My lab is interested in the normal developmental processes that build the lung and how such processes go awry during lung malformations, injury and tumorigenesis. What is unique about my lab is our effort to develop a series of three-dimensional labeling and imaging methods such as optical projection tomography. We develop these novel methods to address a major challenge in studying the lung – its complex three-dimensional architecture including the tree-like airways and honeycomb-like alveoli, making it difficult to compare structures on conventional two-dimensional sections.
Development of the alveolar type 1 cell and bronchopulmonary dysplasia
Bronchopulmonary dysplasia (BPD) is a major chronic lung disease associated with preterm birth and characterized by alveolar simplification with dysmorphic microvasculature. Current BPD research focuses on myofibroblasts, alveolar type 2 (AT2) cells and the endothelium, but seems to leave out the “elephant in the room”, the alveolar type 1 (AT1) cell, which constitutes nearly all the alveolar surface and associates intimately with the microvasculature. We have delineated a two-step AT1 cell morphogenesis process, cell flattening and cell folding, which leads to its ultrathin and yet expansive morphology. We have identified a key transcriptional regulator of this process, without which AT1 cells lose their molecular and cellular characteristics and the lung undergoes alveolar simplification as in BPD. Thus, our findings have implicated AT1 cell development and its regulator in the pathogenesis of BPD. We are dissecting the direct and indirect targets of this regulator. We are also interested in its role in AT1 cell homeostasis and injury in the adult lung.
Novel signaling roles of the alveolar type 1 cell
AT1 cells are traditionally considered a passive structural component of the alveoli while attention has been focused on AT2 cells due to their proposed stem cell function. We have found that AT1 cells, instead of AT2 cells, are the unexpected source of a key angiogenic factor VEGFA and AT1 cell-derived VEGFA is required for alveolar angiogenesis. These findings open the door to a mechanistic understanding of alveolar angiogenesis, which is proposed to occur through intussusceptive angiogenesis, but is poorly understood on the molecular and cellular level. We are studying how lung endothelial cells respond to Vegfa during intussusceptive angiogenesis and whether and how the same Vegfa signaling elicits distinct responses from those during the well-characterized retinal sprouting angiogenesis. We are also interested in the role of AT1 cell-derived VEGFA during homeostasis and tumorigenesis.
Given our demonstration of the unexpected angiogenic role of AT1 cells and their extensive surface area, we hypothesize that AT1 cells have other novel signaling roles. Through a series of transcriptome analyses, we have identified a Wnt ligand specifically expressed in AT1 cells but not AT2 cells, an expression pattern similar to that of Vegfa. We have generated conditional knockout alleles to test whether AT1 cells may also signal to mesenchymal cells such as myofibroblasts.
Maintenance of airway cell mosaicism during growth and regeneration
The lung filters the inhaled air via mucociliary clearance, the failure of which encourages microbial growth and inflammation as in asthma and COPD. The mucociliary clearance function is split between and hence requires coordination of two airway cell types, secretory and ciliated cells, where secretory cells secret mucus to trap inhaled microbes and particles while ciliated cells move the mucus layer rostrally via a conveyor belt-like action. Such functional coordination depends on secretory and ciliated cells intermingled at the proper proportion and pattern, which we call airway cell mosaicism. This airway cell mosaicism is established during embryonic development under the control of Notch signaling, but must be maintained during subsequent airway growth and reestablished during regeneration. Combining clonal analyses, whole-mount imaging and computational modeling, we have found evidence for a robust self-correcting mechanism of maintaining the airway cell mosaicism. We are developing a novel somatic CRISPR method to dissect the underlying molecular mechanisms. We are also interested in extending our analyses to the more complex trachea epithelium with a third airway cell type, basal cells.
A hierarchical gene regulatory network controlling SOX9 epithelial progenitors
We have shown that the entire lung epithelium arises from a group of SOX9 epithelial progenitors. The SOX9 progenitors emerge as the lung primordium just buds off the embryonic foregut, constitute all the tips of the rapidly-branching respiratory tree, and disappear soon after birth as the alveolar region matures. Lineage tracing shows that they differentiate into SOX2 airway cells during early development and alveolar cells during late development. Thus, the maintenance and differentiation of the SOX9 progenitors must be precisely controlled in coordination with branching morphogenesis and developmental timing. Although a number of signaling pathways have been identified to affect the SOX9 progenitors, it is unclear how different signaling pathways interact and what molecular changes they elicit in the SOX9 progenitors. We hypothesize a hierarchical gene regulatory network shapes the epigenetic landscape of the SOX9 progenitors to control their maintenance and differentiation. Using genetic epistasis analysis and genomic bioinformatics, we have assembled an initial gene regulatory network and are developing methods to assay the epigenome of purified SOX9 progenitors. We believe that perturbation or modulation of the gene network in the SOX9 progenitors underlies abnormal/sub-optimal lung development or species-specific lung branching complexity, respectively.