Trisomy 21 alters ciliary localization of Sonic Hedgehog signaling proteins

ABSTRACT Trisomy 21 (T21), the cause of Down syndrome, impairs cilia-dependent Sonic Hedgehog (SHH) signaling. We investigated how T21 affects SHH signaling at the primary cilium and found that Smoothened (SMO) protein - the main effector of SHH signaling - is diminished in T21 primary cilia. Specifically, T21 cells exhibit both decreased SMO localization to cilia and reduced ciliary SMO phosphorylation (pSMO). Our findings suggest that reduced ciliary SMO in T21 results from defective entry into or maintenance in the cilium rather than impaired transport to centrosomes. Moreover, unlike the ciliogenesis defects observed in T21, the SMO defect does not appear to be associated with increased PCNT protein abundance. We also found that GPR161 (a negative SHH regulator), exhibits increased ciliary localization in T21. Notably, prolonged serum depletion to promote primary cilia maturation equalizes ciliary levels of SMO and GPR161 between T21 cells and D21 controls. We propose that delayed primary cilia maturation contributes to defective SHH signaling in T21. SUMMARY T21 impedes primary cilia assembly and disrupts SHH signaling by diminishing ciliary SMO and increasing GPR161, but extended ciliary maturation equalizes SHH protein localization between D21 and T21.