Tissue-specific roles for sonic hedgehog signaling in establishing thymus and parathyroid organ fate.

The thymus and parathyroids develop from third pharyngeal pouch (3rd pp) endoderm. Our previous studies show that Shh null mice have smaller, aparathyroid primordia in which thymus fate specification extends into the pharynx. SHH signaling is active in both dorsal pouch endoderm and neighboring neural crest (NC) mesenchyme. It is unclear which target tissue of SHH signaling is required for the patterning defects in Shh mutants. Here, we used a genetic approach to ectopically activate or delete the SHH signal transducer Smo in either pp endoderm or NC mesenchyme. Although no manipulation recapitulated the Shh null phenotype, manipulation of SHH signaling in either the endoderm or NC mesenchyme had direct and indirect effects on both cell types during fate specification and organogenesis. SHH pathway activation throughout pouch endoderm activated ectopic Tbx1 expression and partially suppressed the thymus-specific transcription factor Foxn1, identifying Tbx1 as a key target of SHH signaling in the 3rd pp. However, ectopic SHH signaling was insufficient to expand the GCM2-positive parathyroid domain, indicating that multiple inputs, some of which might be independent of SHH signaling, are required for parathyroid fate specification. These data support a model in which SHH signaling plays both positive and negative roles in patterning and organogenesis of the thymus and parathyroids.

Ectopic TBX1 suppresses thymic epithelial cell differentiation and proliferation during thymus organogenesis.

The thymus and parathyroid glands arise from a shared endodermal primordium in the third pharyngeal pouch (3rd pp). Thymus fate is specified in the ventral 3rd pp between E9.5 and E11, whereas parathyroid fate is specified in the dorsal domain. The molecular mechanisms that specify fate and regulate thymus and parathyroid development are not fully delineated. Previous reports suggested that Tbx1 is required for thymus organogenesis because loss of Tbx1 in individuals with DiGeorge syndrome and in experimental Tbx1 deletion mutants is associated with thymus aplasia or hypoplasia. However, the thymus phenotype is likely to be secondary to defects in pharyngeal pouch formation. Furthermore, the absence of Tbx1 expression in the thymus-fated domain of the wild-type 3rd pp suggested that Tbx1 is instead a negative regulator of thymus organogenesis. To test this hypothesis, we generated a novel mouse strain in which expression of a conditional Tbx1 allele was ectopically activated in the thymus-fated domain of the 3rd pp. Ectopic Tbx1 expression severely repressed expression of Foxn1, a transcription factor that marks the thymus-fated domain and is required for differentiation and proliferation of thymic epithelial cell (TEC) progenitors. By contrast, ectopic Tbx1 did not alter the expression pattern of Gcm2, a transcription factor restricted to the parathyroid-fated domain and required for parathyroid development. Ectopic Tbx1 expression impaired TEC proliferation and arrested TEC differentiation at an early progenitor stage. The results support the hypothesis that Tbx1 negatively regulates TEC growth and differentiation, and that extinction of Tbx1 expression in 3rd pp endoderm is a prerequisite for thymus organogenesis.