Syndecans in heart fibrosis

Heart disease is a deadly syndrome affecting millions worldwide. It reflects an unmet clinical need, and the disease mechanisms are poorly understood. Cardiac fibrosis is central to heart disease. The four-membered family of transmembrane proteoglycans, syndecan-1 to -4, is believed to regulate fibrosis. We review the current literature concerning syndecans in cardiac fibrosis. Syndecan expression is up-regulated in response to pro-inflammatory stimuli in various forms of heart disease with fibrosis. Mice lacking syndecan-1 and -4 show reduced activation of pro-fibrotic signaling and increased cardiac rupture upon infarction indicating an important role for these molecules. Whereas the short cytoplasmic tail of syndecans regulates signaling, their extracellular part, substituted with heparan sulfate glycosaminoglycan chains, binds a plethora of extracellular matrix (ECM) molecules involved in fibrosis, e.g., collagens, growth factors, cytokines, and immune cell adhesion proteins. Full-length syndecans induce pro-fibrotic signaling, increasing the expression of collagens, myofibroblast differentiation factors, ECM enzymes, growth factors, and immune cell adhesion molecules, thereby also increasing cardiac stiffness and preventing cardiac rupture. Upon pro-inflammatory stimuli, syndecan ectodomains are enzymatically released from heart cells (syndecan shedding). Shed ectodomains affect the expression of ECM molecules, promoting ECM degradation and cardiac rupture upon myocardial infarction. Blood levels of shed syndecan-1 and -4 ectodomains are associated with hospitalization, mortality, and heart remodeling in patients with heart failure. Improved understanding of syndecans and their modifying enzymes in cardiac fibrosis might contribute to the development of compounds with therapeutic potential, and enzymatically shed syndecan ectodomains might constitute a future prognostic tool for heart diseases with fibrosis.