The Airway Surface Layer (ASL) as an Important Part of Respiratory Function

The airway surface layer (ASL) is an important part of the respiratory function as it functions to defend against inhaled pathogens which are introduced to the body via the lungs. The body's primary defense mechanism against pathogens and infection is mucociliary clearance (MCC). The structure and function of the ASL are important for MCC, the cilia's ability to beat properly, and the antimicrobial capacity of the airway cells. In this essay, I will write about how ASL works in the context of the disease cystic fibrosis (CF).

The ASL is a thin layer of fluid that lies above the apical membrane of the airway cell and consists of two separate layers. One of the layers, the periciliary layer (PCL) lies adjacent to the apical membrane and surrounds the cilia. The other layer, the mucous layer lies on top of the PCL. Mucins are found in the mucous layer, which help trap respiratory pathogens in the mucous for MCC. Cilia project from the apical membrane and by beating, move the mucous layer up the respiratory tract. The risk of infection increases if the ability of cilia to beat up liquid is disrupted and as a result, respiratory function is compromised.

CF is an example of a disease where the function of the ASL is affected and so the respiratory function is impaired. CF is a genetic disease that affects more than 70,000 people worldwide. CF occurs when there is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene leading to a defective CFTR protein being made. CFTR is a Cl- channel on the apical membrane which controls the secretion of Cl-. There are more than 1700 mutations, so depending on the combination of the mutations, it affects how severe CF is in a patient. In the upper airway, CF mutations impact on the chloride secretion in the CFTR and so when CFTR is functioning normally this means that the epithelial sodium channel (ENaC) is inhibited. However, in the mutated CFTR, there is a loss of chloride secretion and an enhanced function of ENaC.

The ASL is controlled by two, passive and active mechanisms. The mucous layer is a vital component of the passive mechanism because it acts as a reservoir. When there is too much liquid in the PCL, it will move passively into the mucous layer. So, when there is a lack of liquid in the PCL, the liquid will move out of the mucous layer back into the PCL. The active mechanism is where the salt, Na, and Cl-, levels in the PCL are controlled by the active transport of ions. ENaC and CFTR are both important ion channels that help maintain the optimum volume and function of ASL. Figure 1 shows that when bumetanide is added, the height of the ASL decreases. This decrease in the ASL layer is due to the fact that bumetanide inhibits CFTR and so blocks the active transport of Cl-. Therefore, this is confirmation that CFTR plays a critical role in determining the height of the ASL.

In the upper airway epithelial cell, the basolateral membrane has a sodium-potassium pump (Na K -ATPase) and a basolateral potassium channel. These are important for setting the driving force so that there will be a negative membrane potential and low intracellular Na . This will lead to the uptake of Na, K and 2 Cl- via the Na-K-Cl cotransporter (NKCC1). Na enters the cell on the basolateral membrane through the NKCC1 and recycles via the Na K -ATPase. This allows for Na absorption from the liquid lining of the epithelial cells to occur, which is then followed by Na reabsorption and water. Cl- accumulates inside the cell resulting in high intracellular Cl-. When the apical chloride channel CFTR opens, the Cl- moves down its electrochemical gradient and leaves the cell. The height of the PCL is increased because when chloride secretion occurs, water and Na follow between the cells. The height of the PCL is decreased when the CFTR channel closes and the epithelial sodium channel (ENaC), on the apical membrane, opens. As there is a low concentration of Na in the cell, the negative membrane potential allows the movement of Na through ENaC and into the cell. The Na will eventually leave the cells across the basolateral membrane through the Na K -ATPase, driving water out in the opposite direction and lowering the height of the PCL.

A driving force for the movement of water is generated when ions move because this creates an osmotic gradient. Therefore, to increase the height of the ASL, the concentration of the ions needs to be higher inside the PCL so that water will be drawn in. To decrease the height of the ASL, the ions need to move out of the PCL so that there is an osmotic gradient generated in the opposite direction for the water to leave. The height of the ASL is very important in aiding ciliary function because when the height is decreased, the cilia bend over and are less efficient in beating the mucous up the respiratory tract. When the height of the ASL is increased more than the ideal level, the cilia cannot beat the mucous because of there is too much of it.

The pH and the hydration state of the ASL is important in the regulation of antimicrobial activity against respiratory pathogens. The transport of apical Cl- and HCO3- is disrupted in CF patients and so this leads to a dehydrated and acidic environment. The function of ENaC is upregulated in CF and leads to the increased viscosity of the mucus and dehydrated ASL due to more Na being absorbed than normal. An acidic ASL means that the antimicrobial properties of the ASL become reduced, however, this can be re-established when the pH is brought back to its original value as shown in Figures 1 and 2. This leads to the cilia not being able to clear away the respiratory pathogens stuck in the mucous. The mucous layer becomes too concentrated when the ASL is depleted, which results in water moving out of the mucous layer and the PCL in order to combat the dehydrated ASL. This leads to the mucous layer pressing down onto the PCL and disrupting the cilia's ability to beat the mucous up the respiratory tract. This ultimately affects MCC and so CF patients are more prone to respiratory infections and inflammation, which leads to more serious lung disease.

The ASL and the components that make up the ASL, the PCL, and mucous layer, are very important for respiratory function. The PCL contains cilia which project into it and there is a liquid layer with mucous on top of it. These layers are the body's innate defense mechanism against inhaled pathogens and infections. Active ion transport of Na and Cl- via ENaC and CFTR, is essential for maintaining the height of the ASL. The height of the ASL layer is important in determining ciliary function so that the mucous on the uppermost layer can move up the respiratory tract. This will allow the body to swallow the mucous, with the pathogens, and allow it to be destroyed in the digestive system. When the function of ENaC and CFTR is impaired in genetic diseases such as CF, it affects the ability of the cilia to beat and clear away the mucous due to the dehydrated and acidic ASL. MCC cannot happen and so the mucous traps the pathogens which lead to chronic respiratory infections, damage to the lungs, inflammation, and respiratory failure. The antimicrobial properties of the ASL are impaired due to the lowering of pH due to the CFTR mutation. The lack of bicarbonate secretion by the mutated CFTR creates an acidic environment which changes the mucous' ability to clear away the pathogens.

This can have a significant impact on patients as they accumulate more damage to the lungs due to inflammation and chronic respiratory infections. CF cannot be cured however there are ways to alleviate the symptoms via drugs. The ASL plays a critical role in this disease because it affects the function and properties of the mucous. Therefore, treatments such as nebulized hypertonic saline can hydrate the dehydrated ASL which will in turn improve MCC and help remove the respiratory pathogens.