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ERK1/2 antagonizes glycogen synthase kinase-3beta-induced apoptosis in cortical neurons
Michal Hetman1, Shih-Ling Hsuan, Agata Habas
1Departments of Environmental Health and Pharmacology, University of Washington, Seattle, WA 98195-7234, USA.
Abstract:
Inhibition of glycogen synthase kinase-3beta (GSK3beta) is one of the mechanisms by which phosphatidylinositol 3-kinase (PI3K) activation protects neurons from apoptosis. Here, we report that inhibition of ERK1/2 increased the basal activity of GSK3beta in cortical neurons and that both ERK1/2 and PI3K were required for brain-derived neurotrophic factor (BDNF) suppression of GSK3beta activity. Moreover, cortical neuron apoptosis induced by expression of recombinant GSK3beta was inhibited by coexpression of constitutively active MKK1 or PI3K. Activation of both endogenous ERK1/2 and PI3K signaling pathways was required for BDNF to block apoptosis induced by expression of recombinant GSK3beta. Furthermore, cortical neuron apoptosis induced by LY294002-mediated activation of endogenous GSK3beta was blocked by expression of constitutively active MKK1 or by BDNF via stimulation of the endogenous ERK1/2 pathway. Although both PI3K and ERK1/2 inhibited GSK3beta activity, neither had an effect on GSK3beta phosphorylation at Tyr-216. Interestingly, PI3K (but not ERK1/2) induced the inhibitory phosphorylation of GSK3beta at Ser-9. Significantly, coexpression of constitutively active MKK1 (but not PI3K) still suppressed neuronal apoptosis induced by expression of the GSK3beta(S9A) mutant. These data suggest that activation of the ERK1/2 signaling pathway protects neurons from GSK3beta-induced apoptosis and that inhibition of GSK3beta may be a common target by which ERK1/2 and PI3K protect neurons from apoptosis. Furthermore, ERK1/2 inhibits GSK3beta activity via a novel mechanism that is independent of Ser-9 phosphorylation and likely does not involve Tyr-216 phosphorylation.
Insights
Extracellular signal-regulated kinase 1/2 (ERK1/2) activation protects neurons from apoptosis by inhibiting glycogen synthase kinase-3beta (GSK3beta) via a novel mechanism, independent of Ser-9 phosphorylation.
Area of Science:
- Neuroscience
- Cell Biology
- Molecular Biology
Background:
- Glycogen synthase kinase-3beta (GSK3beta) inhibition is a known neuroprotective mechanism mediated by phosphatidylinositol 3-kinase (PI3K).
- The role of extracellular signal-regulated kinase 1/2 (ERK1/2) in regulating GSK3beta activity and neuronal apoptosis is less understood.
Purpose of the Study:
- To investigate the interplay between ERK1/2, PI3K, and GSK3beta in neuronal apoptosis.
- To elucidate the specific mechanisms by which ERK1/2 and PI3K signaling pathways modulate GSK3beta activity and confer neuroprotection.
Main Methods:
- Utilized primary cortical neurons.
- Manipulated signaling pathways using recombinant proteins, constitutively active mutants (MKK1, PI3K), and specific inhibitors (LY294002).
- Assessed neuronal apoptosis, GSK3beta activity, and phosphorylation states (Ser-9, Tyr-216).
Main Results:
- ERK1/2 inhibition increased basal GSK3beta activity.
- Both ERK1/2 and PI3K were required for brain-derived neurotrophic factor (BDNF)-mediated suppression of GSK3beta activity.
- ERK1/2 and PI3K activation protected neurons from GSK3beta-induced apoptosis.
- ERK1/2 inhibited GSK3beta activity independently of Ser-9 and Tyr-216 phosphorylation.
- PI3K, but not ERK1/2, induced inhibitory GSK3beta phosphorylation at Ser-9.
- Constitutively active MKK1, but not PI3K, suppressed apoptosis induced by a GSK3beta(S9A) mutant.
Conclusions:
- ERK1/2 signaling pathway activation protects neurons from GSK3beta-induced apoptosis.
- GSK3beta inhibition is a common downstream target for both ERK1/2 and PI3K neuroprotective mechanisms.
- ERK1/2 exerts neuroprotection through a novel mechanism involving GSK3beta inhibition, independent of Ser-9 phosphorylation.