Sonic hedgehog (Shh) and its main receptor, Patched (Ptc), are implicated in both neural development and tumorigenesis1,2. Besides its classic morphogenic activity, Shh is also a survival factor3,4. Along this line, Ptc has been shown to function as a dependence receptor; it induces apoptosis in the absence of Shh, whereas its pro-apoptotic activity is blocked in the presence of Shh5. Here we show that, in the absence of its ligand, Ptc interacts with the adaptor protein DRAL (downregulated in rhabdomyosarcoma LIM-domain protein; also known as FHL2). DRAL is required for the pro-apoptotic activity of Ptc both in immortalized cells and during neural tube development in chick embryos. We demonstrate that, in the absence of Shh, Ptc recruits a protein complex that includes DRAL, one of the caspase recruitment (CARD)-domain containing proteins TUCAN (family member, 8) or NALP1 (NLR family, pyrin domain containing 1) and apical caspase-9. Ptc triggers caspase-9 activation and enhances cell death through a caspase-9-dependent mechanism. Thus, we propose that in the absence of its ligand Shh the dependence receptor Ptc serves as the anchor for a caspase-activating complex that includes DRAL, and caspase-9.
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We thank J. Briscoe and N. Le Douarin for advice and reagents, B. W. Schafer, C. Sardet and R. Sadoul for reagents, K. Cywinska for technical work on immunoprecipitation experiments and L. Kremer for the 5e1 hybridoma cell line (protein tools service at the Centro Nacional de Biotecnologia of Madrid). This work was supported by the Ligue Contre le Cancer, the Agence Nationale de la Recherche, the Institut National du Cancer, the Rhône-Alpes Region, the Centre National de la Recherche Scientifique, the EU grants Hermione and APO-SYS, NIH grants to P.M. (NS45093) and J.C.R. (AI56324), and a Rhône-Alpes Region fellowship and an Association pour la Recherche sur le Cancer fellowship to F.M.
The authors declare no competing financial interests.
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