Toddler: An Embryonic Signal That Promotes Cell Movement via Apelin Receptors

Abstract

It has been assumed that most, if not all, signals regulating early development have been identified. Contrary to this expectation, we identified 28 candidate signaling proteins expressed during zebrafish embryogenesis, including Toddler, a short, conserved, and secreted peptide. Both absence and overproduction of Toddler reduce the movement of mesendodermal cells during zebrafish gastrulation. Local and ubiquitous production of Toddler promote cell movement, suggesting that Toddler is neither an attractant nor a repellent but acts globally as a motogen. Toddler drives internalization of G protein–coupled APJ/Apelin receptors, and activation of APJ/Apelin signaling rescues toddler mutants. These results indicate that Toddler is an activator of APJ/Apelin receptor signaling, promotes gastrulation movements, and might be the first in a series of uncharacterized developmental signals.

Apela Regulates Fluid Homeostasis by Binding to the APJ Receptor to Activate Gi Signaling

“…Apela Binds to APJ with High Affinity-Apela induces the internalization of APJ in transfected cell lines overexpressing APJ (13,14). However, binding kinetics between apela and APJ has not been demonstrated.…”

“…Because the APJ-null zebrafish shares the same phenotype as apela mutants during embryogenesis, apela was identified as the cognate peptide ligand for the APJ receptor (13,14). In addition, apela binds to APJ and induces the internalization of APJ in receptor-transfected cell lines (13,14).…”

“…Two groups independently showed that apela is an embryonic hormone capable of regulating heart development and promoting the movement of mesendodermal cells during gastrulation, respectively (13,14). Because the APJ-null zebrafish shares the same phenotype as apela mutants during embryogenesis, apela was identified as the cognate peptide ligand for the APJ receptor (13,14).…”

Hidden Peptides Encoded by Putative Noncoding RNAs

“…The Apela transcript had been annotated as a lncRNA in zebrafish (ENSDARG00000094729), mouse (Gm10664), and human (LOC100506013), but was also found to be a peptide-coding mRNA in zebrafish (Chng et al, 2013;Pauli et al, 2014). Apela consists of 54 amino acids that include a signal peptide sequence and is highly conserved among vertebrates.…”

“…Although lncRNAs do not harbor apparent open reading frames (ORFs) for proteins of >100 amino acids, recent studies have identified a subset of lncRNAs that actually encode polypeptides of <100 amino acids (Fig. 1A) (Anderson et al, 2015;Kondo et al, 2010;Matsumoto et al, 2017;Nelson et al, 2016;Pauli et al, 2014).…”