Distinct compartments of the proepicardial organ give rise to coronary vascular endothelial cells

Abstract

The proepicardial organ is an important transient structure that contributes cells to various cardiac lineages. However, its contribution to the coronary endothelium has been disputed, with conflicting data arising in chick and mouse. Here we resolve this conflict by identifying two proepicardial markers, Scleraxis (Scx) and Semaphorin3D (Sema3D), that genetically delineate heretofore uncharacterized proepicardial subcompartments. In contrast to previously fate-mapped Tbx18/WT-1-expressing cells that give rise to vascular smooth muscle, Scx- and Sema3D-expressing proepicardial cells give rise to coronary vascular endothelium both in vivo and in vitro. Furthermore, Sema3D(+) and Scx(+) proepicardial cells contribute to the early sinus venosus and cardiac endocardium, respectively, two tissues linked to vascular endothelial formation at later stages. Taken together, our studies demonstrate that the proepicardial organ is a molecularly compartmentalized structure, reconciling prior chick and mouse data and providing a more complete understanding of the progenitor populations that establish the coronary vascular endothelium.

Figure 1. Sema3D and Scx mark proepicardial and migrating epicardial cells

A-D, Radioactive RNA section in situ hybridization (ISH) of Sema3D in mouse embryos E9.5-E14.5. Sema3D is restricted to the proepicardium at E9.5 (A) and migrating epicardial cells (B-D) until E12.5 after which expression can also be seen in the valve cushions (D). E-F, bright field (E) and direct GFP fluorescence (F) in Sema3D^GFPCre E9.5 mouse embryos. G-I, immunhistochemistry (IHC) with an anti-GFP antibody on Sema3D^GFPCre embryos. Sema3D^GFPCre recapitulates endogenous Sema3D expression in the early proepicardium and epicardium (F-I). J, whole-mount ISH of Scx in wild type mouse embryos at E9.5. K-L, section ISH to Scx (K) and Tbx18 (L) at E9.5. Scx is restricted to only a subdomain of the PEO as compared to Tbx18 which is expressed throughout the PEO and extends into the septum transversum (notched arrow in L). M-N, RNA ISH to Scx in E10.5 ScxGFP+/- mouse embryos (M) and at higher magnification (N). O-Q, IHC with an anti-GFP antibody in ScxGFP mouse embryo sections, E10.5-E12.5. Scx expression is restricted to the proepicardium and epicardium prior to E11.5 when it comes on only in the developing valves (notched arrow in P and Q). By E12.5, Scx is largely gone from the epicardium (Q). LV/RV – left/right ventricle, LV/RA – left/right atrium, PE-proepicardium, EP-epicardium, A/V- atrium/ventricle. See also Supplemental Figure 1.

Figure 2. Sema3D and Scx expressing proepicardial subdomains are largely distinct from Tbx18 and WT-1 expression

A-F, magnified view of the PEO of Sema3D^GFPCre E9.5 mouse embryos with IHC to GFP (A,D), WT-1 (B) and Tbx18 (E). The merged view of GFP and WT-1 (C) or Tbx18 (F) is shown. A portion of Sema3D expressing proepicardial cells are distinct from the WT-1 and Tbx18 expressing populations, as depicted by the green arrows. Red arrows represent cells expressing only WT-1(C) or Tbx18 (F). Yellow arrows represent cells that are double positive. G-L, proepicardia of ScxGFP E9.5 mouse embryos with IHC to GFP (G,J), WT-1 (H) and Tbx18 (K). Merged images (I,L) show that Scx expression is restricted to a subdomain largely distinct from WT-1 or Tbx18. Arrows represent single positive or double positive cells as described above. M-N, IHC to ß-galactosidase on Tbx18floxednLacZ mice at E12.5 (N, higher magnification of M). O-P, IHC to ß-galactosidase on Tbx18floxednLacZ;ScxCre mice at E12.5 (P, higher magnification of O). Q, by E12.5, 49% of Tbx18 expressing cells have never coexpressed Scx. Error bars represent the mean ± S.D. p≤0.05. RA/LA – right/left atrium, RV/LV – right/left ventricle. See also Supplemental Figure 2.

Figure 3. Sema3D and Scx lineage tracing in the embryonic heart

A-D, section X-gal staining of Sema3D^GFPCre;R26R^LacZ embryonic hearts, E9.5-E14.5. Migrating epicardial cells are seen on the surface of the heart at E10.5 (B) with cells first noted within the heart in the interventricular septum at E12.5 (C, inset) and spread throughout all four chambers (D, inset) and developing vasculature (black arrows in inset) by E14.5. E-J Co-immunostaining with ß-gal as a marker of Sema3D lineage traced cells (E, H inset) and Cardiac Troponin T (F, I inset) at E14.5 and E16.5. The merged images (G, J inset) show that Sema3D lineage traced cells give rise to cardiomyocytes. Shown here are double positive cells in the right ventricle. Note the developed sarcomere morphology in (J). Bright yellow cells in (G, lower magnification) are autofluorescent red blood cells. K-M, Co-immunostaining with ß-gal (K) and smooth muscle actin (L) at E14.5 shows that large numbers of Sema3D lineage traced cells give rise to vascular smooth muscle cells (merged image in M). N-Q, Section X-gal staining of ScxCre;R26R^LacZ mouse embryos E10-E12.5. Scx lineage traced cells are present on the surface of the heart at E10 (N) and by E10.5 are observed within the heart. R-T, Co -immunostaining with ß-gal (R) and Cardiac Troponin T (S) shows that Scx lineage traced cells contribute to the cardiomyocte lineage (merged image shown in T). U-W, Co-immunostaining with ß-gal (U) and Smooth Muscle Myosin (V) reveals that unlike Tbx18, WT-1, and Sema3D, Scx lineage traced cells do not contribute in large quantities to the smooth muscle fate by E13.5 (merged image in W) though contribution is observed at later stages. PE-proepicardium, A/V – atrium/ventricle, RA/LA – right/left atrium, RV/LV – right/left ventricle. See also Supplemental Figure 3.

Figure 4. Endothelial cells derive from Sema3D and Scx lineage traced proepicardial cells

A-F, Co-immunostain of Flk-1 (A) and Pecam-1 (D) with ß-gal (B,E indicating Sema3D lineage traced cells) in Sema3D^GFPCre;R26R^LacZ E14.5 vessels shows that Sema3D lineage traced cells give rise to vascular endothelial cells (C is the merged image of A-B, F is the merged image of D-E). Notched arrows in (C,F) indicate double positive cells. These images are from the free wall of the right ventricle. G-H, the sinus venosus (SV) is morphologically distinct by E10.5 (G), but immunostaining for GFP on Sema3D^GFPCre embryos shows no expression of Sema3D in this structure (H). I-J, Sema3D^GFPCre;R26R^LacZ embryos with X-gal stain of the SV (J is a magnified view of the boxed region in I) and immunostaining with Pecam-1 (K) and ß-gal (L) on adjacent sections of the SV valve leaflets shows that Sema3D lineage traced cells contribute to the endothelial lining of the SV (Notched arrows in L). M, X-gal stain of P5 hearts shows Sema3D lineage traced cells in the vascular tree. N, Transverse section of (M). O-T, Section immunostaining on ScxCre;R26R^LacZ hearts at E11.0. Immunostaining of Flk-1 (O) and Pecam-1 (R), ß-gal (P,S – red cells) and Nfatc1 (P,S – blue cells). Nfatc1 marks endocardium, and is used for exclusionary purposes. (Q) and (T) are merged composites of (O-P) and (R-S), respectively. Notched arrows in (Q) and (T) show that Scx lineage traced epicardial cells give rise to endothelial cells on the surface of the heart as indicated by ß-gal/Flk1 or ß-gal/Pecam-1 double positive stain and a lack of Nfatc1 staining. Additionally, notched arrow in (P) shows that a few Scx lineage traced cells colocalize with Nfatc1, indicating that Scx lineage traced cells can also give rise to endocardial cells. SV-sinus venosus, RA - right atrium, RV- right ventricle, V-ventricle EP- epicardium, EC-endocardium. See also Supplemental Figure 4.

Figure 5. Mouse PEO cells can give rise to endothelial cells both in vivo and in vitro

A-L, sections though a chick heart that has been transplanted with E9.5 mouse PEO cells. Prior to transplantation, mouse cells were co-labeled with diI (A, F). WT-1 transplanted cells do not express Pecam-1 (B) but do express Smooth Muscle Myosin (C). Merged image shown in (D). E-H, transplants with E9.5 proepicardia from ScxGFP mice. DAPI staining of a chick ventricle is shown in (E). Transplanted Scx cells give rise to endothelium as marked by Pecam-1 (G). Merge of diI and Pecam shown in (H). Insets in panels (F-H) show higher magnifications of the boxed regions. I-L, Transplants from Sema3D^GFPCre+;Z/EG mice. DAPI staining of the chick atrioventricular sulcus (I). Sema3D lineage traced cells express GFP following Cre recombination at E9.5 (J). These cells are detected within the heart and are seen to co-express Pecam-1 (K). Merged image of (J) and (K) shown in (L). M-N, E9.5 Scx expressing PEO cells can give rise to endothelial cells in culture as indicated by Pecam-1 expression. The PEO was sorted into a Scx+ fraction (M) and a Scx- fraction (N). Comparative quantification of each fraction is shown in (O). Error bars represent the mean ± S.D. p≤ 0.05. V-ventricle, AVS – atrioventricular sulcus.