The Hillock: A Newly Discovered Airway Epithelial Structure and its Relationship with Squamous Metaplasia

Abstract Four years ago, the Rajagopal lab identified the airway hillock, a novel murine airway epithelial structure of unknown function. Hillocks have since been reported in human airways, suggesting that they are evolutionarily conserved. Most of the airway is covered by pseudostratified epithelium, composed primarily of basal cells, secretory club cells, and ciliated cells. The hillock is distinguished from this epithelium in three major ways: (1) the presence of stratified-appearing layers of flat KRT13+ (a cytokeratin) cells which sit atop hillock basal stem cells (2) the lack of ciliated cells, and (3) the enhanced replication of the hillock basal stem cells. We have preliminary data demonstrating that the hillock is resistant to multiple forms of airway injury including naphthalene, a key component of tobacco smoke, and sulfur dioxide gas. We have evidence to suggest that hillocks serve as a source of reserve stem cells. The stratified nature of the hillock resembles the pathologic finding of squamous metaplasia that is also characteristically stratified, and which is thought to be a precursor of squamous lung cancer. Vitamin A deficiency represents a reproducible model of squamous metaplasia in the trachea, uterine cervix, and renal pelvis in mice and humans. In the setting of vitamin A deficiency, trachea develop flat non-ciliated cell clusters that express the classic hillock marker KRT13. I therefore hypothesize that hillocks are the previously unidentified source of squamous metaplasia. I will use genetic lineage tracing of different cell types including the hillock in the setting of vitamin A deficiency to determine the cell of origin of vitamin A deficiency-induced squamous metaplasia. Since retinoic acid (RA) is the active metabolite of vitamin A, I will define the role of RA on hillock expansion and differentiation. To investigate whether hillocks expand in response to RA deficiency, I will express a dominant negative allele of the RA receptor using a hillock specific lineage driver. I will also add RA and RA receptor inhibitor to regeneration assays to examine the effects of RA on hillock-mediated injury repair. Finally, I hypothesize that hillocks are the primary stem cell that re-epithelializes the airway post-injury. Indeed, previous work has shown a variety of cell populations capable of expanding to resurface the injured airway. Therefore, it is unclear if hillocks are required for efficient epithelial repair and if they are the primary reserve stem cell of the epithelium. To investigate this hypothesis, I will genetically ablate hillocks prior to naphthalene injury, a chemical found in cigarette smoke, to determine if hillocks are necessary for proper regeneration.