PP2A mRNA expression is quantitatively decreased in Alzheimer's disease hippocampus.

Since abnormal tau phosphorylation may play a role in neurofibrillary tangle (NFT) formation in aging and Alzheimer's disease (AD), we probed the distribution and abundance of protein phosphatase 2A (PP2A) catalytic (Calpha) and regulatory (PR55alpha and gamma, PR61varepsilon and delta) subunit mRNA in control and AD hippocampus using in situ hybridization. Quantitation of grain density per neuron area of PP2A subunits and beta-actin was determined for the CA3 region of hippocampus and cerebellum, while a qualitative assessment was performed for CA1, CA4, and dentate gyrus. All subunits are expressed in neurons, while PR55gamma and PR55alpha mRNA are also evident in glia. The expression levels of Calpha, all PP2A regulatory subunits studied, and beta-actin were similar in control and AD cerebellum. beta-Actin mRNA was, however, reduced in AD hippocampus. In addition to the generalized reduction of mRNA, as indicated by decreased beta-actin signal, there was a significant loss of Calpha, PR55gamma, and PR61epsilon mRNA in the CA3 hippocampus of AD. This study delineates the distribution of critical PP2A mRNAs and reveals a neuron- and subunit-specific reduction in PP2A catalytic and regulatory mRNA in AD hippocampus. This could result in decreased protein expression and phosphatase activity, leading to the hyperphosphorylation of tau and the formation of NFTs, as well as neuron degeneration in AD.

Loss of brain tau defines novel sporadic and familial tauopathies with frontotemporal dementia.

Dementia lacking distinctive histopathology (DLDH) or frontotemporal lobe degeneration (FTLD) is the most common neuropathological diagnosis for sporadic frontotemporal dementias (FTDs). The hallmarks of DLDH are neuron loss and gliosis in the absence of any disease-specific brain lesion. Similar brain pathology is also seen in a familial FTD pedigree known as hereditary dysphasic disinhibition dementia 2 (HDDD2). Abnormality in the function or isoform composition of the microtubule binding protein tau is a prominent feature in the brains of many patients with sporadic and hereditary FTDs. Therefore, we studied the tau protein in different brain regions from DLDH and HDDD2 patients. Our results indicate that a selective loss of all six tau isoforms, but not tau mRNA, occurs in these brains compared to normal control and Alzheimer's disease brains. Loss of tau protein was identified by Western blot analysis of protein extracts from DLDH and HDDD2 brains in regions both with and without neuronal degeneration. Functionally, this loss of tau protein may be equivalent to pathogenic mutations in the tau gene identified in familial FTD with parkinsonism linked to chromosome 17 (FTDP-17). Thus, DLDH and HDDD2 are novel tauopathies with a unique mechanism of pathogenesis. The presence of tau mRNA in these brains suggests that the level of tau protein may be controlled posttranscriptionally, at the level of either translation or mRNA stability.

Distinct FTDP-17 missense mutations in tau produce tau aggregates and other pathological phenotypes in transfected CHO cells.

Multiple tau gene mutations are pathogenic for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), with filamentous tau aggregates as the major lesions in the CNS of these patients. Recent studies have shown that bacterially expressed recombinant tau proteins with FTDP-17 missense mutations cause functional impairments, i.e., a reduced ability of mutant tau to bind to or promote the assembly of microtubules. To investigate the biological consequences of FTDP-17 tau mutants and assess their ability to form filamentous aggregates, we engineered Chinese hamster ovary cell lines to stably express tau harboring one or several different FTDP-17 mutations and showed that different tau mutants produced distinct pathological phenotypes. For example, delta K, but not several other single tau mutants (e.g., V337 M, P301L, R406W), developed insoluble amorphous and fibrillar aggregates, whereas a triple tau mutant (VPR) containing V337M, P301L, and R406W substitutions also formed similar aggregates. Furthermore, the aggregates increased in size over time in culture. Significantly, the formation of aggregated delta K and VPR tau protein correlated with reduced affinity of these mutants to bind microtubules. Reduced phosphorylation and altered proteolysis was also observed in R406W and delta K tau mutants. Thus, distinct pathological phenotypes, including the formation of insoluble filamentous tau aggregates, result from the expression of different FTDP-17 tau mutants in transfected Chinese hamster ovary cells and implies that these missense mutations cause diverse neurodegenerative FTDP-17 syndromes by multiple mechanisms.

Mutation-specific functional impairments in distinct tau isoforms of hereditary FTDP-17.

Tau proteins aggregate as cytoplasmic inclusions in a number of neurodegenerative diseases, including Alzheimer's disease and hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Over 10 exonic and intronic mutations in the tau gene have been identified in about 20 FTDP-17 families. Analyses of soluble and insoluble tau proteins from brains of FTDP-17 patients indicated that different pathogenic mutations differentially altered distinct biochemical properties and stoichiometry of brain tau isoforms. Functional assays of recombinant tau proteins with different FTDP-17 missense mutations implicated all but one of these mutations in disease pathogenesis by reducing the ability of tau to bind microtubules and promote microtubule assembly.