SMRT-mediated co-shuttling enables export of class IIa HDACs independent of their CaM kinase phosphorylation sites

摘要

The Class IIa histone deacetylases (HDAC)4 and HDAC5 play a role in neuronal survival and behavioral adaptation in the CNS. Phosphorylation at 2/3 N-terminal sites promote their nuclear export. We investigated whether non-canonical signaling routes to Class IIa HDAC export exist because of their association with the co-repressor Silencing Mediator Of Retinoic And Thyroid Hormone Receptors (SMRT). We found that, while HDAC5 and HDAC4 mutants lacking their N-terminal phosphorylation sites (HDAC4MUT, HDAC5 MUT) are constitutively nuclear, co-expression with SMRT renders them exportable by signals that trigger SMRT export, such as synaptic activity, HDAC inhibition, and Brain Derived Neurotrophic Factor (BDNF) signaling. We found that SMRT's repression domain 3 (RD3) is critical for co-shuttling of HDAC5 MUT, consistent with the role for this domain in Class IIa HDAC association. In the context of BDNF signaling, we found that HDAC5WT, which was more cytoplasmic than HDAC5MUT, accumulated in the nucleus after BDNF treatment. However, co-expression of SMRT blocked BDNF-induced HDAC5WT import in a RD3-dependent manner. In effect, SMRT-mediated HDAC5WT export was opposing the BDNF-induced HDAC5 nuclear accumulation observed in SMRT's absence. Thus, SMRT's presence may render Class IIa HDACs exportable by a wider range of signals than those which simply promote direct phosphorylation. Nuclear export of repressors is an important mechanism of transcriptional regulation. Class IIa HDACs are known to exit the nucleus in Ca2+ dependent manner by CaM Kinase-dependent phosphorylation (a). Here we uncover, in cortical neurons, a novel mechanism of class IIa HDAC nuclear export which is independent of CaM Kinase-dependent phosphorylation, mediated by co-shuttling with co-repressor SMRT. SMRT's class IIa HDAC-interacting domain RD3 is key for co-shuttling (b). We go on to show that this finding is physiologically relevant in the BDNF signaling (c). We find that BDNF promotes the direct nuclear import of HDAC5, but the nuclear export of SMRT. The effect of SMRT-mediated HDAC5 co-shuttling means that the presence of SMRT effectively prevents BDNF-induced direct HDAC5 import. (a) Phospho-mutant forms of class IIa HDACs cannot be exported by Ca2+ signals. (b) SMRT renders phospho-mutant forms of HDACs exportable by Ca2+ signals. (c) SMRT-mediated HDAC5 export opposes BDNF-induced HDAC5 nuclear accumulation.