ECM-mediated positional cues are able to induce pattern, but not new positional information, during axolotl limb regeneration

Integration failure of regenerated limb tissue is associated with incongruencies in positional information in the Mexican axolotl

Introduction: Little is known about how the newly regenerated limb tissues in the Mexican axolotl seamlessly integrate with the remaining stump tissues to form a functional structure, and why this doesn't occur in some regenerative scenarios. In this study, we evaluate the phenomenological and transcriptional characteristics associated with integration failure in ectopic limb structures generated by treating anterior-located ectopic blastemas with Retinoic Acid (RA) and focusing on the "bulbus mass" tissue that forms between the ectopic limb and the host site. We additionally test the hypothesis that the posterior portion of the limb base contains anterior positional identities. Methods: The positional identity of the bulbus mass was evaluated by assaying regenerative competency, the ability to induce new pattern in the Accessory Limb Model (ALM) assay, and by using qRTPCR to quantify the relative expression of patterning genes as the bulbus mass deintegrates from the host site. We additionally use the ALM and qRTPCR to analyze the distribution of anterior and posterior positional identities along the proximal/distal limb axis of uninjured and regenerating limbs. Results: The bulbus mass regenerates limb structures with decreased complexity when amputated and is able to induce complex ectopic limb structure only when grafted into posterior-located ALMs. Expressional analysis shows significant differences in FGF8, BMP2, TBX5, Chrdl1, HoxA9, and HoxA11 expression between the bulbus mass and the host site when deintegration is occuring. Grafts of posterior skin from the distal limb regions into posterior ALMs at the base of the limb induce ectopic limb structures. Proximally-located blastemas express significantly less HoxA13 and Ptch1, and significantly more Alx4 and Grem1 than distally located blastemas. Discussion: These findings show that the bulbus mass has an anterior-limb identity and that the expression of limb patterning genes is mismatched between the bulbus mass and the host limb. Our findings additionally show that anterior positional information is more abundant at the limb base, and that anterior patterning genes are more abundantly expressed in proximally located blastemas compared to blastemas in the more distal regions of the limb. These experiments provide valuable insight into the underlying causes of integration failure and further map the distribution of positional identities in the mature limb.

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

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.

Positional Memory in Vertebrate Regeneration: A Century's Insights from the Salamander Limb

Salamanders, such as axolotls and newts, can regenerate complex tissues including entire limbs. But what mechanisms ensure that an amputated limb regenerates a limb, and not a tail or unpatterned tissue? An important concept in regeneration is positional memory-the notion that adult cells "remember" spatial identities assigned to them during embryogenesis (e.g., "head" or "hand") and use this information to restore the correct body parts after injury. Although positional memory is well documented at a phenomenological level, the underlying cellular and molecular bases are just beginning to be decoded. Herein, we review how major principles in positional memory were established in the salamander limb model, enabling the discovery of positional memory-encoding molecules, and advancing insights into their pattern-forming logic during regeneration. We explore findings in other amphibians, fish, reptiles, and mammals and speculate on conserved aspects of positional memory. We consider the possibility that manipulating positional memory in human cells could represent one route toward improved tissue repair or engineering of patterned tissues for therapeutic purposes.

Epimorphic regeneration of the mouse digit tip is finite

Background

Structural regeneration of amputated appendages by blastema-mediated, epimorphic regeneration is a process whose mechanisms are beginning to be employed for inducing regeneration. While epimorphic regeneration is classically studied in non-amniote vertebrates such as salamanders, mammals also possess a limited ability for epimorphic regeneration, best exemplified by the regeneration of the distal mouse digit tip. A fundamental, but still unresolved question is whether epimorphic regeneration and blastema formation is exhaustible, similar to the finite limits of stem-cell mediated tissue regeneration.

Methods

In this study, distal mouse digits were amputated, allowed to regenerate and then repeatedly amputated. To quantify the extent and patterning of the regenerated digit, the digit bone as the most prominent regenerating element in the mouse digit was followed by in vivo µCT.

Results

Analyses revealed that digit regeneration is indeed progressively attenuated, beginning after the second regeneration cycle, but that the pattern is faithfully restored until the end of the fourth regeneration cycle. Surprisingly, when unamputated digits in the vicinity of repeatedly amputated digits were themselves amputated, these new amputations also exhibited a similarly attenuated regeneration response, suggesting a systemic component to the amputation injury response.

Conclusions

In sum, these data suggest that epimorphic regeneration in mammals is finite and due to the exhaustion of the proliferation and differentiation capacity of the blastema cell source.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02741-2.