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First published online 25 May 2006
doi: 10.1242/dev.02420

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TBX5 is required for embryonic cardiac cell cycle progression

¹ Carolina Cardiovascular Biology Center, 5109 Neuroscience Research Building, Chapel Hill, NC 27599-7126, USA.
² Department of Biology, Fordham Hall, UNC-Chapel Hill, Chapel Hill, NC 27599-3280, USA.
³ Department of Genetics, Fordham Hall, UNC-Chapel Hill, Chapel Hill, NC 27599-3280, USA.

View larger version (73K): [in a new window]   Fig. 1. TBX5 is required for cardiac proliferation. Whole-mount antibody staining with tropomyosin (Tmy) of stage 37 (A) control morpholino (CMO) or (B) TBX5 morpholino (T5MO) embryos. Transverse heart sections through (C,E) stage 33 and (D,F) stage 37 embryos stained with Tmy, to mark cardiac tissue, and DAPI, to mark cell nuclei; (C,D) CMO-derived tissue; (E,F) T5MO-derived tissue. (G-L) Examples of proliferating cardiac cells in transverse heart sections from (G-I) CMO and (J-L) T5MO embryos sectioned through the cardiac region at stages 29, 33 and 37, as indicated. Proliferating cardiomyocytes are identified as those positive for Tmy (myofibrilis shown as green) and anti-phosphohistone H3 (pH3; localized to the nucleus and shown in red). (M-R) Examples of cardiac cells undergoing apoptosis in transverse heart sections from (M-O) CMO and (P-R) T5MO embryos at stages 29, 33 and 37, as indicated. Apoptotic cardiomyocytes are identified as those positive for Tmy (myofibrilis shown as green) and anti-cleaved caspase 3 (CC3; localized to the nucleus and shown in red). Quantification of results from (S) total cardiomyocyte cell numbers, (T) mitotic index, and (U) programmed cell death. In all cases, bars represent the average of at least three embryos: CMO, red bar and T5MO, blue bar. Error bars denote the standard deviation, and * denotes a statistically significant difference (at P<0.05) between CMO and T5MO embryos at a given stage. Results are derived from a single set of experiments, all experiments being repeated at least once with an independent batch of embryos. Scale bars: 50 µm. a, atrium; i, inflow tract; o, outflow tract; v, ventricle.

View larger version (55K): [in a new window]   Fig. 2. TBX5 depletion results in dramatic upregulation of proteins associated with G1/S-phase within the heart. (A) Expression profile of cell-cycle associated proteins, as determined by western blot analysis performed using lysate from either whole stage 33 embryos (emb), or corresponding heart tissue (heart). (B) Relative differences in embryonic cardiac CDK and cyclin proteins between CMO- and T5MO-derived heart tissue (stage 33). Note increased levels of Cyclin E2, CDC6, CDK2. (C) Relative differences in embryonic cardiac S-phase associated proteins SLBP and PCNA between CMO- and T5MO-derived heart tissue (stage 33). Colored bars denote the stage of the cell cycle at which the proteins are expressed, as indicated in the key.

View larger version (70K): [in a new window]   Fig. 3. Terminally differentiated cardiomyocytes retain the capacity to undergo cell division. Transverse sections of (A-C) CMO or (E-G) T5MO heart tissues at stage 37, showing cells coexpressing pH3 (red), DAPI (blue) and either (A,E) actin, (B,F) Tmy or (C,G) MHC (all shown in green). (D) Schematic of a CMO heart displaying the relative positions of each panel. A and B were imaged from an area corresponding with `A,B'; C was imaged from a region corresponding with `C'. (H) Schematic of a T5MO heart displaying the relative positions of each panel. E corresponds with `E'; F and G correspond with `F,G'. White arrows denote sarcomeric bundles. Scale bar: 10 µm. a, atrium; v, ventricle.

View larger version (90K): [in a new window]   Fig. 4. The timing of the cardiac differentiation program is altered in TBX5-depleted embryos. (A) RT-PCR analysis of the expression of heart-specific isoforms of MHC, troponin and tropomyosin; and skeletal muscle-specific genes MyoD and muscle actin, throughout early and mid-tadpole stages of development in CMO (`C') and T5MO (`T') stage-matched embryos. All samples are derived from a single batch of eggs, and identical results were achieved in at least two independent sets of experiments for each marker. EF1-Alpha was used as a loading control for all RT-PCR reactions. (B-I) Images depicting embryos injected with (B-E) CMO, or (F-I) T5MO and immunostained for Tmy, showing delayed expression of Tmy in the hearts of T5MO embryos. Shown are representative sibling embryos imaged at the indicated stages. White arrows denote expression of Tmy within the heart. (J-Q) Images of living cardiac actin:GFP transgenic embryos, showing a delay in the onset of cardiac actin expression in the heart. Representative sibling embryos obtained from a single batch of embryos were injected with (J-M) CMO or (N-Q) T5MO and imaged at the indicated stages. Shown is a representative pair of embryos, while identical results were observed in over 50 embryos. White arrows denote expression of GFP within the heart field.

View larger version (98K): [in a new window]   Fig. 5. TBX5 depletion leads to a disruption in cardiac myofibril structure. Cardiomyocyte structure in transverse sections through the hearts of (A,C,E,G) CMO or (B,D,F,H) T5MO stage 37 embryos, as detected by immunostaining for (A,B) cardiac troponin T (cTNT), (C,D) MHC, (E,F) actin or (G,H) Tmy. (I,K,M) Stage 37 CMO or (J,L,N) T5MO embryos double-immunostained for tropomyosin (green) and (I,J) fibronectin, (K,L) fibrillin or (M,N) ß-catenin, all shown in red. Note increase in fibrillin staining on the walls of the chamber of T5MO hearts relative to CMO (compare panel K to L, white arrows) and ectoptic expression of fibronectin, shown by white arrow, in the dorsal portion of the heart in panel J relative to panel I. (O,P) High magnification confocal images of hearts from (O) CMO or (P) T5MO stage 37 embryos. Note that formation of organized cardiac muscle bundles in T5MO hearts is limited to a single cluster adjacent to the cardiac lumen. (Q-S) Representative transmission electron micrographs of transverse images of stage 37 embryos taken from (Q) CMO cardiac tissue or (R) T5MO cardiac tissue adjacent to the pericardial cavity and (S) T5MO cardiac tissue adjacent to the cardiac lumen. Cardiac muscle fibrils are shown pseudo-colored in yellow. Note that sarcomeres in T5MO hearts can only be identified adjacent to the cardiac lumen (compare R with S) and only found in concentric arrays. By contrast, CMO-derived hearts show both longitudinal and concentric arrays (compare Q with S). High-magnification TEM images reveal the ultrastructures of (T) CMO and (U) T5MO cardiac sarcomeres. Arrows denote A-bands. Note the smaller, non-continuous A-bands in the T5MO-derived sarcomeres (U). (V) Traces of the heart sections from CMO and T5MO embryos imaged by TEM are depicted schematically. Yellow circles represent the location of TEM imaging. Scale bars: 50 µm in A-N; 2 µm in Q-S; 0.2 µm in T,U.

View larger version (81K): [in a new window]   Fig. 6. Tbx5 misexpression leads to changes in cardiac proliferation and morphology. The overall morphology of stage 40 (A) uninjected embryos or (B,C) embryos injected with increasing amounts of Tbx5 RNA, as indicated. Arrows denote the location of the heart. (D-G) Whole-mount in-situ hybridization showing expression of (D,E) Nkx2.5 and (F,G) myosin light chain (MLC) in (D,F) uninjected stage 37 embryos and (E,G) stage-matched embryos injected with 1 ng of Tbx5 RNA. (H) Tbx5 misexpression leads to an increase in the cardiac mitotic index at stage 37. Mitotic index was calculated as the percentage of cardiac cells labeled with pH3. The data represents the mean of at least three different embryos. Error bars denote the standard deviation, and * denotes a statistically significant difference between Tbx5-injected and control embryos (at P<0.05). (I) Mitotic index for sections of the neural tube corresponding to the same position as the heart along the anterior-posterior axis. The data represents the mean mitotic index of four different embryos per condition, with four sections analyzed per embryo. Error bars denote the standard deviation. (J) Tbx5 misexpression leads to an alteration in the timing and order of the cardiac differentiation program. RT-PCR analysis of the expression of Nkx2.5 as well as heart-specific isoforms of MHC, troponin and tropomyosin, and skeletal-muscle-specific genes, MyoD and muscle actin, throughout early and mid-gestation stages of development in control (`C') or Tbx5-injected (`T') stage-matched embryos. All samples are derived from a single batch of eggs and identical results were achieved in at least two independent sets of experiments for each marker. EF1-alpha was used as a control for all RT-PCR reactions.

View larger version (19K): [in a new window]   Fig. 7. Model for potential mechanisms by which TBX5 functions to control embryonic cardiac cell cycle progression. Schematic of embryonic cardiac cell cycle shown in left hand panel and potential roles for TBX5 in regulating G1/S progression. (1) TBX5 may function to induce the expression of a growth factor, for example EGF or FGF required for cell cycle G1 progression. (2) TBX5 may function to regulate E2F activity by controlling the phosphorylation state of pRB. (3) TBX5 could also negatively regulate general cell cycle inhibitors such as p27Xic. (4) Alternatively TBX5 may function to regulate the expression of a key component of the CDK-cyclin complexes required for G1/S progression.