Commentary: Regulating proNGF action: multiple targets for therapeutic intervention

Valproic acid upregulates the expression of the p75NTR/sortilin receptor complex to induce neuronal apoptosis

The antiepileptic and mood stabilizer agent valproic acid (VPA) has been shown to exert anti-tumour effects and to cause neuronal damage in the developing brain through mechanisms not completely understood. In the present study we show that prolonged exposure of SH-SY5Y and LAN-1 human neuroblastoma cells to clinically relevant concentrations of VPA caused a marked induction of the protein and transcript levels of the common neurotrophin receptor p75NTR and its co-receptor sortilin, two promoters of apoptotic cell death in response to proneurotrophins. VPA induction of p75NTR and sortilin was associated with an increase in plasma membrane expression of the receptor proteins and was mimicked by cell treatment with several histone deacetylase (HDAC) inhibitors. VPA and HDAC1 knockdown decreased the level of EZH2, a core component of the polycomb repressive complex 2, and upregulated the transcription factor CASZ1, a positive regulator of p75NTR. CASZ1 knockdown attenuated VPA-induced p75NTR overexpression. Cell treatment with VPA favoured proNGF-induced p75NTR/sortilin interaction and the exposure to proNGF enhanced JNK activation and apoptotic cell death elicited by VPA. Depletion of p75NTR or addition of the sortilin agonist neurotensin to block proNGF/sortilin interaction reduced the apoptotic response to VPA and proNGF. Exposure of mouse cerebellar granule cells to VPA upregulated p75NTR and sortilin and induced apoptosis which was enhanced by proNGF. These results indicate that VPA upregulates p75NTR apoptotic cell signalling through an epigenetic mechanism involving HDAC inhibition and suggest that this effect may contribute to the anti-neuroblastoma and neurotoxic effects of VPA.

Diabetic retinal neurodegeneration as a form of diabetic retinopathy

PURPOSE

To review the evidence supporting diabetic retinal neurodegeneration (DRN) as a form of diabetic retinopathy.

METHOD

Review of literature.

RESULTS

DRN is recognized to be a part of retinopathy in patients with diabetes mellitus (DM), in addition to the well-established diabetic retinal vasculopathy (DRV). DRN has been noted in the early stages of DM, before the onset of clinically evident diabetic retinopathy. The occurrence of DRN has been confirmed in animal models of DM, histopathological examination of donor's eyes from diabetic individuals and assessment of neural structure and function in humans. DRN involves alterations in retinal ganglion cells, photoreceptors, amacrine cells and bipolar cells, and is thought to be driven by glutamate, oxidative stress and dysregulation of neuroprotective factors in the retina. Potential therapeutic options for DRN are under evaluation.

CONCLUSIONS

Literature is divided on the temporal relation between DRN and DRV, with evidence of both precedence and simultaneous occurrence. The relationship between DRN and multi-system neuropathy in DM is yet to be evaluated critically.

2,5-Hexanedione induced apoptosis in rat spinal cord neurons and VSC4.1 cells via the proNGF/p75NTR and JNK pathways

Increasing evidence suggests that n-hexane induces nerve injury via neuronal apoptosis induced by its active metabolite 2,5-hexanedione (HD). However, the underlying mechanism remains unknown. Studies have confirmed that pro-nerve growth factor (proNGF), a precursor of mature nerve growth factor (mNGF), might activate apoptotic signaling by binding to p75 neurotrophin receptor (p75NTR) in neurons. Therefore, we studied the mechanism of the proNGF/p75NTR pathway in HD-induced neuronal apoptosis. Sprague-Dawley (SD) rats were injected with 400 mg/kg HD once a day for 5 weeks, and VSC4.1 cells were treated with 10, 20, and 40 mM HD in vitro. Results showed that HD effectively induced neuronal apoptosis. Moreover, it up-regulated proNGF and p75NTR levels, activated c-Jun N-terminal kinase (JNK) and c-Jun, and disrupted the balance between B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). Our findings revealed that the proNGF/p75NTR signaling pathway was involved in HD-induced neuronal apoptosis; it can serve as a theoretical basis for further exploration of the neurotoxic mechanisms of HD.

Deleterious impact of nerve growth factor precursor (proNGF) on bladder urothelial and smooth muscle cells

The nerve growth factor precursor (proNGF) activates p75^NTR receptor and promotes cell death in different tissues, yet this pathophysiological effect is not fully described in the bladder. The aim of this study was to identify the biological effect of proNGF/p75^NTR activation on urothelial and smooth muscle (SM) cells of rodents' bladder. Cell viability was assessed by MTT assay which showed a significant reduction in urothelial viability after 24 h of incubation with proNGF in culture medium [5 or 10 nM], an effect not seen in SM cells. Western blot analysis on cellular protein extracts showed increased expression of the transmembrane TNF-α and activation of RhoA in urothelial cells exposed to proNGF with no evidence of a nuclear translocation of NF-κB assessed by western blotting on nuclear extracts and immunofluorescence. The activation of p75^NTR-death domain related pathways in urothelial cells such as TNF-α or RhoA had a downstream effect on NO release and the junctional protein occludin, as estimated respectively by colorimetric and western blotting. On the other hand, proNGF did not induce TNF-α or RhoA expression in SM cells, but induced a significant NF-κB nuclear translocation. ProNGF had a different impact on SM as evidenced by a significant dose- and time-dependent increase in SM proliferation and migration examined by MTT test and cell migration assay. Together, our results indicate that activation of proNGF/p75^NTR axis induces degenerative changes to the urothelial layer impacting its barrier and signaling integrity, while promoting adaptive proliferative changes in detrusor SM cells that can interfere with the contractile phenotype essential for proper bladder function.

Can Growth Factors Cure Parkinson's Disease?

Growth factors (GFs) hold considerable promise for disease modification in neurodegenerative disorders because they can protect and restore degenerating neurons and also enhance their functional activity. However, extensive efforts applied to utilize their therapeutic potential in humans have achieved limited success so far. Multiple clinical trials with GFs were performed in Parkinson's disease (PD) patients, in whom diagnostic symptoms of the disease are caused by advanced degeneration of nigrostriatal dopamine neurons (DNs), but the results of these trials are controversial. This review discusses recent developments in the field of therapeutic use of GFs, problems and obstacles related to this use, suggests the ways to overcome these issues, and alternative approaches that can be used to utilize the potential ofGFsin PD management.

Antagonism of proNGF or its receptor p75NTR reverses remodelling and improves bladder function in a mouse model of diabetic voiding dysfunction

AIMS/HYPOTHESIS

Although 80% of diabetic patients will suffer from voiding difficulties and urinary symptoms, defined as diabetic voiding dysfunction (DVD), therapeutic targets and treatment options are limited. We hypothesise that the blockade of the pro-nerve growth factor (NGF)/p75 neurotrophin receptor (p75NTR) axis by an anti-proNGF monoclonal antibody or by a small molecule p75NTR antagonist (THX-B) can restore bladder remodelling (represented by bladder weight) in an animal model of DVD. Secondary outcomes of the study include improvements in bladder compliance, contractility and morphology, as well as in voiding behaviour, proNGF/NGF balance and TNF-α expression.

METHODS

In a streptozotocin-induced mouse model of diabetes, diabetic mice received either a blocking anti-proNGF monoclonal antibody or a p75NTR antagonist small molecule as weekly systemic injections for 4 weeks. Animals were tested at baseline (at 2 weeks of diabetes induction), and after 2 and 4 weeks of treatment. Outcomes measured were voiding function with voiding spot assays and cystometry. Bladders were assessed by histological, contractility and protein expression assays.

RESULTS

Diabetic mice showed features of DVD as early as 2 weeks after diabetes diagnosis (baseline) presented by hypertrophy, reduced contractility and abnormal cystometric parameters. Following treatment initiation, a twofold increase (p < 0.05) in untreated diabetic mouse bladder weight and thickness compared with non-diabetic controls was observed, and this change was reversed by p75NTR antagonism (37% reduction in bladder weight compared with untreated diabetic mice [95% CI 14%, 60%]) after 4 weeks of treatment. However, blocking proNGF did not help to reverse bladder hypertrophy. While diabetic mice had significantly worse cystometric parameters and contractile responses than non-diabetic controls, proNGF antagonism normalised bladder compliance (0.007 [Q1-Q3; 0.006-0.009] vs 0.015 [Q1-Q3; 0.014-0.029] ml/cmH2O in untreated diabetic mice, representing 62% reduction [95% CI 8%, 110%], p < 0.05) and contractility to KCl, carbachol and electrical field stimulation (p < 0.05 compared with the diabetic group) after 2 weeks of treatment. These effects were not observed after 4 weeks of treatment with proNGF antagonist. p75NTR antagonism did not show important improvements in cystometric parameters after 2 weeks of treatment. Slightly improved bladder compliance (0.01 [Q1-Q3; 0.009-0.012] vs 0.013 [Q1-Q3; 0.011-0.016] ml/cmH2O for untreated diabetic mice) was seen in the p75NTR antagonist-treated group after 4 weeks of treatment with significantly stabilised contractile responses to KCl, carbachol and electric field stimulation (p < 0.05 for each) compared with diabetic mice. Bladder dysfunction observed in diabetic mice was associated with a significant increase in bladder proNGF/NGF ratio (3.1 [±1.2] vs 0.26 [±0.04] ng/pg in control group, p < 0.05 at week 2 of treatment) and TNF-α (p < 0.05). The proNGF/NGF ratio was partially reduced (about 60% reduction) with both treatments (1.03 [±0.6] ng/pg for proNGF antibody-treated group and 1.4 [±0.76] ng/pg for p75NTR blocker-treated group after 2 weeks of treatment), concomitant with a significant decrease in the bladder levels of TNF-α (p < 0.05), despite persistent hyperglycaemia.

CONCLUSIONS/INTERPRETATION

Our findings indicate that blockade of proNGF and the p75NTR receptor in diabetes can impede the development and progression of DVD. The reported improvements in morphological and functional features in our DVD model validates the proNGF/p75NTR axis as a potential therapeutic target in this pathology. Graphical abstract.

Imbalance of nerve growth factor metabolism in aging women with overactive bladder syndrome

PURPOSE

Given the disputable link between nerve growth factor (NGF) and overactive bladder syndrome (OAB) and the lack of studies on its precursor (proNGF) in OAB, the aim of the study was to identify changes in the urinary levels of NGF and its proteolytic enzymes in aging women with OAB.

METHODS

We examined the urinary proNGF/NGF ratio and its processing enzymes in aging women (50-80 years), comparing 20 controls and 20 subjects with OAB.

RESULTS

In contrast to previous reports correlating NGF to OAB symptoms, we found that proNGF/NGF ratio in the OAB group was twice as high compared to controls (p = 0.009) with a lower NGF levels in women with OAB without statistical significance [1.36 (Q1, Q3: 0.668, 2.39) vs. 1.7 (Q1, Q3: 1.27, 3.045) pg/mg creatinine in control group, p = 0.05]. Enzymatic activity of MMP-7, the main enzyme for extracellular proNGF maturation, was significantly increased in the OAB group and correlated positively with scores of OAB symptoms questionnaires. However, this was counteracted by several-folds increase in the MMP-9 enzyme responsible for NGF proteolysis. While these findings highlight the importance of changes in the proteolytic enzymes to maintain proNGF/NGF balance in OAB, analysis of covariates showed that these changes were attributed to age, insulin resistance and renal function.

CONCLUSION

NGF proteolysis imbalance can be clinically meaningful in OAB related to aging, rendering it as a potential therapeutic target. However, other age-related factors such as insulin resistance and renal function may contribute to the relationship between NGF and aging-related OAB phenotype.

Pro-Nerve Growth Factor Induces Activation of RhoA Kinase and Neuronal Cell Death

We have previously shown that the expression of pro-nerve growth factor (proNGF) was significantly increased, nerve growth factor (NGF) level was decreased, and the expression of p75^NTR was enhanced in Alzheimer’s disease (AD) hippocampal samples. NGF regulates cell survival and differentiation by binding TrkA and p75^NTR receptors. ProNGF is the precursor form of NGF, binds to p75^NTR, and induces cell apoptosis. The objective of this study is to determine whether the increased p75^NTR expression in AD is due to the accumulation of proNGF and Rho kinase activation. PC12 cells were stimulated with either proNGF or NGF. Pull-down assay was carried out to determine the RhoA kinase activity. We found the expression of p75^NTR was enhanced by proNGF compared to NGF. The proNGF stimulation also increased the RhoA kinase activity leading to apoptosis. The expression of active RhoA kinase was found to be increased in human AD hippocampus compared to control. The addition of RhoA kinase inhibitor Y27632 not only blocked the RhoA kinase activity but also reduced the expression of p75^NTR receptor and inhibited the activation of JNK and MAPK induced by proNGF. This suggests that overexpression of proNGF in AD enhances p75^NTR expression and activation of RhoA, leading to neuronal cell death.

A Novel Neuroprotective Mechanism for Lithium That Prevents Association of the p75NTR-Sortilin Receptor Complex and Attenuates proNGF-Induced Neuronal Death In Vitro and In Vivo

Neurotrophins play critical roles in the survival, maintenance and death of neurons. In particular, proneurotrophins have been shown to mediate cell death following brain injury induced by status epilepticus (SE) in rats. Previous studies have shown that pilocarpine-induced seizures lead to increased levels of proNGF, which binds to the p75NTR-sortilin receptor complex to elicit apoptosis. A screen to identify compounds that block proNGF binding and uptake into cells expressing p75 and sortilin identified lithium citrate as a potential inhibitor of proNGF and p75NTR-mediated cell death. In this study, we demonstrate that low, submicromolar doses of lithium citrate effectively inhibited proNGF-induced cell death in cultured neurons and protected hippocampal neurons following pilocarpine-induced SE in vivo. We analyzed specific mechanisms by which lithium citrate afforded neuroprotection and determined that lithium citrate prevented the association and internalization of the p75NTR-sortilin receptor complex. Our results demonstrate a novel mechanism by which low-dose treatments of lithium citrate are effective in attenuating p75NTR-mediated cell death in vitro and in vivo.

Inducible overexpression of endothelial proNGF as a mouse model to study microvascular dysfunction

Impaired maturation of nerve growth factor precursor (proNGF) and its accumulation has been reported in several neurodegenerative diseases, myocardial infarction and diabetes. To elucidate the direct impact of proNGF accumulation identified the need to create a transgenic model that can express fully mutated cleavage-resistant proNGF. Using Cre-Lox technology, we developed an inducible endothelial-specific proNGF transgenic mouse (proNGF^Loxp) that overexpresses GFP-conjugated cleavage-resistant proNGF123 when crossed with VE-cadherin-CreERT2 (Cre). Expression of proNGF, inflammatory mediators, NGF and VEGF was evaluated by PCR, Western blot and immunohistochemistry. EC-proNGF overexpression was confirmed using colocalization of anti-proNGF within retinal vasculature. EC-proNGF did not cause retinal neurotoxicity or marked glial activation at 4-weeks. Microvascular preparation from Cre-proNGF mice showed significant imbalance of proNGF/NGF ratio, enhanced expression of TNF-α and p75^NTR, and tendency to impair TrkA phosphorylation compared to controls. EC-proNGF overexpression triggered mRNA expression of p75^NTR and inflammatory mediators in both retina and renal cortex compared to controls. EC-proNGF expression induced vascular permeability including breakdown of BRB and albuminuria in the kidney without affecting VEGF level at 4-weeks. Histopathological changes were assessed after 8-weeks and the results showed that EC-proNGF triggered formation of occluded (acellular) capillaries, hall mark of retinal ischemia. EC-proNGF resulted in glomerular enlargement and kidney fibrosis, hall mark of renal dysfunction. We have successfully created an inducible mouse model that can dissect the contribution of autocrine direct action of cleavage-resistant proNGF on systemic microvascular abnormalities in both retina and kidney, major targets for microvascular complication.

Ghrelin ameliorates nerve growth factor Dysmetabolism and inflammation in STZ-induced diabetic rats

Diabetic encephalopathy is characterized by cognitive impairment and neuroinflammation, deficient neurotrophic support, and neuronal and synaptic loss. Ghrelin, a 28 amino acid peptide, is associated with neuromodulation and cognitive improvement, which has been considered as a potential protective agent for several neurodegenerative diseases. Here we sought to investigate the role of ghrelin in preventing diabetic-related neuropathology. We found that ghrelin attenuated astrocytic activation and reduced levels of interleukin-6 and tumor necrosis factor-α in streptozotocin-induced diabetic rats. In addition, ghrelin inhibited p38 mitogen-associated protein kinase activation. The upregulation of nerve growth factor (NGF) precursor and matrix metalloproteinase (MMP)-9 and downregulation of mature NGF and MMP-7 in the diabetic brain were reversed by ghrelin. Treatment with ghrelin elevated synaptophysin expression and synaptic density in diabetic rats. Taken together, our results demonstrate that ghrelin ameliorates diabetes-related neurodegeneration by preventing NGF dysmetabolism and synaptic degeneration through regulating MMP levels as well as inhibiting neuroinflammation.

Neutralizing IL-6 reduces heart injury by decreasing nerve growth factor precursor in the heart and hypothalamus during rat cardiopulmonary bypass

AIMS

To investigate whether the expression of nerve growth factor precursor (proNGF) changes during cardiopulmonary bypass (CPB) and whether neutralizing interleukin-6 (IL-6) during CPB has cardiac benefits.

MAIN METHODS

Thirty patients undergoing CPB were recruited and their serum proNGF and troponin-I (TNI) were detected. In addition, rats were divided into three groups: CPB group, CPB with cardiac ischemia-reperfusion (IR) group, and a control group. The pre-CPB standard deviation of N-N intervals (SDNN) and post-CPB SDNN were compared. At the end of CPB, nerve peptide Y (NPY), acetylcholinesterase, cell apoptosis, and proNGF protein expression were measured in the heart and hypothalamus. Another rat cohort undergoing CPB was divided into two groups: an anti-IL-6 group with IL-6 antibody and a control group with phosphate buffer solution. At the end of CPB, serum hs-troponin-T and cardiac caspases 3 and 9 were detected. NPY and proNGF in the heart and hypothalamus were detected.

KEY FINDINGS

In patients, serum proNGF increased during CPB, and the concentration was positively correlated with TNI. In rats, cardiac autonomic nervous function was disturbed during CPB. More apoptotic cells and higher levels of proNGF were found in the heart and hypothalamus in the CPB groups than in the control groups. Neutralizing IL-6 was beneficial to lower cardiac injury by decreasing proNGF and apoptosis.

SIGNIFICANCE

CPB induced changes in proNGF in the heart and hypothalamus. Suppressing inflammation attenuated myocardial apoptosis and autonomic nerve function disturbance in CPB rats, likely due in part to regulation of proNGF in the heart and hypothalamus.

Expression and signaling of NGF in the healthy and injured retina

This review summarizes the present knowledge concerning the retinal localization of the nerve growth factor (NGF), its precursor proNGF, and the receptors TrkA and p75NTR in the developing and mature rodent retina. We further discuss the changes in the expression of NGF and the receptors in experimental models of retinal disorders and diseases like inherited retinitis pigmentosa, retinal detachment, glaucoma, and diabetic retinopathy. Since proNGF is now recognized as a bioactive signaling molecule which induces cell death through p75NTR activation, the role of proNGF in the induction of retinal cell loss under neurodegenerative conditions is also highlighted. In addition, we present the evidences for a potential therapeutic intervention with NGF for the treatment of retinal neurodegenerative diseases. Different strategies have been developed and experimentally tested in mice and rats in order to reduce cell loss and Müller cell gliosis, e.g., increasing the availability of endogenous NGF, administration of exogenous NGF, activation of TrkA, and inhibition of p75NTR. Here, we discuss the several lines of evidence supporting a protective effect of NGF on retinal cell loss, with specific emphasis on photoreceptor and retinal ganglion cell degeneration. A better understanding of the mechanisms underlying the effects of NGF and proNGF in the modulation of neurodegeneration and gliosis in the retina will help to develop efficient therapeutic strategies for various retinal diseases.

Minocycline targets multiple secondary injury mechanisms in traumatic spinal cord injury

Minocycline hydrochloride (MH), a semi-synthetic tetracycline derivative, is a clinically available antibiotic and anti-inflammatory drug that also exhibits potent neuroprotective activities. It has been shown to target multiple secondary injury mechanisms in spinal cord injury, via its anti-inflammatory, anti-oxidant, and anti-apoptotic properties. The secondary injury mechanisms that MH can potentially target include inflammation, free radicals and oxidative stress, glutamate excitotoxicity, calcium influx, mitochondrial dysfunction, ischemia, hemorrhage, and edema. This review discusses the potential mechanisms of the multifaceted actions of MH. Its anti-inflammatory and neuroprotective effects are partially achieved through conserved mechanisms such as modulation of p38 mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K)/Akt signaling pathways as well as inhibition of matrix metalloproteinases (MMPs). Additionally, MH can directly inhibit calcium influx through the N-methyl-D-aspartate (NMDA) receptors, mitochondrial calcium uptake, poly(ADP-ribose) polymerase-1 (PARP-1) enzymatic activity, and iron toxicity. It can also directly scavenge free radicals. Because it can target many secondary injury mechanisms, MH treatment holds great promise for reducing tissue damage and promoting functional recovery following spinal cord injury.

Neurotrophic factors and their inhibitors in chronic pain treatment

Chronic pain affects more than 20% of the UK population. Neurotrophic factors have been identified as therapeutic targets to improve current treatments of chronic pain. This review article focuses on nerve growth factor (NGF) and interleukin-6 (IL-6) as potential therapeutic targets. In this review we highlight the mechanisms of action and the current progress of targeted therapies in clinical trials.

Differential regulation of macrophage phenotype by mature and pro-nerve growth factor

To characterize the role of neurotrophin receptors on macrophages, we investigated the ability of nerve growth factor (NGF) and its precursor, proNGF, to regulate human macrophage phenotype. The p75 neurotrophin receptor (p75(NTR)) and TrkA were concentrated within overlapping domains on membrane ruffles. NGF stimulation of macrophages increased membrane ruffling, calcium spiking, phagocytosis and growth factor secretion. In contrast, proNGF induced podosome formation, increased migration, suppressed calcium spikes and increased neurotoxin secretion. These results demonstrate opposing roles of NGF and proNGF in macrophage regulation providing new avenues for pharmacological intervention during neuroinflammation.

Silencing p75(NTR) prevents proNGF-induced endothelial cell death and development of acellular capillaries in rat retina

Accumulation of the nerve growth factor precursor (proNGF) and its receptor p75(NTR) have been associated with several neurodegenerative diseases in both brain and retina. However, whether proNGF contributes to microvascular degeneration remain unexplored. This study seeks to investigate the mechanism by which proNGF/p75(NTR) induce endothelial cell (EC) death and development of acellular capillaries, a surrogate marker of retinal ischemia. Stable overexpression of the cleavage-resistant proNGF and molecular silencing of p75(NTR) were utilized in human retinal EC and rat retinas in vivo. Stable overexpression of proNGF decreased NGF levels and induced retinal vascular cell death evident by 1.9-fold increase in acellular capillaries and activation of JNK and cleaved-PARP that were mitigated by p75(NTR)shRNA. In vitro, overexpression of proNGF did not alter TNF-α level, reduced NGF, however induced EC apoptosis evident by activation of JNK and p38 MAPK, cleaved-PARP. Silencing p75(NTR) using siRNA restored expression of NGF and TrkA activation and prevented EC apoptosis. Treatment of EC with human-mutant proNGF induced apoptosis that coincided with marked protein interaction and nuclear translocation of p75(NTR) and the neurotrophin receptor interacting factor. These effects were abolished by a selective p75(NTR) antagonist. Therefore, targeting p75(NTR) represents a potential therapeutic strategy for diseases associated with aberrant expression of proNGF.

Effects of sustained proNGF blockade on attentional capacities in aged rats with compromised cholinergic system

Disruption in nerve growth factor (NGF) signaling via tropomyosin-related kinase A (trkA) receptors compromises the integrity of the basal forebrain (BF) cholinergic system, yielding cognitive, specifically attentional, impairments in Alzheimer's disease (AD). Although normal aging is considered a risk factor for AD, the mechanisms underlying the selective vulnerability of the aging cholinergic system to trkA disruption is not clear. The levels of proNGF, a proneurotrophin that possesses higher affinity for p75 receptors, increase in aging. The present study was designed to test the hypothesis that cholinergic and attentional dysfunction in aged rats with reduced BF trkA receptors occurs due to the overactivation of endogenous proNGF signaling. We employed a viral vector that produced trkA shRNA to suppress trkA receptors in the corticopetal cholinergic neurons of aged rats. BF trkA suppression impaired animals' performance on signal trials in both the sustained attention task (SAT) and the cognitively taxing distractor version of SAT (dSAT) and these deficits were normalized by chronic intracerebroventricular administration of proNGF antibody. Moreover, depolarization-evoked acetylcholine (ACh) release and the density of cortical cholinergic fibers were partially restored in these animals. However, SAT/dSAT scores reflecting overall performance did not improve following proNGF blockade in trkA knockdown rats due to impaired performance in non-signal trials. Sustained proNGF blockade alone did not alter baseline attentional performance but produced moderate impairments during challenging conditions. Collectively, our findings indicate that barring proNGF-p75 signaling may exert some beneficial effects on attentional capacities specifically when BF trkA signaling is abrogated. However, endogenous proNGF may also possess neurotrophic effects and blockade of this proneurotrophin may not completely ameliorate attentional impairments in AD and potentially hinder performance during periods of high cognitive load in normal aging.

Modulation of p75(NTR) prevents diabetes- and proNGF-induced retinal inflammation and blood-retina barrier breakdown in mice and rats

AIMS/HYPOTHESIS

Diabetic retinopathy is characterised by early blood-retina barrier (BRB) breakdown and neurodegeneration. Diabetes causes imbalance of nerve growth factor (NGF), leading to accumulation of the NGF precursor (proNGF), as well as the NGF receptor, p75 neurotrophin receptor (p75(NTR)), suggesting a possible pathological role of the proNGF-p75(NTR) axis in the diabetic retina. To date, the role of this axis in diabetes-induced retinal inflammation and BRB breakdown has not been explored. We hypothesised that modulating p75(NTR) would prevent diabetes- and proNGF-induced retinal inflammation and BRB breakdown.

METHODS

Diabetes was induced by streptozotocin in wild-type and p75(NTR) knockout (p75KO) mice. After 5 weeks, the expression of inflammatory mediators, ganglion cell loss and BRB breakdown were determined. Cleavage-resistant proNGF was overexpressed in rodent retinas with and without p75(NTR) short hairpin RNA or with pharmacological inhibitors. In vitro, the effects of proNGF were investigated in retinal Müller glial cell line (rMC-1) and primary Müller cells.

RESULTS

Deletion of p75(NTR) blunted the diabetes-induced decrease in retinal NGF expression and increases in proNGF, nuclear factor κB (NFκB), p-NFκB and TNF-α. Deletion of p75(NTR) also abrogated diabetes-induced glial fibrillary acidic protein expression, ganglion cell loss and vascular permeability. Inhibited expression or cleavage of p75(NTR) blunted proNGF-induced retinal inflammation and vascular permeability. In vitro, proNGF induced p75(NTR)-dependent production of inflammatory mediators in primary wild-type Müller and rMC-1 cultures, but not in p75KO Müller cells.

CONCLUSIONS/INTERPRETATION

The proNGF-p75(NTR) axis contributes to retinal inflammation and vascular dysfunction in the rodent diabetic retina. These findings underscore the importance of p75(NTR) as a novel regulator of inflammation and potential therapeutic target in diabetic retinopathy.

proNGF inhibits proliferation and oligodendrogenesis of postnatal hippocampal neural stem/progenitor cells through p75NTR in vitro

Neural stem/progenitor cells (NSCs) proliferate and differentiate under tight regulation by various factors in the stem cell niche. Recent studies have shown that the precursor of nerve growth factor (NGF), proNGF, abounds in the central nervous system (CNS) and that its expression level in the brain is substantially elevated with aging as well as in several types of CNS disorders. In this study, we found for the first time that proNGF inhibited the proliferation of NSCs isolated from postnatal mouse hippocampus and caused cell cycle arrest in the G0/G1 phase without affecting apoptosis. In addition, proNGF reduced the differentiation of NSCs to oligodendrocytes. The effects of proNGF were blocked by the fusion protein of p75 neurotrophin receptor extracellular domain and human IgG Fc fragment (p75NTR/Fc), and by p75NTR knockout, suggesting that proNGF/p75NTR interaction was involved in the effects of proNGF on NSC proliferation and differentiation. proNGF decreased the phosphorylation level of extracellular signal responsive kinase 1/2 (ERK 1/2) in a p75NTR-dependent manner under both self-renewal and differentiation conditions. The inhibition of ERK 1/2 phosphorylation by U0126 significantly reduced the proliferation and oligodendrogenesis of NSCs, indicating that ERK 1/2 inhibition by proNGF partially explains its effects on NSC proliferation and oligodendrogenesis. These results suggest that the proNGF/p75NTR signal plays a key role in the regulation of NSCs' behavior.

Presence of proNGF-sortilin signaling complex in nigral dopamine neurons and its variation in relation to aging, lactacystin and 6-OHDA insults

Growing evidence has shown that proNGF-p75NTR-sortilin signaling might be a crucial factor in neurodegeneration, but it remains unclear if it may function in nigral neurons under aging and disease. The purpose of this study is to examine and quantify proNGF and sortilin expression in the substantia nigra and dynamic changes of aging in lactacystin and 6-hydroxydopamine (6-OHDA) rat models of Parkinson’s disease using immunofluorescence, electronic microscopy, western blot and FLIVO staining methods. The expression of proNGF and sortilin was abundantly and selectively identified in tyrosine hydroxylase (TH)-containing dopamine neurons in the substantia nigra. These proNGF/TH, sortilin/TH-positive neurons were densely distributed in the ventral tier, while they were less distributed in the dorsal tier, where calbindin-D28K-containing neurons were numerously located. A correlated decrease of proNGF, sortilin and TH was also detected during animal aging process. While increase of proNGF, sortilin and cleaved (active) caspase-3 expression was found in the lactacystin model, dynamic proNGF and sortilin changes along with dopamine neuronal loss were demonstrated in the substantia nigra of both the lactacystin and 6-OHDA models. This study has thus revealed the presence of the proNGF-sortilin signaling complex in nigral dopamine neurons and its response to aging, lactacystin and 6-OHDA insults, suggesting that it might contribute to neuronal apoptosis or neurodegeneration during pathogenesis and disease progression of Parkinson’s disease; the underlying mechanism and key signaling pathways involved warrant further investigation.

proNGF inhibits neurogenesis and induces glial activation in adult mouse dentate gyrus

Recent studies have shown that the precursor of nerve growth factor (proNGF) is highly elevated in aging brains and in the brains of patients with Alzheimer's Disease. proNGF accumulates in hippocampus which is an important neurogenic region related to learning and memory. However, it remains unclear whether proNGF has an influence on hippocampal neurogenesis. In this study, we demonstrated that the high-affinity receptor of proNGF, p75 neurotrophic factor (p75NTR), was expressed both on cells undergoing mitosis and postmitotic mature cells in mouse hippocampus. proNGF infusion into adult mouse hippocampus significantly reduced the density of BrdU-incorporating cells and the density of BrdU/Doublecortin double positive cells in the subgranular zone of hippocampus, indicating an inhibitory effect of proNGF on hippocampal neurogenesis. proNGF infusion also induced prominent cell apoptosis and activated residential astrocyte and microglia, which might further impair the hippocampal neurogenesis. These results implied that proNGF played a pivotal role in regulating the hippocampal neurogenesis and might account for the memory deficit and cognitive impairment.

Of proneurotrophins and their antineurotrophic effects

Neurotrophins perform essential processes throughout neural development. They signal through Trk receptor proteins, typically in association with a "low affinity" p75(NTR) pan-neurotrophin co-receptor. Neurotrophins are synthesized as proproteins; the pro domains are removed proteolytically to yield the mature, presumably functional forms of the neurotrophins. Recent findings, however, have revealed a positive role for the proneurotrophins themselves. The proproteins bind with high affinity to the p75(NTR) pan-neurotrophin receptor in the absence of Trks to initiate a separate set of signaling cascades that actively oppose the effects of the mature growth factors. These experiments suggest that the balance between pro- and mature neurotrophin plays a critical role in tuning downstream signaling. This view changes the neurotrophin field substantially and also points to the broader idea that the potential activities of precursor proteins deserve a closer look.

Electroporation-mediated gene delivery of cleavage-resistant pro-nerve growth factor causes retinal neuro- and vascular degeneration

PURPOSE

Neurotrophins, including nerve growth factor (NGF), are secreted by glia as a pro-form (proNGF) that is normally cleaved into the mature ligand. Increases of proNGF has been well documented in retinal neurodegenerative diseases. Since systemic overexpression of proNGF exhibits embryonic lethality, we aimed to establish a model that specifically and stably overexpresses a cleavage-resistant mutant of proNGF (proNGF123) plasmid in the retina using electroporation.

METHODS

Male Sprague-Dawley rats were injected intravitreally with pGFP or pGFP-proNGF123 plasmids, then electroporated with various settings for optimization. Retinal cell death and ganglion cell count were assessed by TUNEL and immunostaining with anti-Brn3. Expression of proNGF, NGF, and their receptors was examined by western blot. Retinal vascular permeability was assessed by extravasation of bovine serum albumin-fluorescein. Development of acellular capillaries was assessed by periodic acid-Schiff and hematoxylin staining.

RESULTS

Successful pGFP-proNGF123 gene delivery and expression of proNGF was demonstrated by western blot and extensive proNGF immunostaining in retina sections. Overexpression of proNGF reduced NGF expression while inducing the expression of neurotrophin receptors, including p75(NTR) and tyrosine receptor kinase A, but not sortilin. Overexpression of proNGF resulted in ~50% reduction in ganglion cell count and fivefold increase in TUNEL-positive cells in rat retina. In addition, overexpression of proNGF induced breakdown of the blood-retina barrier evident by twofold increase in extravasation of bovine serum albumin-fluorescein after 1 week and induced the development of acellular capillaries after 4 weeks.

CONCLUSIONS

Electroporation can successfully incorporate and express biologically active cleavage-resistant proNGF locally in rat retinas. Overexpression of cleavage-resistant proNGF can be a useful tool to investigate specific molecular mechanisms by which proNGF causes neurodegeneration and vascular injury in the retina.

ProNGF, a cytokine induced after myocardial infarction in humans, targets pericytes to promote microvascular damage and activation

proNGF and p75NTR are induced following fatal myocardial infraction and are required for the development of microvascular injury.

Treatment of acute cardiac ischemia focuses on reestablishment of blood flow in coronary arteries. However, impaired microvascular perfusion damages peri-infarct tissue, despite arterial patency. Identification of cytokines that induce microvascular dysfunction would provide new targets to limit microvascular damage. Pro–nerve growth factor (NGF), the precursor of NGF, is a well characterized cytokine in the brain induced by injury. ProNGF activates p75 neurotrophin receptor (p75^NTR) and sortilin receptors to mediate proapoptotic responses. We describe induction of proNGF by cardiomyocytes, and p75^NTR in human arterioles after fatal myocardial infarction, but not with unrelated pathologies. After mouse cardiac ischemia-reperfusion (I-R) injury, rapid up-regulation of proNGF by cardiomyocytes and p75^NTR by microvascular pericytes is observed. To identify proNGF actions, we generated a mouse expressing a mutant Ngf allele with impaired processing of proNGF to mature NGF. The proNGF-expressing mouse exhibits cardiac microvascular endothelial activation, a decrease in pericyte process length, and increased vascular permeability, leading to lethal cardiomyopathy in adulthood. Deletion of p75^NTR in proNGF-expressing mice rescues the phenotype, confirming the importance of p75^NTR-expressing pericytes in the development of microvascular injury. Furthermore, deficiency in p75^NTR limits infarct size after I-R. These studies identify novel, nonneuronal actions for proNGF and suggest that proNGF represents a new target to limit microvascular dysfunction.

Diabetes-induced peroxynitrite impairs the balance of pro-nerve growth factor and nerve growth factor, and causes neurovascular injury

AIMS/HYPOTHESIS

Diabetic retinopathy, the leading cause of blindness in working-age Americans, is characterised by reduced neurotrophic support and increased proinflammatory cytokines, resulting in neurotoxicity and vascular permeability. We sought to elucidate how oxidative stress impairs homeostasis of nerve growth factor (NGF) and its precursor, proform of NGF (proNGF), to cause neurovascular dysfunction in the eye of diabetic patients.

METHODS

Levels of NGF and proNGF were examined in samples from human patients, from retinal Müller glial cell line culture cells and from streptozotocin-induced diabetic animals treated with and without atorvastatin (10 mg/kg daily, per os) or 5,10,15,20-tetrakis (4-sulfonatophenyl)porphyrinato iron (III) chloride (FeTPPs) (15 mg/kg daily, i.p.) for 4 weeks. Neuronal death and vascular permeability were assessed by TUNEL and extravasation of BSA-fluorescein.

RESULTS

Diabetes-induced peroxynitrite formation impaired production and activity of matrix metalloproteinase-7 (MMP-7), which cleaves proNGF extracellularly, leading to accumulation of proNGF and reducing NGF in samples from diabetic retinopathy patients and experimental models. Treatment of diabetic animals with atorvastatin exerted similar protective effects that blocked peroxynitrite using FeTPPs, restoring activity of MMP-7 and hence the balance between proNGF and NGF. These effects were associated with preservation of blood-retinal barrier integrity, preventing neuronal cell death and blocking activation of RhoA and p38 mitogen-activated protein kinase (p38MAPK) in experimental and human samples.

CONCLUSIONS/INTERPRETATION

Oxidative stress plays an unrecognised role in causing accumulation of proNGF, which can activate a common pathway, RhoA/p38MAPK, to mediate neurovascular injury. Oral statin therapy shows promise for treatment of diabetic retinopathy.

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.

Alteration of growth factors and neuronal death in diabetic retinopathy: what we have learned so far

PURPOSE

Diabetic retinopathy (DR) is a leading cause of blindness in American adults. Over the years, DR has been perceived as a vascular disease characterized by vascular permeability, macular edema, and neovascularization that can lead to blindness. Relatively new research on neurodegeneration is expanding our views of the pathogenesis of DR. Evidence has begun to point to the fact that even before vascular complications begin to manifest, neuronal cell death and dysfunction have already begun. Based on the literature and our own studies, we address whether neuronal death is associated with loss of neurotrophic support due to less production of a given growth factor or due to impairment of its signaling events regardless of the level of the growth factor itself.

METHODS

In this article we aimed to review the literature that looks at the neuronal side of DR and whether retinal neurons are adversely affected due to the lack of neurotrophic levels or activity. In particular, we examine the research looking at insulin, insulin-like growth factor, vascular endothelial growth factor, pigment epithelium-derived growth factor, brain-derived neurotrophic factor, and nerve growth factor.

RESULTS

Research shows that insulin has neurotrophic properties and that the loss of its pro-survival pathways may have a role in diabetic retinopathy. There is also evidence to suggest that exogenously administered insulin may have a role in the treatment of DR. Insulin-like growth factor has been shown to have a role in retinal neurogenesis and there is early evidence that it may also have neuroprotective effects. While there is evidence of neuroprotective effects of vascular endothelial growth factor, paradoxically, there is also an increased amount of apoptotic activity in retinal neurons despite an increased level of VEGF in the diabetic eye. Further research is necessary to elucidate the exact mechanisms involved. Pigment epithelium derived growth factor has retinal neuroprotective effects and shows evidence that it may be an avenue for future therapeutic use in DR. Brain-derived growth factor has been shown to have neuroprotective effects in the retina and there is also some evidence in diabetic rats that it may have some therapeutic potential in treating DR. Nerve growth factor has also been shown to have neuroprotective effects and research has begun to elucidate some of the pathways and mechanisms through which these effects occur.

CONCLUSIONS

Research has shown that there is some degree of neuronal death involved in DR. It is also evident that there are many growth factors involved in this process. Some of these growth factors have shown some potential as future therapeutic targets in DR. These findings should encourage further investigation into the mechanism of these growth factors, their potential for therapy, and the possibility of a new horizon in the clinical care of DR.

Age-dependent alterations in nerve growth factor (NGF)-related proteins, sortilin, and learning and memory in rats

The objective of this study was to evaluate the effects of aging on the performance of specific memory-related tasks in rats as well as to determine the levels of several nerve growth factor (NGF)-related proteins in relevant brain regions. The results indicated age-related impairments in spatial learning in a water maze task as well as deficits in recognition memory in a Spontaneous Novel Object Recognition task. In the prefrontal cortex and hippocampus, aged rats (compared to young controls) had elevated levels of the proneurotrophin, proNGF (+1.8-1.9 fold), p75(NTR) receptors (+1.6-1.8 fold) and sortilin (+1.8-2.1 fold), and decreased levels of mature NGF (-36 to 44%), and phospho-TrkA receptors (-45 to 49%). The results of this study support the argument that NGF signaling is altered in the aging brain, and that such alterations may contribute to an age-related decline in cognitive function. These results may also help to identify specific components of the NGF-signaling pathway that could serve as targets for novel drug discovery and development for age-related disorders of cognition (e.g., Alzheimer's disease).

Understanding proneurotrophin actions: Recent advances and challenges

Neurotrophins are initially synthesized as larger precursors (proneurotrophins), which undergo proteolytic cleavage to yield mature forms. Although the functions of the mature neurotrophins have been well established during neural development and in the adult nervous system, roles for the proneurotrophins in developmental and injury-induced cell death, as well as in synaptic plasticity, have only recently been appreciated. Interestingly, both mature neurotrophins and proneurotrophins utilize dual-receptor complexes to mediate their actions. The mature neurotrophin coreceptors consist of the Trk receptor tyrosine kinases and p75(NTR), wherein Trk transduces survival and differentiative signaling, and p75(NTR) modulates the affinity and selectivity of Trk activation. On the other hand, proneurotrophins engage p75(NTR) and the structurally distinct coreceptor sortilin, to initiate p75(NTR)-dependent signal transduction cascade. Although the specificity of mature neurotrophins vs. proneurotrophins actions is due in part to the formation of distinct coreceptor complexes, a number of recent studies highlight how different p75(NTR)-mediated cellular actions are modulated. Here, we review emerging evidence for a novel transmembrane mechanism for ligand-specific p75(NTR) activation and several mechanisms by which p75(NTR)-dependent apoptotic and nonapoptotic responses can be selective activated.