Melanophores in damselfish neurofibromatosis: Alterations in morphology and melanosome aggregation responses

Damselfish neurofibromatosis (DNF) is a neoplastic disease affecting bicolor damselfish Pomacentrus partitus on south Florida reefs. This disease exhibits many traits in common with neurofibromatosis type-1 in humans, including multiple plexiform neurofibromas and areas of hyperpigmentation. An important issue in understanding the pathogenesis of DNF is the relation between chromatophores and other cell types that give rise to tumors in this disease. Melanophores cultured from tumors of fish with DNF were compared in vitro with cells from normal skin of healthy fish to determine if these cells were morphologically or physiologically altered in this disease. Melanophores from tumors were significantly larger in cross-sectional perimeter and area than normal counterparts. Pigment translocation responses were measured by bath application of norepinephrine. At drug concentrations of 5 × 10^-9 to 5 × 10^-5 significantly fewer tumorderived cells than normal cells exhibited complete melanosome aggregation, and mean response times for tumor-derived cells were significantly longer. Aggregation times were not positively correlated with cell perimeter or surface area for either cell type. Two types of response fatigue were observed with repeated exposure protocols. A loss of ability to spontaneously disperse pigment after repeated aggregations induced by high doses of norepinephrine (5 × 10^-5 M) was observed in both normal and tumor-derived melanophores. In contrast, at lower concentrations of norepinephrine (5 × 10^-7 M), an increasing proportion of tumor-derived, but not normal cells, failed to aggregate pigment as exposures were repeated. However, fatigue affected only the degree of pigment aggregation and not the rate of melanosome movement. These data demonstrate that melanophores present in damselfish neurofibromatosis are significantly different from their healthy counterparts in both morphology and physiology in vitro. This system should be useful for understanding how the agent responsible for DNF alters either specific hormone receptors, second messenger pathways, pigment organelles, cytoskeletal components, mechanochemical events, or a combination of these factors in these cells.