Thyroid Hormone Receptor α Controls Developmental Timing and Regulates the Rate and Coordination of Tissue-Specific Metamorphosis in Xenopus tropicalis

Abstract

Thyroid hormone (T3) receptors (TRs) mediate the effects of T3 on organ metabolism and animal development. There are two TR genes, TRα and TRβ, in all vertebrates. During animal development, TRα expression is activated earlier than zygotic T3 synthesis and secretion into the plasma, implicating a developmental role of TRα both in the presence and absence of T3. Using T3-dependent amphibian metamorphosis as a model, we previously proposed a dual-function model for TRs, in particular TRα, during development. That is, unliganded TR represses the expression of T3-inducible genes during premetamorphosis to ensure proper animal growth and prevent premature metamorphosis, whereas during metamorphosis, liganded TR activates target gene transcription to promote the transformation of the tadpole into a frog. To determine if TRα has such a dual function, we generated homozygous TRα-knockout animal lines. We show that, indeed, TRα knockout affects both premetamorphic animal development and metamorphosis. Surprisingly, we observed that TRα is not essential for amphibian metamorphosis, given that homozygous knockout animals complete metamorphosis within a similar time period after fertilization as their wild-type siblings. On the other hand, the timing of metamorphosis for different organs is altered by the knockout; limb metamorphosis occurs earlier, whereas intestinal metamorphosis is completed later than in wild-type siblings. Thus, our studies have demonstrated a critical role of endogenous TRα, not only in regulating both the timing and rate of metamorphosis, but also in coordinating temporal metamorphosis of different organs.

Role of Thyroid Hormone Receptor in Amphibian Development

“…If the mutation rate is high, grow up the remaining injected embryos for phenotypic analysis of the Fo knockdown animals, or use the Fo animals to produce total knockout animals for analysis ( see [35, 38]. …”

“…Such high efficiency suggested that we had nearly a complete knockout of the TRα gene and could study the phenotype in the Fo animals. Indeed, the findings in the Fo knockdown animals were subsequently confirmed by studies on complete TRα knockout animals derived from germ line breeding of the knockdown animals [38, 39]. …”

“…2) [34, 35, 36, 37, 38, 39]. Stage 54 is considered to be the onset of metamorphosis and is determined based on hindlimb morphology.…”

“…Stage 54 is considered to be the onset of metamorphosis and is determined based on hindlimb morphology. Based on this, both groups observed that the TRα knockdown/knockout tadpoles initiated metamorphosis at younger age, with concurrent upregulation (derepression) of several well-known T3-inducible genes [35, 36, 38, 39]. In addition, the TRα knockdown/knockout tadpoles had reduced response to exogenous T3 treatment in terms of both gene regulation and morphological changes, indicating that TRα is important both for controlling the timing of the onset of metamorphosis and for mediating the response to T3.…”

“…Consistently, when the knockdown/knockout animals were allowed to undergo natural metamorphosis, it was found that TRα knockdown/knockout animals took much longer time to develop from stage 54, the onset of metamorphosis, to stage 58, the early metamorphic climax. Furthermore, the studies on total TRα knockout animals also led to several novel observations [38, 39]. First, TRα knockout appears to have no gross effect on embryogenesis.…”