Cellular localization of Nicastrin affects amyloid β species production

BIN1 in the Pursuit of Ousting the Alzheimer's Reign: Impact on Amyloid and Tau Neuropathology

Alzheimer's disease contributes to 60-70% of all dementia cases in the general population. Belonging to the BIN1/amphiphysin/RVS167 (BAR) superfamily, the bridging integrator (BIN1) has been identified to impact two major pathological hallmarks in Alzheimer's disease (AD), i.e., amyloid beta (Aβ) and tau accumulation. Aβ accumulation is found to increase by BIN1 knockdown in cortical neurons in late-onset AD, due to BACE1 accumulation at enlarged early endosomes. Two BIN1 mutants, KR and PL, were identified to exhibit Aβ accumulation. Furthermore, BIN1 deficiency by BIN1-related polymorphisms impairs the interaction with tau, thus elevating tau phosphorylation, altering synapse structure and tau function. Even though the precise role of BIN1 in the neuronal tissue needs further investigation, the authors aim to throw light on the potential of BIN1 and unfold its implications on tau and Aβ pathology, to aid AD researchers across the globe to examine BIN1, as an appropriate target gene for disease management.

[Identification of novel regulators involved in AD pathogenesis using the CRISPR-Cas9 system]

The production of amyloid β peptide (Aβ) is an important process relating to the pathogenesis of Alzheimer disease (AD). It is widely known that the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases lead to the production of Aβ. However, the precise regulatory mechanism for Aβ production remains unclear. We have established a CRISPR-Cas9 based screening system to identify the novel regulators of Aβ production. Calcium and integrin-binding protein 1 (CIB1) was identified as a novel potential negative regulator of Aβ production. The knockdown and knockout of Cib1 significantly increased Aβ levels. In addition, immunoprecipitation showed that CIB1 interacts with the γ-secretase complex but did not alter its enzymatic activity. Moreover, Cib1 disruption specifically reduced the cell-surface localization of the γ-secretase complex. Finally, the single-cell RNA-seq analysis in the human brain demonstrated that early-stage AD patients have lower neuronal CIB1 mRNA levels compared to healthy controls. Taken together, we have shown that CIB1 controls the subcellular localization of γ-secretase, resulting in the regulation of Aβ production, suggesting the involvement of CIB1 in the development of AD pathogenesis.

Alzheimer's disease BIN1 coding variants increase intracellular Aβ levels by interfering with BACE1 recycling

Genetic studies have identified BIN1 as the second most important risk locus associated with late-onset Alzheimer's disease (LOAD). However, it is unclear how mutation of this locus mechanistically promotes Alzheimer's disease (AD) pathology. Here we show the consequences of two coding variants in BIN1 (rs754834233 and rs138047593), both in terms of intracellular beta-amyloid (iAbeta) accumulation and early endosome enlargement, two interrelated early cytopathological AD phenotypes, supporting their association with LOAD risk. We previously found that Bin1 deficiency potentiates iAbeta production by enabling BACE1 cleavage of the amyloid precursor protein in enlarged early endosomes due to decreased BACE1 recycling. Here, we discovered that the expression of the two LOAD mutant forms of Bin1 does not rescue the iAbeta accumulation and early endosome enlargement induced by Bin1 knockdown and recovered by wild-type Bin1. Moreover, the overexpression of Bin1 mutants, but not wild-type Bin1, increased the iAbeta42 fragment by reducing the recycling of BACE1, which accumulated in early endosomes, recapitulating the phenotype of Bin1 knockdown. We showed that the mutations in Bin1 reduced its interaction with BACE1. The endocytic recycling of transferrin was similarly affected, indicating that Bin1 is a general regulator of endocytic recycling. These data demonstrate that the LOAD-coding variants in Bin1 lead to a loss of function in endocytic recycling, which may be an early causal mechanism of LOAD.

Bin1 and CD2AP polarise the endocytic generation of beta-amyloid

The mechanisms driving pathological beta-amyloid (Aβ) generation in late-onset Alzheimer's disease (AD) are unclear. Two late-onset AD risk factors, Bin1 and CD2AP, are regulators of endocytic trafficking, but it is unclear how their endocytic function regulates Aβ generation in neurons. We identify a novel neuron-specific polarisation of Aβ generation controlled by Bin1 and CD2AP We discover that Bin1 and CD2AP control Aβ generation in axonal and dendritic early endosomes, respectively. Both Bin1 loss of function and CD2AP loss of function raise Aβ generation by increasing APP and BACE1 convergence in early endosomes, however via distinct sorting events. When Bin1 levels are reduced, BACE1 is trapped in tubules of early endosomes and fails to recycle in axons. When CD2AP levels are reduced, APP is trapped at the limiting membrane of early endosomes and fails to be sorted for degradation in dendrites. Hence, Bin1 and CD2AP keep APP and BACE1 apart in early endosomes by distinct mechanisms in axon and dendrites. Individuals carrying variants of either factor would slowly accumulate Aβ in neurons increasing the risk for late-onset AD.

Mutations in nicastrin protein differentially affect amyloid beta-peptide production and Notch protein processing

The γ-secretase complex is responsible for intramembrane processing of over 60 substrates and is involved in Notch signaling as well as in the generation of the amyloid β-peptide (Aβ). Aggregated forms of Aβ have a pathogenic role in Alzheimer disease and, thus, reducing the Aβ levels by inhibiting γ-secretase is a possible treatment strategy for Alzheimer disease. Regrettably, clinical trials have shown that inhibition of γ-secretase results in Notch-related side effects. Therefore, it is of great importance to find ways to inhibit amyloid precursor protein (APP) processing without disturbing vital signaling pathways such as Notch. Nicastrin (Nct) is part of the γ-secretase complex and has been proposed to be involved in substrate recognition and selection. We have investigated how the four evenly spaced and conserved cysteine residues in the Nct ectodomain affect APP and Notch processing. We mutated these cysteines to serines and analyzed them in cells lacking endogenous Nct. We found that two mutants, C213S (C2) and C230S (C3), differentially affected APP and Notch processing. Both the formation of Aβ and the intracellular domain of amyloid precursor protein (AICD) were reduced, whereas the production of Notch intracellular domain (NICD) was maintained on a high level, although C230S (C3) showed impaired complex assembly. Our data demonstrate that single residues in a γ-secretase component besides presenilin are able to differentially affect APP and Notch processing.

An E3 ubiquitin ligase, Synoviolin, is involved in the degradation of immature nicastrin, and regulates the production of amyloid beta-protein

The presenilin complex, consisting of presenilin, nicastrin, anterior pharynx defective-1 and presenilin enhancer-2, constitutes gamma-secretase, which is required for the generation of amyloid beta-protein. In this article, we show that Synoviolin (also called Hrd1), which is an E3 ubiquitin ligase implicated in endoplasmic reticulum-associated degradation, is involved in the degradation of endogenous immature nicastrin, and affects amyloid beta-protein generation. It was found that the level of immature nicastrin was dramatically increased in synoviolin-null cells as a result of the inhibition of degradation, but the accumulation of endogenous presenilin, anterior pharynx defective-1 and presenilin enhancer-2 was not changed. This was abolished by the transfection of exogenous Synoviolin. Moreover, nicastrin was co-immunoprecipitated with Synoviolin, strongly suggesting that nicastrin is the substrate of Synoviolin. Interestingly, amyloid beta-protein generation was increased by the overexpression of Synoviolin, although the nicastrin level was decreased. Thus, Synoviolin-mediated ubiquitination is involved in the degradation of immature nicastrin, and probably regulates amyloid beta-protein generation. Structured digital abstract: * MINT-7255352: Synoviolin (uniprotkb:Q9DBY1) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti tag coimmunoprecipitation (MI:0007) * MINT-7255377: Ubiquitin (uniprotkb:P62991) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti bait coimmunoprecipitation (MI:0006) * MINT-7255363: NCT (uniprotkb:P57716) physically interacts (MI:0915) with Synoviolin (uniprotkb:Q9DBY1) by anti bait coimmunoprecipitation (MI:0006).