Signaling of the neurotrophin receptor p75 in relation to Alzheimer's disease

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

A comprehensive p75 neurotrophin receptor gene network and pathway analyses identifying new target genes

P75 neurotrophic receptor (p75NTR) is an important receptor for the role of neurotrophins in modulating brain plasticity and apoptosis. The current understanding of the role of p75NTR in cellular adaptation following pathological insults remains blurred, which makes p75NTR’s related signaling networks an interesting and challenging initial point of investigation. We identified p75NTR and related genes through extensive data mining of a PubMed literature search including published works related to p75NTR from the past 20 years. Bioinformatic network and pathway analyses of identified genes (n = 235) were performed using ReactomeFIViz in Cytoscape based on the highly reliable Reactome functional interaction network algorithm. This approach merges interactions extracted from human curated pathways with predicted interactions from machine learning. Genome-wide pathway analysis showed total of 16 enriched hierarchical clusters. A total of 278 enriched single pathways were also identified (p < 0.05, false discovery rate corrected). Gene network analyses showed multiple known and new targets in the p75NTR gene network. This study provides a comprehensive analysis and investigation into the current knowledge of p75NTR signaling networks and pathways. These results also identify several genes and their respective protein products as involved in the p75NTR network, which have not previously been clearly studied in this pathway. These results can be used to generate novel hypotheses to gain a greater understanding of p75NTR in acute brain injuries, neurodegenerative diseases and general response to cellular damage.

Towards the PET radiotracer for p75 neurotrophin receptor: [(11)C]LM11A-24 shows biological activity in vitro, but unfavorable ex vivo and in vivo profile

Mature neurotrophins as well as their pro forms are critically involved in the regulation of neuronal functions. They are signaling through three distinct types of receptors: tropomyosin receptor kinase family (TrkA/B/C), p75 neurotrophin receptor (p75(NTR)) and sortilin. Aberrant expression of p75(NTR) in the CNS is implicated in a variety of neurodegenerative diseases, including Alzheimer's disease. The goal of this work was to evaluate one of the very few reported p75(NTR) small molecule ligands as a lead compound for development of novel PET radiotracers for in vivo p75(NTR) imaging. Here we report that previously described ligand LM11A-24 shows significant inhibition of carbachol-induced persistent firing (PF) of entorhinal cortex (EC) pyramidal neurons in wild-type mice via selective interaction with p75(NTR). Based on this electrophysiological assay, the compound has very high potency with an EC50<10nM. We optimized the radiosynthesis of [(11)C]LM11A-24 as the first attempt to develop PET radioligand for in vivo imaging of p75(NTR). Despite some weak interaction with CNS tissues, the radiolabeled compound showed unfavorable in vivo profile presumably due to high hydrophilicity.

A novel inhibitor of p75-neurotrophin receptor improves functional outcomes in two models of traumatic brain injury

The p75 neurotrophin receptor is important in multiple physiological actions including neuronal survival and neurite outgrowth during development, and after central nervous system injury. We have discovered a novel piperazine-derived compound, EVT901, which interferes with p75 neurotrophin receptor oligomerization through direct interaction with the first cysteine-rich domain of the extracellular region. Using ligand binding assays with cysteine-rich domains-fused p75 neurotrophin receptor, we confirmed that EVT901 interferes with oligomerization of full-length p75 neurotrophin receptor in a dose-dependent manner. Here we report that EVT901 reduces binding of pro-nerve growth factor to p75 neurotrophin receptor, blocks pro-nerve growth factor induced apoptosis in cells expressing p75 neurotrophin receptor, and enhances neurite outgrowth in vitro. Furthermore, we demonstrate that EVT901 abrogates p75 neurotrophin receptor signalling by other ligands, such as prion peptide and amyloid-β. To test the efficacy of EVT901 in vivo, we evaluated the outcome in two models of traumatic brain injury. We generated controlled cortical impacts in adult rats. Using unbiased stereological analysis, we found that EVT901 delivered intravenously daily for 1 week after injury, reduced lesion size, protected cortical neurons and oligodendrocytes, and had a positive effect on neurological function. After lateral fluid percussion injury in adult rats, oral treatment with EVT901 reduced neuronal death in the hippocampus and thalamus, reduced long-term cognitive deficits, and reduced the occurrence of post-traumatic seizure activity. Together, these studies provide a new reagent for altering p75 neurotrophin receptor actions after injury and suggest that EVT901 may be useful in treatment of central nervous system trauma and other neurological disorders where p75 neurotrophin receptor signalling is affected.

Small molecule p75NTR ligands reduce pathological phosphorylation and misfolding of tau, inflammatory changes, cholinergic degeneration, and cognitive deficits in AβPP(L/S) transgenic mice

The p75 neurotrophin receptor (p75NTR) is involved in degenerative mechanisms related to Alzheimer's disease (AD). In addition, p75NTR levels are increased in AD and the receptor is expressed by neurons that are particularly vulnerable in the disease. Therefore, modulating p75NTR function may be a significant disease-modifying treatment approach. Prior studies indicated that the non-peptide, small molecule p75NTR ligands LM11A-31, and chemically unrelated LM11A-24, could block amyloid-β-induced deleterious signaling and neurodegeneration in vitro, and LM11A-31 was found to mitigate neuritic degeneration and behavioral deficits in a mouse model of AD. In this study, we determined whether these in vivo findings represent class effects of p75NTR ligands by examining LM11A-24 effects. In addition, the range of compound effects was further examined by evaluating tau pathology and neuroinflammation. Following oral administration, both ligands reached brain concentrations known to provide neuroprotection in vitro. Compound induction of p75NTR cleavage provided evidence for CNS target engagement. LM11A-31 and LM11A-24 reduced excessive phosphorylation of tau, and LM11A-31 also inhibited its aberrant folding. Both ligands decreased activation of microglia, while LM11A-31 attenuated reactive astrocytes. Along with decreased inflammatory responses, both ligands reduced cholinergic neurite degeneration. In addition to the amelioration of neuropathology in AD model mice, LM11A-31, but not LM11A-24, prevented impairments in water maze performance, while both ligands prevented deficits in fear conditioning. These findings support a role for p75NTR ligands in preventing fundamental tau-related pathologic mechanisms in AD, and further validate the development of these small molecules as a new class of therapeutic compounds.

An Investigation on the Fundamental Interaction between Abeta Peptides and the AT-Rich DNA

DNA damage is ubiquitous in all mammalian cells with the occurrence of more than 60,000 times per day per cell. In particular, DNA damage in neurons is found to accumulate with age and has been suggested to interfere with the synthesis of functional proteins. Moreover, recent studies have found through transgenic mice that human amyloid precursor protein causes an increase in DNA double-strand breaks (DSBs) with the effect of a prolongation in DNA repair. It is surmised that amyloid β (Aβ) exacerbates the DNA DSBs in neurons, possibly engendering neuronal dysfunction as a result. However, a good understanding on the holistic interaction mechanisms and the manner in which Aβ intertwines with DNA damage is still in its infancy. In our study, we found that DNA with an AT-rich sequence has a very low binding affinity toward Aβ by means of molecular dynamics simulation. While we have pursued a particular sequence of DNA in this study, other DNA sequences are expected to affect the interaction and binding affinity between DNA and Aβ, and will be pursued in our further research. Nonetheless, we have uncovered favorable interaction between the positively charged side chain of Aβ and the two ends of DNA. The latest experiment reveals that many of the double-stranded breaks in neurons can be fixed via DNA repair mechanisms but not in the case that Aβs are present. It is found that the increased numbers of DSBs prevail in active neurons. Here, on the basis of the favorable interaction between Aβ and the two ends of DNA, we propose the possibility that Aβ prevents DNA repair via binding directly to the break ends of the DNA, which further exacerbates DNA damage. Moreover, we have found that the base pair oxygen of the DNA has a greater preference to form hydrogen bonds than the backbone oxygen with Aβ at the two ends. Thus, we postulate that Aβ could serve to prevent the repair of AT-rich DNA, and it is unlikely to cause its breakage or affect its binding toward histone. Another important observation from our study is that AT-rich DNA has very little or no influence on Aβ oligomerization. Finally, even though we do not observe any dramatic DNA conformational change in the presence of Aβ, we do observe an increase in diversity of the DNA structural parameters such as groove width, local base step, and torsional angles in lieu of Aβ interactions.

A small molecule p75(NTR) ligand protects neurogenesis after traumatic brain injury

The p75 neurotrophin receptor (p75(NTR)) influences the proliferation, survival, and differentiation of neuronal precursors and its expression is induced in injured brain, where it regulates cell survival. Here, we test the hypotheses that pharmacologic modulation of p75(NTR) signaling will promote neural progenitor survival and proliferation, and improve outcomes of traumatic brain injury (TBI). LM11A-31, an orally available, blood-brain barrier-permeant small-molecule p75(NTR) signaling modulator, significantly increased proliferation and survival, and decreased JNK phosphorylation, in hippocampal neural stem/progenitor cells in culture expressing wild-type p75(NTR), but had no effect on cells expressing a mutant neurotrophin-unresponsive form of the receptor. The compound also enhanced the production of mature neurons from adult hippocampal neural progenitors in vitro. In vivo, intranasal administration of LM11A-31 decreased postinjury hippocampal and cortical neuronal death, neural progenitor cell death, gliogenesis, and microglial activation, and enhanced long-term hippocampal neurogenesis and reversed spatial memory impairments. LM11A-31 diminished the postinjury increase of SOX2-expressing early progenitor cells, but protected and increased the proliferation of endogenous polysialylated-neural cell adhesion molecule positive intermediate progenitors, and restored the long-term production of mature granule neurons. These findings suggest that modulation of p75(NTR) actions using small molecules such as LM11A-31 may constitute a potent therapeutic strategy for TBI.

A small molecule p75NTR ligand, LM11A-31, reverses cholinergic neurite dystrophy in Alzheimer's disease mouse models with mid- to late-stage disease progression

Degeneration of basal forebrain cholinergic neurons contributes significantly to the cognitive deficits associated with Alzheimer's disease (AD) and has been attributed to aberrant signaling through the neurotrophin receptor p75 (p75NTR). Thus, modulating p75NTR signaling is considered a promising therapeutic strategy for AD. Accordingly, our laboratory has developed small molecule p75NTR ligands that increase survival signaling and inhibit amyloid-β-induced degenerative signaling in in vitro studies. Previous work found that a lead p75NTR ligand, LM11A-31, prevents degeneration of cholinergic neurites when given to an AD mouse model in the early stages of disease pathology. To extend its potential clinical applications, we sought to determine whether LM11A-31 could reverse cholinergic neurite atrophy when treatment begins in AD mouse models having mid- to late stages of pathology. Reversing pathology may have particular clinical relevance as most AD studies involve patients that are at an advanced pathological stage. In this study, LM11A-31 (50 or 75 mg/kg) was administered orally to two AD mouse models, Thy-1 hAPPLond/Swe (APPL/S) and Tg2576, at age ranges during which marked AD-like pathology manifests. In mid-stage male APPL/S mice, LM11A-31 administered for 3 months starting at 6-8 months of age prevented and/or reversed atrophy of basal forebrain cholinergic neurites and cortical dystrophic neurites. Importantly, a 1 month LM11A-31 treatment given to male APPL/S mice (12-13 months old) with late-stage pathology reversed the degeneration of cholinergic neurites in basal forebrain, ameliorated cortical dystrophic neurites, and normalized increased basal forebrain levels of p75NTR. Similar results were seen in female Tg2576 mice. These findings suggest that LM11A-31 can reduce and/or reverse fundamental AD pathologies in late-stage AD mice. Thus, targeting p75NTR is a promising approach to reducing AD-related degenerative processes that have progressed beyond early stages.

Memantine improves spatial learning and memory impairments by regulating NGF signaling in APP/PS1 transgenic mice

Memantine (MEM) is used for improving the cognitive impairments of the patients suffering from Alzheimer's disease (AD) by multiple neuroprotective mechanisms. However, it is still not clear whether nerve growth factor (NGF) signaling is involved in the mechanisms of MEM. The present study investigated the neuroprotective effects of MEM treatment on the cognitive performance and amyloidosis in APP/PS1 transgenic mice, and disclosed the NGF-related mechanism of MEM. We found that MEM treatment improved the cognitive performance by decreasing the escape latency and path length in the navigation test, by shortening the duration in target quadrant and reducing the frequency to pass through the target in probe trial, and by prolonging the latency and decreasing the frequencies of entering the dark compartment in passive avoidance test. The over-expressions of Aβ(1-42) and amyloid precursor protein (APP) were also decreased in the brains of APP/PS1 mice. Interestingly, MEM treatment improved the decreased NGF levels in APP/PS1 mice. Furthermore, NGF/TrkA signaling was activated by increasing the phosphorylation levels of tyrosine kinase (TrkA), proto-oncogene serine/threonine-protein kinase, Raf1 (c-Raf), extracellular regulated protein kinases (ERK)1/2 and cAMP-response element binding protein (CREB) after MEM treatment. Simultaneously, MEM also inhibited NGF/p75(NTR) signaling via decreasing the cleavage substrate of p75(NTR), increasing the JNK2 phosphorylation and decreasing the levels of p53 and cleaved-caspase 3. Therefore, the dual-regulation on NGF signaling was attributed to the improvements of cognitive deficits and Aβ depositions in APP/PS1 mice. In conclusion, MEM treatment activated the NGF/TrkA signaling, and inhibited the p75(NTR) signaling in APP/PS1 mice to ameliorate the behavioral deficits and amyloidosis, indicating that NGF signaling was a new potential target of MEM treatment for AD therapy.

The dendritic hypothesis for Alzheimer's disease pathophysiology

Converging evidence indicates that processes occurring in and around neuronal dendrites are central to the pathogenesis of Alzheimer's disease. These data support the concept of a "dendritic hypothesis" of AD, closely related to the existing synaptic hypothesis. Here we detail dendritic neuropathology in the disease and examine how Aβ, tau, and AD genetic risk factors affect dendritic structure and function. Finally, we consider potential mechanisms by which these key drivers could affect dendritic integrity and disease progression. These dendritic mechanisms serve as a framework for therapeutic target identification and for efforts to develop disease-modifying therapeutics for Alzheimer's disease.

Mapping of neurotrophins and their receptors in the adult mouse brain and their role in the pathogenesis of a transgenic murine model of bovine spongiform encephalopathy

Neurotrophins are a family of growth factors that act on neuronal cells. The neurotrophins include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-3, -4 and -5. The action of neurotrophins depends on two transmembrane-receptor signalling systems: (1) the tropomyosin-related kinase (Trk) family of tyrosine kinase receptors (Trk A, Trk B and Trk C) and (2) the p75 neurotrophin receptor (p75(NTR)). The interaction between neurotrophic factors and their receptors may be involved in the mechanisms that regulate the differential susceptibility of neuronal populations in neurodegenerative diseases. The aim of the present study was to evaluate the role of neurotrophins in the pathogenesis of bovine spongiform encephalopathy (BSE) using a transgenic mouse overexpressing bovine prnp (BoTg 110). Histochemistry for Lycopersicum esculentum agglutinin, haematoxylin and eosin staining and immunohistochemistry for the abnormal isoform of the prion protein (PrP(d)), glial fibrillary acidic protein (GFAP), NGF, BDNF, NT-3 and the receptors Trk A, Trk B, Trk C and p75(NTR) was performed. The lesions and the immunolabelling patterns were assessed semiquantitatively in different areas of the brain. No significant differences in the immunolabelling of neurotrophins and their receptors were observed between BSE-inoculated and control animals, except for p75(NTR), which showed increased expression correlating with the distribution of lesions, PrP(d) deposition and gliosis in the BSE-inoculated mice.

Small molecule p75NTR ligand prevents cognitive deficits and neurite degeneration in an Alzheimer's mouse model

The p75 neurotrophin receptor (p75(NTR)) is associated with multiple mechanisms linked to Alzheimer's disease (AD); hence, modulating its function might confer therapeutic effects. In previous in vitro work, we developed small molecule p75(NTR) ligands that inhibited amyloid-β-induced degenerative signaling and prevented neurite degeneration. In the present study, a prototype p75(NTR) ligand, LM11A-31, was administered orally to the Thy-1 hAPP(Lond/Swe) (APP(L/S)) AD mouse model. LM11A-31 reached brain concentrations known to inhibit degenerative signaling without toxicity or induction of hyperalgesia. It prevented deficits in novel object recognition after 2.5 months and, in a separate cohort, deficits in Y-maze performance after 3 months of treatment. Stereology studies found that the number and size of basal forebrain cholinergic neurons, which are normal in APP(L/S) mice, were unaffected. Neuritic dystrophy, however, was readily apparent in the basal forebrain, hippocampus and cortex, and was significantly reduced by LM11A-31, with no effect on amyloid levels. These studies reveal that p75(NTR) is an important and tractable in vivo drug target for AD, with LM11A-31 representing a novel class of therapeutic candidates.

C-terminal peptide of γ-enolase impairs amyloid-β-induced apoptosis through p75(NTR) signaling

γ-Enolase acts as a neurotrophic-like factor promoting growth, differentiation, survival and regeneration of neurons. It is shown in this study to exert a protective effect against amyloid-β-peptide (Aβ)-induced neurotoxicity in rat pheochromocytoma PC12 cells. Aβ-induced toxicity was abolished in the presence of the active C-terminal peptide of γ-enolase (γ-Eno) as measured by cell viability, lactate dehydrogenase release, sub-G1 cell population, intracellular reactive oxygen species, mitochondrial functions and apoptotic morphology. γ-Eno caused downregulation of the pro-apoptotic protein Bax and upregulation of the anti-apoptotic protein Bcl-2, as well as reduced caspase-3 activation. Exposure to Aβ increased surface expression of p75 neurotrophin receptor (p75(NTR)), and the increase was abolished in the presence of γ-Eno peptide. Further, pretreatment with γ-Eno suppressed the activation of mitogen-activated protein kinases p38 and Jun-N-terminal kinase, which are p75(NTR) downstream effectors in apoptotic signaling. Moreover, Aβ triggered γ-enolase co-immunoprecipitation with p75(NTR) as well as their strong association in the perimembrane region as shown by confocal microscopy, which further supports the interaction between these two proteins in cells insulted by Aβ peptide. Our results indicate the possible use of γ-enolase C-terminal peptide for treating or preventing Alzheimer's disease.

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

BACKGROUND

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSION

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

A role for the tyrosine kinase ACK1 in neurotrophin signaling and neuronal extension and branching

Neurotrophins are involved in many crucial cellular functions, including neurite outgrowth, synapse formation, and plasticity. Although these events have long been known, the molecular determinants underlying neuritogenesis have not been fully characterized. Ack1 (activated Cdc42-associated tyrosine kinase) is a non-receptor tyrosine kinase that is highly expressed in the brain. Here, we demonstrate that Ack1 is a molecular constituent of neurotrophin signaling cascades in neurons and PC12 cells. We report that Ack1 interacts with Trk receptors and becomes tyrosine phosphorylated and its kinase activity is increased in response to neurotrophins. Moreover, our data indicate that Ack1 acts upstream of the Akt and MAPK pathways. We show that Ack1 overexpression induces neuritic outgrowth and promotes branching in neurotrophin-treated neuronal cells, whereas the expression of Ack1 dominant negatives or short-hairpin RNAs counteract neurotrophin-stimulated differentiation. Our results identify Ack1 as a novel regulator of neurotrophin-mediated events in primary neurons and in PC12 cells.

Postsynaptic Receptors for Amyloid-β Oligomers as Mediators of Neuronal Damage in Alzheimer's Disease

The neurotoxic effect of amyloid-β peptide (Aβ) over the central synapses has been described and is reflected in the decrease of some postsynaptic excitatory proteins, the alteration in the number and morphology of the dendritic spines, and a decrease in long-term potentiation. Many studies has been carried out to identify the putative Aβ receptors in neurons, and is still no clear why the Aβ oligomers only affect the excitatory synapses. Aβ oligomers bind to neurite and preferentially to the postsynaptic region, where the postsynaptic protein-95 (PSD-95) is present in the glutamatergic synapse, and interacts directly with the N-methyl-D-aspartate receptor (NMDAR) and neuroligin (NL). NL is a postsynaptic protein which binds to the presynaptic protein, neurexin to form a heterophilic adhesion complex, the disruption of this interaction affects the integrity of the synaptic contact. Structurally, NL has an extracellular domain homolog to acetylcholinesterase, the first synaptic protein that was found to interact with Aβ. In the present review we will document the interaction between Aβ and the extracellular domain of NL-1 at the excitatory synapse, as well as the interaction with other postsynaptic components, including the glutamatergic receptors (NMDA and mGluR5), the prion protein, the neurotrophin receptor, and the α7-nicotinic acetylcholine receptor. We conclude that several Aβ oligomers receptors exist at the excitatory synapse, which could be the responsible for the neurotoxic effect described for the Aβ oligomers. The characterization of the interaction between Aβ receptors and Aβ oligomers could help to understand the source of the neurologic damage observed in the brain of the Alzheimer's disease patients.

p75NTR is mainly responsible for Aβ toxicity but not for its internalization: a primary study

Accumulating evidence indicates that the intraneuronal accumulation of beta-amyloid peptide (Aβ) is earlier than the formation of extraneuronal amyloid plaque but the mechanism of the accumulation remains unclear. p75NTR is a receptor for Aβ and interacts with Aβ in vitro and in vivo but whether p75NTR mediates Aβ internalization and intraneuronal accumulation is not known. In this study, we aim to determine if p75NTR mediates Aβ internalization, which might provide new insights into Aβ metabolism and toxicity. FRET analysis in PC12 cells showed that internalized Aβ was close to p75NTR. Aβ1-42 could be internalized in PC12 cells in a concentration-dependent manner but the antibody to the p75NTR extracellular domain did not prevent its internalization. Aβ1-42 could also be internalized in mouse neonatal cortical neurons and the deletion of p75NTR in these neurons did not prevent its internalization but prevented Aβ neurotoxicity. Cholesterol at 10 μM significantly increased Aβ1-42 internalization in PC12 cells. Internalized Aβ1-42 is mainly co-localized with Beclin-1 (a biomarker of autophagosomes) but not with endosomal and lysomal markers. p75NTR may not play a main role in Aβ internalization at the concentrations tested but is responsible for Aβ induced toxicity in primary neurons. Internalized Aβ is mainly sorted to autophagosomes for metabolism.

Neurotrophins role in depression neurobiology: a review of basic and clinical evidence

Depression is a neuropsychiatric disorder affecting a huge percentage of the active population especially in developed countries. Research has devoted much of its attention to this problematic and many drugs have been developed and are currently prescribed to treat this pathology. Yet, many patients are refractory to the available therapeutic drugs, which mainly act by increasing the levels of the monoamines serotonin and noradrenaline in the synaptic cleft. Even in the cases antidepressants are effective, it is usually observed a delay of a few weeks between the onset of treatment and remission of the clinical symptoms. Additionally, many of these patients who show remission with antidepressant therapy present a relapse of depression upon treatment cessation. Thus research has focused on other possible molecular targets, besides monoamines, underlying depression. Both basic and clinical evidence indicates that depression is associated with several structural and neurochemical changes where the levels of neurotrophins, particularly of brain-derived neurotrophic factor (BDNF), are altered. Antidepressants, as well as other therapeutic strategies, seem to restore these levels. Neuronal atrophy, mostly detected in limbic structures that regulate mood and cognition, like the hippocampus, is observed in depressed patients and in animal behavioural paradigms for depression. Moreover, chronic antidepressant treatment enhances adult hippocampal neurogenesis, supporting the notion that this event underlies antidepressants effects. Here we review some of the preclinical and clinical studies, aimed at disclosing the role of neurotrophins in the pathophysiological mechanisms of depression and the mode of action of antidepressants, which favour the neurotrophic/neurogenic hypothesis.

The toxicity of amyloid β oligomers

In this review, we elucidate the mechanisms of Aβ oligomer toxicity which may contribute to Alzheimer's disease (AD). In particular, we discuss on the interaction of Aβ oligomers with the membrane through the process of adsorption and insertion. Such interaction gives rises to phase transitions in the sub-structures of the Aβ peptide from α-helical to β-sheet structure. By means of a coarse-grained model, we exhibit the tendency of β-sheet structures to aggregate, thus providing further insights to the process of membrane induced aggregation. We show that the aggregated oligomer causes membrane invagination, which is a precursor to the formation of pore structures and ion channels. Other pathological progressions to AD due to Aβ oligomers are also covered, such as their interaction with the membrane receptors, and their direct versus indirect effects on oxidative stress and intraneuronal accumulation. We further illustrate that the molecule curcumin is a potential Aβ toxicity inhibitor as a β-sheet breaker by having a high propensity to interact with certain Aβ residues without binding to them. The comprehensive understanding gained from these current researches on the various toxicity mechanisms show promises in the provision of better therapeutics and treatment strategies in the near future.

A synthetic amino acid substitution of Tyr10 in Aβ peptide sequence yields a dominant negative variant in amyloidogenesis

Alzheimer's disease (AD) is the most common cause of dementia in elderly people, and age is the major nongenetic risk factor for sporadic AD. A hallmark of AD is the accumulation of amyloid in the brain, which is composed mainly of the amyloid beta-peptide (Aβ) in the form of oligomers and fibrils. However, how aging induces Aβ aggregation is not yet fully determined. Some residues in the Aβ sequence seem to promote Aβ-induced toxicity in association with age-dependent risk factors for AD, such as (i) increased GM1 brain membrane content, (ii) altered lipid domain in brain membrane, (iii) oxidative stress. However, the role of Aβ sequence in promoting aggregation following interaction with the plasma membrane is not yet demonstrated. As Tyr10 is implicated in the induction of oxidative stress and stabilization of Aβ aggregation, we substituted Tyr 10 with a synthetic amino acid that abolishes Aβ-induced oxidative stress and shows an accelerated interaction with GM1. This variant peptide shows impaired aggregation properties and increased affinity for GM1. It has a dominant negative effect on amyloidogenesis in vitro, in cellulo, and in isolated synaptosomes. The present study shed new light in the understanding of Aβ-membrane interactions in Aβ-induced neurotoxicity. It demonstrates the relevance of Aβ sequence in (i) Aβ-membrane interaction, underlining the role of age-dependent enhanced GM1 content in promoting Aβ aggregation, (ii) Aβ aggregation, and (iii) Aβ-induced oxidative stress. Our results open the way for the design of peptides aimed to inhibit Aβ aggregation and neurotoxicity.

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.

New developments on the role of NMDA receptors in Alzheimer's disease

Since the initial findings that NMDA receptors play important roles in cellular models of learning as well as neurotoxicity, abnormal function of this receptor has been considered a potential mechanism in the pathophysiology underlying Alzheimer's disease. Treatment of Alzheimer's disease with an NMDA receptor antagonist began several years ago, with some limited success. More recent mechanistic studies have examined the role of NMDA receptors in the synaptic effects of beta amyloid (Aβ).

Differences in abundances of cell-signalling proteins in blood reveal novel biomarkers for early detection of clinical Alzheimer's disease

BACKGROUND

In November 2007 a study published in Nature Medicine proposed a simple test based on the abundance of 18 proteins in blood to predict the onset of clinical symptoms of Alzheimer's Disease (AD) two to six years before these symptoms manifest. Later, another study, published in PLoS ONE, showed that only five proteins (IL-1, IL-3, EGF, TNF- and G-CSF) have overall better prediction accuracy. These classifiers are based on the abundance of 120 proteins. Such values were standardised by a Z-score transformation, which means that their values are relative to the average of all others.

METHODOLOGY

The original datasets from the Nature Medicine paper are further studied using methods from combinatorial optimisation and Information Theory. We expand the original dataset by also including all pair-wise differences of z-score values of the original dataset ("metafeatures"). Using an exact algorithm to solve the resulting Feature Set problem, used to tackle the feature selection problem, we found signatures that contain either only features, metafeatures or both, and evaluated their predictive performance on the independent test set.

CONCLUSIONS

It was possible to show that a specific pattern of cell signalling imbalance in blood plasma has valuable information to distinguish between NDC and AD samples. The obtained signatures were able to predict AD in patients that already had a Mild Cognitive Impairment (MCI) with up to 84% of sensitivity, while maintaining also a strong prediction accuracy of 90% on a independent dataset with Non Demented Controls (NDC) and AD samples. The novel biomarkers uncovered with this method now confirms ANG-2, IL-11, PDGF-BB, CCL15/MIP-1; and supports the joint measurement of other signalling proteins not previously discussed: GM-CSF, NT-3, IGFBP-2 and VEGF-B.

Control of neuronal ploidy during vertebrate development

Somatic tetraploid neurons are present in different structures of the vertebrate nervous system, including cortex and retina. In this chapter, we provide evidence that these neurons can be widely detected in the chick nervous system. We also discuss mechanisms creating neuronal tetraploidy in vertebrates, concluding that the neurotrophin receptor p75 could be responsible for the generation of these neurons in most neural tissues, as previously observed in the retina. Somatic tetraploidy in the chick retina correlates with increased neurons' soma size and dendritic arborization, giving rise to neurons known to innervate a specific layer of the optic tectum. Tetraploidy could therefore account for neuronal diversity in the normal nervous system. De novo generation of tetraploid neurons has been shown to occur in Alzheimer's disease. This suggests that the morphological changes expected to occur in the affected neurons could lead to altered neuronal function, thus providing a basis for neurodegeneration.

The p75 neurotrophin receptor is localized to primary cilia in adult murine hippocampal dentate gyrus granule cells

The densely ciliated granule cell layer of the adult murine hippocampal dentate gyrus is one of two sites of adult neurogenesis. The granule cells have already been proven to localize their SSTR3 (somatostatin receptor 3) receptors to their so-called primary cilia. Here we show for the first time that 70-90% of these cells in 7-18 months-old wild-type and 3×Tg-AD (Alzheimer disease transgenic) mice also load p75(NTR) receptors into the structures containing SSTR3, i.e., their primary cilia. On the other hand, p75(NTR')s TrkA co-receptors were not localized to cilia but conventionally distributed throughout the cell surface. Significantly fewer cells (20-40%) in the hippocampal CA1 and CA3 regions and cerebral cortex have p75(NTR) containing cilia. While we don't know what the impact of the cilial localization of p75(NTR) on dentate gyral adult neurogenesis and memory encoding might be, the cilia's amyloid β-activatable p75(NTR) receptors could be damaging or lethal to the hippocampal functioning of amyloid β-accumulating Alzheimer brain.

Curating the innate immunity interactome

BACKGROUND

The innate immune response is the first line of defence against invading pathogens and is regulated by complex signalling and transcriptional networks. Systems biology approaches promise to shed new light on the regulation of innate immunity through the analysis and modelling of these networks. A key initial step in this process is the contextual cataloguing of the components of this system and the molecular interactions that comprise these networks. InnateDB (http://www.innatedb.com) is a molecular interaction and pathway database developed to facilitate systems-level analyses of innate immunity.

RESULTS

Here, we describe the InnateDB curation project, which is manually annotating the human and mouse innate immunity interactome in rich contextual detail, and present our novel curation software system, which has been developed to ensure interactions are curated in a highly accurate and data-standards compliant manner. To date, over 13,000 interactions (protein, DNA and RNA) have been curated from the biomedical literature. Here, we present data, illustrating how InnateDB curation of the innate immunity interactome has greatly enhanced network and pathway annotation available for systems-level analysis and discuss the challenges that face such curation efforts. Significantly, we provide several lines of evidence that analysis of the innate immunity interactome has the potential to identify novel signalling, transcriptional and post-transcriptional regulators of innate immunity. Additionally, these analyses also provide insight into the cross-talk between innate immunity pathways and other biological processes, such as adaptive immunity, cancer and diabetes, and intriguingly, suggests links to other pathways, which as yet, have not been implicated in the innate immune response.

CONCLUSIONS

In summary, curation of the InnateDB interactome provides a wealth of information to enable systems-level analysis of innate immunity.