Single-Cell Lineage Tracing Reveals that Oriented Cell Division Contributes to Trabecular Morphogenesis and Regional Specification

Abstract

The cardiac trabeculae are sheet-like structures extending from the myocardium that function to increase surface area. A lack of trabeculation causes embryonic lethality due to compromised cardiac function. To understand the cellular and molecular mechanisms of trabecular formation, we genetically labeled individual cardiomyocytes prior to trabeculation via the brainbow multicolor system and traced and analyzed the labeled cells during trabeculation by whole-embryo clearing and imaging. The clones derived from labeled single cells displayed four different geometric patterns that are derived from different patterns of oriented cell division (OCD) and migration. Of the four types of clones, the inner, transmural, and mixed clones contributed to trabecular cardiomyocytes. Further studies showed that perpendicular OCD is an extrinsic asymmetric cell division that putatively contributes to trabecular regional specification. Furthermore, N-Cadherin deletion in labeled clones disrupted the clonal patterns. In summary, our data demonstrate that OCD contributes to trabecular morphogenesis and specification.

Figure 1. Whole embryo clearing to image a single clone inside a heart

(A) Nkx2.5^Cre/+; mTmG embryos at E12.5 were cleared in CUBIC reagents or in PBS as a control. H: Heart; L: Liver. (B) Single GFP cells inside the SCALE treated Mef2c-Cre; mTmG heart at E9.5 can be imaged. LV: Left ventricle. (C) Single iCre;Conf control clone of a 6 st embryo is shown. (D) The number of clones per ventricle 24 hours after induction is not significantly different from that 72 hours after induction. (E) The number of clones is tamoxifen dosage dependent. Scale bars in A are 200 µm and in B&C are 20 µm.

Figure 2. The labeled clones display four types of geometric patterns

(A) The number of clones of each cardiac cell type was identified. (B) The different zones of the heart were schematically defined, including O.C.Z. (Outer compact zone), I.C.Z. (Inner compact zone) and T.Z. (Trabecular zone). (C–G) Sections A & B are two representative sections from a Z stack of the same clone. The Z stack was reconstructed to a 3D image. Scheme is an illustration to show the geometric pattern of the clone. (C) An endocardial cell clone. (D) A transmural clone. (E) A surface clone. (F) A mixed clone. (G) An inner clone. (H) shows the percentage distribution of different types of clones. E.C.: endocardial cells; C.M.: cardiomyocytes; Epi: epicardial cells. Scale bars in C–G are 50 µm.

Figure 3. Inner and transmural clones contribute to trabeculation

(A–C) Comparison of the values of x, y and z in different types of clones between control and iCdh2 null clones. z is the depth from the heart surface to the cell that is the furthest away from the heart surface, and x and y are the lengths the clone spans in the x and y axis, respectively. For the inner clone, z1 and z2 are respectively the distances of the furthest cell and the closest cells to the heart surface. (D) Shows the number of cells in 4 different types of control clones. 10 clones from each type were presented. (E) The distribution of the 4 control clones: 1, surface clone; 2, transmural clone; 3, inner clone; and 4, mixed clone in the heart. RV: right ventricle; LV: left ventricle; RA: right atrium; LA: Left atrium. AVC: atria-ventricular cushion.

Figure 4. Myocardial cells undergo parallel and perpendicular divisions

(A) E9.0-9.5 hearts were whole mount stained with acetylated α-tubulin and PECAM. Images show that cardiomyocytes undergo perpendicular (perp.) and parallel divisions. (B) The mitotic spindle orientation patterns of cardiomyocytes from control and Cdh2 CKO at E9.0-E9.5 hearts. The horizontal line represents the axis of heart surface and the each blue line represents an angle between mitotic spindle and heart surface. The number between blue lines indicates the percentage of cells whose mitotic spindle angle is between the two angles represented by the blue lines. (C–E) iCre; Conf embryos were induced with tamoxifen at E7.75 and were examined 24 hours after induction. The labeled cells could undergo perpendicular division (C), parallel division (D), and directional migration (E). There are four quadrants in C-E. c1, d1 and e1 are the PECAM staining; c2, d2 and e2 show the fluorescent protein; c3, d3 and e3 show the merged pictures; and c4, d4 and e4 show the enlarged quadrant in c3, d3, and e3, respectively. Scale bar in A is 10 µm and in C–E are 50 µm.

Figure 5. OCD and directional migration contribute to trabeculation in an N-CAD dependent manner

(A–B) show sagittal sections of Cdh2 heterozygous or CKO hearts at E9.5. Arrow points to a trabecula. (C) N-CAD was not detected in iCdh2 clone indicated by an arrow. (D&E) show the clonal patterns of control and iCdh2. The control is the same as Fig. 2H. (E) Cells in the iCdh2 were not dispersed. Pictures show two sections from a Z stack and a reconstructed 3D picture. (F) The clonal sizes of iCdh2 clones are significantly smaller than the control. Scale bars in A–B are 100 µm and in C&E 10 µm. A: atrium; V: ventricle.

Figure 6. Extrinsic asymmetric cell division might contribute to asymmetric expression of Hey2 and BMP10 in trabecular and compact zones

(A–B) Hey2 and BMP10 are asymmetrically distributed to compact and trabecular zones with Hey2 enriched in the compact zone (A) and BMP10 enriched in the trabecular zone (B). (C–H) All show one low magnification picture to the left and one section of the same cells with higher magnification on the right. (C) Hey2 and (E) BMP10 are not asymmetrically distributed in perpendicular dividing cells. After division, when the two daughter cells are separated but still connected at the mid-body, both (D) Hey2 and (F) BMP10 are asymmetrically distributed, with the daughter closer to cardiac jelly displaying less Hey2 and more BMP10 than the daughter at the heart surface. In parallel division, both Bmp10 and Hey2 are symmetrically distributed (G&H) to the two daughter cells. (I&J) shows the ratio of Hey2 and BMP10 between the twodomains/ daughter cells in perpendicular/parallel mitotic or post-division cells (Three sections for each cell and n=8 for each type). (K) Q-PCR data to show the differential expression between trabecular and compact cardiomyocytes harvested by LCM of an E9.5 heart. The experiments were repeated three times. Scale bars in A&B are 100 µm and in C–H 10 µm. Lu: luminal; ba: basal; Mi: mitotic; P.D.: post-division. E.C.: Endocardial cells; LV: left ventricle; AVC: atrioventricular canal; PEO: pro-epicardial organ.

Figure 7. Models of trabecular initiation and trabecular regional specification

(A&B) OCD and directional migration contribute to trabecular initiation in an N-CAD dependent manner. Cdh2 null cells failed to undergo OCD and directional migration. (C) Cardiac jelly and endocardium might provide instructive cues for trabecular regional specification.