Distinct phases of cardiomyocyte differentiation regulate growth of the zebrafish heart

Abstract

Amongst animal species, there is enormous variation in the size and complexity of the heart, ranging from the simple one-chambered heart of Ciona intestinalis to the complex four-chambered heart of lunged animals. To address possible mechanisms for the evolutionary adaptation of heart size, we studied how growth of the simple two-chambered heart in zebrafish is regulated. Our data show that the embryonic zebrafish heart tube grows by a substantial increase in cardiomyocyte number. Augmented cardiomyocyte differentiation, as opposed to proliferation, is responsible for the observed growth. By using transgenic assays to monitor developmental timing, we visualized for the first time the dynamics of cardiomyocyte differentiation in a vertebrate embryo. Our data identify two previously unrecognized phases of cardiomyocyte differentiation separated in time, space and regulation. During the initial phase, a continuous wave of cardiomyocyte differentiation begins in the ventricle, ends in the atrium, and requires Islet1 for its completion. In the later phase, new cardiomyocytes are added to the arterial pole, and this process requires Fgf signaling. Thus, two separate processes of cardiomyocyte differentiation independently regulate growth of the zebrafish heart. Together, our data support a model in which modified regulation of these distinct phases of cardiomyocyte differentiation has been responsible for the changes in heart size and morphology among vertebrate species.

Fig. 1.

Insufficient proliferation in the myocardium to account for the increase of cardiomyocyte number in the zebrafish heart. (A,C,E) To quantify cardiomyocyte numbers at various stages of development, Tg(cmlc2:dsred2-nuc) embryos of 24, 36 and 48 hpf were fixed, stained using an α-DsRed antibody and imaged by confocal microscopy. Maximum projections of the DsRed signal is shown in these panels. The dotted line outlines the heart tube; the arterial pole (outflow) is to the top and the venous pole (inflow) to the bottom. Schematics of embryos at the various developmental stages are shown in the left bottom of panels A, C and E; white outlines the embryo and red represents the forming heart tube at each stage. The 24 hpf embryo is shown in dorsal view, the 36 and 48 hpf embryos in anterior view. V, ventricle; A, atrium. (B,D,F) Brightfield images of the embryos shown in A, C and E; dotted lines outline the eyes. (G) Phospho-His staining (brown) on transverse sections through the heart at 36 hpf. The dotted line indicates the myocardium; white arrowhead indicates a phospho-His-positive cell in the neural tube. (H) The total number of phospho-His-positive myocardial cells/embryo at 30 (n=3), 36 (n=3) and 48 hpf (n=6). Bars represent mean±s.e.m. (I,J) BrdU labeling from 24-48 hpf. White arrowheads indicate BrdU-positive (brown) endocardial cells; red arrowhead indicates a BrdU-positive blood cell; black arrow indicates BrdU-positive myocardial cells. (J) Enlargement of the black box in I indicating BrdU-positive cells in the arterial pole of the 48 hpf zebrafish heart; dotted lines outline the myocardium. Scale bars: 50 μm in A,C,E; 25 μm in G; 10 μm in I.

Fig. 2.

Delayed cardiomyocyte differentiation first arises at the venous pole and later also contributes cells to the arterial pole. (A-I) Developmental timing assay images: reconstruction of confocal z-stacks of eGFP (A,D,G), DsRed (B,E,H), and overlay (C,F,I) from Tg(cmlc2:eGFP)/Tg(cmlc2:dsred2-nuc) embryos. The arterial pole is to the top left; dotted lines outline the eGFP positive heart tubes. Scale bars: 50 μm. (A-C) At 24 hpf, eGFP fluorescence was visible in the entire heart tube. At this time point, no DsRed fluorescence was observed. (D-F) At 36 hpf, the eGFP fluorescence was visible in the entire heart tube. DsRed fluorescence was detected only in the ventricle and at the inner curvature of the atrium. Arrows indicate the regions in the heart tube that are negative for the DsRed signal. (G-I) At 48 hpf, both GFP and DsRed fluorescence were observed in the heart tube. The DsRed signal remained absent from the poles (arrows), which were positive for eGFP. (J,K) Single confocal scan from Tg(cmlc2:GFP)/Tg(cmlc2:dsred2-nuc) embryos at 48 hpf at the level of the arterial pole. Dotted lines indicates the eGFP signal shown in J; arrows in K indicate the most distal outflow tract cells that are positive for eGFP but negative for DsRed. The signal within the heart lumen detected in K but not in J is derived from background fluorescence from red blood cells. Scale bar: 25 μm. (L) Schematic of the location of the eGFP^posDsRed^pos cells (yellow) and the eGFP^posDsRed^neg (green) cells in the arterial pole. (M-R) Reconstruction of confocal z-stacks of Tg(cmlc2:Kaede) embryos with the arterial pole of the heart tube to the top. (M-O) Tg(cmlc2:Kaede) embryos photoconverted at 19 hpf and imaged at 26 hpf with the green unconverted signal shown in gray in M, the converted red signal in gray in N and a merged image in O. The arrow indicates the location of atrial cells that are green but not red, indicating that they were not expressing the transgene at the time of photoconversion. The dotted line indicates the green signal at 26 hpf. V, ventricle; A, atrium. (P-R) Tg(cmlc2:Kaede) embryos photoconverted at 34 hpf and imaged at 48 hpf with the green unconverted signal shown in gray in P, the converted red signal in gray in Q and a merged image in R. Images are of a partial reconstruction of confocal z-stacks, allowing a view into the ventricle chamber. The dotted line indicates the green signal at 48 hpf; the arrow in Q indicates the location of distal outflow tract cells that are green but not red, indicating that they were not expressing the transgene at the time of photoconversion.

Fig. 3.

Isl localization in cardiomyocytes of the future atrium. (A-F) Isl is expressed in cardiac cells located at lateral positions in the cardiac disk. Single z-scans of confocal images of Tg(cmlc2:eGFP) embryos at 23 somites immunofluorescently stained with α-eGFP and α-Isl antibodies. C and F show an overlay with eGFP in green and Isl in red. Images in D-F are magnified views of one side of the cardiac disk. Arrows indicate the eGFP^posIsl^pos cells located at lateral positions (future atrium) of the cardiac disk. The white arrowhead indicates Isl-positive cells in the endocardium and the green arrowhead indicates the Isl signal in the trigeminal sensory ganglion. Scale bars: 50 μm.

Fig. 4.

Cardiac defects of isl1^K88X mutant embryos. (A) Cartoon of the Isl1 protein with the location of the premature stop codon (K88X) identified in the isl1 mutant. (B) DNA sequence of a homozygous wild-type sibling (top) and a homozygous isl1^K88X mutant (bottom) at the site of the mutation (AAG>TAG) that results in a premature stop codon. (C) The number of heart beats per minute measured at 24, 48 and 72 hpf of wild-type sibling (black squares) and isl1 mutant (white squares) embryos. (D,E) In situ hybridization for bmp4 on 48 hpf wild-type sibling (D) and isl1 mutant (E) embryos. Black arrow (D) indicates bmp4 expression in the sinus venosus of a wild-type embryo; red arrow (E) indicates the inflow area of a representative isl1 mutant embryo lacking bmp4 expression. Embryos are shown as frontal-ventral views (OFT to the top).

Fig. 5.

Reduced cardiomyocyte differentiation at the venous pole in isl1 mutant embryos. (A-F) Single z-scan of the atrium of a representative isl1^K88X sibling (A-C) and mutant (D-F) embryo. White arrow (B) indicates eGFP^posDsRed^neg cardiomyocytes at the venous pole of the wild-type sibling heart; red arrow (E) indicates the venous pole of the isl1 mutant where only very few eGFP^posDsRed^neg cells are present. (G,H) Single z-scan of a confocal image at the level of the arterial pole showing the eGFP^posDsRed^neg cardiomyocytes in the isl1 mutant (white arrow). The region shown is similar to that of the control embryo shown in Fig. 2G-I. V, ventricle: A, atrium. Scale bar: 50 μm. (I) Schematic of the location of the eGFP^posDsRed^pos cells (yellow) and the eGFP^posDsRed^neg (green) cells in the arterial pole of the isl1 mutant. (J,K) The number of eGFP^posDsRed^pos (J) and eGFP^posDsRed^neg (K) cardiomyocytes per embryo. (K) The eGFP^posDsRed^neg cardiomyocytes are subdivided into eGFP^posDsRed^neg cells being present at either the arterial or the venous pole. Note the significant reduction of eGFP^posDsRed^neg cells at the venous pole in the isl1 mutants. Bars represent mean±s.e.m. ^**P<0.01.

Fig. 6.

Fgf signaling is required for the addition of cardiomyocytes to the arterial pole. (A-D) In situ hybridization for sprouty4 (spry4). (A,B) Lateral (A) and dorsal (B) view of a 24 hpf embryo. The arrows indicate the region of spry4 expression near the arterial pole of the heart tube, which could contribute cells to the heart tube. The asterisk indicates the midbrain-hindbrain boundary. In B the heart tube is indicated by the dotted line. (C,D) Dorsal views of 36 hpf embryos that had been treated from 24 hpf until 36 hpf with DMSO (C) or SU5402 (D); spry4 expression in the DMSO-treated embryo is outlined by the dotted line. (E-J) Images of the developmental timing assay (similar to Fig. 2A-I). The arterial pole is at the top left. V, ventricle; A, atrium. The dotted line outlines the eGFP-positive heart tubes. Scale bars: 50 μm. (E-G) A representative fgf8 MO-injected embryo showing reduced heart size. The white arrows indicate the eGFP^posDsRed^neg cells at the poles of the heart. (H-J) A representative embryo treated with SU5402 from 24-48 hpf. The red arrow indicates the arterial pole, in which eGFP^posDsRed^neg are absent; the white arrow indicates the eGFP^posDsRed^neg cells at the venous pole. The peripheral red dots are background signal from yolk granules; the diffuse red signal within the heart tube is due to background fluorescence of red blood cells accumulated within the heart lumen. (K,L) Single z-scan of a confocal image taken from a Tg(cmlc2:eGFP)/Tg(cmlc2:dsred2-nuc) embryo treated with SU5402 from 24-48 hpf. A complete overlap of the eGFP (K) and DsRed (L) signals at the distal tip of the outflow (red arrow) was observed, in contrast to the control embryos represented in Fig. 2J,K. Scale bar: 25 μm. (M) Schematic of the location of the eGFP^posDsRed^pos cells after SU5402 treatment. (N,O) The number of eGFP^posDsRed^pos (N) and eGFP^posDsRed^neg (O) cardiomyocytes/embryo. (O) The eGFP^posDsRed^neg cardiomyocytes are subdivided into those present at the arterial pole and those present at the venous pole. Bars represent mean±s.e.m. Note the reduction in the number of eGFP^posDsRed^neg cells located at the arterial pole after fgf8 MO-injection or SU5402 treatment. ^*P<0.05; ^**P<0.01.