Maherali et al., 2007; Meissner et al., 2008). Furthermore, the numerous cell divisions required for the reprogramming process and differentiation may dilute any accumulated macromolecular damage. The direct transcription factor-based conversion of fibroblasts into induced neurons (iNs) represents an alternative avenue for generating human neurons in vitro (Pang et al., 2011; Vierbuchen et al., 2010). Induction of only two transcription factors in combination with a cocktail of small molecular enhancers was shown to directly yield functional iNs from human fibroblasts with high efficiencies (Ladewig et al., 2012; Liu et al., 2013). As iNs circumvent the pluripotent state as well as any cell division, we hypothesize that direct conversion preserves the cellular signatures of aging and results in neurons that show age equivalence with their human donor. In this study, we set out to analyze primary cells from young and old human donors to identify the key factors relevant to human aging and to subsequently program these cells into iNs to generate an age-equivalent in vitro model for neuronal cell aging.
