Identification of the probable structure of the sAPPα-GABABR1a complex and theoretical solutions for such cases

Aggregation Dynamics Characteristics of Seven Different Aβ Oligomeric Isoforms-Dependence on the Interfacial Interaction

The aggregation of β-amyloid (Aβ) peptides has been confirmed to be associated with the onset of Alzheimer's disease (AD). Among the three phases of Aβ aggregation, the lag phase has been considered to be the best time for early Aβ pathological deposition clinical intervention and prevention for potential patients with normal cognition. Aβ peptide exists in various lengths in vivo, and Aβ oligomer in the early lag phase is neurotoxic but polymorphous and metastable, depending on Aβ length (isoform), molecular weight, and specific phase, and therefore hardly characterized experimentally. To cope with the problem, molecular dynamics simulation was used to investigate the aggregation process of five monomers for each of the seven common Aβ isoforms during the lag phase. Results showed that Aβ(1-40) and Aβ(1-38) monomers aggregated faster than their truncated analogues Aβ(4-40) and Aβ(4-38), respectively. However, the aggregation rate of Aβ(1-42) was slower than that of its truncated analogues Aβ(4-42) rather than that of Aβpe(3-42). More importantly, Aβ(1-38) is first predicted as more likely to form stable hexamer than the remaining five Aβ isoforms, as Aβ(1-42) does. It is hydrophobic interaction mainly (>50%) from the interfacial β1 and β2 regions of two reactants, pentamer and monomer, aggregated by Aβ(1-38)/Aβ(1-42) rather than by other Aβ isoforms, that drives the hexamer stably as a result of the formation of the effective hydrophobic collapse. This paper provides new insights into the aggregation characteristics of Aβ with different lengths and the conditions necessary for Aβ to form oligomers with a high molecular weight in the early lag phase, revealing the dependence of Aβ hexamer formation on the specific interfacial interaction.

Identification and characterization of the conformation and size of amyloid-β (42) oligomers targeting the receptor LilrB2

Experimental observations revealed that the amyloid-β 42 oligomer (AβO) can directly bind to the LilrB2 D1D2(LDD) receptor with nanomolar-affinity, leading to changes in synaptic plasticity and cognitive deficits. However, the dependence of neurotoxicity on the morphology, size, and aggregation stage (SP1, SP2) of AβO, as well as the specific molecular mechanism of AβO-LDD interaction, remain uncertain. To address these uncertainties, we investigated the interaction between the LDD neuroreceptor and AβO with different Aβ42 species (nontoxic species, toxic species, and protofibril) and sizes. Our results showed that the LDD selectively binds AβO species rather than the Aβ42 monomer, accommodating various Aβ42 dimers and trimers as well as SP2 AβO, in a specific pose in the pocket of the LDD receptor (region I). Additionally, protofibrils with exposed β1/β2 regions can also bind to region I of the LDD receptor, as observed experimentally (Cao, et al., Nat. Chem., 2018, 10, 1213; and Aim et al., Nat. Commun., 2021, 12, 3451). More extensively, we identified two additional regions of the LDD receptor, regions II and III, suitable for binding to larger AβO species at the SP1 with different molecular weights and conformations, accounting for the stronger binding strength obtained experimentally. We suggest that the two regions are more competitive than region I in causing toxicity by AβO binding. The detailed and systematic characterization for the complexes generated between the LDD receptor and various AβO species, including the protofibril, offers deep insight into the dependence of neurotoxicity on the AβO size and conformation at the molecular level, and provides novel and specific targets for drug design of Alzheimer's disease.

Age-Dependent Dysregulation of APP in Neuronal and Skin Cells from Fragile X Individuals

Fragile X syndrome (FXS) is the most common form of monogenic intellectual disability and autism, caused by the absence of the functional fragile X messenger ribonucleoprotein 1 (FMRP). FXS features include increased and dysregulated protein synthesis, observed in both murine and human cells. Altered processing of the amyloid precursor protein (APP), consisting of an excess of soluble APPα (sAPPα), may contribute to this molecular phenotype in mice and human fibroblasts. Here we show an age-dependent dysregulation of APP processing in fibroblasts from FXS individuals, human neural precursor cells derived from induced pluripotent stem cells (iPSCs), and forebrain organoids. Moreover, FXS fibroblasts treated with a cell-permeable peptide that decreases the generation of sAPPα show restored levels of protein synthesis. Our findings suggest the possibility of using cell-based permeable peptides as a future therapeutic approach for FXS during a defined developmental window.

Soluble amyloid-β precursor peptide does not regulate GABAB receptor activity

Amyloid-β precursor protein (APP) regulates neuronal activity through the release of secreted APP (sAPP) acting at cell surface receptors. APP and sAPP were reported to bind to the extracellular sushi domain 1 (SD1) of GABA_B receptors (GBRs). A 17 amino acid peptide (APP17) derived from APP was sufficient for SD1 binding and shown to mimic the inhibitory effect of sAPP on neurotransmitter release and neuronal activity. The functional effects of APP17 and sAPP were similar to those of the GBR agonist baclofen and blocked by a GBR antagonist. These experiments led to the proposal that sAPP activates GBRs to exert its neuronal effects. However, whether APP17 and sAPP influence classical GBR signaling pathways in heterologous cells was not analyzed. Here, we confirm that APP17 binds to GBRs with nanomolar affinity. However, biochemical and electrophysiological experiments indicate that APP17 does not influence GBR activity in heterologous cells. Moreover, APP17 did not regulate synaptic GBR localization, GBR-activated K⁺ currents, neurotransmitter release, or neuronal activity in vitro or in vivo. Our results show that APP17 is not a functional GBR ligand and indicate that sAPP exerts its neuronal effects through receptors other than GBRs.

Toxicity Mechanism of Aβ42 Oligomer in the Binding between the GABABR1a sushi1 Domain and Amyloid Precursor Protein 9mer: A Mechanism like Substitution Reaction

Amyloid-β peptide (Aβ), characterized by its abnormal folding into neurotoxic aggregates, impairs synaptic plasticity and causes synaptic loss associated with Alzheimer's disease (AD). The neurotoxicity of Aβ oligomers via the binding to various cell-surface receptors was frequently observed experimentally; however, the toxic mechanism still remains unknown. In this paper, we study the intervention of Aβ oligomers to the receptor-peptide binding in the GABA_BR1a sushi1-APP 9mer complex, a key node in increasing short-term synaptic facilitation in the mouse hippocampus and decreasing neuronal activity by inhibiting neurotransmitter release by molecular dynamics simulations. The residue types of Aβ42 oligomers involved in the intervention and core contact areas of the receptor were first identified, by which an unprecedented toxicity mechanism of Aβ42 oligomers is proposed. These involved residues of Aβ42 oligomers are positively charged residues Asp and Glu, and the core area of GABA_BR1a sushi1 domain is the Coil one, sharing the rich negatively charged residues R19/R21/R25/R45 with the pocket, in which APP 9mer is locked. The presence of an Aβ42 oligomer rather than of a monomer stretches these key residues in the core area and consequently "unlocks and releases" the APP 9mer from its initial pocket, unsteadying the sushi1 domain and taking into toxic effect. It looks like a chemical "substitution" reaction, Aβ42 oligomer + GABA_BR1a sushi1-APP 9mer complex → Aβ42 oligomer-GABA_BR1a sushi1 complex + APP 9mer. Further analysis reveals that the toxicity of Aβ42 oligomer to GABA_BR1a sushi1 domain stability depends on the residue number of the contact area and the size of Aβ42 oligomer, in which semi-extended trimeric Aβ42 oligomer is identified as the most toxic one. This work provides a novel insight into the mechanism of Aβ oligomeric toxicity to neuroreceptors and sets an important precedent for dealing with Aβ oligomeric toxicity to other receptors at the molecular level.