Targeting WNT, protein kinase B, and mitochondrial membrane integrity to foster cellular survival in the nervous system

Abstract

Targeting essential cellular pathways that determine neuronal and vascular survival can foster a successful therapeutic platform for the treatment of a wide variety of degenerative disorders in the central nervous system. In particular, oxidative cellular injury can precipitate several nervous system disorders that may either be acute in nature, such as during cerebral ischemia, or more progressive and chronic, such as during Alzheimer disease. Apoptotic injury in the brain proceeds through two distinct pathways that ultimately result in the early externalization of membrane phosphatidylserine (PS) residues and the late induction of genomic DNA fragmentation. Degradation of DNA may acutely impact cellular survival, while the exposure of membrane PS residues can lead to microglial phagocytosis of viable cells, cellular inflammation, and thrombosis in the vascular system. Through either independent or common pathways, the Wingless/Wnt pathway and the serine-threonine kinase Akt serve central roles in the maintenance of cellular integrity and the prevention of the phagocytic disposal of cells "tagged" by PS exposure. By selectively governing the activity of specific downstream substrates that include GSK-3beta, Bad, and beta-catenin, Wnt and Akt serve to foster neuronal and vascular survival and block the induction of programmed cell death. Novel to Akt is its capacity to protect cells from phagocytosis through the direct modulation of membrane PS exposure. Intimately linked to the activation of Wnt signaling and Akt is the maintenance of mitochondrial membrane potential and the regulation of Bcl-xL, mitochondrial energy metabolism, and cytochrome c release that can lead to specific cysteine protease activation.

Fig. 1

Wnt-1 maintains genomic DNA integrity and membrane phosphatidylserine (PS) asymmetry during oxygen-glucose deprivation (OGD). Representative images illustrate DNA fragmentation with terminal deoxynucleotidyl transferase nick end labeling (TUNEL) and phosphatidylserine (PS) exposure with annexin V phycoerythrin labeling in wildtype (Wt/OGD) and Wnt-1 transfected (Wnt-1/OGD) hippocampal neurons 24 hours following exposure to OGD. OGD was performed by replacing media with glucose-free HBSS containing 116 mM NaCl, 5.4 mM KCl, 0.8 mM MgSO₄, 1mM NaH₂PO₄, 0.9 mM CaCl₂, and 10 mg/L phenol red (pH 7.4) and cultures were maintained in an anoxic environment (95% N₂ and 5% CO₂) at 37 °C for 3 hours. OGD induced DNA fragmentation and membrane PS exposure in wildtype cells (Wt/NO) while there was no injury present in Wnt-1 transfected cells.

Fig. 2

Cellular modulators of degenerative disease in the central nervous system. Apoptotic injury in either neuronal or vascular cells proceeds through two independent pathways that ultimately result in the early externalization of membrane phosphatidylserine (PS) residues and the late induction of genomic DNA fragmentation. Externalization of membrane PS residues can precipitate microglial activation, the phagocytosis of injured cells, and thrombosis in the vascular system. Essential to the maintenance of genomic DNA integrity are the signaling pathways of the proto-oncogene Wnt and the serine-threonine kinase Akt. Through either distinct or common pathways, Wnt and Akt act upon downstream substrates, such as GSK-3β, Bad, and b-catenin to block the induction of programmed cell death. Unique to Akt is its ability to prevent phagocytosis of cells through the direct modulation of cellular membrane PS externalization. Closely tied to the protective capacities of both Wnt and Akt is the maintenance of ΔΨ_m and the central modulation of Bcl-x_L, mitochondrial energy reserves, and cytochrome c release that can lead to specific cysteine protease activation of caspase 1, 3, and 9.