Nerve growth factor stimulates MAPK via the low affinity receptor p75(LNTR)

Alzheimer's Disease and Frontotemporal Dementia: A Review of Pathophysiology and Therapeutic Approaches

Alzheimer's disease (AD) is a devastating form of dementia, with the number of affected individuals rising sharply. The main hallmarks of the disease include amyloid-beta plaque deposits and neurofibrillary tangles consisting of hyperphosphorylated tau protein, besides other pathological features that contribute to the disease's complexity. The causes of sporadic AD are multifactorial and mostly age-related and involve risk factors such as diabetes and cardiovascular or cerebrovascular disorders. Frontotemporal dementia (FTD) is another type of dementia characterized by a spectrum of behaviors, memory, and motor abnormalities and associated with abnormal depositions of protein aggregation, including tau protein. Currently approved medications are symptomatic, and no disease-modifying therapy is available to halt the disease progression. Therefore, the development of multi-targeted therapeutic approaches could hold promise for the treatment of AD and other neurodegenerative disorders, including tauopathies. In this article, we will discuss the pathophysiology of AD and FTD, the proposed hypotheses, and current therapeutic approaches, highlighting the development of novel drug candidates and the progress of clinical trials in this field of research.

A Generalizable Optogenetic Strategy to Regulate Receptor Tyrosine Kinases during Vertebrate Embryonic Development

Ligand-independent activation of receptor tyrosine kinases (RTKs) allows for dissecting out the receptor-specific signaling outcomes from the pleiotropic effects of the ligands. In this regard, RTK intracellular domains (ICD) are of interest due to their ability to recapitulate signaling activity in a ligand-independent manner when fused to chemical or optical dimerizing domains. A common strategy for synthetic activation of RTKs involves membrane tethering of dimerizer-RTK ICD fusions. Depending on the intrinsic signaling capacity, however, this approach could entail undesirable baseline signaling activity in the absence of stimulus, thereby diminishing the system's sensitivity. Here, we observed toxicity in early Xenopus laevis embryos when using such a conventional optogenetic design for the fibroblast growth factor receptor (FGFR). To surpass this challenge, we developed a cytoplasm-to-membrane translocation approach, where FGFR ICD is recruited from the cytoplasm to the plasma membrane by light, followed by its subsequent activation via homo-association. This strategy results in the optical activation of FGFR with low background activity and high sensitivity, which allows for the light-mediated formation of ectopic tail-like structures in developing X. laevis embryos. We further generalized this strategy by developing optogenetic platforms to control three neurotrophic tropomyosin receptor kinases, TrkA, TrkB, and TrkC. We envision that these ligand-independent optogenetic RTKs will provide useful toolsets for the delineation of signaling sub-circuits in developing vertebrate embryos.

Optogenetic Delineation of Receptor Tyrosine Kinase Subcircuits in PC12 Cell Differentiation

Nerve growth factor elicits signaling outcomes by interacting with both its high-affinity receptor, TrkA, and its low-affinity receptor, p75NTR. Although these two receptors can regulate distinct cellular outcomes, they both activate the extracellular-signal-regulated kinase pathway upon nerve growth factor stimulation. To delineate TrkA subcircuits in PC12 cell differentiation, we developed an optogenetic system whereby light was used to specifically activate TrkA signaling in the absence of nerve growth factor. By using tyrosine mutants of the optogenetic TrkA in combination with pathway-specific pharmacological inhibition, we find that Y490 and Y785 each contributes to PC12 cell differentiation through the extracellular-signal-regulated kinase pathway in an additive manner. Optogenetic activation of TrkA eliminates the confounding effect of p75NTR and other potential off-target effects of the ligand. This approach can be generalized for the mechanistic study of other receptor-mediated signaling pathways.

Genetic evidence for p75NTR-dependent tetraploidy in cortical projection neurons from adult mice

A subpopulation of chick retinal projection neurons becomes tetraploid during development, an event prevented by blocking antibodies against p75 neurotrophin receptor (p75(NTR)). We have used an optimized flow cytometric assay, based on the analysis of unfixed brain cell nuclei, to study whether p75(NTR)-dependent neuronal tetraploidization takes place in the cerebral cortex, giving rise to projection neurons as well. We show that 3% of neurons in both murine neocortex and chick telencephalic derivatives are tetraploid, and that in the mouse ~85% of these neurons express the immediate early genes Erg-1 and c-Fos, indicating that they are functionally active. Tetraploid cortical neurons (65-80%) express CTIP2, a transcription factor specific for subcortical projection neurons in the mouse neocortex. During the period in which these neurons are born, p75(NTR) is detected in differentiating neurons undergoing DNA replication. Accordingly, p75(NTR)-deficient mice contain a reduced proportion of both NeuN and CTIP2-positive neocortical tetraploid neurons, thus providing genetic evidence for the participation of p75(NTR) in the induction of neuronal tetraploidy in the mouse neocortex. In the striatum tetraploidy is mainly associated with long-range projection neurons as well since ~80% of tetraploid neurons in this structure express calbindin, a marker of neostriatal-matrix spiny neurons, known to establish long-range projections to the substantia nigra and globus pallidus. In contrast, only 20% of tetraploid cortical neurons express calbindin, which is mainly expressed in layers II-III, where CTIP2 is absent. We conclude that tetraploidy mainly affects long-range projection neurons, being facilitated by p75(NTR) in the neocortex.

Nerve growth factor stimulation of ERK1/2 phosphorylation requires both p75NTR and α9β1 integrin and confers myoprotection towards ischemia in C2C12 skeletal muscle cell model

The functions of nerve growth factor (NGF) in skeletal muscles physiology and pathology are not clear and call for an updated investigation. To achieve this goal we sought to investigate NGF-induced ERK1/2 phosphorylation and its role in the C2C12 skeletal muscle myoblasts and myotubes. RT-PCR and western blotting experiments demonstrated expression of p75(NTR), α9β1 integrin, and its regulator ADAM12, but not trkA in the cells, as also found in gastrocnemius and quadriceps mice muscles. Both proNGF and βNGF induced ERK1/2 phosphorylation, a process blocked by (a) the specific MEK inhibitor, PD98059; (b) VLO5, a MLD-disintegrin with relative selectivity towards α9β1 integrin; and (c) p75(NTR) antagonists Thx-B and LM-24, but not the inactive control molecule backbone Thx. Upon treatment for 4 days with either anti-NGF antibody or VLO5 or Thx-B, the proliferation of myoblasts was decreased by 60-70%, 85-90% and 60-80% respectively, indicative of trophic effect of NGF which was autocrinically released by the cells. Exposure of myotubes to ischemic insult in the presence of βNGF, added either 1h before oxygen-glucose-deprivation or concomitant with reoxygenation insults, resulted with about 20% and 33% myoprotection, an effect antagonized by VLO5 and Thx-B, further supporting the trophic role of NGF in C2C12 cells. Cumulatively, the present findings propose that proNGF and βNGF-induced ERK1/2 phosphorylation in C2C12 cells by functional cooperation between p75(NTR) and α9β1 integrin, which are involved in myoprotective effects of autocrine released NGF. Furthermore, the present study establishes an important trophic role of α9β1 in NGF-induced signaling in skeletal muscle model, resembling the role of trkA in neurons. Future molecular characterization of the interactions between NGF receptors in the skeletal muscle will contribute to the understanding of NGF mechanism of action and may provide novel therapeutic targets.

Nerve growth factor-induced cell cycle reentry in newborn neurons is triggered by p38MAPK-dependent E2F4 phosphorylation

Cumulative evidence indicates that activation of cyclin D-dependent kinase 4/6 (cdk4/6) represents a major trigger of cell cycle reentry and apoptosis in vertebrate neurons. We show here the existence of another mechanism triggering cell cycle reentry in differentiating chick retinal neurons (DCRNs), based on phosphorylation of E2F4 by p38(MAPK). We demonstrate that the activation of p75(NTR) by nerve growth factor (NGF) induces nuclear p38(MAPK) kinase activity, which leads to Thr phosphorylation and subsequent recruitment of E2F4 to the E2F-responsive cdc2 promoter. Inhibition of p38(MAPK), but not of cdk4/6, specifically prevents NGF-dependent cell cycle reentry and apoptosis in DCRNs. Moreover, a constitutively active form of chick E2F4 (Thr261Glu/Thr263Glu) stimulates G(1)/S transition and apoptosis, even after inhibition of p38(MAPK) activity. In contrast, a dominant-negative E2F4 form (Thr261Ala/Thr263Ala) prevents NGF-induced cell cycle reactivation and cell death in DCRNs. These results indicate that NGF-induced cell cycle reentry in neurons depends on the activation of a novel, cdk4/6-independent pathway that may participate in neurodegeneration.

Polyubiquitination of the neurotrophin receptor p75 directs neuronal cell survival

Specific binding of nerve growth factor (NGF) to p75 neurotrophin receptor (p75(NTR)) leads to p75(NTR) polyubiquitination and its subsequent interaction with TRAF6 resulting in neuronal cell survival. However, when the binding of NGF to p75(NTR) was blocked with p75 antiserum, p75(NTR) polyubiquitination and neuronal cell survival were impaired. Results showed that tyrosine phosphorylation of p75(NTR) increased the polyubiquitination of p75(NTR) and contributed to the observed apparent neuroprotective effects. Similar to p75(NTR) polyubiquitination, interaction of TRAF6 with p75(NTR) was NGF/tyrosine phosphorylation dependent suggesting that TRAF6 might function as an E3 ubiquitin ligase. In sum, the results show that specific binding of NGF to p75(NTR) mediates neuronal cell survival.

Cholinergic system during the progression of Alzheimer's disease: therapeutic implications

Alzheimer's disease (AD) is characterized by a progressive phenotypic downregulation of markers within cholinergic basal forebrain (CBF) neurons, frank CBF cell loss and reduced cortical choline acetyltransferase activity associated with cognitive decline. Delaying CBF neurodegeneration or minimizing its consequences is the mechanism of action for most currently available drug treatments for cognitive dysfunction in AD. Growing evidence suggests that imbalances in the expression of NGF, its precursor proNGF and the high (TrkA) and low (p75(NTR)) affinity NGF receptors are crucial factors underlying CBF dysfunction in AD. Drugs that maintain a homeostatic balance between TrkA and p75(NTR) may slow the onset of AD. A NGF gene therapy trial reduced cognitive decline and stimulated cholinergic fiber growth in humans with mild AD. Drugs treating the multiple pathologies and clinical symptoms in AD (e.g., M1 cholinoceptor and/or galaninergic drugs) should be considered for a more comprehensive treatment approach for cholinergic dysfunction.

Retinoic acid and nerve growth factor induce differential regulation of nicotinic acetylcholine receptor subunit expression in SN56 cells

Retinoic acid (RA) and nerve growth factor (NGF) have multiple functions in the regulation of neuronal development. In the present study, we characterized the expression of different nicotinic acetylcholine receptor (nAChR) subtypes in the cholinergic SN56 cell line and investigated the roles of RA and NGF in the expression of choline acetyltransferase (ChAT) and different nAChR subtypes. The nAChR agonist [(3)H]epibatidine was bound to two sites, with apparent affinities of 13 and 380 pM. RT-PCR analysis revealed expression of alpha3, alpha4, alpha5, alpha7, beta2, and beta4 nAChR subunits. RA treatment induced morphological changes, and the mRNA level of ChAT was maximally elevated after 4 days of exposure. The density of [(3)H]epibatidine binding sites and the mRNA and protein level of the alpha3 and beta2 nAChR subunits were also increased by RA-induced differentiation. RA down-regulated the mRNA and protein level of the alpha4 nAChR subunit, whereas no significant change was observed in the mRNA and protein level of the alpha7 nAChR subunit. NGF treatment increased the mRNA and protein level of the alpha3 and beta2 nAChR subunits. No morphological effects of NGF were observed, and the mRNA level of ChAT and mRNA and protein level of the alpha4 and alpha7 nAChR subunits were not significantly altered. Validation was performed with real-time RT-PCR. The present results show that RA and NGF have different effects on the expression of ChAT and the morphology and the expression pattern of different nAChR subunits in cholinergic SN56 cells.

Cell type specific signalling by hematopoietic growth factors in neural cells

Correct timing and spatial location of growth factor expression is critical for undisturbed brain development and functioning. In terminally differentiated cells distinct biological responses to growth factors may depend on cell type specific activation of signalling cascades. We show that the hematopoietic growth factors thrombopoietin (TPO) and granulocyte colony-stimulating factor (GCSF) exert cell type specific effects on survival, proliferation and the degree of phosphorylation of Akt1, ERK1/2 and STAT3 in rat hippocampal neurons and cortical astrocytes. In neurons, TPO induced cell death and selectively activated ERK1/2. GCSF protected neurons from TPO- and hypoxia-induced cell death via selective activation of Akt1. In astrocytes, neither TPO nor GCSF had any effect on cell viability but inhibited proliferation. This effect was accompanied by activation of ERK1/2 and inhibition of STAT3 activity. A balance between growth factors, their receptors and signalling proteins may play an important role in regulation of neural cell survival.

Ethanol inhibits brain-derived neurotrophic factor stimulation of extracellular signal-regulated/mitogen-activated protein kinase in cerebellar granule cells

Brain-derived neurotrophic factor (BDNF) has emerged as a prominent mediator of neuronal development and synaptic plasticity. BDNF activates multiple signal transduction cascades that regulate cellular function through phosphorylation, transcription, and translation. Ethanol is known to inhibit neurotrophin signaling, but a thorough pharmacological analysis of the effect of ethanol on BDNF signaling in developing neurons has not been performed. These experiments were undertaken to determine the interactions between membrane depolarization, BDNF concentration, and ethanol concentration on extracellular signal-regulated protein kinase (ERK) activation in neurons. We examined cerebellar granule cells grown under physiological (5mM) or elevated (25mM) potassium culture conditions after 3 days in vitro. BDNF-stimulated ERK phosphorylation (pERK) within 10min and supported stimulation from 20 to 60min. Ethanol decreased basal pERK and reduced the magnitude of BDNF stimulation of ERK under both conditions. The NMDA receptor antagonist 2-amino-5-phosphonovalerate did not effect basal pERK or inhibit BDNF stimulation of ERK, suggesting that NMDA receptors do not modulate BDNF stimulation of ERK in short-term cultures. These data characterize the pharmacological effects of ethanol on growth factor signaling and provide the basis of a model for further characterization of the biochemical mechanisms of ERK inhibition by ethanol. Perturbation of BDNF signal transduction by ethanol may underlie some of the cognitive deficits and developmental abnormalities resulting from ethanol exposure.

Sensory neurons from Nf1 haploinsufficient mice exhibit increased excitability

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by tumor formation. People with NF1 also can experience more intense painful responses to stimuli, such as minor trauma, than normal. NF1 results from a heterozygous mutation of the NF1 gene, leading to decreased levels of neurofibromin, the protein product of the NF1 gene. Neurofibromin is a guanosine triphosphatase activating protein (GAP) for Ras and accelerates the conversion of active Ras-GTP to inactive Ras-GDP; therefore mutation of the NF1 gene frequently results in an increase in activity of the Ras transduction cascade. Using patch-clamp electrophysiological techniques, we examined the excitability of capsaicin-sensitive sensory neurons isolated from the dorsal root ganglia of adult mice with a heterozygous mutation of the Nf1 gene (Nf1+/-), analogous to the human mutation, in comparison to wildtype sensory neurons. Sensory neurons from adult Nf1+/- mice generated a more than twofold higher number of action potentials in response to a ramp of depolarizing current as wild-type neurons. Consistent with the greater number of action potentials, Nf1+/- neurons had lower firing thresholds, lower rheobase currents, and shorter firing latencies than wild-type neurons. Interestingly, nerve growth factor augmented the excitability of wild-type neurons in a concentration-related manner but did not further alter the excitability of the Nf1+/- sensory neurons. These data clearly suggest that GAPs, such as neurofibromin, can play a key role in the excitability of nociceptive sensory neurons. This increased excitability may explain the painful conditions experienced by people with NF1.

Low concentrations of aggregated β-amyloid induce neurite formation via the neurotrophin receptor p75

Aggregated beta-amyloid (Abeta) binds to the neurotrophin receptor p75 and induces signaling. We examined this signaling process in different cell lines which express p75 either naturally (Schwannoma RN22 cells) or which are stably transfected with wild-type p75 (MDCKwt and PCNA cells) or with a truncated form of p75 comprising only extracellular and transmembrane domains (MDCKtm cells). While Abeta in higher concentrations (10-100 microM) is known to cause apoptosis via p75, our experiments focused on the effects of low concentrations of Abeta (25 nM) which may occur in early stages of Alzheimer disease. Application of Abeta caused tyrosine phosphorylation of wild-type p75 and induced the Ras-ERK pathway as has been reported for nerve growth factor (NGF). Since Ras activation and ERK phosphorylation (via MEK) could not be observed in MDCKtm cells and since they were clearly reduced in cells transfected with a p75 antisense construct, these effects should have been mediated by p75. Abeta also induced Ras and ERK activation in cerebellar neurons of 2-day-old rats which express p75 at that developmental stage but not TrkA; other Trk receptors were inhibited by K252a. In these neurons, Abeta led to quick formation, branching and elongation of processes. But while NGF distinctly promoted neurite branching and elongation, Abeta was less effective in neurite elongation and counts of small processes and of growth cones remained clearly elevated after 24-h stimulation; these peculiarities might be linked to aberrant neuronal connections reported for an animal model of Alzheimer disease. Essentially, the observed effects were mediated by interaction of Abeta and p75.

The neurotrophin receptor p75NTR modulates long-term depression and regulates the expression of AMPA receptor subunits in the hippocampus

Neurotrophins are involved in the modulation of synaptic transmission, including the induction of long-term potentiation (LTP) through the receptor TrkB. Because previous studies have revealed a bidirectional mode of neurotrophin action by virtue of signaling through either the neurotrophin receptor p75NTR or the Trk receptors, we tested the hypothesis that p75NTR is important for longterm depression (LTD) to occur. Although LTP was found to be unaffected in hippocampal slices of two different strains of mice carrying mutations of the p75NTR gene, hippocampal LTD was impaired in both p75NTR-deficient mouse strains. Furthermore, the expression levels of two (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits, GluR2 and GluR3, but not GluR1 or GluR4, were found to be significantly altered in the hippocampus of p75NTR-deficient mice. These results implicate p75NTR in activity-dependent synaptic plasticity and extend the concept of functional antagonism of the neurotrophin signaling system.

Inactivation and activation of Ras by the neurotrophin receptor p75

The neurotrophin receptor p75 induces neurotrophic and/or apoptotic signalling pathways and can also cooperate with the neurotrophic Trk receptor tyrosine kinases. Its intracellular part encloses a so-called 'death domain' with a segment similar to the wasp venom mastoparan which binds small GTPases such as Rho. To study possible interactions of p75 and Ras (and Rho) we used wild-type and mutant genes of p75 stably expressed by MDCK cells which normally have neither Trk nor p75. We found that p75 can directly bind the GTPases Ras and Rho and that the unstimulated p75 inactivates total cellular Ras through a differential influence on the dissociation of GDP and GTP from Ras and an exchange of bound Ras.GDP for free Ras.GTP. These properties of p75 could also be demonstrated in vitro and should therefore be cell type-independent. Stimulation of p75 with nerve growth factor causes Ras activation via adapter proteins known from Trk signalling and induces rapid outgrowth of cellular processes. Both inactivation and activation of Ras by p75 are controlled by the phosphorylation state of the receptor's two intracellular tyrosines. p75 also influences Rho activation and inactivation, and the combined interactions of the receptor with the two GTPases Ras and Rho can regulate neurite formation in an efficient, synergistic way.

c-Cbl binds to tyrosine-phosphorylated neurotrophin receptor p75 and induces its ubiquitination

The p75 neurotrophin receptor (p75NTR) has dual functions in cell survival and cell death but its intracellular signalling pathways are not understood. Here we describe that in rat brain and in pervanadate-stimulated PCNA and HEK293 cells p75NTR is phosphorylated at a single tyrosine residue within the cytosolic C-terminus. Phosphorylated tyrosine 308 constitutes a binding site for the ubiquitin ligase c-Cbl. This interaction is a prerequisite for ubiquitination of p75NTR. Our data suggest a c-Cbl-dependent ubiquitination of p75NTR involved in the regulation of p75NTR signalling.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

TrkA and mitogen-activated protein kinase phosphorylation are enhanced in sympathetic neurons lacking functional p75 neurotrophin receptor expression

This study examined the effects of hypomorphic p75 neurotrophin receptor (p75NTR) expression and high levels of nerve growth factor (NGF) on trkA phosphorylation and downstream activation of p44/42 mitogen-activated protein kinase (MAPK). Post-ganglionic sympathetic neurons from postnatal day 1 p75NTR exon III null mutant (p75(-/-)) and 129/SvJ mice were cultured in the presence of 50 ng/mL NGF and analysed by Western blotting. Levels of phosphorylated trkA are increased in p75(-/-) neurons compared with 129/SvJ neurons, and these higher levels are maintained with continuous exposure to NGF. MAPK is also phosphorylated to a greater extent in p75(-/-) neurons than in 129/SvJ neurons, both within 10 min of exposure to NGF, and with continuous NGF treatment for 5 days. These data provide new insight into the mechanism underlying enhanced neurite outgrowth in p75(-/-) neurons, demonstrating that trkA and MAPK signalling in sympathetic neurons are increased when p75NTR function is disrupted.

A ligand of the p65/p95 receptor suppresses perforant path kindling, kindling-induced mossy fiber sprouting, and hilar area changes in adult rats

Kindling, an animal model of epilepsy, results in an increased volume of the hilus of the dentate gyrus and sprouting of the mossy fiber pathway in the hippocampus. Our previous studies have revealed that chronic infusion of neurotrophins can regulate not only seizure development, but also these kindling-induced structural changes. Kindling, in turn, can alter the expression of neurotrophins and their receptors. We previously showed that intraventricular administration of a synthetic peptide that interferes with nerve growth factor stability and thus its binding to TrkA and p75(NTR) receptors suppressed kindling and sprouting. However, the precise involvement of TrkA, p75(NTR), and downstream signaling effectors of neurotrophins on kindling, sprouting and hilar changes are unknown. One of these downstream effectors is Ras. In the present study, we find that intraventricular infusion of the synthetic peptide Reo3Y, which binds to p65/p95 receptors and causes a rapid inactivation of Ras protein, impairs development of perforant path kindling, reduces the growth in afterdischarge duration, blocks kindling-induced mossy fiber sprouting in area CA3 of hippocampus and in inner molecular layer of the dentate gyrus, and prevents kindling-induced increases in hilar area. These results are consistent with a mediation of neurotrophin effects on kindling, hilar area, and axonal sprouting via Trk receptors, and suggest important roles for Ras in kindling and in kindling-induced structural changes.

Activation of the mitogen-activated protein kinase pathway through p75NTR: a common mechanism for the neurotrophin family

Neurotrophins interact with two distinct classes of cell-surface receptors, the Trk receptor tyrosine kinase family and the common neurotrophin receptor p75(NTR). For many years, the biological role of p75(NTR) remained obscure, being relegated to modulating Trk binding of neurotrophins. Recently, the importance of p75(NTR) as a signaling receptor in itself has become increasingly clear. The signals initiated by p75(NTR) are likely to be as complex as those for the Trk family and probably depend on the cell system in which such signaling is being studied. In this study, all members of the neurotrophin family were demonstrated to be capable of stimulating p75(NTR)-mediated activation of the mitogen-activated protein kinase (MAPK) family (ERK1,2). This activation is rapid and transient, peaking at 5-15 min, depending on the cell system. The classical MAPK cascade consists of the reaction series Ras-Raf-MEK-MAPK. The p75(NTR)-induced MAPK activation is MEK dependent but Raf independent. This result implies that neurotrophin activation of p75(NTR) results in some cascade (as yet unknown) that bypasses Raf and converges on MEK to result in activation of MAPK. This activated MAPK is then able to translocate to the nucleus. The effect of this MAPK activation on cell survival is dependent on cell type. These results support the concept that signaling from the p75(NTR) receptor is more diverse and extensive than previously believed.

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

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

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.

Extracellular ATP and nerve growth factor intensify hypoglycemia-induced cell death in primary neurons: role of P2 and NGFRp75 receptors

In this study, we monitored the direct expression of P2 receptors for extracellular ATP in cerebellar granule neurons undergoing metabolism impairment. Glucose deprivation for 30-60 min inhibited P2Y1 receptor protein, only weakly modulated P2X1, P2X2 and P2X3, and up-regulated by about two-fold P2X4, P2X7 and P2Y4. The P2X/Y antagonist basilen blue, protecting cerebellar neurons from hypoglycemic cell death, maintained within basal levels only the expression of P2X7 and P2Y4 proteins, but not P2X4 or P2Y1. Glucose starvation transiently increased (up to three-fold) the expression of NGFRp75 receptor protein and strongly stimulated the extracellular release of nerve growth factor (NGF; about 10-fold). Exogenously added NGF then augmented hypoglycemic neuronal death by about 60%, increasing the percentage of Höechst-positive nuclei (from approximately 62 to 95%), reducing lactate dehydrogenase (LDH) release (from about 50 to 14%) and significantly overstimulating the hypoglycemia-induced expression of P2X7 and P2Y4. Conversely, extracellular ATP augmented hypoglycemic neuronal death by about 80%, reducing the number of Höechst-positive nuclei (from approximately 62% to 14%), augmenting LDH outflow (by about 30%) and further increasing the hypoglycemia-induced expression of NGFRp75. Our results indicate that P2 and NGFRp75 receptors are modulated during glucose starvation and that extracellular ATP and NGF drive features of, respectively, necrotic and apoptotic hypoglycemic cell death, aggravating the consequences of metabolism impairment in cerebellar primary neurons.

The neurotrophin receptor p75(NTR): novel functions and implications for diseases of the nervous system

Neurotrophins have long been known to promote the survival and differentiation of vertebrate neurons. However, these growth factors can also induce cell death through the p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor receptor superfamily. Consistent with a function in controlling the survival and process formation of neurons, p75(NTR) is mainly expressed during early neuronal development. In the adult, p75(NTR) is re-expressed in various pathological conditions, including epilepsy, axotomy and neurodegeneration. Potentially toxic peptides, including the amyloid beta- (Abeta-) peptide that accumulates in Alzheimer's disease, are ligands for p75(NTR). Recent work also implicates p75(NTR) in the regulation of both synaptic transmission and axonal elongation. It associates with the Nogo receptor, a binding protein for axonal growth inhibitors, and appears to be the transducing subunit of this receptor complex.

Evidence for a role of the N-terminal domain in subcellular localization of the neuronal connexin36 (Cx36)

The expression and functional properties of connexin36 (Cx36) have been investigated in two neuroblastoma cell lines (Neuro2A, RT4-AC) and primary hippocampal neurons transfected with a Cx36-enhanced green fluorescent protein (EGFP) expression vector. Transfected cells express Cx36-EGFP mRNA, and Cx36-EGFP protein is localized in the perinuclear area and cell membrane. Upon differentiation of cell lines, Cx36-EGFP protein was detectable in processes with both axonal and dendritic characteristics. Small gap junction plaques were found between adjacent cells, and electrophysiological recordings demonstrated that the electrical properties of these gap junctions were virtually indistinguishable from those reported for native Cx36. Mutagenesis of Cx36 led to the identification of a structural element that interferes with normal protein localization. In contrast, site directed mutagenesis of putative protein phosphorylation motifs did not alter subcellular localization. This excludes phosphorylation/dephosphorylation as a major regulatory step in Cx36 protein transport.

Dual role for p75(NTR) signaling in survival and cell death: can intracellular mediators provide an explanation?

Several recent reports support a dual role of p75(NTR) in cell death, as well as survival, depending on the physiological or developmental stage of the cells. Coexpression of the TrkA receptor with p75(NTR) further enhances the complexity of nerve growth factor (NGF) signaling. Recent identification of serine/threonine kinases that interact with the p75(NTR) provides an explanation for the lack of an apparent kinase domain needed for signaling. In this report, we review the possible roles of the intracellular proteins that directly interact with the p75(NTR), atypical protein kinase C (PKC) binding protein, p62 and second messengers in the functional antagonism exhibited by TrkA and p75(NTR) with an emphasis on the nuclear factor-kappa B activation pathway.

Characterization of a p75(NTR) apoptotic signaling pathway using a novel cellular model

The p75 neurotrophin receptor (p75(NTR)) belongs to the tumor necrosis factor receptor/nerve growth factor receptor superfamily. In some cells derived from neuronal tissues it causes cell death through a poorly characterized pathway. We developed a neuronal system using conditionally immortalized striatal neurons, in which the expression of p75(NTR) is inducibly controlled by the ecdysone receptor. In these cells p75(NTR) induces apoptosis through its death domain in a nerve growth factor-independent manner. Caspases 9, 6, and 3 are activated by receptor expression indicating the activation of the common effector pathway of apoptosis. Cell death is blocked by a dominant negative form of caspase 9 and Bcl-X(L) consistent with a pathway that involves mitochondria. Significantly, the viral flice inhibitory protein E8 protects from p75(NTR)-induced cell death indicating that death effector domains are involved. A p75(NTR) construct with a deleted death domain dominantly interferes with p75(NTR) signaling, implying that receptor multimerization is required. However, in contrast to the other receptors of the family, p75(NTR)-mediated apoptosis does not involve the adaptor proteins Fas-associated death domain protein or tumor necrosis factor-associated death domain protein, and the apical caspase 8 is not activated. We conclude that p75(NTR) signals apoptosis by similar mechanisms as other death receptors but uses different adaptors and apical caspases.

A novel p75NTR signaling pathway promotes survival, not death, of immunopurified neocortical subplate neurons

Subplate neurons of mammalian neocortex undergo pronounced cell death postnatally, long after they have matured and become incorporated into functional cortical circuits. They express the p75 neurotrophin receptor (p75NTR), which is known to signal cell death in some types of neurons via the activation of sphingomyelinase and the concomitant increase in the sphingolipid ceramide. To evaluate the role of p75NTR in subplate neurons, they were immunopurified and cultured in vitro. Contrary to its known function as a death receptor, ligand binding to p75NTR promotes subplate neuron survival. Moreover, p75NTR-dependent survival is blocked by inhibition of ceramide synthesis and rescued by addition of its precursor sphingomyelin. Inhibition of Trk signaling does not block survival, nor is Trk signaling alone sufficient to promote survival. Thus, ligand-dependent p75NTR regulation of the ceramide pathway mediates survival in certain neurons and may represent an important target for neuroprotective drugs in degenerative diseases involving p75NTR-expressing neurons, such as Alzheimer's disease.

Zinc inhibits p75NTR-mediated apoptosis in chick neural retina

It has previously been documented that Zn2+ inhibits TrkA-mediated effects of NGF. To evaluate the ability of Zn2+ to attenuate the biological activities of NGF mediated by p75NTR, we characterized the effects of this transition metal cation on both binding and the pro-apoptotic properties of the NGF-p75NTR interaction. Binding of NGF to p75NTR displayed higher affinity in embryonic chick retinal cells than in PC12 cells. NGF induced apoptosis in dissociated cultures of chick neural retina. The addition of 100 microM Zn2+ inhibited binding and chemical cross-linking of 125I-NGF to p75NTR, and also attenuated apoptosis mediated by this ligand-receptor interaction. These studies lead to the conclusion that Zn2+ antagonizes NGF/p75NTR-mediated signaling, suggesting that the effect of this transition metal cation can be either pro- or anti-apoptotic depending on the cellular context.

Differential regulation of NGF receptors in primary sensory neurons by adjuvant-induced arthritis in the rat

In the adult brain, neurotrophins play a key role in adaptive processes linked to increased neuronal activity. A growing body of evidence suggests that chronic pain results from long-term plasticity of central pathways involved in nociception. We have investigated the involvement of nerve growth factor (NGF) in adaptive responses of primary sensory neurons during the course of a long-lasting inflammatory pain model. The amount and distribution of the NGF receptors p75(NTR) and TrkA were measured in the dorsal horn and dorsal root ganglia (DRG) of animals subjected to Freund's adjuvant-induced arthritis (AIA). We observed an increased immunoreactivity of both receptors in the central terminals of primary sensory neurons in the arthritic state. The increases were seen in the same population of afferent terminals in deep dorsal horn laminae. These changes paralleled the variations of clinical and behavioral parameters that characterize the course of the disease. They occurred in NGF-sensitive, but not GDNF-sensitive, nerve terminals. However, p75(NTR) and TrkA protein levels in the DRG (in the cell body of these neurons) showed different response patterns. An immediate rise of p75(NTR) was seen in parallel with the initial inflammation that developed after administration of Freund's adjuvant in hindpaws. In contrast, increases of the mature (gp140(trk)) form of TrkA occurred later and seemed to be linked to the development of the long-lasting inflammatory response. The changes in receptor expression were observed exclusively at lumbar levels, L3-L5, somatotopically appropriate for the inflammation. Together, these results implicate NGF in long-term mechanisms accompanying chronic inflammatory pain, via the up-regulation of its high affinity receptor, and offer additional evidence for differential processes underlying short- versus long-lasting inflammatory pain.

p75 neurotrophin receptor signaling: mechanisms for neurotrophic modulation of cell stress?

The recent recognition that the p75 neurotrophin receptor, p75((NTR)), can induce apoptotic signals has contributed to the perception that it acts primarily as a death receptor. Although the molecular mechanisms of p75(NTR) signaling remain to be fully characterized, many of the currently identified pathways activated by p75(NTR) may be generally characterized as stress response signals. This review describes recent advances in identifying the molecular components involved in p75(NTR) signal transduction and suggests that p75(NTR) signaling may more aptly serve as a general mechanism for the transduction and modulation of stress signals.