¹Cutaneous Biology Research Center, Massachusetts General Hospital, Charlestown, Massachusetts, USA

Correspondence: Dr Ethan A. Lerner, Building 149, Street 13, Charlestown, Massachusetts 02129, USA. E-mail: ethan.lerner@cbrc2.mgh.harvard.edu

Abstract

McNeill et al. (2007, this issue) dissect the potential role of TRPM7 ion channels in pigment cells by studying the phenotype of zebrafish trpm7 mutant embryos. They demonstrate that 1-phenyl-2-thiourea, a known melanin synthesis inhibitor, prevents melanophore cell death in these mutants. This suggests a potential functional link between TRPM7 signaling and the detoxification of melanin synthesis intermediates.

The transient receptor potential melastatin (TRPM) ion channel subfamily, one of the six classes of TRP channels, has been discovered and investigated in the past decade. It has a variety of functions from cold sensing (TRPM8) or oxidative stress sensing (TRPM2) to calcium (TRPM4, TRPM5), magnesium (TRPM6, TRPM7), or osmohomeostasis (TRPM3). The first member discovered—TRPM1, or melastatin—is a tumor-suppressor protein expressed at high levels in benign melanocytes but at low levels in melanoma cells with highly metastatic behavior (see the review by Harteneck, 2005). The main topic of this discussion, TRPM7, is a ubiquitously expressed nonselective cation channel permeable for calcium, magnesium, and trace metals. A role for TRPM7 in pigment cell survival has been previously suggested (Elizondo et al., 2005) and is further investigated by McNeill et al. (2007, this issue).

The trpm7 zebrafish mutant was initially identified in a chemical mutagenesis screen (Rawls et al., 2003; Arduini and Henion, 2004; Cornell et al., 2004). At 48 hours postfertilization (hpf) these mutants are lighter in color because they have fewer melanophores and altered melanophore morphology (punctate versus stellate) as compared with the wild-type phenotype. Surprisingly, melanophores reappear at 72 hpf, which suggests that a compensatory mechanism takes place after this stage. In addition, the trpm7 mutants develop temporary touch response impairment during embryogenesis (Cornell et al., 2004) and kidney stones and skeletal defects during the larval stage (Elizondo et al., 2005).

In this study, McNeill et al. (2007) shed light on the mechanism of melanophore cell death and find methods to rescue this phenotype. Thus, magnesium chloride is shown to significantly increase the number of surviving melanophores in the trmp7 mutant embryos. This suggests that TRPM7 is functionally linked to melanophore survival. Calcium chloride gave variable results (McNeill et al., 2007; Elizondo et al., 2005). A broad-spectrum caspase inhibitor had no effect on melanophore viability in trpm7 mutants but prevented melanophore cell death in kit mutants. The authors also provide electron microscopic images showing that, in the trpm7 mutant embryos, melanophores—at least in part—undergo necrosis. For instance, they bring evidence for ruptured melanophore plasma membrane, a feature of necrosis. These data suggest that, in these mutants most melanophores suffer a nonapoptotic cell death. A small fraction of the melanophores in the trpm7 mutants does undergo apoptosis (Arduini and Henion, 2004).

The authors further show that 1-phenyl-2-thiourea (PTU), a known melanin synthesis inhibitor, rescues the melanophore cell death phenotype in the trpm7 mutant, as shown by dopachrome tautomerase (dct) expression. Melanophore cell death in the kit mutant was unaffected by PTU. To confirm this finding, by studying homozygous albino/trmp7 double mutant embryos in comparison with homozygous albino siblings, McNeill et al. (2007) showed that the number of dct-expressing cells was similar. They argue that the albino mutant embryos display low levels of melanin synthesis and thus are protected from melanophore cell death, the consequence of the trpm7 background. An alternative approach using trpm7 antisense morpholino oligonucleotide-injected albino and golden mutants generated similar results.

The zebrafish trpm7 mutant embryos, despite melanophore loss, have intact xanthophores and iridophores (Arduini and Henion, 2004). However, in light of these new data provided by McNeill et al. (2007), only melanophores are affected because they accumulate toxic melanin synthesis intermediates. Xanthophores contain yellow pteridine pigments, while iridophores have plates of crystalline guanine responsible for the diffraction of light, which generates iridescent colors.

The effect of trpm7 knockdown on adult melanophores and whether magnesium or calcium chloride supplementation alters this effect are not known. This could be determined in fish scales cultured in vitro or in Xenopus laevis melanophore cell lines. The effect of trpm7 knockdown on melanocytes survival and whether melanoma cells are more sensitive than benign melanocytes and other cell types to this intervention should be examined. This information may create a basis for the development of novel and specific treatments for melanoma. Such studies could also test the hypothesis that TRPM7-specific inhibitors could have a therapeutic role in decreasing depigmentation.

McNeill et al. (2007) note that the temporary paralysis between 48 and 72 hpf displayed by the zebrafish trpm7 mutant embryos correlates with that of a form of Parkinson's dementia associated with a partial loss of function mutation of TRPM7 (Hermosura et al., 2005). Parkinson's disease is characterized by the loss of dopamine-synthesizing melanin-containing cells in the midbrain—more precisely, in the pars compacta of the substantia nigra. As suggested in the zebrafish model, accumulation of melanin synthesis toxic intermediates could be responsible for pigment cell death in the substantia nigra. Extrapolating from the work done in zebrafish, magnesium or calcium supplementation may have a protective effect in some forms of Parkinson's disease. Taken together, these observations suggest that TRPM7 may be involved in melanin-bearing cell survival in humans.

There may be other associations between TRP channel mutations and human diseases involving pigment cell loss. For instance, a calcium-uptake defect has also been suggested in vitiliginous melanocytes (Schallreuter-Wood et al., 1996), which may reflect, at least in that patient, the presence of an ion-channel-related effect. Some hypopigmentation disorders have been associated with hearing impairment, presumably because of pigment cell loss in the inner ear (Price and Fisher, 2001). Furthermore, magnesium supplementation is known to be protective in noise-induced hearing loss. Magnesium-permeable channels, such as TRPM7, could be involved. A mouse TRPM7 knockout would allow further evaluation of the role of this ion channel in pigmentation, neurodegenerative diseases, and possibly hearing.