Evidence of post‐transcriptional regulation in the maintenance of a partial muscle phenotype by electrogenic cells of S.macrurus

Abstract

Electrocytes, the current-producing cells of electric organs (EOs) in electric fish, are unique in that they derive from striated muscle and they possess biochemical characteristics of both muscle and non-muscle cells. In the freshwater teleost Sternopygus macrurus, electrocytes are multinucleated cells that do not contract yet retain expression of some proteins common to skeletal muscle cells. Given the role that transcriptional regulation plays in the activation of the myogenic program in vertebrates, we examined the expression patterns of several genes associated with multiple functions of skeletal muscle in mature electrocytes of S. macrurus. Our expression analyses detected transcripts for alpha-actin, alpha-acetylcholine (ACh) receptor (alpha-AChR), desmin, muscle creatine kinase (MCK), myosin heavy chain (MHC) isoforms, titin, tropomyosin, and troponin-T genes in the EO. However, immunolabeling studies revealed that electrocytes do not contain MCK, MHCs, or tropomyosin or troponin-T proteins. These results underscore the contribution of gene regulatory mechanisms in the maintenance of the muscle-like phenotype of EO that may be transcriptional-independent. We also report the classification and frequency of distinct transcripts from a random selection of 420 clones from an EO cDNA library. This is the first characterization of expressed genes in an EO, and it is an important step toward identifying mechanisms that affect different muscle protein systems for the evolution of highly specialized noncontractile tissues. Evidence of post-transcriptional regulation in the maintenance of a partial muscle phenotype by electrogenic cells of S. macrurus.

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

“…For each specimen, the caudal peduncle or ~1cm of axial muscle posterior to the operculum and dorsal to the lateral line was removed and flash frozen in liquid nitrogen-chilled isopentane (Cuellar et al, 2006), then mounted on a dry ice-chilled chuck using Cryo-M-Bed (Bright Instrument Company, Huntingdon, UK). Tissues were cut at 20m using a cabinet cryostat onto freshly subbed slides and dried overnight at room temperature.…”

“…For the purposes of later discussion, we assigned putative functional roles based on the known function of these genes in skeletal muscle: transcription factor, Ca 2+ binding, sarcomeric proteins, ion pump and other (Fig.2) Spatial distribution of proteins is similar but not identical between SM and EO Cuellar and colleagues described the existence of several mRNAs present in the EO of gymnotiforms (Cuellar et al, 2006), but their corresponding proteins were not found using either immunohistochemistry or western blotting, suggesting the existence of a post-transcriptional mechanism of regulating gene expression. Given the similar identities of the mRNAs detected by Cuellar and colleagues (Cuellar et al, 2006) and the differentially expressed ESTs described in this study, we wished to determine whether protein products like those differentially expressed ESTs described above were translated in mormyrid EO. We determined protein presence and distribution in the mormyrid EO and SM using widely cross-reactive antibodies for a subset of seven proteins: MHC, Na…”

“…assumptions regarding the identities of specific genes that might be differentially expressed (Patterson and Zakon, 1996;Patterson and Zakon, 1997;Cuellar et al, 2006;Kim et al, 2008;Kim et al, 2009;Kim et al, 2004;Unguez and Zakon, 1998a;Unguez and Zakon, 1998b;Zakon and Unguez, 1999). Unlike these prior approaches, our use of the SSH technique allows for the construction of a dataset for which fewer assumptions have been made.…”

“…Several studies over the past two decades have contributed to our understanding of gene expression and development in gymnotiform EOs, in particular with regard to the role of innervation (Cuellar et al, 2006;Kim et al, 2008;Kim et al, 2009;Kim et al, 2004;Unguez and Zakon, 1998a;Unguez and Zakon, 1998b;Zakon and Unguez, 1999;Patterson and Zakon, 1996;Patterson and Zakon, 1997). Comparative studies between gymnotiforms and mormyrids, however, are presently limited because no studies have described gene expression in the mormyrid EO, which is the focus of the present study.…”

Electric fish: new insights into conserved processes of adult tissue regeneration

“…We use this model system because skeletal muscle cells of electric fish express a unique ability to undergo changes in their biochemical and morphological properties and give rise to electrocytes, the current-producing cells of the electric organ (Bennett, 1971;Fox and Richardson, 1978;Fox and Richardson, 1979), and we have begun to determine how the developmental program for striated muscle has been altered to produce an electrogenic, noncontractile cell that retains a partial muscle phenotype Zakon and Unguez, 1999;Unguez and Zakon, 2002;Kim et al, 2004;Kim et al, 2008;Cuellar et al, 2006). We have exploited the inexhaustible regenerative capacity of adult S. macrurus to study the effects of neural input on the differentiation of muscle fibers and their phenotypic conversion into electrocytes.…”