NGF and BDNF signaling control amyloidogenic route and Abeta production in hippocampal neurons

Basic approach on the protective effects of hesperidin and naringin in Alzheimer's disease

OBJECTIVES

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive impairment. This situation imposes a great burden on individuals, both economically and socially. Today, an effective method for treating the disease and protective approach to tau accumulation has not been developed yet. Studies have been conducted on the effects of hesperidin and naringin flavonoids found in citrus fruits on many diseases.

METHODS

In this review, the pathophysiology of AD is defined, and the effects of hesperidin and naringin on these factors are summarized.

RESULTS

Studies have shown that both components may potentially affect AD due to their antioxidative and anti-inflammatory properties. Based on these effects of the components, it has been shown that they may have ameliorative effects on Aβ, α-synuclein aggregation, tau pathology, and cognitive functions in the pathophysiology of AD.

DISCUSSION

There are studies suggesting that hesperidin and naringin may be effective in the prevention/treatment of AD. When these studies are examined, it is seen that more studies should be conducted on the subject.

New BDNF and NT-3 Cyclic Mimetics Concur with Copper to Activate Trophic Signaling Pathways as Potential Molecular Entities to Protect Old Brains from Neurodegeneration

A low level of Neurotrophins (NTs), their Tyrosine Kinase Receptors (Trks), Vascular Endothelial Growth Factors (VEGFs) and their receptors, mainly VEGFR1 and VEGFR2, characterizes AD brains. The use of NTs and VEGFs as drugs presents different issues due to their low permeability of the blood-brain barrier, the poor pharmacokinetic profile, and the relevant side effects. To overcome these issues, different functional and structural NT mimics have been employed. Being aware that the N-terminus domain as the key domain of NTs for the binding selectivity and activation of Trks and the need to avoid or delay proteolysis, we herein report on the mimicking ability of two cyclic peptide encompassing the N-terminus of Brain Derived Growth Factor (BDNF), (c-[HSDPARRGELSV-]), cBDNF(1-12) and of Neurotrophin3 (NT3), (c-[YAEHKSHRGEYSV-]), cNT3(1-13). The two cyclic peptide features were characterized by a combined thermodynamic and spectroscopic approach (potentiometry, NMR, UV-vis and CD) that was extended to their copper(II) ion complexes. SH-SY5Y cell assays show that the Cu2+ present at the sub-micromolar level in the complete culture media affects the treatments with the two peptides. cBDNF(1-12) and cNT3(1-13) act as ionophores, induce neuronal differentiation and promote Trks and CREB phosphorylation in a copper dependent manner. Consistently, both peptide and Cu2+ stimulate BDNF and VEGF expression as well as VEGF release; cBDNF(1-12) and cNT3(1-13) induce the expression of Trks and VEGFRs.

Recent advances in Alzheimer's disease: Mechanisms, clinical trials and new drug development strategies

Alzheimer's disease (AD) stands as the predominant form of dementia, presenting significant and escalating global challenges. Its etiology is intricate and diverse, stemming from a combination of factors such as aging, genetics, and environment. Our current understanding of AD pathologies involves various hypotheses, such as the cholinergic, amyloid, tau protein, inflammatory, oxidative stress, metal ion, glutamate excitotoxicity, microbiota-gut-brain axis, and abnormal autophagy. Nonetheless, unraveling the interplay among these pathological aspects and pinpointing the primary initiators of AD require further elucidation and validation. In the past decades, most clinical drugs have been discontinued due to limited effectiveness or adverse effects. Presently, available drugs primarily offer symptomatic relief and often accompanied by undesirable side effects. However, recent approvals of aducanumab (1) and lecanemab (2) by the Food and Drug Administration (FDA) present the potential in disrease-modifying effects. Nevertheless, the long-term efficacy and safety of these drugs need further validation. Consequently, the quest for safer and more effective AD drugs persists as a formidable and pressing task. This review discusses the current understanding of AD pathogenesis, advances in diagnostic biomarkers, the latest updates of clinical trials, and emerging technologies for AD drug development. We highlight recent progress in the discovery of selective inhibitors, dual-target inhibitors, allosteric modulators, covalent inhibitors, proteolysis-targeting chimeras (PROTACs), and protein-protein interaction (PPI) modulators. Our goal is to provide insights into the prospective development and clinical application of novel AD drugs.

Cell-Penetrating Peptides: Promising Therapeutics and Drug-Delivery Systems for Neurodegenerative Diseases

Currently, one of the most significant and rapidly growing unmet medical challenges is the treatment of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). This challenge encompasses the imperative development of efficacious therapeutic agents and overcoming the intricacies of the blood-brain barrier for successful drug delivery. Here we focus on the delivery aspect with particular emphasis on cell-penetrating peptides (CPPs), widely used in basic and translational research as they enhance drug delivery to challenging targets such as tissue and cellular compartments and thus increase therapeutic efficacy. The combination of CPPs with nanomaterials such as nanoparticles (NPs) improves the performance, accuracy, and stability of drug delivery and enables higher drug loads. Our review presents and discusses research that utilizes CPPs, either alone or in conjugation with NPs, to mitigate the pathogenic effects of neurodegenerative diseases with particular reference to AD and PD.

Histone acetylation in an Alzheimer's disease cell model promotes homeostatic amyloid-reducing pathways

Alzheimer's Disease (AD) is a disorder characterized by cognitive decline, neurodegeneration, and accumulation of amyloid plaques and tau neurofibrillary tangles in the brain. Dysregulation of epigenetic histone modifications may lead to expression of transcriptional programs that play a role either in protecting against disease genesis or in worsening of disease pathology. One such histone modification, acetylation of histone H3 lysine residue 27 (H3K27ac), is primarily localized to genomic enhancer regions and promotes active gene transcription. We previously discovered H3K27ac to be more abundant in AD patient brain tissue compared to the brains of age-matched non-demented controls. In this study, we use iPSC-neurons derived from familial AD patients with an amyloid precursor protein (APP) duplication (APPDup neurons) as a model to study the functional effect of lowering CBP/P300 enzymes that catalyze H3K27ac. We found that homeostatic amyloid-reducing genes were upregulated in the APPDup neurons compared to non-demented controls. We lowered CBP/P300 to reduce H3K27ac, which led to decreased expression of numerous of these homeostatic amyloid-reducing genes, along with increased extracellular secretion of a toxic amyloid-β species, Aβ(1-42). Our findings suggest that epigenomic histone acetylation, including H3K27ac, drives expression of compensatory genetic programs in response to AD-associated insults, specifically those resulting from APP duplication, and thus may play a role in mitigating AD pathology in neurons.

Mechanisms of the Beneficial Effects of Exercise on Brain-Derived Neurotrophic Factor Expression in Alzheimer's Disease

Brain-derived neurotrophic factor (BDNF) is a key molecule in promoting neurogenesis, dendritic and synaptic health, neuronal survival, plasticity, and excitability, all of which are disrupted in neurological and cognitive disorders such as Alzheimer's disease (AD). Extracellular aggregates of amyloid-β (Aβ) in the form of plaques and intracellular aggregates of hyperphosphorylated tau protein have been identified as major pathological insults in the AD brain, along with immune dysfunction, oxidative stress, and other toxic stressors. Although aggregated Aβ and tau lead to decreased brain BDNF expression, early losses in BDNF prior to plaque and tangle formation may be due to other insults such as oxidative stress and contribute to early synaptic dysfunction. Physical exercise, on the other hand, protects synaptic and neuronal structure and function, with increased BDNF as a major mediator of exercise-induced enhancements in cognitive function. Here, we review recent literature on the mechanisms behind exercise-induced BDNF upregulation and its effects on improving learning and memory and on Alzheimer's disease pathology. Exercise releases into the circulation a host of hormones and factors from a variety of peripheral tissues. Mechanisms of BDNF induction discussed here are osteocalcin, FNDC5/irisin, and lactate. The fundamental mechanisms of how exercise impacts BDNF and cognition are not yet fully understood but are a prerequisite to developing new biomarkers and therapies to delay or prevent cognitive decline.

Neurocognitive impairment in females with breast cancer treated with endocrine therapy and CDK4/6 inhibitors: a pharmacovigilance study using the World Health Organization's database

Importance: Endocrine therapies (ETs) and inhibitors of cyclin-dependent kinases-4/6 (iCDK4/6s) are a standard treatment in breast cancer. However, data on potential neurocognitive impacts remain inconsistent for ET and are scarce for iCDK4/6s. Objective: To evaluate whether ET and iCDK4/6s are associated with neurocognitive impairment (NCI). Methods: We used observational, real-world cases of NCI from the World Health Organization's database VigiBase® to perform disproportionality analysis. Cases were defined as any symptom of NCI in females treated with ETs or iCDK4/6s. The study period was from the date of the first adverse event reported in VigiBase® with iCDK4/6s (1 January 2014) until the date of data extraction (16 March 2022). In our primary analysis, we calculated the reporting odds ratio (ROR) adjusted for age to identify a potential association between NCI and individual ETs in isolation or in combination with iCDK4/6s. We also performed subgroup analyses by the NCI class. Results: We identified 2.582 and 1.943 reports of NCI associated with ETs and iCDK4/6s, respectively. NCI was significantly associated with each ET [anastrozole: n = 405, aROR = 1.52 (95% CI: 1.37-1.67); letrozole: n = 741, aROR = 1.37 (95% CI: 1.27-1.47); exemestane: n = 316, aROR = 1.37 (95% CI: 1.22-1.53); tamoxifen: n = 311, aROR = 1.25 (95% CI: 1.12-1.40); and fulvestrant: n = 319, aROR = 1.19 (95% CI: 1.06-1.33)] and only with palbociclib for iCDK4/6s [n = 1,542, aROR = 1.41 (95% CI: 1.34-1.48)]. Conclusion: These findings suggest that in females treated for breast cancer, all ETs may be associated with NCI. However, amongst iCDK4/6s, NCI may be specific to palbociclib. NCI most frequently involved learning and memory as well as language. Neurocognitive impact of treatments requires better consideration and management.

Naringin enhances long-term potentiation and recovers learning and memory deficits of amyloid-beta induced Alzheimer's disease-like behavioral rat model

Alzheimer's disease (AD), as the most typical type of dementia, is a chronic neurodegenerative disorder characterized by progressive learning and memory impairment. It is known that the main causes of AD are the accumulation of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFT) containing hyperphosphorylated tau protein. Naringin is a flavonoid from citrus fruits, especially in grapefruit, which has anti-inflammatory, antioxidant, anti-apoptotic, and neuroprotective activities. However, the effect of naringin in AD caused by Aβ has not been clearly studied, and there are few studies on the electrophysiological aspect. Thus, we investigated the ex vivo neuroprotective effect of naringin through the long-term potentiation (LTP) on organotypic hippocampal slice cultures. We evaluated the in vivo effects of naringin (100 mg/kg/day) orally treated for 20 days on learning, memory, and cognition which was impaired by bilateral CA1 subregion injection of Aβ. Cognitive behaviors were measured 2 weeks after Aβ injection using behavioral tests and the hippocampal expression of apoptotic and neurotrophic regulators were measured by immunoblotting. In hippocampal tissue slices, naringin dose-dependently increased the field excitatory postsynaptic potential (fEPSP) after theta burst stimulation and attenuated Aβ-induced blockade of fEPSP in the hippocampal CA1 area. In Aβ injected rats, naringin improved object recognition memory in the novel object test, avoidance memory in the passive avoidance test and spatial recognition memory in the Morris water maze test. In the hippocampus, naringin attenuated the Aβ-induced cyclooxygenase-2, Bax activation and Bcl-2, CREB, BDNF and TrkB inhibition. These results suggest that naringin has therapeutic potential to reduce neuronal inflammation and apoptosis induced by Aβ related with the BDNF/TrkB/CREB signaling.

Cross interactions between Apolipoprotein E and amyloid proteins in neurodegenerative diseases

Three common Apolipoprotein E isoforms, ApoE2, ApoE3, and ApoE4, are key regulators of lipid homeostasis, among other functions. Apolipoprotein E can interact with amyloid proteins. The isoforms differ by one or two residues at positions 112 and 158, and possess distinct structural conformations and functions, leading to isoform-specific roles in amyloid-based neurodegenerative diseases. Over 30 different amyloid proteins have been found to share similar characteristics of structure and toxicity, suggesting a common interactome. The molecular and genetic interactions of ApoE with amyloid proteins have been extensively studied in neurodegenerative diseases, but have not yet been well connected and clarified. Here we summarize essential features of the interactions between ApoE and different amyloid proteins, identify gaps in the understanding of the interactome and propose the general interaction mechanism between ApoE isoforms and amyloid proteins. Perhaps more importantly, this review outlines what we can learn from the interactome of ApoE and amyloid proteins; that is the need to see both ApoE and amyloid proteins as a basis to understand neurodegenerative diseases.

AChE as a spark in the Alzheimer's blaze - Antagonizing effect of a cyclized variant

The amyloid precursor protein (APP), presenilin 1 (PS1), amyloid beta (Aβ), and GSK3 are the effectors, which are significantly associated with progression of Alzheimer's Disease (AD) and its symptoms. A significant protein, acetylcholinesterase (AChE) becomes dysfunctional as a result of cholinergic neuronal loss in AD pathology. However, certain associated peptides potentiate the release of primary neuropathological hallmarks, i.e., senile plaque and neurofibrillary tangles (NFTs), by modulating the alpha 7 acetylcholinesterase receptor (α7nAChR). The AChE variants, T30 and T14 have also been found to be elevated in AD patients and mimic the toxic actions of pathological events in patients. The manuscript discusses the significance of AChE inhibitors in AD therapeutics, by indicating the disastrous role of molecular alterations and elevation of AChE, accompanied with the downstream effects instigated by the peptide, supported by clinical evidence and investigations. The cyclized variant of AChE peptide, NBP14 has been identified as a novel candidate that reverses the harmful effects of T30, T14 and Aβ, mainly calcium influx, cell viability and AChE release. The review aims to grab the attention of neuro-researchers towards the significance of triggering effectors in propagating AD and role of AChE in regulating them, which can potentially ace the development of reliable therapeutic candidates, similar to NBP14, to mitigate neurodegeneration.

Reduced levels of NGF shift astrocytes toward a neurotoxic phenotype

Nerve growth factor (NGF) is critical for neuronal physiology during development and adulthood. Despite the well-recognized effect of NGF on neurons, less is known about whether NGF can actually affect other cell types in the central nervous system (CNS). In this work, we show that astrocytes are susceptible to changes in ambient levels of NGF. First, we observe that interfering with NGF signaling in vivo via the constitutive expression of an antiNGF antibody induces astrocytic atrophy. A similar asthenic phenotype is encountered in an uncleavable proNGF transgenic mouse model (TgproNGF#72), effectively increasing the brain proNGF levels. To examine whether this effect on astrocytes is cell-autonomous, we cultured wild-type primary astrocytes in the presence of antiNGF antibodies, uncovering that a short incubation period is sufficient to potently and rapidly trigger calcium oscillations. Acute induction of calcium oscillations by antiNGF antibodies is followed by progressive morphological changes similar to those observed in antiNGF AD11 mice. Conversely, incubation with mature NGF has no effect on either calcium activity nor on astrocytic morphology. At longer timescales, transcriptomic analysis revealed that NGF-deprived astrocytes acquire a proinflammatory profile. In particular, antiNGF-treated astrocytes show upregulation of neurotoxic transcripts and downregulation of neuroprotective mRNAs. Consistent with that data, culturing wild-type neurons in the presence of NGF-deprived astrocytes leads to neuronal cell death. Finally, we report that in both awake and anesthetized mice, astrocytes in layer I of the motor cortex respond with an increase in calcium activity to acute NGF inhibition using either NGF-neutralizing antibodies or a TrkA-Fc NGF scavenger. Moreover, in vivo calcium imaging in the cortex of the 5xFAD neurodegeneration mouse model shows an increased level of spontaneous calcium activity in astrocytes, which is significantly reduced after acute administration of NGF. In conclusion, we unveil a novel neurotoxic mechanism driven by astrocytes, triggered by their sensing and reacting to changes in the levels of ambient NGF.

Finding memo: versatile interactions of the VPS10p-Domain receptors in Alzheimer’s disease

The family of VPS10p-Domain (D) receptors comprises five members named SorLA, Sortilin, SorCS1, SorCS2 and SorCS3. While their physiological roles remain incompletely resolved, they have been recognized for their signaling engagements and trafficking abilities, navigating a number of molecules between endosome, Golgi compartments, and the cell surface. Strikingly, recent studies connected all the VPS10p-D receptors to Alzheimer’s disease (AD) development. In addition, they have been also associated with diseases comorbid with AD such as diabetes mellitus and major depressive disorder. This systematic review elaborates on genetic, functional, and mechanistic insights into how dysfunction in VPS10p-D receptors may contribute to AD etiology, AD onset diversity, and AD comorbidities. Starting with their functions in controlling cellular trafficking of amyloid precursor protein and the metabolism of the amyloid beta peptide, we present and exemplify how these receptors, despite being structurally similar, regulate various and distinct cellular events involved in AD. This includes a plethora of signaling crosstalks that impact on neuronal survival, neuronal wiring, neuronal polarity, and synaptic plasticity. Signaling activities of the VPS10p-D receptors are especially linked, but not limited to, the regulation of neuronal fitness and apoptosis via their physical interaction with pro- and mature neurotrophins and their receptors. By compiling the functional versatility of VPS10p-D receptors and their interactions with AD-related pathways, we aim to further propel the AD research towards VPS10p-D receptor family, knowledge that may lead to new diagnostic markers and therapeutic strategies for AD patients.

The relationships between neuroglial and neuronal changes in Alzheimer's disease, and the related controversies II:

Since the original description of Alzheimer´s disease (AD), research into this condition has mainly focused on assessing the alterations to neurons associated with dementia, and those to the circuits in which they are involved. In most of the studies on human brains and in many models of AD, the glial cells accompanying these neurons undergo concomitant alterations that aggravate the course of neurodegeneration. As a result, these changes to neuroglial cells are now included in all the "pathogenic cascades" described in AD. Accordingly, astrogliosis and microgliosis, the main components of neuroinflammation, have been integrated into all the pathogenic theories of this disease, as discussed in this part of the two-part monograph that follows an accompanying article on gliopathogenesis and glioprotection. This initial reflection verified the implication of alterations to the neuroglia in AD, suggesting that these cells may also represent therapeutic targets to prevent neurodegeneration. In this second part of the monograph, we will analyze the possibilities of acting on glial cells to prevent or treat the neurodegeneration that is the hallmark of AD and other pathologies. Evidence of the potential of different pharmacological, non-pharmacological, cell and gene therapies (widely treated) to prevent or treat this disease is now forthcoming, in most cases as adjuncts to other therapies. A comprehensive AD multimodal therapy is proposed in which neuronal and neuroglial pharmacological treatments are jointly considered, as well as the use of new cell and gene therapies and non-pharmacological therapies that tend to slow down the progress of dementia.

Amyloidogenesis and Neurotrophic Dysfunction in Alzheimer’s Disease: Do They have a Common Regulating Pathway?

The amyloid cascade hypothesis has predominately been used to describe the pathogenesis of Alzheimer’s disease (AD) for decades, as Aβ oligomers are thought to be the prime cause of AD. Meanwhile, the neurotrophic factor hypothesis has also been proposed for decades. Accumulating evidence states that the amyloidogenic process and neurotrophic dysfunction are mutually influenced and may coincidently cause the onset and progress of AD. Meanwhile, there are intracellular regulators participating both in the amyloidogenic process and neurotrophic pathways, which might be the common original causes of amyloidogenesis and neurotrophic dysfunction. In this review, the current understanding regarding the role of neurotrophic dysfunction and the amyloidogenic process in AD pathology is briefly summarized. The mutual influence of these two pathogenesis pathways and their potential common causal pathway are further discussed. Therapeutic strategies targeting the common pathways to simultaneously prevent amyloidogenesis and neurotrophic dysfunction might be anticipated for the disease-modifying treatment of AD.

Regulation of the Soluble Amyloid Precursor Protein α (sAPPα) Levels by Acetylcholinesterase and Brain-Derived Neurotrophic Factor in Lung Cancer Cell Media

In comparing two human lung cancer cells, we previously found lower levels of acetylcholinesterase (AChE) and intact amyloid-β40/42 (Aβ), and higher levels of mature brain-derived neurotrophic factor (mBDNF) in the media of H1299 cells as compared to A549 cell media. In this study, we hypothesized that the levels of soluble amyloid precursor protein α (sAPPα) are regulated by AChE and mBDNF in A549 and H1299 cell media. The levels of sAPPα were higher in the media of H1299 cells. Knockdown of AChE led to increased sAPPα and mBDNF levels and correlated with decreased levels of intact Aβ40/42 in A549 cell media. AChE and mBDNF had opposite effects on the levels of Aβ and sAPPα and were found to operate through a mechanism involving α-secretase activity. Treatment with AChE decreased sAPPα levels and simultaneously increased the levels of intact Aβ40/42 suggesting a role of the protein in shifting APP processing away from the non-amyloidogenic pathway and toward the amyloidogenic pathway, whereas treatment with mBDNF led to opposite effects on those levels. We also show that the levels of sAPPα are regulated by protein kinase C (PKC), extracellular signal-regulated kinase (ERK)1/2, phosphoinositide 3 Kinase (PI3K), but not by protein kinase A (PKA).

Clathrin adaptor AP-1-mediated Golgi export of amyloid precursor protein is crucial for the production of neurotoxic amyloid fragments

One of the hallmarks of Alzheimer's disease is the accumulation of toxic amyloid-β (Aβ) peptides in extracellular plaques. The direct precursor of Aβ is the carboxyl-terminal fragment β (or C99) of the amyloid precursor protein (APP). C99 is detected at elevated levels in Alzheimer's disease brains, and its intracellular accumulation has been linked to early neurotoxicity independently of Aβ. Despite this, the causes of increased C99 levels are poorly understood. Here, we demonstrate that APP interacts with the clathrin vesicle adaptor AP-1 (adaptor protein 1), and we map the interaction sites on both proteins. Using quantitative kinetic trafficking assays, established cell lines and primary neurons, we also show that this interaction is required for the transport of APP from the trans-Golgi network to endosomes. In addition, disrupting AP-1-mediated transport of APP alters APP processing and degradation, ultimately leading to increased C99 production and Aβ release. Our results indicate that AP-1 regulates the subcellular distribution of APP, altering its processing into neurotoxic fragments.

Neurotrophin Crosstalk in the Etiology and Treatment of Neuropsychiatric and Neurodegenerative Disease

This article reviews the current progress in our understanding of the mechanisms by which growth factors, including brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), and select neurotrophin-regulated gene products, such as VGF (non-acronymic) and VGF-derived neuropeptides, function in the central nervous system (CNS) to modulate neuropsychiatric and neurodegenerative disorders, with a discussion of the possible therapeutic applications of these growth factors to major depressive disorder (MDD) and Alzheimer’s disease (AD). BDNF and VEGF levels are generally decreased regionally in the brains of MDD subjects and in preclinical animal models of depression, changes that are associated with neuronal atrophy and reduced neurogenesis, and are reversed by conventional monoaminergic and novel ketamine-like antidepressants. Downstream of neurotrophins and their receptors, VGF was identified as a nerve growth factor (NGF)- and BDNF-inducible secreted protein and neuropeptide precursor that is produced and trafficked throughout the CNS, where its expression is greatly influenced by neuronal activity and exercise, and where several VGF-derived peptides modulate neuronal activity, function, proliferation, differentiation, and survival. Moreover, levels of VGF are reduced in the CSF of AD subjects, where it has been repetitively identified as a disease biomarker, and in the hippocampi of subjects with MDD, suggesting possible shared mechanisms by which reduced levels of VGF and other proteins that are similarly regulated by neurotrophin signaling pathways contribute to and potentially drive the pathogenesis and progression of co-morbid neuropsychiatric and neurodegenerative disorders, particularly MDD and AD, opening possible therapeutic windows.

The Rab11-regulated endocytic pathway and BDNF/TrkB signaling: Roles in plasticity changes and neurodegenerative diseases

Neurons are highly polarized cells that rely on the intracellular transport of organelles. This process is regulated by molecular motors such as dynein and kinesins and the Rab family of monomeric GTPases that together help move cargo along microtubules in dendrites, somas, and axons. Rab5-Rab11 GTPases regulate receptor trafficking along early-recycling endosomes, which is a process that determines the intracellular signaling output of different signaling pathways, including those triggered by BDNF binding to its tyrosine kinase receptor TrkB. BDNF is a well-recognized neurotrophic factor that regulates experience-dependent plasticity in different circuits in the brain. The internalization of the BDNF/TrkB complex results in signaling endosomes that allow local signaling in dendrites and presynaptic terminals, nuclear signaling in somas and dynein-mediated long-distance signaling from axons to cell bodies. In this review, we briefly discuss the organization of the endocytic pathway and how Rab11-recycling endosomes interact with other endomembrane systems. We further expand upon the roles of the Rab11-recycling pathway in neuronal plasticity. Then, we discuss the BDNF/TrkB signaling pathways and their functional relationships with the postendocytic trafficking of BDNF, including axonal transport, emphasizing the role of BDNF signaling endosomes, particularly Rab5-Rab11 endosomes, in neuronal plasticity. Finally, we discuss the evidence indicating that the dysfunction of the early-recycling pathway impairs BDNF signaling, contributing to several neurodegenerative diseases.

Targeting the Cation-Chloride Co-Transporter NKCC1 to Re-Establish GABAergic Inhibition and an Appropriate Excitatory/Inhibitory Balance in Selective Neuronal Circuits: A Novel Approach for the Treatment of Alzheimer's Disease

GABA, the main inhibitory neurotransmitter in the adult brain, depolarizes and excites immature neurons because of an initially higher intracellular chloride concentration [Cl-]i due to the delayed expression of the chloride exporter KCC2 at birth. Depolarization-induced calcium rise via NMDA receptors and voltage-dependent calcium channels is instrumental in shaping neuronal circuits and in controlling the excitatory (E)/inhibitory (I) balance in selective brain areas. An E/I imbalance accounts for cognitive impairment observed in several neuropsychiatric disorders. The aim of this review is to summarize recent data on the mechanisms by which alterations of GABAergic signaling alter the E/I balance in cortical and hippocampal neurons in Alzheimer's disease (AD) and the role of cation-chloride co-transporters in this process. In particular, we discuss the NGF and AD relationship and how mice engineered to express recombinant neutralizing anti-NGF antibodies (AD11 mice), which develop a neurodegenerative pathology reminiscent of that observed in AD patients, exhibit a depolarizing action of GABA due to KCC2 impairment. Treating AD and other forms of dementia with bumetanide, a selective KCC2 antagonist, contributes to re-establishing a proper E/I balance in selective brain areas, leading to amelioration of AD symptoms and the slowing down of disease progression.

Brain-Derived Neurotropic Factor in Neurodegenerative Disorders

Globally, neurodegenerative diseases cause a significant degree of disability and distress. Brain-derived neurotrophic factor (BDNF), primarily found in the brain, has a substantial role in the development and maintenance of various nerve roles and is associated with the family of neurotrophins, including neuronal growth factor (NGF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). BDNF has affinity with tropomyosin receptor kinase B (TrKB), which is found in the brain in large amounts and is expressed in several cells. Several studies have shown that decrease in BDNF causes an imbalance in neuronal functioning and survival. Moreover, BDNF has several important roles, such as improving synaptic plasticity and contributing to long-lasting memory formation. BDNF has been linked to the pathology of the most common neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease. This review aims to describe recent efforts to understand the connection between the level of BDNF and neurodegenerative diseases. Several studies have shown that a high level of BDNF is associated with a lower risk for developing a neurodegenerative disease.

Functionalized nanoparticles for brain targeted BDNF gene therapy to rescue Alzheimer's disease pathology in transgenic mouse model

Brain-derived neurotrophic factor (BDNF) is actively produced and utilized in cortical circuits throughout life to sustain neuronal function and synaptic plasticity. In animal models of Alzheimer's Disease (AD), highly invasive BDNF gene therapy using viral vectors has successfully shown enhanced synaptic protein expression, proliferation of neurons and attenuation of amyloidogenic processes. However, to eliminate virus-related safety issues and invasive procedures, our present study has explored brain-targeted lipid-based nanoparticles that can deliver plasmid encoding BDNF to brain in a safe and efficient manner. Efficacy of these nanoparticles was tested in early (6-months) and advanced stage (9-months) transgenic APP/PS1 AD mice. Liposomes were surface-functionalized with brain targeting ligand, mannose, and cell-penetrating peptides (rabies virus-derived peptide or penetratin). These bifunctionalized nanoparticles enhanced BDNF expression by ~2 times and resulted in >40% (p < 0.05) reduction in toxic amyloid-beta peptides in 6- and 9-months old APP/PS1 mice brains compared to their age-matched untreated controls. Plaque load was reduced ~7 and ~3 times (p < 0.05), respectively, whereas synaptic proteins, synaptophysin and PSD-95, were found to be increased >90% (p < 0.05) in both age groups of transgenic mice treated with bifunctionalized nanoparticles. No untoward adverse effects were observed throughout treatment, suggesting a safe and effective strategy to rescue AD pathology.

Brain-derived neurotrophic factor in Alzheimer's disease and its pharmaceutical potential

Synaptic abnormalities are a cardinal feature of Alzheimer's disease (AD) that are known to arise as the disease progresses. A growing body of evidence suggests that pathological alterations to neuronal circuits and synapses may provide a mechanistic link between amyloid β (Aβ) and tau pathology and thus may serve as an obligatory relay of the cognitive impairment in AD. Brain-derived neurotrophic factors (BDNFs) play an important role in maintaining synaptic plasticity in learning and memory. Considering AD as a synaptic disorder, BDNF has attracted increasing attention as a potential diagnostic biomarker and a therapeutical molecule for AD. Although depletion of BDNF has been linked with Aβ accumulation, tau phosphorylation, neuroinflammation and neuronal apoptosis, the exact mechanisms underlying the effect of impaired BDNF signaling on AD are still unknown. Here, we present an overview of how BDNF genomic structure is connected to factors that regulate BDNF signaling. We then discuss the role of BDNF in AD and the potential of BDNF-targeting therapeutics for AD.

Brothers in arms: proBDNF/BDNF and sAPPα/Aβ-signaling and their common interplay with ADAM10, TrkB, p75NTR, sortilin, and sorLA in the progression of Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) is an important modulator for a variety of functions in the central nervous system (CNS). A wealth of evidence, such as reduced mRNA and protein level in the brain, cerebrospinal fluid (CSF), and blood samples of Alzheimer's disease (AD) patients implicates a crucial role of BDNF in the progression of this disease. Especially, processing and subcellular localization of BDNF and its receptors TrkB and p75 are critical determinants for survival and death in neuronal cells. Similarly, the amyloid precursor protein (APP), a key player in Alzheimer's disease, and its cleavage fragments sAPPα and Aβ are known for their respective roles in neuroprotection and neuronal death. Common features of APP- and BDNF-signaling indicate a causal relationship in their mode of action. However, the interconnections of APP- and BDNF-signaling are not well understood. Therefore, we here discuss dimerization properties, localization, processing by α- and γ-secretase, relevance of the common interaction partners TrkB, p75, sorLA, and sortilin as well as shared signaling pathways of BDNF and sAPPα.

Aberrant role of pyruvate kinase M2 in the regulation of gamma-secretase and memory deficits in Alzheimer's disease

Toxic amyloid beta (Aβ) species cause synaptic dysfunction and neurotoxicity in Alzheimer's disease (AD). As of yet, however, there are no reported regulators for gamma-secretase, which links a risky environment to amyloid accumulation in AD. Here, we report that pyruvate kinase M2 (PKM2) is a positive regulator of gamma-secretase under hypoxia. From a genome-wide functional screen, we identify PKM2 as a gamma-secretase activator that is highly expressed in the brains of both patients and murine models with AD. PKM2 regulates Aβ production and the amount of active gamma-secretase complex by changing the gene expression of aph-1 homolog. Hypoxia induces PKM2 expression, thereby promoting gamma-secretase activity. Moreover, transgenic expression of PKM2 in 3xTg AD model mice enhances hippocampal production of Aβ and exacerbates the impairment of spatial and recognition memory. Taken together, these findings indicate that PKM2 is an important gamma-secretase regulator that promotes Aβ production and memory impairment under hypoxia.

A Review of Brain-Targeted Nonviral Gene-Based Therapies for the Treatment of Alzheimer's Disease

Diseases of the central nervous system (CNS) are difficult to treat owing to the complexity of the brain and the presence of a natural blood-brain-barrier (BBB). Alzheimer's disease (AD) is one of the major progressive and currently incurable neurodegenerative disorders of the CNS, which accounts for 60-80% of cases of dementia. The pathophysiology of AD involves the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain. Additionally, synaptic loss and imbalance of neuronal signaling molecules are characterized as important markers of AD. Existing treatments of AD help in the management of its symptoms and aim toward the maintenance of cognitive functions, behavior, and attenuation of gradual memory loss. Over the past decade, nonviral gene therapy has attracted increasing interest due to its various advantages over its viral counterparts. Moreover, advancements in nonviral gene technology have led to their increasing contributions in clinical trials. However, brain-targeted nonviral gene delivery vectors come across various extracellular and intracellular barriers, limiting their ability to transfer the therapeutic gene into the target cells. Chief barriers to nonviral gene therapy have been discussed briefly in this review. We have also highlighted the rapid advancement of several nonviral gene therapies for AD, which are broadly categorized into physical and chemical methods. These methods aim to modulate Aβ, beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), apolipoprotein E, or neurotrophic factors' expression in the CNS. Overall, this review discusses challenges and recent advancements of nonviral gene therapy for AD.

Challenges in LC-MS-based metabolomics for Alzheimer's disease early detection: targeted approaches versus untargeted approaches

BACKGROUND

Alzheimer's disease (AD) is one of the most common causes of dementia in old people. Neuronal deficits such as loss of memory, language and problem-solving are severely compromised in affected patients. The molecular features of AD are Aβ deposits in plaques or in oligomeric structures and neurofibrillary tau tangles in brain. However, the challenge is that Aβ is only one piece of the puzzle, and recent findings continue to support the hypothesis that their presence is not sufficient to predict decline along the AD outcome. In this regard, metabolomic-based techniques are acquiring a growing interest for either the early diagnosis of diseases or the therapy monitoring. Mass spectrometry is one the most common analytical platforms used for detection, quantification, and characterization of metabolic biomarkers. In the past years, both targeted and untargeted strategies have been applied to identify possible interesting compounds.

AIM OF REVIEW

The overall goal of this review is to guide the reader through the most recent studies in which LC-MS-based metabolomics has been proposed as a powerful tool for the identification of new diagnostic biomarkers in AD. To this aim, herein studies spanning the period 2009-2020 have been reported. Advantages and disadvantages of targeted vs untargeted metabolomic approaches have been outlined and critically discussed.

Targeting both BDNF/TrkB pathway and delta-secretase for treating Alzheimer's disease

Alzheimer's disease (AD) is the most common dementia, and no disease-modifying therapeutic agents are currently available. BDNF/TrkB signaling is impaired in AD and is associated with prominent delta-secretase (δ-secretase, also known as asparaginyl endopeptidase or legumain) activation, which simultaneously cleaves both APP and Tau and promotes Aβ production and neurofibrillary tangles (NFT) pathologies. Here we show that the optimized δ-secretase inhibitor (#11a) or TrkB receptor agonist (CF3CN) robustly blocks δ-secretase activity separately, and their combination synergistically blunts δ-secretase, exhibiting promising therapeutic efficacy in 3xTg AD mouse model. The optimal δ-secretase inhibitor reveals demonstrable brain exposure and oral bioavailability, suppressing APP N585 and Tau N368 cleavage by δ-secretase. Strikingly, CF3CN treatment evidently escalates BDNF levels. Both #11a and CF3CN display strong in vivo PK/PD properties and ability to suppress δ-secretase activity in the brain. Orally administrated CF3CN strongly activates TrkB that triggers active Akt to phosphorylate δ-secretase T322, preventing its proteolytic activation and mitigating AD pathologies. #11a or CF3CN significantly diminishes AD pathogenesis and improves cognitive functions with the combination exhibiting the maximal effect. Thus, our data support that these derivatives are strong pharmaceutical candidates for the treatment of AD.

Optimized TrkB Agonist Ameliorates Alzheimer's Disease Pathologies and Improves Cognitive Functions via Inhibiting Delta-Secretase

BDNF/TrkB neurotropic pathway, essential for neural synaptic plasticity and survival, is deficient in neurodegenerative diseases including Alzheimer’s disease (AD). Our previous works support that BDNF diminishes AD pathologies by inhibiting delta-secretase, a crucial age-dependent protease that simultaneously cleaves both APP and Tau and promotes AD pathologies, via Akt phosphorylation. Small molecular TrkB receptor agonist 7,8-dihydroxyflavone (7,8-DHF) binds and activates the receptor and its downstream signaling, exerting therapeutic efficacy toward AD. In the current study, we optimize 7,8-DHF pharmacokinetic characteristics via medicinal chemistry to obtain a synthetic derivative CF₃CN that interacts with the TrkB LRM/CC2 domain. CF₃CN possesses improved druglike features, including oral bioavailability and half-life, compared to those of the lead compound. CF₃CN activates TrkB neurotrophic signaling in primary neurons and mouse brains. Oral administration of CF₃CN blocks delta-secretase activation, attenuates AD pathologies, and alleviates cognitive dysfunctions in 5xFAD. Notably, chronic treatment of CF₃CN reveals no demonstrable toxicity. Hence, CF₃CN represents a promising preclinical candidate for treating the devastating neurodegenerative disease.

TrkB receptor cleavage by delta-secretase abolishes its phosphorylation of APP, aggravating Alzheimer's disease pathologies

Neurotrophins promote neuronal survival and synaptic plasticity via activating the tropomyosin receptor kinases. BDNF and its high-affinity receptor TrkB are reduced in Alzheimer's disease (AD), contributing to progressive cognitive decline. However, how the signaling mediates AD pathologies remains incompletely understood. Here we show that the TrkB receptor binds and phosphorylates APP, reducing amyloid-β production, which are abrogated by δ-secretase cleavage of TrkB in AD. Remarkably, BDNF stimulates TrkB to phosphorylate APP Y687 residue that accumulates APP in the TGN (Trans-Golgi Network) and diminishes its amyloidogenic cleavage. Delta-secretase cleaves TrkB at N365 and N486/489 residues and abolishes its neurotrophic activity, decreasing p-APP Y687 and altering its subcellular trafficking. Notably, both TrkB and APP are robustly cleaved by δ-secretase in AD brains, accompanied by mitigated TrkB signaling and reduced p-Y687. Blockade of TrkB cleavage attenuates AD pathologies in 5xFAD mice, rescuing the learning and memory. Viral expression of TrkB 1-486 fragment in the hippocampus of APP/PS1 mice facilitates amyloid pathology and mitigates cognitive functions. Hence, δ-secretase cleaves TrkB and blunts its phosphorylation of APP, facilitating AD pathogenesis.

Alzheimer’s Disease, Neural Plasticity, and Functional Recovery

Alzheimer’s disease (AD) is the most common and devastating neurodegenerative condition worldwide, characterized by the aggregation of amyloid-β and phosphorylated tau protein, and is accompanied by a progressive loss of learning and memory. A healthy nervous system is endowed with synaptic plasticity, among others neural plasticity mechanisms, allowing structural and physiological adaptations to changes in the environment. This neural plasticity modification sustains learning and memory, and behavioral changes and is severely affected by pathological and aging conditions, leading to cognitive deterioration. This article reviews critical aspects of AD neurodegeneration as well as therapeutic approaches that restore neural plasticity to provide functional recoveries, including environmental enrichment, physical exercise, transcranial stimulation, neurotrophin involvement, and direct electrical stimulation of the amygdala. In addition, we report recent behavioral results in Octodon degus, a promising natural model for the study of AD that naturally reproduces the neuropathological alterations observed in AD patients during normal aging, including neuronal toxicity, deterioration of neural plasticity, and the decline of learning and memory.

Brain-Derived Neurotrophic Factor Signaling in the Pathophysiology of Alzheimer's Disease: Beneficial Effects of Flavonoids for Neuroprotection

The function of the brain-derived neurotrophic factor (BDNF) via activation through its high-affinity receptor Tropomyosin receptor kinase B (TrkB) has a pivotal role in cell differentiation, cell survival, synaptic plasticity, and both embryonic and adult neurogenesis in central nervous system neurons. A number of studies have demonstrated the possible involvement of altered expression and action of the BDNF/TrkB signaling in the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). In this review, we introduce an essential role of the BDNF and its downstream signaling in neural function. We also review the current evidence on the deregulated the BDNF signaling in the pathophysiology of AD at gene, mRNA, and protein levels. Further, we discuss a potential usefulness of small compounds, including flavonoids, which can stimulate BDNF-related signaling as a BDNF-targeting therapy.

Nerve Growth Factor Peptides Bind Copper(II) with High Affinity: A Thermodynamic Approach to Unveil Overlooked Neurotrophin Roles

Nerve growth factor (NGF) is a protein essential to neurons survival, which interacts with its receptor as a non-covalent dimer. Peptides belonging to NGF N-terminal domain are able to mimic the activity of the whole protein. Such activity is affected by the presence of copper ions. The metal is released in the synaptic cleft where proteins, not yet identified, may bind and transfer to human copper transporter 1 (hCtr1), for copper uptake in neurons. The measurements of the stability constants of copper complexes formed by amyloid beta and hCtr1 peptide fragments suggest that beta-amyloid (Aβ) can perform this task. In this work, the stability constant values of copper complex species formed with the dimeric form of N-terminal domain, sequence 1–15 of the protein, were determined by means of potentiometric measurements. At physiological pH, NGF peptides bind one equivalent of copper ion with higher affinity of Aβ and lower than hCtr1 peptide fragments. Therefore, in the synaptic cleft, NGF may act as a potential copper chelating molecule, ionophore or chaperone for hCtr1 for metal uptake. Copper dyshomeostasis and mild acidic environment may modify the balance between metal, NGF, and Aβ, with consequences on the metal cellular uptake and therefore be among causes of the Alzheimer’s disease onset.

Intranasal delivery of BDNF rescues memory deficits in AD11 mice and reduces brain microgliosis

A decrease in brain-derived neurotrophic factor (BDNF), a neurotrophin essential for synaptic function, plasticity and neuronal survival, is evident early in the progression of Alzheimer's disease (AD), being apparent in subjects with mild cognitive impairment or mild AD, and both proBDNF and mature BDNF levels are positively correlated with cognitive measures. BDNF delivery is, therefore, considered of great interest as a potentially useful therapeutic strategy to contrast AD. Invasive BDNF administration has indeed been recently used in animal models of AD with promising results in rescuing memory deficits, synaptic density and cell loss. Here, we tested whether non-invasive intranasal administration of different BDNF concentrations after the onset of cognitive and anatomical deficits (6 months of age) could rescue neuropathological and memory deficits in AD11 mice, a model of NGF deprivation-induced neurodegeneration. In addition to AD hallmarks, we investigated BDNF effects on microglia presence in the brain of AD11 mice, since alterations in microglia activation have been associated with ageing-related cognitive decline and with the progression of neurodegenerative diseases, including AD. We found that intranasal delivery of 42 pmol BDNF (1 μM), but not PBS, was sufficient to completely rescue performance of AD11 mice both in the object recognition test and in the object context test. No further improvement was obtained with 420 pmol (10 μM) BDNF dose. The strong improvement in memory performance in BDNF-treated mice was not accompanied by an amelioration of AD-like pathology, Aβ burden, tau hyperphosphorylation and cholinergic deficit, but there was a dramatic decrease of CD11b immunoreactive brain microglia. These results reinforce the potential therapeutic uses of BDNF in AD and the non-invasive intranasal route as an effective delivery strategy of BDNF to the brain. They also strengthen the connection between neuroinflammation and neurodegenerative dementia and suggest microglia as a possible mediator of BDNF therapeutic actions in the brain.

Dual-target compounds for Alzheimer's disease: Natural and synthetic AChE and BACE-1 dual-inhibitors and their structure-activity relationship (SAR)

Alzheimer's disease (AD) is a chronic neurodegenerative disease and represents the major cause of dementia worldwide. Currently, there are no available treatments capable to deliver disease-modifying effects, and the available drugs can only alleviate the symptoms. The exact pathology of AD is not yet fully understood and several hallmarks such as the presence of amyloid-β (Aβ) senile plaques, neurofibrillary tangles (NFTs) as well as the loss of cholinergic function have been associated to AD. Distinct pharmacological targets have been validated to address AD, with acetylcholinesterase (AChE) and β-secretase-1 (BACE-1) being two of the most explored ones. A great deal of research has been devoted to the development of new AChE and BACE-1 effective inhibitors, tackled separately or in combination of both. The multi-factorial nature of AD conducted to the development of multi-target directed ligands (MTDLs), defined as single molecules capable to modulate more than one biological target, as an alternative approach to the old paradigm one-target one-drug. In this context, this review describes a collection of natural and synthetic compounds with dual-inhibitory properties towards both AChE and BACE-1 in the MTDLs context. Furthermore, this review also provides a critical comprehensive analysis of structure-activity relationships (SAR) of the synthetic compounds.

Sirt-1 Regulates Physiological Process and Exerts Protective Effects against Oxidative Stress

BACKGROUND

Recent studies suggest a correlation between the reduced Sirt-1 expression with Alzheimer's diseases (AD) and depression, respectively, suggesting a possible pathogenic role of the altered Sirt-1 expression in neuronal degenerative diseases, such as AD and depression. However, the molecular mechanisms underlying how Sirt-1 reduction impairs neuronal functions remain unknown.

METHODS

We used the SK-N-SH neuroblastoma cells to study the role of Sirt-1 expression on physiological roles in neuronal cells. Gain of Sirt-1 was achieved by transiently transfecting Sirt-1 expression plasmid. Sirt-1-specific shRNA was used to elucidate the role of Sirt-1 loss of function. CCK-8 (Cell Counting Kit-8) assay and flow cytometry were used to evaluate cell proliferation. Semiquantitative western blotting was used to detect relative protein levels. A further luciferase reporter gene assay was employed to examine the effect of Sirt-1 expression on the transcriptional activity of p53. RT-qPCR was used to determine the mRNA levels of p21, Bax, and Bcl-2, which were the downstream target genes of p53.

RESULTS

Sirt-1 suppressed the p53 downstream gene p21 transcription, while shRNA-mediated Sirt-1 knockdown resulted in a significant increase in p21 expression, implying a possibility that Sirt-1 promotes neuron proliferation through suppressing p53 transcriptional activity. The mRNA and protein levels of p53 were not affected by the altered Sirt-1 expression, suggesting that Sirt-1 regulates the transcriptional regulatory activity of p53 rather than p53 expression. Indeed, we further confirmed that Sirt-1 appeared to inhibit p53 transcriptional activity by attenuating its acetylation and resulted in a decrease of p53's binding to the p21 promoter. Overexpressed Sirt-1 scavenged reactive oxygen species (ROS) production in SK-N-SH with H2O2. Knockdown of Sirt-1 presented opposite effect; the addition of EX527 (Sirt-1 inhibitor) increased ROS accumulation.

CONCLUSIONS

Oxidative stress induces Sirt-1 in neuron cells, and Sirt-1 promotes proliferation in SK-N-SH cells, which protects them from oxidative stress-induced cell death, potentially via suppressing the transcriptional activity of p53. These results provide a molecular explanation underlying how the reduced Sirt-1 potentially causes the AD and depression-related diseases, supporting the idea that Sirt-1 can possibly be used as a diagnostic biomarker and/or therapeutic drug target for the AD and depression-related diseases.

Alzheimer's Disease and Protein Kinases

Alzheimer's disease (AD) is the most common neurodegenerative disorder and accounts for more than 60-80% of all cases of dementia. Loss of pyramidal neurons, extracellular amyloid beta (Abeta) accumulated senile plaques, and neurofibrillary tangles that contain hyperphosphorylated tau constitute the main pathological alterations in AD.Synaptic dysfunction and extrasynaptic N-methyl-D-aspartate receptor (NMDAR) hyperactivation contributes to excitotoxicity in patients with AD. Amyloid precursor protein (APP) and Abeta promoted neurodegeneration develop through the activation of protein kinase signaling cascade in AD. Furthermore, ultimate neuronal death in AD is under control of protein kinases-related signaling pathways. In this chapter, critical check-points within the cross-talk between neuron and protein kinases have been defined regarding the initiation and progression of AD. In this context, amyloid cascade hypothesis, neuroinflammation, oxidative stress, granulovacuolar degeneration, loss of Wnt signaling, Abeta-related synaptic alterations, prolonged calcium ions overload and NMDAR-related synaptotoxicity, damage signals hypothesis and type-3 diabetes are discussed briefly.In addition to clinical perspective of AD pathology, recommendations that might be effective in the treatment of AD patients have been reviewed.

NGF and the Amyloid Precursor Protein in Alzheimer's Disease: From Molecular Players to Neuronal Circuits

Alzheimer's disease (AD), one of the most common causes of dementia in elderly people, is characterized by progressive impairment in cognitive function, early degeneration of basal forebrain cholinergic neurons (BFCNs), abnormal metabolism of the amyloid precursor protein (APP), amyloid beta-peptide (Aβ) depositions, and neurofibrillary tangles. According to the cholinergic hypothesis, dysfunction of acetylcholine-containing neurons in the basal forebrain contributes markedly to the cognitive decline observed in AD. In addition, the neurotrophic factor hypothesis posits that the loss nerve growth factor (NGF) signalling in AD may account for the vulnerability to atrophy of BFCNs and consequent impairment of cholinergic functions. Though acetylcholinesterase inhibitors provide only partial and symptomatic relief to AD patients, emerging data from in vivo magnetic resonance imaging (MRI) and positron emission tomography (PET) studies in mild cognitive impairment (MCI) and AD patients highlight the early involvement of BFCNs in MCI and the early phase of AD. These data support the cholinergic and neurotrophic hypotheses of AD and suggest new targets for AD therapy.Different mechanisms account for selective vulnerability of BFCNs to AD pathology, with regard to altered metabolism of APP and tau. In this review, we provide a general overview of the current knowledge of NGF and APP interplay, focusing on the role of APP in regulating NGF receptors trafficking/signalling and on the involvement of NGF in modulating phosphorylation of APP, which in turn controls APP intracellular trafficking and processing. Moreover, we highlight the consequences of APP interaction with p75NTR and TrkA receptor, which share the same binding site within the APP juxta-membrane domain. We underline the importance of insulin dysmetabolism in AD pathology, in the light of our recent data showing that overlapping intracellular signalling pathways stimulated by NGF or insulin can be compensatory. In particular, NGF-based signalling is able to ameliorates deficiencies in insulin signalling in the medial septum of 3×Tg-AD mice. Finally, we present an overview of NGF-regulated microRNAs (miRNAs). These small non-coding RNAs are involved in post-transcriptional regulation of gene expression , and we focus on a subset that are specifically deregulated in AD and thus potentially contribute to its pathology.

Involvement of Bradykinin Receptor 2 in Nerve Growth Factor Neuroprotective Activity

Neurotrophin nerve growth factor (NGF) has been demonstrated to upregulate the gene expression of bradykinin receptor 2 (B2R) on sensory neurons, thus facilitating nociceptive signals. The aim of the present study is to investigate the involvement of B2R in the NGF mechanism of action in nonsensory neurons in vitro by using rat mixed cortical primary cultures (CNs) and mouse hippocampal slices, and in vivo in Alzheimer’s disease (AD) transgenic mice (5xFAD) chronically treated with NGF. A significant NGF-mediated upregulation of B2R was demonstrated by microarray, Western blot, and immunofluorescence analysis in CNs, indicating microglial cells as the target of this modulation. The B2R involvement in the NGF mechanism of action was also demonstrated by using a selective B2R antagonist which was able to reverse the neuroprotective effect of NGF in CNs, as revealed by viability assay, and the NGF-induced long-term potentiation (LTP) in hippocampal slices. To confirm in vitro observations, B2R upregulation was observed in 5xFAD mouse brain following chronic intranasal NGF treatment. This study demonstrates for the first time that B2R is a key element in the neuroprotective activity and synaptic plasticity mediated by NGF in brain cells.

Signalling Pathways Implicated in Alzheimer's Disease Neurodegeneration in Individuals with and without Down Syndrome

Down syndrome (DS), the most common cause of intellectual disability of genetic origin, is characterized by alterations in central nervous system morphology and function that appear from early prenatal stages. However, by the fourth decade of life, all individuals with DS develop neuropathology identical to that found in sporadic Alzheimer's disease (AD), including the development of amyloid plaques and neurofibrillary tangles due to hyperphosphorylation of tau protein, loss of neurons and synapses, reduced neurogenesis, enhanced oxidative stress, and mitochondrial dysfunction and neuroinflammation. It has been proposed that DS could be a useful model for studying the etiopathology of AD and to search for therapeutic targets. There is increasing evidence that the neuropathological events associated with AD are interrelated and that many of them not only are implicated in the onset of this pathology but are also a consequence of other alterations. Thus, a feedback mechanism exists between them. In this review, we summarize the signalling pathways implicated in each of the main neuropathological aspects of AD in individuals with and without DS as well as the interrelation of these pathways.

Deficiency in BDNF/TrkB Neurotrophic Activity Stimulates δ-Secretase by Upregulating C/EBPβ in Alzheimer's Disease

BDNF/TrkB neurotrophic signaling regulates neuronal development, differentiation, and survival, and deficient BDNF/TrkB activity underlies neurodegeneration in Alzheimer’s disease (AD). However, exactly how BDNF/TrkB participates in AD pathology remains unclear. Here, we show that deprivation of BDNF/TrkB increases inflammatory cytokines and activates the JAK2/STAT3 pathway, resulting in the upregulation of transcription factor C/EBPβ. This, in turn, results in increased expression of δ-secretase, leading to both APP and Tau fragmentation by δ-secretase and neuronal loss, which can be blocked by expression of STAT3 Y705F, knockdown of C/EBPβ, or the δ-secretase enzymatic-dead C189S mutant. Inhibition of this pathological cascade can also rescue impaired synaptic plasticity and cognitive dysfunctions. Importantly, reduction in BDNF/TrkB neurotrophic signaling is inversely coupled with an increase in JAK2/STAT3, C/EBPβ, and δ-secretase escalation in human AD brains. Therefore, our findings provide a mechanistic link between BDNF/TrkB reduction, C/EBPβ upregulation, δ-secretase activity, and Aβ and Tau alterations in murine brains.

Deficient BDNF/TrkB activity underlies AD pathogenesis. Wang et al. report that deprivation of BDNF/TrkB increases inflammatory cytokines and activates the JAK2/STAT3 pathway, resulting in the upregulation of C/EBPβ/AEP signaling. Reduction of BDNF is inversely coupled with the aforementioned pathway in AD brains. Inhibition of JAK2/STAT3/C/EBPβ/AEP prevents BDNF-depletion-mediated pathology.

Fyn Tyrosine Kinase as Harmonizing Factor in Neuronal Functions and Dysfunctions

Fyn is a non-receptor or cytoplasmatic tyrosine kinase (TK) belonging to the Src family kinases (SFKs) involved in multiple transduction pathways in the central nervous system (CNS) including synaptic transmission, myelination, axon guidance, and oligodendrocyte formation. Almost one hundred years after the original description of Fyn, this protein continues to attract extreme interest because of its multiplicity of actions in the molecular signaling pathways underlying neurodevelopmental as well as neuropathologic events. This review highlights and summarizes the most relevant recent findings pertinent to the role that Fyn exerts in the brain, emphasizing aspects related to neurodevelopment and synaptic plasticity. Fyn is a common factor in healthy and diseased brains that targets different proteins and shapes different transduction signals according to the neurological conditions. We will primarily focus on Fyn-mediated signaling pathways involved in neuronal differentiation and plasticity that have been subjected to considerable attention lately, opening the fascinating scenario to target Fyn TK for the development of potential therapeutic interventions for the treatment of CNS injuries and certain neurodegenerative disorders like Alzheimer’s disease.

Nerve growth factor in metabolic complications and Alzheimer's disease: Physiology and therapeutic potential

As the population ages, obesity and metabolic complications as well as neurological disorders are becoming more prevalent, with huge economic burdens on both societies and families. New therapeutics are urgently needed. Nerve growth factor (NGF), first discovered in 1950s, is a neurotrophic factor involved in regulating cell proliferation, growth, survival, and apoptosis in both central and peripheral nervous systems. NGF and its precursor, proNGF, bind to TrkA and p75 receptors and initiate protein phosphorylation cascades, resulting in changes of cellular functions, and are associated with obesity, diabetes and its complications, and Alzheimer's disease. In this article, we summarize changes in NGF levels in metabolic and neuronal disorders, the signal transduction initiated by NGF and proNGF, the physiological and pathophysiological relevance, and therapeutic potential in treating chronic metabolic diseases and cognitive decline.

SERP1 is an assembly regulator of γ-secretase in metabolic stress conditions

The enzyme γ-secretase generates β-amyloid (Aβ) peptides by cleaving amyloid protein precursor (APP); the aggregation of these peptides is associated with Alzheimer's disease (AD). Despite the development of various γ-secretase regulators, their clinical use is limited by coincident disruption of other γ-secretase-regulated substrates, such as Notch. Using a genome-wide functional screen of γ-secretase activity in cells and a complementary DNA expression library, we found that SERP1 is a previously unknown γ-secretase activator that stimulates Aβ generation in cells experiencing endoplasmic reticulum (ER) stress, such as is seen with diabetes. SERP1 interacted with a subcomplex of γ-secretase (APH1A/NCT) through its carboxyl terminus to enhance the assembly and, consequently, the activity of the γ-secretase holoenzyme complex. In response to ER stress, SERP1 preferentially recruited APP rather than Notch into the γ-secretase complex and enhanced the subcellular localization of the complex into lipid rafts, increasing Aβ production. Moreover, SERP1 abundance, γ-secretase assembly, and Aβ production were increased both in cells exposed to high amounts of glucose and in diabetic AD model mice. Conversely, Aβ production was decreased by knocking down SERP1 in cells or in the hippocampi of mice. Compared to postmortem samples from control individuals, those from patients with AD showed increased SERP1 expression in the hippocampus and parietal lobe. Together, our findings suggest that SERP1 is an APP-biased regulator of γ-secretase function in the context of cell stress, providing a possible molecular explanation for the link between diabetes and sporadic AD.

Focused ultrasound delivery of a selective TrkA agonist rescues cholinergic function in a mouse model of Alzheimer's disease

The degeneration of cholinergic neurons is a prominent feature of Alzheimer's disease (AD). In animal models of injury and aging, nerve growth factor (NGF) enhances cholinergic cell survival and function, contributing to improved memory. In the presence of AD pathology, however, NGF-related therapeutics have yet to fulfill their regenerative potential. We propose that stimulating the TrkA receptor, without p75^NTR activation, is key for therapeutic efficacy. Supporting this hypothesis, the selective TrkA agonist D3 rescued neurotrophin signaling in TgCRND8 mice, whereas NGF, interacting with both TrkA and p75^NTR, did not. D3, delivered intravenously and noninvasively to the basal forebrain using MRI-guided focused ultrasound (MRIgFUS)-mediated blood-brain barrier (BBB) permeability activated TrkA-related signaling cascades and enhanced cholinergic neurotransmission. Recent clinical trials support the safety and feasibility of MRIgFUS BBB modulation in AD patients. Neuroprotective agents targeting TrkA, combined with MRIgFUS BBB modulation, represent a promising strategy to counter neurodegeneration in AD.

Brain-derived neurotrophic factor (BDNF) and TrkB hippocampal gene expression are putative predictors of neuritic plaque and neurofibrillary tangle pathology

INTRODUCTION

Downregulation of brain-derived neurotrophic factor (BDNF) and its cognate neurotrophin receptor, TrkB, were observed during the progression of dementia, but whether the Alzheimer's disease (AD) pathological lesions diffuse plaques, (DPs), neuritic plaques (NPs), and neurofibrillary tangles (NFTs) are related to this alteration remains to be clarified.

METHODS

Negative binomial (NB) regressions were performed using gene expression data accrued from a single population of CA1 pyramidal neurons and regional hippocampal dissections obtained from participants in the Rush Religious Orders Study (RROS).

RESULTS

Downregulation of Bdnf is independently associated with increased entorhinal cortex NPs. Downregulation of TrkB is independently associated with increased entorhinal cortex NFTs and CA1 NPs during the progression of AD.

DISCUSSION

Results indicate that BDNF and TrkB dysregulation contribute to AD neuropathology, most notably hippocampal NPs and NFTs. These data suggest attenuating BDNF/TrkB signaling deficits either at the level of BDNF, TrkB, or downstream of TrkB signaling may abrogate NPs and/or NFTs.

Brain-Derived Neurotrophic Factor: A Key Molecule for Memory in the Healthy and the Pathological Brain

Brain Derived Neurotrophic Factor (BDNF) is a key molecule involved in plastic changes related to learning and memory. The expression of BDNF is highly regulated, and can lead to great variability in BDNF levels in healthy subjects. Changes in BDNF expression are associated with both normal and pathological aging and also psychiatric disease, in particular in structures important for memory processes such as the hippocampus and parahippocampal areas. Some interventions like exercise or antidepressant administration enhance the expression of BDNF in normal and pathological conditions. In this review, we will describe studies from rodents and humans to bring together research on how BDNF expression is regulated, how this expression changes in the pathological brain and also exciting work on how interventions known to enhance this neurotrophin could have clinical relevance. We propose that, although BDNF may not be a valid biomarker for neurodegenerative/neuropsychiatric diseases because of its disregulation common to many pathological conditions, it could be thought of as a marker that specifically relates to the occurrence and/or progression of the mnemonic symptoms that are common to many pathological conditions.

The Y682ENPTY687 motif of APP: Progress and insights toward a targeted therapy for Alzheimer's disease patients

Alzheimer's disease (AD) is a devastating neurodegenerative disorder for which no curative treatments, disease modifying strategies or effective symptomatic therapies exist. Current pharmacologic treatments for AD can only decelerate the progression of the disease for a short time, often at the cost of severe side effects. Therefore, there is an urgent need for biomarkers able to diagnose AD at its earliest stages, to conclusively track disease progression, and to accelerate the clinical development of innovative therapies. Scientific research and economic efforts for the development of pharmacotherapies have recently homed in on the hypothesis that neurotoxic β-amyloid (Aβ) peptides in their oligomeric or fibrillary forms are primarily responsible for the cognitive impairment and neuronal death seen in AD. As such, modern pharmacologic approaches are largely based on reducing production by inhibiting β and γ secretase cleavage of the amyloid precursor protein (APP) or on dissolving existing cerebral Aβ plaques or to favor Aβ clearance from the brain. The following short review aims to persuade the reader of the idea that APP plays a much larger role in AD pathogenesis. APP plays a greater role in AD pathogenesis than its role as the precursor for Aβ peptides: both the abnormal cleavage of APP leading to Aβ peptide accumulation and the disruption of APP physiological functions contribute to AD pathogenesis. We summarize our recent results on the role played by the C-terminal APP motif -the Y₆₈₂ENPTY₆₈ motif- in APP function and dysfunction, and we provide insights into targeting the Tyr₆₈₂ residue of APP as putative novel strategy in AD.

Inflammatory markers in Alzheimer's disease and mild cognitive impairment: a meta-analysis and systematic review of 170 studies

OBJECTIVE

Inflammation plays a crucial role in the pathogenesis of mild cognitive impairment (MCI) and Alzheimer's disease (AD). Our study aimed to analyse previous inconsistent results of inflammatory markers in AD and MCI quantitatively.

METHODS

Studies reporting concentrations of peripheral or cerebrospinal fluid (CSF) markers were included, and eligible data on AD, MCI and control were extracted. Pooled Hedges's g was adopted to illustrate comparisons, and various confounding factors were used to explore sources of heterogeneity.

RESULTS

A total of 170 studies were included in the meta-analysis and systematic review, which demonstrated increased peripheral levels of high-sensitivity C reactive protein (Hedges's g 0.281, p<0.05), interleukin-6 (IL-6) (0.429, p<0.005), soluble tumour necrosis factor receptor 1 (sTNFR1) (0.763, p<0.05), soluble tumour necrosis factor receptor 2 (sTNFR2) (0.354, p<0.005), alpha1-antichymotrypsin (α1-ACT) (1.217, p<0.005), IL-1β (0.615, p<0.05) and soluble CD40 ligand (0.868, p<0.005), and CSF levels of IL-10 (0.434, p<0.05), monocyte chemoattractant protein-1 (MCP-1) (0.798, p<0.005), transforming growth factor-beta 1 (1.009, p<0.05), soluble triggering receptor expressed on myeloid cells2 (sTREM2) (0.587, p<0.001), YKL-40 (0.849, p<0.001), α1-ACT (0.638, p<0.001), nerve growth factor (5.475, p<0.005) and visinin-like protein-1 (VILIP-1) (0.677, p<0.005), in AD compared with the control. Higher levels of sTNFR2 (0.265, p<0.05), IL-6 (0.129, p<0.05) and MCP-1 (0.779, p<0.05) and lower levels of IL-8 (-1.293, p<0.05) in the periphery, as well as elevated concentrations of YKL-40 (0.373, p<0.05), VILIP-1 (0.534, p<0.005) and sTREM2 (0.695, p<0.05) in CSF, were shown in MCI compared with the control. Additionally, increased peripheral sTNFR1 (0.582, p<0.05) and sTNFR2 (0.254, p<0.05) levels were observed in AD compared with MCI.

CONCLUSION

Significantly altered levels of inflammatory markers were verified in comparison between AD, MCI and control, supporting the notion that AD and MCI are accompanied by inflammatory responses in both the periphery and CSF.