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For more than 15 years, the CLCA (chloride channel regulator, calcium-activated) protein family has been in the focus of several research groups worldwide due to their strong implication in several important animal and human diseases.
However, the functions of these molecules in normal tissues and their exact roles in diseases are still incompletely understood. Some CLCA proteins possess a well established role in inflammatory airway diseases with mucus overproduction, such as asthma, cystic fibrosis and chronic obstructive pulmonary disease.
In the respiratory tract, the human hCLCA1 and its mouse ortholog mCLCA3 are selectively expressed in mucus cells and have directly been linked to the trait of mucus cell metaplasia, a common feature of these diseases.
In addition to mCLCA3, the murine mCLCA5 has also been associated with airway mucus cell metaplasia and a redundant or overlapping function of the two murine members was previously proposed. However, the cell types that express mCLCA5 in the airways were unknown.
Consequently, in this study the cellular expression pattern of mCLCA5 was determined under healthy and challenged conditions in murine lungs. Since differences in expression patterns between different species have previously been observed for other CLCA proteins, the expression patterns of the mCLCA5 orthologous proteins in humans and pigs, hCLCA2 and pCLCA2, respectively, were also established to allow for a better understanding of animal models for human diseases. In healthy mice, mCLCA5 was found to be uniquely expressed in highly select regions of bronchial epithelial cells and in submucosal glands (SMG), consistent with the canonical anatomical locations of progenitor cell niches.
Since club cells were the predominantly mCLCA5 expressing cell type, followed by fewer mucus cells and ciliated cells, it appears unlikely that mCLCA5 has a fully redundant function with mCLCA3 which is expressed in mucus cells only.
Under conditions of challenge including instillation of phosphate buffered saline (PBS), Staphylococcus aureus (S. aureus), Streptococcus pneumoniae (S. pneumoniae) or influenza virus, mCLCA5 mRNA and protein expression strongly declined.
Protein reappearance was observed only in models retaining intact epithelial cells (PBS, S. aureus). The unique localization of mCLCA5 to murine airway epithelial progenitor cell niches and the observation that mCLCA5 but not mCLCA3 is present in club cells as putative progenitors for mucus cells suggest that mCLCA5 but not mCLCA3 is the prime CLCA protein involved in mucus cell differentiation from precursor cells in mice.
Of note, normal human and porcine bronchial epithelial cells did not express their respective mCLCA5 orthologs and SMG of both species had fewer expressing cells, indicative of fundamental differences in mice on the one side versus human and pigs on the other.
In addition to their modulation of mucus production, the human hCLCA1 and possibly other CLCA proteins have also been implicated in vitro in the regulation of tissue inflammation in the innate immune response. Consequently, early immune responses were characterized in this study in vivo using a mouse model that lacks expression of mCLCA3, the mouse ortholog to hCLCA1. A S. aureus pneumonia model was employed in mClca3 knockout (mClca3-/-) mice and wild-type (WT) littermates. Experimental readouts included clinical symptoms, bacterial clearance, leukocyte immigration and cytokine responses in the bronchoalveolar compartment, pulmonary vascular permeability and histopathological changes. Furthermore, effects on mucus cell number and mucin gene expression levels as well as possibly compensatory differential regulation of other murine CLCA homologs were determined. Deficiency of mCLCA3 resulted in decreased neutrophilic immune cell infiltration into the bronchoalveolar space after S. aureus infection when compared to WT controls. Only the cytokines IL-17 and the murine CXCL-8 homolog CXCL-1, also termed KC, were decreased on mRNA and protein levels in infected mClca3-/- mice compared to WT controls.
However, no differences were observed in clinical outcome, histopathology or mucus cell metaplasia. No evidence was found for regulation of other CLCA homologs that would putatively compensate for the lack of mCLCA3. In summary, this in vivo study clearly revealed that mCLCA3 plays a significant role in the early innate immune response in a S. aureus pneumonia mouse model via induction of select cytokines with subsequent immune cell recruitment. These data confirm and extend the functional understanding of previous in vitro observations on its human ortholog hCLCA1.
Taken together, the results gained from this study substantially add to our knowledge on the expression patterns, functions in healthy and diseased airways as well as differences in expression between mice, humans and pigs for the two CLCA members most relevant for respiratory diseases. Still, other important challenges remain and several new questions were raised. Future studies should more closely define the putative role of mCLCA5 as modulator of progenitor cells in mucus cell differentiation and mucus cell metaplasia. Furthermore, the mechanisms, target cells and pathways of mCLCA3 modulating the inflammatory innate immune response will have to be established.
Finally, we hope that this research on CLCA molecules and their roles in such devastating respiratory diseases will contribute to the development of more powerful therapeutic approaches in the future.