Unravelling the limb regeneration mechanisms of Polypedates maculatus, a sub-tropical frog, by transcriptomics

Abstract

Background Regeneration studies help to understand the strategies that replace a lost or damaged organ and provide insights into approaches followed in regenerative medicine and engineering. Amphibians regenerate their limbs effortlessly and are indispensable models to study limb regeneration. Xenopus and axolotl are the key models for studying limb regeneration but recent studies on non-model amphibians have revealed species specific differences in regeneration mechanisms. Results The present study describes the de novo transcriptome of intact limbs and three-day post-amputation blastemas of tadpoles and froglets of the Asian tree frog Polypedates maculatus, a non-model amphibian species commonly found in India. Differential gene expression analysis between early tadpole and froglet limb blastemas discovered species-specific novel regulators of limb regeneration. The present study reports upregulation of proteoglycans, such as epiphycan, chondroadherin, hyaluronan and proteoglycan link protein 1, collagens 2,5,6, 9 and 11, several tumour suppressors and methyltransferases in the P. maculatus tadpole blastemas. Differential gene expression analysis between tadpole and froglet limbs revealed that in addition to the expression of larval-specific haemoglobin and glycoproteins, an upregulation of cysteine and serine protease inhibitors and downregulation of serine proteases, antioxidants, collagenases and inflammatory genes in the tadpole limbs were essential for creating an environment that would support regeneration. Dermal myeloid cells were GAG+, EPYC+, INMT+, LEF1+ and SALL4+ and seemed to migrate from the unamputated regions of the tadpole limb to the blastema. On the other hand, the myeloid cells of the froglet limb blastemas were few and probably contributed to sustained inflammation resulting in healing. Conclusions Studies on non-model amphibians give insights into alternate tactics for limb regeneration which can help devise a plethora of methods in regenerative medicine and engineering.

The salamander blastema within the broader context of metazoan regeneration

“…These abilities are not only lost or diminished in amniotes, but also in frogs, which share a more recent common ancestor with salamanders ( Anderson et al, 2008 ). Frog tadpoles can regenerate limbs but gradually lose this ability over the course of metamorphosis ( Suzuki et al, 2006 ; Simon and Tanaka, 2013 ; Mahapatra et al, 2023 ). Given this observation, it is tempting to assume that axolotl regenerative abilities arise from their neotenic lifestyle, but extensive regenerative capacities are found in all studied post-metamorphic salamanders, including both closely related ambystomids, as well as the most basal salamander groups Cryptobranchidae and Hynobiidae ( Young et al, 1983 ; Griffin, 1995 ; Shen et al, 2013 ; Geng et al, 2015 ).…”