The microtubule-associated protein tau is the main component of the paired helical filaments (PHFs) of Alzheimer's disease, the most common senile dementia. To understand the origin of tau's abnormal assembly we have studied the influence of other cytosolic components. Here we report that PHF assembly is strongly enhanced by RNA. The RNA-induced assembly of PHFs is dependent on the formation of intermolecular disulfide bridges involving Cys 322 in the third repeat of tau, and it includes the dimerization of tau as an early intermediate. Three-repeat constructs polymerize most efficiently, two repeat constructs are the minimum number required for assembly, and even all six full-length isoforms of tau can be induced to form PHFs by RNA.Key words: Paired helical filament; Alzheimer's disease; Microtubule-associated protein; RNA such factors in the cytoplasm. An initial hint was provided by the tubulin molecule, the natural partner of tau. Tubulin associates with tau, polymerizes into microtubules, and thus prevents tau's interaction with itself. Tubulin's C-terminus, to which tau binds [15], is unusually acidic, suggesting that tau might respond to other polyanionic molecules. Other prominent polyanions in the cytosol are the various RNA species. It turns out that these molecules have the capacity of promoting PHF assembly. In this respect, they are similar to polyanions of the extracellular matrix, such as heparin or heparan sulfate, whose effect on PHF assembly has been reported recently [8,19]. While it is conceptually difficult to imagine how components of the extracellular matrix might interact with cytosolic proteins, the potential role of cytosolic polyanions seems straightforward, making the RNA-PHF connection an attractive model for further investigation.

In Alzheimer's disease and related dementias, human tau protein aggregates into paired helical filaments and neurofibrillary tangles. However, such tau aggregates have not yet been demonstrated in transgenic mouse models of the disease. One of the possible explanations would be that mouse tau has different properties which prevents it from aggregating. We have cloned several murine tau isoforms, containing three or four repeats and different combinations of inserts, expressed them in Escherichia coli and show here that they can all be assembled into paired helical filaments similar to those in Alzheimer's disease, using the same protocols as with human tau. Therefore, the absence of pathologically aggregated tau in transgenic mice cannot be explained by intrinsic differences in mouse tau protein and instead must be explained by other as yet unknown factors.z 1999 Federation of European Biochemical Societies.