The Sonic Hedgehog (SHH) protein is involved in limb differentiation during embryonic development. It acts through a signaling pathway that involves the activation of a transcription factor called Gli.
In the developing limb bud, SHH is produced by a group of cells known as the zone of polarizing activity (ZPA) located at the posterior margin of the limb bud. The SHH protein diffuses from the ZPA to the anterior part of the limb bud, where it interacts with the cells in a concentration-dependent manner.
When SHH binds to its receptor, Patched (PTCH1), it releases another receptor, Smoothened (SMO), from inhibition. This leads to the activation of a downstream signaling cascade that ultimately activates the transcription factor Gli.
Gli then enters the nucleus of the target cells and regulates the expression of various genes involved in limb development, such as the homeobox transcription factor genes. These genes determine the identity of the cells in the developing limb bud and dictate the formation of specific structures such as bones, muscles, and tendons.
Thus, SHH acts as a morphogen, controlling the pattern and identity of cells in the developing limb bud, and its expression and activity are critical for the proper development of limbs.