Influenza influences ion channels.

The symptoms of influenza virus A are multiple and include fever, cough, sore throat, body aches, headache, chills and fatigue. This pathogen is responsible for the seasonal outbreak of the flu, with mild airway involvement and low mortality. However, in a more virulent form (the Spanish flu of 1918) influenza was responsible for some 50–100 million deaths world wide (Taubenberger & Morens, 2006). Influenza virus is transmitted by aerosol; once inhaled, the virus attaches to sialic acid residues on the apical membrane of respiratory epithelia via haemagglutinin present in the viral membrane. Viral content enters the cell where the cell's machinery is utilized to produce more viral particles which then bud from the epithelial cell membrane to infect adjacent cells. In rapidly progressing cases (typically one week) mortality is in part due to viral pneumonia (inflammation and filling of the lung with fluid) and is a consequence of viral-dependent cell necrosis or apoptosis, and an immune response involving leukocytes, cytokines and chemokines, with a resultant accumulation of fluid and cell debris in the alveoli and upper airway spaces. Upper and lower airway epithelia maintain a low fluid volume in the respiratory tract by balancing fluid secretion (active chloride secretion) with fluid absorption (active sodium absorption) (see Sartori & Matthay, 2002). In the lower airways, disruption of the absorptive component, stimulation of the secretory component, or both, can result in fluid accumulation in the alveolar space, increased airway resistance and consequent reduction in blood oxygenation. Fluid absorption occurs by the diffusion of sodium into the cell through apical membrane epithelial sodium channels (ENaCs), the sodium is transported out of the cell across the basolateral membrane by the sodium–potassium ATPase, and the potassium that enters the cell exits across the basolateral membrane through potassium-selective channels. Does the influenza A virus cause fluid accumulation by altering epithelial cell transport?Kunzelmann et al. (2000) demonstrated in tracheal epithelium treated with influenza A virus a reduced rate of sodium absorption with a half-time of about 60 min. This inhibition was triggered by the binding of viral haemagglutinin to the epithelial cell membrane receptor, resulting in an increased activity of protein kinase C. This inhibition of absorption did not require viral endocytosis. Wolk et al. (2008) confirmed these findings and found that long-term viral exposure (2 days) resulted in an up-regulation of chloride secretion. This up-regulation of secretion along with the partial inhibition of absorption would result in increased fluid accumulation. Is the binding of the haemagglutinin to the cell surface the only signal needed for altered epithelial ion transport? This question is addressed by Gallacher et al. (2009) in this issue of The Journal of Physiology. NS1 (a non-structural protein of influenza A) is a multifunctional protein (see Hale et al. 2008) that is produced inside the epithelial cell and is responsible for viral replication, host innate and adaptive responses and also intracellular signalling pathways. In this latter function NS1 has been previously shown to activate phosphoinositide-3-kinase (PI-3-K) by binding to its regulatory subunit. Others have shown that PI-3-K is involved in ENaC regulation. The authors clearly demonstrate that transfection of H441 (a human lung adenocarcinoma cell line) with NS1 induced a non-selective cation conductance in the cell membrane (the potassium permeability is twice that of sodium), while mock transfected cells do not exhibit this conductance. However, a mutant NS1, which does not activate PI-3-K, induced a cation conductance but to a lesser extent. This mutant NS1 response could be rescued by transfecting these cells with a chimeric protein known to activate PI-3-K. These results clearly demonstrate that NS1 activates a cation conductance, but it does it by both a PI-3-K-dependent pathway and a PI-3-K-independent pathway. Of importance is that similar increases in cell membrane cation conductance were measured in cells exposed to influenza A virus when compared to transfection with NS1. Also mutant influenza A virus (mutated NS1) had a smaller increase in cell membrane cation conductance similar to the mutated NS1. Thus protein and virus produce similar effects suggesting that the viral effect is via NS1. The last and interesting observation is that epithelial cells typically communicate with each other via gap junctions; these authors reported that infection with the virus (whether mutated or not) resulted in uncoupling of the cells which might lead to loss of epithelial barrier function allowing fluid to flow from blood to alveolar space. How does one explain the apparent paradox between these observations of an increase in cation conductance, which presumably would lead to an increase in absorption, and previous results (Kunzelmann et al. 2000; Wolk et al. 2008), which demonstrate a decreased absorption? It might not be a paradox; if the cation channel is in the basolateral membrane it will decrease absorption by decreasing the driving forces favouring sodium absorption. The apical membrane presence of this conductance (in which potassium is twice as permeable as sodium) would increase sodium absorption but will also increase potassium secretion resulting in an overall decrease in net ion and water absorption. As stressed by the authors, one cannot look at a single component and predict how the system will respond; rather, one must look at all of the individual responses and integrate these into a single output. This paper has provided a key component to such an integrative approach.