Genetic Knock-Down of HDAC7 Does Not Ameliorate Disease Pathogenesis in the R6/2 Mouse Model of Huntington's Disease

Huntington's disease (HD) is an inherited, progressive neurological disorder caused by a CAG/polyglutamine repeat expansion, for which there is no effective disease modifying therapy. In recent years, transcriptional dysregulation has emerged as a pathogenic process that appears early in disease progression. Administration of histone deacetylase (HDAC) inhibitors such as suberoylanilide hydroxamic acid (SAHA) have consistently shown therapeutic potential in models of HD, at least partly through increasing the association of acetylated histones with down-regulated genes and by correcting mRNA abnormalities. The HDAC enzyme through which SAHA mediates its beneficial effects in the R6/2 mouse model of HD is not known. Therefore, we have embarked on a series of genetic studies to uncover the HDAC target that is relevant to therapeutic development for HD. HDAC7 is of interest in this context because SAHA has been shown to decrease HDAC7 expression in cell culture systems in addition to inhibiting enzyme activity. After confirming that expression levels of Hdac7 are decreased in the brains of wild type and R6/2 mice after SAHA administration, we performed a genetic cross to determine whether genetic reduction of Hdac7 would alleviate phenotypes in the R6/2 mice. We found no improvement in a number of physiological or behavioral phenotypes. Similarly, the dysregulated expression levels of a number of genes of interest were not improved suggesting that reduction in Hdac7 does not alleviate the R6/2 HD-related transcriptional dysregulation. Therefore, we conclude that the beneficial effects of HDAC inhibitors are not predominantly mediated through the inhibition of HDAC7.


Introduction
Huntington's disease (HD) is an autosomal dominant late-onset progressive neurodegenerative disorder with a mean age of onset of 40 years. Symptoms include psychiatric disturbances, motor disorders, cognitive decline and weight loss, disease duration is 15-20 years and there are no effective disease modifying treatments [1]. The HD mutation is an expanded CAG trinucleotide repeat in the HD gene that is translated into a polyglutamine (polyQ) repeat in the huntingtin (Htt) protein [2]. Neuropathologically, the disease is characterized by neuronal cell loss in the striatum, cortex and other brain regions and the deposition of nuclear and cytoplasmic polyQ aggregates [3,4]. The R6/2 mouse model expresses exon 1 of the human HD gene with more than 150 CAG repeats [5,6]. The R6/2 phenotype has an early onset and rapid and reproducible phenotype progression that recapitulates many features of the human disease. Motor and cognitive abnormalities can be detected before 6 weeks of age [7,8], and mice are rarely kept beyond 15 weeks. PolyQ aggregates are clearly apparent in some brain regions from 3 to 4 weeks of age and striatal cell loss has been documented at later stages [9]. This suggests that the mouse phenotype is predominantly caused by neuronal dysfunction.
Transcriptional dysregulation occurs early in the molecular pathology of HD and has been recapitulated across multiple HD model systems (reviewed in [10]). RNA Affymetrix expression profiles of brain regions and muscle from both the R6/2 transgenic mouse and knock-in mouse models of HD show high correlation to expression profiles from HD post-mortem tissue [11][12][13]. The molecular mechanisms that underlie these selective transcriptional disturbances are unknown and remain the subject of investigation. The control of eukaryotic gene expression in part depends on the modification of histone proteins associated with specific genes with the acetylation and deacetylation of histones playing a critical role in gene expression [14][15][16]. Studies in numerous HD models have shown that mutant huntingtin expression leads to a change in histone acetyltransferase (HAT) activity and suggest that aberrant HAT activity may contribute to transcriptional dysregulation in HD [17][18][19]. Supporting this view, administration of histone deacetylase (HDAC) inhibitors such as suberoylanilide hydroxamic acid (SAHA) consistently shows therapeutic potential in HD models [20][21][22][23][24][25][26][27][28], at least partly through increasing the association of acetylated histones with down-regulated genes and correcting mRNA abnormalities [29].
There are three major classes of mammalian HDACs, based on their structural homology to the three Saccharomyces cerevisiae HDACs: rpd3 (class I), hda1 (class II) and sir2 (class III). Class I comprises HDAC1, -2, -3 and -8, class IIa HDAC4, -5, -7 and -9, class IIb HDAC6 and -10 and HDAC11 (class IV) shows homology to both rpd3 and hda1 [30]. Pan-HDAC inhibitors such as SAHA target the zinc-dependent HDACs 1-11 and not the NAD+ dependent class III HDACs (the seven sirtuins, SIRT1-7) [31]. In order to gain insight into which HDACs must be inhibited in order to alleviate HD-related phenotypes, genetic approaches have been used to complement pharmacology in Drosophila melanogaster and Caenorhabditis elegans HD models [19,32]. However, as HDACs show differential levels of evolutionary conservation [33], the extent to which these studies will inform drug development in man has yet to be demonstrated.
The molecular mechanisms by which SAHA exerts its beneficial and toxic effects are currently not clear, nor whether the beneficial and toxic effects can be dissociated. We therefore need to systematically interrogate known targets of SAHA genetically in order to identify the HDAC(s) that present therapeutic targets relevant to HD. As a consequence, we have embarked on a serried of genetic crosses between the R6/2 mouse and specific HDAC knock-out mouse lines. It has previously been shown that in addition to enzyme inhibition, SAHA treatment selectively suppresses expression of HDAC7 in vitro [34], thus representing a potential molecular avenue by which pan-HDAC inhibition exerts a beneficial effect. We confirmed that chronic administration of SAHA decreases Hdac7 mRNA expression levels in mouse brain irrespective of the HD genotype. Hdac7 knockout mice were previously generated through a targeted inactivation of the endogenous murine Hdac7 gene [35]. Hdac7 null mice are embryonic lethal and die at E11.0. However, Hdac7 heterozygote knockout mice were found to be viable and fertile, with no overt phenotype. We demonstrate that Hdac7 mRNA and protein levels are reduced in Hdac7+/2 heterozygous knock-out mice and that Hdac7 expression is not altered by the presence of the R6/2 transgene. We performed a genetic cross between R6/2 mice and Hdac7+/2 heterozygotes and found that genetic knock-down of Hdac7 fails to confer any improvement to a number of physiological, behavioral and transcriptional phenotypes. We conclude that neither inhibition of HDAC7 nor its downregulation contributed significantly to the beneficial effects that we observed upon administration of SAHA.

Results
The expression of Hdac7 is decreased in R6/2 brain in response to chronic treatment with SAHA SAHA has been described as a pan-HDAC enzyme inhibitor [36,37]. In addition, it was recently shown that treatment of a number of cell lines with SAHA resulted in the specific downregulation of HDAC7 at the mRNA level [34]. To determine whether the expression of Hdac7 might be similarly altered by the administration of SAHA in vivo, we established a quantitative real time PCR assay (RT-qPCR) for murine Hdac7. We had previously conducted an efficacy trial to assess the effects of the administration of SAHA, when complexed with hydroxypropyl-b-cyclodextrin at a concentration of 0.67 g/L in the drinking water, and observed a considerable improvement in RotaRod performance [23]. The WT and R6/2 mice that had been treated with SAHA or vehicle in this experiment had been sacrificed at 13 weeks of age and we still had access to cerebellar cDNA that had been prepared from the brains of these mice. We were able to show that SAHA significantly decreased the expression of Hdac7 in the cerebellum of both wild type (WT) and R6/2 mice ( Figure 1A). Therefore, SAHA has a similar effect on the expression of Hdac7 in vivo as has been described in cell culture and this could contribute to the beneficial effects of SAHA administration.
Hdac7 expression level is unaffected by the expression of the R6/2 transgene We found heterozygous Hdac7 knockout mice to be viable and fertile, with no overt phenotype. Therefore, prior to interpreting the results of a genetic cross to determine whether heterozygous knock-down of Hdac7 has a beneficial effect on R6/2 HD-related phenotypes, it was important to show that (1) Hdac7 expression is reduced in Hdac7+/2 knock-out mice: that Hdac7 is not autoregulated to wild type levels as is the case for Hdac1 [38] and (2) that the presence of the R6/2 transgene does not alter Hdac7 expression levels. RT-qPCR was performed on cDNA prepared from the striatum and cerebellum of 15 week old wild type (WT), Hdac7 heterozygote (Hdac7+/2), R6/2 transgenic (R6/2) and R6/ 2 mice heterozygote for Hdac7 (R6/2-Hdac7+/2) mice. We found that Hdac7 expression levels were significantly decreased in the striatum and cerebellum of Hdac7+/2 heterozygote mice, irrespective of the presence of the R6/2 transgene ( Figure 1B and C). Furthermore, we saw no difference in Hdac7 expression levels between WT and R6/2 mice in either brain region. Western blotting was performed on whole cell lysates from 15 week cortices. We found HDAC7 protein levels to be in agreement with the RNA expression profiling ( Figure 1D and E).
Hdac7 is expressed in neuronal populations in the mouse brain An important feature of the class IIa HDACs is their ability to shuttle between the nucleus and the cytoplasm. Precise regulation of the subcellular distribution of class IIa HDACs is intimately linked to the control of their activity and plays a pivotal role in cellular processes and organ development [39]. Previous observations have highlighted a differential subcellular localization of HDAC7 in different cell lines and body tissues, suggesting that the control of subcellular localization and function of HDAC7 differs with respect to the cell type. To explore the expression pattern of Hdac7 in the mouse brain we performed immunohistochemistry to coronal sections of WT and R6/2 brains from animals at 14 weeks of age. HDAC7 is present in both the nucleus and cytoplasm in the striatum (Figure 2), cortex and cerebellum (data not shown) of both WT and R6/2 brains. It is present in all neurons, but interestingly appears to be absent from the nuclei of at least some non-neuronal cell populations.

Genetic reduction of Hdac7 does not modify the R6/2 phenotype
We have previously established a set of quantitative tests with which to monitor progressive behavioral phenotypes in R6/2 mice [40][41][42]. To generate mice for this analysis, male R6/2 mice were bred with female Hdac7 heterozygote knock-out mice (Hdac7+/2) to produce at least 10 female mice from each genotype (WT, n = 10; Hdac7+/2, n = 13; R6/2, n = 14; R6/2-Hdac7+/2, n = 12), which were born over a period of 5 days. The CAG repeat size was well matched between the R6/2 and R6/2-Hdac7+/2 groups (P = 0.102) ( Table 1). Weight gain, RotaRod performance, grip strength and exploratory activity were monitored from 4 to 15 weeks of age, and in each case, a specific test was performed on the same day and at the same time during the weeks in which measurements were taken.
The body temperature of the mice was recorded at 14 and 15 weeks of age by rectal probe ( Figure 3B). R6/2 mice were found to be hypothermic [F (1,45) = 69.781, P,0.001], which worsened over the course of the week between measurements [F (1,45)  RotaRod performance is a sensitive indicator of balance and motor coordination, which has been reliably shown to decline in R6/2 mice [40]. Using this test, we found that R6/2 and R6/2-Hdac7+/2 mice performed similarly with age and that the performance of Hdac7+/2 and WT mice was equivalent ( Figure 3C). Consistent with previous results, the overall RotaRod performance of R6/2 was impaired as compared to WT [F (1 ,45) = 40.634, P,0.001] and deteriorated with age [F (2 ,900) = 9.119, P,0.001]. Overall, the performance of Hdac7+/2 mice did not differ from WT [F (1,45) = 0.047, P = 0.83] but did change with age [F (2,900) = 4.427, P = 0.009] however, this is most likely the result of the exceptionally good, but atypical, performance of the WT mice at four weeks. There was no overall effect of Hdac7 knock-down on the RotaRod performance of the R6/2 mice [F (1 ,45) = 0.194, P = 0.661] and examination of the data ( Figure 3C) indicates that the statistically significant interaction between the genotypes over the course of the experiment [F (2,900) = 4.516, P = 0.009] is, once again, due to the performance of the WT mice at 4 weeks of age and not a reflection of an alteration in the R6/2 phenotype. Blots were probed with an antibody that recognizes Hdac7 (120 kDa) and a non-specific band (70 kDa). (E) Quantification of Hdac7 protein expression levels in WT (white) and Hdac7+/2 (grey) mice. Quantification was performed on blots containing four samples per genotype using the non-specific band for reference. Blots were additionally probed with an antibody to a-tubulin to confirm equal protein loading (data not shown). Error bars represent standard deviation from the mean (n = 4). * p,0.05, ** p,0.01, *** p,0.001. doi:10.1371/journal.pone.0005747.g001 Exploratory activity was assessed fortnightly from 5 to 13 weeks of age as described previously [41] and analyzed by repeated measures general linear model (GLM) ANOVA. Mice were assessed for a period of 60 min for total activity, mobility and rearing and the P-values obtained through the analyses are displayed in Table 2 for each parameter. Mice of all genotypes exhibit most activity during the first 15 minutes of the assessment period [41]. R6/2 mice show an overall hypoactivity relative to WT mice from 11 weeks ( Table 2, R6/2) although the pattern of activity over the course of the 60 min period was significantly different between R6/2 and WT mice (R6/2*time) by five weeks of age. Hdac7+/2 mice were indistinguishable from WT mice, both in overall activity (HDAC7) and in the 60 minute pattern of activity (HDAC7*time). There was no overall improvement in R6/2 hypoactivity through genetic reduction of Hdac7 (R6/ 2*HDAC7), nor was the R6/2 pattern of hypoactivity changed (R6/2*HDAC7*time). In summary, Hdac7 genetic reduction does not improve hypoactivity in the R6/2 mice.
Hdac7 genetic reduction does not ameliorate the dysregulated expression of genes of interest in R6/2 mouse brains As HDAC inhibitors have been shown to ameliorate the dysregulation of gene expression in HD models systems [28,29,43], we used RT-qPCR to measure the level of expression of a set of genes of interest in the striatum and cerebellum of WT, R6/2, Hdac7+/2 and R6/2-Hdac7+/2 mice aged 15 weeks. These included striatal genes that are consistently down-regulated in mouse models of HD and in HD patient brains and cerebellar genes that have consistently altered expression patterns in both R6/2 and the HdhQ150 knock-in mouse model of HD [13,42,44]. We found that Hdac7 reduction did not ameliorate the transgenemediated transcriptional dysregulation in the striatum ( Figure 4A) or the cerebellum ( Figure 4B) of 14 week old mice. However, the expression of Igfbp5 was increased with HDAC7 reduction in the cerebellum of WT (P = 0.047) but not R6/2 mice (P = 0.690) ( Figure 4B). Concomitantly, we found no difference in R6/2 transgene expression in the striatum (P = 0.620) ( Figure 4C) and the cerebellum (P = 0.240) ( Figure 4D) of R6/2 and R6/2-Hdac7+/2 mice.

Discussion
We have previously shown that chronic administration of the HDAC inhibitor SAHA to R6/2 mice significantly improves some motor and neuropathological phenotypes [23]. However, the therapeutic index of SAHA in mice is very narrow, and therapeutic doses show considerable toxicity resulting in weight loss in both WT and R6/2 mice. SAHA has been reported to inhibit the eleven Zn 2+ dependent HDAC enzymes [37] and therefore dissection of the mechanism through which SAHA exerts its beneficial effects is complex. In order to identify the HDAC enzyme(s) that are therapeutic targets for HD and in an attempt to separate the beneficial effects of SAHA from its toxicity, we have embarked on a series of genetic crosses to mice that have been genetically engineered to knock-out specific HDAC enzymes. HDAC7 is of additional interest because SAHA has been shown to decrease the level of expression of HDAC7 in a number of cell lines [34]. In this report we showed that these results extend to an in vivo system and that, after chronic administration, SAHA downregulates Hdac7 mRNA levels in the brains of WT and R6/2 mice.
As nullizygosity for Hdac7 is embryonic lethal [35], it is not possible to investigate the effects of knocking-out the Hdac7 gene on the R6/2 phenotype. However, reduced levels of Hdac7 might be more akin to the effects to the pharmacological inhibition of this enzyme. We therefore established whether an investigation of the effects of knocking-down Hdac7 levels using Hdac7+/2 knock-out mice would be feasible. This was necessary as it has been shown that the effects of the genetic knock-down of Hdac1 cannot be studied because Hdac1 mRNA and protein levels autoregulate to that of WT mice in Hdac1+/2 mice [38] and furthermore, nullizygosity for Hdac1 is embryonic lethal [45]. We thus confirmed that Hdac7 expression is reduced in both the striatum and cerebellum of Hdac7+/2 mice and that the presence of the R6/2 transgene does not alter Hdac7 expression levels in either Hdac7+/+ or Hdac7+/2 mice at the mRNA or protein levels. We went on to show that genetic reduction of Hdac7 levels did not impact on the body weight, body temperature, RotaRod performance, grip strength or exploratory activity of R6/2 mice. Similarly, decreased Hdac7 expression did not ameliorate the HDrelated dysregulated expression levels of a number of specific genes of interest.
Very little is currently known about the function of HDAC7 in brain. Hdac7 has previously been shown to be expressed throughout the rat brain by in situ hybridization [46]. Crucially, we established that Hdac7 is present in neurons, but were surprised to find that it could not be detected by immunohistochemistry in at least a proportion of non-neuronal brain cells. HDAC7 together with HDACs 4, -5 and -9 comprise the class IIa HDACs, which share a high degree of homology at their Cterminal catalytic domain [47]. The class IIa HDACs shuttle between the nucleus and cytoplasm and precise regulation of their subcellular distribution plays a pivotal role in modulating their function, via post-translational modifications such as phosphorylation, and the formation of nucleocytoplasmic shuttling complexes [31,[47][48][49][50][51][52][53][54][55][56][57][58][59][60]. Of the class IIa HDACs, HDAC7 is the most divergent [47,61]. HDAC7 has been shown to associate with transcription co-repressors and factors such as CtBP, MEF2, HP1a, SMRT, N-CoR, mSin3A, and HIF1a [16,31,39,47,49,57]. These interactions are consistent with the role of HDAC7 in regulating gene expression either as a co-activator or a corepressor. In vitro studies with recombinant HDAC7 protein have suggested that histones may be substrates of HDAC7 deacetylase activity [62]. However, it has been shown that modulating HDAC7 levels in vitro by siRNA knockdown or overexpression is associated with growth arrest without detectable changes in histone acetylation or p21 gene expression [34]. This would be consistent with HDACs having many protein substrates, in addition to histones, involved in the regulation of gene expression, cell proliferation, and cell death and thus HDACs can be considered to be ''lysine deactelylases''. Although the functions of HDAC7 in brain are unknown and remain to be elucidated, our genetic studies lead us to conclude that inhibition of Hdac7 is not a major mediator of the beneficial effects that we obtained upon administration of SAHA to R6/2 mice and HDAC7 should not be prioritized as a therapeutic target for HD.

Phenotype analysis
Mice were weighed weekly to the nearest 0.1 g. Motor coordination was assessed using an Ugo Basile 7650 accelerating RotaRod (Linton Instrumentation, UK), modified as previously described [40]. At 4 weeks of age, mice were tested on four consecutive days, with three trials per day. At 8, 10, 12 and 14 weeks of age, mice were tested on three consecutive days with three trials per day. Forelimb grip strength was measured once a week at 4, 7, 9, 11 and 13 weeks using a San Diego Instruments Grip Strength Meter (San Diego, CA, USA) as described [40]. Exploratory, spontaneous motor activity was recorded and assessed every two weeks at 5, 7, 9, 11 and 13 weeks of age for 60 min during the day using AM1053 activity cages, as described previously [41]. Briefly, activity (total number of beam breaks in the lower level), mobility (at least two consecutive beam breaks in the lower level) and rearing (number of rearing beam breaks) were measured. The data were collected and analyzed as described previously.

RNA extraction and real-time PCR expression analysis
RNA extraction and reverse transcription of 4 mg of total cerebellar RNA and 1 mg total striatal RNA was performed as previously described [44]. The RT reaction was diluted 10-fold in nuclease free water (Sigma) and 5 ml was used in a 25 ml reaction containing Precision MasterMix (PrimerDesign), 400 nM primers and 300 nM probe using the Opticon 2 real-time PCR machine (MJ Research). Estimation of mRNA copy number was determined in duplicate for each RNA sample by comparison to the geometric mean of two or three endogenous housekeeping genes as described [44]. Primer and probe sequences are available in Supplementary Table S1.

Immunohistochemistry and confocal microscopy
Whole brains were frozen in isopentane, stored at 80uC and 15 mm thick sections were cut using a cryostat (Bright Instruments Ltd.). Sections were fixed for 10 min in methanol at 220uC and washed twice in 0.1 M phosphate buffered saline pH 7.4 (PBS) for 15 min before blocking in PBS containing 2% bovine serum albumin (BSA) and 0.1% Triton-X for 15 min. Sections were incubation in primary antibodies in PBS with BSA overnight at 4uC, washed twice in PBS for 15 min, incubated in secondary fluorescent antibodies in PBS with BSA for 1 hr at room temperature and washed twice in PBS for 15 min. Primary antibodies were: HDAC7 (rabbit polyclonal Sigma H2662) (1:50) and NeuN (mouse monoclonal Chemicon MAB377) (1:200) and secondary antibodies were: Alex-555 donkey anti-rabbit (1:1000) and Alexa 488 goat antimouse (1:1000) respectively (Molecular Probes). Nuclei were visualized using TO-PRO-3 (Molecular Probes) (1:1000). Slides were mounted in Mowial and antibody location was visualized using an LSM150 Meta confocal microscope (Zeiss).

Statistical analysis
Statistic analysis was performed by Student's t-test (Excel or SPSS), one-way ANOVA, two-way ANOVA and repeated measures GLM ANOVA, with the Greenhouse-Geisser correction for non-sphericity using SPSS.