Deletion of the thyroid hormone receptor alpha 1 prevents the structural alterations of the cerebellum induced by hypothyroidism

Abstract

Thyroid hormone (T3) controls critical aspects of cerebellar development, such as migration of postmitotic granule cells and terminal differentiation of Purkinje cells. T3 acts through nuclear receptors (TR) of two types, TRalpha1 and TRbeta, that either repress or activate gene expression. We have analyzed the cerebellar structure of developing mice lacking the TRalpha1 isoform, which normally accounts for about 80% of T3 receptors in the cerebellum. Contrary to what was expected, granule cell migration and Purkinje cell differentiation were normal in the mutant mice. Even more striking was the fact that when neonatal hypothyroidism was induced, no alterations in cerebellar structure were observed in the mutant mice, whereas the wild-type mice showed delayed granule cell migration and arrested Purkinje cell growth. The results support the idea that repression by the TRalpha1 aporeceptor, and not the lack of thyroid hormone, is responsible for the hypothyroid phenotype. This conclusion was supported by experiments with the TRbeta-selective compound GC-1. Treatment of hypothyroid animals with T3, which binds to TRalpha1 and TRbeta, prevents any defect in cerebellar structure. In contrast, treatment with GC-1, which binds to TRbeta but not TRalpha1, partially corrects Purkinje cell differentiation but has no effect on granule cell migration. Our data indicate that thyroid hormone has a permissive effect on cerebellar granule cell migration through derepression by the TRalpha1 isoform.

Figure 1

Cerebellar structure in mice. Toluidine blue-stained sagittal sections of P11 and P21 mice cerebella. The figure shows sections through lobule VII of intact wild-type mice (TRα^+/+), hypothyroid wild-type mice (TRα^+/+ H), intact TRα1-deficient mice (TRα^−/−), and hypothyroid TRα1-deficient mice (TRα^−/−H). The arrows point to the presence of spindle-shaped, migrating cells. egl, external germinal layer; ml, molecular layer; and igl, internal granule cell layer. (Bar = 50 μm.)

Figure 2

Morphology of the Purkinje cells in mice. Purkinje cells were immunostained for calbindin in sagittal sections of lobules V, VII, and X of P11 mice cerebella. Symbols are the same as in Fig. 1. The arrows show the presence of perisomatic terminals in Purkinje cells of hypothyroid wild-type mice. Arrowheads show elongated primary dendrites. (Bar = 50 μm.)

Figure 3

Effects of hypothyroidism and T3 or GC-1 treatment on granular cell migration and Purkinje cell differentiation in mice. (a) Toluidine blue-stained sagittal sections of lobule VII from P21 mice cerebella. Arrows point to spindle-shaped migrating cells in hypothyroid and GC-1-treated mice. egl, external germinal layer; ml, molecular layer; and igl, internal granule cell layer. (b) Purkinje cells immunostained for calbindin in sagittal sections of lobules V, VII, and X of P11 mice cerebella. The arrows show the presence of perisomatic terminals in Purkinje cells from hypothyroid and GC-1-treated mice. Arrowheads show elongated primary dendrites. (c Left) Northern blot of cerebellum RNA probed with PCP-2 and GAPDH cDNA probes. (Right) Densitometric measurements of the autoradiographs. C, Control mice; H, hypothyroid mice; H + T3, hypothyroid mice treated with T3; H + GC-1, hypothyroid mice treated with GC-1. (Bar = 50 μm.)

Figure 4

Effects of hypothyroidism, and T3 or GC-1 treatment on granular cell migration and Purkinje cell differentiation in rats. (a) Toluidine blue-stained sagittal sections of lobule VII from P25 rat cerebella. The arrows show the presence of migrating cells from hypothyroid and GC-1-treated mice. egl, external germinal layer; ml, molecular layer; and igl, internal granule cell layer. (b) Purkinje cells immunostained for calbindin in sagittal sections of lobules V, VII, and X of P16 mice cerebella. Arrows show the presence of perisomatic terminals in Purkinje cells from hypothyroid and GC-1-treated rats. Arrowheads show elongated primary dendrites. (Bar = 25 μm.)