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Spatial and temporal expression of syndecan--2 (fibroglycan) in chick heart development.

Article Excerpt
Syndecan-2 is but one member of the syndecan family of heparan sulfate proteoglycans. Four syndecan isoforms occur in higher vertebrates and a single isoform in invertebrates including Drosophila and the nematode Caenorhabditis elegans. Regardless of species or isoform, all syndecans share structural homology. In addition to a hydrophobic transmembrane domain, the cytoplasmic domain is organized into a membrane proximal constant domain (C1), a variable domain (V) with shared homology only between species, and a COOH terminal constant domain (C2) ending with a PDZ-binding motif (Lopes et al. 2006). The extracellular domain (aka. ectodomain) is the most variable of the regions and is thought to contribute to the distinct functions of the different syndecan isoforms (Fears & Woods 2006). Recent evidence indicates that the ectodomain core protein sequences are not only critical for the specification and display of glycosaminoglycans (Zhang et al. 1995), but are also essential for mediating cellular activity (McFall & Rapraeger 1998; Langford et al. 2005).

Syndecans are present on a wide variety of cell types and commonly exhibit a developmentally regulated pattern (Rapraeger 2001). Functional studies demonstrate that syndecans mediate cellular activities including cell adhesion, binding and modulation of growth factor activity, interacting with and arranging the cytoskeleton and organizing of the extracellular matrix. These processes are critical during morphogenesis and become reiterated in tissues undergoing wound healing following injury.

During cardiac morphogenesis, many of the biological activities mediated by syndecans are active in shaping cardiac tissue into a functioning complex organ. Cellular proliferation and differentiation are likely regulated to a large degree by the array of growth factors present, including basic fibroblast growth factor (bFGF) (Sheikh et al. 2004), transforming growth factor beta (TGF[beta]) (Akhurst et al. 1990) bone morphogenetic protein (BMP) (Neuhaus et al. 1999) and thrombospondin (Corless et al. 1992). Construction of the extracellular environment and directed cell migration are additional processes that require coordinated regulation among all the cells within the developing heart. Clearly, growth factors and adhesion molecules such as the integrins play a role in shaping the heart (Kim et al. 1999); however, the presence of the syndecans suggests that they are active participants in cardiac morphogenesis as well.

Proteoglycans are common constituents present within the developing heart (Handler et al. 1997; Litwack et al. 1998; Zanin et al. 1999) including syndecan-2 in avian and rodent cardiac tissue. Northern blot analysis has been used to show that late embryonic stage chick (Chen et al. 2002) and rat hearts (Asundi et al. 1997) contain syndecan-2 mRNA. The latter study extended these findings by suggesting that cardiomyocytes possess the potential to express syndecan-2. Cultures of cardiomyocytes were found to express low levels of syndecan-2 mRNA. While studies such as these demonstrate the ability of cardiac tissue to express syndecan-2, the pattern of syndecan-2 expression during early stages of cardiac morphogenesis has not been elucidated.

This study seeks to describe the pattern of expression within the developing chick heart using a polyclonal antibody shown to recognize chicken syndecan-2 (Chen et al. 2002). The data presented here demonstrate that syndecan-2 is expressed during Hamburger and Hamilton (HH) stage 15-25 within the developing myocardium of the heart. Earlier stages of HH 12-14 showed no detectable syndecan-2. Additionally, during no stage assayed was...

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