Murine CLCA5 is uniquely expressed in distinct niches of airway epithelial cells

The murine mCLCA5 protein is a member of the chloride channel regulators, calcium-activated (CLCA) family and is suspected to play a role in airway mucus cell differentiation. Although mCLCA5 mRNA was previously found in total lung extracts, the expressing cells and functions in the naive murine respiratory tract are unknown. Therefore, mCLCA5 protein expression was identified by immunohistochemistry and confocal laser scanning microscopy using entire lung sections of naive mice. Moreover, we determined mRNA levels of functionally related genes (mClca3, mClca5, Muc5ac and Muc5b) and quantified mCLCA5-, mCLCA3- and CC10-positive cells and periodic acid-Schiff-positive mucus cells in naive, PBS-treated or Staphylococcus aureus-infected mice. We also investigated mCLCA5 protein expression in Streptococcus pneumoniae and influenza virus lung infection models. Finally, we determined species-specific differences in the expression patterns of the murine mCLCA5 and its human and porcine orthologs, hCLCA2 and pCLCA2. The mCLCA5 protein is uniquely expressed in highly select bronchial epithelial cells and submucosal glands in naive mice, consistent with anatomical locations of progenitor cell niches. Under conditions of challenge (PBS, S. aureus, S. pneumoniae, influenza virus), mRNA and protein expression strongly declined with protein recovery only in models retaining intact epithelial cells. In contrast to mice, human and porcine bronchial epithelial cells do not express their respective mCLCA5 orthologs and submucosal glands had fewer expressing cells, indicative of fundamental differences in mice versus humans and pigs.