TrkA NGF receptor plays a role in the modulation of p75NTR expression

NGF mediates protection of mesenchymal stem cells-conditioned medium against 2,5-hexanedione-induced apoptosis of VSC4.1 cells via Akt/Bad pathway

It has been shown that the conditioned medium of bone mesenchymal stem cells (BMSC-CM) can inhibit apoptosis of neural cells exposed to 2,5-hexanedione (HD), but its protective mechanism remains unclear. To investigate the underlying mechanism, VSC4.1 cells were given HD and 5, 10 and 15% BMSC-CM (v/v) in the current experiment. Our data showed that BMSC-CM concentration-dependently attenuated HD-induced cell apoptosis. Moreover, BMSC-CM remarkably decreased the mitochondrial cytochrome c (Cyt C) release and the caspase-3 activity in HD-given VSC4.1 cells. Given a relatively high expression of NGF in BMSCs and BMSC-CM, we hypothesized that NGF might be an important mediator of the protection of BMSC-CM against apoptosis induced by HD. To verify our hypothesis, the VSC4.1 cells were administrated with NGF and anti-NGF antibody in addition to HD. As expected, NGF could perfectly mimic BMSC-CM's protective role and these beneficial effects were abolished by anti-NGF antibody intervention. To further explore its mechanism, inhibitors of TrkA and Akt were given to the VSC4.1 cells and NGF/Akt/Bad pathway turned out to be involved in anti-apoptotic role of BMSC-CM. Based on these findings, it was revealed that BMSC-CM beneficial role was mediated by NGF and relied on the Akt/Bad pathway.

Nerve Growth Factor Expression and Its Receptors TrkA and p75NTR in Peri-Implantitis Lesions

BACKGROUND

Nerve growth factor (NGF) can, through its receptors TrkA and p75NTR, convey signals for cell survival or cell differentiation. These proteins are also involved in inflammation and in bone resorption. The aim of this study is to evaluate, for the first time, the expression of NGF and its receptors TrkA and p75NTR in peri-implantitis lesions.

MATERIALS AND METHODS

Fifteen biopsy specimens from patients with chronic peri-implantitis and 4 of healthy oral mucosa were immunostained with antibodies against NGF, TrkA, and p75NTR. The staining intensity and percentage of stained cells were semi-quantitatively evaluated and results were compared between the 2 groups.

RESULTS

In the peri-implant pocket epithelium and gingival epithelium, NGF and TrkA expressions were similar to the healthy oral mucosa, however, a decreased expression of p75NTR was observed. In all cases, more than 75% of the inflammatory cells stained positively for NGF and TrkA, and p75NTR was negatively expressed.

CONCLUSION

The intense expression of NGF and TrkA in the inflammatory cell infiltrate associated with decreased expression of p75NTR in both gingival and pocket epithelium suggests that these proteins may have a role in peri-implantitis lesions.

Chronic TrkB agonist treatment in old age does not mitigate diaphragm neuromuscular dysfunction

Previously, we found that brain-derived neurotrophic factor (BDNF) signaling through the high-affinity tropomyosin-related kinase receptor subtype B (TrkB) enhances neuromuscular transmission in the diaphragm muscle. However, there is an age-related loss of this effect of BDNF/TrkB signaling that may contribute to diaphragm muscle sarcopenia (atrophy and force loss). We hypothesized that chronic treatment with 7,8-dihydroxyflavone (7,8-DHF), a small molecule BDNF analog and TrkB agonist, will mitigate age-related diaphragm neuromuscular transmission failure and sarcopenia in old mice. Adult male TrkBF616A mice (n = 32) were randomized to the following 6-month treatment groups: vehicle-control, 7,8-DHF, and 7,8-DHF and 1NMPP1 (an inhibitor of TrkB kinase activity in TrkBF616A mice) cotreatment, beginning at 18 months of age. At 24 months of age, diaphragm neuromuscular transmission failure, muscle-specific force, and fiber cross-sectional areas were compared across treatment groups. The results did not support our hypothesis in that chronic 7,8-DHF treatment did not improve diaphragm neuromuscular transmission or mitigate diaphragm muscle sarcopenia. Taken together, these results do not exclude a role for BDNF/TrkB signaling in aging-related changes in the diaphragm muscle, but they do not support the use of 7,8-DHF as a therapeutic agent to mitigate age-related neuromuscular dysfunction.

Implication of the neurotrophin receptor p75NTR in vascular diseases: beyond the eye

INTRODUCTION

The p75 neurotrophin receptor (p75^NTR) is a member of TNF-α receptor superfamily that bind all neurotrophins, mainly regulating their pro-apoptotic actions. Ischemia is a common pathology in different cardiovascular diseases affecting multiple organs, however the contribution of p75^NTR remains not fully addressed. The aim of this work is to review the current evidence through published literature studying the impact of p75^NTR receptor in ischemic vascular diseases.

AREAS COVERED

In the eye, several ischemic ocular diseases are associated with enhanced p75^NTR expression. Ischemic retinopathy including diabetic retinopathy, retinopathy of prematurity and retinal vein occlusion are characterized initially by ischemia followed by excessive neovascularization. Beyond the eye, cerebral ischemia, myocardial infarction and critical limb ischemia are ischemic cardiovascular diseases that are characterized by altered expression of neurotrophins and p75^NTR expression. We surveyed both clinical and experimental studies that examined the impact of p75^NTR receptor in ischemic diseases of eye, heart, brain and peripheral limbs.

EXPERT COMMENTARY

p75^NTR receptor is a major player in multiple ischemic vascular diseases affecting the eye, brain, heart and peripheral limbs with significant increases in its expression accompanying neuro-vascular injury. This has been addressed in the current review along with the beneficial vascular outcomes of p75^NTR inhibition.

Neurotrophic factors and their effects in the treatment of multiple sclerosis

Neurotrophins are small molecules of polypeptides, which include nerve growth factor (NGF) family, glial cell line–derived neurotrophic factor (GDNF) family ligands, and neuropoietic cytokines. These factors have an important role in neural regeneration, remyelination, and regulating the development of the peripheral and central nervous systems (PNS and CNS, respectively) by intracellular signaling through specific receptors. It has been suggested that the pathogenesis of human neurodegenerative disorders may be due to an alteration in the neurotrophic factors and their receptors. The use of neurotrophic factors as therapeutic agents is a novel strategy for restoring and maintaining neuronal function during neurodegenerative disorders such as multiple sclerosis. Innate and adaptive immune responses contribute to pathology of neurodegenerative disorders. Furthermore, autoimmune and mesenchymal stem cells, by the release of neurotrophic factors, have the ability to protect neuronal population and can efficiently suppress the formation of new lesions. So, these cells may be an alternative source for delivering neurotrophic factors into the CNS.

Induction of a reactive state in perineuronal satellite glial cells akin to that produced by nerve injury is linked to the level of p75NTR expression in adult sensory neurons

Satellite glial cells (SGCs) surrounding primary sensory neurons are similar to astrocytes of the central nervous system in that they buffer the extracellular environment via potassium and calcium channels and express the intermediate filament glial fibrillary acidic protein (GFAP). Peripheral nerve injury induces a reactive state in SGCs that includes SGC proliferation, increased SGC/SGC coupling via gap junctions, decreased inward rectifying potassium channel 4.1 (Kir 4.1) expression and increased expression of GFAP and the common neurotrophin receptor, p75NTR. In contrast, neuronal p75NTR expression, normally detected in ∼80% of adult rat sensory neurons, decreases in response to peripheral axotomy. Given the differential regulation of p75NTR expression in neurons versus SGCs with injury, we hypothesized that reduced signaling via neuronal p75NTR contributes to the induction of a reactive state in SGCs. We found that reducing neuronal p75NTR protein expression in uninjured sensory neurons by intrathecal subarachnoid infusion of p75NTR-selective anti-sense oligodeoxynucleotides for one week was sufficient to induce a "reactive-like" state in the perineuronal SGCs akin to that normally observed following peripheral nerve injury. This reactive state included significantly increased SGC p75NTR, GFAP and gap junction protein connexin-43 protein expression, increased numbers of SGCs surrounding individual sensory neurons and decreased SGC Kir 4.1 channel expression. Collectively, this supports the tenet that reductions in target-derived trophic support leading to, or as a consequence of, reduced neuronal p75NTR expression plays a critical role in switching the SGC to a reactive state.

Three new neuritogenic steroidal saponins from Ophiopogon japonicus (Thunb.) Ker-Gawl

Three new steroidal saponins (1-3) and a known saponin (4) were isolated from Ophiopogon japonicus (Thunb.) Ker-Gawl. Their structures were determined by spectroscopic analyses and chemical derivatization. The isolated compounds (1-4) were potent inducers of neuritogenesis on PC12 cells. Compound 1 showed the highest neuritogenic activity of 46% at 1 μM. The study of structure-activity relationships suggests that aglycone is important for the neuritogenic activity of the compounds. Specific inhibitor experiments and Western blot analysis suggest that 1-induced neuritogenic activity depends on the activation of mitogen-activated protein kinase kinase (MEK)/extracellular signal regulated kinase (ERK) signaling pathway on PC12 cells.

Targeted delivery of TrkB receptor to phrenic motoneurons enhances functional recovery of rhythmic phrenic activity after cervical spinal hemisection

Progressive recovery of rhythmic phrenic activity occurs over time after a spinal cord hemisection involving unilateral transection of anterolateral funiculi at C₂ (SH). Brain-derived neurotrophic factor (BDNF) acting through its full-length tropomyosin related kinase receptor subtype B (TrkB.FL) contributes to neuroplasticity after spinal cord injury, but the specific cellular substrates remain unclear. We hypothesized that selectively targeting increased TrkB.FL expression to phrenic motoneurons would be sufficient to enhance recovery of rhythmic phrenic activity after SH. Several adeno-associated virus (AAV) serotypes expressing GFP were screened to determine specificity for phrenic motoneuron transduction via intrapleural injection in adult rats. GFP expression was present in the cervical spinal cord 3 weeks after treatment with AAV serotypes 7, 8, and 9, but not with AAV2, 6, or rhesus-10. Overall, AAV7 produced the most consistent GFP expression in phrenic motoneurons. SH was performed 3 weeks after intrapleural injection of AAV7 expressing human TrkB.FL-FLAG or saline. Delivery of TrkB.FL-FLAG to phrenic motoneurons was confirmed by FLAG protein expression in the phrenic motor nucleus and human TrkB.FL mRNA expression in microdissected phrenic motoneurons. In all SH rats, absence of ipsilateral diaphragm EMG activity was confirmed at 3 days post-SH, verifying complete interruption of ipsilateral descending drive to phrenic motoneurons. At 14 days post-SH, all AAV7-TrkB.FL treated rats (n = 11) displayed recovery of ipsilateral diaphragm EMG activity compared to 3 out of 8 untreated SH rats (p<0.01). During eupnea, AAV7-TrkB.FL treated rats exhibited 73±7% of pre-SH root mean squared EMG vs. only 31±11% in untreated SH rats displaying recovery (p<0.01). This study provides direct evidence that increased TrkB.FL expression in phrenic motoneurons is sufficient to enhance recovery of ipsilateral rhythmic phrenic activity after SH, indicating that selectively targeting gene expression in spared motoneurons below the level of spinal cord injury may promote functional recovery.

Messenger RNA expression patterns of p75 neurotrophin receptor and tropomyosin-receptor-kinase A following spinal cord injury

BACKGROUND

Induction of p75 neurotrophin receptor (p75NTR) could be one of the first steps that initiate apoptotic cascade after injury, or it may indicate regeneration responses undertaken by the injured system, possibly in collaboration with resident tropomyosin-receptor-kinase (Trk).

OBJECTIVE

To measure quantitative changes in messenger RNA (mRNA) expression levels of p75NTR, Trk A, and caspase-9 in rat's injured spinal cord (SCI). The reciprocal interaction between Trk and p75NTR signaling pathways can dictate cellular responses to neurotrophins. p75NTR can regulate Trk-dependent responses, but the role of Trk in regulating p75NTR-dependent signaling is not well documented.

DESIGN

Using real-time polymerase chain reaction, this study analyzed changes in the mRNA abundance of the mentioned genes at 6, 24, and 72 hours and 7 and 10 days after SCI in adult male rats. SCI was induced at T9 level by transsection.

RESULTS

Results show a complicated temporal and spatial pattern of alteration with different degrees and direction (up- or down-regulation) in p75NTR, Trk A, and caspase-9 mRNA expression levels after SCI. The greatest variation was seen in center regions following SCI. This study shows that alteration in p75NTR, Trk A, and caspase-9 expression starts as early as 6 hours after SCI. Alterations in p75NTR, Trk A, and caspase-9 expression within the spinal cord may play a key role in the apoptotic cell death.

CONCLUSION

Results suggest that the role of p75NTR is to eliminate damaged cells by activating the apoptotic machinery, especially at the center of damage and during first week after injury.

Memory-enhancing effect of Mori Fructus via induction of nerve growth factor

Fruits rich in phytochemicals have been shown to improve memory by protecting or enhancing neuronal functions mediated by neurotrophic factors, such as nerve growth factor (NGF), in the hippocampus. Mori Fructus (Morus alba L., Moraceae), also called mulberry, is used as a food, dietary supplement and an anti-ageing agent in traditional Oriental medicine. It is also known to contain abundant flavonoid compounds and to exhibit various pharmacological effects. The present study was performed to evaluate the memory-enhancing effect of Mori Fructus extract (ME) in mice, with a focus on NGF regulation. ME (20, 100 and 500 mg/kg per d for 7 d, per os) dose-dependently promoted NGF release in the mouse hippocampus, leading to phosphorylation of extracellular signal-regulated kinases and cyclic AMP response element-binding protein. ME significantly increased pre- and post-synapse formation, acetylcholine synthesisation, neuronal cell differentiation, neurite outgrowth and neuronal cell proliferation in the mouse hippocampus. Furthermore, ME significantly increased latency time in the passive avoidance task (P< 0·001) and recognition time of novel objects in the object recognition test (P< 0·05), indicating improvements in learning and memory. Taken together, these data suggest that ME exhibits a memory-enhancing effect via up-regulation of NGF.

Infusion of BDNF into the nucleus accumbens of aged rats improves cognition and structural synaptic plasticity through PI3K-ILK-Akt signaling

To investigate the involvement of the nucleus accumbens (NAc) in cognitive impairment and the therapeutic effects of brain-derived neurotrophic factor (BDNF) in an animal model of cognitive deficit, we infused BDNF into the NAc of cognitively impaired aged rats. Cognition was evaluated by Morris water maze test. Structural synaptic plasticity was measured by Golgi staining. Brain tissue homogenization was used to measure the changes in signal molecules. Cultured PC-12 cells expressing tyrosine kinase receptor (Trk) B/p75 neurotrophin receptor (p75(NTR)), p75(NTR) or TrkA/p75(NTR) receptors were used for BDNF stimulation assays. Significant decreases in the levels of BDNF, phosphatidylinositol-3-kinase (PI3K) and integrin-linked kinase (ILK) activity, protein kinase B (Akt) Ser⁴⁷³ phosphorylation, dendritic branching, and density of dendritic spines on medium spiny neurons were observed in the NAc. Importantly, infusion of BDNF restored cognition, synaptic plasticity, and cell signaling. In cultured PC-12 cells, BDNF activated PI3K/Akt signaling through the TrkB receptor, whereas stimulation of ILK/Akt occurred through TrkA/p75(NTR) heteroreceptor. Our study suggested that the decreased BDNF level and its downstream signaling as well as loss of synaptic plasticity in the NAc are associated with cognitive impairments in aged rats. The BDNF-activated PI3K-Akt and ILK-Akt signaling play a key role in structural synaptic plasticity. Our study also suggested that BDNF could be a mechanism-based treatment for dementia.

MKK7γ1 reverses nerve growth factor signals: proliferation and cell death instead of neuritogenesis and protection

c-Jun N-terminal kinases (JNKs) are the exclusive downstream substrates of mitogen-activated protein kinase kinase 7 (MKK7). Recently, we have shown that a single MKK7 splice variant, MKK7γ1, substantially changes the functions of JNKs in naïve PC12 cells. Here we provide evidence that MKK7γ1 blocks NGF-mediated differentiation and sustains proliferation by interfering with the NGF-triggered differentiation programme at several levels: (i) down-regulation of the NGF receptors TrkA and p75; (ii) attenuation of the differentiation-promoting pathways ERK1/2 and AKT; (iii) increase of JNK1 and JNK2, especially the JNK2 54kDa splice variants; (iv) repression of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1), which normally supports NGF-mediated cell cycle arrest; (v) strong induction of the cell cycle promoter CyclinD1, and (vi) profound changes of p53 functions. Moreover, MKK7γ1 substantially changes the responsiveness to stress. Whereas NGF differentiation protects PC12 cells against taxol-induced apoptosis, MKK7γ1 triggers an escape from cell cycle arrest and renders transfected cells sensitive to taxol-induced death. This stress response completely differs from naïve PC12 cells, where MKK7γ1 protects against taxol-induced cell death. These novel aspects on the regulation of JNK signalling emphasise the importance of MKK7γ1 in its ability to reverse basic cellular programmes by simply using JNKs as effectors. Furthermore, our results highlight the necessity for the cells to balance the expression of JNK activators to ensure precise intracellular processes.

Epicatechin blocks pro-nerve growth factor (proNGF)-mediated retinal neurodegeneration via inhibition of p75 neurotrophin receptor expression in a rat model of diabetes [corrected]

AIMS/HYPOTHESIS

Accumulation of pro-nerve growth factor (NGF), the pro form of NGF, has been detected in neurodegenerative diseases. However, the role of proNGF in the diabetic retina and the molecular mechanisms by which proNGF causes retinal neurodegeneration remain unknown. The aim of this study was to elucidate the role of proNGF in neuroglial activation and to examine the neuroprotective effects of epicatechin, a selective inhibitor of tyrosine nitration, in an experimental rat model of diabetes.

METHODS

Expression of proNGF and its receptors was examined in retinas from streptozotocin-induced diabetic rats, and in retinal Müller and retinal ganglion cells (RGCs). RGC death was assessed by TUNEL and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays in diabetic retinas and cell culture. Nitrotyrosine was determined using Slot-blot. Activation of the tyrosine kinase A (TrkA) receptor and p38 mitogen-activated protein kinase (p38MAPK) was assessed by western blot.

RESULTS

Diabetes-induced peroxynitrite impaired phosphorylation of TrkA-Y490 via tyrosine nitration, activated glial cells and increased expression of proNGF and its receptor, p75 neurotrophin receptor (p75(NTR)), in vivo and in Müller cells. These effects were associated with activation of p38MAPK, cleaved poly-(ADP-ribose) polymerase and RGC death. Treatment of diabetic animals with epicatechin (100 mg kg(-1) day(-1), orally) blocked these effects and restored neuronal survival. Co-cultures of RGCs with conditioned medium of activated Müller cells significantly reduced RGC viability (44%). Silencing expression of p75(NTR) by use of small interfering RNA protected against high glucose- and proNGF-induced apoptosis in RGC cultures.

CONCLUSIONS/INTERPRETATION

Diabetes-induced peroxynitrite stimulates p75(NTR) and proNGF expression in Müller cells. It also impairs TrkA receptor phosphorylation and activates the p75(NTR) apoptotic pathway in RGCs, leading to neuronal cell death. These effects were blocked by epicatechin, a safe dietary supplement, suggesting its potential therapeutic use in diabetic patients.

Chemical genetics identifies small-molecule modulators of neuritogenesis involving neuregulin-1/ErbB4 signaling

Genetic findings have suggested that neuregulin-1 (Nrg1) and its receptor v-erb-a erythroblastic leukemia viral oncogene homolog 4 (ErbB4) may play a role in neuropsychiatric diseases. However, the downstream signaling events and relevant phenotypic consequences of altered Nrg1 signaling in the nervous system remain poorly understood. To identify small molecules for probing Nrg1-ErbB4 signaling, a PC12-cell model was developed and used to perform a live-cell, image-based screen of the effects of small molecules on Nrg1-induced neuritogenesis. By comparing the resulting phenotypic data to that of a similar screening performed with nerve growth factor (NGF), this multidimensional screen identified compounds that directly inhibit Nrg1-ErbB4 signaling, such as the 4-anilino-quinazoline Iressa (gefitinib), as well as compounds that potentiate Nrg1-ErbB4 signaling, such as the indolocarbazole K-252a. These findings provide new insights into the regulation of Nrg1-ErbB4 signaling events and demonstrate the feasibility of using such a multidimensional, chemical-genetic approach for discovering probes of pathways implicated in neuropsychiatric diseases.

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

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

Neurotrophin-induced upregulation of p75NTR via a protein kinase C-delta-dependent mechanism

Neurotrophins exert their biological effects via p75NTR and Trk receptors. Functional interplay between these two receptors has been widely explored with respect to p75NTR enhancing the activation and signalling of Trk, but few studies address the bidirectional aspects. We have previously demonstrated that the expression of p75NTR can be differentially modulated by different Trk receptor mutations. Here we investigate the mechanism of Nerve Growth Factor (NGF)-induced upregulation of p75NTR expression. We utilize pharmacological inhibition to investigate the role of various TrkA-associated signalling intermediates in this regulatory cascade. Notably, the inhibition of phospholipase C-gamma (PLC-gamma) using U73122, prevented the NGF-induced upregulation of p75NTR protein and mRNA. The inhibition of protein kinase C-delta (PKC-delta) activation by rottlerin, a selective PKC-delta inhibitor, and by small interfering RNA (siRNA) directed against PKC-delta also inhibited this NGF-induced upregulation. Finally, we also show that in cerebellar granule neurons, BDNF acting via TrkB increases p75NTR expression in a PKC-delta dependent manner. These results indicate the importance of Trk-dependent PLC-gamma and PKC-delta activation for downstream regulation of p75NTR protein expression in response to neurotrophin stimulation, a process that has implications to the survival and growth of the developing nervous system.

Expression of acid-sensing ion channel 3 (ASIC3) in nucleus pulposus cells of the intervertebral disc is regulated by p75NTR and ERK signaling

Although a recent study has shown that skeletal tissues express ASICs, their function is unknown. We show that intervertebral disc cells express ASIC3; moreover, expression is uniquely regulated and needed for survival in a low pH and hypoeromsotic medium. These findings suggest that ASIC3 may adapt disc cells to their hydrodynamically stressed microenvironment.

INTRODUCTION

The nucleus pulposus is an avascular, hydrated tissue that permits the intervertebral disc to resist compressive loads to the spine. Because the tissue is hyperosmotic and avascular, the pH of the nucleus pulposus is low. To determine the mechanisms by which the disc cells accommodate to the low pH and hypertonicity, the expression and regulation of the acid sensing ion channel (ASIC)3 was examined.

MATERIALS AND METHODS

Expression of ASICs in cells of the intervertebral disc was analyzed. To study its regulation, we cloned the 2.8-kb rat ASIC3 promoter and performed luciferase reporter assays. The effect of pharmacological inhibition of ASICs on disc cell survival was studied by measuring MTT and caspase-3 activities.

RESULTS

ASIC3 was expressed in discal tissues and cultured disc cells in vitro. Because studies of neuronal cells have shown that ASIC3 expression and promoter activity is induced by nerve growth factor (NGF), we examined the effect of NGF on nucleus pulposus cells. Surprisingly, ASIC3 promoter activity did not increase after NGF treatment. The absence of induction was linked to nonexpression of tropomyosin-related kinase A (TrkA), a high-affinity NGF receptor, although a modest expression of p75NTR was seen. When treated with p75NTR antibody or transfected with dominant negative-p75NTR plasmid, there was significant suppression of ASIC3 basal promoter activity. To further explore the downstream mechanism of control of ASIC3 basal promoter activity, we blocked p75NTR and measured phospho extracellular matrix regulated kinase (pERK) levels. We found that DN-p75NTR suppressed NGF mediated transient ERK activation. Moreover, inhibition of ERK activity by dominant negative-mitogen activated protein kinase kinase (DN-MEK) resulted in a dose-dependent suppression of ASIC3 basal promoter activity, whereas overexpression of constitutively active MEK1 caused an increase in ASIC3 promoter activity. Finally, to gain insight in the functional importance of ASIC3, we suppressed ASIC activity in nucleus pulposus cells. Noteworthy, under both hyperosmotic and acidic conditions, ASIC3 served to promote cell survival and lower the activity of the pro-apoptosis protein, caspase-3.

CONCLUSIONS

Results of this study indicate that NGF serves to maintain the basal expression of ASIC3 through p75NTR and ERK signaling in discal cells. We suggest that ASIC3 is needed for adaptation of the nucleus pulposus and annulus fibrosus cells to the acidic and hyperosmotic microenvironment of the intervertebral disc.

A method to assess multiple aspects of the motile behaviour of adherent PC12 cells on applied biological substrates

Cellular migration is central to a wide range of biological and pathological processes in vivo. In vitro cell migration assays can be used to obtain invaluable information relating to the mechanism of cell movement, but current available methods can be limiting. Here we describe a novel motility assay that allows the simultaneous investigation of both quantitative and qualitative aspects of a population of motile cells as they move across a variety of substrates. By plating cells in a confluent monolayer on a coverslip, the monolayer can then be inverted to migrate over a larger substrate-coated coverslip, which can subsequently be reliably quantified, and subjected to immunocytochemistry and confocal imaging. This assay can be used to assess multiple aspects of motility, including distance, quantity, morphology, polarization and component colocalization. To demonstrate the utility of this assay, it was applied to the study of a stimulator of PC12 cell migration, nerve growth factor (NGF), and how this migration is influenced by the extracellular substrate, laminin. Furthermore, since mutations to the NGF receptor, TrkA, have been noted to alter the behaviour of PC12 cells in response to NGF, a PC12 subline that expresses a mutated TrkA receptor was utilized to illustrate that a Y785F mutation in the cytoplasmic tail of TrkA results in increased migration in response to the stimulus compared to the control PC12s.