Role of nerve growth factor in the expression of trkA mRNA in cultured embryonic rat basal forebrain cholinergic neurons

Loss of PKCδ/Prkcd prevents cartilage degeneration in joints but exacerbates hyperalgesia in an experimental osteoarthritis mouse model

Pain is the prime symptom of osteoarthritis (OA) that directly affects the quality of life. Protein kinase Cδ (PKCδ/Prkcd) plays a critical role in OA pathogenesis; however, its significance in OA-related pain is not entirely understood. The present study investigated the functional role of PKCδ in OA pain sensation. OA was surgically induced in control (Prkcdfl/fl), global- (Prkcdfl/fl; ROSACreERT2), and sensory neuron-specific conditional knockout (cKO) mice (Prkcdfl/fl; NaV1.8/Scn10aCreERT2) followed by comprehensive analysis of longitudinal behavioral pain, histopathology and immunofluorescence studies. GlobalPrkcd cKO mice prevented cartilage deterioration by inhibiting matrix metalloproteinase-13 (MMP13) in joint tissues but significantly increased OA pain. Sensory neuron-specificdeletion of Prkcd in mice did not protect cartilage from degeneration but worsened OA-associated pain. Exacerbated pain sensitivity observed in global- and sensory neuron-specific cKO of Prkcd was corroborated with markedly increased specific pain mediators in knee synovium and dorsal root ganglia (DRG). These specific pain markers include nerve growth factor (NGF) and vascular endothelial growth factor (VEGF), and their cognate receptors, including tropomyosin receptor kinase A (TrkA) and vascular endothelial growth factor receptor-1 (VEGFR1). The increased levels of NGF/TrkA and VEGF/VEGFR1 were comparable in both global- and sensory neuron-specific cKO groups. These data suggest that the absence of Prkcd gene expression in the sensory neurons is strongly associated with OA hyperalgesia independent of cartilage protection. Thus, inhibition of PKCδ may be beneficial for cartilage homeostasis but could aggravate OA-related pain symptoms.

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.

CNS Gene Therapy and a Nexus of Complexity: Systems and Biology at a Crossroads

Gene therapy is a potentially promising new treatment for neurodegenerative disorders such as Alzheimer's disease (AD), which has been difficult to treat with conventional therapeutics. Viral vector-mediated somatic gene therapy is a rapidly developing methodology for providing never before achieved capability to deliver specific genes to the CNS in a highly localized and controlled manner. With the advent and refinements of this technology one focus is directed to which genes are the most appropriate to select for specific disease indications. Nerve growth factor (NGF), a potent survival factor for critical cell populations that degenerate in AD, has been chosen already for clinical gene therapy trials in human AD patients. Much knowledge about the pathophysiological underpinnings of AD is still lacking to make clear which patients may benefit from a gene therapy approach. Moreover, a detailed understanding of sustained NGF action in the normal and diseased CNS needs to be resolved before conclusions can be drawn regarding the utility of NGF gene therapy. Systematic efforts to acquire this new knowledge should compel clinically and biologically sophisticated efforts to advance gene therapy for neurodegenerative diseases.

PKCδ null mutations in a mouse model of osteoarthritis alter osteoarthritic pain independently of joint pathology by augmenting NGF/TrkA-induced axonal outgrowth

OBJECTIVES

A key clinical paradox in osteoarthritis (OA), a prevalent age-related joint disorder characterised by cartilage degeneration and debilitating pain, is that the severity of joint pain does not strictly correlate with radiographic and histological defects in joint tissues. Here, we determined whether protein kinase Cδ (PKCδ), a key mediator of cartilage degeneration, is critical to the mechanism by which OA develops from an asymptomatic joint-degenerative condition to a painful disease.

METHODS

OA was induced in 10-week-old PKCδ null (PKCδ-/-) and wild-type mice by destabilisation of the medial meniscus (DMM) followed by comprehensive examination of the histology, molecular pathways and knee-pain-related-behaviours in mice, and comparisons with human biopsies.

RESULTS

In the DMM model, the loss of PKCδ expression prevented cartilage degeneration but exacerbated OA-associated hyperalgesia. Cartilage preservation corresponded with reduced levels of inflammatory cytokines and of cartilage-degrading enzymes in the joints of PKCδ-deficient DMM mice. Hyperalgesia was associated with stimulation of nerve growth factor (NGF) by fibroblast-like synovial cells and with increased synovial angiogenesis. Results from tissue specimens of patients with symptomatic OA strikingly resembled our findings from the OA animal model. In PKCδ null mice, increases in sensory neuron distribution in knee OA synovium and activation of the NGF-tropomyosin receptor kinase (TrkA) axis in innervating dorsal root ganglia were highly correlated with knee OA hyperalgesia.

CONCLUSIONS

Increased distribution of synovial sensory neurons in the joints, and augmentation of NGF/TrkA signalling, causes OA hyperalgesia independently of cartilage preservation.

Nerve growth factor: an update on the science and therapy

OBJECTIVE

Nerve growth factor (NGF) is a key regulator of nociceptive pain and thus appears to be an interesting target molecule for an innovative class of analgesic medication. We set out to review the principles of neurogenic inflammation and results of anti-NGF regimens in animal studies as well as clinical trials with patients with back pain and osteoarthritis (OA).

DESIGN

We searched using Google Scholar Search and Pubmed as well as through conference reports for articles and abstracts related to NGF and clinical trials using anti-NGF regimens. We report on efficacy findings and adverse events (AEs) related to these agents in this review.

RESULTS

We identified five full articles and eight abstract reports relating to anti-NGF agents studied for use in back pain and in OA.

CONCLUSIONS

Anti-NGF agents either alone or in combination with non-steroidal anti-inflammatory agents (NSAIDs) were more efficacious for the treatment of pain in a number of trials of knee and hip pain compared to NSAIDs alone. However, adverse effects that included rapidly progressive OA and joint replacement were more common in patients treated with anti-NGF and NSAIDs than either treatment alone. Anti-NGF treatment related neurologic symptoms including paresthesias, and potentially other types of adverse effects were usually transient but warrant additional investigation.

NGF - the TrkA to successful pain treatment

Chronic pain arising from various pathological conditions such as osteoarthritis, low back or spinal injuries, cancer, and urological chronic pelvic pain syndromes presents significant challenges in diagnosis and treatment. Specifically, since the underlying cause of these pain syndromes is unknown or heterogeneous, physicians diagnose and treat patients based on the symptoms presented. Nerve growth factor (NGF) has been recognized as an important mediator of chronic pain in many pathological conditions, and has been shown to be upregulated in a subset of individuals suffering from such pain syndromes. These findings have led to the development of anti-NGF monoclonal antibodies such as tanezumab as potentially effective therapeutics for chronic pain. Although tanezumab has reached Phase II and III clinical trials, the trials of anti-NGF antibodies were halted due to safety concerns. Some of these trials of anti-NGF treatment have had statistically significant decreases in pain, while others have yielded inconclusive results. These findings are suggestive of, though do not prove, target (NGF) neutralization in chronic pain syndromes. A biomarker-driven anti-NGF clinical study layout is proposed that incorporates NGF measurements in the relevant samples before and after treatment, in addition to collecting the pain scores. This approach might not only confirm the mechanism of tanezumab's action in these chronic pain patients, but should establish NGF levels as a predictive biomarker for patients who can benefit from anti-NGF treatment, thereby creating a personalized approach to pain treatment.

Nerve growth factor survival signaling in cultured hippocampal neurons is mediated through TrkA and requires the common neurotrophin receptor P75

The role of the common neurotrophin receptor p75 (p75NTR) in neuronal survival and cell death remains controversial. On the one hand, p75NTR provides a positive modulatory influence on nerve growth factor (NGF) signaling through the high affinity neurotrophin receptor TrkA, and hence increases NGF survival signaling. However, p75NTR may also signal independently of TrkA, causing cell death or cell survival, depending on the cell type and stage of development. Here we demonstrate that TrkA is expressed in primary cultures of hippocampal neurons and is activated by NGF within 10 min of exposure. In primary hippocampal cultures neuroprotection by NGF against glutamate toxicity was mediated by NF-kappaB and accompanied by an increased expression of neuroprotective NF-kappaB target genes Bcl-2 and Bcl-xl. In mouse hippocampal cells lacking p75NTR (p75NTR-/-) activation of TrkA by NGF was not detectable. Moreover, neuroprotection by NGF against glutamate toxicity was abolished in p75NTR-/- neurons, and the expression of bcl-2 and bcl-xl was markedly reduced as compared to wildtype cells. NGF increased TrkA phosphorylation in hippocampal neurons and provided protection that required phosphoinositol-3-phosphate (PI3)-kinase activity and Akt phosphorylation, whereas the mitogen-activated protein kinases (MAPK), extracellular-regulated kinases (Erk) 1/2, were not involved. P75NTR signaling independent of TrkA, such as increased neutral sphingomyelinase (NSMase) activity causing enhanced levels of ceramide, were not detected after exposure of hippocampal neurons to NGF. Interestingly, inhibition of sphingosine-kinase blocked the neuroprotective effect of NGF, suggesting that sphingosine-1-phosphate was also involved in NGF-mediated survival in our cultured hippocampal neurons. Overall, our results indicate an essential role for p75NTR in supporting NGF-triggered TrkA signaling pathways mediating neuronal survival in hippocampal neurons.

Decreased expression of hippocampal cholinergic neurostimulating peptide precursor protein mRNA in the hippocampus in Alzheimer disease

Hippocampal cholinergic neurostimulating peptide (HCNP) is involved in the phenotype development of the septo-hippocampal system. HCNP precursor protein (HCNP-pp) is known to interact with other molecules including phosphatidylethanolamine and Raf-1 kinase, and is also known as phosphatidylethanolamine-binding protein and raf kinase-inhibitory protein. To assess whether HCNP-pp is involved in the pathogenesis of Alzheimer disease (AD), the expression levels of its mRNA in the hippocampus of autopsy brains from patients with dementia (including AD and ischemic vascular dementia) were compared with those of non-demented control subjects. The in situ hybridization analysis revealed that the expression of HCNP-pp mRNA in patients with clinically late-onset AD was decreased in the hippocampal CA1 field, but not in the CA3 field or the dentate gyrus. The early-onset AD patients showed a wide range of expression levels in the hippocampal sub-regions. Northern blot analysis of HCNP-pp mRNA in brain tissue supported these observations. Since HCNP is known to stimulate the enzymatic activity of choline acetyltransferase in neurons, its low expression in the CAI field of AD patients may explain the downregulation of cholinergic neurons seen in these patients and may thus contribute to the pathogenic processes underlying AD.

The precursor pro-nerve growth factor is the predominant form of nerve growth factor in brain and is increased in Alzheimer's disease

Nerve growth factor (NGF) is important for regulation, differentiation, and survival of peripheral and central nervous system neurons, including basal forebrain cholinergic neurons (BFCN) which degenerate in Alzheimer's disease (AD). Mature NGF protein is processed from a larger precursor, proNGF. We demonstrate that proNGF is the predominant form of NGF in mouse, rat, and human brain tissue, whereas little or no mature NGF is detected. Previous reports showed NGF protein, measured by ELISA, is increased in AD BFCN target regions such as hippocampus and cortex. Using Western blotting, we demonstrate a twofold increase in proNGF in AD parietal cortex compared to controls, indicating that it is this precursor form, proNGF, that accumulates in AD. This increase may reflect either a role for biologically active proNGF or posttranslational disturbances in NGF biosynthesis that decrease the processing of proNGF to mature NGF in AD.

Inhibition of nerve growth factor signaling by peroxynitrite

The reactive oxygen species peroxynitrite has been implicated in mediating oxidative damage within the brain, and in particular in those regions associated with the pathology of Alzheimer disease. Evidence for peroxynitrite damage includes the abundance of nitrated tyrosine residues within proteins of neural cells. Potential sites for peroxynitrite-induced cytotoxicity are the tyrosine residues of tyrosine kinase receptors that are crucial for the maintenance of cholinergic neurons. The peroxynitrite generator 3-morpholinosydnonmine (SIN-1) was used to examine the effects of peroxynitrite generation on nerve growth factor (NGF)/TrkA signaling in PC12 pheochromocytoma cells that express a cholinergic phenotype. NGF produced a concentration-dependent increase in PC12 cellular metabolism (EC(50) = 15.2 ng/ml) measured in a microphysiometer. This action of NGF was inhibited in a concentration-dependent manner up to 67% of control by a brief (20 min) exposure of the cells to SIN-1. This inhibition of the NGF cellular response by SIN-1 was not related to generalized cellular toxicity. In fact, the peroxynitrite scavenger uric acid significantly attenuated the inhibitory actions of SIN-1. Pretreatment with SIN-1 also resulted in a decrease in the NGF-induced phosphorylation of TrkA protein. Furthermore, SIN-1 treatment reduced the activity of mitogen activated protein kinase (MAPK), a downstream kinase activated by TrkA receptor stimulation. These data suggest that SIN-1 treatment inhibits NGF signaling by inactivating TrkA receptors through the formation of nitrotyrosine residues on the receptor. The inactivation of TrkA receptors may contribute to the initial insult that eventually leads to neuronal cell death.

Heterologous upregulation of nerve growth factor-TrkA receptors in PC12 cells by pituitary adenylate cyclase-activating polypeptide (PACAP)

The capacity for the neurotrophic factor PACAP38 to regulate expression of nerve growth factor (NGF)-trkA receptors in PC12 cells has been examined. Treatment of PC12 cells with 5 nM PACAP38 for 48 h elicited a 2.5-fold increase in 125I-NGF binding sites. FACS and Western analysis of trkA receptor protein indicate an abundance of receptors. The PACAP38-selective antagonist PACAP 6-38 blocked trkA receptor upregulation elicited by PACAP38. The expression of epidermal growth factor receptors was not affected by PACAP38 suggesting that upregulation of trkA represents a selective effect of this neurotrophic peptide. Similarly, expression of the pan-neurotrophin binding receptor p75 was not altered by PACAP38 treatment. In addition to effects on trkA observed in wild-type PC12 cells, PACAP38 stimulated an increase in the level of expressed human trkA receptors stably transfected into PC12 cells. PACAP38 provoked an increase in basal and NGF-stimulated phosphorylation of trkA. Enhanced phosphorylation of trkA was detected as early as 6 h following addition of PACAP38 and was maximal at 48 h. Increased incorporation of phosphate occurs on both serine and tyrosine residues of trkA. These results suggest that PACAP38 is able to promote upregulation of trkA receptors, an event associated with elevated serine/tyrosine phosphorylation of trkA.

Pancreatic beta cells synthesize and secrete nerve growth factor

Differentiation and function of pancreatic beta cells are regulated by a variety of hormones and growth factors, including nerve growth factor (NGF). Whether this is an endocrine or autocrine/paracrine role for NGF is not known. We demonstrate that NGF is produced and secreted by adult rat pancreatic beta cells. NGF secretion is increased in response to elevated glucose or potassium, but decreased in response to dibutyryl cAMP. Moreover, steady-state levels of NGF mRNA are down-regulated by dibutyryl cAMP, which is opposite to the effect of cAMP on insulin release. NGF-stimulated changes in morphology and function are mediated by high-affinity Trk A receptors in other mammalian cells. Trk A receptors are present in beta cells and steady-state levels of Trk A mRNA are modulated by NGF and dibutyryl cAMP. Taken together, these findings suggest endocrine and autocrine roles for pancreatic beta-cell NGF, which, in turn, could be related to the pathogenesis of diabetes mellitus where serum NGF levels are diminished.