Thyroid hormone mediates syndecan expression in rat neonatal cerebellum

Thyroid hormone (T-3) plays an essential role in the central nervous system development. Astrocytes mediate many of the T-3 effects in the growth and differentiation of cerebellum. In culture, T-3 induces cerebellar astrocytes to secrete growth factors, mainly FGF(2), and alters the expression and organization of the extracellular matrix (ECM) proteins, laminin, and fibronectin. In addition, T-3-treated astrocytes promote neuronal differentiation. In this study, we have investigated whether other ECM molecules, such as syndecans, are involved in T-3 action. Thus, we analyzed the expression of syndecans (1-4) by RT-PCR in astrocyte cultures from cerebellum, cortex, and hippocampus of newborn rats. Our results showed that syndecans (1-4) are expressed in astrocytes of cerebellum and cortex, whereas in hippocampus only syndecans 2 and 4 were detected. Semi-quantitative RT-PCR analysis revealed the reduced expression of syndecans 1, 2, and 4, and increased expression of syndecan 3 in hypothyroid cerebellum, when compared to the euthyroid tissue. Furthermore, we observed a reduced expression of syndecans 2 and 3 in T-3-treated cerebellar astrocytes, when compared to control cultures. This balance of proteoglycans may be involved in T-3 action mediated by FGF(2) signaling, possibly affecting the formation of the trimeric signaling receptor complex composed by syndecan/FGF/FGF-receptor (FGFR), which is essential for FGFR dimerization, activation, and subsequent cell signaling.