Histones—which act like spools to wrap and compact DNA in the cell—keep genes relatively quiet until acetyl groups are added to the proteins and the DNA unwinds. Histone deacetylases (HDACs)—18 types in all—yank off those gene-freeing acetyl groups to quiet gene expression again. If HDACs are too active, histone acetylation may run amok and important genes stay repressed.

Tsai's lab has probed the role of HDACs in AD for some years. Previously, the team found that general HDAC inhibitors, which lead to increased histone acetylation, restored lost long-term memories and enhanced learning in a mouse model of AD (ARF related news story on Fischer et al., 2007). The lab then showed that a particular class I HDAC called HDAC2 was important in learning and memory. Overexpression in mice quashed synaptic density and memory formation, while underexpression did the opposite (see ARF related news story on Guan et al., 2009). It was unclear, however, how HDAC2 exerted its effects.

To probe HDAC2's action, first author Johannes Gräff and colleagues looked at expression in the CK-p25 mouse developed in Tsai’s lab. This transgenic model of neurodegenerative disease expresses p25, a truncated form of p35 that activates cyclin-dependent kinase 5 (Cdk5). The mice develop Aβ and tau pathology, and reduced synaptic density and neuron loss (ARF related news story). Comparing this model to wild-type mice, Gräff and colleagues found higher levels of HDAC2 in hippocampal area CA1 and in the prefrontal cortex, while related class I HDACs (HDAC1 and HDAC3, also involved in memory formation) remained normal. A different, 5xFAD mouse model showed similar elevation in HDAC2. Chromatin immunoprecipitation revealed enriched HDAC2 binding to genes involved in learning and memory, including genes that are tuned down in humans with AD. These included immediate-early genes such as Homer1 and Arc, and genes related to synaptic plasticity, such as certain glutamate receptor subunits and synaptophysin. Histones associated with those genes sported fewer acetyl groups than usual and expression of those genes dropped off. HDAC2 seemed to be silencing genes involved in neuroplasticity.

Would knocking down HDAC2 restore gene expression? To find out, the team injected HDAC2-specific short hairpin RNAs (shRNA) into the hippocampal CA1 areas of CK-p25 mice. They injected, as controls, scrambled, non-specific shRNAs into CK-p25 and wild-type mice. The shRNA squashed HDAC2 levels and boosted mRNA expression of the target genes involved in learning and memory. Synaptic plasticity and abundance of dendrites, as visualized by MAP2 immunoreactivity, exceeded that of control CK-p25 mice and nearly matched levels of wild-type mouse controls. Knocking down HDAC2 did not boost the number of surviving neurons in CK-p25 mice. However, compared to controls, the animals’ associative memory in fear-conditioning tasks returned to normal and the mice also found refuge on the hidden platform in the Morris water maze more quickly. Treated mice spent as much time as wild-type mice in the correct quadrant in probe trials the next day. "The good news is that, even though there's an epigenetic blockade, this is potentially reversible," said Tsai.

To find Aβ's link to a surge in HDAC2 expression, the researchers exposed cultured hippocampal neurons to synthetic peptide. HDAC2 mRNA levels rose. What drove the increased expression? In the promoter region of HDAC2, Gräff and colleagues found a recognition element for glucocorticoid receptor 1 (GR1), a transcriptional activator when phosphorylated during stress. They wondered if stress brought on by Aβ might activate GR1. Sure enough, after application of Aβ42 to cultured wild-type hippocampal neurons, GR1 displayed more phosphate and bound to the HDAC2 promoter, enhancing HDAC2 transcription. Phosphorylated GR1 also decorated HDAC2 promoters in CK-p25 hippocampi, where the researchers saw an increase in HDAC2 transcription. This provides a path between neurotoxic Aβ and HDAC2 expression, said the authors.

"I think now we offer a novel explanation for the pathogenesis of Alzheimer's disease, and how the Aβ peptide can exert a long-term effect in the brain," said Tsai. "It can increase HDAC2 and cause a chronic blockade of expression of genes necessary for memory formation." Upon examination of postmortem human brains with AD, the team then found that HDAC2—but not HDAC1 or HDAC3—was present at higher levels in the CA1 and entorhinal areas, even in early stages of the disease. "This advocates for the development of HDAC2 inhibitors," added Tsai, who reported no involvement in companies working on HDAC inhibitors.

The implication that human AD deficits may be reversible "should create excitement in the patient community, especially given the fact that HDAC inhibitors are currently being explored in the clinic for a variety of conditions," said Randal Tibbetts, University of Wisconsin, Madison, in an e-mail to ARF. HDAC inhibitors are already used in cancer treatment, and companies including Envivo Pharmaceuticals are pursuing them as potential AD therapeutic targets. Since the authors only looked for certain genes that are suppressed by HDAC2, more research is needed to determine if others are affected as well, he added. Tsai said she and her team plan to do that next.

This provides an intriguing mechanism for one of Aβ's detrimental effects, said Jang-Ho Cha of Merck, North Wales, Pennsylvania. "One of the big gaps in Alzheimer's study is understanding what Aβ is doing," Cha said. "It is not clear that this is the only thing it is doing, but this pathway they are mapping out is really elegant." Addition and removal of acetyl groups are fairly transient forms of gene regulation, but like blood attracts sharks, HDACs attract enzymes that more permanently alter the accessibility of DNA's coils. For instance, some enzymes methylate DNA. Such enzymes might provide more selective drug targets than HDAC inhibitors, which may affect more than one HDAC, Cha pointed out.—Gwyneth Dickey Zakaib.

Reference:
Gräff J, Rei D, Guan JS, Wang WY, Seo J, Hennig KM, Nieland TJF, Fass DM, Kao PF, Kahn M, Su SC, Samiei A, Joseph N, Haggarty SJ, Delalle I, Tsai LH. An epigenetic blockade of cognitive functions in the neurodegenerating brain. Nature. 1 March 2012; 483(7387). Abstract