Thyroid hormone receptor beta is critical for intestinal remodeling during Xenopus tropicalis metamorphosis.

Background: Thyroid hormone (T3) is critical for development in all vertebrates. The mechanism underlying T3 effect has been difficult to study due to the uterus-enclosed nature of mammalian embryos. Anuran metamorphosis, which is dependent on T3 but independent of maternal influence, is an excellent model to study the roles of T3 and its receptors (TRs) during vertebrate development. We and others have reported various effects of TR knockout (TRalpha and TRbeta) during Xenopus tropicalis development. However, these studies were largely focused on external morphology. Results: We have generated TRbeta knockout animals containing an out-frame-mutation of 5 base deletion by using the CRISPR/Cas9 system and observed that TRbeta knockout does not affect premetamorphic tadpole development. We have found that the basal expression of direct T3-inducible genes is increased but their upregulation by T3 is reduced in the intestine of premetamorphic homozygous TRbeta knockout animals, accompanied by reduced target binding by TR. More importantly, we have observed reduced adult stem cell proliferation and larval epithelial apoptosis in the intestine during T3-induced metamorphosis. Conclusions: Our data suggest that TRbeta plays a critical role in intestinal remodeling during metamorphosis.