Central CRF system perturbation in an Alzheimer's disease knockin mouse model

Abstract

Alzheimer's disease (AD) is often accompanied by changes in mood as well as increases in circulating cortisol levels, suggesting that regulation of the stress responsive hypothalamic-pituitary-adrenal (HPA) axis is disturbed. Here, we show that amyloid precursor protein (APP) is endogenously expressed in important limbic, hypothalamic, and midbrain nuclei that regulate hypothalamic-pituitary-adrenal axis activity. Furthermore, in a knockin mouse model of AD that expresses familial AD (FAD) mutations of both APP with humanized amyloid beta (hAβ), and presenilin 1 (PS1), in their endogenous patterns (APP/hAβ/PS1 animals), corticotropin releasing factor (CRF) levels are increased in key stress-related nuclei, resting corticosteroid levels are elevated, and animals display increased anxiety-related behavior. Endocrine and behavioral phenotypes can be normalized by loss of 1 copy of CRF receptor type-1 (Crfr1), consistent with a perturbation of central CRF signaling in APP/hAβ/PS1 animals. However, reductions in anxiety and corticosteroid levels conferred by heterozygosity of CRF receptor type-1 do not improve a deficit in working memory observed in APP/hAβ/PS1 mice, suggesting that perturbations of the CRF system are not the primary cause of decreased cognitive performance.

Introduction

Alzheimer's disease (AD) is characterized by the progressive loss of cognitive ability and eventual dementia. On autopsy, AD patients present with amyloid plaques and neurofibrillary tangles in addition to neurodegeneration, together considered the pathological hallmarks of AD. The most well characterized outcome of these neuropathologies is the progressive loss of memory, however, other brain systems have been shown to misfunction with the onset of AD. For example, AD patients exhibit elevated cortisol levels (Davis et al., 1986, Hartmann et al., 1997), indicating a progressive imbalance in the function of the hypothalamic-pituitary adrenal (HPA) axis, the endocrine axis that is initiated by corticotropin releasing factor (CRF) release by the hypothalamus and coordinates the release of corticosteroids by the adrenal glands (Vale et al., 1981). Imbalance in HPA axis function can directly impact emotional status, responses to stress, and cognitive ability, and increased HPA axis activity in many cases accompanies depression (Owens and Nemeroff, 1991). Interestingly, changes in mood, anxiety, and depression often precede or present concomitantly with the earliest signs of memory loss or mild cognitive impairment (MCI) in AD, and are found to precede dementia in many families carrying familial Alzheimer's disease (FAD) mutations in amyloid precursor protein (APP) and presenilin 1 (PS1), leading many to propose that depression is perhaps one of the earliest signs of progressing AD pathology (Feldman et al., 2004, Gabryelewicz et al., 2004, Kasuga et al., 2009, Mullan et al., 1993, Panza et al., 2010, Rozzini et al., 2008). Indeed, patients with higher levels of corticosteroids suffer more rapid progression of dementia (Csernansky et al., 2006, Davis et al., 1986, Lupien et al., 1998, Weiner et al., 1997). However, it remains unclear whether stress and increased cortisol accelerate dementia or are in fact a symptom of progressing AD pathogenesis.

While the cause of increased anxiety and corticosteroid levels in AD is still unknown, the impact of stress and corticosteroids on the development and progression of AD pathogenesis is more thoroughly understood. In transgenic animal models of AD, increased stress leads to higher amyloid beta (Aβ) and Aβ oligomer levels, increased phosphorylated tau, and accelerated amyloid plaque deposition (Dong et al., 2004, Rissman et al., 2007, Rothman et al., 2011, Touma et al., 2004). This may be due to stress-induced elevations in corticosteroid levels, which have been shown to have many of the same effects (Budas et al., 1999, Catania et al., 2009, Green et al., 2006). These studies suggest that animals predisposed to succumb to AD are sensitized to the effects of stress and stress hormones, which can severely provoke the progression of AD pathogenesis. However, reports on the status of stress responsive circuitry and the HPA axis at a baseline state, before stress has been given have been inconsistent. In various AD transgenic mouse models, anxiety-related behavior has been reported to be reduced (Gil-Bea et al., 2007, Harris et al., 2010, Jawhar et al., 2012, Lalonde et al., 2003), increased (Dong et al., 2008, Lee et al., 2004, Sterniczuk et al., 2010, Touma et al., 2004), or not affected (Arendash et al., 2001). Changes in corticosteroid levels have also varied in reports using different AD model animals (Dong et al., 2008, Gil-Bea et al., 2007, Touma et al., 2004). These differences might be partially explained by differences between the heterologous promoters used to misexpress APP, which do not necessarily recapitulate the endogenous expression pattern of APP, and might lead to APP gain-of-function phenotypes not typically present in human AD.

To examine stress responsive neural and endocrine pathways in the context of endogenous expression of APP mutations, we have taken advantage of a knockin AD mouse model (APP/hAβ/PS1) in which a mutant APP allele carrying the disease causing FAD Swedish (K670N/M671L) and London (V717F) mutations with a humanized Aβ (hAβ) sequence has replaced mouse APP (Köhler et al., 2005). Wild type PS1 has been replaced by mouse PS1 carrying the M146V FAD mutation (Guo et al., 1999). APP/hAβ/PS1 mice display endogenous temporal and spatial expression of mutant forms of APP and PS1, and model AD in the absence of high levels of misexpression.