Co-operative Bmp- and Fgf-signaling inputs convert skin wound healing to limb formation in urodele amphibians

Abstract

Urodele amphibians have remarkable organ regeneration capability, and their limb regeneration capability has been investigated as a representative phenomenon. In the early 19th century, nerves were reported to be an essential tissue for the successful induction of limb regeneration. Nerve substances that function in the induction of limb regeneration responses have long been sought. A new experimental system called the accessory limb model (ALM) has been established to identify the nerve factors. Skin wounding in urodele amphibians results in skin wound healing but never in limb induction. However, nerve deviation to the wounded skin induces limb formation in ALM. Thus, nerves can be considered to have the ability to transform skin wound healing to limb formation. In the present study, co-operative Bmp and Fgf application, instead of nerve deviation, to wounded skin transformed skin wound healing to limb formation in two urodele amphibians, axolotl (Ambystoma mexicanum) and newt (Pleurodeles waltl). Our findings demonstrate that defined factors can induce homeotic transformation in postembryonic bodies of urodele amphibians. The combination of Bmp and Fgf(s) may contribute to the development of novel treatments for organ regeneration.

Mechanisms of urodele limb regeneration

“…Both are expressed in DRG neurons and are detectable in peripheral limb nerve axons in vivo (Satoh, Makanae, Nishimoto, & Mitogawa, 2016). Furthermore, they can substitute for the nerve in the outgrowth of a supernumerary axolotl limb blastema in the Lheureux model (Makanae, Mitogawa, & Satoh, 2014a). …”

Looking Ahead to Engineering Epimorphic Regeneration of a Human Digit or Limb

“…This cell population expresses both transcription factors involved in maintaining an undifferentiated phenotype (Msx1 and Msx2 46,52,53 ) and markers of mesenchymal cells in developing limbs (Prrx1 46,54 ). In addition, genes expressed include those involved in developing limb patterning (tbx5 55 , hoxa-9, and -13 54,56 ) and key factors for initiation, outgrowth, and patterning during limb regeneration (fgfs 57,58 ).…”

“…39 After denervation, however, the regenerative process can be rescued by the exogenous delivery of FGF and bone morphogenetic protein (BMP) cytokines. 53 Nerves also secrete other growth factors that are relevant for regeneration: for example, keratinocyte growth factor upregulates the expression of the epidermal embryonic marker sp9 in the AEC, 38 FGF2 promotes MMP expression in surrounding fibroblasts and keratinocytes, 65 and newt anterior gradient has been suggested to be involved in the proximaldistal patterning process during the regrowth stage. 77 Thus, together the blastema and the surrounding tissues interact to generate a permissive microenvironment that ultimately allows for tissue regrowth.…”

The Axolotl Limb Regeneration Model as a Discovery Tool for Engineering the Stem Cell Niche

“…This later step is dependent on signals from the nerve that interact with the newly formed wound epithelium to induce formation of a signaling center that recruits mesenchymal cells from the adjacent connective tissue (see [5,21,22]. This step that is dependent on nerve-associated signals in vivo can be regulated by the therapeutic delivery of a defined cocktail of growth factors [23-25]. In so doing, a deviated nerve is no longer required, and the downstream pathways (next steps in the regeneration cascade) leading to blastema formation can be targeted directly.…”

“…human recombinant growth factors induce ectopic blastema formation in the axolotl [23,25]). The lack of a regenerative response in humans may be a consequence of a failure to produce the necessary signals at the right time or place, but it is unlikely that there is a problem in the underlying pathways themselves.…”