Human alveolar Type 2 epithelium transdifferentiates into metaplastic KRT5+ basal cells during alveolar repair

By using different co-culturing cells in vitro, the authors have found that AT2 cells are able to transdifferentiate into basal cells, the cells that can give rise to any cell type in the lung. They show that by transplanting AT2 cells to injured mice lungs, the same transdifferentiation occurs.

 

Background

The lung is a complex organ in charge of the gas exchange between the air and our blood. It has two differentiated structures: airways and alveoli. These two structures have specific functions that make the respiratory system very efficient in gas exchange, while protecting the lung epithelia from microorganisms and particles that are inhaled on a daily basis. Alveolar type 2 (AT2) cells are the epithelial cells in the lung responsible for surfactant production. They can self-renew and give rise to alveolar type 1 (AT1) cells, the epithelial cells that are in charge of gas exchange in the lung. Upon injury, it is well described how AT2 cells give rise to AT1 cells to regenerate the alveoli and recover the lung function (Barkauskas et al., 2013). The same way AT2 cells maintain the homeostasis of the alveoli, basal cells are the cells that maintain the airway of the lung in homeostasis and injury. Basal cells can differentiate into any cell type in the airway (ciliated, club, goblet cells…) (Rock et al., 2010). At the same time, club cells and BASC, differentiated from basal cells, can give rise to AT2 (Kim et al., 2005; Zheng et al., 2017). So far, this is the way the field knew the AT2 cell differentiation path worked.

 

Key findings

The authors show that:

Human AT2 cells can transdifferentiate into basal cells in vitro: hAT2 cells are usually cultured with lung embryonic fibroblast (MrC5), that allow for hAT2 organoid growth. In this work, the authors co-cultured the hAT2 with adult human lung mesenchyme (AHLM) cells. This change in the feeder cells generates a loss of AT2 markers in the organoids, that at the same time gain KRT5 and KRT14, markers of basal cells.

Niche factors are crucial for AT2 to basal cell transdifferentiation: Transcriptome analysis of hAT2 organoids cultured both in MrC5 and AHML showed that BMP activation is downregulated in hAT2 co-cultured with AHML. Using BMP4 on the hAT2 culture showed decreased KRT5 expression. At the same time, HHIP, a decoy receptor of hedgehog (Hh) that sequesters Hh ligands is highly expressed in mature hAT2. HHIP treatment of the hAT2 organoids also decreased the expression of KRT5, in line with the BMP4 treatment.

Human AT2 engrafted into fibrotic mice also transdifferentiate into basal cells: The authors transplanted hAT2 cells into immune-defficient (NSG) mice with fibrotic lungs. Engraftments were analyzed 20 days after transplantation and patches of human cells were observed within the mouse lungs. A specific antibody against human cells (HNA) was used to distinguish human from mouse cells. Those patches of human cells were either KRT5+ (basal) or SPC+ (AT2), and markers for alveolar-basal intermediate (ABI) within the SPC+ population. These results demonstrate that this transdifferentiation can also occur in vivo in a model of lung fibrosis.

 

What I like about this preprint

The description of AT2 cells giving rise to basal cells is something that changes the downstream paradigm of lung cell differentiation from basal cells downwards. The authors not only describe this transdifferentiation in vitro, but also show that it happens upon lung transplantation in fibrotic mice. It seems that this type of transdifferentiation could be a good model to study idiopathic pulmonary fibrosis (IPF), which is a deadly disease for which we still don’t have a good in vitro model. In this preprint, a new intermediate population is revealed, the ABI cells that are the ones that give rise to metaplastic basal cells. Altogether, this work provides an interesting tool to study IPF and uncovers a newfound plasticity of the AT2 cells that was previously unknown.

 

Questions for the authors

The first question I had when I first read this preprint was how functional were these transdifferentiated basal cells. Therefore, my questions to the authors are regarding this subject.

Would you say that this transdifferentiation occurs always until the basal cell is finally generated? Or could the intermediates give rise to other cells such as club cells or ciliated cells?

Do the transdifferentiated basal cells occupy the upper airway in the transplanted mice or any lung site preferentially?

 

 

 

Bibliography

Barkauskas, C. E., Cronce, M. J., Rackley, C. R., Bowie, E. J., Keene, D. R., Stripp, B. R., Randell, S. H., Noble, P. W., & Hogan, B. L. M. (2013). Type 2 alveolar cells are stem cells in adult lung. Journal of Clinical Investigation, 123(7), 3025–3036. https://doi.org/10.1172/JCI68782

Kim, C. F. B., Jackson, E. L., Woolfenden, A. E., Lawrence, S., Babar, I., Vogel, S., Crowley, D., Bronson, R. T., & Jacks, T. (2005). Identification of Bronchioalveolar Stem Cells in Normal Lung and Lung Cancer. Cell, 121(6), 823–835. https://doi.org/10.1016/j.cell.2005.03.032

Rock, J. R., Randell, S. H., & Hogan, B. L. M. (2010). Airway basal stem cells: A perspective on their roles in epithelial homeostasis and remodeling. Disease Models & Mechanisms, 3(9–10), 545–556. https://doi.org/10.1242/dmm.006031

Zheng, D., Soh, B.-S., Yin, L., Hu, G., Chen, Q., Choi, H., Han, J., Chow, V. T. K., & Chen, J. (2017). Differentiation of Club Cells to Alveolar Epithelial Cells In Vitro. Scientific Reports, 7. https://doi.org/10.1038/srep41661

Tags: at2, basal cell, fibrosis, ipf, lung, lung repair, stem cell, transdifferentiation

doi: https://doi.org/10.1242/prelights.23417

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