Year: 2005
Type: editorial
Abstract: The primary aim of melanocytes is to generate pigment. To do so efficiently the various enzymes responsible for pigment synthesis are housed in melanosomes. The advantages of producing melanin in melanosomes are manifold and include variously the ability to couple closely the various steps involved in pigment synthesis, the ability to protect the cell from melanin-associated toxicity by housing melanin in a membrane-bound structure, and also the capacity to attach and move melanosomes via the cytoskeleton along dendritic processes for transfer to surrounding cells. The role of melanosome associated proteins in melanin synthesis and melanosome architecture has attracted considerable interest, and melanosomes have become a major model for examining organelle transport. In the review by Theos et al., one of the more enigmatic melanosome components, the multi-aliased product of the silver locus Pmel17 is placed under the spotlight. Despite some controversy surrounding several aspects of Pmel17, it plays a critical role in melanosome biogenesis, most likely in forming fibrillar arrays that may serve as a melanosome scaffold, but as the review highlights several key questions remain to be addressed the most notable being perhaps what exactly is the role of the scaffold. One of the key roles of melanocytes in humans is to increase the manufacture of melanosomes and pigment in response to UV irradiation and to transfer the melanosomes to surrounding keratinocytes in the skin as protection from UV-induced DNA damage, a process known as the tanning response. Many of the key factors implicated in transducing the UV signal to melanocytes are known and includes the keratinocyte-derived signalling molecules described in the review from Hirobe in the first issue of this year. Two signalling pathways are particularly important, p38 signalling that impacts on both the USF and Mitf transcription factors that regulate tyrosinase expression, as well as cAMP signalling lying downstream from the MC1R, a seven transmembrane domain G-protein coupled receptor that will be discussed in an upcoming review from Jose Carlos Garcia Borron. Mammalian melanocytes respond to cAMP signalling by increasing their dendricity and upregulating Mitf expression, and cAMP also plays a critical role in regulating melanosome function. A new twist on this story was provided by Ban et al. using erythrophores from the Nile tilapia as a model system. These erythrophores, unlike other pigment cells, respond differently to different wavelengths of light with pigment being dispersed at 500 nm but aggregated at 600 nM. Using these cells as a model system the authors now show that cAMP signalling downstream from Gi or Gs G-proteins, most likely coupled to red or green opsins identified in the fish fins mediate the bi-directional pigment movement. The paper provides a key insight into how cAMP signalling can be modulated to provide different outputs. While Nile tilapia erythorphores provide an nice model system for studying signalling pathways regulating pigment dispersal or aggregation, a model organism, Medaka fish is increasingly popular as a system for the analysis of genes implicated in pigment cell development as reviewed by Lynn Lamoreux et al. in the April issue of Pigment Cell Research this year. Now Iida et al. have demonstrated that an albino phenotype caused by an insertion of a Tol2 element into the promoter region of the tyrosinase gene can be subject to reversion. The results are significant as they suggest that Medaka may possess an active transposase able to mediate the excision/integration of transposons like Tol2. No doubt further examples of active transposons will be reported in the near future.
Source: Pigment Cell Research
Author: Colin Goding
Language: English
Subfield: Cell Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W1986098957