This application explores the paradigm shifting hypothesis that post-translational modification (sialylation) of a cell surface (tethered) mucin, MUC4, drives terminal differentiation and senescence of airway epithelial cells (AECs) through inhibition of epidermal growth factor receptor (EGFR) family pathways, worsening epithelial wound repair and asthma severity. Our proposed studies will be the first to specifically test tethered mucins and their post-translational N-glycosylation/sialylation for a role in AEC terminal differentiation and senescence. Our published data (Zhou et al) demonstrate that sialylation of the tethered mucin, MUC4?, i.e., MUC4?SA? controls goblet cell terminal differentiation as a function of ?-galactoside ?2,6-sialyltransferase-1 (ST6GAL1) activity. Both ST6GAL1 and MUC4? critically lower AEC proliferative capacity, with additional published data (Inoue et al) linking diminished AEC proliferation to abnormal wound repair in vitro. Reduced AEC proliferative capacity may reflect reduced activation of the EGFR family member, ErbB2, a known receptor for MUC4?. New data confirm that elevations in MUC4 and ST6GAL1 are present in severe exacerbation-prone asthma and in house dust mite challenged mice. Elevations in both biologically associate with low intracellular glutathione (GSH) to oxidized glutathione (GSSG) ratios (oxidative stress) and lower mitochondrial and senescence gene expression. Parallel in vitro AEC data show T2 inflammation/IL-13 alters intracellular metabolism and mitochondria, decreases GSH/GSSG, and further decreases wound repair. Yet, the overall impact of MUC4? and/or its sialylation to epithelial cell phenotypes, cell senescence, and the mechanisms by which these changes contribute to severe asthma are unclear. The studies proposed here will comprehensively evaluate the intersection of mucins, mucin sialylation, and senescence pathway(s), while robustly testing their functional importance using primary human AEC cell cultures and transgenic mouse models. The three proposed aims will: 1) elucidate the mechanistic and functional impact of MUC4? and MUC4?SA on AEC terminal differentiation, senescence and wound repair in vitro, 2) test the functional impact of Muc4?SA-AEC interactions on mucin secretion, goblet cell hyperplasia/terminal differentiation, senescence, and wound repair in an asthma mouse model and 3) define the relationship of MUC4?SA to senescence and AEC phenotypes in human asthma patients. These concurrent aims will iteratively develop data that link mucins, their sialylation and fundamental senescence-related epithelial processes to identify highly novel targets for treatment of severe asthma.
Proteins are well appreciated to drive human biology, with alterations in expression contributing to disease. However, proteins exist in many forms, including addition of carbohydrate groups (glycosylation), which greatly impact their function. The studies proposed here will determine the role of N-glycosylation of a common mucin, MUC4? to epithelial (goblet cell) differentiation, metabolic stress, poor wound repair and, finally, severe asthma. These N-glycoslylation pathways offer highly novel targets for asthma treatment.