August 21, 2003
Tipping the Balance of Prion Infectivity
Two important questions face biologists studying the infectious
proteins called prions: What stops prions that infect one species from
infecting another species and what causes the invisible transmission
barrier between species to fail sometimes?
In experiments with yeast prions reported in this week's issue of
Nature, Howard Hughes Medical Institute researchers have shown
how point mutations in prions — which do not compromise their
infectivity — can nevertheless cause prions to alter the specificity
of the yeast strain that they infect.
“Our studies of yeast prions argue in a very concrete and definitive way that this mutational effect on conformation is a major mechanism driving the origin of species barriers.”
Jonathan S. Weissman
According to the researchers, their findings point the way to
studies that could begin to clarify the factors that determine whether
a prion specific to cattle that causes bovine spongiform encephalopathy
(BSE), or mad cow disease, might become infectious to humans.
The studies also suggest a new approach for treating disorders such
as Alzheimer's disease that involve aberrant protein folding, said the
researchers. It might be possible to develop drugs that would influence
toxic proteins that aggregate into brain-clogging plaque to fold into
less toxic versions, they said.
The researchers, which included Howard Hughes Medical Institute
(HHMI) investigator Jonathan
Weissman, Peter Chien, HHMI predoctoral fellow Angela DePace and
Sean Collins at the University of California, San Francisco, reported
their findings in the August 21, 2003, issue of the journal
Nature.
Unlike bacteria and viruses, prions consist only of aberrant
proteins that misfold themselves into forms that, in turn, induce their
normal counterparts to misfold. In mammalian prion infections, these
abnormal, insoluble proteins trigger protein clumping that can kill
brain cells. In humans, clumping causes fatal brain-destroying human
diseases such as Creutzfeldt-Jakob disease and kuru, and in animals it
causes BSE and scrapie.
In the yeast cells used as research models by Weissman and his
colleagues, the insoluble prion merely alters a cell's metabolism. In
previous studies of yeast prions, Weissman and his colleagues created a
“chimeric” prion consisting of stitched-together pieces of
prions that infected either of two yeast strains — Saccharomyces
cerevisiae (Sc) or Candida albicans (Ca). The
researchers found this chimeric protein to be “promiscuous”
— capable of infecting either strain of yeast, depending on which one
it was introduced into. The chimeric protein gave the researchers an
opportunity to explore in detail why transmission barriers exist in
yeast prions, which may help researchers understand the basis of
species barriers that affect mammalian prions.
“It was known that very small mismatches, only a few amino
acids, in a prion protein could cause a transmission barrier,”
said Weissman. “It was also known that some proteins can misfold
into multiple different types of prions, and that the specific shape of
a prion is a key determinant of transmission barriers. But what wasn't
understood was why, when you change the sequence, you would get a new
transmission barrier.”
In their initial experiments, working with pure proteins, the
researchers found that even changes in temperature could affect which
infective form their chimeric prion assumed. Thus, they theorized,
subtle mutations could cause species specificity by favoring one folded
form over another.
“We hypothesized that if something as minor as a slight
temperature change could affect which misfolded form the prion went
into, if we could slow down which folding route the prion took, we
could change the specificity of its infectivity,” said
Weissman.
“It's like the Pachinko game in which a ball flipped into play
can fall into one of a number of wells,” said. “A mutation
in the prion produces a preferred misfolding — like tipping the
Pachinko ball one way or another so that it affects which well the ball
tends to fall into.”
To explore their hypothesis, the researchers created subtle
mutations in the chimeric prion. These mutations caused the prion to be
slower in adopting the folded conformation that infected either the
Sc or Ca strains of yeast. They found that these
mutations created a transmission barrier — such that for example, the
chimeric prion mutated to favor the Sc-infecting form no longer
infected the Ca yeast strain. Importantly, the researchers found
this effect both in test tube mixtures of the prions and in the yeast
cell cultures themselves.
The findings emphasize the importance of looking beyond just the
sequence of a prion protein in asking whether species barriers might be
crossed. “Practically speaking, these findings mean that you
can't just ask the question of whether people are protected from
mad-cow disease because cows are different from people,” Weissman
said. “Rather, the answer depends on which type of cow prion it
is. Studies must focus as much on the strain of the misfolded form as
on what animal it is coming from.
“Our studies of yeast prions argue in a very concrete and
definitive way — together with the extensive animal studies of
mammalian prions — that this mutational effect on conformation is a
major mechanism driving the origin of species barriers. And these
findings begin to answer some of the questions of why new species
barriers arise so quickly,” said Weissman.
Since the aggregation of misfolded amyloid proteins into
pathological plaques also causes Alzheimer's and Parkinson's disease,
said Weissman, the studies may suggest a new route to treating such
disorders. Rather than seeking to prevent formation of amyloid plaques,
drug treatments might aim at influencing the amyloid proteins to form
less toxic products.
“The thinking in the field has now evolved to recognize that
not all misfolded proteins are equally bad,” said Weissman.
“So, a general strategy for treating or preventing diseases of
misfolding might concentrate on small-molecule compounds that influence
protein folding to favor non-toxic over toxic misfolded
forms.”
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