By David N. Leff
What does water in the lungs have to do with asthma?
Of your average young man¿s total body weight, 60 percent consists of water ¿ two-thirds inside cells, one-third outside. It¿s less in the obese and elderly.
¿The mammalian body is very highly fluidized,¿ observed molecular geneticist Anil Menon, at the University of Cincinnati. ¿There¿s a lot of water in our bodies, and it has to be very efficiently kept in different corporeal compartments ¿ not like one big bucketful of water, but a whole pile of little sub-compartments, like a honeycomb. And the amount of water in each cell of the honeycomb is very precisely regulated.
¿The average human lung, when you spread out all its cells and airways, has a huge surface,¿ Menon pointed out. ¿That surface for an adult human is as big as a tennis court. And that lung has to be properly fluidized. It can¿t be too dry and it can¿t be too moist. So there¿s a very precise control of lung fluidization. That control,¿ he continued, ¿is managed by proteins called aquaporins. What we think happens is that the genes involved in water transport express these aquaporins, which may have some role not just in normal pulmonary fluidization but also in lung diseases such as asthma. Traditionally,¿ Menon observed, ¿asthma has been looked upon as a disease that involves inflammation and the immune system. But there¿s probably another component to it, and that is the overall water balance of the lungs.¿
Menon is senior author of an article in the Proceedings of the National Academy of Sciences (PNAS), dated Nov. 20, 2001, but released electronically Nov. 12. Its title: ¿Aquaporin 5-deficient mouse lungs are hyperresponsive to cholinergic stimulation.¿
¿Our finding,¿ he told BioWorld Today, ¿is that a new class of mechanisms involved in water balance of the lungs may play a role in asthma. Its novel element,¿ Menon went on, ¿is that there¿s a whole new class of proteins and genes which had previously not been known. People studying asthma have looked very hard at inflammatory factors, immune factors and signaling factors such as adrenergic receptors. What this PNAS paper does,¿ he added, ¿is put on the table a new avenue, which may play a role in asthma, and potentially be modulated by therapeutic interventions.¿
Aquaporin¿s Role In Water-Transport Diseases
¿Aquaporins,¿ Menon explained, ¿are newly identified proteins, discovered eight or nine years ago at the Johns Hopkins University School of Medicine. Right now people are trying to understand what the aquaporin proteins do. It¿s known that many of them are involved in water transport, and the challenge is going to be trying to figure out how diseases of various kinds, which involve water transport, might be affected by variations of the aquaporin genes that express those proteins.¿
He cited edema as one common disorder of water balance. ¿For example, edema occurs when people get injuries of various kinds. If you hurt your hand, it swells up, and that¿s because there¿s differential water transport; it¿s swollen with water. In the same way, you can have that phenomenon in the lungs, which fill with water. This may happen ¿ and I¿m speculating here ¿ because of defects in the mechanism of water transport, in which the aquaporins play an important role. And there¿s a possibility that an asthma-associated disorder called chronic obstructive pulmonary disease ¿ COPD ¿ may be affected to some degree by the lung fluidization.¿ (A decade ago, COPD ranked as the fourth-highest cause of mortality in the U.S., with 95,910 deaths in 1993 ¿ more than double that in 1979.)
¿A total of 11 aquaporins have been identified to date,¿ Menon related. ¿The one that we¿re describing in PNAS, aquaporin 5 ¿ which is the major lung form of the aquaporin gene ¿ is located on the long arm of human chromosome 12. It encodes a membrane protein that transports water and electrolytes from one side of the cell membrane to the other.¿
In in vivo experiments, Menon and his co-authors tested the effects of aquaporin 5 in asthma. ¿What we did,¿ he recounted, ¿was create genetically engineered mice, in which we made a defective aquaporin gene; that is, we knocked it out. These KO animals no longer produced the aquaporin protein in their lungs, and we found that they were very susceptible to bronchial constriction, which is one of the major symptoms of asthma.
¿We injected both KO and normal mice with methacholine, which is a cholinergic stimulus,¿ Menon went on. ¿This is a standard way to examine how many bronchial constrictions occurred. The aquaporin-minus mice were two times more sensitive to methacholine challenge than the unmodified wild-type animals. Human asthmatic patients, in fact, are routinely tested with this methacholine challenge, to see how much of their asthma is caused by bronchial constriction. Some have severe physiobronchial constriction while others have milder or very little tightening.
¿Someone suffering an asthma attack can¿t breathe, feels congested,¿ Menon explained. ¿The lungs can¿t expand properly. All the tubes in the lungs, the bronchi, are closed. The trachea bifurcates, then keeps branching out repeatedly. That whole respiratory system of bronchial tubes subdivides into bronchioles and smaller and smaller branches and twigs.¿
From Mouse Experience To Human Condition
¿So we used the same logic to test these mouse cohorts,¿ Menon continued. ¿The KO mice we injected with methacholine responded very strongly. They experienced a tremendous bronchial constriction ¿ twice that of the same experiment in the wild-type animals. That data looked to be statistically valid, with equal numbers of animals,¿ Menon recalled, ¿so we presented it in our paper.
¿The question we have to address in the future,¿ he observed, ¿is to see if in fact there¿s a correlation between variations in the aquaporin genes, and severity of asthma. Eventually if strong proof implicates aquaporins in human asthma, then there are good reasons to start looking for candidate compounds that either activate or stimulate those proteins. Our own lab is working in this direction.
¿I think we¿re quite far away from a Phase I human trial at the moment,¿ he foresaw, ¿because what we found so far is simply in an animal model. I think these studies have to be repeated in humans to make sure that they are holding up. Beyond that,¿ Menon concluded, ¿it will take some work.¿