Abstract
Neurotrophins and modifiers of chromatin acetylation and deacetylation participate in regulation of transcription during neuronal maturation and maintenance. The striatal medium spiny neuron is supported by cortically-derived brain derived neurotrophic factor and is the most vulnerable neuron in Huntington’s disease, in which growth factor and histone deacetylase activity are both disrupted. We examined the ability of three histone deacetylase inhibitors, trichostatin A, valproic acid and Compound 4 b, alone and combined with brain derived neurotrophic factor (BDNF), to promote phenotypic maturation of striatal medium spiny neurons in vitro. Exposure of these neurons to each of the three compounds led to an increase in overall histone H3 and H4 acetylation, dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (DARPP-32) mRNA and protein, and mRNA levels of other markers of medium spiny neuron maturation. We were, however, unable to prove that HDAC inhibitors directly lead to remodeling of Ppp1r1b chromatin. In addition, induction of DARPP-32 by brain-derived neurotrophic factor was inhibited by histone deacetylase inhibitors. Although BDNF-induced increases in pTrkB, pAkt, pERK and Egr-1 were unchanged by combined application with VPA, the increase in DARPP-32 was relatively diminished. Strikingly, the NGF1A-binding protein, Nab2, was induced by BDNF, but not in the presence of VPA or TSA. Gel shift analysis showed that α-Nab2 super-shifted a band that is more prominent with extract derived from BDNF-treated neurons than with extracts from cultures treated with VPA alone or VPA plus BDNF. In addition, overexpression of Nab2 induced DARPP-32. We conclude that histone deacetylase inhibitors inhibit the induction of Nab2 by BDNF, and thereby the relative induction of DARPP-32.
Highlights
Transcriptional regulatory mechanisms specifying neuronal subtype differentiation and gene expression remain enigmatic for most neurons in the central nervous system
To determine whether all subtypes of medium-sized spiny neurons (MSNs) respond to histone deacetylase (HDAC) inhibitors, we assayed 1) Regulator of Calcium Signaling, which like DARPP-32 is a marker of all MSN subtypes [2]; 2) calbindin, a marker of the matrix compartment; 3) D1 type receptors (D1R) and dynorphin (DYN), markers of the direct pathway; and 4) D2 type receptors (D2R) and enkephalin (ENK), representative of the indirect pathway
We examined the effects of HDAC inhibitors, trichostatin A (TSA), valproic acid (VPA) and HDACi4b, on the phenotypic maturation of MSNs in vitro, both alone and in combination with Brain-derived neurotrophic factor (BDNF)
Summary
Transcriptional regulatory mechanisms specifying neuronal subtype differentiation and gene expression remain enigmatic for most neurons in the central nervous system. Dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (DARPP-32), encoded by the ppp1r1b gene, is expressed in 98% of the MSNs and modulates their response to dopamine and other first messengers [2]. Brain-derived neurotrophic factor (BDNF) is a major regulator of MSN phenotype during development and in the adult, and multiple molecular mechanisms via which it regulates ppp1r1b transcription have been well characterized [4,5,6,7]. Many factors that contribute to neuronal differentiation and plasticity, including BDNF [8], lead to alterations of chromatin structure via core histone modifications, and inhibition of histone deacetylase (HDAC) activity promotes this process [9]. Histone acetyltransferases (HATs) acetylate lysine residues on the amino terminal tails of core histones (H2A, H2B, H3 and H4), thereby relaxing chromatin structure and allowing for transcriptional activation. HDAC inhibitors activate multiple signal transduction pathways and may directly or indirectly lead to transcriptional repression [10,11,12]
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