Melanocyte stem cell subpopulations show distinct pigmentation and regenerative potential

Abstract

Melanocyte stem cells (McSCs) are key components of the hair follicle (HF) stem cell system. They are derived from neural crest during embryogenesis and are responsible for regeneration of differentiated melanocytes during successive HF cycles. We have described McSC subsets that can be distinguished by CD34 expression. CD34+ McSCs are located within the bulge/lower permanent portion (LPP) while CD34- McSCs are in the secondary hair germ (SHG) during the resting stage (telogen). Whether these two cell subpopulations are maintained separately or exist in a developmental hierarchy is not yet known. The goal of my thesis is to explore whether (a) the two McSC subpopulations are functionally distinct, (b) if they are maintained independently throughout the HF cycle and (c) their role in generation of mature melanocytes. To study McSCs, we engineered the Dct-H2BGFP bitransgenic mouse. We confirm that this animal model accurately identifies melanoblasts, McSCs and mature melanocytes by constitutive GFP expression that can be regulated by doxycycline. Using our Dct-H2BGFP mouse, we compared the transcriptomes of bulge and SHG McSC subsets by genome-wide expression profiling (RNA-seq). This study, along with functional in vitro and in vivo assays, confirms that CD34+/bulge share characteristics of neural crest stem cells with multilineage potential while CD34-/SHG McSCs represent a stem cell population that is more committed to melanocyte differentiation. To further understand the relationship between two McSC subpopulations, we traced their proliferation throughout the HF cycle and found that proliferation of SHG McSCs gives rise to mature melanocytes. The analysis also surprisingly revealed quiescent CD34- melanocytes maintained outside of the HF bulge region throughout anagen retaining the stem cell phenotype, identifying a SHG McSC-like population outside telogen suggesting independent or quasi-independent maintenance of the two McSC subpopulations. Taken together, our study identifies heterogeneous McSC subpopulations with distinct pigmentation and regenerative potential for the first time. Strikingly, CD34+/bulge McSCs exhibited the ability to myelinate neurons in vivo, revealing a novel therapeutic possibility for demyelinating disorders and traumatic nerve injury.