Zebrafish melanophilin facilitates melanosome dispersion by regulating dynein.
BACKGROUND
Fish melanocytes aggregate or disperse their melanosomes in response to the level of intracellular cAMP. The role of cAMP is to regulate both melanosome travel along microtubules and their transfer between microtubules and actin. The factors that are downstream of cAMP and that directly modulate the motors responsible for melanosome transport are not known. To identify these factors, we are characterizing melanosome transport mutants in zebrafish.


RESULTS
We report that a mutation (allele j120) in the gene encoding zebrafish melanophilin (Mlpha) interferes with melanosome dispersion downstream of cAMP. Based on mouse genetics, the current model of melanophilin function is that melanophilin links myosin V to melanosomes. The residues responsible for this function are conserved in the zebrafish ortholog. However, if linking myosin V to melanosomes was Mlpha's sole function, elevated cAMP would cause mlpha(j120) mutant melanocytes to hyperdisperse their melanosomes. Yet this is not what we observe. Instead, mutant melanocytes disperse their melanosomes much more slowly than normal and less than halfway to the cell margin. This defect is caused by a failure to suppress minus-end (dynein) motility along microtubules, as shown by tracking individual melanosomes. Disrupting the actin cytoskeleton, which causes wild-type melanocytes to hyperdisperse their melanosomes, does not affect dispersion in mutant melanocytes. Therefore, Mlpha regulates dynein independently of its putative linkage to myosin V.


CONCLUSIONS
We propose that cAMP-induced melanosome dispersion depends on the actin-independent suppression of dynein by Mlpha and that Mlpha coordinates the early outward movement of melanosomes along microtubules and their later transfer to actin filaments.
Sheets
Lavinia
L
Department of Cell and Developmental Biology, Oregon Health and Science University, Basic Science Building Room 5365, 3181 SW Sam Jackson Park Road, Portland, Oregon 97201-3098, USA.
Ransom
David G
DG
Mellgren
Eve M
EM
Johnson
Stephen L
SL
Schnapp
Bruce J
BJ
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001693:Biological Transport, Active / Q000187:drug effects; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D017931:DNA Primers / Q000235:genetics; D004398:Dyneins / Q000502:physiology*; D007028:Hypothalamic Hormones / Q000494:pharmacology; D008543:Melanins / Q000494:pharmacology; D009074:Melanocyte-Stimulating Hormones / Q000494:pharmacology; D008544:Melanocytes / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure; D020460:Melanosomes / Q000187:drug effects / Q000502:physiology*; D008870:Microtubules / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation; D010802:Phylogeny; D010858:Pigmentation / Q000187:drug effects / Q000235:genetics / Q000502:physiology; D010907:Pituitary Hormones / Q000494:pharmacology; D017386:Sequence Homology, Amino Acid; D015027:Zebrafish / Q000235:genetics / Q000502:physiology*; D029961:Zebrafish Proteins / Q000235:genetics / Q000502:physiology*
D002352:Carrier Proteins; D017931:DNA Primers; D007028:Hypothalamic Hormones; D008543:Melanins; D010907:Pituitary Hormones; D029961:Zebrafish Proteins; C019463:melanin-concentrating hormone; D009074:Melanocyte-Stimulating Hormones; D004398:Dyneins