Regulated Shuttling of the Histone Deacetylase HDAC5 to the Nucleus May Put a Brake on Cocaine Addiction
Authors: Anne E. West
Journal: Neuron (2012)

The histone deacetylase HDAC5 has been shown to regulate behavioral adaptations to cocaine. In this issue of Neuron, Taniguchi et al., 2012Taniguchi M. Carreira M.B. Smith L.N. Zirlin B.C. Neve R.L. Cowan C.W. Neuron. 2012; 73 (this issue): 108-120Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar describe a cAMP-dependent signaling pathway that regulates nuclear accumulation of HDAC5, suggesting a mechanism to couple cocaine with changes in HDAC5 function. The histone deacetylase HDAC5 has been shown to regulate behavioral adaptations to cocaine. In this issue of Neuron, Taniguchi et al., 2012Taniguchi M. Carreira M.B. Smith L.N. Zirlin B.C. Neve R.L. Cowan C.W. Neuron. 2012; 73 (this issue): 108-120Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar describe a cAMP-dependent signaling pathway that regulates nuclear accumulation of HDAC5, suggesting a mechanism to couple cocaine with changes in HDAC5 function. Psychostimulant drugs of abuse very rapidly increase extracellular levels of dopamine in the brain; however, repeated exposure to these drugs over long periods of time is necessary to produce the persistent alterations in behavior that can lead to drug addiction (Hyman et al., 2006Hyman S.E. Malenka R.C. Nestler E.J. Annu. Rev. Neurosci. 2006; 29: 565-598Crossref PubMed Scopus (1949) Google Scholar). This temporal distinction between the pharmacological and the behavioral actions of psychostimulants has long suggested that molecular mechanisms downstream of dopamine receptor activation, which include the induction of new gene transcription, are critical for mediating behavioral adaptations induced by these drugs. In the two decades since it was discovered that transcription of the immediate-early gene Fos is rapidly induced in striatal neurons after cocaine or amphetamine administration (Graybiel et al., 1990Graybiel A.M. Moratalla R. Robertson H.A. Proc. Natl. Acad. Sci. USA. 1990; 87: 6912-6916Crossref PubMed Scopus (876) Google Scholar), a wealth of molecular genetic studies have defined the functional contributions of key transcriptional regulatory pathways to psychostimulant-induced behaviors (Robison and Nestler, 2011Robison A.J. Nestler E.J. Nat. Rev. Neurosci. 2011; 12: 623-637Crossref PubMed Scopus (696) Google Scholar). Among these mechanisms, a growing body of evidence causally implicates epigenetic mechanisms of chromatin regulation, which include processes such as histone modifications and DNA methylation, in the persistent behavioral adaptations induced by chronic exposure to these drugs. Acetylation of histones is positively correlated with gene transcription, and psychostimulant administration has been shown to increase histone acetylation at the promoters of inducibly transcribed genes (Kumar et al., 2005Kumar A. Choi K.H. Renthal W. Tsankova N.M. Theobald D.E. Truong H.T. Russo S.J. Laplant Q. Sasaki T.S. Whistler K.N. et al.Neuron. 2005; 48: 303-314Abstract Full Text Full Text PDF PubMed Scopus (620) Google Scholar). Histone acetylation is a dynamic posttranslational modification that is regulated at steady state by the balance in activity between histone acetyltransferases (HATs) and histone deacetylases (HDACs) that are locally recruited to chromatin (McKinsey et al., 2001McKinsey T.A. Zhang C.L. Olson E.N. Curr. Opin. Genet. Dev. 2001; 11: 497-504Crossref PubMed Scopus (350) Google Scholar). Diversity in the large HDAC family may allow for specificity in the regulation of histone acetylation. The eleven "classical" HDAC proteins are classified into three families (class I, class IIa/b, and class IV) based on their structure, enzymatic function, and pattern of expression (Haberland et al., 2009Haberland M. Montgomery R.L. Olson E.N. Nat. Rev. Genet. 2009; 10: 32-42Crossref PubMed Scopus (1928) Google Scholar). All of the nuclear HDACs regulate specific target genes by associating with sequence-specific DNA binding transcription factors. However, class IIa HDACs (HDACs 4, 5, 7, and 9) are distinguished by the fact that they shuttle between the nucleus and the cytoplasm in a stimulus-dependent fashion, providing an important mechanism to regulate the function of their transcription factor partners (McKinsey et al., 2001McKinsey T.A. Zhang C.L. Olson E.N. Curr. Opin. Genet. Dev. 2001; 11: 497-504Crossref PubMed Scopus (350) Google Scholar). In 2007, Renthal and colleagues presented the first genetic evidence that HDAC5 can modulate behavioral responses to chronic cocaine (Renthal et al., 2007Renthal W. Maze I. Krishnan V. Covington 3rd, H.E. Xiao G. Kumar A. Russo S.J. Graham A. Tsankova N. Kippin T.E. et al.Neuron. 2007; 56: 517-529Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). This study showed that viral overexpression of HDAC5 in the nucleus accumbens (NAc) of adult mice decreased preference for the cocaine-paired chamber in a conditioned place preference assay (CPP). Conversely, Hdac5 knockout mice showed increased preference for the cocaine-paired chamber compared with their wild-type littermates in a modified CPP paradigm that assessed preference after prior sensitization to cocaine. On the basis of these data the authors concluded that HDAC5 is an essential regulator of the actions of chronic cocaine on reward. However, this study also raised the important question of whether HDAC5 was acting as a direct downstream target of regulation by cocaine-activated signaling cascades in striatal neurons. In this issue of Neuron, Taniguchi et al., 2012Taniguchi M. Carreira M.B. Smith L.N. Zirlin B.C. Neve R.L. Cowan C.W. Neuron. 2012; 73 (this issue): 108-120Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar report that they have elucidated the signaling cascades that regulate the nuclear localization, and thus presumably the activity, of HDAC5 in striatal neurons. The nuclear accumulation of HDAC5 is governed by the balance between the activity of an N-terminal nuclear localization signal (NLS) and a C-terminal nuclear export signal. Because activation of cAMP signaling enhanced the nuclear accumulation of HDAC5 in striatal neurons, Taniguchi hypothesized that cAMP-regulated posttranslational modifications of HDAC5 mediate this change in subcellular distribution. Previous studies had identified two CaMK/PKD-inducible sites of serine phosphorylation on HDAC5 (S259 and S498) that promote the cytoplasmic retention of HDAC5 by enhancing its binding to the cytoplasmic anchoring protein 14-3-3 (McKinsey et al., 2000McKinsey T.A. Zhang C.L. Lu J. Olson E.N. Nature. 2000; 408: 106-111Crossref PubMed Scopus (874) Google Scholar). However, Taniguchi and colleagues focused on phosphorylation of HDAC5 at a different site, S279, because this amino acid lies within the NLS and thus is well positioned to directly regulate nuclear import. Using a phospho-specific antibody, the authors showed that HDAC5 S279 is constitutively phosphorylated in neurons of the adult striatum and that HDAC5 is primarily cytoplasmic under these conditions. However, administration of cocaine drove a rapid and transient decrease in HDAC5 S279 phosphorylation that was correlated with a similarly rapid and transient increase in the fraction of HDAC5 recovered in the nucleus. On the basis of these data the authors propose that cocaine-induced dephosphorylation of S279 is a mechanism to enhance the nuclear import of HDAC5. Although cyclin-dependent protein kinase 5 (Cdk5), protein kinase A (PKA) and p38 MAP kinase were all identified in silico as potential S279 kinases, in cultured striatal neurons only the addition of the Cdk5 inhibitor roscovitine reduced basal S279 phosphorylation of HDAC5, suggesting that Cdk5 is responsible for constitutive phosphorylation at this site. By contrast, treatment of striatal neurons with forskolin, which elevates cAMP and activates PKA, led to the rapid dephosphorylation of HDAC5 at S279, demonstrating that in striatal neurons, HDAC5 is not a direct target of phosphorylation by endogenous PKA. Previous studies had shown that the phosphatase PP2A is a direct target of regulation by PKA in striatal neurons (Ahn et al., 2007Ahn J.H. McAvoy T. Rakhilin S.V. Nishi A. Greengard P. Nairn A.C. Proc. Natl. Acad. Sci. USA. 2007; 104: 2979-2984Crossref PubMed Scopus (213) Google Scholar), and consistent with a role for this pathway in regulation of HDAC5, the authors found that addition of the phosphatase inhibitor okadaic acid to striatal neurons blocked the cAMP-induced dephosphorylation of S279. A summary of the pathways that couple cocaine to HDAC5 is diagrammed in Figure 1. Consistent with the hypothesis that dephosphorylation of S279 is required for nuclear import, the authors found that changing the serine at this site to a glutamic acid (S279E), which mimics the phosphorylated state of HDAC5, blocked nuclear accumulation of HDAC5 after forskolin treatment. However, mutation of S279 to a nonphosphorylable alanine residue (S279A) had no effect on the nuclear-cytoplasmic distribution, demonstrating that dephosphorylation of S279 alone is not sufficient to drive nuclear accumulation of HDAC5. The authors suggest that this is probably because phosphorylation of the two other identified phosphorylation sites, S259 and S498, traps HDAC5 in the cytoplasm via their association with 14-3-3. Interestingly, just like Ser279, both S259 and S498 were rapidly dephosphorylated in striatal neurons after treatment with either forskolin or cocaine. Thus these data suggest that phosphorylation-dependent nuclear translation of HDAC5 provides a mechanism through which integration of the activation of multiple signaling cascades is transduced into a change in nuclear histone acetylation. Having identified a mechanism of cocaine-dependent regulation of HDAC5, the authors seized the opportunity to test the biological requirements for HDAC5 regulation in behavioral adaptations to cocaine. Using stereotaxic injection of viruses into the NAc of adult mice, the authors found that overexpression of the S279A HDAC5 mutant, which cannot be phosphorylated at S279, inhibited CPP. These findings are consistent with previous evidence implicating HDAC5 in the inhibition of reward (Renthal et al., 2007Renthal W. Maze I. Krishnan V. Covington 3rd, H.E. Xiao G. Kumar A. Russo S.J. Graham A. Tsankova N. Kippin T.E. et al.Neuron. 2007; 56: 517-529Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). However, they further suggest that regulation of HDAC5 phosphorylation at S279 is an essential part of this mechanism. Unfortunately, how the S279A mutation disrupts HDAC5 function in CPP is not entirely clear, since the authors uncovered no differences in nucleocytoplasmic shuttling between this mutant and wild-type HDAC5 in cultured striatal neurons. Though it remains possible that mutation of S279 to alanine could selectively affect HDAC5 trafficking in adult striatal neurons in vivo, an alternative explanation is that this mutation affects the ability of HDAC5 to act as a corepressor through mechanisms that remain to be identified. The work of Taniguchi and colleagues substantially enhances understanding of the molecular players that lie between exposure to cocaine and a key enzyme that regulates histone acetylation. However, the specific findings of this study also raise important new questions about the downstream consequences of HDAC5 regulation for behavior. For example, Renthal and collaborators identified a large set of gene transcripts that were dysregulated in Hdac5 knockout mice compared with their wild-type littermates (Renthal et al., 2007Renthal W. Maze I. Krishnan V. Covington 3rd, H.E. Xiao G. Kumar A. Russo S.J. Graham A. Tsankova N. Kippin T.E. et al.Neuron. 2007; 56: 517-529Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar); however, whether these are direct or indirect targets of HDAC5 regulation remains unknown. Taniguchi and colleagues propose that repression of MEF2-dependent transcription is an essential function of HDAC5 and point out that the phenotype of the HDAC5 S279A mutant in CPP is opposite of that seen upon viral overexpression of a constitutively active MEF2 (Pulipparacharuvil et al., 2008Pulipparacharuvil S. Renthal W. Hale C.F. Taniguchi M. Xiao G. Kumar A. Russo S.J. Sikder D. Dewey C.M. Davis M.M. et al.Neuron. 2008; 59: 621-633Abstract Full Text Full Text PDF PubMed Scopus (210) Google Scholar). However, Renthal reported that deletion of the MEF2 binding domain in HDAC5 had no effect on HDAC5-dependent inhibition of CPP (Renthal et al., 2007Renthal W. Maze I. Krishnan V. Covington 3rd, H.E. Xiao G. Kumar A. Russo S.J. Graham A. Tsankova N. Kippin T.E. et al.Neuron. 2007; 56: 517-529Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). Thus, further experiments will be needed to clarify the gene regulatory pathways that require HDAC5. It will also be important to determine which striatal neuron classes utilize HDAC5 regulation. Given the requirement for cAMP elevation in the cascade that leads to S279 dephosphorylation, it is likely that the D1-class dopamine receptor-expressing medium spiny neurons are a major site of HDAC5 regulation in this study. Nonetheless, future analysis of the cell-type-specific regulation of HDAC5 in the striatum in vivo may suggest new hypotheses of how modulation of this pathway impacts the striatal circuit plasticities that underlie reward-related behaviors. Finally, questions are raised by this study about the importance of HDAC5 regulation at different phases in the behavioral response to cocaine. The Taniguchi study proposes that after both acute and repeated exposure to cocaine, the dephosphorylation-induced nuclear import of HDAC5 functions in the NAc to drive homeostatic compensations that limit the rewarding effects of psychostimulants, similar to the actions of many other transcriptional regulators including CREB (Carlezon et al., 1998Carlezon Jr., W.A. Thome J. Olson V.G. Lane-Ladd S.B. Brodkin E.S. Hiroi N. Duman R.S. Neve R.L. Nestler E.J. Science. 1998; 282: 2272-2275Crossref PubMed Scopus (640) Google Scholar), MEF2 (Pulipparacharuvil et al., 2008Pulipparacharuvil S. Renthal W. Hale C.F. Taniguchi M. Xiao G. Kumar A. Russo S.J. Sikder D. Dewey C.M. Davis M.M. et al.Neuron. 2008; 59: 621-633Abstract Full Text Full Text PDF PubMed Scopus (210) Google Scholar), and MeCP2 (Deng et al., 2010Deng J.V. Rodriguiz R.M. Hutchinson A.N. Kim I.-H. Wetsel W.C. West A.E. Nat. Neurosci. 2010; 13: 1128-1136Crossref PubMed Scopus (163) Google Scholar). By contrast, Renthal and colleagues showed enhanced Ser259 phosphorylation and export of HDAC5 from nuclei of striatal neurons after repeated exposure to cocaine and suggested that this decrease in the nuclear activity of HDAC5 positively mediates reward (Renthal et al., 2007Renthal W. Maze I. Krishnan V. Covington 3rd, H.E. Xiao G. Kumar A. Russo S.J. Graham A. Tsankova N. Kippin T.E. et al.Neuron. 2007; 56: 517-529Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). Given the high degree of sequence conservation surrounding Ser259, 279, and 489 among all the class IIa HDACs, it is possible that the Renthal study could have been detecting these changes on HDAC4 or HDAC9 rather than HDAC5. Alternatively, the regulation of HDAC5 phosphorylation may change as cocaine administration passes from repeated to chronic, perhaps even as a direct result of the early homeostatic cellular adaptations to acute cocaine exposure. If this is the case, then future elucidation of the mechanism of this regulatory switch might reveal maladaptive responses to chronic cocaine that could underlie the transition to addiction. Histone Deacetylase 5 Limits Cocaine Reward through cAMP-Induced Nuclear ImportTaniguchi et al.NeuronJanuary 12, 2012In BriefTaniguchi et al. discover that cocaine and cAMP signaling regulate the transient nuclear accumulation of the histone deacetylase HDAC5. Nuclear HDAC5 limits the development of cocaine reward-associated behaviors, likely by providing a braking mechanism for drug-induced gene expression programs. Full-Text PDF Open Archive