David L. Armstrong & Lutz
Birnbaumer talk with ScienceWatch.com and answer a
few questions about this month's Fast Breaking Paper in the
Multidisciplinary field .
Article Title: Orai proteins interact with TRPC
channels and confer responsiveness to store
Authors: Liao, Y;Erxleben, C;Yildirim, E;Abramowitz,
Journal: PROC NAT ACAD SCI USA
Year: MAR 13 2007
* NIEHS, Div Intramural Res, Neurobiol Labs, NIH, POB
12233, Res Triangle Pk, NC 27709 USA.
(addresses have been truncated)
Why do you think your paper is highly
Because it unifies two independent lines of research into the identity of
the ion channel proteins that are responsible for calcium entry into cells
in response to G protein-mediated depletion of intracellular calcium
One line of research, which originated from studies of fruit fly vision,
implicated TRPC channels as the mediators of store-operated calcium entry.
A second more recent line of research, which involved genetic screens of
calcium entry in the Drosophila Schneider (S2) cell line,
implicated Orai proteins in store-operated entry, even though Orai proteins
have different secondary structure than any previously identified ion
Nevertheless, mutations in Orai proteins that produced
immunodeficiency in humans were shown to block store-operated entry in
lymphocytes (Patrick G. Hogan and Anjana Rao, "Dissecting ICRAC,
a store-operated calcium current," Trends Biochem. Sci. 32:235-45,
2007). Our study provided some of the earliest evidence, and an unambiguous
summary of the hypothesis in Fig. 5, that the two classes of protein could
act synergistically to mediate calcium entry.
Does it describe a new discovery,
methodology, or synthesis of knowledge?
Our paper describes a new discovery that resulted when we carried out a
classic experiment somewhat differently than previous investigators. By
restricting expression of both TRPC and Orai proteins, we observed a
synergy that had not been reported previously, although there were already
examples in the literature of accessory proteins affecting the properties
of other ion channel proteins.
Would you summarize the significance of your
paper in layman's terms?
Calcium ions trigger many of the fundamental cellular processes on which
human health depends, but too much calcium is toxic to cells, so cells
sequester calcium ions in intracellular organelles and strictly regulate
their release. Many proteins are involved in regulating calcium entry into
the cell, but, until recently, one of the most mysterious proteins was the
one mediating calcium entry in response to depletion of intracellular
calcium stores. Without this entry from outside, cells would exhaust their
calcium stores and stop signaling. Knowing what proteins are responsible
for entry provides a therapeutic target for diseases that involve
disruption of calcium signaling.
How did you become involved in this
research, and were there any problems along the way?
Dr. Lutz Birnbaumer has been studying G protein-dependent signaling since
he was a postdoctoral fellow at NIH in the lab of Martin Rodbell, who won
the Nobel Prize in medicine in 1994 with Alfred Gilman for their discovery
of G proteins (See: Birnbaumer L, "Expansion of signal transduction by G
proteins. The second 15 years or so: from 3 to 16 a subunits plus ß?
dimmers," Biochim. Biophys. Acta. 1768:756–71, 2007).
Many years later, Dr. Birnbaumer's own student, Ching-Fong Liao, at the
Baylor College of Medicine, discovered that G protein-coupled M5 muscarinic
receptors stimulate the enzyme phospholipase C (PLC) which hydrolyses
membrane phospholipids and triggers both the initial release of calcium
from stores and the subsequent calcium entry.
trigger many of the
processes on which human
health depends, but too much
calcium is toxic to cells, so
cells sequester calcium ions
in intracellular organelles
and strictly regulate their
At the same time Israeli scientists had postulated that an ion channel
protein in flies, called "transient receptor potential" or TRP, because of
the effects of mutating it on phototransduction, which also depends on
signaling through phospholipase C, might be the mysterious calcium channel
responsible for store-operated calcium entry into mammalian cells.
Therefore, Dr. Birnbaumer and his postdoctoral fellow, Michael Xi Zhu, set
out to identify the mammalian orthologue of TRP. They discovered that
mammals express a large family of TRP proteins (See: Xi Zhu et
al., "trp, a novel mammalian gene family
essential for agonist-activated capacitative Ca2+ entry,"Cell 85:661-71, 1996), which were subsequently named TRPC to
reflect their close homology with the canonical TRP from
However, when the TRPC proteins were expressed heterologously, they did not
show the calcium selectivity that was predicted from electrophysiological
studies of native store-operated channels. Nevertheless, in the absence of
other plausible candidates, TRPCs became the focus of many people in the
field, and there were numerous reports from many laboratories with indirect
evidence implicating TRPC channels in store-operated calcium entry. It was
somewhat surprising, therefore, that any role for TRPC proteins disappeared
when Orai was discovered (Taylor CW, "Store-operated Ca(2+) entry: a
STIMulating stOrai" Trends Biochem. Sci. 31: 597–601, 2006).
The study reported by Liao et al. (see below) was an attempt to
reconcile these two disparate lines of research.
Where do you see your research leading in
Earlier this year we published a second paper (Y Liao et al.,
"Functional interactions among Orai1, TRPCs, and STIM1 suggest a
STIM-regulated heteromeric Orai/TRPC model for SOCE/Icrac channels,"
Proc. Natl. Acad. Sci. USA 105: 2895-2900, 2008) that provides
additional electrophysiological evidence for the participation of TRPC
proteins in Orai-dependent, store-operated calcium entry in human embryonic
kidney (HEK) cells.
This paper also has new genetic evidence that mutants of Orai proteins,
which are associated with immune disease, not only disrupt store-operated
entry but also disrupt receptor-operated calcium entry, which, as more
people already agree, also involves TRPC channels. Nevertheless, no one yet
has been able to provide direct evidence for the structure of the channel
or its function at the molecular level, so that is where our efforts are
Now that the Human Genome Project has provided us with a catalogue of human
proteins, many important areas of biological research are currently
shifting from identifying proteins to understanding how they assemble and
work together to carry out cellular processes.
Do you foresee any social or political
implications for your research?
The implications of our research are primarily in understanding human
disease. Additional physiological roles for TRPC channels and Orai proteins
in asthma, diabetes, hypertension, and psychiatric disorders are being
discovered by many groups using mice which have specific genes deleted. On
the other hand, the implications of our discovery for human health do
illustrate how essential basic research support is to translational
medicine, so in that sense it does have political implications for science