Proteolytic stress causes heat shock protein induction, tau ubiquitination, and the recruitment of ubiquitin to tau-positive aggregates in oligodendrocytes in culture.

Abstract

Molecular chaperones and the ubiquitin-proteasome system are participants in the defense against unfolded proteins and provide an effective protein quality control system that is essential for cellular functions and survival. Ubiquitinated tau-positive inclusion bodies containing the small heat shock protein alphaB-crystallin in oligodendrocytes are consistent features of a variety of neurodegenerative diseases, and defects in the proteasome system might contribute to the aggregation process. Oligodendrocytes, the myelin-forming cells of the CNS, are specifically sensitive to stress situations. Here we can show that in cultured rat brain oligodendrocytes proteasomal inhibition by MG-132 or lactacystin caused apoptotic cell death and the induction of heat shock proteins in a time- and concentration-dependent manner. Specifically, alphaB-crystallin was upregulated, and ubiquitinated proteins accumulated. After incubation with MG-132 the tau was dephosphorylated, which enhanced its microtubule-binding capacity. Proteasomal inhibition led to ubiquitination of tau and its association with alphaB-crystallin and to the occurrence of thioflavine S-positive aggregates in the oligodendroglial cytoplasm. These aggregates were positive for tau and also contained ubiquitin and alphaB-crystallin; hence they resembled the glial cytoplasmic inclusions observed in white matter disease and frontotemporal dementias with parkinsonism linked to chromosome 17 (FTDP-17). In summary, the data underscore the specific sensitivity of oligodendrocytes to stress situations and point to a causal relationship of proteasomal impairment and inclusion body formation.