Intracellular Aβ pathology and early cognitive impairments in a transgenic rat overexpressing human amyloid precursor protein: a multidimensional study

M Florencia Iulita, Simon Allard, Luise Richter, Lisa-Marie Munter, Adriana Ducatenzeiler, Christoph Weise, Sonia Do Carmo, William L Klein, Gerhard Multhaup, A Claudio Cuello

Numerous studies have implicated the abnormal accumulation of intraneuronal amyloid-beta (Abeta) as an important contributor to Alzheimer's disease (AD) pathology, capable of triggering neuroinflammation, tau hyperphosphorylation and cognitive deficits. However, the occurrence and pathological relevance of intracellular Abeta remain a matter of controversial debate. In this study, we have used a multidimensional approach including high-magnification and super-resolution microscopy, cerebro-spinal fluid (CSF) mass spectrometry analysis and ELISA to investigate the Abeta pathology and its associated cognitive impairments, in a novel transgenic rat model overexpressing human APP. Our microscopy studies with quantitative co-localization analysis revealed the presence of intraneuronal Abeta in transgenic rats, with an immunological signal that was clearly distinguished from that of the amyloid precursor protein (APP) and its C-terminal fragments (CTFs). The early intraneuronal pathology was accompanied by a significant elevation of soluble Abeta42 peptides that paralleled the presence and progression of early cognitive deficits, several months prior to amyloid plaque deposition. Abeta38, Abeta39, Abeta40 and Abeta42 peptides were detected in the rat CSF by MALDI-MS analysis even at the plaque-free stages; suggesting that a combination of intracellular and soluble extracellular Abeta may be responsible for impairing cognition at early time points. Taken together, our results demonstrate that the intraneuronal development of AD-like amyloid pathology includes a mixture of molecular species (Abeta, APP and CTFs) of which a considerable component is Abeta; and that the early presence of these species within neurons has deleterious effects in the CNS, even before the development of full-blown AD-like pathology.