Absence of p75NTR causes increased basal forebrain cholinergic neuron size, choline acetyltransferase activity, and target innervation

Ngfr+ cholinergic projection from SI/nBM to mPFC selectively regulates temporal order recognition memory

Acetylcholine regulates various cognitive functions through broad cholinergic innervation. However, specific cholinergic subpopulations, circuits and molecular mechanisms underlying recognition memory remain largely unknown. Here we show that Ngfr+ cholinergic neurons in the substantia innominate (SI)/nucleus basalis of Meynert (nBM)-medial prefrontal cortex (mPFC) circuit selectively underlies recency judgements. Loss of nerve growth factor receptor (Ngfr-/- mice) reduced the excitability of cholinergic neurons in the SI/nBM-mPFC circuit but not in the medial septum (MS)-hippocampus pathway, and impaired temporal order memory but not novel object and object location recognition. Expression of Ngfr in Ngfr-/- SI/nBM restored defected temporal order memory. Fiber photometry revealed that acetylcholine release in mPFC not only predicted object encounters but also mediated recency judgments of objects, and such acetylcholine release was absent in Ngfr-/- mPFC. Chemogenetic and optogenetic inhibition of SI/nBM projection to mPFC in ChAT-Cre mice diminished mPFC acetylcholine release and deteriorated temporal order recognition. Impaired cholinergic activity led to a depolarizing shift of GABAergic inputs to mPFC pyramidal neurons, due to disturbed KCC2-mediated chloride gradients. Finally, potentiation of acetylcholine signaling upregulated KCC2 levels, restored GABAergic driving force and rescued temporal order recognition deficits in Ngfr-/- mice. Thus, NGFR-dependent SI/nBM-mPFC cholinergic circuit underlies temporal order recognition memory.

The Neurotrophin System in the Postnatal Brain-An Introduction

Neurotrophins can bind to and signal through specific receptors that belong to the class of the Trk family of tyrosine protein kinase receptors. In addition, they can bind and signal through a low-affinity receptor, termed p75NTR. Neurotrophins play a crucial role in the development, maintenance, and function of the nervous system in vertebrates, but they also have important functions in the mature nervous system. In particular, they are involved in synaptic and neuronal plasticity. Thus, it is not surprisingly that they are involved in learning, memory and cognition and that disturbance in the neurotrophin system can contribute to psychiatric diseases. The neurotrophin system is sensitive to aging and changes in the expression levels correlate with age-related changes in brain functions. Several polymorphisms in genes coding for the different neurotrophins or neurotrophin receptors have been reported. Based on the importance of the neurotrophins for the central nervous system, it is not surprisingly that several of these polymorphisms are associated with psychiatric diseases. In this review, we will shed light on the functions of neurotrophins in the postnatal brain, especially in processes that are involved in synaptic and neuronal plasticity.

Accelerated neurodegeneration of basal forebrain cholinergic neurons in HIV-1 gp120 transgenic mice: Critical role of the p75 neurotrophin receptor

Human Immunodeficiency Virus-1 (HIV) infection of the brain induces HIV-associated neurocognitive disorders (HAND). The set of molecular events employed by HIV to drive cognitive impairments in people living with HIV are diverse and remain not completely understood. We have shown that the HIV envelope protein gp120 promotes loss of synapses and decreases performance on cognitive tasks through the p75 neurotrophin receptor (p75NTR). This receptor is abundant on cholinergic neurons of the basal forebrain and contributes to cognitive impairment in various neurological disorders. In this study, we examined cholinergic neurons of gp120 transgenic (gp120tg) mice for signs of degeneration. We observed that the number of choline acetyltransferase-expressing cells is decreased in old (12-14-month-old) gp120tg mice when compared to age matched wild type. In the same animals, we observed an increase in the levels of pro-nerve growth factor, a ligand of p75NTR, as well as a disruption of consolidation of extinction of conditioned fear, a behavior regulated by cholinergic neurons of the basal forebrain. Both biochemical and behavioral outcomes of gp120tg mice were rescued by the deletion of the p75NTR gene, strongly supporting the role that this receptor plays in the neurotoxic effects of gp120. These data indicate that future p75NTR-directed pharmacotherapies could provide an adjunct therapy against synaptic simplification caused by HIV.

Cholinergic neurodegeneration and cholesterol metabolism dysregulation by constitutive p75NTR signaling in the p75exonIII-KO mice

Degeneration of basal forebrain cholinergic neurons (BFCNs) is a hallmark of Alzheimer's disease (AD). However, few mouse models of AD recapitulate the neurodegeneration of the cholinergic system. The p75 neurotrophin receptor, p75NTR, has been associated with the degeneration of BFCNs in AD. The senescence-accelerated mouse prone number 8 (SAMP8) is a well-accepted model of accelerated and pathological aging. To gain a better understanding of the role of p75NTR in the basal forebrain during aging, we generated a new mouse line, the SAMP8-p75exonIII-/-. Deletion of p75NTR in the SAMP8 background induces an increase in the number of BFCNs at birth, followed by a rapid decline during aging compared to the C57/BL6 background. This decrease in the number of BFCNs correlates with a worsening in the Y-maze memory test at 6 months in the SAMP8-p75exonIII-/-. We found that SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice expressed constitutively a short isoform of p75NTR that correlates with an upregulation of the protein levels of SREBP2 and its targets, HMGCR and LDLR, in the BF of both SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice. As the neurodegeneration of the cholinergic system and the dysregulation of cholesterol metabolism are implicated in AD, we postulate that the generated SAMP8-p75exonIII-/- mouse strain might constitute a good model to study long-term cholinergic neurodegeneration in the CNS. In addition, our results support the role of p75NTR signaling in cholesterol biosynthesis regulation.

Neurotrophic Factors and Dendritic Spines

Dendritic spines are highly dynamic structures that play important roles in neuronal plasticity. The morphologies and the numbers of dendritic spines are highly variable, and this diversity is correlated with the different morphological and physiological features of this neuronal compartment. Dendritic spines can change their morphology and number rapidly, allowing them to adapt to plastic changes. Neurotrophic factors play important roles in the brain during development. However, these factors are also necessary for a variety of processes in the postnatal brain. Neurotrophic factors, especially members of the neurotrophin family and the ephrin family, are involved in the modulation of long-lasting effects induced by neuronal plasticity by acting on dendritic spines, either directly or indirectly. Thereby, the neurotrophic factors play important roles in processes attributed, for example, to learning and memory.

An Alternative View of Familial Alzheimer's Disease Genetics

Probabilistic and parsimony-based arguments regarding available genetics data are used to propose that Hardy and Higgin's amyloid cascade hypothesis is valid but is commonly interpreted too narrowly to support, incorrectly, the primacy of the amyloid-β peptide (Aβ) in driving Alzheimer's disease pathogenesis. Instead, increased activity of the βCTF (C99) fragment of AβPP is the critical pathogenic determinant altered by mutations in the APP gene. This model is consistent with the regulation of APP mRNA translation via its 5' iron responsive element. Similar arguments support that the pathological effects of familial Alzheimer's disease mutations in the genes PSEN1 and PSEN2 are not exerted directly via changes in AβPP cleavage to produce different ratios of Aβ length. Rather, these mutations likely act through effects on presenilin holoprotein conformation and function, and possibly the formation and stability of multimers of presenilin holoprotein and/or of the γ-secretase complex. All fAD mutations in APP, PSEN1, and PSEN2 likely find unity of pathological mechanism in their actions on endolysosomal acidification and mitochondrial function, with detrimental effects on iron homeostasis and promotion of "pseudo-hypoxia" being of central importance. Aβ production is enhanced and distorted by oxidative stress and accumulates due to decreased lysosomal function. It may act as a disease-associated molecular pattern enhancing oxidative stress-driven neuroinflammation during the cognitive phase of the disease.

P75 neurotrophin receptor as a therapeutic target for drug development to treat neurological diseases

The interaction of neurotrophins with their receptors is involved in the pathogenesis and progression of various neurological diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury and acute and chronic cerebral damage. The p75 neurotrophin receptor (p75NTR) plays a pivotal role in the development of neurological dysfunctions as a result of its high expression, abnormal processing and signalling. Therefore, p75NTR represents as a vital therapeutic target for the treatment of neurodegeneration, neuropsychiatric disorders and cerebrovascular insufficiency. This review summarizes the current research progress on the p75NTR signalling in neurological deficits. We also summarize the present therapeutic approaches by genetically and pharmacologically targeting p75NTR for the attenuation of pathological changes. Based on the evolving knowledge, the role of p75NTR in the regulation of tau hyperphosphorylation, Aβ metabolism, the degeneration of motor neurons and dopaminergic neurons has been discussed. Its position as a biomarker to evaluate the severity of diseases and as a druggable target for drug development has also been elucidated. Several prototype small molecule compounds were introduced to be crucial in neuronal survival and functional recovery via targeting p75NTR. These small molecule compounds represent desirable agents in attenuating neurodegeneration and cell death as they abolish activation-induced neurotoxicity of neurotrophins via modulating p75NTR signalling. More comprehensive and in-depth investigations on p75NTR-based drug development are required to shed light on effective treatment of numerous neurological disorders.

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.

The role of brain-derived neurotrophic factor and the neurotrophin receptor p75NTR in age-related brain atrophy and the transition to Alzheimer's disease

Alzheimer's disease is a neurodegenerative condition that is potentially mediated by synaptic dysfunction before the onset of cognitive impairments. The disease mostly affects elderly people and there is currently no therapeutic which halts its progression. One therapeutic strategy for Alzheimer's disease is to regenerate lost synapses by targeting mechanisms involved in synaptic plasticity. This strategy has led to promising drug candidates in clinical trials, but further progress needs to be made. An unresolved problem of Alzheimer's disease is to identify the molecular mechanisms that render the aged brain susceptible to synaptic dysfunction. Understanding this susceptibility may identify drug targets which could halt, or even reverse, the disease's progression. Brain derived neurotrophic factor is a neurotrophin expressed in the brain previously implicated in Alzheimer's disease due to its involvement in synaptic plasticity. Low levels of the protein increase susceptibility to the disease and post-mortem studies consistently show reductions in its expression. A desirable therapeutic approach for Alzheimer's disease is to stimulate the expression of brain derived neurotrophic factor and potentially regenerate lost synapses. However, synthesis and secretion of the protein are regulated by complex activity-dependent mechanisms within neurons, which makes this approach challenging. Moreover, the protein is synthesised as a precursor which exerts the opposite effect of its mature form through the neurotrophin receptor p75NTR. This review will evaluate current evidence on how age-related alterations in the synthesis, processing and signalling of brain derived neurotrophic factor may increase the risk of Alzheimer's disease.

In vivo functions of p75NTR: challenges and opportunities for an emerging therapeutic target

The p75 neurotrophin receptor (p75^NTR) functions at the molecular nexus of cell death, survival, and differentiation. In addition to its contribution to neurodegenerative diseases and nervous system injuries, recent studies have revealed unanticipated roles of p75^NTR in liver repair, fibrinolysis, lung fibrosis, muscle regeneration, and metabolism. Linking these various p75^NTR functions more precisely to specific mechanisms marks p75^NTR as an emerging candidate for therapeutic intervention in a wide range of disorders. Indeed, small molecule inhibitors of p75^NTR binding to neurotrophins have shown efficacy in models of Alzheimer's disease (AD) and neurodegeneration. Here, we outline recent advances in understanding p75^NTR pleiotropic functions in vivo, and propose an integrated view of p75^NTR and its challenges and opportunities as a pharmacological target.

Ablation of p75NTR signaling strengthens gamma-theta rhythm interaction and counteracts Aβ-induced degradation of neuronal dynamics in mouse hippocampus in vitro

Gamma and theta brain rhythms play important roles in cognition and their interaction can affect gamma oscillation features. Hippocampal theta oscillations depend on cholinergic and GABAergic input from the medial septum-diagonal band of Broca. These projecting neurons undergo degeneration during aging and maintain high levels of neurotrophin receptor p75 (p75^NTR). p75^NTR mediates both apoptosis and survival and its expression is increased in Alzheimer's disease (AD) patients. Here, we investigate the importance of p75^NTR for the cholinergic input to the hippocampus. Performing extracellular recordings in brain slices from p75^NTR knockout mice (p75^-/-) in presence of the muscarinic agonist carbachol, we find that gamma oscillation power and rhythmicity are increased compared to wild-type (WT) mice. Furthermore, gamma activity is more phase-locked to the underlying theta rhythm, which renders a stronger coupling of both rhythms. On the cellular level, we find that fast-spiking interneurons (FSNs) fire more synchronized to a preferred gamma phase in p75^-/- mice. The excitatory input onto FSN is more rhythmic displaying a higher similarity with the concomitant gamma rhythm. Notably, the ablation of p75^NTR counteracts the Aβ-induced degradation of gamma oscillations and its nesting within the underlying theta rhythm. Our results show that the lack of p75^NTR signaling could promote stronger cholinergic modulation of the hippocampal gamma rhythm, suggesting an involvement of p75^NTR in the downregulation of cognition-relevant hippocampal network dynamics in pathologies. Moreover, functional data provided here suggest p75^NTR as a suitable target in the search for efficacious treatments to counteract the loss of cognitive function observed in amyloid-driven pathologies such as AD.

Cholinergic regulation of adult hippocampal neurogenesis and hippocampus-dependent functions

The production and circuit integration of new neurons is one of the defining features of the adult mammalian hippocampus. A wealth of evidence has established that adult hippocampal neurogenesis is exquisitely sensitive to neuronal activity-mediated regulation. How these signals are interpreted and contribute to neurogenesis and hippocampal functions has been a subject of immense interest. In particular, neurotransmitters, in addition to their synaptic roles, have been shown to offer important trophic support. Amongst these, acetylcholine, which has a prominent role in cognition, has been implicated in regulating neurogenesis. In this review, we appraise the evidence linking the contribution of cholinergic signalling to the regulation of adult hippocampal neurogenesis and hippocampus-dependent functions. We discuss open questions that need to be addressed to gain a deeper mechanistic understanding of the role and translational potential of acetylcholine and its receptors in regulating this form of cellular neuroplasticity.

Aging-relevant human basal forebrain cholinergic neurons as a cell model for Alzheimer's disease

Background

Alzheimer’s disease (AD) is an adult-onset mental disorder with aging as a major risk factor. Early and progressive degeneration of basal forebrain cholinergic neurons (BFCNs) contributes substantially to cognitive impairments of AD. An aging-relevant cell model of BFCNs will critically help understand AD and identify potential therapeutics. Recent studies demonstrate that induced neurons directly reprogrammed from adult human skin fibroblasts retain aging-associated features. However, human induced BFCNs (hiBFCNs) have yet to be achieved.

Methods

We examined a reprogramming procedure for the generation of aging-relevant hiBFCNs through virus-mediated expression of fate-determining transcription factors. Skin fibroblasts were obtained from healthy young persons, healthy adults and sporadic AD patients. Properties of the induced neurons were examined by immunocytochemistry, qRT-PCR, western blotting, and electrophysiology.

Results

We established a protocol for efficient generation of hiBFCNs from adult human skin fibroblasts. They show electrophysiological properties of mature neurons and express BFCN-specific markers, such as CHAT, p75NTR, ISL1, and VACHT. As a proof-of-concept, our preliminary results further reveal that hiBFCNs from sporadic AD patients exhibit time-dependent TAU hyperphosphorylation in the soma and dysfunctional nucleocytoplasmic transport activities.

Conclusions

Aging-relevant BFCNs can be directly reprogrammed from human skin fibroblasts of healthy adults and sporadic AD patients. They show promises as an aging-relevant cell model for understanding AD pathology and may be employed for therapeutics identification for AD.

Small molecule modulation of the p75 neurotrophin receptor suppresses age- and genotype-associated neurodegeneration in HIV gp120 transgenic mice

The persistence of HIV in the central nervous system leads to cognitive deficits in up to 50% of people living with HIV even with systemic suppression by antiretroviral treatment. The interaction of chronic inflammation with age-associated degeneration places these individuals at increased risk of accelerated aging and other neurodegenerative diseases and no treatments are available that effectively halt these processes. The adverse effects of aging and inflammation may be mediated, in part, by an increase in the expression of the p75 neurotrophin receptor (p75^NTR) which shifts the balance of neurotrophin signaling toward less protective pathways. To determine if modulation of p75^NTR could modify the disease process, we treated HIV gp120 transgenic mice with a small molecule ligand designed to engage p75^NTR and downregulate degenerative signaling. Daily treatment with 50 mg/kg LM11A-31 for 4 months suppressed age- and genotype-dependent activation of microglia, increased microtubule associated protein-2 (MAP-2), reduced dendritic varicosities and slowed the loss of parvalbumin immunoreactive neurons in the hippocampus. An age related accumulation of microtubule associated protein Tau was identified in the hippocampus in extracellular clusters that co-expressed p75^NTR suggesting a link between Tau and p75^NTR. Although the significance of the relationship between p75^NTR and Tau is unclear, a decrease in Tau-1 immunoreactivity as gp120 mice entered old age (>16 months) suggests that the Tau may transition to more pathological modifications; a process blocked by LM11A-31. Overall, the effects of LM11A-31 are consistent with strong neuroprotective and anti-inflammatory actions that have significant therapeutic potential.

Cholinergic System and NGF Receptors: Insights from the Brain of the Short-Lived Fish Nothobranchius furzeri

Nerve growth factor (NGF) receptors are evolutionary conserved molecules, and in mammals are considered necessary for ensuring the survival of cholinergic neurons. The age-dependent regulation of NTRK1/NTRKA and p75/NGFR in mammalian brain results in a reduced response of the cholinergic neurons to neurotrophic factors and is thought to play a role in the pathogenesis of neurodegenerative diseases. Here, we study the age-dependent expression of NGF receptors (NTRK1/NTRKA and p75/NGFR) in the brain of the short-lived teleost fish Nothobranchius furzeri. We observed that NTRK1/NTRKA is more expressed than p75/NGFR in young and old animals, although both receptors do not show a significant age-dependent change. We then study the neuroanatomical organization of the cholinergic system, observing that cholinergic fibers project over the entire neuroaxis while cholinergic neurons appear restricted to few nuclei situated in the equivalent of mammalian subpallium, preoptic area and rostral reticular formation. Finally, our experiments do not confirm that NTRK1/NTRKA and p75/NGFR are expressed in cholinergic neuronal populations in the adult brain of N. furzeri. To our knowledge, this is the first study where NGF receptors have been analyzed in relation to the cholinergic system in a fish species along with their age-dependent modulation. We observed differences between mammals and fish, which make the African turquoise killifish an attractive model to further investigate the fish specific NGF receptors regulation.

Targeted Deletion of PTEN in Kisspeptin Cells Results in Brain Region- and Sex-Specific Effects on Kisspeptin Expression and Gonadotropin Release

Kisspeptin-expressing neurons in the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (ARC) of the hypothalamus relay hormonal and metabolic information to gonadotropin-releasing hormone neurons, which in turn regulate pituitary and gonadal function. Phosphatase and tensin homolog (PTEN) blocks phosphatidylinositol 3-kinase (PI3K), a signaling pathway utilized by peripheral factors to transmit their signals. However, whether PTEN signaling in kisspeptin neurons helps to integrate peripheral hormonal cues to regulate gonadotropin release is unknown. To address this question, we generated mice with a kisspeptin cell-specific deletion of Pten (Kiss-PTEN KO), and first assessed kisspeptin protein expression and gonadotropin release in these animals. Kiss-PTEN KO mice displayed a profound sex and region-specific kisspeptin neuron hyperthrophy. We detected both kisspeptin neuron hyperthrophy as well as increased kisspeptin fiber densities in the AVPV and ARC of Kiss-PTEN KO females and in the ARC of Kiss-PTEN KO males. Moreover, Kiss-PTEN KO mice showed a reduced gonadotropin release in response to gonadectomy. We also found a hyperactivation of mTOR, a downstream PI3K target and central regulator of cell metabolism, in the AVPV and ARC of Kiss-PTEN KO females but not males. Fasting, known to inhibit hypothalamic kisspeptin expression and luteinizing hormone levels, failed to induce these changes in Kiss-PTEN KO females. We conclude that PTEN signaling regulates kisspeptin protein synthesis in both sexes and that its role as a metabolic signaling molecule in kisspeptin neurons is sex-specific.

3D Reconstruction of the Neurovascular Unit Reveals Differential Loss of Cholinergic Innervation in the Cortex and Hippocampus of the Adult Mouse Brain

Increasing evidence supports a role for cerebrovasculature dysfunction in the etiology of Alzheimer’s disease (AD). Blood vessels in the brain are composed of a collection of cells and acellular material that comprise the neurovascular unit (NVU). The NVU in the hippocampus and cortex receives innervation from cholinergic neurons that originate in the basal forebrain. Death of these neurons and their nerve fibers is an early feature of AD. However, the effect of the loss of cholinergic innervation on the NVU is not well characterized. The purpose of this study was to evaluate the effect of the loss of cholinergic innervation of components of the NVU at capillaries, arteries and veins in the hippocampus and cortex. Adult male C57BL/6 mice received an intracerebroventricular injection of the immunotoxin p75NTR mu-saporin to induce the loss of cholinergic neurons. Quadruple labeling immunohistochemistry and 3D reconstruction were carried out to characterize specific points of contact between cholinergic fibers and collagen IV, smooth muscle cells and astrocyte endfeet. Innate differences were observed between vessels of the hippocampus and cortex of control mice, including a greater amount of cholinergic contact with perivascular astrocytes in hippocampal capillaries and a thicker basement membrane in hippocampal veins. Saporin treatment induced a loss of cholinergic innervation at the arterial basement membrane and smooth muscle cells of both the hippocampus and the cortex. In the cortex, there was an additional loss of innervation at the astrocytic endfeet. The current results suggest that cortical arteries are more strongly affected by cholinergic denervation than arteries in the hippocampus. This regional variation may have implications for the etiology of the vascular pathology that develops in AD.

Exploring the Pathogenesis of Alzheimer Disease in Basal Forebrain Cholinergic Neurons: Converging Insights From Alternative Hypotheses

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary financial and emotional burdens (Apostolova, 2016). Studies of pathogenesis are essential for defining critical molecular and cellular events and for discovering therapies to prevent or mitigate their effects. Through studies of neuropathology, genetic and cellular, and molecular biology recent decades have provided many important insights. Several hypotheses have been suggested. Documentation in the 1980s of selective loss of cholinergic neurons of the basal forebrain, followed by clinical improvement in those treated with inhibitors of acetylycholinesterase, supported the "cholinergic hypothesis of age-related cognitive dysfunction" (Bartus et al., 1982). A second hypothesis, prompted by the selective loss of cholinergic neurons and the discovery of central nervous system (CNS) neurotrophic factors, including nerve growth factor (NGF), prompted the "deficient neurotrophic hypothesis" (Chen et al., 2018). The most persuasive hypothesis, the amyloid cascade hypothesis first proposed more than 25 years ago (Selkoe and Hardy, 2016), is supported by a wealth of observations. Genetic studies were exceptionally important, pointing to increased dose of the gene for the amyloid precursor protein (APP) in Down syndrome (DS) and a familial AD (FAD) due to duplication of APP and to mutations in APP and in the genes for Presenilin 1 and 2 (PSEN1, 2), which encode the γ-secretase enzyme that processes APP (Dorszewska et al., 2016). The "tau hypothesis" noted the prominence of tau-related pathology and its correlation with dementia (Kametani and Hasegawa, 2018). Recent interest in induction of microglial activation in the AD brain, as well as other manifestations of inflammation, supports the "inflammatory hypothesis" (Mcgeer et al., 2016). We place these findings in the context of the selective, but by no means unique, involvement of BFCNs and their trophic dependence on NGF signaling and speculate as to how pathogenesis in these neurons is initiated, amplified and ultimately results in their dysfunction and death. In so doing we attempt to show how the different hypotheses for AD may interact and reinforce one another. Finally, we address current attempts to prevent and/or treat AD in light of advances in understanding pathogenetic mechanisms and suggest that studies in the DS population may provide unique insights into AD pathogenesis and treatment.

Neurotrophin Receptor p75 mRNA Level in Peripheral Blood Cells of Patients with Alzheimer's Disease

The neurotrophin receptor p75 (p75NTR) plays important roles in regulating amyloid-beta (Aβ) metabolism in the brain. The expression of p75NTR is altered in the brain of patients with Alzheimer's disease (AD). In this study, we aimed to evaluate whether p75NTR mRNA level in the peripheral blood cells is changed among AD patients and its potential to be a biomarker for AD. The study subjects included 26 patients with AD (PiB-PET positive) and 28 cognitively normal controls (PiB-PET negative). RNA was extracted from peripheral blood cells of fast blood. p75NTR mRNA was measured using quantitative real-time PCR assay. p75NTR mRNA levels in blood cells were comparable between AD patients and controls. p75NTR mRNA levels in blood cells were not correlated with MMSE scores, ApoE genotypes, gender, and age. p75NTR mRNA expression in blood cells is not changed in AD patients and is unlikely to be a biomarker for AD.

Modulation of the p75 neurotrophin receptor suppresses age-related basal forebrain cholinergic neuron degeneration

Age-related degeneration of basal forebrain cholinergic neurons (BFCNs) is linked to cognitive impairment. The p75 neurotrophin receptor (p75^NTR) has been proposed to mediate neuronal degeneration in aging. Therefore, we tested the hypothesis that modifying p75^NTR function would prevent or reverse aging-related neuronal degeneration using LM11A-31, a small molecule p75^NTR modulator that downregulates degenerative and upregulates trophic receptor-associated signaling. Morphological analysis in mice showed loss of BFCN area detectable by 18 months of age. Oral administration of LM11A-31 from age 15 to 18 months resulted in a dose-related preservation of BFCN area and one month of treatment from 17 to 18 months also preserved cell area. To evaluate reversal of established neuronal atrophy, animals were treated from 21 to 25 months of age. Treatment was associated with an increase of cell size to a mean area larger than that observed at 18 months, accompanied by increases in mean MS/VDB neurite length, as well as increased cholinergic fiber density and synaptophysin pre-synaptic marker levels in the hippocampus. These findings support the idea that modulation of p75^NTR activity can prevent and potentially reverse age-associated BFCN degeneration. Moreover, this may be achieved therapeutically with orally bioavailable agents such as LM11A-31.

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

ProNGF and Neurodegeneration in Alzheimer's Disease

Profound and early basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Alzheimer's disease (AD). Loss of synapses between basal forebrain and hippocampal and cortical target tissue correlates highly with the degree of dementia and is thought to be a major contributor to memory loss. BFCNs depend for their survival, connectivity and function on the neurotrophin nerve growth factor (NGF) which is retrogradely transported from its sites of synthesis in the cortex and hippocampus. The form of NGF found in human brain is proNGF. ProNGF binds to the NGF receptors TrkA and p75^NTR, but it binds more strongly to p75^NTR and more weakly to TrkA than does mature NGF. This renders proNGF more sensitive to receptor balance than mature NGF. In the healthy brain, where BFCNs express both TrkA and p75^NTR, proNGF is neurotrophic, activating TrkA-dependent signaling pathways such as MAPK and Akt-mTOR and eliciting cell survival and neurite outgrowth. However, if TrkA is lost or if p75^NTR is increased, proNGF activates p75^NTR-dependent apoptotic pathways such as JNK. This receptor sensitivity serves as a neurotrophic/apoptotic switch that eliminates BFCNs that cannot maintain TrkA/p75^NTR balance and therefore synaptic connections with their targets. TrkA is increasingly lost in mild cognitive impairment (MCI) and AD. In addition, proNGF accumulates at BFCN terminals in cortex and hippocampus, reducing the amount of trophic factor that reaches BFCN cell bodies. The loss of TrkA and accumulation of proNGF occur early in MCI and correlate with cognitive impairment. Increased levels of proNGF and reduced levels of TrkA lead to BFCN neurodegeneration and eventual p75NTR-dependent apoptosis. In addition, in AD BFCNs suffer from reduced TrkA-dependent retrograde transport which reduces neurotrophic support. Thus, BFCNs are particularly vulnerable to AD due to their dependence upon retrograde trophic support from proNGF signaling and transport.

Nerve Growth Factor Is Responsible for Exercise-Induced Recovery of Septohippocampal Cholinergic Structure and Function

Exercise has been shown to improve or rescue cognitive functioning in both humans and rodents, and the augmented actions of neurotrophins within the hippocampus and associated regions play a significant role in the improved neural plasticity. The septohippocampal circuit is modified by exercise. Beyond an enhancement of spatial working memory and a rescue of hippocampal activity-dependent acetylcholine (ACh) efflux, the re-emergence of the cholinergic/nestin neuronal phenotype within the medial septum/diagonal band (MS/dB) is observed following exercise (Hall and Savage, 2016). To determine which neurotrophin, brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), is critical for exercise-induced cholinergic improvements, control and amnestic rats had either NGF or BDNF sequestered by TrkA-IgG or TrkB-IgG coated microbeads placed within the dorsal hippocampus. Hippocampal ACh release within the hippocampus during spontaneous alternation was measured and MS/dB cholinergic neuronal phenotypes were assessed. Sequestering NGF, but not BDNF, abolished the exercise-induced recovery of spatial working memory and ACh efflux. Furthermore, the re-emergence of the cholinergic/nestin neuronal phenotype within the MS/dB following exercise was also selectively dependent on the actions of NGF. Thus, exercise-induced enhancement of NGF within the septohippocampal pathway represents a key avenue for aiding failing septo-hippocampal functioning and therefore has significant potential for the recovery of memory and cognition in several neurological disorders.

Cholinergic basal forebrain neurons regulate fear extinction consolidation through p75 neurotrophin receptor signaling

Cholinergic basal forebrain (cBF)-derived neurotransmission plays a crucial role in regulating neuronal function throughout the cortex, yet the mechanisms controlling cholinergic innervation to downstream targets have not been elucidated. Here we report that removing the p75 neurotrophin receptor (p75NTR) from cBF neurons induces a significant impairment in fear extinction consolidation. We demonstrate that this is achieved through alterations in synaptic connectivity and functional activity within the medial prefrontal cortex. These deficits revert back to wild-type levels upon re-expression of the active domain of p75NTR in adult animals. These findings demonstrate a novel role for cholinergic neurons in fear extinction consolidation and suggest that neurotrophic signaling is a key regulator of cholinergic-cortical innervation and function.

Complexity and Selectivity of γ-Secretase Cleavage on Multiple Substrates: Consequences in Alzheimer's Disease and Cancer

The processing of the amyloid-β protein precursor (AβPP) by β- and γ-secretases is a pivotal event in the genesis of Alzheimer's disease (AD). Besides familial mutations on the AβPP gene, or upon its overexpression, familial forms of AD are often caused by mutations or deletions in presenilin 1 (PSEN1) and 2 (PSEN2) genes: the catalytic components of the proteolytic enzyme γ-secretase (GS). The "amyloid hypothesis", modified over time, states that the aberrant processing of AβPP by GS induces the formation of specific neurotoxic soluble amyloid-β (Aβ) peptides which, in turn, cause neurodegeneration. This theory, however, has recently evidenced significant limitations and, in particular, the following issues are debated: 1) the concept and significance of presenilin's "gain of function" versus "loss of function"; and 2) the presence of several and various GS substrates, which interact with AβPP and may influence Aβ formation. The latter consideration is suggestive: despite the increasing number of GS substrates so far identified, their reciprocal interaction with AβPP itself, even in the AD field, is significantly unexplored. On the other hand, GS is also an important pharmacological target in the cancer field; inhibitors or GS activity are investigated in clinical trials for treating different tumors. Furthermore, the function of AβPP and PSENs in brain development and in neuronal migration is well known. In this review, we focused on a specific subset of GS substrates that directly interact with AβPP and are involved in its proteolysis and signaling, by evaluating their role in neurodegeneration and in cell motility or proliferation, as a possible connection between AD and cancer.

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

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

Effects of p75NTR deficiency on cholinergic innervation of the amygdala and anxiety-like behavior

The p75 neurotrophin receptor (p75NTR) is a low-affinity receptor that is capable of binding neurotrophins. Two different p75NTR knockout mouse lines are available either with a deletion in Exon III (p75NTRExIII-/- ) or in Exon IV (p75NTRExIV-/- ). In p75NTRExIII knockout mice, only the full-length p75NTR is deleted, whereas in p75NTRExIV knockout mice, the full-length as well as the truncated isoform of the receptor is deleted. Deletion of p75NTR has been shown to affect, among others, the septohippocampal cholinergic innervation pattern and neuronal plasticity within the hippocampus. We hypothesize that deletion of p75NTR also alters the morphology and physiology of a further key structure of the limbic system, the amygdala. Our results indicate that deletion of p75NTR also increases cholinergic innervation in the basolateral amygdala in adult as well as aged p75NTRExIII-/- and p75NTRExIV-/- mice. The p75NTRExIV-/- mice did not display altered long-term potentiation (LTP) in the basolateral amygdala as compared to age-matched control littermates. However, p75NTRExIII-/- mice display stronger LTP in the basolateral amygdala compared to age-matched controls. Bath-application of K252a (a trk antagonist) did not inhibit the induction of LTP in the basolateral amygdala, but reduced the level of LTP in p75NTRExIII-/- mice to levels seen in respective controls. Moreover, p75NTRExIII-/- mice display altered behavior in the dark/light box. Thus, deletion of p75NTR in mice leads to physiological and morphological changes in the amygdala and altered behavior that is linked to the limbic system.

Effect of Estradiol on Neurotrophin Receptors in Basal Forebrain Cholinergic Neurons: Relevance for Alzheimer's Disease

The basal forebrain is home to the largest population of cholinergic neurons in the brain. These neurons are involved in a number of cognitive functions including attention, learning and memory. Basal forebrain cholinergic neurons (BFCNs) are particularly vulnerable in a number of neurological diseases with the most notable being Alzheimer's disease, with evidence for a link between decreasing cholinergic markers and the degree of cognitive impairment. The neurotrophin growth factor system is present on these BFCNs and has been shown to promote survival and differentiation on these neurons. Clinical and animal model studies have demonstrated the neuroprotective effects of 17β-estradiol (E2) on neurodegeneration in BFCNs. It is believed that E2 interacts with neurotrophin signaling on cholinergic neurons to mediate these beneficial effects. Evidence presented in our recent study confirms that altering the levels of circulating E2 levels via ovariectomy and E2 replacement significantly affects the expression of the neurotrophin receptors on BFCN. However, we also showed that E2 differentially regulates neurotrophin receptor expression on BFCNs with effects depending on neurotrophin receptor type and neuroanatomical location. In this review, we aim to survey the current literature to understand the influence of E2 on the neurotrophin system, and the receptors and signaling pathways it mediates on BFCN. In addition, we summarize the physiological and pathophysiological significance of E2 actions on the neurotrophin system in BFCN, especially focusing on changes related to Alzheimer's disease.

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.

Neurotrophin receptor agonists and antagonists as therapeutic agents: An evolving paradigm

Neurodegenerative disorders are prevalent, complex and devastating conditions, with very limited treatment options currently available. While they manifest in many forms, there are commonalities that link them together. In this review, we will focus on neurotrophins - a family of related factors involved in neuronal development and maintenance. Neurodegenerative diseases often present with a neurotrophin imbalance, in which there may be decreases in trophic signaling through Trk receptors for example, and/or increases in pro-apoptotic activity through p75. Clinical trials with neurotrophins have continuously failed due to their poor pharmacological properties as well as the unavoidable activation of p75. Thus, there is a need for drugs without such setbacks. Small molecule neurotrophin mimetics are favorable options since they can selectively activate Trks or inactivate p75. In this review, we will initially present a brief outline of how these molecules are synthesized and their mechanisms of action; followed by an update in the current state of neurotrophins and small molecules in major neurodegenerative diseases. Although there has been significant progress in the development of potential therapeutics, more studies are needed to establish clear mechanisms of action and target specificity in order to transition from animal models to the assessment of safety and use in humans.

Reduced Anxiety-Like Behavior and Altered Hippocampal Morphology in Female p75NTR(exon IV-/-) Mice

The presence of the p75 neurotrophin receptor (p75NTR) in adult basal forebrain cholinergic neurons, precursor cells in the subventricular cell layer and the subgranular cell layer of the hippocampus has been linked to alterations in learning as well as anxiety- and depression- related behaviors. In contrast to previous studies performed in a p75NTR^{exon III−/−} model still expressing the short isoform of the p75NTR, we focused on locomotor and anxiety–associated behavior in p75NTR^{exon IV−/−} mice lacking both p75NTR isoforms. Comparing p75NTR^{exon IV−/−} and wildtype mice for both male and female animals showed an anxiolytic-like behavior as evidenced by increased central activities in the open field paradigm and flex field activity system as well as higher numbers of open arm entries in the elevated plus maze test in female p75NTR knockout mice. Morphometrical analyses of dorsal and ventral hippocampus revealed a reduction of width of the dentate gyrus and the granular cell layer in the dorsal but not ventral hippocampus in male and female p75NTR^{exon IV−/−} mice. We conclude that germ-line deletion of p75NTR seems to differentially affect morphometry of dorsal and ventral dentate gyrus and that p75NTR may play a role in anxiety-like behavior, specifically in female mice.

Treatment of beta amyloid 1-42 (Aβ(1-42))-induced basal forebrain cholinergic damage by a non-classical estrogen signaling activator in vivo

In Alzheimer's disease (AD), there is a loss in cholinergic innervation targets of basal forebrain which has been implicated in substantial cognitive decline. Amyloid beta peptide (Aβ(1-42)) accumulates in AD that is highly toxic for basal forebrain cholinergic (BFC) neurons. Although the gonadal steroid estradiol is neuroprotective, the administration is associated with risk of off-target effects. Previous findings suggested that non-classical estradiol action on intracellular signaling pathways has ameliorative potential without estrogenic side effects. After Aβ(1-42) injection into mouse basal forebrain, a single dose of 4-estren-3α, 17β-diol (estren), the non-classical estradiol pathway activator, restored loss of cholinergic cortical projections and also attenuated the Aβ(1-42)-induced learning deficits. Estren rapidly and directly phosphorylates c-AMP-response-element-binding-protein and extracellular-signal-regulated-kinase-1/2 in BFC neurons and restores the cholinergic fibers via estrogen receptor-α. These findings indicated that selective activation of non-classical intracellular estrogen signaling has a potential to treat the damage of cholinergic neurons in AD.

Ventromedial hypothalamic expression of Bdnf is required to establish normal patterns of afferent GABAergic connectivity and responses to hypoglycemia

OBJECTIVE

The ventromedial nucleus of the hypothalamus (VMH) controls energy and glucose homeostasis through direct connections to a distributed network of nuclei in the hypothalamus, midbrain, and hindbrain. Structural changes in VMH circuit morphology have the potential to alter VMH function throughout life, however, molecular signals responsible for specifying its neural connections are not fully defined. The VMH contains a high density of neurons that express brain-derived neurotrophic factor (BDNF), a potent neurodevelopmental effector known to regulate neuronal survival, growth, differentiation, and connectivity in a number of neural systems. In the current study, we examined whether BDNF impacts the afferent and efferent connections of the VMH, as well as energy homeostatic function.

METHODS

To determine if BDNF is required for VMH circuit formation, a transgenic mouse model was used to conditionally delete Bdnf from steroidogenic factor 1 (SF1) expressing neurons of the VMH prior to the onset of establishing neural connections with other regions. Projections of SF1 expressing neurons were visualized with a genetically targeted fluorescent label and immunofluorescence was used to measure the density of afferents to SF1 neurons in the absence of BDNF. Physiological changes in body weight and circulating blood glucose were also evaluated in the mutant mice.

RESULTS

Our findings suggest that BDNF is required to establish normal densities of GABAergic afferents onto SF1 neurons located in the ventrolateral part of the VMH. Furthermore, loss of BDNF from VMH SF1 neurons results in impaired physiological responses to insulin-induced hypoglycemia.

CONCLUSION

The results of this study indicate that BDNF is required for formation and/or maintenance of inhibitory inputs to SF1 neurons, with enduring effects on glycemic control.

Deletion of p75NTR enhances the cholinergic innervation pattern of the visual cortex

The cholinergic system is involved in cortical plasticity, attention, and learning. Within the visual cortex the cholinergic system seems to play a role in visual perception. The cholinergic neurons which project into the visual cortex are located in the basal forebrain. It has been shown that mice deficient for the low-affinity neurotrophin receptor p75NTR display increased numbers of cholinergic neurons in the basal forebrain and a denser cholinergic innervation of the hippocampus. This prompted us to analyze whether the cholinergic system is altered in adult p75NTR deficient mice. By analyzing the densities of cholinergic fibers within layer IV as well as within layer V of the visual cortex, we found that adult p75NTR deficient mice display increased cholinergic fiber densities. However, this increase was not accompanied by an increase in the density of local cholinergic neurons within the visual cortex. This indicates that the enhanced cholinergic innervation of the visual cortex is due to alteration of the cholinergic neurons located in the basal forebrain, projecting to the visual cortex. The increased cholinergic innervation of the visual cortex makes the p75NTR deficient mice an attractive model to study the necessity of the cholinergic system for the visual cortex.

Impact of a deletion of the full-length and short isoform of p75NTR on cholinergic innervation and the population of postmitotic doublecortin positive cells in the dentate gyrus

Analyses of mice carrying a deletion of the pan-neurotrophin receptor p75NTR have allowed identifying p75NTR as an important structural regulator of the hippocampus. Most of the previous analyses were done using p75NTR (ExIII) knockout mice which still express the short isoform of p75NTR. To scrutinize the role of p75NTR in the hippocampus, we analyzed adult and aged p75NTR (ExIV) knockout mice, in which both, the short and the full-length isoform are deleted. Deletion of these isoforms induced morphological alterations in the adult dentate gyrus (DG), leading to an increase in the thickness of the molecular and granular layer. Based on these observations, we next determined the morphological substrates that might contribute to this phenotype. The cholinergic innervation of the molecular and granular layer of the DG was found to be significantly increased in the knockout mice. Furthermore, adult neurogenesis in the DG was found to be significantly altered with increased numbers of doublecortin (DCX) positive cells and reduced numbers of apoptotic cells in p75NTR (ExIV) knockout mice. However, cell proliferation as measured by phosphohiston H3 (PH3) positive cell numbers was not affected. These morphological alterations (number of DCX-positive cells and increased cholinergic fiber densities) as well as reduced cell death in the DG are likely to contribute to the observed thickening of the granular layer in p75NTR (ExIV) knockout mice. In addition, Sholl-analysis of DCX-positive neurons revealed a higher dendritic complexity and could thus be a possible morphological correlate for the increased thickness of the molecular layer in p75NTR deficient animals. Our data clearly demonstrate that deletion of both, the short and the full-length isoform of p75NTR affects DG morphology, due to alterations of the cholinergic system and an imbalance between neurogenesis and programmed cell death within the subgranular zone.

Reduction of p75 neurotrophin receptor ameliorates the cognitive deficits in a model of Alzheimer's disease

Alzheimer's disease (AD) is an extremely prevalent cause of dementia. It is characterized by progressive memory loss, confusion, and other behavioral and physiological problems. The amyloid-β (Aβ) protein is thought to be involved in the pathogenesis of AD, and there is evidence that Aβ may act through the p75 neurotrophin receptor (p75) to mediate its pathogenic effects. This raises the possibility that reducing levels of p75 could be a treatment for AD by preventing the effects of Aβ. In this study, we have crossed the transgenic AD model mice, Tg2576, with p75(-/-) mice to generate Tg2576/p75(+/-) mice with reduced levels of p75. These mice are rescued from the deficits in learning and memory and hippocampal function which were found in the Tg2576 mice. These findings suggest that reduction of p75 can ameliorate some of the primary symptoms of AD.

The Binding Receptors of Aβ: an Alternative Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders, which causes the deterioration of memory and other cognitive abilities of the elderly. Previous lines of research have shown that Aβ is an essential factor in AD pathology and the soluble oligomeric species of Aβ peptide is presumed to be the drivers of synaptic impairment in AD. However, the exact mechanisms underlying Aβ-induced synapse dysfunction are still not fully understood. Recently, increasing evidence suggests that some potential receptors which bind specifically with Aβ may play important roles in inducing the toxicity of the neurons in AD pathology. These receptors include the cellular prion protein (PrPc), the α7 nicotinic acetylcholine receptor (α7nAChR), the p75 neurotrophin receptor (p75(NTR)), the beta-adrenergic receptors (β-ARs), the Eph receptors, the paired immunoglobulin-like receptor B (PirB), the PirB's human ortholog receptor (LilrB2), and the Fcγ receptor II-b (FcγRIIb). This review summarizes the characters of these prominent receptors and how the bindings of them with Aβ inhibit the LTP, decrease the number of dendritic spine, damage the neurons, and so on in AD pathogenesis. Blocking or rescuing these receptors may have significant importance for AD treatments.

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.

TNF-α/TNFR1 signaling is required for the development and function of primary nociceptors

Primary nociceptors relay painful touch information from the periphery to the spinal cord. Although it is established that signals generated by receptor tyrosine kinases TrkA and Ret coordinate the development of distinct nociceptive circuits, mechanisms modulating TrkA or Ret pathways in developing nociceptors are unknown. We have identified tumor necrosis factor (TNF) receptor 1 (TNFR1) as a critical modifier of TrkA and Ret signaling in peptidergic and nonpeptidergic nociceptors. Specifically, TrkA+ peptidergic nociceptors require TNF-α-TNFR1 forward signaling to suppress nerve growth factor (NGF)-mediated neurite growth, survival, excitability, and differentiation. Conversely, TNFR1-TNF-α reverse signaling augments the neurite growth and excitability of Ret+ nonpeptidergic nociceptors. The developmental and functional nociceptive defects associated with loss of TNFR1 signaling manifest behaviorally as lower pain thresholds caused by increased sensitivity to NGF. Thus, TNFR1 exerts a dual role in nociceptor information processing by suppressing TrkA and enhancing Ret signaling in peptidergic and nonpeptidergic nociceptors, respectively.

Implications of p75NTR for dentate gyrus morphology and hippocampus-related behavior revisited

The pan-neurotrophin receptor p75NTR is expressed in the adult brain in a discrete pattern. Although numerous studies have addressed its implications for hippocampal functions, the generated sets of data are surprisingly conflicting. We have therefore set out to re-investigate the impact of a deletion of the full-length p75NTR receptor on several parameters of the dentate gyrus (DG), including neurogenesis and hippocampus-related behavior by using p75NTR(ExIII) knockout mice. Moreover, we investigated further parameters of the DG (cholinergic innervation, dendritic spines). In addition, we analyzed on the morphological level the impact of aging by comparing adult and aged p75NTR(ExIII) mice and their age-matched littermates. Adult (4-6 months old), but not aged (20 months old), p75NTR(ExIII) knockout mice display an enhanced volume of the DG. However, adult neurogenesis within the adult DG was unaffected in both adult and aged p75NTR(ExIII) knockout mice. We could further demonstrate that the change in the volume of the DG was accompanied by an increased cholinergic innervation and increased spine densities of granule cells in adult, but not aged p75NTR deficient mice. These morphological changes in the adult p75NTR deficient mice were accompanied by specific alterations in their behavior, including altered behavior in the Morris water maze test, indicating impairments in spatial memory retention.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

TAp73 knockout mice show morphological and functional nervous system defects associated with loss of p75 neurotrophin receptor

Total and N-terminal isoform selective p73 knockout mice show a variety of central nervous system defects. Here we show that TAp73 is a transcriptional activator of p75 neurotrophin receptor (p75(NTR)) and that p75(NTR) mRNA and protein levels are strongly reduced in the central and peripheral nervous systems of p73 knockout mice. In parallel, primary cortical neurons from p73 knockout mice showed a reduction in neurite outgrowth and in nerve growth factor-mediated neuronal differentiation, together with reduced miniature excitatory postsynaptic current frequencies and behavioral defects. p73 null mice also have impairments in the peripheral nervous system with reduced thermal sensitivity, axon number, and myelin thickness. At least some of these morphological and functional impairments in p73 null cells can be rescued by p75(NTR) re-expression. Together, these data demonstrate that loss of p75(NTR) contributes to the neurological phenotype of p73 knockout 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.

The p75 neurotrophin receptor has nonapoptotic antineurotrophic actions in the basal forebrain

Because of controversy about the role of the p75 neurotrophin receptor (p75(NTR) ) in the cholinergic basal forebrain (CBF), we investigated this region in p75(NTR) third exon knockout mice that were congenic with 129/Sv controls. They express a shortened intracellular form of p75(NTR) , permitting detection of p75(NTR) -expressing cells. We performed separate counts of choline acetyltransferase (ChAT)-expressing and p75(NTR) -expressing neurons. In agreement with past reports, the number of ChAT-immunoreactive neurons in knockout mice was greater than in wild-type mice, and this was evident in each of the main anatomical divisions of the CBF. In contrast, the number of p75(NTR) -immunoreactive neurons did not differ between genotypes. The biggest increase in ChAT neurons (27%) was in the horizontal limb of the diagonal band of Broca (HDB), in which region the number of p75(NTR) -positive neurons was unchanged. Double staining revealed that some neurons in wild-type mice expressed p75(NTR) but not ChAT. In the knockout mice, all p75(NTR) -expressing neurons expressed ChAT. The increase in cholinergic neurons, therefore, was at least partially attributable to a higher proportion of ChAT immunoreactivity within the population of p75(NTR) -expressing neurons. Cholinergic neurons were also larger in knockout mice than in controls. In the hippocampal CA1 region, knockout mice had a greater number of cholinergic fibers. There was a 77% increase in hippocampal ChAT activity in knockout mice and a 38% increase in heterozygotes. The data do not support an apoptotic role but indicate a broad antineurotrophic role of p75(NTR) in the cholinergic basal forebrain.

Postlesion estradiol treatment increases cortical cholinergic innervations via estrogen receptor-α dependent nonclassical estrogen signaling in vivo

17β-Estradiol (E2) treatment exerts rapid, nonclassical actions via intracellular signal transduction system in basal forebrain cholinergic (BFC) neurons in vivo. Here we examined the effect of E2 treatment on lesioned BFC neurons in ovariectomized mice and the role of E2-induced nonclassical action in this treatment. Mice given an N-methyl-d-aspartic acid (NMDA) injection into the substantia innominata-nucleus basalis magnocellularis complex (SI-NBM) exhibited cholinergic cell loss in the SI-NBM and ipsilateral cholinergic fiber loss in the cortex. A single injection of E2 after NMDA lesion did not have an effect on cholinergic cell loss in the SI-NBM, but it restored the ipsilateral cholinergic fiber density in the cortex in a time- and dose-dependent manner. The most effective cholinergic fiber restoration was observed with 33 ng/g E2 treatment at 1 h after NMDA lesion. The E2-induced cholinergic fiber restoration was absent in neuron-specific estrogen receptor-α knockout mice in vivo. Selective activation of nonclassical estrogen signaling in vivo by estren induced E2-like restorative actions. Selective blockade of the MAPK or protein kinase A pathway in vivo prevented E2's ability to restore cholinergic fiber loss. Finally, studies in intact female mice revealed an E2-induced restorative effect that was similar to that of E2-treated ovariectomized mice. These observations demonstrate that a single E2 treatment restores the BFC fiber loss in the cortex, regardless of endogenous E2 levels. They also reveal the critical role of nonclassical estrogen signaling via estrogen receptor-α and protein kinase A-MAPK pathways in E2-induced restorative action in the cholinergic system in vivo.

The role of tumor necrosis factor receptor superfamily members in mammalian brain development, function and homeostasis

Tumor necrosis factor receptor superfamily (TNFRSF) members were initially identified as immunological mediators, and are still commonly perceived as immunological molecules. However, our understanding of the diversity of TNFRSF members' roles in mammalian physiology has grown significantly since the first discovery of TNFRp55 (TNFRSF1) in 1975. In particular, the last decade has provided evidence for important roles in brain development, function and the emergent field of neuronal homeostasis. Recent evidence suggests that TNFRSF members are expressed in an overlapping regulated pattern during neuronal development, participating in the regulation of neuronal expansion, growth, differentiation and regional pattern development. This review examines evidence for non-immunological roles of TNFRSF members in brain development, function and maintenance under normal physiological conditions. In addition, several aspects of brain function during inflammation will also be described, when illuminating and relevant to the non-immunological role of TNFRSF members. Finally, key questions in the field will be outlined.

Apoptosis and in vitro Alzheimer disease neuronal models

Alzheimer disease (AD) is a human neurodegenerative disease characterized by co-existence of extracellular senile plaques (SP) and neurofibrillary tangles (NFT) associated with an extensive neuronal loss, primarily in the cerebral cortex and hippocampus. Several studies suggest that caspase(s)-mediated neuronal death occurs in cellular and animal AD models as well as in human brains of affected patients, although an etiologic role of apoptosis in such neurodegenerative disorder is still debated. This review summarizes the experimental evidences corroborating the possible involvement of apoptosis in AD pathogenesis and discusses the usefulness of ad hoc devised in vitro approaches to study how caspase(s), amyloidogenic processing and tau metabolism might reciprocally interact leading to neuronal death.