Body Electric

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 each other.”

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 fast.

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.”

Ion channel proteins are
in every cell membrane

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 what happened.

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.

Ion channel proteins are
in every cell membrane

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.

“Most people 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.”


To support Dr. Clapham’s ion channel research contact Karen Ann Engelbourg in the Children’s Hospital Trust at (617) 355-8863 or karen.engelbourg@chtrust.org
http://www.childrenshospital.org/dream/summer2003/body.html

Lungs In cystic fibrosis, defective chloride channels fail to let chloride ions exit lung cells, so the cells don’t secrete enough fluid. The result is thick, dried-out mucus that clogs the airways.

Brain In epilepsy, malfunctioning ion channels disrupt electrical activity in the brain and play a major role in seizures.

The Body Electric

A natural voltage within a growing embryo may teach it left from right
John Travis

An electric field inside an embryo may tell it whether to place an internal organ on its left or right side.
http://www.sciencenews.org/articles/20030920/bob9ref.asp

Robinson, K.R., and M.A. Messerli. 2003. Left/right, up/down: The role of endogenous electrical fields as directional signals in development, repair and invasion. BioEssays 25(August):759-766. Abstract available at http://dx.doi.org/10.1002/bies.10307.

Electricity Makes It Happen
By Janice Valverde

When your brain is stimulated, brain cells send millions of fast-moving electrical signals along the pathways of your central nervous system. These paths are nerves that branch out into all your muscles. Whenever you move a muscle, it is powered by electricity running through your nervous system!

Move your fingers. Blink your eyes. It happens so fast that it seems automatic. People with healthy bodies hardly have to think about moving a muscle at all. But this is not true for people who are completely paralyzed.

A spinal cord injury, a stroke, or other serious condition can lock people in their bodies. Their brains still produce electrical signals when stimulated. They are alert, intelligent, have memory, and can learn, but since their nervous systems are badly damaged, electrical signals cannot reach their muscles. They can’t move their arms or legs or even speak. People with this “locked-in syndrome” have great difficulty communicating with others.

Dr. Philip Kennedy is a neurologist, a doctor specializing in the nervous system. His remarkable work helps people with locked-in syndrome. Dr. Kennedy invented the “neurotrophic electrode,” a tiny electrical conductor that is implanted above the ear in a patient’s skull.

One of the first people to have Dr. Kennedy’s electrode implanted in his brain was Johnny Ray. Before Johnny was paralyzed by a stroke in 1997, he was a construction worker and musician. His stroke locked him up in a body he can’t move at all.

Now, when Johnny thinks, the implanted electrode carries his brain’s electrical impulses to a computer instead of into his nervous system. While his brain signals can no longer move his fingers to write, or his mouth and tongue to speak, they can control a computer’s cursor the way they once controlled his muscles. He can even create music again, not by playing an instrument with his mouth or hands, but by thinking into his instrument, the computer!

Connecting the electrical impulses of the human brain to the electronic signals of the computer unlocks paralyzed people from their bodies and gives them a tool to more easily communicate with their families and friends­and the world.

http://www.santeecooperkids.com/culver/sse_root/body/

Activity: Keep Your Ion the Ball

Background
Did you ever wonder why your sweat and tears taste salty? It’s because the water that makes up nearly 70 percent of your body has salts dissolved in it. These salts­which include compounds of sodium, potassium, magnesium, and calcium­are necessary for good health. A loss of electrolytes, called an electrolyte imbalance, can slow down the transmission of nerve impulses, impair muscle function, and cause an irregular heartbeat.

Your body loses electrolytes when you sweat. Normally you get all the electrolytes you need from the food you eat. (Bananas and potatoes, for example, contain a lot of potassium.) Because they sweat a great deal, professional and endurance athletes can lose a large quantity of electrolytes during practice and competition. They will often turn to sports drinks to quickly restore the balance. That’s because sports drinks contain a lot of sodium and potassium.

In this experiment you will test the conductivity (ability to conduct electricity) of a variety of beverages to see which ones contain a higher concentration of electrolytes. The beverage that has the highest concentration of electrolytes will have the greatest conductivity. For more information on conductors and insulators, see “Who Can Resist?”

http://www.santeecooperkids.com/culver/sse_root/body/ion.html

Activity: Nervous Energy

Background
Nerve impulses travel from one neuron (nerve cell) to another in the form of electrical signals. Each neuron consists of a cell body, short threadlike projections called dendrites, and one longer thread called an axon. The electrical signals are received by the dendrites of a neuron and then passed along the axon to the dendrites of adjacent neurons.

Interestingly, axons and dendrites don’t actually touch. There is a space between them, called a synapse. So how does the electrical signal “jump” the gap? You could say the energy changes form. The electrical current causes chemicals in the axon tip to be released. These chemicals, called neurotransmitters, flow across the synapse and lock on to the dendrite of the next neuron, where they cause new electrical signals to be generated and passed on in the same manner.

You can use common electronic components to model how nerve impulses get relayed from one neuron to another in the body.

Materials
-4 AA batteries
-2 battery holders [RadioShack Cat. No. 270-408]
-3-volt DC buzzer [RadioShack Cat. No. 273-053A]
-1 infrared phototransistor [RadioShack Cat. No. 276-145A]
-1 jumbo super-bright LED (light-emitting diode) [RadioShack Cat. No. 276-086A]
-electrical tape

Steps
1. Set up the equipment as shown. Make sure the shorter lead of the LED is connected to the black wire of the battery holder. Similarly, make sure the shorter lead of the phototransistor is connected to the red wire of the other battery holder. Wrap a small piece of electrical tape around each connection.

2. You should have two circuits. The circuit on the left contains batteries, wire, and an LED. The circuit on the right contains batteries, wire, a phototransistor, and a buzzer. Electricity travels in a loop called a circuit. Every circuit has an energy source, wires, a load, and a switch.

3. Line up the LED with the phototransistor, leaving about a half-inch of space between them. Then touch the end of the loose red wire to the long lead of the LED. The LED should light up and the buzzer should sound. If the buzzer doesn’t sound, check the alignment of the LED and the phototransistor and then repeat until it does.

http://www.santeecooperkids.com/culver/sse_root/body/nervous.html

Leave a Reply

*