Objective: ApoM enriches S1P (sphingosine-1-phosphate) within HDL (high-density lipoproteins) and facilitates the activation of the S1P 1 (S1P receptor type 1) by S1P, thereby preserving endothelial barrier function. Many protective functions exerted by HDL in extravascular tissues raise the question of how S1P regulates transendothelial HDL transport. Approach and Results: HDL were isolated from plasma of wild-type mice, Apom knockout mice, human apoM transgenic mice or humans and radioiodinated to trace its binding, association, and transport by bovine or human aortic endothelial cells. We also compared the transport of fluorescently-labeled HDL or Evans Blue, which labels albumin, from the tail vein into the peritoneal cavity of apoE-haploinsufficient mice with (apoE-haploinsufficient mice with endothelium-specific knockin of S1P 1 ) or without (control mice, ie, apoE-haploinsufficient mice without endothelium-specific knockin of S1P 1 ) endothelium-specific knockin of S1P 1 . The binding, association, and transport of HDL from Apom knockout mice and human apoM-depleted HDL by bovine aortic endothelial cells was significantly lower than that of HDL from wild-type mice and human apoM-containing HDL, respectively. The binding, uptake, and transport of 125 I-HDL by human aortic endothelial cells was increased by an S1P 1 agonist but decreased by an S1P 1 inhibitor. Silencing of SR-BI (scavenger receptor BI) abrogated the stimulation of 125 I-HDL transport by the S1P 1 agonist. Compared with control mice, that is, apoE-haploinsufficient mice without endothelium-specific knockin of S1P 1 , apoE-haploinsufficient mice with endothelium-specific knockin of S1P 1 showed decreased transport of Evans Blue but increased transport of HDL from blood into the peritoneal cavity and SR-BI expression in the aortal endothelium. Conclusions: ApoM and S1P 1 promote transendothelial HDL transport. Their opposite effect on transendothelial transport of albumin and HDL indicates that HDL passes endothelial barriers by specific mechanisms rather than passive filtration.

Heterogeneous high-density lipoprotein (HDL) particles, which can contain hundreds of proteins, affect human health and disease through dynamic molecular interactions with cell surface proteins. How HDL mediates its long-range signaling functions and interactions with various cell types is largely unknown. Due to the complexity of HDL, we hypothesize that multiple receptors engage with HDL particles resulting in condition-dependent receptor-HDL interaction clusters at the cell surface. Here we used the mass spectrometry-based and light-controlled proximity labeling strategy LUX-MS in a discovery-driven manner to decode HDL-receptor interactions. Surfaceome nanoscale organization analysis of hepatocytes and endothelial cells using LUX-MS revealed that the previously known HDL-binding protein scavenger receptor SCRB1 is embedded in a cell surface protein community, which we term HDL synapse. Modulating the endothelial HDL synapse, composed of 60 proteins, by silencing individual members showed that the HDL synapse can be assembled in the absence of SCRB1 and that the members are interlinked. The aminopeptidase AMPN (also known as CD13) was identified as an HDL synapse member that directly influences HDL uptake into the primary human aortic endothelial cells (HAECs). Our data indicate that preformed cell surface residing protein complexes modulate HDL function and suggest new theragnostic opportunities.

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