The intracellular domain of the epilepsy protein PCDH19 regulates spine density in cortical neurons in vivo via Xlr genes.

Mosaic mutations in the X-linked cell adhesion molecule Protocadherin 19 (PCDH19) lead to epilepsy with cognitive impairment, whereas complete absence of functional protein does not elicit any symptoms. It is believed that mosaic expression of PCDH19 leads to defective neuronal communication, but whether further roles beyond cell adhesion are critical for PCDH19 function in the cortex is currently unknown. We confirm that the proteolytic processing of PCDH19, previously described in hippocampal neurons, also takes place in cortical neurons in vivo and show that nuclear transport of its intracellular domain is mediated by importins. RNAseq analysis further indicates that the intracellular domain of PCDH19 leads to broad transcriptomic changes. Finally, we use in utero electroporation to provide the first in vivo data about the role of this cleaved intracellular domain in upper layer cortical neurons, where it reduces spine density through an increase in Xlr gene expression without affecting overall dendritic morphology. Our results suggest that PCDH19 could act as an activity sensor in a synapse to nucleus signalling pathway involved in synaptic homeostasis.