Current prevention of cutaneous melanoma is passive. In part, this is because the early biological and molecular events that transform normal melanocytes to melanomas are poorly understood. Based on the cell-of-origin model of tumor initiation, a given melanomagenic stimulus may be more or less transforming depending on the type of melanocyte exposed to it. To develop sophisticated strategies for melanoma prevention, understanding is essential of how melanocytes develop, are maintained, and respond to mitogenic stimuli.
Towards this, we performed scRNAseq on CKIT+ human skin melanocytes purified by flow cytometry. One cell cluster displayed increased expression of NTRK2, which is linked to neural progenitor function and oncogenesis. The gene expression profile of NTRK2-expressing cells had progenitor cell characteristics including increased ribosome biogenesis signatures. Further, splicing isoform analysis and western blot revealed that the isoform of NTRK2 expressed in human melanocytes is the short-truncated form TrkB.T1.
NTRK2+ melanocytes were detected in the epidermis and in the hair follicle in human and mouse skin in immunohistochemistry. NTRK2+ melanocytes were less pigmented and more clonogenic than NTRK2- ones and dominated primary melanocyte in vitro culture. NTRK2+ melanocytes had increased expression of nucleolin (NCL), a regulator of ribosome biogenesis and a marker of proliferative cells. When NTRK2 was suppressed with siNTRK2s in melanocytes, expressions of NCL and Internally Transcribed Spacer 2 (ITS2) of human 47S pre-rRNA were suppressed. Further, survival of siNTRK2-treated melanocytes after UVB-irradiation was inhibited in vitro, indicating potential upstream regulation by NTRK2 of ribosome biogenesis and resistance to UV-stress. Moreover NTRK2+ melanocytes were increased shortly after UVB-irradiation in ex vivo whole skin culture and in vivo after depilation in mice.
These data implicate NTRK2+ melanocytes as progenitors that are activated by mitogenic stimuli.