Figure 3Cartoon illustrating at which levels GPR56 may enhance canonical Wnt signaling (turquoise dashed arrows): at the level of ligand-receptor/co-receptor interaction, β-catenin, downstream of β-catenin, indirectly via activation of Rho/SRF through Gɑ12/13. CHIR99021 is a GSK3 inhibitor that prevents degradation of β-catenin thus enhancing Wnt signaling. MAL/MKL1 (Megakaryocytic Acute Leukemia) is a strong activator of the Rho/SRF 3DA Luc reporter. Cartoon illustrating GPR56 full length (FL, left) and the C-terminal fragment (CTF, right). Amino acid changes introduced in the intracellular loops are indicated by circles and letters. GAIN: GPCR autoproteolysis- inducing domain, GPS: GPCR proteolytic site, PLL: Pentraxin/Laminin/neurexin/sex-hormone-binding-globulin-Like SuperTop reporter assay showing fold change of Relative Luminescence Units (RLU) in Wnt3a conditioned culture media normalized to empty pcDNA3.1+ in the respective control (Ctrl) media after transfection of HEK293T cells with empty vector (violet), GPR56 FL (light turquoise) and GPR56 CTF (turquoise). Shown are means, standard deviation (stdev), and individual values of four technical replicates performed in either complete media or 1:3 diluted Wnt3a conditioned media. Unpaired t-test, **** p<0.0001, *** p<0.0005, ** p<0.005.   β-catenin protein expression in K562 cells infected with shGPR56strong or shLuc negative control in presence and absence of Wnt3a (representative Western Blot (above) and quantification after normalization to GAPDH (below). Shown is fold-change compared to shLuc in control media, bars and error bars represent mean and standard deviation of three biological replicates, unpaired t-test).   SuperTop reporter assay showing fold-change of RLU in presence of Wnt3a normalized to shSCR in control media after transfection with shSCR, shRNAs against GPR56 with or without additional transfection with 1ng or 2ng of β-catenin plasmid. Overexpression of β-catenin fully rescues the SuperTop signal reduction caused by shRNAs against GPR56. Unpaired t-test, bars and error bars represent mean and standard deviation of four biological replicates, **** p<0.0001, *** p<0.0005, ** p<0.005, * p<0.05. SuperTop reporter assay indicating fold change in RLU normalized to control media after transfection of HEK293T cells with empty vector, GPR56-FL, or 5 GPR56-CTF mutants. Unpaired t-test, bars and error bars represent mean and standard deviation of four biological replicates, **** p<0.0001, ** p<0.005. Left: SMO-GFP reporter RPE cell line or RPE-1 wild type cells were infected with shGPR56strong or shLuc control followed by administration of doxycycline or the SMO agonist SAG as indicated. GPR56 KD significantly reduces the baseline and doxycycline induced SMO-GFP signal. Right: representative FACS histogram plots showing GFP intensity in shLuc versus shGPR56strong RPE cells with and without doxycycline. Unpaired t-test, bars and error bars represent mean and standard deviation of three biological replicates, *** p<0.0005, ** p<0.005, * p<0.05. Left: Representative immunofluorescence (IF) images showing primary cilium formation in RPE cells upon starvation after infection with shLuc (above) or shGPR56strong (below). The non-motile primary cilium is visualized using antibodies against γ-tubulin, which stains the basal body, and the ciliary membrane marker ARL13B. DAPI indicates the nucleus. Images on the right show selected cells at 3 x magnification. Brightness was increased in all images to enhance visibility of cilia. Right: Percentage of RPE-1 cells with primary cilium in shLuc vs. shGPR56strong RPE-1 cells at different time points after the start of serum starvation. Unpaired t-test, bars and error bars represent mean and stdev of three biological replicate wells. **** p<0.0001, *** p<0.0005, ** p<0.005, * p<0.05.