Sonic Hedgehog (Shh) is a signaling molecule that undergoes post-translational modifications resulting in the addition two lipophilic moieties to the protein, rendering it obligatory membrane associated. Despite these modifications, during development, in stem cell niches, and in tumors Shh signals to cells often several cell diameters away from the sites of synthesis. Two related molecules, Disp1 and Ptc1, are required for the secretion of Shh and its uptake in neighboring cells respectively. It is hypothesized that transport of Shh involves the reiterated and complementary actions of Disp1 and Ptc1 generating and absorbing Shh containing complexes (either lipoprotein particles or exosomes) thus mediating long-range transport while generating a Shh activity gradient. Three central observations support this hypothesis, i/ cells without Disp1 do not secrete Shh, ii/ Ptch1 in surrounding cells is required for the uptake of Shh and iii/ tissues without Disp1 are less able to transport Shh. Ptch1 and Disp1 are members of a large family of trimeric, proton driven antiporters, and in the first aim it is tested if Ptch1 and Disp1 function is required for planar transcytosis of Shh.
In aim 2 the apicobasal polarity of the transport as well as the response is addressed, to reconcile basolateral transport with the apical localization of the primary cilium.
Aims 3 and 4 address the Shh itinerary as well as the role of the putative proton driven pump activity of Ptch1 and Disp1 in the transport of Shh, and the associated relocation of Smoothened.
Cell to cell signaling mediated by Sonic Hedgehog (Shh) is critically important for normal embryonic development and correct function of many stem cell niches. Furthermore, inappropriate expression of Shh is a determining step in the formation of many common cancers. We propose a series of experiments that address the molecular mechanism by which Shh is secreted and transported to neighboring cells.
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