Date Published: June 13, 2019
Publisher: Public Library of Science
Author(s): Ricardo F. Frausto, Doug D. Chung, Payton M. Boere, Vinay S. Swamy, Huong N. V. Duong, Liyo Kao, Rustam Azimov, Wenlin Zhang, Liam Carrigan, Davey Wong, Marco Morselli, Marina Zakharevich, E. Maryam Hanser, Austin C. Kassels, Ira Kurtz, Matteo Pellegrini, Anthony J. Aldave, Jung Weon Lee.
The zinc finger e-box binding homeobox 1 (ZEB1) transcription factor is a master regulator of the epithelial to mesenchymal transition (EMT), and of the reverse mesenchymal to epithelial transition (MET) processes. ZEB1 plays an integral role in mediating cell state transitions during cell lineage specification, wound healing and disease. EMT/MET are characterized by distinct changes in molecular and cellular phenotype that are generally context-independent. Posterior polymorphous corneal dystrophy (PPCD), associated with ZEB1 insufficiency, provides a new biological context in which to understand and evaluate the classic EMT/MET paradigm. PPCD is characterized by a cadherin-switch and transition to an epithelial-like transcriptomic and cellular phenotype, which we study in a cell-based model of PPCD generated using CRISPR-Cas9-mediated ZEB1 knockout in corneal endothelial cells (CEnCs). Transcriptomic and functional studies support the hypothesis that CEnC undergo a MET-like transition in PPCD, termed endothelial to epithelial transition (EnET), and lead to the conclusion that EnET may be considered a corollary to the classic EMT/MET paradigm.
The zinc finger e-box binding homeobox 1 (ZEB1) gene encodes a transcription factor involved in epithelial and endothelial cell plasticity critical in development, wound healing and cancer . ZEB1 is a master regulator of cell state transitions (CSTs), namely epithelial to mesenchymal (EMT) or the reverse process, mesenchymal to epithelial (MET). EMT is characterized by distinct molecular and morphologic changes in which epithelial cells lose an epithelial-associated gene expression profile, apicobasal polarity and intercellular adhesions, and gain a mesenchymal-associated gene expression profile and increased migratory capacity. Conversely, the reverse of the EMT process effectively characterizes MET. EMT and MET are tightly regulated CST processes involving the regulation of many genes in a cell-type-independent manner, and for which stable transition states have been identified [2–8]. For example, the cadherin-switch, a well-described feature of EMT, involves the repression of cadherin 1 (CDH1; E-cadherin) and activation of cadherin 2 (CDH2; N-cadherin) gene expression, with the reverse being observed in MET. In addition, an inverse correlation is observed between the mesenchymal-associated transcription factor ZEB1 and two epithelial-associated transcription factors, ovo-like 2 (OVOL2) and grainy head-like transcription factor 2 (GRHL2), known to directly repress ZEB1 transcription [6, 9–13].
Contact inhibited and quiescent  corneal endothelial cells, when dissociated from the cornea and grown in culture, have demonstrated re-initiation of the cell cycle and transition towards a fibroblast-like (i.e., mesenchymal) phenotype [30, 31], which is associated with an increase in ZEB1 expression . This process is termed endothelial to mesenchymal transition (EnMT) and can be induced by various growth factors and cytokines . As such, there is overwhelming evidence that the terminally differentiated and quiescent corneal endothelial cells retain the potential to undergo a CST towards a fibroblast-like phenotype. Similarly, vascular endothelium has also been observed to undergo EnMT , and this is in addition to an EMT-like (epithelial to endothelial) transition that may form the basis for vascular mimicry in cancer . Besides the global effects elicited by transcription factors (e.g. ZEB1 or OVOL2), signaling molecules, such as the mesenchymal epithelial transition (MET) receptor tyrosine kinase, may play key roles in regulating corneal endothelial cell state identity [35, 36]. Notably, increased MET expression was previously observed in PPCD endothelium . Taken together, these findings raise the possibility that endothelial cells possess the potential to transition to an epithelial-like state, (i.e., endothelial to epithelial transition, EnET), and the study of EnET may provide new insights into other cell state transitions important in development, wound healing, stem cells and cancer cells [8, 37].