway causes sympathetic neurons to fire in a phasic pattern showing marked accommodation. Signaling through the NGF-specific TrkA, on the other hand, causes cells to fire tonically. Neurons switch rapidly between firing patterns, on the order of minutes to hours. We show that changes in firing patterns are caused by neurotrophin-dependent regulation of at least four voltage-gated currents: the sodium current and the M-type, delayed rectifier, and calcium-dependent potassium currents. Neurotrophin release, and thus receptor activation, varies among somatic tissues and physiological state. Thus, these data suggest that target-derived neurotrophins may be an important determinant of the characteristic electrical properties of sympathetic neurons and therefore regulate the functional output of the sympathetic nervous system.

p75 Neurotrophin receptor signaling regulates hepatic myofibroblast proliferation and apoptosis in recovery from rodent liver fibrosis

Hepatic myofibroblast apoptosis is critical to resolution of liver fibrosis. We show that human hepatic myofibroblasts co-express p75(NTR) (p75 neurotrophin receptor) and sortilin, thus facilitating differential responses to mature and pro nerve growth factor (proNGF). Although mature NGF is proapoptotic, proNGF protects human hepatic myofibroblasts from apoptosis. Moreover, in recovery from experimental liver fibrosis, the decrease in proNGF parallels loss of hepatic myofibroblasts by apoptosis. Macrophage-derived matrix metalloproteinase 7 (MMP7) cleaves proNGF in a concentration-dependent manner, and its expression in the liver coincides with falling proNGF levels. To define the dominant effect of p75(NTR)-mediated events in experimental liver fibrosis, we have used a mouse lacking the p75(NTR) ligand-binding domain but expressing the intracellular domain. We show that absence of p75(NTR) ligand-mediated signals leads to significantly retarded architectural resolution and reduced hepatic myofibroblast loss by apoptosis. Lack of the ligand-competent p75(NTR) limits hepatocyte and oval cell proliferative capacity in vivo without preventing hepatic stellate cell transdifferentiation.

CONCLUSION

NGF species have a differential effect on hepatic myofibroblast survival. Our data suggest that cleavage of proNGF by MMP7 during the early phase of recovery from liver fibrosis alters the pro/mature NGF balance to facilitate hepatic myofibroblast loss. Whereas fibrosis develops in the absence of p75(NTR) signaling, the dominant effects of loss of p75(NTR) ligand-mediated events are the retardation of liver fibrosis resolution via regulation of hepatic myofibroblast proliferation and apoptosis, and the reduction of hepatocyte and oval cell proliferation.

Expression profile of nerve growth factor after muscle incision in the rat

BACKGROUND

Previous studies have demonstrated that nerve growth factor (NGF) is an important mediator of pathologic pain. Many studies have focused on cutaneous mechanisms for NGF-induced hyperalgesia; few have examined its contribution in deeper tissues like muscle. This study examined pain behaviors and the expression of NGF in incised hind paw flexor digitorum brevis muscle.

METHODS

Adult Sprague-Dawley rats were pretreated with anti-NGF peptibody and underwent skin or skin plus deep fascia and muscle incision. Guarding pain behaviors were measured. Muscle NGF messenger RNA (mRNA) was measured by reverse-transcriptase polymerase chain reaction. Changes in NGF protein expression were measured using Western blot, enzyme-linked immunosorbent assay, and immunohistochemistry. In situ hybridization for NGF mRNA was also performed.

RESULTS

Pretreatment with anti-NGF peptibody (100 mg/kg) decreased the guarding behavior caused by deep fascia and muscle incision. Muscle NGF mRNA increased abruptly 2 h after incision and was the same as control by postoperative day 1. NGF protein increased from 4 h after incision and was sustained for several days. NGF was localized in many calcitonin gene-related peptide-positive axons, few N52-positive axons, but not isolectin B4-positive axons in incised muscle. The sources of NGF mRNA included keratinocytes in epidermis and fibroblasts in deeper tissues.

CONCLUSION

Fibroblasts adjacent to the injury are sources of NGF in incised muscle. NGF is upregulated by incision of muscle and contributes to guarding pain behavior.

Changes in NGF and NT-3 protein species in the superior cervical ganglion following axotomy of postganglionic axons

Mature sympathetic neurons in the superior cervical ganglion (SCG) are regulated by target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3). High molecular weight NGF species and mature NT-3 are the predominant NGF and NT-3 protein isoforms in the SCG, yet it is unknown whether the presence of these species is dependent on intact connection with the target tissues. In an attempt to determine the role of peripheral targets in regulating the neurotrophin species found in the SCG, we investigated the NGF and NT-3 protein species present in the SCG following axotomy (transection) or injury of the post-ganglionic axons. Following a 7 day axotomy, the 22-24 kDa NGF species and the mature 14 kDa NT-3 species in the SCG were significantly reduced by 99% and 66% respectively, suggesting that intact connection with the target is necessary for the expression of these protein species. As expected, tyrosine hydroxylase (TH) protein in the SCG was significantly reduced by 80% at 7 days following axotomy. In order to distinguish between the effects of injury and loss of target connectivity, the SCG was examined following compression injury to the post-ganglionic nerves. Following injury, no reduction in the 22-24 kDa NGF or 14 kDa mature NT-3 species was observed in the SCG. TH protein was slightly, yet significantly, decreased in the SCG following injury. The findings of this study suggest that the presence of the 22-24 kDa NGF and mature 14 kDa NT-3 species in the SCG is dependent on connection with peripheral targets and may influence, at least in part, TH protein expression in adult sympathetic neurons.

Components of the NGF signaling complex are altered in mdx mouse superior cervical ganglion and its target organs

We previously reported that in the superior cervical ganglion (SCG) of dystrophic mdx mice, which lack full-length dystrophin, there is a loss of neurons projecting to SCG muscular targets, like the iris. Nonetheless, surviving neurons, innervating either iris or submandibular gland (SuGl), a SCG non-muscular target, underwent reduced axon defasciculation and terminal branching. Here we report that, during early post-natal development, levels of pro-apoptotic proNGF in mdx mouse iris, but not in the SuGl, are higher than in the wild-type. This increase, along with reduced levels of NGF receptors (TrkA and p75NTR) in SCG, may be partly responsible for the observed loss of neurons projecting to the iris. These alterations, combined with a reduction in polysialylated-NCAM and neurofilament protein levels in SCG, may also account for reduced axon defasciculation and terminal branching in mdx mouse SCG targets.

Proneurotrophins require endocytosis and intracellular proteolysis to induce TrkA activation

The uncleaved, pro-form of nerve growth factor (proNGF) functions as a pro-apoptotic ligand for the p75 neurotrophin receptor (p75NTR). However, some reports have indicated that proneurotrophins bind and activate Trk receptors. In this study, we have examined proneurotrophin receptor binding and activation properties in an attempt to reconcile these findings. We show that proNGF readily binds p75NTR expressed in HEK293T cells but does not interact with TrkA expressed under similar circumstances. Importantly, proNGF activates TrkA tyrosine phosphorylation, induces Erk and Akt activation, and causes PC12 cell differentiation. We show that inhibiting endocytosis or furin activity reduced TrkA activation induced by proNGF but not that induced by mature NGF and that proNGF123, a mutant form of NGF lacking dibasic cleavage sites in the prodomain, does not induce TrkA phosphorylation in PC12 cells. Therefore, endocytosis and cleavage appear to be prerequisites for proNGF-induced TrkA activity. We also found that proBDNF induces activation of TrkB in cerebellar granule neurons and that proBDNF cleavage by furin and metalloproteases facilitates this effect. Taken together, these data indicate that under physiological conditions, proneurotrophins do not directly bind or activate Trk receptors. However, endocytosis and cleavage of proneurotrophins produce processed forms of neurotrophins that are capable of inducing Trk activation.

Up-regulation of pro-nerve growth factor, neurotrophin receptor p75, and sortilin is associated with retrovirus-induced spongiform encephalomyelopathy

The progressive spongiform encephalomyelopathy caused by ts1, a neuropathogenic temperature-sensitive mutant of Moloney murine leukemia virus (MoMuLV-ts1), results in motor neuronal loss without direct neuronal infection. We have previously reported that ts1-mediated neuronal degeneration in mice has a multifactorial pathogenesis. Here, we report that in the ts1-infected central nervous system (CNS) activated neural cells showed intense immunoreactivity for pro-nerve growth factor (proNGF), neurotrophin receptor p75 (p75(NTR)), and sortilin in the areas showing spongiform changes. Since recent studies suggested that proNGF is more active than mature NGF in inducing neuronal death after binding to co-receptors p75(NTR)/sortilin, we hypothesized that overexpression of proNGF, sortilin and p75(NTR) play a role in ts1-induced neurodegeneration. We found that proNGF and p75(NTR), but not sortilin, mRNA and protein were significantly elevated in ts1-infected brainstem compared to non-infected control tissue. There was extensive tyrosine phosphorylation of p75(NTR), a marker for its activation, in ts1-infected brainstem with abundance in degenerating neurons. We explored whether the increase in the in vivo proNGF expression also occurs in cultured immortalized C1 astrocytes infected by ts1 virus. The proNGF level was significantly increased in infected C1 cells compared to control cells only after addition of fibroblast growth factor (FGF-1). We also showed increased expression of FGF-1 in the CNS of ts1-infected mice. Our findings suggest that the FGF-1 signaling pathway may be responsible for the overexpression of proNGF in neural cells during pathogenesis of ts1-induced neurodegeneration. This study provides new in vivo insights into the possible role of proNGF and its receptors in ts1-induced neurodegeneration.

The nerve growth factor and its receptors in airway inflammatory diseases

The nerve growth factor (NGF) belongs to the neurotrophin family and induces its effects through activation of 2 distinct receptor types: the tropomyosin-related kinase A (TrkA) receptor, carrying an intrinsic tyrosine kinase activity in its intracellular domain, and the receptor p75 for neurotrophins (p75NTR), belonging to the death receptor family. Through activation of its TrkA receptor, NGF activates signalling pathways, including phospholipase Cgamma (PLCgamma), phosphatidyl-inositol 3-kinase (PI3K), the small G protein Ras, and mitogen-activated protein kinases (MAPK). Through its p75NTR receptor, NGF activates proapoptotic signalling pathways including the MAPK c-Jun N-terminal kinase (JNK), ceramides, and the small G protein Rac, but also activates pathways promoting cell survival through the transcription factor nuclear factor-kappaB (NF-kappaB). NGF was first described by Rita Levi-Montalcini and collaborators as an important factor involved in nerve differentiation and survival. Another role for NGF has since been established in inflammation, in particular of the airways, with increased NGF levels in chronic inflammatory diseases. In this review, we will first describe NGF structure and synthesis and NGF receptors and their signalling pathways. We will then provide information about NGF in the airways, describing its expression and regulation, as well as pointing out its potential role in inflammation, hyperresponsiveness, and remodelling process observed in airway inflammatory diseases, in particular in asthma.

Nerve Growth Factor Expression after Plantar Incision in the Rat

Background:

Postoperative pain control remains a significant problem. Advances will proceed if we can further reveal the underlying mechanisms of incisional pain and its mediators. Previous studies have demonstrated that nerve growth factor (NGF) is released in incised tissue and contributes to hyperalgesia in incisional pain. The purpose of this study is to examine the expression of NGF in skin after planter incision.

Methods:

Adult Sprague-Dawley rats underwent incision at the plantar aspect of hind paw. The NGF messenger RNA (mRNA) was measured at various times after incision by polymerase chain reaction. NGF protein expression was detected by Western blot and immunohistochemistry in incisions.

Results:

NGF mRNA increased from 2 to 4 h after incision and was the same as control by postoperative day 1. A large-molecular-weight form of NGF, approximately 75 kd, was found in normal skin. The large-molecular-weight NGF protein increased 4 h after incision and returned to baseline on postoperative day 7. The skin immediately adjacent to the incision had the greatest NGF expression. Immunohistochemical staining for NGF was present adjacent to the incision and localized in Schwann cells and axons.

Conclusion:

NGF mRNA is increased and a large-molecular-weight form of NGF protein is expressed in the region adjacent to the incision. NGF immunoreactivity is present in nerve bundles; both Schwann cells and axons are labeled. Immunoreactive NGF in axons is likely taken up into cut axons. This study suggests some common mechanisms for neuropathic and incisional pain.

Regulation of NGF and NT-3 protein expression in peripheral targets by sympathetic input

Nerve growth factor (NGF) and neurotrophin-3 (NT-3) are target-derived proteins that regulate innervating sympathetic neurons. Here, we used western blot analysis to investigate changes in NGF and NT-3 protein in several peripheral tissues following loss of sympathetic input. Following removal of the superior cervical ganglion (SCG), large molecular weight (MW) NGF species, including proNGF-A, were increased in distal intracranial SCG targets, such as pineal gland and extracerebral blood vessels (bv). Mature NGF was a minor species in these tissues and unchanged following sympathectomy. Large MW NGF species also were increased when sympathectomy was followed by in vivo NGF administration. Mature NT-3, which was abundant in controls, was significantly decreased in these targets following sympathetic denervation. The decrease in mature NT-3 was enhanced following NGF administration. The trigeminal ganglion, which provides sensory input to these targets, showed increased NGF, but decreased NT-3, in these treatments, demonstrating that decreased NT-3 at the targets did not result from enhanced NT-3 uptake. Unlike pineal gland and extracerebral bv, the external carotid artery, an extracranial proximal SCG target, showed no change in NGF following denervation, and mature NT-3 was significantly increased. Following NGF administration, NT-3 was significantly decreased. We provide evidence for sympathetic regulation of NGF and NT-3 in peripheral targets and that elevated NGF can depress NT-3. The differential response in distal and proximal adult targets is consistent with the idea that neurons innervating proximal and distal targets may serve different roles in regulating neurotrophin protein. In addition, we conclude that previous ELISA results showing increased NGF protein following sympathetic denervation may have resulted from increases in large MW species, rather than an increase in mature NGF.

Pro-region of neurotrophins determines the processing efficiency

Neurotrophins are synthesized as precursors called pro-neurotrophins and then mature neurotrophins are formed proteolytically from them. Recent findings revealed that pro- and mature neurotrophins elicit opposite functional effects on cell survival, highlighting the importance of this processing step. Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) belong to the neurotrophin family and are mutually homologous, but BDNF is less efficiently processed. In order to find the reason for this, we examined some possibilities by using PC12 cells, and found that the pro-region, especially the last half of it, affected very much the processing efficiency of these neurotrophins.

Estrogen regulation of neurotrophin expression in sympathetic neurons and vascular targets

We hypothesize that estrogen exerts a modulatory effect on sympathetic neurons to reduce neural cardiovascular tone and that these effects are modulated by nerve growth factor (NGF), a neurotrophin that regulates sympathetic neuron survival and maintenance. We examined the effects of estrogen on NGF and tyrosine hydroxylase (TH) protein content in specific vascular targets. Ovariectomized, adult Sprague-Dawley rats were implanted with placebo or 17beta-estradiol (release rate, 0.05 mg/day). Fourteen days later, NGF levels in the superior cervical ganglia (SCG) and its targets, the heart, external carotid artery, and the extracerebral blood vessels, as well as estrogen receptor alpha (ERalpha) content levels in the heart, were determined using semi-quantitative Western blot analysis. TH levels in the SCG and extracerebral blood vessels were determined by Western blotting and immunocytochemistry, respectively. Circulating levels of 17beta-estradiol and prolactin (PRL) were quantified by RIA. Estrogen replacement significantly decreased NGF protein in the SCG and its targets, the external carotid artery, heart and extracerebral blood vessels. TH protein associated with the extracerebral blood vessels was also significantly decreased, but ERalpha levels were significantly increased in the heart following estrogen replacement. These results indicate that estrogen reduces NGF protein content in sympathetic vascular targets, which may lead to decreased sympathetic innervations to these targets, and therefore reduced sympathetic regulation. In addition, the estrogen-induced increase in ERalpha levels in the heart, a target tissue of the SCG, suggests that estrogen may sensitize the heart to further estrogen modulation, and possibly increase vasodilation of the coronary vasculature.

Processing of nerve growth factor: the role of basic amino acid clusters in the pro-region

Neurotrophins are synthesized first as precursors called pro-neurotrophins, and their propeptides are then proteolytically removed to form mature neurotrophins. However, a significant proportion of total neurotrophins has been shown to be secreted as pro-neurotrophins. Furthermore, pro- and mature neurotrophins have been shown to elicit opposite effects on cell survival. Thus, the processing step of neurotrophins is very important. In order to understand the mechanism of neurotrophin processing, we focused on the two basic amino acid clusters in the pro-region of nerve growth factor (NGF). Various NGFs mutated at basic amino acids in the pro-region were introduced in COS7 and PC12 cells. The results indicated that these basic amino acid clusters were actually cleaved in the cells by furin, but that their cleavage contributed little to the production of mature NGF. However, one of the two sites was considered to contribute to mature NGF production depending on conditions used.

Increased NGF proforms in aged sympathetic neurons and their targets

Target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3) regulate sympathetic neuron survival. Here, NGF and NT-3 protein and transcript were examined in sympathetic neurons and targets in order to determine their role in age-related neuronal atrophy. One obvious alteration was a dramatic increase (up to 50-fold) in NGF protein forms, corresponding to proNGF-B, in the superior cervical ganglion (SCG) and targets where sympathetic innervation shows atrophy. In the iris, where sympathetic innervation is protected into old age, proNGF-B was decreased. Alterations in NGF transcript paralleled changes in NGF protein, albeit to a lesser degree. Though significantly increased in aged SCG, NT-3 protein, found primarily as the 'mature' form, showed only minor changes in most tissues, though NT-3 mRNA generally was decreased. In contrast, both NT-3 transcript and NT-3 precursors were increased in iris. The dramatic increases in proNGF, together with minimal changes in NT-3, suggest that alterations in NGF regulation may contribute to the loss of sympathetic innervation observed in many aged peripheral targets.