Figure 2.

Long non-coding RNA acting within the NODAL signalling pathway. The TGFβ family protein NODAL (green) binds to a transmembrane receptor (ActRI, ActRII). Binding requires the co-factor EGF-CFC2 and can be inhibited by LEFTY1/2 or DAND5 (red). Receptor activation leads to phosphorylation of SMAD2/3, which recruits SMAD4 and translocates across the nuclear membrane (dashed line). SMAD2/3 can be inhibited by SMAD7. The SMAD2/3/4 complex binds DNA in cooperation with tissue-specific transcription factors such as FOXH1, leading to transcription of target genes including Pitx2c, Nodal, Lefty2 and Smad7. mRNA are indicated in the diagram in blue, while long non-coding RNA (lncRNA) are indicated in green and miRNA in red. Cytoplasmic lncRNA known to regulate NODAL pathway genes include Gas5 (growth arrest specific), which can bind to and inhibit SMADs through its RNA SMAD binding element (rSBE) or act as an endogenous competitor to inhibit miRNAs targeting Nodal, Lefty2 or Acvr1. Nuclear lncRNA are illustrated in the lower part of the diagram. Fendrr (fetal-lethal noncoding developmental regulatory RNA) can recruit PRC2 (polycomb repressive complex 2) to the Pitx2 locus, leading to trimethylation of histone H3 lysine 27. This inhibits binding by SMAD2/3/4 and prevents transcription. Meg3 (maternal expressed gene) similarly recruits PRC2 to the Smad2 locus. Smad7 expression is regulated by an enhancer RNA (eRNA) which promotes expression. Playrr (Pitx2 locus-asymmetric regulated RNA) mediates a chromatin-looping mechanism to regulate Pitx2 expression.