Nuclear polyglutamine-containing protein aggregates as active proteolytic centers

Abstract

Protein aggregates and nuclear inclusions (NIs) containing components of the ubiquitin–proteasome system (UPS), expanded polyglutamine (polyQ) proteins, and transcriptional coactivators characterize cellular responses to stress and are hallmarks of neurodegenerative diseases. The biological function of polyQ-containing aggregates is unknown. To analyze proteasomal activity within such aggregates, we present a nanoparticle (NP)-based method that enables controlled induction of sodium dodecyl sulfate–resistant inclusions of endogenous nuclear proteins while normal regulatory mechanisms remain in place. Consistent with the idea that the UPS maintains quality control, inhibition of proteasomal proteolysis promotes extra large protein aggregates (1.4–2 μm), whereas formation of NP-induced NIs is found to be inversely correlated to proteasome activation. We show that global proteasomal proteolysis increases in NP-treated nuclei and, on the local level, a subpopulation of NIs overlaps with focal domains of proteasome-dependent protein degradation. These results suggest that inclusions in the nucleus constitute active proteolysis modules that may serve to concentrate and decompose damaged, malfolded, or misplaced proteins.

Nuclear Aggregation of Polyglutamine-expanded Ataxin-3

“…On the other hand, evidence has been provided that nuclear inclusions are active proteolytic centers (31). The latter finding would support the idea that inclusion formation in the nucleus is a rather physiological response of cells to the accumulation of misfolded proteins.…”

Protein and DNA destabilization by osmolytes: The other side of the coin

“…Since destabilizing osmolytes are like denaturants, one could easily speculate that these osmolytes will be potentially useful for removing protein aggregates produced by mutant proteins. In agreement, destabilizing osmolytes like arginine and lysine are commonly used to solubilize inclusion bodies and insoluble protein aggregates (Das et al, 2007;Chen et al, 2008;Bajorunaite et al, 2007).…”

Pathology and function of nuclear amyloid

“…Examples are changes of thermal conditions as observed in heat shock responses or experimentally induced fluctuations. Consistently, several reports describe controlled nucleation of protein fibrillation by heat shock, metals, or nanoparticles in vitro, in mammalian cell culture as well as in whole organisms 10 , 22 - 25 . The analysis of intracellular amyloid formation has advanced in recent years due to the diagnostic value of amyloid fibril localization within cells and tissues 26 .…”

“…Likewise are heat shock proteins, 20S proteasomes and the 19S proteasome regulator units (Table 1). 38 - 41 With all the players on the field and the demonstration of proteasome-dependent proteolytic activity in NIs a picture is emerging where at least a subpopulation of intranuclear protein aggregates operate as proteolytic centers 10 . Here, formation of NIs may serve recruitment of excess, mutated, or misfolded nuclear proteins to their degradation.…”

“…A common feature of such inclusions is the recruitment of mutated, misfolded, and waste proteins that leave solution because of the tendency of normally buried hydrophobic domains to associate with one another 9 . The presence of proteasome-dependent proteolytic activity in NIs indicates that misfolded or excess polypeptides form intracellular aggregates before their degradation 10 . Yet, the role of nuclear protein aggregates in disease pathology is still unknown, and their assumed function ranges from being cytotoxic to benign or even neuroprotective 11 - 13 .…”