Alternative transcription generates multiple Mitf isoforms with different expression patterns and activities in medaka

Abstract

Microphthalmia-associated transcription factor (Mitf) is best known for distinct functions in multiple cell lineages including melanocytes, mast cells, and osteoclasts. In mammals, mitf produces multiple Mitf isoforms by alternative transcription and splicing. The fish medaka has two mitf genes, mitf1 and mitf2. Here, we report differential expression and activities of medaka Mitf isoforms. Molecular cloning identified four mitf1 variants encoding isoforms Mitf1A, MitfB, MitfH, and MitfM, and only one mitf2RNA encoding Mitf2M, which exhibited differential expression. Mitf1 isoforms and Mitf2M differed dramatically in activating the dazl and tyrosinase promoters DAZ and TYR. Interestingly, Mitf1ΔN, an N-terminus-less Mitf1 mutant form, retained activity to activate TYR but not DAZ. Importantly, Mitf1B was also sufficient for inducing melanocyte differentiation and endogenous tyrosinase RNA expression in medaka embryonic stem cells. Intriguingly, Mitf1 isoforms possessed considerable differences in inducing the expression of multiple cell lineage marker genes. Therefore, alternative mitf transcription is a conserved mechanism from fish to mammals, and medaka Mitf1 isoforms show differences in expression and activity. We conclude that differential expression of isoforms contributes to multiple distinct functions of Mitf in vertebrates.