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A myocyte enhancer factor 2D (MEF2D) kinase activated during neuronal apoptosis is a novel target inhibited by
Daniel A Linseman1, Brandon J Cornejo, Shoshona S Le
1Department of Pharmacology, University of Colorado Health Sciences Center and the Denver Veterans Affairs Medical Center, Denver, Colorado 80262, USA.
Abstract:
Depolarization promotes the survival of cerebellar granule neurons via activation of the transcription factor myocyte enhancer factor 2D (MEF2D). Removal of depolarization induces hyperphosphorylation of MEF2D on serine/threonine residues, resulting in its decreased DNA binding and susceptibility to caspases. The subsequent loss of MEF2-dependent gene transcription contributes to the apoptosis of granule neurons. The kinase(s) that phosphorylates MEF2D during apoptosis is currently unknown. The serine/threonine kinase, glycogen synthase kinase-3 beta (GSK-3 beta), plays a pro-apoptotic role in granule neurons. To investigate a potential role for GSK-3 beta in MEF2D phosphorylation, we examined the effects of lithium, a non-competitive inhibitor of GSK-3 beta, on MEF2D activity in cultured cerebellar granule neurons. Lithium inhibited caspase-3 activation and chromatin condensation in granule neurons induced to undergo apoptosis by removal of depolarizing potassium and serum. Concurrently, lithium suppressed the hyperphosphorylation and caspase-mediated degradation of MEF2D. Moreover, lithium sustained MEF2 DNA binding and transcriptional activity in the absence of depolarization. Lithium also attenuated MEF2D hyperphosphorylation and apoptosis induced by calcineurin inhibition under depolarizing conditions, a GSK-3 beta-independent model of neuronal death. In contrast to lithium, MEF2D hyperphosphorylation was not inhibited by forskolin, insulin-like growth factor-I, or valproate, three mechanistically distinct inhibitors of GSK-3 beta. These results demonstrate that the kinase that phosphorylates and inhibits the pro-survival function of MEF2D in cerebellar granule neurons is a novel lithium target distinct from GSK-3 beta.
Insights
This study reveals a novel kinase targeting myocyte enhancer factor 2D (MEF2D) in cerebellar granule neurons. This kinase, distinct from GSK-3 beta, is inhibited by lithium, protecting neurons from apoptosis.
Area of Science:
- Neuroscience
- Molecular Biology
- Cell Death Research
Background:
- Cerebellar granule neurons (CGNs) survival is promoted by depolarization via myocyte enhancer factor 2D (MEF2D) activation.
- Apoptosis in CGNs is linked to MEF2D hyperphosphorylation, reduced DNA binding, and caspase degradation upon depolarization removal.
- The specific kinase responsible for MEF2D phosphorylation during apoptosis remains unidentified.
Purpose of the Study:
- To investigate the role of glycogen synthase kinase-3 beta (GSK-3 beta) in MEF2D phosphorylation during CGN apoptosis.
- To identify the kinase that phosphorylates and inhibits MEF2D's pro-survival function.
Main Methods:
- Cultured cerebellar granule neurons were treated with lithium, a GSK-3 beta inhibitor.
- Apoptosis markers (caspase-3 activation, chromatin condensation) were assessed.
- MEF2D phosphorylation, DNA binding, and transcriptional activity were analyzed.
- Experiments utilized both depolarization-induced and calcineurin inhibition-induced apoptosis models.
Main Results:
- Lithium treatment inhibited apoptosis markers and suppressed MEF2D hyperphosphorylation and degradation.
- Lithium sustained MEF2D DNA binding and transcriptional activity without depolarization.
- Lithium attenuated MEF2D hyperphosphorylation and apoptosis in a GSK-3 beta-independent model.
- Other GSK-3 beta inhibitors (forskolin, IGF-I, valproate) did not inhibit MEF2D hyperphosphorylation.
Conclusions:
- A novel kinase, distinct from GSK-3 beta, phosphorylates and inhibits MEF2D in CGNs.
- This novel kinase represents a potential therapeutic target for neuroprotection.