Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss and cognitive decline, and is the most common form of dementia worldwide. The hallmark pathological features of AD involve misfolding and aggregation of 2 proteins including the extracellular accumulation of amyloid-β peptide (Aβ) in plaques and aggregation of hyper-phosphorylated tau in neurofibrillary tangles. Although genetic and biochemical studies suggest a cardinal role for Aβ and its expression precedes dementia by many years (Davis et al., 1999), the mechanism by which Aβ induces neurodegeneration is unclear. Only a minimal percentage of all AD cases are early onset and arise through autosomal inheritance of 1 of several causative genetic mutations, including the genetic precursor to Aβ, the amyloid precursor protein (APP) gene (Selkoe, 2001). The majority of AD cases are sporadic and are not attributed to specific genetic mutations.
The etiology and neuropathological progression of sporadic AD is associated with multiple environmental events and mechanisms. During the progression of AD pathophysiology, the interplay between genetic and environmental factors may play a crucial role. Gene expression can be modulated without changes to DNA sequence, by epigenetic mechanisms such as histone modifications, binding of non-histone proteins and DNA methylation. Although epigenetic changes are often heritable, they may also be induced spontaneously and in response to environmental factors, and such modifications may critically contribute to the progression of AD pathophysiology. (Bannister and Kouzarides, 2011; Waddington, 1957).
It is known that epigenetic modifications by DNA methylation are altered in the promoter region of neuronal genes in AD (Lee and Ryu, 2010), and histone methylation and phosphorylation are increased in AD brain as well as in response to numerous signaling events (Lee and Ryu, 2010; Ogawa et al., 2003; Sweatt, 2010). In addition, AD and AD mouse models (associated with aberrant APP processing) demonstrate altered gene transcription and histone acetylation (Chouliaras et al., 2010; Kilgore et al., 2010; Robakis 2003; Walker et al., 2012). Using both in vitro and in vivo model systems, we sought to identify and localize changes in epigenetic marks at the histone level associated with Aβ secretion and exposure using SH-SY5YAPPSWE cells, primary cortico/hippocampal neurons, Tg2576 (a human APPSWE transgenic mouse model), and AD occipital cortex. During the early stages of AD, even in prodromal AD, it is a challenge to detect Aβ in brain and serum and to identify the prevalent structure (i.e., monomeric, dimeric, trimeric, oligomeric, etc.) (Bao et al., 2012; Fukumoto et al., 2010; Gong et al., 2003; Klyubin et al., 2008; Lesne et al., 2006; McDonald et al., 2012; Selkoe, 2008). A significant Aβ load in brain, measured biochemically or by PET imaging, is highly correlated with progression from mild cognitive impairment to AD (Brys et al., 2009; Forsberg et al., 2008; Hansson et al., 2006; Mattsson et al., 2009; Nordberg, 2011; Okello et al., 2009;Visser et al., 2009; Waragai et al., 2009; Wolk et al., 2009). PET amyloid imaging (using Pittsburgh compound B, a thioflavin T fluorescent analog) also indicates that amyloid deposition in the brain occurs in non-demented elderly individuals and before the onset of cognitive symptoms in AD patients (Chetelat et al., 2011; Pike et al., 2007; Villemagne et al., 2008). Moreover, increased cortical PIB is associated with discrete episodic memory impairments in healthy (i.e. non-demented) elderly subjects (Pike et al., 2007). In addition, many AD mouse models demonstrate the same trends in amyloid load (Duyckaerts et al., 2008; Morrissette et al., 2009). Many studies demonstrate that soluble Aβ (Aβsol) oligomers cause early-onset structural and functional changes to neurons and may be the trigger for cognitive dysfunction and decline observed in AD (Cleary et al., 2005; Hu et al., 2008; Jin et al., 2011; Klyubin et al., 2008; Lacor et al., 2004; Lacor et al., 2007; Lambert et al., 1998; Lesne et al., 2006; Mucke et al., 2000; Renner et al., 2010; Selkoe 2002; Selkoe 2008; Shankar et al., 2007; Walsh et al., 2002; Zempel et al., 2010).
Our hypothesis is that Aβsol induces disruption of histone homeostasis early during the progression of AD and, in turn, is involved in the progressive cognitive decline observed in AD patients. Work from our laboratory and others’ has established that epigenetic mechanisms at the histone level, particularly histone H3 (Chwang et al., 2006; Levenson et al., 2004), are important for learning and memory function (Day and Sweatt, 2011b). The objective of the present studies was to determine whether epigenetic changes at the histone level were associated with Aβsol oligomers, and furthermore, whether changes are detectable before or after the onset of plaques and memory dysfunction in an AD murine model and human AD brain. We narrowed our focus to investigate total histone H3 (TH3) and 3 key modifications associated with regulating transcriptional activation and inhibition: acetylated lysine 14 (AcH3), phosphorylated serine (S) 10 (PH3), and dimethylated lysine 9 (2MeH3). Our results indicate that Aβsol oligomers may be a potent signaling molecules indirectly modulating transcriptional activity by acetylating and methylating H3 lysine residues.
