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A twenty-eight-day old
Physcomitrella (left) moss gametophyte has a surprising link to an
amyloid plaque, (right) found in brains that have Alzheimer's
disease.
Plaque image courtesy of NIH |
This enzyme, with PS at its core, mediates two
cellular decisions. One is to cut APP and, as a byproduct, generate
the bad peptide associated with Alzheimer's; the other is to cut the
Notch protein in response to specific stimuli. Notch is then free to
enter the nucleus of cells where it partakes in regulating normal gene
expression. Without Notch activity, a mammal has no chance of living.
Notch is a part of a short-range mammalian communication channel, and
for years it has been known to have a working relationship with PS.
However, Notch is absent in plant cells, and PS function in plants
remained mysterious until Quatrano's post-doctoral researcher, Abha
Khandelwal, Ph.D., arrived at WUSTL and was interested in
understanding signal transduction in plants.
"When I searched the literature, the plant signal transduction
pathways were not very well documented compared to the mammalian
counterparts such as Notch," said Khandelwal. "Meanwhile, my husband,
Dilip Chandu, Ph.D., was working in the Kopan lab on ways to study
functions of PS without interference from its predominant substrate
Notch."
This encouraged Khandelwal to search for the PS gene in the genomes of
plants including the recently sequenced Physcomitrella patens genome,
to which the Quatrano lab had access. In addition to the known
Arabidopsis PS, she found the gene in Physcomitrella and asked, "What
is PS doing in moss? Is it acting as an enzyme or does it have a
different function?"
Forming a collaboration
"Moss, like yeast, has this great ability where you can actually
select a gene and remove it, mutate it, or replace it with another
gene from any source. This approach allows us to discern a gene's
value and function in moss," said Quatrano, who was a world leader in
the sequencing of the moss genome. "It is an excellent system to
experimentally discern gene function because of this property as well
as others that we and a worldwide consortium have developed over the
last several years."
Thus, a collaboration was born. By engaging the expertise of the team
in the Kopan lab, the Quatrano lab proceeded to experiment with PS in
moss, which finally resulted in a fruitful collaboration recently
reported in the Proceedings of the National Academy of Science.
Khandelwal proceeded to remove PS, and the result was an obvious
change - a phenotype. Moss lacking PS looked
different, growing with straight, rigid filaments instead of curved
and bent filaments like the parent moss with the PS gene intact.
"That showed the gene has an obvious function that clearly did not
require Notch. We just don't know exactly what it is yet, but we have
proposed a hypothesis to be tested," Quatrano said.
The phenotype piqued Kopan's interest: He saw the potential of looking
at the role of PS independent of Notch. Khandelwal and Chandu took the
phenotype, switched out a mammalian form of PS into the phenotype and
rescued it. Similarly, inserting the moss gene in mammalian cells
resulted in reversing some of the losses experienced by animal cells
lacking PS function, testifying that the human and moss proteins had
an evolutionary conserved function.
"In the moss, the proteins were very nearly interchangeable," Quatrano
said. "This suggested that PS has a role outside the Notch pathway and
may provide clues in mammalian systems as to its primary role,
independent of its substrate in mammalian cells."
"We were amazed to realize that genes from moss and humans were not
only structurally conserved but also shared similar functions,"
Khandelwal said.
Moonlighting protein in mammals
"We spent a lot of time trying to find an activity of PS to circumvent
cleavage of APP, which has been very difficult, "Kopan said. "Importantly,
the human protein acted in plant cells even if its enzymatic activity
was removed by mutation. We stumbled upon an observation that PS
proteins in mammals can perform other functions besides the enzymatic
ones, that is, outside its role as gamma secretase. We're now looking
closely to define these moonlighting functions and determine their
contribution to disease."
In moss, the mutant phenotypes suggest PS might
play a role in signal gathering, cytoskeleton organization and/or cell
wall composition and organization. Quatrano and Khandelwal are
investigating. Kopan, Chandu and others are searching for PS's
moonlighting activities in mammalian cells.
"As a developmental biologist, my job is to translate the genetic code
as if it were a manufacturer's manual, and that is accomplished by
gaining detailed understanding of genes and protein function," Kopan
said. "Unfortunately, we're doing it one gene at a time, slowly
building networks, figuring out what the context is. We can't think of
all of it at once. We have to look at a small subset of genes and how
they work with their 'friends', and hope that our observations will
fit together in one coherent network."
Quatrano said the collaboration between the two labs is a reflection
of what the Genomic Age can do.
"Today, sitting at your computer, you can data mine genomes from
hundreds of microorganisms, animals, fungi, insects and plants, and
you're seeing more evidence of genes being conserved in widely
different organisms," Quatrano said. "This collaboration is a perfect
example of bringing two labs together that on the surface have nothing
in common other than one protein and two people who were aware of the
interests of the other. It's led to a significant contribution that
hopefully will lead to further clues as to the function of PS."
With this study, the Kopan and Quatrano labs and others could use this
outstanding plant model not only to understand some of the off-target
affects during Alzheimer's Disease therapy, but also to unravel novel
interactions and pathways in plants.
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