Upregulation Of Mu 3a Drives Homeostatic Plasticity By Rerouting Ampar Into The Recycling Endosomal Pathway

Synaptic scaling is a form of homeostatic plasticity driven by transcription-dependent changes in AMPA-type glutamate receptor (AMPAR) trafficking. To uncover the pathways involved, we performed a cell-type-specific screen for transcripts persistently altered during scaling, which identified the mu subunit (mu 3A) of the adaptor protein complex AP-3A. Synaptic scaling increased mu 3A (but not other AP-3 subunits) in pyramidal neurons and redistributed dendritic mu 3A and AMPAR to recycling endosomes (REs). Knockdown of mu 3A prevented synaptic scaling and this redistribution, while overexpression (OE) of full-length m3A or a truncated m3A that cannot interact with the AP-3A complex was sufficient to drive AMPAR to REs. Finally, OE of mu 3A acted synergistically with GRIP1 to recruit AMPAR to the dendritic membrane. These data suggest that excess mu 3A acts independently of the AP-3A complex to reroute AMPAR to RE, generating a reservoir of receptors essential for the regulated recruitment to the synaptic membrane during scaling up.