Activation of Pax7-Positive Cells in a Non-Contractile Tissue Contributes to Regeneration of Myogenic Tissues in the Electric Fish S. macrurus

Abstract

The ability to regenerate tissues is shared across many metazoan taxa, yet the type and extent to which multiple cellular mechanisms come into play can differ across species. For example, urodele amphibians can completely regenerate all lost tissues, including skeletal muscles after limb amputation. This remarkable ability of urodeles to restore entire limbs has been largely linked to a dedifferentiation-dependent mechanism of regeneration. However, whether cell dedifferentiation is the fundamental factor that triggers a robust regeneration capacity, and whether the loss or inhibition of this process explains the limited regeneration potential in other vertebrates is not known. Here, we studied the cellular mechanisms underlying the repetitive regeneration of myogenic tissues in the electric fish S. macrurus. Our in vivo microinjection studies of high molecular weight cell lineage tracers into single identified adult myogenic cells (muscle or noncontractile muscle-derived electrocytes) revealed no fragmentation or cellularization proximal to the amputation plane. In contrast, ultrastructural and immunolabeling studies verified the presence of myogenic stem cells that express the satellite cell marker Pax7 in mature muscle fibers and electrocytes of S. macrurus. These data provide the first example of Pax-7 positive muscle stem cells localized within a non-contractile electrogenic tissue. Moreover, upon amputation, Pax-7 positive cells underwent a robust replication and were detected exclusively in regions that give rise to myogenic cells and dorsal spinal cord components revealing a regeneration process in S. macrurus that is dependent on the activation of myogenic stem cells for the renewal of both skeletal muscle and the muscle-derived electric organ. These data are consistent with the emergent concept in vertebrate regeneration that different tissues provide a distinct progenitor cell population to the regeneration blastema, and these progenitor cells subsequently restore the original tissue.

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

“…Hence, we began to study the cellular mechanisms underlying the repetitive regeneration of myogenic tissues in S. macrurus using ultrastructural analysis, immunolabeling detection of the adult muscle stem cells called satellite cells, and in vivo microinjection studies of high molecular weight cell lineage tracers into single muscle fibers or electrocytes. First, our ultrastructural studies verified the anatomical presence of myogenic satellite cells in both muscle and the electric organ (Weber et al, 2012). (B)Pax7-positive cells are localized in blastema regions that give rise to muscle, electric organ and dorsal spinal cord.…”

“…We have studied tail regeneration in the adult gymnotiform electric fish Sternopygus macrurus Unguez and Zakon, 2002;Weber et al, 2012). 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).…”

“…Recently, we have also begun to address some aspects of the cellular and molecular mechanisms underlying the regeneration of complex tail structures such as skeletal muscle and the electric organ in this teleost fish. Tail regeneration in gymnotiforms has been most studied in S. macrurus (Fig.2) (Patterson and Zakon, 1993;Patterson and Zakon, 1996;Patterson and Zakon, 1997;Unguez and Zakon, 2002;Weber et al, 2012). Like all gymnotiforms studied to date, tail regeneration in S. macrurus begins by epidermal cells covering the wound followed by formation of a blastema (Fig.2).…”

“…Pax7 labeling was also detected in dorsal spinal cord (SC). Reproduced from Weber et al (Weber et al, 2012). Regeneration in electric fish (Fig.3).…”

“…For example, generation of reliable cell markers have led to recent findings that resident, tissuespecific stem cells also respond to injury and contribute to blastema formation in some vertebrate species (Li et al, 2007;Morrison et al, 2006;Poss et al, 2003;Singh et al, 2012;Weber et al, 2012). Hence, the extent to which cell dedifferentiation versus stem cell activation occurs in response to injury in vertebrates warrants further investigation.…”