Epithelial L1


Airway Surface Liquid (ASL), Influenza, and Ion Channels

Aim and Objectives

  • Aim: To evaluate the significance of Airway Surface Liquid (ASL) and the effects of influenza on the epithelial sodium channel (ENaC) and cystic fibrosis transmembrane conductance regulator (CFTR).

  • Objectives:

    • Discuss the generation, regulation, and importance of ASL.

    • Describe how influenza affects transepithelial potential (Vte) and short-circuit current (Isc).

    • Analyze the effects on ENaC and CFTR functions.

    • Evaluate evidence of influenza's impact on these channels.

Airways Defense Against Respiratory Pathogens

Components of Airway Defense

  • Mucous Clearance: The clearance of mucus is vital for trapping and expelling pathogens.

    • The mucous layer and the periciliary layer (PCL) are critical for effective mucus clearance, where the height of the PCL is important.

  • Airway Surface Liquid (ASL): It is a critical component in innate immunity found in the airways.

Problem of Small Airways

  • Size and Surface Area: Small airways cover approximately 2 m² for proximal regions and converge to about 50 cm² in the bronchial region. Understanding airway surface liquid movement is crucial for respiratory health.

Control of ASL

Mechanisms of ASL Regulation

  • Passive Mechanisms:

    • The mucous layer acts as a reservoir, allowing water to move in and out freely.

  • Active Transport Mechanisms:

    • Ion transport through cellular mechanisms plays a predominant role in controlling the salt levels within the PCL and ASL.

Ion Transport in ASL

Ion Channels and Their Functions

  1. Chloride Ions (Cl-): Important for regulating ASL volume and ionic composition.

  2. Sodium Ions (Na+):

    • ENaC is responsible for the absorption of Na+.

  3. Potassium Ions (K+):

    • K+ ions play a supportive role in the functioning of ENaC.

Evidence and Impact of Influenza on Ion Channels

Influenza Statistics

  • Influenza is responsible for approximately 12,000 to 61,000 deaths annually in the USA, with 600 to 13,000 deaths in the UK. Approximately 10% of hospital admissions during flu season are attributable to:

    1. InfluenzaLinked Bronchopneumonia and Edema.

    2. Acquired Respiratory Distress Syndrome.

Mechanisms of Influenza Impact

  • Influenza is shown to alter the functioning of ENaC and CFTR, leading to significant impacts on ASL levels.

Experimental Evidence from Studies

Key Findings

  • Nearly~Gone Proteins During Infection: Studies show that during influenza infection, the presence of specific proteins, such as GFP-NS1, results in a transient but significant inhibition of ENaC and CFTR channel currents.

  • Single Channel Analysis: Experimental setups using murine nasal tissues reveal the kinetics of ENaC and CFTR under infected conditions.

Inhibition Mechanics

  1. Reduction in Current Density: Both ENaC and CFTR exhibit a significant reduction in their peak currents during infection conditions.

  2. Formula of Ion Current Regulation:

    • The current can be modeled as I = N.Po.g.(V-Vrev), where:

      • I = current through channel

      • N = number of channels

      • Po = open probability

      • g = conductance

      • V = membrane potential

      • Vrev = reversal potential.

Overall Impact of Influenza on ASL

  1. ENaC Inhibition: Results in less sodium absorption, leading to increased ASL volume due to water retention.

  2. CFTR Inhibition: Results in less chloride secretion, which detrimentally lowers ASL volume.

  3. Predicted Outcome: Ultimately, the balance of these effects suggests that the reduction of ASL predominates due to sustained CFTR inhibition, despite transient ENaC reductions.

Research Sources and References

  • Research Articles

    • Brand et al. (2018), JCI Insight, provides critical insights into influenza effects on ion channels.

  • Literature Conflicts: Variations observed in studies may arise from different cell sources, influenza virus properties, and lab conditions, leading to questions about in vivo relevance.