Amyloid Beta Peptides Block New Synapse Assembly by Nogo Receptor-Mediated Inhibition of T-Type Calcium Channels

Updates in Alzheimer's disease: from basic research to diagnosis and therapies

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized pathologically by extracellular deposition of β-amyloid (Aβ) into senile plaques and intracellular accumulation of hyperphosphorylated tau (pTau) as neurofibrillary tangles. Clinically, AD patients show memory deterioration with varying cognitive dysfunctions. The exact molecular mechanisms underlying AD are still not fully understood, and there are no efficient drugs to stop or reverse the disease progression. In this review, we first provide an update on how the risk factors, including APOE variants, infections and inflammation, contribute to AD; how Aβ and tau become abnormally accumulated and how this accumulation plays a role in AD neurodegeneration. Then we summarize the commonly used experimental models, diagnostic and prediction strategies, and advances in periphery biomarkers from high-risk populations for AD. Finally, we introduce current status of development of disease-modifying drugs, including the newly officially approved Aβ vaccines, as well as novel and promising strategies to target the abnormal pTau. Together, this paper was aimed to update AD research progress from fundamental mechanisms to the clinical diagnosis and therapies.

Neurotoxic β-amyloid oligomers cause mitochondrial dysfunction-the trigger for PANoptosis in neurons

As the global population ages, the incidence of elderly patients with dementia, represented by Alzheimer's disease (AD), will continue to increase. Previous studies have suggested that β-amyloid protein (Aβ) deposition is a key factor leading to AD. However, the clinical efficacy of treating AD with anti-Aβ protein antibodies is not satisfactory, suggesting that Aβ amyloidosis may be a pathological change rather than a key factor leading to AD. Identification of the causes of AD and development of corresponding prevention and treatment strategies is an important goal of current research. Following the discovery of soluble oligomeric forms of Aβ (AβO) in 1998, scientists began to focus on the neurotoxicity of AβOs. As an endogenous neurotoxin, the active growth of AβOs can lead to neuronal death, which is believed to occur before plaque formation, suggesting that AβOs are the key factors leading to AD. PANoptosis, a newly proposed concept of cell death that includes known modes of pyroptosis, apoptosis, and necroptosis, is a form of cell death regulated by the PANoptosome complex. Neuronal survival depends on proper mitochondrial function. Under conditions of AβO interference, mitochondrial dysfunction occurs, releasing lethal contents as potential upstream effectors of the PANoptosome. Considering the critical role of neurons in cognitive function and the development of AD as well as the regulatory role of mitochondrial function in neuronal survival, investigation of the potential mechanisms leading to neuronal PANoptosis is crucial. This review describes the disruption of neuronal mitochondrial function by AβOs and elucidates how AβOs may activate neuronal PANoptosis by causing mitochondrial dysfunction during the development of AD, providing guidance for the development of targeted neuronal treatment strategies.

Clarity on the blazing trail: clearing the way for amyloid-removing therapies for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with a complex pathogenesis. Senile plaques composed of the amyloid-β (Aβ) peptide in the brain are the core hallmarks of AD and a promising target for the development of disease-modifying therapies. However, over the past 20 years, the failures of clinical trials directed at Aβ clearance have fueled a debate as to whether Aβ is the principal pathogenic factor in AD and a valid therapeutic target. The success of the recent phase 3 trials of lecanemab (Clarity AD) and donanemab (Trailblazer Alz2), and lessons from previous Aβ clearance trials provide critical evidence to support the role of Aβ in AD pathogenesis and suggest that targeting Aβ clearance is heading in the right direction for AD treatment. Here, we analyze key questions relating to the efficacy of Aβ targeting therapies, and provide perspectives on early intervention, adequate Aβ removal, sufficient treatment period, and combinatory therapeutics, which may be required to achieve the best cognitive benefits in future trials in the real world.

How does Nogo receptor influence demyelination and remyelination in the context of multiple sclerosis?

Multiple sclerosis (MS) can progress with neurodegeneration as a consequence of chronic inflammatory mechanisms that drive neural cell loss and/or neuroaxonal dystrophy in the central nervous system. Immune-mediated mechanisms can accumulate myelin debris in the disease extracellular milieu during chronic-active demyelination that can limit neurorepair/plasticity and experimental evidence suggests that potentiated removal of myelin debris can promote neurorepair in models of MS. The myelin-associated inhibitory factors (MAIFs) are integral contributors to neurodegenerative processes in models of trauma and experimental MS-like disease that can be targeted to promote neurorepair. This review highlights the molecular and cellular mechanisms that drive neurodegeneration as a consequence of chronic-active inflammation and outlines plausible therapeutic approaches to antagonize the MAIFs during the evolution of neuroinflammatory lesions. Moreover, investigative lines for translation of targeted therapies against these myelin inhibitors are defined with an emphasis on the chief MAIF, Nogo-A, that may demonstrate clinical efficacy of neurorepair during progressive MS.

Suspension TRAPping Filter (sTRAP) Sample Preparation for Quantitative Proteomics in the Low µg Input Range Using a Plasmid DNA Micro-Spin Column: Analysis of the Hippocampus from the 5xFAD Alzheimer's Disease Mouse Model

Suspension TRAPping filter (sTRAP) is an attractive sample preparation method for proteomics studies. The sTRAP protocol uses 5% SDS that maximizes protein solubilization. Proteins are trapped on a borosilicate glass membrane filter, where SDS is subsequently removed from the filter. After trypsin digestion, peptides are analyzed directly by LC-MS. Here, we demonstrated the use of a low-cost plasmid DNA micro-spin column for the sTRAP sample preparation of a dilution series of a synapse-enriched sample with a range of 10-0.3 µg. With 120 ng tryptic peptides loaded onto the Evosep LC system coupled to timsTOF Pro 2 mass spectrometer, we identified 5700 protein groups with 4% coefficient of variation (CoV). Comparing other sample preparation protocols, such as the in-gel digestion and the commercial Protifi S-TRAP with the plasmid DNA micro-spin column, the last is superior in both protein and peptide identification numbers and CoV. We applied sTRAP for the analysis of the hippocampal proteome from the 5xFAD mouse model of Alzheimer's disease and their wildtype littermates, and revealed 121 up- and 54 down-regulated proteins. Protein changes in the mutant mice point to the alteration of processes related to the immune system and Amyloid aggregation, which correlates well with the known major Alzheimer's-disease-related pathology. Data are available via ProteomeXchange with the identifier PXD041045.

Inhibition of Nogo-A rescues synaptic plasticity and associativity in APP/PS1 animal model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and cognitive decline. Synaptic impairment is one of the first events to occur in the progression of this disease. Synaptic plasticity and cellular association of various plastic events have been shown to be affected in AD models. Nogo-A, a well-known axonal growth inhibitor with a recently discovered role as a plasticity suppressor, and its main receptor Nogo-66 receptor 1 (NGR1) have been found to be overexpressed in the hippocampus of Alzheimer's patients. However, the role of Nogo-A and its receptor in the pathology of AD is still widely unknown. In this work we set out to investigate whether Nogo-A is working as a plasticity suppressor in AD. Our results show that inhibition of the Nogo-A pathway via the Nogo-R antibody in an Alzheimer's mouse model, APP/PS1, leads to the restoration of both synaptic plasticity and associativity in a protein synthesis and NMDR-dependent manner. We also show that inhibition of the p75^NTR pathway, which is strongly associated with NGR1, restores synaptic plasticity as well. Mechanistically, we propose that the restoration of synaptic plasticity in APP/PS1 via inhibition of the Nogo-A pathway is due to the modulation of the RhoA-ROCK2 pathway and increase in plasticity related proteins. Our study identifies Nogo-A as a plasticity suppressor in AD models hence targeting Nogo-A could be a promising strategy to understanding AD pathology.

A Function of Amyloid-β in Mediating Activity-Dependent Axon/Synapse Competition May Unify Its Roles in Brain Physiology and Pathology

Amyloid-β protein precursor (AβPP) gives rise to amyloid-β (Aβ), a peptide at the center of Alzheimer's disease (AD). AβPP, however, is also an ancient molecule dating back in evolution to some of the earliest forms of metazoans. This suggests a possible ancestral function that may have been obscured by those that evolve later. Based on literature from the functions of Aβ/AβPP in nervous system development, plasticity, and disease, to those of anti-microbial peptides (AMPs) in bacterial competition as well as mechanisms of cell competition uncovered first by Drosophila genetics, I propose that Aβ/AβPP may be part of an ancient mechanism employed in cell competition, which is subsequently co-opted during evolution for the regulation of activity-dependent neural circuit development and plasticity. This hypothesis is supported by foremost the high similarities of Aβ to AMPs, both of which possess unique, opposite (i.e., trophic versus toxic) activities as monomers and oligomers. A large body of data further suggests that the different Aβ oligomeric isoforms may serve as the protective and punishment signals long predicted to mediate activity-dependent axonal/synaptic competition in the developing nervous system and that the imbalance in their opposite regulation of innate immune and glial cells in the brain may ultimately underpin AD pathogenesis. This hypothesis can not only explain the diverse roles observed of Aβ and AβPP family molecules, but also provide a conceptual framework that can unify current hypotheses on AD. Furthermore, it may explain major clinical observations not accounted for and identify approaches for overcoming shortfalls in AD animal modeling.

LOTUS suppresses amyloid β-induced dendritic spine elimination through the blockade of amyloid β binding to PirB

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide but has no effective treatment. Amyloid beta (Aβ) protein, a primary risk factor for AD, accumulates and aggregates in the brain of patients with AD. Paired immunoglobulin-like receptor B (PirB) has been identified as a receptor of Aβ and Aβ-PirB molecular interactions that cause synapse elimination and synaptic dysfunction. PirB deletion has been shown to suppress Aβ-induced synaptic dysfunction and behavioral deficits in AD model mice, implying that PirB mediates Aβ-induced AD pathology. Therefore, inhibiting the Aβ-PirB molecular interaction could be a successful approach for combating AD pathology. We previously showed that lateral olfactory tract usher substance (LOTUS) is an endogenous antagonist of type1 Nogo receptor and PirB and that LOTUS overexpression promotes neuronal regeneration following damage to the central nervous system, including spinal cord injury and ischemic stroke. Therefore, in this study, we investigated whether LOTUS inhibits Aβ-PirB interaction and Aβ-induced dendritic spine elimination.

METHODS

The inhibitory role of LOTUS against Aβ-PirB (or leukocyte immunoglobulin-like receptor subfamily B member 2: LilrB2) binding was assessed using a ligand-receptor binding assay in Cos7 cells overexpressing PirB and/or LOTUS. We assessed whether LOTUS inhibits Aβ-induced intracellular alterations and synaptotoxicity using immunoblots and spine imaging in a primary cultured hippocampal neuron.

RESULTS

We found that LOTUS inhibits the binding of Aβ to PirB overexpressed in Cos7 cells. In addition, we found that Aβ-induced dephosphorylation of cofilin and Aβ-induced decrease in post-synaptic density-95 expression were suppressed in cultured hippocampal neurons from LOTUS-overexpressing transgenic (LOTUS-tg) mice compared with that in wild-type mice. Moreover, primary cultured hippocampal neurons from LOTUS-tg mice improved the Aβ-induced decrease in dendritic spine density. Finally, we studied whether human LOTUS protein inhibits Aβ binding to LilrB2, a human homolog of PirB, and found that human LOTUS inhibited the binding of Aβ to LilrB2 in a similar manner.

CONCLUSIONS

This study implied that LOTUS improved Aβ-induced synapse elimination by suppressing Aβ-PirB interaction in rodents and inhibited Aβ-LilrB2 interaction in humans. Our findings revealed that LOTUS may be a promising therapeutic agent in counteracting Aβ-induced AD pathologies.

Protofibrillar Amyloid Beta Modulation of Recombinant hCaV2.2 (N-Type) Voltage-Gated Channels

Cav2.2 channels are key regulators of presynaptic Ca²⁺ influx and their dysfunction and/or aberrant regulation has been implicated in many disease states; however, the nature of their involvement in Alzheimer's disease (AD) is less clear. In this short communication, we show that recombinant hCav2.2/b_1b/a₂d₁ channels are modulated by human synthetic AD-related protofibrillar amyloid beta Ab_1-42 peptides. Structural studies revealed a time-dependent increase in protofibril length, with the majority of protofibrils less than 100 nm at 24 h, while at 48 h, the majority were longer than 100 nm. Cav2.2 modulation by Ab_1-42 was different between a 'low' (100 nM) and 'high' (1 µM) concentration in terms of distinct effects on individual biophysical parameters. A concentration of 100 nM Ab_1-42 caused no significant changes in the measured biophysical properties of Cav2.2 currents. In contrast, 1 µM Ab_1-42 caused an inhibitory decrease in the current density (pA/pF) and maximum conductance (Gmax), and a depolarizing shift in the slope factor (k). These data highlight a differential modulation of Cav2.2 channels by the Ab_1-42 peptide. Discrete changes in the presynaptic Ca²⁺ flux have been reported to occur at an early stage of AD; therefore, this study reveals a potential mechanistic link between amyloid accumulation and Ca_v2.2 channel modulation.

Altered pain sensitivity in 5×familial Alzheimer disease mice is associated with dendritic spine loss in anterior cingulate cortex pyramidal neurons

ABSTRACT

Chronic pain is highly prevalent. Individuals with cognitive disorders such as Alzheimer disease are a susceptible population in which pain is frequently difficult to diagnosis. It is still unclear whether the pathological changes in patients with Alzheimer disease will affect pain processing. Here, we leverage animal behavior, neural activity recording, optogenetics, chemogenetics, and Alzheimer disease modeling to examine the contribution of the anterior cingulate cortex (ACC) neurons to pain response. The 5× familial Alzheimer disease mice show alleviated mechanical allodynia which can be regained by the genetic activation of ACC excitatory neurons. Furthermore, the lower peak neuronal excitation, delayed response initiation, as well as the dendritic spine reduction of ACC pyramidal neurons in 5×familial Alzheimer disease mice can be mimicked by Rac1 or actin polymerization inhibitor in wild-type (WT) mice. These findings indicate that abnormal of pain sensitivity in Alzheimer disease modeling mice is closely related to the variation of neuronal activity and dendritic spine loss in ACC pyramidal neurons, suggesting the crucial role of dendritic spine density in pain processing.

MCU knockdown in hippocampal neurons improves memory performance of an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a progressive and degenerative disorder accompanied by cognitive decline, which could be promoted by mitochondrial dysfunction induced by mitochondrial Ca ²⁺ (mCa ²⁺) homeostasis Mitochondrial calcium uniporter (MCU), a key channel of mCa ²⁺ uptake, may be a target for AD treatment. In the present study, we reveal for the first time that MCU knockdown in hippocampal neurons improves the memory performance of APP/PS1/tau mice through radial arm maze task. Western blot analysis, transmission electron microscopy (TEM), Golgi staining, immunohistochemistry (IHC) and ELISA results demonstrate that MCU knockdown in hippocampal neurons upregulates the levels of postsynaptic density protein 95 (PSD95) and synaptophysin (SYP), and increases the numbers of synapses and dendritic spines. Meanwhile, MCU knockdown in hippocampal neurons decreases the neuroinflammatory response induced by astrogliosis and high levels of IL-1β and TNF-α, and improves the PINK1-Parkin mitophagy signaling pathway and increases the level of Beclin-1 but decreases the level of P62. In addition, MCU knockdown in hippocampal neurons recovers the average volume and number of mitochondria. These data confirm that MCU knockdown in hippocampal neurons improves the memory performance of APP/PS1/tau mice through ameliorating the synapse structure and function, relieving the inflammation response and recovering mitophagy, indicating that MCU inhibition has the potential to be developed as a novel therapy for AD.

Trio and Kalirin as unique enactors of Rho/Rac spatiotemporal precision

Rac1 and RhoA are among the most widely studied small GTPases. The classic dogma surrounding their biology has largely focused on their activity as an "on/off switch" of sorts. However, the advent of more sophisticated techniques, such as genetically-encoded FRET-based sensors, has afforded the ability to delineate the spatiotemporal regulation of Rac1 and RhoA. As a result, there has been a shift from this simplistic global view to one incorporating the precision of spatiotemporal modularity. This review summarizes emerging data surrounding the roles of Rac1 and RhoA as cytoskeletal regulators and examines how these new data have led to a revision of the traditional dogma which placed Rac1 and RhoA in antagonistic pathways. This more recent evidence suggests that rather than absolute activity levels, it is the tight spatiotemporal regulation of Rac1 and RhoA across multiple roles, from oppositional to complementary, that is necessary to execute coordinated cytoskeletal processes affecting cell structure, function, and migration. We focus on how Kalirin and Trio, as dual GEFs that target Rac1 and RhoA, are uniquely designed to provide the spatiotemporally-precise shifts in Rac/Rho balance which mediate changes in neuronal structure and function, particularly by way of cytoskeletal rearrangements. Finally, we review how alterations in Trio and/or Kalirin function are associated with cellular abnormalities and neuropsychiatric disease.

Conformational and functional changes of the native neuropeptide somatostatin occur in the presence of copper and amyloid-β

The progression of neurodegenerative disorders can lead to impaired neurotransmission; however, the role of pathogenic factors associated with these diseases and their impact on the structures and functions of neurotransmitters have not been clearly established. Here we report the discovery that conformational and functional changes of a native neuropeptide, somatostatin (SST), occur in the presence of copper ions, metal-free amyloid-β (Aβ) and metal-bound Aβ (metal-Aβ) found as pathological factors in the brains of patients with Alzheimer's disease. These pathological elements induce the self-assembly of SST and, consequently, prevent it from binding to the receptor. In the reverse direction, SST notably modifies the aggregation profiles of Aβ species in the presence of metal ions, attenuating their cytotoxicity and interactions with cell membranes. Our work demonstrates a loss of normal function of SST as a neurotransmitter and a gain of its modulative function against metal-Aβ under pathological conditions.

Cav3 T-Type Voltage-Gated Ca2+ Channels and the Amyloidogenic Environment: Pathophysiology and Implications on Pharmacotherapy and Pharmacovigilance

Voltage-gated Ca2+ channels (VGCCs) were reported to play a crucial role in neurotransmitter release, dendritic resonance phenomena and integration, and the regulation of gene expression. In the septohippocampal system, high- and low-voltage-activated (HVA, LVA) Ca2+ channels were shown to be involved in theta genesis, learning, and memory processes. In particular, HVA Cav2.3 R-type and LVA Cav3 T-type Ca2+ channels are expressed in the medial septum-diagonal band of Broca (MS-DBB), hippocampal interneurons, and pyramidal cells, and ablation of both channels was proven to severely modulate theta activity. Importantly, Cav3 Ca2+ channels contribute to rebound burst firing in septal interneurons. Consequently, functional impairment of T-type Ca2+ channels, e.g., in null mutant mouse models, caused tonic disinhibition of the septohippocampal pathway and subsequent enhancement of hippocampal theta activity. In addition, impairment of GABA A/B receptor transcription, trafficking, and membrane translocation was observed within the septohippocampal system. Given the recent findings that amyloid precursor protein (APP) forms complexes with GABA B receptors (GBRs), it is hypothesized that T-type Ca2+ current reduction, decrease in GABA receptors, and APP destabilization generate complex functional interdependence that can constitute a sophisticated proamyloidogenic environment, which could be of potential relevance in the etiopathogenesis of Alzheimer's disease (AD). The age-related downregulation of T-type Ca2+ channels in humans goes together with increased Aβ levels that could further inhibit T-type channels and aggravate the proamyloidogenic environment. The mechanistic model presented here sheds new light on recent reports about the potential risks of T-type Ca2+ channel blockers (CCBs) in dementia, as observed upon antiepileptic drug application in the elderly.

Role of Calcium Homeostasis in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease associated with senile plaques (SP) and neurofibrillary tangles (NFTs) in the brain. With aging of the population, AD has become the most common form of dementia. However, the mechanisms leading to AD are still under investigation, and there are currently no specific drugs for its treatment. Therefore, further study on the pathogenesis of AD to develop new drugs for AD treatment remains a top priority. Several studies have suggested that intracellular calcium homeostasis is dysregulated in AD, and this has been implicated in the deposition of amyloid β (Aβ), hyperphosphorylation of tau protein, abnormal synaptic plasticity, and apoptosis, all of which are involved in the occurrence and development of AD. In addition, some based on pathways linking calcium homeostasis and AD have achieved results in AD treatment. This review comprehensively explores the relationship between calcium homeostasis and the pathogenesis of AD to provide a theoretical basis for the future exploration of AD and the development of novel therapeutic drugs.

A Kalirin missense mutation enhances dendritic RhoA signaling and leads to regression of cortical dendritic arbors across development

Normally, dendritic size is established prior to adolescence and then remains relatively constant into adulthood due to a homeostatic balance between growth and retraction pathways. However, schizophrenia is characterized by accelerated reductions of cerebral cortex gray matter volume and onset of clinical symptoms during adolescence, with reductions in layer 3 pyramidal neuron dendritic length, complexity, and spine density identified in multiple cortical regions postmortem. Nogo receptor 1 (NGR1) activation of the GTPase RhoA is a major pathway restricting dendritic growth in the cerebral cortex. We show that the NGR1 pathway is stimulated by OMGp and requires the Rho guanine nucleotide exchange factor Kalirin-9 (KAL9). Using a genetically encoded RhoA sensor, we demonstrate that a naturally occurring missense mutation in Kalrn, KAL-PT, that was identified in a schizophrenia cohort, confers enhanced RhoA activitation in neuronal dendrites compared to wild-type KAL. In mice containing this missense mutation at the endogenous locus, there is an adolescent-onset reduction in dendritic length and complexity of layer 3 pyramidal neurons in the primary auditory cortex. Spine density per unit length of dendrite is unaffected. Early adult mice with these structural deficits exhibited impaired detection of short gap durations. These findings provide a neuropsychiatric model of disease capturing how a mild genetic vulnerability may interact with normal developmental processes such that pathology only emerges around adolescence. This interplay between genetic susceptibility and normal adolescent development, both of which possess inherent individual variability, may contribute to heterogeneity seen in phenotypes in human neuropsychiatric disease.

Emerging targets in drug discovery against neurodegenerative diseases: Control of synapsis disfunction by the RhoA/ROCK pathway

Synaptic spine morphology is controlled by the activity of Rac1, Cdc42 and RhoA, which need to be finely balanced, and in particular RhoA/ROCK prevents the formation of new protrusions by stabilizing actin formation. These processes are crucial to the maturation process, slowing the de novo generation of new spines. The RhoA/ROCK also influences plasticity processes, and selective modulation by ROCK1 of MLC-dependent actin dynamics leads to neurite retraction, but not to spine retraction. ROCK1 is also responsible for the reduction of the readily releasable pool of synaptic vesicles. These and other evidences suggest that ROCK1 is the main isoform acting on the presynaptic neuron. On the other hand, ROCK2 seems to have broad effects on LIMK/cofilin-dependent plasticity processes such as cofilin-dependent PSD changes. The RhoA/ROCK pathway is an important factor in several different brain-related pathologies via both downstream and upstream pathways. In the aggregate, these evidences show that the RhoA/ROCK pathway has a central role in the etiopathogenesis of a large group of CNS diseases, which underscores the importance of the pharmacological modulation of RhoA/ROCK as an important pathway to drug discovery in the neurodegenerative disease area. This article aims at providing the first review of the role of compounds acting on the RhoA/ROCK pathway in the control of synaptic disfunction.

Aβ receptors specifically recognize molecular features displayed by fibril ends and neurotoxic oligomers

Several cell-surface receptors for neurotoxic forms of amyloid-β (Aβ) have been described, but their molecular interactions with Aβ assemblies and their relative contributions to mediating Alzheimer's disease pathology have remained uncertain. Here, we used super-resolution microscopy to directly visualize Aβ-receptor interactions at the nanometer scale. We report that one documented Aβ receptor, PrPC, specifically inhibits the polymerization of Aβ fibrils by binding to the rapidly growing end of each fibril, thereby blocking polarized elongation at that end. PrPC binds neurotoxic oligomers and protofibrils in a similar fashion, suggesting that it may recognize a common, end-specific, structural motif on all of these assemblies. Finally, two other Aβ receptors, FcγRIIb and LilrB2, affect Aβ fibril growth in a manner similar to PrPC. Our results suggest that receptors may trap Aβ oligomers and protofibrils on the neuronal surface by binding to a common molecular determinant on these assemblies, thereby initiating a neurotoxic signal.

Amyloid-β (25-35) regulates neuronal damage and memory loss via SIRT1/Nrf2 in the cortex of mice

Alzheimer's disease (AD) is the most common type of dementia. AD is pathologically characterized by synaptic dysfunction and cognitive decline due to the aggregation of a large amount of amyloid-β (Aβ) protein in the brain. However, recent studies have discovered that the Aβ is produced as an antimicrobial peptide that acts against bacteria and viruses. This has renewed interest in the effect of Aβ on AD. Thus, in this study, we investigated the different concentrations of Aβ_25-35 on neuroprotection and further explore the related mechanisms. Firstly, we detected the cognitive function using the Y-maze test, novel object recognition memory task and Morris water maze test. Then, we analyzed the ultrastructure of synapses and mitochondria, in addition to evaluating SOD levels. We also examined the effect of Aβ_25-35 on the viability and structure of the primary neurons. The western blot analysis was used to measure the protein levels. The results showed that mice treated with high concentration of Aβ_25-35 impaired the learning-memory ability and disordered the structure of neurons and mitochondria. Meanwhile, high concentration of Aβ_25-35 decreased the SIRT1/Nrf2 related antioxidant capacity and induced apoptosis. In contrast, mice treated with low concentration of Aβ_25-35 increased SOD levels and SIRT1/Nrf2 expressions, and induced autophagy. Our data suggest that low concentration of Aβ_25-35 may increase SOD levels through SIRT1/Nrf2 and induce autophagy.

LOTUS, an endogenous Nogo receptor antagonist, is involved in synapse and memory formation

The Nogo signal is involved in impairment of memory formation. We previously reported the lateral olfactory tract usher substance (LOTUS) as an endogenous antagonist of the Nogo receptor 1 that mediates the inhibition of axon growth and synapse formation. Moreover, we found that LOTUS plays an essential role in neural circuit formation and nerve regeneration. However, the effects of LOTUS on synapse formation and memory function have not been elucidated. Here, we clearly showed the involvement of LOTUS in synapse formation and memory function. The cultured hippocampal neurons derived from lotus gene knockout (LOTUS-KO) mice exhibited a decrease in synaptic density compared with those from wild-type mice. We also found decrease of dendritic spine formation in the adult hippocampus of LOTUS-KO mice. Finally, we demonstrated that LOTUS deficiency impairs memory formation in the social recognition test and the Morris water maze test, indicating that LOTUS is involved in functions of social and spatial learning and memory. These findings suggest that LOTUS affects synapse formation and memory function.

Lentivirus-Mediated Expression of Human Secreted Amyloid Precursor Protein-Alpha Promotes Long-Term Induction of Neuroprotective Genes and Pathways in a Mouse Model of Alzheimer's Disease

BACKGROUND

Secreted amyloid precursor protein-alpha (sAPPα) can enhance memory and is neurotrophic and neuroprotective across a range of disease-associated insults, including amyloid-β toxicity. In a significant step toward validating sAPPα as a therapeutic for Alzheimer's disease (AD), we demonstrated that long-term overexpression of human sAPPα (for 8 months) in a mouse model of amyloidosis (APP/PS1) could prevent the behavioral and electrophysiological deficits that develop in these mice.

OBJECTIVE

To explore the underlying molecular mechanisms responsible for the significant physiological and behavioral improvements observed in sAPPα-treated APP/PS1 mice.

METHODS

We assessed the long-term effects on the hippocampal transcriptome following continuous lentiviral delivery of sAPPα or empty-vector to male APP/PS1 mice and wild-type controls using Affymetrix Mouse Transcriptome Assays. Data analysis was carried out within the Affymetrix Transcriptome Analysis Console and an integrated analysis of the resulting transcriptomic data was performed with Ingenuity Pathway analysis (IPA).

RESULTS

Mouse transcriptome assays revealed expected AD-associated gene expression changes in empty-vector APP/PS1 mice, providing validation of the assays used for the analysis. By contrast, there were specific sAPPα-associated gene expression profiles which included increases in key neuroprotective genes such as Decorin, betaine-GABA transporter and protocadherin beta-5, subsequently validated by qRT-PCR. An integrated biological pathways analysis highlighted regulation of GABA receptor signaling, cell survival and inflammatory responses. Furthermore, upstream gene regulatory analysis implicated sAPPα activation of Interleukin-4, which can counteract inflammatory changes in AD.

CONCLUSION

This study identified key molecular processes that likely underpin the long-term neuroprotective and therapeutic effects of increasing sAPPα levels in vivo.

[Role of T-type Calcium Channels in Regulating Neuronal Function]

T-type calcium channels are low-threshold voltage-gated calcium channel and characterized by unique electrophysiological properties such as fast inactivation and slow deactivation kinetics. All subtypes of T-type calcium channel (Cav3.1, 3.2 and 3.3) are widely expressed in the central nerve system, and they have an important role in homeostasis of sleep, pain response, and development of epilepsy. Recently, several reports suggest that T-type calcium channels may mediate neuronal plasticity in the mouse brain. We succeeded to develop T-type calcium channel enhancer ethyl 8'-methyl-2',4-dioxo-2-(piperidin-1-yl)-2'H-spiro[cyclopentane-1,3'-imidazo[1,2-a]pyridine]-2-ene-3-carboxylate (SAK3) which enhances Cav3.1 and 3.3 currents in each-channel expressed neuro2A cells. SAK3 can promote acetylcholine (ACh) release in the mouse hippocampus via enhancing T-type calcium channel. In this review, we have introduced the role of T-type calcium channel, especially Cav3.1 channel in the mouse hippocampus based on our previous data using SAK3 and Cav3.1 knockout mice.

Chondroitin sulfate proteoglycans: key modulators of neuronal plasticity, long-term memory, neurodegenerative, and psychiatric disorders

The chondroitin sulfate proteoglycans (CSPGs) are large groups of heterogenous proteoglycans that are mainly expressed by reactive astrocytes in the central nervous system (CNS). They share similar core proteins and are post-transcriptionally modified by chondroitin sulfate glycosaminoglycans. CSPGs are the major components of the perineuronal nets (PNN) that regulate the opening and closure of the critical period. Mounting reports have documented the crucial roles of CSPGs in restricting neuronal plasticity, axonal growth, and pathfinding during development as well as axonal regeneration after CNS injury. Moreover, CSPGs and PNNs modulate long-term memory, which impairments frequently happened in several neurodegenerative and psychiatric disorders. This review will shortly introduce the expression patterns of CSPGs during development and after injury, the PNNs constitutions, the roles of CSPGs and PNNs in axonal regrowth, discuss the most recently identified roles of CSPGs and PNNs in mediating long-term memory and their correlation with brain disorders, and finally, propose a short perspective of future investigations. Hopefully, further explorations may validate the therapeutic potentials of PNNs and CSPGs.

SAK3 Administration Improves Spine Abnormalities and Cognitive Deficits in AppNL-G-F/NL-G-F Knock-in Mice by Increasing Proteasome Activity through CaMKII/Rpt6 Signaling

Alzheimer's disease (AD) is the most common form of dementia and is characterized by neuropathological hallmarks consisting of accumulation of extracellular amyloid-β (Aβ) plaques and intracellular neurofibrillary tangles (NFT). Recently, we have identified a new AD therapeutic candidate, ethyl-8'-methyl-2',4-dioxo-2-(piperidin-1-yl)-2'H-spiro[cyclopentane-1,3'-imidazo [1,2-a] pyridin]-2-ene-3-carboxylate (SAK3), which ameliorates the AD-like pathology in AppNL-F/NL-F knock-in mice. However, the detailed mechanism underlying the therapeutic effects of SAK3 remains unclear. In this study, we found that SAK3 administration improved the reduced proteasome activity through the activation of CaMKII/Rpt6 signaling in AppNL-F/NL-F knock-in (NL-G-F) mice. Moreover, spine abnormalities observed in NL-G-F mice were significantly reversed by SAK3 administration. Along with this, cognitive impairments found in NL-G-F mice were markedly ameliorated by SAK3. In summary, our data suggest that SAK3 administration increases the activity of the proteasome via activation of the CaMKII/Rpt6 signaling pathway, contributing to improvements in spine abnormalities and cognitive deficits in NL-G-F mice. Overall, our findings suggest that SAK3 might be a new attractive drug candidate, representing a new mechanism for the treatment of AD pathology.

APP Osaka Mutation in Familial Alzheimer's Disease-Its Discovery, Phenotypes, and Mechanism of Recessive Inheritance

Alzheimer’s disease is believed to begin with synaptic dysfunction caused by soluble Aβ oligomers. When this oligomer hypothesis was proposed in 2002, there was no direct evidence that Aβ oligomers actually disrupt synaptic function to cause cognitive impairment in humans. In patient brains, both soluble and insoluble Aβ species always coexist, and therefore it is difficult to determine which pathologies are caused by Aβ oligomers and which are caused by amyloid fibrils. Thus, no validity of the oligomer hypothesis was available until the Osaka mutation was discovered. This mutation, which was found in a Japanese pedigree of familial Alzheimer’s disease, is the deletion of codon 693 of APP gene, resulting in mutant Aβ lacking the 22nd glutamate. Only homozygous carriers suffer from dementia. In vitro studies revealed that this mutation has a very unique character that accelerates Aβ oligomerization but does not form amyloid fibrils. Model mice expressing this mutation demonstrated that all pathologies of Alzheimer’s disease can be induced by Aβ oligomers alone. In this review, we describe the story behind the discovery of the Osaka mutation, summarize the mutant’s phenotypes, and propose a mechanism of its recessive inheritance.

Interaction of Aβ42 with Membranes Triggers the Self-Assembly into Oligomers

The self-assembly of amyloid β (Aβ) proteins into oligomers is the major pathogenic event leading to Alzheimer’s disease (AD). Typical in vitro experiments require high protein concentrations, whereas the physiological concentration of Aβ is in the picomolar to low nanomolar range. This complicates the translation of results obtained in vitro to understanding the aggregation process in vivo. Here, we demonstrate that Aβ42 self-assembles into aggregates on membrane bilayers at low nanomolar concentrations - a pathway in which the membrane plays the role of a catalyst. Additionally, physiological ionic conditions (150 mM NaCl) significantly enhance on-membrane aggregation, leading to the rapid formation of oligomers. The self-assembly process is reversible, so assembled aggregates can dissociate from the membrane surface into the bulk solution to further participate in the aggregation process. Molecular dynamics simulations demonstrate that the transient membrane-Aβ interaction dramatically changes the protein conformation, facilitating the assembly of dimers. The results indicate peptide–membrane interaction is the critical step towards oligomer formation at physiologically low protein concentrations.

Activity-Induced Amyloid-β Oligomers Drive Compensatory Synaptic Rearrangements in Brain Circuits Controlling Memory of Presymptomatic Alzheimer's Disease Mice

BACKGROUND

A consistent proportion of individuals at risk for Alzheimer's disease show intact cognition regardless of the extensive accumulation of amyloid-β (Aβ) peptide in their brain. Several pieces of evidence indicate that overactivation of brain regions negative for Aβ can compensate for the underactivation of Aβ-positive ones to preserve cognition, but the underlying synaptic changes are still unexplored.

METHODS

Using Golgi staining, we investigate how dendritic spines rearrange following contextual fear conditioning (CFC) in the hippocampus and amygdala of presymptomatic Tg2576 mice, a genetic model for Aβ accumulation. A molecular biology approach combined with intrahippocampal injection of a γ-secretase inhibitor evaluates the impact of Aβ fluctuations on spine rearrangements.

RESULTS

Encoding of CFC increases Aβ oligomerization in the hippocampus but not in the amygdala of Tg2576 mice. The presence of Aβ oligomers predicts vulnerability to network dysfunctions, as low c-Fos activation and spine maturation are detected in the hippocampus of Tg2576 mice upon recall of CFC memory. Rather, enhanced c-Fos activation and new spines are evident in the amygdala of Tg2576 mice compared with wild-type control mice. Preventing Aβ increase in the hippocampus of Tg2576 mice restores CFC-associated spine changes to wild-type levels in both the hippocampus and amygdala.

CONCLUSIONS

Our study provides the first evidence of neural compensation consisting of enhanced synaptic activity in brain regions spared by Aβ load. Furthermore, it unravels an activity-mediated feedback loop through which neuronal activation during CFC encoding favors Aβ oligomerization in the hippocampus and prevents synaptic rearrangements in this region.

The kinetic mechanisms of fast-decay red-fluorescent genetically encoded calcium indicators

Genetically encoded calcium indicators (GECIs) are useful reporters of cell-signaling, neuronal, and network activities. We have generated novel fast variants and investigated the kinetic mechanisms of two recently developed red-fluorescent GECIs (RGECIs), mApple-based jRGECO1a and mRuby-based jRCaMP1a. In the formation of fluorescent jRGECO1a and jRCaMP1a complexes, calcium binding is followed by rate-limiting isomerization. However, fluorescence decay of calcium-bound jRGECO1a follows a different pathway from its formation: dissociation of calcium occurs first, followed by the peptide, similarly to GCaMP-s. In contrast, fluorescence decay of calcium-bound jRCaMP1a occurs by the reversal of the on-pathway: peptide dissociation is followed by calcium. The mechanistic differences explain the generally slower off-kinetics of jRCaMP1a-type indicators compared with GCaMP-s and jRGECO1a-type GECI: the fluorescence decay rate of f-RCaMP1 was 21 s-1, compared with 109 s-1 for f-RGECO1 and f-RGECO2 (37 °C). Thus, the CaM-peptide interface is an important determinant of the kinetic responses of GECIs; however, the topology of the structural link to the fluorescent protein demonstrably affects the internal dynamics of the CaM-peptide complex. In the dendrites of hippocampal CA3 neurons, f-RGECO1 indicates calcium elevation in response to a 100 action potential train in a linear fashion, making the probe particularly useful for monitoring large-amplitude, fast signals, e.g. those in dendrites, muscle cells, and immune cells.

Effects of NMDAR Antagonist on the Regulation of P-MARCKS Protein to Aβ1-42 Oligomers Induced Neurotoxicity

Alzheimer's disease (AD) is a well-known neurodegenerative disease. Deposition of β-amyloid protein (Aβ) oligomers plays a crucial role in the disease progression. Previous studies showed that toxicity induced by Aβ oligomers in cultured neurons and adult rat brain was partially mediated by activation of glutamatergic N-methyl-D-aspartate receptors (NMDAR). Additionally, memantine, a noncompetitive NMDAR antagonist, can significantly improve cognitive functions in some AD patients. However, little is currently known about the potential role of NMDAR antagonist on the regulation of P-MARCKS protein to Aβ_1-42 oligomers induced neurotoxicity. The protective effect and mechanism of NMDAR antagonist on primary neurons exposed to Aβ_1-42 oligomers were investigated in the study. We have defined that the Aβ_1-42 treatment decreased cell viability and increased apoptosis. Moreover, Aβ_1-42 oligomers exposure increased P-MARCKS and PIP2 expressions, while decreased SYP expression. However, NMDAR antagonist pretreatment ameliorates Aβ_1-42 oligomers induced neuronal apoptosis and partially reverses the expression of P-MARCKS, PIP2 and SYP. In conclusion, NMDAR antagonist may ameliorate neurotoxicity induced by Aβ_1-42 oligomers through reducing neuronal apoptosis and protecting synaptic plasticity in rat primary neurons. The mechanism involved may be mediated by the variation of protein P-MARCKS.

Excitatory Dendritic Mitochondrial Calcium Toxicity: Implications for Parkinson's and Other Neurodegenerative Diseases

Dysregulation of calcium homeostasis has been linked to multiple neurological diseases. In addition to excitotoxic neuronal cell death observed following stroke, a growing number of studies implicate excess excitatory neuronal activity in chronic neurodegenerative diseases. Mitochondria function to rapidly sequester large influxes of cytosolic calcium through the activity of the mitochondrial calcium uniporter (MCU) complex, followed by more gradual release via calcium antiporters, such as NCLX. Increased cytosolic calcium levels almost invariably result in increased mitochondrial calcium uptake. While this response may augment mitochondrial respiration, limiting classic excitotoxic injury in the short term, recent studies employing live calcium imaging and molecular manipulation of calcium transporter activities suggest that mitochondrial calcium overload plays a key role in Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and related dementias [PD with dementia (PDD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD)]. Herein, we review the literature on increased excitatory input, mitochondrial calcium dysregulation, and the transcriptional or post-translational regulation of mitochondrial calcium transport proteins, with an emphasis on the PD-linked kinases LRRK2 and PINK1. The impact on pathological dendrite remodeling and neuroprotective effects of manipulating MCU, NCLX, and LETM1 are reviewed. We propose that shortening and simplification of the dendritic arbor observed in neurodegenerative diseases occur through a process of excitatory mitochondrial toxicity (EMT), which triggers mitophagy and perisynaptic mitochondrial depletion, mechanisms that are distinct from classic excitotoxicity.

Abnormal dendritic calcium activity and synaptic depotentiation occur early in a mouse model of Alzheimer's disease

Background

Alzheimer’s disease (AD) is characterized by amyloid deposition, tangle formation as well as synapse loss. Synaptic abnormalities occur early in the pathogenesis of AD. Identifying early synaptic abnormalities and their underlying mechanisms is likely important for the prevention and treatment of AD.

Methods

We performed in vivo two-photon calcium imaging to examine the activities of somas, dendrites and dendritic spines of layer 2/3 pyramidal neurons in the primary motor cortex in the APPswe/PS1dE9 mouse model of AD and age-matched wild type control mice. We also performed calcium imaging to determine the effect of Aβ oligomers on dendritic calcium activity. In addition, structural and functional two-photon imaging were used to examine the link between abnormal dendritic calcium activity and changes in dendritic spine size in the AD mouse model.

Results

We found that somatic calcium activities of layer 2/3 neurons were significantly lower in the primary motor cortex of 3-month-old APPswe/PS1dE9 mice than in wild type mice during quiet resting, but not during running on a treadmill. Notably, a significantly larger fraction of apical dendrites of layer 2/3 pyramidal neurons showed calcium transients with abnormally long duration and high peak amplitudes during treadmill running in AD mice. Administration of Aβ oligomers into the brain of wild type mice also induced abnormal dendritic calcium transients during running. Furthermore, we found that the activity and size of dendritic spines were significantly reduced on dendritic branches with abnormally prolonged dendritic calcium transients in AD mice.

Conclusion

Our findings show that abnormal dendritic calcium transients and synaptic depotentiation occur before amyloid plaque formation in the motor cortex of the APPswe/PS1dE9 mouse model of AD. Dendritic calcium transients with abnormally long durations and high amplitudes could be induced by soluble Aβ oligomers and contribute to synaptic deficits in the early pathogenesis of AD.

Electronic supplementary material

The online version of this article (10.1186/s13024-017-0228-2) contains supplementary material, which is available to authorized users.