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Abstract:
Zebrafish owes its name to a pattern of dark stripes and light interstripes, which develops during metamorphosis replacing a larval pattern. The stripes are formed by black melanophores and reflecting iridophores whereas interstripes are formed by yellow xanthophores and dense iridophores. The processes ruling pigment cell migration and positioning in the skin during metamorphosis remain largely unknown and existing models are frequently controversial. Mutagenesis screens have identified a number of genes influencing distinct aspects of stripe pattern formation. We are interested in mutants displaying various levels of stripe disruption, from interruptions to a complete dispersal of melanophores. In this study we have characterized schachbrett (German word for chessboard), a mutant displaying interruptions of the stripes by dense iridophores and xanthophores of the interstripe. The schachbrett mutant has a premature stop codon in a gene coding Tight Junction Protein 1a. Subsequently several new alleles with the same phenotype were identified, all of which had premature stops in tjp1a. Immunohistochemical analysis with antisera raised against two distinct parts of Tjp1a, allowed to detect expression in dense iridophores but no other pigment cell types. In wildtype, after establishment of the first interstripe, iridophores disperse as a loose net along the dorso-ventral axis to contribute to loose iridophores of the dark stripes and new interstripes. Time-lapse imaging of metamorphic animals showed that the formation of the first interstripe in schachbrett is normal, but later iridophores invade the developing dark stripes as dense sheets at multiple points along the anterior-posterior axis, changing melanophore dynamics and leading to interruptions of the stripes. While several transmembrane proteins, such as connexins and ion channels, were shown to affect the patterning, their intracellular partners remain unidentified. Since Tjp1a is a scaffold protein which provides an interaction platform for membrane and cytoskeletal proteins, we postulate that the Tjp1 could facilitate the intracellular arm of cell-cell communication downstream of the identified proteins involved in pattern formation.
