The Amyloid Precursor Protein Intracellular Domain Is an Effector Molecule of Metaplasticity

The amyloid precursor protein: a converging point in Alzheimer's disease

The decades of evidence that showcase the role of amyloid precursor protein (APP), and its fragment amyloidβ (Aβ), in Alzheimer's disease (AD) pathogenesis are irrefutable. However, the absolute focus on the single APP metabolite Aβ as the cause for AD has resulted in APP and its other fragments that possess toxic propensity, to be overlooked as targets for treatment. The complexity of its processing and its association with systematic metabolism suggests that, if misregulated, APP has the potential to provoke an array of metabolic dysfunctions. This review discusses APP and several of its cleaved products with a particular focus on their toxicity and ability to disrupt healthy cellular function, in relation to AD development. We subsequently argue that the reduction of APP, which would result in a concurrent decrease in Aβ as well as all other toxic APP metabolites, would alleviate the toxic environment associated with AD and slow disease progression. A discussion of those drug-like compounds already identified to possess this capacity is also included.

Multi-Arm PEG/Peptidomimetic Conjugate Inhibitors of DR6/APP Interaction Block Hematogenous Tumor Cell Extravasation

The binding of amyloid precursor protein (APP) expressed on tumor cells to death receptor 6 (DR6) could initiate the necroptosis pathway, which leads to necroptotic cell death of vascular endothelial cells (ECs) and results in tumor cells (TCs) extravasation and metastasis. This study reports the first inhibitor of DR6/APP interaction as a novel class of anti-hematogenous metastatic agent. By rationally utilizing three combined strategies including selection based on phage display library, d-retro-inverso modification, and multiple conjugation of screened peptidomimetic with 4-arm PEG, the polymer-peptidomimetic conjugate PEG-tAHP-DRI (tetra-(D-retro-inverso isomer of AHP-12) substitued 4-arm PEG5k ) is obtained as the most promising agent with the strongest binding potency (KD  = 51.12 × 10-9  m) and excellent pharmacokinetic properties. Importantly, PEG-tAHP-DRI provides efficient protection against TC-induced ECs necroptosis both in vitro and in vivo. Moreover, this ligand exhibits prominent anti-hematogenous metastatic activity in serval different metastatic mouse models (B16F10, 4T1, CT26, and spontaneous lung metastasis of 4T1 orthotopic tumor model) and displays no apparent detrimental effects in preliminary safety evaluation. Collectively, this study demonstrates the feasibility of exploiting DR6/APP interaction to regulate hematogenous tumor cells transendothelial migration and provides PEG-tAHP-DRI as a novel and promising inhibitor of DR6/APP interaction for developments of anti-hematogenous metastatic therapies.

System-Level Analysis of Alzheimer's Disease Prioritizes Candidate Genes for Neurodegeneration

Alzheimer’s disease (AD) is a debilitating neurodegenerative disorder. Since the advent of the genome-wide association study (GWAS) we have come to understand much about the genes involved in AD heritability and pathophysiology. Large case-control meta-GWAS studies have increased our ability to prioritize weaker effect alleles, while the recent development of network-based functional prediction has provided a mechanism by which we can use machine learning to reprioritize GWAS hits in the functional context of relevant brain tissues like the hippocampus and amygdala. In parallel with these developments, groups like the Alzheimer’s Disease Neuroimaging Initiative (ADNI) have compiled rich compendia of AD patient data including genotype and biomarker information, including derived volume measures for relevant structures like the hippocampus and the amygdala. In this study we wanted to identify genes involved in AD-related atrophy of these two structures, which are often critically impaired over the course of the disease. To do this we developed a combined score prioritization method which uses the cumulative distribution function of a gene’s functional and positional score, to prioritize top genes that not only segregate with disease status, but also with hippocampal and amygdalar atrophy. Our method identified a mix of genes that had previously been identified in AD GWAS including APOE, TOMM40, and NECTIN2(PVRL2) and several others that have not been identified in AD genetic studies, but play integral roles in AD-effected functional pathways including IQSEC1, PFN1, and PAK2. Our findings support the viability of our novel combined score as a method for prioritizing region- and even cell-specific AD risk genes.

Transcranial magnetic stimulation reveals diminished homoeostatic metaplasticity in cognitively impaired adults

Homoeostatic metaplasticity is a neuroprotective physiological feature that counterbalances Hebbian forms of plasticity to prevent network destabilization and hyperexcitability. Recent animal models highlight dysfunctional homoeostatic metaplasticity in the pathogenesis of Alzheimer's disease. However, the association between homoeostatic metaplasticity and cognitive status has not been systematically characterized in either demented or non-demented human populations, and the potential value of homoeostatic metaplasticity as an early biomarker of cognitive impairment has not been explored in humans. Here, we report that, through pre-conditioning the synaptic activity prior to non-invasive brain stimulation, the association between homoeostatic metaplasticity and cognitive status could be established in a population of non-demented human subjects (older adults across cognitive spectrums; all within the non-demented range). All participants (n = 40; age range, 65-74, 47.5% female) underwent a standardized neuropsychological battery, magnetic resonance imaging and a transcranial magnetic stimulation protocol. Specifically, we sampled motor-evoked potentials with an input/output curve immediately before and after repetitive transcranial magnetic stimulation to assess neural plasticity with two experimental paradigms: one with voluntary muscle contraction (i.e. modulated synaptic activity history) to deliberately introduce homoeostatic interference, and one without to serve as a control condition. From comparing neuroplastic responses across these experimental paradigms and across cohorts grouped by cognitive status, we found that (i) homoeostatic metaplasticity is diminished in our cohort of cognitively impaired older adults and (ii) this neuroprotective feature remains intact in cognitively normal participants. This novel finding suggests that (i) future studies should expand their scope beyond just Hebbian forms of plasticity that are traditionally assessed when using non-invasive brain stimulation to investigate cognitive ageing and (ii) the potential value of homoeostatic metaplasticity in serving as a biomarker for cognitive impairment should be further explored.

Uniting homeostatic plasticity and exosome biology: A revision of the conceptual framework for drug discovery in neurodegenerative diseases?

Neurodegenerative diseases (NDDs) are in need of new drug discovery approaches. Our previous systematic analyses of Huntington's Disease (HD) literature for protein-protein interactors (PPIs) and modifiers of mutant Huntingtin-driven phenotypes revealed enrichment for PPIs of genes required for homeostatic synaptic plasticity (HSP) and exosome (EV) function and exosomal proteins, which in turn highly overlapped each other and with PPIs of genes associated with other NDDs. We proposed that HSP and EVs are linked to each other and are also involved in NDD pathophysiology. Recent studies showed that HSP is indeed altered in HD and AD, and that presynaptic homeostatic plasticity in motoneurons compensates for ALS pathology. Eliminating it causes earlier degeneration and death. If this holds true in other NDDs, drug discovery in animal models should then include elucidation of homeostatic compensation that either masks phenotypes of physiologically expressed mutant genes or are overridden by their overexpression. In this new conceptual framework, enhancing such underlying homeostatic compensation forms the basis for novel therapeutic strategies to slow progression of NDDs. Moreover, if EVs are linked to HSP, then their ability to penetrate the brain, target cell types, deliver miRNA and other molecules can be leveraged to develop attractive drug modalities. Testing this new framework is posed as four questions on model development and mechanistic studies progressing from higher throughput platforms to mouse models. Similar approaches may apply to other CNS disorders including schizophrenia, autism, Rett and Fragile X syndromes due to potential links of their susceptibility genes to HSP and EVs.

Microglia and Wnt Pathways: Prospects for Inflammation in Alzheimer's Disease

Alzheimer’s disease (AD) has been a major health issue for more than one century since it was first reported in 1906. As one of the most common neurodegenerative diseases, AD is characterized by the presence of senile plaques and neurofibrillary tangles (NFTs) in the affected brain area. Microglia are the major regulators of neuroinflammation in the brain, and neuroinflammation has become recognized as the core pathophysiological process of various neurodegenerative diseases. In the central nervous system (CNS), microglia play a dual role in AD development. For one thing, they degrade amyloid β (Aβ) to resist its deposition; for another, microglia release pro-inflammatory and inflammatory factors, contributing to neuroinflammation as well as the spreading of Aβ and tau pathology. Wnt pathways are important regulators of cell fate and cell activities. The dysregulation of Wnt pathways is responsible for both abnormal tau phosphorylation and synaptic loss in AD. Recent studies have also confirmed the regulatory effect of Wnt signaling on microglial inflammation. Thus, the study of microglia, Wnt pathways, and their possible interactions may open up a new direction for understanding the mechanisms of neuroinflammation in AD. In this review, we summarize the functions of microglia and Wnt pathways and their roles in AD in order to provide new ideas for understanding the pathogenesis of AD.

Amyloid, APP, and Electrical Activity of the Brain

The Amyloid Precursor Protein (APP) is infamous for its proposed pivotal role in the pathogenesis of Alzheimer’s disease (AD). Much research on APP focusses on potential contributions to neurodegeneration, mostly based on mouse models with altered expression or mutated forms of APP. However, cumulative evidence from recent years indicates the indispensability of APP and its metabolites for normal brain physiology. APP contributes to the regulation of synaptic transmission, plasticity, and calcium homeostasis. It plays an important role during development and it exerts neuroprotective effects. Of particular importance is the soluble secreted fragment APPsα which mediates many of its physiological actions, often counteracting the effects of the small APP-derived peptide Aβ. Understanding the contribution of APP for normal functions of the nervous system is of high importance, both from a basic science perspective and also as a basis for generating new pathophysiological concepts and therapeutic approaches in AD. In this article, we review the physiological functions of APP and its metabolites, focusing on synaptic transmission, plasticity, calcium signaling, and neuronal network activity.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.