Using In Vivo Models to Understand Melanogenesis

The diversity of hair and skin color is a direct result of melanin synthesis occurring in melanocytes. Melanogenesis is a set of highly regulated processes that occur in a specialized organelle called the melanosome. Two components that are essential for melanogenesis to occur are 1) transcriptional regulation of the genes required for pigment production by MITF, and 2) proper trafficking of proteins and enzyme components to the melanosomes. While extensive studies have identified over 100 genes, there are still gaps in understanding how MITF isoforms alter melanogenesis-specific gene expression and how proteins control the formation of the melanosome. While these two parts of melanogenesis are distinct, in vivo models are a standard way to understand how these pathways contribute to melanogenesis as a whole. We recently identified Pikfyve, a gene that encodes a phosphoinositide kinase, as a regulator of melanogenesis. Conditional loss of Pikfyve in murine melanocytes causes greying of the coat. Isolation and culture of melanocytes from Pikfyve mice revealed defects in melanosome maturation due to failure of protein trafficking to the melanosome, defining a new lipid signaling pathway that controls the early stages of melanogenesis. Another phosphoinositide binding protein, Wipi1, subtly alters pigmentation after constitutive loss in mice, with more pronounced phenotypes occurring at the cellular level. In addition to this work, we completed additional studies trying to understand which Mitf isoform regulates melanogenesis and when and where it does. As Mitf-A and Mitf-M are the primary Mitf isoforms in melanocytes, we used CRISPR/Cas9 technology to generate both Mitf-A and Mitf-M isoform-specific knockout mice. While loss of Mitf-A had only subtle impacts on pigmentation, loss of Mitf-M drastically alters pigmentation in the hair, skin, and eyes due to loss of melanocytes in adults. Intriguingly, Mitf-M knockout mice also have enlarged kidneys, revealing new roles for this transcription factor in other tissues. Moreover, we have recently revealed that some Mitf-M knockout mice can develop spontaneous repigmentation on parts of their body, a mechanism that is a result of spontaneous reactivation of melanocytes. Taken together, these studies outline an approach that can be used to better understand how melanogenesis is specified in both place (intracellularly and in the organism) and time (development and hair stage)