No abstract

Two well-characterized nongenomic actions of thyroid hormone in cultured brain tissues are: 1) regulation of type 2 iodothyronine 5'deiodinase (D2) activity and 2) regulation of actin polymerization. In particular, the latter is likely to have profound effects on neuronal migration in the developing brain. In this study, we determined whether these nongenomic actions also occurred in vivo during brain development. Neonatal hypothyroidism was induced by propylthiouracil given to pregnant dams beginning on d17 of gestation and continued throughout the neonatal period. On postnatal d 14, rats were injected with either cold or [(125)I]-labeled iodothyronines and killed sequentially after injection. In contrast to reports in the adult rat, all three iodothyronines readily and equally entered developing brain tissues. As expected, cerebrocortical D2 activity was markedly elevated in the hypothyroid brain and both reverse T(3) (rT(3)) and T(4) rapidly decreased D2 to euthyroid levels within 3 h. Furthermore, cerebellar G-actin content in the hypothyroid rat was approximately 5-fold higher than in the euthyroid rat. Again, both rT(3) and T(4) rapidly decreased the G-actin content by approximately 50%, with a reciprocal increase in F-actin content to euthyroid levels without altering total actin. Neither T(3) nor vehicle had any effect on D2 activity in the cortex or G- or F-actin content in the cerebellum. The thyroid hormone-dependent regulation of actin polymerization in the rat brain provides a mechanism by which this morphogenic hormone can influence neuronal migration independent of the need for altered gene transcription. Furthermore, these data suggest a prominent role for rT(3) during brain development.

In the rat cerebellum, migration of neurons from the external granular layer to the internal granular layer occurs postnatally and is dependent upon the presence of thyroid hormone. In hypothyroidism, many neurons fail to complete their migration and die. Key guidance signals to these migrating neurons are provided by laminin, an extracellular matrix protein that is fixed to the surface of astrocytes. Expression of laminin in the brain is developmentally timed to coincide with neuronal growth spurts. In this study, we examined the role of thyroid hormone on the expression and distribution of laminin in the rat cerebellum. We show that laminin content steadily increased 2- to 3-fold from birth to maximal levels on postnatal day 8-10 then steadily decreased to a plateau by postnatal day 12 in the euthyroid cerebellum. Immunoreactive laminin appeared in the molecular layer of the euthyroid cerebellum by postnatal day 4, reached maximal intensity by postnatal day 8-10, and was gone by postnatal day 14. In contrast, laminin content in the hypothyroid cerebellum remained unchanged from birth until postnatal day 10 and then increased to maximal levels over the next two days; maximal levels were approximately 35% less than those levels in the euthyroid cerebellum. Laminin staining did not appear in the molecular layer of the hypothyroid rat cerebellum until postnatal day 10, reached maximal intensity by postnatal day 15 and disappeared by postnatal day 18, despite the continued presence granular neurons in the external granular layer. These data indicate that the disruption of the timing of the appearance and regional distribution of laminin in the absence of thyroid hormone may play a major role in the profound derangement of neuronal migration observed in the cretinous brain.

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 T4 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 T4-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 T3-treated cells until 36 h after seeding and then was restricted to punctate deposits. Secretion of laminin into the medium by hormone-deficient and T3-treated cells was significantly greater than that by T4-treated cells. Conversely, deposition of laminin into the extracellular matrix was significantly greater in T4-treated cells than in hormone-deficient and T3-treated cells. Thyroid hormone had no effect on the production of laminin by astrocytes. These data show that T4 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.