Radial construction of an arterial wall

Abstract

Some of the most serious diseases involve altered size and structure of the arterial wall. Elucidating how arterial walls are built could aid understanding of these diseases, but little is known about how concentric layers of muscle cells and the outer adventitial layer are assembled and patterned around endothelial tubes. Using histochemical, clonal, and genetic analysis in mice, here we show that the pulmonary artery wall is constructed radially, from the inside out, by two separate but coordinated processes. One is sequential induction of successive cell layers from surrounding mesenchyme. The other is controlled invasion of outer layers by inner layer cells through developmentally regulated cell reorientation and radial migration. We propose that a radial signal gradient controls these processes and provide evidence that PDGF-B and at least one other signal contribute. Modulation of such radial signaling pathways may underlie vessel-specific differences and pathological changes in arterial wall size and structure.

Figure 1. Pulmonary artery smooth muscle cells derive from lung mesenchyme

A, Ventral view of whole mount lung at embryonic day (E) 12.5 immunostained for E-cadherin (airway epithelium, green) and PECAM (endothelium, red). Box, portion of left pulmonary artery (PA) investigated here, from carina (ca) to LL1 (first lateral secondary branch off left primary bronchus) (Metzger et al., 2008). LL2 and LL3, second and third lateral secondary branches. B, C, Longitudinal sections of left PA at E18.5 stained with haematoxylin and eosin (H&E) (B) or immunostained for α-smooth muscle actin (SMA, red), PDGFR-β (green) and counterstained with DAPI (nuclei, blue) (C). E, endothelial cell (EC) layer; 1, first (inner) smooth muscle cell (SMC) layer; 2, second SMC layer; A, adventitial cell layer. Lu, PA lumen. D, Schematic of B and C, showing ECs (black), SMCs (red) and adventitial cells (green). E, F, Lineage tracing using the Wt1-CreER^T2 and the Tbx4-CreER^T2 transgenes as indicated with the mTmG Cre reporter to determine the contribution of the mesothelium and lung mesenchyme, respectively, to the PA wall. Longitudinal sections are shown of one side of the E18.5 left PA wall stained for the Cre reporter (green) and PECAM (white), SMA (red), and DAPI (blue) to visualize the PA wall. Pregnant dams were injected with a single dose of tamoxifen (3 mg at E9.5 in E; 1.5 mg at E10.5 in F) to activate the Cre(ER) recombinase. Inset in E, shows in the same specimen lineage-labeled cells in the lung mesothelium, outside the PA. Lineage trace of lung mesenchyme (F; also see Fig. S1C), but not mesothelium (E), labels PA wall cells, and the fraction of lineage labeled PA wall cells with the Tbx4-CreER^T2 in F is similar to the overall lung mesenchyme labeling efficiency (data not shown). G–I, PDGFR-β expression around developing PA. Whole mount lungs at indicated ages immunostained for PDGFR-β (G, I) and PECAM (endothelium, H). I′, Confocal optical section of boxed region of left PA in I showing both PDGFR-β and PECAM staining. I″, Schematic of I′ showing zone of PDGFR-β (green) cells surrounding developing left PA ECs (black). J, Lineage trace (as in E, F) of PDGFR-β expressing cells. All cells of E18.5 PA wall are marked by lineage tag (green). Scale bars, 100 μm (A, G–I) and 10 μm (B, C, E, F, I′, J). (See also Figs. S1, S2A, C, D.)

Figure 2. Radial development of the PA wall

Longitudinal sections through the left PA wall at the embryonic ages indicated, stained for SMA (red), PDGFR-β (green), and nuclei (DAPI, blue) as indicated. Arrowheads, nuclei elongated longitudinally. The inset in E shows a transverse section through the left PA wall. Schematics at bottom summarize SMA (red) and PDGFR-β (green) expression (orange, co-expression of SMA and PDGFR-β) and cell shape and orientation changes in the forming layers (E, 1, 2, A) of the developing PA wall. Lu, PA lumen. Square cells, circumferentially-oriented cells. Scale bars, 10 μm. (See also Fig. S2.)

Figure 3. Radial patterning of cell division in the developing PA wall

A, Representative confocal optical section of left PA at E14.5 stained for the mitosis marker phosphohistone H3 (pH3, white) and SMA (red), PDGFR-β (green) and nuclei (DAPI, blue) to assign cell layer (1, 2, A) of each dividing cell. The inset shows a close up of a dividing cell and its assigned cell layer. B, Distribution of dividing cells among the three cell layers. Values shown are the fraction of all dividing (pH3⁺) left PA wall cells located in each cell layer, for each embryonic age indicated. The total number of PA walls analyzed and dividing cells scored were: E11.5 (16 walls, 32 dividing cells), E12.5 (12, 31), E13.5 (14, 37) and E14.5 (15, 70). C, Marker expression pattern of dividing PA wall cells. Values shown are the fraction of all proliferating (pH3⁺) left PA wall cells that were SMA⁺ or PDGFR-β⁺/SMA⁻. The number of PA walls analyzed and dividing cells scored were: E13.5 (18, 48) and E14.5 (16, 80). D, Proliferative index of PA wall cells. Values shown are the proliferative index (fraction pH3⁺ cells) of SMA⁺ cells and PDGFR-β⁺/SMA⁻ cells of the PA wall at the ages indicated. The number of left PA walls analyzed and cells scored were: E13.5 (18 walls, 1286 SMA⁺ cells, 1111 PDGFR-β⁺/SMA⁻ cells) and E14.5 (16, 3024, 519). Errors bars, s.e.m. *p=0.019 by Student’s t-test. E, Schematic (top) and representative confocal images of stained lungs (bottom) showing longitudinal and radial axes of cell division in developing PA wall. Lungs were stained for pH3, SMA, and DAPI as indicated. Dashed circles, daughter chromosomes of dividing cell. F, Fraction of longitudinal vs. radial cell divisions in the indicated layers of the PA wall at the indicated ages. After E13.5, layer 1 (L1) cells switch from predominantly longitudinal to predominantly radial division. The “0” in the panel indicates no detected radial cell divisions in the L2 and A layers at E14.5. The number of mitotic figures scored at E11.5–13.5 was 22 in L1 and at E14.5 was 15 in L1 and 12 in L2 and A. **p=0.003, ***p=0.0002 by Fisher’s exact test. Scale bars, 10 μm.

Figure 4. Clonal analysis of inner layer cells of the PA wall

A, Clonal analysis scheme showing early (E11.5) marking of an inner layer cell and four possible patterns of its proliferation and migration: longitudinal (L), circumferential (C), and radial (R) expansion, and longitudinal with mixing with unlabeled cells (L, M). B, A GFP-marked left PA clone (#9, Table S1) in a SMMHC-CreER^T2, Rosa26R^mTmG/+ embryo, induced by administration of a limiting dose of tamoxifen at E11.5 and analyzed at E13.5 after staining for clone marker (membrane-localized GFP, abbreviated “mG”, green) and for SMA (red), PECAM (white) and nuclei (DAPI, blue). An individual coronal confocal optical section (dorsal view, anterior up) of the four-cell clone is shown (left panel) along with a maximal projection (center panel). 1–4, cells of clone; Lu, PA lumen. In the clone schematic (right panel), the positions of marked cells are indicated by circles color coded to highlight the layer in which the cell resides: green (layer 1), red (layer 2), and blue (adventitia). For cells located superficial (white circles) or deep (grey circles) to the lumen, we were unable to determine which layer they reside in (nd, not determined). This clone expanded longitudinally (L) and circumferentially (C), with mixing (M). ca, position of carina; LL1, position of LL1 airway branch. C, Six cell clone (#11), induced and analyzed as in B. Two confocal optical sections are shown, along with a maximal projection and schematic. Dashed box, portion of clone shown in the confocal images. D, Sixteen cell clone (#30), induced and analyzed as in B, except clone marker was multicolor (Rainbow, abbreviated “Rb”) ROSA26R^Rb Cre reporter and the clone was analyzed at E18.5. Left panel, Cerulean channel of confocal image (ventral view) of coronal cryosection of the clone: bright cells are Cerulean-expressing cells of clone (numbered). Faint background staining shows the rest of the SMCs of PA wall. Center panel, Cerulean, mOrange and mCherry channels of the same cryosection. All labeled cells in left PA express Cerulean marker, confirming clonality. Clone expanded in all three axes (L, C, and R), with some cells (red in schematic) having invaded layer 2. For clarity, only every other cell in the schematic is numbered. E, Schematics (ventral views) of other representative clones. Although layer 1 clones can expand and mix extensively within layer 1, they do not expand radially into layer 2 until E14.5. By E18.5, they have spread extensively in layers 1 and 2, but have not invaded the adventitial layer. mG, mGFP clone marker; Rb, Rainbow (multicolor) clone marker. Scale bars, 10 μm. (See also Table S1 and Figs. S3, S4.)

Figure 5. Signal gradient model for PA wall patterning and involvement of PDGF pathway

A, Summary of cellular and molecular events in radial construction of PA wall. Cell wall progenitor zone surrounding PA endothelial tube is initially marked by PDGFR-β expression (green cells). Innermost cells (adjacent to endothelium, E) initiate smooth muscle marker expression and morphogenesis (orange cells) and then down-regulate PDGFR-β (red cells) to form layer 1. Smooth muscle cycle repeats to form layer 2, and third cycle initiates to form adventitial (A) layer. As layer 1 cells mature, they radially reorient cell division and migration, sending daughter cells outward to supplement layer 2. R (radial), L (longitudinal), C (circumferential) cell division and migration. B, Gradient of a hypothetical radial signal that sequentially induces cell layers surrounding the endothelium to differentiate into the vascular wall. Different identity thresholds (1, 2, 3) result in differences among layers (e.g., smooth muscle vs. adventitia, or high vs. low elastin expression). The signaling gradient could also control the radial reorientation of cell division and migration. C, Whole mount lung in situ hybridization of the ligand PDGF-B, a candidate for the radial signal. Gene is expressed in ECs of developing PA (arrowheads). D, E, Confocal images of whole mount lungs from wild type littermates implanted with a control or PDGF-B-soaked bead as indicated, cultured for 72 hours and then stained for PDGFR-β (green), SMA (red), and E-cadherin (blue) and counterstained with DAPI (not shown). a, airway with associated smooth muscle; *, bead location. Phosphohistone H3 staining (not shown) did not demonstrate any obvious differences in cell division surrounding the beads. F, Close up of the region in E around the PDGF-B bead that has initiated arterial wall formation as shown by induction of PDGFR-β and SMA expression. G–I, Confocal optical sections of left PAs from whole mount E14 wild type (G) and PDGF-B^(−/−) (H) littermate lungs stained for SMA, PDGFR-β and nuclei (DAPI) as indicated. Marker expression patterns are quantified in I, and values shown are fraction of cells in each layer with the indicated marker expression profile. Three wild type PAs (n=96 cells) and two PDGF-B^(−/−) PAs (48 cells) were scored. There is no detectable phenotype in the mutant. J–M, Confocal images of left PAs from whole mount E12 lungs of the indicated genotypes, immunostained for SMA. Because PDGFR-α^(−/−), PDGFR-β^(−/−) double mutants die before the PA wall forms, double mutant shown in L is a lung-mesenchyme-specific (LM) double knock-out of genotype PDGFR-α^(flox/−), PDGFR-β^(−/−), Tbx4-Cre. Airway branching, which is stereotypically correlated with PA development, is maintained in these mutants (data not shown). Genotype of littermate control in M is PDGFR-α^(flox/+), PDGFR-β^(+/+), Tbx4-Cre. Compound PDGFR-α^(+/−), PDGFR-β^(−/−) embryos at E11.5–12.5 did not show lung developmental delay or a PA phenotype (data not shown). Scale bars, 100 μm (C, E) and 10 μm (F, H, K, M). (See also Fig. S5.)

Figure 6. Radial patterning in arterial diversity and disease

A–D, Transverse sections of the left anterior descending coronary artery and the left pulmonary and left common carotid arteries (CCA) and aorta at ~E17, stained for SMA (red), EC marker (VE-cadherin in A and PECAM in B–D; white) and DAPI (nuclei, blue). Walls range from one to five smooth muscle layers thick. E, Differences in the radial signal gradient (see Fig. 5B) could determine the thickness of specific developing arteries. F–J, Sections through the developing left CCA (panel C) at the embryonic ages indicated and stained with SMA (red), PECAM (white) and DAPI (nuclei, blue). (Panels F-H are longitudinal sections of the third aortic arch artery, which later becomes the CCA, and panels I, J are transverse sections of the CCA.) Note the gradual increase in SMA⁺ cell layers from rare first layer cells at E10.5 to four continuous circumferentially elongated layers at E16.5. K, L, Fate mapping of CCA first layer SMC marker⁺ cells in SMMHC-CreER^T2, Rosa26R^mTmG/+ embryos induced with a single low dose of tamoxifen (0.5mg) at E10.25. First layer CCA SMCs give rise to outer layer SMCs (marked by *), as for the PA (Figs. 4D, E, S4). Despite the different sizes and origins of the vascular wall cells (PA from mesenchyme, see Figs. 1F, J, S1C; CCA from neural crest, see ((Jiang et al., 2000; Le Lievre and Le Douarin, 1975)), radial invasion occurs in both vessels. M, N, Transverse sections of descending aortic walls stained for SMA, PECAM and nuclei (DAPI) in control(panel M, SM22-Cre) and a conditional PDGFR-β gain-of-function mutation activated specifically in smooth muscle (panel N, PDGFR-β^(+/J), SM22-Cre indicated by PDGFR-β*). (The PDGFR-β^J allele, a V536A mutation that disrupts an inhibitory juxtamembrane region and results in constitutive PDGFR-β activity, isa knock-in at the endogenous PDGFR-β locus (Olson and Soriano, 2011)). Note that the mutant wall has more than twice the normal number of smooth muscle layers but the normal number of elastic layers (i, e, o) (N; Olson and Soriano, 2011), whereas in the PA there was no observable phenotype (see Fig. S5D). O, Changes in the radial signaling gradient may contribute to diseases characterized by vessel wall thickening or thinning. E, endothelial layer; 1–9, smooth muscle layers 1–9; Lu, vessel lumen; i, internal elastic lamina; e, elastic lamina; o, external elastic lamina. Scale bars, 10 μm.