Perisomatic granules (non-plaque dystrophic dendrites) of hippocampal CA1 neurons in Alzheimer's disease and Pick's disease: a lesion distinct from granulovacuolar degeneration.

A number of pathological changes have been reported in relation to CA1 pyramidal cells in Alzheimer's disease (AD), among them hyperphosphorylation of tau protein followed by the formation of filamentous tau lesions, granulovacuolar degeneration (GVD), Hirano bodies and spindle-shaped dilatations of distal apical dendrites. Juxtacellular clusters of glutamate receptor (GluR)-positive granules around pyramidal cells of the CA1 sector have been recently reported under the term "non-plaque dystrophic dendrites". We independently found that CA1 pyramidal cells in AD patients are regularly surrounded by ubiquitin-positive granules measuring 1-4 microns in diameter, which we have termed perisomatic granules (PSG). Using confocal microscopy, ubiquitin- and GluR-reactive granules were found to largely coincide and to correspond to the same structure. By immunoelectron microscopy PSG were found to consist of GluR1-2-reactive enlarged synaptic boutons containing tubulo-filamentous or floccular material. PSG were found to be consistently associated with pyramidal (principal) cells but not with interneurons of the CA1 sector. Dual-labeling experiments have shown that PSG are preferentially associated with tau-immunoreactive "pretangle" neurons but not with cells containing filamentous tau inclusions or with tau-negative nerve cell bodies. The number of PSG was found to increase with the severity of AD changes with almost no PSG found in Braak stages I and II and few in stage III. Furthermore, PSG were not AD specific, as shown by their presence around CA1 pyramidal cells in Pick's disease. The reasons for GluR reactivity and ubiquitin complex formation in enlarged perisomatic boutons are unclear. Marked changes in GluR subunits have been observed in association with even moderate AD pathology in hippocampal pyramidal cells in AD and our findings suggest a pathogenic link between PSG and early tau pathology in CA1 neurons. PSG might represent residual and abnormally clustered GluR subunits in degenerating perisomatic neurites. Our work confirms and extend previous study on perisomatic "non-plaque dystrophic dendrites" in AD and establish PSG as a pathological entity distinct from GVD. In addition PSG should be acknowledged among main histological changes associated with hippocampal neurons in AD and Pick's disease.

Mouse models of alpha-synucleinopathy and Lewy pathology.

The discovery of two missense mutations (A53T and A30P) in the gene encoding the presynaptic protein alpha-synuclein (alphaSN) that are genetically linked to rare familial forms of Parkinson's disease and its accumulation in Lewy bodies and Lewy neurites has triggered several attempts to generate transgenic mice overexpressing human alphaSN. Analogous to a successful strategy for the production of transgenic animal models for Alzheimer's disease we generated mice expressing wildtype and the A53T mutant of human alphaSN in the nervous system under control of mouse Thy1 regulatory sequences. These animals develop neuronal alpha-synucleinopathy, striking features of Lewy pathology, neuronal degeneration and motor defects. Neurons in brainstem and motor neurons appeared particularly vulnerable. Motor neuron pathology included axonal damage and denervation of neuromuscular junctions, suggesting that alphaSN may interfere with a universal mechanism of synapse maintenance. Thy1-transgene expression of wildtype human alphaSN resulted in comparable pathological changes thus supporting a central role for mutant and wildtype alphaSN in familial and idiopathic forms of diseases with neuronal alpha-synucleinopathy and Lewy pathology. The mouse models provide means to address fundamental aspects of alpha-synucleinopathy and to test therapeutic strategies.

Neuropathology in mice expressing human alpha-synuclein.

The presynaptic protein alpha-synuclein is a prime suspect for contributing to Lewy pathology and clinical aspects of diseases, including Parkinson's disease, dementia with Lewy bodies, and a Lewy body variant of Alzheimer's disease. alpha-Synuclein accumulates in Lewy bodies and Lewy neurites, and two missense mutations (A53T and A30P) in the alpha-synuclein gene are genetically linked to rare familial forms of Parkinson's disease. Under control of mouse Thy1 regulatory sequences, expression of A53T mutant human alpha-synuclein in the nervous system of transgenic mice generated animals with neuronal alpha-synucleinopathy, features strikingly similar to those observed in human brains with Lewy pathology, neuronal degeneration, and motor defects, despite a lack of transgene expression in dopaminergic neurons of the substantia nigra pars compacta. Neurons in brainstem and motor neurons appeared particularly vulnerable. Motor neuron pathology included axonal damage and denervation of neuromuscular junctions in several muscles examined, suggesting that alpha-synuclein interfered with a universal mechanism of synapse maintenance. Thy1 transgene expression of wild-type human alpha-synuclein resulted in similar pathological changes, thus supporting a central role for mutant and wild-type alpha-synuclein in familial and idiotypic forms of diseases with neuronal alpha-synucleinopathy and Lewy pathology. These mouse models provide a means to address fundamental aspects of alpha-synucleinopathy and test therapeutic strategies.

Axonopathy and amyotrophy in mice transgenic for human four-repeat tau protein.

Coding region and intronic mutations in the tau gene cause frontotemporal dementia and parkinsonism linked to chromosome 17. Some of these mutations lead to an overproduction of tau isoforms with four microtubule-binding repeats. Here we have expressed the longest four-repeat human brain tau isoform in transgenic mice under the control of the murine Thy1 promoter. Transgenic mice aged 3 weeks to 25 months overexpressed human tau protein in nerve cells of brain and spinal cord. Numerous abnormal, tau-immunoreactive nerve cell bodies and dendrites were seen. In addition, large numbers of pathologically enlarged axons containing neurofilament- and tau-immunoreactive spheroids were present, especially in spinal cord. Signs of Wallerian degeneration and neurogenic muscle atrophy were observed. When motor function was tested, transgenic mice showed signs of muscle weakness. Taken together, these findings demonstrate that overexpression of human four-repeat tau leads to a central and peripheral axonopathy that results in nerve cell dysfunction and amyotrophy.

Distinct role of protein phosphatase 2A subunit Calpha in the regulation of E-cadherin and beta-catenin during development.

Protein phosphatase 2A (PP2A) plays central roles in development, cell growth and transformation. Inactivation of the gene encoding the PP2A catalytic subunit Calpha by gene targeting generates a lethal embryonic phenotype. No mesoderm is formed in Calpha(-/-) embryos. Here, we found that during normal early embryonic development Calpha was predominantly present at the plasma membrane whereas the highly homologous isoform Cbeta was localized to the cytoplasm and nuclei, suggesting the inability of Cbeta to compensate for vital functions of Calpha in Calpha(-/-) embryos. In addition, PP2A was found in a complex containing the PP2A substrates E-cadherin and beta-catenin. In Calpha(-/-) embryos, E-cadherin and beta-catenin were redistributed from the plasma membrane to the cytosol. Cytosolic concentrations of beta-catenin were low. Our results suggest that Calpha is required for stabilization of E-cadherin/beta-catenin complexes at the plasma membrane.

Argyrophilic grains of Braak: occurrence in dendrites of neurons containing hyperphosphorylated tau protein.

Braak's argyrophilic grains (ArGs) are spindle-shaped neuropil structures originally found in patients afflicted with adult onset dementia. We recently observed that tau protein is hyperphosphorylated in most nerve cells in areas rich in ArGs, suggesting that these grains may be a morphological expression of tau protein pathology in local neurons. The aim of this study was therefore to determine in three cases with ArGs whether grains are associated with individual neurons containing hyperphosphorylated tau. A combination of Gallyas silver staining and AT8 immunocytochemistry was used. AT8 is a monoclonal antibody that recognizes tau in a phosphorylation-dependent manner. Up to 80% of pyramidal cells of sector CA1 showed diffuse AT8 staining of their cell bodies and dendrites. Most grains were freely scattered throughout the neuropil. However, some were clearly located in side-branches of apical dendrites of AT8 immunoreactive pyramidal neurons. Dendritic branches often formed bush-like ramifications containing clusters of ArGs. Other dendrites consisted of a single stump containing one or two large grains at their tips. Spheroidal enlargements of dendritic branches, with a size corresponding to ArGs, were also found in Golgi Cox preparations of cases with ArGs but not in Alzheimer's disease cases or in controls. Our results show that some ArGs are formed within dendrites of neurons whose most obvious pathology is a diffuse hyperphosphorylation of the tau protein. Furthermore, morphology of dendrites containing grains suggests that a process of progressive shrinkage of dendrites is taking place in neurons bearing ArGs.

Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology.

Mutations in the amyloid precursor protein (APP) gene cause early-onset familial Alzheimer disease (AD) by affecting the formation of the amyloid beta (A beta) peptide, the major constituent of AD plaques. We expressed human APP751 containing these mutations in the brains of transgenic mice. Two transgenic mouse lines develop pathological features reminiscent of AD. The degree of pathology depends on expression levels and specific mutations. A 2-fold overexpression of human APP with the Swedish double mutation at positions 670/671 combined with the V717I mutation causes A beta deposition in neocortex and hippocampus of 18-month-old transgenic mice. The deposits are mostly of the diffuse type; however, some congophilic plaques can be detected. In mice with 7-fold overexpression of human APP harboring the Swedish mutation alone, typical plaques appear at 6 months, which increase with age and are Congo Red-positive at first detection. These congophilic plaques are accompanied by neuritic changes and dystrophic cholinergic fibers. Furthermore, inflammatory processes indicated by a massive glial reaction are apparent. Most notably, plaques are immunoreactive for hyperphosphorylated tau, reminiscent of early tau pathology. The immunoreactivity is exclusively found in congophilic senile plaques of both lines. In the higher expressing line, elevated tau phosphorylation can be demonstrated biochemically in 6-month-old animals and increases with age. These mice resemble major features of AD pathology and suggest a central role of A beta in the pathogenesis of the disease.