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Mucus hypersecretion in asthma: causes and effects
Christopher M Evans1, Kyubo Kim, Michael J Tuvim
1Department of Pulmonary Medicine, University of Texas M. D. Anderson Cancer Center, Houston, TX 77030-1402, USA.
Purpose Of Review:
Airway mucus plugging has long been recognized as a principal cause of death in asthma. However, molecular mechanisms of mucin overproduction and secretion have not been understood until recently. These mechanisms are reviewed together with ongoing investigations relating them to lung pathophysiology.
Recent Findings:
Of the five secreted gel-forming mucins in mammals, only MUC5AC and MUC5B are produced in significant quantities in intrapulmonary airways. MUC5B is the principal gel-forming mucin at baseline in small airways of humans and mice, and therefore likely performs most homeostatic clearance functions. MUC5AC is the principal gel-forming mucin upregulated in airway inflammation and is under negative control by forkhead box a2 (Foxa2) and positive control by hypoxia inducible factor-1 (HIF-1). Mucin secretion is regulated separately from production, principally by extracellular triphosphate nucleotides that bind P2Y2 receptors on the lumenal surface of airway secretory cells, generating intracellular second messengers that activate the exocytic proteins, Munc13-2 and synaptotagmin-2.
Summary:
Markedly upregulated production of MUC5AC together with stimulated secretion leads to airflow obstruction in asthma. As MUC5B appears to mediate homeostatic functions, it may be possible to selectively inhibit MUC5AC production without impairing airway function. The precise roles of mucin hypersecretion in asthma symptoms such as dyspnea and cough and in physiologic phenomena such as airway hyperresponsiveness remain to be defined.
Insights
Asthma mucus plugging is linked to MUC5AC overproduction and secretion. Targeting MUC5AC may treat asthma without affecting normal airway function, as MUC5B is crucial for homeostasis.
Area of Science:
- Pulmonary Medicine
- Molecular Biology
- Respiratory Physiology
Background:
- Airway mucus plugging is a major cause of asthma mortality.
- The molecular mechanisms of mucin overproduction and secretion in asthma were previously unclear.
- Understanding these pathways is crucial for managing lung pathophysiology.
Purpose of the Study:
- To review the molecular mechanisms of mucin overproduction and secretion in the airways.
- To investigate the roles of specific mucins (MUC5AC and MUC5B) in airway homeostasis and inflammation.
- To explore potential therapeutic strategies targeting mucin pathways in asthma.
Main Methods:
- Review of existing literature on airway mucin production and secretion.
- Analysis of the roles of MUC5AC and MUC5B in human and mouse airways.
- Investigation of regulatory factors such as Foxa2 and HIF-1 in mucin production.
- Examination of the signaling pathways involved in mucin secretion, including P2Y2 receptors and triphosphate nucleotides.
Main Results:
- MUC5B is the primary gel-forming mucin in normal small airways, essential for homeostasis.
- MUC5AC is upregulated during airway inflammation and is regulated by Foxa2 and HIF-1.
- Mucin secretion is regulated independently of production, primarily by extracellular triphosphate nucleotides acting on P2Y2 receptors.
- Specific proteins like Munc13-2 and synaptotagmin-2 are involved in activating mucin exocytosis.
Conclusions:
- Upregulated MUC5AC production and secretion contribute significantly to airflow obstruction in asthma.
- Selective inhibition of MUC5AC may offer a therapeutic approach for asthma without compromising essential homeostatic functions mediated by MUC5B.
- Further research is needed to define the exact roles of mucin hypersecretion in asthma symptoms and physiological changes.
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