Dynamics of Lgr6+ Progenitor Cells in the Hair Follicle Sebaceous Gland and Interfollicular Epidermis

摘要

class="head no_bottom_margin" id="sec1title">IntroductionSkin contains several epithelial structures that undergo constant self-renewal, including the hair follicle (HF), sebaceous gland (SG), and interfollicular epidermis (IFE), making it an ideal organ to study the lineage potential of stem cells in a compartmentalized tissue (, ). Hair is generated in recurring cycles of growth (anagen), regression (catagen), and resting (telogen) phases of the lower HF part (). In contrast, the IFE and SG continuously produce cells that differentiate into stratified epithelium or mature sebocytes, respectively (). The murine pilosebaceous unit (PSU), encompassing the HF and SG, has a well-defined anatomy that consists of several subcompartments. The bulge, containing mainly quiescent progenitor cells, and the hair germ, which is in direct contact with the dermal papilla, contribute to the hair lineages during anagen (, , ). The permanent part of the HF that does not participate in hair regeneration comprises the isthmus, spanning from the bulge to the infundibulum, the junctional zone, which is the upper region of the isthmus adjacent to the SG, and the infundibulum, linking the PSU and the IFE ().Genetic fate-mapping studies using lineage tracing in vivo demonstrated that molecularly defined basal cells that possess stem cell properties reside in most of these regions (, ). Apart from self-renewal, their contribution to other structures seems to remain locally restricted, although contribution of HF cells to SG maintenance during homeostasis has been proposed (, ). Because of the large variety of genetic markers used to study the individual stem cell populations, direct comparisons of the results have been difficult. Thus, the interrelation between different stem cell pools within the PSU remains to be elucidated.The IFE is less well characterized in terms of its cellular heterogeneity and molecular markers. However, the mode of stem cell renewal in the IFE has been a subject of intense research (). Based on genetic lineage tracing studies, it has been proposed that the maintenance of IFE tissue relies on the turnover of a single population of progenitors, which follows a pattern of balanced stochastic fate (, ). In this paradigm, the division of basal progenitors results in stochastic fate outcome, with cells choosing stochastically between symmetrical duplication, asymmetrical division, and terminal division. However, to achieve homeostasis, the chance of terminal cell division is balanced with the frequency of symmetric duplication so that, on average, cell division results in asymmetric fate outcome. A similar mode of stochastic self-renewal, termed population asymmetry (href="#bib45" rid="bib45" class=" bibr popnode">Watt and Hogan, 2000), was also observed in other tissues, such as the male germline (href="#bib34" rid="bib34" class=" bibr popnode">Nakagawa et al., 2007), the small intestine (href="#bib31" rid="bib31" class=" bibr popnode">Lopez-Garcia et al., 2010, href="#bib43" rid="bib43" class=" bibr popnode">Snippert et al., 2010b), the esophagus (href="#bib12" rid="bib12" class=" bibr popnode">Doupé et al., 2012), and the stomach (href="#bib25" rid="bib25" class=" bibr popnode">Leushacke et al., 2013). Moreover, a study based on separate targeted promoters provided evidence of proliferative heterogeneity in IFE, with the committed progenitor cell pool underpinned by a second quiescent or slow-cycling stem cell population, which becomes mobilized on injury (href="#bib32" rid="bib32" class=" bibr popnode">Mascré et al., 2012). However, no studies have been performed to determine whether compartments within the PSU are maintained by population asymmetry or whether stem cell self-renewal follows a process of invariant asymmetry in which each and every stem cell division results in an asymmetric fate outcome.Expression of Lgr6 (leucine-rich repeat-containing G protein-coupled receptor 6) was initially reported to be a specific marker of stem cells located in the HF isthmus that were thought to give rise to cells of the HF, SG, and IFE lineage (href="#bib42" rid="bib42" class=" bibr popnode">Snippert et al., 2010a). More recent reports have uncovered that Lgr6 is additionally present in basal cells of the IFE and the SG (href="#bib28" rid="bib28" class=" bibr popnode">Liao and Nguyen, 2014, href="#bib36" rid="bib36" class=" bibr popnode">Page et al., 2013), thus questioning the origin of the Lgr6⁺-derived clones in the first study.The aim of this study was to define the stem cell properties of the Lgr6⁺ cell populations in the HF, SG, and IFE and understand their interrelation. Tracking the fate of Lgr6⁺ cells using multicolor lineage tracing, the current study revealed that Lgr6⁺ cells in the IFE are able to maintain IFE tissue without contribution from Lgr6⁺ PSU cells. In the isthmus and SG, local Lgr6⁺ cell populations exhibited the potential for long-term maintenance of their respective compartment. Furthermore, quantitative analysis of clonal dynamics revealed that local Lgr6⁺ progenitors renew all three compartments through the process of population asymmetry. Finally, genome-wide mRNA profiling uncovered that the transcriptome of Lgr6⁺ keratinocytes is mainly determined by the cellular location, rather than by a gene signature specific for Lgr6⁺ cells.