Degeneration of β-amyloid-associated cholinergic structures in transgenic APP_SW mice

Abstract

Cholinergic dysfunction is a consistent feature of Alzheimer’s disease, and the interrelationship between β-amyloid deposits, inflammation and early cholinergic cell loss is still not fully understood. To characterize the mechanisms by which β-amyloid and pro-inflammatory cytokines may exert specific degenerating actions on cholinergic cells ultrastructural investigations by electron microscopy were performed in brain sections from transgenic Tg2576 mice that express the Swedish double mutation of the human amyloid precursor protein and progressively develop β-amyloid plaques during aging. Both light and electron microscopical investigations of the cerebral cortex of 19-month-old transgenic mice revealed a number of pathological tissue responses in close proximity of β-amyloid plaques, such as activated microglia, astroglial proliferation, increased number of fibrous astrocytes, brain edema, degeneration of nerve cells, dendrites and axon terminals. Ultrastructural detection of choline acetyl transferase (ChAT)-immunostaining in cerebral cortical sections of transgenic mice clearly demonstrated degeneration of ChAT-immunoreactive fibres in the environment of β-amyloid plaques and activated glial cells suggesting a role of β-amyloid and/or inflammation in specific degeneration of cholinergic synaptic structures.

Introduction

Cholinergic dysfunction is a consistent feature of Alzheimer’s disease [9]. Co-localization of acetylcholinesterase (AChE) with β-amyloid deposits in Alzheimer brains [25], [26] as well as the capability of AChE to affect processing of amyloid precursor protein (APP) [27] and aggregation of β-amyloid peptides [15] strongly suggested a link between APP metabolism and cholinergic neurotransmission (see, e.g., Ref. [5]). This was recently emphasized by using transgenic approaches. Transgenic mice expressing the C-terminal fragment of the amyloid precursor protein demonstrated increased tissue levels of AChE [29], while double transgenic mice expressing both mutations of human APP and α-synuclein showed a prominent age-dependent degeneration of cholinergic neurons in the nucleus basalis and caudate-putamen [21]. In the APP23 mouse model of cerebral amyloidosis, modest β-amyloid plaque-associated cholinergic changes in the aged neocortex were observed with no loss of cholinergic basal forebrain neurons [6]. In contrast, a significant increase in p75-neurotrophin receptor-immunoreactive cells has been observed in basal forebrain of doubly transgenic mice containing both the Swedish mutation of human APP and a mutated presenilin1 as transgenes, as compared to non-transgenic or single transgenic mice, suggesting a neurotrophic role of mutant APP and presenilin in cholinergic cells [17].

The extracellular deposition of β-amyloid plaques in Alzheimer’s disease is also accompanied by a significant plaque-associated micro- and astrogliosis, complemented by the expression of a number of inflammatory cytokines (see, e.g., Ref. [1]). The capacity of β-amyloid to induce pro-inflammatory cytokines such as interleukin (IL)-1 in activated glial cells has been demonstrated in a number of studies including transgenic approaches with expression of human APP and/or presenilin-1 as transgenes [2], [12], [17]. On the other hand, IL-1 has also been found to up-regulate the APP expression, to stimulate the amyloidogenic pathway of APP processing [7], [8], [10], [11], [28] and to induce expression and phosphorylation of the tau-protein [32] pointing to a very complex role of cytokines in the pathogenesis of Alzheimer’s disease. This is further emphasized from studies in post mortem brain tissue from Alzheimer patients demonstrating a correlation of enhanced glial IL-1 expression with the progressive accumulation of neuritique plaques and neurodegeneration [31], and from studies in transgenic APP_SW mice suggesting a role of glial cells in fibrillary plaque evolution [34]. However, IL-1 has also been found to promote activity and expression of AChE both in cell cultures and in vivo [20] suggesting also a link between IL-1 expression and cholinergic degeneration. This is further emphasized by our recent study in transgenic Alzheimer-like Tg2576 mice demonstrating plaque-associated gliosis that is accompanied by subtle and differential changes in cholinergic synaptic markers [3]. To further characterize the mechanisms by which β-amyloid and pro-inflammatory cytokines may exert specific degenerating actions on cholinergic cells ultrastructural investigations by electron microscopy may represent a useful tool. In particular, the question should be addressed, whether the amyloid plaque-associated neuropathology including glial reactions and brain edema may result into specific degenerations of cholinergic structures.