Insights into the cellular and molecular adaptations of the skeletal muscle program in mature electrocytes of Sternopygus macrurus .

摘要

In most electric fish species, the electric organ (EO) derives from striated muscle cells that suppress many muscle properties. In the gymnotiform Sternopygus macrurus, electrocytes, the current-producing cells of the EO, derive from type II (fast-twitch) muscle fibers during tail regeneration in adulthood. Mature electrocytes do not contain sarcomeres, yet they continue to make some cytoskeletal and sarcomeric proteins and the muscle transcription factors (MTFs) that induce their expression. How mature electrocytes regulate components of the muscle program including contraction-associated genes and metabolic pathways is not well known. The studies in this dissertation tested the hypothesis that the myogenic features retained by mature electrocytes are those of type II fibers and are maintained independently of transcriptional control. In an effort to comprehensively examine the regulation of genes associated with the skeletal muscle phenotype in the non-contractile EO, we performed qRT-PCR and high-throughput sequencing of muscle transcripts and microRNAs in EO and muscle, as well as protein detection and ultrastructural studies of mitochondria in skeletal muscle and EO from adult S. macrurus . Our transcript expression data show that: 1) components associated with the homeostasis of the sarcomere and sarcomere-sarcolemma linkage were transcribed in EO at levels similar to those in muscle; 2) MTF families associated with activation of the skeletal muscle program were not differentially expressed between these tissues; and 3) a set of miRNAs that are implicated in regulation of the muscle phenotype were enriched in EO. The similar expression of all factors associated with regulating both slow-twitch and fast-twitch fibers in EO and muscle contrast with our stated hypothesis, but these results do provide evidence that the skeletal muscle program is regulated in a transcription-independent fashion in EO. With the knowledge that the EO of S. macrurus expresses all regulatory factors found in both fast- and slow-twitch fibers, we characterized the metabolic phenotype in the electric organ. The data from our metabolic studies demonstrated that: 1) glycolytic genes were differentially expressed in EO and muscle; 2) oxidative genes were not differentially expressed in EO and muscle; and 3) mitochondrial morphology and density varied in different sub-cellular regions of electrocytes. Taken together, these results suggest that electrocytes might be less reliant on glycolytic metabolism, which does not support our hypothesis. In sum, these results provide an interesting picture of one mechanism of regulating the skeletal muscle phenotype to derive a non-contractile, highly active electrogenic cell.