Stability and plasticity of positional memory during limb regeneration in Ambystoma mexicanum

Molecular basis of positional memory in limb regeneration

The amputation of a salamander limb triggers anterior and posterior connective tissue cells to form distinct signalling centres that together fuel regeneration1. Anterior and posterior identities are established during development and are thought to persist for the whole life in the form of positional memory2. However, the molecular basis of positional memory and whether positional memory can be altered remain unknown. Here, we identify a positive-feedback loop that is responsible for posterior identity in the limb of an axolotl (Ambystoma mexicanum). Posterior cells express residual Hand2 transcription factor from development, and this primes them to form a Shh signalling centre after limb amputation. During regeneration, Shh signalling is also upstream of Hand2 expression. After regeneration, Shh is shut down but Hand2 is sustained, safeguarding posterior memory. We used this regeneration circuitry to convert anterior cells to a posterior-cell memory state. Transient exposure of anterior cells to Shh during regeneration kick-started an ectopic Hand2-Shh loop, leading to stable Hand2 expression and lasting competence to express Shh. Our results implicate positive-feedback in the stability of positional memory and reveal that positional memory is reprogrammed more easily in one direction (anterior to posterior) than in the other. Modifying positional memory in regenerative cells changes their signalling outputs, which has implications for tissue engineering.

Neural regulation of H3K27me3 during the induction of patterning competency in regenerating Axolotl limb cells

Limb regeneration in the Mexican axolotl relies on the dedifferentiation of mature limb cells into blastema cells, which gain the ability to respond to patterning signals that guide tissue regeneration. While limb nerves are essential to make the blastema cells competent to pattern, the mechanisms remain unclear due to the complex and overlapping signals in amputated limbs. To overcome this challenge, we developed the Competency Accessory Limb Model (CALM), a simplified limb regeneration assay to study the induction and maintenance of patterning competency. Using CALM, here we show specific temporal windows during which cells acquire competency and associate this state with distinct H3K27me3 chromatin signatures. Furthermore, a combination of FGF and BMP signaling is sufficient to induce patterning competency in limb wound cells, and the ErBB signaling pathway is a downstream epigenetic target of these signals. These findings offer new insights into the molecular regulation of regenerative patterning.

Optimized toolkit for the manipulation of immortalized axolotl fibroblasts

The axolotl salamander model has broad utility for regeneration studies, but this model is limited by a lack of efficient cell-culture-based tools. The Axolotl Limb-1 (AL-1) fibroblast line, the only available immortalized axolotl cell line, was first published over 20 years ago, but many established molecular biology techniques, such as lipofectamine transfection, CRISPR-Cas9 mutagenesis, and antibiotic selection, work poorly or remain untested in AL-1 cells. Innovating technologies to manipulate AL-1 cells in culture and study their behavior following transplantation into the axolotl will complement in-vivo studies, decrease the number of animals used, and enable the faster, more streamlined investigation of regenerative biology questions. Here, we establish transfection, mutagenesis, antibiotic selection, and in-vivo transplantation techniques in axolotl AL-1 cells. These techniques will enable efficient culture with AL-1 cells and guide future tool development for the culture and manipulation of other salamander cell lines.

Allometry in limb regeneration and scale-invariant patterning as the basis of normal morphogenesis from different sizes of blastemas

Axolotl (Ambystoma mexicanum) limb regeneration begins with blastemas of various sizes, in contrast to the limb developmental process. Despite this size variation, normal limb morphology, consistent with a limb stump size, is regenerated. This outcome suggests the existence of underlying scale-invariant mechanisms. To identify such mechanisms, we examined the allometric relationships between blastema size, and Sonic Hedgehog (Shh) and Fibroblast Growth Factor 8 (Fgf8) expression patterns against limb stump size. We found that all factors showed allometric rather than isometric scaling; specifically, their relative sizes decrease with an increase in limb stump size. However, the ratio of Shh/Fgf8 signaling dominant region was nearly constant, independent of blastema/body size. Furthermore, the relative spatial patterns of cell density and proliferation activity, and the relative position of first digit formation were scale invariant in the summed Shh/Fgf8 crosstalk region. This scale-invariant nature may underlie the morphogenesis of normal limbs from different sizes of blastemas.

Making a new limb out of old cells: exploring endogenous cell reprogramming and its role during limb regeneration

Cellular reprogramming is characterized by the induced dedifferentiation of mature cells into a more plastic and potent state. This process can occur through artificial reprogramming manipulations in the laboratory such as nuclear reprogramming and induced pluripotent stem cell (iPSC) generation, and endogenously in vivo during amphibian limb regeneration. In amphibians such as the Mexican axolotl, a regeneration permissive environment is formed by nerve-dependent signaling in the wounded limb tissue. When exposed to these signals, limb connective tissue cells dedifferentiate into a limb progenitor-like state. This state allows the cells to acquire new pattern information, a property called positional plasticity. Here, we review our current understanding of endogenous reprogramming and why it is important for successful regeneration. We will also explore how naturally induced dedifferentiation and plasticity were leveraged to study how the missing pattern is established in the regenerating limb tissue.

Organ Regeneration Without Relying on Regeneration-Dedicated Stem Cells

The classic conception of tissue regeneration assumed the existence of tissue-proper regeneration stem cells that are set aside during normal tissue development and reserved as stem cells for regeneration. However, modern studies using cell tracing and other approaches have ruled out the presence of regeneration-proper stem cells in most cases in vertebrate tissue regeneration. The only experimentally validated regeneration-dedicated reserve cells are the satellite cells in skeletal muscle (e.g., Michele 2022) (see Sect. 5.2.3 ). Here, we will first discuss examples of large-scale tissue regeneration, liver regeneration in mammals, and lens and limb regeneration in newts. Then, attempts to widen the tissue regeneration capacity in mammals with exogenous transcription factor genes will be reviewed.

How might we build limbs in vitro informed by the modular aspects and tissue-dependency in limb development?

Building limb morphogenesis in vitro would substantially open up avenues for research and applications of appendage development. Recently, advances in stem cell engineering to differentiate desired cell types and produce multicellular structures in vitro have enabled the derivation of limb-like tissues from pluripotent stem cells. However, in vitro recapitulation of limb morphogenesis is yet to be achieved. To formulate a method of building limbs in vitro, it is critically important to understand developmental mechanisms, especially the modularity and the dependency of limb development on the external tissues, as those would help us to postulate what can be self-organized and what needs to be externally manipulated when reconstructing limb development in vitro. Although limbs are formed on the designated limb field on the flank of embryo in the normal developmental context, limbs can also be regenerated on the amputated stump in some animals and experimentally induced at ectopic locations, which highlights the modular aspects of limb morphogenesis. The forelimb-hindlimb identity and the dorsal-ventral, proximal-distal, and anterior-posterior axes are initially instructed by the body axis of the embryo, and maintained in the limb domain once established. In contrast, the aspects of dependency on the external tissues are especially underscored by the contribution of incoming tissues, such as muscles, blood vessels, and peripheral nerves, to developing limbs. Together, those developmental mechanisms explain how limb-like tissues could be derived from pluripotent stem cells. Prospectively, the higher complexity of limb morphologies is expected to be recapitulated by introducing the morphogen gradient and the incoming tissues in the culture environment. Those technological developments would dramatically enhance experimental accessibility and manipulability for elucidating the mechanisms of limb morphogenesis and interspecies differences. Furthermore, if human limb development can be modeled, drug development would be benefited by in vitro assessment of prenatal toxicity on congenital limb deficiencies. Ultimately, we might even create a future in which the lost appendage would be recovered by transplanting artificially grown human limbs.

Limb blastema formation: How much do we know at a genetic and epigenetic level?

Regeneration of missing body parts is an incredible ability which is present in a wide number of species. However, this regenerative capability varies among different organisms. Urodeles (salamanders) are able to completely regenerate limbs after amputation through the essential process of blastema formation. The blastema is a collection of relatively undifferentiated progenitor cells that proliferate and repattern to form the internal tissues of a regenerated limb. Understanding blastema formation in salamanders may enable comparative studies with other animals, including mammals, with more limited regenerative abilities and may inspire future therapeutic approaches in humans. This review focuses on the current state of knowledge about how limb blastemas form in salamanders, highlighting both the possible roles of epigenetic controls in this process as well as limitations to scientific understanding that present opportunities for research.

The Accessory Limb Model Regenerative Assay and Its Derivatives

When the Accessory Limb Model (ALM) regenerative assay was first published by Endo, Bryant, and Gardiner in 2004, it provided a robust system for testing the cellular and molecular contributions during each of the basic steps of regeneration: the formation of the wound epithelium, neural induction of the apical epithelial cap, and the formation of a positional disparity between blastema cells. The basic ALM procedure was developed in the axolotl and involves deviating a limb nerve into a lateral wound and grafting skin from the opposing side of the limb axis into the site of injury. In this chapter, we will review the studies that lead to the conception of the ALM, as well as the studies that have followed the development of this assay. We will additionally describe in detail the standard ALM surgery and how to perform this surgery on different limb positions.

Embryonic Tissue and Blastema Transplantations

Embryo grafts have been an experimental pillar in developmental biology, and particularly, in amphibian biology. Grafts have been essential in constructing fate maps of different cell populations and migratory patterns. Likewise, autografts and allografts in older larvae or adult salamanders have been widely used to disentangle mechanisms of regeneration. The combination of transgenesis and grafting has widened even more the application of this technique.In this chapter, we provide a detailed protocol for embryo transplants in the axolotl (Ambystoma mexicanum ). The location and stages to label connective tissue, muscle, or blood vessels in the limb and blood cells in the whole animal. However, the potential of embryo transplants is enormous and impossible to cover in one chapter. Furthermore, we provide a protocol for blastema transplantation as an example of allograft in older larvae.

Lmx1b activation in axolotl limb regeneration

BACKGROUND

Axolotls can regenerate their limbs. In their limb regeneration process, developmental genes are re-expressed and reorganize the developmental axes, in which the position-specific genes are properly re-expressed. However, how such position specificity is reorganized in the regeneration processes has not been clarified. To address this issue, we focused on the reactivation process of Lmx1b, which determines the limb dorsal identity in many animals.

RESULTS

Here, we show that Lmx1b expression is maintained in the dorsal skin before amputation and is activated after amputation. Furthermore, we demonstrate that only cells located in the dorsal side prior to limb amputation could reactivate Lmx1b after limb amputation. We also found that Lmx1b activation was achieved by nerve presence. The nerve factors, BMP2 + FGF2 + FGF8 (B2FF), consistently reactivate Lmx1b when applied to the dorsal skin.

CONCLUSIONS

These results imply that the retained Lmx1b expression in the intact skin plays a role in positional memory, which instruct cells about the spatial positioning before amputation. This memory is reactivated by nerves or nerve factors that can trigger the entire limb regeneration process. Our findings highlight the role of nerves in amphibian limb regeneration, including both the initiation of limb regeneration and the reactivation of position-specific gene expression. This article is protected by copyright. All rights reserved.

The Use of Larval Sea Stars and Sea Urchins in the Discovery of Shared Mechanisms of Metazoan Whole-Body Regeneration

The ability to regenerate is scattered among the metazoan tree of life. Further still, regenerative capacity varies widely within these specific organisms. Numerous organisms, all with different regenerative capabilities, have been studied at length and key similarities and disparities in how regeneration occurs have been identified. In order to get a better grasp on understanding regeneration as a whole, we must search for new models that are capable of extensive regeneration, as well as those that have been under sampled in the literature. As invertebrate deuterostomes, echinoderms fit both of these requirements. Multiple members regenerate various tissue types at all life stages, including examples of whole-body regeneration. Interrogations in two highly studied echinoderms, the sea urchin and the sea star, have provided knowledge of tissue and whole-body regeneration at various life stages. Work has begun to examine regeneration in echinoderm larvae, a potential new system for understanding regenerative mechanisms in a basal deuterostome. Here, we review the ways these two animals' larvae have been utilized as a model of regeneration.