Observations obtained in several transgenic mice expressing FAD-linked mutant PS1 suggest that alterations in neurogenesis in these mice are apparent only following introduction of an external neurogenic stimulus, such as enriched environment. Thus, Choi and colleagues (2008) as well as Feng and colleagues (2001) report impairments in enriched environment-induced neurogenesis, but no impairment without stimulus (Feng et al., 2001; Choi et al., 2008). While Feng and colleagues used a conditional PS1 KO, Choi and colleagues used transgenic mice harboring mutant PS1. These results may suggest that either PS1 plays a role in regulation of the neurogenic response to enriched environment or that without stimulus, the effect of the mutation on neurogenesis is milder. Transgenic mice expressing FAD-linked PS1 P117L that were maintained in either standard housing or environmentally enriched conditions exhibit compromised survival of new neurons. In PS1 P117L mice maintained in standard housing conditions, survival of new astrocytes is compromised as well (Wen et al., 2004). Using PS1M146V knockin mice Wang and colleagues show that the number of newly formed cells and neurons in the hippocampus of these mice
