A soluble form of DDR1 functions dominant negatively in neurite extension of granule cells cultured on pial cells. Granule cells were placed at a low density on a pial monolayer expressing either DDR1–Fc (A,C) or ssFc (B,D) and cultured for 36 hr. Pial cells infected with GFP viruses expressing recombinant Fc-fusion proteins were visualized by GFP epifluorescence, confirming that almost all pial cells express a uniform level of fusion proteins. After fixation, granule cells (red) were visualized by antibody staining using either Tuj-1 (A,B) or anti-TAG-1 antibody (C,D) with a Cy-3 conjugated secondary antibody. Fewer granule cells extended neurites in the presence of DDR1–Fc, as opposed to in the presence of ssFc. Arrowheads indicate granule cells positive for antibody staining. Expression of recombinant Fc-fusion proteins in the pial culture supernatant was confirmed by immunoblotting using an anti-human IgG1–Fc antibody (E). DDR1–Fc migrated at 98 kD, and the ssFc migrated at 27 kD. (F) The percentage of Tuj-1-positive granule cells with neurites >20 μm in length, placed on the pial monolayer expressing either DDR1–Fc (5 ± 2%) or ssFc (70 ± 6%), was scored. Scale bars, 10 μm in A,B and 20 μm in C,D.