EXPRESSION OF DDR2 DURING CARDIAC DEVELOPMENT M.O. Morales, J.D. Potts, R.L. Price, T.K. Borg, and E.C. Goldsmith Department of Cell and Developmental Biology and Anatomy, University of South Carolina School of Medicine, Columbia, SC 29209 The extracellular matrix (ECM) of the heart consists of a dynamic, 3-dimensional arrangement of collagens, proteoglycans, glycoproteins and other matricellular components. The collagen network in the heart interconnects myocytes, connects myocytes to capillaries and forms a weave network that surrounds and mechanically couples groups of myocytes (1). This network, composed mainly of collagen types I and III (2), develops rapidly after birth concomitant with increases in load (3). Analysis of collagen type I and III expression during heart development demonstrated that mRNA for both collagens increases after birth and that both have similar localization to valve leaflets and cells neighboring blood vessels (4). The primary producers of interstitial collagen in the heart are fibroblasts, which account for approximately two-thirds of the cells in the heart (5). Until recently, integrins were regarded as the primary receptors for collagen and other components of the extracellular matrix (ECM) in the heart. The expression of β 1 integrins, the predominant subunit present in the heart, has been well characterized during cardiac development (6,7). Receptor tyrosine kinases are another family of proteins involved in the conversion of extracellular stimuli into cellular response. These receptors mediate a variety of cell functions including growth, migration, morphology and differentiation. The Discoidin Domain Receptors, DDR1 and DDR2, represent a relatively new family of collagen specific receptor tyrosine kinases (8,9). While the tissue distribution of these receptors varies and can be mutually exclusive (10,11), DDR2 expression has been detected in both rat and mouse heart (12,13). Using a combination of confocal microscopy and in situ hybridization we have examined the localization of DDR2 in the developing heart. Analysis by RT-PCR demonstrated that DDR2 is expressed in the heart from embryonic day (ED) 11.5 through ED 20 and that expression of this receptor continues in the neonatal heart. DDR2 expression was also detected in hearts isolated from adult animals. Localization of DDR2 in the heart was demonstrated with in situ hybridization and confocal microscopy (Figure 1). Early in embryonic development, DDR2 expression was detected mainly in cardiac cushions on activated endothelial cells and migrating mesenchymal cells. In later development (ED 19 and later), DDR2 staining was observed on fibroblasts surrounding cardiac myocytes and in areas located around vasculature. However, no staining was observed on myocytes, smooth muscle cells or endothelial cells. In addition, staining of DDR2 on isolated cardiac fibroblasts indicates that this receptor localizes to regions of the cell in contact with underlying substrate. This distribution is similar to what has been observed for β 1 integrin localization into focal adhesions. REFERENCES 1. Caulfield JB, Borg TK. Lab. Invest. 1979. 40 (1979) 364. 2. Bishop JE, Laurent, GJ. Euro. Heart J. 16C (1995) 38. 3. Borg TK, Caulfield, JB. Texas Rep. Biol. Med. 39 (1979) 321. 4. Carver W et al. Anat. Rec. 236 (1993) 511. Microsc Microanal 10(Suppl 2), 2004 Copyright 2004 Microscopy Society of America DOI: 10.1017/S1431927604885155 1396