A Erythrophore Intracellular Motility and Actin in

The Holocentrus erythrophore, a red pigment cell, represents a model system for the study of organized intracellular transport. We have investigated the possibility that microtubules and actin are integral components of the pigment translocating motil ity machine. By creating cells that have total or partial loss of the microtubule framework we have demonstrated that the presence of microtubules is essential for organized, radial transport of the pigment granules. However, in the absence of microtubules, some undirected movement of the pigment can be stimulated; this suggests that a nonmicrotubular component of the cytoplast is responsible, at least in part, for the generation of motive force. In order to test the hypothesis that this component consists of actin or actomyosin, we examined the effects of probes for these classical motil ity proteins. Neither microinjection of phalloidin, DNase I or Nethylmaleimide-modified heavy meromyosin nor exogenous application of cytochalasin B has any effect on pigment motion, although these materials do block the actin-mediated motility of other systems in our hands. Therefore, intracellular particle transport in erythrophores does not appear to be actin or actomyosin-based. The color changes that an organism can undergo in response to environmental stimuli are frequently mediated by the movement of pigment granules within cells called chromatophores. Erythrophores, or red chromatophores, from the squirrelfish Holocentrus ascensionis display such pigment granule movement in response to neural or humoral stimulation (6). This cell type represents a unique model system for the study of intracellular motility since it exhibits both saltatory and uniform velocity granule movements. Dispersion of the pigment is characteristically slow and saltatory, and by these criteria is analogous to axonal transport, intra-axopodial particle movements, and some secretory events. Aggregation, on the other hand, occurs with uniform velocity, a feature that it shares with the process of anaphase chromosome segregation, although pigment movement is much faster (16, 21). One advantage of the erythrophore lies in the fact that the saltatory and uniform velocity movements are separated temporally, thus they can be studied as mechanistically distinct processes. Both erythrophores (6) and melanophores (4, 24, 36) contain radially arrayed microtubules that extend from the cell center to its margin, establishing channels within which the pigment granules course. Numerous investigators have attempted to define the role of these microtubules in pigment granule transport. Disruption of melanophore microtubules by a variety of 354 means affects motility (e.g. 15, 24, 33) suggesting that these structures are required for pigment migration, but possible nonspecific (i.e., nonmicrotubule) effects of these treatments cannot be eliminated as the cause of the decline in motility noted in these studies. For example, it has been reported that both lumicolchicine and colchicine alter aggregative motility in melanophores (26). In this paper we present evidence that an intact microtubule framework is required for organized pigment motion in erythrophores. Nevertheless, some slow movement of pigment granules can be stimulated in melanophores in the absence of microtubules, suggesting that nonmicrotubular components can generate motive force in these cells (26, 35). Because actin is involved in a wide variety of nonmuscle motile events (18, 25, 28, 40) it represents a prime candidate for participation in pigment granule transport. Actin microfilaments have been identified in melanophores by heavy meromyosin decoration (27, 37), but their role in pigment migration remains controversial. Obika and co-workers (27) report an increase in the number of decoratable filaments in aggregated Fundulus melanophores and suggest, as do other investigators (reference 1, for swordtail erythrophores) that these filaments are involved in the motility function. Other data, however, indicate that the filamentous actin present in melanophores is largely confined to the cell THE JOURNAL OF CELL BIOLOGY. VOLUME 96 FEBRUARY 1983 354-362 © The Rockefeller University Press • 0021-9525/83/02/0354/09 $1.00 on A ril 0, 2017 D ow nladed fom Published February 1, 1983