Megalin and the neurodevelopmental biology of sonic hedgehog and retinol.

Megalin is a receptor expressed by embryonic epithelia that mediates endocytosis of numerous ligands, including sonic hedgehog (Shh) and retinol, the precursor to retinoic acid (RA). The importance of Shh and RA signaling in neurodevelopment, combined with the fact that megalin-deficient mice show profound neurodevelopmental abnormalities, has raised questions as to the possible role of megalin in Shh and RA signaling. Several mechanisms could explain how megalin influences Shh and RA signaling in the context of neurodevelopment. These include the involvement of megalin in the transport of Shh and retinol within neuroepithelia, as well as direct signal transduction as a response to binding of Shh and retinol to megalin.