Tril targets Smad7 for degradation to allow hematopoietic specification in Xenopus embryos

Abstract

In Xenopus laevis, Bone morphogenetic proteins induce expression of the transcription factor Gata2 during gastrulation, and Gata2 is required in both ectodermal and mesodermal cells to enable mesoderm to commit to a hematopoietic fate. In the current studies, we identify tril as a Gata2 target gene that is required in both ectoderm and mesoderm for primitive hematopoiesis to occur. Tril is a transmembrane protein that functions as a co-receptor for Toll like receptors to mediate innate immune responses in the adult brain, but developmental roles for this molecule have not been identified. We show that Tril function is required both upstream and downstream of Bmp receptor mediated Smad1 phosphorylation for induction of Bmp target genes. Mechanistically, Tril triggers degradation of the Bmp inhibitor, Smad7. Tril-dependent down regulation of Smad7 relieves repression of endogenous Bmp signaling during gastrulation and this enables mesodermal progenitors to commit to a blood fate. Thus, Tril is a novel component of a Bmp-Gata2 positive feedback loop that plays an essential role in hematopoietic specification.

Fibronectin type III and intracellular domains of Toll-like receptor 4 interactor with leucine-rich repeats (Tril) are required for developmental signaling

“…1B). These results replicate our published studies showing that endogenous Tril triggers degradation of Smad7 protein, particularly within the nuclear compartment (6).…”

“…1F) in the nuclei of cells ( Fig. S2), phenocopying what is seen in Tril morphants (6). Thus, the FN domain is essential for Tril function, and ectopic Tril∆FN dominantly suppresses signaling by endogenous Tril, possibly by serving as a sink for ligand or signal transduction components.…”

“…In vivo activity of wild type and deletion mutant forms of Xenopus Tril. We analyzed the subcellular localization and steady state levels of Smad7 in Xenopus embryos in which Tril expression was reduced by injection of a well characterized translation blocking antisense morpholino (MO) (6). Since we are unable to detect endogenous Smad7 using available antibodies we instead analyzed ectopically expressed epitope tagged Smad7.…”

“…We have recently shown that the transmembrane protein Toll-like receptor 4 (Tlr4) interactor with leucine-rich repeats (Tril) is required to augment Bmp signaling during gastrulation in Xenopus embryos (6). Tril promotes degradation of Smad7 protein, thereby relieving repression of endogenous Bmp signaling.…”

“…This is essential to enable the mesoderm to be specified as blood. When expression of endogenous Tril is knocked down in Xenopus embryos using antisense morpholinos, high levels of Smad7 protein accumulate, particularly in the nuclear compartment (6). As a consequence, pSmad1/5/8 levels are reduced and development of blood is impaired.…”