December 22, 1999
Untangling a Link Between Normal Protein Folding and Alzheimer's Disease
An enzyme that snips apart proteins that form brain-clogging plaques
in people with Alzheimer's disease also appears to regulate enzymes
that fold new proteins into their working forms in healthy cells.
The discovery offers new hints about how mutations or exposure to
chemicals that affect the regulation of protein-folding machinery might
stimulate a protein-snipping enzyme, called presenilin-1, which has
been implicated in the pathogenesis of Alzheimer's disease. Such a
link, if further confirmed, could have important implications for
understanding and treating Alzheimer's disease, say the
researchers.
“Environmental agents or toxins could cause protein misfolding through the UPR might induce presenilin-1 activity, which in turn might activate a proteolytic cascade that could also lead to increased amyloid deposits.”
Peter Walter
The research team, which included Howard Hughes Medical Institute
(HHMI) investigators Peter
Walter of the University of California, San Francisco (UCSF) and
Randall Kaufman of the University of Michigan, as well as UCSF
colleagues Maho Niwa and Carmela Sidrauski, reported its findings in an
article in the December 22, 1999, issue of the journal Cell.
The researchers began their studies in hopes of learning more about
how proteins involved in the "unfolded protein response" (UPR) detect
the amount of unfolded proteins in a cell and signal genes to either
increase or decrease the production of protein-folding enzymes. Such
signals are critical because newly synthesized proteins, which are
essentially linear strings of amino acids, are functionally useless
unless they are folded into a three-dimensional form.
The researchers knew that the UPR hinges on a protein, called Ire1,
that senses the amount of unfolded proteins and switches on
protein-folding genes. They suspected that Ire1 works in the nucleus
where it cuts a specific messenger RNA (mRNA) at two places, so that it
can be restitched into a gene-activating form by another enzyme.
Since studies of Ire1 splicing had only been done in yeast, the
scientists first wanted to see whether such splicing occurred in
mammalian cells. Thus, in their initial experiments they inserted the
yeast mRNA into human cells and found that it was cut and spliced just
as in yeast cells.
"This is a first report showing that salient features of this highly
unusual signaling pathway are conserved in mammalian cells," said
Walter.
The scientists next wanted to learn how Ire1, which is normally
nestled in the membrane of the endoplasmic reticulum (ER), extends its
activity all the way to the cell's nucleus. The enzyme appears to
extend a "sensor" into the protein-synthesizing region of the ER, where
it detects unfolded proteins. The other end of Ire1 that bears the
RNA-splicing machinery extends toward the cytosol.
The scientists theorized that Ire1's RNA-splicing end might actually
be snipped off, and like an enzymatic guided missile, enter and
penetrate deep into the nucleus to splice the target mRNA.
In experiments designed to track the location of the Ire1-cleaved
fragment within mammalian cells, the scientists found that the fragment
did, indeed, invade the nucleus, and not remain part of the
membrane-bound protein.
"This was a complete surprise that when we induced UPR in these
cells, a large fraction of Ire1 localizes to the nucleus," said
Walter.
"However, it was not all or nothing," he emphasized. "Even uninduced
cells showed some Ire1 in the nucleus, so there are many complexities
of the process we don't understand." Walter said that the two similar
forms of Ire1, dubbed alpha and beta, might be regulated differently
and have slightly different functions.
The scientists next sought the identity of the enzyme that snipped
off the "missile" portion of Ire1, in a protein-cleaving reaction
called proteolysis. Beginning with a hunch that presenilin-1 might be
that enzyme, they obtained engineered cells that lacked presenilin-1
from Dennis Selkoe's laboratory at Harvard Medical School. They then
studied the cells to see what effect, if any, the loss of presenilin-1
activity would have on Ire1, and found that cells without presenilin-1
did not show proper movement of Ire1 into the nucleus.
"While this analysis establishes a link between presenilin-1 and
Ire1 processing, it is not necessarily a direct one, since we have not
figured out its biochemistry," emphasized Walter.
Presenilin-1's apparent role in protein folding links this normal
cellular process to Alzheimer's disease because researchers had
previously shown that presenilin-1 is part of the machinery that slices
apart "amyloid precursor protein" to produce the amyloid plaques that
clog the brains of Alzheimer's patients. Presenilin-1's dual role could
aid in both understanding and treating Alzheimer's disease, said
Walter.
For example, he said, abnormally activated Ire1 — perhaps through
genetic mutation — could overstimulate presenilin-1, which could act
to create amyloid plaque deposition.
"Also, environmental agents or toxins could cause protein misfolding
through the UPR might induce presenilin-1 activity, which in turn might
activate a proteolytic cascade that could also lead to increased
amyloid deposits," said Walter.
"Finally, this indication that presenilin-1 plays a role in normal
protein processing makes it unlikely that Alzheimer's disease could be
treated using drugs to block this pathway without severe side effects
for normal physiology," said Walter.
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