Alzheimer disease paired helical filament core structures contain glycolipid

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

Neurochemical profile of dementia pugilistica

Dementia pugilistica (DP), a suite of neuropathological and cognitive function declines after chronic traumatic brain injury (TBI), is present in approximately 20% of retired boxers. Epidemiological studies indicate TBI is a risk factor for neurodegenerative disorders including Alzheimer disease (AD) and Parkinson disease (PD). Some biochemical alterations observed in AD and PD may be recapitulated in DP and other TBI persons. In this report, we investigate long-term biochemical changes in the brains of former boxers with neuropathologically confirmed DP. Our experiments revealed biochemical and cellular alterations in DP that are complementary to and extend information already provided by histological methods. ELISA and one-dimensional and two dimensional Western blot techniques revealed differential expression of select molecules between three patients with DP and three age-matched non-demented control (NDC) persons without a history of TBI. Structural changes such as disturbances in the expression and processing of glial fibrillary acidic protein, tau, and α-synuclein were evident. The levels of the Aβ-degrading enzyme neprilysin were reduced in the patients with DP. Amyloid-β levels were elevated in the DP participant with the concomitant diagnosis of AD. In addition, the levels of brain-derived neurotrophic factor and the axonal transport proteins kinesin and dynein were substantially decreased in DP relative to NDC participants. Traumatic brain injury is a risk factor for dementia development, and our findings are consistent with permanent structural and functional damage in the cerebral cortex and white matter of boxers. Understanding the precise threshold of damage needed for the induction of pathology in DP and TBI is vital.

Potential structure/function relationships of predicted secondary structural elements of tau

The microtubule-associated protein tau is believed to be a natively unfolded molecule with virtually no secondary structure. However, this protein self-associates into filamentous forms in various neurodegenerative diseases. Since these filamentous forms show a remarkable degree of higher order due to their regular widths and periodicity, it is widely speculated that tau does contain secondary structures that come together to form tertiary and quaternary structures in the filamentous form. The purpose of this review is to use the primary sequence of tau along with predictive methods in an effort to identify potential secondary structural elements that could be involved in its normal and pathological functions. Although there are few predicted structural elements in the tau molecule, these analyses should lead to a better understanding of the structure/function relationships that regulate the behavior of tau.

Assembly of Alzheimer-like, insoluble filaments from brain cerebrosides

Amphiphiles are molecules which contain a polar head and a hydrophobic tail. When the head contains a chiral center, amphiphiles, incubated in the presence of some di- and trivalent metal ions, have been shown to form large fibrous molecular aggregates. In this study, naturally occurring brain cerebroside was tested to determine if it had sufficient amphiphilic properties to form similar supramolecular structures. When galactocerebroside was heated in the presence of magnesium, it was able to form tubules, vesicles and filaments. The filaments included long fibrils that aggregated into dense bundles and short fibrils that were associated to form smaller bundles. These fibrils were shown to be resistant to solubilization in boiling SDS in the presence of reducing agents. This is the first report of a naturally occurring glycolipid being able to form filaments. Since their structural and physical properties are similar to the paired helical filaments of Alzheimer's disease, they may serve as an experimental model for their assembly.

Analysis of the core components of Alzheimer paired helical filaments. A gas chromatography/mass spectrometry characterization of fatty acids, carbohydrates and long-chain bases

We have carried out a fatty acid and carbohydrate compositional analysis of the protease-resistant core of paired helical filaments (prcPHF) isolated from six Alzheimer's diseased brains. Fatty acids, long-chain bases and monosaccharides were characterized by gas chromatography/mass spectrometry (GC/MS) of fatty acid methyl esters, trimethylsilylated long-chain bases, peracetylated alditol acetates and trimethylsilyl methyl glycosides. Glucose and mannose were found to be the only carbohydrate components. Four of the six prcPHF samples contained only glucose while the remaining two samples contained between 30-40% mannose in addition to glucose. None of the samples were found to contain either hydroxylated fatty acids or long-chain bases. The average fatty acid profile of prcPHF was highest in stearic (C18:0) and palmitic acids (C16:0) with less than 10% unsaturated fatty acids. By comparing the carbohydrate and lipid composition of prcPHF to similar data for other brain glycolipids, it was determined that prcPHF is a unique glycolipid, distinct from cerebrosides, gangliosides or brain phospholipids. The fatty acid and carbohydrate composition of a glycolipid isolated from a population of normal brains according to the prcPHF protocol was found to be identical to that of prcPHF glycolipid. It is possible that subtle differences in structure or indigenous factors are responsible for the initiation of PHF formation in vivo.

X-ray probe microanalysis of Alzheimer disease soluble and insoluble paired helical filaments

The paired helical filaments of Alzheimer disease, which have been shown to consist of both soluble and insoluble forms, were examined by X-ray probe microanalysis in order to determine if there existed differences in their elemental composition. The soluble paired helical filaments contained both sulfur and phosphorus, supporting their composition being enriched in a phosphorylated protein. The insoluble paired helical filament core structures, which retained their morphology after extensive protease digestion, contained only a small amount of sulfur over background, which suggests that they are not composed entirely of protein. This significant difference in sulfur and phosphorus indicates a difference in composition between the soluble and insoluble paired helical filaments, and that the paired helical filament core structures may attribute their insolubility to their being predominately non-proteinaceous.

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.