Culturing Alveolar Type I, Alveolar Type II, and Mixed Cell Populations by Altering the Extracellular Matrix Promotes Changes in Intercellular Signaling Mechanisms

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The alveolar epithelium consists of two cell types: alveolar type I (AT-I) and alveolar type II (AT-II) cells. Isolated AT-II cells cultured for 7 days on a rat tail collagen/fibronectin matrix develop phenotypic characteristics of AT-I cells (morphologically flat without lamellar bodies, positive staining for aquaporin-5, and negative staining for surfactant protein-C). Isolated AT-II cells cultured for 7 days on rat tail collagen matrix supplemented with laminin-5 (with or without fibronectin) maintain phenotypic characteristics of AT-II cells (morphologically cuboidal with lamellar bodies, aquaporin-5 negative, and surfactant protein-C positive). Previously, cultured rat AT-II cells have been shown to respond to secretogogues and stretch with increases in intracellular [Ca2+], and have been shown to be dye coupled; most recently, AT-I cells have been shown to communicate intracellular [Ca2+] changes to AT-II cells in situ. However, the mechanism of communication among and between AT-I and AT-II cells in vivo remains ill-defined. Both cell types can communicate intercellular Ca2+ changes after mechanical stimulation. However, AT-I–like cells coordinate Ca2+ changes via gap junctions, and AT-II–like cells coordinate Ca2+ changes via extracellular nucleotide triphosphate release. AT-I–like and AT-II–like cultured cells are additionally distinct in their Lucifer Yellow dye coupling and gap junctional protein (connexin) profile, indicative of further distinct intercellular signaling mechanisms. The parallel changes in Ca2+ wave propagation with cell differentiation suggests cell signaling mechanisms are an intrinsic component of alveolar cell phenotype. We have now modified the extracellular matrix to allow for co-cultures of AT-I and AT-II cells to better model communication in vivo.