Reintroduction of Psen1 into Psen1-/- fibroblasts. Psen1-/- fibroblasts were infected with lentiviruses carrying the human Psen1 cDNA (hPS1-LV) for 24 hours and grown in complete medium for 48 hours. The infected cells were then treated for 4 hours with 100 μM cobalt chloride along with non-infected Psen1-/- cells grown in parallel. The cells were lysed and analyzed for HIF-1α induction by Western blot using anti HIF-1α polyclonal antibodies. Blots were reprobed for anti β-tubulin as a loading control. Psen1 expression was analyzed using the NT.1 mouse monoclonal antibody. FL: full length, NTF: N-terminal fragment. A representative blot is shown in panel A. Panel B shows quantitation of the HIF-1α response (n = 3 samples per group non-infected; n = 4 infected). P values (unpaired t-tests) are indicated for selected comparisons which are discussed further in the text.