SPDEFending the Lung through Mucin Expression

Each day, the lungs are exposed to billions of particles whose accumulation could result in tissue injury, inflammation, or infection if not trapped and cleared by airway mucus. Protection requires robust defense that must occur with minimal disruption of airflow and gas exchange. Accordingly, a thin mucus layer in both upper and lower airways effectively traps inhaled materials, which are transported to the mouth, where they are eliminated by expectoration or swallowing. Although mucus function is critical for health, in lung diseases—including asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis—airflow limitation, inflammation, and tissue injury are associated with mucus dysfunction. The balance between effective and defective host defense is tightly linked to the regulation of mucus function.

The overall function of airway mucus is dictated by its physical hydrogel properties and thus depends upon the secreted gel-forming mucin glycoproteins MUC5AC and MUC5B (1). In secretions from healthy humans, MUC5B levels predominate relative to MUC5AC. Likewise, Muc5b is the predominant mucin present in healthy mucus secretions of mice, where it is required for innate defense in the upper and lower airways (2). MUC5AC gene expression is dramatically induced in human diseases, including type 2-high asthma and chronic obstructive pulmonary disease, and it contributes to lower airway pathophysiology in animal models (3, 4). Accordingly, because of its tight regulation and significant functions, the control of MUC5AC/Muc5ac transcriptional induction is well studied (5).

A growing base of knowledge shows a key role for the transcription factor SAM pointed domain containing ETS transcription factor (SPDEF) in MUC5AC gene regulation. Upregulation of MUC5AC transcription by the type 2 cytokine IL-13 is SPDEF dependent in human airway cells, and allergically inflamed lungs in Spdef gene–knockout mice show diminished histochemically detectable mucous metaplasia, which is directly linked to decreased levels of Muc5ac mucin induction (6). In contrast, MUC5B gene expression has been observed to be downregulated by IL-13 stimulation of human cells, and MUC5B gene expression also decreases in airway brushings of patients with type 2-high asthma. Because of its role in homeostasis in mice, and because of the recent association of MUC5B promoter genetic variants with the development of pulmonary fibrosis in humans, the control of MUC5B gene expression has also emerged as an important topic (7).

In this issue of the Journal (pp. 383–396), Chen and colleagues test whether mouse pulmonary Muc5b gene expression and mucus functions are regulated by Spdef (8). They report that the effects of Spdef gene deficiency depend on location, age, and health status. Spdef was required for Muc5b production in the lower airways of neonatal mice, and it was partly required for normal levels of Muc5b in the nasopharynx and proximal bronchi of adult mice. In a transgenic (Tg) mouse model of mucoobstructive lung disease caused by epithelial sodium transport dysregulation (Scnn1b overexpression), loss of Spdef was beneficial in early life, when Muc5ac-rich tracheal plugs are a major cause of death. However, at later time points, Spdef deficiency had little impact on pathological Muc5b-dependent mucus plugging. Taken together, these data point to complex regulation of Muc5ac and Muc5b gene expression in mice, which could be due to the use of multiple gene expression networks as well as to tissue-specific programming potentially related to the ontogeny of mucin-expressing cells.

Although the molecular basis for the control of MUC5B gene expression in health and disease remains obscure, this study defines important principles to frame future investigations. In alignment with prior reports (9), Chen and colleagues demonstrate that normal expression of Muc5b depends on SPDEF, with evidence supporting an SPDEF–Foxa3 axis controlling nonpathologic Muc5b airway expression. However, this pathway was also shown to be dispensable for Muc5b gene regulation in Scnn1b-Tg mouse airway mucus obstruction and inflammation.

The juxtaposition of these findings has important implications for the understanding of Muc5b gene regulation. First, many genes require developmentally regulated chromatin modifications to ensure appropriate expression. In the case of pathologic gene expression of Muc5b, either specific chromatin modifications are not required or the SPDEF–Foxa3 axis does not participate in generating lineage-specific chromatin architecture in the airway required for Muc5b expression. Second, efforts to identify regulators of the MUC5B gene that may contribute to pathology in various human diseases, such as idiopathic pulmonary fibrosis, will need to extend beyond interrogating pathways involved in developmental specification of airway epithelial fate. Indeed, analysis of MUC5B levels and localization in distal airway cells in idiopathic pulmonary fibrosis has suggested that SPDEF expression is not associated with MUC5B expression in pulmonary fibrosis (10), whereas a role for SPDEF in regulating MUC5B gene expression in cancer has been demonstrated (11). Thus, whether SPDEF and FOXA3 are dispensable for pathologic MUC5B gene regulation in specific human disease contexts requires further investigation.

Although mucin-secreting cells are generally viewed as a single airway epithelial cell type, the results here may be reflective of very distinct mucin-expressing cells in the lungs based on airway location and disease state. Lineage-tracing studies in allergic mouse models clearly show club cells that activate SPDEF undergo a metaplastic process to differentiate into Muc5ac-expressing cells (6). It is possible that SPDEF-independent metaplasia of a nonmucous population at baseline explains the Muc5b produced in Scnn1b-Tg mice. Higher-resolution analyses, such as single-cell sequencing, should help comprehensively define different mucin-expressing populations in the epithelium and whether the frequencies of these populations shift or new ones emerge in disease models such as the one studied by Chen and colleagues (8). The elucidation of mucin-expressing epithelial cell subtypes and accompanying marker genes will also aid in the development of lineage-tracing study designs that should help close the gap in the understanding of how mucin expression is regulated in the healthy and diseased airway epithelium.