Cross-species transcriptome profiling identifies new alveolar epithelial type I cell-specific genes

Diseases involving the distal lung alveolar epithelium include chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF) and lung adenocarcinoma. Accurate labeling of specific cell types is critical for determining the contribution of each to pathogenesis of these diseases. The distal lung alveolar epithelium is comprised of two cell types, alveolar epithelial type 1 (AT1) and type 2 (AT2) cells. While cell type-specific markers, most prominently surfactant protein C (SFTPC), have allowed detailed lineage tracing studies of AT2 cell differentiation and their roles in disease, studies of AT1 cells have been hampered by lack of genes with expression unique to AT1 cells. In this study, we performed genome-wide expression profiling of multiple rat organs alongside purified rat AT2, AT1 and in vitro differentiated AT1-like cells, resulting in identification of 54 candidate AT1 cell markers. Cross-referencing with genes upregulated in human in vitro differentiated AT1-like cells narrowed the potential list to 18 candidate genes. Testing the top four candidate genes at RNA and protein levels revealed GRAM domain 2 (GRAMD2), a protein of unknown function, as highly specific to AT1 cells. RNAseq confirmed that GRAMD2 is transcriptionally silent in human AT2 cells. Immunofluorescence verified that GRAMD2 expression is restricted to the plasma membrane of AT1 cells and is not expressed in bronchial epithelial cells, while RT-PCR confirmed that it is not expressed in endothelial cells. Utilizing GRAMD2 as a new AT1 cell-specific gene will enhance AT1 cell isolation, investigation of alveolar epithelial cell differentiation potential, and contribution of AT1 cells to distal lung diseases. Two preps of purified AT1 cells (92%, and 93% pure, respectively) were isolated from Sprague-Dawley male rats and RNA was extracted using Trizol. In addition, RNA was extracted from bronchoalveolar lavage cells and 14 rat tissues: kidney, spleen, ileum, duodenum, colon, stomach, skin, brain, testis, skeletal muscle, trachea, heart, salivary gland, and liver.