Dr. Andy McMahon

Cell Fate Regulation in the Neural Tube

Vokes SA, Ji H, McCuine S, Tenzen T, Giles S, Zhong S, Longabaugh WJ, Davidson EH, Wong WH, McMahon AP. 2007. Genomic characterization of Gli-activator targets in sonic hedgehog-mediated neural patterning.Development. 134(10):1977-89.

Question 1

Briefly tell us a bit about yourself, your career path over the years, and specifically what led you to begin working on Hedgehog signaling and neural tube patterning.

Michael Barresi

Question 2

You show in your paper that Gas1 functions as a Shh pathway activator, but simultaneously is negatively regulated by Shh.  This seems counter intuitive.  Can you please explain what benefit there is for neural tube patterning to have an activator of Shh signaling be negatively regulated by that same signaling system?

Trina Campbell

Question 3

You mention time as an important factor to consider when examining the co-expression of Gas1 and Cdo, which occurs during a brief temporal period. Can you explain what impact this has on our current models of Hh signaling?

Krista Riihimaki

Question 4

You show that Gas1 cooperates with Cdo to promote Shh signaling, however the phenotypes of a Gas1/Cdo double knockout are not as severe as the Shh null.  First, what is the nature of the cooperation of Gas1 and Cdo?  Do they directly interact and signal along the same pathway or do they promote Shh target activation through independent pathways?  Second, what else compensates for the difference in phenotypes between the double Gas1/Cdo KO and Shh KO alone?

Cecilia Flores

Question 5

You suggest that the possible reasons why little to no defects are seen in the limb of the Gas1/Cdo double knock-out embryo is because 1) Boc makes up for the loss of Cdo/Gas1, or 2) the limb structures that depend on Shh signaling in the limb are made up of Shh secreting cells (unlike in neural tube patterning). Has there been any work to support either of your proposed mechanisms?

Susy Ramos

Question 6

When you examined the possible cooperation of Gas1 and Cdo, the double knockout mutant showed no Shh expression at the midline, not in the floorplate or even in the notochord. However, when you labeled the notochord with CAIII, notochord development, while not completely normal, still continued. What does this suggest? How is it that Gas1 and Cdo actually influence the expression of Shh in the notochord?

Elizabeth Deschene

Question 7

In your paper, Genomic characterization of Gli-activator targets in sonic hedgehog-mediated neural patterning, you used an epitope-tagged Gli1 for ChIP analysis.  This being a relatively new technique, can you please walk us through your experimental design?  You used Embryoid Bodies for this study, is there any chance it can be done with Neural Tube tissues?  If it could, do you predict the results would be any different?

Sanna Cahn

Question 8

The MCA analysis predicts multiple gli sites within a given locus, where only a subset were bound by other Gli factors.  You state that you are not sure if this is because different sites may have differential affinities, and that there may be a mechanism by which one could impart specificity.  Can you explain this mechanism, and what potential impact this new information may have?

Monica Severson

Question 9

Currently in your opinion what are the most pressing questions in the field of Hedgehog signaling and Neural Tube Patterning?  What steps is your lab taking to address those questions?

Michael Barresi