PHLPP1 Gene Deletion Protects the Brain from Ischemic Injury

Abstract

A recently discovered protein phosphatase PHLPP (PH domain Leucine-rich repeat Protein Phosphatase) has been shown to dephosphorylate Akt on its hydrophobic motif (Ser473) thereby decreasing Akt kinase activity. We generated PHLPP1 knockout (KO) mice and used them to explore the ability of enhanced in vivo Akt signaling to protect the brain against ischemic insult. Brains from KO mice subjected to middle cerebral artery occlusion (MCAO) for 2 hours showed significantly greater increases in Akt activity and less neurovascular damage after reperfusion than wild-type (WT) mice. Remarkably, infarct volume in the PHLPP1 KO was significantly reduced compared with WT (12.7±2.7% versus 22.9±3.1%) and this was prevented by Akt inhibition. Astrocytes from KO mice and neurons in which PHLPP1 was downregulated showed enhanced Akt activation and diminished cell death in response to oxygen-glucose deprivation. Thus, deletion of PHLPP1 can enhance Akt activation in neurons and astrocytes, and can significantly increase cell survival and diminish infarct size after MCAO. Inhibition of PHLPP could be a therapeutic approach to minimize damage after focal ischemia.

Pharmacological Inhibition of Pleckstrin Homology Domain Leucine-Rich Repeat Protein Phosphatase Is Neuroprotective: Differential Effects on Astrocytes

“…PHLPP1 KD increased AKT but not ERK activation in astrocytes under serum growth conditions (i.e., 10% FBS). This is consistent with work of Chen et al (2013), who showed that astrocytes isolated from PHLPP1 KO mice had higher AKT but not ERK phosphorylation after acute IGF-1 stimulation.…”

“…However, ERK activation may not be detrimental in all brain injury paradigms. PHLPP1 KO mice were recently shown to have reduced brain infarct volumes after stroke injury (Chen et al, 2013). Administering an AKT inhibitor before ischemia completely reversed the protective KO phenotype.…”

“…In addition, PHLPP1 (2/2) KO mice had elevated AKT and were protected from experimental stroke induced by middle cerebral artery occlusion. Pretreatment with an AKT inhibitor completely prevented the protective phenotype (Chen et al, 2013).…”

“…Recent studies confirm that PHLPP1 promotes CNS injury by inhibiting AKT. PHLPP1 KD in HT22 cells (an immortalized hippocampal neuron-derived cell line) activated AKT and protected against oxygen-glucose deprivation injury (Chen et al, 2013). In addition, PHLPP1 (2/2) KO mice had elevated AKT and were protected from experimental stroke induced by middle cerebral artery occlusion.…”

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

“…PHLPP1 exists in neurons in two isoforms, PHLPP1α and PHLPP1β, which are synthesized as splice variants of the same gene and share amino acid sequence homology but differ in size (140 kDa and 190 kDa, respectively). The PHLPP1α isoform dephosphorylates Akt in neurons and its downregulation is linked with a reduced infarct volume in brain ischemia, as found in a transgenic animal model lacking PHLPP1 subjected to MCAO (Chen et al, 2013). On the other hand, PHLPP1β binds and traps the small GTP-binding protein Ras in its inactive form, inhibiting the phosphorylation of ERK and its downstream signaling activity ( Figure 2) (Shimizu et al, 2007;Wang et al, 2013;Wang et al, 2014b).…”