To function, each living cell needs both to build new and to degrade old or damaged proteins. To accomplish that, a number of intracellular systems work in concert to keep the cell healthy and from clogging up with damaged proteins. When proteins or peptides mutate, they can present major problems to the clearing up of the intracellular environment. In Huntington's disease (HD) the disease provoking mutation in the huntingtin gene eventually causes the cell to build up intranuclear and cellular inclusions of protein-aggregates, made up primarily of huntingtin. One cellular organelle with a central role of clearing such protein build up in the cell is the ubiquitin proteasome system (UPS).

In Huntington's disease (HD) brains and other tissues, UPS activity is inhibited and intraneuronal nuclear protein aggregates of mutant huntingtin in HD brains indicate dysfunction of the UPS. From these results, we hypothesized that enhancing UPS function would improve catalytic degradation of abnormal proteins in HD. We first genetically engineered proteasome activators involved in either non-ubiquitinated protein degradation pathways (PA28ƒ×) or subunits of PA700, the 26S proteasome ubiquitinated pathway (S5a) into transducible lentiviral vectors. To address the therapeutic hypothesis experimentally, we transduced UPS subunits into HD skin fibroblasts or HD mutant protein expressing striatum-derived neurons. We determined how this intervention altered cell survival after exposure to toxins known to simulate pathological mechanisms in HD.

The manuscript shows that cellular changes due to expression of huntingtin protein with longer CAG repeats can reduce the ubiquitin proteasome system (UPS) function in Huntington¡¦s disease cells. Following compromise of the UPS, the overexpression of proteasome activator PA28 can specifically recover proteasome function and improve cell viability in both HD model and patient cells.

These remarkable results demonstrate for the first time that it is possible to intervene therapeutically in the proteolytic pathways and organelles that participate in the specific degradation of misfolded and abnormal proteins.

In patients with Huntington's disease (HD), the proteolytic activity of the ubiquitin proteasome system (UPS) is reduced in the brain and other tissues. The pathological hallmark of HD is the intraneuronal nuclear protein aggregates of mutant huntingtin. We determined how to enhance UPS function and influence catalytic protein degradation and cell survival in HD. Proteasome activators involved in either the ubiquitinated or the non-ubiquitinated proteolysis were overexpressed in HD patients' skin fibroblasts or mutant huntingtin-expressing striatal neurons. Following compromise of the UPS, overexpression of the proteasome activator subunit PA28γ, but not subunit S5a, recovered proteasome function in the HD cells. PA28γ also improved cell viability in mutant huntingtin-expressing striatal neurons exposed to pathological stressors, such as the excitotoxin quinolinic acid and the reversible proteasome inhibitor MG132. These results demonstrate the specific functional enhancements of the UPS that can provide neuroprotection in HD cells.

Written from a news release by PLoS One.