Retinoic Acid Receptors and the Control of Positional Information in the Regenerating Axolotl Limb.

Trey Polvadore,Malcolm Maden

doi:10.3390/cells10092174

Abstract

We know little about the control of positional information (PI) during axolotl limb regeneration, which ensures that the limb regenerates exactly what was amputated, and the work reported here investigates this phenomenon. Retinoic acid administration changes the PI in a proximal direction so that a complete limb can be regenerated from a hand. Rather than identifying all the genes altered by RA treatment of the limb, we have eliminated many off-target effects by using retinoic acid receptor selective agonists. We firstly identify the receptor involved in this respecification process as RARα and secondly, identify the genes involved by RNA sequencing of the RARα-treated blastemal mesenchyme. We find 1177 upregulated genes and 1403 downregulated genes, which could be identified using the axolotl genome. These include several genes known to be involved in retinoic acid metabolism and in patterning. Since positional information is thought to be a property of the cell surface of blastemal cells when we examine our dataset with an emphasis on this aspect, we find the top canonical pathway is integrin signaling. In the extracellular matrix compartment, we find a MMP and several collagens are upregulated; several cell membrane genes and secretory factors are also upregulated. This provides data for future testing of the function of these candidates in the control of PI during limb regeneration.

Highlights

Unlike development, where concentration gradients of extracellular morphogens are considered to be the means by which positional information is encoded, in regeneration, the cell surface of connective tissue blastemal cells has become the focus of attention
Sequence similarities between human and axolotl retinoic acid receptors (RARs) are high
Much work will be required to test the function of the individual genes we have identified above in controlling positional information, their pathways from the genome to the cell surface and extracellular matrix (ECM) and how these molecules interact, but our results described here provide continuing supporting for the concept that in regeneration, the cell surface is the seat of positional information recognition

Summary

Introduction

Regardless of where the amputation occurs along the proximal–distal (PD; shoulder to hand) limb axis, only the missing tissue is regrown. This indicates that cells in the limb blastema, a conical assembly of proliferating cells that will form the regenerated tissue, are somehow aware of their position along the PD axis. This property is reflected in the rule of distal transformation, which states that regeneration can only occur in the proximal-to-distal direction, as blastema cells can only create cells with an identity more distal than their own.

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Discussion

Conclusion