Myoendothelial Junctions of Mature Coronary Vessels Express Notch Signaling Proteins

Abstract

The Notch signaling pathway, via cell‐cell communication, is critical during normal vascular development and homeostasis, and it is known to regulate vascular smooth muscle (VSMC) phenotype, including proliferation; however, few studies have addressed the role of Notch signaling in the mature vasculature. The current study tested the hypothesis that Notch signaling is present and active at mature myoendothelial junctions (MEJs), signaling hubs that occur at breaks in the internal elastic lamina (IEL). In mature, intact normal adult mouse coronary resistance microvessels (CRMs) embedded within heart sections, we observed Notch1, Notch2, Notch3, and Jagged1 expression within the fenestrae of the CRM IEL where MEJs are known to occur. Immunofluorescence clearly shows bridging from the VSMCs to endothelial cells (ECs). Immunofluorescence in larger mouse left anterior descending (LAD) coronary arteries demonstrate the expression Notch2 and Notch3 protein immunofluorescence within fenestrae of the IEL. We found that, on average, 39% and 33% of IEL fenestrae were positive for Notch2 and Notch3 expression, respectively. We also confirmed the presence of Notch3 expression in the MEJ by immunogold electron microscopy. In normal mouse femoral arteries, we found that, on average, 46%, 51%, and 58% of IEL fenestrae were positive for Notch1, Notch3, and Jagged1 expression, respectively. To determine whether Notch signaling components were localized to the MEJ, we co‐cultured primary human coronary ECs (hcECs) and primary human coronary VSMCs (hcVSMCs) across transwell inserts, and we utilized fluorescent microscopy to immunostain for Pai‐1, Jagged1, Notch1, and Notch2. PAI‐1 immunofluorescence was observed on the EC side of the transwells and within the transwell pores, indicating the formation of in vitro MEJs. We also identified distinct staining of Notch1, Notch2, and Jagged1 in the pores of the co‐culture transwells, which co‐localized with PAI‐1 within the pores. To assess whether Notch signaling was active at the MEJ, we co‐cultured hcECs and hcVSMCs on transwells to create in vitro MEJs. Smooth muscle cells were first transfected with a Notch‐sensor luciferase plasmid followed by transwell co‐culture of the transfected cells with endothelial cells. Notch activity was stimulated by 3‐fold across the MEJ in hcVSMC (1.00 ± 0.15 vs. 3.02 ± 0.43, p<0.001), which was completely abrogated by the non‐selective Notch γ‐secretase inhibitor, DAPT (1.15 ± 0.16, p<0.01). In this study, we demonstrate for the first time that Notch receptors and ligands are expressed within and are active at mature coronary MEJs, demonstrating a previously unrecognized mode of Notch signaling regulation between the endothelium and smooth muscle.Support or Funding InformationFunding: EW was supported by NIH R25HD086885. NIH R00HL116769, R21EB026518, and S10OD023438 to AJT.