A Positive Feedback Loop between Glycogen Synthase Kinase 3β and Protein Phosphatase 1 after Stimulation of NR2B NMDA Receptors in Forebrain Neurons

Abstract

N-methyl-D-aspartate receptors (NMDARs) are critical for neuronal plasticity and survival, whereas their excessive activation produces excitotoxicity and may accelerate neurodegeneration. Here, we report that stimulation of NMDARs in cultured rat hippocampal or cortical neurons and in the adult mouse brain in vivo disinhibited glycogen synthase kinase 3␤ (GSK3␤) by protein phosphatase 1(PP1)-mediated dephosphorylation of GSK3␤ at the serine 9 residue. NMDA-triggered GSK3␤ activation was mediated by NMDAR that contained the NR2B subunit. Interestingly, GSK3␤ inhibition reduced inhibitory phosphorylation of the PP1 inhibitor 2 (I2) and attenuated serine 9 dephosphorylation by PP1. These data suggest existence of a feedback loop between GSK3␤ and PP1 that results in amplification of PP1 activation by GSK3␤. In addition, GSK3␤ inhibition decreased PP1-mediated dephosphorylation of the cAMPresponse element-binding protein (CREB) at the serine 133 residue in NMDA-stimulated neurons. Conversely, overexpression of GSK3␤ abolished non-NR2B-mediated activation of CRE-driven transcription. These data suggest that cross-talk between GSK3␤ and PP1 contributes to NR2B NMDAR-induced inhibition of CREB signaling by non-NR2B NMDAR. The excessive activation of NR2B-PP1-GSK3␤-PP1 circuitry may contribute to the deficits of CREBdependent neuronal plasticity in neurodegenerative diseases.

The protein phosphatase-1/inhibitor-2 complex differentially regulates GSK3 dephosphorylation and increases sarcoplasmic/endoplasmic reticulum calcium ATPase 2 levels

“…In this case, the phosphorylation and inhibition of I-2 by GSK3 results in increased PP1 activity and decreased Ser21/9 phosphorylation of GSK3. Szatmari et al [41] reported that NMDA treatment of cortical and hippocampal neurons caused the dephosphorylation of Ser9 on GSK3β via the activation of PP1 [41] which was blocked by lithium treatment. Furthermore, they reported that inhibition of GSK3β by lithium reduced I-2 phosphorylation and increased GSK3β Ser9 phosphorylation.…”

“…The regulation of Ser21/9 phosphorylation has been attributed to the actions of protein phosphatase-2A (PP2A) [40,43,27,38] and protein phosphatase-1 (PP1) [46,34,41]. The PP1 holoenzyme is comprised of a 37 kDa catalytic subunit which associates with inhibitory subunits and targeting subunits.…”

“…SERCA2a and b are expressed in the brain, in skeletal and cardiac muscle as well as in all non-muscle tissues [45]. Interestingly, treatments that cause increases in intracellular [Ca 2+ ] can also increase the activity of GSK3β through the dephosphorylation of its inhibitory Ser9 site [38,27,41]. Active GSK3β can in turn regulate the storage of calcium in the ER.…”

Regulation and Function of Glycogen Synthase Kinase-3 Isoforms in Neuronal Survival

“…It has also been reported that serine phosphorylation of GSK-3 is mediated by protein kinases, including AKT, protein kinase A, protein kinase C, ERK1/2/p90 RSK , p70 S6K , and integrin-linked kinase (2-4), whereas protein phosphatases, including PP1 and PP2A, have been implicated in GSK-3 serine dephosphorylation (24,56,57). The autophosphorylation of GSK-3␤ Ser 9 elicited by GSK-3 inhibition or gene silencing is proposed to be due to the inhibition of PP1 activity and a positive feedback loop between GSK-3␤ and PP1 (24,58). Thus, a reduction in GSK-3 activity results in a decrease in Thr 72 phosphorylation of PP1 inhibitor-2 (and hence, activation of its inhibitory effect), causing suppression of PP1 activity and induction of GSK-3/PP1 inhibitor-2/PP1/GSK-3 autoregulatory circuitry.…”

Full Reversal of Alzheimer's Disease-Like Phenotype in a Mouse Model with Conditional Overexpression of Glycogen Synthase Kinase-3

“…GSK-3 activity has been shown to be modulated by phosphorylation, subcellular localization, protein complex formation, and priming phosphorylation of substrates. Furthermore, an amplification mechanism that affects both inhi-bition (Jope, 2003;Zhang et al, 2003) and activation (Szatmari et al, 2005) of GSK-3 has been reported. It is, therefore, possible that in vivo sustained inhibitions or activations might persist after cessation of the initial trigger.…”