Increased protein hydrophobicity in response to aging and Alzheimer disease.

Increased levels of misfolded and damaged proteins occur in response to brain aging and Alzheimer disease (AD), which presumably increase the amount of aggregation-prone proteins via elevations in hydrophobicity. The proteasome is an intracellular protease that degrades oxidized and ubiquitinated proteins, and its function is known to be impaired in response to both aging and AD. In this study we sought to determine the potential for increased levels of protein hydrophobicity occurring in response to aging and AD, to identify the contribution of proteasome inhibition to increased protein hydrophobicity, and last to identify the contribution of ubiquitinated and oxidized proteins to the pool of hydrophobic proteins. In our studies we identified that aging and AD brain exhibited increases in protein hydrophobicity as detected using Bis ANS, with dietary restriction (DR) significantly decreasing age-related increases in protein hydrophobicity. Affinity chromatography purification of hydrophobic proteins from aging and AD brains identified increased levels of oxidized and ubiquitinated proteins in the pool of hydrophobic proteins. Pharmacological inhibition of the proteasome in neurons, but not astrocytes, resulted in an increase in protein hydrophobicity. Taken together, these data indicate that there is a relationship between increased protein oxidation and protein ubiquitination and elevations in protein hydrophobicity within the aging and the AD brain, which may be mediated in part by impaired proteasome activity in neurons. Our studies also suggest a potential role for decreased oxidized and hydrophobic proteins in mediating the beneficial effects of DR.