Conditional RAR alpha knockout mice reveal acute requirement for retinoic acid and RAR alpha in homeostatic plasticity

All-trans retinoic acid (RA) plays important roles in brain development through regulating gene transcription. Recently, a novel post-developmental role of RA in mature brain was proposed. Specifically, RA rapidly enhanced excitatory synaptic transmission independent of transcriptional regulation. RA synthesis was induced when excitatory synaptic transmission was chronically blocked, and RA then activated dendritic protein synthesis and synaptic insertion of homomeric GluAl AMPA receptors, thereby compensating for the loss of neuronal activity in a homeostatic fashion. This action of RA was suggested to be mediated by its canonical receptor RAR alpha but no genetic evidence was available. Thus, we here tested the fundamental requirement of RAR alpha in homeostatic plasticity using conditional RAFict knockout (KO) mice, and additionally performed a structure-function analysis of RAR alpha. We show that acutely deleting RAR alpha in neurons eliminated RAs effect on excitatory synaptic transmission, and inhibited activity blockade induced homeostatic synaptic plasticity. By expressing various RAR alpha rescue constructs in RAR alpha KO neurons, we found that the DNA-binding domain of RAR alpha was dispensable for its role in regulating synaptic strength, further supporting the notion that RA and RAR alpha act in a non-transcriptional manner in this context. By contrast, the ligand-binding domain (LBD) and the mRNA-binding domain (F domain) are both necessary and sufficient for the function of RAR alpha in homeostatic plasticity. Furthermore, we found that homeostatic regulation performed by the LBD/F-domains leads to insertion of calcium-permeable AMPA receptors. Our results confirm with unequivocal genetic approaches that RA and RAFiu perform essential non transcriptional functions in regulating synaptic strength, and establish a functional link between the various domains of RAR alpha and their involvement in regulating protein synthesis and excitatory synaptic transmission during homeostatic plasticity.