Regeneration of spinal electrocyte fibers in Sternarchus albifrons: development of axon-Schwann cell relationships and nodes of Ranvier

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

Biology is replete with examples of regeneration, the process that allows animals to replace or repair cells, tissues and organs. As on land, vertebrates in aquatic environments experience the occurrence of injury with varying frequency and to different degrees. Studies demonstrate that ray-finned fishes possess a very high capacity to regenerate different tissues and organs when they are adults. Among fishes that exhibit robust regenerative capacities are the neotropical electric fishes of South America (Teleostei: Gymnotiformes). Specifically, adult gymnotiform electric fishes can regenerate injured brain and spinal cord tissues and restore amputated body parts repeatedly. We have begun to identify some aspects of the cellular and molecular mechanisms of tail regeneration in the weakly electric fish Sternopygus macrurus (long-tailed knifefish) with a focus on regeneration of skeletal muscle and the muscle-derived electric organ. Application of in vivo microinjection techniques and generation of myogenic stem cell markers are beginning to overcome some of the challenges owing to the limitations of working with non-genetic animal models with extensive regenerative capacity. This review highlights some aspects of tail regeneration in S. macrurus and discusses the advantages of using gymnotiform electric fishes to investigate the cellular and molecular mechanisms that produce new cells during regeneration in adult vertebrates.

Teleost fish as a model system to study successful regeneration of the central nervous system

Traumatic brain injury and spinal cord injury are devastating conditions that may result in death or long-term disability. A promising strategy for the development of effective cell replacement therapies involves the study of regeneration-competent organisms. Among this group, teleost fish are distinguished by their excellent potential to regenerate nervous tissue and to regain function after injury to the central nervous system. In this chapter, we summarize our current understanding of the cellular processes that mediate this regenerative potential, and we show that several of these processes are shared with the normal development of the intact central nervous system; we describe how the spontaneous self-repair of the teleostean central nervous system leads to functional recovery, at physiological and behavioral levels; we discuss the possible function of molecular factors associated with the degenerative and regenerative processes after injury; and, finally, we speculate on evolutionary aspects of adult neurogenesis and neuronal regeneration, and on how a better understanding of these aspects could catalyze the development of therapeutic strategies to overcome the regenerative limits of the mammalian CNS.

Spinal cord repair in regeneration-competent vertebrates: adult teleost fish as a model system

Spinal cord injuries in mammals, including humans, have devastating long-term consequences. Despite substantial research, therapeutic approaches developed in mammalian model systems have had limited success to date. An alternative strategy in the search for treatment of spinal cord lesions is provided by regeneration-competent vertebrates. These organisms, which include fish, urodele amphibians, and certain reptiles, have a spinal cord very similar in structure to that of mammals, but are capable of spontaneous structural and functional recovery after spinal cord injury. The present review aims to provide an overview of the current status of our knowledge of spinal cord regeneration in one of these groups, teleost fish. The findings are discussed from a comparative perspective, with reference to other taxa of regeneration-competent vertebrates, as well as to mammals.

Developing nodes of Ranvier are defined by ankyrin-G clustering and are independent of paranodal axoglial adhesion

Nodes of Ranvier are excitable regions of axonal membranes highly enriched in voltage-gated sodium channels that propagate action potentials. The mechanism of protein clustering at nodes has been a source of controversy. In this study, developmental analysis of nodes of Ranvier in optic nerve axons reveals that early node intermediates are defined by ankyrin-G. Other node components, including beta IV spectrin, voltage-gated sodium channels, and the L1 cell adhesion molecule neurofascin, are subsequently recruited to sites of ankyrin-G clustering. The role of intact paranodes in protein clustering was examined in the dysmyelinating mouse mutant jimpy. Jimpy mice do not have intact paranodal axoglial contacts, which is indicated by a complete lack of neurexin/contactin-associated protein/paranodin clustering in paranodes. In the absence of intact paranodes, ankyrin-G was still able to cluster, although fewer ankyrin clusters were seen in jimpy optic nerves than in wild-type optic nerves. Recruitment of Na(v)1.2, Na(v)1.6, beta IV spectrin, and neurofascin to sites of ankyrin-G clustering is unimpaired in jimpy mice, indicating that node formation occurs independent of intact paranodal axoglial contacts.

Axon-glia interactions: building a smart nerve fiber

Myelinated nerve fibers are designed in an optimal manner which requires tuning of conduction time with millisecond precision. This involves the highly coordinated differentiation of axons and myelin-forming glial cells; the nature of the signals involved in this axon-glial cell dance are beginning to be elucidated.

Formation of new sensory cells in deafferented tuberous organs of the gymnotid fish Eigenmannia virescens

The cutaneous electroreceptor "tuberous organs" of the lateral line system of Eigenmannia virescens were studied at light and electron microscopic levels with immunohistochemical and autoradiographic techniques after sectioning of the posterior branch of the lateral line nerve. After deafferentation total degeneration of the sensory cells was observed. The accessory cells of the basal platform, however, remain intact and undergo a process of differentiation. The cytoplasms and nuclei of these cells increase in volume, and the nuclei incorporate tritiated thymidine. In control tuberous organs with intact innervation, tritiated thymidine is absorbed by the nuclei of the elongated epidermal cells surrounding the sensory cavity. The newly formed sensory cells, but not those of the intact organs, are substance P immunoreactive. They have synaptic bars surrounded by vesicles and their free membrane surface is covered with microvilli. The new sensory cells are fully differentiated 35 days after the lateral line was cut. These results demonstrate that in the tuberous organs of E. virescens new sensory cells are formed in the absence of an innervation.

Neurogenesis in adult vertebrate spinal cord in situ and in vitro: a new model system

Phylogenetically "lower" species in some cases use different biological strategies for recovery after injury to the CNS than do "higher" species. One approach that we have taken in our laboratory has been to study the mechanisms of functional recovery of the CNS after injury in those vertebrate species where recovery does occur. The present report reviews recent studies on a model system, the spinal electromotor system of the gymnotiform teleost Sternarchus albifrons, which exhibits regeneration and neurogenesis after injury. Regeneration in this system leads to a recapitulation of relatively normal morphologic structure by the damaged or extirpated spinal cord. In Sternarchus, new spinal cord is generated from ependymal cells; some ependymal cells in the adult remain pluripotent and retain the capability to generate new neurons. The Sternarchus spinal cord thus represents an especially useful model for the study of neurogenesis after injury to the CNS. Recent studies in our laboratory indicate that neurogenesis in adult Sternarchus spinal cord tissue occurs both in vivo and in vitro. Neurogenesis has been demonstrated by incorporation of tritiated thymidine into explant cultures from the spinal cord of adult Sternarchus. Autoradiography reveals the presence of thymidine-labeled neurons. Neuronal identity of 3H-labeled cells has been confirmed by positive staining with neuron-specific monoclonal antibodies. Thymidine labeling occurs in cultured neurons derived from both normal (histologically and functionally mature) and regenerating spinal cord of adult Sternarchus albifrons. These results provide evidence that some cells in spinal cord of adult Sternarchus retain the ability to incorporate thymidine and undergo neuronal differentiation in vitro. This system provides a new model in which neurogenesis from adult tissue can be studied in vivo and in vitro.

Cell death of asynaptic neurons in regenerating spinal cord

The weakly electric fish Sternarchus albifrons possesses a unique class of asynaptic neurons, the electromotor neurons, whose axons constitute the electric organ. The cell bodies of origin of the electrocyte axons are located in the spinal cord. Both spinal cord and electromotor neurons ( electrocytes ) regenerate after amputation of the tail. Sternarchus spinal cords which have regenerated for 1 or more years show a progression in number of perikarya of electromotor neurons along the rostro-caudal axis. The most recently regenerated region of the cord is at the caudal end, which consists of a tube of ependyma. Progressing rostrally along regenerated spinal cord from the caudal end, numerous cells are generated and large numbers of electromotor neurons differentiate. The maximum number of electromotor neurons per transverse section of regenerated cord is five times higher than in normal mature cord. Rostral to this, the number of electromotor neurons decreases gradually to the normal number near the transition zone (the border with unregenerated cord). As the more rostral regenerated cord has presumably had a longer period of regeneration, we conclude that excess numbers of electromotor neurons are generated initially, and that subsequently the number of these neurons is decreased by cell death. This conclusion is supported by the fact that younger regenerates (2-4 months) have larger-than-normal numbers of perikarya of electromotor neurons extending up to the transition zone (Anderson and Waxman , 1981). No evidence of migration or depletion of electromotor neurons from unregenerated cord rostral to the amputation has been observed. Since the axons of the electromotor neurons in Sternarchus do not normally form any synapses, this study provides evidence that factors other than synaptic competition must be responsible for determining cell death during regeneration of these spinal neurons.