En1 and Lmx1b do not recapitulate embryonic dorsal-ventral limb patterning functions during mouse digit tip regeneration

Abstract

The mouse digit tip is a complex tissue that is capable of regeneration after amputation. How the regenerated digit tip is patterned is unknown, but a long-standing hypothesis in the field of regeneration proposes that developmental patterning mechanisms are re-used during regeneration. The digit tip bone exhibits strong dorsal-ventral (DV) polarity, so we focus on Engrailed 1 (En1) and LIM homeobox transcription factor 1B (Lmx1b), two well-studied transcription factors necessary for DV patterning during limb development. We investigate if En1 and Lmx1b are re-expressed during regeneration in a developmental-like spatially restricted pattern, and if they direct DV morphology of the regenerated digit tip. We find that both En1 and Lmx1b are expressed in the regenerating mouse digit tip epithelium and mesenchyme, respectively, but without DV polarity. We use conditional genetics and quantitative analysis of digit tip bone morphology to determine that genetic deletion of En1 or Lmx1b in adult digit tip regeneration modestly reduces bone regeneration but does not affect DV patterning of the regenerate. Collectively, our data suggest that while En1 and Lmx1b are re-expressed during mouse digit tip regeneration, they do not define the DV axis during regeneration.

Failure of digit tip regeneration in the absence of Lmx1b suggests Lmx1b functions disparate from dorsoventral polarity

“…In summary, we show that double ventral digits with no Lmx1b expression do not elicit a regenerative response, but that very likely this is not due to the lack of DV polarity as doubledorsal digits do regenerate. Similar findings have been recently reported using conditional knockouts of Lmx1b and En1 (Johnson et al 2022). Even considering that the lack of a fully developed nail organ might impair the regenerative outcome, our results suggest that the expression of Lmx1b , which normally persists in the connective tissue underlying the nail through adulthood, may be required in the onychodermis to promote blastemal progression to regenerate the digit tip.…”