Data from this study show that BDNF stimulates MEF2 transcriptional activity through a novel molecular mechanism. Thus, by inducing the expression, phosphorylation, and nuclear translocation of SIK1, BDNF phosphorylates HDAC5 and triggers its nuclear export resulting in enhanced MEF2-dependent transcription in cortical neurons.
Previous studies in different cell types have shown that the expression of SIK1 is upregulated by the cAMP/PKA signaling pathway. For instance, forskolin induces SIK1 expression in primary myocytes, C2C12 myoblasts and hypothalamic neurons 
. Other studies have reported that the β-adrenergic receptor agonist isoproterenol upregulates SIK1 expression in skeletal muscle and pinealocytes 
. In developing cortical neurons, evidence indicates that membrane depolarization by KCl induces SIK1 expression, an effect that is abolished by the L-type voltage-gated calcium channel blocker nifedipine 
. In the present study we show, for the first time, that the expression of SIK1 is upregulated in developing neurons by the neurotrophic factor BDNF ().
In addition to inducing SIK1 expression, BDNF triggers the phosphorylation and nuclear translocation of SIK1 (). Interestingly, it has been shown that phosphorylation of SIK1 at Thr182 is mediated by the liver kinase B1 (LKB1) 
, suggesting that BDNF increases SIK1 phosphorylation at Thr182 by activating LKB1. Consistent with this hypothesis, BDNF has been shown to stimulate LKB1 activity in cortical and hippocampal neurons 
. In addition, BDNF-induced SIK1 phosphorylation at Thr182 is associated with its nuclear translocation (). In line with these findings, it is interesting to note that intraperitoneal injection of cocaine induces both phosphorylation of SIK1 at Thr182 and translocation of SIK1 from the cytoplasm to the nucleus of rat striatal neurons 
Importantly, our data show that induction of SIK1 expression, phosphorylation and, nuclear translocation by BDNF is followed by the increased phosphorylation and nuclear export of HDAC5 (–), as well as by the activation of MEF2 transcriptional activity () and MEF2-dependent gene expression (). Interestingly, previous studies have shown that the SIK1-HDAC5-MEF2 pathway is also activated in skeletal and cardiac muscle cells. Thus, in skeletal myocytes and C2C12 myoblasts, induction of SIK1 expression by forskolin or isoproterenol increases the phosphorylation and nuclear export of HDAC5, thereby enhancing MEF2-dependent transcription 
. In line with these data, overexpression of SIK1 was found to induce nuclear export of HDAC5 and activation of MEF2C in C2C12 myoblasts 
. In addition, recent evidence indicates that induction of SIK1 expression and activity by increases in intracellular sodium concentration in a cardiac myocyte cell line activates MEF2 through HDAC5 phosphorylation 
. Together, these observations provide evidence that regulation of MEF2 activity by the induction of SIK1 expression and phosphorylation of HDAC5 is not limited to neuronal cells but extend to other cell types such as skeletal and cardiac muscle cells.
The transcriptional activity of MEF2 is regulated by direct phosphorylation and by indirect mechanisms 
. In particular, activation of p38 MAPK and ERK5 was shown to increase MEF2 transcriptional activity through the phosphorylation of MEF2 transactivation domain 
. In addition, it is well established that phosphorylation of class II HDACs leads to their nuclear-to-cytoplasmic shuttling and to the subsequent derepression of MEF2 
. Although most of these studies are related to skeletal muscle differentiation or cardiac growth 
, there is also evidence that KCl-induced depolarization of cerebellar granule and hippocampal neurons induces nuclear export of HDAC5, resulting in increased MEF2 activity 
. In this context, our study provides the first evidence of the stimulation of MEF2 activity by a neurotrophic factor through the increased phosphorylation and nuclear export of HDAC5 (–, ).
Interestingly, BDNF was previously shown to stimulate MEF2-dependent transcription through the activation of ERK5 in cortical and cerebellar granule neurons 
. More recently, BDNF was found to activate MEF2C-mediated transcription in cortical neurons through ERK1/2-p90 ribosomal S6 kinase 2 (RSK2) signaling pathway 
. These findings, together with our data, indicate that activation of ERK1/2 by BDNF can stimulate MEF2-dependent transcription by inducing SIK1 expression () followed by the nuclear export of HDAC5 () and by the direct phosphorylation of MEF2C by RSK2 
. These results support the view that SIK1-mediated inactivation of HDAC5 may act in cooperation with RSK2-dependent phosphorylation of MEF2 to activate MEF2 transcriptional activity in response to BDNF.
In addition to regulating MEF2-dependent transcription, previous studies have revealed an important role for SIK1 in controlling CREB-mediated transcription. Thus, SIK1 inhibits CREB transcriptional activity by phosphorylating CRTCs, triggering their nuclear export and cytoplasmic sequestration 
. These data provide evidence that MEF2- and CREB-mediated transcription are regulated in opposite directions by SIK1 through the phosphorylation of HDAC5 and CRTCs, respectively.
Together, our data identify a novel signaling pathway by which BDNF stimulates MEF2 activity. Thus, by inducing the expression, phosphorylation and, nuclear translocation of SIK1, BDNF increases HDAC5 phosphorylation and nuclear export, resulting in derepression of MEF2 activity. Because BDNF controls many aspects of neuronal development and synaptic plasticity, characterization of the role of SIK1-mediated activation of MEF2 in the effects of BDNF on neuronal survival, differentiation and synaptic transmission should improve our understanding of the mechanisms by which BDNF regulates neuronal development and function.