Having established that APP deficiency results in reduced dendritic spine numbers both in vitro and in vivo, we next sought to establish whether this is a consequence of a lack of sAPP, which is shed following α- or β-secretase cleavage. A number of studies have shown that sAPPα exhibits neurotrophic properties (Ring et al., 2007; Weyer et al., 2011; Zheng and Koo, 2011) but whether the primary function of APP requires the transmembrane or cytoplasmic domain is debated (Barbagallo et al., 2011; Lee et al., 2010; Li et al., 2010). To test whether lack of sAPP is responsible for spine loss in APP−/− neurons in culture, conditioned medium from wild type neurons (APP WT-CM) was added to APP−/− primary hippocampal neurons starting at DIV 2 (with repeated addition every other day). As before, the neurons were transfected with eGFP at DIV 16-21 and spine density was assessed. In cultures with wild type neuron conditioned medium, the deficit in spine density seen in APP−/− neurons was essentially restored to normal levels (Fig. 5). In addition, the number of branch points in
