Forced expression of a dominant-negative chimeric tropomyosin causes abnormal motile behavior during cell division.

Forced expression of the chimeric human fibroblast tropomyosin 5/3 (hTM5/3) in CHO cell was previously shown to affect cytokinesis [Warren et al., 1995: J. Cell Biol. 129:697-708]. To further investigate the phenotypic consequences of misexpression, we have compared mitotic spindle organization and dynamic 2D and 3D shape changes during mitosis in normal cells and in a hTM5/3 misexpressing (mutant) cell line. Immunofluorescence microscopy of wild type and mutant cells stained with monoclonal anti-tubulin antibody revealed that the overall structures of mitotic spindles were not significantly different. However, the axis of the mitotic spindle in mutant cells was more frequently misaligned with the long axis of the cell than that of wild type cells. To assess behavioral differences during mitosis, wild type and mutant cells were reconstructed in 2D and 3D and motion analyzed with the computer-assisted 2D and 3D Dynamic Image Analysis Systems (2D-DIAS, 3D-DIAS). Mutant cells abnormally formed large numbers of blebs during the later stages of mitosis and took longer to proceed from the start of anaphase to the start of cytokinesis. Furthermore, each mutant cell undergoing mitosis exhibited greater shape complexity than wild type cells, and in every case lifted one of the two evolving daughter cells off the substratum and abnormally twisted. These results demonstrate that misexpression of hTM5/3 in CHO cells leads to morphological instability during mitosis. Misexpression of hTM5/3 interferes with normal tropomyosin function, suggesting in turn that tropomyosin plays a role through its interaction with actin microfilaments in the regulation of the contractile ring, in the localized suppression of blebbing, in the maintenance of polarity and spatial symmetry during cytokinesis, and in cell spreading after cytokinesis is complete.

Requirement of a novel gene, Xin, in cardiac morphogenesis.

A novel gene, Xin, from chick (cXin) and mouse (mXin) embryonic hearts, may be required for cardiac morphogenesis and looping. Both cloned cDNAs have a single open reading frame, encoding proteins with 2,562 and 1,677 amino acids for cXin and mXin, respectively. The derived amino acid sequences share 46% similarity. The overall domain structures of the predicted cXin and mXin proteins, including proline-rich regions, 16 amino acid repeats, DNA-binding domains, SH3-binding motifs and nuclear localization signals, are highly conserved. Northern blot analyses detect a single message of 8.9 and 5.8 kilo base (kb) from both cardiac and skeletal muscle of chick and mouse, respectively. In situ hybridization reveals that the cXin gene is specifically expressed in cardiac progenitor cells of chick embryos as early as stage 8, prior to heart tube formation. cXin continues to be expressed in the myocardium of developing hearts. By stage 15, cXin expression is also detected in the myotomes of developing somites. Immunofluorescence microscopy reveals that the mXin protein is colocalized with N-cadherin and connexin-43 in the intercalated discs of adult mouse hearts. Incubation of stage 6 chick embryos with cXin antisense oligonucleotides results in abnormal cardiac morphogenesis and an alteration of cardiac looping. The myocardium of the affected hearts becomes thickened and tends to form multiple invaginations into the heart cavity. This abnormal cellular process may account in part for the abnormal looping. cXin expression can be induced by bone morphogenetic protein (BMP) in explants of anterior medial mesoendoderm from stage 6 chick embryos, a tissue that is normally non-cardiogenic. This induction occurs following the BMP-mediated induction of two cardiac-restricted transcription factors, Nkx2.5 and MEF2C. Furthermore, either MEF2C or Nkx2.5 can transactivate a luciferase reporter driven by the mXin promoter in mouse fibroblasts. These results suggest that Xin may participate in a BMP-Nkx2.5-MEF2C pathway to control cardiac morphogenesis and looping.

Expression of type VI collagen in the developing mouse heart.

During development, the embryonic atrioventricular (AV) endocardial cushions undergo a morphogenic process to form mature valve leaflets and the membranous septa in the heart. Several extracellular matrix (ECM) proteins are expressed in the developing AV endocardial cushions, but it remains to be established if any specific ECM proteins are necessary for normal cushion morphogenesis. Abnormal development of the cardiac AV valves is a frequent cause of congenital heart defects, particularly in infants with trisomy 21 (Down syndrome). The genes encoding the alpha1 and alpha2 chains of type VI collagen are located on human chromosome 21 within the region thought to be critical for congenital heart defects in trisomy 21 infants. This suggests that the type VI collagen alpha1(VI) and alpha2(VI) chains may be important in normal AV valve morphogenesis. As a first step in understanding the role of type VI collagen in valve development, the authors examined the normal spatial and temporal expression patterns of mRNA and protein for type VI collagen in the embryonic mouse heart. Ribonuclease protection assay analysis demonstrates cardiac expression of the type VI collagen for alpha1(VI), alpha2(VI), and alpha3(VI) transcripts beginning at embryonic days 11-11.5 of mouse development. In situ hybridization studies demonstrate a coordinated pattern of cardiac expression within the AV valves for each type VI collagen chain from embryonic day 11.5 through the neonatal period. Immunohistochemical studies confirm a concentrated type VI collagen localization pattern in the endocardial cushions from the earliest stages of valve development through the neonatal period. These data indicate that type VI collagen is expressed in the developing AV canal in a pattern consistent with cushion tissue mesenchymal cell migration and proliferation, and suggest that type VI collagen plays a role in the morphogenesis of the developing cardiac AV endocardial cushions into the valve leaflets and membranous septa of the heart.

Type VI collagen in the cardiac valves and connective tissue septa during heart development.

A variety of extracellular matrix (ECM) proteins have been shown to be present in the embryonic heart during the morphogenesis of the valves and membranous septa. It is not known if any specific ECM protein is required for the normal morphogenesis of these tissues, but this is of great interest since there is a high incidence of congenital malformations which affect valvular and septal tissues. Interestingly, the alpha 1 and alpha 2 genes of type VI collagen are located within the region of human chromosome 21 thought to be involved in the congenital heart defect phenotype associated with trisomy 21 (Down's syndrome). In this study we examined the distribution and investigated the function of type VI collagen in the cardiac valves and septa of chicken and mouse embryos during various stages of development. Immunohistochemical and in situ hybridization studies revealed a pattern of cardiac expression of type VI collagen which is present from the earliest stages of valve and septum development through the neonatal period. Results from an in vitro bioassay suggest that type VI collagen may play a role in the formation and migration of specific cells in the forming valves and septa. These data support molecular genetic studies which have indicated that type VI collagen is involved in the heart defect phenotype seen in trisomy 21.