Chemical and genetic control of melanocyte development, proliferation and regeneration in zebrafish
Marie, Kerrie Leanne
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Melanocytes are pigment-producing cells that colour our hair, skin and eyes. Melanocytes are evolutionary conserved in vertebrates, and in addition to contributing to pigmentation and pattern formation, can contribute to background adaptation (zebrafish) and protection against harmful UV irradiation (humans). Many of the processes involved in melanocyte development – such as migration, proliferation and differentiation - are misregulated in melanoma. Here, I use chemical biology in zebrafish to identify targetable pathways in melanocyte development and regeneration, with a view to how these processes may be misregulated in melanoma and other pigmentation syndromes. We first wanted to address the potential for small molecules to regulate multiple stages of melanocyte development and differentiation. In Chapter 3, I describe my work involved in a small molecule screen for clinically active compounds that alter melanocyte biology (Colanesi et al., 2012). In this work we have identified small-molecules that affect melanocyte migration, differentiation, survival, morphology and number. This is important as it highlights new pathways essential for normal melanocyte development and consequently provides further tools in which to study melanocytes. Identifying the target of small molecules in vivo is a challenge in chemical biology. In Chapter 4, I describe my contributions to understanding how 5-nitrofuran compounds act in zebrafish (Zhou et al., 2012). My work has contributed to understanding the activity of 5-nitrofurans is dependent upon its nitrofuran ring structure. I have also helped confirm a conserved interaction between 5-nitrofurans and ALDH2, which may contribute to the off-target effects observed in the clinic. These results are important as they aid further understand of the 5-nitrofuran class of drugs and give evidence to support combination therapy of 5-nitrofurans with ALDH2 inhibitors as a way to overcome clinical side effects. Additionally I show that NFN1 treatment limits ensuing melanocyte regeneration thereby suggesting a role at the Melanocyte Stem Cell (MSC), which provides me with a key tool to study melanocyte regeneration in zebrafish. How tissue specific cell numbers are specified and maintained is a key question in developmental biology. In Chapter 5, I describe the identification of the MITF gene in the maintenance of cell cycle arrest in differentiated melanocytes (Taylor et al., 2011). We show that the human melanoma mutation MITF4TΔ2B promotes melanocyte division, thereby suggesting a role for melanocyte division in the pathogenesis of melanoma. This work is valuable because it highlights Mitf as a molecular rheostat that controls melanocyte proliferation and differentiation in living vertebrates, and helps us to understand the role of MITF in melanoma progression. Little is known about the pathways that control melanocyte stem cells in animals. To identify new melanocyte stem cell pathways, I used NFN1 as the basis for a small molecule screen for enhancers of melanocyte regeneration (Chapter 6). I find that chemical inhibition of Phosphatase of Regenerating Liver-3 (Prl-3) in zebrafish can enhance melanocyte regeneration. Importantly, I have found that there are an increased number of melanocyte progenitor cells in PRL3-inhibitor treated zebrafish. I propose that PRL-3 may control progenitor cell number in melanocyte regeneration. This is significant because it identifies PRL-3 as a novel molecular target controlling melanocyte progenitor cells, and identifies a new chemical tool with which to study melanocyte differentiation from a progenitor population. In the final chapter, I discuss how this work relates to the larger field of melanocyte developmental biology, and the new insight it provides into the fundamental processes of how organisms control cell number and pattern formation. In addition, I discuss how this work may have implications for understanding and treating melanocyte diseases, such as vitiligo (loss of melanocytes) and melanoma (cancer of the melanocyte).