Evaluation of Histone Deacetylases as Drug Targets in HuntingtonTMs Disease models ΠPLOS Currents Huntington Disease

The family of histone deacetylases (HDACs) has recently emerged as important drug targets for treatment of slow progressive neurodegenerative disorders, including Huntington’s disease (HD). Broad pharmaceutical inhibition of HDACs has shown neuroprotective effects in various HD models. Here we examined the susceptibility of HDAC targets for drug treatment in affected brain areas during HD progression. We observed increased HDAC1 and decreased HDAC4, 5 and 6 levels, correlating with disease progression, in cortices and striata of HD R6/2 mice. However, there were no significant changes in HDAC protein levels, assessed in an age-dependent manner, in HD knock-in CAG140 mice and we did not observe significant changes in HDAC1 levels in human HD brains. We further assessed acetylation levels of ?-tubulin, as a biomarker of HDAC6 activity, and found it unchanged in cortices from R6/2, knock-in, and human subjects at all disease stages. Inhibition of deacetylase activities was identical in cortical extracts from R6/2 and wild-type mice treated with a class II-selective HDAC inhibitor. Lastly, treatment with class Iand II-selective HDAC inhibitors showed similar responses in HD and wild-type rat striatal cells. In conclusion, our results show that class I and class II HDAC targets are present and accessible for chronic drug treatment during HD progression and provide impetus for therapeutic development of brain-permeable classor isoform-selective inhibitors. Introduction The HDAC family includes eleven Zn++-dependent deacetylases belonging to three structural classes [1] [2]. HDAC class I and class II deacetylases share significant structural homology, especially within the highly conserved catalytic domains. Ubiquitously expressed class I HDACs 1, 2, and 3 are components of stable transcriptional repressor complexes, involved in global transcriptional regulation. Class II includes HDAC4, HDAC5, HDAC7 and HDAC9, exhibiting distinct tissue-specific patterns of expression, and the ubiquitously expressed cytoplasmic microtubule (?-tubulin) deacetylase, HDAC6. In the past years numerous studies have demonstrated neuroprotection by small molecule HDAC inhibitors, broadly modulating all HDAC enzymes, in various human disease models, including Huntington’s disease (HD) [3] [4] [5]. HD is an autosomaldominant neurodegenerative disorder, caused by the expansion of a CAG-triplet repeat within the coding sequences of the HD gene, IT15 [6]. The mutant huntingtin protein, containing a pathologically expanded polyglutamine sequence near the Nterminus, causes a progressive and fatal neurological phenotype [7]. Dysfunction and degeneration of cerebral cortical and striatal neurons underlie the symptoms of HD and the progressive functional decline that occurs [8] [9]. The precise mechanism(s) of neurodegeneration remain unknown, however neuronal homeostasis is profoundly perturbed by transcriptional dysregulation, abnormal histone acetylation and chromatin remodeling, aberrant protein interactions, mutant protein misfolding and aggregation, defects in axonal transport, and synaptic dysfunction. Alterations in transcriptional regulation have been proposed to be especially significant for HD [10] [11] [12] [13], and HDAC inhibition has been suggested as a therapeutic strategy for modulating transcriptional pathology. The neuroprotective effects of HDAC inhibition have been well-documented in both invertebrate and mouse models of HD [3] [5] [14] [15] [16]. The involvent of HDACs in HD is shown in Table 1. The observed benefits have been attributed to a general HDAC-mediated chromatin remodeling and amelioration of transcriptional dysregulation by targeting HDACs 1-3 [17]. Alternatively, efficacy could be related to specific HDAC targets, as suggested by experimental data in invertebrate HD models. In C. elegans it has been shown that HDAC3 (HAD-3), but not HDAC1 (HAD-1), modulates polyglutamine-associated toxicity [18], whereas in Drosophila , neuroprotection was achieved by specific knockdown of rpd3 (the fly ortholog of HDAC1/2) [19]. 1 PLOS Currents Huntington Disease Table 1: Therapeutic effects of inhibiting HDAC isoforms in HD models. HDAC Function(s) Effect in HD Model Ref. Class I, HDAC1 Global regulation of transcription Neuroprotective in fly HD model [19] Class I, HDAC2 Global regulation of transcription Neuroprotective in fly HD model [19] Class I, HDAC3 Global regulation of transcription Neuroprotective in C.elegans HD model [18] Class IIa, HDAC4 Transcriptional repression n.d. Class IIa, HDAC5 Transcriptional repression n.d. Class IIa, HDAC7 Transcriptional repression No effect in R6/2 crossed with HDAC7 knock-out mice [20] Class IIb, HDAC6 Microtubule transport, autophagy Ameliorates microtubule transport defect, increases BDNF release in HD neurons in vitro [21] It has also been demonstrated that inhibition of the activity of the class IIb deacetylase HDAC6 may compensate for the microtubule-dependent transport deficit in HD by increasing ?-tubulin acetylation [21]. Further, recent results implicate HDAC6 as a regulatory protein for the major cellular protein degradation pathways: the ubiquitin-proteasome system and autophagy [22] , both of which are linked with aggregation and clearance of mutant huntingtin. Since HD is an age-dependent and slowly progressive disorder, neuroprotective therapy would require chronic drug treatment. Thus, in this study we have assessed, in an age-dependent manner, the presence and availability of HDAC targets for pharmacological treatment.