Differential expression of genes and proteins between electric organ and skeletal muscle in the mormyrid electric fish Brienomyrus brachyistius

Electric organs (EOs) have evolved independently in vertebrates six times from skeletal muscle (SM). The transcriptional changes accompanying this developmental transformation are not presently well understood. Mormyrids and gymnotiforms are two highly convergent groups of weakly electric fish that have independently evolved EOs: while much is known about development and gene expression in gymnotiforms, very little is known about development and gene expression in mormyrids. This lack of data limits prospects for comparative work. We report here on the characterization of 28 differentially expressed genes between SM and EO tissues in the mormyrid Brienomyrus brachyistius, which were identified using suppressive subtractive hybridization (SSH). Forward and reverse SSH was performed on tissue samples of EO and SM resulting in one cDNA library enriched with mRNAs expressed in EO, and a second library representing mRNAs unique to SM. Nineteen expressed sequence tags (ESTs) were identified in EO and nine were identified in SM using BLAST searching of Danio rerio sequences available in NCBI databases. We confirmed differential expression of all 28 ESTs using RT-PCR. In EO, these ESTs represent four classes of proteins: (1) ion pumps, including the α- and β-subunits of Na(+)/K(+)-ATPase, and a plasma membrane Ca(2+)-ATPase; (2) Ca(2+)-binding protein S100, several parvalbumin paralogs, calcyclin-binding protein and neurogranin; (3) sarcomeric proteins troponin I, myosin heavy chain and actin-related protein complex subunit 3 (Arcp3); and (4) the transcription factors enhancer of rudimentary homolog (ERH) and myocyte enhancer factor 2A (MEF2A). Immunohistochemistry and western blotting were used to demonstrate the translation of seven proteins (myosin heavy chain, Na(+)/K(+)-ATPase, plasma membrane Ca(2+)-ATPase, MEF2, troponin and parvalbumin) and their cellular localization in EO and SM. Our findings suggest that mormyrids express several paralogs of muscle-specific genes and the proteins they encode in EOs, unlike gymnotiforms, which may post-transcriptionally repress several sarcomeric proteins. In spite of the similarity in the physiology and function of EOs in mormyrids and gymnotiforms, this study indicates that the mechanisms of development in the two groups may be considerably different.