Neural regeneration is successfully achieved in vertebrate models such as zebrafish and some amphibians. Among non-chordate Deuterostomes, echinoderms can efficiently regenerate their nervous system. A recent paper from the laboratory of José E. García-Arrarás (http://www.ncbi.nlm.nih.gov/pubmed/23597108) describes the important role of radial glial cells in this process. Studying how similarly zebrafish and amphibians regenerate their nervous system compared to the non-chordate echinoderms may provide insights into: i) up to what extend the cellular and molecular mechanisms and events during neural regeneration have been evolutionary conserved among Deuterostomes, and ii) trying to improve the poor regenerative abilities of the mammalian CNS.
Echinoderm and chordate radial glia are similar morphologically and immunocytochemically. In adult echinoderms, radial glial cells retain their proliferative capabilities suggesting a role in neurogenesis. The authors here use sea cucumbers as a model. These animals possess 5 radial nerve cords (RNC) that are joined at the oral side of the body. Each RNC is formed by two bands, the ectoneural and the hyponeural neuroepithelia separated by a thin layer of connective tissue. These neuroepithelia are formed by radial glial cells and interspersed neurons. In their paradigm the authors cut one of the RNC at the mid-body level and analyze how both truncated ends regenerate and get re-connected.
Thus, RNC regeneration is divided into 4 stages: 1) early post-injury phases (days 1-2), 2) late-post injury phase (days 6-8), 3) growth phase (days 8-12) and 4) late regenerate (+21 days). During the early phase the radial glial cells close to the wound start dedifferentiating. These cells lose their basal processes while their cell bodies maintain the epithelial organization in the apical region. During the late phase, the coelomic epithelium migrates over the injury site sealing the wound. Also, at this stage, there is an expansion of the region of the neuroepithelium where the radial glial cells dedifferentiate. Those dedifferentiated radial glial cells do not only show changes in morphology but also show a much-reduced expression of a typical material that is recognized by antibodies against Reissner’s substance (RS), which is typically produced by glial cells also in vertebrates. Next, during the growth phase, the two truncated ends of the RNC at each side of the wound start growing towards each other. The ectoneural and hyponeural bands of the RNC form separate tubular rudiments that grow parallel to each other. Finally, in the late regenerate phase the continuity of the RNC is restored. New radial glial cells re-differentiate adopting their typical morphology and producing again RS-like material. Also, this new neural cord gets populated with neuronal cell bodies and processes.
Then, the authors combined BrdU pulse-chase experiments with labelling with radial glia specific markers. In adult non-regenerating CNS of sea cucumbers a basal level of BrdU incorporation had been previously reported. Now, after injury, no significant increase in BrdU incorporation takes place during the early phase. However, at the late post-injury phase there is a significant increase mainly in the ectoneural part of the RNC. During the growth phase this increase in BrdU-positive cells peaks in both ectoneural and hyponeural cords. From this stage the levels start decreasing until reaching again typical basal levels in the late regenerate phase. Remarkably, most of the BrdU positive cells (90-100%) are positive also for a radial glia-specific marker, indicating the dominant role of radial glia in cell division during RNC regeneration. It is also worthy to mention that whereas in the intact CNS the BrdU-positive cells are randomly scattered during neural regeneration most of the BrdU-positive cells localize in the dedifferentiated glia at the tip of the regenerate.
Finally, the authors show that at the very late regenerate phase (day 51 post-injury) 45% of the BrdU-positive cells are not labelled with the specific radial glia-specific marker and that some of these cells express now different neuronal markers. This suggests that a percentage of the progeny of radial glia differentiate into neurons.
In summary, this paper reports how radial glia is involved not only in mediating the bridging of the wound gap and axonal outgrowth but also acts as a source of new neurons during RNC regeneration in sea cucumber. This leading role for the radial glia resembles very much the role of the chordate radial glia in those regeneration-competent species, which opens the possibility that these conserved mechanisms for neural regeneration are also present in non-regenerating vertebrates but need to be somehow awaken.