Importantly, however, HDAC inhibitors activate multiple signal transduction pathways and may also directly or indirectly lead to transcriptional repression. Loss of striatal, cortically-derived BDNF and aberrant histone regulation are two major factors leading to transcriptional dysregulation in Huntington��s disease. A combination of BDNF and an HDAC inhibitor is a potential therapeutic cocktail for HD, and is already used to derive mature MSNs from iPS and ES cells. We sought to determine the ability of HDAC inhibitors to promote expression of DARPP-32 and other markers of MSN maturation in vitro, either alone or in combination with BDNF. We therefore focused on HDAC inhibitors that have been evaluated in models of polyglutamine disease and in the promotion of the MSN phenotype from iPS and ES cells. Two classical, broadspectrum HDAC inhibitors, trichostatin A, a hydroxamic acid, and the small carboxylate molecule, valproic acid, increase acetylation of histone H3 in HD, thereby restoring levels of some of the dysregulated transcripts. Compound 4 b is a pimeloylanilide Temozolomide derivative that targets acetylated histone H4, and also corrects transcription and behavior abnormalities in R6/2- 300Q transgenic mice. VPA is utilized in almost all iPSC differentiation protocols for MSNs, but the direct effect of HDACi��s on MSNs has not been assayed. We report that treatment with all three HDAC inhibitors results in a two-to-threefold increase in DARPP-32 protein levels in MSNs in vitro, but surprisingly, reduces the induction of differentiation by BDNF without interfering with any of the heretofore identified signal transduction pathways utilized by BDNF in this process. We determine that BDNF also up-regulates the NGF1A binding protein, Nab2, and that opposite effects on induction of Nab2 by VPA appear to mediate this apparent antagonistic action. We examined the effects of HDAC inhibitors, TSA, VPA and HDACi4b, on the phenotypic maturation of MSNs in vitro, both alone and in combination with BDNF. HDCAi��s, particularly VPA, and BDNF are utilized for induction of mature neuronal phenotypes from iPS and ES cells, and are candidate treatments for HD. Our main findings include: 1) TSA, VPA and HDACi4b increase the overall level of histone acetylation in MSNs; 2) acetylated H3 histone association is relatively enriched within 1 Kb downstream of the ppp1r1b TSS in striatal chromatin in vivo, but we were unable to demonstrate chromatin modification following in vitro treatment of MSNs with HDACi; induction of DARPP-32 and other markers of the mature MSN by HDAC inhibitors does not require phosphorylation of Akt or ERK; and 4) BDNF induces Nab2, which is required for maximal induction of DARPP-32, and the induction of which is inhibited by VPA and TSA. The data BAY 73-4506 side effects presented herein do not demonstrate that HDACi��s induce DARPP-32 via chromatin remodeling and in fact, do not definitively identify the mechanism via which this induction occurs. We previously identified two groups of transcription start sites in the ppp1r1b gene. One is grouped 400 ntd 59 to the translation initiation codon, and the second 200�C300 ntd further upstream. We also showed that the sequences both 1 Kb downstream and upstream of the TSS are unable to direct transgene expression to MSNs in vivo. Therefore, the hyperacetylation of the 1 Kb downstream of the TSS relative to the non-expressing NIH-3T3 cells is likely marking the core promoter region, i.e. an area of active transcription, but not necessarily a cell-specific enhancer.