Here we demonstrated that extracellular, not intracellular, amyloid-beta (Abeta) and the associated cytotoxic glial neuroinflammatory response are major contributors to early neuronal loss in a PS1xAPP model. A significant loss of principal (27%) and SOM/NPY (56-46%) neurons was found in the entorhinal cortex at 6 months of age. Loss of principal cells occurred selectively in deep layers (primarily layer V) whereas SOM/NPY cell loss was evenly distributed along the cortical column. Neither layer V pyramidal neurons nor SOM/NPY interneurons displayed intracellular Abeta immunoreactivity, even after formic acid retrieval; thus, extracellular factors should be preferentially implicated in this selective neurodegeneration. Amyloid deposits were mainly concentrated in deep layers at 4-6 months, and of relevance was the existence of a potentially cytotoxic inflammatory response (TNFalpha, TRAIL, and iNOS mRNAs were upregulated). Moreover, non-plaque associated activated microglial cells and reactive astrocytes expressed TNFalpha and iNOS, respectively. At this age, in the hippocampus of same animals, extracellular Abeta induced a non-cytotoxic glial activation. The opposite glial activation, at the same chronological age, in entorhinal cortex and hippocampus strongly support different mechanisms of disease progression in these two regions highly affected by Abeta pathology.
The detection of the early phenotypic modifications of Alzheimer's disease (AD) models is fundamental to understand the progression and identify pharmacologic targets of this pathology. However, a large variability within different models and between age-matched mice from the same model has been observed. This variability could be due to heterogeneity in the Abeta production. Present results showed the existence of a large variability in the Abeta deposition in both hippocampus and cortex in 6-month-old PS1xAPP mice. This variability was not due to the expression of hAPP751SL, however, linear relationship between hPS1M146L mRNA and Abeta production was identified. The Abeta content was related to the incorporation of the hPS1M146L into functional gamma-secretase complexes, detected by the presence of the corresponding human or endogenous PS1-CTFs. Animals expressing low amount of hPS1M146L mRNA, displayed low hPS1-CTF incorporation and produced a low amount of Abeta peptides. Conversely, mice with relatively high hPS1 mRNA expression displayed high hPS1-CTF and high Abeta deposition. Furthermore, the Abeta total and Abeta1-42 content was increased dramatically by the expression of hPS1M146L (as compared with transgenic APPsl littermates). Therefore, variations in the expression of transgenic form of hPS1M146L in this model, or even between different models, influenced strongly the incorporation of the mutated PS1 into functional gamma-secretase complexes, the production of Abeta peptides and, in consequence, the detrimental effects of Abeta peptides. These data might implicate an "apparent gain-of-function" of the gamma-secretase complex by the expression of the mutated PS1M146L.
The presence of two heterologous alpha subunits and a single benzodiazepine binding site in the GABA(A) receptor implicates the existence of pharmacologically active and inactive alpha subunits. This fact raises the question of whether a particular alpha subtype could predominate performing the benzodiazepine binding site. The hippocampal formation expresses high levels of alpha subunits with different benzodiazepine binding properties (alpha1, alpha2 and alpha5). Thus, we first demonstrated the existence of alpha2-alpha1 (36.3 +/- 5.2% of the alpha2 population) and alpha2-alpha5 (20.2 +/- 2.1%) heterologous receptors. A similar alpha2-alpha1 association was observed in cortex. This association allows the direct comparison of the pharmacological properties of heterologous native GABA(A) receptors containing a common (alpha2) and a different (alpha1 or alpha5) alpha subunit. The alpha2 subunit pharmacologically prevailed over the alpha1 subunit in both cortex and hippocampus (there was an absence of high-affinity binding sites for Cl218,872, zolpidem and [3H]zolpidem). This prevalence was directly probed by zolpidem displacement experiments in alpha2-alpha1 double immunopurified receptors (K(i) = 295 +/- 56 nM and 200 +/- 8 nM in hippocampus and cortex, respectively). On the contrary, the alpha5 subunit pharmacologically prevailed over the alpha2 subunit (low- and high-affinity binding sites for zolpidem and [3H]L-655,708, respectively). This prevalence was probed in alpha2-alpha5 double immunopurified receptors. Zolpidem displayed a single low-affinity binding site (K(i) = 1.73 +/- 0.54 microM). These results demonstrated the existence of a differential dominance between the different alpha subunits performing the benzodiazepine binding sites in the native GABA(A) receptors.
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