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Vertebrate-Like Regeneration in the Invertebrate Chordate Amphioxus

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Vertebrate-Like Regeneration in the Invertebrate Chordate Amphioxus Vertebrate-like regeneration in the invertebrate chordate amphioxus Ildikó M. L. Somorjaia,b,1,2, Rajmund L. Somorjaic, Jordi Garcia-Fernàndezb,1, and Hector Escrivàa,1 aObservatoire Océanologique de Banyuls-sur-Mer, Centre National de la Recherche Scientifique and Université Pierre et Marie Curie Paris VI, F-66650 Banyus-sur-Mer, France; bDepartament de Genètica, Facultat de Biologia, Universitat de Barcelona, 08028 Barcelona, Spain; and cNational Research Council of Canada, Institute for Biodiagnostics, Winnipeg, MB, R3B 1Y6, Canada Edited* by Sean B. Carroll, University of Wisconsin, Madison, WI, and approved November 23, 2011 (received for review January 5, 2011) An important question in biology is why some animals are able to mechanisms in chordates at the invertebrate–vertebrate transi- regenerate, whereas others are not. The basal chordate amphioxus tion. It has been reported that cephalochordates regenerate is uniquely positioned to address the evolution of regeneration. major axial structures (1–3); however, although there is some We report here the high regeneration potential of the European morphological evidence for wound healing and tail regeneration amphioxus Branchiostoma lanceolatum. Adults regenerate both (10–13), no systematic study has been undertaken, and neither anterior and posterior structures, including neural tube, notochord, molecular nor comparative data exist. fin, and muscle. Development of a classifier based on tail regener- Here, we perform a comprehensive study of regeneration in the ation profiles predicts the assignment of young and old adults to European amphioxus B. lanceolatum. Notably, we show a re- their own class with >94% accuracy. The process involves loss of markably high capacity to regenerate not only posterior but also differentiated characteristics, formation of an msx-expressing blas- anterior structures. Tail regeneration seems to be epimorphic and tema, and neurogenesis. Moreover, regeneration is linked to the involve the loss of differentiated characteristics. Moreover, activation of satellite-like Pax3/7 progenitor cells, the extent of reexpression of embryonic markers in the blastema and de novo which declines with size and age. Our results provide a framework neurogenesis occur. At the population level, we show that young for understanding the evolution and diversity of regeneration and old adults can be distinguished based on early tail re- fi mechanisms in vertebrates. generation pro les. Moreover, we show that blastema formation involves the activation of proliferative Pax3/7+ satellite-like pro- invertebrate chordate-vertebrate transition | stem cells | cephalochordate genitor cells, the anterior expression of which declines with size and age. These data support the hypothesis that many compo- nents central to the regeneration process in vertebrates were al- egeneration, as an evolutionary trait, is distributed widely and ready present in the ancestral invertebrate chordate. Rnonuniformly across the Metazoa; it is also highly variable in fi quality and structural speci city. Recently, there has been a re- Results surgence of interest in the evolutionary distribution and basis of regeneration (1). A question of general interest is why certain Amphioxus Adults Regenerate Anterior as well as Posterior Structures. We first examined the putative regenerative ability of adult am- lineages have lost or reduced regenerative capacity relative B. lanceolatum to their regeneration-competent sister taxa. In particular, the phioxus after postanal tail amputation. This start- properties of CNS regeneration in more basal vertebrates may ing point was considered appropriate, because bilaterians with shed light on the reasons for reduced capacity in mammals and regenerative capabilities typically show posterior regenerative birds (2). Despite the plethora of hypotheses that attempt to ex- ability; also, it has been argued that natural selection will favor plain the evolutionary significance of regenerative ability, the regeneration only when noncritical structures are amputated (14). unresolved central issue is whether the ability to regenerate is We found that, under sterile conditions, 100% of tails healed and adaptive or simply a byproduct of selection on other metabolic or formed a blastema-like structure. We then amputated amphioxus – developmental processes. at various levels along the anterior posterior axis to determine the B Studies across phyla indicate that there is broad conservation of limits of their regenerative abilities (Fig. 1 ). Amphioxus are able the developmental signaling pathways involved in regeneration to regenerate the most anterior and most posterior structures with (3), but the signals that initiate regeneration on injury and the high fidelity, particularly the anterior notochord, neural elements downstream targets that they induce have proven more elusive. and fin, buccal cilia, and all structures of the tail (Fig. 1, boxes 1–7 Moreover, the historical distinction between invertebrate- and and Fig. S1). Mortality is higher and regenerative ability is lower vertebrate-type regeneration can make comparisons difficult. In with proximity of the amputation to the pharynx; damage to the planaria and Hydra, for example, the contribution of pluripotent branchial basket or hepatic diverticulum produced defects from stem cells has been the predominant focus of regeneration re- wound healing without regeneration to death. Because we are search. In contrast, among vertebrate models, studies have interested in the evolution of chordate characters, we focused highlighted dedifferentiation of existing structures or the role of exclusively on postanal tail regeneration, which was most easily muscle satellite cells (3). Bridging the gap, ambulacrarian deu- reproducible. terostomes and urochordates show considerable regenerative capacity (1, 4–6) (Fig. 1A). However, their anatomy is not readily comparable with the vertebrate body plan, making it difficult to Author contributions: I.M.L.S. and R.L.S. designed research; I.M.L.S. and R.L.S. performed draw conclusions from such derived phyla. The origins and evo- research; I.M.L.S., R.L.S., J.G.-F., and H.E. contributed new reagents/analytic tools; I.M.L.S. lution of chordate regeneration mechanisms are, therefore, still and R.L.S. analyzed data; and I.M.L.S., R.L.S., J.G.-F., and H.E. wrote the paper. unresolved. The authors declare no conflict of interest. The cephalochordate amphioxus, the most basal living chor- *This Direct Submission article had a prearranged editor. date (7), possesses many ancestral anatomical characters, where- Freely available online through the PNAS open access option. fi as many derived vertebrate features, such as bona de neural 1To whom correspondence may be addressed. E-mail: [email protected], jordigarcia@ crest, are absent (8). Sequencing the Branchiostoma floridae ge- ub.edu, or [email protected]. nome has uncovered a prototypical genomic architecture and 2Present address: Centre for Organismal Studies (COS), University of Heidelberg, 69120 gene complement devoid of many of the duplicated genes char- Heidelberg, Germany. EVOLUTION acteristic of vertebrate genomes (9). Amphioxus is, therefore, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ideal to address questions about the evolution of regenerative 1073/pnas.1100045109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1100045109 PNAS | January 10, 2012 | vol. 109 | no. 2 | 517–522 Downloaded by guest on October 2, 2021 2 and 7 d postamputation (dpa), formation of a blister-like swell- ing by 10 dpa, and finally, formation of a cone-shaped blastema bud as early as 14 dpa, often with an identifiable neural ampulla (Fig. 2A). The muscle fibers in the transected myomeres pro- gressively degrade, although not uniformly, leaving gaps anterior to the amputation plane. Over the next week (up to 21 dpa), elongation of the neural tube and notochord occurs, and the fin begins to extend to the tip of the forming regenerate. Starting at week 4, the regenerate fills out, continuing to elongate and flatten laterally. The holes in the muscle begin to repair them- selves, and the shape of the new notochord becomes more reg- ular. At 6 wk, new muscle fibers and notochord lamellae can be observed with the naked eye; by 15 wk, albeit smaller in size, the regenerate is virtually indistinguishable from the original. By 10 dpa, histology reveals accumulation of connective tissue and mesenchymal cells beneath the wound epithelium (Fig. 2B) and muscle fiber fragmentation at the amputation plane. After a clear blastema forms, the neural ampulla can also be identified, and the notochord stump is filled with flattened, stellate-shaped, and apparently undifferentiated cells (week 2). By week 3, de- differentiation of the notochord, here defined as loss of differ- entiated characters (loss of coin-stack organization, change to a mesenchymal cell shape, and disconnection of lamellae from interior of sheath), has proceeded a considerable distance (≥100 μm) anterior to the amputation plane. The new neural tube shows distinct anterior–posterior patterning, which is similar to the original neural tube (Fig. S2). After the first two phases of debris clearing/wound healing and dedifferentiating have oc- curred, tail elongation slows, and repatterning begins. By 6 wk, undifferentiated notochord cells realign distally to form lamellae, and mature muscle fibers can be identified in the regenerate (Fig. 2 B and C and Fig. S3). These morphological
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