Biochemical and molecular characterization of neurofibrillary degeneration in frontotemporal dementias.

Neurofibrillary degeneration (NFD) is a degenerating process characterized by the intraneuronal aggregation of abnormal tau proteins. These proteins have a biochemical signature which is disease-specific. They also have a neocortical distribution which is typical of the disease. Pathological tau proteins have been analyzed qualitatively and quantitatively in all diseases that may present the clinical symptoms of frontotemporal dementias. In Alzheimer's disease, a disease with sometimes a frontal predominance, paired helical filaments (PHF) of neurofibrillary tangles are made of hyperphosphorylated tau, named PHF-tau. Their electrophoretic profile consists of four main bands (tau 55, 64, 69, 74 kD), resulting from the presence of the six tau isoforms. In Pick's disease the phosphorylated tau from Pick bodies are made of two major components (tau 55, 64 kD) and a minor 69 kD resulting from the lack of tau isoforms with the translated exon 10 (E10-). Corticobasal degeneration (CBD) also has a different pattern of tau variants, with tau 64, 69 components and a minor tau 74. Pathological tau proteins that aggregate in CBD (and progressive supranuclear palsy) are exclusively made of E10+ tau isoforms. In frontotemporal dementias non-Alzheimer, non-Pick (Lund and Manchester criteria), we did not observe the presence of pathological tau proteins in 2 cases, but a third one presented a particular pattern of tau, with soluble pathological tau in frontotemporal areas. These data show that this group could be heterogeneous. In conclusion, the biochemical signature of tau distinguishes four classes of frontotemporal dementia. The characteristic tau phenotypes observed are linked to the specific neuronal networks that are affected in each disease.