Intraneuronal APP, not free Aß peptides in 3xTg-AD mice: implications for tau versus Aß-mediated Alzheimer neurodegeneration.

Alzheimer's disease (AD) is characterized by the accumulation of intraneuronal tau and extracellular amyloid-ß (Aß) peptide. A triple transgenic (Tg) mouse (3xTg-AD) was reported to develop Aß plaques and tau inclusions as well as remarkable accumulations of intracellular Aß that were suggested to be the initiators of AD pathogenesis. However, it was unclear whether the anti-Aß antibodies were able to distinguish Aß peptide from the same Aß epitopes within the amyloid precursor protein (APP). To further elucidate the identity of the immunoreactive intraneuronal material in 3xTg-AD mice, we conducted immunohistochemical, biochemical, and ultrastructural studies using a well characterized panel of antibodies that distinguish Aß within APP from cleaved Aß peptides. We found that the intraneuronal material shared epitopes with full-length APP but not free Aß. To demonstrate unequivocally that this intraneuronal material was not free Aß peptide, we generated 3xTg-AD mice deficient for ß-secretase (BACE), the protease required for Aß generation from APP. In the absence of Aß production, robust intraneuronal APP immunostaining was detected in the 3xTg-AD/BACE(-/-) mice. Finally, we found that the formation of tau lesions was not different between 3xTg-AD versus 3xTg-AD/BACE(-/-) mice, thereby demonstrating that tau pathology forms independently from Aß peptide generation in this mouse model. Although we cannot corroborate the presence of intraneuronal Aß peptide in 3xTg-AD mice, our findings warrant further study as to the role of aberrant APP accumulation in this unique model of AD.

Disturbance of nuclear and cytoplasmic TAR DNA-binding protein (TDP-43) induces disease-like redistribution, sequestration, and aggregate formation.

TAR DNA-binding protein 43 (TDP-43) is the disease protein in frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) and amyotrophic lateral sclerosis (ALS). Although normal TDP-43 is a nuclear protein, pathological TDP-43 is redistributed and sequestered as insoluble aggregates in neuronal nuclei, perikarya, and neurites. Here we recapitulate these pathological phenotypes in cultured cells by altering endogenous TDP-43 nuclear trafficking and by expressing mutants with defective nuclear localization (TDP-43-DeltaNLS) or nuclear export signals (TDP-43-DeltaNES). Restricting endogenous cytoplasmic TDP-43 from entering the nucleus or preventing its exit out of the nucleus resulted in TDP-43 aggregate formation. TDP-43-DeltaNLS accumulates as insoluble cytoplasmic aggregates and sequesters endogenous TDP-43, thereby depleting normal nuclear TDP-43, whereas TDP-43-DeltaNES forms insoluble nuclear aggregates with endogenous TDP-43. Mutant forms of TDP-43 also replicate the biochemical profile of pathological TDP-43 in FTLD-U/ALS. Thus, FTLD-U/ALS pathogenesis may be linked mechanistically to deleterious perturbations of nuclear trafficking and solubility of TDP-43.