BRI2 Interacts with BACE1 and Regulates Its Cellular Levels by Promoting its Degradation and Reducing Its mRNA Levels

A Novel c.800G>C Variant of the ITM2B Gene in Familial Korean Dementia

Mutations in ITM2B have been reported to be associated with several familial dementias, such as Familial British dementia and familial Danish dementia. These are autosomal dominant disorders characterized by progressive dementia with an onset at around the fifth decade of life. We describe a family with cognitive impairment caused by a novel ITM2B p.*267Serext*11 mutation. The probands presented with cognitive impairment and cerebral infarction. MRI revealed diffuse white matter hyperintensity and microbleeds. Amyloid deposition was not observed on amyloid positron emission tomography. Our case suggests that the BRI2 mutation impacts cognition regardless of amyloid-β accumulation.

The role of the integral type II transmembrane protein BRI2 in health and disease

BRI2 is a type II transmembrane protein ubiquitously expressed whose physiological function remains poorly understood. Although several recent important advances have substantially impacted on our understanding of BRI2 biology and function, providing valuable information for further studies on BRI2. These findings have contributed to a better understanding of BRI2 biology and the underlying signaling pathways involved. In turn, these might provide novel insights with respect to neurodegeneration processes inherent to BRI2-related pathologies, namely Familial British and Danish dementias, Alzheimer's disease, ITM2B-related retinal dystrophy, and multiple sclerosis. In this review, we provided a state-of-the-art outline of BRI2 biology, both in physiological and pathological conditions, and discuss the proposed molecular underlying mechanisms. Overall, the BRI2 knowledge here reviewed is of extreme importance and may contribute to propose BRI2 and/or BRI2 proteolytic fragments as novel therapeutic targets for neurodegenerative diseases, such as Alzheimer's disease.

NRBP1-Containing CRL2/CRL4A Regulates Amyloid β Production by Targeting BRI2 and BRI3 for Degradation

Alzheimer's disease (AD) is a progressive neurodegenerative disease caused by accumulations of Aβ peptides. Production and fibrillation of Aβ are downregulated by BRI2 and BRI3, which are physiological inhibitors of amyloid precursor protein (APP) processing and Aβ oligomerization. Here, we identify nuclear receptor binding protein 1 (NRBP1) as a substrate receptor of a Cullin-RING ubiquitin ligase (CRL) that targets BRI2 and BRI3 for degradation. Moreover, we demonstrate that (1) dimerized NRBP1 assembles into a functional Cul2- and Cul4A-containing heterodimeric CRL through its BC-box and an overlapping cryptic H-box, (2) both Cul2 and Cul4A contribute to NRBP1 CRL function, and (3) formation of the NRBP1 heterodimeric CRL is strongly enhanced by chaperone-like function of TSC22D3 and TSC22D4. NRBP1 knockdown in neuronal cells results in an increase in the abundance of BRI2 and BRI3 and significantly reduces Aβ production. Thus, disrupting interactions between NRBP1 and its substrates BRI2 and BRI3 may provide a useful therapeutic strategy for AD.

Identification and characterization of the BRI2 interactome in the brain

BRI family proteins are ubiquitous type II transmembrane proteins but BRI2 is highly expressed in some neuronal tissues. Possible BRI2 functions include neuronal maturation and differentiation. Protein complexes appear to be important in mediating its functions. Previously described BRI2 interactors include the Alzheimer's amyloid precursor protein and protein phosphatase 1, but clearly the identification of novel interactors provides an important tool to understand the role and function of BRI2. To this end three rat brain regions (cerebellum, hippocampus, and cerebral cortex) were processed by BRI2 immunoprecipitation; co-precipitating proteins were identified by Nano-HPLC-MS/MS. The pool of the brain regions resulted in 511 BRI2 interacting proteins (BRI2 brain interactome) of which 120 were brain specific and 49 involved in neuronal differentiation. Brain region-specific analyses were also carried out for cerebellum, hippocampus, and cerebral cortex. Several novel BRI2 interactors were identified among them DLG4/PSD-95, which is singularly important as it places BRI2 in the postsynaptic compartment. This interaction was validated as well as the interaction with GAP-43 and synaptophysin. In essence, the resulting BRI2 brain interactome, associates this protein with neurite outgrowth and neuronal differentiation, as well as synaptic signalling and plasticity. It follows that further studies should address BRI2 particularly given its relevance to neuropathological conditions.

The Bri2 and Bri3 BRICHOS Domains Interact Differently with Aβ42 and Alzheimer Amyloid Plaques

Alzheimer’s disease (AD) is the most common form of dementia and there is no successful treatment available. Evidence suggests that fibril formation of the amyloid β-peptide (Aβ) is a major underlying cause of AD, and treatment strategies that reduce the toxic effects of Aβ amyloid are sought for. The BRICHOS domain is found in several proteins, including Bri2 (also called integral membrane protein 2B (ITM2B)), mutants of which are associated with amyloid and neurodegeneration, and Bri3 (ITM2C). We have used mouse hippocampal neurons and brain tissues from mice and humans and show Bri3 deposits dispersed on AD plaques. In contrast to what has been shown for Bri2, Bri3 immunoreactivity is decreased in AD brain homogenates compared to controls. Both Bri2 and Bri3 BRICHOS domains interact with Aβ₄₀ and Aβ₄₂ present in neurons and reduce Aβ₄₂ amyloid fibril formation in vitro, but Bri3 BRICHOS is less efficient. These results indicate that Bri2 and Bri3 BRICHOS have different roles in relation to Aβ aggregation.

Signal peptide peptidase and SPP-like proteases - Possible therapeutic targets?

Signal peptide peptidase (SPP) and the four homologous SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 are GxGD-type intramembrane-cleaving proteases (I-CLIPs). In addition to divergent subcellular localisations, distinct differences in the mechanistic properties and substrate requirements of individual family members have been unravelled. SPP/SPPL proteases employ a catalytic mechanism related to that of the γ-secretase complex. Nevertheless, differential targeting of SPP/SPPL proteases and γ-secretase by inhibitors has been demonstrated. Furthermore, also within the SPP/SPPL family significant differences in the sensitivity to currently available inhibitory compounds have been reported. Though far from complete, our knowledge on pathophysiological functions of SPP/SPPL proteases, in particular based on studies in mice, has been significantly increased over the last years. Based on this, inhibition of distinct SPP/SPPL proteases has been proposed as a novel therapeutic concept e.g. for the treatment of autoimmunity and viral or protozoal infections, as we will discuss in this review. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Transthyretin and BRICHOS: The Paradox of Amyloidogenic Proteins with Anti-Amyloidogenic Activity for Aβ in the Central Nervous System

Amyloid fibrils are physiologically insoluble biophysically specific β-sheet rich structures formed by the aggregation of misfolded proteins. In vivo tissue amyloid formation is responsible for more than 30 different disease states in humans and other mammals. One of these, Alzheimer's disease (AD), is the most common form of human dementia for which there is currently no definitive treatment. Amyloid fibril formation by the amyloid β-peptide (Aβ) is considered to be an underlying cause of AD, and strategies designed to reduce Aβ production and/or its toxic effects are being extensively investigated in both laboratory and clinical settings. Transthyretin (TTR) and proteins containing a BRICHOS domain are etiologically associated with specific amyloid diseases in the CNS and other organs. Nonetheless, it has been observed that TTR and BRICHOS structures are efficient inhibitors of Aβ fibril formation and toxicity in vitro and in vivo, raising the possibility that some amyloidogenic proteins, or their precursors, possess properties that may be harnessed for combating AD and other amyloidoses. Herein, we review properties of TTR and the BRICHOS domain and discuss how their abilities to interfere with amyloid formation may be employed in the development of novel treatments for AD.

Role of human GKN1 on APP processing in gastric cancer

Gastrokine 1 (GKN1) is highly expressed in gastric tissue and is secreted into the stomach but is not expressed in gastric cancer. GKN1 belongs to the BRICHOS domain family and plays a major role in maintaining gastric mucosa integrity. We previously demonstrated that a recombinant human GKN1 protein was able to interact with the amyloid precursor protein (APP) and was endowed with an anti-amyloidogenic property because it inhibited polymerization of the Aβ(1-40) peptide released from APP upon its partial hydrolysis. Here, we report that GKN1 can act as a physiological suppressor of Aβ production in gastric cancer cells. GKN1 blocked the access of γ-secretase to APP, thereby facilitating the cleavage of APP by α- and β-secretases. GKN1 directly interacted with APP C-terminal fragments, C83 and C99. In addition, it did not affect γ-secretase activity in gastric cancer cells because it did not alter Notch1 processing. GKN1-mediated inhibition of APP processing might represent a new approach for the prevention and therapy of Alzheimer's disease (AD).

Post-translational regulation of the β-secretase BACE1

β-Secretase, widely known as β-site APP cleaving enzyme 1 (BACE1), is a membrane-associated protease that cleaves amyloid precursor protein (APP) to generate amyloid β-protein (Aβ). As this cleavage is a pathologically relevant event in Alzheimer's disease, BACE1 is considered a viable therapeutic target. BACE1 can be regulated at the transcriptional, post-transcriptional, translational, and post-translational levels. Elucidation of the regulatory pathways of BACE1 is critical, not only for understanding the pathological mechanisms of AD but also developing effective therapeutic strategies to inhibit activity of the protease. This mini-review focuses on the post-translational regulation of BACE1, as modulation at this level is closely associated with both physiological and pathological conditions. Current knowledge on the mechanisms underlying such BACE1 regulation and their implications for therapy are discussed.

BRI2 ectodomain affects Aβ42 fibrillation and tau truncation in human neuroblastoma cells

Alzheimer's disease (AD) is pathologically characterized by the presence of misfolded proteins such as amyloid beta (Aβ) in senile plaques, and hyperphosphorylated tau and truncated tau in neurofibrillary tangles (NFT). The BRI2 protein inhibits Aβ aggregation via its BRICHOS domain and regulates critical proteins involved in initiating the amyloid cascade, which has been hypothesized to be central in AD pathogenesis. We recently detected the deposition of BRI2 ectodomain associated with Aβ plaques and concomitant changes in its processing enzymes in early stages of AD. Here, we aimed to investigate the effects of recombinant BRI2 ectodomain (rBRI276-266) on Aβ aggregation and on important molecular pathways involved in early stages of AD, including the unfolded protein response (UPR), phosphorylation and truncation of tau, as well as apoptosis. We found that rBRI276-266 delays Aβ fibril formation, although less efficiently than the BRI2 BRICHOS domain (BRI2 residues 113-231). In human neuroblastoma SH-SY5Y cells, rBRI276-266 slightly decreased cell viability and increased up to two-fold the Bax/Bcl-2 ratio and the subsequent activity of caspases 3 and 9, indicating activation of apoptosis. rBRI276-266 upregulated the chaperone BiP but did not modify the mRNA expression of other UPR markers (CHOP and Xbp-1). Strikingly, rBRI276-266 induced the activation of GSK3β but not the phosphorylation of tau. However, exposure to rBRI276-266 significantly induced the truncation of tau, indicating that BRI2 ectodomain can contribute to NFT formation. Since BRI2 can also regulate the metabolism of Aβ, the current data suggests that BRI2 ectodomain is a potential nexus between Aβ, tau pathology and neurodegeneration.

Accumulation of BRI2-BRICHOS ectodomain correlates with a decreased clearance of Aβ by insulin degrading enzyme (IDE) in Alzheimer's disease

The precursor protein BRI2 that in its mutated form is associated with British and Danish dementia, can regulate critical processes involved in AD pathogenesis including not only the metabolism of amyloid precursor protein (APP) and formation of Aβ, but also the levels of secreted insulin degrading enzyme (IDE), an enzyme involved in Aβ clearance. We recently observed increased levels of a 45kDa BRI2 form as well as BRI2 ectodomain deposits in Aβ plaques in human AD hippocampus, which may affect BRI2 functional activity. Since BRI2 regulated the levels of secreted IDE and subsequent degradation of Aβ in human cell culture models, we explored if BRI2 changes could affect the Aβ degradation capacity of IDE in human hippocampus (n=28). We observed that IDE is the main enzyme involved in Aβ degradation, and both IDE levels as well as Aβ degradation tend to be decreased in AD. Interestingly, the levels of the 45kDa BRI2 form and BRI2 deposits in hippocampal tissue were inversely correlated with IDE protein levels (r=-0.52, p=0.005; r=-0.4, p=0.045) and IDE activity (r=-0.5935, p=0.0004; r=-0.4, p=0.03). Taken together, the current results suggest a relationship between BRI2 protein changes, IDE activity and Aβ levels in human hippocampus. Thus, the formation and accumulation high of molecular weight BRI2 forms observed in AD may impair IDE functioning and consequently lead to impaired Aβ clearance and to the accumulation of Aβ.

BRI2-BRICHOS is increased in human amyloid plaques in early stages of Alzheimer's disease

BRI2 protein binds amyloid precursor protein to halt amyloid-β production and inhibits amyloid-β aggregation via its BRICHOS-domain suggesting a link between BRI2 and Alzheimer's disease (AD). Here, we investigate the possible involvement of BRI2 in human AD pathogenesis. BRI2 containing BRICHOS-domain was increased up to 3-fold in AD hippocampus (p = 0.003, n = 14/group). Immunohistochemistry showed BRI2 deposits associated with amyloid-β plaques in early pathologic stages (Braak-III; Thal-2/3). We observed a decrease in the protein levels of ADAM10 (p = 0.02) and furin (p = 0.066), as well as an increase in SPPL2b (p < 0.0001) in AD hippocampus. Because these enzymes are involved in BRI2 processing, their changes may lead to aberrant processing of BRI2 promoting its deposition and likely affecting BRI2 function. Loss of BRI2 function in AD was supported by the decreased presence of BRI2-amyloid precursor protein complexes in the hippocampus of AD patients compared with control subjects. In conclusion, our data obtained from human samples indicate that in early stages of AD there is an increased deposition of BRI2, which likely leads to impaired BRI2 function thereby influencing AD pathophysiology.