FMRP-dependent Mdm2 dephosphorylation is required for MEF2-induced synapse elimination

Area of Science:

• Neuroscience
• Molecular Biology
• Developmental Biology

Background:

• Myocyte Enhancer Factor 2 (MEF2) transcription factors regulate excitatory synapse number.
• MEF2 promotes the degradation of postsynaptic density protein 95 (PSD-95), a key synaptic scaffold protein.
• This MEF2-mediated synapse elimination process is impaired in the mouse model of Fragile X Syndrome (Fmr1 KO).

Purpose of the Study:

• To elucidate the molecular mechanisms by which MEF2 activation leads to PSD-95 degradation.
• To understand why this process is defective in Fragile X Syndrome (FXS) neurons.
• To identify the role of Mdm2 phosphorylation and its interaction with EF1α in FXS.

Main Methods:

• Investigated the role of Protein phosphatase 2A (PP2A) in Mdm2 dephosphorylation.
• Analyzed Mdm2 localization and PSD-95 degradation in wild-type and Fmr1 KO neurons.
• Examined the interaction between Mdm2 and Eukaryotic Elongation Factor 1α (EF1α).
• Utilized a dephosphomimetic Mdm2 mutant to rescue FXS phenotypes.

Main Results:

• MEF2 induces PP2A-mediated dephosphorylation of Mdm2, leading to its nuclear export and PSD-95 degradation.
• In Fmr1 KO neurons, Mdm2 is hyperphosphorylated and remains in the nucleus, preventing PSD-95 degradation.
• Elevated EF1α levels in Fmr1 KO neurons enhance Mdm2 interaction, contributing to Mdm2 hyperphosphorylation.
• Expressing a dephosphomimetic Mdm2 mutant restored PSD-95 ubiquitination, degradation, and synapse elimination in Fmr1 KO neurons.

Conclusions:

• MEF2-mediated Mdm2 dephosphorylation is a critical mechanism for synapse elimination.
• Dysregulation of Mdm2 phosphorylation and its interaction with EF1α contribute to synaptic deficits in Fragile X Syndrome.
• Targeting Mdm2 phosphorylation offers a potential therapeutic strategy for FXS and other neurodevelopmental disorders.