Cholinotrophic basal forebrain system alterations in 3xTg-AD transgenic mice

The cholinotrophic system, which is dependent upon nerve growth factor and its receptors for survival, is selectively vulnerable in Alzheimer's disease (AD). But, virtually nothing is known about how this deficit develops in relation to the hallmark lesions of this disease, amyloid plaques and tau containing neurofibrillary tangles. The vast majority of transgenic models of AD used to evaluate the effect of beta amyloid (Aβ) deposition upon the cholinotrophic system over-express the amyloid precursor protein (APP). However, nothing is known about how this system is affected in triple transgenic (3xTg)-AD mice, an AD animal model displaying Aβ plaque- and tangle-like pathology in the cortex and hippocampus, which receive extensive cholinergic innervation. We performed a detailed morphological and biochemical characterization of the cholinotrophic system in young (2–4 months), middle-aged (13–15 months) and old (18–20 months) 3xTg-AD mice. Cholinergic neuritic swellings increased in number and size with age, and were more conspicuous in the hippocampal–subicular complex in aged female than in 3xTg-AD male mice. Stereological analysis revealed a reduction in choline acetyltransferase (ChAT) positive cells in the medial septum/vertical limb of the diagonal band of Broca in aged 3xTg-AD mice. ChAT enzyme activity levels decreased significantly in the hippocampus of middle-aged 3xTg-AD mice compared to age-matched non-transgenic (or wild type) mice. ProNGF protein levels increased in the cortex of aged 3xTg-AD mice, whereas TrkA protein levels were reduced in a gender-dependent manner in aged mutant mice. In contrast, p75NTR protein cortical levels were stable but increased in the hippocampus of aged 3xTg-AD mice. These data demonstrate that cholinotrophic alterations in 3xTg-AD mice are age- and gender-dependent and more pronounced in the hippocampus, a structure more severely affected by Aβ plaque pathology.

Research Highlights
► This article is the first qualitative and quantitative biochemical analysis of the cholinergic system in the cortex and hippocampus in young, middle-aged and aged 3xTg-AD mice.
► This article is the first quantitative biochemical analysis of changes NGF/proNGF and its high (TrkA) and low (p75NTR) receptors in the cortex and hippocampus of young, middle-aged and aged 3xTg-AD mice.
► This article is the first stereological unbiased counting analysis demonstrating an age-related reduction in cholinergic medial septal diagonal band neurons in 3xTg-AD mice.
► The current study is the first to demonstrate that cholinotrophic alterations are age- and gender-dependent and more pronounced in the hippocampus, a structure more severely affected by Aβ plaque pathology than the cortex in 3xTg-AD mice.