FoxK1 Splice Variants Show Developmental Stage-specific Plasticity of Expression with Temperature in the Tiger Pufferfish

FoxK1 is a member of the highly conserved forkhead/winged helix (Fox) family of transcription factors and it is known to play a key role in mammalian muscle development and myogenic stem cell function. The tiger pufferfish (Takifugu rubripes) orthologue of mammalian FoxK1 (TFoxK1) has seven exons and is located in a region of conserved synteny between pufferfish and mouse. TFoxK1 is expressed as three alternative transcripts: TFoxK1-alpha, TFoxK1-gamma and TFoxK1-delta. TFoxK1-alpha is the orthologue of mouse FoxK1-alpha, coding for a putative protein of 558 residues that contains the forkhead and forkhead-associated domains typical of Fox proteins and shares 53% global identity with its mammalian homologue. TFoxK1-gamma and TFoxK1-delta arise from intron retention events and these transcripts translate into the same 344-amino acid protein with a truncated forkhead domain. Neither are orthologues of mouse FoxK1-beta. In adult fish, the TFoxK1 splice variants were differentially expressed between fast and slow myotomal muscle, as well as other tissues, and the FoxK1-alpha protein was expressed in myogenic progenitor cells of fast myotomal muscle. During embryonic development, TFoxK1 was transiently expressed in the developing somites, heart, brain and eye. The relative expression of TFoxK1-alpha and the other two alternative transcripts varied with the incubation temperature regime for equivalent embryonic stages and the differences were particularly marked at later developmental stages. The developmental expression pattern of TFoxK1 and its localisation to mononuclear myogenic progenitor cells in adult fast muscle indicate that it may play an essential role in myogenesis in T. rubripes.