Neurotoxin-induced selective ubiquitination and regulation of MEF2A isoform in neuronal stress response

The myocyte enhancer factor 2A-D (MEF2) proteins are members of the MCM1-agamous-deficiens-serum response factor family of transcription factors. Various MEF2 isoform proteins are enriched in neurons and exhibit distinct patterns of expression in different regions of the brain. In neurons, MEF2 functions as a converging factor to regulate many neuronal functions including survival. MEF2 activities are tightly controlled in neurons in response to stress. Whether stress signal may differentially regulate MEF2s remains largely unknown. In this work, we showed that MEF2A, but not MEF2C or MEF2D, was modified by ubiquitination in dopaminergic neuronal cell line SN4741 cells. MEF2A was ubiquitinated at its N'-terminus, and the ubiquitination of MEF2A was first detectable in the nuclear compartment and later in the cytoplasm. Ubiquitination of MEF2A correlated with reduced DNA-binding activity and transcriptional activity. More importantly, disturbing the degradation of ubiquitinated MEF2A through proteasome pathway by neurotoxins known to induce Parkinson's disease features in model animals caused accumulation of ubiquitinated MEF2A, reduced MEF2 activity, and impaired cellular viability. Our work thus provides the first evidence to demonstrate an isoforms-specific regulation of MEF2s by ubiquitination-proteasome pathway in dopaminergic neuronal cell by neurotoxins, suggesting that stress signal and cellular context-dependent dysregulation of MEF2s may underlie the loss of neuronal viability.