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 resemble those generated by the η-secretase or MT5-MMP, which was described to generate so called Aη fragments. The similarity in large APP C-terminal fragments between both proteases raised the question whether RHBDL4 may contribute to η-secretase activity and Aη-like fragments. Here, we identified two cleavage sites of RHBDL4 in APP by mass spectrometry, which, intriguingly, lie in close proximity to the MT5-MMP cleavage sites. Indeed, we observed that RHBDL4 generates Aη-like fragments in vitro without contributions of α-, ß-, or γ-secretases. Such Aη-like fragments are likely generated in the ER since RHBDL4-derived APP-C-terminal fragments do not reach the cell surface. Inherited, familial APP mutations appear to not affect this processing pathway. In RHBDL4 knockout mice, we observed increased cerebral full length APP in comparison to wild type (WT) in support of RHBDL4 being a physiologically relevant protease for APP. Furthermore, we found secreted Aη fragments in dissociated mixed cortical cultures from WT mice, however significantly less Aη fragments in RHBDL4 knockout cultures. Our data underscores that RHBDL4 contributes to η-secretease-like processing of APP and that RHBDL4 is a physiologically relevant protease for APP.

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. Our analyses corroborated several of the expected protein interactors such as the transitional endoplasmic reticulum (ER) ATPase VCP/p97 (TERA), but they also described novel putative interactors including IRS4, PGAM5, and GORS2. Using proximity-ligation assays, we validated VCP/p97, COPB, and VRK2 as proteins that are in proximity to RHBDL4. Overall, our results support the emerging functions of RHBDL4 in ER quality control and also point toward putative RHBDL4 functions in protein membrane insertion and membrane organization and trafficking.

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. Currently, the molecular mechanisms induced because of CETP activity are not clear. To understand how CETP activity affects the brain, we utilized CETP transgenic (CETPtg) mice that show elevated LDL levels upon induction of CETP expression through a high-cholesterol diet. CETPtg mice on a high-cholesterol diet showed up to 22% higher cholesterol levels in the brain. Using a microarray on mostly astrocyte-derived mRNA, we found that this cholesterol increase is likely not because of elevated de novo synthesis of cholesterol. However, cholesterol efflux is decreased in CETPtg mice along with an upregulation of the complement factor C1Q, which plays a role in neuronal cholesterol clearance. Our data suggest that CETP activity affects brain health through modulating cholesterol distribution and clearance. Therefore, we propose that CETPtg mice constitute a valuable research tool to investigate the impact of cholesterol metabolism on brain function.

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. using a raising technology called MALDI IMS to identify up- or downregulation of specific lipids in and around the amyloid plaque, one of the pathological hallmarks of Alzheimer's disease. Interestingly, such lipid changes were paralleled with disrupted myelin structure only at the border between white and gray matter. The sequestration of apolipoprotein E towards the amyloid plaque may provide a clue towards the underlying mechanisms leading to disrupted lipid profiles. This study highlights the necessity to increase research activities related to lipid metabolism in Alzheimer's disease and demonstrates that the technological progress now facilitates the advancement of this area.

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. In view of our recent finding that human RHBDL4 efficiently cleaves the amyloid precursor protein (APP), a key protein in the pathology of Alzheimer's disease, we used established reagents to modulate the cellular cholesterol content and analyzed the effects of this modulation on RHBDL4-mediated processing of endogenous APP. We discovered that lowering membrane cholesterol levels increased the levels of RHBDL4-specific endogenous APP fragments, whereas high cholesterol levels had the opposite effect. Direct binding of cholesterol to APP did not mediate these modulating effects of cholesterol. Instead, using homology modeling, we identified two potential cholesterol-binding motifs in the transmembrane helices 3 and 6 of RHBDL4. Substitution of the essential tyrosine residues of the potential cholesterol-binding motifs to alanine increased the levels of endogenous APP C-terminal fragments, reflecting enhanced RHBDL4 activity. In summary, we provide evidence that the activity of RHBDL4 is regulated by cholesterol likely through a direct binding of cholesterol to the enzyme.

The discovery of proteases and intramembrane proteolysis 1.

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. Ongoing research on proteases addresses their biological roles, small molecule inhibitors for therapeutic uses, and protein engineering to modify their activities. The discovery of intramembrane proteases is more recent, beginning with the discovery of site-2 protease in 1997. Since then, different mechanistic classes of intramembrane proteases have been characterized, and many of these act in regulated intramembrane proteolysis in signaling pathways. Furthermore, the rhomboid intramembrane proteases were discovered by genetic and biochemical experiments in Drosophila and then in human cells. Research on the intramembrane proteases is expanding, as their biological importance is recognized.

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. Here, we demonstrate that the introduction of positively charged amino acid residues in the TMS redirects the RHBDL4-mediated cleavage of APP from its ectodomain closer towards the TMS, possibly inducing an ER-associated degradation (ERAD) of the substrate. In addition, we concluded that the cytoplasmic tail and proposed palmitoylation sites in the ectodomain of APP are not essential for the RHBDL4-mediated APP processing. In summary, our previously identified APP ectodomain cleavages by RHBDL4 are a subsidiary mechanism to the proposed RHBDL4-mediated ERAD of substrates likely through a single cleavage near or within the TMS.

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. There is evidence that the membrane composition itself can regulate intramembrane protease activity and specificity. In this review, we highlight the work on γ-secretase and rhomboid proteases and summarize several studies as to how different lipids impact on enzymatic activity.

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 ß (Aß) 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. RHBDL4 efficiently cleaves APP inside the cell, thus bypassing APP from amyloidogenic processing, leading to reduced Aß levels. RHBDL4 cleaves APP multiple times in the ectodomain, resulting in several N- and C-terminal fragments that are not further degraded by classical APP secretases. Knockdown of endogenous RHBDL4 results in decreased levels of C-terminal fragments derived from endogenous APP. Similarly, we found the APP family members APLP1 and APLP2 to be substrates of RHBDL4. We conclude that RHBDL4-mediated APP processing provides insight into APP and rhomboid physiology and qualifies for further investigations to elaborate its impact on Alzheimer disease pathology.

How many amyloid-ß 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 Aß oligomer causing neuronal cell death. While most studies so far aimed to determine the Aß oligomer with highest toxicity using preformed and characterized Aß oligomers added to cell cultures, this study established an approach to analyze Aß 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. The authors' procedure allows to monitor the effects of different Aß oligomers in parallel, constituting an important advancement in the research field. Read the highlighted article 'Membrane bound tetramer and trimer Aß oligomeric species correlate with toxicity towards cultured neurons' on page 594.

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

Amyloid-ß (Aß) peptides are likely the molecular cause of neurodegeneration observed in Alzheimer's disease. In the brain, Aß42 and Aß40 are toxic and the most important proteolytic fragments generated through sequential processing of the amyloid precursor protein (APP) by ß- and γ-secretases. Impeding the generation of Aß42 and Aß40 is thus considered as a promising strategy to prevent Alzheimer's disease. We therefore wanted to determine key parameters of the APP transmembrane sequence enabling production of these Aß species. Here we show that the hydrophilicity of amino acid residues G33, T43, and T48 critically determines the generation of Aß42 and Aß40 peptides (amino acid numbering according to Aß nomenclature starting with aspartic acid 1). First, we performed a comprehensive mutational analysis of glycine residue G33 positioned within the N-terminal half of the APP transmembrane sequence by exchanging it against the 19 other amino acids. We found that hydrophilicity of the residue at position 33 positively correlated with Aß42 and Aß40 generation. Second, we analyzed two threonine residues at positions T43 and T48 in the C-terminal half of the APP-transmembrane sequence. Replacement of single threonine residues by hydrophobic valines inversely affected Aß42 and Aß40 generation. We observed that threonine mutants affected the initial γ-secretase cut, which is associated with levels of Aß42 or Aß40. Overall, hydrophilic residues of the APP transmembrane sequence decide on the exact initial γ-cut and the amounts of Aß42 and Aß40.

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 Aß 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. We demonstrated previously that RHBLD4 cleaves APP in HEK293T cells, leading to decreased total APP and Aß. More recently, we showed that RHBDL4 processes APP in mouse primary mixed cortical cultures as well. Here, we aim to examine the physiological relevance of RHBDL4 in the brain. We first found that brain samples from AD patients and an AD mouse model (APPtg) showed increased RHBDL4 mRNA and protein expression. To determine the effects of RHBDL4's absence on APP physiology in vivo, we crossed APPtg mice to a RHBDL4 knockout (R4 KO) model. RHBDL4 deficiency in APPtg mice led to increased total cerebral APP and Aß levels when compared to APPtg controls. Contrary to expectations, as assessed by cognitive tests, RHBDL4 absence rescued cognition in 5-month-old female APPtg mice. Informed by unbiased RNAseq data, we demonstrated in vitro and in vivo that RHBDL4 absence leads to greater levels of active ß-catenin due to decreased proteasomal clearance. Decreased ß-catenin activity is known to underlie cognitive defects in APPtg mice and AD. Our work suggests that RHBDL4's increased expression in AD, in addition to regulating APP levels, leads to aberrant degradation of ß-catenin, contributing to cognitive impairment.

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. By studying APP-TMS dimerization we previously showed that an attenuated interaction similarly decreased Abeta42 levels and concomitantly increased Abeta38 levels. However, the precise molecular mechanism by which GSMs modulate Abeta production is still unclear. In this study, using a reporter gene-based dimerization assay, we found that APP-TMS dimers are destabilized by sulindac sulfide and related Abeta42-lowering compounds in a concentration-dependent manner. By surface plasmon resonance analysis and NMR spectroscopy, we show that sulindac sulfide and novel sulindac-derived compounds directly bind to the Abeta sequence. Strikingly, the attenuated APP-TMS interaction by GSMs correlated strongly with Abeta42-lowering activity and binding strength to the Abeta sequence. Molecular docking analyses suggest that certain GSMs bind to the GxxxG dimerization motif in the APP-TMS. We conclude that these GSMs decrease Abeta42 levels by modulating APP-TMS interactions. This effect specifically emphasizes the importance of the dimeric APP-TMS as a promising drug target in Alzheimer's disease.

CETP inhibitor evacetrapib enters mouse brain tissue.

High levels of plasma cholesterol, especially high levels of low-density lipoprotein cholesterol (LDL-C), have been associated with an increased risk of Alzheimer's disease. The cholesteryl ester transfer protein (CETP) in plasma distributes cholesteryl esters between lipoproteins and increases LDL-C in plasma. Epidemiologically, decreased CETP activity has been associated with sustained cognitive performance during aging, longevity, and a lower risk of Alzheimer's disease. Thus, pharmacological CETP inhibitors could be repurposed for the treatment of Alzheimer's disease as they are safe and effective at lowering CETP activity and LDL-C. Although CETP is mostly expressed by the liver and secreted into the bloodstream, it is also expressed by astrocytes in the brain. Therefore, it is important to determine whether CETP inhibitors can enter the brain. Here, we describe the pharmacokinetic parameters of the CETP inhibitor evacetrapib in the plasma, liver, and brain tissues of CETP transgenic mice. We show that evacetrapib crosses the blood-brain barrier and is detectable in brain tissue 0.5 h after a 40 mg/kg i.v. injection in a non-linear function. We conclude that evacetrapib may prove to be a good candidate to treat CETP-mediated cholesterol dysregulation in Alzheimer's disease.

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

The amyloid-ß (Aß) peptide is contained within the C-terminal fragment (ß-CTF) of the amyloid precursor protein (APP) and is intimately linked to Alzheimer's disease. In vivo, Aß is generated by sequential cleavage of ß-CTF within the γ-secretase module. To investigate γ-secretase function, in vitro assays are in widespread use which require a recombinant ß-CTF substrate expressed in bacteria and purified from inclusion bodies, termed C100. So far, little is known about the conformation of C100 under different conditions of purification and refolding. Since C100 dimerization influences the efficiency and specificity of γ-secretase cleavage, it is also of great interest to determine the secondary structure and the oligomeric state of the synthetic substrate as well as the binding properties of small molecules named γ-secretase modulators (GSMs) which we could previously show to modulate APP transmembrane sequence interactions [Richter et al. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 14597-14602]. Here, we use circular dichroism and continuous-wave electron spin resonance measurements to show that C100 purified in a buffer containing SDS at micelle-forming concentrations adopts a highly stable α-helical conformation, in which it shows little tendency to aggregate or to form higher oligomers than dimers. By surface plasmon resonance analysis and molecular modeling we show that the GSM sulindac sulfide binds to C100 and has a preference for C100 dimers.

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. The combination of the APP-GxxxG mutation G33A with APP-FAD mutations yielded a constant 60% decrease of Abeta42 levels and a concomitant 3-fold increase of Abeta38 levels compared with the Gly(33) wild-type as determined by ELISA. In the presence of PS1-FAD mutations, the effects of G33A were attenuated, apparently attributable to a different mechanism of PS1-FAD mutants compared with APP-FAD mutants. Our results contribute to a general understanding of the mechanism how APP is processed by the gamma-secretase module and strongly emphasize the potential of the GxxxG motif in the prevention of sporadic AD as well as FAD.

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. Abeta peptides with alanine or isoleucine substitutions of G33 displayed an increased propensity to form higher oligomers, which we could attribute to conformational changes. Importantly, the oligomers of G33 variants were much less toxic than Abeta(42) wild type (WT), in vitro and in vivo. Also, whereas Abeta(42) WT is known to inhibit LTP, Abeta(42) G33 variants had lost the potential to inhibit LTP. Our findings reveal that conformational changes induced by G33 substitutions unlink toxicity and oligomerization of Abeta on the molecular level and suggest that G33 is the key amino acid in the toxic activity of Abeta. Thus, a specific toxic conformation of Abeta exists, which represents a promising target for therapeutic interventions.

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). Intracellular Cu levels could be increased up to 3-fold above normal with a sublethal dose of carriers. Both types of carriers released their bound guest molecules into the cytosolic compartment where they were accessible for the Cu-dependent enzyme SOD1. In particular, CS-NPs reduced Abeta levels and targeted intracellular organelles more efficiently than CMS-NPs. Fluorescently labeled CMS-NPs unraveled a cellular uptake mechanism, which depended on clathrin-mediated endocytosis in an energy-dependent manner. In contrast, the transport of CS-NPs was most likely driven by a concentration gradient. Overall, nanocarriers depending on the nature of the surrounding shell functioned by mediating import of Cu across cellular membranes, increased levels of bioavailable Cu, and affected Abeta turnover. Our studies illustrate that Cu-charged nanocarriers can achieve a reasonable metal ion specificity and represent an alternative to metal-complexing agents. The results demonstrate that carrier strategies have potential for the treatment of metal ion deficiency disorders.

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 APPL17C (residue 17 in Aß sequence) construct, which is associated with a 30% increase in APP dimer expression, on APP's neurite outgrowth promoting activity. Overexpression of APPL17C in SH-SY5Y cells decreased neurite outgrowth upon retinoic acid differentiation as compared to overexpressing APPWT cells. The APPL17C phenotype was rescued by replacing the APPL17C media with conditioned media from APPWT cells, indicating that the APPL17C mutant is impairing the secretion of a neuritogenic promoting factor. APPL17C had altered transport and was localised in the endoplasmic reticulum. Defining the molecular basis of the APPL17C phenotype showed that RhoA GTPase activity, a negative regulator of neurite outgrowth, was increased in APPL17C cells. RhoA activity was decreased after APPWT conditioned media rescue. Moreover, treatment with the RhoA inhibitor, Y27632, restored a wild-type morphology to the APPL17C cells. Small RNAseq analysis of APPL17C and APPWT cells identified several differentially expressed miRNAs relating to neurite outgrowth. Of these, miR-34a showed the greatest decrease in expression. Lentiviral-mediated overexpression of miR-34a rescued neurite outgrowth in APPL17C cells to APPWT levels and changed RhoA activation. This study has identified a novel link between APP dimerisation and its neuritogenic activity which is mediated by miR-34a expression.

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. Such cross-inhibition may indicate heterodimerization of these receptors. Therefore, this study investigated the molecular and functional interplay between MAS and the AT2R. Molecular interactions were assessed by fluorescence resonance energy transfer and by cross correlation spectroscopy in human embryonic kidney-293 cells transfected with vectors encoding fluorophore-tagged MAS or AT2R. Functional interaction of AT2R and MAS was studied in astrocytes with CX3C chemokine receptor-1 messenger RNA expression as readout. Coexpression of fluorophore-tagged AT2R and MAS resulted in a fluorescence resonance energy transfer efficiency of 10.8 ± 0.8%, indicating that AT2R and MAS are capable to form heterodimers. Heterodimerization was verified by competition experiments using untagged AT2R and MAS. Specificity of dimerization of AT2R and MAS was supported by lack of dimerization with the transient receptor potential cation channel, subfamily C-member 6. Dimerization of the AT2R was abolished when it was mutated at cysteine residue 35. AT2R and MAS stimulation with the respective agonists, Compound 21 or angiotensin-(1-7), significantly induced CX3C chemokine receptor-1 messenger RNA expression. Effects of each agonist were blocked by an AT2R antagonist (PD123319) and also by a MAS antagonist (A-779). Knockout of a single of these receptors made astrocytes unresponsive for both agonists. Our results suggest that MAS and the AT2R form heterodimers and that-at least in astrocytes-both receptors functionally depend on each other.