Nerve growth factor (NGF)-induced calcium influx and intracellular calcium mobilization in 3T3 cells expressing NGF receptors

A bone to pick-cellular and molecular mechanisms of bone pain in sickle cell disease

The bone is one of the most commonly affected organs in sickle cell disease (SCD). Repeated ischemia, oxidative stress and inflammation within the bone is largely responsible for promoting bone pain. As more individuals with SCD survive into adulthood, they are likely to experience a synergistic impact of both aging and SCD on their bone health. As bone health deteriorates, bone pain will likely exacerbate. Recent mechanistic and observational studies emphasize an intricate relationship between bone remodeling and the peripheral nervous system. Under pathological conditions, abnormal bone remodeling plays a key role in the propagation of bone pain. In this review, we first summarize mechanisms and burden of select bone complications in SCD. We then discuss processes that contribute to pathological bone pain that have been described in both SCD as well as non-sickle cell animal models. We emphasize the role of bone-nervous system interactions and pitfalls when designing new therapies especially for the sickle cell population. Lastly, we also discuss future basic and translational research in addressing questions about the complex role of stress erythropoiesis and inflammation in the development of SCD bone complications, which may lead to promising therapies and reduce morbidity in this vulnerable population.

Therapeutic Strategies Targeting Mitochondrial Calcium Signaling: A New Hope for Neurological Diseases?

Calcium (Ca²⁺) is a versatile secondary messenger involved in the regulation of a plethora of different signaling pathways for cell maintenance. Specifically, intracellular Ca²⁺ homeostasis is mainly regulated by the endoplasmic reticulum and the mitochondria, whose Ca²⁺ exchange is mediated by appositions, termed endoplasmic reticulum-mitochondria-associated membranes (MAMs), formed by proteins resident in both compartments. These tethers are essential to manage the mitochondrial Ca²⁺ influx that regulates the mitochondrial function of bioenergetics, mitochondrial dynamics, cell death, and oxidative stress. However, alterations of these pathways lead to the development of multiple human diseases, including neurological disorders, such as amyotrophic lateral sclerosis, Friedreich's ataxia, and Charcot-Marie-Tooth. A common hallmark in these disorders is mitochondrial dysfunction, associated with abnormal mitochondrial Ca²⁺ handling that contributes to neurodegeneration. In this work, we highlight the importance of Ca²⁺ signaling in mitochondria and how the mechanism of communication in MAMs is pivotal for mitochondrial maintenance and cell homeostasis. Lately, we outstand potential targets located in MAMs by addressing different therapeutic strategies focused on restoring mitochondrial Ca²⁺ uptake as an emergent approach for neurological diseases.

Investigational neuroprotective compounds in clinical trials for retinal disease

INTRODUCTION

Retinal neurodegeneration causes irreversible vision loss, impairing quality of life. By targeting neurotoxic conditions, such as oxidative stress and ischemia, neuroprotectants can slow or stop sight loss resulting from eye disease. Despite limimted clinical use of neuroprotectants, there are several promising compounds in early clinical trials (pre-phase III) which may fulfil new therapeutic roles. Search terms relating to neuroprotection and eye disease were used on ClinicalTrials.gov to identify neuroprotective candidates.

AREAS COVERED

Research supporting neuroprotection in eye diseases is focused on, ranging from preclinical to phase II, according to the ClinicalTrials.gov database. The compounds discussed are explored in terms of future clinical applications.

EXPERT OPINION

The major challenge in neuroprotection research is translation from basic research to the clinic. A number of potential neuroprotectants have progressed to ophthalmology clinical trials in recent years, with defined mechanisms of action - saffron and CoQ10 - targeting mitochondria, and both CNTF and NGF showing anti-apoptotic effects. Enhancements in trial design and patient cohorts in proof-of-concept trials with enriched patient populations and surrogate endpoints should accelerate drug development. A further important consideration is optimising drug delivery to improve individualised management and patient compliance. Progress in these areas means that neuroprotective strategies have a much improved chance of translational success.

Nerve Growth Factor Signaling and Its Contribution to Pain

Nerve growth factor (NGF) is a neurotrophic protein essential for the growth, differentiation, and survival of sympathetic and sensory afferent neurons during development. A substantial body of evidence, based on both animal and human studies, demonstrates that NGF plays a pivotal role in modulation of nociception in adulthood. This has spurred development of a variety of novel analgesics that target the NGF signaling pathway. Here, we present a narrative review designed to summarize how NGF receptor activation and downstream signaling alters nociception through direct sensitization of nociceptors at the site of injury and changes in gene expression in the dorsal root ganglion that collectively increase nociceptive signaling from the periphery to the central nervous system. This review illustrates that NGF has a well-known and multifunctional role in nociceptive processing, although the precise signaling pathways downstream of NGF receptor activation that mediate nociception are complex and not completely understood. Additionally, much of the existing knowledge derives from studies performed in animal models and may not accurately represent the human condition. However, available data establish a role for NGF in the modulation of nociception through effects on the release of inflammatory mediators, nociceptive ion channel/receptor activity, nociceptive gene expression, and local neuronal sprouting. The role of NGF in nociception and the generation and/or maintenance of chronic pain has led to it becoming a novel and attractive target of pain therapeutics for the treatment of chronic pain conditions.

Topical recombinant human Nerve growth factor (rh-NGF) is neuroprotective to retinal ganglion cells by targeting secondary degeneration

Optic neuropathy is a major cause of irreversible blindness worldwide, and no effective treatment is currently available. Secondary degeneration is believed to be the major contributor to retinal ganglion cell (RGC) death, the endpoint of optic neuropathy. Partial optic nerve transection (pONT) is an established model of optic neuropathy. Although the mechanisms of primary and secondary degeneration have been delineated in this model, until now how this is influenced by therapy is not well-understood. In this article, we describe a clinically translatable topical, neuroprotective treatment (recombinant human nerve growth factor, rh-NGF) predominantly targeting secondary degeneration in a pONT rat model. Topical application of rh-NGF twice daily for 3 weeks significantly improves RGC survival as shown by reduced RGC apoptosis in vivo and increased RGC population in the inferior retina, which is predominantly affected in this model by secondary degeneration. Topical rh-NGF also promotes greater axonal survival and inhibits astrocyte activity in the optic nerve. Collectively, these results suggest that topical rh-NGF exhibits neuroprotective effects on retinal neurons via influencing secondary degeneration process. As topical rh-NGF is already involved in early clinical trials, this highlights its potential in multiple indications in patients, including those affected by glaucomatous optic neuropathy.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

The role of soluble adenylyl cyclase in neurite outgrowth

Axon regeneration in the mature central nervous system is limited by extrinsic inhibitory signals and a postnatal decline in neurons' intrinsic growth capacity. Neuronal levels of the second messenger cAMP are important in regulating both intrinsic growth capacity and neurons' responses to extrinsic factors. Approaches which increase intracellular cAMP in neurons enhance neurite outgrowth and facilitate regeneration after injury. Thus, understanding the factors which affect cAMP in neurons is of potential therapeutic importance. Recently, soluble adenylyl cyclase (sAC, ADCY10), the ubiquitous, non-transmembrane adenylyl cyclase, was found to play a key role in neuronal survival and axon growth. sAC is activated by bicarbonate and cations and may translate physiologic signals from metabolism and electrical activity into a neuron's decision to survive or regenerate. Here we critically review the literature surrounding sAC and cAMP signaling in neurons to further elucidate the potential role of sAC signaling in neurite outgrowth and regeneration. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Design of a truncated cardiotoxin-I analogue with potent insulinotropic activity

Insulin secretion by pancreatic β-cells in response to glucose or other secretagogues is tightly coupled to membrane potential. Various studies have highlighted the prospect of enhancing insulin secretion in a glucose-dependent manner by blocking voltage-gated potassium channels (K(v)) and calcium-activated potassium channels (K(Ca)). Such strategy is expected to present a lower risk for hypoglycemic events compared to KATP channel blockers. Our group recently reported the discovery of a new insulinotropic agent, cardiotoxin-I (CTX-I), from the Naja kaouthia snake venom. In the present study, we report the design and synthesis of [Lys(52)]CTX-I(41-60) via structure-guided modification, a truncated, equipotent analogue of CTX-I, and demonstrate, using various pharmacological inhibitors, that this derivative probably exerts its action through Kv channels. This new analogue could represent a useful pharmacological tool to study β-cell physiology or even open a new therapeutic avenue for the treatment of type 2 diabetes.

Activation of the phospholipase C signaling pathway in nerve growth factor-treated neurons by carbon nanotubes

Low concentrations of carbon nanotubes (CNTs) promoted the number of nerve growth factor (NGF)-treated neurons with neurite outgrowth by activating extracellular signal-regulated kinase (ERK), even when MEK inhibitor was added to the neuron culture medium. We speculated that CNTs may activate ERK through the phospholipase C (PLC) signaling pathway independent of the Ras/Raf/MEK cascade involved in the ERK signaling pathway. CNTs enhanced phosphorylation of PLC-γ1 in NGF-treated neurons but failed to increase the number and length of neurites of NGF-treated neurons with neurite outgrowth when a PLC inhibitor, an inositol triphosphate receptor (IP3R) inhibitor, or an inhibitor of protein kinase C (PKC) in the PLC signaling pathway were added to the neuron culture medium. Furthermore, intracellular Ca(++) levels of cells treated with CNTs+NGF were higher than those of cells treated with NGF alone. Although the combination of CNTs and NGF increased the concentration of phosphorylated ERK (p-ERK) in MEK inhibitor-treated neurons, CNTs did not induce phosphorylation of ERK in PLC inhibitor-treated neurons. These data suggest that PKC in the PLC signaling pathway may activate ERK independent of the Ras/Raf/MEK cascade. In summary, we identified a role of PLC signaling in mediating neurite outgrowth of NGF-treated neurons in the presence of CNTs.

Stable release of BDNF from the fibroblast cell line NIH3T3 grown on silicone elastomers enhances survival of spiral ganglion cells in vitro and in vivo

The treatment of choice for profound sensorineural hearing loss (SNHL) is direct electrical stimulation of spiral ganglion cells (SGC) via a cochlear implant (CI). The number and excitability of SGC seem to be critical for the success that can be achieved via CI treatment. However, SNHL is associated with degeneration of SGC. Long-term drug delivery to the inner ear for improving SGC survival may be achieved by functionalisation of CI electrodes with cells providing growth factors. Therefore, the capacity of brain-derived neurotrophic factor (BDNF)-secreting NIH3T3 cells grown on cylindrically shaped silicone elastomers (SE) to exert local and sustained neuroprotective effects was assessed in vitro and in vivo. An in vitro model to investigate adhesion and cell growth of lentivirally modified NIH3T3 cells synthesising BDNF on SE was established. The bioactivity of BDNF was characterised by co-cultivation of SGC with cell-coated SE. In addition, cell-coated SE were implanted into deafened guinea pigs. The recombinant NIH3T3 cells proliferated on silicone surfaces during 14 days of cultivation and expressed significantly increasing BDNF levels. Enhanced survival rates and neurite outgrowth of SGC demonstrated the bioactivity of BDNF in vitro. Implantation of SE with adhering BDNF-secreting NIH3T3 cells into the cochleae of systemically deafened guinea pigs induced a significant increase in SGC survival in comparison to SE without cell coating. Our data demonstrate a novel approach of cell-based long-term drug delivery to support SGC survival in vitro and in vivo. This therapeutic strategy--once transferred to cells suitable for clinical application--may improve CI performance.

TACE‐induced cleavage of NgR and p75 NTR in dorsal root ganglion cultures disinhibits outgrowth and promotes branching of neurites in the presence of inhibitory CNS myelin

After binding, central nervous system (CNS) myelin-derived axon growth inhibitory ligands, the Nogo-66 receptor (NgR), complexes with LINGO-1 and either the low-affinity neurotrophin receptor (p75(NTR)) or TROY to initiate growth cone collapse via a Rho-A inhibitory signaling pathway and/or Ca(2+)-dependent activation of epidermal growth factor receptor (EGFR) through an unknown signaling pathway. We have shown that axon growth through CNS myelin is disinhibited after neurotrophic factor administration by 1) initiating intramembranous proteolysis (RIP) of p75(NTR), leading to cleavage of the extracellular (p75(ECD)) and intracellular domains (p75(ICD)) by alpha- and gamma-secretase, respectively, thereby paralyzing inhibitory signaling; 2) shedding of soluble NgR(ECD), which acts as a competitive antagonist to NgR for binding of inhibitory ligands; and 3) antagonizing NgR/p75(NTR) clustering by competitive p75(ECD)/NgR interaction. Here, we report that TNF-alpha converting enzyme (TACE) (a disintegrin and metalloproteinase 17, ADAM17) induces disinhibition of FGF2-stimulated neurite outgrowth of dorsal root ganglion neurons (DRGN) cultured in the presence of a predetermined concentration of inhibitory CNS myelin-derived ligands. After addition of TACE (which has alpha-secretase activity) to mitotically arrested adult rat mixed DRG cultures, we demonstrate 1) NgR(ECD) shedding; 2) release of p75(ECD) and p75(ICD) by RIP of p75(NTR); 3) blockade of Rho-A activation; 4) reduced EGFR phosphorylation; and 5) increased FGF2-stimulated DRGN neurite outgrowth and branching in the presence of CNS myelin-derived inhibitory ligands. Thus, TACE-induced cleavage of NgR and RIP of p75(NTR) abrogates axon growth inhibitory signaling, thereby disinhibiting CNS axon/neurite growth.

A role for brain-derived neurotrophic factor in B cell development

In the present study, we demonstrated a significant reduction of B lymphocytes in the blood, spleen and bone marrow of BDNF deficient mice. The observed developmental block in bone marrow B cell development was linked specifically to the Pre-BII stage. B lymphocytes express the BDNF receptors p75NTR and TrkB(gp95), while no BDNF expression was found. However, a strong BDNF expression was demonstrated in bone marrow stromal cells. An increase of intracellular free calcium [Ca2+]i in B lymphocytes after BDNF application confirms a direct responsiveness of B lymphocytes to BDNF. In conclusion, these results suggest a role of BDNF for normal B lymphocyte development through paracrine effects in the bone marrow.

Diverse functions of the p75 neurotrophin receptor

The pan-neurotrophin receptor p75NTR belongs to a large family of receptors, which includes tumor necrosis factor receptors, Fas and approximately 25 other members. The p75NTR is the first receptor to be cloned molecularly. Recent years have seen the emergence of a consensus regarding the signaling pathways activated by p75NTR and its potential biological function, although receptor characterization had not been targeted for some years. We now know that p75NTR has surprisingly diverse effects, ranging from cell death to regulation of axon elongation. This diversity can be explained by the complex formation of p75NTR with other receptors and multiple signaling molecules that interact with the intracellular domain of p75NTR.

Multiple acute effects on the membrane potential of PC12 cells produced by nerve growth factor (NGF)

We studied whether nerve growth factor (NGF) can affect the membrane potential and conductance of PC12 cells. We demonstrate that NGF depolarizes the membrane of PC12 cells within a minute and by using transfected NIH 3T3-Trk and -p75 cells we show that both the high affinity NGF receptor p140(trk) and the low affinity NGF receptor or p75(NGF) may be involved in the depolarization. Tyrosine kinase inhibitor, K252a, partially inhibited the depolarization, but two agents affecting intracellular calcium movements, Xestospongin C (XeC) and thapsigargin, did not. The early depolarization was eliminated in Na+ free solutions and under this condition, a 'prolonged' (> 2 min) hyperpolarization was observed in PC12 cells in response to NGF. This hyperpolarization was also induced in PC12 cells by epidermal growth factor (EGF). Voltage clamp experiments showed that NGF produced a late (> 2 min) increase in membrane conductance. The Ca2+-dependent BK-type channel blocker, iberiotoxin, and the general Ca2+-dependent K+ channel blocker, TEA, attenuated or eliminated the hyperpolarization produced by NGF in sodium free media. Under pretreatment with the non-selective cation channel blockers La3+ and Gd3+, NGF hyperpolarized the membrane of PC12 cells. These results suggest that three different currents are implicated in rapid NGF-induced membrane voltage changes, namely an acutely activated Na+ current, Ca2+-dependent potassium currents and non-selective cation currents.

Neurotrophin-dependent tyrosine phosphorylation of Ras guanine-releasing factor 1 and associated neurite outgrowth is dependent on the HIKE domain of TrkA

Ras guanine-releasing factor 1 (RasGrf1), a guanine nucleotide exchange factor for members of the Ras and Rho family of GTPases, is highly expressed in the brain. It is regulated by two separate mechanisms, calcium regulation through interaction with its calcium/calmodulin-binding IQ domain and serine and tyrosine phosphorylation. RasGrf1 is activated downstream of G-protein-coupled receptors and the non-receptor tyrosine kinases, Src and Ack1. Previously, we demonstrated a novel interaction between the intracellular domain of the nerve growth factor-regulated TrkA receptor tyrosine kinase and an N-terminal fragment of RasGrf1. We now show that RasGrf1 is phosphorylated and interacts with TrkA, -B, and -C in co-transfection studies. This interaction and phosphorylation of RasGrf1 is dependent on the HIKE domain of TrkA (a region shown to interact with pleckstrin homology domains) but not on any of the phosphotyrosine residues that act as docking sites for intracellular signaling molecules such as Shc and FRS-2. The PH1 domain alone of RasGrf1 is sufficient for phosphorylation by the TrkA receptor. A potential role for Trk activation of RasGrf1 is suggested through transfection studies in PC12 cells in which RasGrf1 significantly increases neurite outgrowth at low doses of neurotrophin stimulation. Notably, this neurite outgrowth is dependent on an intact HIKE domain, as nnr5-S10 cells expressing a TrkA HIKE domain mutant do not exhibit potentiated neurite outgrowth in the presence of RasGrf1. These studies identify RasGrf1 as a novel target of neurotrophin activation and suggest an additional pathway whereby neurotrophin-stimulated neurite outgrowth may be regulated.

The role of neurotransmission and the Chopper domain in p75 neurotrophin receptor death signaling

The role of p75 neurotrophin receptor (p75NTR) in mediating cell death is now well characterized, however, it is only recently that details of the death signaling pathway have become clearer. This review focuses on the importance of the juxtamembrane Chopper domain region of p75NTR in this process. Evidence supporting the involvement of K+ efflux, the apoptosome (caspase-9, apoptosis activating factor-1, APAF-1, and Bcl-xL), caspase-3, c-jun kinase, and p53 in the p75NTR cell death pathway is discussed and regulatory roles for the p75NTR ectodomain and death domain are proposed. The role of synaptic activity is also discussed, in particular the importance of neutrotransmitter-activated K+ channels acting as the gatekeepers of cell survival decisions during development and in neurodegenerative conditions.

p75 neurotrophin receptor signaling in the nervous system

The neurotrophin receptor p75(NTR) has long been known as a receptor for neurotrophins that promote survival and differentiation. Consistent with the role of neurotrophins, p75(NTR) is expressed during the developmental stages of the nervous system. However, p75(NTR) is re-expressed in various pathological conditions in the adult. We now know that p75(NTR) has the ability to elicit bi-directional signals, that result in the inhibition as well as the promotion of the neurite outgrowth. p75(NTR) is a key receptor for myelin-derived inhibitory cues that contribute to the lack of regeneration of the central nervous system.

PKA phosphorylates the p75 receptor and regulates its localization to lipid rafts

Although a large number of studies have been carried out on the diverse effects mediated by the common neurotrophin receptor p75(NTR), little is known about the molecular mechanisms by which p75(NTR) initiates intracellular signal transduction. We identified a variant of the beta catalytic subunit of cAMP-dependent protein kinase (PKACbeta) as a p75(NTR)-interacting protein, which phosphorylates p75(NTR) at Ser304. Intracellular cAMP in cerebellar neurons was accumulated transiently by ligand binding to p75(NTR). Activation of cAMP-PKA is required for translocation of p75(NTR) to lipid rafts, and for biochemical and biological activities of p75(NTR), such as inactivation of Rho and the neurite outgrowth. Proper recruitment of activated p75(NTR) to lipid rafts, structures that represent specialized signaling organelles, is of fundamental importance in determining p75(NTR) bioactivity.

Functional inhibition of the p75 receptor using a small interfering RNA

The neurotrophin receptor p75(NTR) mediates a wide variety of biological effects. Consistent with the function in controlling the survival and neurite formation, p75(NTR) is expressed during the developmental stages of the nervous system. Importantly, p75(NTR) is re-expressed in various pathological conditions and is suggested to contribute to the inhibition of neuronal regeneration and the death of the neurons. Here we develop a tool to knock down the expression of p75(NTR) by employing a small interfering RNA (siRNA). The siRNA for p75(NTR) effectively reduces the expression of endogenous p75(NTR) both in Schwann cells and dorsal root ganglion neurons in vitro. NGF-induced cell death in Schwann cells and the neurite retraction in DRG neurons induced by myelin-associated glycoprotein are attenuated by the siRNA. Inhibition of p75(NTR) in specific pathological conditions by the siRNA may provide a potential therapeutic agent.

Rapid effects of neurotrophic factors on calcium homeostasis in rat visceral afferent neurons

Neurotrophic factors maintain and modulate neuron function in adults. We tested the hypothesis that neurotrophic factors rapidly alter intracellular calcium concentrations, thereby affecting neuron excitability. The majority of rat nodose neurons express TrkA, TrkB and TrkC receptor after 1 day in culture. Addition of nerve growth factor, brain derived neurotrophic factor or glial derived neurotrophic factor increased cytosolic calcium in about one third of the neurons within less than 10 min. This increase was due to calcium release from intracellular stores and could be blocked by the tyrosine kinase inhibitor K252a. The rapid effect of neurotrophic factors suggests a role of these molecules in the early response after inflammation as potential mediators for sensitization of afferent neurons.

Modulation of neurotransmitter release induced by brain-derived neurotrophic factor in rat brain striatal slices in vitro

This study examined the influence of brain-derived neurotrophic factor (BDNF) on the basal and depolarisation-induced release of the neurotransmitters GABA, dopamine and serotonin from rat striatal brain slices in vitro. BDNF potentiated the potassium or veratrine-stimulated release of GABA, dopamine and serotonin. This potentiation was shown to be dependent on activation of the high-affinity tyrosine kinase-linked receptor TrkB, as K252a (a potent TrkB antagonist) largely prevented the effects. BDNF potentiated the release of each neurotransmitter to similar extents irrespective of the type of depolarising stimulus used. In all cases the potentiation of neurotransmitter release caused by BDNF was dependent on membrane depolarisation as BDNF alone was incapable of causing potentiation. These results, obtained using striatal slices in vitro, suggest that BDNF may be acting via the specific receptor TrkB to modulate synaptic performance in the corpus striatum in vivo.

A rapid switch in sympathetic neurotransmitter release properties mediated by the p75 receptor

Cardiac function is modulated by norepinephrine release from innervating sympathetic neurons. These neurons also form excitatory connections onto cardiac myocytes in culture. Here we report that brain-derived neurotrophic factor (BDNF) altered the neurotransmitter release properties of these sympathetic neuron-myocyte connections in rodent cell culture, leading to a rapid shift from excitatory to inhibitory cholinergic transmission in response to neuronal stimulation. Fifteen minutes of BDNF perfusion was sufficient to cause this shift to inhibitory transmission, indicating that BDNF promotes preferential release of acetylcholine in response to neuronal stimulation. We found that p75(-/-) neurons did not release acetylcholine in response to BDNF and that neurons overexpressing p75 showed increased cholinergic transmission, indicating that the actions of BDNF are mediated through the p75 neurotrophin receptor. Our findings indicate that p75 is involved in modulating the release of distinct neurotransmitter pools, resulting in a functional switch between excitatory and inhibitory neurotransmission in individual neurons.

Inhibitors of nitric oxide synthase attenuate nerve growth factor-mediated increases in choline acetyltransferase expression in PC12 cells

NGF can regulate nitric oxide synthase (NOS) expression and nitric oxide (NO) can modulate NGF-mediated neurotrophic responses. To investigate the role of NO in NGF-activated expression of cholinergic phenotype, PC12 cells were treated with either the nonselective NOS inhibitor L-NAME (N (omega)-nitro-L-arginine methylester) or the inducible NOS selective inhibitor MIU (s-methylisothiourea), and the effect on NGF-stimulated ChAT mRNA levels and ChAT specific activity was determined. NGF increased steady-state levels of mRNA and protein for both inducible and constitutive isozymes of NOS in PC12 cells, and led to enhanced NOS activity and NO production. MIU and, to a lesser extent, L-NAME blocked neurite outgrowth in nerve growth factor (NGF)-treated PC12 cells. Both L-NAME and MIU attenuated NGF-mediated increases in choline transferase (ChAT)-specific activity and prevented the increase in expression of ChAT mRNA normally produced by NGF treatment of PC12 cells. The present study indicates that NO may be involved in the modulation of signal transduction pathways by which NGF leads to increased ChAT gene expression in PC12 cells.

A double cysteine trkA mutant exhibiting reduced NGF binding and delayed Erk signaling

The NGF receptor trkA is a tyrosine kinase receptor comprising an extracellular domain with a ligand-binding site, a transmembrane-spanning domain (TMD), and an intracellular domain composed of a juxtamembrane region (JMR), a tyrosine kinase domain, and a short carboxy-terminal tail. Nerve growth factor (NGF) binds and activates this receptor, leading to phosphorylation of signaling substrates involved in neuronal proliferation, differentiation, and survival. Human trkA contains one cysteine residue in the TMD (C423) and another, separated by 12 residues, in the JMR (C436). We hypothesized that the removal of one or both of the cysteines would affect NGF-induced signaling of the trkA receptor. Here we show that NGF induces rapid receptor autophosphorylation in a wild-type, trkA-expressing clone (WT11), in a single cysteine trkA mutants (C423T or C436A), but lower autophosphorylation activity in a double-cysteine trkA mutant (C423T/C436A). WT11 and SM cells had similar binding affinity, but that of DM cells was lower, according to the NGF radioreceptor assay. NGF-induced Erk phosphorylation was rapid in WT11 and C423T cells, but delayed in C436A and C423T/C436A cells. NGF induced [3H]thymidine incorporation into WT11 and SM cells, but had no effect on DM cells. However, basic fibroblast growth factor (bFGF) induced rapid phosphorylation of Erk1/2, and [3H]thymidine incorporation in NIH3T3, WT11, single mutant (SM), and double mutant (DM) cells, suggesting that the impaired NGF-induced Erk phosphorylation and thymidine incorporation observed in DM cells are due to the double-cysteine mutations in the trkA receptor. Cumulatively, our findings support a model in which Cys436 of the trkA is responsible for the rapid transfer of the transmembrane occupancy signal to the SHC adaptor protein for activation of the Ras-Erk pathway and DNA synthesis.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Modifications of intracellular Ca2+ signalling during nerve growth factor-induced neuronal differentiation of rat adrenal chromaffin cells

Postnatal sympathetic neurons (SNs) and chromaffin cells (CCs) derive from neural crest precursors. CCs can differentiate in vitro into SN-like cells after nerve growth factor (NGF) exposure. This study examines changes of intracellular Ca2+ homeostasis and dynamics of CCs under conditions that promote a neuronal phenotype. Spontaneous Ca2+ fluctuations, a frequent observation in early cultures of CCs, diminished after > 10 days in vitro in control cells and ceased in NGF-treated ones. At the same time, Ca2+ rises resulting from entry upon membrane depolarization, gradually increased both their size and peak d[Ca2+]i/dt, resembling those recorded in SNs. Concomitantly, caffeine-induced Ca2+ rises, resulting from Ca2+ release from intracellular stores, increased their size and their peak d[Ca2+]i/dt by > 1000%, and developed transient and sustained release components, similar to those of SNs. The transient component, linked to regenerative Ca2+ release, appeared after > 10 days of NGF treatment, suggesting a delayed steep enhancement of Ca2+-induced Ca2+ release (CICR). Immunostaining showed that proteins coded by the three known isoforms of ryanodine receptors (RyRs) are present in CCs, but that only RyR2 increased significantly after NGF treatment. Since the transient release component increased more steeply than RyR2 immunostaining, we suggest that the development of robust CICR requires both an increased expression of RyRs and more efficient functional coupling among them. NGF-induced transdifferentiation of chromaffin cells involves the enhancement of both voltage-gated Ca2+ influx and Ca2+ release from intracellular stores. These modifications are likely to complement the extensive morphological and functional reorganization required for the replacement of the endocrine phenotype with the neuronal one.

The atypical protein kinase C-interacting protein p62 is a scaffold for NF-kappaB activation by nerve growth factor

Nerve growth factor (NGF) binding to both p75 and TrkA neurotrophin receptors activates the transcription factor nuclear factor kappaB (NF-kappaB). Here we show that the atypical protein kinase C-interacting protein, p62, which binds TRAF6, selectively interacts with TrkA but not p75. In contrast, TRAF6 interacts with p75 but not TrkA. We demonstrate the formation of a TRAF6-p62 complex that serves as a bridge linking both p75 and TrkA signaling. Of functional relevance, transfection of antisense p62-enhanced p75-mediated cell death and diminished NGF-induced differentiation occur through a mechanism involving inhibition of IKK activity. These findings reveal a new function for p62 as a common platform for communication of both p75-TRAF6 and TrkA signals. Moreover, we demonstrated that p62 serves as a scaffold for activation of the NF-kappaB pathway, which mediates NGF survival and differentiation responses.

Venom from the platypus, Ornithorhynchus anatinus, induces a calcium-dependent current in cultured dorsal root ganglion cells

The platypus (Ornithorhynchus anatinus), a uniquely Australian species, is one of the few living venomous mammals. Although envenomation of humans by many vertebrate and invertebrate species results in pain, this is often not the principal symptom of envenomation. However, platypus envenomation results in an immediate excruciating pain that develops into a very long-lasting hyperalgesia. We have previously shown that the venom contains a C-type natriuretic peptide that causes mast cell degranulation, and this probably contributes to the development of the painful response. Now we demonstrate that platypus venom has a potent action on putative nociceptors. Application of the venom to small to medium diameter dorsal root ganglion cells for 10 s resulted in an inward current lasting several minutes when the venom was diluted in buffer at pH 6.1 but not at pH 7.4. The venom itself has a pH of 6.3. The venom activated a current with a linear current-voltage relationship between -100 and -25 mV and with a reversal potential of -11 mV. Ion substitution experiments indicate that the current is a nonspecific cationic current. The response to the venom was blocked by the membrane-permeant Ca(2+)-ATPase inhibitor, thapsigargin, and by the tyrosine- and serine-kinase inhibitor, k252a. Thus the response appears to be dependent on calcium release from intracellular stores. The identity of the venom component(s) that is responsible for the responses we have described is yet to be determined but is probably not the C-type natriuretic peptide or the defensin-like peptides that are present in the venom.

BDNF, NT-3 and NGF induce distinct new Ca2+ channel synthesis in developing hippocampal neurons

Neurotrophins exert short- and long-term effects on synaptic transmission. The mechanism underlying these forms of synaptic plasticity is unknown although it is likely that intracellular Ca2+ and presynaptic Ca2+ channels play a critical role. Here we show that BDNF, NGF and NT-3 (10-100 ng/mL) exhibit a selective long-term up-regulation of voltage-gated Ca2+ current densities in developing hippocampal neurons of 6-20 days in culture. NGF and NT-3 appear more effective in up-regulating L-currents, while BDNF predominantly acts on non-L-currents (N, P/Q and R). The effects of the three neurotrophins were time- and dose-dependent. The EC50 was comparable for BDNF, NGF and NT-3 (10-16 ng/mL) while the time of half-maximal activation was significantly longer for NGF compared to BDNF (58 vs. 25 h). Despite the increased Ca2+ current density, the neurotrophins did not alter the voltage-dependence of channel activation, the kinetics parameters or the elementary properties of Ca2+ channels (single-channel conductance, probability of opening and mean open time). Neurotrophin effects were completely abolished by coincubation with the nonspecific Trk-receptor inhibitor K252a, the protein synthesis blocker anisomycin and the MAP-kinase inhibitor PD98059, while cotreatment with the PLC-gamma blocker, U73122, was without effect. Immunocytochemistry and Western blotting revealed that neurotrophins induced an increased MAP-kinase phosphorylation and its translocation to the nucleus. The present findings suggest that on a long time scale different neurotrophins can selectively up-regulate different Ca2+ channels. The action is mediated by Trk-receptors/MAP-kinase pathways and induces an increased density of newly available Ca2+ channels with unaltered gating activity.

Chopper, a new death domain of the p75 neurotrophin receptor that mediates rapid neuronal cell death

The cytoplasmic juxtamembrane region of the p75 neurotrophin receptor (p75(NTR)) has been found to be necessary and sufficient to initiate neural cell death. The region was named "Chopper" to distinguish it from CD95-like death domains. A 29-amino acid peptide corresponding to the Chopper region induced caspase- and calpain-mediated death in a variety of neural and non-neural cell types and was not inhibited by signaling through Trk (unlike killing by full-length p75(NTR)). Chopper triggered cell death only when bound to the plasma membrane by a lipid anchor, whereas non-anchored Chopper acted in a dominant-negative manner, blocking p75(NTR)-mediated death both in vitro and in vivo. Removal of the ectodomain of p75(NTR) increased the potency of Chopper activity, suggesting that it regulates the association of Chopper with downstream signaling proteins.

Voltage-sensitive calcium currents are acutely increased by nerve growth factor in PC12 cells

Whole cell calcium currents were recorded from PC12 cells with the perforated patch technique. Currents were evoked by step depolarization from a holding potential of -90 mV. Nerve growth factor (NGF) increased calcium currents through L-type calcium channels by >75% within 3-5 min. This increase was inhibited by K-252a, by nifedipine, and by inhibition or down-regulation of kinase C. Brain-derived neurotrophic factor (BDNF) also increased calcium current, but to a smaller extent. Thus increases in calcium current can be linked to activation of either the high- or the low-affinity nerve growth factor receptor. Increases in presynaptic calcium uptake appear to be a crucial element in the short-term actions of the neurotrophins on neurotransmitter release leading to long-term potentiation. Also, the control of calcium uptake is likely to be an important factor in the long-term actions of the neurotrophins on neuronal survival and neuronal protection. The present data indicate that the PC12 cell may be a useful model for studying the effect of the neurotrophins on calcium uptake.