Gidalevitz T, Ben-Zvi A, Ho KH, Brignull HR, Morimoto RI.
Progressive disruption of cellular protein folding in models of polyglutamine diseases.
Science. 2006 Mar 10;311(5766):1471-4.
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Cellular homeostasis is an exceedingly complex process. Conceptually, one may
consider two regimes within which the phenomenon operates, extra- and intracellular.
The extracellular regime requires dynamic responses of the cell to
external stimuli. The intracellular regime involves metabolic processes that neurologists
might refer to as “activities of daily living,” those processes that the
cell must execute continuously to function normally. One of these activities is
the synthesis and folding of proteins. This activity is highly efficient overall,
but imperfect. A significant percentage of nascent proteins fold improperly,
even with the help of folding chaperones, and thus must be “recycled” through
proteolysis in the proteasome system. What happens if the capacity of the
protein folding and degradation machinery is exceeded?
In a paper published on 9 February in Sciencexpress, Morimoto and colleagues
at Northwestern University address the general question raised above from the
perspective of diseases of protein aggregation that cause neurodegenerative disorders.
These diseases include Alzheimer’s, Parkinson’s, prion, and Huntington’s.
The last disease is an archetypal member of a family of diseases caused
by an increase in the number of contiguous glutamine residues within normal
proteins, the “polyglutamine diseases.” A number of pathogenetic mechanisms
have been postulated to explain the cellular and organismal effects of polyglutamine
expansion. Morimoto et al. sought mechanistic insights through an
examination of the effects of simultaneous expression in the worm C. elegans
of temperature-sensitive (ts) protein mutants and fluorescent proteins bearing
varying numbers of polyglutamine tracts. The key strategic kernel was the
study of phenotype at permissive temperatures. In a visually and intellectually
beautiful set of experiments, effects were observed in sarcomere morphology,
movement, body shape, egg-laying, and early development. Affected animals
displayed large numbers of protein aggregates, and evidence was presented that
proteins expressed in the presence of a polyglutamine-containing protein acquired
increased protease resistance (one characteristic of amyloids and other
All proteins constantly sample different areas of conformational space. In the ts
mutants studied, amino acid substitutions alter the energy-dependence of this
sampling process, allowing the protein to fold into pathologic conformations if
sufficient activation energy can be obtained (at the non-permissive temperature).
One intriguing part of the work of Morimoto et al. is the finding that
polyglutamine tracts can decrease this activation energy, allowing the ts proteins
to fold pathologically at relatively low temperatures. Importantly, this effect is
bidirectional. The ts proteins themselves contribute to the self-assembly of
the polyglutamine proteins, as evidenced by a synergistic increase in aggregate
number when both proteins are expressed.
These results suggest two answers to the question posed above, one obvious
from first principles and one new and unexpected. The obvious answer is that
the quality and quantity of pathologically folded proteins can overwhelm
a cell’s homeostatic mechanisms and cause disease. The novel insight is that
pathologically folded proteins may act relatively nonspecifically to alter folding
pathways of many other nascent proteins—an effect opposite to that of folding
chaperones—and thus affect many different physiologic processes.
How does the work further our understanding of pathogenetic mechanisms of
polyglutamine diseases in particular, vis-à-vis the beneficial or detrimental effect
of aggregation (a question posed by Dr. Eddie Koo at UC, San Diego). In
my opinion, the work increases the complexity of the problem and emphasizes
the difficulty of ascribing a single pathologic process to a protein-linked disease.
Specifically, macroscopic aggregates of polyglutamine-containing proteins
may be “protective” through their ability to sequester toxic protein polymers.
However, monomers, low-order oligomers, or protofibrils, prior to accretion on
deposits, all may contribute to the effects observed by Morimoto et al. If so,
strategies to eliminate deposits may fail because they do not target smaller,
intracellular, toxic assemblies.
The authors have elegantly demonstrated the importance of the presence of intracellular misfolded proteins in mediating cellular dysfunction in neurodegenerative disease. Coexpressing the temperature-sensitive (ts) mutants with polyQ in C. elegans at permissive conditions resulted in phenotypes similar to those exhibited by ts mutants under restrictive conditions. This conversion of relatively harmless ts mutants into those which exhibit mutant phenotypes under permissive conditions is a fascinating and enlightening observation. The experiments with various other strains of ts mutants make the case that the expression of aggregation-prone polyQ protein meddles with the structure and function of unrelated proteins. Specifically, the authors suggest that the levels of polyQ influence the folding of ts protein and that perhaps the opposite is also true, as though a positive feedback mechanism exists to augment the imbalance in cellular folding.
In interpreting the results, the authors propose that marginally stable proteins do not in and of themselves cause disease; rather, they misfold and subsequently modify the aggregation and toxicity of aggregation-prone proteins associated with conformational disease, and this overwhelms the balance of cellular folding and clearing processes. However, if in the presence of aggregation-prone proteins these metastable proteins misfold, this hints at a possible interaction between the two that perpetuates their mutual misfolding and accumulation. To expand on the authors’ interpretation, perhaps the expression of the aggregation-prone protein somehow induces the misfolding of the metastable mutant proteins, contributing to the progressive disruption of cellular processes that maintain the folding environment.
This imbalance of clearance and folding may be initially caused by the early accumulation of such aggregation-prone proteins as polyQ, which not only stress the overall folding capacity of the cell, but also play a role in stimulating the misfolding of metastable proteins. In fact, in our hands, many aggregation-prone proteins in vitro are capable of interacting with other marginally stable proteins and inducing subsequent conformational changes and aggregation.
Alternatively, the expression of aggregation-prone proteins might in some way stimulate the synthesis of more of these metastable proteins, thus increasing the chances for even more misfolding and increasing their accumulation. If that is the case, again, the cellular clearance processes may become overwhelmed and tilt the delicate balance of protein homeostasis.
Whatever the case may be, clearly these data suggest that disruption of cellular processes does not result from a single defect. Rather, in combination, aggregation-prone and metastable proteins can affect protein homeostasis and help to explain the gradual accumulation of damaged proteins observed in misfolding diseases. It would be interesting to try these same experiments with other aggregation-prone proteins.