Novel Interactions Involving the Mas Receptor Show Potential of the Renin–Angiotensin system in the Regulation of Microglia Activation: Altered Expression in Parkinsonism and Dyskinesia

Abstract

The renin–angiotensin system (RAS) not only plays an important role in controlling blood pressure but also participates in almost every process to maintain homeostasis in mammals. Interest has recently increased because SARS viruses use one RAS component (ACE2) as a target-cell receptor. The occurrence of RAS in the basal ganglia suggests that the system may be targeted to improve the therapy of neurodegenerative diseases. RAS-related data led to the hypothesis that RAS receptors may interact with each other. The aim of this paper was to find heteromers formed by Mas and angiotensin receptors and to address their functionality in neurons and microglia. Novel interactions were discovered by using resonance energy transfer techniques. The functionality of individual and interacting receptors was assayed by measuring levels of the second messengers cAMP and Ca²⁺ in transfected human embryonic kidney cells (HEK-293T) and primary cultures of striatal cells. Receptor complex expression was assayed by in situ proximity ligation assay. Functionality and expression were assayed in parallel in primary cultures of microglia treated or not with lipopolysaccharide and interferon-γ (IFN-γ). The proximity ligation assay was used to assess heteromer expression in parkinsonian and dyskinetic conditions. Complexes formed by Mas and the angiotensin AT₁ or AT₂ receptors were identified in both a heterologous expression system and in neural primary cultures. In the heterologous system, we showed that the three receptors—MasR, AT₁R, and AT₂R—can interact to form heterotrimers. The expression of receptor dimers (AT₁R-MasR or AT₂R-MasR) was higher in microglia than in neurons and was differentially affected upon microglial activation with lipopolysaccharide and IFN-γ. In all cases, agonist-induced signaling was reduced upon coactivation, and in some cases just by coexpression. Also, the blockade of signaling of two receptors in a complex by the action of a given (selective) receptor antagonist (cross-antagonism) was often observed. Differential expression of the complexes was observed in the striatum under parkinsonian conditions and especially in animals rendered dyskinetic by levodopa treatment. The negative modulation of calcium mobilization (mediated by AT₁R activation), the multiplicity of possibilities on RAS affecting the MAPK pathway, and the disbalanced expression of heteromers in dyskinesia yield new insight into the operation of the RAS system, how it becomes unbalanced, and how a disbalanced RAS can be rebalanced. Furthermore, RAS components in activated microglia warrant attention in drug-development approaches to address neurodegeneration.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13311-020-00986-4.

Introduction

The renin–angiotensin system (RAS) has been widely studied for its role in controlling blood pressure. Protein components of RAS are angiotensin-converting enzyme 1 (ACE1), which produces angiotensin II (Ang II); angiotensin-converting enzyme 2 (ACE2), which converts Ang II to angiotensin 1–7 (Ang(1-7)); the Ang II receptors AT₁R and AT₂R; and the Ang(1-7) receptor Mas. For several decades, the balance between the prooxidative, proinflammatory, and the antioxidative, anti-inflammatory RAS arms was thought to be centered in the functional equilibrium of AT₁R and AT₂R. Now, the role of the ACE2/Mas receptor function has been revealed as a crucial element of RAS balance and RAS function. The Mas receptor (MasR) was identified as a product of an oncogene and, because of its resemblance to the mitochondrial assembly gene from Saccharomyces cerevisiae, it was first known as Mas-related proto-oncogene. All RAS receptors belong to the superfamily of G protein-coupled receptors (GPCRs). A novel family of RAS-related receptors are named Mas-related GPCRs (Mrgprs); they respond to Ang (1-7) but their endogenous agonist is one of the newest members of the RAS system, alamandine [1–5]. ACE2 is considered the canonical cell surface receptor for SARS-CoV-2, the virus that causes COVID-19. In fact, ACE2 was serendipitously identified as the receptor for viruses of the SARS family [6–10].

RAS has a relevant role in controlling neurotransmission in both the central and peripheral nervous systems. Identification of Ang II receptors in neural cells of the substantia nigra and other regions within the basal ganglia has uncovered novel therapeutic approaches to address neurodegeneration in Parkinson’s disease (PD) and other synucleinopathies [11–18]. For obvious reasons, we must consider the brain RAS to understand the neurological manifestations of some patients infected with SARS-CoV-2. In fact, some cases of COVID-19 have neurological symptoms as diverse as encephalitis and seizures [19–21]. In some cases, the basal ganglia have been identified as mediating virus-induced central nervous system (CNS) alterations [13, 22, 23].

Although angiotensin I has little biological activity, ACE1 converts it into Ang II, the principal RAS effector, which activates AT₁R and AT₂R, whose actions in CNS development have been clearly delineated. However, in adults, the scenario is quite complex because AT₁R and AT₂R seemingly mediate opposite actions, whereas AT₁R activation usually leads to production of reactive oxygen species, activation of AT₂R counterbalances this noxious effect. In macrophages and microglia, AT₁R is thought to contribute to inflammation, whereas AT₂R attenuates inflammation and contributes to neuroprotection. These data reinforce the hypothesis that RAS balance is important in illnesses such as PD, which involves glial activation and neuroinflammation [14, 24–27]. MasR, whose endogenous agonist is Ang (1-7), adds complexity by mediating antioxidant and anti-inflammatory effects of AT₂R [24, 28]. In summary, RAS action in a given cell or brain circuit depends on peptide production by ACE1 and ACE2 and the expression of RAS receptors. Overall, the mechanisms underlying RAS balance in healthy conditions and RAS unbalance in aging [14] or disease remain unknown.

The physiological action of GPCRs is often mediated by heteromers; that is, by complexes having more than one receptor. Consensus is that receptor heteromers are novel functional units—their properties are different from those of the individually expressed receptors [29–31]. The first reported heteromer consisted of two GPCRs with the same endogenous agonist; namely, the μ-δ opioid receptor heteromer [32]. Further examples of heteromers formed by two receptors for the same endogenous ligand are adenosine A₁-A_2A and adenosine A_2A-A_2B receptors. In these cases, the presence and/or activation of one of the receptors blunts the response arising in the partner receptor, and structural information can explain how the trans blockade may happen [33–38]. Importantly, it has been reported that AT₁R and AT₂R and MasR and AT₂R form heteromers [39–41]. The aim of this study was to demonstrate the presence of Mas and AT₁/AT₂ receptor heterodimerization in neurons and microglia. The function of the MasR-AT₁R/AT₂R complexes in resting and activated microglia was investigated to complement our recent findings on AT₁R-AT₂R heteromer function in neural cells [42]. Importantly, we assessed the expression of heteroreceptor complexes in animal models of parkinsonian and dyskinesia. We focused on the striatum because of its relevance in PD. Our results provide a holistic model to understand RAS action, especially under conditions in which microglia become activated.

Materials and Methods

Reagents

Lipopolysaccharide (LPS), interferon-γ (IFN-γ), Ang II, CGP-42112A, Ang [1–7], candesartan, PD123319, and A779 were purchased from Sigma-Aldrich (St. Louis, MO, USA). Forskolin was purchased from Tocris (Bristol, UK). The concentrations of ligands used for signaling assays were selected on the basis of the dose–response experiments shown in Supplementary Figure S1.

Cell Culture

Human embryonic kidney (HEK-293T) cells were grown in Dulbecco’s modified Eagle’s medium (DMEM; Gibco/Thermo Fisher Scientific, Paisley, UK) supplemented with 2 mM l-glutamine, 100 μg/mL sodium pyruvate, 100 U/mL penicillin/streptomycin, MEM Non-Essential Amino Acids Solution (1/100) and 5% (v/v) heat-inactivated fetal bovine serum (FBS) (all supplements were from Invitrogen/Thermo Fisher Scientific, Paisley, UK). Cells were maintained at 37 °C in a humid atmosphere of 5% CO₂.

As mentioned in the “Introduction,” in this study, we focused on brain striatum. To prepare mice striatal primary microglial cultures, the brain was removed from C57BL/6 mice at 2 to 4 days of age. Microglial cells were isolated as described in [43] and grown in DMEM supplemented with 2 mM L-glutamine, 100 U/mL penicillin/streptomycin, MEM non-essential amino acids preparation (1/100), and 5% (v/v) heat-inactivated FBS. Briefly, striatum tissue was dissected, carefully stripped of its meninges, and digested with 0.25% trypsin for 20 min at 37 °C. Digestion was stopped by washing the tissue. A cell suspension was prepared by passing the cells through a 100-μm pore mesh. Glial cells were resuspended in medium and seeded at a density of 1 × 10⁶ cells/mL in 6-well plates for cyclic adenylic acid (cAMP) assays, in 12-well plates with coverslips for in situ proximity ligation assays (PLA), and in 96-well plates for mitogen-activated protein kinase (MAPK) experiments. Cultures were maintained at 37 °C in a humidified 5% CO₂ atmosphere, and unless otherwise stated, the medium was replaced once a week.

For neuronal primary cultures, the striatum from mouse embryos (E19) was removed and the neurons were isolated, as described by [44], and plated at a density of approximately 120,000 cells/cm². The cells were grown in a neurobasal medium supplemented with 2 mM l-glutamine, 100 U/mL penicillin/streptomycin, and 2% (v/v) B27 supplement (Gibco) in a 6-, 12-, or 96-well plate for 19 to 21 days. Cultures were maintained at 37 °C in a humidified 5% CO₂ atmosphere, and the medium was replaced every 4 to 5 days.

Immunodetection of specific markers (NeuN for neurons and CD-11b for microglia) showed that neuronal preparations contained > 98% neurons and that microglia preparations contained at least 98% microglial cells [45, 46]. Contamination by astrocytes was negligible; in our hands, the passage of the suspension through a syringe disrupts astroglial cells, and in addition, the culture medium used does not favor astrocyte survival (checked in every culture).

PD Model Generation, Levodopa Treatment, and Dyskinesia Assessment

All experiments were carried out in accordance with EU directives (2010/63/EU and 86/609/CEE) and were approved by the ethical committee of the University of Santiago de Compostela. Similar to the approach elsewhere described [47], our experimental design using male Wistar rats aimed to obtain four groups of animals as described below. Animals were 8 weeks old at the beginning of the experimental procedure.

Details of model generation and the protocol of drug administration and behavioral analysis, performed by a blinded investigator, are given elsewhere [48, 49]. Surgery was performed on rats anesthetized with ketamine/xylazine (1% ketamine, 75 mg/kg, and 2% xylazine, 10 mg/kg). Lesions were produced in the right medial forebrain bundle to achieve a complete degeneration of the nigrostriatal pathway. The rats were injected with 12 μg of 6-hydroxydopamine (6-OH-DA) (to provide 8 μg of 6-hydroxy-DA free base; Sigma-Aldrich) in 4 μL of sterile saline containing 0.2% ascorbic acid. These were considered “lesioned” animals. Injection of the vehicle led to the generation of naïve (or non-lesioned) animals.

The 6-OH-DA hemilesioned rat is considered a PD model. Amphetamine-induced rotation was tested in a bank of 8 automated rotometer bowls (Rota-count 8, Columbus Instruments, Columbus, OH, USA) by monitoring full (360°) body turns in either direction. Right and left full body turns were recorded over 90 min following an injection of d-amphetamine (2.5 mg/kg i.p.) dissolved in saline. Rats that displayed more than 6 full body turns/min ipsilateral to the lesion were included in the study (this rate would correspond to > 90% depletion of dopamine fibers in the striatum [50]).

Spontaneous use of forelimb can be measured by the cylinder test [51, 52]. Rats were placed individually in a glass cylinder (20 cm in diameter) and the number of left or right forepaw contacts was scored by an observer blinded to the animals’ identity and presented as left (impaired) touches as a percentage of total touches. A control animal would thus receive an unbiased score of 50%, whereas the lesion usually reduces the performance of the impaired paw to less than 20% of total wall contacts.

Of the lesioned animals displaying parkinsonism-like behavior according to the above described tests (18 in total), 12 were chronically treated with l-DOPA daily for 3 weeks. A mixture of l-DOPA methyl ester (6 mg/kg) plus benserazide (10 mg/kg) was administered subcutaneously. The treatment reliably induces dyskinetic movements in some rats. As described in a previous report [47], abnormal involuntary movements were evaluated according to the rat dyskinesia scale described in detail elsewhere [48, 53–56]. The severity of each abnormal involuntary movement (AIM) subtype (limb, orolingual, and axial) was assessed using scores from 0 to 4 (1 = occasional, present < 50% of the time; 2 = frequent, present > 50% of the time; 3 = continuous but interrupted by strong sensory stimuli; 4 = continuous, not interrupted by strong sensory stimuli). Rats were classified as “dyskinetic” if they displayed a score ≥ 2 per monitoring period on at least two AIM subtypes. Animals classified as “non-dyskinetic” exhibited either no l-DOPA-induced abnormal involuntary movements or very mild/occasional ones [57]. Animals with low scores, either non-dyskinetic or dyskinetic, were excluded. In summary, four groups of animals were obtained: [1] non-lesioned; [2] lesioned, treated with vehicle; [3] lesioned and became dyskinetic when treated with l-DOPA; and [4] lesioned and did not become dyskinetic upon l-DOPA treatment. Tyrosine hydroxylase immunostaining was performed in every animal from sections taken postmortem [18, 49]; selected animals undergoing 6-OH-DA treatment showed, in the lesioned hemisphere, >95% nigral dopaminergic denervation. Overall, 4 animals (those with better scores) were selected in each of the following 4 groups: naïve, lesioned, lesioned/l-DOPA dyskinetic, and lesioned/l-DOPA non-dyskinetic. The PLA analysis (see below) was performed in different fields of striatal sections from each of the 16 selected animals. The striatum was delimited in sections using a bright field, and images were captured within delimitation coordinates.

Fusion proteins

Human cDNAs for AT₁, AT₂, Mas, and σ₁ receptors cloned into pcDNA3.1 were amplified without their stop codons using sense and antisense primers harboring either BamHI and HindIII restriction sites to amplify AT₁R and AT₂R or with BamHI and EcoRI restriction sites to amplify Mas and σ₁ receptors. Amplified fragments were then subcloned to be in frame with an enhanced yellow fluorescent protein (pEYFP-N1; Clontech, Heidelberg, Germany) or a Rluc (pRluc-N1; PerkinElmer, Wellesley, MA) on the C-terminal end of the receptor to produce AT₁R-YFP, AT₁R-Rluc, AT₂R-YFP, Mas-YFP, Mas-Rluc, and σ₁R-Rluc fusion proteins.

Cell Transfection

HEK-293T cells were transiently transfected with the corresponding cDNA by means of the poly-ethylenimine (PEI; Sigma-Aldrich) method, as previously described [58]. Briefly, the corresponding cDNA diluted in 150 mM NaCl was mixed with PEI (5.5 mM in nitrogen residues) also prepared in 150 mM NaCl for 10 min. The cDNA-PEI complexes were transferred to HEK-293T cells, which were incubated for 4 h in serum-starved medium. Then, the medium was replaced by fresh supplemented culture medium and cells were maintained at 37 °C in a humid atmosphere of 5% CO₂. Forty-eight hours after transfection, cells were washed, detached, and resuspended in the assay buffer.

Immunocytochemistry

HEK-293T cells were seeded on glass coverslips in 12-well plates. After 24 h of culture, cells were transfected with AT₁R-YFP cDNA (1 μg) and Mas-Rluc cDNA (1 μg) or with AT₂R-Rluc cDNA (1 μg) and Mas-Rluc cDNA (1 μg). Forty-eight hours later, cells were fixed in 4% paraformaldehyde for 15 min and washed twice with PBS containing 20 mM glycine before permeabilization with PBS-glycine containing 0.2% Triton X-100 (5 min incubation). A blocking solution consisting of PBS containing 1% bovine serum albumin was added (1 h). HEK-293T cells were then labeled with a mouse anti-Rluc antibody (1/100; Millipore, Darmstadt, Germany) and subsequently treated with Cy3-conjugated anti-mouse (1/200; Jackson ImmunoResearch, West Grove, PA, USA; red) antibody (1 h each). The expression of Mas-YFP and AT₂R-YFP was detected by their YFP fluorescence. Nuclei were stained with Hoechst (1/100 from 1 mg/mL stock; Sigma-Aldrich). Samples were washed several times and mounted with 30% Mowiol (Calbiochem, Merck, Darmstadt, Germany). Images were obtained in a Zeiss LSM 880 confocal microscope (Zeiss, Jena, Germany) with a × 63 oil objective.

Bioluminescence Resonance Energy Transfer Assay

HEK-293T cells were transiently cotransfected with a constant amount of cDNA encoding Mas-Rluc (0.75 μg) and with increasing amounts of cDNAs corresponding to AT₁R-YFP (0.5 to 2.5 μg) or AT₂R-YFP (0.5 to 3 μg). When indicated cDNA coding for AT₁R fused to Rluc was used. For negative control, HEK-293T cells were transiently cotransfected with a constant amount of cDNA encoding σ₁-Rluc (0.75 μg) and with increasing amounts of cDNA corresponding to AT₂R-YFP (0.5 to 3 μg). To control the cell number, sample protein concentration was determined using a Bradford assay kit (Bio-Rad, Munich, Germany) using bovine serum albumin (BSA) dilutions as standards. To quantify fluorescent proteins, cells (20 μg of total protein) were distributed in 96-well microplates (black plates with transparent bottoms) and fluorescence was read in a Fluostar Optima Fluorimeter (BMG Labtech, Offenburg, Germany) equipped with a high-energy xenon flash lamp, using a 10-nm bandwidth excitation filter at 485 nm. For bioluminescence resonance energy transfer (BRET) measurements, the equivalent of 20 μg of total protein cell suspension was distributed in 96-well white microplates with white bottoms (Corning 3600; Corning Inc., Corning, NY, USA). BRET was determined 1 min after adding coelenterazine H (Molecular Probes, Eugene, OR), using a Mithras LB 940 reader (DLReady, Berthold Technologies, Bad Wildbad, Germany), which allows the integration of the signals detected in the short-wavelength filter at 485 nm and the long-wavelength filter at 530 nm. To quantify Mas-Rluc expression, luminescence readings were obtained 10 min after the addition of 5 μM coelenterazine H. MilliBRET units (mBU) are defined as follows:

$$\mathrm{mBU}=\left[\frac{{\lambda }_{530}\left(\text{long}-\text{wavelength emission}\right)}{{\lambda }_{485}\left(\text{short}-\text{wavelength emission}\right)}-C_{\mathrm{f}}\right]\times 1000$$

in which C_f corresponds to [(long-wavelength emission)/(short-wavelength emission)] for the Rluc construct expressed alone in the same experiment.

Sequential BRET-FRET Assay

HEK-293T cells were transiently cotransfected with a constant amount of cDNA for MasR-Rluc (0.6 μg) and for AT₁R-GFP² (1 μg) and increasing amounts of cDNA for AT₂R-YFP (0.5 to 4.1 μg). For the negative control, HEK-293T cells were transiently cotransfected with a constant amount of σ₁R-Rluc (0.3 μg) and AT₁R-GFP² (1 μg) and increasing amounts of cDNA for AT₂R-YFP (0.5 to 4.1 μg). To control the cell number, sample protein concentration was determined using a Bradford assay kit (Bio-Rad) using BSA dilutions as standards and adjusted to 0.2 mg/mL. To quantify fluorescent proteins, cells (20 μg of total protein) were distributed in 96-well microplates (black plates with transparent bottoms) and fluorescence was read in a Fluostar Optima Fluorimeter (BMG Labtech) equipped with a high-energy xenon flash lamp, using a 10-nm bandwidth excitation filter with a 485-nm excitation filter for YFP or 410-nm excitation filter for GFP². For SRET measurements, the equivalent of 20 μg of total protein cell suspension was distributed in 96-well white microplates (Corning 3600). SRET was determined 30 s after the addition of Deepblue C (5 μM) (Molecular Probes), using a Mithras LB 940 reader (Berthold Technologies), which allows the integration of the signals detected in the short-wavelength filter at 400 nm and the long-wavelength filter at 530 nm. To quantify MasR-Rluc or σ₁R-Rluc expression, luminescence readings were obtained 10 min after the addition of 5 μM coelenterazine H.

Assessment of Cytoplasmic Calcium Ion Levels

HEK-293T cells were cotransfected with the cDNAs for AT₁ (1 μg) and/or AT₂ (1 μg), and/or MasR (1 μg) and/or the GCaMP6 calcium sensor (1 μg) [59] by the use of PEI method, as described above. Forty-eight hours after transfection, HEK-293T cells plated in 6-well plates (black, clear-bottomed) were incubated with Mg²⁺-free Locke’s buffer (154 mM NaCl, 5.6 mM KCl, 3.6 mM NaHCO₃, 2.3 mM CaCl₂, 5.6 mM glucose, 5 mM HEPES, 10 μM glycine, pH 7.4). Receptor antagonists were added 10 min before readings and receptor agonists were added a few seconds before readings. Fluorescence emission intensity of GCaMP6 was recorded at 515 nm upon excitation at 488 nm on an EnSpire Multimode Plate Reader (PerkinElmer, Waltham, MA, USA) for 150 s every 5 s at 100 flashes per well.

Determination of cAMP Level

The analysis of cAMP levels in primary neural cells or in transfected HEK-293T was performed using the Lance® Ultra cAMP kit (PerkinElmer). Two hours before the experiment, cells were placed in serum-starved DMEM. Cells growing in the medium containing 50 μM zardaverine were distributed in 384-well microplates (2000 HEK-293T cells or 4000 striatal neurons or microglial cells per well) and pretreated with the AT₁R, AT₂R, and MasR antagonists candesartan, PD123319, and A779, respectively, or with the vehicle at room temperature for 15 min, and then stimulated with the AT₁R, AT₂R, and MasR agonists Ang II, CGP-42112A, and Ang(1-7), respectively, for 15 min before adding 0.5 μM forskolin or vehicle for an additional 15 min. Homogeneous time-resolved fluorescence energy transfer (HTRF) measures were performed using the Lance® Ultra cAMP kit (PerkinElmer). Fluorescence at 665 nm was measured on a PHERAstar Flagship microplate reader equipped with an HTRF optical module (BMG Labtech). A standard curve for (cAMP) was obtained in each experiment.

Extracellular Signal-Regulated Kinases 1/2 Phosphorylation

To determine ERK1/2 phosphorylation, 40,000 HEK-293T cells expressing MasR and either AT₁R or AT₂R, or 50,000 striatal neurons, or microglial primary cultures were plated in transparent Deltalab 96-well microplates. Two hours before the experiment, the medium was substituted with serum-starved DMEM. Cells were treated or not for 10 min with the selective antagonists (300 nM candesartan, 1 μM PD123319, or 500 nM A779) followed by 7-min treatment with the selective agonists (Ang II, CGP-42112A, or Ang(1-7). Cells were then washed twice with cold PBS before the addition of lysis buffer (15-min treatment). Ten microliters of each supernatant was placed in white ProxiPlate 384-well microplates, and ERK1/2 phosphorylation was determined using an AlphaScreen SureFire kit (PerkinElmer) following the instructions of the supplier and using an EnSpire Multimode Plate Reader (PerkinElmer).

Dynamic Mass-Redistribution Label-Free Assays

Cell signaling was explored using an EnSpire Multimode Plate Reader (PerkinElmer) by using label-free technology. Cellular cytoskeleton redistribution movement induced upon receptor activation was detected by illuminating the underside of the plate with polychromatic light and measured as changes in the wavelength of the reflected monochromatic light, which is a sensitive function of the index of refraction. The magnitude of this wavelength shift (in picometers) is directly proportional to the amount of dynamic mass-redistribution (DMR). To determine the label-free DMR signal, 10,000 HEK-293T cells cotransfected with cDNAs for AT₁R (1 μg) or AT₂R (1 μg) and MasR or 10,000 striatal neuronal or microglial primary cultures cells were plated on transparent 384-well fibronectin-coated microplates to obtain 70 to 80% confluent monolayers, and kept in the incubator for 24 h. Before the assay, cells were washed twice with assay buffer (Hanks’ balanced salt solution with 20 mM HEPES, pH 7.15, 0.1% dimethyl sulfoxide) and incubated in the reader for 2 h in 30 μL/well of assay buffer at 24 °C. Then, the sensor plate was scanned and a baseline optical signature was recorded for 10 min before adding 10 μL of test antagonist (candesartan, PD123319, or A779) dissolved in assay buffer, followed by the addition of 10 μL of selective agonist (Ang II, CGP-42112A, or Ang(1-7)) also dissolved in assay buffer. The DMR responses induced by the agonist were monitored for a minimum of 3600 s.

Proximity Ligation Assay

Detection in natural sources of clusters formed by AT₁ and Mas receptors or AT₂ and Mas receptors was addressed in primary cultures of microglia or striatal neurons and in brain sections. Cells grown on glass coverslips were fixed in 4% paraformaldehyde for 15 min, washed twice with PBS containing 20 mM glycine to quench the aldehyde groups, permeabilized with the same buffer containing 0.05% Triton X-100 for 5 to 15 min, and washed with PBS. After a 1-h incubation at 37 °C with the blocking solution in a preheated humidity chamber, samples were incubated overnight at 4 °C with a mixture of a rabbit monoclonal anti-AT₁R antibody (1/100, ab124734, Abcam, Cambridge, UK) and a mouse monoclonal anti-MasR antibody (1/100, sc-390453, Santa Cruz Biotechnology, Santa Cruz, CA, USA) or a mixture of a rabbit monoclonal anti-AT₂R antibody (1/100, ab92445, Abcam) and the mouse monoclonal anti-MasR antibody (1/100, sc-390453, Santa Cruz Biotechnology). Nuclei were stained with Hoechst (1/100 from 1 mg/mL stock; Sigma-Aldrich). The antibodies were validated following the method in the technical brochure of the vendor and also by immunofluorescence in HEK-293T cells (transfected vs non-transfected). Cells were further processed using PLA probes detecting primary antibodies (Duolink In Situ PLA probe anti-mouse plus and Duolink In Situ PLA probe Anti-Rabbit minus; all probes from Sigma-Aldrich) (1/5 v:v for 1 h at 37 °C). Ligation and amplification were done as indicated by the supplier (Sigma-Aldrich), and cells were mounted using the mounting medium 30% Mowiol (Calbiochem, Merck). To detect red dots corresponding to AT₁-MasHets or AT₂-MasHets, samples were observed in a Zeiss LSM 880 confocal microscope equipped with an apochromatic 63× oil-immersion objective, and 405- and 561-nm laser lines. For each field of view, a stack of 2 channels (one per staining) and 3 Z-planes with a step size of 1 μm were acquired. Andy’s algorithm, a specific ImageJ (National Institutes of Health, Bethesda, MD, USA) macro for reproducible and high-throughput quantification of the total PLA foci dots and total nuclei, was used for data analysis [60].

The specificity of antibodies against angiotensin receptors has been questioned, even though different laboratories have reported excellent performance of different antibodies see [58–61]. All antibodies used in this study were monoclonal; however, to check the specificity of the antibodies used, we performed experiments in HEK-293T cells, either non-transfected or expressing AT₁R or AT₂R. Signal was negligible in AT₂R-expressing cells and non-transfected cells when the anti-AT₁R antibody was used and negligible in AT₁R-expressing cells and non-transfected cells when the anti-AT₂R antibody was used (Supplementary Figure S2). These results confirm the antibody specificity as previously reported [62].

Data Analysis

All data were obtained from at least five independent experiments and are expressed as the mean ± standard error of the mean (SEM). GraphPad Prism 8 software (GraphPad Inc., San Diego, CA, USA) was used for data fitting and statistical analysis. One-way ANOVA and post hoc Bonferroni’s test were used when comparing multiple values. When a pair of values was compared, Student’s t test was used. Significant differences were considered when the p value was < 0.05.

Results

The Mas Receptor Physically Interacts with AT₁ or AT₂ Receptors

Colocalization of MasR with AT₁R or AT₂R was first addressed using a heterologous expression system. Immunocytochemical assays performed in HEK-293T cells transfected with cDNAs encoding MasR fused to YFP and AT₁ fused to Rluc (Fig. 1A–C) indicated a marked degree of colocalization (yellow in Fig. 1C). A lower degree of colocalization was observed after immunocytochemical assays performed in HEK-293T cells transfected with cDNAs encoding MasR fused to Rluc and AT₂R fused to YFP (Fig. 1D–F). In those experiments, red fluorescence was due to a secondary Cy3-conjugated antibody, whereas green fluorescence was due to YFP (see “Materials and Methods”); colocalization can be observed by yellow in the merged image. It should be noted that the images are taken near the glass surface, that is, most of the labeling was due to proteins in the cell membrane proximal to the slide.

Fig. 1.

Human AT₁ and AT₂ receptors interact with Mas receptor (MasR) in transfected HEK-293T cells. Immunocytochemistry assays were performed in HEK-293T cells expressing Mas-YFP and AT₁-Rluc receptors (1 μg of cDNA each) (A–C), or Mas-Rluc and AT₂R-YFP (1 μg of cDNA each) (D–F). Images were taken using a Zeiss 880 confocal microscope. Receptors fused to YFP were detected by yellow fluorescence (green), and receptors fused to Rluc were detected by a mouse anti-Rluc antibody and a secondary Cy3 anti-mouse antibody (red). Colocalization is shown in yellow. Cell nuclei were stained with Hoechst (blue). Scale bar: 2 μm. G, H BRET assays were performed in HEK-293T cells transfected with a constant amount of cDNA for Mas-Rluc (0.75 μg) or σ₁R-Rluc (0.75 μg) (as negative control) and increasing amounts of cDNA for AT₁R-YFP (0.5 to 2.5 μg) or AT₂R-YFP (0.5 to 3 μg). Values are the mean ± SEM of 8 independent experiments performed in duplicates. I, J Schematic representation of BRET assays: energy transfer and fluorescence emission at 530 nM only occurs if the BRET donor (Rluc) and the BRET acceptor (YFP) are close

Immunocytochemistry assays do not demonstrate physical interactions because they occur within distances of  200 nm. Thus, we addressed potential interactions using BRET in HEK-293T cells expressing a constant amount of MasR-Rluc and increasing amounts of AT₁R-YFP. The saturable BRET curve (BRET_max 60 ± 2 mBU; BRET₅₀ 13 ± 4 mBU) indicates specific interactions between Mas and AT₁ receptors (Fig. 1G). A saturable curve was obtained when a similar experiment was carried out in HEK-293T cells expressing a constant amount of MasR-Rluc and increasing amounts of AT₂R-YFP (BRET_max 32 ± 1; BRET₅₀ 7 ± 2), indicating that the two receptors do physically interact in living cells (Fig. 1H), confirming the results previously reported in HEK-293T cells using fluorescence resonance energy transfer and cross-correlation spectroscopy [40]. In contrast, experiments in cells coexpressing σ₁R-Rluc and AT₂-YFP showed a linear non-specific signal (negative control in Fig. 1H).

We addressed the potential formation of trimers by taking advantage of the sequential resonance energy transfer (SRET; [63]) technique using cells expressing MasR-Rluc, AT₁R-GFP², and AT₂R-YFP. The saturable curve, which was not detected in the negative control (σ₁R-Rluc, AT₁R-GFP², and AT₂R-YFP), indicated that the three RAS receptors interacted in a heterologous system, forming trimers (Fig. 2A). Trimer formation led to a structural rearrangement; in fact, increased expression of MasR led to a higher BRET signal due to the AT₁R-Rluc and AT₂R-YFP pair. These results are consistent with the reduced distance between Rluc fused to AT₁R and YFP fused to AT₂R when forming receptor heterotrimers with MasR (Fig. 2B).

Fig. 2.

All three RAS receptors interact in transfected HEK-293T cells. A Sequential resonance energy transfer (SRET²) assays were performed in HEK-293T cells transfected with a constant amount of cDNA for MasR-Rluc (0.6 μg) or σ₁R-Rluc (0.3 μg) (as negative control), a constant amount of cDNA for AT₁R-GFP² (1 μg), and increasing amounts of cDNA for AT₂R-YFP (0.5 to 4.1 μg). Values are the mean ± SEM of 8 independent experiments performed in duplicate. B BRET assays were performed in HEK-293T cells transfected with a constant amount of cDNA for AT₂R-Rluc (0.5 μg), a constant amount of cDNA for AT₁R-YFP (3 μg), and increasing amounts of cDNA for MasR (0 to 1 μg). Values are the mean ± SEM of 5 independent experiments performed in triplicate. One-way ANOVA followed by Bonferroni’s multiple comparison post hoc tests were used for statistical analysis. C: Schematic representation of the SRET² assay. *p < 0.05, ***p < 0.0001 versus absence of MasR

Functional Characterization of AT₁R-MasR Heteromeric Complexes in HEK-293T Cells

Upon identification of AT₁-MasHets and AT₂-MasHets in cotransfected HEK-293T cells, we characterized the functionality of these complexes. It is well established that the AT₁ receptor couples to G_q proteins, increasing inositol trisphosphate (IP₃) and diacylglycerol (DAG) levels by breaking phosphatidylinositol 4,5-bisphosphate (PIP₂) and subsequently releasing calcium (II) ion from endoplasmic reticulum channels, but it also couples to G_i proteins, inhibiting adenylate cyclase and decreasing cAMP levels. In contrast, the MasR signal transduction pathway has not yet been fully characterized. Calcium measurements were first addressed in HEK-293T cells transfected with cDNAs for AT₁ and Mas receptors (1 μg of cDNA each) and with the cDNA for the GCaMP6 calcium sensor (1 μg of cDNA). After treating cells with the AT₁R agonist Ang II (100 nM), a characteristic curve of cytoplasmic transient [Ca²⁺] increase was recorded. This ion mobilization was completely blocked in cells pretreated with the AT₁R antagonist candesartan (300 nM; Fig. 3A). Interestingly, when cells were pretreated with the MasR antagonist A779 (500 nM) followed by Ang II stimulation, we observed a marked increase in the calcium mobilization signal. Thus, MasR blockade potentiated AT₁R-mediated signaling in the AT₁-MasHet context. The MasR agonist Ang(1-7) (250 nM) induced no effect, indicating that the MasR receptor does not couple with G_q.

Fig. 3.

Functional analysis of AT₁R-MasR and AT₂R-MasR complexes in HEK-293T cells. HEK-293T cells were transfected with the cDNAs for MasR (1 μg) and for either AT₁R (1 μg) or AT₂R (1 μg) and, in assays of Ca²⁺ determination, with the cDNA for an engineered calcium sensor, GCaMP6 (1 μg). Transfected cells were pretreated with the selective antagonists (300 nM candesartan for AT₁R, 1 μM PD123319 for AT₂R, and 500 nM A779 for MasR) and subsequently treated with selective agonists (100 nM Ang II for AT₁R, 300 nM CGP-42112A for AT₂R, and 500 nM Ang [1–7] for MasR). Thereafter, cytosolic calcium increases (A, B), intracellular cAMP levels (C, D), ERK1/2 phosphorylation (E, F), and the time-dependent DMR signal (G, H) were determined. Values are the mean ± SEM of 6 independent experiments performed in triplicate. One-way ANOVA followed by Bonferroni’s multiple comparison post hoc test were used for statistical analysis. *p < 0.05, **p < 0.01, ***p < 0.001 versus forskolin treatment in cAMP determinations or versus vehicle in pERK and DMR assays

Because of the controversy existing around the pathways engaged by MasR activation, we assayed whether the receptor agonist could affect the cytoplasmic levels of cAMP. As can be observed in Fig. 3C, activation of MasR in HEK-293T cells coexpressing AT₁ and Mas receptors (1 μg of cDNA each) did not have any effect on either basal or forskolin-induced cAMP levels, indicating that MasR does not couple with either G_s or G_i. Similar results were obtained in cells only expressing MasR (Supplementary Figure S3). Furthermore, Ang II stimulation induced no effect in cells expressing AT₁-MasHets (Fig. 3C).

We next addressed activation of the MAPK pathway, which is linked to the action of many GPCRs. When HEK-293T cells expressing AT₁-MasHets were treated with Ang II or Ang(1-7), observed an increase in ERK1/2 phosphorylation. However, when cells were simultaneously stimulated with both agonists, the signal was reduced (Fig. 3E). This phenomenon—in which the signal in a combined treatment is lower than the sum of individual activation—is known as negative cross-talk and it can be used to detect AT₁-MasHets in native tissues. Furthermore, when cells were pretreated with selective antagonists, we observed that A779 treatment blocked not only the MasR-induced signal but also the AT₁R-induced signal. Similarly, candesartan blocked both AT₁R-induced and MasR-induced signals (Fig. 3E). This phenomenon—by which the antagonist of one receptor in the heteromer blocks the signaling of the other protomer in the complex—is known as cross-antagonism and is a common print found for different GPCR heteromers.

Finally, when AT₁-MasR complex signaling was assayed by using label-free DMR, a technique that detects cytoskeletal rearrangements upon receptor activation, negative cross-talk was detected when cells were costimulated; cross-antagonism was unidirectional because it was only detected with candesartan (Fig. 3G).

Functional Characterization of AT₂R-MasR Heteromeric Complexes in HEK-293T Cells

After confirming that MasR and AT₁R arrange into a functional unit with novel properties, we proceeded to analyze the possibility of a similar scenario for AT₂R. The assays were similar to those described in the previous section. On the one hand, HEK-293T cells expressing AT₂-MasHet and the GCaMP6 calcium sensor did not respond to the MasR agonist Ang [1–7] (250 nM) or to the AT₂R agonist CGP-42112A (300 nM). These results fit with a lack of G_q coupling (Fig. 3B). On the other hand, assays to determine cAMP levels showed that CGP-42112A decreased the forskolin-induced effect by approximately 20%, whereas Ang [1–7] produced no effect (Fig. 3D). Similar results were obtained in cells expressing either AT₂R (CGP-42112A) or MasR (Ang [1–7]) (Supplementary Figure S3). However, MasR activation partially blocked the AT₂R signal. Interestingly, when HEK-293T cells expressing AT₂-MasHets were pretreated with the MasR antagonist A779 followed by AT₂R activation, potentiation of AT₂R-mediated signaling was detected. Thus, these results show that MasR stimulation blocked AT₂R-induced signaling, whereas, remarkably, MasR blockade potentiated AT₂R functionality. To further characterize signaling in AT₁-MasHet-expressing cells, ERK1/2 phosphorylation and DMR were determined. Equivalent results were found in both assays, namely negative cross-talk when receptors were simultaneously activated (Fig. 3F, H) and partial cross-antagonism in MAPK phosphorylation when cells were pretreated with selective antagonists. This partial cross-antagonism was not detectable in DMR assays (Fig. 3H). MasR blockade did not potentiate AT₂R functionality in MAPK activation or DMR assays.

AT₁R-MasR and AT₂R-MasR Heteromeric Complexes in Neuronal Primary cultures

Parkinson’s disease is characterized by neuronal death and neuroinflammation, mainly affecting the indirect pathway of the basal ganglia, where angiotensin receptors are expressed. Thus, we isolated primary cultures of brain striatum to look for expression of angiotensin and Mas receptor complexes.

We first identified AT₁-MasHets and AT₂-MasHets in primary neurons by in situ PLA. Clusters of receptor pairs were identifiable as red fluorescent dots (Fig. 4A–D); 1.44 red dots/cell were counted using primary antibodies against AT₁R and MasR, and 0.63 red dots/cell were counted using primary antibodies against AT₂R and MasR. The non-specific signal was equivalent to 0.17 red dots/cell in the negative control.

Fig. 4.

AT₁-MasHet and AT₂-MasHet expression and function in primary cultures of striatal neurons. A–C Expression of AT₁-MasHets and AT₂-MasHets heteromers was determined by proximity ligation assay (PLA), which was performed using specific primary antibodies against AT₁, AT₂, and Mas receptors. Confocal microscopy images (stacks of 4 consecutive planes) show heteroreceptor complexes; nuclei are Hoechst-stained (blue). Scale bar: 20 μm. D Bar graph showing the percentage of AT₁R-MasR and AT₂R-MasR clusters as red dots/cell compared with the negative control (*p < 0.05, **p < 0.01; Student’s t test vs the negative control condition). For cAMP (E–G) or ERK1/2 phosphorylation (H, I), cells were pretreated with selective receptor antagonists (300 nM candesartan for AT₁R, 1 μM PD123319 for AT₂R, and 500 nM A779 for MasR) and subsequently treated with selective agonists (100 nM Ang II for AT₁R, 300 nM CGP-42112A for AT₂R, and 250 nM Ang(1-7) for MasR). Values are the mean ± SEM of 5 independent experiments performed in triplicate. One-way ANOVA followed by Bonferroni’s multiple comparison post hoc tests were used for statistical analysis. *p < 0.05, **p < 0.01, ***p < 0.001 versus forskolin treatment in cAMP determinations or versus vehicle treatment (basal) in pERK determinations

Once expression of heteromers was demonstrated, we addressed their functionality in striatal neurons. Receptors in primary cultures of striatal neurons were treated with selective antagonists (candesartan for AT₁R, PD123319 for AT₂R, and A779 for MasR) and activated with agonists (Ang II for AT₁R, CGP-42112A for AT₂R, and Ang(1-7) for MasR), and cAMP levels and MAPK activation were analyzed.

In the cAMP assays, we observed that only AT₂R activation produced a significant decrease in forskolin-induced cAMP levels (Fig. 4E–G). This effect was partially counteracted by MasR activation and thus congruent with the data obtained in the heterologous expression system. In addition, pretreatment with MasR antagonist potentiated AT₂R signaling.

Finally, except for AT₂R in cells expressing AT₂-MasHets, individual activation of receptors led to ERK1/2 phosphorylation. Furthermore, AT₂R activation blunted the effect of MasR activation. Antagonists allowed us to identify unidirectional cross-antagonism, that is, the MasR antagonist blocked the AT₁R-induced but not the AT₂R-induced effect, whereas angiotensin receptor antagonists did not affect the link of MasR and the MAPK pathway (Fig. 4H, I). Compared with results in transfected HEK-293T cells, the cross-antagonism of AT₁ and AT₂ receptors over MasR was not observed in neurons.

AT₂R-MasR Heteromeric Complex Expression in Microglia Treated or Not with LPS and IFN-γ

In pathological conditions, microglia migrate to the injury site, releasing pro- and anti-inflammatory factors, and becoming key actors in regulating the neurodegenerative/neuroprotective balance [64]. First, expression of the AT₁R-MasR and AT₂R-MasR heteromeric complexes was detected by PLA in primary microglia, both resting and activated. Microglia were activated by treating the cells for 48 h with 1 μM LPS and 200 U/mL IFN-γ. As can be observed in Fig. 5, 3.8 and 4.4 red dots/cell were counted for AT₁-MasHets (Fig. 5A) and AT₂-MasHets (Fig. 5D), respectively, in resting microglia; in the negative control using the MasR primary antibody, only 0.2 red dots/cells were counted (Fig. 5C). These results indicate an expression level of AT₁-MasHets and AT₂-MasHets that seems markedly higher in resting microglia than in striatal neurons (see previous section). Interestingly, when the same experiment was performed in activated microglia, a significant decrease was observed in expression of the AT₂R-MasR complex (3.1 red dots/cell) (Fig. 5E) but not in expression of the AT₁R-MasR complex (4.2 red dots/cell) (Fig. 5B).

Fig. 5.

AT₁-MasHet and AT₂-MasHet expression in microglial primary cultures treated with LPS and IFN-γ. A–E Expression of AT₁R-MasR and AT₂R-MasR heteromers in primary microglial cultures was determined by proximity ligation assay (PLA), which was performed using specific primary antibodies against AT₁, AT₂, and Mas receptors. Confocal microscopy images (stacks of 4 consecutive planes) show heteroreceptor complexes; nuclei are Hoechst-stained (blue). Microglial cultures were incubated for 48 h in the absence (A, C, D) or in the presence (B, E) of 1 μM LPS and 200 U/mL IFN-γ. Scale bar: 20 μm. F: Bar graph showing the percentage of AT₁R-MasR and AT₂R-MasR clusters as red dots/cell. **p < 0.01; Student’s t test versus resting cells

AT₁R-MasR Complexes Show Negative Cross-talk in cAMP and MAPK Signaling Pathways in Microglia Treated or Not with LPS and IFN-γ

Signaling outputs were used to look for any differential functionality of heteromers in resting versus activated microglial cells. As observed in Fig. 6A, B, neither AT₁R nor MasR activation resulted in any cAMP level alteration in resting microglia. Also, activation of each receptor led to MAPK activation (Fig. 6C). Moreover, simultaneous coactivation of resting microglia with the two agonists induced negative cross-talk, whereas experiments with antagonists showed cross-antagonism. Interestingly, when primary cultures were treated for 48 h with 1 mM LPS and 200 U/mL IFN-γ, Ang II led to a significant G_i-mediated effect, indicating that the blockade of AT₁R that occurred when forming complexes with MasR disappeared in activated microglia. This effect was potentiated when receptors were coactivated and cross-antagonism was not detected (Fig. 6D). By analyzing ERK1/2 phosphorylation data, we observed a similar effect of the MasR agonist in both resting and activated microglia. However, as in resting microglia, negative cross-talk and cross-antagonism were observed (Fig. 6E). To summarize, in activated microglia, the AT₁-MasHets seem to undergo structural reorganization favoring the action of Ang II on G_i-coupled AT₁R.

Fig. 6.

AT₁-MasHet function in primary cultures of microglia. A–E Microglia in primary cultures were incubated for 48 h with 1 μM LPS and 200 U/mL IFN-γ (D, E) or vehicle (A–C). Then, cultures were pretreated with selective receptor antagonists (300 nM candesartan for AT₁R, 1 μM PD123319 for AT₂R, and 500 nM A779 for MasR) and subsequently treated with selective agonists (100 nM Ang II for AT₁R, 300 nM CGP-42112A for AT₂R, and 250 nM Ang [1–7] for MasR). Cytosolic cAMP levels (A, B, D) and ERK1/2 phosphorylation (C, E) were subsequently determined. Values are the mean ± SEM of 5 independent experiments performed in triplicate. One-way ANOVA followed by Bonferroni’s multiple comparison post hoc test were used for statistical analysis. *p < 0.05, **p < 0.01, ***p < 0.001 versus basal treatment (A) or forskolin treatment (B, D) in cAMP measurements or versus vehicle in pERK measurements (C, E)

AT₂R-MasR Complexes Show Negative Cross-talk in cAMP and MAPK Signaling Pathways in microglia Treated or Not with LPS and IFN-γ

As in the previous section, we observed differential effects in resting and activated microglia when the AT₂R-MasR couple was analyzed with a functional perspective. The cAMP data indicated a non-significant effect upon AT₂R or MasR activation in resting cells (Fig. 7A). Again, in resting microglia, a blockade of G_i-mediated AT₂R function was detected when AT₂R was coexpressed with MasR. In resting cells, only MasR was linked to MAPK pathway activation but with marked negative cross-talk when the AT₂R agonist was present and with a cross-antagonism effect (Fig. 7B). These results are similar to those obtained in the heterologous expression system.

Fig. 7.

AT₂-MasHet functionality in primary cultures of microglia. A–D Microglia in primary cultures were incubated for 48 h with 1 μM LPS and 200 U/mL IFN-γ (C, D) or vehicle (A, B). Microglial cells were pretreated with selective receptor antagonists (300 nM candesartan for AT₁R, 1 μM PD123319 for AT₂R, and 500 nM A779 for MasR) and subsequently treated with selective agonists (100 nM Ang II for AT₁R, 300 nM CGP-42112A for AT₂R, and 250 nM Ang [1–7] for MasR). Cytosolic cAMP levels (A, C) and ERK1/2 phosphorylation (B, D) were subsequently determined. Values are the mean ± SEM of 5 independent experiments performed in triplicate. One-way ANOVA followed by Bonferroni’s multiple comparison post hoc test were used for statistical analysis. *p < 0.05, **p < 0.01, ***p < 0.001 versus forskolin treatment in cAMP measurements (A, C) or versus vehicle in pERK assays (B, D)

In microglia activated for 48 h with 1 mM LPS and 200 U/mL IFN-γ, we observed that the AT₂R agonist CGP-42112A induced a significant decrease in forskolin-induced cAMP levels (Fig. 7C). Activation of microglia provoked structural alterations in AT₂-MasHets because cross-antagonism and negative cross-talk were not found; that is, there was no blockade by MasR over AT₂R. In terms of ERK1/2 phosphorylation, the results were similar to those found in resting cells: only MasR was linked to MAPK pathway activation, with marked negative cross-talk and cross-antagonism (Fig. 7D).

Expression of Heteroreceptor Complexes in Parkinsonian and Dyskinetic Animals

In situ PLA is instrumental in detecting receptor heteromer clusters in natural sources. We performed PLA in striatal sections from the 6-OH-dopamine-based PD model. Assays were performed in sections from non-lesioned and lesioned hemispheres in three animal groups (see “Materials and Methods”): untreated, levodopa-treated non-dyskinetic, and levodopa-treated dyskinetic. Both AT₁R-MasR and AT₂R-MasR clusters were expressed but at relatively low levels (Fig. 8) and their expression was markedly enhanced in the lesioned hemisphere (Fig. 8). Levodopa treatment per se did not lead to significant modification of expression in parkinsonian animals; however, in dyskinetic animals (i.e., animals that developed dyskinesias upon treatment with the drug), expression was modified. Importantly, the expression in dyskinetic animals (with respect to non-dyskinetic animals) was lower in the case of the AT₂R-MasR heteromer but significantly higher in the case of AT₁R-MasR (Fig. 8).

Fig. 8.

AT₁-MasHet and AT₂-MasHet complex expression in the striatum of lesioned and levodopa-treated animals. Heteroreceptor expression was assessed by the in situ proximity ligation assay (PLA) in the striatum of the different groups of animals treated as described in Methods. Graphs in the right represent the quantification of the red labeling in the different groups calculated using Andy’s algorithm. *p < 0.05, **p < 0.01, ***p < 0.001 versus non-lesioned animals

Discussion

At the beginning of the twenty-first century, research on GPCRs uncovered a novel property resulting from the formation of heteroreceptor complexes. Receptor–receptor interactions lead to new functional units whose properties are different from those displayed by the individually acting receptors. Interestingly, consideration of receptor heteromers was first suggested for neurodegenerative conditions such as those occurring in PD. Heteromers containing adenosine and cannabinoid receptors are among those relevant to striatal function [65–71]. The occurrence of angiotensin receptors in the nigra and the striatum was followed by the identification of complexes formed by AT₁R and AT₂R [39, 42]. In this paper, we showed that the Mas receptor forms heteroreceptor complexes with AT₁R and AT₂R and that such complexes are relevant for better understanding the role of RAS in neuroinflammation.

The RAS imbalance has been well studied in relation to blood pressure alterations, which are exacerbated by aging [14, 72, 73]. However, alterations in RAS balance upon aging affects almost any physiological process. RAS regulates functions in the kidneys, lungs, and brain and thus is key for maintaining homeostasis both in the periphery and in the CNS. Unbalanced RAS upon aging is seen as an opportunity to intervene using ligands of RAS receptors to combat neurological diseases whose main risk factor is age. Parkinson’s and Alzheimer’s diseases are the most prevalent aging-related diseases affecting the CNS [74–76]. It is tempting to speculate that RAS is center stage in understanding the greater severity of COVID-19 in older patients and the variety of symptoms ranging from asymptomatic to serious lung, kidney, immunological, and/or neurological manifestations [19, 21, 22, 77].

A review in 2009 questioned whether results associated with the RAS system were due to cross-talk or to heteromerization [78]. Both AT₁R-AT₂R [39, 79] and AT₂R-MasR interactions had previously been reported [4, 40]. In 2018, it was suggested that MasR and AT₂R were “joining forces” to counteract deleterious actions on blood pressure mediated by AT₁R and that AT₂-MasHets could help explain the functionality of AT₁R [41]. A scenario of RAS receptors interacting with each other raised numerous questions, particularly to understand why receptors mediating opposing effects would “join forces.” To approach the issue, two lines of inquiry are necessary: [1] assessing the expression levels of the different components of the system in targeted cells/tissues, heteromers included; and [2] deciphering the properties of the units resulting from receptor–receptor interactions. The functionality studies reported here show differential behaviors depending on the signaling pathway and inter-receptor interactions that lead to negative cross-talk, even reaching full blockade and cross-antagonism. Such counterbalancing effects occurred for both AT₁-MasHets and AT₂-MasHets. Cross-antagonism was reported for AT₂-MasHet expressed in mouse astrocytes using the mRNA level of the CX3C chemokine receptor-1 as a read-out [40]. Apart from cross-antagonism, which is often a characteristic of heteromer expression, a novel feature that is also explained by inter-receptor interaction is that the antagonist of MasR increased the calcium peak elicited by the AT₁R agonist [31, 80, 81]. The properties of these heteromers must be considered to understand the efficacy of antihypertensive medication targeting RAS and for repurposing such approved drugs.

Unbalanced RAS associated with Parkinson’s and Alzheimer’s diseases opens new therapeutic perspectives. Pharmacological manipulation of RAS components has potential in PD [82]; it is suggested that antagonists already used to treat hypertension and able to cross the blood–brain barrier may be repurposed to treat PD [83]. On the one hand, the system has been extensively characterized in relation to nigral neurodegeneration [27, 72, 83–85]. On the other hand, there are confounding factors in assessing the risk of PD in individuals taking antihypertensive medication. This is due to the multiplicity of therapeutic choices but also to the fact that some drugs targeting RAS enter the brain (e.g., candesartan) [86], whereas others cannot cross the blood–brain barrier. Therefore, we cannot assess with certainty whether chronic administration of RAS-related antihypertensives is neuroprotective or whether they can delay neurodegeneration once the disease shows clinical symptoms. Selecting the most appropriate cell to be targeted is also important. It is feasible to target activated microglia to prevent neurodegeneration if the disease has a neuroinflammatory component. In addition, MasR is key for microglia-driven development of the retinal vasculature [87]. The results presented here and those related to expression of AT₁R-AT₂R heteromers in striatal cells [42] suggest that MasR-related therapies to delay progression of PD should consider RAS components in microglia, with the ultimate goal of attenuating inflammation or skewing microglia toward the M2 neuroprotective phenotype [64]. RAS is well positioned to be targeted to polarization of microglia activated upon neuroinflammation [82]. In contrast to results in transfected HEK-293T cells and activated microglia, we did not observe cross-antagonism of AT₁ and AT₂ receptors over MasR in neurons. This, together with the lesser expression of MasR-containing heteromers, suggests that MasR functionality in neurons is not efficiently regulated by Ang II acting on AT₁ or AT₂ receptors.

Further parameters to consider are the expression of ACE1 and ACE2 and the possibility that these enzymes also interact with RAS receptors, thus affecting the local concentration of agonists of RAS receptors. In relation to COVID-19 management, it has been demonstrated that ACE2 interacts with AT₁R [88]. Combining these results with the results presented here, we cannot rule out the occurrence of a functional unit constituted by an enzyme, two receptors, and the corresponding G-coupled proteins. The functionality of equivalent macromolecular complexes and the 3-dimensional structure underlying the particular functional features has been demonstrated for adenosine A₁-A_2A and A_2A-A_2B receptors ( [33–38] and data in preparation). Both the allosteric modulation of ACE2 activity that results from the interaction with SARS-CoV viruses and the effect of viral infection on the surface expression of RAS components, heteromers included, need to be addressed.

Mechanistically, the two most relevant findings are the MasR-mediated regulation of cytosolic calcium mobilization triggered by AT₁R agonists and the differential link between RAS and MAPK activation, depending on which heteromers are expressed in a given cell type and which agonist(s) is (are) affecting RAS receptors. Very few mechanisms lead to radical changes in the activation of the MAPK pathway. However, we present here data that show that one of the properties of RAS receptor heteromers is the possibility to engage or not this relevant pathway, depending on the overall RAS balance. Several years ago, we reported that histamine H₃ receptor activation could not lead to ERK phosphorylation unless it formed a heteromer with the dopamine D₁ receptor. D₁-H₃ receptor heteromers are unique devices directing histaminergic and dopaminergic signaling toward the MAPK pathway in a G_i-dependent but G_s-independent manner [89]. Remarkably, MasR in activated microglia is linked to MAPK pathway activation with marked negative cross-talk and cross-antagonism. In a complementary recent study, we showed that AT₁-AT₂ receptor heteromers are expressed in microglia where [1] they are upregulated in both parkinsonian conditions and in l-DOPA-induced dyskinesias, and [2] their activation is seemingly neuroprotective. The data shown in Fig. 8 suggest that expression of heteroreceptor complexes formed by MasR and either AT₁R or AT₂R is higher in the striatum of the lesioned hemisphere in the rat 6-OH-DA-based PD model. The marked increase in the case of AT₁R-MasR heteromers in dyskinetic animals open new therapeutic avenues for the assessment of this well-known side effect of antiparkinsonian medication. In microglia, a differential pharmacological trend, when Ang II receptor heteromers are compared with AT₁-MasHets or AT₂-MasHets, is cross-antagonism, which is not found in the former but in the latter. This is promising from a therapeutic point of view, because cross-antagonism in AT₁R-AT₂R heteromers would lead to a dead end in terms of neuroprotection; instead, the antagonist of one Ang II receptor releases the brake on activation of the partner Ang II receptor within the AT₁R-AT₂R heteromer [42].

Funding

This work was partially supported by grants from the Spanish Ministry of Science and Innovation (MICINN) and/or Science, Innovation, and Universities; they may include EU FEDER funds (BFU2015-64405-R, SAF2016-77830-R, AARFD-17-503612, SAF2017-84117-R, RTI2018-094204-B-I00, and RTI2018-098830-B-I00). The laboratory of the University of Barcelona is considered of excellence by the Regional Catalonian Government (grup consolidat #2017 SGR 1497), which does not provide any specific funding for personnel, equipment, and reagents or for payment of services.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

Statement of Ethics

Under the current legislation, obtaining protocol approval is not needed if animals are sacrificed to obtain a specific tissue. Animal handling, sacrifice and further experiments were conducted according to the guidelines set in Directive 2010/63/EU of the European Parliament and the Council of the European Union that is enforced in Spain by National and Regional organisms; also the 3R rule (replace, refine, reduce) for animal experimentation was taken into account.