Two-step activation of FOXO3 by AMPK generates a coherent feed-forward loop determining excitotoxic cell fate

Abstract

Cerebral ischemia and excitotoxic injury induce transient or permanent bioenergetic failure, and may result in neuronal apoptosis or necrosis. We have previously shown that ATP depletion and activation of AMP-activated protein kinase (AMPK) during excitotoxic injury induces neuronal apoptosis by transcription of the pro-apoptotic BH3-only protein, Bim. AMPK, however, also exerts pro-survival functions in neurons. The molecular switches that determine these differential outcomes are not well understood. Using an approach combining biochemistry, single-cell imaging and computational modeling, we here demonstrate that excitotoxic injury activated the bim promoter in a FOXO3-dependent manner. The activation of AMPK reduced AKT activation, and led to dephosphorylation and nuclear translocation of FOXO3. Subsequent mutation studies indicated that bim gene activation during excitotoxic injury required direct FOXO3 phosphorylation by AMPK in the nucleus as a second activation step. Inhibition of this phosphorylation prevented Bim expression and protected neurons against excitotoxic and oxygen/glucose deprivation-induced injury. Systems analysis and computational modeling revealed that these two activation steps defined a coherent feed-forward loop; a network motif capable of filtering any effects of short-term AMPK activation on bim gene induction. This may prevent unwanted AMPK-mediated Bim expression and apoptosis during transient or physiological bioenergetic stress. Excitotoxicity has been implicated in the pathogenesis of cerebral ischemia and several neurodegenerative disorders, 1-3 and results from abnormal levels of the excitatory neurotransmitter glutamate (GLUT) in the synaptic cleft. [4][5][6]

Metformin Alleviates Radiation-Induced Skin Fibrosis via the Downregulation of FOXO3

“…Numerous studies have also suggested that metformin inhibits cardiac and renal fibrosis by regulating the TGFβ signaling pathway, which is important in fibrosis [12,42]. FOXO3, which is regulated by the AMPK signaling pathway [43,44], has been found to be an important mediator of TGFβ gene expression [45]. Moreover, the PI3K-AKT signaling pathway negatively regulates the pro-apoptotic forkhead transcription factors (FOXO) [16,17,19].…”

Serine 574 phosphorylation alters transcriptional programming of FOXO3 by selectively enhancing apoptotic gene expression

“…Specific PTMs regulate the partitioning of FOXO3 between the cytosol and the nucleus [8][9][10][11][12][13][14][15] and its stability and degradation, 16 but the links between specific PTMs and FOXO3 transcriptional specificity are less well understood.…”

Role of the AMPK signaling pathway in early brain injury after subarachnoid hemorrhage in rats

“…Several upstream kinases have been implicated in the phosphorylation and activation of AMPK, including LKB1 and Ca 2+ /calmodulin-dependent kinase kinase β (CaMKKβ) [14]. The phosphorylation of AMPK by LKB1 is enhanced by AMP [15], and this may be an important pathway by which AMPK can be activated to maintain metabolic homeostasis. In particular, LKB1 appears to predominantly target the a2 isoform [13].…”

“…All motor function and reflexes as typical neurological manifestations of brain injury were evaluated using the neurological severity score (NSS). Neurological function was graded on a scale of 0-18 (normal score, 0; mild injury score, 1-6; moderate injury score, 7-12; severe injury score, [13][14][15][16][17][18]. The NSS consists of motor, sensory, reflex and balance tests.…”