The Spark of Life: Electricity in the Human Body

by Ashcroft, Frances

  Current treatments for cystic fibrosis involve simply managing the symptoms: fighting lung infections with antibiotics, preventing the build-up of mucous in the lungs by physiotherapy, and replacing the missing digestive enzymes. But new research aims to correct the defective channel itself. About 4 per cent of patients have a mutation in CFTR (known as G551D) that reduces the time the channel spends open. Recently, a drug known as ivacaftor has been shown to coax such sleepy channels into functioning normally, and preliminary studies suggest it may be of clinical benefit. While there is still a long way to go, it is a promising new approach for treating people with the G551D mutation. Most patients, however, have a different variant of CFTR, known as F508del, that prevents the channel from ever reaching the surface membrane of the cell. In this case, drugs that correct the defective targeting of the channel are needed.

  Cystic fibrosis is extremely rare in Orientals and black Africans and is highest in individuals of northern European extraction, where it is one of the most common inherited single gene diseases. Around 9,000 people in the UK have the disease and one in twenty-five of the population – over two million people – carry one copy of the faulty gene: although they are asymptomatic themselves, when two of them have a child there is a 25 per cent chance the child will have cystic fibrosis. This high frequency suggests there may be a selective advantage in having a single copy of the gene. One possibility is that such ‘carriers’ may be more resistant to the effects of diarrhoeal diseases, such as cholera. Vibrio cholerae, the bacterium responsible for cholera, produces a toxin that leads to opening of CFTR channels in gut cells, so that chloride rushes out of the cell, dragging water with it. This causes massive fluid loss from the gut, which results in severe diarrhoea and rapid death from dehydration. Individuals with a lower complement of CFTR channels may secrete less chloride and thus potentially be less susceptible to dehydration.

  The cholera bacterium is transmitted in faeces, and any natural disaster that leads to a breakdown of sanitation, such as an earthquake or floods, brings with it the risk of a cholera outbreak. The 2010 earthquake in Haiti was no exception and was quickly followed by an epidemic of the disease. Although cholera is no longer a disease of northern Europe, being mainly confined to third-world countries, this was not always the case. One of the most notable successes of public hygiene was the removal of the handle of a water pump in Broad Street, London, by Dr John Snow in the summer of 1853.

  During a severe fourteen-week-long cholera outbreak, Snow noticed that there were about ten times as many deaths in the district of Southwark than in Lambeth. He was of the opinion that cholera was spread in the water, whereas others contended it was the ‘foul miasma’ seeping from the sewers. Diligent study led Snow to discover that in one area of London the pipes from two different water companies were intermingled so that people were exposed to the same air and environment, but not necessarily the same water. By removing the handle of the pump that supplied infected water, he contained the outbreak of cholera and confirmed his hypothesis that the disease was spread in the water supply. The outbreak was eventually traced to Frances Lewis, a five-month-old baby who died of an attack of violent diarrhoea. Her mother poured the water used to rinse her daughter’s infected clothes into the gutter outside her house, which leaked into the Broad Street well and contaminated the water supply. It was a fatal mistake.

  The Cell’s Plumbing System

  Ultimately, both ENaC and CFTR produce disease by affecting transcellular water fluxes. For many years scientists puzzled about how water could cross cell membranes. As they are made of lipids (fats), they should be largely impervious to water, so how was it possible for water to penetrate the lipid barrier in such large amounts that it produces tears, saliva, sweat and urine? The answer is that most cells have specialized water channels known as aquaporins that conduct water across the membrane into and out of the cell. They were discovered serendipitously by Peter Agre. He called his finding, which eventually led to the award of a Nobel Prize, ‘sheer blind luck’. Having a suspicion that the protein he had discovered might be the long-sought cell water channel, he tested its ability to transport water using frog eggs, which normally live very happily in freshwater. To his excitement, frog eggs engineered to express water channels in their membranes swelled up and burst when they were placed in freshwater.

  Agre’s experiment was a perfect demonstration of the power of osmosis – the tendency of water to flow from a region of low salt concentration to one of a higher concentration. Because freshwater has far fewer salts than those inside the cell, water will always attempt to penetrate frogs’ eggs but it is normally prevented from doing so by the lipid membrane. Increase the water permeability of that membrane, however (for example, by adding lots of water channels as Agre did) and water will rush in, causing the egg to swell and eventually burst.

  It turns out that there are many different kinds of aquaporin channels and they are present in many types of cells, including brain cells and red blood cells, and even the cells of plants and microorganisms. One of the most important (known as aquaporin 2) sits in the collecting ducts of the kidney tubules and is responsible for reabsorbing the final thirty-five litres of water that the kidney filters every day, and thus for our ability to make a concentrated urine.1 Approximately three billion water molecules a second pass through a single aquaporin channel. It is highly selective as, due to the unique architecture of the pore, only water – and not ions – can pass through. Water channels are also unusual in that they do not open and close like ion channels, but are permanently locked open: instead the amount of water taken up is regulated by shuttling the channels in and out of the cell membrane. When the body needs to conserve water, extra water channels are inserted. Conversely, if you drink too much fluid, water channels are removed, so that less of the water filtered by the kidney is reabsorbed and it simply flows away as urine. This endless cycling of water channels into and out of the cell membrane is under hormonal control and occurs continuously. It is happening in your own kidneys, right now.

  Interestingly, the process can be disrupted by alcohol. A few pints of beer prevent the release of the anti-diuretic hormone that causes water channels to be inserted into the kidney tubules, which is why you produce copious amounts of dilute urine. The result is that the morning after a binge you wake up in a partially dehydrated state, which contributes to the headache. As all the alcohol has now been metabolized (one hopes), hormone levels will be higher, water channels will be mobilized into the tubule membranes, and the increased water uptake will result in a concentrated urine. You can even observe the phenomenon for yourself for the concentrated urine you produce the morning after an evening out is a far darker colour than the dilute pee of the night before.

  People who lack functioning aquaporin 2 channels produce large amounts of dilute urine – as much as 25 litres a day – and quickly become seriously dehydrated and very thirsty. This can happen because of a rare genetic mutation, in which case the disease manifests at birth; it can be hard for parents to spot, for urine-soaked nappies are far from uncommon in babies.

  Lethal Agents

  Ion channels are not only crucial at the start of life – they are also intimately involved in its end. Many cells and organisms use ion channels as offensive weapons. These act as molecular hole-punches, inserting themselves into the membrane of the target cell and forming a huge hole – a giant pore so big that not only ions but also small molecules and essential nutrients can leave the cell. Water rushes in, causing the cell to swell up so much that it eventually explodes (lyses) and dies. Channels used as lethal agents in this way are particularly interesting as they are packaged within the aggressor cell in an inactive form in which they do no harm. Once released, they reassemble themselves into a structure that is able to embed itself in the membrane of their prey. They are true transformers, shape-shifting from a harmless inactive form to a highly lethal one in matter of seconds.

  Such channel-forming molecules play
important roles in our immune system, defending us against invading pathogens. One type, appropriately named defensins, is found in our skin and the lining of the airways, where they act as natural antibiotics with a broad spectrum of action against bacteria, fungi and some viruses. Others are released by specialized white blood cells known as killer T-cells (or natural killer cells). Killer T-cells kill viruses and bacteria in a number of different ways, but one of them is by releasing perforins – ion channels that punch holes in alien cell membranes. Another pore-forming weapon in the arsenal of our immune system is complement, which produces even larger perforations in invading cells.

  Bacteria also indulge in incessant chemical warfare with one another, secreting channel-forming proteins that kill other bacteria. Unfortunately, some of these also attack human cells. Alpha toxin, secreted by Staphylococcus aureus, is one of the largest, most lethal and most beautiful of all. It is a mushroom-shaped channel, with the stalk spanning the membrane and the cap resting on its outer surface, projecting out from the cell. To avoid damaging the bacterium itself the channel is made of seven separate subunits, which are secreted individually and subsequently co-assemble to form a giant pore that punctures the target cell. Staphylococcus bacteria cause skin infections such as carbuncles, boils, abscesses, wound infections, and, most seriously of all, systemic infections in which the bloodstream carries the toxin and bacteria to all tissues and both red and white blood cells may be damaged (causing blood poisoning). The ability of alpha toxin to lyse red blood cells gives rise to its alternative name, haemolysin.

  Staphylococcus pyrogenes, the bug that causes scarlet fever, also a produces a toxin that bursts red blood cells, causing a characteristic fine red rash all over the body and a bright strawberry-red tongue. It can be fatal – the mother of the nineteenth-century American novelist Louisa May Alcott died of the disease, a traumatic event that the writer subsequently used in her novel Little Women. Other ion channels, such as those released by the protozoan that causes amoebic dysentery, wreak havoc in our guts.

  Battling Bugs

  Humans have harnessed such channel-forming bacterial toxins for their own purposes. Some, which attack bacterial cells but not mammalian ones, have been exploited as antibiotics. Others are used as insecticides. The best known is that secreted by the bacterium Bacillus thurigiensis, which inserts itself into the cells lining an insect’s gut, causing them to lyse, so that the insect eventually dies of dehydration. The toxin is released as an inactive precursor that must be activated in the insect gut and so is harmless to humans.

  Bacillus thurigiensis is widely used as a biological control agent to limit caterpillar populations in commercial greenhouses, to destroy mosquito larvae, and to kill the blackflies that carry river blindness. More recently, the gene that codes for the bacterium’s toxin has been engineered into plants, which then manufacture the toxin themselves. Pesticide-producing strains of maize, potato and cotton are commonly grown in the USA and enable the use of synthetic insecticides to be dramatically reduced. This has had clear environmental benefits. Nevertheless, the practice has been quite controversial, in part because of anxiety about genetically modified crops. Another concern is that continual exposure of insects to the pesticide creates a strong evolutionary selection pressure that favours toxin-resistant insects. Any insect developing a mutant receptor that does not bind the toxin has a clear reproductive advantage, and insects resistant to the pesticide have already been reported. As is the case for antibiotics, countering resistance is a constant battle.

  Cell Suicide

  Long ago, before you were born, you had webbed hands and feet like those of a duck. As you developed inside your mother’s womb, the cells that made up the web of soft tissue between your digits were killed off in a process known as programmed cell death (or apoptosis) so that you ended up with separate fingers and toes. If this process of body sculpting fails, as occasionally happens, you end up born with webbed fingers.

  Everyone who has kept tadpoles has seen such cell suicide in action for the gradual disappearance of the tadpole’s tail as it develops into a baby frog occurs by apoptosis and reabsorption of the dying cells. Similarly, apoptosis is drawn to the attention of a woman every month, for the sloughing off of the lining of the womb that occurs at the start of her period is also the result of programmed cell death. Perhaps most important of all, cell suicide plays a key role in the development of the nervous system and in how your brain is wired up. Early in development, many nerve cells are born and send forth their axons towards their destination in an exploratory manner. If they find their correct targets, a tentative connection is established, impulses speed excitedly down the lines, chemical kisses are exchanged, and the link is cemented. Nerve cells whose axons fail to find their correct targets produce more feeble impulse activity and simply wither away through lack of use. Many die during brain development and without such cell suicide the brain could not function correctly. Apoptosis is also a way to ensure that damaged cells that might threaten an organism’s survival are eliminated. Your immune system can kill cells infected with viruses this way, and cells whose DNA is damaged are encouraged to commit suicide to prevent cancers forming.

  At the cellular level, then, death is far from being a negative event. It is an essential part of the life of every multicellular organism and every day several billion cells in our bodies die by apoptosis. Without it, multicellular life is not possible. If we are no closer to understanding the meaning of life, at the cellular level, at least, we might claim to understand the meaning of death.

  A Time to Live, a Time to Die

  When a cell commits suicide it shrinks, its membrane lifting away from the underlying cytoplasm in ugly bubble-like blebs. The DNA is broken down so that no more proteins can be produced, and the mitochondria, the cell’s powerhouses, are disabled. Specific lipids appear on the surface of the cell membrane that signal to scavenger cells to come and gobble up the broken fragments of the dying cell for recycling.

  There are several ways in which a cell can self-destruct but, as you have probably guessed, one of them is mediated by an ion channel. It also involves the mitochondria, tiny intracellular organelles, about the size of a bacterium, that are found in almost every cell of your body. The ancestors of mitochondria were once free-living entities, rather similar to the blue-green algae (the cyanobacteria) that form the familiar green scum on lakes in hot summers, but around two billion years ago these ancestral mitochondria gave up the solitary life and became incorporated within early cells. Thus like the Star Trek aliens known as the Trill, we live our lives in partnership with another organism – but this is no science fiction and our symbionts are microscopic. Almost all plant and animal cells contain mitochondria and they are essential for life: without them, multicellular organisms could not function, as mitochondria act as molecular furnaces where fuels such as sugar and fats are burned with oxygen to produce chemical energy. Cells that require a lot of energy, like muscle cells, have large numbers of mitochondria.

  But mitochondria also have their dark side. They are surrounded by two membranes, whose integrity is important for the ability of the mitochondrion to produce energy. When a cell decides to commit suicide a large pore forms in the outer mitochondrial membrane known as the mitochondrial apoptosis-induced channel. The hole is so big that relatively large chemicals can leak out of the mitochondria into the cytoplasm, where they create mayhem, triggering a cascade of events that leads inexorably to cell death. Importantly, however, the decision to commit suicide is not decided by the mitochondria itself. It is a process that is initiated and tightly controlled by the cell, which simply co-opts the mitochondrial machinery to serve its own ends.

  Blighted Harvest

  Mitochondria are also targeted by the Southern Corn Leaf Blight toxin, which wreaks such havoc on cytoplasmic male sterile (CMS) strains of maize. CMS maize plants are sterile because they possess a unique ion channel that sits within their inner mitochondrial membrane. Like a silent
timebomb, this channel is normally closed and does not affect organelle function. However, binding of the Southern Corn Leaf Blight toxin activates the timebomb, opening the channel and destroying the ability of the mitochondria to make energy. Without energy, the cell dies. As the fungus spreads, the toxin destroys the plant, cell by cell. Only those plants that possess the ion channel gene, that is the CMS varieties, are susceptible. It is an inescapable association, for toxin sensitivity and male sterility result from the same process. Even in the absence of the toxin, the ion channel is activated in the mitochondria of the cells that supply the developing pollen grains with nutrients, and when these cells wither and die, so too does the pollen.

  Despite the wide devastation caused by Southern Corn Leaf Blight in 1970 in the USA, the country was lucky. More than 85 per cent of maize plants at that time carried the gene. Only the dry September in the northern and western states, which limited the spread of the fungus, prevented an almost total destruction of the crop. As Paul Raeburn points out in his thought-provoking book The Last Harvest, the size of the Southern Corn Leaf Blight epidemic and its enormous economic impact resulted from the fact that the Corn Belt in the USA was largely planted with a single variety of maize. The genetic uniformity of modern crops and the practice of planting only one or two varieties over a wide area means that if one plant is susceptible to a new disease, all plants will be. Consequently, the whole crop is at risk. More traditional methods of agriculture, which use many different local varieties, preserve considerable genetic variability so that although some plants may succumb to infection, many others will be resistant. A good reason, then, for preserving as many wild crop species and indigenous cultivars as we can, for without the genes that these plants contain, plant breeders may be unable to adapt crops to the new dangers we will assuredly encounter in the future.