Histone deacetylase 6: Biological Overview | References
Gene name - Histone deacetylase 6
Cytological map position - 13B6-13B6
Function - enzyme
Keywords - protein degradation, stress response, compensatory autophagy, 'histone deacetylase'
Symbol - HDAC6
FlyBase ID: FBgn0026428
Genetic map position - X: 15,226,988..15,234,388 [+]
Classification - protein deacetylase and zf-UBP
Cellular location - cytoplasmic
|Recent literature||Perry, S., Kiragasi, B., Dickman, D. and Ray, A.
(2017). The role of Histone
Deacetylase 6 in synaptic plasticity and memory. Cell Rep 18:
1337-1345. PubMed ID: 28178513
Histone deacetylases (HDACs) have been extensively studied as drug targets in neurodegenerative diseases, but less is known about their role in healthy neurons. This study tested zinc-dependent HDACs using RNAi in Drosophila melanogaster and found memory deficits with RPD3 and HDAC6. HDAC6 was found to be required in both the larval and adult stages for normal olfactory memory retention. Neuronal expression of HDAC6 rescues memory deficits, and the N-terminal deacetylase (DAC) domain is required for this ability. This suggests that deacetylation of synaptic targets associated with the first DAC domain, such as the active-zone scaffold Bruchpilot, is required for memory retention. Finally, electrophysiological experiments at the neuromuscular junction reveal that HDAC6 mutants exhibit a partial block of homeostatic plasticity, suggesting that HDAC6 may be required for the stabilization of synaptic strength. The learning deficit observed in HDAC6 mutants could be a behavioral consequence of these synaptic defects.
A prominent feature of late-onset neurodegenerative diseases is accumulation of misfolded protein in vulnerable neurons. When levels of misfolded protein overwhelm degradative pathways, the result is cellular toxicity and neurodegeneration. Cellular mechanisms for degrading misfolded protein include the ubiquitin-proteasome system (UPS), the main non-lysosomal degradative pathway for ubiquitinated proteins, and autophagy, a lysosome-mediated degradative pathway. The UPS and autophagy have long been viewed as complementary degradation systems with no point of intersection. This view has been challenged by two observations suggesting an apparent interaction: impairment of the UPS induces autophagy in vitro, and conditional knockout of autophagy in the mouse brain leads to neurodegeneration with ubiquitin-positive pathology. It is not known whether autophagy is strictly a parallel degradation system, or whether it is a compensatory degradation system when the UPS is impaired; furthermore, if there is a compensatory interaction between these systems, the molecular link is not known. This study shows that autophagy acts as a compensatory degradation system when the UPS is impaired in Drosophila melanogaster, and that histone deacetylase 6 (HDAC6; Barlow, 2001), a microtubule-associated deacetylase that interacts with polyubiquitinated proteins (Kawaguchi, 2003), is an essential mechanistic link in this compensatory interaction. Compensatory autophagy was induced in response to mutations affecting the proteasome and in response to UPS impairment in a fly model of the neurodegenerative disease spinobulbar muscular atrophy. Autophagy compensates for impaired UPS function in an HDAC6-dependent manner. Furthermore, expression of HDAC6 os sufficient to rescue degeneration associated with UPS dysfunction in vivo in an autophagy-dependent manner. This study suggests that impairment of autophagy (for example, associated with ageing or genetic variation) might predispose to neurodegeneration. Moreover, these findings suggest that it may be possible to intervene in neurodegeneration by augmenting HDAC6 to enhance autophagy (Pandey, 2007).
DTS7 is a temperature sensitive, dominant negative mutant of the β2 subunit of the proteasome (Smyth, 1999). Using the UAS/GAL4 system12, DTS7 expression was targeted to the Drosophila eye to cause tissue-restricted proteasome impairment. At 22°C, proteasome function is intact and eye morphology was normal (Fig. 1a). However, at 28°C substantial degeneration of the retina occurs owing to proteasome impairment. To investigate the role of HDAC6 in the setting of misfolded protein stress, transgenic flies were generated expressing wild-type Drosophila HDAC6 as well as wild-type and mutant versions of human HDAC6. Expression of either Drosophila HDAC6 or human HDAC6 strongly suppresses the degenerative phenotype associated with proteasome impairment. However, expression of a catalytically dead mutant of human HDAC6 (H216A;H611A) failed to modify the degenerative phenotype, indicating that the deacetylase function of HDAC6 is required for suppression. To assess the role of endogenous HDAC6, RNAi knockdown was used. Targeted knockdown of Drosophila HDAC6 did not noticeably alter eye morphology on its own, but strongly enhanced degeneration when the proteasome was impaired. HDAC6 did not modify the rough eye phenotype caused by ectopic expression of the positive regulator of cell death reaper, indicating that HDAC6 is not a general suppressor of cell death pathways. Ectopic expression of Drosophila HDAC3 and Drosophila HDAC11 did not suppress degeneration caused by proteasome impairment, indicating that this is not a general response of HDACs (Pandey, 2007).
Impaired UPS function has been implicated in a broad array of neurodegenerative disorders, but in vivo evidence is lacking. Spinobulbar muscular atrophy (SBMA) is an inherited neurodegenerative disease that is caused by polyglutamine (polyQ) repeat expansion in the androgen receptor (AR). Like most adult-onset neurodegenerative diseases, SBMA pathology features accumulation of ubiquitin-positive protein aggregates in vulnerable neurons. To develop a Drosophila model of SBMA, transgenic flies were generated expressing full-length human AR with 12-121 glutamine repeats using the UAS/GAL4 system. Flies expressing polyQ-expanded AR recapitulate key features of human SBMA, including ligand-dependent, polyQ length-dependent degeneration (Pandey, 2007).
To evaluate UPS function in this fly model of SBMA, transgenic flies were generated expressing a fluorescent reporter of UPS function. CL1-GFP is a fusion protein created by introducing a degradation signal to otherwise stable green fluorescent protein (GFP). This protein is rapidly degraded by the UPS, and its steady state levels reflect the functional status of this pathway. When stable GFP was expressed in eye imaginal discs from third-instar larvae, a robust fluorescent signal was detected by confocal microscopy. In contrast, eye imaginal discs from control flies expressing the CL1-GFP reporter emitted a low fluorescent signal, reflecting an active UPS. To test the ability of the UPS reporter flies to detect proteasome impairment in vivo, CL1-GFP was co-expressed in the eye with DTS7. At 22°C, CL1-GFP reporter levels remained low in eye imaginal discs co-expressing DTS7, consistent with normal proteasome function. In contrast, at 28°C, there was a significant increase in the CL1-GFP signal, demonstrating the ability of the reporter to detect proteasome impairment associated with a degenerative phenotype in vivo. UPS reporter RNA levels were not altered by the conditions used in these experiments (Pandey, 2007).
The CL1-GFP reporter in SBMA flies. In AR121 flies not exposed to ligand, fluorescent signal from the UPS reporter remained low, indicating that proteasome function was normal despite high expression of polyQ-expanded AR. However, flies reared on dihydrotestosterone (DHT), the natural ligand of AR, exhibited a significant increase in reporter signal, indicating proteasome impairment in association with induction of toxicity. The ligand-dependent nature of this finding indicates that UPS impairment is not merely a consequence of overexpressed AR121. Proteasome impairment by AR expression is a polyQ length-dependent phenomenon, because no impairment was observed in flies expressing AR12. The finding of proteasome impairment in SBMA flies is consistent with a prior report that polyQ toxicity in vivo is enhanced by proteasome mutations (Pandey, 2007).
The determination that there is impairment of the UPS in SBMA flies led to an examination of the ability of HDAC6 to modify the degenerative phenotype in this model of human neurodegenerative disease. Consistent with the results using proteasome mutant flies, ectopic expression of either Drosophila or human HDAC6 suppressed the ligand-dependent degenerative phenotype in flies expressing polyQ-expanded AR. Expression of the catalytically dead mutant of human HDAC6 (H216A;H611A) failed to modify the degenerative phenotype, indicating that the deacetylase function of HDAC6 is also required for suppression of polyQ toxicity. Knockdown of endogenous HDAC6 with RNAi enhanced ligand-dependent degeneration in AR52 flies. Thus, endogenous HDAC6 also plays a role in protecting cells from polyQ toxicity (Pandey, 2007).
Induction of autophagy and sequestration of polyQ-expanded AR in autophagic vacuoles has been reported in vitro (Taylor, 2003). Induction of autophagy in vitro in response to proteasome impairment has also been described. To determine whether autophagy is induced in vivo when the UPS is impaired, ultrastructural evaluation was performed by transmission electron microscopy (TEM) in the DTS7 and SBMA flies. In both cases, a significant increase was found in morphological features of autophagy. These included autophagic vacuoles such as early autophagosomes in which membranes surrounded cytoplasmic components, more mature autophagic vacuoles, multilamellar bodies and multivesicular bodies (Pandey, 2007).
To assess the role of autophagy when the UPS is impaired, autophagy was inhibited by RNAi knockdown of the autophagy genes atg6 and atg12. Knockdown of either atg6 or atg12 did not affect eye morphology, indicating that the Drosophila eye can tolerate reduced autophagy when UPS function is intact, at least in 1-day-old flies. In contrast, knocking down either atg6 or atg12 strongly enhanced the rough eye phenotype associated with UPS impairment in DTS7 flies reared at 28°C and in AR52 flies reared on DHT. From these data, it can be inferred that the autophagy induced by UPS impairment is compensatory (Pandey, 2007).
It was hypothesized that ectopic expression of HDAC6 suppressed degeneration by promoting autophagic degradation of aberrant protein. Thus, AR levels were examined in vivo, and it was determined that expression of HDAC6 leads to lower steady state levels of polyQ-expanded AR in vivo, whereas inhibition of autophagy by knockdown of atg6 or atg12 resulted in higher steady state levels. These altered steady state levels occurred despite no significant change in RNA levels, suggesting that HDAC6 accelerates the rate of AR degradation. To investigate this further, the inducible Geneswitch expression system was adapted to monitor protein turnover. In elav-GS;UAS-AR52 flies, no expression was detected before exposure to the inducing agent RU486. To induce expression, starved flies were fed sucrose media containing RU486 for one hour, which resulted in a pulse of expression that became detectable within 2 h, peaked after approximately 10 h, and then gradually decayed with a half-life of 100 min. In elav-GS;UAS-AR52;UAS-dHDAC6 flies, there was a parallel induction of AR52 expression, but an accelerated rate of decay, with a half-life of 50 min. Importantly, co-expression of Drosophila HDAC6 accelerated the turnover of not only AR52 monomers, but also high molecular weight aggregates that were trapped in the stacking gel (Pandey, 2007).
It was determined that treatment with rapamycin suppressed degeneration caused by either proteasome impairment or polyQ toxicity. This finding is consistent with a prior report, in which rapamycin suppressed degeneration in fly and mouse models of Huntington's disease. Rescue by rapamycin has been attributed to inhibition of TOR and induction of autophagy, although a role for other TOR-regulated pathways could not be excluded. This study found that knockdown of atg12 blocked the ability of rapamycin to suppress degeneration when the proteasome was impaired, verifying that rapamycin rescue is autophagy-dependent. Importantly, it was also determined that knockdown of Drosophila HDAC6 blocks the ability of rapamycin to suppress degeneration, indicating that Drosophila HDAC6 is essential in order for induction of autophagy to compensate for proteasome impairment. Furthermore, it was determined that the ability of Drosophila HDAC6 to suppress degeneration was autophagy-dependent, since rescue was blocked by knockdown of atg12. Thus, HDAC6 is integral to rescue of degeneration by autophagy and essential for autophagy to compensate for impaired UPS function (Pandey, 2007).
These findings extend previous studies in three important ways. First, it was determined that induction of autophagy is sufficient to rescue degeneration associated with UPS impairment, dramatically illustrating the compensatory relationship between autophagy and the UPS. Second, it was determined that HDAC6 activity is essential for autophagy to compensate for impaired UPS function. Finally, it was determined that ectopic expression of HDAC6 alone is sufficient to rescue degeneration caused by proteasome mutations and polyQ toxicity, and does so in an autophagy-dependent manner. These observations are consistent with a mechanism in which HDAC6 facilitates turnover of aberrant proteins by autophagy, lowering their steady state levels and mitigating toxicity. It was recently determined that overexpression of HDAC6 also suppressed degenerative phenotypes in additional models of neurodegenerative disease, including flies expressing pathologic A fragments and other polyQ-expanded proteins. Thus, the HDAC6-mediated pathway of protein clearance may have broad relevance to degenerative proteopathies (Pandey, 2007).
Although the current study indicates that the mechanism of HDAC6 rescue involves accelerated turnover of misfolded protein by autophagy, further study is required to determine the precise details of how this occurs. The mechanism could involve modulation of HSP90 activity, since this chaperone is a substrate of HDAC6 deacetylase activity (Kovacs, 2005). Alternatively, HDAC6 may be involved in shuttling polyubiquitinated substrates to a location conducive to engulfment by autophagosomes, consistent with a known role for HDAC6 in the formation of aggresomes in vitro (Kawaguchi, 2003). A third possibility is that HDAC6 may contribute to the transport of lysosomes to the site of autophagy, as suggested by the observation that HDAC6 knockdown results in dispersal of lysosomes away from the microtubule organizing centre (Iwata, 2005). Elucidating the precise role of HDAC6 in linking autophagy and the UPS promises substantial insights into cellular management of misfolded protein (Pandey, 2007).
An essential part of the cellular response to environmental stress is a reversible translational suppression, taking place in dynamic cytoplasmic structures called stress granules (SGs). In a study carried out in murine cultured cells, it was discovered that HDAC6, a cytoplasmic deacetylase that acts on tubulin and HSP90 and also binds ubiquitinated proteins with high affinity, is a novel critical SG component. HDAC6 interacts with another SG protein, G3BP (Ras-GTPase-activating protein SH3 domain-binding protein 1), and localizes to SGs under all stress conditions tested. Pharmacological inhibition or genetic ablation of HDAC6 abolishes SG formation. Intriguingly, it was found that the ubiquitin-binding domain of HDAC6 is essential and that SGs are strongly positive for ubiquitin. Moreover, disruption of microtubule arrays or impairment of motor proteins also prevents formation of SGs. These findings identify HDAC6 as a central component of the stress response, and suggest that it coordinates the formation of SGs by mediating the motor-protein-driven movement of individual SG components along microtubules (Kwon, 2007).
Reversible protein acetylation has emerged in recent years as one of the major forms of protein modifications whose importance has been particularly well documented in the case of the N-terminal histone tails, and of a few transcription factors such as p53 and STAT3. Acetylation and deacetylation are catalyzed by histone acetylases (HATs) and histone deacetylases (HDACs). HDAC6 is a unique class II deacetylase that contains two catalytic domains and also a C-terminal zinc finger domain (ZnF-UBP) binding with high-affinity free ubiquitin as well as mono- and polyubiquitinated proteins. HDAC6 is actively maintained in the cytoplasm (Verdel, 2000; Bertos, 2004), where it is found partly associated with the microtubule network. HDAC6 can deacetylate tubulin as well as the microtubule network in vivo (Hubbert, 2002; Matsuyama, 2002; Zhang, 2003). HDAC6 associates with the chaperone-like AAA ATPase p97/VCP, a protein that is critical for proteasomal degradation of misfolded proteins. Thereby, the ratio of HDAC6 and p97/VCP modulates the levels of polyubiquitinated aggregates (Boyault, 2006). HDAC6 also facilitates the clearance of misfolded ubiquitinated proteins, promoting their accumulation in an aggresome, and protects cells from apoptosis following stress induced by misfolded proteins (Kawaguchi, 2003). At the same time, HDAC6 also controls the induction of heat-shock proteins in response to the accumulation of ubiquitinated protein aggregates (Boyault, 2007). Furthermore, HDAC6 can deacetylate the chaperone Hsp90 and regulate its activity (Bali, 2005; Kovacs, 2005). Consequently, these different biochemical functions of HDAC6 impinge on diverse cellular processes. For example, HDAC6 function was found to be necessary for the formation of an immune synapse between antigen-presenting cells and T lymphocytes and also for nuclear translocation and transcription activation by the glucocorticoid receptor. Mice lacking HDAC6 are viable despite having highly elevated tubulin acetylation in multiple organs; in addition, they exhibit a moderately impaired immune response and also a mild phenotype in the bone (Kwon, 2007 and references therein).
One of the most immediate responses to cellular stress is a reversible block of mRNA translation, triggered by phosphorylation of the translation initiation factor eIF2α under the action of several stress-sensing kinases such as PKR or PERK. Thereby, translationally stalled mRNAs are sequestered in dynamic cytoplasmic structures called stress granules (SGs). These granules represent a complex assembly of initiation factors, such as eIF3 or eIF4E; proteins involved in translation control, such as T-cell intracellular antigen (TIA-1) or Fragile X mental retardation protein (FMR1); proteins implicated in RNA remodeling or degradation, such as HuR, tristetraproline (TTP), or Staufen; as well as 40S ribosome subunits. In addition, SGs also contain various polyadenylated mRNAs whose translation has been arrested. However, mRNAs encoding stress-induced proteins, such as heat-shock proteins, are excluded from SGs and are spared from translational inhibition. It is thought that SGs are sites where triage takes place in order to direct RNAs to degradation in processing bodies (PBs), or to recycle mRNAs for translation. In addition, very recent evidence suggests that parts of the microRNA pathway may also take place in SGs that contain Argonaute proteins and microRNAs such as let-7 (Kwon, 2007 and references therein).
This study reports the identification of G3BP-1 (RasGTPase-activating protein SH3 domain-binding protein 1, hereafter G3BP), an SG component, as a novel protein interacting with HDAC6 in vivo and in vitro. This protein is conserved between species, and orthologs are found in Drosophila, humans, and mice. G3BP has been implicated in modulating Ras activity and the cell cycle, by binding to the RasGAP protein. The precise function of G3BP is not understood yet, but it appears to be essential in the mouse where inactivation of the G3BP gene leads to embryonic lethality and growth retardation. G3BP has attracted attention recently as it was found to increase decay of the c-Myc RNA and to localize to SGs. This study shows that HDAC6 is recruited to SGs and that pharmacological HDAC inhibition leads to impaired SG assembly. Indeed, HDAC6-deficient mouse embryo fibroblasts (MEFs) fail to form SGs, although they exhibit normal phosphorylation of eIF2α in response to stress. Furthermore, inactivating mutations in the catalytic domains or in the ZnF-UBP domain of HDAC6 impair SG assembly. Moreover, SG formation is abolished by disruption of microtubule arrays or by impairment of dynein motor proteins. HDAC6 is required for the cells to recover from oxidative stress: In the absence of intact HDAC6 function, cells that have been treated with arsenite undergo apoptosis. Based on these results, it is proposed that HDAC6 is a central component of the stress response, regulating SG formation and potentially contributing to the control of RNA metabolism and translation (Kwon, 2007).
Search PubMed for articles about Drosophila Hdac6
Bali, P., et al. (2005). Inhibition of histone deacetylase 6 acetylates and disrupts the chaperone function of heat shock protein 90: A novel basis for antileukemia activity of histone deacetylase inhibitors. J. Biol. Chem. 280: 26729-26734. PubMed ID: 15937340
Barlow, A. L., et al. (2001). dSIR2 and dHDAC6: two novel, inhibitor-resistant deacetylases in Drosophila melanogaster. Exp. Cell Res. 265(1): 90-103. PubMed ID: 11281647
Bertos, N. R., Gilquin, B., Chan, G. K., Yen, T. J., Khochbin, S. and Yang, X. J. (2004). Role of the tetradecapeptide repeat domain of human histone deacetylase 6 in cytoplasmic retention. J. Biol. Chem. 279: 48246-48254. PubMed ID: 15347674
Boyault, C., Gilquin, B., Zhang, Y., Rybin, V., Garman, E., Meyer-Klaucke, W., Matthias, P., Muller, C. W. and Khochbin, S. (2006). HDAC6-p97/VCP controlled polyubiquitin chain turnover. EMBO J. 25: 3357-3366. PubMed ID: 16810319
Boyault, C., et al. (2007). HDAC6 controls major cell response pathways to cytotoxic accumulation of protein aggregates. Genes Dev. 21: 2172-2181. PubMed ID: 17785525
Hubbert, C., Guardiola, A., Shao, R., Kawaguchi, Y., Ito, A., Nixon, A., Yoshida, M., Wang, X. F., and Yao, T. P. (2002). HDAC6 is a microtubule-associated deacetylase. Nature 417: 455-458. PubMed ID: 12024216
Iwata, A., Riley, B. E., Johnston, J. A. and Kopito, R. R. (2005). HDAC6 and microtubules are required for autophagic degradation of aggregated huntingtin. J. Biol. Chem. 280: 40282-40292. PubMed ID: 16192271
Kawaguchi, Y. et al. (2003). The deacetylase HDAC6 regulates aggresome formation and cell viability in response to misfolded protein stress. Cell 115: 727-738. PubMed ID: 14675537
Kovacs, J. J. et al. (2005). HDAC6 regulates Hsp90 acetylation and chaperone-dependent activation of glucocorticoid receptor. Mol. Cell 18: 601-607. PubMed ID: 15916966
Kwon, S., Zhang, Y. and Matthias, P. (2007). The deacetylase HDAC6 is a novel critical component of stress granules involved in the stress response. Genes Dev. 21(24): 3381-94. PubMed ID: 18079183
Matsuyama, A., Shimazu, T., Sumida, Y., Saito, A., Yoshimatsu, Y., Seigneurin-Berny, D., Osada, H., Komatsu, Y., Nishino, N., Khochbin, S., et al. (2002). In vivo destabilization of dynamic microtubules by HDAC6-mediated deacetylation. EMBO J. 21: 6820-6831. PubMed ID: 12486003
Pandey, U. B., et al. (2007). HDAC6 rescues neurodegeneration and provides an essential link between autophagy and the UPS. Nature 447(7146): 859-63. PubMed ID: 17568747
Smyth, K. A. and Belote, J. M. (1999). The dominant temperature-sensitive lethal DTS7 of Drosophila melanogaster encodes an altered 20S proteasome -type subunit. Genetics 151: 211-220. PubMed ID: 9872961
Taylor, J. P. et al. (2003). Aggresomes protect cells by enhancing the degradation of toxic polyglutamine-containing protein. Hum. Mol. Genet. 12: 749-757. PubMed ID: 12651870
Verdel, A., Curtet, S., Brocard, M.P., Rousseaux, S., Lemercier, C., Yoshida, M., and Khochbin, S. (2000). Active maintenance of mHDA2/mHDAC6 histone-deacetylase in the cytoplasm. Curr. Biol. 10: 747-749. PubMed ID: 10873806
Zhang, Y., Li, N., Caron, C., Matthias, G., Hess, D., Khochbin, S. and Matthias, P. (2003). HDAC-6 interacts with and deacetylates tubulin and microtubules in vivo. EMBO J. 22: 1168-1179. PubMed ID: 12606581
date revised: 22 June 2008
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