Elsevier

Brain Research

Volume 1216, 24 June 2008, Pages 92-103
Brain Research

Research Report
Females exhibit more extensive amyloid, but not tau, pathology in an Alzheimer transgenic model

https://doi.org/10.1016/j.brainres.2008.03.079Get rights and content

Abstract

Epidemiological studies indicate that women have a higher risk of Alzheimer's disease (AD) even after adjustment for age. Though transgenic mouse models of AD develop AD-related amyloid beta (Abeta) and/or tau pathology, gender differences have not been well documented in these models. In this study, we found that female 3xTg-AD transgenic mice expressing mutant APP, presenilin-1 and tau have significantly more aggressive Abeta pathology. We also found an increase in beta-secretase activity and a reduction of neprilysin in female mice compared to males; this suggests that a combination of increased Abeta production and decreased Abeta degradation may contribute to higher risk of AD in females. In contrast to significantly more aggressive Abeta pathology in females, gender did not affect the levels of phosphorylated tau in 3xTg-AD mice. These results point to the involvement of Abeta pathways in the higher risk of AD in women. In addition to comparison of pathology between genders at 9, 16 and 23 months of age, we examined the progression of Abeta pathology at additional age points; i.e., brain Abeta load, intraneuronal oligomeric Abeta distribution and plaque load, in male 3xTg-AD mice at 3, 6, 9, 12, 16, 20 and 23 months of age. These findings confirm progressive Abeta pathology in 3xTg-AD transgenic mice, and provide guidance for their use in therapeutic research.

Introduction

Alzheimer's disease (AD) is a neurodegenerative disease that is the most common form of dementia. Only a small portion (< 1%) of cases have known genetic causes, while the majority of AD cases are sporadic (Cummings, 2004). The most significant non-genetic risk factor for AD is age; the risk of AD doubles between the age groups of 65–70 and 70–74. In addition to age, various factors, such as medical history (head injury, stroke, hypertension, hypercholesterolemia, stress, etc.), life style (diet, lack of exercise, alcohol consumption, smoking, etc) and education, may be associated with AD (McDowell, 2001). Epidemiological studies also indicate that women have a higher risk of AD (Brookmeyer et al., 1998) even after adjusting for age (Hy and Keller, 2000). The precise cause of the higher risk of AD in women is unknown.

The pathological hallmarks of AD are amyloid plaques in the extracellular space and intraneuronal neurofibrillary tangles (Hyman, 1997). Amyloid beta (Abeta), the primary component of amyloid plaques, is generated from the amyloid precursor protein (APP) by sequential proteolytic cleavage at the beta and gamma sites (Hardy and Selkoe, 2002). Freshly generated Abeta forms oligomers on and within neurons (Walsh et al., 2000) and compromises hippocampal long-term potentiation in vivo (Walsh et al., 2002). Neurofibrillary tangles are composed of hyperphosphorylated tau, a neuronal microtubule-associated protein. Phosphorylation of tau regulates its ability to promote microtubule assembly (Lindwall and Cole, 1984); hyperphosphorylation interferes with the normal biological functions of tau by reducing its ability to bind to and stabilize microtubules (Trojanowski and Lee, 1995). All genetic mutations that cause familial AD are linked to the Abeta cascade, while tau mutations are associated with AD and several other forms of dementia including frontotemporal dementia and progressive supranuclear palsy (Hardy and Selkoe, 2002). The clinical progression of AD is closely related to tau pathology (Braak and Braak, 1995). In animal models, modulation of Abeta or tau cascades alter pathology and influence cognitive function (Barten et al., 2005, SantaCruz et al., 2005).

To investigate the mechanisms of these pathological events and explore therapeutic strategies, various lines of transgenic mice have been created (McGowan et al., 2006). Overexpression of mutant APP elevates Abeta production leading to plaque formation (Games et al., 1995, Hsiao et al., 1996). Overexpression of mutant presenilin (PS)-1 elevates the level of endogenous Abeta slightly, but does not induce Abeta plaque deposition (Duff et al., 1996). Mice obtained by crossing APP and PS-1 transgenic mice show dramatically accelerated Abeta pathology (Holcomb et al., 1998) associated with oxidative stress and neuroinflammation (Matsuoka et al., 2001a, Matsuoka et al., 2001b). Overexpression of mutant tau causes hyperphosphorylation and tangle formation (Lewis et al., 2000). In many tau transgenic mice, tauopathy is more evident in the brainstem and spinal cord and is associated with motor dysfunction. Some tau transgenic mice, including the one used in this study, show tau pathology in the brain along with cognitive impairment (Ramsden et al., 2005).

Currently available AD mouse models are well-characterized, but the effects of gender on Abeta and tau pathology in animal models have not been carefully examined. Previously, gender differences have been studied in Tg2576 APP transgenic mice, which develop Abeta, but not tau pathology. In that study, female mice showed more histologically-determined Abeta plaques at 15 and 19 months of age and ELISA-quantified Abeta 1–40 (but not Abeta 1–42) load at 15 months of age (Callahan et al., 2001). Mice at these two time points bear Abeta plaques; the effect of gender at the pre-plaque stage was not evaluated. Abeta and tau pathology influence each other, but gender influences on the co-occurrence of these two aspects of AD pathology have not been examined. In this study, we examined the progression of AD-related pathology in male and female mice during the course of their lifespan using transgenic mice which develop both Abeta and tau pathology. In addition, we also investigated the progression of Abeta pathology in male mice at additional age points.

Section snippets

Soluble and total Abeta 1–40 and 1–42 are increased with aging after plaque formation

Our Abeta ELISAs were composed of N and C-terminus end-specific antibodies to quantify full-length Abeta 1–40 and 1–42 (Horikoshi et al., 2004). Both the Abeta 1–40 and 1–42 ELISAs provide linear (r2 > 0.99) standard curves over the range of 1–800 fmol/ml (pM). In this study, results from multiple plates needed to be combined; therefore, to assure quality control, we used a higher cut-off, 10 fmol/ml, which is equivalent to 220 and 6500 fmol/g tissue in diethylamine (DEA) and formic acid (FA)

Discussion

Epidemiological studies indicate that women have a higher risk of AD even after adjustment for age (Brookmeyer et al., 1998), the most important risk factor for AD (Hy and Keller, 2000). Transgenic mice expressing pathogenic genes of AD develop Abeta and/or tau pathology (McGowan et al., 2006); however, previous studies of AD transgenic mouse models have not necessarily reflected a consistent gender difference. In Tg2576 mice, the Abeta 1–40 level has been reported to be higher in females than

Animals and sampling

We used 3xTg-AD mice (Oddo et al., 2003) generated by co-microinjection of mutant APP (K670M/N671L) and tau (P301L) transgenes under the control of Thy 1.2 promoter into mutant PS-1 (M146V) knock-in mice (Guo et al., 1999). 3xTg-AD mice were created by a group at the University California, Irvine, in collaboration with a group at the National Institute on Aging. The colony was established at Georgetown University using breeding pairs provided by the National Institute on Aging after

Acknowledgments

We thank Ms. Hibiki Takenouchi, Department of Neurology, Georgetown University, for her technical assistance. Ms. Hibiki is a visiting student from Shiga University of Medical Science, Japan. We also thank Dr. Mary Ann Ottinger, University of Maryland, and Drs. Donna M. Barten and Margi Goldstein, Bristol-Myers Squibb for sharing their findings in the 3xTg-AD mouse colony maintained at their institute. Antibodies used for Abeta ELISA and Abeta immunostaining were generously provided by Dr.

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