Expression of OR51E1 in the human heart
A variety of ORs are expressed in diverse non-olfactory human tissues, including the brain, the lung, the kidney and the prostate [
25,
28,
98,
115]. However, the functional role of these ORs has only been investigated in a few tissues [
8,
67,
90,
99,
114].
OR51E1 was primarily identified in the prostate and is overexpressed in prostate cancer [
31,
104,
105]. Further studies have demonstrated the expression of OR51E1 in various healthy and pathologically altered tissues, whereas the function still remains unknown [
19,
28,
34,
50].
In the human heart, a broad spectrum of ORs was detected at the RNA level, whereas the odorant receptor
OR51E1 was identified as the highest expressed OR by transcriptome analysis [
28]. In addition, our transcriptome analysis results showed that
OR51E1 is also the highest expressed OR in the human fetal heart. The prenatal expression of ectopic OR was described for the rodent developing heart, suggesting a potential role during cardiac development, and for avian embryos, indicating an important role in cell recognition during embryogenesis [
22,
23,
26]. The occurrence of
OR51E1 mRNA during the early stages of fetal development suggests a possible role in heart development. In this study, we focused on the expression of OR51E1 in the adult human cardiac ventricles and stem cell-derived cardiomyocytes, which we used as a cardiac in vitro model for the functional analysis of the odorant receptor OR51E1 at the cellular and molecular levels. We detected transcripts of
OR51E1 by RT-PCR in human heart tissue and stem cell-derived cardiomyocytes but not in pluripotent stem cells. Additionally, the expression of OR51E1 could also be validated at the protein level in human heart tissue by immunocytochemical analysis and western blot. Compared to prostate tissue, in the human heart tissue we detected beside the monomeric form of OR51E1 also receptor oligomeres. The detection of higher molecular weight receptor dimers and oligomers are features commonly found after SDS-PAGE for ORs and for GPCR in general [
6,
17,
66,
100,
107]. The physiological significance of OR oligomerization is still unknown. Interestingly, different oligomerization states can be observed for the same receptor also in other tissues. The OR51E1 paralogous olfactory receptor 51E2 was found as monomer in the prostate and in human pigment cells in a monomeric and dimeric form [
33,
67].
Activation of OR51E1 in cardiomyocytes
We observed that stimulation of cardiomyocytes with OR51E1 agonist leads to inhibition of the spontaneous Ca2+ transients. We showed that this negative chronotropic effect is dose-dependent over the micromolar-to-millimolar range. Detailed analysis revealed that regardless of which type of stem cell-derived cardiomyocytes (hESC- and hIPSC-derived) was investigated, the nonanoic acid-induced effect remained unchanged. We tested whether other medium-chain fatty acids (MCFA) also reduce the frequency of Ca2+ spikes and found that the OR51E1 agonists decanoic, dodecanoic and tetradecanoic acid elicited similar effects, whereas compounds that were inactive on the heterologously expressed OR51E1 did not affect the Ca2+ spike frequency.
We next aimed to investigate the OR51E1-induced signaling cascade. In olfactory receptor neurons, activation of an olfactory receptor leads to an activation of adenylyl cyclase III via the Gα
olf protein. AC-III, in turn, is activated via cAMP CNG-channels [
3,
45,
65]. The OR signal transduction pathway in non-olfactory tissues appears to vary depending on the tissue. For most non-olfactory tissues, which express ORs, the initiated pathway is still unknown, but the few examples of pathways that have been determined differ from the classical olfactory signaling cascade [
8,
13,
89].
Our results suggest that OR51E1 couples predominantly to the stimulatory G protein, probably to the olfactory G protein. Moreover, the inhibitory G protein could also be a putative interaction partner of OR51E1 due to the finding that Gα
i protein was weakly co-precipitated with OR51E1 protein. Recently, it was shown that beside Gα
olf also Gα
o is functionally coupled to ORs in olfactory sensory neurons [
84].
Furthermore, we could observe the involvement of the βγ subunit of the G protein in the nonanoic acid-induced effect. The G protein βγ subunits interact with effector molecules, such as phospholipases, adenylyl cyclase and ion channels, in a manner that leads to their activation or inhibition [
16]. Therefore, it is not surprising that the βγ subunit is discussed as a drug target,
inter alia, for preventing heart failure [
53,
88]. OR signaling via βγ subunits was previously described in olfactory sensory neurons and was also demonstrated for ectopically expressed OR51E2, a paralog of OR51E1, in prostate cancer cells [
82,
96]. Classical cardiac GPCRs such as the muscarinic acetylcholine receptor M2 and adrenergic receptors also act via the βγ subunit in the heart [
87]. Interestingly, bitter taste receptor agonists elicit G protein βγ-dependent negative inotropy in the murine heart [
29]. Thus, we suggest that OR51E1 activation leads to GIRK channel opening via G
βγ subunit. This opening leads to a hyperpolarization of the cell, which counteracts the HCN channel-induced depolarization, resulting in a reduction of the Ca
2+ spike frequency.
However, we could not deduce whether the interaction between G protein βγ subunits and other effector molecules primarily mediates the negative chronotropic effect or if the Gβγ subunit only enhances the effect without playing a key role.
We also found that OR51E1 agonists evoked negative inotropic effects on explanted heart preparations. The reduction of contractility of adult human myocardium stimulated with MCFA was dose‐dependent and occurred over a similar dose range as the negative chronotropic effect in stem cell-derived cardiomyocytes, possibly indicating the involvement of a similar G protein βγ-mediated signal transduction. It is well recognized that βγ subunits contribute to the depression of contractility in failing myocardium by targeting the β-adrenergic receptor kinase to the membrane-bound receptors, thereby mediating autologous β-receptor desensitization [
74,
83,
85]. However, it is unlikely that this mechanism would be responsible for the reduction in contractility provoked by MCFA in our study because these responses developed with fast kinetics and in the absence of exogenous β-receptor stimulation. There are alternative targets by which G protein βγ-subunits reduce contractility of failing myocardium in a manner independently of β-receptor sensitization [
52]. G protein βγ-coupled receptors may activate phospholipase C-β, ERK1/2 and PI3-kinase γ isoform [
46,
64], the latter of which has been identified as a negative regulator of cardiac contractility [
18]. It is, therefore, tempting to speculate that the negative inotropic effect of the G protein βγ-coupled OR51E1 receptor may be mediated by PI3-kinase γ activation. In addition, it should be noted that the measurements were conducted with myocardium of patients with a heart failure. Human healthy heart tissue was not available in this study due to practical and ethical limitations. Therefore, we could not ensure that cardiac diseases influence the nonanoic acid-induced effect.
Possible role of OR51E1 in the heart
De-orphanization studies on the recombinant OR51E1 revealed nonanoic acid, which has an unpleasant rancid odor, as an activating ligand [
1,
80]. In the present study, we could identify, in addition to nonanoic acid, several MCFA as specific OR51E1 activator. The specificity of recombinant OR51E1 for MCFA was determined by testing a verity of structurally related substances that did not activate the receptor. OR51E1-activating MCFA consist of a 4–14 carbon chain and are mainly provided by food digestion and lipolysis in adipose tissue. In contrast to long-chain fatty acids (LCFA), the dietary intake of short- and medium-chain fatty acids are not significantly associated with the risk of coronary heart disease [
41]. A number of studies have indicated that fatty acids not only serve as energy sources but can also act as signaling molecules [
43,
68].
We determined the fatty acid profile in human plasma and epicardial adipose tissue as a potential storage site for OR51E1-activating FFA. Recent data indicate a possible role of adipose tissue in modulating cardiac function [
12,
42,
69]. It was reported that cardiac adipocytes are able to release substances that suppress the contraction of cardiomyocytes by attenuating intracellular Ca
2+ levels [
49]. We detected medium-chain free fatty acids, which may act as OR51E1 agonists in epicardial adipose tissue. We, therefore, hypothesize that, in addition to dietary intake, OR51E1-activating MCFA in plasma may be released from adipocytes. In plasma of diabetic patients medium-chain free fatty acid concentrations were elevated, whereas the fatty acid profile of triacylglyceride in the fat compartment was not different between both groups. Previously, the fatty profile in epicardial adipose tissue has been determined; however, the MCFA content has not been reported in detail [
73]. Thus, OR51E1 is one of the few de-orphanized ORs with an odorant that is also present in the human body.
A great number of reports have shown that FFA-sensing GPCRs play important roles in mediating a variety of physiological processes, such as regulation of energy metabolism mediated by the secretion of hormones and by the regulation of the sympathetic nerve systems and taste preferences, and have demonstrated potential as therapeutic targets for various metabolic and inflammatory disorders, including obesity, type 2 diabetes, atherosclerosis, cardiovascular diseases, ulcerative colitis, Crohn’s disease and irritable bowel disease [
5,
21,
38,
43,
61,
91,
93,
97].
OR51E1 is the highest expressed orphan MCFA-sensing receptor according to our transcriptome data (Table S3). The ligand spectrum of the free fatty acid receptors (FFAR) known to date is only partially overlapping with that of OR51E1. Among the FFAR family, FFAR1 (known as GPR40) and FFAR4 (known as GPR120 and O3FAR1) are classified as medium- to long-chain fatty acid-activated receptors (FFAR1: > C12; FFAR4: C14–C18) [
9,
40]. FFAR2 and FFAR3 (known as GPR43 and GPR41) respond to short-chain fatty acids that have fewer than five carbon atoms [
10]. Nonetheless, a potential role of FFAR4 in the cardioprotective effect of eicosapentaenoic acid was recently described [
24]. Only the free fatty acid receptor GPR84 (C9–C14) is activated by nonanoic acid, whereby nonanoic acid displays the weakest ligand [
102]. According to our transcriptome analysis and as previously described, all five receptors are not or only very weakly expressed on mRNA level in the human ventricular myocardium [
40]. Because the OR51E1 antagonist and knock-down experiments abolished the nonanoic acid-induced effect completely, we propose that the observed action of MCFA on cardiomyocytes is primarily mediated by OR51E1, but the involvement of other FFARs in sensing OR51E1-activating MCFA cannot be fully excluded in an in vivo situation.
Effects of MCFA on cardiac function were previously reviewed by Francois Labarthe and colleagues [
47]. The authors suggested that MCFA not only provide a highly efficient source of energy production as contrary to LCFA they enter the cells by free diffusion and are preferentially directed toward oxidation rather than storage. MCFA also may positively modulate cardiac disease progression when considering the heart’s energy status and contractile dysfunction. In animal heart models, MCFA were reported to improve diabetic cardiomyopathy, prevent cardiac hypertrophy and recover metabolism and contractile function after transient ischemia [
2,
15,
27,
48,
55,
63]. The role of OR51E1 in MCFA-mediated benefits for cardiac function or disease progression remains elusive but would be interesting to examine in further studies. Moreover, polyunsaturated fatty acids (PUFAs), such as arachidonic and eicosapentaenoic acid, have been shown to affect Ca
2+ handling of cardiomyocytes in vitro [
54,
57,
110]. PUFAs induce negative inotropy and inhibit Ca
2+ transients, which leads to the consideration of PUFAs as antiarrhythmic agents. Furthermore, arachidonic acid was described to counteract β-adrenergic receptor-induced stimulation and was consequently considered to perform a cardiac protective function [
57,
72].
The results of this study indicate at negative inotropic effects of OR51E1 activation in vivo, however, do not allow for straightforward interpretation of a clinical relevance as further preclinical and eventually clinical evaluation are required to qualify OR51E1 as a cardiovascular drug target. Preclinical testing in animal models should be conducted in future studies.
Interestingly, the OR51E1 and OR51E2 mouse orthologs were found to be expressed in the carotid body and the OR51E2 mouse ortholog, namely Olfr78, acts as a hypoxia sensor. A decreasing oxygen level leads to the production of lactate that in turn affects the breathing circulation via Olfr78 [
14]. This finding represents an alternative physiological function of an OR by detecting an endogenous ligand. In the human heart, OR51E1 may act as dietary sensor, which may influence the mechanical and electrical heart function and may possibly be involved in the regulation of the energy metabolism.
In conclusion, our data demonstrate a significant progress towards the characterization of the functional role of ORs in the human heart. We could show that activation of OR51E1 by MCFA induces negative inotropy in human explanted heart preparations and leads to negative chronotropy in stem cell-derived cardiomyocytes, which could be reversed by our identified OR51E1 antagonist. Based on our results, we hypothesize that OR51E1 may play a role in the metabolic regulation of cardiac function. The involvement of heart diseases in pathophysiological processes can only be speculated in the absence of data from animal models or patient studies.