To determine at the tissue level whether the proteasome (Ps), a unique nonlysosomal protease, is involved in the metabolism of ubiquitinated proteins, we examined for the first time the immunocytochemical localizations of both Ps and ubiquitin (Ub) in sections of various abnormal structures that are known to be ubiquitinated in various neurodegenerative diseases and in the elderly.

Concomitant increases of Ps and Ub were observed at the sites of most dystrophic neurites in Alzheimer disease (AD) and parkinsonism-dementia complex on Guam (PDC) and in Lewy bodies in Parkinson's disease and diffuse Lewy body disease, but not in neurofibrillary tangles in AD or PDC, in filamentous inclusions within anterior horn cells in sporadic motor neuron disease, or in eosinophilic granules in the olivary nucleus of the elderly. These results at the tissue level indicated that Ps is involved in the metabolism of some, but not all, ubiquitinated proteins and structures in various neurodegenerative disorders. This suggests that the involvement of Ps in the metabolism of ubiquitinated structures differs in different cases and at different stages of disease.

These results and our previous immunocytochemical studies of lysosomal cathepsin proteases suggest that both nonlysosomal and lysosomal systems are involved in the metabolism of various ubiquitinated proteins and that their involvements differ in different structures and at different stages of degeneration of the structures.

Author notes

This study was supported in part by grants from the Amyotrophic Lateral Sclerosis Association (USA), The Ministry of Education, Science and Culture (Japan, 06670194), and Takeda Medical Research Foundation (Japan).