Solubility of neurofibrillary tangles and ultrastructure of paired helical filaments in sodium dodecylsulphate

Quantitative solubilization and analysis of insoluble paired helical filaments from Alzheimer disease

In this study, we evaluate the ability of several solvents to solubilize insoluble paired helical filaments (PHF) of Alzheimer disease. Specifically, we use protein extraction and reduction in the volume of insoluble material as quantitative assays to establish solvents of PHF. Using sequential categories of protein solvent to analyze insoluble PHF, only alkali or exhaustive proteolysis are effective in completely solubilizing PHF, while a variety of denaturants are ineffective. Alkali does not affect the phosphorylation state of PHF and complete dephosphorylation of PHF with hydrofluoric acid does not affect PHF solubility. These findings suggest that the 'hyperphosphorylation' of PHF proteins is not responsible for PHF insolubility. However the in vitro glycation of tau generates PHF that are insoluble in SDS and soluble in alkali. These findings suggest that protein crosslinks, including advanced glycation endproduct-derived crosslinks which were recently described in Alzheimer disease, play a major role in effecting PHF insolubility in vivo.

Developmental patterns of neuronal thread protein gene expression in Down syndrome

Neuronal thread proteins (NTP) are a group of immunologically related molecules expressed in brain and neuroectodermal tumor cell lines. NTP gene expression is up-regulated and NTP molecules accumulate in Alzheimer's disease (AD) brains, pathological states associated with regenerative neuritic sprouting, and during brain development. To investigate the role of NTP over-expression in AD, we examined NTP immunoreactivity in brains from differently aged individuals with Down syndrome, since patients with Down syndrome nearly always develop AD neuropathology and dementia. Using SMI monoclonal antibodies to neurofilament protein, we detected age-associated increases in neurofilament immunoreactive (SMI-positive) neurites in Layers I and II of the cerebral cortex beginning at 1 year of age, followed by SMI-positive neurofibrillary tangles beginning at age 5 years, and then SMI-positive plaques beginning in the third decade. Increased NTP immunoreactivity in Down syndrome brains began in the second decade, prior to establishment of widespread AD neurodegeneration (Down syndrome + AD), and at an age when low-level or absent NTP expression was observed in control brains. Analysis of SDS and Triton X-100-treated histological sections and tissue extracts demonstrated that a largely insoluble, denaturation-resistant form of NTP accumulates in both Down syndrome + AD and AD brains. The findings provide further evidence that abnormal NTP expression and accumulation in brain may be an early marker of AD neurodegeneration in Down syndrome.

Solubilization and fractionation of paired helical filaments

Paired helical filaments isolated from brains of two different patients with Alzheimer's disease were extensively treated with the ionic detergent, sodium dodecyl sulphate. Filaments were solubilized at different extents, depending on the brain examined, thus suggesting the existence of two types of paired helical filaments: sodium dodecyl sulphate-soluble and insoluble filaments. In the first case, the number of structures resembling paired helical filaments greatly decreased after the detergent treatment, as observed by electron microscopy. Simultaneously, a decrease in the amount of sedimentable protein was also observed upon centrifugation of the sodium dodecyl sulfate-treated paired helical filaments. A sodium dodecyl sulphate-soluble fraction was isolated as a supernatant after low-speed centrifugation of the sodium dodecyl sulphate-treated paired helical filaments. The addition of the non-ionic detergent Nonidet-P40 to this fraction resulted in the formation of paired helical filament-like structures. When the sodium dodecyl sulphate-soluble fraction was further fractionated by high-speed centrifugation, three subfractions were observed: a supernatant, a pellet and a thin layer between these two subfractions. No paired helical filaments were observed in any of these subfractions, even after addition of Nonidet P-40. However, when they were mixed back together, the treatment with Nonidet P-40 resulted in the visualization of paired helical filament-like structures. These results suggest that at least two different components are needed for the reconstitution of paired helical filaments as determined by electron microscopy. The method described here may allow the study of the components involved in the formation of paired helical filaments and the identification of possible factors capable of blocking this process.

Extracellular neurofibrillary tangles associated with degenerating neurites and neuropil threads in Alzheimer-type dementia

We examined the cellular components of extracellular neurofibrillary tangles (E-NFT) in the hippocampal areas in cases with Alzheimer-type dementia. Immunohistochemically, the E-NFT were labeled for the C terminus of tau and glial fibrillary acidic protein. Moreover, the majority of the E-NFT was associated with intensely argyrophilic rods and with tau- and ubiquitin-immunoreactive dots. Ultrastructurally, the E-NFT consisted mainly of extracellular paired helical filaments (PHF) and astroglial processes. The extracellular PHF tended to be straighter and thinner. One third of the E-NFT was associated with small degenerating neurites containing many dense bodies and with neuropil threads containing PHF. These findings suggested that extracellular PHF promote both intense astroglial reaction and neuritic alteration, and that the E-NFT are continuously changing their morphology.