MRM Insights: There and back again – the fantastic journey of melanocyte stem cells
Every month, in the MRM Insights, a member of the MRM Network writes about stem cells and regenerative medicine from a different perspective. This month, Dr. Katie Cockburn, Assistant Professor in the Department of Biochemistry at the Rosalind and Morris Goodman Cancer Institute, discusses the fantastic journey of melanocyte stem cells.
There and back again: the fantastic journey of melanocyte stem cells
The remarkable cellular turnover in the human body is sustained by populations of tissue-resident stem cells. Although they are highly distinct in terms of their sites of residence, their behavioral kinetics and their gene expression profiles, these populations are often thought to operate under a unifying set of principles centered around a hierarchical relationship between stem cells and their more differentiated daughters (1). Recent work from Sun et al. is helping to turn this dogma on its head, at least in the case of melanocyte stem cells (2).
The traditional view of tissue-resident stem cells, originally established through the study of the hematopoietic system, proposes that this population is unique in its ability to both self-renew and give rise to differentiated progeny (3). The latter of these two functions, differentiation, is typically thought to occur in a unidirectional manner, with stem cells capable of generating differentiated cells but not vice versa. In the last decade however, data from multiple tissues has begun to reveal that this is not the whole story. In the small intestine, stomach, and trachea, damaged stem cells can be replaced by nearby differentiating cells that reverse course and revert into bona fide stem cells (4–6). Importantly however, these examples of de-differentiation have primarily been associated with injury and tissue repair. In homeostatic tissue, the transition from differentiated cell “backwards” toward a stem cell has not been considered a normal part of the regenerative program.
Enter the melanocyte stem cell (McSC). In mice, McSCs reside in hair follicles, where they give rise to the mature melanocytes that give hair its color. Traditional lineage tracing approaches have supported a model in which McSCs in the upper part of the hair follicle (a region known as the bulge) give rise to melanocytes that migrate downwards towards the region of hair production (called the hair bulb), where they secrete pigment to color the nascent hair shaft (7, 8). However, this model comes from traditional static approaches, where snapshots of individual hair follicles at different points in time are used to infer lineage relationships and cell movements that are occurring over space and time. Although these approaches have led to incredible insights into tissue regeneration across a huge range of systems, there are necessarily some aspects of stem cell behavior that may be missed.
To interrogate this process from another angle, Sun et al. genetically labelled single McSCs and used longitudinal intravital imaging to follow the same hair follicles and the same labelled cells over periods of time up to two years (2). Through this incredible feat, they discovered an unexpected series of events: it is in fact the McSCs themselves that move downwards and away from the bulge, and as they do so, they start to differentiate. Sun et al. confirmed this differentiation both at the level of cell morphology, which becomes highly dendritic, but also at the transcriptional level through single-cell RNA sequencing and functionally by demonstrating that these cells can secrete pigment. Strikingly however, these McSCs then de-differentiate, losing their dendritic morphology and turning off genes associated with pigmentation as they migrate back up towards the bulge region of the follicle. They do this not once but again and again, shuttling between stem cell and differentiated states continuously for two years or perhaps even longer. The demonstration that these cells repeatedly differentiate and then de-differentiate in homeostatic tissue, and that this cyclical process is essential for their physiological function, is an entirely new paradigm for how stem cells can behave.
How do these migrating McSCs couple their changing position in the follicle to their differentiation status? Sun et al. demonstrate that this occurs through differences in Wnt signaling in the upper vs lower regions of the follicle. Wnt signaling has long been known to drive melanocyte differentiation, and Wnt ligands are specifically secreted by epithelial cells in the lower part of the follicle during hair growth (7, 9). By constitutively activating Wnt in McSCs, Sun et al. show that these cells can still migrate upwards but that they fail to differentiate and maintain production of pigment. Through further elegant genetics, Sun et al. demonstrate that it is indeed epithelial cells in the follicle that secrete the key Wnt ligands that cause McSCs to initiate differentiation specifically while they remain in the lower part of the follicle.
Finally, Sun et al. show that the cycling behavior of McSCs has implications for hair color during aging. They find that over time, the ability of McSCs to home back to the correct location in the upper follicle can go awry, and they increasingly begin to end up in a slightly higher location than they should be. In this new location, they are much more likely to remain quiescent and don’t produce the pigment-generating melanocytes that are needed to maintain hair color. The more times they undergo a cycle of de-differentiation and migration, the more likely these cells are to end up in the wrong place and lose the ability to generate functional melanocytes. Thus, the unique life history of McSCs may be one of the reasons that hair greying is one of the first signs of aging (2).
What can the story of McSCs and their remarkable journey tell us? First, on a practical level, this unprecedented knowledge of how McSC function goes awry has important implications for aging. If we can figure out how to get McSCs to more accurately home back to the right location in the hair follicle as they migrate upwards, we may be able to come up with new treatments to prevent hair greying. But on a more philosophical level, the findings of Sun et al. provide further support for the emerging view that there is really no one way to be a stem cell (1, 10). In this case, the dogma that cells can only become more differentiated over time clearly does not apply. Instead, the unique structure, organization and kinetics of each tissue likely dictate how resident cells renew themselves over time.
And finally, this author’s favorite take-home message from the work of Sun et al: you never know for sure until you look! Even when there are well-established models for how a particular biological process occurs, we can almost always learn something new by putting our preconceived notions to the side and simply watching what happens. The continually expanding use of intravital imaging as a tool to follow stem cells over time in living tissues (11, 12) is sure to reveal more unexpected journeys in years to come.
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12. S. Park, V. Greco, K. Cockburn, Current opinion in cell biology. 43, 30–37 (2016).