Analysis of fat3KO retinas at P3 revealed striking changes in AC morphology. In WT retina, calretinin immunolabeling marks several classes of unipolar ACs that extend processes into the nascent IPL ( Figure 2A). Yet in fat3KOs, 23% (+/−8%) of these cells have two neurites: a normal process projecting into the IPL and a second process extending into the INL ( Figures 2B and 2C). Unaffected cells are likely starburst cells that also express ChAT ( Gábriel and Witkovsky, 1998) and appear normal in the fat3KO retina ( Figures 5I–5J). Changes in neurite number are also evident in thy1::YFP-H transgenic mice ( Feng et al., 2000), which

express YFP in subsets of isolated retinal cells at these early postnatal stages, allowing independent confirmation that ACs extend ectopic neurites toward the outer retina in fat3KOs ( Figures 2D and 2E). To determine this website the origin of the extra neurites, we examined

Ptf1a-cre;Z/EG–labeled ACs during migration in fat3KO retinas. BIBW2992 molecular weight Initially, mutant ACs transform from multipolar to bipolar with clear leading and trailing processes, similar to controls (Figures 1B and 2F). In contrast, mutant neurons frequently maintain two processes after reaching the IPL ( Figure 2G). Most WT ACs are unipolar at this stage, although 14% extend a short trailing process that is likely in the process of retraction (mean = 5.5 ± 0.5 μm; n = 38 cells). In contrast, 35% of mutant neurons retain a trailing process. The length of these extra neurites is much longer than PAK6 in controls (mean = 24.5 ± 0.5 μm; n = 58 cells) and is similar to the length of the trailing process in migrating WT neurons at this stage (mean = 35.3 ± 1.7 μm; n = 50 cells). The simplest explanation for these observations is that fat3KO ACs develop abnormal shapes because of a failure to retract the trailing process upon reaching the IPL. During normal development, most ACs retain a single neurite that develops as a primary

dendrite and arborizes in the IPL. However, it is not known whether this neurite becomes a dendrite by default or if additional cues are involved. Therefore, we asked whether retention of an extra neurite in fat3KOs is sufficient to promote dendrite development by examining different classes of ACs in the mature fat3KO retina. We found that multiple types of ACs develop extra dendrites that project away from the IPL and stratify in a single layer dividing the INL, as visualized by staining for calretinin ( Figure 3B). Furthermore, although GFP-positive ACs in Ptf1a-cre;Z/EG retinas are unipolar ( Figure 3C), extra dendrites extend away from mutant cells and into the INL (arrow, Figure 3D). Ectopic arborization is easiest to appreciate in the dopaminergic (TH-positive) ACs, which extend multiple processes away from the cell soma, through the INL and toward the OPL.