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Processed Sonic hedgehog - an active signaling protease?

Subject Area Developmental Biology
Biochemistry
Evolutionary Cell and Developmental Biology (Zoology)
Term from 2013 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 246417003
 
Final Report Year 2020

Final Report Abstract

Although our original attempts to prove the putative roles of Shh proteolytic activity in Hh gradient formation in vivo were unsuccessful, observations made in this project initiated or contributed to other projects. One of them currently addresses the newly discovered possibility that catalytic Shh processing off its lipidated tags is required for its release and signaling during early development, as outlined above. Two other now published projects are of particular importance because they address the current dogma that lipidated Hh is released from the plasma membrane of producing cells by the extracting activity of the RND efflux pump Dispatched (Disp), is transported by Scube2 and signals to Ptc receptors on receiving cells via both lipids. Yet, despite its popularity, this model not only lacks true in vivo support, it is even refuted by the available data. For example, overexpressed mouse Disp can functionally replace Drosophila Dispatched in vivo, despite the fact that flies do not express Scube2 orthologs, and sterol-sensing domains (SSD) in Disp-related SSD proteins NPC1 and Ptc are involved in different aspects of homeostasis of free cellular cholesterol, but not in the extraction of lipidated proteins. Finally, Scube2 is not strictly required for Hh signaling in vertebrates, because the zebrafish you (=Scube2 KO) phenotype is the weakest in its class of mutants that disrupt the Hh signal transduction pathway, and because Shh overexpression can bypass Hh patterning defects as a consequence of Scube1-3 depletion in vivo. Consistent with this, overexpressed Hh can also be released independent of Disp function. Therefore, the question of Hh release and transport is all but settled, and the progress made in our in vitro and in vivo studies supports Disp- and Scube2-regulated shedding as the underlying Hh release mode. In addition, methodological work has contributed to progressing the field of proteomics with regard to protease specificity determination and quantitative feature determination.

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