Critical Role for Akt1 in the Modulation of Apoptotic Phosphatidylserine Exposure and Microglial Activation

Biological targets for neurodegenerative disease that focus on the intrinsic maintenance of cellular integrity and the extrinsic prevention of phagocytic cellular disposal offer the greatest promise for therapeutic intervention. Protein kinase B (Akt1), a serine-threonine kinase closely involved in cell growth and survival, offers a strong potential to address both intrinsic and extrinsic mechanisms of neuronal injury. We demonstrate that overexpression of a constitutively active form of Akt1 (myristoylated Akt1) in differentiated SH-SY5Y neuronal cells provides intrinsic cellular protection against apoptotic genomic DNA destruction and membrane phosphatidylserine (PS) exposure. Transfection of SH-SY5Y cells with a plasmid encoding a kinase-deficient dominant-negative Akt1 eliminates cytoprotection, suggesting that activation of Akt1 is necessary and sufficient to prevent apoptotic destruction. Apoptotic neuronal membrane PS exposure provides a unique pathway for Akt1 to offer extrinsic cellular protection and block microglial activation, because independent cotreatment with an anti-PS receptor neutralizing antibody could also prevent microglial proliferation. Akt1 maintains nuclear DNA integrity and membrane PS exposure through the specific inhibition of caspase 3-, 8-, and 9-like activities that were linked to mitochondrial membrane potential and cytochrome c release. Our work elucidates a novel capacity for Akt1 to maintain cellular integrity through a series of cysteine protease pathways and to uniquely regulate microglial activation through the modulation of membrane PS residue externalization.