Activation of Pax7-Positive Cells in a Non-Contractile Tissue Contributes to Regeneration of Myogenic Tissues in the Electric Fish S. macrurus Christopher M. Weber1¤, Mark Q. Martindale2, Stephen J. Tapscott3, Graciela A. Unguez1* 1 Biology Department, New Mexico State University, Las Cruces, New Mexico, United States of America, 2 Kewalo Marine Lab, University of Hawaii, Honolulu, Hawaii, United States of America, 3 Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America 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. Citation: Weber CM, Martindale MQ, Tapscott SJ, Unguez GA (2012) Activation of Pax7-Positive Cells in a Non-Contractile Tissue Contributes to Regeneration of Myogenic Tissues in the Electric Fish S. macrurus. PLoS ONE 7(5): e36819. doi:10.1371/journal.pone.0036819 Editor: Marcello Rota, Brigham & Women’s Hospital - Harvard Medical School, United States of America Received August 25, 2011; Accepted April 15, 2012; Published May 31, 2012 Copyright: 2012 Weber et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was funded by National Institutes of Health grants S06-GM008136 (GAU), RR16480 (GAU), and U56-CA96286 (GAU and SJT). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: gunguez@nmsu.edu ¤ Current address: Basic Sciences Division, Fred Hutchinson Cancer Research Center and Molecular and Cellular Biology Program, University of Washington, Seattle, Washington, United States of America Introduction A major challenge in the study of mammalian regeneration remains the identification of the limiting factors that constrain the restoration of a tissue with the purpose of modifying these factors to boost the regeneration potential [1,2]. Many vertebrate animals repair and remodel their injured or lost tissues primarily through the activation of stem cells and/or the process of dedifferentiation, i.e., a process by which mature cells lose their differentiated phenotype to produce progenitor cells that can reenter the cell cycle. Multiple regeneration mechanisms may come into play in one animal in response to injury or disease. For example, in humans liver tissue regenerates through a dedifferentiation process [3], whereas skeletal muscle regenerates through the activation of myogenic stem cells [4,5] suggesting that programs of tissue regeneration might be broadly conserved but their expression is regulated by tissue-specific constraints. Although one mechanism might be favored under certain conditions, it is not possible to exclude some contribution from an alternative mode of regeneration. Nevertheless, the idea that a greater potential for regeneration is associated with a correspond- ing potential for cell dedifferentiation remains broadly accepted [6,7,8]. Support for this idea is based largely on the vast body of evidence from experiments in urodele amphibians such as the newt and axolotls, which are regarded as the champions of regeneration among vertebrates. Urodeles are able to regenerate limbs, tail, jaws, ocular tissues like retina and lens, and some portions of the heart. Studies have suggested that their ability to regenerate these tissues depends largely on dedifferentiation of cells at the site of injury to reenter the cell cycle [6,8]. In contrast, there is less evidence for the existence of adult stem cell populations and their contribution to restored tissues in urodeles [9,10]. Some studies using urodele amphibians have indicated that regeneration does not proceed in the absence of cell dedifferen- tiation [6,11,12,13]. Whether cell dedifferentiation is the funda- mental factor that triggers a robust regeneration capacity, and whether the loss or inhibition of this process explains the limited PLoS ONE | www.plosone.org 1 May 2012 | Volume 7 | Issue 5 | e36819