P3.79 Friday, Jan. 6 Sonic Hedgehog's negative autoregulatory properties in salamander limb development STOPPER, Geffrey F.*; GRZYB, Amanda; PERLEE, Brooke; SWIFT, Shannon; ENGEL, Ashley; HARTMAN, Brittany; Sacred Heart University; Sacred Heart University; Sacred Heart University; Sacred Heart University; Sacred Heart University; Sacred Heart University firstname.lastname@example.org
Sonic hedgehog (Shh) is a diffusible morphogen that is expressed in the posterior of tetrapod limbs and patterns the anterior-posterior axis of the limb. Shh is thought to interact in a positive feedback loop with signaling molecules at the distal tip of the limb that direct proximal-distal outgrowth. Recent experiments support that Shh has negative autoregulatory properties, causing its expression to increase when its signaling is blocked. Previous studies in the salamander Ambystoma mexicanum have shown that a 10-day exposure to cyclopamine, which allows Shh expression but blocks its signaling, results in a loss of nearly all Shh patterning function from the onset of exposure, yielding limbs with reduced numbers of digits. Here we investigate the effects of a shorter 2-day exposure to cyclopamine. A simple positive feedback loop predicts a collapse of the loop in the absence of function of one of the loop elements; a limb with Shh signaling blocked for 2 days should result in a morphology similar to a limb with Shh signaling blocked for 10 days. In contrast, a negative autoregulatory loop predicts an increase in expression in response to blocked signaling; a limb with Shh signaling blocked for 2 days should look much more similar to a normal limb. Many of our experimental limbs with 2-day cyclopamine exposures show the same morphologies as normal limbs, with a few showing slight abnormalities consistent with reduced Shh function. This supports that Shh has strong negative autoregulatory properties that allow its signaling to recover from, and possibly even compensate for, short periods of interruption, with little to no morphological effect.