The heterochronic gene Lin28 regulates amphibian metamorphosis through disturbance of thyroid hormone function

Abstract

Metamorphosis is a classic example of developmental transition, which involves important morphological and physiological changes that prepare the organism for the adult life. It has been very well established that amphibian metamorphosis is mainly controlled by Thyroid Hormone (TH). Here, we show that the heterochronic gene Lin28 is downregulated during Xenopus laevis metamorphosis. Lin28 overexpression before activation of TH signaling delays metamorphosis and inhibits the expression of TH target genes. The delay in metamorphosis is rescued by incubation with exogenous TH, indicating that Lin28 works upstream or parallel to TH. High-throughput analyses performed before any delay on metamorphosis or change in TH signaling showed that overexpression of Lin28 reduces transcript levels of several hormones secreted by the pituitary, including the Thyroid-Stimulating Hormone (TSH), and regulates the expression of proteins involved in TH transport, metabolism and signaling, showing that Lin28 disrupts TH function at different levels. Our data demonstrates that the role of Lin28 in controlling developmental transitions is evolutionary conserved and establishes a functional interaction between Lin28 and thyroid hormone function introducing a new regulatory step in perinatal development with implications for our understanding of endocrine disorders.

Overexpression of Lin28a delays Xenopus metamorphosis and down‐regulates albumin independently of its translational regulation domain

“…As a first approach to characterize the role of Lin28 on let‐7 biogenesis during Xenopus metamorphosis, we previously studied the expression of lin28 and the mature form of let‐7a in the CNS by quantitative reverse transcription PCR (RT‐qPCR). Expression levels of Lin28a and Lin28b decreased in the CNS during metamorphosis concomitant with an increase in the levels of mature let‐7a . To determine if the increase of the mature form of let‐7 is also a consequence of an increase in the transcription of the let‐7 precursors, we evaluated the expression of the precursors pri‐let7a and pri‐let7c by RT‐qPCR in brain samples.…”

“…Developmental transitions are regulated by heterochronic genes, such as in larval transitions in Caenorhabditis elegans , or by hormones such as in the metamorphosis of insects and amphibians . The molecular interaction between heterochronic genes and hormones has only recently been explored …”

“…Since the discovery of Lin28 as a regulator of larval transitions in C. elegans , the role of Lin28 has been studied in other transitions. Overexpression of Lin28a delays puberty and increases size in mice, overexpression of Lin28 results in lethality of Drosophila pupa during metamorphosis and overexpression of Lin28a or Lin28b delays Xenopus metamorphosis . However, the molecular mechanisms underlying the interaction between Lin28 and hormonal axes during developmental transitions are not understood.…”

“…To study the role of Lin28 in frog metamorphosis, we prepared transgenic lines in X. laevis to overexpress Lin28a or Lin28b under the control of a heat‐shock (HS) inducible promoter . We found that global overexpression of Lin28a or Lin28b during premetamorphosis and prometamorphosis significantly delays the progression of metamorphosis compared to control animals.…”

“…Proteomic analyses in tail and limbs showed that the overexpression of Lin28a for 3 weeks from premetamorphosis (stage 50) changes the levels of several proteins involved in TH homeostasis . Interestingly, serum albumin protein levels are down‐regulated by Lin28a overexpression in tail and limbs.…”