The body electric
By Nancy Fliesler
In the late 18th century,
Italian anatomist and physician Luigi Galvani demonstrated the existence
of "animal electricity" when he touched the nerve of one frog to
the exposed muscle of another frog, and caused the muscle to contract.
Today, animal electricity is proving to be more important than anyone
ever thought, and Children's Hospital Boston researcher David Clapham,
MD, PhD, is at the forefront of studying it.
Put simply, says Clapham, "all cells are batteries,"
and fully 30 percent of a cell's energy is spent keeping those batteries
charged. The cell stores up its energy by keeping some electrically
charged particles, or ions, inside its boundaries, and keeping other
ions out. Clapham, director of Basic Cardiac Research at Children's
and a Howard Hughes investigator, has devoted his career to studying
these cellular batteries, particularly the "switches"
that turn them on and off.
These switches, better known as ion channels, are roughly donut-shaped
proteins that straddle the membrane of every cell in the body. They're
the gatekeepers that let ions flow in and out of the cell. Any given
cell might have hundreds or thousands of channels. The right stimulus„a
messenger molecule, a change in voltage, or even a change in acidity
or temperature„can throw a channel open, allowing ions to pass through.
As in a battery, opposites attract: positively-charged ions move
toward a negative charge, and vice versa, and the surge of ions
across the cell membrane generates tiny electrical currents that
orchestrate a multitude of bodily functions.
"The working of every cell in the body requires ion channels,"
Clapham says. "They govern where cells go and how they signal
First discovered in the 1940s, ion channels have been implicated
in a long list of diseases, including cystic fibrosis, diabetes,
cardiac arrhythmias, neurologic and psychiatric diseases, gastrointestinal
disorders, and hypertension (see figure below). A malfunctioning
channel can throw off the timing of a heartbeat, cause a brain cell
to "talk" too much or too little to its neighbors, or
constrict a blood vessel too tightly. Many drugs on the market today
act on ion channels, either directly or indirectly, including Valium,
Glucotrol for diabetes, Robitussin cough medicine, and even the
hair-loss drug Rogaine.
Channels come in several types, based on the ions that tunnel through
them: positively charged sodium, calcium, or potassium ions, or
negatively charged chloride ions. Sodium channels are excitatory,
tending to trigger fast physiologic responses; potassium channels
are inhibitory, tending to slow things down. Regardless of their
effect, the signals that pass through ion channels are lightning
Using ion channels, cells continually adjust
their inside and outside electrical charges. Normal levels of calcium
ions, for example, are 20,000 times higher outside the cell than
inside. Heart cells, using special voltage-sensitive calcium channels,
can quickly upset this ratio, generating the spike of energy needed
for a heartbeat. Through the controlled movement of ions across
its membrane, the cell returns to its resting state and then recharges
its battery to begin the next cycle.
Clapham, who originally studied electrical engineering, wants to
know what triggers different kinds of channels to open and close,
how channels detect these triggers, how channels "know"
to let one kind of ion pass but not another, and how they physically
open and close.
His work began in the 1970s, as the field was undergoing dramatic
change. He did his postdoctoral studies in Germany with Erwin Neher,
who together with another researcher named Bert Sakmann, would win
the Nobel Prize for developing the patch clamp technique. Patch
clamping allows researchers to study the real-time electrical behavior
of a single cell„and sometimes even a single ion channel„by directly
measuring the current.
"Many biologists are used to looking at things that are dead,
frozen, fixed, or as pieces of the whole," Clapham says. "An
ion channel is something that's live in its environment„you can
see it working, while it's doing its job."
the cell membrane, ion channels are built from four repeated
units of up to six segments each. Together, the segments form
the channel opening (pore), the sensor that triggers the pore
to open and a filter that lets only select ions
through (in this case, potassium, or K+).
Scott Ramsey, a postdoctoral fellow in Clapham's lab, demonstrates
the patch clamp technique. Using a joystick, and peering through
a microscope, he carefully eases a needle-thin probe against the
membrane of a living embryonic kidney cell, forming a tight seal.
A small jagged line of current shows up on his computer screen,
amplified to make it visible. He then adds a chemical known to activate
ion channels. As the chemical diffuses in, there's a spike of current
on the screen„a few trillionths of an amp„that Ramsey measures and
records. "That's how the channels talk to us," he says.
"That's their language."
The 1980s brought a second key advance: the ability to clone ion
channels and determine the makeup of their genes. This led to an
explosion of research, as investigators tampered with ion channel
genes„or completely disabled them„to alter the channels and watch
A third revolution came in 1998, when
a team led by Rod MacKinnon, a researcher at Rockefeller University,
took an ion channel protein (in this case a potassium channel),
grew it in crystals with a lattice-like structure, and aimed an
X-ray beam through the crystals. The way the X-rays bounced off
the lattice revealed, for the first time, the channel's three-dimensional
shape. It was known that ion channels have sensing mechanisms that
pick up triggering cues, a pore through which the ions flow, and
a selectivity filter that allows only certain types of ions through,
but those structures had never been seen. The 3-D, extremely high-resolution
images from X-ray crystallography are accurate down to the atom.
Clapham praises the work of his friend and colleague, MacKinnon,
but adds, "There's so much more to do." First on his list
is to crystallize the "NaChBac" channel, which his lab
discovered and cloned. NaChBac (short for sodium [Na] channel of
bacteria) is activated by changes in voltage. If his lab can coax
NaChBac to grow in crystals, Clapham hopes that X-ray imaging will
reveal, structurally, how the voltage sensor works. It should also
reveal the workings of the sodium selectivity filter, which admits
sodium but excludes ions like potassium that are very close in size.
The sodium filter is key to the signaling of "excitable cells"
like nerve and muscle cells, which not only use electricity internally
but can fire off electrical impulses (known as action potentials)
to their neighbors.
In Clapham's lab, and in others throughout Children's, ion channel
research is touching areas as wide-ranging as sickle cell disease,
brain development and craniofacial development (see sidebar). But
Clapham's most recent discovery is a calcium channel found only
in the tails of sperm. Dubbed CatSper, it provides the wriggle and
thrust that propel sperm toward the egg; without it, sperm are incapable
of fertilization. Hydra Biosciences, Inc., co-founded by Clapham,
is now developing a male contraceptive that would specifically target
CatSper. Since CatSper is unique to sperm, Clapham says, such a
drug shouldn't have side effects.
Some of the hottest ion channel research
involves non-excitable cells„cells with no obvious electrical activity.
Clapham is now focusing on a family of channels known as transient
receptor potential (TRP) channels. Preliminary research indicates
that many TRP channels are involved in sensory functions like smell,
taste, hearing, primitive forms of vision and even pheromone sensing.
Last year, Clapham's lab reported that TRP channel TRPV3 is activated
by subtle temperature changes. Found in skin, hair follicles, and
nerve cells, it may help regulate body temperature. A still-mysterious
group of TRP channels seem to influence cells' ability to move and
travel in the body, potentially affecting functions as diverse as
wound healing, infection fighting, embryonic development and even
the metastasis of cancers.
Since ion channels are major drug targets, the pharmaceutical industry
has invested heavily in studying them. Clapham and collaborator Dejian
Ren, a researcher from the University of Pennsylvania, recently filed
a patent application for the use of NaChBac, the bacterial channel,
as a template for testing channel blockers and openers. NaChBac is
easy to manipulate in the lab. Through genetic engineering, selected
channel components can be removed, and their counterparts from human
(or other mammalian) channels can be inserted in their place and tested.
Alternatively, a sodium channel could be converted into, say, a calcium
channel by changing the pore and filter. Such a tool would be convenient
not just for drug discovery, but for learning more about how channels
work, Clapham says.
Yet despite the commercial interest, ion channels are surprisingly
under-appreciated by the general medical community. For a start, people
who gravitate to life science tend not to be interested in electronics
or physical science, says Clapham. And the study of ion channels requires
electrophysiology tools and techniques that are unfamiliar to researchers
more used to cloning and expressing genes.
in biology don't like to think about ion channels because they involve
using electrodes and knowing what volts and amps are," Clapham
says. "But they are how we work."