Retinal degeneration is rescued in transgenic rd mice by expression of the cGMP phosphodiesterase ß subunit

The ß subunit of the cGMP phosphodiesterase (PDE) gene has been identified as the candidate gene for retinal degeneration in the rd mouse. To study the molecular mechanisms underlying degeneration and the potential for gene repair, we have expressed a functional bovine cGMP PDE ß subunit in transgenic rd mice. One transgenic mouse line showed complete photoreceptor rescue across the entire span of the retina. A second independently derived line showed partial rescue in which photoreceptors in the superior but not the inferior hemisphere of the retina were rescued. In the latter animals, intermediate stages of degeneration were observed in the transition zone between rescued and diseased photoreceptors. Pathologic changes in the retina ranged from vesiculation of the basalmost outer segment discs in otherwise structurally intact rod cells to photoreceptors with highly disorganized outer segments and intact inner segments. Totally or partially rescued retinas showed a corresponding restoration of cGMP PDE activity, whereas nonrescued retinas had minimal enzyme activity, characteristic of the rd phenotype. These transgenic animals provide models for studying the molecular basis of retinal degenerative disease and conclusively demonstrate that the phenotype of rd mice is produced by a defect in the ß subunit of cGMP PDE

Thyroid hormone action in the absence of thyroid hormone receptor DNA-binding in vivo

Thyroid hormone action is mediated by thyroid hormone receptors (TRs), which are members of the nuclear hormone receptor superfamily. DNA-binding is presumed to be essential for all nuclear actions of thyroid hormone. To test this hypothesis in vivo, the DNA-binding domain of TR-β was mutated within its P-box (GS mutant) using gene targeting techniques. This mutation in vitro completely abolishes TR-β DNA-binding, while preserving ligand (T(3)) and cofactor interactions with the receptor. Homozygous mutant (TR-β(GS/GS)) mice displayed abnormal T(3) regulation of the hypothalamic-pituitary-thyroid axis and retina identical to abnormalities previously observed in TR-β KO (TR-β(–/–)) mice. However, TR-β(GS/GS) mutant mice maintained normal hearing at certain frequencies and did not display significant outer hair cell loss, in contrast to TR-β(–/–) mice. DNA-binding, therefore, is essential for many functions of the TR, including retinal development and negative feedback regulation by thyroid hormone of the hypothalamic-pituitary-thyroid axis. Inner ear development, although not completely normal, can occur in the absence of TR DNA-binding, suggesting that an alternative and perhaps novel thyroid hormone-signaling pathway may mediate these effects