Eta-secretase-like processing of the amyloid precursor protein (APP) by the rhomboid protease RHBDL4.

The amyloid precursor protein (APP) is a key protein in Alzheimer's disease synthesized in the endoplasmic reticulum (ER) and translocated to the plasma membrane where it undergoes proteolytic cleavages by several proteases. Conversely, to other known proteases, we previously elucidated rhomboid protease RHBDL4 as a novel APP processing enzyme where several cleavages likely occur already in the ER. Interestingly, the pattern of RHBDL4-derived large APP C-terminal fragments resembles those generated by the η-secretase or MT5-MMP, which was described to generate so-called Aη fragments.

Loss of the APP regulator RHBDL4 preserves memory in an Alzheimer's disease mouse model.

Characteristic cerebral pathological changes of Alzheimer's disease (AD) such as glucose hypometabolism or the accumulation of cleavage products of the amyloid precursor protein (APP), known as Aβ peptides, lead to sustained endoplasmic reticulum (ER) stress and neurodegeneration. To preserve ER homeostasis, cells activate their unfolded protein response (UPR). The rhomboid-like-protease 4 (RHBDL4) is an enzyme that participates in the UPR by targeting proteins for proteasomal degradation.

Putative Protein Interactome of the Rhomboid Protease RHBDL4.

The physiological functions of the rhomboid-related protein 4 (RHBDL4) are emerging, but their molecular details remain unclear. Because increased expression of RHBDL4 has been clinically linked to poorer outcomes in cancer patients, this association urgently demands a better understanding of RHBDL4. To elucidate the molecular interactions and pathways that RHBDL4 may be involved in, we conducted proximity-dependent biotin identification (BioID) assays.

The cholesteryl ester transfer protein (CETP) raises cholesterol levels in the brain.

The cholesteryl ester transfer protein (CETP) is a lipid transfer protein responsible for the exchange of cholesteryl esters and triglycerides between lipoproteins. Decreased CETP activity is associated with longevity, cardiovascular health, and maintenance of good cognitive performance. Interestingly, mice lack the CETP-encoding gene and have very low levels of LDL particles compared with humans.

The lipid component of Alzheimer's disease research: An Editorial Highlight for "Brain region-specific amyloid plaque-associated myelin lipid loss, APOE deposition and disruption of the myelin sheath in familial Alzheimer's disease mice" on page 84.

It may not be surprising that the brain as a lipid-rich organ shows perturbed lipid profiles in neurodegenerative conditions such as Alzheimer's disease. It is, however, more challenging to detect these changes as they may only occur in a spatially small area. This Editorial highlights the work by Kaya et al.

An alternative processing pathway of APP reveals two distinct cleavage modes for rhomboid protease RHBDL4.

Since the first genetic description of a rhomboid in Drosophila melanogaster, tremendous efforts have been geared towards elucidating the proteolytic mechanism of this particular class of intramembrane proteases. In particular, mammalian rhomboid proteases sparked our interest and we aimed to investigate the human homologue RHBDL4. In light of our recent finding of the amyloid precursor protein (APP) family as efficient substrates of RHBDL4, we were enticed to further study the specific proteolytic mechanism of this enzyme by comparing cleavage patterns of wild type APP and APP TMS chimeras.

Membrane cholesterol as regulator of human rhomboid protease RHBDL4.

In the last decade, intramembrane proteases have gained increasing attention because of their many links to various diseases. Nevertheless, our understanding as to how they function or how they are regulated is still limited, especially when it comes to human homologues. In this regard, here we sought to unravel mechanisms of regulation of the protease rhomboid-like protein-4 (RHBDL4), one of five active human serine intramembrane proteases.

The discovery of proteases and intramembrane proteolysis .

Proteases carry out a wide variety of physiological functions. This review presents a brief history of protease research, starting with the original discovery of pepsin in 1836. Following the path of time, we revisit how proteases were originally classified based on their catalytic mechanism and how chemical and crystallographic studies unravelled the mechanism of serine proteases.

Amyloid Precursor Protein Dimerisation Reduces Neurite Outgrowth.

The amyloid precursor protein (APP) undergoes extensive metabolism, and its transport and proteolytic processing can be modulated by its ability to form a homodimer. We have investigated the functional consequences of stabilised APP dimer expression in cells by studying the engineered dimerisation of the APP (residue 17 in Aβ sequence) construct, which is associated with a 30% increase in APP dimer expression, on APP's neurite outgrowth promoting activity. Overexpression of APP in SH-SY5Y cells decreased neurite outgrowth upon retinoic acid differentiation as compared to overexpressing APP cells.

Embedded in the Membrane: How Lipids Confer Activity and Specificity to Intramembrane Proteases.

Proteases, sharp yet unforgivable tools of every cell, require tight regulation to ensure specific non-aberrant cleavages. The relatively recent discovered class of intramembrane proteases has gained increasing interest due to their involvement in important signaling pathways linking them to diseases including Alzheimer's disease and cancer. Despite tremendous efforts, their regulatory mechanisms have only started to unravel.

Evidence for Heterodimerization and Functional Interaction of the Angiotensin Type 2 Receptor and the Receptor MAS.

The angiotensin type 2 receptor (AT2R) and the receptor MAS are receptors of the protective arm of the renin-angiotensin system. They mediate strikingly similar actions. Moreover, in various studies, AT2R antagonists blocked the effects of MAS agonists and vice versa.

Alternative Processing of the Amyloid Precursor Protein Family by Rhomboid Protease RHBDL4.

The amyloid precursor protein (APP) is an ubiquitously expressed cell surface protein and a key molecule in the etiology of Alzheimer disease. Amyloidogenic processing of APP through secretases leads to the generation of toxic amyloid β (Aβ) peptides, which are regarded as the molecular cause of the disease. We report here an alternative processing pathway of APP through the mammalian intramembrane rhomboid protease RHBDL4.

How many amyloid-β peptides can a neuron bind before it dies?

This Editorial highlights a study by Jana and coworkers published in the current issue of Journal of Neurochemistry, in which the authors performed a detailed, quantitative analysis to identify the Aβ oligomer causing neuronal cell death. While most studies so far aimed to determine the Aβ oligomer with highest toxicity using preformed and characterized Aβ oligomers added to cell cultures, this study established an approach to analyze Aβ oligomers bound to primary neurons. This may shed new light on how oligomeric status changes at the cell surface and if minor oligomeric species may account for measured effects.

Impact of amyloid precursor protein hydrophilic transmembrane residues on amyloid-beta generation.

Amyloid-β (Aβ) peptides are likely the molecular cause of neurodegeneration observed in Alzheimer's disease. In the brain, Aβ42 and Aβ40 are toxic and the most important proteolytic fragments generated through sequential processing of the amyloid precursor protein (APP) by β- and γ-secretases. Impeding the generation of Aβ42 and Aβ40 is thus considered as a promising strategy to prevent Alzheimer's disease.

Intracellular Aβ pathology and early cognitive impairments in a transgenic rat overexpressing human amyloid precursor protein: a multidimensional study.

Numerous studies have implicated the abnormal accumulation of intraneuronal amyloid-β (Aβ) as an important contributor to Alzheimer's disease (AD) pathology, capable of triggering neuroinflammation, tau hyperphosphorylation and cognitive deficits. However, the occurrence and pathological relevance of intracellular Aβ remain a matter of controversial debate. In this study, we have used a multidimensional approach including high-magnification and super-resolution microscopy, cerebro-spinal fluid (CSF) mass spectrometry analysis and ELISA to investigate the Aβ pathology and its associated cognitive impairments, in a novel transgenic rat model overexpressing human APP.

The proton-pump inhibitor lansoprazole enhances amyloid beta production.

A key event in the pathogenesis of Alzheimer's disease (AD) is the accumulation of amyloid-β (Aβ) species in the brain, derived from the sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. Based on a systems biology study to repurpose drugs for AD, we explore the effect of lansoprazole, and other proton-pump inhibitors (PPIs), on Aβ production in AD cellular and animal models. We found that lansoprazole enhances Aβ37, Aβ40 and Aβ42 production and lowers Aβ38 levels on amyloid cell models.

The amyloid precursor protein and its homologues: structural and functional aspects of native and pathogenic oligomerization.

Over the last 25 years, remarkable progress has been made not only in identifying key molecules of Alzheimer's disease but also in understanding their meaning in the pathogenic state. One hallmark of Alzheimer pathology is the amyloid plaque. A major component of the extracellular deposit is the amyloid-β (Aβ) peptide which is generated from its larger precursor molecule, i.

The amyloid precursor protein C-terminal fragment C100 occurs in monomeric and dimeric stable conformations and binds γ-secretase modulators.

The amyloid-β (Aβ) peptide is contained within the C-terminal fragment (β-CTF) of the amyloid precursor protein (APP) and is intimately linked to Alzheimer's disease. In vivo, Aβ is generated by sequential cleavage of β-CTF within the γ-secretase module. To investigate γ-secretase function, in vitro assays are in widespread use which require a recombinant β-CTF substrate expressed in bacteria and purified from inclusion bodies, termed C100.

Amyloid beta 42 peptide (Abeta42)-lowering compounds directly bind to Abeta and interfere with amyloid precursor protein (APP) transmembrane dimerization.

Following ectodomain shedding by beta-secretase, successive proteolytic cleavages within the transmembrane sequence (TMS) of the amyloid precursor protein (APP) catalyzed by gamma-secretase result in the release of amyloid-beta (Abeta) peptides of variable length. Abeta peptides with 42 amino acids appear to be the key pathogenic species in Alzheimer's disease, as they are believed to initiate neuronal degeneration. Sulindac sulfide, which is known as a potent gamma-secretase modulator (GSM), selectively reduces Abeta42 production in favor of shorter Abeta species, such as Abeta38.

Aberrant amyloid precursor protein (APP) processing in hereditary forms of Alzheimer disease caused by APP familial Alzheimer disease mutations can be rescued by mutations in the APP GxxxG motif.

The identification of hereditary familial Alzheimer disease (FAD) mutations in the amyloid precursor protein (APP) and presenilin-1 (PS1) corroborated the causative role of amyloid-beta peptides with 42 amino acid residues (Abeta42) in the pathogenesis of AD. Although most FAD mutations are known to increase Abeta42 levels, mutations within the APP GxxxG motif are known to lower Abeta42 levels by attenuating transmembrane sequence dimerization. Here, we show that aberrant Abeta42 levels of FAD mutations can be rescued by GxxxG mutations.

Role of amyloid-beta glycine 33 in oligomerization, toxicity, and neuronal plasticity.

The aggregation of the amyloid-beta (Abeta) peptide plays a pivotal role in the pathogenesis of Alzheimer's disease, as soluble oligomers are intimately linked to neuronal toxicity and inhibition of hippocampal long-term potentiation (LTP). In the C-terminal region of Abeta there are three consecutive GxxxG dimerization motifs, which we could previously demonstrate to play a critical role in the generation of Abeta. Here, we show that glycine 33 (G33) of the central GxxxG interaction motif within the hydrophobic Abeta sequence is important for the aggregation dynamics of the peptide.

Cellular copper import by nanocarrier systems, intracellular availability, and effects on amyloid beta peptide secretion.

Studies in animals have reported that normalized or elevated Cu levels can inhibit or even remove Alzheimer's disease-related pathological plaques and exert a desirable amyloid-modifying effect. We tested engineered nanocarriers composed of diverse core-shell architectures to modulate Cu levels under physiological conditions through bypassing the cellular Cu uptake systems. Two different nanocarrier systems were able to transport Cu across the plasma membrane of yeast or higher eukaryotic cells, CS-NPs (core-shell nanoparticles) and CMS-NPs (core-multishell nanoparticles).

Subcellular localization and dimerization of APLP1 are strikingly different from APP and APLP2.

The molecular association between APP and its mammalian homologs has hardly been explored. In systematically addressing this issue, we show by live cell imaging that APLP1 mainly localizes to the cell surface, whereas APP and APLP2 are mostly found in intracellular compartments. Homo- and heterotypic cis interactions of APP family members could be detected by FRET and co-immunoprecipitation analysis and occur in a modular mode.