β-Secretase (BACE1) is the rate-limiting protease for the generation of the amyloid β-peptide (Aβ) in Alzheimer disease. Mice in which the bace1 gene is inactivated are reported to be healthy. However, the presence of a homologous gene encoding BACE2 raises the possibility of compensatory mechanisms. Therefore, we have generated bace1, bace2, and double knockout mice. We report here that BACE1 mice display a complex phenotype. A variable but significant number of BACE1 offspring died in the first weeks after birth. The surviving mice remained smaller than their littermate controls and presented a hyperactive behavior. Electrophysiologically, subtle alterations in the steady-state inactivation of voltage-gated sodium channels in BACE1-deficient neurons were observed. In contrast, bace2 knockout mice displayed an overall healthy phenotype. However, a combined deficiency of BACE2 and BACE1 enhanced the bace1^–/– lethality phenotype. At the biochemical level, we have confirmed that BACE1 deficiency results in an almost complete block of Aβ generation in neurons, but not in glia. As glia are 10 times more abundant in brain compared with neurons, our data indicate that BACE2 could indeed contribute to Aβ generation in the brains of Alzheimer disease and, in particular, Down syndrome patients. In conclusion, our data challenge the general idea of BACE1 as a safe drug target and call for some caution when claiming that no major side effects should be expected from blocking BACE1 activity.

We investigated whether peroxisome proliferator-activated receptor γ (PPARγ) could be involved in the modulation of the amyloid cascade causing Alzheimer's disease. Inducing expression or activating PPARγ using synthetic agonists of the thiazolinedione family results in a dramatic decrease in the levels of the amyloid-β (Aβ) peptide in the conditioned medium of neuronal and non-neuronal cells. PPARγ does not affect expression or activity of any of the secretases involved in the generation of the Aβ peptide but induces a fast, cell-bound clearing mechanism responsible for the removal of the Aβ peptide from the medium. Although PPARγ expression is generally low in the CNS, induction of PPARγ expression during inflammation could be beneficial for inducing Aβ clearance. We confirm that the Aβ clearance mechanism can indeed be induced by PPARγ activation in primary murine-mixed glia and cortical neuronal cultures. Our results suggest that PPARγ-controlled mechanisms should be explored further as potential drug targets for Alzheimer's disease treatment.