Thyroid hormone regulates the extracellular organization of laminin on astrocytes

Astrocytes produce laminin, a key extracellular matrix guidance molecule in the developing brain. Laminin is bound to transmembrane receptors on the surface of astrocytes known as integrins, which are, in turn, bound to the microfilament meshwork inside the astrocyte. Previous studies have shown that T-4 regulates the pattern of integrin distribution in astrocytes by modulating the organization of the microfilaments. In this study, the effect of thyroid hormone on the secretion and topology of laminin in astrocytes was examined. Linear arrays of secreted laminin were observed on the surface of the T-4-treated astrocytes within 10 h after seeding the cells onto poly-D-lysine-coated coverslips and became an organized meshwork by 24 h. In contrast, little if any laminin was identified on the surface of either hormone-deficient or T-3-treated cells until 36 h after seeding and then was restricted to punctate deposits. Secretion of laminin into the medium by hormone-deficient and T-3-treated cells was significantly greater than that by T-4-treated cells. Conversely, deposition of laminin into the extracellular matrix was significantly greater in T-4-treated cells than in hormone-deficient and T-3-treated cells. Thyroid hormone had no effect on the production of laminin by astrocytes. These data show that T-4 regulates the extracellular deposition and organization of laminin on the surface of astrocytes and provide a mechanism by which this morphogenic hormone can influence neuronal migration and axonal projection in the developing brain.