Activity-induced MeCP2 phosphorylation regulates retinogeniculate synapse refinement [RNA-Seq]
Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelop- mental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MeCP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here, we show that MeCP2 is phosphorylated at four residues in the brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from the synapse refinement defect previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period. Overall design: Bulk nucleus RNA-sequencing was performed from WT and MeCP2 QKI littermates at P15 (n=6), P16 (n=10), and P30 (n=6). Visual cortex and dLGN were dissected in either ACSF (P15, P16) or potassium gluconate electrophyisology solution (P30). Bulk RNA-seq was performed from visual cortex of WT and MeCP2 T308A littermates that were housed in the dark from 6 to 8 weeks of age, then exposed to light for 0 or 6 hours (WT 0hr n=3, T308A 0hr n=3, WT 6hr n=5, T308A 6hr n=10). Bulk RNA-seq was performed from dLGN and visual cortex of WT and MeCP2 QKI littermates that were housed in the dark from P20 to P30, then exposed to light for 0 to 4 hours (n=4).