The theory is a variation on the old theme that aggregation-prone proteins overtax the cell’s protein folding machinery and send the cell into a destructive spiral (see ARF related Live Discussion). But first authors Tali Gidalevitz, Anat Ben-Zvi, and colleagues turn the tables on this idea. Using the roundworm Caenorhabditis elegans as a model, they show that not only do polyglutamine (polyQ)-expanded proteins turn metastable proteins into flagrantly unstable ones, but that it also works the other way around—the metastable proteins enhance aggregation of proteins with polyQ expansions.

The metastable proteins in question are temperature-sensitive (ts) mutants conferring distinct phenotypes that only emerge at “restrictive” temperatures. At “permissive” temperatures, worms with the mutations appear normal. The ts mutants are good indicators of how healthy a worm’s protein folding machinery is, because the mutated proteins rely heavily on that machinery for proper folding.

To test how polyQ affects folding of ts mutants, the researchers expressed yellow fluorescent protein (YFP) containing various polyQ expansions in worms harboring a paramyosin ts mutant. At the restrictive temperature, this mutation disrupts muscle formation, leading to premature death of both embryos and larvae. Though at the permissive temperature (15 degrees C) these worms normally survive as well as the wild-type, Gidalevitz, Ben-Zvi, and colleagues found that in the presence of YFP with a 40-glutamine stretch (PolyQ40m), embryos behaved as if they were grown at the restrictive temperature (25 degrees C). That is, almost half of the paramyosin ts embryos failed to hatch or even move at 15 degrees when the polyQ was present. Interestingly, expression of the PolyQ40m in a wild-type background had no effect on either embryos or larvae, indicating that the metastable protein and aggregation-prone protein together make a lethal mix.

To show that this is a general phenomenon, and not just a quirk of paramyosin, the researchers tested various other mutants. They found that in the presence of polyQ40, ts mutants of the neuronal protein dynamin-1 caused severe paralysis at normally permissive temperatures, while phenotypes evoked by mutations in homologs of human myosin, perlecan, and ras-1 were also evident at permissive temperatures in the presence of the polyQ YFP.

Given that polyQ40m, which has no effect on wild-type worms, has such a drastic impact on the ts mutants, Gidalevitz and colleagues next asked what effect the ts proteins might have on aggregation-prone polyglutamine proteins. They found that the number of visible polyQ40m aggregates dramatically increased from around 10 or fewer to around 60 if larvae also expressed the temperature-sensitive paramyosin. Importantly, the expression of loss-of-function paramyosin mutants had no effect on polyQ aggregation, indicating that it is not loss of paramyosin activity per se that exacerbates the process, but rather the instability of the ts protein. “Our data identify the presence of marginally stable or folding defective protein in the genetic background of conformational disease as potent extrinsic factors that modify aggregation and toxicity. Given the prevalence of polymorphisms in the human genome, they could contribute to variability of disease onset and progression,” write the authors.

The authors did not test any non-polyQ proteins in their model. It would be interesting to see what effect ts mutants might have on aggregation of amyloid-β, α-synuclein, tau, and other proteins that contribute to neurodegenerative pathologies.—Tom Fagan.

Reference:
Gidalevitz T, Ben-Zvi A, Ho KH, Brignull HR, Morimoto RI. Progressive disruption of cellular protein folding in models of polyglutamine diseases. Science Express February 9, 2006. Abstract