The molecular chaperone heat shock protein 90 (Hsp90) and its accessory cochaperones function by facilitating the structural maturation and complex assembly of client proteins, including steroid hormone receptors and selected kinases. By promoting the activity and stability of these signaling proteins, Hsp90 has emerged as a critical modulator in cell signaling. Here, we present evidence that Hsp90 chaperone activity is regulated by reversible acetylation and controlled by the deacetylase HDAC6. We show that HDAC6 functions as an Hsp90 deacetylase. Inactivation of HDAC6 leads to Hsp90 hyperacetylation, its dissociation from an essential cochaperone, p23, and a loss of chaperone activity. In HDAC6-deficient cells, Hsp90-dependent maturation of the glucocorticoid receptor (GR) is compromised, resulting in GR defective in ligand binding, nuclear translocation, and transcriptional activation. Our results identify Hsp90 as a target of HDAC6 and suggest reversible acetylation as a unique mechanism that regulates Hsp90 chaperone complex activity.

Genomic imprinting causes parental origin–specific monoallelic gene expression through differential DNA methylation established in the parental germ line. However, the mechanisms underlying how specific sequences are selectively methylated are not fully understood. We have found that the components of the PIWI-interacting RNA (piRNA) pathway are required for de novo methylation of the differentially methylated region (DMR) of the imprinted mouse Rasgrf1 locus, but not other paternally imprinted loci. A retrotransposon sequence within a noncoding RNA spanning the DMR was targeted by piRNAs generated from a different locus. A direct repeat in the DMR, which is required for the methylation and imprinting of Rasgrf1, served as a promoter for this RNA. We propose a model in which piRNAs and a target RNA direct the sequence-specific methylation of Rasgrf1.

SummaryHuman adult spermatogonial stem cells (hSSCs) must balance self-renewal and differentiation. To understand how this is achieved, we profiled DNA methylation and open chromatin (ATAC-seq) in SSEA4+ hSSCs, analyzed bulk and single-cell RNA transcriptomes (RNA-seq) in SSEA4+ hSSCs and differentiating c-KIT+ spermatogonia, and performed validation studies via immunofluorescence. First, DNA hypomethylation at embryonic developmental genes supports their epigenetic “poising” in hSSCs for future/embryonic expression, while core pluripotency genes (OCT4 and NANOG) were transcriptionally and epigenetically repressed. Interestingly, open chromatin in hSSCs was strikingly enriched in binding sites for pioneer factors (NFYA/B, DMRT1, and hormone receptors). Remarkably, single-cell RNA-seq clustering analysis identified four cellular/developmental states during hSSC differentiation, involving major transitions in cell-cycle and transcriptional regulators, splicing and signaling factors, and glucose/mitochondria regulators. Overall, our results outline the dynamic chromatin/transcription landscape operating in hSSCs and identify crucial molecular pathways that accompany the transition from quiescence to proliferation and differentiation.

A system of five purified proteins that assembles stable glucocorticoid receptor (GR)-hsp90 heterocomplexes has been reconstituted from reticulocyte lysate. Two proteins, hsp90 and hsp70, are required for the activation of steroid binding activity that occurs with heterocomplex assembly, and three proteins, Hop, hsp40, p23, act as co-chaperones that enhance activation and assembly (Morishima, Y., Kanelakis, K. C., Silverstein, A. M., Dittmar, K. D., Estrada, L., and Pratt, W. B. (2000) J. Biol. Chem. 275, 6894 -6900). Here we demonstrate that the first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent binding of hsp70 to the GR. After elimination of free hsp70, these preformed GR⅐hsp70 complexes can be activated to the steroid binding state by the hsp70 free assembly system in a second ATP-dependent step. hsp90 is required for opening of the steroid binding pocket and is converted to its ATP-dependent conformation during this second step. We predict that hsp70 in its ATP-dependent conformation binds initially to the folded receptor and is then converted to the ADP-dependent form with high affinity for hydrophobic substrate. This conversion initiates the opening of the hydrophobic steroid binding pocket such that it can now accept the hydrophobic binding form of hsp90, which in turn must be converted to its ATP-dependent conformation for the pocket to be accessible by steroid.Unliganded steroid receptors exist in cytosols in heterocomplexes with the abundant, ubiquitous, and essential heat shock protein hsp90 1 (for review, see Ref. 1). The glucocorticoid receptor (GR) must be in heterocomplex with hsp90 for it to have steroid binding activity (2, 3). The ligand binding domain (LBD) is the region of the receptor that interacts with hsp90 (1), and biochemical data (4) coupled with data from GR mutants (5, 6) support the notion (3) that formation of a complex with hsp90 opens up a hydrophobic pocket in the LBD to access by steroid. Steroid receptor⅐hsp90 heterocomplexes are formed in an ATP-dependent process by a multiprotein chaperone system that has been studied most extensively in reticulocyte lysate (7,8) but is present in lysates of both animal and plant cells (9).The receptor⅐hsp90 heterocomplex assembly system has now been reconstituted (10 -14), and five purified proteins, including hsp90, hsp70, 2 Hop (hsp organizer protein), hsp40, and p23, are required for optimally efficient assembly (for review of heterocomplex assembly, see Refs. 15 and 16). Only two of these proteins, hsp70 and hsp90, are absolutely required for opening the steroid binding cleft in the GR LBD, and the other three proteins act as co-chaperones that increase the overall efficiency of GR⅐hsp90 heterocomplex assembly (17).Hop binds independently to hsp90 and hsp70 to form an hsp90⅐Hop⅐hsp70 complex (18). Although Hop is not required for opening of the steroid binding cleft in the GR LBD, it increases the rate of the process (17). The peptide binding activity of hsp70 is coupled to the binding of ADP versus ATP (for review...

Unliganded steroid receptors are assembled into heterocomplexes with heat-shock protein (hsp) 90 by a multiprotein chaperone machinery. In addition to binding the receptors at the chaperone site, hsp90 binds cofactors at other sites that are part of the assembly machinery, as well as immunophilins that connect the assembled receptor-hsp90 heterocomplexes to a protein trafficking pathway. The hsp90-/hsp70-based chaperone machinery interacts with the unliganded glucocorticoid receptor to open the steroid-binding cleft to access by a steroid, and the machinery interacts in very dynamic fashion with the liganded, transformed receptor to facilitate its translocation along microtubular highways to the nucleus. In the nucleus, the chaperone machinery interacts with the receptor in transcriptional regulatory complexes after hormone dissociation to release the receptor and terminate transcriptional activation. By forming heterocomplexes with hsp90, the chaperone machinery stabilizes the receptor to degradation by the ubiquitin-proteasome pathway of proteolysis.

The fate and function of epigenetic marks during the germline-to-embryo transition is a key issue in developmental biology, with relevance to stem cell programming and transgenerational inheritance. In zebrafish, DNA methylation patterns are programmed in transcriptionally quiescent cleavage embryos; paternally inherited patterns are maintained, whereas maternal patterns are reprogrammed to match the paternal. Here, we provide the mechanism by demonstrating that "Placeholder" nucleosomes, containing histone H2A variant H2A.Z(FV) and H3K4me1, virtually occupy all regions lacking DNA methylation in both sperm and cleavage embryos and reside at promoters encoding housekeeping and early embryonic transcription factors. Upon genome-wide transcriptional onset, genes with Placeholder become either active (H3K4me3) or silent (H3K4me3/K27me3). Notably, perturbations causing Placeholder loss confer DNA methylation accumulation, whereas acquisition/expansion of Placeholder confers DNA hypomethylation and improper gene activation. Thus, during transcriptionally quiescent gametic and embryonic stages, an H2A.Z(FV)/H3K4me1-containing Placeholder nucleosome deters DNA methylation, poising parental genes for either gene-specific activation or facultative repression.

It is known that inhibition of histone deacetylases (HDACs) leads to acetylation of the abundant protein chaperone hsp90. In a recent study, we have shown that knockdown of HDAC6 by a specific small interfering RNA leads to hyperacetylation of hsp90 and that the glucocorticoid receptor (GR), an established hsp90 "client" protein, is defective in ligand binding, nuclear translocation, and gene activation in HDAC6-deficient cells (Kovacs, J. J., Murphy, P. J. M., Gaillard, S., Zhao, X., Wu, J-T., Nicchitta, C. V., Yoshida, M., Toft, D. O., Pratt, W. B., and Yao, T-P. (2005) Mol. Cell 18, 601-607). Using human embryonic kidney wild-type and HDAC6 (small interfering RNA) knockdown cells transiently expressing the mouse GR, we show here that the intrinsic properties of the receptor protein itself are not affected by HDAC6 knockdown, but the knockdown cytosol has a markedly decreased ability to assemble stable GR⅐hsp90 heterocomplexes and generate stable steroid binding activity under cell-free conditions. HDAC6 knockdown cytosol has the same ability to carry out dynamic GR⅐hsp90 heterocomplex assembly as wild-type cytosol. Addition of purified hsp90 to HDAC6 knockdown cytosol restores stable GR⅐hsp90 heterocomplex assembly to the level of wild-type cytosol. hsp90 from HDAC6 knockdown cytosol has decreased ATP-binding affinity, and it does not assemble stable GR⅐hsp90 heterocomplexes when it is a component of a purified five-protein assembly system. Incubation of knockdown cell hsp90 with purified HDAC6 converts the hsp90 to wild-type behavior. Thus, acetylation of hsp90 results in dynamic GR⅐hsp90 heterocomplex assembly/disassembly, and this is manifest in the cell as a ϳ100-fold shift to the right in the steroid dose response for gene activation.The hsp90/hsp70-based chaperone machinery regulates a wide variety of proteins involved in cellular signaling through the assembly of client protein⅐hsp90 heterocomplexes (reviewed in Ref. 1). The chaperones hsp90 and hsp70 play a key role in the balance between maintenance of protein integrity and degradation by the ubiquitin-proteasomal pathway, and inhibition of hsp90 function leads to degradation of hsp90 client proteins (2, 3). Yu et al. (4) have reported that treatment of cells with an inhibitor of histone deacetylases (see Refs. 5 and 6 for a review of deacetylase inhibitors) leads to acetylation of hsp90 and depletion of several hsp90 client proteins, including Raf-1, ErbB2, and mutant p53. This suggests that hsp90 function is affected by acetylation/deacetylation.Histone deacetylase (HDAC) 3 6 is a cytoplasmic HDAC that is associated with microtubules (7) and has been shown to regulate aggresome formation in response to misfolded protein stress (8). The cystic fibrosis transmembrane conducting regulator (CFTR) is a client protein of hsp90 (9), and a mutant form, CFTR-⌬508, is prone to misfolding and aggresome formation (reviewed in Ref. 10). Misfolded CFTR-⌬508 moves in a dynein-dependent fashion along microtubules to form the perinuclear aggresome (10). HDAC6 is ...

Numerous health consequences of tobacco smoke exposure have been characterized, and smoking’s effects on traditional measures of male fertility are well described. However, a growing body of data indicates that pre-conception paternal smoking also confers increased risk for a number of morbidities on offspring. The mechanism for this increased risk has not been elucidated, but it is likely mediated, at least in part, through epigenetic modifications transmitted through sperm. In this study, we investigated the impact of cigarette smoke exposure on sperm DNA methylation patterns in 78 men who smoke and 78 never-smokers using the Infinium HumanMethylation450 beadchip. We investigated two models of DNA methylation alterations: (1) consistently altered methylation at specific CpGs or within specific genomic regions and (2) stochastic DNA methylation alterations manifest as increased variability in genome-wide methylation patterns in men who smoke. We identified 141 significantly differentially methylated CpGs associated with smoking. In addition, we identified a trend toward increased variance in methylation patterns genome-wide in sperm DNA from men who smoke compared with never-smokers. These findings of widespread DNA methylation alterations are consistent with the broad range of offspring heath disparities associated with pre-conception paternal smoke exposure and warrant further investigation to identify the specific mechanism by which sperm DNA methylation perturbation confers risk to offspring health and whether these changes can be transmitted to offspring and transgenerationally.