Rapid remodeling of the actin cytoskeleton in the pre- and/or post-synaptic compartments is responsible for the regulation of neuronal plasticity,which is an important process for learning and memory. Cofilin1 plays an essential role in these processes and a dysregulation of its activity was associated with the cognitive decline observed during normal aging and Alzheimer's disease (AD). To understand the mechanism(s) regulating Cofilin1 activity we evaluated changes occurring with regard to Cofilin1 and its up-stream regulators Lim kinase-1 (LIMK1) and Slingshot phosphatase-1 (SSH1) in (i) human AD brain, (ii) 1-, 4-, and 10-months old APP/PS1 mice, (iii) wildtype 3-, 8-, 12-, 18- and 26-months old mice, as well as in cellular models including (iv) mouse primary cortical neurons (PCNs, cultured for 5, 10, 15 and 20 days in vitro) and (v) mouse embryonic fibroblasts (MEF). Interestingly,we found an increased Cofilin1 phosphorylation/inactivation with age and AD pathology, both in vivo and in vitro. These changes were associated with a major inactivation of SSH1. Interestingly, inhibition of ã-secretase activity with Compound-E (10 ìM) prevented Cofilin1 phosphorylation/inactivation through an increase of SSH1 activity in PCNs. Similarly, MEF cells double knock-out for ã-secretase catalytic subunits presenilin-1 and -2(MEFDKO) showed a strong decrease of both Cofilin1 and SSH1 phosphorylation,which were rescued by the over expression of human ã-secretase. Together, these results shed new light in understanding the molecular mechanisms promoting Cofilin1 dysregulation, both during aging and AD. They further have the potential to impact the development of therapies to safely treat AD.