Sphingosine-1-phosphate receptor-1 (S1PR1) signalling: the homeostatic pathway of the heart

This editorial refers to ‘Vascular endothelial S1pr1 ameliorates adverse cardiac remodelling via stimulating reparative macrophage proliferation after myocardial infarction’ by Y. Kuang et al., pp. 585–599.

In 1884, Johann Ludwig Thudichum, a German physician and biochemist, identified sphingolipids as abundant constituents of the brain, and because of their enigmatic nature, he named them after the sphinx. Sphingolipids are essential components of eukaryotic cell membranes and potent signalling molecules. Sphingosine-1-phosphate (S1P), one of the best characterized bioactive lipids, exerts different biological functions, including cell survival and migration, which are mediated by five different G-protein-coupled receptors, named S1P receptor (S1PR1-5). S1PR1 is highly expressed in endothelial cells and lymphocytes and is mainly accountable for the S1P-regulated immune cell trafficking, vascular development and homeostasis.^1,2

Circulating levels of S1P are in the high nanomolar range (∼1 µM), and red blood cells, endothelial cells, and platelets are major sources. On the contrary S1P levels are markedly decreased in the lymph (∼10 nM) and in the interstitial space (∼1 nM). This S1P gradient is fundamental to drive lymphocyte egress from secondary lymphoid organs via activation of S1PR1, a biological function that has been therapeutically exploited. In 2010, the U.S. Food and Drug Administration (FDA) approved Fingolimod (Gilenya), targeting all the S1PRs, except S1PR2, to treat the relapsing form of multiple sclerosis.³ Fingolimod inhibits the egress of lymphocytes into the lymph, via downregulation of S1P1 expression, therefore defined ‘functional antagonist’.³ However, fingolimod side effects include bradycardia, macular oedema, and increased blood pressure,³ which result from the disruption of beneficial cardiovascular effects of S1P signalling. S1P1-3 are expressed in the cardiovascular system. Interestingly, Siponimod, targeting only S1PR1 and S1PR5, showed similar side effects of fingolimod in clinical trials, suggesting that antagonism of S1PR1 is accountable for cardiovascular adverse effects.⁴ This is not surprising considering the important cardiovascular functions of S1PR1.

While necessary for vascular and heart development during embryogenesis,^5,6 S1PR1 signalling also contributes to post-natal cardiovascular homeostasis. For instance, endothelial S1PR1 controls blood flow and pressure via endothelial-derived nitric oxide production,⁷ and heightens endothelial barrier functions.² In cardiomyocytes, S1PR1 signalling is critical to maintain ion homeostasis, and cardiac functions.⁸

In this elegant study, Kuang et al. uncovered a novel role for endothelial S1P1 signalling in the context of myocardial infarction (MI). They demonstrated that endothelial S1P1 activation ‘orchestrates’ the proliferation of reparative macrophages, thus counterbalancing the pathological cardiac remodelling post-MI.⁹

By using a mouse model of MI induced by left anterior descending ligation, the authors demonstrated a downregulation of endothelial S1PR1 expression in the ischaemic heart, and severe cardiac remodelling and dysfunction post-MI when endothelial S1PR1 was genetically deleted. These findings evidenced that endothelial S1PR1 signalling is of critical importance following MI. This is not the first study reporting the cardioprotective role of S1PR1 signalling. Keul et al.⁸ showed that S1PR1 in cardiomyocytes protects the heart in ischaemic preconditioning by regulating Ca⁺⁺ sensitivity and Na⁺/K⁺ exchange. Zhang et al. reported a protective role of endothelial-derived S1P-S1PR1 signalling in pathological cardiac hypertrophy induced by haemodynamic stress.¹⁰ By using pharmacological approaches, additional studies have demonstrated beneficial effects of S1P pathway on the heart during ischaemic event.¹¹

Considering that inflammation can contribute to adverse left ventricle (LV) remodelling post-MI, the authors performed a longitudinal assessment of the inflammatory response. Interestingly, the loss of endothelial S1PR1 did not alter the number of inflammatory cells recruited into the infarcted hearts, such as neutrophils, CD3+ lymphocytes, and monocytes/macrophages, nor did alter circulating lymphocytes.

This is quite unexpected since the loss of endothelial S1PR1 disrupts the barrier function, which refrains circulating cells from extravating,² and thus would result in an increased margination of inflammatory cells. However, following MI endothelial S1PR1 expression is also reduced in wild type (WT) mice, which could account for the similar magnitude of inflammatory cells recruited to the heart of S1pr1^ECKO mice vs. controls.

Monocytes/macrophages (Mo/MΦ) are known to accumulate into the ischaemic hearts in biphasic mode. Hilgendorf et al. demonstrated that inflammatory Ly-6C^high Mo/MΦ, dominating the early phase (3 days, in grey in Figure 1), differentiate into Ly-6C^low macrophages, which prevail in the second phase (7 days, in light blue in Figure 1) and impact the healing and remodelling of the infarcted hearts.¹² Based on this knowledge, the authors further characterized the Mo/MΦ populations post-MI. Interestingly, while the loss of endothelial S1PR1 does not influence the number of Ly-6C^high Mo/MΦ in the first phase, dramatically reduces the number of Ly-6C^low Mo/MΦ in the second phase. These data were also supported by RNA microarray showing a significant reduction of Ly-6C^low macrophage-relative gene expression (i.e. Arg1, Vegf, Tgf-β) in ischaemic myocardial tissue from S1pr1^ECKO mice compared to controls.

Figure 1

Schematic showing the protective role of endothelial S1PR1 signalling in myocardial infarction. Circulating and locally produced S1P can activate the endothelial S1PR1 and contribute to the differentiation of reparative Ly-6C^low MΦ following MI, via ERK/CSF-1 signalling and cell contacts. Endothelial S1PR1 signalling improves cardiac function and adverse remodelling post-MI. CSF-1, colony stimulating factor 1; S1PR1, sphingosine-1- phosphate receptor 1.

It has been suggested that tissue-resident macrophages in the heart can contribute to inflammation post-MI.¹³ Thus, to dissect whether endothelial S1PR1 affects Ly-6C^low MΦ post-MI by influencing the infiltration of Mo and/or local MΦ as sources, the authors performed a parabiosis experiment, by conjoining the circulation of green fluorescent protein (GFP)-positive and negative mice. Interestingly, the percentage of Ki67 positive F4/80⁺Ly-6C^low GFP⁻ was not affected by the loss of endothelial S1PR1, thus arguing against the resident myocardial MΦ as a source of reparative F4/80⁺Ly-6C^low MΦ.

Endothelial cells can impart a selective growth and differentiation of macrophages towards the M2-like phenotype, a process that requires cellular contacts and, in part, the colony-stimulating factor-1 (CSF-1).¹⁴ Thus, Kuang et al. attempt to further dissect the mechanistic basis by which endothelial S1PR1 contribute to the F4/80⁺Ly-6C^low MΦ differentiation. By combining cellular and molecular approaches, the authors demonstrated that physical contacts between the endothelial cells and MΦ were necessary for the S1PR1 signalling to impart the differentiation towards F4/80⁺Ly-6C^low MΦ. Furthermore, co-culturing THP-1 (human monocyte cell line) and human umbilical vein endothelial cells (HUVEC) overexpressing S1PR1, in the presence and absence of CSF-1 signalling inhibitor (GW2580), they demonstrated that CSF-1 was required for endothelial S1PR1-mediated increase in MΦ differentiation towards F4/80⁺Ly-6C^low.

A deeper investigation of the molecular mechanisms also revealed that endothelial CSF-1 expression is regulated by S1PR1-extracellular signal-regulated kinase (ERK) signalling axis in a cell-contact manner. As a conclusive experiment of this study, Kuang et al. tested whether S1PR1 signalling could be therapeutically exploited to induce reparative macrophage differentiation in ischaemic hearts. Interestingly, following MI, the treatment of the mice with SEW2871, an agonist of S1PR1, was able to increase the reparative F4/80⁺Ly-6C^low MΦ, and significantly improve cardiac functions and adverse remodelling. Lastly, the cardioprotective effects of SEW2871 were abolished by GW2580 treatment, underlining the importance CSF-1 signalling downstream of S1PR1 activation.

The major finding of this study is that endothelial S1PR1 signalling can ameliorate cardiac remodelling post-MI by promoting the differentiation of reparative macrophages, which is a novel and previously unidentified function. From a translational point of view, previous and current findings from this study point out to S1PR1 as a potential target to treat the adverse inflammation and cardiac remodelling after MI. The activation of endothelial S1PR1 also improves blood flow and pressure,⁷ which can contributes to the beneficial effects of S1PR1 signalling on the vasculature and the heart.

In a recent study, Poirier et al. reported a novel biased agonist of S1PR1, SAR247799, which is able to activate G-protein signalling to a greater extent than β-arrestin, thus minimizing S1P1 internalization and desensitization.¹⁵ The advantage of this biased agonist vs. current clinically available molecules is the activation of S1PR1 without compromising the immune response while exploiting the beneficial cardiovascular effects of this pathway. The study of Kuang et al. definitely set a strong foundation to exploit SAR247799, and other more selective S1PR1 drugs, in the treatment of MI, and other cardiovascular conditions where S1PR1 signalling is impaired.

Conflict of interest: none declared.

Funding

National Heart, Lung, and Blood Institute of the National Institutes of Health grant R01 HL126913 to A.D.L.

The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.