Interview part 1- Mechanical instability of cell junctions regulates fate of hair follicle stem cells

Ritusree Biswas’s interview with Bio Patrika hosting “Vigyaan Patrika”, a series of author interviews. Ritusree is a PhD student in the lab of Dr. Srikala Raghavan at inStem, Bengaluru. She completed her Master’s degree in Applied Microbiology from Vellore Institute of Technology, Vellore, during which she undertook master’s project training for 6 months in Dr. Yashoda Ghanekar’s lab at inStem, and was inspired by the work culture there. Ritusree joined Dr. Srikala Raghavan’s lab as a Junior research fellow in 2015. Her research focuses on the mechanistic role of factors that regulates mammalian(mouse) skin stem cells. Ritusree seeks to explore other stem cell models in the field of mechanotransduction. Apart from research, she is a trained Kathak dancer and is passionate about dancing, artwork, gardening and loves playing badminton. Here, Ritusree talks about her first co-author paper titled “Mechanical instability of adherens junctions overrides intrinsic quiescence of hair follicle stem cells” published in Dev Cell.

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How would you explain your paper’s key results to the non-scientific community?

Our skin constantly turnover every three weeks to replenish the outermost layer. This is very important for maintaining stability and development in this organ. The specialized cells that contribute to this process are known as the stem cells that reside in particular areas called the niche. One such niche is the bulge compartment of the hair follicle, which contains multipotent stem cells. Though there are several studies about intrinsic factors regulating the fate of these stem cells much less is known about extrinsic factors that controls the niche of these cells. In our recent publication in Developmental Cell, we identify the role of cell-cell junctions within the bulge stem cells, in maintaining their fate.

We first started by deleting a gene called vinculin, from the epidermis (the outer layer of skin) and hair follicles of mice. The protein vinculin has been previously shown to be very critical in development. It is present at both the cell-cell junction as well as cell-substratum junction, and is activated upon force generation. Vinculin depleted mice (vinculin KO) displayed sparse hair, otherwise being perfectly normal. This intriguing observation then led us to look at the stem cell compartment as they contribute to regeneration of hair follicles during normal development. The analysis of the hair follicle stem cells revealed these animals failed to maintain the quiescent state of the stem cells. The regulation of this quiescent or resting state is very important for the stem cells to maintain tissue lifelong.

Model. Image credits Avinanda. WT bulge stem cells: Normal cell junctions, Quiescent, Cytoplasmic YAP. KO bulge stem cells: Abnormal cell junctions, Non-quiescent, Increased nuclear localization of YAP (green) providing mechanism for increased proliferation, Abbreviations: AJ-Adherens Junction, D-Desmosomes, IF-Intermediate filaments, HD-Hemidesmosomes, FA- Focal adhesion, N-Nucleus, ECM-Extra cellular matrix.

Next, we focused on the factors that led to the loss of quiescence and were able to pin down to the cell-cell junctions’ role in regulating quiescence. Measurement of the forces generated at the junctions of these vinculin KO animals were almost half compared to its corresponding control. This data suggested, in absence of vinculin, the junctions in between the cells were weak, thereby, leading to loss of quiescence.

To further investigate the mechanism behind the loss of quiescence, a global gene expression profiling was performed. One of the transcription factors, YAP1 and its targets were highly upregulated. YAP1 is a potent regulator of cell proliferation and is sequestered at strong cell-cell junctions in resting stem cells. In vinculin, KO animals YAP1 was translocated to nucleus and resulted in increased cell proliferation causing loss of quiescence in the stem cells.

this study provides novel insights into the role of junction proteins in regulating stem cell quiescence.

What are the possible consequences of these findings for your research area?

From determining the shape of multicellular organisms to normal development of adult tissues require a fine tuning of the cell-cell junction responses and transmission of the applied mechanical load at these junctions to its stem cell niche. Elucidating the molecular mechanisms by which stem cells use mechanical signals to communicate with their environment, might pave the way for understanding how perturbations to these interactions can promote various disease states that arise from the dysregulation of stem cells.

In this study, we show an important link between loss of mechanically stable junctions and the failure to maintain stem cell properties, in absence of vinculin from mouse skin stem cells. Given the importance of stem cell niche in maintaining normal tissue development, this study provides novel insights into the role of junction proteins in regulating stem cell quiescence. 

What was the exciting moment (eureka moment) during your research?

Initially after focusing our attention to cell-cell junction’s role in regulating quiescence of stem cells, we measured the forces generated by the vinculin KO cells at the cell-cell junction. From the force measurement experiment, performed by my colleague Avinanda in collaboration with Prof. Yan Jie’s group, at the Mechanobiology Institute (MBI), Singapore, we discovered the forces generated by the cells at the junction was only half the amount generated by the control cells. This was very exciting and surprising at the same time. Since, we had already seen before, that the junction protein levels were much higher in the vinculin KO cells. This experiment finally revealed as to why higher levels of junction protein were unable to keep the stem cells in quiescent state.

What do you hope to do next?

In the vinculin KO system, I would like to investigate the immune response in the skin due to loss of vinculin, as we have some interesting preliminary data in this context. In addition, I would like to extrapolate the force transduction role of vinculin in a more mechanically loaded system like muscle stem cells.

Where do you seek scientific inspiration?

My scientific role model is my mentor Dr. Srikala Raghavan. I have been extremely fortunate to be under her guidance. The way she extends her support in terms of scientific discussions to constant motivation is truly inspiring. She has helped me develop my scientific thinking and communication, as well as, experimental skills. Being a part of an esteemed institution and such a brilliant scientific community, I am also constantly learning from my peers as well. One thing that my PhD journey has taught me is to embrace the failures and keep on improving. This is what I have learnt from my colleagues and friends here and I think that has shaped my intellect to a greater extent.

How do you intend to help Indian science improve?

One thing that I have learnt during my graduate training is that communicating your science to both scientific and non-scientific community is very important. Talking about your research not only helps to get quality inputs but also motivates young scientist’s mind to go forward with their work. It also influences school and college students, the future of our nation, to develop interest in scientific research. Therefore, in addition to going forward in my career with translational research I would like to contribute to science communication to bridge the gap between scientists and general population.

Reference

Biswas R, Banerjee A, Lembo S, Zhao Z, Lakshmanan V, Lim R, Le S, Nakasaki M, Kutyavin V, Wright G, Palakodeti D, Ross RS, Jamora C, Vasioukhin V, Jie Y, Raghavan S. Mechanical instability of adherens junctions overrides intrinsic quiescence of hair follicle stem cells. Dev Cell. 2021;56(6):761-780.e7. doi: 10.1016/j.devcel.2021.02.020.

Dr. Srikala Raghavan lab: https://www.instem.res.in/faculty/srikala

Edited by: Nivedita Kamath

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