title: Persistent Muscle Fiber Regeneration in Long Term Denervation. Past, Present, Future.
abstract: Despite the ravages of long term denervation there is structural and ultrastructural evidence for survival of muscle fibers in mammals, with some fibers surviving at least ten months in rodents and 3-6 years in humans. Further, in rodents there is evidence that muscle fibers may regenerate even after repeated damage in the absence of the nerve, and that this potential is maintained for several months after denervation. While in animal models permanently denervated muscle sooner or later loses the ability to contract, the muscles may maintain their size and ability to function if electrically stimulated soon after denervation. Whether in mammals, humans included, this is a result of persistent de novo formation of muscle fibers is an open issue we would like to explore in this review. During the past decade, we have studied muscle biopsies from the quadriceps muscle of Spinal Cord Injury (SCI) patients suffering with Conus and Cauda Equina syndrome, a condition that fully and irreversibly disconnects skeletal muscle fibers from their damaged innervating motor neurons. We have demonstrated that human denervated muscle fibers survive years of denervation and can be rescued from severe atrophy by home-based Functional Electrical Stimulation (h-bFES). Using immunohistochemistry with both non-stimulated and the h-bFES stimulated human muscle biopsies, we have observed the persistent presence of muscle fibers which are positive to labeling by an antibody which specifically recognizes the embryonic myosin heavy chain (MHCemb). Relative to the total number of fibers present, only a small percentage of these MHCemb positive fibers are detected, suggesting that they are regenerating muscle fibers and not pre-existing myofibers re-expressing embryonic isoforms. Although embryonic isoforms of acetylcholine receptors are known to be re-expressed and to spread from the end-plate to the sarcolemma of muscle fibers in early phases of muscle denervation, we suggest that the MHCemb positive muscle fibers we observe result from the activation, proliferation and fusion of satellite cells, the myogenic precursors present under the basal lamina of the muscle fibers. Using morphological features and molecular biomarkers, we show that severely atrophic muscle fibers, with a peculiar cluster reorganization of myonuclei, are present in rodent muscle seven-months after neurectomy and in human muscles 30-months after complete Conus-Cauda Equina Syndrome and that these are structurally distinct from early myotubes. Beyond reviewing evidence from rodent and human studies, we add some ultrastructural evidence of muscle fiber regeneration in long-term denervated human muscles and discuss the options to substantially increase the regenerative potential of severely denervated human muscles not having been treated with h-bFES. Some of the mandatory procedures, are ready to be translated from animal experiments to clinical studies to meet the needs of persons with long-term irreversible muscle denervation. An European Project, the trial Rise4EU (Rise for You, a personalized treatment for recovery of function of denervated muscle in long-term stable SCI) will hopefully follow.
authors.affiliation: Department of Neurorehabilitation, Foundation San Camillo Hospital , I.R.C.C.S., Venice, Italy.
authors.forename: Ugo
authors.initials: U
authors.lastname: Carraro
authors.affiliation: CeSI, Center for Research on Aging, Department of Neuroscience, Imaging and Clinical Sciences, University G. d'Annunzio of Chieti , Italy.
authors.forename: Simona
authors.initials: S
authors.lastname: Boncompagni
authors.affiliation: C.N.R. Institute of Neuroscience, Department of Biomedical Science, University of Padova , Italy.
authors.forename: Valerio
authors.initials: V
authors.lastname: Gobbo
authors.affiliation: Translational Myology, Interdepartmental Research Center of Myology of the University of Padova CIR-Myo, Department of Biomedical Science, Padova, Italy; Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Vienna, Austria.
authors.forename: Katia
authors.initials: K
authors.lastname: Rossini
authors.affiliation: Translational Myology, Interdepartmental Research Center of Myology of the University of Padova CIR-Myo, Department of Biomedical Science, Padova, Italy; Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Vienna, Austria.
authors.forename: Sandra
authors.initials: S
authors.lastname: Zampieri
authors.affiliation: Translational Myology, Interdepartmental Research Center of Myology of the University of Padova CIR-Myo, Department of Biomedical Science, Padova, Italy; Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Vienna, Austria.
authors.forename: Simone
authors.initials: S
authors.lastname: Mosole
authors.affiliation: Translational Myology, Interdepartmental Research Center of Myology of the University of Padova CIR-Myo, Department of Biomedical Science, Padova, Italy; Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Vienna, Austria.
authors.forename: Barbara
authors.initials: B
authors.lastname: Ravara
authors.affiliation: Translational Myology, Interdepartmental Research Center of Myology of the University of Padova CIR-Myo , Department of Biomedical Science, Padova, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Nori
authors.affiliation: CIR-Myo, Department of Medicine, Radiology Unit, University of Padova , Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Stramare
authors.affiliation: Antalgic Laboratory, Department of Medicine, University of Padova , Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Ambrosio
authors.affiliation: Department of Neurorehabilitation, Foundation San Camillo Hospital , I.R.C.C.S., Venice, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Piccione
authors.affiliation: CIR-Myo, Department of Neuroscience, Rehabilitation Unit, University of Padova , Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Masiero
authors.affiliation: CIR-Myo, Department of Neuroscience, Plastic Surgery Unit, University of Padova , Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Vindigni
authors.affiliation: Department of Science, Education, Innovation, Landspitali University Hospital , Reykjavik, Iceland.
authors.forename: Paolo
authors.initials: P
authors.lastname: Gargiulo
authors.affiliation: CeSI, Center for Research on Aging, Department of Neuroscience, Imaging and Clinical Sciences, University G. d'Annunzio of Chieti , Italy.
authors.forename: Feliciano
authors.initials: F
authors.lastname: Protasi
authors.affiliation: Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Vienna, Austria; Department of Physical Medicine, Wilhelminenspital, Vienna, Austria.
authors.forename: Helmut
authors.initials: H
authors.lastname: Kern
authors.affiliation: Anatomy Department, Southern Illinois University , School of Medicine, Carbondale, Illinois, USA.
authors.forename: Amber
authors.initials: A
authors.lastname: Pond
authors.affiliation: Department of Neurorehabilitation, Foundation San Camillo Hospital , I.R.C.C.S., Venice, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Marcante
keywords: Conus-Cauda Equina syndrome; Home-based functional electrical stimulation (h-b FES); Human muscle; Muscle fiber regeneration; Myogenic stem cells; Permanent muscle denervation; Recovery of tetanic contractility; Severe atrophy and nuclear clumpings; Spinal cord injury