Scientists at the University of Cologne in Germany have identified a protein that blocks accumulation of the toxic protein aggregates that cause neurodegeneration in Huntington’s disease. Studies by research head David Vilchez, Ph.D.,and colleagues at the University of Cologne’s Cluster of Excellence for Aging Research (CECAD), found that a protein known as UBR5 promotes degradation of the mutant huntingtin protein (HTT) in induced pluripotent stem cells (IPSCs) derived from human Huntington's disease (HD) patients. Their experiments in invertebrate models of the disease also showed that while lack of the worm's equivalent protein promotes neurotoxicity, overexpression induced breakdown of mutant HTT.
The findings, reported in Nature Communications, could help scientists to better understand how Huntington's disease develops, and potentially lead to new therapeutic approaches. The team’s published paper is titled, “The ubiquitin ligase UBR5 suppresses proteostasis collapse in pluripotent stem cells from Huntington’s disease patients.”
Embryonic stem cells and induced pluripotent stem cells (iPSCs) reprogrammed from adult cells can continue to replicate indefinitely in an undifferentiated state, and this requires “stringent quality control mechanisms” to ensure that errors in DNA and protein production are corrected between and during each cell division, the authors explained. This control mechanism includes processes involved in proteostasis, a finely tuned network of integrated cellular systems that act to constantly maintain all of the cell’s expressed proteins – the proteome – and correct any damaging changes to protein concentration, location, folding, and interaction. Defects in proteostasis can, for example, lead to the accumulation of damaged, misfolded, and aggregated proteins.
Proteostasis is maintained just about perfectly in stem cells that continue to replicate indefinitely. Once the cells start to differentiate, however, the proteostasis mechanisms start to become less effective and protein errors can start to creep in. “Remarkably,” the team wrote, while pluripotent stem cells can maintain enhanced proteostasis while proliferating indefinitely in an undifferentiated state, as soon as the cells start to differentiate, the proteostasis network undergoes a “rewiring” that reduces the cells’ ability to sustain proteome integrity.
In fact, the body’s own progenitor and somatic stem cells also demonstrate a reduced ability to maintain protein folding and clearance as a natural part of aging. “This demise of proteostasis is linked with the onset of age-related disorders such as Alzheimer’s, Parkinson’s, and Huntington’s disease," the authors wrote. Reprogram somatic cells back to a dedifferentiated iPSC state and proteostasis is completely restored.
HD is an inherited, and fatal neurodegenerative disorder that is caused by mutations in the huntingtin gene, which result in an HTT protein that carries an abnormally long repeated stretch. This PolyQ-expanded HTT tends to aggregate into clumps, and this aggregation s directly linked with neurodegeneration. The longer the polyQ expanded repeat, the earlier that HD symptoms tend to develop.
Interestingly, while the mutant HTT protein is still produced in IPSCs derived from HD patients, in these cells the protein doesn’t aggregate. Differentiate the HD patient-derived IPSCs back into neurons and again, while the cells demonstrate the same HD gene expression, the HD neurons lack polyQ aggregates and are not prone to neurodegeneration.
To look at how this is possible the researchers looked more closely at IPSCs derived from HD patients (HD-iPSs), and found that activity of the cell’s natural protein degrading machinery, the proteasome, was key to preventing HTT aggregation in HD-derived iPSCs. Chemically inhibiting the proteasome led to mutant HTT aggregation. Further analyses showed that in particular, the ubiquitin ligase UBR5 was upregulated in HD-iPSCs and downregulated during differentiation of human embryonic stem cells. “In all the lines tested, we observed that UBR5 downregulation is already significant when iPSCs differentiate into neural progenitor cells,” they wrote. Significantly, knocking down UBR5 led to increased levels of normal HTT in control human pluripotent stem cells and human embryonic stem cells, and also increased levels of mutant HTT and aggregation in HD-derived iPSC lines. “This was striking to see,” says Dr. Vilchez. “From nothing, the cells went to huge amounts of aggregates.”
The authors suggest their results indicate that “intrinsic high expression of UBR5 determines HTT levels in iPSCs, a process that could contribute to the remarkable ability of these cells to maintain proteostasis of mutant HTT.”
Interestingly, while HD-iPSCs could be differentiated into neural cells that had no accumulation of mutant HTT aggregates, neural cells derived from HD-iPSCs with impaired UBR5 expression did exhibit increased levels of mutant HTT and aggregate accumulation. “Overall, our results indicate that intrinsic high levels of UBR5 are essential to suppress mutant HTT aggregation in iPSCs, contributing to their ability to generate neural progenitor cells with no detectable polyQ-expanded aggregates,” the authors stated.
Subsequent experiments in a nematode worm model that expresses polyQ repeats showed that knocking down the animal’s ubr-5 gene, which is equivalent to the human UBR5 gene, resulted in aggregation of polyQ-expanded proteins in their nervous system. And in a final set of tests, the researchers showed that overexpression of UBR5 decreased levels of mutant HTT in human cell lines that express polyQ-expanded HTT protein. “Taken together, our data indicate that the ubiquitin ligase activity of UBR5 modulates proteasomal degradation of mutant HTT, a process that ameliorates polyQ-expanded aggregation,” the authors concluded.
They note that at least 30 different diseases are directly associated with aberrant protein folding and aggregation. The approach of studying patient-derived pluripotent stem cells could help to identify “super-vigilant” proteostasis mechanisms that could feasibly be harnessed as therapeutics. However, the team also acknowledges that the prospect of therapeutics is still a long way down the line. “It’s not like you discover something new and then there is a cure, it's more difficult,” comments CECAD co-author Isabel Saez, Ph.D. “But in some years there might be a therapy.”