RTK signaling pathway overview
- RTK receptors dimerize when both ligands interact with receptors.
- Phosphorylation occurs on one of the receptors (in cytoplasm), where adaptor protein binds to phos domain.
- Intermediate domain is then activated, such as GNRP, which leads to GTP activating Ras
- Ras bound to GTP results in activation of RAF, then MEK, followed by ERK in the nucleus.
- ERK then aids in binding to activate a transcription factor which can now aid in transcription
- *Note, Phos domain of receptor binds to GAP, which aids in stopping Ras activation by aiding in GTP hydrolysis from Ras
DEVELOPMENTAL EVENT THAT UTLIZES RTK PATWHAY
- Plays a role in limb development, specifically with the mesenchymal cells involved in limb bud formation, and leading to AER formation during limb development.
- Cells expressing Fgf-10 are injected into flank of chick embryo and ectopic limbs are formed. It is observed that limb bud formation signals come from the LPM and mesenchymal cells in the LPM cause the outgrowth. These Mesenchymal cells secrete Fgf-10, and this signal aids in formation of AER in the overlying ectoderm.
- Fgf8 is expressed in cells of the AER, and Fgf10 from LPM induces Fgf8 in AER. Fgf8 is also expressed during limb initiation in the IM, next to the LPM
- Fgf signaling can REPRESS GremI expression, if Fgf levels are high. However, Fgf REPRESSION is also dependent upon Grem1, acting in this positive feedback loop including Shh (the loop causes outgrowth, but the highFgf/Grem1 inhibitory loop TERMINATES outgrowth signals.
1st EXPERIMENTAL APPROACHES FROM THE PAPER THAT SUPPORTS A ROLE FOR RTK signaling IN THIS DEVELOPMENTAL EVENT:
In Verheyden et al. paper, one test involves looking at Fgf8 repression, to see if it represses Fgf4 VIA Germ1 INHIBITION. To do this they sought to block expression using a null allele for Grem1, and inserting it into a Fgf8-KO. With this system, no skeletal limbs were observed. And although AER was present, Fgf4 expression was not maintained. This showed Fgf8 repression of Fgf4 is DEPENDENT on Grem1.
2nd EXPERIMENTAL APPROACHES FROM THE PAPER THAT SUPPORTS A ROLE FOR RTK signaling IN THIS DEVELOPMENTAL EVENT:
A second test involves AER-FGF signaling. The authors wanted to see if Fgf2 specifically represses Grem1 in a dose dependent manner. At low doses of Fgf2 bead injection into embryos, there is not a repression of Grem1. However as Fgf2 dosages increased, there was repression of Grem1 observed distal to the beads, confirming that Fgf at high concentrations acts to repress Grem1.