Pre- and postsynaptic MEF2C promote experience-dependent, input-specific development of Layer 4 to Layer 2/3 excitatory synapses

Experience and activity-dependent transcription is a candidate mechanism to mediate development and refinement of specific cortical circuits. Here we provide evidence that the activity-dependent transcription factor Myocyte-Enhancer Factor 2C (MEF2C) is required in both presynaptic layer 4 (L4) and postsynaptic L2/3 somatosensory (S1) cortical neurons for development of their synaptic connection. In contrast, synaptic inputs from local L2/3, contralateral S1, or ipsilateral frontal/motor cortex are unaffected by postsynaptic Mef2c deletion in L2/3 neurons. Postsynaptic MEF2C is required for L4 to L2/3 synapse function during, but not after, an early postnatal, experience-dependent period. Furthermore, homozygous and heterozygous Mef2c deletion in presynaptic L4 neurons weakens L4 to L2/3 excitatory synaptic inputs by decreasing presynaptic release probability. Our results suggested that experience or activity-dependent transcriptional activation of MEF2C promotes development of L4→L2/3 synapses. In support of this idea, expression of transcriptionally active MEF2C (MEF2C-VP16) rescued weak L4 to L2/3 synaptic strength in sensory deprived mice. Consistent with a presynaptic function for MEF2C, we find that MEF2C regulates the activity-dependent expression of many presynaptic assembly genes, including transsynaptic cell adhesion proteins and regulators of neurotransmitter release. This work provides mechanistic insight into the experience-dependent development of specific cortical circuits. Organotypic slice cultures were prepared from P6 pups that were either: CaMKii-α Cre +/-; Mef2c fl/fl or Mef2c fl/fl. These cultures were grown for 8 days in-vitro (DIV) and then treated with either: 1% DMSO and 1% water or 100 μM Picrotoxin and 100 μM 4-aminopyridine. Then RNA was made and turned into 150 basepair paired end cDNA libraries and sequenced on a NextSeq2000 to give ~80-100 million reads per sample