Molecular characterization of melanocyte stem cells in their niche

Abstract

Emerging evidence from stem cell (SC) research has strengthened the idea that SC fate is determined by a specialized environment, known as the SC niche. However, because of the difficulty of identifying individual stem cells and their surrounding components in situ, the exact mechanisms underlying SC regulation by the niche remain elusive. To overcome this difficulty, we employed melanocyte stem cells (MSCs), which allow the identification of individual SCs in the niche, the lower permanent portion of the hair follicle(HF). Here, we present molecular makers that can distinguish MSCs from other melanocyte (MC) subsets in the HF. We also describe a simple and robust method that allows gene expression profiling in individual SCs. After isolating individual MSCs from transgenic mice in which the MCs are marked by green fluorescence protein (GFP), we performed single-cell transcript analysis to obtain the molecular signature of individual MSCs in the niche. The data suggest the existence of a mechanism that induces the downregulation of various key molecules for MC proliferation or differentiation in MSCs located in the niche. By integrating these data, we propose that the niche is an environment that insulates SCs from various activating stimuli and maintains them in a quiescent state.

The proliferating field of neural crest stem cells

“…The apparent discrepancy of Sox10 Ϫ SKPs (Toma et al, 2001;Fernandes et al, 2004) is, therefore, enigmatic, especially because similar cells expressing Sox10 have also been well-characterized (Wong et al, 2006). MSCs have been characterized as Sox10 Ϫ (Osawa et al, 2005), which would be consistent with the assumption that these cells are unipotent, but again highlights the significance of a direct test of MSC potential and self-renewal. Interestingly, nestin and p75 are both markers of neural stem cells (Lendahl et al, 1990;Andressen et al, 2001).…”

“…The ability to identify MSCs in their niche has allowed some characterization of their expression profile in vivo (Nishimura et al, 2002;Lang et al, 2005;Osawa et al, 2005). Using both immunofluorescent staining of sectioned mouse skin and single-cell expression profiling of microdissected, genetically labeled MSCs, a molecular…”

“…Ϫ , and Mki67 Ϫ ) has been proposed as distinguishing MSCs from melanoblasts or melanocytes, which were generally positive for all these markers (Osawa et al, 2005). It is perhaps surprising that MSCs express the melanocyte marker Dct, while not expressing key activators of Dct expression, Mitf and Sox10 (Ludwig et al, 2004).…”

“…This finding suggests that Dct expression can be induced independently of Mitf and Sox10 by means of another pathway. It has been proposed that MSCs are derived from Sox10 ϩ , Mitf ϩ , Dct ϩ embryonic melanoblasts, and hence, once activated, the Dct chromatin may adopt a config-uration in which expression becomes Sox10-and Mitf-independent (Osawa et al, 2005). Alternatively, Dct expression may be activated by another combination of transcription factors in these cells.…”

“…Currently, most NC-derived stem cells have been examined in a slightly ad hoc manner for marker genes and this obscures any generalities that may exist. Few studies have explicitly attempted to profile these stem cells and only two have done so in an unbiased way (Molofsky et al, 2003;Hu et al, 2006), but in addition, a wide range of candidate markers have also been exam-ined for MSCs (Osawa et al, 2005). Markers identified by these studies are summarized in Figure 2, but reveal few commonalities.…”