Vertebrate-like regeneration in the invertebrate chordate amphioxus

Abstract

An important question in biology is why some animals are able to regenerate, whereas others are not. The basal chordate amphioxus is uniquely positioned to address the evolution of regeneration. We report here the high regeneration potential of the European amphioxus Branchiostoma lanceolatum. Adults regenerate both anterior and posterior structures, including neural tube, notochord, fin, and muscle. Development of a classifier based on tail regeneration profiles predicts the assignment of young and old adults to their own class with >94% accuracy. The process involves loss of differentiated characteristics, formation of an msx-expressing blastema, and neurogenesis. Moreover, regeneration is linked to the activation of satellite-like Pax3/7 progenitor cells, the extent of which declines with size and age. Our results provide a framework for understanding the evolution and diversity of regeneration mechanisms in vertebrates.invertebrate chordate-vertebrate transition | stem cells | cephalochordate R egeneration, as an evolutionary trait, is distributed widely and nonuniformly across the Metazoa; it is also highly variable in quality and structural specificity. Recently, there has been a resurgence of interest in the evolutionary distribution and basis of regeneration (1). A question of general interest is why certain lineages have lost or reduced regenerative capacity relative to their regeneration-competent sister taxa. In particular, the properties of CNS regeneration in more basal vertebrates may shed light on the reasons for reduced capacity in mammals and birds (2). Despite the plethora of hypotheses that attempt to explain the evolutionary significance of regenerative ability, the unresolved central issue is whether the ability to regenerate is adaptive or simply a byproduct of selection on other metabolic or developmental processes.Studies across phyla indicate that there is broad conservation of the developmental signaling pathways involved in regeneration (3), but the signals that initiate regeneration on injury and the downstream targets that they induce have proven more elusive. Moreover, the historical distinction between invertebrate-and vertebrate-type regeneration can make comparisons difficult. In planaria and Hydra, for example, the contribution of pluripotent stem cells has been the predominant focus of regeneration research. In contrast, among vertebrate models, studies have highlighted dedifferentiation of existing structures or the role of muscle satellite cells (3). Bridging the gap, ambulacrarian deuterostomes 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 draw conclusions from such derived phyla. The origins and evolution of chordate regeneration mechanisms are, therefore, still unresolved.The cephalochordate amphioxus, the most basal living chordate (7), possesses many ancestral anatomical characters, whereas many derived vertebrate features, such as bona fi...

Amphioxus regeneration: evolutionary and biomedical implications

“…Zhang and colleagues (2009) use posterior blastemas in lieu of embryos to successfully make metaphase spreads for chromosome counts, but otherwise do not discuss regeneration in these species. In contrast, the recent, more detailed analyses of B. lanceolatum show that it has considerable tail regeneration capacity (Bone 1992; Pegeta 1992; Somorjai et al, 2012a), even after multiple rounds of amputation (Somorjai et al, 2012b). Interestingly, Bone (1992) states that animals allowed to bury in the gravel or maintained in the dark do not regenerate, in stark contrast to the successful regenerates observed by Pegeta (1992) and Somorjai et al, 2012a, b) under the aforementioned conditions.…”

“…Of the small B. lanceolatum (2.3 cm) amputated anteriorly, wound healing was observed in only a single small adult lancelet, and none at all in larger (2.8 cm) animals (Biberhofer 1906). Recent work confirms that only extreme anterior axial amputations result in any appreciable regeneration, even under prime conditions that promote complete tail regeneration (Somorjai et al, 2012a), suggesting this is not an artefact of environmental or physiological conditions. Exceptions to this are the oral cirri, the non-mineralised skeletal rods surrounding the mouth opening, which regenerate well in both B. japonicum and B. lanceolatum (Kaneto and Wada 2011;Somorjai et al, 2012a).…”

“…Recent work confirms that only extreme anterior axial amputations result in any appreciable regeneration, even under prime conditions that promote complete tail regeneration (Somorjai et al, 2012a), suggesting this is not an artefact of environmental or physiological conditions. Exceptions to this are the oral cirri, the non-mineralised skeletal rods surrounding the mouth opening, which regenerate well in both B. japonicum and B. lanceolatum (Kaneto and Wada 2011;Somorjai et al, 2012a). Data are lacking on anterior regeneration outside Branchiostoma, and no regenerative abilities have been reported for the third cephalochordate genus, Epigonichthys.…”

“…He documents the caudal regenerative ability in this species, indicating a full recovery of all structures including the notochord and nerve cord (Andrews 1893). Direct or indirect evidence of posterior regeneration in Branchiostoma has also been shown in recent years in B. lanceolatum (Bone 1992;Pegeta 1992;Somorjai et al, 2012a), B. platae (Silva et al, 1998), B. japonicum (previously conflated with B. belcheri) and B. belcheri (Zhang et al, 2009). Zhang and colleagues (2009) use posterior blastemas in lieu of embryos to successfully make metaphase spreads for chromosome counts, but otherwise do not discuss regeneration in these species.…”

“…The cellular events occurring during regeneration have been described in most detail during post-anal tail regeneration in the European species, B. lanceolatum (Somorjai et al, 2012a). After posterior amputation, humoral fluids are released, and cellular debris around the amputation plane, including fragments of transected muscle fibres, are sloughed away (stage 0).…”